Mersana Therapeutics, Inc. - Annual Report: 2017 (Form 10-K)

 

 

UNITED STATES

SECURITIES AND EXCHANGE COMMISSION

Washington, D.C. 20549

 

FORM 10-K

(Mark One)

☒ANNUAL REPORT PURSUANT TO SECTION 13 OR 15(d) OF THE SECURITIES EXCHANGE ACT OF 1934

 

For the fiscal year ended December 31, 2017.

 

OR

 

☐TRANSITION REPORT PURSUANT TO SECTION 13 OR 15(d) OF THE SECURITIES EXCHANGE ACT OF 1934

 

For the transition period from                    to                   

 

Commission file number 001-38129

 

Mersana Therapeutics, Inc.

(Exact name of registrant as specified in its charter)

 

 

Registrant’s telephone number, including area code (617) 498-0020

 

Securities registered pursuant to Section 12(b) of the Act:

 

 

Securities registered pursuant to Section 12(g) of the Act:

 

NONE

 

Indicate by check mark if the registrant is a well-known seasoned issuer, as defined in Rule 405 of the Securities Act.Yes  ☐  No  ☒

 

Indicate by check mark if the registrant is not required to file reports pursuant to Section 13 or Section 15(d) of the Act.Yes  ☐  No  ☒

 

Indicate by check mark whether the registrant (1) has filed all reports required to be filed by Section 13 or 15(d) of the Securities Exchange Act of 1934 during the preceding 12 months (or for such shorter period that registrant was required to file such reports), and (2) has been subject to such filing requirements for the past 90 days. Yes  ☒  No  ☐

 

Indicate by check mark whether the registrant has submitted electronically and posted on its corporate Website, if any, every Interactive Data File required to be submitted and posted pursuant to Rule 405 of Regulation S-T (§232.405 of this chapter) during the preceding 12 months (or for such shorter period that the registrant was required to submit and post such files).Yes  ☒  No  ☐

 

Indicate by check mark if disclosure of delinquent filers pursuant to Item 405 of Regulation S-K (§299.405 of this chapter) is not contained herein, and will not be contained, to the best of the registrant’s knowledge, in definitive proxy or information statements incorporated by reference in Part III of this Form 10-K or any amendment to this Form 10-K.☒

 

Indicate by check mark whether the registrant is a large accelerated filer, an accelerated filer, a non-accelerated filer, a smaller reporting company, or an emerging growth company. See the definitions of “large accelerated filer,” “accelerated filer,” “smaller reporting company,” and “emerging growth company” in Rule 12b-2 of the Exchange Act.

 

 

If an emerging growth company, indicate by check mark if the registrant has elected not to use the extended transition period for complying with any new or revised financial accounting standards provided pursuant to Section 13(a) of the Exchange Act.☒

 

Indicate by check mark whether the registrant is a shell company (as defined by Rule 12b-2 of the Exchange Act).Yes  ☐  No  ☒

 

As of June 30, 2017, the last business day of the registrant’s most recently completed second fiscal quarter, the aggregate market value of the registrant’s common stock held by non-affiliates was $19,147,953, based on the last reported sale price of such stock on the NASDAQ Global Select Market as of such date. 

 

As of March 20, 2018, the registrant had 22,845,916 shares of common stock outstanding at a par value $0.0001 per share.

 

DOCUMENTS INCORPORATED BY REFERENCE

Portions of the registrant’s definitive proxy statement that will be filed for the 2018 Annual Meeting of Stockholders are incorporated by reference in Part III.

 

 

 

 

TABLE OF CONTENTS

 

 

 

 

PART I

FORWARD LOOKING STATEMENTS

This Annual Report on Form 10-K contains forward‑looking statements. Forward‑looking statements are neither historical facts nor assurances of future performance. Instead, they are based on our current beliefs, expectations and assumptions regarding the future of our business, future plans and strategies, our clinical results and other future conditions. The words “anticipate,” “believe,” “estimate,” “expect,” “goal,” “intend,” “may,” “seek,” “plan,” “predict,” “project,” “target,” “potential,” “will,” “would,” “possible,” “could,” “should,” “continue,” “contemplate” or the negative of these terms or other similar expressions are intended to identify forward‑looking statements, although not all forward‑looking statements contain these identifying words.

These forward‑looking statements include, among other things, statements about:

We may not actually achieve the plans, intentions or expectations disclosed in our forward‑looking statements, and you should not place undue reliance on our forward‑looking statements. Actual results or events could differ materially from the plans, intentions and expectations disclosed in the forward‑looking statements we make. We have included important factors in the cautionary statements included in this Annual Report on Form 10-K, particularly in the “Risk factors” section, that we believe could cause actual results or events to differ materially from the forward‑looking statements that we make. Our forward‑looking statements do not reflect the potential impact of any future acquisitions, mergers, dispositions, joint ventures or investments we may make.

The forward‑looking statements contained herein represent our views as of the date of this Annual Report on Form 10-K. We anticipate that subsequent events and developments will cause our views to change. However, although we may elect to update these forward‑looking statements at some point in the future, we have no current intention of doing so except to the extent required by applicable law. You should, therefore, not rely on these forward‑looking statements as representing our views as of any date subsequent to the date of this Annual Report on Form 10-K.

ITEM 1.              BUSINESS

We are a clinical stage biopharmaceutical company focused on developing antibody drug conjugates, or ADCs, that offer a clinically meaningful benefit for cancer patients with significant unmet need. We have leveraged 20 years of industry learning in the ADC field to develop proprietary technologies that enable us to design ADCs to have improved efficacy, safety and tolerability relative to existing ADC therapies. Our most advanced platform, Dolaflexin, has been used to generate a pipeline of proprietary ADC product candidates to address patient populations that are not currently amenable to treatment with traditional ADC‑based therapies. Our lead product candidate, XMT‑1522, is a HER2‑targeted ADC

currently in a Phase 1 dose escalation study primarily in breast cancer patients as well as non‑small cell lung cancer (NSCLC) and gastric cancer. Upon the completion of dose escalation, we plan to expand clinical development of XMT‑1522 into additional breast cancer, NSCLC, and gastric cancer patient populations, all of which are not addressed by existing HER2 therapies. Our second product candidate, XMT‑1536, is an ADC targeting NaPi2b, an antigen broadly expressed in ovarian cancer and NSCLC. XMT‑1536 entered clinical development in late 2017 and is currently in a Phase 1 dose escalation study. Beyond our two lead product candidates, we continue to invest in our earlier stage product candidates and in our ADC technologies. In addition, we have established a strategic partnership with Takeda Pharmaceutical Company Limited, or Takeda, under which they obtained rights to XMT‑1522 outside of the United States and Canada. We have also established strategic research and development partnerships with Takeda and Merck KGaA for the development and commercialization of additional ADC product candidates against a limited number of targets selected by our partners based on our Dolaflexin platform. We believe the potential of our ADC technologies, supported by our world‑class management team and protected by our robust intellectual property portfolio, will allow us to develop targeted and highly tailored therapies to help cancer patients become cancer survivors. Our current product candidates all based on our Dolaflexin platform, are summarized in the chart below:

ADCs are an established therapeutic approach in oncology used to selectively deliver a highly potent chemotherapeutic payload directly to tumors thereby minimizing toxicity to surrounding healthy tissue. An ADC consists of an antibody attached to a chemotherapeutic “payload” via a molecule known as a linker. The antibody provides targeting capability against a distinct antigen expressed preferentially on a tumor cell, which restricts the ADC binding only to those cells that express the target antigen. Upon binding to the tumor cell antigen, the ADC is internalized by the tumor cell and the payload is released, killing the cell in a targeted manner. Currently, there are four approved ADCs, (i) brentuximab vedotin marketed by Seattle Genetics, Inc., or Seattle Genetics, and Takeda; (ii) ado‑trastuzumab emtansine marketed by Genentech, Inc., or Genentech, a member of the Roche Group, or Roche; (iii) gemtuzumab ozogamicin, marketed by Pfizer Inc., or Pfizer; and (iv) inotuzumab ozogamicin, also marketed by Pfizer. Brentuximab vedotin and ado-trastuzumab emtansine achieved combined worldwide net sales in excess of $1 billion in 2017. Gemtuzumab ozogamicin and inotuzumab ozogamicin were both approved for sale in the United States in 2017. In addition to these four marketed ADCs, there are six ADCs that have been granted FDA Breakthrough Therapy Designation based on promising clinical data. There are also approximately 74 ADCs presently in development in over 300 clinical studies, the vast majority of which are focused on the treatment of cancer. We believe the commercial success of previously approved ADCs, combined with the number of ADCs currently in clinical development, demonstrates the potential of ADCs to become a mainstay of cancer treatment.

Despite the promise of ADCs, companies in the field have faced certain challenges in developing product candidates that achieve the optimal therapeutic index, or the balance between efficacy and tolerability. These challenges are characterized as follows:

Our proprietary and highly differentiated Dolaflexin platform is designed to overcome these challenges and achieve improved efficacy, safety and tolerability, hence improving the therapeutic index, compared to traditional ADC technologies. Unlike traditional ADCs, where the payload is attached directly to the antibody via a linker, our ADCs feature antibodies attached to multiple units of Dolaflexin, which each consist of our Fleximer polymer scaffold conjugated to several proprietary auristatin payload molecules. As a result, we believe our ADCs offer the following benefits relative to traditional ADCs:

The benefits of the Dolaflexin platform have resulted in tolerable doses in our preclinical models well in excess of the efficacious doses. Based on these findings, we have advanced XMT‑1522 and XMT-1536 into Phase 1 development. We believe these advantageous characteristics of our Dolaflexin platform provide a substantial opportunity to develop

clinically meaningful ADC therapies with potential to address a broader range of cancers than traditional ADC‑based approaches.

We have assembled a management team with extensive, relevant experience, including specific ADC experience, from prior work at leading pharmaceutical companies such as Millennium Pharmaceuticals, Inc., Takeda, Biogen, Inc., MedImmune, Inc., Bayer AG and Vertex Pharmaceuticals, Inc. We are supported by our board of directors and scientific advisory board, who offer complementary experience in drug discovery and development, as well as expertise in building public companies, management and business development. Our key investors include funds managed by New Enterprise Associates, Arrowpoint Partners, Cormorant Asset Management, F‑Prime Capital Partners, Rock Springs Capital and Wellington Management, as well as Pfizer and our strategic partner, Takeda. We successfully completed an Initial Public Offering of 5 million shares at $15 per share in June 2017 raising gross proceeds of $75 million.  We believe that our highly differentiated platform, together with the team we have assembled, positions us well to generate best‑in‑class ADCs with the potential to transform the lives of cancer patients.

Our strategy

Our goal is to become a leading oncology company by leveraging the potential of our innovative and differentiated ADC technologies. Our strategy to achieve this goal is as follows:

Background on antibody drug conjugates (ADCs) for cancer

Overview

ADCs for cancer traditionally consist of an antibody attached to a chemotherapeutic “payload” via a chemical known as a linker. The antibody provides targeting capability against a distinct antigen selectively expressed on a tumor cell, resulting in the ADC binding only to those cells that express the target antigen. Upon binding to the antigen, the ADC is internalized by the tumor cell and the payload is released through either cleavage of the linker or degradation of the antibody. Cell death results once the threshold level of payload has been internalized by the target cell. Figure 1 illustrates the general mechanism by which ADCs kill tumor cells. The individual components of an ADC dictate the efficacy, safety and tolerability of the treatment. Historically, ADC development has involved making compromises between features which may improve efficacy at the expense of safety and tolerability, and vice versa. The challenge of optimizing this balance is exemplified by the dearth of approved ADC products, despite the technology having existed for over 20 years.

Figure 1.

Monoclonal antibodies

The first component of an ADC is a monoclonal antibody, which is the highly specific targeting agent enabling binding to the tumor antigen and internalization of the ADC into the tumor cell. Antibodies themselves are a well established therapeutic modality, with $85.4 billion in worldwide sales in 2015.

In the context of an ADC, two factors are considered in the selection of the antigen to which the antibody is targeted: (i) preferential expression on tumor cells with as limited as possible expression on healthy tissues and (ii) level of antigen expression on tumor cells. The amount of payload delivered to the tumor cell is related to the binding of the ADC to the antigen and internalization, and as a result, it is generally recognized that very high and consistent (or homogeneous) antigen expression throughout the tumor correlates with higher efficacy. For example, ado‑trastuzumab emtansine is indicated for HER2‑positive late stage metastatic breast cancer. The HER2 antigen expression levels in the tumors of these patients is very high, and it has been reported that patients with the highest levels of HER2 expression derive the most therapeutic benefit. The ability to achieve a therapeutic concentration of payload in the tumor quickly diminishes as the level of antigen expression decreases, which may explain why current ADC approaches have only demonstrated efficacy in a limited range of tumors with relatively high expression of a target antigen.

Chemotherapeutic payloads

The second component of an ADC is a chemotherapeutic payload, or cell‑killing agent, too potent to be delivered as a standalone therapy. In the context of an ADC, the payload, which is conjugated to the antibody, is selectively delivered to the tumor as a result of the targeting ability of the antibody thereby limiting toxicity to healthy tissues.

The majority of payloads currently used in ADCs fall within one of two categories, based on mechanism of action: anti‑tubulin agents or DNA damaging agents. Many of the ADCs in clinical development use anti‑tubulin linker‑payload platforms developed by ImmunoGen, Inc., or ImmunoGen (SMCC‑DM1 and SPDB‑DM4) and Seattle Genetics (mc‑MMAF and vc‑MMAE). Anti‑tubulin payloads are preferentially toxic to dividing cells versus resting cells, a feature that is beneficial for ADCs where the target antigen is also expressed on healthy resting cells. Anti‑tubulins typically have potencies of 0.1 to 10 nM but are not effective against certain tumors, such as colorectal. More recently, in order to increase potency and potentially expand addressable indications, the field has seen an emergence of novel DNA damaging payload classes, such as pyrrolobenzodiazepine, or PBD, dimers, with potencies 100 to 1000 times higher than the anti‑tubulins. These payloads bind to the cell’s DNA, leading to cell death. To date, ADCs utilizing PBD dimers have been shown to be highly potent in early clinical development, however due to toxicities, the dose and duration of these ADCs have been limited.

After internalization by the targeted tumor cell, some ADC payloads have an additional ability to passively diffuse into and kill neighboring cells. This bystander effect can be very useful in enhancing the efficacy of these ADCs in tumors with heterogeneous antigen expression by providing a mechanism to kill neighboring tumor cells which do not express the target antigen. While the bystander effect can be beneficial in terms of efficacy, it can also be detrimental in terms of tolerability, as it allows for cell‑killing independent of targeting.

Chemical linkers

A third critical component of an ADC is the chemical linker used to attach the payload to the antibody, as it directly affects efficacy, safety and tolerability. Ideally, a linker provides a stable connection between the payload and the antibody in systemic circulation. Premature release of the payload in systemic circulation can cause significant off‑target toxicity.

Upon internalization of the ADC by the targeted tumor cell, the linker should release the payload from the antibody to promote rapid and efficient killing of the tumor. Linkers used for ADCs fall into one of two categories: cleavable or non‑cleavable. In general, cleavable linkers are designed to be stable in the circulation and to be selectively cleaved as a result of an inherent property of the tumor, such as degradation by tumor‑specific enzymes. In contrast, non‑cleavable linkers are resistant to this type of degradation and instead rely on the degradation of the antibody to release the payload. As a result, the released linker‑payload remains attached to a fragment of the antibody, which limits the cell permeability and bystander effect. The solubility of the linker‑payload combination employed also has a significant influence on the properties of the resulting ADC. Many linkers and payloads used in traditional ADCs are highly insoluble, which limit DAR to three to four due to aggregation and poor drug‑like properties of ADCs. Because existing conjugation approaches use direct conjugation, the site of payload attachment can also influence the stability and performance of the ADC, as the microenvironments surrounding each attachment site can differ and affect the properties of the linker‑payload.

Dolaflexin platform

Our proprietary and highly differentiated Dolaflexin platform is designed to increase the efficacy, safety and tolerability of ADCs by overcoming key limitations of existing technologies based on direct conjugation. Dolaflexin consists of Fleximer, a biodegradable, highly biocompatible, water soluble polymer, to which are attached multiple copies of our proprietary auristatin drug payload, using a linker specifically optimized for use with our polymer. The high water solubility of the Fleximer polymer compensates for the low solubility of the payload, surrounding the payload and protecting it from aggregation. Multiple copies of this Dolaflexin polymer‑drug conjugate can then be attached to an antibody of choice, which significantly increases the payload capacity of the resulting ADC. As shown in the schematic in Figure 2, this approach differs from most other ADC technologies where the payload is directly conjugated to the antibody via a linker. Using the Dolaflexin platform, we have been able to generate ADCs with DAR between 10 to 15 while maintaining acceptable pharmacokinetics and drug‑like properties in animal models. This represents a three to four fold increase in DAR relative to the traditional ADC approach.

Figure 2.

 

Below is a summary of key advantages that we believe our proprietary Dolaflexin platform offers over other existing ADC technologies. We believe these properties will enable us to develop ADCs with an improved therapeutic index that may broaden the scope of addressable cancer patients for which ADC therapies are amenable.

Figure 3. Accumulation of AF‑HPA/AF in Tumor Consistent with Efficacy and Tolerability

 

The more limited exposure of free AF to healthy tissues corresponds to lower drug toxicities, such as neutropenia, seen in safety studies of Dolaflexin ADCs compared to competitor technologies (e.g., Seattle Genetics vc‑MMAE), with seven out of nine ADCs that have reported Phase 1 results showing dose‑limiting neutropenia. As shown in Figure 4, neutrophil counts did not decline in either rats or monkeys at Dolaflexin ADC doses above the maximum doses that can be administered of vc‑MMAE ADCs, which are frequently dose‑limited by neutropenia and sepsis.

Figure 4. Neutrophil Counts as a Function of Dolaflexin ADC Dose (in Auristatin Equivalents)

 

 

Our product candidates

We are leveraging our platform to develop a robust pipeline of clinically meaningful cancer therapies. Our pipeline strategy focuses on targets that have been biologically validated (either as ADCs or through another modality) and where the advantages of our platform can lead to a clinically superior therapeutic. Our lead product candidates, XMT‑1522 and XMT-1536, are in Phase 1 dose escalation studies. A robust discovery stage pipeline supports our objective of bringing one new product candidate into clinical development every 12 to 24 months. In addition, our partners have multiple ADC product candidates leveraging our technology in late discovery.

XMT‑1522: our HER2‑targeted ADC

Program description

Our lead product candidate, XMT‑1522, is a Dolaflexin ADC targeting HER2‑expressing tumors. It is currently in Phase 1 clinical development. HER2 belongs to a family of signaling molecules that are highly and preferentially expressed on the surface of various cancer cells and are known to play a role in promoting tumor cell growth. XMT‑1522 is composed of a proprietary fully human anti‑HER2 antibody, selected for its advantageous internalization properties and its ability to bind to a unique epitope distinct from the epitopes of trastuzumab and pertuzumab, two approved therapies that also target HER2. The development of XMT‑1522 leverages the differentiating aspects of our Dolaflexin platform to focus on HER2‑expressing patient populations that have the highest unmet medical need because they are not served by the existing HER2 therapies currently on the market. We are actively recruiting and dosing primarily breast cancer patients with a HER2 IHC score of 1+ or greater, NSCLC patients with a HER2 IHC score greater than 2+ or gene-amplified, and gastric cancer patients that are HER2-positive.

Unmet need and epidemiology

Currently approved HER2‑targeted therapies are indicated only for breast or gastric cancer patients who are considered HER2‑positive based on well established, Food and Drug Administration, or FDA, approved tests that rely on immunohistochemistry, or IHC, or genetic methods. Patients are classified by their level of HER2 expression on a scale

ranging from 0 to 3+, with 3+ representing the highest level of HER2 expression. Patients with HER2 3+ expression or who have gene amplification that results in them having multiple copies of the HER2 gene are considered HER2‑positive. There is a significantly larger population of patients with HER2 expression of 1+ or 2+ and without gene amplification, and for those patients, there are currently no approved HER2‑targeted therapies in breast, gastric or other cancers.

Our development plan is supported by extensive preclinical data demonstrating XMT‑1522’s increased potency compared to currently marketed HER2 therapies, including against HER2 1+ and 2+ breast and gastric cancers where existing therapies are not approved and HER2-expressing breast, NSCLC and gastric cancers where existing therapies have failed. The following chart shows the initial therapeutic focus for our XMT‑1522 product candidate. We are focused in areas that leverage the advantages of XMT‑1522 and where patients have limited treatment options.

 

Among breast cancer patients, approximately 55% express HER2 at the 1+ or 2+ level without HER2 gene amplification. These patients are not eligible to receive existing HER2 therapies (trastuzumab, pertuzumab or ado‑trastuzumab emtansine) and have limited other options. Initially, we are studying XMT‑1522 in advanced or metastatic breast cancer patients who express HER2 at the 1+ and 2+ levels (whether hormone negative or have become hormone resistant or refractory) and have progressed on at least one line of chemotherapy. If proof‑of‑concept is established in this patient population, opportunities exist to move to an earlier stage of treatment in this hard‑to‑treat patient population. We are also planning to develop XMT‑1522 for HER2-positive breast cancer patients whose tumors have progressed after treatment with other HER2 therapies, such as ado‑trastuzumab emtansine and pertuzumab, and have limited other treatment options.

Among patients with NSCLC, expression of the HER2 protein at the 1+, 2+ or 3+ level has been shown to occur at a rate of approximately 50%. We are developing XMT‑1522 in HER2 1+, 2+ and 3+ patients who have previously been treated with a platinum‑containing regimen. Unlike HER2‑positive breast cancer, HER2 expression in NSCLC is not a dominant driver of tumor growth and hence HER2‑targeted antibodies have failed in this setting. If proof‑of‑concept is established in this population, opportunities exist to move earlier in the treatment paradigm or consider combination treatment with PD‑1/PD‑L1 antibodies, the emerging standard of care in front line NSCLC. Our emerging preclinical data appear to also support the potential for synergy with immune checkpoint inhibitors.

Among gastric cancer patients, approximately 15% to 20% are HER2‑positive. Trastuzumab is approved for this patient population but ado‑trastuzumab emtansine has failed to demonstrate a survival benefit. We are developing XMT‑1522 in HER2‑positive patients who have received prior therapy with trastuzumab. If proof‑of‑concept is established in this population, opportunities exist to address gastric cancer patients expressing HER2 at the 1+ and 2+ levels.

Clinical development plan and timeline

XMT‑1522 is in a Phase 1, open label, multi‑center study and is administered as an intravenous infusion once every three weeks. There are two parts to the Phase 1 study: (i) a dose escalation primarily in breast cancer patients with a HER2 score of 1+ or greater and (ii) a dose expansion in five parallel patient cohorts. At the request of investigators we amended the protocol for the dose escalation study to allow for enrollment of NSCLC patients expressing HER2 by IHC (2+ or 3+) or with gene amplification, and gastric patients that are HER2-positive post trastuzumab treatment. The primary objective of the dose escalation part of the study is to establish the maximum‑tolerated dose and a recommended Phase 2 dose. The objective of the cohort expansion stage is to further assess tolerability at the recommended Phase 2 dose and to estimate the objective response rate and durability of response in five patient cohorts.

The dose escalation part of the study utilizes a 3+3 design with a three week evaluation period for dose limiting toxicity, or DLT. A Safety Review Committee will review the data after each dose cohort of three patients completes the DLT evaluation period and will recommend three patients be enrolled at the next dose level if a dose is reasonably well‑tolerated. After the first cycle, patients may continue to receive XMT‑1522 until disease progression, provided the drug is well‑tolerated and patients continue to derive clinical benefit in the opinion of the investigator.

XMT-1522 has been evaluated in 22 patients through dose level 6. Treatment-related adverse events have generally been mild or moderate and reversible. There have been early signals of efficacy. The pharmacokinetic profile of XMT-1522 has been generally consistent with the favorable profile seen in animal studies. The objective of the study is to establish maximum tolerated dose and use that information to select a Phase 2 dose. The program has progressed to dose level 7. An abstract of interim results has been submitted to the American Society of Clinical Oncology’s (ASCO) annual meeting.

After completion of dose escalation, the expansion part of the study will be opened in five cohorts of approximately 20 patients each:

The expansion part of the study is designed to provide an initial estimate of the response rate for XMT‑1522 in each cohort and the durability of the observed responses. These data will be used to support end‑of‑Phase 1 interactions with regulatory authorities and to inform the design of subsequent studies. We anticipate that observation of a clinically meaningful rate of durable responses in any of the cohorts could be used to support the initiation of pivotal studies to support approval in the indication.

Preclinical efficacy studies

We have studied the efficacy of XMT‑1522 in xenograft as well as in patient‑derived models representing diverse levels of HER2 expression and tumor types. The data are summarized in the waterfall plot below, showing the best tumor response to XMT‑1522 across 15 tumor models representing six indications (Figure 5). These indications informed our clinical development plan. Each column represents an individual tumor model and measures the best overall change in tumor volume relative to the measured tumor volume on the first day of XMT‑1522 administration. A more negative value represents greater anti‑tumor efficacy of XMT‑1522, with a 100% reduction in tumor volume corresponding to complete regression of the tumor to the point where it was no longer measurable. In these experiments, XMT‑1522 was given in

doses of 3 mg/kg or below, either as a single dose on Day 0 of the experiment or in three weekly doses on Days 0, 7 and 14. Experiments were allowed to run until at least Day 60, or at least 45 days following the last administration of XMT‑1522. As depicted in the graph, XMT‑1522 was able to achieve complete or near‑complete tumor regressions in 11 out of the 15 models. Of the 11 models that achieved complete or near‑complete regression, the regressions were sustained until Day 60 in 10 of the models even in the absence of additional therapy, showing the durability of tumor regressions induced by XMT‑1522 (Figure 6).

Figure 5. Waterfall Plot of Best Tumor Response to XMT‑1522 in Xenograft Models

Figure 6. Day 60 Tumor Volumes in Models Achieving Complete or Near‑Complete Regression After Treatment with XMT‑1522

 

To evaluate the relative efficacy of XMT‑1522 compared to ado‑trastuzumab emtansine, we conducted studies in tumor models representing high, medium and low levels of HER2 expression (Figure 7). In the high HER2‑expressing model (NCI‑N87 gastric cancer, HER2 gene amplified, HER2 3+), XMT‑1522 induced complete tumor regressions after a single 1 mg/kg dose on Day 1. As we expected, ado‑trastuzumab emtansine was similarly active in HER2 high expressing tumors after a single dose of 10 mg/kg. However, in the medium‑ and low‑expressing models, XMT‑1522 was still able to induce

durable complete tumor regressions where ado‑trastuzumab emtansine failed to do so, even at doses at least 10‑fold higher than the XMT‑1522 dose. XMT‑1522 was also capable of inducing complete tumor regressions in models of acquired resistance to ado‑trastuzumab emtansine, both in a model generated in the laboratory and in a tumor model obtained from a patient who responded to ado‑trastuzumab emtansine but then experienced disease progression. In contrast, in the model obtained from a patient, lapatinib/gemcitabine, the current standard of care, did not have material impact on tumor growth. These data suggest that our ADCs may have improved efficacy relative to traditional ADCs, even in tumors where the target antigen is expressed at moderate to low levels.

Figure 7. Comparing XMT‑1522 to Ado‑Trastuzumab Emtansine in Models Representing High, Medium and Low Levels of HER2 Expression

 

To evaluate the potential of XMT‑1522 in combination with other agents, we conducted preclinical studies with other HER2‑targeted therapies and checkpoint inhibitors currently used in the treatment of cancer. Since XMT‑1522 binds to an epitope distinct from the HER2 epitopes to which trastuzumab and pertuzumab bind, it does not compete with either of those antibodies for HER2 binding. We have shown that the triple combination of XMT‑1522 with trastuzumab and pertuzumab is more active than XMT‑1522 alone or the trastuzumab/pertuzumab doublet in a HER2‑driven tumor model (N87 HER2‑positive gastric cancer) (Figure 8). In this experiment, XMT‑1522 was administered at a dose lower than the maximally efficacious dose to manifest the triplet synergy. Consequently, we believe XMT‑1522 has the potential to be combined with either or both of those monoclonal antibodies, even at doses of trastuzumab and pertuzumab over 20‑fold higher than the dose of XMT‑1522, to promote more complete inhibition of HER2 signaling while not interfering with delivery of the XMT‑1522 chemotherapeutic payload.

Figure 8. Synergy Seen in Triple Combination with Trastuzumab and Pertuzumab

ADC payloads, including AF‑HPA, have been shown to induce immunogenic cell death, or ICD. Chemotherapeutic compounds that induce ICD are hypothesized to increase the presentation of tumor antigens in the tumor microenvironment and to generate an immune response to the tumor, resulting in increased sensitivity of the tumor to immune checkpoint drugs such as the PD‑1 or PD‑L1 inhibitors. We have tested this hypothesis with XMT‑1522 in a mouse model carrying a humanized immune system and a patient‑derived NSCLC tumor expressing HER2. In this experiment, we tested the PD‑1 antibody pembrolizumab alone, XMT‑1522 alone and the combination of XMT‑1522 with pembrolizumab. As shown in Figure 9, the combination of XMT‑1522 with pembrolizumab is more active than either therapy alone. We believe these data support the potential to combine Dolaflexin ADCs with immune checkpoint inhibitors in cancer indications where checkpoint inhibitors are active.

Figure 9. XMT‑1522 in Combination with Pembrolizumab Results in Greater Efficacy than Either Treatment Alone

Preclinical safety studies

We have evaluated the safety and tolerability of XMT‑1522 in both non‑human primates and rats. Based on these studies, we have established that the XMT‑1522 plasma concentrations necessary for efficacy in the variety of models studied are below the highest tolerated dose in non‑human primates. Furthermore, we have established that XMT‑1522 is stable in circulation, has predictable pharmacokinetics and has a safety profile acceptable for Phase 1 testing in patients with advanced cancer. The plasma concentrations of XMT‑1522 ADC and the monoclonal antibody were similar over the course of the study and the concentration of free AF‑HPA payload was less than 0.05% the concentration of antibody conjugated AF‑HPA at all time points, indicating the stability of the ADC in circulation. Plasma exposure to free AF payload was also low and peaked at a later time point compared to free AF‑HPA, consistent with the metabolism of our AF‑HPA payload. There was no evidence of cardiotoxicity in non‑human primates at any dose tested, including at doses significantly above the highest tolerated dose in dose finding studies. The most pronounced hematologic finding in non‑human primates was a transient decrease in platelet counts not associated with clinically‑significant bleeding. Neutropenia was not observed in either species. Opthalmological evaluation was performed in preclinical studies in both species. Adverse ocular events related to XMT‑1522 were seen only at the highest dose tested in the rat, associated with plasma exposure of XMT‑1522 greater than eight fold higher than the exposure at the highest non‑severely toxic dose in non‑human primates. Gastrointestinal toxicity was the primary toxicity associated with XMT‑1522 and was seen only in non‑human primates. These effects were fully reversible at tolerated doses.

XMT‑1536: our NaPi2b‑targeted ADC

Program description

Our second product candidate, XMT‑1536, is a Dolaflexin ADC targeting NaPi2b‑expressing tumors. NaPi2b is an antigen highly expressed in 60 to 90% of both non‑squamous NSCLC and epithelial ovarian cancer. However, the expression of NaPi2b in normal tissue is restricted to a limited subset of cell types, rendering it an ideal antigen for ADC development. XMT‑1536 is composed of a proprietary anti‑NaPi2b antibody, selected for its advantageous internalization properties. XMT‑1536 entered clinical development in late 2017 and is currently in a Phase 1 dose escalation study.

Genentech’s lifastuzumab vedotin, an ADC targeting NaPi2b utilizing the Seattle Genetics vc‑MMAE platform, provided encouraging results in Phase 1 studies in ovarian cancer, where a 41% confirmed objective response rate by RECIST criteria was achieved without evidence of target‑mediated toxicities. However, in a randomized Phase 2 study in platinum‑resistant ovarian cancer, lifastuzumab vedotin failed to demonstrate a statistically‑significant benefit to liposomal doxorubicin, the comparator, on the primary endpoint of progression free survival, or PFS, despite a numerically superior response rate and improvement in median progression‑free survival. Surprisingly, responses in NSCLC patients were also limited despite widespread expression of the NaPi2b target in the Phase 1 patients. Genentech has since discontinued development of lifastuzumab vedotin. The partial validation of the NaPi2b target provided by these studies forms the basis of our rationale to develop XMT‑1536 as a potentially clinically meaningful ADC for the treatment of epithelial ovarian cancer and non‑squamous NSCLC. Based on our preclinical data, we believe that XMT‑1536 may offer improved efficacy and a wider therapeutic index in these patients.

Unmet need and epidemiology

Ovarian cancer patients who progress during or within six months of completion of platinum‑based therapy are considered to have platinum‑resistant disease. These patients have limited treatment options other than single agent platinum‑based chemotherapies (e.g., docetaxel, paclitaxel) or targeted therapies, such as bevacizumab (in patients who have not received beviciaumab for treatment of earlier stage disease), olaparib (for patients carrying germline mutations in the BRCA1 and BRCA2 genes) and rucaparib (for patients carrying germline and somatic mutations in the BRCA1 and BRCA2 genes), which have either shown limited overall survival benefit (e.g., bevacizumab) or have yet to demonstrate survival benefit (e.g., olaparib and rucaparib). We plan to initially test XMT‑1536 in patients with platinum‑resistant ovarian cancer. If proof‑of‑concept is established, there are opportunities to address treatment of primary ovarian cancer and recurrent, platinum‑sensitive disease where platinum‑based chemotherapy regimens remain the standard of care.

Given the breadth of NaPi2b expression in non‑squamous NSCLC, we believe XMT‑1536 also has the potential to treat a broad population of NSCLC patients. Initially, we plan to test XMT‑1536 in platinum‑resistant NSCLC patients. If proof‑of‑concept is established in this population, we believe that there are opportunities to move earlier in the treatment paradigm or consider combination treatment with PD‑1/PD‑L1 antibodies, the emerging standard of care in front line NSCLC. Our preclinical data indicating that the AF‑HPA payload used in XMT‑1536 induced immunogenic cell death support the potential for synergy with immune checkpoint inhibitors.

There are currently no FDA‑approved tests to measure NaPi2b expression on tumor cells, however given the prevalence of its expression on epithelial ovarian and non‑squamous NSCLC tumors, our initial clinical studies of XMT‑1536 will be conducted without prospective identification of patients with NaPi2b‑expressing tumors. Nonetheless, we have developed and validated an immunohistochemistry assay to measure NaPi2b expression which we intend to use retrospectively to confirm the broad prevalence of NaPi2b expression in our target patient populations while correlating those expression levels with the efficacy observed in such patients. To date, data generated using our assay has found NaPi2b expression in 12 out of 20 and 16 out of 20 samples of ovarian and lung cancer, respectively. If results are sufficiently robust, we believe there is an opportunity to develop XMT‑1536 without the need for a companion diagnostic, or with the inclusion of the NaPi2b assay in the label as a complementary diagnostic to guide physician decision making. If a companion diagnostic is required for the label for XMT‑1536, we may seek approval for our validated assay as a companion diagnostic or we may contract with third parties to create and obtain approval for a companion diagnostic.

Preclinical studies

XMT‑1536 induced complete tumor regressions in the OVCAR3 ovarian cancer model after a single dose of 5 mg/kg or three weekly doses of 3 mg/kg. In comparison, lifastuzumab vedotin administered via three weekly doses of 3 mg/kg failed to achieve tumor regressions (Figure 10). Genentech published regressions in this model at doses of 6 mg/kg and above, but, given the dose‑limiting neutropenia seen in monkeys at doses above 3 mg/kg, these higher doses are unlikely to be translationally relevant.

Figure 10. Comparison of XMT‑1536 to Lifastuzumab Vedotin in the OVCAR3 Ovarian

Cancer Xenograft Model

Established CTG‑0852 patient‑derived NSCLC xenograft tumors were treated with XMT‑1536, lifastuzumab vedotin or non‑binding IgG1‑dolaflexin control ADC at a 3 mg/kg dose once weekly for three weeks and tumor volume was measured for 60 days. XMT‑1536 treatment resulted in nearly complete regression of the treated tumors that was durable for 45 days after cessation of treatment. In contrast, treatment with the non‑binding ADC control or lifastuzumab vedotin led to modest tumor growth control without achieving tumor regression.

Figure 11. Comparison of XMT‑1536 to Lifastuzumab Vedotin in the CTG‑0852 NSCLC Xenograft Model

 

XMT‑1536 was also tested in eight patient‑derived tumor models of NSCLC adenocarcinoma, where it led to complete or near‑complete tumor regressions in five of eight models and significant tumor growth delay in two of the remaining three models (Figure 12). All models were treated with three weekly doses of 3 mg/kg or less. The models were not pre‑selected

for NaPi2b expression and represented a range of tumor genotypes frequently observed in NSCLC adenocarcinoma, including RAS/RAF mutant tumors, EGFR mutant tumors, ALK‑translocated tumors and tumors not carrying known oncogenic drivers. As with the data presented above, each column represents an individual tumor model, and the more negative the value, the greater the degree of XMT‑1536 efficacy, with negative 100% representing complete tumor regression. In these experiments, the last dose of XMT‑1536 was administered on Day 14 and tumor volumes were measured until Day 60 to evaluate durability of the regressions. The regressions were maintained until Day 60 in four of the five models achieving complete or near‑complete regression after a 45 day treatment‑free interval, indicating good durability of the tumor regressions (Figure 13).

 

XMT-1536 was tested at 3 mg/kg three times weekly in a series of 19 human primary xenograft models derived from serous ovarian or fallopian tube cancers (n=3 animals/group). Models were not preselected for NaPi2B target expression.  Growth effects were evaluated by calculating median best response relative to day 0, at any time-point. An immunohistochemistry (IHC) assay to detect NaPi2b was established using a primary anti-NaPi2b antibody, that consisted of a human/rabbit chimera of XMT-1535, the antibody included in XMT-1536. A tumor block from one untreated study animal, representing each tumor model, was evaluated to determine an efficacy/staining pattern relationship, and IHC values were reported as an “H” Score.

Median best response (Figure 14) calculation showed 10/19 models with a median best response of -50% to - 100%. Considering models with a 50% or greater median best response after XMT-1536 treatment, all had a NaPi2b IHC  H-score of ≥70 Amongst tumors with H-score ≥70, 10/12 (83%) models achieved 50% or greater reduction in tumor volume after XMT-1536 treatment, vs 0/7 (0%) models with H-score <70. There was an association between NaPi2b IHC H-score and tumor volume change after XMT-1536 treatment (Spearman rank coefficient 0.76).

Figure 14. Median Best Response to XMT-1536 in an Unselected Series of Human Primary Ovarian and Fallopian Tube Cancer Xenografts; Gray Scale by NaPi2b IHC H Score

Regression Observed in 14/19 Models

 

 

Preclinical tolerability data and therapeutic index

XMT‑1536 is cross‑reactive with cynomolgous monkey and rat NaPi2b, allowing an informative evaluation of whether XMT‑1536 retains good tolerability in these commonly used safety species. In the exploratory repeat dose NHP study as well as the IND-enabling study, there was no evidence of neutropenia at payload doses that were at least four times the maximum tolerated dose of lifastuzumab vedotin and at least two times the dose that caused fatal neutropenia and sepsis in monkeys treated with lifastuzumab vedotin. Further, there was no evidence of significant pulmonary toxicity. We believe these data, combined with the strong efficacy data for XMT‑1536 in models of NSCLC and ovarian cancer, are indicative of a favorable therapeutic index and supported moving into Phase 1 trials in cancer patients.

Clinical development plan and timeline

The Phase 1 study is an open label, multi‑center study of XMT‑1536 administered as an intravenous infusion once every three weeks. The dose escalation part of the study is expected to establish a recommended Phase 2 dose for XMT‑1536 in patients primarily with advanced epithelial ovarian cancer and non‑squamous NSCLC. We expect the study will not require molecular testing for eligibility and will be open to all patients regardless of NaPi2b expression. Upon completion of dose escalation, the cohort expansion segment of the study is expected to consist of three parallel cohorts of patients to estimate the objective response rate and durability of responses. Retrospective analysis of tumor response and durability of response as a function of NaPi2b expression will be performed to determine the necessity of developing a companion diagnostic for NaPi2b expression in subsequent studies.

XMT-1536 entered clinical development in late 2017. The first two dose levels were cleared without dose limiting toxicity. Enrollment of the third dose cohort is complete with patients currently being evaluated for dose limiting toxicity. Treatment-related AEs have generally been mild or moderate and reversible. Dose escalation is ongoing in accordance with the study protocol. The objective of the study is to establish maximum tolerated dose and use that information to select a Phase 2 dose.

Platform development

We intend to establish a leading position in the field of ADCs by continuing to advance platform innovations that further broaden the potential of our ADCs to deliver clinically meaningful benefit for cancer patients. Our areas of focus include the development of alternative scaffolds to drive homogeneity of our ADCs, alternative payloads to address additional indications and drug resistance and alternative targeting moieties to improve tumor penetration and biodistributuion. We believe these efforts may lead to improved efficacy and tolerability as well as expansion of the addressable patient population. 

Strategic partnerships

Strategic partnerships with leading biopharmaceutical companies to advance Fleximer ADC product candidates

We believe that our ADC platform has broad applicability across a number of targets. We have used strategic partnering to accelerate bringing Fleximer ADCs to patients. Since 2012, we have entered into a strategic partnership for XMT‑1522 with Takeda, through its wholly owned subsidiary Millennium Pharmaceuticals, Inc., and strategic research and development partnerships with Takeda, Merck KGaA and Asana BioSciences, LLC (by assignment from Endo Pharmaceuticals Inc.) to enable development of certain ADC product candidates utilizing Fleximer. In establishing each of these partnerships, our primary objectives were to collaborate with leading biopharmaceutical companies to validate the potential of ADC product candidates utilizing Fleximer, gain meaningful near‑term funding and drive significant long‑term value. Under each of our partnerships, we own the rights to any improvements to our ADC platform. The details of our material existing strategic partnerships are as follows:

Takeda XMT‑1522 strategic partnership

In January 2016, we entered into a collaboration agreement with Takeda for the development and commercialization of XMT‑1522. Under this agreement, we granted Takeda an exclusive license under certain of our ADC‑related patents and know‑how to commercialize XMT‑1522 outside of the United States and Canada. We will conduct certain Phase 1 development activities for XMT‑1522, including the ongoing Phase 1 clinical study, at our own expense, and Takeda may also conduct Phase 1 development activities at its own expense. The parties will collaborate on the further development of XMT‑1522 in accordance with a global development plan. In addition, the parties will share equally all clinical‑stage manufacturing costs and any post‑Phase 1 development costs incurred in connection with obtaining regulatory approval in either the United States or Canada and in certain major markets in the rest of the world. Each party will be responsible for all post‑Phase 1 development costs specific to such party’s territory incurred for the purpose of obtaining regulatory approval in such party’s territory. Subject to certain restrictions, each party may conduct independent development of XMT‑1522 and the other party may elect to use any resulting data if it agrees to share the development costs equally and pays a premium for previously incurred costs.

During 2016, we received an upfront payment of $26.5 million and a milestone payment of $20 million under this agreement. We are entitled to receive future development, regulatory and commercial milestones of up to $288 million and tiered royalties in the low‑ to mid‑teen percentages on net sales of XMT‑1522 in Takeda’s territory during the applicable royalty term, if XMT‑1522 is successfully developed and commercialized. Pursuant to this Agreement, Takeda invested approximately $10 million in our Series C‑1 financing in June 2016 and $10 million in our initial public offering.

Unless earlier terminated, this agreement will expire upon the expiration of the last royalty term for XMT‑1522 under the agreement in all countries. The royalty term for XMT‑1522 means, on a country‑by‑country basis, the period commencing upon the first commercial sale of XMT‑1522 and ending upon the later to occur of: (i) expiration of the last Mersana or jointly‑owned patent right that covers XMT‑1522 in such country, (ii) expiration of any exclusive marketing right, data exclusivity right, orphan drug designation or other country‑wide exclusive right or status conferred by any governmental authority with respect to XMT‑1522 in such country, other than a patent right, or (iii) 15 years from the date of first commercial sale of XMT‑1522 in such country. Upon the expiration of the royalty term for XMT‑1522 on a country‑by‑country basis, Takeda will have a perpetual, exclusive license to XMT‑1522 in such country. Takeda may terminate this agreement in its entirety for convenience upon 30 days’ prior written notice at any time up to the initiation of the first Phase 2 clinical study of XMT‑1522 or upon 90 days’ prior written notice following the initiation of the first

Phase 2 clinical study of XMT‑1522. Each party may terminate this agreement in its entirety upon bankruptcy or similar proceedings of the other party and in its entirety or on a country‑by‑country basis upon an uncured material breach of the agreement by the other party. Following any such termination, all rights in XMT‑1522 licensed to Takeda will revert to us for further development and commercialization.

Takeda strategic research and development partnership

In March 2014, we entered into a collaboration agreement with Takeda for the development and commercialization of ADC product candidates utilizing Fleximer. We formed a strategic partnership with Takeda because of their industry expertise in oncology drug development and their experience developing and commercializing brentuximab vedotin outside of the United States, one of only four approved ADCs. Under this agreement, Takeda received rights to select up to seven target antigens, of which it has selected four to date. Takeda is responsible for generating antibodies against the target antigens, and we are responsible for generating Fleximer and our proprietary payloads and conjugating this to the antibody to create the ADC product candidates. With respect to each target antigen selected by Takeda, we granted Takeda an exclusive, worldwide license under certain of our Fleximer ADC‑related patents and know‑how to develop, manufacture and commercialize ADC product candidates directed to such target antigen. Takeda is then responsible for the further development, manufacture and commercialization of these ADC product candidates. Takeda selected its first Dolaflexin ADC in 2017 and initiated full preclinical IND enabling studies.

Takeda is responsible for its own costs in the development, commercialization and manufacture of ADC product candidates and reimburses us for our costs incurred in performing our research activities under this agreement, except in the event that we exercise our opt‑in right as described below.

Through December 31, 2017, we have received $24.8 million in upfront payments and option fees under this agreement. If products are successfully developed and commercialized against all seven potential target antigens, we are entitled to receive future development, regulatory and commercial milestones of up to $1.063 billion, except in the event that we exercise our opt‑in right as described below. During the applicable royalty term, we are entitled to receive tiered royalties in the mid‑single digit percentages on net sales of each product targeting Takeda’s first or second target antigen and in the mid‑ to high‑single digit percentages on net sales of each product targeting Takeda’s third through seventh target antigens if products are successfully developed and commercialized by Takeda and except in the event that we exercise our opt‑in right as described below.

In addition, we have an option to co‑develop and co‑commercialize one product targeting one of Takeda’s third through seventh target antigens in the United States for a payment of $15 million in cash or in our common stock, and we may exercise such option with respect to an applicable product no later than 30 days after initiation of a Phase 2 clinical study for such product or at an earlier time if Takeda intends to grant rights to such product to a third party. If we elect to exercise the option to co‑develop and co‑commercialize a product, we will share development costs related to such product in the United States equally with Takeda and we will be responsible for 30% of the global development costs for such product. If we elect to exercise the option to co‑develop and co‑commercialize a product, we will share the profits and losses related to such product in the United States equally with Takeda in lieu of certain milestones and royalties on the net sales in the United States.

Unless earlier terminated, this agreement will expire upon the expiration of the last royalty term for a product under the agreement in all countries. The royalty term means, on a product‑by‑product and country‑by‑country basis, the period commencing upon the first commercial sale of a product and ending upon the later to occur of: (i) the later of expiration of the last Mersana patent right that would be infringed by the manufacture or commercialization of such product in such country and the expiration of the first‑to‑expire patent right claiming the composition of matter of the ADC contained in such product, or (ii) 10 years from the date of first commercial sale of such product in such country. Upon the expiration of each royalty term for each product on a country‑by‑country basis, Takeda’s exclusive license will convert to a perpetual, non‑exclusive, royalty‑free license with respect to such product in such country. Except with respect to the target antigen of a product for which we exercised our option to co‑develop and co‑commercialize in the United States, Takeda may terminate this agreement in its entirety or with respect to any target antigen for convenience upon 45 days’ prior written notice. Each party may terminate this agreement in its entirety upon bankruptcy or similar proceedings of the other party

or upon an uncured material breach of the agreement by the other party. However, if such breach only relates to one target antigen, the agreement may only be terminated with respect to such target antigen.

Merck KGaA strategic research and development partnership

In June 2014, we entered into a collaboration agreement with Merck KGaA for the development and commercialization of ADC product candidates utilizing Fleximer for up to six target antigens. We formed a strategic partnership with Merck KGaA because of their expertise in oncology drug development. Under this agreement, we are responsible for generating ADC product candidates against Merck KGaA‑selected target antigens. Merck KGaA received rights to select up to six target antigens, of which it has selected all six. Merck KGaA is responsible for generating antibodies against the target antigens, and we are responsible for generating Fleximer and our proprietary payloads and conjugating this to such antibodies to create the ADC product candidates. With respect to each target antigen selected by Merck KGaA, we granted Merck KGaA an exclusive, worldwide license under certain of our Fleximer ADC‑related patents and know‑how to develop, manufacture and commercialize ADC product candidates directed to such target antigen. Merck KGaA is then responsible for the further development and commercialization of these ADC product candidates. In addition, if Merck KGaA advances candidates, we are responsible for manufacturing these ADC product candidates for GLP toxicology studies and Phase 1 clinical studies at Merck KGaA’s expense and Merck KGaA is responsible for all further manufacture of these ADC product candidates. Merck KGaA is required to pay its own costs in the development, commercialization and manufacture of these ADC product candidates and to reimburse us for our costs incurred in performing our research activities under this agreement. The most advanced product candidates in this partnership are in the lead optimization stage.

Through  December 31, 2017, we have received an upfront payment of $12 million and milestone payments of $3 million under this agreement. If products are successfully developed and commercialized against all six target antigens, we are entitled to receive future development, regulatory and commercial milestones of up to $778 million. We are entitled to receive tiered royalties in the low‑ to mid‑single digit percentages on net sales of products targeting Merck KGaA’s target antigens during the applicable royalty term if products are successfully developed and commercialized by Merck KGaA under this agreement.

Unless earlier terminated, this agreement will expire upon the expiration of the last royalty term for a product under the agreement in all countries or, if Merck KGaA does not designate any ADC product candidates produced by us under the agreement as preclinical development candidates, upon the expiration of the last‑to‑expire research program. The royalty term means, on a product‑by‑product and country‑by‑country basis, the period commencing upon the first commercial sale of a product and ending upon the later to occur of: (i) the expiration of the last Mersana patent right that covers or claims the exploitation of such product in such country, or (ii) 10 years from the date of first commercial sale of such product in such country. Upon the expiration of each royalty term for each product on a country‑by‑country basis, Merck KGaA’s exclusive license will convert to a perpetual, non‑exclusive, royalty‑free license with respect to such product in such country. Merck KGaA may terminate this agreement in its entirety or with respect to any target antigen for convenience upon 60 days’ prior written notice. Each party may terminate this agreement in its entirety upon an uncured material breach of the agreement by the other party.

Strategic partnerships to access antibodies to progress our proprietary pipeline

Our focus is to progress our proprietary pipeline of Fleximer based ADCs. For this reason, we have partnered with biotechnology companies that have the capability to generate high quality antibodies or that have existing antibodies that we can license for inclusion in our ADCs. These strategic partnerships have facilitated the acceleration of our proprietary pipeline.

Adimab strategic partnership for the antibody in XMT‑1522

In July 2012, we entered into a collaboration agreement with Adimab, LLC, or Adimab. We formed a strategic partnership with Adimab because we believe they have industry leading capabilities in antibody discovery, as evidenced by their existing partnerships with numerous significant pharmaceutical and biotechnology companies. The initial focus of this partnership was for the discovery of antibodies by Adimab directed to two targets, which would then be conjugated to our

Dolaflexin platform technology. Our HER2‑targeted antibody used in XMT‑1522 was the result of this partnership. We exercised an option under this agreement to acquire Adimab’s interest in this antibody and certain other antibodies developed under this partnership. Through exercising this option, we have also acquired Adimab’s interests in patents and know‑how arising from its work that were solely related to such antibodies and obtained a non‑exclusive, worldwide license to Adimab’s background technology to exploit ADCs containing these antibodies. Under the agreement, we are responsible for all development, manufacture and commercialization activities related to ADCs containing these antibodies, including XMT‑1522, and we must use commercially reasonable efforts to develop or commercialize one such ADC or our rights to these antibodies will revert to Adimab. During 2014, we paid an option exercise fee of $1.5 million under this agreement and are obligated to pay Adimab up to $26.5 million in development and regulatory milestones for each product containing one of these antibodies and a low‑single digit percentage royalty on net sales of each product during the applicable royalty term if this product is successfully developed and commercialized. The royalty term for XMT‑1522 means, on a country‑by‑country basis, the period during which (i) the sale of XMT‑1522 in the country of sale, or the manufacture of XMT‑1522 in the country of manufacture, is covered by a licensed patent in such country or (ii) XMT‑1522 has regulatory exclusivity granted by the FDA or any other regulatory authority in such country providing a period of marketing exclusivity or data exclusivity. During 2017, we made a milestone payment of $1.5 million to Adimab with respect to XMT‑1522.

Recepta license for the antibody in XMT‑1536

In July 2015, we entered into a license agreement with Recepta Biopharma S.A., or Recepta, a Brazilian biopharmaceutical company, licensing Recepta’s NaPi2b antibody for use in XMT‑1536 and granting Recepta the exclusive right to commercialize XMT‑1536 in Brazil. Under this agreement, Recepta granted us an exclusive license and sub‑license with respect to certain patents licensed by Recepta from Ludwig Institute for Cancer Research and technology owned by Recepta to develop and exploit products containing Recepta’s NaPi2b antibody, including XMT‑1536, worldwide for the diagnosis, prophylaxis and treatment of human cancer. We granted Recepta an exclusive license under our rights in such patents and technology and certain of our ADC‑related patents and technology to commercialize any such products developed by us, including XMT‑1536, in Brazil. We are responsible for the worldwide development and commercialization of products under this agreement at our own expense in certain major markets, including at least one study site in our Phase 3 clinical studies in Brazil. Recepta may conduct development activities in Brazil at its own expense after providing us the opportunity to first conduct such activities at Recepta’s expense. If a product is successfully developed and commercialized by Recepta in Brazil, we will use diligent efforts to enter into an agreement for the supply of such products to Recepta for sale in Brazil.

Under this agreement, we paid Recepta an upfront payment of $1 million during the year ended December 31, 2015 and are obligated to pay Recepta up to $65.5 million in development, regulatory and commercial milestones and tiered royalties in the low‑single digit percentages on net sales of products outside of Brazil until the expiration of the royalty term if products are successfully developed and commercialized. We are entitled to receive tiered royalties in the low‑ to mid‑single digit percentages on net sales of products in Brazil until the expiration of the royalty term if products are successfully developed and commercialized. The royalty term means, on a product‑by‑product and country‑by‑country basis, the period ending upon the later of (i) with respect to products commercialized by Mersana, the expiration of the last‑to‑expire Recepta patent that covers the product in such country (including the term of any applicable supplementary protection certificate) or with respect to products commercialized by Recepta, the expiration of the last‑to‑expire Mersana Patent that covers the product in Brazil (including the term of any applicable supplementary protection certificate) or (ii) 10 years from the date of first commercial sale of such product in such country. Upon the expiration of each royalty term in each country for each applicable product, the exclusive licenses granted to each party under the agreement will become fully‑paid up and royalty‑free. This agreement will remain in effect until otherwise terminated as set forth below. We may terminate this agreement for convenience in its entirety or on a country‑by‑country basis (except with respect to Brazil) or product‑by‑product basis upon 180 days’ prior written notice for a termination in its entirety or upon 45 days’ prior written notice for a termination in part. Each party may terminate this agreement in its entirety upon bankruptcy or similar proceedings of the other party, upon a patent challenge by the other party or upon an uncured material breach of the agreement by the other party. However, if such breach only relates to one country, the agreement may only be terminated with respect to such country.  During 2017, we made aggregate milestone payments of $1.25 million to Recepta with respect to XMT‑1536.

Manufacturing

We do not own or operate and currently have no plans to establish any cGMP compliant manufacturing facilities. We currently rely, and expect to continue to rely, on external Contract Manufacturing Organizations, or CMOs, for the manufacture of product to support clinical testing. In the future, we expect to use CMOs to manufacture commercial supply of our products. The Dolaflexin manufacturing process involves readily available starting materials and uses unit operations that are well‑precedented in the field of chemical/pharmaceutical production.

Government regulation

Government authorities in the United States, at the federal, state and local level, and in other countries and jurisdictions, including the European Union, extensively regulate, among other things, the research, development, clinical and preclinical testing, manufacture, packaging, storage, recordkeeping, labeling, advertising, promotion, distribution, marketing and import and export of pharmaceutical products. The processes for obtaining regulatory approvals in the United States and in foreign countries and jurisdictions, along with subsequent compliance with applicable statutes and regulations and other regulatory authorities, require the expenditure of substantial time and financial resources. Failure to comply with the applicable requirements at any time during the product development process, approval process or after approval may subject an applicant and/or sponsor to a variety of administrative or judicial sanctions, including imposition of a clinical hold, refusal to approve marketing applications, withdrawal of an approval, import/export delays, issuance of warning letters and other types of enforcement letters, product recalls, product seizures, total or partial suspension of production or distribution, injunctions, fines, refusals of government contracts, restitution, disgorgement of profits or civil or criminal investigations and penalties.

Review and approval in the United States

In the United States, our ADC product candidates are subject to regulation by the FDA as biologics. The FDA regulates biologics under the Federal Food, Drug, and Cosmetic Act, or FDCA, the Public Health Service Act, or PHS Act, and associated implementing regulations. The failure to comply with the FDCA, the PHS Act and other applicable U.S. requirements at any time during the product development process, approval process or after approval may subject an applicant and/or sponsor to a variety of administrative or judicial sanctions, including refusal by the FDA to approve pending applications, withdrawal of an approval, imposition of a clinical hold, issuance of warning letters and other types of enforcement‑related letters, product recalls, product seizures, total or partial suspension of production or distribution, injunctions, fines, refusals of government contracts, restitution, disgorgement of profits or civil or criminal investigations and penalties brought by the FDA and the Department of Justice, or DOJ, or other governmental entities.

The steps before a biological product may be approved for marketing in the United States generally include:

The testing and approval process requires substantial time, effort and financial resources, and the receipt and timing of any approval is uncertain.

Preclinical studies

Preclinical studies include laboratory evaluation of the product candidate, as well as in vitro and animal studies to assess the potential safety and efficacy of the product candidate for use in humans. The conduct of preclinical studies is subject to federal regulations and requirements, including GLP regulations. The results of the preclinical studies, together with manufacturing information, analytical data, any available clinical data or literature and plans for clinical studies, among other things, are submitted to the FDA as part of an IND. Additional preclinical testing, such as toxicity studies, may continue after the IND is submitted.

Clinical studies

Clinical studies involve the administration of the product candidate to human subjects under the supervision of qualified investigators in accordance with GCP requirements. GCP requirements include, among other things, conducting the study in accordance with a written protocol, obtaining informed consent from study subjects and approval and ongoing review of the study by an IRB at each site where the study will be conducted.

A protocol for each clinical study and any subsequent protocol amendments must be submitted to the FDA as part of the IND. An IND automatically becomes effective 30 days after receipt by the FDA, unless before that time the FDA raises concerns or questions related to a proposed clinical study or places the study on clinical hold. In such a case, the IND sponsor and the FDA must resolve any outstanding concerns before the clinical study can begin.

Clinical studies are typically conducted in three sequential phases prior to approval, which may overlap or be combined:

Phase 1:  The product candidate is initially introduced into healthy human subjects or, in some cases, patients with the target disease (e.g., cancer) or condition. In Phase 1, the product candidate is typically tested for safety, dosage tolerance, absorption, metabolism, distribution, excretion and, if possible, to gain an early indication of its effectiveness and to determine optimal dosage.

Phase 2:  The product candidate is administered to a limited patient population to preliminarily evaluate the efficacy of the product for specific targeted diseases, to identify possible adverse effects and safety risks and to determine dosage tolerance and optimal dosage.

Phase 3:  The product candidate is administered to an expanded patient population, generally at geographically dispersed clinical study sites, in well‑controlled clinical studies to generate enough data to statistically evaluate the efficacy and safety of the product for approval, to establish the overall risk‑benefit profile of the product and to provide adequate information for the labeling of the product.

Phase 4 clinical studies may be conducted after initial marketing approval. These studies are used to gain additional experience from the treatment of patients in the intended therapeutic indication and to document a clinical benefit in the case of products approved under accelerated approval regulations or when otherwise requested by the FDA in the form of post‑market requirements or commitments.

Clinical studies at each phase of development may not be completed successfully within any specified period, or at all. Furthermore, the FDA, an IRB, the sponsor or the data monitoring committee, if applicable, may suspend or terminate a clinical study at any time on various grounds, including a finding that the research subjects are being exposed to an unacceptable health risk. The FDA will typically inspect one or more clinical sites to assure compliance with GCP and the integrity of the clinical data submitted.

Submission of a marketing application to the FDA

Assuming successful completion of required clinical testing and other requirements, the results of the preclinical studies and clinical studies, together with detailed information relating to the product’s chemistry, manufacture, controls and proposed labeling, among other things, are submitted to the FDA as part of a BLA requesting approval to market the product for one or more indications.

BLA pathway

Our ADC product candidates must be licensed via FDA approval of a BLA under Section 351 of the PHS Act on the basis of a demonstration that the product is safe, pure and potent. Once a BLA has been accepted for filing, the FDA’s goal is to review BLAs within ten months of the filing date for standard review or six months of the filing date for priority review. The review process is often significantly extended by FDA requests for additional information or clarification. The FDA may refer the application to an advisory committee for review, evaluation and recommendation as to whether the application should be approved. The FDA is not bound by the recommendation of an advisory committee, but it generally follows such recommendations.

Before approving the BLA, the FDA will inspect the facilities at which the biological product is manufactured and will not approve the product unless the facility is compliant with cGMPs. Additionally, the FDA will typically inspect one or more clinical study sites for compliance with GCP and integrity of the data supporting safety and efficacy.

During the approval process, the FDA also will determine whether to require post‑approval testing, including Phase 4 clinical studies and surveillance programs to monitor the effect of approved biologics after they are commercialized. In addition, the FDA will determine whether the biologic will require a REMS to ensure that the benefits of the product outweigh its risks, which could include medication guides, physician communication plans or elements to assure safe use, such as restricted distribution methods, patient registries and other risk minimization tools.

On the basis of the FDA’s evaluation of the BLA and accompanying information, including the results of the inspection of the manufacturing facilities, the FDA will issue either an approval of the BLA or a Complete Response Letter, detailing the deficiencies in the submission and the additional testing or information required for reconsideration of the application. Even with submission of this additional information, the FDA ultimately may decide that the application does not satisfy the regulatory criteria for approval.

If the FDA approves a product, it may limit the approved indications for use for the product, require that contraindications, warnings or precautions be included in the product labeling, require that post‑approval studies, including Phase 4 clinical studies, be conducted to further assess the product’s safety after approval, require testing and surveillance programs to monitor the product after commercialization or impose other conditions, including a REMS, which can materially affect the potential market and profitability of the product. The FDA may prevent or limit further marketing of a product based on the results of post‑market studies or surveillance programs. After approval, many types of changes to the approved product, such as adding new indications, manufacturing changes and additional labeling claims, are subject to further testing requirements and FDA review and approval.

Fast track, breakthrough therapy and priority review designations

The FDA is authorized to designate certain products for expedited review if they are intended to address an unmet medical need in the treatment of a serious or life‑threatening disease or condition. These programs are fast track designation, breakthrough therapy designation and priority review designation.

First, the FDA may designate a product for “fast track” review if it is intended for the treatment of a serious or life‑threatening disease or condition and it demonstrates the potential to address unmet medical needs for such disease or condition. For fast track products, sponsors may have greater interactions with the FDA, and the FDA may initiate review of sections of a fast track product’s BLA before the application is complete. This “rolling review” is available if the FDA determines, after preliminary evaluation of clinical data submitted by the sponsor, that a fast track product may be effective. The sponsor must also provide, and the FDA must approve, a schedule for the submission of the remaining information and the sponsor must pay applicable user fees.

Second, the FDA may designate a product as a breakthrough therapy if it is intended to treat a serious or life‑threatening disease or condition and preliminary clinical evidence indicates that the product may demonstrate substantial improvement over existing therapies on one or more clinically significant endpoints. The FDA may take certain actions with respect to breakthrough therapies, including holding meetings with the sponsor throughout the development process, providing timely advice to the product sponsor regarding development and approval, involving more senior staff in the review process, assigning a cross‑disciplinary project lead for the review team and taking other steps to design the clinical studies in an efficient manner.

Third, the FDA may designate a product for priority review if it treats a serious condition and, if approved, would provide a significant improvement in safety or effectiveness. A priority designation is intended to direct overall attention and resources to the evaluation of such applications and shortens the FDA’s goal for taking action on a marketing application from ten months to six months from the filing date.

Accelerated approval pathway

The FDA may grant accelerated approval to a product for a serious or life‑threatening condition that provides meaningful therapeutic advantage to patients over existing treatments. A product eligible for accelerated approval may be approved on the basis of either a surrogate endpoint that is reasonably likely to predict clinical benefit, or on a clinical endpoint that can be measured earlier than irreversible morbidity or mortality, that is reasonably likely to predict an effect on irreversible morbidity or mortality or other clinical benefit, taking into account the severity, rarity or prevalence of the condition and the availability or lack of alternative treatments.

The accelerated approval pathway is most often used in settings in which the course of a disease is long and an extended period of time is required to measure the intended clinical benefit of a product, even if the effect on the surrogate or intermediate clinical endpoint occurs rapidly. Thus, accelerated approval has been used extensively in the development and approval of products for treatment of a variety of cancers in which the goal of therapy is generally to improve survival or decrease morbidity and the duration of the typical disease course requires lengthy and sometimes large studies to demonstrate a clinical or survival benefit.

The accelerated approval pathway is usually contingent on a sponsor’s agreement to conduct, in a diligent manner, additional post‑approval confirmatory studies to verify and describe the product’s clinical benefit. As a result, a product candidate approved on this basis is subject to rigorous post‑marketing compliance requirements, including the completion of Phase 4 or post‑approval clinical studies to confirm the effect on the clinical endpoint. Failure to conduct required post‑approval studies, or to confirm a clinical benefit during post‑marketing studies, would allow the FDA to withdraw the product from the market on an expedited basis. All promotional materials for product candidates approved under accelerated regulations are subject to prior review by the FDA.

Post‑approval requirements

Products manufactured or distributed pursuant to FDA approvals are subject to pervasive and continuing regulation by the FDA, including, among other things, requirements relating to recordkeeping, periodic reporting, product sampling and distribution, advertising and promotion and reporting of adverse experiences with the product. After approval, most changes to the approved product, such as adding new indications or other labeling claims, are subject to prior FDA review and approval. There also are continuing, annual user fee requirements for any marketed products and the establishments at which such products are manufactured, as well as new application fees for certain supplemental applications.

In addition, manufacturers and other entities involved in the manufacture and distribution of approved products are required to register their establishments with the FDA and state agencies and are subject to periodic unannounced inspections by the FDA and these state agencies for compliance with cGMP requirements. Changes to the manufacturing process are strictly regulated and often require prior FDA approval before being implemented. FDA regulations also require investigation and correction of any deviations from cGMP and impose reporting and documentation requirements upon the sponsor and any third‑party manufacturers that the sponsor may decide to use. Accordingly, manufacturers must continue to expend time, money and effort in the area of production and quality control to maintain cGMP compliance.

Once an approval is granted, the FDA may withdraw the approval if compliance with regulatory requirements and standards is not maintained or if problems occur after the product reaches the market. Later discovery of previously unknown problems with a product, including adverse events of unanticipated severity or frequency, or with manufacturing processes, or failure to comply with regulatory requirements, may result in revisions to the approved labeling to add new safety information, imposition of post‑market studies or clinical studies to assess new safety risks or imposition of distribution or other restrictions under a REMS program. Other potential consequences include, among other things:

The FDA strictly regulates marketing, labeling, advertising and promotion of products that are placed on the market. Such products may be promoted only for the approved indications and in accordance with the provisions of the approved label. The FDA and other agencies actively enforce the laws and regulations prohibiting the promotion of off‑label uses, and a company that is found to have improperly promoted off‑label uses may be subject to significant liability.

Biosimilars and exclusivity

The Patient Protection and Affordable Care Act, as amended by the Health Care and Education Reconciliation Act, signed into law on March 23, 2010, or the Health Care Reform Act, includes a subtitle called the Biologics Price Competition and Innovation Act of 2009, or the BPCIA, which created an abbreviated approval pathway for biological products shown to be biosimilar to, or interchangeable with, an FDA‑licensed reference biological product. Biosimilarity requires a showing that the product is “highly similar” to the reference product notwithstanding minor differences in clinically inactive components and that there are no clinically meaningful differences between the biological product and the reference product in terms of safety, purity and potency. Interchangeability requires that a product is biosimilar to the reference product and the product must demonstrate that it can be expected to produce the same clinical results as the reference product in any given patient and, for products administered multiple times, the biologic and the reference biologic may be switched after one has been previously administered without increasing safety risks or risks of diminished efficacy

relative to exclusive use of the reference biologic. To date, the FDA has approved a number of biosimilars and has issued several guidance documents outlining its approach to the review and approval of biosimilars.

A reference biologic is entitled to 12 years of exclusivity from the time of first licensure of the product. In addition, the first biological product submitted under the abbreviated approval pathway that is determined to be interchangeable with, not just biosimilar to, the reference product has exclusivity against other biologics submitting under the abbreviated approval pathway for the lesser of (i) one year after the first commercial marketing, (ii) 18 months after approval if there is no legal challenge, (iii) 18 months after the resolution in the applicant’s favor of a lawsuit challenging the biologics’ patents if an application has been submitted or (iv) 42 months after the application has been approved if a lawsuit is ongoing within the 42‑month period.

The BPCIA is complex and is still being implemented by the FDA.  In addition, recent government proposals have sought to reduce the 12-year reference product exclusivity period. Other aspects of the BPCIA, some of which may impact the BPCIA exclusivity provisions, have also been the subject of recent litigation. As a result, the ultimate impact, implementation and meaning of the BPCIA is subject to significant uncertainty.

Pediatric studies and exclusivity

Under the Pediatric Research Equity Act of 2003, all applications for new active ingredients, new indications, new dosage forms, new dosing regimens or new routes of administration are required to contain an assessment of the safety and effectiveness of the product for the claimed indication in pediatric patients unless this requirement is waived, deferred or inapplicable.

Under the Best Pharmaceuticals for Children Act, a product may be eligible for pediatric exclusivity, which, if granted, adds six months to existing exclusivity periods and patent terms. This six‑month exclusivity, which runs from the end of other exclusivity protection or patent term, may be granted based on the voluntary completion of a pediatric study in accordance with an FDA‑issued written request for such a study.

Orphan drug designation and exclusivity

Under the Orphan Drug Act, the FDA may designate a product, including a biological product, as an “orphan drug” if it is intended to treat a rare disease that affects fewer than 200,000 individuals in the United States or, if it affects more than 200,000 individuals in the United States, a disease for which there is no reasonable expectation that the cost of developing and making the product for this type of disease or condition will be recovered from sales in the United States.

A product that receives the first FDA approval for a product for the indication for which it has orphan designation is entitled to orphan drug exclusivity, which means the FDA may not approve any other application to market the same product for the same indication for a period of seven years, except in limited circumstances, such as a showing of clinical superiority over the product with orphan exclusivity.

Patent term restoration

A patent claiming a new product may be eligible for a limited patent term extension under the Hatch‑Waxman Amendments, which permits a patent restoration of up to five years for patent term lost during product development and the FDA regulatory review. The restoration period granted is typically one‑half the time between the effective date of an IND and the submission date of a BLA, plus the time between the submission date of a BLA and the ultimate approval date. Patent term restoration cannot be used to extend the remaining term of a patent past a total of 14 years from the product’s approval date. Only one patent applicable to an approved drug product is eligible for the extension, and the application for the extension must be submitted prior to the expiration of the patent in question. The USPTO, reviews and approves the application for any patent term extension or restoration in consultation with the FDA.

Review and approval outside the United States

In order to market any product outside of the United States, we would need to comply with numerous and varying regulatory requirements of other countries and jurisdictions governing, among other things, clinical studies, marketing authorization, commercial sales and distribution of our products. Whether or not we obtain FDA approval for a product, we would need to obtain the necessary approvals by the comparable foreign regulatory authorities before we can commence clinical studies or marketing of the product in foreign countries and jurisdictions. Although many of the issues discussed above with respect to the United States apply similarly in the context of the European Union and other geographies, the approval process varies between countries and jurisdictions and can involve additional product testing and additional administrative review periods. The time required to obtain approval in other countries and jurisdictions might differ from and be longer than that required to obtain FDA approval. Regulatory approval in one country or jurisdiction does not ensure regulatory approval in another, but a failure or delay in obtaining regulatory approval in one country or jurisdiction may negatively impact the regulatory process in others.

Pharmaceutical coverage, pricing and reimbursement

Significant uncertainty exists as to the coverage and reimbursement status of products approved by the FDA and other government authorities. Sales of pharmaceutical products depend in significant part on the availability and adequacy of third‑party reimbursement. Third‑party payors include government health administrative authorities, including authorities at the U.S. federal and state level, managed care providers, private health insurers and other organizations. Third‑party payors are increasingly challenging the prices charged for, examining the medical necessity of and assessing the cost‑effectiveness of medical products and services.

In order to secure coverage and reimbursement for any product that might be approved for sale, a company may need to conduct expensive pharmacoeconomic studies in order to demonstrate the medical necessity and cost‑effectiveness of the product, in addition to the costs required to obtain FDA or other comparable regulatory approvals. A payor’s decision to provide coverage for a product does not imply that an adequate reimbursement rate will be approved. Third‑party reimbursement may not be sufficient to maintain price levels high enough to realize an appropriate return on investment in product development.

The containment of healthcare costs also has become a priority of federal, state and foreign governments and the prices of drugs and biologics have been a focus in this effort. Governments have shown significant interest in implementing cost‑containment programs, including price controls, restrictions on reimbursement and requirements for substitution of generic products. Adoption of price controls and cost‑containment measures, and adoption of more restrictive policies in jurisdictions with existing controls and measures, could further limit a company’s revenue generated from the sale of any approved products. Coverage policies and third‑party reimbursement rates may change at any time. Even if favorable coverage and reimbursement status is attained for one or more products for which a company or its collaborators receive marketing approval, less favorable coverage policies and reimbursement rates may be implemented in the future.

In the European Union, pricing and reimbursement schemes vary widely from country to country. Some countries provide that products may be marketed only after a reimbursement price has been agreed. Some countries may require the completion of additional studies that compare the cost‑effectiveness of a particular product candidate to currently available therapies, or so called health technology assessments, to obtain reimbursement or pricing approval. For example, the European Union provides options for its member states to restrict the range of products for which their national health insurance systems provide reimbursement and to control the prices of medicinal products for human use. European Union member states may approve a specific price for a product or may instead adopt a system of direct or indirect controls on the profitability of the company.

The downward pressure on healthcare costs in general, particularly prescription drugs and biologics, has become intense. As a result, increasingly high barriers are being erected to the entry of new products. In addition, in some countries, cross‑border imports from low‑priced markets exert competitive pressure that may reduce pricing within a country. Any country that has price controls or reimbursement limitations for products may not allow favorable reimbursement and pricing arrangements.

Healthcare law and regulation

Our current and future arrangements with healthcare professionals, principal investigators, consultants, customers and third‑party payors are and will be subject to various federal, state and foreign fraud and abuse laws and other healthcare laws and regulations. These laws and regulations may impact, among other things, our arrangements with third‑party payors, healthcare professionals who participate in our clinical research programs, healthcare professionals and others who purchase, recommend or prescribe our approved products and our proposed sales, marketing, distribution and education programs. The federal and state healthcare laws and regulations that may affect our ability to operate include, without limitation, the following:

We will be required to spend substantial time and money to ensure that our business arrangements with third parties comply with applicable healthcare laws and regulations. Violations of these laws can subject us to criminal, civil and administrative sanctions including monetary penalties, damages, fines, disgorgement, individual imprisonment and exclusion from participation in government funded healthcare programs, such as Medicare and Medicaid, additional reporting requirements and oversight if we become subject to a corporate integrity agreement or similar agreement to resolve allegations of non‑compliance with these laws, and reputational harm. Additionally, we may be required to curtail or

restructure our operations. Moreover, we expect that there will continue to be federal and state laws and regulations, proposed and implemented, that could impact our future operations and business.

Healthcare reform

Our revenue and operations could be affected by changes in healthcare spending and policy in the United States and elsewhere. We operate in a highly regulated industry and new laws, regulations or judicial decisions, or new interpretations of existing laws, regulations or decisions, related to healthcare availability, the method of delivery or payment for healthcare products and services could negatively impact our business, operations and financial condition. As noted above, the U.S. Congress, state legislatures and foreign regulators from time to time propose and adopt initiatives aimed at cost containment, which could impact our ability to sell our products profitably. For example, the Health Care Reform Act substantially changed the way healthcare is financed by both governmental and private insurers. The law contains a number of provisions that affect coverage and reimbursement of drug products and/or that could potentially reduce the demand for our products such as:

Since its enactment, there have been judicial and Congressional challenges to certain aspects of the Health Care Reform Act, and we expect there will be additional challenges and amendments to the Health Care Reform Act in the future. The current Presidential administration and members of the U.S. Congress have indicated that they may continue to seek to modify, repeal or otherwise invalidate all, or certain provisions of, the Health Care Reform Act. Most recently, on December 22, 2017, the Tax Cuts and Jobs Act was enacted, which, among other things, removes penalties for not complying with the individual mandate to carry health insurance. It is uncertain the extent to which any such changes may impact our business or financial condition. We cannot predict the ultimate content, timing or effect of any changes to the Health Care Reform Act or other federal and state reform efforts. There is no assurance that federal or state healthcare reform will not adversely affect our business and financial results, and we cannot predict how future federal or state legislative, judicial or administrative changes relating to healthcare reform will affect our business.

In addition, other legislative changes have been proposed and adopted since the Health Care Reform Act was enacted. The Budget Control Act of 2011 includes provisions to reduce the federal deficit. The Budget Control Act, as amended, resulted in the imposition of 2% reductions in Medicare payments to providers which began in April, 2013, and will remain in effect through 2024 unless additional Congressional action is taken. Any significant spending reductions affecting Medicare, Medicaid or other publicly funded or subsidized health programs that may be implemented and/or any significant taxes or fees that may be imposed on us, as part of any broader deficit reduction effort or legislative replacement to the Budget Control Act, could have an adverse impact on our results of operations.

Additional regulation

In addition to the foregoing, state and federal laws regarding environmental protection and hazardous substances, including the Occupational Safety and Health Act, the Resource Conservation and Recovery Act and the Toxic Substances Control Act, affect our business. These and other laws govern the use, handling and disposal of various biologic, chemical and radioactive substances used in, and wastes generated by, operations. If our operations result in contamination of the environment or expose individuals to hazardous substances, we could be liable for damages and governmental fines. Equivalent laws have been adopted in foreign countries that impose similar obligations.

Intellectual property

We actively seek to protect the proprietary technology that we consider important to our business, including pursuing patents that cover our ADC platform, proprietary composition of matter, ADC product candidates and methods of using and manufacturing the same, as well as any other relevant inventions and improvements that are considered commercially important to the development of our business. We also rely on trade secrets, know‑how and continuing technological innovation to develop and maintain our proprietary and intellectual property position.

Our commercial success will depend significantly on our ability to obtain and maintain patent and other proprietary protection for the technology, inventions and improvements we consider important to our business, and to defend our patents, preserve the confidentiality of our trade secrets and operate without infringing the patents and proprietary rights of third parties. Our policy is to seek to protect our proprietary and intellectual property position by, among other methods, filing U.S., international (under Patent Cooperation Treaty, or PCT) and foreign patent applications related to our proprietary technology, inventions and improvements that we consider to be important to the development and implementation of our business.

The term of individual patents depends upon the legal term for patents in the countries in which they are obtained. In most countries, including the United States, the patent term is 20 years from the earliest filing date of a non‑provisional patent application. In the United States, a patent’s term may be lengthened by patent term adjustment, which compensates a patentee for administrative delays by the USPTO in examining and granting a patent, or may be shortened if a patent is terminally disclaimed over an earlier filed patent. The term of a patent that covers a drug or biological product may also be eligible for patent term extension when FDA approval is granted, provided statutory and regulatory requirements are met. In the future, if and when our drug candidates receive approval by the FDA or foreign regulatory authorities, we expect to apply for patent term extensions on issued patents covering those drugs, depending upon the length of the clinical studies for each drug and other factors. There can be no assurance that any of our pending patent applications will issue or that we will benefit from any patent term extension or favorable adjustments to the terms of any of our patents.

As with other biotechnology and pharmaceutical companies, our ability to maintain and solidify our proprietary and intellectual property position for our drug candidates and technologies will depend on our success in obtaining effective patent claims and enforcing those claims if granted. However, our pending patent applications, and any patent applications that we may in the future file or license from third parties, may not result in the issuance of patents. We also cannot predict the breadth of claims that may be allowed or enforced in our patents. Any issued patents that we may currently own or license or may receive in the future may be challenged, invalidated, circumvented or have the scope of their claims narrowed. For example, we cannot be certain of the priority of inventions covered by pending third‑party patent applications. If third parties prepare and file patent applications in the United States that also claim technology or therapeutics to which we have rights, we may have to participate in interference proceedings in the USPTO to determine priority of invention, which could result in substantial costs to us, even if the eventual outcome is favorable to us, which is highly unpredictable. In addition, because of the extensive time required for clinical development and regulatory review of a drug candidate we may develop, it is possible that, before any of our drug candidates can be commercialized, any related patent may expire or remain in force for only a short period following commercialization, thereby limiting the protection such patent would afford the respective product and any competitive advantage such patent may provide. For more information regarding the risks related to our intellectual property, please see “Risk factors—Risks related to our intellectual property.”

As of February 28, 2018, we owned, in all of our patent portfolios, 13 issued U.S. patents, 13 pending non‑provisional U.S. patent applications (including one allowed U.S. patent application), 10 pending provisional U.S. patent applications, 25 issued foreign patents, five pending PCT patent applications and 80 pending foreign patent applications in a number of jurisdictions, including, but being not limited to, Australia, Brazil, Canada, China, Europe, Eurasia, Gulf Cooperation Council, Hong Kong, Israel, India, Indonesia, Iran, Japan, Mexico, Macau, New Zealand, Russia, South Korea, South Africa and Taiwan. Our seven issued U.S. patents covering our Fleximer ADC platform are projected to expire in 2032, excluding any additional term for patent term adjustments or patent term extensions; our one issued U.S. patent covering our Dolaflexin ADC platform is projected to expire in 2034, excluding any additional term for patent term adjustments or patent term extensions; our two issued U.S. patents covering our XMT-1522 ADC are projected to expire in 2035, excluding any additional term for patent term adjustments or patent term extensions; our additional two issued U.S. patents

are projected to expire in 2033 and 2035, excluding any additional term for patent term adjustments or patent term extensions; and any patent that may issue from our pending U.S. applications is projected to expire between 2032 and 2038, in each case, excluding any additional term for patent term adjustments or patent term extensions. In addition, we have exclusively in‑licensed three issued U.S. patents, one pending U.S. patent application and one issued European patent for the NaPi2b antibody from Recepta. These in‑licensed issued U.S. and foreign patents are projected to expire in 2029, excluding any additional term for patent term adjustments or patent term extensions. We have so far not filed for patent protection in all national and regional jurisdictions where such protection may be available. In addition, we may decide to abandon national and regional patent applications before they are granted. Finally, the grant proceeding of each national or regional patent is an independent proceeding which may lead to situations in which applications might in some jurisdictions be refused by the relevant registration authorities, while granted by others. It is also quite common that depending on the country, various scopes of patent protection may be granted on the same product candidate or technology.

The intellectual property portfolio of our ADC platform, our ADC product candidates and components thereof are summarized below. Some of these portfolios are in very early stages and prosecution has yet to commence on most of the pending patent applications. Prosecution is a lengthy process, during which the scope of the claims initially submitted for examination by the USPTO may be narrowed (sometimes significantly) by the time they issue, if they issue at all. We expect this to be the case with respect to our pending patent applications referred to below.

Fleximer ADC platform

The intellectual property portfolio for our Fleximer ADC platform is directed to compositions of matter for the Fleximer ADCs, as well as methods of using and making these novel conjugates, compositions of matter for Fleximer drug conjugates prior to conjugation with the antibody or antibody fragment and methods of making the same and compositions of matter for our proprietary auristatin compounds (and by extension our proprietary DolaLock feature) and conjugates thereof (e.g., to Fleximer and/or an antibody or antibody fragment). As of February 28, 2018, we owned seven issued U.S. patents, three pending U.S. patent applications (including one allowed U.S. patent application), 25 issued foreign patents and 18 pending foreign patent applications in a number of jurisdictions, including Australia, Brazil, Canada, China, Europe, Hong Kong, Israel, India, Japan, Macau, Mexico, Russia, South Korea, and Taiwan. Any U.S. or foreign patent issuing from the pending applications covering the Fleximer ADC platform is projected to expire in June 2032, excluding any additional term for patent term adjustments or patent term extensions.

Dolaflexin ADC platform

The intellectual property portfolio for our Dolaflexin ADC platform is directed to compositions of matter for the Dolaflexin ADCs, as well as methods of using and making these novel conjugates, compositions of matter for Dolaflexin drug conjugates prior to conjugation with the antibody or antibody fragment and methods of making the same. As of February 28, 2018, we owned one issued U.S. patent, four pending U.S. patent applications (including three pending provisional U.S. patent applications) and 13 pending foreign patent applications in a number of jurisdictions, including Australia, Brazil, Canada, China, Eurasia, Europe, Israel, India, Japan, South Korea, Mexico and South Africa. Any U.S. or foreign patent issuing from the pending applications covering Dolaflexin ADC platform is projected to expire in October 2034, and any U.S. or foreign patent issuing from the pending applications covering the method of making the Dolaflexin ADC is projected to expire in June 2038, excluding any additional term for patent term adjustments or patent term extensions.

XMT‑1522 ADC

The intellectual property portfolio for our HER2 ADC product candidate, XMT‑1522, is directed to compositions of matter for our novel HER2 antibody or fragment thereof and conjugates and combinations thereof (including XMT‑1522) based on our Dolaflexin platform, as well as methods of using and making these novel conjugates. This intellectual property portfolio covering the novel HER2 antibody or fragment thereof is assigned to us from Adimab. As of February 28, 2018, we owned two issued U.S. patents, three pending U.S. patent applications (including one pending provisional U.S. patent application), one pending PCT patent application, and 38 pending foreign patent applications in a number of jurisdictions, including Algeria, African Regional Intellectual Property Organization, or the ARIPO, Argentina, Australia, Brazil, Canada, Chile, China, Columbia, Costa Rica, Dominican Republic, Ecuador, Egypt, Eurasia, Europe, Georgia, Gulf Cooperation Council, Israel, India, Indonesia, Iran, Japan, Pakistan, South Korea, Malaysia, Mexico, New Zealand, Peru,

Philippines, Singapore, South Africa, Thailand, Taiwan, Tunisia, Ukraine, Uzbekistan and Vietnam. Any U.S. or foreign patent issuing from the pending applications covering XMT‑1522 ADC platform is projected to expire in June 2035, and any U.S. or foreign patent issuing from the pending applications covering the XMT‑1522 ADC platform in combination with checkpoint inhibitors is projected to expire in February 2038, excluding any additional term for patent term adjustments or patent term extensions.

XMT‑1536 ADC

The intellectual property portfolio for our NaPi2b ADC product candidate, XMT‑1536, is directed to compositions of matter for our novel ADC based on exclusively in‑licensed NaPi2b antibody and our Dolaflexin platform, as well as methods of using and making these novel conjugates. As of February 28, 2018, we owned three pending U.S. applications (including two pending provisional U.S. patent applications), four pending foreign patent applications, and one pending PCT application directed to the composition of matter for XMT‑1536, methods of using and making same and companion diagnostics for XMT-1536 ADC. We also intend to enter the national/regional phase of the pending PCT patent application in foreign jurisdictions, including Australia, Brazil, Canada, China, Eurasia, Europe, Israel, India, Japan, South Korea, Mexico and South Africa. Any U.S. or foreign patent issuing from the pending applications covering XMT‑1536 is projected to expire in March 2037, and any U.S. or foreign patent issuing from the pending applications covering XMT‑1536 companion diagnostics is projected to expire in September 2038, excluding any additional term for patent term adjustments or patent term extensions.

In addition, we have exclusively in‑licensed three issued U.S. patents, one pending U.S. patent application and one issued European patent for the novel NaPi2b antibody from Recepta, which Recepta licensed from Ludwig Institute for Cancer Research. These in‑licensed issued U.S. and European patents are projected to expire in 2029, excluding any additional term for patent term adjustments or patent term extensions. Recepta still owns one pending Brazilian patent application for the NaPi2b antibody, which is not licensed to us. A patent issuing from this Brazilian patent application is projected to expire in 2029.

Novel DNA Alkylators and Novel Scaffolds

The intellectual property portfolio for our novel DNA alkylators and novel scaffold platforms is directed to compositions of matter for the novel DNA alkylators, ADCs thereof, novel scaffolds, as well as methods of using and making these novel conjugates, scaffolds and compositions of matter. As of February 28, 2018, we owned five pending U.S. patent applications (including three pending provisional U.S. patent applications), three pending foreign patent applications, and three pending PCT patent applications. We intend to enter the national/regional phase of the PCT patent applications in a number of jurisdictions, including Australia, Brazil, Canada, China, Europe, Hong Kong, Israel, India, Japan, Macau, Mexico, Russia, South Korea, and Taiwan. Any U.S. or foreign patent issuing from the pending applications covering the novel DNA alkylators and novel scaffold platforms is projected to expire between 2037 and 2038, excluding any additional term for patent term adjustments or patent term extensions.

In addition to patents, we rely upon unpatented trade secrets and know‑how and continuing technological innovation to develop and maintain our competitive position. We seek to protect our proprietary information, in part, by executing confidentiality agreements with our collaborators and scientific advisors and non‑competition, non‑solicitation, confidentiality and invention assignment agreements with our employees and consultants. We have also executed agreements requiring assignment of inventions with selected scientific advisors and collaborators. The confidentiality agreements we enter into are designed to protect our proprietary information and the agreements or clauses requiring assignment of inventions to us are designed to grant us ownership of technologies that are developed through our relationship with the respective counterparty. We cannot guarantee, however, that we will have executed such agreements with all applicable employees and contractors, or that these agreements will afford us adequate protection of our intellectual property and proprietary information rights. With respect to the building of our proprietary compound library, we consider trade secrets and know‑how to be our primary intellectual property. Trade secrets and know‑how can be difficult to protect. In particular, we anticipate that with respect to this technology platform, these trade secrets and know‑how will over time be disseminated within the industry through independent development and public presentations describing the methodology. For more information regarding the risks associated with our trade secrets, please see “Risk

factors—Risks related to our intellectual property—Confidentiality agreements with employees and third parties may not prevent unauthorized disclosure of trade secrets and other proprietary information.”

Competition

The biotechnology and biopharmaceutical industries, and the oncology subsector, are characterized by rapid evolution of technologies, fierce competition and strong defense of intellectual property. Any product candidates that we successfully develop and commercialize will have to compete with existing therapies and new therapies that may become available in the future. While we believe that our proprietary ADC platform and scientific expertise provide us with competitive advantages, a wide variety of institutions, including large biopharmaceutical companies, specialty biotechnology companies, academic research departments and public and private research institutions, are actively developing potentially competitive products and technologies. These competitors generally fall within the following categories:

New cancer treatments:  Many global pharmaceutical companies, as well as medium and small biotechnology companies, are pursuing new cancer treatments whether small molecules, biologics or ADCs. Any of these treatments could prove to be superior clinically to our products.

ADC platforms:  Although Dolaflexin and the new platform initiatives we have underway are highly differentiated and proprietary, many companies continue to invest in innovation in the ADC field including new payload classes, new conjugation approaches and new targeting moieties. Any of these initiatives could lead to a platform that has superior properties to ours. We are aware of multiple companies with ADC technologies that may be competitive to our ADC platforms, including Astellas, AstraZeneca, Bristol‑Myers Squibb, Daiichi Sankyo, ImmunoGen, Immunomedics, Pfizer and Seattle Genetics. These companies or their partners, including AbbVie, Genentech, Lilly, Novartis, Sanofi and Takeda, may develop ADCs based on these ADC technologies which compete in the same indications as our current and future ADC product candidates. We expect to compete on improved efficacy, safety and tolerability compared to other ADCs and if our products are not demonstrably superior in these respects compared to other approved therapeutics, we may not be able to compete effectively.

One of the four currently approved ADC therapies in the United States, ado‑trastuzumab emtansine marketed by Genentech, is a HER2‑targeted ADC approved for use in HER2‑positive patients and, even though we are developing, and expect to get approval for, XMT‑1522 for lower expressing HER2 patients, ado‑trastuzumab emtansine may compete with our HER2‑targeted ADC, XMT‑1522, if XMT‑1522 is approved. In addition, other companies are exploring treatments for patients with low HER2 expression, or may do so in the future.

Many of our competitors, either alone or with strategic partners, have substantially greater financial, technical and human resources than we do. Accordingly, our competitors may be more successful than us in obtaining approval for treatments and achieving widespread market acceptance, rendering our treatments obsolete or non‑competitive. Accelerated merger and acquisition activity in the biotechnology and biopharmaceutical industries may result in even more resources being concentrated among a smaller number of our competitors. These companies also compete with us in recruiting and retaining qualified scientific and management personnel, establishing clinical study sites and patient registration for clinical studies and acquiring technologies complementary to, or necessary for, our programs. Smaller or early‑stage companies may also prove to be significant competitors, particularly through collaborative arrangements with large and established companies. Our commercial opportunity could be substantially limited in the event that our competitors develop and commercialize products that are more effective, safer, less toxic, more convenient or less expensive than our comparable products. In geographies that are critical to our commercial success, competitors may also obtain regulatory approvals before us, resulting in our competitors building a strong market position in advance of our products’ entry. We believe the factors determining the success of our programs will be the efficacy, safety and tolerability of our product candidates

Employees

As of February 28, 2018, we had 78 full time employees, including 62 with M.D., Ph.D. or other advanced degrees. Of these full time employees, 64 are engaged in research and development and 14 are engaged in general and administrative

activities. None of our employees are represented by a labor union or covered by a collective bargaining agreement. We consider our relationship with our employees to be good.

Facilities

We occupy approximately 34,000 square feet of office and laboratory space in Cambridge, MA under a lease that expires in early 2021. We have an option to extend the lease term for an additional five years. We believe that this office and laboratory space is sufficient to meet our current needs and that suitable additional space will be available as and when needed.

Legal proceedings

From time to time, we may be subject to various legal proceedings and claims that arise in the ordinary course of our business activities. Although the results of litigation and claims cannot be predicted with certainty, as of the date of this Annual Report on Form 10-K, we do not believe we are party to any claim or litigation, the outcome of which, if determined adversely to us, would individually or in the aggregate be reasonably expected to have a material adverse effect on our business. Regardless of the outcome, litigation can have an adverse impact on us because of defense and settlement costs, diversion of management resources and other factors.

ITEM 1A.           RISK FACTORS

The following risk factors and other information included in this Annual Report on Form 10-K should be carefully considered. The risks and uncertainties described below are not the only ones we face. Additional risks and uncertainties not presently known to us or that we currently believe to be immaterial may also adversely affect our business. If any of the following risks occur, our business, financial condition, results of operations and future growth prospects could be materially and adversely affected.

Risks related to our financial position and need for additional capital

We have incurred net losses since our inception, we have no products approved for commercial sale and we anticipate that we will continue to incur substantial operating losses for the foreseeable future. We may never achieve or sustain profitability.

We have incurred net losses since our inception. Our net loss was $38.7 million for the year ended December 31, 2017. As of December 31, 2017, we had an accumulated deficit of $97.9 million. We do not know when or whether we will become profitable. To date, we have not commercialized any products and therefore have never generated any revenues from the sale of products, and we do not expect to generate any product revenues in the foreseeable future. Our losses have resulted principally from costs incurred in our discovery and development activities. Our net losses may fluctuate significantly from quarter to quarter and year to year.

We have devoted most of our financial resources to research and development, including our clinical and preclinical development activities. To date, we have financed our operations primarily through the sale of equity securities and the receipt of funds through strategic partnerships with third parties. The amount of our future net losses will depend, in part, on the rate of our future expenditures. We have not completed pivotal clinical studies for any product candidate and only have two product candidates in clinical studies. It will be several years, if ever, before we have a product candidate ready for commercialization. Even if we obtain regulatory approval to market a product candidate, our future revenues would depend upon the size of the market or markets in which our product candidates received such approval and our ability to achieve sufficient market acceptance, reimbursement from third‑party payors and adequate market share for our product candidates in those markets.

We expect to continue to incur significant expenses and increasing net losses for at least the next several years. We expect our expenses will increase substantially in connection with our ongoing activities, as we:

If we are required by the United States Food and Drug Administration, or FDA, or any equivalent foreign regulatory authority to perform clinical studies or studies in addition to those we currently expect to conduct, or if there are any delays in completing the clinical studies of XMT‑1522 or XMT‑1536, our expenses could increase.

To become and remain profitable, we must succeed in developing our ADC product candidates, obtaining regulatory approval for them, and manufacturing, marketing and selling those products for which we may obtain regulatory approval. We may not succeed in these activities, and we may never generate revenue from product sales or strategic partnerships in an amount sufficient to achieve profitability. Even if we achieve profitability in the future, we may not be able to sustain profitability in subsequent periods. Our failure to become or remain profitable would depress our market value and could impair our ability to raise capital, expand our business, discover or develop other ADC product candidates or continue our operations.

We will require substantial additional financing to achieve our goals, and a failure to obtain this necessary capital when needed could force us to delay, limit, reduce or terminate our product development or commercialization efforts.

Our cash, cash equivalents and marketable securities were $125.2 million as of December 31, 2017. We have utilized substantial amounts of cash since our inception and expect that we will continue to expend substantial resources for the foreseeable future developing XMT‑1522, XMT‑1536 and any future ADC product candidates. These expenditures may include costs associated with research and development, conducting preclinical studies and clinical studies, potentially obtaining regulatory approvals and manufacturing products, as well as marketing and selling products approved for sale, if any, and potentially acquiring new technologies. In addition, other unanticipated costs may arise. Because the outcome of our planned and anticipated clinical studies is highly uncertain, we cannot reasonably estimate the actual amounts necessary to successfully complete the development and commercialization of our ADC product candidates. Our costs will increase if we experience any delays in our clinical studies for XMT‑1522 or XMT‑1536, including delays in enrollment of patients. We also incur costs associated with operating as a public company, hiring additional personnel and expanding our facilities.

Our future capital requirements depend on many factors, including:

Based on our current operating plan, we estimate that our existing cash, cash equivalents and marketable securities will be sufficient to fund our projected operating requirements through at least mid-2019 and to fund our Phase 1 clinical studies for XMT‑1522 and XMT‑1536. Our operating plan, however, may change as a result of many factors currently unknown to us and we may need additional funds sooner than planned. Additional funds may not be available when we need them on terms that are acceptable to us, or at all. If adequate funds are not available to us on a timely basis, we may be required to delay, limit, reduce or terminate preclinical studies, clinical studies or other development activities for one or more of our ADC product candidates or delay, limit, reduce or terminate our establishment of sales and marketing capabilities or other activities that may be necessary to commercialize our ADC product candidates. In addition, we may seek additional capital due to favorable market conditions or strategic considerations even if we believe we have sufficient funds for our current or future operating plans.

Raising additional capital may cause dilution to our existing stockholders, restrict our operations or require us to relinquish rights to our technologies or ADC product candidates on unfavorable terms to us.

We may seek additional capital through a variety of means, including through private and public equity offerings and debt financings. To the extent that we raise additional capital through the sale of equity or convertible debt securities, the ownership interests of our common stockholders will be diluted, and the terms of such equity or convertible debt securities may include liquidation or other preferences that are senior to or otherwise adversely affect the rights of our common stockholders. Debt financing, if available, may involve agreements that include covenants limiting or restricting our ability to take certain actions, such as incurring additional debt, making capital expenditures, declaring dividends or encumbering our assets to secure future indebtedness, each of which could adversely impact our ability to conduct our business and execute our operating plan. If we raise additional funds through strategic partnerships with third parties, we may have to relinquish valuable rights to our technologies, including our ADC platforms, or ADC product candidates, or grant licenses on terms that are not favorable to us. If we are unable to raise additional funds through equity or debt financing when needed, we may be required to delay, limit, reduce or terminate our product development or commercialization efforts for XMT‑1522, XMT‑1536 or any other ADC product candidate, or grant rights to third parties to develop and market ADC product candidates that we would otherwise prefer to develop and market ourselves.

We may expend our resources to pursue a particular product candidate and fail to capitalize on product candidates that may be more profitable or for which there is a greater likelihood of success.

Because we have limited financial and managerial resources, we focus on specific product candidates. As a result, we may forgo or delay pursuit of opportunities with other product candidates that later prove to have greater commercial potential. Our resource allocation decisions may cause us to fail to capitalize on viable commercial products or profitable market opportunities. Failure to properly assess potential product candidates could result in our focus on product candidates with low market potential, which would harm our business and financial condition. Our spending on current and future research and development programs and product candidates for specific indications may not yield any commercially viable product candidates. If we do not accurately evaluate the commercial potential or target market for a particular product candidate, we may relinquish valuable rights to that product candidate through partnering, licensing or other royalty arrangements in cases in which it would have been more advantageous for us to retain sole development and commercialization rights to such product candidate.

Risks related to development and approval of our ADC product candidates

Failure of a discovery program or product candidate may occur at any stage of preclinical or clinical development, and, because our and our partner’s discovery programs and our product candidates are in an early stage of preclinical or clinical development, there is a relatively higher risk of failure and we or our partners may never succeed in generating revenue from such discovery programs or product candidates.

Our early encouraging preclinical results for XMT‑1522 and XMT‑1536 are not necessarily predictive of the results of our ongoing or future discovery programs or clinical studies. Promising results in preclinical studies of a drug candidate may not be predictive of similar results in later‑stage preclinical studies or in humans during clinical studies. Many companies in the pharmaceutical and biotechnology industries have suffered significant setbacks in late‑stage clinical studies after achieving positive results in early‑stage development, including early‑stage clinical studies, and we cannot be certain that we will not face similar setbacks. These companies’ setbacks have been caused by, among other things, preclinical findings made while clinical studies were underway or safety or efficacy observations made in preclinical studies and clinical studies, including previously unreported adverse events.

Any clinical studies that we may conduct may not demonstrate the efficacy and safety necessary to obtain regulatory approval to market our product candidates. If the results of our ongoing or future clinical studies are inconclusive with respect to the efficacy of our ADC product candidates or if we do not meet the clinical endpoints with statistical significance or if there are safety concerns or adverse events associated with our ADC product candidates, we may be prevented or delayed in obtaining marketing approval for our ADC product candidates. There can be significant variability in safety or efficacy results between different clinical studies of the same product candidate due to numerous factors, including changes in study procedures set forth in protocols, differences in the size and type of the patient populations, changes in and adherence to the clinical study protocols and the rate of dropout among clinical study participants. Moreover, preclinical and clinical data are often susceptible to varying interpretations and analyses, and many companies that believed their product candidates performed satisfactorily in preclinical studies and clinical studies nonetheless failed to obtain FDA approval.

Alternatively, even if we obtain regulatory approval, that approval may be for indications or patient populations that are not as broad as intended or desired or may require labeling that includes significant use or distribution restrictions or safety warnings. We may also be required to perform additional or unanticipated clinical studies to obtain approval or be subject to additional post‑marketing testing requirements to maintain regulatory approval. In addition, regulatory authorities may withdraw their approval of a product or impose restrictions on its distribution, such as in the form of a modified REMS. The failure to obtain timely regulatory approval of product candidates, any product marketing limitations or a product withdrawal would negatively impact our business, results of operations and financial condition.

We currently have only two ADC product candidates, XMT‑1522 and XMT-1536, in clinical studies. A failure of these product candidates in clinical development would adversely affect our business and may require us to discontinue development of other ADC product candidates based on the same technology.

XMT‑1522 and XMT-1536 are our only clinical‑stage development product candidates. While we have certain other preclinical programs in development and we intend to develop other product candidates, it will take additional investment and time for such programs to reach the same stage of development as XMT‑1522 and XMT-1536. In addition, we have other product candidates in our current pipeline that are based on the same ADC platform. If XMT‑1522 or XMT-1536 fails in development as a result of any underlying problem with our ADC platform, then we may be required to discontinue development of the ADC product candidates that are based on the same technology. If we were required to discontinue development of XMT‑1522 or XMT-1536 or if XMT‑1522 or XMT-1536 were to fail to receive regulatory approval or were to fail to achieve sufficient market acceptance, we could be prevented from or significantly delayed in achieving profitability.

Delays in the commencement, enrollment or completion of clinical studies of our ADC product candidates could result in increased costs to us as well as a delay or failure in obtaining regulatory approval, or prevent us from commercializing our ADC product candidates on a timely basis, or at all.

We cannot guarantee that clinical studies, including our ongoing Phase 1 clinical studies and anticipated additional clinical studies for XMT‑1522 and XMT-1536, will be conducted as planned or completed on schedule, if at all. A failure of one or more clinical studies can occur at any stage of testing. Events that may prevent successful or timely commencement, enrollment or completion of clinical development include, among others:

Delays, including delays caused by the above factors, can be costly and could negatively affect our ability to complete a clinical study. If we or our partners are not able to successfully complete clinical studies, we or they will not be able to obtain regulatory approval and will not be able to commercialize our ADC product candidates or our partners’ ADC product candidates based on our technology.

An inability to enroll sufficient numbers of patients in our clinical studies could result in increased costs and longer development periods for our product candidates.

Clinical studies require sufficient patient enrollment, which is a function of many factors, including:

Challenges in recruiting and enrolling suitable patients to participate in clinical studies that meet the criteria of the protocol for clinical studies could increase costs and result in delays to our current development plan for XMT‑1522, XMT‑1536 or any other future ADC product candidate.

We may seek a Breakthrough Therapy Designation or Fast Track Designation by the FDA for any of our product candidates, and we may be unsuccessful. If we are successful, the designation may not actually lead to a faster development or regulatory review or approval process, and it does not increase the likelihood that any product candidate would receive marketing approval.

We may seek a Breakthrough Therapy Designation or Fast Track Designation for any of our product candidates. A breakthrough therapy is defined as a drug that is intended, alone or in combination with one or more other drugs, to treat a serious or life-threatening disease or condition, and preliminary clinical evidence indicates that the drug may demonstrate substantial improvement over currently approved therapies on one or more clinically significant endpoints, such as substantial treatment effects observed early in clinical development. Fast Track Designation may be available if a product is intended for the treatment of a serious or life-threatening condition and preclinical or clinical data demonstrate the potential to address an unmet medical need for this condition. Drugs that receive Breakthrough Therapy Designation or Fast Track Designation by the FDA are eligible for accelerated approval and priority review.

The FDA has broad discretion whether or not to grant Breakthrough Therapy Designation or Fast Track Designation. Even if we receive Breakthrough Therapy Designation or Fast Track Designation for a product candidate, such designation may not result in a faster development process, review or approval compared to conventional FDA procedures and does not assure ultimate approval by the FDA. In addition, even if any of our product candidates receives Breakthrough Therapy Designation or Fast Track Designation, the FDA may later decide that the drugs no longer meet the conditions for qualification and rescind the designation.

Clinical development, regulatory review and approval by the FDA and comparable foreign authorities are lengthy, time consuming and inherently unpredictable. If we or our partners are ultimately unable to obtain regulatory approval for our product candidates, our business will be substantially harmed.

The preclinical studies and clinical studies of our product candidates are, and the manufacturing and marketing of our product candidates will be, subject to extensive and rigorous review and regulation by numerous government authorities in the United States and in other countries where we intend to test and, if approved, market any such product candidate. These government regulations relate to, among other things, development, clinical studies, manufacturing and commercialization. In order to obtain regulatory approval for the commercial sale of any ADC product candidates, we or our partners must demonstrate through extensive preclinical studies and clinical studies that the ADC product candidate is safe and effective for use in each target indication.

The time required to obtain approval by the FDA and comparable foreign authorities is unpredictable, typically takes many years following the commencement of clinical studies and depends upon numerous factors. Of the large number of drugs in development in the United States, only a small percentage will successfully complete the FDA regulatory approval process and will be commercialized. Accordingly, even if we are able to obtain the requisite financing to continue to fund our development and preclinical studies and clinical studies, we cannot be assured that any of our product candidates will be successfully developed or commercialized.

In addition, approval policies, regulations or the type and amount of clinical data necessary to gain approval may change during the course of a product candidate’s clinical development and may vary among jurisdictions, which may cause delays in the approval of or the decision not to approve an application. Regulatory approval has not been obtained for any product candidate based on our ADC technology, and it is possible that none of our existing product candidates or any product candidates we may seek to develop in the future will ever obtain regulatory approval. In addition, we may gain regulatory approval for XMT‑1522, XMT‑1536 or any other ADC product candidate in some but not all of the territories for which we seek approval or some but not all of the target indications, resulting in limited commercial opportunity for the approved ADC product candidates.

Applications for our or our partners’ product candidates could be delayed or could fail to receive regulatory approval for many reasons, including, but not limited to the following:

Any of these factors, many of which are beyond our control, may result in our failing to obtain regulatory approval to market any of our product candidates, which would significantly harm our business, results of operations and prospects.

If we fail to obtain regulatory approval in jurisdictions outside the United States, we will not be able to market our products in those jurisdictions.

We intend to market our ADC product candidates, including XMT‑1522 and XMT‑1536, if approved, in international markets either directly or through partnerships. We have entered into an agreement with Takeda to commercialize XMT‑1522 outside of the United States and Canada. Such marketing will require separate regulatory approvals in each market and compliance with numerous and varying regulatory requirements. The approval procedures vary from country to country and may require additional testing that we are not required to perform to obtain regulatory approval in the United States. Moreover, the time required to obtain approval in countries outside the United States may differ from that required to obtain FDA approval. In addition, in many countries outside the United States, an ADC drug must be approved for reimbursement before it can be approved for sale in that country. Approval by the FDA does not ensure approval by regulatory authorities in other countries or jurisdictions, and approval by one foreign regulatory authority does not ensure approval by regulatory authorities in other foreign countries or by the FDA. The foreign regulatory approval process may include all of the risks associated with obtaining FDA approval. We or our partners may not obtain foreign regulatory approvals on a timely basis, if at all. We or our partners may not be able to file for regulatory approvals and may not receive necessary approvals to commercialize our products in any market. If we or any existing or future partner are unable to obtain regulatory approval for XMT‑1522 or XMT‑1536 in one or more significant foreign jurisdictions, then the commercial opportunity for XMT‑1522 or XMT‑1536, as applicable, and our financial condition, will be adversely affected.

Even if we receive regulatory approval for our ADC product candidates, such products will be subject to ongoing regulatory review, which may result in significant additional expense. Additionally, our ADC product candidates, if approved, could be subject to labeling and other restrictions, and we may be subject to penalties if we fail to comply with regulatory requirements or experience unanticipated problems with our products.

Any regulatory approvals that we receive for our ADC product candidates may also be subject to limitations on the approved indicated uses for which the product may be marketed or to conditions of approval, or contain requirements for potentially costly post‑marketing testing and surveillance to monitor safety and efficacy. In addition, if the FDA or any other governing regulatory body approves any of our ADC product candidates, the manufacturing processes, labeling, packaging, distribution, adverse event reporting, storage, advertising, promotion and recordkeeping for the product will be subject to extensive and ongoing regulatory requirements. These requirements include submissions of safety and other post‑marketing information and reports, registration, as well as continued compliance with cGMP and GCP, for any clinical studies that we conduct post‑approval.

Later discovery of previously unknown problems with an approved ADC drug, including adverse events of unanticipated severity or frequency, or with manufacturing operations or processes, or failure to comply with regulatory requirements, may result in, among other things:

The policies of the FDA or any other governing regulatory body may change and additional government regulations may be enacted that could prevent, limit or delay regulatory approval of our ADC product candidates. We cannot predict the likelihood, nature or extent of government regulation that may arise from future legislation or administrative action, either in the United States or abroad. If we are slow or unable to adapt to changes in existing requirements or the adoption of new requirements or policies, or not able to maintain regulatory compliance, we may lose any marketing approval that may have been obtained and we may not achieve or sustain profitability, which would adversely affect our business.

Our ADC product candidates or ADCs developed or commercialized by our competitors may cause undesirable side effects or have other properties that delay or prevent regulatory approval of our ADC product candidates or limit their commercial potential.

Undesirable side effects caused by our ADC product candidates or ADCs being developed or commercialized by our competitors could cause us or regulatory authorities to interrupt, delay or halt clinical studies and could result in a more restrictive label or the denial of regulatory approval by the FDA or other regulatory authorities and potential product liability claims. Further, clinical studies by their nature utilize a sample of the potential patient population. With a limited number of subjects and limited duration of exposure, rare and severe side effects of our product candidates or those of our competitors may only be uncovered with a significantly larger number of patients exposed to the drug. SAEs deemed to be caused by our ADC product candidates or those of our competitors, either before or after receipt of marketing approval, could have a material adverse effect on the development of our ADC product candidates and our business as a whole.

If we or others identify undesirable side effects caused by our ADC product candidates or those of our competitors either before or after receipt of marketing approval, a number of potentially significant negative consequences could result, including:

Any of these events could prevent us from achieving or maintaining market acceptance of our ADC product candidates and could substantially increase commercialization costs.

If we or our third‑party collaborators are unable to successfully develop and commercialize any required companion diagnostics for our product candidates or engage a third party to do so, or we or they experience significant delays in doing so, we may not realize the full potential of our product candidates.

If a companion diagnostic is required for the label for XMT‑1536 or any of our future product candidates, therefore conditioning our ability to market such product candidates on the commercial availability of an approved companion diagnostic, we may seek approval for our validated assay as a companion diagnostic or we may contract with third parties to create and obtain approval for a companion diagnostic. To be successful in developing and commercializing such a companion diagnostic, we need to address a number of scientific, technical and logistical challenges. We have little experience in the development and commercialization of diagnostics and may not be successful in developing and commercializing appropriate diagnostics to pair with XMT‑1536 or any of our other product candidates. Companion diagnostics are subject to regulation by the FDA and equivalent foreign regulatory authorities as medical devices and require separate regulatory approval prior to commercialization. Given our limited experience in developing and commercializing diagnostics, we may rely in part or in whole on third parties for their design, manufacture and commercialization. We, our collaborators or such third parties may encounter difficulties in developing and obtaining approval for the companion diagnostics, including issues relating to selectivity/specificity, analytical validation, reproducibility or clinical validation. Any delay or failure by us, our collaborators or such third parties to develop or obtain regulatory approval of the companion diagnostics could delay or prevent approval of our product candidates. If we, or any third parties that we may contract with to assist us, are unable to successfully develop and commercialize companion diagnostics for our product candidates, or experience delays in doing so:

As a result, our business would be harmed, possibly materially.

In addition, third‑party collaborators may encounter production difficulties that could constrain the supply of the companion diagnostics, and both they and we may have difficulties gaining acceptance of the use of the companion diagnostics in the clinical community. If such companion diagnostics fail to gain market acceptance, it would have an adverse effect on our ability to derive revenues from sales of our product candidates, if approved. In addition, any diagnostic company with whom we contract may decide to discontinue selling or manufacturing the companion diagnostic that we anticipate using in connection with development and commercialization of our product candidates or our relationship with such diagnostic company may otherwise terminate. We may not be able to enter into arrangements with another diagnostic company to obtain supplies of an alternative diagnostic test for use in connection with the development and commercialization of our product candidates or do so on commercially reasonable terms, which could adversely affect and/or delay the development or commercialization of our product candidates.

We or our partners may fail to discover and develop additional potential product candidates.

Our and our partners’ research programs to identify new product candidates will require substantial technical, financial and human resources, and we or our partners may be unsuccessful in our or their efforts to identify new product candidates. If we or our partners are unable to identify suitable additional product candidates for preclinical and clinical development, our or their ability to develop product candidates and our ability to obtain revenues from commercializing our products or to receive royalties from our partners’ sales of their products in future periods could be compromised, which could result in significant harm to our financial position and adversely impact our stock price.

Risks related to our reliance on third parties

Because we rely on third‑party manufacturing and supply partners, our supply of research and development, preclinical and clinical development materials may become limited or interrupted or may not be of satisfactory quantity or quality.

We rely on third‑party contract manufacturers to manufacture our preclinical and clinical study product supplies, and we lack the internal resources and the capability to manufacture any product candidates on a clinical or commercial scale. The facilities used by our contract manufacturers to manufacture the active pharmaceutical ingredient and final drug product must be acceptable to the FDA and other comparable foreign regulatory agencies pursuant to inspections that would be conducted after we submit our marketing application or relevant foreign regulatory submission to the applicable regulatory agency. There can be no assurance that our preclinical and clinical development product supplies will be sufficient, uninterrupted or of satisfactory quality or continue to be available at acceptable prices. If our contract manufacturers cannot successfully manufacture material that conforms to our specifications and the strict regulatory requirements of the FDA or applicable foreign regulatory agencies, they will not be able to secure or maintain regulatory approval for their manufacturing facilities. Any replacement of our manufacturers could require significant effort and expertise because there may be a limited number of qualified replacements.

The manufacturing process for an ADC product candidate is subject to FDA and foreign regulatory authority review. Suppliers and manufacturers must meet applicable manufacturing requirements and undergo rigorous facility and process validation tests required by regulatory authorities in order to comply with regulatory standards, such as cGMP. We have no direct control over our contract manufacturers’ ability to maintain adequate quality control, quality assurance and qualified personnel. In the event that any of our manufacturers fails to comply with regulatory requirements or to perform its obligations to us in relation to quality, timing or otherwise, or if our supply of components or other materials becomes limited or interrupted for other reasons, we may be forced to manufacture the materials ourselves, for which we currently do not have the capabilities or resources, or enter into an agreement with another third party, which we may not be able to do on reasonable terms, if at all. In some cases, the technical skills or technology required to manufacture our ADC product candidates may be unique or proprietary to the original manufacturer and we may have difficulty transferring such skills or technology to another third party and a feasible alternative may not exist. These factors would increase our reliance on such manufacturer or require us to obtain a license from such manufacturer in order to have another third-party manufacture our ADC product candidates. If we are required to change manufacturers for any reason, we will be required to verify that the new manufacturer maintains facilities and procedures that comply with quality standards and with all applicable regulations and guidelines. The delays associated with the verification of a new manufacturer could negatively affect our ability to develop ADC product candidates in a timely manner or within budget. Our reliance on contract manufacturers also exposes us to the possibility that they, or third parties with access to their facilities, will have access to and may appropriate our trade secrets or other proprietary information.

We expect to continue to rely on third‑party manufacturers if we receive regulatory approval for any ADC product candidate. To the extent that we have existing, or enter into future, manufacturing arrangements with third parties, we will depend on these third parties to perform their obligations in a timely manner consistent with contractual and regulatory requirements, including those related to quality control and assurance. If we are unable to obtain or maintain third‑party manufacturing for ADC product candidates, or to do so on commercially reasonable terms, we may not be able to develop and commercialize our ADC product candidates successfully. Our or a third party’s failure to execute on our manufacturing requirements and comply with cGMP could adversely affect our business in a number of ways, including:

We, or our third‑party manufacturers, may be unable to successfully scale‑up manufacturing of our ADC product candidates in sufficient quality and quantity, which would delay or prevent us from developing our ADC product candidates and commercializing approved products, if any.

In order to conduct clinical studies of our ADC product candidates and commercialize any approved ADC product candidates, we, or our manufacturing partners, will need to manufacture them in large quantities. We, or our manufacturing partners, may be unable to successfully increase the manufacturing capacity for any of our product candidates in a timely or cost‑effective manner, or at all. In addition, quality issues may arise during scale‑up activities. If we, or any manufacturing partners, are unable to successfully scale up the manufacture of our ADC product candidates in sufficient quality and quantity, the development, testing and clinical studies of that ADC product candidates may be delayed or infeasible, and regulatory approval or commercial launch of any resulting product may be delayed or not obtained, which could significantly harm our business. We have evaluated which third‑party manufactures to engage for scale‑up to commercial supply of our ADC product candidates, including XMT‑1522 and XMT‑1536 and we have begun transfer and scale-up of certain manufacturing activities. If we are unable to obtain or maintain third‑party manufacturing for commercial supply of ADC product candidates, or to do so on commercially reasonable terms, we may not be able to develop and commercialize our ADC product candidates successfully.

We rely on third parties to conduct preclinical studies and clinical studies for our ADC product candidates, including XMT‑1522 and XMT‑1536, and if they do not properly and successfully perform their obligations to us, we may not be able to obtain regulatory approvals for XMT‑1522 or XMT‑1536 or any other ADC product candidates that we may develop in the future.

We have designed the Phase 1 clinical studies for XMT‑1522 and XMT-1536 and intend to design any future clinical study for any future unpartnered ADC product candidates that we may develop if preclinical studies are successful. However, we rely on CROs and other third parties to assist in managing, monitoring and otherwise carrying out many of these studies. As a result, we have less direct control over the conduct, timing and completion of these clinical studies and the management of data developed through clinical studies than would be the case if we were relying entirely upon our own staff. These CROs and other third parties are not our employees and we have limited control over the amount of time and resources that they dedicate to our programs. We compete with many other companies for the resources of these third parties. These third parties may have contractual relationships with other entities, some of which may be our competitors, which may draw time and resources from our programs. The third parties with whom we contract might not be diligent, careful or timely in conducting our preclinical studies or clinical studies, resulting in the preclinical studies or clinical studies being delayed or unsuccessful.

The third parties on whom we rely generally may terminate their engagements at any time, and having to enter into alternative arrangements would delay development and commercialization of our ADC product candidates. Communicating with outside parties can also be challenging, potentially leading to mistakes as well as difficulties in coordinating activities. Outside parties may:

The FDA and comparable foreign regulatory authorities require compliance with regulations and standards, including GCP, for designing, conducting, monitoring, recording, analyzing and reporting the results of clinical studies to assure that the data and results are credible and accurate and that the rights, integrity and confidentiality of study participants are protected. Although we rely, and intend to continue to rely, on third parties to conduct our clinical studies, they are not our employees, and we are responsible for ensuring that each of these clinical studies is conducted in accordance with its general investigational plan, protocol and other requirements. Our reliance on these third parties for research and development activities will reduce our control over these activities but will not relieve us of our responsibilities.

If these third parties do not successfully carry out their duties under their agreements, if the quality or accuracy of the data they obtain is compromised due to their failure to adhere to clinical study protocols or to regulatory requirements, or if they otherwise fail to comply with clinical study protocols or meet expected deadlines, the clinical studies of our ADC product candidates may not meet regulatory requirements. The FDA enforces GCP regulations through periodic inspections of clinical study sponsors, principal investigators and study sites. If we or our CROs fail to comply with applicable GCPs or other regulatory requirements, the clinical data generated in our clinical studies may be deemed unreliable, third parties may need to be replaced and preclinical development activities or clinical studies may be extended, delayed, suspended or terminated. If any of these events occur, we may not be able to obtain regulatory approval of our ADC product candidates on a timely basis or at all.

We depend on strategic partnerships with other companies to assist in the research, development and commercialization of our ADC platforms and ADC product candidates. If our existing partners do not perform as expected, this may negatively affect our ability to commercialize our ADC product candidates, including XMT‑1522, or generate revenues through technology licensing, or may otherwise negatively affect our business.

We have established strategic partnerships and intend to continue to establish strategic partnerships with third parties to research, develop and commercialize our ADC platforms and existing and future ADC product candidates. We entered into a collaboration agreement with Takeda in January 2016 for the co‑development of XMT‑1522 that granted Takeda rights to commercialize XMT‑1522 outside of the United States and Canada. We also have entered into another collaboration agreement with Takeda and a collaboration agreement with Merck KGaA for the development and commercialization of other ADC product candidates. For certain of these programs, we will depend on our partners to design and conduct their clinical studies. As a result, we may not be able to conduct these programs in the manner or on the time schedule we currently contemplate, which may negatively impact our business operations. In addition, if any of these partners withdraw support for these programs or proposed products or otherwise impair their development, our business could be negatively affected.

Our partners may terminate their agreements with us for cause under certain circumstances or at will in certain cases and discontinue use of our technologies. In addition, we cannot control the amount and timing of resources our partners may devote to products utilizing or incorporating our technology. Moreover, our relationships with our partners may divert significant time and effort of our scientific staff and management team and require effective allocation of our resources to multiple internal and collaborative projects. Our partners may fail to perform their obligations under the collaboration agreements or may not perform their obligations in a timely manner. If conflicts arise between our partners and us, the other party may act in a manner adverse to us and could limit our ability to implement our strategies. If any of our partners terminate or breach our agreements with them, or otherwise fail to complete their obligations in a timely manner, it may have a detrimental effect on our financial position by reducing or eliminating the potential for us to receive technology access and license fees, milestones and royalties, reimbursement of development costs, as well as possibly requiring us to devote additional efforts and incur costs associated with pursuing internal development of product candidates. Furthermore, if our partners do not prioritize and commit sufficient resources to programs associated with our product

candidates or collaboration product candidates, we or our partners may be unable to commercialize these product candidates, which would limit our ability to generate revenue and become profitable.

In particular, if Takeda were to terminate the XMT‑1522 strategic partnership, we would not receive milestone payments, co‑funded development payments or, following approval, royalties for the sale of XMT‑1522 outside the United States and Canada. As a result of such termination, we would have to engage another strategic partner to complete the XMT‑1522 development process and to commercialize XMT‑1522 outside the United States and Canada, or to complete the development process and undertake commercializing XMT‑1522 outside the United States and Canada ourselves, either of which could significantly delay the continued development and commercialization of XMT‑1522 and would increase our costs. In turn, this could significantly harm our financial position, adversely affect our stock price and require us to incur all the costs of developing and commercializing XMT‑1522, which are now being co‑funded by Takeda.

Our partners may separately pursue competing products, therapeutic approaches or technologies to develop treatments for the diseases targeted by us or our partners. Competing products, either developed by the partners or to which the partners have rights, may result in the withdrawal of partner support for our product candidates. Even if our partners continue their contributions to the strategic partnerships, they may nevertheless determine not to actively pursue the development or commercialization of any resulting products. Any of these developments could harm our product development efforts.

To date, we have depended on a small number of partners for a substantial portion of our revenue. The loss of any one of these partners could result in a material decline in our revenue.

We have strategic partnerships with a limited number of companies. To date, a substantial portion of our revenue has resulted from payments made under agreements with our strategic partners, and we expect that a portion of our revenue will continue to come from strategic partnerships. If XMT‑1522 receives regulatory approval, our revenues will still depend in part on Takeda’s ability and willingness to market the approved product outside of the United States and Canada. The loss of any of our partners, especially Takeda, or the failure of our partners to perform their obligations under their agreements with us, including paying license or technology fees, milestone payments, royalties or reimbursements, could have a material adverse effect on our financial performance. Payments under our existing and future strategic partnerships are also subject to significant fluctuations in both timing and amount, which could cause our revenue to fall below the expectations of securities analysts and investors and cause a decrease in our stock price.

We may not be successful in establishing and maintaining additional strategic partnerships, which could adversely affect our ability to develop and commercialize products, negatively impacting our operating results.

We continue to strategically evaluate our partnerships and, as appropriate, we expect to enter into additional strategic partnerships in the future, including potentially with major biotechnology or biopharmaceutical companies. We face significant competition in seeking appropriate partners for our ADC product candidates, and the negotiation process is time‑consuming and complex. In order for us to successfully partner our ADC product candidates, potential partners must view these ADC product candidates as economically valuable in markets they determine to be attractive in light of the terms that we are seeking and other available products for licensing by other companies. Even if we are successful in our efforts to establish strategic partnerships, the terms that we agree upon may not be favorable to us, and we may not be able to maintain such strategic partnerships if, for example, development or approval of an ADC product candidate is delayed or sales of an approved product are disappointing. Any delay in entering into strategic partnership agreements related to our ADC product candidates could delay the development and commercialization of such candidates and reduce their competitiveness even if they reach the market. If we are not able to generate revenue under our strategic partnerships when and in accordance with our expectations or the expectations of industry analysts, this failure could harm our business and have an immediate adverse effect on the trading price of our common stock.

If we fail to establish and maintain additional strategic partnerships related to our unpartnered ADC product candidates, we will bear all of the risk and costs related to the development of any such ADC product candidate, and we may need to seek additional financing, hire additional employees and otherwise develop expertise, such as regulatory expertise, for which we have not budgeted. If we were not successful in seeking additional financing, hiring additional employees or developing additional expertise, our cash burn rate would increase or we would need to take steps to reduce our rate of

ADC product candidate development. This could negatively affect the development of any unpartnered ADC product candidate.

Risks related to commercialization of our ADC product candidates

Our future commercial success depends upon attaining significant market acceptance of our ADC product candidates, if approved, among physicians, patients and health care payors.

Even if we obtain regulatory approval for XMT‑1522, XMT‑1536 or any other ADC product candidates that we may develop or acquire in the future, the product candidate may not gain market acceptance among physicians, health care payors, patients and the medical community. Market acceptance of any approved products depends on a number of factors, including:

Perceptions of any product are influenced by perceptions of competitors’ products that are in the same class of drugs or have a similar mechanism of action. As a result, adverse public perception of our competitors’ ADC products may negatively impact the market acceptance of our ADC product candidates. Market acceptance is critical to our ability to generate significant revenue and become profitable. Any therapeutic candidate, if approved and commercialized, may be accepted in only limited capacities or not at all. If any approved products are not accepted by the market to the extent that we expect, we may not be able to generate significant revenue and our business would suffer.

The incidence and prevalence for target patient populations of our drug candidates have not been established with precision. If the market opportunities for our drug candidates are smaller than we estimate or if any approval that we obtain is based on a narrower definition of the patient population, our revenue and ability to achieve profitability will be adversely affected, possibly materially.

The precise incidence and prevalence of breast cancer, NSCLC and gastric cancer with low HER2 expression and of epithelial ovarian cancer and non‑squamous NSCLC with NaPi2b expression are unknown. Our projections of both the number of people who have these diseases, as well as the subset of people with these diseases who have the potential to benefit from treatment with our drug candidates, are based on estimates. The total addressable market opportunity for XMT‑1522 for the treatment of patients with breast cancer, NSCLC and gastric cancer with HER2 expression and XMT‑1536 for the treatment of epithelial ovarian cancer and non‑squamous NSCLC with NaPi2b expression will ultimately depend upon, among other things, the diagnosis criteria included in the final label for each of XMT‑1522 and XMT‑1536, if our drug candidates are approved for sale for these indications, acceptance by the medical community and patient access, drug pricing and reimbursement. The number of patients who can be treated with our drug candidates may turn out to be lower than expected, patients may not be otherwise amenable to treatment with our drugs, or new patients

may become increasingly difficult to identify or gain access to, all of which would adversely affect our results of operations and our business.

If we are unable to establish sales, marketing and distribution capabilities, we may not be successful in commercializing our product candidates if and when they are approved.

We do not have a sales or marketing infrastructure and have no experience in the sale, marketing or distribution of products. To achieve commercial success for any product for which we have obtained marketing approval, we will need to establish a sales and marketing organization.

In the future, we expect to build a focused sales and marketing infrastructure to market XMT‑1522 in the United States and Canada, if and when it is approved, and to market XMT‑1536 and other ADC product candidates in the United States and certain foreign jurisdictions, if and when they are approved. There are risks involved with establishing our own sales, marketing and distribution capabilities. For example, recruiting and training a sales force is expensive and time consuming and could delay any product launch. If the commercial launch of a product candidate for which we recruit a sales force and establish marketing capabilities is delayed or does not occur for any reason, we would have prematurely or unnecessarily incurred these commercialization expenses. This may be costly, and our investment would be lost if we cannot retain or reposition our sales and marketing personnel.

Factors that may inhibit our efforts to commercialize our products on our own include:

If we are unable to establish our own sales, marketing and distribution capabilities and enter into arrangements with third parties to perform these services, our product revenues and our profitability, if any, are likely to be lower than if we were to market, sell and distribute any products that we develop ourselves.

In addition, we may not be successful in entering into arrangements with third parties to sell, market and distribute certain of our product candidates outside of the United States or may be unable to do so on terms that are favorable to us. We likely will have little control over such third parties, and any of them may fail to devote the necessary resources and attention to sell and market our products effectively. If we do not establish sales, marketing and distribution capabilities successfully, either on our own or in collaboration with third parties, we will not be successful in commercializing our product candidates.

Reimbursement may be limited or unavailable in certain market segments for our ADC product candidates, which could make it difficult for us to sell our products profitably.

In both domestic and foreign markets, sales of any of our product candidates, if approved, will depend, in part, on the extent to which the costs of our products will be covered by third‑party payors, such as government health programs, commercial insurance and managed health care organizations. These third-party payors decide which drugs will be covered and establish reimbursement levels for those drugs. The containment of health care costs has become a priority of foreign and domestic governments as well as private third-party payors. The prices of drugs have been a focus in this effort. Governments and private third‑party payors have attempted to control costs by limiting coverage and the amount of reimbursement for particular medications, which could affect our ability to sell our product candidates profitably.

Cost‑control initiatives could cause us to decrease the price we might establish for products, which could result in lower than anticipated product revenues.

Reimbursement by a third‑party payor may depend upon a number of factors, including the third‑party payor’s determination that use of a product is:

Adverse pricing limitations may hinder our ability to recoup our investment in XMT‑1522, XMT‑1536 or any future ADC product candidates, even if such product candidates obtain marketing approval.

Obtaining coverage and reimbursement approval for a product from a government or other third‑party payor is a time consuming and costly process that could require us to provide supporting scientific, clinical and cost‑effectiveness data for the use of our products to the payor. Further, there is significant uncertainty related to third‑party payor coverage and reimbursement of newly approved drugs. We may not be able to provide data sufficient to gain acceptance with respect to coverage and reimbursement. We cannot be sure that coverage or adequate reimbursement will be available for any of our ADC product candidates. Also, we cannot be sure that reimbursement amounts will not reduce the demand for, or the price of, our products. If reimbursement is not available or is available only to limited levels, we may not be able to commercialize certain of our products. In addition, in the United States, third‑party payors are increasingly attempting to contain health care costs by limiting both coverage and the level of reimbursement of new drugs. As a result, significant uncertainty exists as to whether and how much third‑party payors will reimburse patients for their use of newly approved drugs, which in turn will put pressure on the pricing of drugs.

Price controls may be imposed in foreign markets, which may adversely affect our future profitability.

In some countries, including member states of the European Union, the pricing of prescription drugs is subject to governmental control. Additional countries may adopt similar approaches to the pricing of prescription drugs. In such countries, pricing negotiations with governmental authorities can take considerable time after receipt of marketing approval for a product. In addition, there can be considerable pressure by governments and other stakeholders on prices and reimbursement levels, including as part of cost containment measures. Political, economic and regulatory developments may further complicate pricing negotiations, and pricing negotiations may continue after reimbursement has been obtained. Reference pricing used by various European Union member states and parallel distribution, or arbitrage between low‑priced and high‑priced member states, can further reduce prices. In some countries, we may be required to conduct a clinical study or other studies that compare the cost‑effectiveness of our ADC product candidates to other available therapies in order to obtain or maintain reimbursement or pricing approval. We cannot be sure that such prices and reimbursement will be acceptable to us or our strategic partners. Publication of discounts by third‑party payors or authorities may lead to further pressure on the prices or reimbursement levels within the country of publication and other countries. If pricing is set at unsatisfactory levels or if reimbursement of our products is unavailable or limited in scope or amount, our revenues from sales by us or our strategic partners and the potential profitability of our ADC product candidates in those countries would be negatively affected.

The impact of health care reform legislation and other changes in the health care industry and in health care spending on us is currently unknown and may adversely affect our business model.

Our revenue prospects could be affected by changes in health care spending and policy in the United States and abroad. We operate in a highly regulated industry and new laws, regulations or judicial decisions, or new interpretations of existing laws, regulations or decisions, related to health care availability, the method of delivery or payment for health care products and services could negatively impact our business, operations and financial condition.

The United States and state governments continue to propose and pass legislation designed to reduce the cost of health care. In March 2010, the U.S. Congress enacted the Patient Protection and Affordable Care Act and the Health Care and Education Reconciliation Act, or the Health Care Reform Act, which include changes to the coverage and reimbursement of drug products under government health care programs such as:

It is likely that federal and state legislatures within the United States and foreign governments will continue to consider changes to existing health care legislation. We cannot predict the reform initiatives that may be adopted in the future or whether initiatives that have been adopted will be repealed or modified. The continuing efforts of the government, insurance companies, managed care organizations and other payors of health care services to contain or reduce costs of health care may adversely affect:

Modifications to or repeal of all or certain provisions of the Health Care Reform Act are expected as a result of the outcome of the 2016 presidential election and Congressional Republicans maintaining control of Congress, consistent with statements made by Donald Trump and members of Congress during the presidential campaign and following the election. Some of the provisions of the Health Care Reform Act have yet to be implemented, and there have been legal and political challenges to certain aspects of the Health Care Reform Act. Since January 2017, President Trump has signed two executive orders and other directives designed to delay, circumvent, or loosen certain requirements mandated by the Health Care Reform Act. Moreover, the Tax Cuts and Jobs Act of 2017 was enacted on December 22, 2017 and includes a provision repealing, effective January 1, 2019, the tax-based shared responsibility payment imposed by the Health Care Reform Act on certain individuals who fail to maintain qualifying health coverage for all or part of a year that is commonly referred to as the “individual mandate.” Congress may consider other legislation to repeal or replace additional elements of the Health Care Reform Act. We continue to evaluate the effect that the Health Care Reform Act, the repeal of the

individual mandate, and any additional possible repeal and replacement efforts may have on our business but expect that the Health Care Reform Act, as currently enacted or as it may be amended in the future, and other healthcare reform measures that may be adopted in the future could have a material adverse effect on our industry generally and on our ability to successfully commercialize our product candidates, if approved. In addition to the Health Care Reform Act, there will continue to be proposals by legislators at both the federal and state levels, regulators and third party payors to address, among other things, healthcare costs and individual healthcare benefits. We cannot predict the ultimate content, timing or effect of any changes to the Healthcare Reform Act, and significant changes to, or repeal of, the Healthcare Reform Act could have a material adverse effect on our business, financial condition and profitability.

In addition, other legislative changes have been proposed and adopted since the 2010 health care reform legislation. The Budget Control Act of 2011, as amended, or the Budget Control Act, includes provisions intended to reduce the federal deficit. The Budget Control Act resulted in the imposition of 2% reductions in Medicare payments to providers beginning in 2013. Legislation extends reductions through 2023. Any significant spending reductions affecting Medicare, Medicaid or other publicly funded or subsidized health programs that may be implemented, or any significant taxes or fees that may be imposed on us, as part of any broader deficit reduction effort or legislative replacement to the Budget Control Act, could have an adverse impact on our anticipated product revenues.

We face substantial competition, which may result in others discovering, developing or commercializing products before, or more successfully than, we do.

The biotechnology and biopharmaceutical industries are characterized by rapidly advancing technologies, intense competition and a strong emphasis on proprietary products. Many third parties compete with us in developing various approaches to cancer therapy. They include pharmaceutical companies, biotechnology companies, academic institutions and other research organizations. Any treatments developed by our competitors could be superior to our ADC product candidates. It is possible that these competitors will succeed in developing technologies that are more effective than our ADC platforms or ADC product candidates or that would render our ADC platforms obsolete or noncompetitive. We anticipate that we will face increased competition in the future as additional companies enter our market and scientific developments surrounding other cancer therapies continue to accelerate.

We are also aware of multiple companies with ADC technologies that may be competitive to our ADC platforms, including Astellas, AstraZeneca, Bristol‑Myers Squibb, Daiichi Sankyo, ImmunoGen, Immunomedics, Pfizer and Seattle Genetics. These companies or their partners, including AbbVie, Genentech, Lilly, Novartis, Sanofi and Takeda, may develop ADC product candidates which compete in the same indications as our current and future ADC product candidates. There are approximately 75 ADC product candidates in active clinical development. There are currently four approved ADC therapies in the United States: brentuximab vedotin, marketed by Seattle Genetics and Takeda, ado‑trastuzumab emtansine, marketed by Genentech, gemtuzumab ozogamicin, marketed by Pfizer; and inotuzumab ozogamicin, also marketed by Pfizer. Ado‑trastuzumab emtansine is a HER2 targeted ADC approved for use in HER2-positive patients and, even though we are developing, and expect to get approval for, XMT‑1522 for lower expressing HER2 patients, ado‑trastuzumab emtansine may compete with our HER2 targeted ADC, XMT‑1522, if XMT‑1522 is approved. We expect to compete on improved efficacy, safety and tolerability compared to other ADC product candidates and if our products are not demonstrably superior in these respects compared to other approved therapeutics, we may not be able to compete effectively.

Many of our competitors have significantly greater financial resources and expertise in research and development, manufacturing, preclinical testing, conducting clinical studies, obtaining regulatory approval and marketing than we do. In addition, many of these competitors are active in seeking patent protection and licensing arrangements in anticipation of collecting royalties for use of technology that they have developed. Smaller or early‑stage companies may also prove to be significant competitors, particularly through strategic partnerships with large and established companies. These third parties compete with us in recruiting and retaining qualified scientific and management personnel, as well as in acquiring technologies complementary to our programs.

In addition, if our product candidates are approved and commercialized, we may face competition from biosimilars. The route to market for biosimilars was established with the passage of the Health Care Reform Act in March 2010. The Health Care Reform Act establishes a pathway for the FDA approval of follow‑on biologics and provides twelve years data

exclusivity for reference products and an additional six months exclusivity period if pediatric studies are conducted. In Europe, the European Medicines Agency has issued guidelines for approving products through an abbreviated pathway, and biosimilars have been approved in Europe. If a biosimilar version of one of our potential products were approved in the United States or Europe, it could have a negative effect on sales and gross profits of the potential product and our financial condition.

With respect to our current and potential future product candidates, we believe that our ability to compete effectively and develop products that can be manufactured cost‑effectively and marketed successfully will depend on our ability to:

Risks related to our intellectual property

If we are unable to obtain or protect intellectual property rights related to our technology and ADC product candidates, or if our intellectual property rights are inadequate, we may not be able to compete effectively.

Our success depends in large part on our ability to obtain and maintain protection with respect to our intellectual property and proprietary technology. We rely upon a combination of patents, trade secret protection and confidentiality agreements to protect the intellectual property related to our ADC platforms and ADC product candidates, XMT‑1522 and XMT‑1536. The patent position of biopharmaceutical companies is generally uncertain because it involves complex legal and factual considerations and has, in recent years, been the subject of much litigation. As a result, the issuance, scope, validity, enforceability and commercial value of our patent rights is highly uncertain. The standards applied by the United States Patent and Trademark Office, or USPTO, and foreign patent offices in granting patents are not always applied uniformly or predictably. For example, there is no uniform worldwide policy regarding patentable subject matter or the scope of claims allowable in patents. In addition, changes in either the patent laws or interpretation of the patent laws in the United States and other countries may diminish the value of our patents or narrow the scope of our patent protection. The patent prosecution process is expensive, complex and time‑consuming, and we may not be able to file, prosecute, maintain, enforce or license all necessary or desirable patents and patent applications at a reasonable cost or in a timely manner. It is also possible that we fail to identify patentable aspects of our research and development output before it is too late to obtain patent protection. There is no assurance that all potentially relevant prior art relating to our patents and patent applications has been found. We may be unaware of prior art that could be used to invalidate an issued patent or prevent our pending patent applications from issuing as patents.

The patent applications that we own or in‑license may fail to result in issued patents, and even if they do issue as patents, such patents may not cover our ADC platforms and ADC product candidates in the United States or in other countries. The issuance of a patent is not conclusive as to its inventorship, scope, validity or enforceability, and our patents may be

challenged in the courts or patent offices in the United States and abroad. Such challenges may result in loss of exclusivity or in patent claims being narrowed, invalidated or held unenforceable, which could limit our ability to stop others from using or commercializing similar or identical technology and products, or limit the duration of the patent protection of our technology and product candidates. For example, even if patent applications we license or own do successfully issue as patents and even if such patents cover our ADC platforms and ADC product candidates, third parties may challenge their validity, enforceability or scope, which may result in such patents being narrowed or invalidated. Furthermore, even if they are unchallenged, our patents and patent applications may not provide adequate protection or exclusivity for our ADC platform or ADC product candidates, prevent others from designing around our claims or otherwise provide us with a competitive advantage. Any of these outcomes could impair our ability to prevent competition from third parties, which may have an adverse impact on our business.

If patent applications we own or have in‑licensed with respect to our ADC platforms or our ADC product candidates fail to issue as patents, if their breadth or strength of protection is threatened, or if they fail to provide meaningful exclusivity, it could dissuade companies from collaborating with us. We cannot offer any assurances about which, if any, patents will issue, the breadth of any such patents or whether any issued patents will be found invalid and unenforceable or will be threatened by third parties. Any successful challenge to these patents or any other patents owned by or licensed to us could deprive us of rights necessary for the successful development and commercialization of any ADC product candidate. Since patent applications in the United States and most other countries are confidential for a period of time after filing, and some remain so until issued, we cannot be certain that we were the first to file any patent application related to an ADC product candidate. Furthermore, if third parties have filed such patent applications, an interference proceeding in the United States can be initiated by the USPTO or a third‑party to determine who was the first to invent any of the subject matter covered by the patent claims of our applications. In addition, patents have a limited lifespan. In the United States, the natural expiration of a patent is generally 20 years after it is filed. Various extensions may be available; however, the life of a patent and the protection it affords is limited. Given the amount of time required for the development, testing and regulatory review of new product candidates, our owned or in‑licensed patents protecting such candidates might expire before or shortly after such candidates are commercialized. If we encounter delays in obtaining regulatory approvals, the period of time during which we could market an ADC drug under patent protection could be further reduced. Even if patents covering our ADC product candidates are obtained, once the patent life has expired for a product, we may be open to competition from similar or generic products. The launch of a generic version of one of our products in particular would be likely to result in an immediate and substantial reduction in the demand for our product, which could have a material adverse effect on our business, financial condition, results of operations and prospects.

On September 16, 2011, the Leahy‑Smith America Invents Act, or the Leahy‑Smith Act, was signed into law, which could increase the uncertainties and costs surrounding the prosecution of our patent applications and the enforcement or defense of our issued patents. The Leahy‑Smith Act includes a number of significant changes to U.S. patent law. These include provisions that affect the way patent applications are prosecuted, redefine prior art, may affect patent litigation and switch the U.S. patent system from a “first‑to‑invent” system to a “first‑to‑file” system. Under a first‑to‑file system, assuming the other requirements for patentability are met, the first inventor to file a patent application generally will be entitled to the patent on an invention regardless of whether another inventor had made the invention earlier. These provisions also allow third‑party submission of prior art to the USPTO during patent prosecution and set forth additional procedures to attack the validity of a patent by the USPTO administered post grant proceedings. The USPTO developed additional regulations and procedures to govern administration of the Leahy‑Smith Act, and many of the substantive changes to patent law associated with the Leahy‑Smith Act, and, in particular, the first‑to‑file provisions, only became effective on March 16, 2013. Accordingly, it is not clear what, if any, impact the Leahy‑Smith Act will have on the operation of our business. The Leahy‑Smith Act and its implementation could increase the uncertainties and costs surrounding the prosecution of our patent applications and the enforcement or defense of our issued patents, all of which could have a material adverse effect on our business, financial condition, results of operations and prospects.

Any loss of patent protection could have a material adverse impact on our business. We may be unable to prevent competitors from entering the market with a product that is similar to or the same as our ADC product candidates.

Issued patents covering XMT‑1522 and XMT‑1536 and any future ADC product candidates could be found invalid or unenforceable if challenged in court or before the USPTO or comparable foreign authority.

If we or one of our licensing partners initiate legal proceedings against a third party to enforce a patent covering one of XMT‑1522, XMT‑1536 or any other future product candidates, the defendant could counterclaim that the patent covering our product candidate is invalid or unenforceable. In patent litigation in the United States, defendant counterclaims alleging invalidity or unenforceability are commonplace, and there are numerous grounds upon which a third party can assert invalidity or unenforceability of a patent. Grounds for a validity challenge could be, among other things, an alleged failure to meet any of several statutory requirements, including lack of novelty, obviousness, lack of written description or non‑enablement. Grounds for an unenforceability assertion could be, among other things, an allegation that someone connected with prosecution of the patent withheld relevant information from the USPTO, or made a misleading statement, during prosecution. Third parties may also raise similar claims before administrative bodies in the United States or abroad, even outside the context of litigation. Such mechanisms include re‑examination, inter partes review, post‑grant review, interference proceedings, derivation proceedings and equivalent proceedings in foreign jurisdictions (e.g., opposition proceedings). Such proceedings could result in revocation, cancellation or amendment to our patents in such a way that they no longer cover and protect our product candidates. The outcome following legal assertions of invalidity and unenforceability is unpredictable. With respect to the validity of our patents, for example, we cannot be certain that there is no invalidating prior art of which we, our licensors, our patent counsel and the patent examiner were unaware during prosecution. If a third party were to prevail on a legal assertion of invalidity or unenforceability, we would lose at least part, and perhaps all, of the patent protection on one or more of our product candidates. Any such loss of patent protection could have a material adverse impact on our business, financial condition, results of operations and prospects.

If we fail to comply with our obligations under any license, strategic partnership or other agreements, we may be required to pay damages and could lose intellectual property rights that are necessary for developing and protecting our product candidates.

We rely, in part, on license, collaboration and other agreements. We may need to obtain additional licenses from others to advance our research or allow commercialization of our product candidates and it is possible that we may be unable to obtain additional licenses at a reasonable cost or on reasonable terms, if at all. The licensing or acquisition of third party intellectual property rights is a competitive area, and several more established companies may pursue strategies to license or acquire third party intellectual property rights that we may consider attractive. These established companies may have a competitive advantage over us due to their size, capital resources and greater clinical development and commercialization capabilities. In addition, companies that perceive us to be a competitor may be unwilling to assign or license rights to use. We also may be unable to license or acquire third-party intellectual property rights on terms that would allow us to make an appropriate return on our investment.

In addition, our existing licenses and collaboration agreements, including our license with Recepta Biopharma S.A., or Recepta, for intellectual property covering the NaPi2b antibody in XMT‑1536 and our agreement with Adimab under which we acquired Adimab’s rights to XMT‑1519, the antibody in XMT‑1522, and were granted a license to certain intellectual property controlled by Adimab to exploit ADC product candidates containing XMT‑1519, including XMT‑1522, impose, and any future licenses, collaborations or other agreements we enter into are likely to impose, various development, commercialization, funding, milestone, royalty, diligence, sublicensing, insurance, patent prosecution and enforcement or other obligations on us. If we breach any of these obligations, or use the intellectual property licensed to us in an unauthorized manner, we may be required to pay damages and the licensor may have the right to terminate the license, including, in the case of our agreement with Recepta, the license for the rights covering the NaPi2b antibody in XMT‑1536. In addition, if we breach certain obligations under our agreement with Adimab, Adimab may have the right to reacquire the rights to XMT‑1519. Any of the foregoing could result in us being unable to develop, manufacture and sell products that are covered by the licensed technology or enable a competitor to gain access to the licensed technology. Disputes may arise regarding intellectual property subject to a licensing, collaboration or other agreements, including:

In addition, the agreements under which we currently license intellectual property or technology to or from third parties are complex, and certain provisions in such agreements may be susceptible to multiple interpretations. The resolution of any contract interpretation disagreement that may arise could narrow what we believe to be the scope of our rights to the relevant intellectual property or technology, or increase what we believe to be our financial or other obligations under the relevant agreement, either of which could have a material adverse effect on our business, financial condition, results of operations and prospects. Moreover, if disputes over intellectual property that we have licensed prevent or impair our ability to maintain our current licensing arrangements on commercially acceptable terms, we may be unable to successfully develop and commercialize the affected product candidates.

In some circumstances, we may not have the right to control the preparation, filing and prosecution of patent applications, or to maintain the patents, covering the technology that we license from third parties. For example, pursuant to our license agreement with Recepta, Ludwig Institute for Cancer Research Ltd., a co‑owner of the intellectual property, retains control of such activities. Therefore, we cannot be certain that these patents and applications will be prosecuted, maintained and enforced in a manner consistent with the best interests of our business. If our licensors fail to obtain or maintain such intellectual property, or lose rights to such intellectual property, the rights we have licensed and our exclusivity may be reduced or eliminated and our right to develop and commercialize any of our products that are subject to such licensed rights could be adversely affected.

Moreover, our rights to our in‑licensed patents and patent applications are dependent, in part, on inter‑institutional or other operating agreements between the joint owners of such in‑licensed patents and patent applications. If one or more of such joint owners breaches such inter‑institutional or operating agreements, our rights to such in‑licensed patents and patent applications may be adversely affected. In addition, while we cannot currently determine the amount of the royalty obligations we would be required to pay on sales of future products, if any, the amounts may be significant. The amount of our future royalty obligations will depend on the technology and intellectual property we use in products that we successfully develop and commercialize, if any. Therefore, even if we successfully develop and commercialize products, we may be unable to achieve or maintain profitability. Any of the foregoing could have a material adverse effect on our competitive position, business, financial conditions, results of operations and prospects.

If we are unable to successfully obtain rights to required third-party intellectual property rights or maintain the existing intellectual property rights we have, we may have to abandon development of the relevant program or product candidate and our business, financial condition, results of operations and prospects could suffer.

We may become involved in lawsuits to protect or enforce our intellectual property or to defend against intellectual property claims, which could be expensive, time consuming and unsuccessful.

Competitors and other third parties may infringe our patents or misappropriate or otherwise violate our owned and in‑licensed intellectual property rights. To counter infringement or unauthorized use, litigation or other intellectual property proceedings may be necessary to enforce or defend our owned and in‑licensed intellectual property rights, to protect our trade secrets or to determine the validity and scope of our own intellectual property rights or the proprietary rights of others. Such litigation or proceedings can be expensive and time consuming, and any such claims could provoke defendants to assert counterclaims against us, including claims alleging that we infringe their patents or other intellectual property rights. We may not have sufficient financial or other resources to adequately conduct such litigation or proceedings. Many of our current and potential competitors have the ability to dedicate substantially greater resources to litigate intellectual property rights than we can and have more mature and developed intellectual property portfolios. Accordingly, despite our efforts, we may not be able to prevent third parties from infringing upon or misappropriating our

intellectual property. Even if resolved in our favor, litigation or other intellectual property proceedings could result in substantial costs and diversion of management attention and resources, which could harm our business and financial results.

In addition, in a litigation or other proceeding, a court or administrative judge may decide that a patent owned by or licensed to us is invalid or unenforceable, or a court may refuse to stop the other party from using the technology at issue on the grounds that our patents do not cover the technology in question. An adverse result in any litigation or other proceeding could put one or more of our patents at risk of being invalidated, held unenforceable or interpreted narrowly. Furthermore, because of the substantial amount of discovery required in connection with intellectual property litigation and other proceedings, there is a risk that some of our confidential information could be compromised by disclosure during this type of litigation. During the course of any patent or other intellectual property litigation or other proceeding, there could be public announcements of the results of hearings, rulings on motions and other interim proceedings or developments and if securities analysts or investors regard these announcements as negative, the perceived value of our ADC product candidates, programs or intellectual property could be diminished. Accordingly, the market price of our common stock may decline. Any of the foregoing could have a material adverse effect on our business, financial conditions, results of operations and prospects.

Third‑party claims of intellectual property infringement or misappropriation may prevent or delay our development and commercialization efforts.

Our commercial success depends in part on our ability and the ability of our strategic partners to develop, manufacture, market and sell product candidates and use our proprietary technologies without infringing, misappropriating or otherwise violating the patents and proprietary rights of third parties. There is a substantial amount of litigation, both within and outside the United States, involving patent and other intellectual property rights in the biopharmaceutical industries, including patent infringement lawsuits, interferences, oppositions, reexamination, inter partes review, derivation and post grant review proceedings before the USPTO and corresponding foreign patent offices. Numerous U.S. and foreign issued patents and pending patent applications owned by third parties exist in the fields in which we are developing and may develop our ADC product candidates. As the biopharmaceutical industries expand and more patents are issued, the risk increases that our ADC product candidates may be subject to claims of infringement of the patent rights of third parties.

Third parties may assert that we, our customers, licensees or parties indemnified by us are employing their proprietary technology without authorization or have infringed upon, misappropriated or otherwise violated their intellectual property or other rights, regardless of their merit. For example, we may be subject to claims that we are infringing the patent, trademark or copyright rights of third parties, or that our employees have misappropriated or divulged their former employers’ trade secrets or confidential information. There may be third‑party patents or patent applications with claims to materials, formulations, methods of manufacture or methods for treatment related to the use or manufacture of our ADC product candidates, that we failed to identify. For example, applications filed before November 29, 2000 and certain applications filed after that date that will not be filed outside the United States remain confidential until issued as patents. Except for certain exceptions, including the preceding exceptions, patent applications in the United States and elsewhere are generally published only after a waiting period of approximately 18 months after the earliest filing, and sometimes not at all. Therefore, patent applications covering our ADC platforms or our ADC product candidates could have been filed by others without our knowledge. Additionally, pending patent applications which have been published can, subject to certain limitations, be later amended in a manner that could cover our ADC platforms, our ADC product candidates or the use or manufacture of our ADC product candidates.

Even if we believe a third party’s claims against us are without merit, a court of competent jurisdiction could hold that such third party’s patent is valid, enforceable and cover aspects of our product candidates, including the materials, formulations, methods of manufacture, methods of analysis, or methods for treatment, in which case, such third party would be able to block our ability to develop and commercialize the applicable technology or product candidate until such patent expired or unless we obtain a license and we may be required to pay such third-party monetary damages, which could be substantial. Such licenses may not be available on acceptable terms, if at all. Even if we were able to obtain a license, the rights may be nonexclusive, which could result in our competitors gaining access to the same intellectual property and it could require us to make substantial licensing and royalty payments. Ultimately, we could be prevented

from commercializing a product, or be forced to cease some aspect of our business operations, if, as a result of actual or threatened patent infringement claims, we are unable to enter into licenses on acceptable terms.

Parties making claims against us may also obtain injunctive or other equitable relief, which could effectively block our ability to further develop and commercialize our ADC technology or one or more of our ADC product candidates. Defending against claims of patent infringement, misappropriation of trade secrets or other violations of intellectual property could be costly and time consuming, regardless of the outcome. Thus, even if we were to ultimately prevail, or to settle at an early stage, such litigation could burden us with substantial unanticipated costs. In addition, litigation or threatened litigation could result in significant demands on the time and attention of our management team, distracting them from the pursuit of other company business. In the event of a successful claim of infringement against us, in addition to potential injunctive relief, we may have to pay substantial damages, including treble damages and attorneys’ fees for willful infringement, pay royalties, redesign our infringing products or obtain one or more licenses from third parties, which may be impossible or require substantial time and monetary expenditure.

We may face a claim of misappropriation if a third party believes that we inappropriately obtained and used trade secrets of such third party. If we are found to have misappropriated a third party’s trade secrets, we may be prevented from further using such trade secrets, limiting our ability to develop our ADC product candidates, we may be required to obtain a license to such trade secrets which may not be available on commercially reasonable terms or at all and may be non‑exclusive, and we may be required to pay damages, which could be substantial. Any of the foregoing could have a material adverse effect on our business, financial condition, results of operations and prospects.

We may not be able to protect our intellectual property and proprietary rights throughout the world.

Filing, prosecuting and defending patents on product candidates in all countries throughout the world where we expect there to be significant markets for our products could be prohibitively expensive, and the laws of foreign countries may not protect our rights to the same extent as the laws of the United States. In addition, our intellectual property license agreements may not always include worldwide rights. For example, certain U.S. and foreign issued patents and patent applications are licensed to us by Recepta on a worldwide basis, except that Recepta retains exclusive rights in such patents and patent applications in Brazil. Consequently, we may not be able to prevent third parties from practicing our inventions in all countries outside the United States. Competitors may use our technologies in jurisdictions where we have not obtained patent protection to develop their own products and, further, may export otherwise infringing products to territories where we have patent protection or licenses but enforcement is not as strong as that in the United States. These products may compete with our products, and our patents or other intellectual property rights may not be effective or sufficient to prevent them from competing.

Additionally, the laws of some foreign countries do not protect intellectual property rights to the same extent as the laws of the United States, and many companies have encountered significant problems in protecting and defending such rights in foreign jurisdictions. The legal systems of certain countries, particularly certain developing countries, do not favor the enforcement of patents and other intellectual property protection, particularly those relating to biotechnology, which could make it difficult for us to stop the infringement of our licensed and owned patents or marketing of competing products in violation of our intellectual property and proprietary rights generally. Proceedings to enforce our intellectual property and proprietary rights in foreign jurisdictions could result in substantial costs and divert our efforts and attention from other aspects of our business, could put our patents at risk of being invalidated or interpreted narrowly, could put our patent applications at risk of not issuing as patents, and could provoke third parties to assert claims against us. We may not prevail in any lawsuits that we initiate, and the damages or other remedies awarded, if any, may not be commercially meaningful. Accordingly, our efforts to enforce our intellectual property and proprietary rights around the world may be inadequate to obtain a significant commercial advantage from the intellectual property that we develop or license.

Many countries have compulsory licensing laws under which a patent owner may be compelled to grant licenses to third parties. In addition, many countries limit the enforceability of patents against government agencies or government contractors. In these countries, the patent owner may have limited remedies, which could materially diminish the value of such patent. If we or any of our licensors is forced to grant a license to third parties with respect to any patents relevant to our business, our competitive position may be impaired, and our business, financial condition, results of operations and prospects may be adversely affected.

Confidentiality agreements with employees and third parties may not prevent unauthorized disclosure of trade secrets and other proprietary information.

In addition to the protection afforded by patents, we rely on trade secret protection and confidentiality agreements to protect proprietary know‑how that is not patentable or that we elect not to patent, processes for which patents are difficult to enforce and any other elements of our platform technology and discovery and development processes that involve proprietary know‑how, information or technology that is not covered by patents. However, trade secrets can be difficult to protect. We seek to protect our proprietary technology and processes, in part, by entering into confidentiality agreements with our employees, consultants and outside scientific advisors, contractors and partners. We cannot guarantee that we have entered into such agreement with each party that may have or have had access to our trade secrets or proprietary technology and processes. Additionally, our confidentiality agreements and other contractual protections may not be adequate to protect our intellectual property from unauthorized disclosure, third‑party infringement or misappropriation. We may not have adequate remedies in the case of a breach of any such agreements, and our trade secrets and other proprietary information could be disclosed to our competitors or others may independently develop substantially equivalent or superior proprietary information and techniques or otherwise gain access to our trade secrets or disclose such technologies.

Enforcing a claim that a third party illegally obtained and is using any of our trade secrets is expensive and time consuming, and the outcome is unpredictable. In addition, some courts outside and within the United States sometimes are less willing to protect trade secrets. Misappropriation or unauthorized disclosure of our trade secrets could impair our competitive position and may have a material adverse effect on our business.

We may be subject to claims by third parties asserting that our licensors, employees, consultants, advisors or we have misappropriated their intellectual property, or claiming ownership of what we regard as our own intellectual property.

Many of our and our licensors’ employees, including our senior management, consultants or advisors are currently, or previously were, employed at universities or other biotechnology or pharmaceutical companies, including our competitors or potential competitors. Some of these employees, including members of our senior management, executed proprietary rights, non‑disclosure and non‑competition agreements, or similar agreements, in connection with such previous employment. Although we try to ensure that our employees, consultants and advisors do not use the proprietary information or know‑how of others in their work for us, we may be subject to claims that we or these individuals have used or disclosed intellectual property, including trade secrets or other proprietary information, of any such individual’s current or former employer. Litigation may be necessary to defend against such claims. If we fail in defending any such claims, in addition to paying monetary damages, we may lose valuable intellectual property rights or personnel or sustain damages. Such intellectual property rights could be awarded to a third party, and we could be required to obtain a license from such third party to commercialize our technology or products. Such a license may not be available on commercially reasonable terms or at all. Even if we are successful in defending against such claims, litigation could result in substantial costs and be a distraction to management. Any of the foregoing may have a material adverse effect on our business, financial condition, results of operations and prospects.

In addition, while it is our policy to require our employees and contractors who may be involved in the conception or development of intellectual property to execute agreements assigning such intellectual property to us, we may be unsuccessful in executing such an agreement with each party who, in fact, conceives or develops intellectual property that we regard as our own. The assignment of intellectual property rights may not be self‑executing or the assignment agreements may be breached, and we may be forced to bring claims against third parties, or defend claims that they may bring against us, to determine the ownership of what we regard as our intellectual property.

If we do not obtain patent term extension and data exclusivity for any product candidates we may develop, our business may be materially harmed.

Depending upon the timing, duration and specifics of any FDA marketing approval of any product candidates we may develop, one or more of our owned or in‑licensed U.S. patents may be eligible for limited patent term extension under the Drug Price Competition and Patent Term Restoration Act of 1984, or Hatch‑Waxman Amendments. The Hatch‑Waxman Amendments permit a patent term extension of up to five years as compensation for the patent term lost during the FDA

regulatory review process. A patent term extension cannot extend the remaining term of a patent beyond a total of 14 years from the date of product approval, only one patent may be extended and only those claims covering the approved drug, a method for using it or a method for manufacturing it may be extended. However, we may not be granted an extension because of, for example, failing to exercise due diligence during the testing phase or regulatory review process, failing to apply within applicable deadlines, failing to apply prior to expiration of relevant patents, or otherwise failing to satisfy applicable requirements. Moreover, the applicable time period or the scope of patent protection afforded could be less than we request. If we are unable to obtain patent term extension or the term of any such extension is less than we request, our competitors may obtain approval of competing products following our patent expiration, and our business, financial condition, results of operations and prospects could be materially harmed.

Obtaining and maintaining our patent protection depends on compliance with various procedural, document submission, fee payment, and other requirements imposed by government patent agencies, and our patent protection could be reduced or eliminated for non‑compliance with these requirements.

Periodic maintenance fees, renewal fees, annuity fees and various other government fees on patents and patent applications will be due to be paid to the USPTO and various government patent agencies outside of the United States over the lifetime of our owned or licensed patents and applications. In certain circumstances, we rely on our licensing partners to pay these fees due to U.S. and non‑U.S. patent agencies. The USPTO and various non‑U.S. government agencies require compliance with several procedural, documentary, fee payment and other similar provisions during the patent application process. We are also dependent on our licensors to take the necessary action to comply with these requirements with respect to our licensed intellectual property. In some cases, an inadvertent lapse can be cured by payment of a late fee or by other means in accordance with the applicable rules. There are situations, however, in which non‑compliance can result in abandonment or lapse of the patent or patent application, resulting in a partial or complete loss of patent rights in the relevant jurisdiction. In such an event, potential competitors might be able to enter the market with similar or identical products or technology, which could have a material adverse effect on our business, financial condition, results of operations and prospects.

Intellectual property rights do not necessarily address all potential threats.

The degree of future protection afforded by our intellectual property rights is uncertain because intellectual property rights have limitations and may not adequately protect our business or permit us to maintain our competitive advantage. For example:

Should any of these events occur, they could have a material adverse effect on our business, financial condition, results of operations and prospects.

Risks related to our business and industry

If we fail to attract and keep senior management and key scientific personnel, we may be unable to successfully develop our ADC product candidates, conduct our clinical studies and commercialize our ADC product candidates.

Our ability to compete in the highly competitive biotechnology and biopharmaceutical industries depends upon our ability to attract, motivate and retain highly qualified managerial, scientific and medical personnel. We are highly dependent on members of our senior management, including Anna Protopapas, our President and Chief Executive Officer. The loss of the services of any of these persons could impede the achievement of our research, development and commercialization objectives. Also, each of these persons may terminate their employment with us at any time. We do not maintain “key person” insurance for any of our executives or other employees.

Recruiting and retaining qualified scientific, clinical, sales and marketing personnel will also be critical to our success. We conduct our operations at our facility in Cambridge, Massachusetts, in a region that is headquarters to many other biopharmaceutical companies and many academic and research institutions. Competition for skilled personnel is intense and the turnover rate can be high, which may limit our ability to hire and retain highly qualified personnel on acceptable terms or at all. We may not be able to attract and retain these personnel on acceptable terms given the competition among numerous pharmaceutical and biotechnology companies for similar personnel. In addition, we rely on consultants and advisors, including scientific and clinical advisors, to assist us in formulating our research and development and commercialization strategy. Our consultants and advisors, may be employed or have commitments under consulting or advisory contracts with other entities that may limit their availability to us.

We may encounter difficulties in managing our growth and expanding our operations successfully.

As we seek to advance our ADC product candidates through clinical studies and commercialization, we will need to expand our development, regulatory, manufacturing, marketing and sales capabilities or contract with third parties to provide these capabilities for us. As our operations expand, we expect that we will need to manage additional relationships with various strategic partners, suppliers and other third parties. Future growth will impose significant added responsibilities on members of management. Our future financial performance and our ability to commercialize our ADC product candidates and to compete effectively will depend, in part, on our ability to manage any future growth effectively. To that end, we must be able to manage our development efforts and clinical studies effectively and hire, train and integrate additional management, administrative and, if necessary, sales and marketing personnel. Due to our limited financial resources and the limited experience of our management team in managing a company with such anticipated growth, we may not be able to accomplish these tasks, and our failure to accomplish any of them could prevent us from successfully growing our company or disrupt our operations.

Our relationships with health care professionals, institutional providers, principal investigators, consultants, customers (actual and potential) and third‑party payors are, and will continue to be, subject, directly and indirectly, to federal and state health care fraud and abuse, false claims, marketing expenditure tracking and disclosure, government price reporting and health information privacy and security laws. If we are unable to comply, or have not fully complied, with such laws, we could face penalties, including, without limitation, civil, criminal and administrative penalties, damages, monetary fines, disgorgement, possible exclusion from participation in Medicare, Medicaid and other federal health care programs, contractual damages, reputational harm, diminished profits and future earnings and curtailment or restructuring of our operations.

Our business operations and activities may be directly or indirectly subject to various federal and state fraud and abuse laws, including, without limitation, the federal Anti‑Kickback Statute and the federal False Claims Act. If we obtain FDA approval for any of our ADC product candidates and begin commercializing those products in the United States, our potential exposure under such laws will increase significantly, and our costs associated with compliance with such laws are also likely to increase. These laws may impact, among other things, our current activities with principal investigators and research subjects, as well as proposed and future sales, marketing and education programs. In addition, we may be subject to patient privacy regulation by the federal government and state governments in which we conduct our business. The laws that may affect our ability to operate include, but are not limited to:

In addition, the regulatory approval and commercialization of any of our product candidates outside the United States will also likely subject us to foreign equivalents of the health care laws mentioned above, among other foreign laws.

The Health Care Reform Act, among other things, amended the intent standard of the federal Anti‑Kickback Statute and criminal health care fraud statutes to a stricter standard such that a person or entity no longer needs to have actual knowledge of this statute or specific intent to violate it. In addition, the Health Care Reform Act codified case law that a claim including items or services resulting from a violation of the federal Anti‑Kickback Statute constitutes a false or fraudulent claim for purposes of the federal False Claims Act.

Efforts to ensure that our business arrangements will comply with applicable health care laws may involve substantial costs. It is possible that governmental and enforcement authorities will conclude that our business practices may not comply with current or future statutes, regulations or case law interpreting applicable fraud and abuse or other health care laws and regulations. If our operations are found to be in violation of any of the laws described above or any other governmental regulations that apply to us, we may be subject to penalties, including, without limitation, civil, criminal and administrative penalties, damages, monetary fines, disgorgement, possible exclusion from participation in Medicare, Medicaid and other federal health care programs, contractual damages, reputational harm, diminished profits and future earnings and curtailment or restructuring of our operations.

Our employees may engage in misconduct or other improper activities, including noncompliance with regulatory standards and requirements and insider trading.

We are exposed to the risk of employee fraud or other misconduct. Misconduct by employees could include intentional failures to comply with FDA regulations, to provide accurate information to the FDA, to comply with manufacturing standards we have established, to comply with federal and state health care fraud and abuse laws and regulations, to report financial information or data accurately or to disclose unauthorized activities to us. In particular, sales, marketing and business arrangements in the health care industry are subject to extensive laws and regulations intended to prevent fraud, kickbacks, self‑dealing and other abusive practices. These laws and regulations may restrict or prohibit a wide range of pricing, discounting, marketing and promotion, sales commission, customer incentive programs and other business arrangements. Employee misconduct could also involve the improper use of information obtained in the course of clinical studies, which could result in regulatory sanctions and serious harm to our reputation. It is not always possible to identify and deter employee misconduct, and the precautions we take to detect and prevent this activity may not be effective in controlling unknown or unmanaged risks or losses or in protecting us from governmental investigations or other actions or lawsuits stemming from a failure to be in compliance with such laws or regulations. If any such actions are instituted against us, and we are not successful in defending ourselves or asserting our rights, those actions could have a significant impact on our business, including the imposition of civil, criminal and administrative penalties, damages, monetary fines, disgorgement, possible exclusion from participation in Medicare, Medicaid and other federal health care programs, contractual damages, reputational harm, diminished profits and future earnings and curtailment or restructuring of our operations, any of which could adversely affect our ability to operate our business and our results of operations.

If product liability lawsuits are brought against us, we may incur substantial liabilities and may be required to limit commercialization of our ADC product candidates.

We face an inherent risk of product liability as a result of the clinical testing of our ADC product candidates and will face an even greater risk if we commercialize any products. For example, we may be sued if any product we develop allegedly causes injury or is found to be otherwise unsuitable during product testing, manufacturing, marketing or sale. Any such product liability claims may include allegations of defects in manufacturing, defects in design, a failure to warn of dangers inherent in the product, negligence, strict liability and a breach of warranties. Claims could also be asserted under state consumer protection acts. If we cannot successfully defend ourselves against product liability claims, we may incur substantial liabilities or be required to limit commercialization of our ADC product candidates. Even a successful defense would require significant financial and management resources. Regardless of the merits or eventual outcome, liability claims may result in:

Failure to obtain and retain sufficient product liability insurance at an acceptable cost to protect against potential product liability claims could prevent or inhibit the commercialization of products we develop. We currently carry product liability insurance covering our clinical studies in the amount of $10 million in the aggregate. Although we maintain such insurance, any claim that may be brought against us could result in a court judgment or settlement in an amount that is not covered, in whole or in part, by our insurance or that is in excess of the limits of our insurance coverage. Our insurance policies also have various exclusions, and we may be subject to a product liability claim for which we have no coverage. In such instance, we might have to pay any amounts awarded by a court or negotiated in a settlement that exceed our coverage limitations or that are not covered by our insurance, and we may not have, or be able to obtain, sufficient capital to pay such amounts. If we are unable to obtain or maintain sufficient insurance coverage at an acceptable cost or to otherwise protect against potential product liability claims, it could prevent or inhibit the development and commercial production and sale of our product candidates, which could adversely affect our business, financial condition, results of operations and prospects.

We and our third‑party contract manufacturers must comply with environmental, health and safety laws and regulations, and failure to comply with these laws and regulations could expose us to significant costs or liabilities.

We and our third‑party manufacturers are subject to numerous environmental, health and safety laws and regulations, including those governing laboratory procedures and the use, generation, manufacture, distribution, storage, handling, treatment, remediation and disposal of hazardous materials and wastes. Hazardous chemicals, including flammable and biological materials, are involved in certain aspects of our business, and we cannot eliminate the risk of injury or contamination from the use, generation, manufacture, distribution, storage, handling, treatment or disposal of hazardous materials and wastes. In the event of contamination or injury, or failure to comply with environmental, health and safety laws and regulations, we could be held liable for any resulting damages and any such liability could exceed our assets and resources. We could also incur significant costs associated with civil or criminal fines and penalties for failure to comply with such laws and regulations.

Although we maintain workers’ compensation insurance to cover us for costs and expenses we may incur due to injuries to our employees resulting from the use of hazardous materials, this insurance may not provide adequate coverage against potential liabilities. We do not maintain insurance for environmental liability or toxic tort claims that may be asserted against us in connection with our storage or disposal of biological, hazardous or radioactive materials.

Environmental, health and safety laws and regulations are becoming increasingly more stringent. We may incur substantial costs in order to comply with current or future environmental, health and safety laws and regulations. These current or future laws and regulations may impair our research, development or production efforts. Our failure to comply with these laws and regulations also may result in substantial fines, penalties or other sanctions.

Further, with respect to the operations of our third‑party contract manufacturers, it is possible that if they fail to operate in compliance with applicable environmental, health and safety laws and regulations or properly dispose of wastes associated with our products, we could be held liable for any resulting damages, suffer reputational harm or experience a disruption in the manufacture and supply of our product candidates or products.

We may acquire assets or form strategic alliances in the future, and we may not realize the benefits of such acquisitions.

We may acquire additional technologies and assets, form strategic alliances or create joint ventures with third parties that we believe will complement or augment our existing business. If we acquire assets with promising markets or technologies, we may not be able to realize the benefit of acquiring such assets if we are unable to successfully integrate them with our existing technologies. We may encounter numerous difficulties in developing, manufacturing and marketing any new products resulting from a strategic alliance or acquisition that delay or prevent us from realizing their expected benefits or enhancing our business. We cannot be assured that, following any such acquisition, we will achieve the expected synergies to justify the transaction.

Our internal computer systems, or those of our strategic partners, third‑party CROs or other contractors or consultants, may fail or suffer security breaches, which could result in a material disruption of our product candidates’ development programs.

Despite the implementation of security measures, our internal computer systems and those of our strategic partners, third‑party CROs and other contractors and consultants are vulnerable to damage from computer viruses, unauthorized access, natural disasters, terrorism, war and telecommunication and electrical failures. If a failure, accident or security breach were to occur and cause interruptions in our or our CROs’ operations, it could result in a material disruption of our programs. For example, the loss of clinical study data for our product candidates could result in delays in our regulatory approval efforts and significantly increase our costs to recover or reproduce the data. To the extent that any disruption or security breach results in a loss of or damage to our data or applications or other data or applications relating to our technology or product candidates, or inappropriate disclosure of confidential or proprietary information, we could incur liabilities and the further development of our product candidates could be delayed.

Unfavorable global economic conditions could adversely affect our business, financial condition or results of operations.

Our results of operations could be adversely affected by general conditions in the global economy and in the global financial markets. For example, the global financial crisis caused extreme volatility and disruptions in the capital and credit markets. A severe or prolonged economic downturn, such as the global financial crisis, could result in a variety of risks to our business, including, weakened demand for our product candidates and our ability to raise additional capital when needed on acceptable terms, if at all. A weak or declining economy could also strain our suppliers, possibly resulting in supply disruption. Any of the foregoing could harm our business and we cannot anticipate all of the ways in which the current economic climate and financial market conditions could adversely impact our business.

Risks related to our common stock

We are an “emerging growth company,” as defined in the JOBS Act, and we cannot be certain if the reduced disclosure requirements applicable to emerging growth companies will make our common stock less attractive to investors.

We are an emerging growth company, as defined in the JOBS Act. For as long as we continue to be an emerging growth company, we may take advantage of exemptions from various reporting requirements that are applicable to other public companies that are not emerging growth companies, including, but not limited to, (1) not being required to comply with the auditor attestation requirements of Section 404 of the Sarbanes‑Oxley Act, (2) reduced disclosure obligations regarding executive compensation in our periodic reports and proxy statements and (3) exemptions from the requirements of holding a non‑binding advisory vote on executive compensation.

We could be an emerging growth company through 2022, although circumstances could cause us to lose that status earlier, including if the market value of our common stock held by non‑affiliates exceeds $700.0 million as of any June 30 before that time or if we have total annual gross revenue of $1.07 billion or more during any fiscal year before that time, in which cases we would no longer be an emerging growth company as of the following December 31 or, if we issue more than $1.00 billion in non‑convertible debt during any three‑year period before that time, we would cease to be an emerging growth company immediately. Even after we no longer qualify as an emerging growth company, we may still qualify as a “smaller reporting company” which would allow us to take advantage of many of the same exemptions from disclosure requirements, including not being required to comply with the auditor attestation requirements of Section 404 of the Sarbanes‑Oxley Act and reduced disclosure obligations regarding executive compensation in our periodic reports and proxy statements. We cannot predict if investors will find our common stock less attractive because we may rely on these exemptions. If some investors find our common stock less attractive as a result, there may be a less active trading market for our common stock and our stock price may be more volatile. When these exemptions cease to apply, we expect to incur additional expenses and devote increased management effort toward ensuring compliance with them, and we cannot predict or estimate the amount or timing of such additional costs.

Under the JOBS Act, emerging growth companies can also delay adopting new or revised accounting standards until such time as those standards apply to private companies. We have irrevocably elected not to avail ourselves of this exemption

from new or revised accounting standards and, therefore, are subject to the same new or revised accounting standards as other public companies that are not emerging growth companies.

If our stock price is volatile, our stockholders could incur substantial losses.

Our stock price has been and may continue to be volatile. Since our initial public offering, or IPO, in June 2017, the price of our common stock as reported on the NASDAQ Select Global Market has ranged from a low of $12.71 on June 29, 2017 to a high of $21.01 on October 3, 2017. The market price of shares of our common stock could be subject to wide fluctuations in response to many risk factors listed in this “Risk Factors” section, and others beyond our control, including:

In addition, the stock market has historically experienced significant volatility, particularly with respect to pharmaceutical, biotechnology and other life sciences company stocks. The volatility of pharmaceutical, biotechnology and other life sciences company stocks often does not relate to the operating performance of the companies represented by the stock. As

a result of this volatility, stockholders may not be able to sell their common stock at or above the price for which they paid for their shares. As we operate in a single industry, we are especially vulnerable to these factors to the extent that they affect our industry or our products, or to a lesser extent our markets. In the past, securities class action litigation has often been initiated against companies following periods of volatility in their stock price. This type of litigation could result in substantial costs and divert our management’s attention and resources, and could also require us to make substantial payments to satisfy judgments or to settle litigation.

Our principal stockholders and management own a significant percentage of our stock and are able to exercise significant influence over matters subject to stockholder approval.

As of December 31, 2017, our executive officers, directors and stockholders who own more than 5% of our outstanding common stock, together with their respective affiliates, beneficially owned a substantial majority of our common stock, including shares subject to outstanding options and warrants that are exercisable within 60 days after such date. Accordingly, these stockholders are able to exert a significant degree of influence over our management and affairs and over matters requiring stockholder approval, including the election of our board of directors and approval of significant corporate transactions. This concentration of ownership could have the effect of entrenching our management or board of directors, delaying or preventing a change in our control or otherwise discouraging a potential acquirer from attempting to obtain control of us, which in turn could have a material and adverse effect on the fair market value of our common stock.

We are incurring and will continue to incur significant increased costs as a result of operating as a public company, and our management is required to devote substantial time to compliance requirements and initiatives.

As a public company, we are incurring and will continue to incur significant legal, insurance, accounting and other expenses that we did not incur as a private company. In addition, the Sarbanes-Oxley Act and rules of the SEC and The NASDAQ Stock Market have imposed various requirements on public companies, including requiring establishment and maintenance of effective disclosure and financial controls. Our management and other personnel need to devote a substantial amount of time to these compliance initiatives. Moreover, these rules and regulations have increased and will continue to increase our legal and financial compliance costs and will make some activities more time-consuming and costly.

We do not expect to pay any cash dividends for the foreseeable future.

We do not anticipate that we will pay any cash dividends to holders of our common stock in the foreseeable future. Instead, we plan to retain any earnings to maintain and expand our operations. In addition, any future debt financing arrangement may contain terms prohibiting or limiting the amount of dividends that may be declared or paid on our common stock. Accordingly, investors must rely on sales of their common stock after price appreciation, which may never occur, as the only way to realize any return on their investment.

Provisions in our amended and restated certificate of incorporation, our amended and restated by‑laws and Delaware law may have anti‑takeover effects that could discourage an acquisition of us by others, even if an acquisition would be beneficial to our stockholders, and may prevent attempts by our stockholders to replace or remove our current management.

Our amended and restated certificate of incorporation, amended and restated by‑laws and Delaware law contain provisions that may have the effect of discouraging, delaying or preventing a change in control of us or changes in our management that stockholders may consider favorable, including transactions in which our stockholders might otherwise receive a premium for their shares. Our amended and restated certificate of incorporation and by‑laws include provisions that:

These provisions, alone or together, could delay or prevent hostile takeovers and changes in control or changes in our management. These provisions could also limit the price that investors might be willing to pay in the future for shares of our common stock, thereby depressing the market price of our common stock.

In addition, because we are incorporated in the State of Delaware, we are governed by the provisions of Section 203 of the General Corporation Law of the State of Delaware, or the DGCL, which prohibits a person who owns in excess of 15% of our outstanding voting stock from merging or combining with us for a period of three years after the date of the transaction in which the person acquired in excess of 15% of our outstanding voting stock, unless the merger or combination is approved in a prescribed manner.

Any provision of our amended and restated certificate of incorporation, amended and restated by‑laws or Delaware law that has the effect of delaying or deterring a change in control could limit the opportunity for our stockholders to receive a premium for their shares of our common stock, and could also affect the price that some investors are willing to pay for our common stock.

The recently passed comprehensive tax reform bill could adversely affect our business and financial condition.

 

On December 22, 2017, the U.S. President signed into law new legislation that significantly revises the Internal Revenue Code of 1986, as amended (the Code).  The newly enacted federal income tax law, among other things, contains significant changes to corporate taxation, including reduction of the corporate tax rate from a top marginal rate of 35% to a rate of 21%, limitation of the tax deduction for interest expense to 30% of adjusted earnings (except for certain small businesses), limitation of the deduction for net operating losses to 80% of current year taxable income in respect of net operating losses generated during or after 2018 and elimination of net operating loss carrybacks, immediate deductions for certain new investments instead of deductions for depreciation expense over time, and modifying or repealing many business deductions and credits.  Notwithstanding the reduction in the corporate income tax rate, the overall impact of the new federal tax law is uncertain and our business and financial condition could be adversely affected.  In addition, it is uncertain if and to what extent various states will conform to the newly enacted federal tax law.  The impact of this tax reform on holders of our common stock is also uncertain and could be adverse.  We urge you to consult with your legal and tax advisors with respect to this legislation and the potential tax consequences of investing in or holding our common stock.

Our ability to use net operating losses and certain tax credit carryforwards may be subject to certain limitations.

For the years ended December 31, 2017 and 2016, the Company recorded no income tax benefit for the net operating losses incurred in each year, due to the uncertainty of realizing a benefit from those items. The Company has incurred net operating losses (NOLs) since its inception. At December 31, 2017, the Company had federal NOLs of  approximately $34.2 million and state NOLs of approximately $33.9 million. The federal and state NOLs will expire at various dates through 2037. At December 31, 2017, the Company had Federal and State research and development tax credit carryforwards of approximately $5.6 million and $2.6 million, respectively, which expire at various dates through 2037. Under the newly enacted federal income tax law, federal NOLs incurred in 2018 and in future years may be carried forward indefinitely, but the deductibility of such federal NOLs is limited. It is uncertain if and to what extent various states will conform to the newly enacted federal tax law. In addition, under Section 382 of the Internal Revenue Code, and corresponding provisions of state law, if a corporation undergoes an “ownership change,” which is generally defined as a greater than 50% change, by value, in its equity ownership over a three-year period, the corporation’s ability to use its pre-change NOLs and other pre-change tax attributes to offset its post-change income or taxes may be limited. Our past issuances of stock and other changes in our stock ownership may have resulted in ownership changes within the meaning of Section 382 of the Code; accordingly, our pre-change NOLs may be subject to limitation under Section 382. If we determine that we have not undergone an ownership change, the Internal Revenue Service could challenge our analysis, and our ability to use our NOLs to offset taxable income could be limited by Section 382 of the Code. Future changes in our stock ownership, including in connection with this offering and some of which are outside of our control, could result in ownership changes under Section 382 of the Code further limiting our ability to utilize our NOLs. Our NOLs may also be impaired under state law. Accordingly, we may not be able to utilize a material portion of our NOLs. The Company has determined that ownership changes have occurred through December 31, 2015 and that certain NOLs and research and development tax credit carryforwards will be subject to limitation.  We may also have incurred subsequent ownership changes. Furthermore, our ability to utilize our NOLs is conditioned upon our attaining profitability and generating U.S. federal taxable income. We have incurred net losses since our inception and anticipate that we will continue to incur significant losses for the foreseeable future; thus, we do not know whether or when we will generate the U.S. federal taxable income necessary to utilize our NOLs. We have recorded a full valuation allowance related to our NOLs and other deferred tax assets due to the uncertainty of the ultimate realization of the future benefits of those assets.

Our amended and restated certificate of incorporation designates the state or federal courts within the State of Delaware as the exclusive forum for certain types of actions and proceedings that may be initiated by our stockholders, which could limit our stockholders’ ability to obtain a favorable judicial forum for disputes with us or our directors, officers or employees.

Our amended and restated certificate of incorporation provides that, subject to limited exceptions, the state or federal courts within the State of Delaware will be exclusive forums for (1) any derivative action or proceeding brought on our behalf, (2) any action asserting a claim of breach of a fiduciary duty owed by any of our directors, officers or other employees to us or our stockholders, (3) any action asserting a claim against us arising pursuant to any provision of the DGCL, our amended and restated certificate of incorporation or our amended and restated by‑laws or (4) any other action asserting a claim against us that is governed by the internal affairs doctrine. Any person or entity that purchases or otherwise acquires any interest in shares of our capital stock shall be deemed to have notice of and to have consented to the provisions of our amended and restated certificate of incorporation described above. This choice of forum provision may limit a stockholder’s ability to bring a claim in a judicial forum that it finds favorable for disputes with us or our directors, officers or other employees, which may discourage such lawsuits against us and our directors, officers and employees. Alternatively, if a court were to find these provisions of our amended and restated certificate of incorporation inapplicable to, or unenforceable in respect of, one or more of the specified types of actions or proceedings, we may incur additional costs associated with resolving such matters in other jurisdictions, which could adversely affect our business and financial condition.

ITEM 1B.           UNRESOLVED STAFF COMMENTS.

None.

ITEM 2.              PROPERTIES.

We occupy approximately 34,000 rentable square feet of office and laboratory space in Cambridge, Massachusetts under a lease that expires on March 31, 2021. We have an option to extend the lease term for an additional five years. We believe that this office and laboratory space is sufficient to meet our current needs and that suitable additional space will be available as and when needed.

ITEM 3.              LEGAL PROCEEDINGS.

None.

ITEM 4.              MINE SAFETY DISCLOSURES.

Not applicable.

PART II

ITEM 5.              MARKET FOR REGISTRANT’S COMMON EQUITY, RELATED STOCKHOLDER MATTERS AND ISSUER PURCHASES OF EQUITY SECURITIES

Certain Information Regarding the Trading of Our Common Stock

Our common stock trades under the symbol “MRSN” on the NASDAQ Global Select Market and has been publicly traded since June 28, 2017. Prior to this time, there was no public market for our common stock. The following table sets forth the high and low sales price of our common stock as reported on the NASDAQ Global Select Market for the periods indicated:

 

Holders of Our Common Stock

As of February 28, 2018, there were approximately 40 holders of record of shares of our common stock.

Dividend Policy

We have never declared nor paid cash dividends on our common stock. We currently intend to retain all of our future earnings, if any, to finance the growth and development of our business. We do not intend to pay cash dividends in respect of our common stock in the foreseeable future. Any future determination to pay cash dividends will be made at the discretion of our board of directors and will depend on restrictions and other factors our board of directors may deem relevant. Investors should not purchase our common stock with the expectation of receiving cash dividends.

Stock Performance Graph

The following performance graph and related information shall not be deemed to be “soliciting material” or to be “filed” with the Securities and Exchange Commission, or SEC, for purposes of Section 18 of the Securities Exchange Act of 1934, as amended, or the Exchange Act, nor shall such information be incorporated by reference into any future filing under the Exchange Act or Securities Act of 1933, as amended, or the Securities Act, except to the extent that we specifically incorporate it by reference into such filing.

The following graph compares the performance of our common stock to the NASDAQ Composite Index and to the NASDAQ Biotechnology Index from June 28, 2017 (the first date that shares of our common stock were publicly traded) through December 29, 2017, which was the last trading day of the year. The comparison assumes $100 was invested in our common stock and in each of the foregoing indices after the market closed on June 28, 2017, and it assumes

reinvestment of dividends, if any. The stock price performance included in this graph is not necessarily indicative of future stock price performance.

Use of Proceeds from Registered Securities

On July 3, 2017, we completed an initial public offering (IPO), in which we issued and sold 5,000,000 shares of our common stock at a public offering price of $15.00 per share, for aggregate gross proceeds of $75 million. We received $67.4 million in net proceeds after deducting $7.6 million of underwriting discounts and commissions and offering costs. On August 2, 2017, we issued and sold 51,977 shares of common stock at $15.00 per share for gross proceeds of $0.8 million upon the partial exercise of the underwriters’ overallotment option. We received net proceeds of $0.7 million after deducting $0.1 million in underwriting discounts and commissions. No offering expenses were paid directly or indirectly to any of our directors or officers (or their associates) or persons owning 10.0% or more of any class of our equity securities or to any other affiliates. 

All of the shares issued and sold in the IPO were registered under the Securities Act pursuant to a Registration Statement on Form S-1 (File No. 333-218412), which was declared effective by the SEC on June 27, 2017. J.P. Morgan Securities LLC, Cowen and Company, LLC and Leerink Partners LLC acted as joint book-running managers of the offering and as representatives of the underwriters.  The offering commenced on June 27, 2017 and did not terminate until the sale of all of the shares offered.

As of December 31, 2017, we estimate that we have used approximately $11.0 million of the net proceeds from the IPO to fund manufacturing and clinical development activities for XMT-1522 and XMT-1536 and other research activities in support of our preclinical programs, and for working capital and other general corporate purposes. We have invested the unused proceeds from the offering in marketable securities and money market accounts. There has been no material change in our planned use of the net proceeds from our IPO as described in our final prospectus filed with the SEC pursuant to Rule 424(b)(4) on June 29, 2017.

 

ITEM 6.              SELECTED FINANCIAL DATA

You should read the following selected financial data together with our financial statements and the related notes appearing elsewhere in this Annual Report on Form 10-K and the information under the heading “Management’s Discussion and Analysis of Financial Condition and Results of Operations.” We have derived the statement of operations data for the years ended December 31, 2017, 2016 and 2015 and the balance sheet data as of December 31, 2017 and 2016 from our audited

financial statements included elsewhere in this Annual Report on Form 10-K. Our historical results are not necessarily indicative of the results that should be expected in the future.

 

ITEM 7.              MANAGEMENT’S DISCUSSION AND ANALYSIS OF FINANCIAL CONDITION AND RESULTS OF OPERATIONS

The following discussion and analysis of our financial condition and results of operations should be read in conjunction with our audited financial statements and related notes appearing elsewhere in this Annual Report on Form 10-K.

Our actual results and timing of certain events may differ materially from the results discussed, projected, anticipated, or indicated in any forward-looking statements. We caution you that forward-looking statements are not guarantees of future performance and that our actual results of operations, financial condition and liquidity, and the development of the industry in which we operate may differ materially from the forward-looking statements contained in this Annual Report. In addition, even if our results of operations, financial condition and liquidity, and the development of the industry in which we operate are consistent with the forward-looking statements contained in this Annual Report, they may not be predictive of results or developments in future periods. 

The following information and any forward-looking statements should be considered in light of factors discussed elsewhere in the Annual Report on Form 10-K, including those risks identified under Part II, Item 1A. Risk Factors.

We caution readers not to place undue reliance on any forward-looking statements made by us, which speak only as of the date they are made. We disclaim any obligation, except as specifically required by law and the rules of the SEC, to publicly update or revise any such statements to reflect any change in our expectations or in events, conditions or circumstances on which any such statements may be based, or that may affect the likelihood that actual results will differ from those set forth in the forward-looking statements.

Overview

We are a clinical stage biopharmaceutical company focused on developing antibody drug conjugates (ADCs), that offer a clinically meaningful benefit for cancer patients with significant unmet need. We have leveraged 20 years of industry learning in the ADC field to develop proprietary technologies that enable us to design ADCs to have improved efficacy, safety and tolerability relative to existing ADC therapies. Our most advanced platform, Dolaflexin, has been used to generate a pipeline of proprietary ADC product candidates to address patient populations that are not currently amenable to treatment with traditional ADC-based therapies. Our lead product candidate, XMT‑1522, is a HER2‑targeted ADC currently in a Phase 1 dose escalation study primarily in breast cancer patients as well as non‑small cell lung cancer (NSCLC) and gastric cancer. Upon the completion of dose escalation, we plan to expand clinical development of XMT‑1522 into additional breast cancer, NSCLC, and gastric cancer patient populations, all of which are not addressed by existing HER2 therapies. Our second product candidate, XMT-1536, is an ADC targeting NaPi2b, an antigen broadly expressed in ovarian cancer and NSCLC. XMT-1536 entered clinical development in late 2017 and is currently in a Phase 1 dose escalation study. Beyond our two lead product candidates, we continue to invest in our earlier stage product candidates and in our ADC technologies. In addition, we have established a strategic partnership with Takeda Pharmaceutical Company Limited, or Takeda, under which they obtained rights to XMT-1522 outside of the United States and Canada. We have also established strategic research and development partnerships with Takeda and Merck KGaA for the development and commercialization of additional ADC product candidates against a limited number of targets selected by our partners based on our Dolaflexin platform. We believe the potential of our ADC technologies, supported by our world-class management team and protected by our robust intellectual property portfolio, will allow us to develop targeted and highly tailored therapies to help cancer patients become cancer survivors.

On July 3, 2017 we closed our IPO of 5,000,000 shares at a price of $15.00 per share for gross proceeds of $75.0 million.  We received approximately $67.4 million after deducting underwriting discounts and commissions and offering costs of approximately $7.6 million. On August 2, 2017, we issued and sold 51,977 shares of common stock at $15.00 per share for gross proceeds of $0.78 million upon the partial exercise of the underwriters’ overallotment option. We received net proceeds of $0.73 million after deducting $0.05 million in underwriting discounts and commissions.

Since inception, our operations have focused on building our platform, identifying potential product candidates, producing drug substance and drug product material for use in preclinical studies, conducting preclinical studies, including Good Laboratory Practice, or GLP, toxicology studies, manufacturing clinical trial material and conducting clinical trials,

establishing and protecting our intellectual property, staffing our company and raising capital. We do not have any products approved for sale and have not generated any revenue from product sales. We have funded our operations primarily through our strategic partnerships, private placements of our convertible preferred stock and our initial public offering.

Since inception, we have incurred significant operating losses. Our net losses were $38.7 million, $13.7 million and $16.4 million for the years ended December 31, 2017, 2016 and 2015, respectively. As of December 31, 2017, we had an accumulated deficit of $97.9 million. We expect to continue to incur significant expenses and operating losses over the next several years. We anticipate that our expenses will increase significantly in connection with our ongoing activities, as we:

Financial Operations Overview

Revenue

To date, all of our revenue has been generated from strategic partnerships. We have not generated any revenue from product sales, and we do not expect to generate any revenue from product sales for the foreseeable future.

In March 2014, we entered into a collaboration agreement with Takeda for the development and commercialization of ADC product candidates utilizing Fleximer. Under this agreement, as amended, Takeda may select up to seven target antigens and has selected four target antigens to date. Takeda is responsible for generating antibodies against the target antigens and we are responsible for generating Fleximer and our proprietary payloads and conjugating this to the antibody to create the ADC product candidates. Takeda then has the exclusive right to and is responsible for the further development, manufacture and commercialization of these ADC product candidates, except that we have an option to co-develop and co-commercialize one product targeting one of Takeda's third through seventh target antigens and may exercise such option with respect to an applicable product no later than 30 days after initiation of a Phase 2 clinical study for such product or at an earlier time if Takeda intends to grant rights to such product to a third party.

In addition, in January 2016, we entered into a collaboration agreement with Takeda for the development and commercialization of XMT-1522. Under this agreement, Takeda is granted the exclusive right and responsibility to commercialize XMT-1522 outside the United States and Canada.

For the years ended December 31, 2017, 2016 and 2015, we recognized revenue of $13.8 million, $21.4 million and $5.5 million, respectively, related to the Takeda agreements.

In June 2014, we entered into a collaboration agreement with Merck KGaA for the development and commercialization of ADC product candidates utilizing Fleximer for up to six target antigens. Merck KGaA is responsible for generating antibodies against the target antigens and we are responsible for generating Fleximer and our proprietary payloads and conjugating this to the antibody to create the ADC product candidates. Merck KGaA then has the exclusive right to and is responsible for the further development and commercialization of these ADC product candidates.

For the years ended December 31, 2017, 2016 and 2015, we recognized revenue of $3.6 million, $3.6 million and $4.6 million, respectively, related to the Merck KGaA agreement.

For the foreseeable future, we expect substantially all of our revenue to be generated from our collaboration agreements with Takeda and Merck KGaA and any other collaboration agreements we may enter into. Given the schedule of potential milestone payments and the uncertain nature and timing of clinical development, we cannot predict when or whether we will receive further milestone payments or any royalty payments under these collaborations.

For information about our revenue recognition policy, see the notes to condensed consolidated financial statements included in this Annual Report on Form 10-K.

Expenses

Research and Development Expenses

Research and development expenses consist primarily of costs incurred for our research and development activities, including our drug discovery efforts, and the development of our product candidates, which include:

Research and development costs are expensed as incurred. Costs of certain activities, such as manufacturing, preclinical studies and clinical trials, are generally recognized based on an evaluation of the progress to completion of specific tasks. Nonrefundable advance payments for goods or services to be received in the future for use in research and development activities are deferred and capitalized. The capitalized amounts are expensed as the related goods are delivered or the services are performed.

We expect research and development costs to increase significantly for the foreseeable future as our product candidate development programs progress. There are numerous factors associated with the successful development and commercialization of any of our product candidates, including future trial design and various regulatory requirements, many of which cannot be determined with accuracy at our current stage of development. Additionally, future commercial and regulatory factors beyond our control may impact our clinical development programs and plans.

A significant portion of our research and development costs have been external costs, which we track on a program-by-program basis following nomination as a product candidate. Our internal research and development costs are primarily personnel-related costs, facility costs, including depreciation and lab consumables. We have not historically tracked all of our internal research and development expenses on a program-by-program basis as they are deployed across multiple projects under development. The following table summarizes our external research and development expenses, by program following nomination as a development candidate for the years ended December 31, 2017, 2016 and 2015. Pre-