ImmunoGen, Inc. - Annual Report: 2010 (Form 10-K)

Use these links to rapidly review the document
ImmunoGen, Inc. Form 10-K TABLE OF CONTENTS
Item 8. Financial Statements and Supplementary Data

Table of Contents

UNITED STATES
SECURITIES AND EXCHANGE COMMISSION
Washington, D.C. 20549

Form 10-K

Commission file number 0-17999

ImmunoGen, Inc.

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

         Indicate by check mark if the registrant is a well-known seasoned issuer, as defined in Rule 405 of the Securities Act. o Yes    ý 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. o Yes    ý 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 the registrant was required to file such reports), and (2) has been subject to such filing requirements for the past 90 days. ý Yes    o 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 (§229.405 of this chapter) during the preceding 12 months (or for such shorter period that the registrant was required to submit and post such files). o Yes    o No

         Indicate by check mark if disclosure of delinquent filers pursuant to Item 405 of Regulation S-K (§229.405 of this chapter) is not contained herein, and will not be contained, to the best of 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. o

         Indicate by check mark whether the registrant is a large accelerated filer, an accelerated filer, a non-accelerated filer or a smaller reporting company. See definitions of "large accelerated filer," "accelerated filer," and "smaller reporting company" in Rule 12b-2 of the Exchange Act. (Check one):

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

         Aggregate market value, based upon the closing sale price of the shares as reported by the NASDAQ Global Market, of voting stock held by non-affiliates at December 31, 2009: $444,794,248 (excludes shares held by executive officers and directors). Exclusion of shares held by any person should not be construed to indicate that such person possesses the power, direct or indirect, to direct or cause the direction of management or policies of the registrant, or that such person is controlled by or under common control with the registrant. Common Stock outstanding at August 24, 2010: 67,949,840 shares.

DOCUMENTS INCORPORATED BY REFERENCE

         Portions of the definitive Proxy Statement to be delivered to shareholders in connection with the Annual Meeting of Shareholders to be held on November 16, 2010 are incorporated by reference into Part III.

Table of Contents

ImmunoGen, Inc.

Form 10-K

TABLE OF CONTENTS

2

Table of Contents

Item 1.    Business

        In this Annual Report on Form 10-K, ImmunoGen, Inc. (ImmunoGen, Inc., together with its subsidiaries, is referred to in this document as "we", "us", "ImmunoGen", or the "Company"), incorporates by reference certain information from parts of other documents filed with the Securities and Exchange Commission. The Securities and Exchange Commission allows us to disclose important information by referring to it in that manner. Please refer to all such information when reading this Annual Report on Form 10-K. All information is as of June 30, 2010 unless otherwise indicated. For a description of the risk factors affecting or applicable to our business, see "Risk Factors," below.

The Company

        We develop novel, targeted therapeutics for the treatment of cancer using our expertise in cancer biology, the development of monoclonal antibodies, the creation of highly potent cytotoxic, or cell-killing, agents, and the engineering of linkers used to attach our cell-killing agents to antibodies.

        Most of the product candidates being developed by us and through our collaborations with others utilize our Targeted Antibody Payload, or TAP, technology. A TAP compound consists of a monoclonal antibody that binds specifically to an antigen target found on cancer cells with one of our highly potent cytotoxic agents attached using one of our linkers. Our linkers are engineered to keep the cytotoxic agent stably attached to the antibody while the TAP compound is in the blood stream and then release it in a fully active form after delivery to a cancer cell. Six TAP compounds and one therapeutic antibody are in clinical testing through our own programs and those of our partners, with more expected to enter the clinic in 2010 and 2011.

        We develop targeted, antibody-based anticancer compounds for our own proprietary pipeline. Our most advanced wholly owned product candidate is lorvotuzumab mertansine (also known as IMGN901). This TAP compound is a potential treatment for cancers that express CD56, which include small-cell lung cancer, Merkel cell carcinoma, ovarian cancer, and multiple myeloma. We have several clinical trials underway with lorvotuzumab mertansine and expect to initiate additional trials going forward. Our second most advanced compound, IMGN388, is a potential treatment for solid tumors including melanomas, sarcomas and many carcinomas. IMGN388 also is a clinical-stage TAP compound. We have three TAP compounds currently in or positioned to begin preclinical toxicology studies. One of these compounds, IMGN529, is being developed for the treatment of certain liquid tumors and we expect to submit an investigational new drug, or IND, application to the FDA for this product candidate in 2011. We expect to submit an IND for another of these in 2012. In addition to our product programs, we continue to invest in our TAP technology, including the development of additional cytotoxic agents and linkers, to maintain a leadership position in our field.

        Part of our business model is to establish collaborations with other companies in order to provide us with cash and revenue short term and potential significant value long term. The collaborations also expand the utilization of our TAP technology. The most advanced TAP compound, Trastuzumab-DM1 or T-DM1, is in development through our collaboration with Genentech, a member of the Roche Group, and it is in Phase III testing for two indications. SAR3419 and SAR650984 are in clinical testing through a collaboration with sanofi-aventis. BIIB015 and BT-062 are in clinical development through our collaborations with Biogen Idec and Biotest, respectively. Companies with licenses to develop TAP compounds to other targets include Amgen, Bayer Schering Pharma, Genentech, and sanofi-aventis.

        We were organized as a Massachusetts corporation in 1981. Our principal offices are located at 830 Winter Street, Waltham, Massachusetts (MA) 02451, and our telephone number is (781) 895-0600. We maintain a website at www.immunogen.com, where certain information about us is available. Please note that information contained on the website is not a part of this document. Our Annual Reports on Form 10-K, Quarterly Reports on Form 10-Q, Current Reports on Form 8-K, and any amendments to

3

Table of Contents

those reports are available free of charge through the "Investor Information" section of our website as soon as reasonably practicable after those materials have been electronically filed with, or furnished to, the Securities and Exchange Commission. We have adopted a Code of Corporate Conduct that applies to all our directors, officers and employees and a Senior Officer and Financial Personnel Code of Ethics that applies to our senior officers and financial personnel. Our Code of Corporate Conduct and Senior Officer and Financial Personnel Code of Ethics are available free of charge through the "Investor Information" section of our website.

Product Candidates

        The following table summarizes the status for compounds in development by us and our collaborators. The results from preclinical testing and early clinical trials may not be predictive of results obtained in subsequent clinical trials and there can be no assurance that our or our

4

Table of Contents

collaborators' clinical trials will demonstrate the level of safety and efficacy of any product candidates necessary to obtain regulatory approval.

⁽¹⁾

Therapeutic antibody; the rest of the product candidates are TAP compounds.

Trastuzumab-DM1 (T-DM1)

        The most advanced compound in our pipeline is T-DM1, which is in global development by Roche for the treatment of advanced HER2+ breast cancer. T-DM1 consists of our DM1 cell-killing agent attached to trastuzumab, which is the active component of the marketed anticancer compound,

5

Table of Contents

Herceptin®. T-DM1 was created under a HER2-specific license agreement established in 2000 between ImmunoGen and Genentech.

        To date, we have earned $13.5 million of a potential $44 million in milestone payments with Genentech/Roche's advancement of T-DM1: $2 million when the IND became effective in January 2006, $5 million when T-DM1 entered Phase II clinical testing in July 2007, and $6.5 million when it began Phase III evaluation in February 2009. Genentech retained us to develop a commercial-scale manufacturing process for T-DM1 in May 2006 and we have completed and transferred this process.

        In early July 2010, Roche announced the submission of a Biologics License Application, or BLA, to the FDA to gain U.S. marketing approval for use of T-DM1 to treat patients with advanced HER2+ breast cancer who had previously received multiple chemotherapies and HER2-targeted medicines; this is also known as 3^rd-line use. The basis of the submission is the single-arm Phase II clinical trial that was reported at the San Antonio Breast Cancer Symposium in December 2009. In August 2010, Roche announced that the FDA issued a Refuse to File letter for this BLA. Roche will continue to work with the FDA and expects to submit a new BLA in mid 2012 based on results of the ongoing EMILIA trial described below.

        In February 2009, patient dosing began in a Phase III trial, EMILIA, being conducted by Roche to assess T-DM1 for 2^nd-line use in advanced HER2+ breast cancer. EMILIA compares T-DM1, used alone, against the marketed anticancer agents, lapatinib and capecitabine, used together. Roche has noted that, if successful, this trial would be used to apply in 2012 for marketing approval of T-DM1 for 2^nd-line use in the U.S. It would also be used to apply in 2012 for marketing approval in Europe.

        In July 2010, patient dosing began in a Phase III trial, MARIANNE, being conducted by Roche to assess T-DM1 for 1st-line use in advanced HER2+ breast cancer. MARIANNE compares T-DM1, used alone, against the marketed agent, trastuzumab, used together with a taxane, in patients not previously treated for metastatic disease. This trial also includes comparison to T-DM1 used with Roche's experimental agent, pertuzumab. Roche has noted that, if successful, MARIANNE would be used to apply some time after 2013 for marketing approval of T-DM1 for 1^st-line use in the U.S. and Europe.

        The first findings with T-DM1 used for 1^st-line treatment of advanced HER2+ breast cancer are expected to be reported at a medical meeting in October 2010. These data would be from a Phase II trial.

        Roche also is assessing T-DM1 used in combination with several approved and experimental agents, and reported the first data on T-DM1 used together with pertuzumab at a medical meeting in June 2010. It expects to report additional data at a medical meeting in December 2010.

Lorvotuzumab mertansine

        Our most advanced wholly owned compound is lorvotuzumab mertansine, also known as IMGN901. The target for this TAP compound, CD56, is found on a number of tumor types, including small-cell lung cancer, ovarian cancer, Merkel cell carcinoma, and the liquid tumor, multiple myeloma. We believe lorvotuzumab mertansine has the potential to be the first effective antibody-based therapy for the treatment of these cancers.

        There is a need for better therapies for a number of CD56+ cancers. For example, patients with advanced small-cell lung cancer can respond to their first treatment regimen, but typically their disease then recurs fairly quickly and, at that stage, patients usually survive for less than 6 months. There are no approved therapies for metastatic Merkel cell carcinoma, and patients with advanced disease typically survive for less than 7 months. In recent years, several new therapies have become available to treat multiple myeloma; however, patients still die from this disease and we believe there is a need for a treatment, such as lorvotuzumab mertansine, that has a different mechanism of action from the approved therapies.

6

Table of Contents

        We are evaluating lorvotuzumab mertansine for the treatment of CD56+ cancers, focusing initially on small-cell lung cancer, Merkel cell carcinoma, ovarian cancer, and multiple myeloma:

Small-cell lung cancer, or SCLC—Patients with recurrent SCLC were enrolled in the first lorvotuzumab mertansine Phase I trial conducted, which has been completed, and are being enrolled in the expansion phase of a second Phase I trial, which is in progress. To assess lorvotuzumab mertansine for 1^st-line treatment of SCLC, we plan to commence a Phase I/II trial by late 2010 assessing it used in combination with etoposide/carboplatin, the standard 1^st-line treatment for this cancer. We have applied for orphan drug designation in SCLC in the U.S. and Europe and are awaiting the decision.

Merkel cell carcinoma, or MCC—A limited number of patients with advanced MCC received lorvotuzumab mertansine in the ongoing Phase I trial referenced previously. Additional patients with advanced MCC are being enrolled in the expansion phase of this trial to gain more experience with lorvotuzumab mertansine in the treatment of this disease. These findings—along with other clinical data and input gained from regulatory agencies—will be used to determine whether or not we will initiate a pivotal Phase II trial for this use during 2011. We have received orphan drug designation in MCC in the U.S. and Europe.

Ovarian cancer—To gain experience with lorvotuzumab mertansine in this disease, patients with CD56+ ovarian cancer are being recruited to the expansion phase of the ongoing Phase I trial referenced previously.

Multiple myeloma, or MM—Two Phase I trials are underway in this indication. One evaluates lorvotuzumab mertansine when used as a single agent and is currently in its expansion phase. The other evaluates the compound used in combination with the standard treatment for this cancer, lenalidomide plus dexamethasone. We expect to report interim data from one or both of these trials at the ASH annual meeting in December 2010. The findings from these trials will inform the future development of lorvotuzumab mertansine for MM.

SAR3419

        We created this TAP compound for the treatment of non-Hodgkin's lymphoma and other B-cell malignancies and licensed it to sanofi-aventis as part of a broader research collaboration. We earned a $1 million milestone payment from sanofi-aventis in October 2007 with the its advancement into clinical testing. SAR3419 is currently in Phase I testing for the treatment of non-Hodgkin's lymphoma. Encouraging findings from the first Phase I trial conducted were reported at a medical meeting in December 2009. Sanofi-aventis is evaluating the compound using a different dosing schedule in a second Phase I trial, and SAR3419 is expected to advance into Phase II testing in the second half of 2011. Like Genentech for T-DM1, sanofi-aventis retained us to develop a commercial-scale manufacturing process for the compound. We have completed and transferred this process.

IMGN388

        We are developing IMGN388 for the treatment of solid tumors. It includes an integrin-targeting antibody developed by Centocor Ortho Biotech, previously Centocor. IMGN388's target occurs on many types of solid tumors and also on vascular endothelial cells in the process of forming new blood vessels, a process that needs to occur for solid tumors to grow. IMGN388 is in Phase I testing and clinical data from this trial were presented at a medical meeting in June 2010. Centocor Ortho Biotech has opt-in rights for this compound.

7

Table of Contents

Other Compounds in Development by Us

        In addition to lorvotuzumab mertansine and IMGN388, we have a number of product candidates at earlier stages in our pipeline. We expect to submit an IND for our third compound, IMGN529, in 2011, and an IND for our fourth compound in 2012. These are both TAP compounds. IMGN529 is a potential treatment for certain types of liquid tumors.

        We also continue to invest in our TAP technology, including the development of additional cytotoxic agents and linkers, to maintain our leadership position in our field.

Other Compounds in Development by Our Partners

        In addition to T-DM1 and SAR3419, other compounds in clinical testing through our collaborations with other companies are BT-062, BIIB015, and SAR650984. We expect 3 - 4 additional TAP compounds to advance into clinical testing during 2010 and 2011 through our existing collaborative partnerships. Companies with licenses to develop TAP compounds other than those already in the clinic include Amgen, Bayer Schering Pharma, Genentech, and sanofi-aventis.

Incidence of Relevant Cancers

        Cancer remains a leading cause of death worldwide, and is the second leading cause of death in the U.S. Based on American Cancer Society estimates, we believe approximately 1.5 million new cases of cancer will be diagnosed in the U.S. in 2010 and that approximately 570,000 people will die from various cancers. The total number of people living with cancer significantly exceeds the number of patients diagnosed with cancer in a given year as patients can live with cancer for a year or longer. Additionally, the potential market for anticancer drugs exceeds the number of patients treated as many types of cancer typically are treated with multiple compounds at the same time. Additionally, patients often receive multiple drug regimens sequentially, either to treat or help prevent recurrence of the disease. In recent years, several antibody-based anticancer drugs have enjoyed considerable commercial success, as have other targeted anticancer agents.

        T-DM1—Based on American Cancer Society and Roche estimates, we believe approximately 42,000-50,000 new cases of HER2+ breast cancer will be diagnosed in 2010. These include diagnoses for both localized disease and advanced, or metastatic, disease. The first approvals of T-DM1 are expected to be for advanced disease. Roche has estimated the 2^nd-line and later patient population in the U.S. to be approximately 13,700 patients.

        Lorvotuzumab mertansine—We are assessing this compound for the treatment of CD56+ solid tumors, including small-cell lung cancer, ovarian cancer and Merkel cell carcinoma, as well as the liquid tumor, multiple myeloma. Based on our own studies and scientific literature, we believe that CD56 is expressed on approximately 100% of small-cell lung cancer and Merkel cell carcinoma cases, 70% of multiple myeloma cases, and 58% of ovarian cancer cases. Based on American Cancer Society estimates and other sources, we believe that approximately 28,000 new cases of small-cell lung cancer will be diagnosed in the U.S. in 2010. Based on American Cancer Society estimates, we also believe that approximately 22,000 new cases of ovarian cancer and 20,000 new cases of multiple myeloma will be diagnosed in the U.S. in 2010. Based on other published data, we believe approximately 1,900 new cases of Merkel cell carcinoma will be diagnosed in the U.S. in 2010.

        We are assessing our IMGN388 compound for the treatment of solid tumors. Cancers of particular interest include melanoma, lung, breast, and ovarian cancers. Based on American Cancer Society estimates, we believe approximately 522,000 new cases of these cancers will be diagnosed in the U.S. in 2010.

8

Table of Contents

Out-licenses and Collaborations

        As part of our business strategy to expand the use of and financial return from our TAP technology, we enter into license agreements with third parties where we grant the other party the exclusive right to use our TAP technology with their antibodies to specific antigen targets. We also had a research collaboration with sanofi-aventis that provided them access to compounds in our preclinical pipeline. As part of these agreements, we are entitled to receive upfront fees, potential milestone payments and royalties on the sales of any resulting products. Our principal out-licenses and collaborative agreements are described below.

Genentech (a member of the Roche Group)

        In May 2000, we entered into two separate agreements with Genentech. The first agreement grants Genentech an exclusive license to our maytansinoid TAP technology for use with antibodies that target HER2, such as trastuzumab. Under the terms of this agreement, Genentech has exclusive worldwide rights to develop and commercialize maytansinoid TAP compounds with antibodies that target HER2. Genentech is responsible for the manufacturing, product development and marketing of any products resulting from the agreement. We are reimbursed for any preclinical and clinical materials that we manufacture under the agreement. We received a $2 million non-refundable payment from Genentech upon execution of the agreement. We also are entitled to receive up to $44 million in milestone payments from Genentech under this agreement, as amended in May 2006, in addition to royalties on the net sales of any resulting products. Genentech and Roche began Phase III evaluation of T-DM1 in February 2009, which triggered a $6.5 million milestone payment to us. Through June 30, 2010, we have received a total of $13.5 million in milestone payments.

        In May 2000 we also entered into a "right-to-test" agreement with Genentech. This agreement provided Genentech with the right to test our maytansinoid TAP technology with antibodies to a defined number of targets on an exclusive basis for specified option periods and to take exclusive licenses to use our maytansinoid TAP technology to develop products directed to individual targets on agreed-upon terms. We received non-refundable technology access fees totaling $5 million for the eight-year term of the agreement. Genentech no longer has the right to designate new targets under this "right-to-test" agreement.

        Under this agreement, Genentech licensed exclusive rights to use our maytansinoid TAP technology with antibodies to four undisclosed targets. The most recent license was taken in December 2008. Under the terms defined in the 2000 "right-to-test" agreement, for each license we received a $1 million license fee and may receive up to $38 million in milestone payments. We are also entitled to receive royalties on the sales of any resulting products. Genentech is responsible for the development, manufacturing, and marketing of any products resulting from these licenses.

Amgen

        In September 2000, we entered into a ten-year "right-to-test" agreement with Abgenix, Inc., which was later acquired by Amgen. The agreement provides Amgen with the right to test our maytansinoid TAP technology with antibodies to a defined number of targets on either an exclusive and non-exclusive basis for specified option periods and to take exclusive or non-exclusive licenses to use our maytansinoid TAP technology to develop products for individual targets on agreed-upon terms. We received a $5 million technology access fee in September 2000. Under the agreement, in September 2009 and November 2009, we entered into two development and license agreements with Amgen and received a $1 million upfront payment with each license taken. In addition to the $1 million upfront payment, we are entitled to earn milestone payments potentially totaling $34 million per target for each compound developed under the "right-to-test" agreement, as well as royalties on the commercial sales of any resulting products. In March 2010, we granted Amgen a non-exclusive option to test our TAP

9

Table of Contents

technology with antibodies to a specific target, for which Amgen paid us a nominal fee. Under this "right-to-test" agreement, there can be option periods in effect that extend beyond the expiration of the agreement in September 2010.

sanofi-aventis

        In July 2003, we entered into a broad collaboration agreement with sanofi-aventis to discover, develop and commercialize antibody-based anticancer therapeutics.

        The agreement provides sanofi-aventis with worldwide commercialization rights to new anticancer therapeutics developed to targets that were included in the collaboration, including the right to use our TAP technology and our humanization technology in the creation of therapeutics to these targets. The product candidates (targets) currently in the collaboration include SAR3419 (CD19), SAR650984 (CD38), SAR566658 (CA6) and additional compounds at earlier stages of development that have yet to be disclosed.

        The collaboration agreement entitles us to receive milestone payments potentially totaling $21.5 million for each therapeutic now included in the collaboration agreement. Through June 30, 2010, we have earned a total of $4 million in milestone payments related to the three product candidates noted above and one target not yet disclosed. We also earned an aggregate of $8 million of milestone payments related to two product candidates previously in the collaboration that have been returned to us along with the rights to the respective targets.

        The agreement also entitles us to royalties on the commercial sales of any resulting products if and when such sales commence. Sanofi-aventis is responsible for the cost of the development, manufacturing and marketing of any products created through the collaboration. We are reimbursed for any preclinical and clinical materials that we make under the agreement. The collaboration agreement also provides us an option to certain co-promotion rights in the U.S. on a product-by-product basis. The terms of the collaboration agreement allow sanofi-aventis to terminate our co-promotion rights if there is a change of control of our company.

        The overall term of the agreement extends to the later of the latest patent to expire or twelve years after the latest launch of any product discovered, developed and/or commercialized under the agreement. Sanofi-aventis paid us an upfront fee of $12.0 million in August 2003. Inclusive of all of its allowed extensions, the agreement enabled us to receive committed research funding totaling $79.3 million over the five years of the research collaboration. The two companies subsequently agreed to extend the date of research funding through October 31, 2008 to enable completion of previously agreed-upon research. We recorded the research funding as it was earned based upon its actual resources utilized in the collaboration. We earned $81.5 million of committed funding over the duration of the research program and are now compensated for research performed for sanofi-aventis on a mutually agreed-upon basis.

        In October 2006, sanofi-aventis licensed non-exclusive rights to use our proprietary resurfacing technology to humanize antibodies to targets not included in the collaboration, including antibodies for non-cancer applications. This license provides sanofi-aventis with the non-exclusive right to use our proprietary humanization technology through August 31, 2011 with the right to extend for one or more additional periods of three years each by providing us with written notice prior to expiration of the then-current license term. Under the terms of the license, we are entitled to a $1 million license fee, half of which was paid upon contract signing and the second half was paid in August 2008, and in addition, we are entitled to receive milestone payments potentially totaling $4.5 million for each antibody humanized under this agreement and also royalties on commercial sales, if any.

        In August 2008, sanofi-aventis exercised its option under a 2006 agreement for expanded access to our TAP technology. The exercise of this option enables sanofi-aventis to evaluate, with certain

10

Table of Contents

restrictions, our maytansinoid TAP technology with antibodies to targets that were not included in the research collaboration between the companies and to license the exclusive right to use the technology to develop products to specific targets based on the terms in the 2006 agreement. We are entitled to earn upfront and milestone payments potentially totaling $32 million per target for each compound developed under the 2006 agreement, as well as royalties on the commercial sales of any resulting products. We are also entitled to manufacturing payments for any materials made on behalf of sanofi-aventis. We received $500,000 in December 2006 with the signing of the option agreement and we received $3.5 million with the exercise of this option in August 2008. The agreement has a three-year term from the date of the exercise of the option and can be renewed by sanofi-aventis for one additional three-year term by payment of a $2 million fee.

Biogen Idec

        In October 2004, we entered into a development and license agreement with Biogen Idec. The agreement grants Biogen Idec exclusive rights to use our maytansinoid TAP technology to develop and commercialize therapeutic compounds to the target Cripto. Biogen Idec is responsible for the research, development, manufacturing, and marketing of any products resulting from the license. We received a $1 million upfront payment upon execution of the agreement. In January 2008, Biogen Idec submitted an IND to the FDA for BIIB015, which was created under this agreement. This event triggered a $1.5 million milestone payment to us. Assuming all benchmarks are met, we could receive up to $42 million in milestone payments under this agreement. We are also entitled to receive royalties on net sales of resulting products. We also receive compensation from Biogen Idec for any product development research done on its behalf, as well as for the production of preclinical and clinical materials.

Biotest

        In July 2006, we entered into a development and license agreement with Biotest. The agreement grants Biotest exclusive rights to use our maytansinoid TAP technology to develop and commercialize therapeutic compounds directed to the target CD138. We received a $1 million upfront payment upon execution of the agreement. In September 2008, Biotest began Phase I evaluation of BT-062, which was created under this agreement. This event triggered a $500,000 milestone payment to us. Assuming all benchmarks are met under this agreement, we could receive up to $35.5 million in milestone payments. We are also entitled to receive royalties on net sales of any resulting products. We receive payments for manufacturing any preclinical and clinical materials made at the request of Biotest.

        The agreement also provides us with the right to elect, at specific stages during the clinical evaluation of any compound created under this agreement, to participate in the U.S. development and commercialization of that compound in lieu of receiving royalties on U.S. sales and the milestone payments not yet earned. We can exercise this right by making a payment to Biotest of an agreed-upon fee of $5 million or $15 million, depending on the stage of development. Upon exercise of this right, we would share equally with Biotest the associated costs of product development and commercialization in the U.S. along with the profit, if any, from U.S. product sales.

Bayer Schering Pharma

        In October 2008, we entered into a development and license agreement with Bayer Schering Pharma AG. The agreement grants Bayer Schering Pharma exclusive rights to use our maytansinoid TAP technology to develop and commercialize therapeutic compounds directed to a specific target. Bayer Schering Pharma is responsible for the research, development, manufacturing and marketing of any products resulting from the license. We received a $4 million upfront payment upon execution of the agreement, and—for each compound developed and marketed by Bayer Schering Pharma under this collaboration—we could potentially receive up to $170.5 million in milestone payments;

11

Table of Contents

additionally, we are entitled to receive royalties on the net sales of any resulting products. In September 2009, Bayer Schering Pharma reached a preclinical milestone which triggered a $1 million payment to us. We also are entitled to receive payments for manufacturing any preclinical and clinical materials at the request of Bayer Schering Pharma as well as for any related process development activities.

In-Licenses

        From time to time we may in-license certain rights to targets or technologies for use in conjunction with our internal efforts to develop both TAP and naked-antibody products and related technologies. In exchange, we may be obligated to pay upfront fees, potential milestone payments and royalties on any product sales.

Centocor Ortho Biotech

        In December 2004, we entered into a development and license agreement with a predecessor to Centocor Ortho Biotech, a wholly owned subsidiary of Johnson & Johnson. Under the terms of this agreement, Centocor was granted exclusive worldwide rights to develop and commercialize anticancer therapeutics that consist of our maytansinoid cell-killing agent attached to an av integrin-targeting antibody that was developed by Centocor. Under the terms of the agreement, we received an upfront payment of $1 million upon execution of the agreement.

        In December 2007, we licensed from Centocor the exclusive, worldwide right to develop and commercialize a TAP compound, IMGN388, that consists of an av integrin-targeting antibody developed by them and one of our maytansinoid cell-killing agents. This license reallocates the parties' respective responsibilities and financial obligations from the license referenced above. Centocor has the right to opt-in on future development and commercialization of IMGN388 at an agreed-upon stage in early clinical testing. Should Centocor not exercise this right, Centocor would be entitled to receive milestone payments potentially totaling $30 million, with the first payment due upon the completion of a successful Phase III trial, and also royalties on IMGN388 sales, if any. In this event, ImmunoGen has the right to obtain a new partner for IMGN388, with certain restrictions. Should Centocor exercise its opt-in right, ImmunoGen would receive an opt-in fee and be released from its obligation to pay Centocor any milestone payments or royalties on sales. Both companies would contribute to the costs of developing the compound. The two companies would share equally any profits on the sales of the compound in the U.S. and ImmunoGen would receive royalties on any international sales. The companies have agreed to share certain third-party payments. In June 2008, the FDA approved the IND application for IMGN388. This event triggered a $1 million milestone payment to a third-party, half of which was paid by ImmunoGen. As of June 30, 2010, the maximum amount that may be payable in the future to such third-parties under this agreement is $11 million.

Other Licenses

        We also have licenses with third parties, including other companies and academic institutions, to gain access to techniques and materials for drug discovery and product development and the rights to use those techniques and materials to make our product candidates. These licenses include rights to certain antibodies.

Patents, Trademarks and Trade Secrets

        Our intellectual property strategy centers on obtaining patent protection for our proprietary technologies and product candidates. As of June 30, 2010, our patent portfolio had a total of 299 issued patents worldwide and 463 pending patent applications worldwide that we own or license from third parties. We seek to protect our TAP technology and our product candidates through a multi-pronged

12

Table of Contents

approach. In this regard, we have patents and patent applications covering antibodies and other cell-binding agents, linkers, maytansinoid and other cell-killing agents, and complete antibody-drug conjugates, or immunoconjugates, comprising these components and methods of making and using each of the above. Typically, multiple issued patents and pending patent applications cover various aspects of each product candidate.

        We consider our maytansinoid technology to be a key component of our overall corporate strategy. We currently own 21 issued U.S. patents covering various embodiments of our maytansinoid technology including claims directed to certain maytansinoids, antibody-maytansinoid conjugates and other cell-binding agents used with maytansinoids, and methods of making and using the same. In all cases, we have received or are applying for comparable patents in other jurisdictions including Europe and Japan. We have issued patents that cover numerous aspects of the manufacture of both our DM1 and DM4 cell-killing agents. These issued patents remain in force until various times between 2020 and 2026. We also have a composition of matter patent on our DM4 cell-killing agent that is expected to remain in force until 2024.

        Our intellectual property strategy also includes pursuing patents directed to linkers, antibodies, conjugation methods, immunoconjugate formulations and the use of specific antibodies and immunoconjugates to treat certain diseases. In this regard, we have issued patents and pending patent applications related to many of our linker technologies. These issued patents, expiring in 2021-2023, and any patents which may issue from the patent applications, cover antibody-maytansinoid conjugates using these linkers. We also have issued U.S. patents and pending patent applications covering methods of assembling immunoconjugates from their constituent antibody, linker and cell-killing agent moieties. These issued patents will expire in 2021, while any patents that may issue from pending patent applications also covering various aspects of these technologies will, if issued, expire between 2021 and 2030. We also have issued patents and pending patent applications related to monoclonal antibodies that may be a component of a TAP compound or may be developed as a therapeutic, or "naked," antibody anticancer compound. Among these patents is an issued U.S. patent claiming a method of humanizing murine antibodies to avoid their detection by the human immune system. We have received patents in other jurisdictions, including Europe and Japan, that correspond to our antibody humanization U.S. patent. These patents will expire between 2013 and 2014.

        We expect our continued work in each of these areas will lead to other patent applications. In all such cases, we will either be the assignee or owner of such patents or have an exclusive license to the technology covered by the patents. For example, we also own issued patents covering proprietary derivatives of non-maytansinoid cell-killing molecules. However, we do not currently consider these additional patent families to be material to our business.

        As described elsewhere in this annual report on Form 10-K under the heading "In-Licenses—Centocor Ortho Biotech," we have in-licensed certain technology from Centocor Ortho Biotech in connection with the development of our IMGN388 product candidate. In addition, we have in-licensed intellectual property relating to our lorvotuzumab mertansine product candidate from Dana-Farber Cancer Institute. We do not believe that the terms of this license are material to our business or prospects.

        We cannot provide assurance that the patent applications will issue as patents or that any patents, if issued, will provide us with adequate protection against competitors with respect to the covered products, technologies or processes. Defining the scope and term of patent protection involves complex legal and factual analyses and, at any given time, the result of such analyses may be uncertain. In addition, other parties may challenge our patents in litigation or administrative proceedings resulting in a partial or complete loss of certain patent rights owned or controlled by ImmunoGen, Inc. Furthermore, as a patent does not confer any specific freedom to operate, other parties may have patents that may block or otherwise hinder the development and commercialization of our technology.

13

Table of Contents

        In addition, many of the processes and much of the know-how that are important to us depend upon the skills, knowledge and experience of our key scientific and technical personnel, which skills, knowledge and experience are not patentable. To protect our rights in these areas, we require that all employees, consultants, advisors and collaborators enter into confidentiality agreements with us. Further, we require that all employees enter into assignment of invention agreements as a condition of employment. We cannot provide assurance, however, that these agreements will provide adequate or any meaningful protection for our trade secrets, know-how or other proprietary information in the event of any unauthorized use or disclosure of such trade secrets, know-how or proprietary information. Further, in the absence of patent protection, we may be exposed to competitors who independently develop substantially equivalent technology or otherwise gain access to our trade secrets, know-how or other proprietary information.

Competition

        We focus on highly competitive areas of product development. Our competitors include major pharmaceutical companies and other biotechnology firms. For example, Pfizer, Seattle Genetics, and Bristol Myers Squibb have programs to attach a proprietary cell-killing small molecule to an antibody for targeted delivery to cancer cells. Pharmaceutical and biotechnology companies, as well as other institutions, also compete with us for promising targets for antibody-based therapeutics and in recruiting highly qualified scientific personnel. Many competitors and potential competitors have substantially greater scientific, research and product development capabilities, as well as greater financial, marketing and human resources than we do. In addition, many specialized biotechnology firms have formed collaborations with large, established companies to support the research, development and commercialization of products that may be competitive with ours.

        In particular, competitive factors within the antibody and cancer therapeutic market include:

•

the safety and efficacy of products;



•

the timing of regulatory approval and commercial introduction;



•

special regulatory designation of products, such as Orphan Drug designation; and



•

the effectiveness of marketing, sales, and reimbursement efforts.

        Our competitive position depends on our ability to develop effective proprietary products, implement clinical development programs, production plans and marketing plans, including collaborations with other companies with greater marketing resources than ours, and to obtain patent protection and secure sufficient capital resources.

        Continuing development of conventional and targeted chemotherapeutics by large pharmaceutical companies and biotechnology companies may result in new compounds that may compete with our product candidates. In addition, antibodies developed by certain of these companies have been approved for use as cancer therapeutics. In the future, additional antibodies may compete with our product candidates. In addition, other companies have created or have programs to create potent cell-killing agents for attachment to antibodies. These companies may compete with us for technology out-license arrangements.

        Because of the acceptance of combination therapy for the treatment of cancer and the variety of genes and targets implicated in cancer incidence and progression, we believe that products resulting from applications of new technologies may be complementary to our own.

        Such new technologies include, but are not limited to:

•

the use of genomics technology to identify new gene-based targets for the development of anticancer drugs;

14

Table of Contents

•

the use of high-throughput screening to identify and optimize lead compounds;



•

the use of gene therapy to deliver genes to regulate gene function; and



•

the use of therapeutic vaccines.

Regulatory Matters

Government Regulation and Product Approval

        Government authorities in the U.S., at the federal, state and local level, and other countries extensively regulate, among other things, the research, development, testing, manufacture, quality control, approval, labeling, packaging, storage, record-keeping, promotion, advertising, distribution, marketing and export and import of products such as those we are developing. A new drug must be approved by the FDA through the new drug application, or NDA, process and a new biologic must be approved by the FDA through the biologics license application, or BLA, process before it may be legally marketed in the U.S.

U.S. Drug Development Process

        In the U.S., the FDA regulates drugs under the federal Food, Drug, and Cosmetic Act, or FDCA, and in the case of biologics, also under the Public Health Service Act, or PHSA, and implementing regulations. The process of obtaining regulatory approvals and the subsequent compliance with appropriate federal, state, local, and foreign statutes and regulations require the expenditure of substantial time and financial resources. Failure to comply with the applicable U.S. requirements at any time during the product development process, approval process or after approval, may subject an applicant to administrative or judicial sanctions. These sanctions could include the FDA's refusal to approve pending applications, withdrawal of an approval, a clinical hold, warning letters, product recalls, product seizures, total or partial suspension of production or distribution, injunctions, fines, refusals of government contracts, restitution, disgorgement, or civil or criminal penalties. Any agency or judicial enforcement action could have a material adverse effect on us. The process required by the FDA before a drug or biologic may be marketed in the U.S. generally involves the following:

•

completion of preclinical laboratory tests, animal studies and formulation studies according to Good Laboratory Practices or other applicable regulations;



•

submission to the FDA of an IND which must become effective before human clinical trials may begin;



•

performance of adequate and well-controlled human clinical trials according to Good Clinical Practices to establish the safety and efficacy of the proposed drug for its intended use;



•

submission to the FDA of an NDA or BLA;



•

satisfactory completion of an FDA inspection of the manufacturing facility or facilities at which the drug is produced to assess compliance with current good manufacturing practice, or cGMP, to assure that the facilities, methods and controls are adequate to preserve the drug's identity, strength, quality and purity; and



•

FDA review and approval of the NDA or BLA.

        The testing and approval process requires substantial time, effort and financial resources, and we cannot be certain that any approvals for our product candidates will be granted on a timely basis, if at all.

        Once a pharmaceutical candidate is identified for development it enters the preclinical testing stage. Preclinical tests include laboratory evaluations of product chemistry, toxicity and formulation, as

15

Table of Contents

well as animal studies. An IND sponsor must submit the results of the preclinical tests, together with manufacturing information and analytical data, to the FDA as part of the IND. The sponsor will also include a protocol detailing, among other things, the objectives of the first phase of the clinical trial, the parameters to be used in monitoring safety, and the effectiveness criteria to be evaluated, if the first phase lends itself to an efficacy evaluation. Some preclinical testing may continue even after the IND is submitted. The IND automatically becomes effective 30 days after receipt by the FDA, unless the FDA, within the 30-day time period, places the clinical trial on a clinical hold. In such a case, the IND sponsor and the FDA must resolve any outstanding concerns before the clinical trial can begin. Clinical holds also may be imposed by the FDA at any time before or during studies due to safety concerns or non-compliance.

        All clinical trials must be conducted under the supervision of one or more qualified investigators in accordance with good clinical practice regulations. These regulations include the requirement that all research subjects provide informed consent. Further, an institutional review board, or IRB, must review and approve the plan for any clinical trial before it commences at any institution. An IRB considers, among other things, whether the risks to individuals participating in the trials are minimized and are reasonable in relation to anticipated benefits. The IRB also approves the information regarding the trial and the consent form that must be provided to each trial subject or his or her legal representative and must monitor the study until completed.

        Each new clinical protocol must be submitted to the FDA for its review, and to the IRBs for approval. Protocols detail, among other things, the objectives of the study, dosing procedures, subject selection and exclusion criteria, and the parameters to be used to monitor subject safety.

        Human clinical trials are typically conducted in three sequential phases that may overlap or be combined:

•

Phase I:  The drug is initially introduced into healthy human subjects and tested for safety, dosage tolerance, absorption, metabolism, distribution and excretion. In the case of some products for severe or life-threatening diseases, such as cancer, especially when the product may be too inherently toxic to ethically administer to healthy volunteers, the initial human testing is often conducted in patients.



•

Phase II:  Involves studies in a limited patient population to identify possible adverse effects and safety risks, to preliminarily evaluate the efficacy of the product for specific targeted diseases and to determine dosage tolerance and optimal dosage.



•

Phase III:  Clinical trials are undertaken to further evaluate dosage, clinical efficacy and safety in an expanded patient population at geographically dispersed clinical study sites. These studies are intended to establish the overall risk-benefit ratio of the product and provide, if appropriate, an adequate basis for product labeling.

        Progress reports detailing the results of the clinical trials must be submitted at least annually to the FDA and safety reports must be submitted to the FDA and the investigators for serious and unexpected adverse events. Phase I, Phase II, and Phase III testing may not be completed successfully within any specified period, if at all. The FDA or the sponsor may suspend a clinical trial at any time on various grounds, including a finding that the research subjects or patients are being exposed to an unacceptable health risk. Similarly, an IRB can suspend or terminate approval of a clinical trial at its institution if the clinical trial is not being conducted in accordance with the IRB's requirements or if the drug has been associated with unexpected serious harm to patients.

16

Table of Contents

        Concurrent with clinical trials, companies usually complete additional animal studies and must also develop additional information about the chemistry and physical characteristics of the drug and finalize a process for manufacturing the product in commercial quantities in accordance with cGMP requirements. The manufacturing process must be capable of consistently producing quality batches of the product candidate and, among other things, the manufacturer must develop methods for testing the identity, strength, quality and purity of the final drug. Additionally, appropriate packaging must be selected and tested and stability studies must be conducted to demonstrate that the product candidate does not undergo unacceptable deterioration over its shelf life.

U.S. Review and Approval Processes

        The results of product development, preclinical studies and clinical trials, along with descriptions of the manufacturing process, analytical tests conducted on the chemistry of the drug, proposed labeling, and other relevant information are submitted to the FDA as part of an NDA or BLA requesting approval to market the product. The submission of an NDA or BLA is subject to the payment of user fees; a waiver of such fees may be obtained under certain limited circumstances.

        In addition, under the Pediatric Research Equity Act of 2007, or PREA, an NDA, BLA and certain types of supplements to an NDA or BLA must contain data to assess the safety and effectiveness of the drug for the claimed indications in all relevant pediatric subpopulations and to support dosing and administration for each pediatric subpopulation for which the drug is safe and effective. The FDA may grant deferrals for submission of data or full or partial waivers. Unless otherwise required by regulation, PREA does not apply to any drug for an indication for which orphan designation has been granted. PREA sunsets on October 1, 2012.

        The FDA reviews all NDAs and BLAs submitted to ensure that they are sufficiently complete for substantive review before it accepts them for filing. The FDA may request additional information rather than accept a NDA or BLA for filing. In this event, the NDA must be resubmitted with the additional information. The resubmitted application also is subject to review before the FDA accepts it for filing. Once the submission is accepted for filing, the FDA begins an in-depth substantive review. FDA may refer the NDA or BLA to an advisory committee for review, evaluation and recommendation as to whether the application should be approved and under what conditions. The FDA is not bound by the recommendation of an advisory committee, but it generally follows such recommendations. The approval process is lengthy and difficult and the FDA may refuse to approve an NDA or BLA if the applicable regulatory criteria are not satisfied or may require additional clinical or other data and information. Even if such data and information is submitted, the FDA may ultimately decide that the NDA or BLA does not satisfy the criteria for approval. Data obtained from clinical trials are not always conclusive and the FDA may interpret data differently than we interpret the same data. The FDA may issue a complete response letter, which may require additional clinical or other data or impose other conditions that must be met in order to secure final approval of the NDA or BLA, or an approved letter following satisfactory completion of all aspects of the review process. The FDA reviews an NDA to determine, among other things, whether a product is safe and effective for its intended use and whether its manufacturing is cGMP-compliant to assure and preserve the product's identity, strength, quality and purity. The FDA reviews a BLA to determine, among other things whether the product is safe, pure and potent and the facility in which it is manufactured, processed, packed or held meets standards designed to assure the product's continued safety, purity and potency. Before approving an NDA, the FDA will inspect the facility or facilities where the product is manufactured.

        NDAs or BLAs receive either standard or priority review. A drug representing a significant improvement in treatment, prevention or diagnosis of disease may receive priority review. In addition, products studied for their safety and effectiveness in treating serious or life-threatening illnesses and that provide meaningful therapeutic benefit over existing treatments may receive accelerated approval and may be approved on the basis of adequate and well-controlled clinical trials establishing that the

17

Table of Contents

drug product has an effect on a surrogate endpoint that is reasonably likely to predict clinical benefit or on the basis of an effect on a clinical endpoint other than survival or irreversible morbidity. As a condition of approval, the FDA may require that a sponsor of a drug receiving accelerated approval perform adequate and well-controlled post-marketing clinical trials. Priority review and accelerated approval do not change the standards for approval, but may expedite the approval process.

        If a product receives regulatory approval, the approval may be significantly limited to specific diseases and dosages or the indications for use may otherwise be limited, which could restrict the commercial value of the product. In addition, the FDA may require us to conduct Phase IV testing which involves clinical trials designed to further assess a drug's safety and effectiveness after NDA or BLA approval, and may require testing and surveillance programs to monitor the safety of approved products which have been commercialized.

Patent Term Restoration and Marketing Exclusivity

        Depending upon the timing, duration and specifics of FDA approval of the use of our drugs, some of our U.S. patents may be eligible for limited patent term extension under the Drug Price Competition and Patent Term Restoration Act of 1984, referred to as the Hatch-Waxman Amendments. The Hatch-Waxman Amendments permit a patent restoration term of up to five years as compensation for patent term lost during product development and the FDA regulatory review process. However, patent term restoration cannot extend the remaining term of a patent beyond a total of 14 years from the product's approval date. The patent term restoration period is generally one-half the time between the effective date of an IND, and the submission date of an NDA or BLA, plus the time between the submission date of an NDA or BLA and the approval of that application. Only one patent applicable to an approved drug is eligible for the extension and the extension must be applied for prior to expiration of the patent. The United States Patent and Trademark Office, in consultation with the FDA, reviews and approves the application for any patent term extension or restoration. In the future, we intend to apply for restorations of patent term for some of our currently owned or licensed patents to add patent life beyond their current expiration date, depending on the expected length of clinical trials and other factors involved in the filing of the relevant NDA or BLA.

        Market exclusivity provisions under the FDCA also can delay the submission or the approval of certain applications. The FDCA provides a five-year period of non-patent marketing exclusivity within the U.S. to the first applicant to gain approval of an NDA for a new chemical entity. A drug is a new chemical entity if the FDA has not previously approved any other new drug containing the same active moiety, which is the molecule or ion responsible for the action of the drug substance. During the exclusivity period, the FDA may not accept for review an abbreviated new drug application, or ANDA, or a 505(b)(2) NDA submitted by another company for another version of such drug where the applicant does not own or have a legal right of reference to all the data required for approval. However, an application may be submitted after four years if it contains a certification of patent invalidity or non-infringement. The FDCA also provides three years of marketing exclusivity for an NDA, 505(b)(2) NDA or supplement to an existing NDA if new clinical investigations, other than bioavailability studies, that were conducted or sponsored by the applicant are deemed by FDA to be essential to the approval of the application, for example, for new indications, dosages, or strengths of an existing drug. This three-year exclusivity covers only the conditions associated with the new clinical investigations and does not prohibit the FDA from approving ANDAs for drugs containing the original active agent. Five-year and three-year exclusivity will not delay the submission or approval of a full NDA; however, an applicant submitting a full NDA would be required to conduct or obtain a right of reference to all of the preclinical studies and adequate and well-controlled clinical trials necessary to demonstrate safety and effectiveness.

        Pediatric exclusivity is another type of exclusivity in the U.S. Pediatric exclusivity, if granted, provides an additional six months to an existing exclusivity or statutory delay in approval resulting from

18

Table of Contents

a patent certification. This six-month exclusivity, which runs from the end of other exclusivity protection or patent delay, may be granted based on the voluntary completion of a pediatric study in accordance with an FDA-issued "Written Request" for such a study. The current pediatric exclusivity provision will sunset on October 1, 2012.

Biologics Price Competition and Innovation Act of 2009

        On March 23, 2010, President Obama signed into law the Patient Protection and Affordable Care Act which included the Biologics Price Competition and Innovation Act of 2009, or BPCIA. The BPCIA amended the PHSA to create an abbreviated approval pathway for two types of "generic" biologics—biosimilars and interchangeable biologic products, and provides for a twelve-year exclusivity period for the first approved biological product, or reference product, against which a biosimilar or interchangeable application is evaluated. A biosimilar product is defined as one that is highly similar to a reference product notwithstanding minor differences in clinically inactive components and for which there are no clinically meaningful differences between the biological product and the reference product in terms of the safety, purity and potency of the product. An interchangeable product is a biosimilar product that may be substituted for the reference product without the intervention of the health care provider who prescribed the reference product.

        The biosimilar applicant must demonstrate that the product is biosimilar based on data from (1) analytical studies showing that the biosimilar product is highly similar to the reference product; (2) animal studies (including toxicity); and (3) one or more clinical studies to demonstrate safety, purity and potency in one or more appropriate conditions of use for which the reference product is approved. In addition, the applicant must show that the biosimilar and reference products have the same mechanism of action for the conditions of use on the label, route of administration, dosage and strength, and the production facility must meet standards designed to assure product safety, purity and potency.

        An application for a biosimilar product may not be submitted until four years after the date on which the reference product was first approved; however if pediatric studies are performed and accepted by the FDA, the twelve-year exclusivity period will be extended for an additional six months. The first approved interchangeable biologic product will be granted an exclusivity period of up to one year after it is first commercially marketed, but the exclusivity period may be shortened under certain circumstances.

Orphan Drug Designation

        Under the Orphan Drug Act, the FDA may grant orphan drug designation to a drug intended to treat a rare disease or condition, which is generally a disease or condition that affects fewer than 200,000 individuals in the U.S., or more than 200,000 individuals in the U.S. and for which there is no reasonable expectation that the cost of developing and making available in the U.S. a drug for this type of disease or condition will be recovered from sales in the U.S. for that drug. Orphan drug designation must be requested before submitting an NDA or BLA. After the FDA grants orphan drug designation, the identity of the therapeutic agent and its potential orphan use are disclosed publicly by the FDA. Orphan drug designation does not convey any advantage in or shorten the duration of the regulatory review and approval process.

        If a product that has orphan drug designation subsequently receives the first FDA approval for the disease for which it has such designation, the product is entitled to orphan product exclusivity, which means that the FDA may not approve any other applications to market the same drug for the same indication, except in very limited circumstances, for seven years. Orphan drug exclusivity, however, also could block the approval of one of our products for seven years if a competitor obtains approval of the

19

Table of Contents

same drug as defined by the FDA or if our product candidate is determined to be contained within the competitor's product for the same indication or disease.

        The FDA also administers a clinical research grants program, whereby researchers may compete for funding to conduct clinical trials to support the approval of drugs, biologics, medical devices, and medical foods for rare diseases and conditions. A product does not have to be designated as an orphan drug to be eligible for the grant program. An application for an orphan grant should propose one discrete clinical study to facilitate FDA approval of the product for a rare disease or condition. The study may address an unapproved new product or an unapproved new use for a product already on the market.

        In March 2010 and August 2010, the FDA granted Orphan Drug designation to our lorvotuzumab mertansine compound when used for the treatment of Merkel cell carcinoma (MCC) and small-cell lung cancer (SCLC), respectively. Orphan drug designation provides ImmunoGen with seven years of market exclusivity that begins once lorvotuzumab mertansine receives FDA marketing approval for the use for which the orphan drug status was granted. Through a separate process, lorvotuzumab mertansine has been granted orphan medicinal product designation for the treatment of MCC in the European Union. The European Medicines Agency Committee for Orphan Medicinal Products rendered a positive opinion in July 2010 on also granting lorvotuzumab mertansine orphan medicinal product status for SCLC, and finalization of that designation is expected in 2010. Orphan medicinal product designation provides ImmunoGen with ten years of market exclusivity that begins once lorvotuzumab mertansine receives European approval for the use for which it was granted. We may pursue these designations for other indications for lorvotuzumab mertansine, and for other product candidates intended for qualifying patient populations.

New Drugs for Serious or Life Threatening Illnesses

        The FDA Modernization Act allows the designation of "Fast Track" status to expedite development of new drugs, including review and approvals, and is intended to speed the availability of new therapies to desperately ill patients. "Fast Track" procedures permit early consultation and commitment from the FDA regarding preclinical studies and clinical trials necessary to gain marketing approval. We may seek "Fast Track" status for some, or all, of our product candidates.

        "Fast Track" status also incorporates initiatives announced by the President of the U.S. and the FDA Commissioner in March 1996 intended to provide cancer patients with faster access to new cancer therapies. One of these initiatives states that the initial basis for approval of anticancer agents to treat refractory, hard-to-treat cancer may be objective evidence of response, rather than statistically improved disease-free and/or overall survival, as had been common practice. The sponsor of a product approved under this accelerated mechanism is required to follow up with further studies on clinical safety and effectiveness in larger groups of patients.

Post-Approval Requirements

        Once an approval is granted, the FDA may withdraw the approval if compliance with regulatory standards is not maintained or if problems occur after the product reaches the market. Later discovery of previously unknown problems with a product may result in restrictions on the product or even complete withdrawal of the product from the market. After approval, some types of changes to the approved product, such as adding new indications, manufacturing changes and additional labeling claims, are subject to further FDA review and approval. Drug manufacturers and other entities involved in the manufacture and distribution of approved drugs are required to register their establishments with the FDA and certain state agencies, and are subject to periodic unannounced inspections by the FDA and certain state agencies for compliance with cGMP and other laws. We rely, and expect to continue to rely, on third parties for the production of clinical and commercial quantities of our products. Future

20

Table of Contents

FDA and state inspections may identify compliance issues at the facilities of our contract manufacturers that may disrupt production or distribution, or require substantial resources to correct.

        Any drug products manufactured or distributed by us pursuant to FDA approvals are subject to continuing regulation by the FDA, including, among other things, record-keeping requirements, reporting of adverse experiences with the drug, providing the FDA with updated safety and efficacy information, drug sampling and distribution requirements, complying with certain electronic records and signature requirements, and complying with FDA promotion and advertising requirements. FDA strictly regulates labeling, advertising, promotion and other types of information on products that are placed on the market. Drugs may be promoted only for the approved indications and in accordance with the provisions of the approved label.

        From time to time, legislation is drafted, introduced and passed in Congress that could significantly change the statutory provisions governing the approval, manufacturing and marketing of products regulated by the FDA. For example, on September 27, 2007, the Food and Drug Administration Amendments Act of 2007, or FDAAA, gave the FDA enhanced postmarket authority, including the authority to require postmarket studies and clinical trials, labeling changes based on new safety information, and compliance with a risk evaluation and mitigation strategy approved by the FDA. Failure to comply with any requirements under the new law may result in significant penalties. The new law also authorizes significant civil money penalties for the dissemination of false or misleading direct-to-consumer advertisements. Additionally, the new law expands the clinical trial registry so that sponsors of all clinical trials, except for Phase I trials, are required to submit certain clinical trial information for inclusion in the clinical trial registry data bank, including summary adverse effect information. In addition, FDA regulations and guidance are often revised or reinterpreted by the agency in ways that may significantly affect our business and our products. It is impossible to predict whether further legislative changes will be enacted, or FDA regulations, guidance or interpretations changed or what the impact of such changes, if any, may be.

Foreign Regulation

        In addition to regulations in the U.S., we will be subject to a variety of foreign regulations governing clinical trials and commercial sales and distribution of our products. Whether or not we obtain FDA approval for a product, we must obtain approval of a product by the comparable regulatory authorities of foreign countries before we can commence clinical trials or marketing of the product in those countries. The approval process varies from country to country and the time may be longer or shorter than that required for FDA approval. The requirements governing the conduct of clinical trials, product licensing, pricing and reimbursement vary greatly from country to country.

        Under European Union regulatory systems, we may submit marketing authorization applications either under a centralized or decentralized procedure. The centralized procedure, which is compulsory for medicines produced by certain biotechnological processes, orphan drugs and products containing a new active substance intended for treatment of specific conditions/illnesses including cancer, provides for the grant of a single marketing authorization that is valid for all European Union member states. If a drug does not fall within a mandatory category, it may still be submitted to the centralized procedure if it contains a new active substance and constitutes a significant therapeutic, scientific or technical innovation. Other new drugs without approval in any Member State, will follow the decentralized procedure which provides for approval by one or more other, or concerned, Member States of an assessment of an application performed by one Member State, known as the reference Member State. Under this procedure, an applicant submits an application, or dossier, and related materials (draft summary of product characteristics, draft labeling and package leaflet) to the reference Member State and concerned Member States. The reference Member State prepares a draft assessment and drafts of the related materials within 120 days after receipt of a valid application. Within 90 days of receiving the reference Member State's assessment report, each concerned Member State must decide whether to

21

Table of Contents

approve the assessment report and related materials. If a Member State cannot approve the assessment report and related materials on the grounds of potential serious risk to public health, the disputed points may eventually be referred to the European Commission, whose decision is binding on all Member States.

Reimbursement

        Sales of pharmaceutical products depend in significant part on the availability of third-party reimbursement. Third-party payors include government healthcare programs, managed care providers, private health insurers and other organizations. We anticipate third-party payors will provide reimbursement for our products. However, these third-party payors are increasingly challenging the price and examining the cost-effectiveness of medical products and services. In addition, significant uncertainty exists as to the reimbursement status of newly approved healthcare products. We may need to conduct expensive pharmacoeconomic studies in order to demonstrate the cost-effectiveness of our products. Our product candidates may not be considered cost-effective. It is time consuming and expensive for us to seek reimbursement from third-party payors. Reimbursement may not be available or sufficient to allow us to sell our products on a competitive and profitable basis.

        The Medicare Prescription Drug, Improvement, and Modernization Act of 2003, or the MMA, imposed new requirements for the distribution and pricing of prescription drugs for Medicare beneficiaries, and included a major expansion of the prescription drug benefit under Medicare Part D. Under Part D, Medicare beneficiaries may enroll in prescription drug plans offered by private entities which provide coverage of outpatient prescription drugs. Part D plans include both stand-alone prescription drug benefit plans and prescription drug coverage as a supplement to Medicare Advantage plans. Unlike Medicare Part A and B, Part D coverage is not standardized. Part D prescription drug plan sponsors are not required to pay for all covered Part D drugs, and each drug plan can develop its own drug formulary that identifies which drugs it will cover and at what tier or level. However, Part D prescription drug formularies must include drugs within each therapeutic category and class of covered Part D drugs, though not necessarily all the drugs in each category or class. Any formulary used by a Part D prescription drug plan must be developed and reviewed by a pharmacy and therapeutic committee.

        It is not clear what effect the MMA will have on the prices paid for currently approved drugs and the pricing options for future approved drugs. Government payment for some of the costs of prescription drugs may increase demand for products for which we receive marketing approval. However, any negotiated prices for our products covered by a Part D prescription drug plan will likely be lower than the prices we might otherwise obtain. Moreover, while the MMA applies only to drug benefits for Medicare beneficiaries, private payors often follow Medicare coverage policy and payment limitations in setting their own payment rates. Any reduction in payment that results from the MMA may result in a similar reduction in payments from non-governmental payors.

        We expect that there will continue to be a number of federal and state proposals to implement governmental pricing controls and limit the growth of healthcare costs, including the cost of prescription drugs. For example, in March 2010, President Obama signed one of the most significant health care reform measures in decades, the Patient Protection and Affordable Care Act, as amended by the Health Care and Education Affordability Reconciliation Act, collectively referred to as the PPACA. The PPACA will significantly impact the pharmaceutical industry. The PPACA will require discounts under the Medicare drug benefit program and increased rebates on drugs covered by Medicaid. In addition, the PPACA imposes an annual fee, which will increase annually, on sales by branded pharmaceutical manufacturers starting in 2011. The financial impact of these discounts, increased rebates and fees and the other provisions of the PPACA on our business is unclear.

22

Table of Contents

        In addition, in some foreign countries, the proposed pricing for a drug must be approved before it may be lawfully marketed. The requirements governing drug pricing vary widely from country to country. For example, the European Union provides options for its member states to restrict the range of medicinal products for which their national health insurance systems provide reimbursement and to control the prices of medicinal products for human use. A member state may approve a specific price for the medicinal product or it may instead adopt a system of direct or indirect controls on the profitability of the company placing the medicinal product on the market. There can be no assurance that any country that has price controls or reimbursement limitations for pharmaceutical products will allow favorable reimbursement and pricing arrangements for any of our products.

Research and Development Spending

        During each of the three years ended June 30, 2010, 2009 and 2008, we spent approximately $50.3 million, $45.9 million and $60.0 million, respectively, on research and development activities. During the year ended June 30, 2010, approximately 1% of our full-time equivalent research and development personnel were dedicated to our sanofi-aventis collaboration compared to 14% and 36% during the years ended June 30, 2009 and 2008, respectively.

Raw Materials and Manufacturing

        We procure certain raw material components of finished conjugate, including antibodies, DM1, DM4, and linker, for ourselves and on behalf of our collaborators. In order to meet our commitments to our collaborators as well as our own needs, we are required to enter into agreements with third parties to produce these components well in advance of our production needs. Our principal suppliers for these components include Cytovance Biologics LLC, SAFC, Inc. and Società Italiana Corticosteroidi S.r.l.

        In addition, we operate a conjugate manufacturing facility. A portion of the cost of operating this facility, including the cost of manufacturing personnel, is reimbursed by our collaborators based on the number of batches of preclinical and clinical materials produced on their behalf. Over the past few years, we have expanded and upgraded the capabilities of our manufacturing facility.

Employees

        As of June 30, 2010, we had 211 full-time employees, of whom 178 were engaged in research and development activities. Seventy-seven research and development employees hold post-graduate degrees, of which 41 hold Ph.D. degrees and four hold M.D. degrees. We consider our relations with our employees to be good. None of our employees is covered by a collective bargaining agreement.

        We have entered into confidentiality agreements with all of our employees, members of our board of directors and consultants. Further, we have entered into assignment of invention agreements with all of our employees.

Third-Party Trademarks

        Herceptin is a registered trademark of Genentech.

23

Table of Contents

Item 1A.    Risk Factors

THE RISKS AND UNCERTAINTIES DESCRIBED BELOW ARE THOSE THAT WE CURRENTLY BELIEVE MAY MATERIALLY AFFECT OUR COMPANY. ADDITIONAL RISKS AND UNCERTAINTIES THAT WE ARE UNAWARE OF OR THAT WE CURRENTLY DEEM IMMATERIAL ALSO MAY BECOME IMPORTANT FACTORS THAT AFFECT OUR COMPANY.

We have a history of operating losses and expect to incur significant additional operating losses.

        We have generated operating losses since our inception. As of June 30, 2010, we had an accumulated deficit of $372.4 million. For the years ended June 30, 2010, 2009, and 2008, we generated losses of $50.9 million, $31.9 million and $32.0 million, respectively. We may never be profitable. We expect to incur substantial additional operating expenses over the next several years as our research, development, preclinical testing, clinical trials and collaborator support activities continue. We intend to continue to invest significantly in our product candidates. Further, we expect to invest significant resources supporting our existing collaborators as they work to develop, test and commercialize TAP and other antibody compounds. We or our collaborators may encounter technological or regulatory difficulties as part of this development and commercialization process that we cannot overcome or remedy. We may also incur substantial marketing and other costs in the future if we decide to establish marketing and sales capabilities to commercialize our product candidates. None of our or our collaborators' product candidates has generated any commercial revenue and our only revenues to date have been primarily from upfront and milestone payments, research and development support and clinical materials reimbursement from our collaborative partners. We do not expect to generate revenues from the commercial sale of our internal product candidates in the near future, and we may never generate revenues from the commercial sale of internal products. Even if we do successfully develop products that can be marketed and sold commercially, we will need to generate significant revenues from those products to achieve and maintain profitability. Even if we do become profitable, we may not be able to sustain or increase profitability on a quarterly or annual basis.

If we are unable to obtain additional funding when needed, we may have to delay or scale back some of our programs or grant rights to third parties to develop and market our product candidates.

        We will continue to expend substantial resources developing new and existing product candidates, including costs associated with research and development, acquiring new technologies, conducting preclinical studies and clinical trials, obtaining regulatory approvals and manufacturing products as well as providing certain support to our collaborators in the development of their products. We believe that our current working capital and expected future payments from our collaboration arrangements will be sufficient to meet our current and projected operating and capital requirements for fiscal year 2011 and approximately the first half of fiscal 2012. However, we may need additional financing sooner due to a number of factors including:

•

if either we incur higher than expected costs or we or any of our collaborators experience slower than expected progress in developing product candidates and obtaining regulatory approvals;



•

lower revenues than expected under our collaboration agreements; or



•

acquisition of technologies and other business opportunities that require financial commitments.

        Additional funding may not be available to us on favorable terms, or at all. We may raise additional funds through public or private financings, collaborative arrangements or other arrangements. Debt financing, if available, may involve covenants that could restrict our business activities. If we are unable to raise additional funds through equity or debt financing when needed, we may be required to delay, scale back or eliminate expenditures for some of our development programs or grant rights to develop and market product candidates that we would otherwise prefer to internally

24

Table of Contents

develop and market. If we are required to grant such rights, the ultimate value of these product candidates to us may be reduced.

If our TAP technology does not produce safe, effective and commercially viable products, our business will be severely harmed.

        Our TAP technology yields novel product candidates for the treatment of cancer. To date, no TAP product candidate has obtained regulatory approval and the most advanced TAP product candidate is in Phase III clinical testing. Our TAP product candidates and/or our collaborators' TAP product candidates may not prove to be safe, effective or commercially viable treatments for cancer and our TAP technology may not result in any future meaningful benefits to us or for our current or potential collaborative partners. Furthermore, we are aware of only one compound that is a conjugate of an antibody and a cytotoxic small molecule that has obtained approval by the FDA and is based on technology similar to our TAP technology. This product was recently taken off the market by its owner due to toxicity concerns. If our TAP technology fails to generate product candidates that are safe, effective and commercially viable treatments for cancer, or fails to obtain FDA approval, our business will be severely harmed.

Clinical trials for our and our collaborative partners' product candidates will be lengthy and expensive and their outcome is uncertain.

        Before obtaining regulatory approval for the commercial sale of any product candidates, we and our collaborative partners must demonstrate through clinical testing that our product candidates are safe and effective for use in humans. Conducting clinical trials is a time-consuming, expensive and uncertain process and typically requires years to complete. The most advanced product candidate incorporating our TAP technology is in Phase III clinical testing. In our industry, the results from preclinical studies and early clinical trials often are not predictive of results obtained in later-stage clinical trials. Some compounds that have shown promising results in preclinical studies or early clinical trials subsequently fail to establish sufficient safety and efficacy data necessary to obtain regulatory approval. At any time during the clinical trials, we, our collaborative partners, or the FDA might delay or halt any clinical trials of our product candidates for various reasons, including:

•

occurrence of unacceptable toxicities or side effects;

ineffectiveness of the product candidate;

•

insufficient drug supply;



•

negative or inconclusive results from the clinical trials, or results that necessitate additional studies or clinical trials;



•

delays in obtaining or maintaining required approvals from institutions, review boards or other reviewing entities at clinical sites;



•

delays in patient enrollment;



•

insufficient funding or a reprioritization of financial or other resources; or



•

other reasons that are internal to the businesses of our collaborative partners, which they may not share with us.

        Any failure or substantial delay in successfully completing clinical trials and obtaining regulatory approval for our product candidates or our collaborative partners' product candidates could severely harm our business.

25

Table of Contents

We and our collaborative partners are subject to extensive government regulations and we and our collaborative partners may not be able to obtain necessary regulatory approvals.

        We and our collaborative partners may not receive the regulatory approvals necessary to commercialize our product candidates, which would cause our business to be severely harmed. Pharmaceutical product candidates, including those in development by us and our collaborative partners, are subject to extensive and rigorous government regulation. The FDA regulates, among other things, the development, testing, manufacture, safety, record-keeping, labeling, storage, approval, advertising, promotion, sale and distribution of pharmaceutical products. If our potential products or our collaborators' potential products are marketed abroad, they will also be subject to extensive regulation by foreign governments. None of our product candidates has been approved for sale in the U.S. or any foreign market. The regulatory review and approval process, which includes preclinical studies and clinical trials of each product candidate, is lengthy, complex, expensive and uncertain. Securing FDA approval requires the submission of extensive preclinical and clinical data and supporting information to the FDA for each indication to establish the product candidate's safety and efficacy. Data obtained from preclinical studies and clinical trials are susceptible to varying interpretation, which may delay, limit or prevent regulatory approval. The approval process may take many years to complete and may involve ongoing requirements for post-marketing studies. In light of the limited regulatory history of monoclonal antibody-based therapeutics, regulatory approvals for our or our collaborative partners' product candidates may not be obtained without lengthy delays, if at all. Any FDA or other regulatory approvals of our or our collaborative partners' product candidates, once obtained, may be withdrawn. The effect of government regulation may be to:

•

delay marketing of potential products for a considerable period of time;



•

limit the indicated uses for which potential products may be marketed;



•

impose costly requirements on our activities; and



•

place us at a competitive disadvantage to other pharmaceutical and biotechnology companies.

        We may encounter delays or rejections in the regulatory approval process because of additional government regulation from future legislation or administrative action or changes in FDA policy during the period of product development, clinical trials and FDA regulatory review. Failure to comply with FDA or other applicable regulatory requirements may result in criminal prosecution, civil penalties, recall or seizure of products, total or partial suspension of production or injunction, as well as other regulatory action against our product candidates or us. Outside the U.S., our ability to market a product is contingent upon receiving clearances from the appropriate regulatory authorities. The foreign regulatory approval process includes similar risks to those associated with the FDA approval process. In addition, we are, or may become, subject to various federal, state and local laws, regulations and recommendations relating to safe working conditions, laboratory and manufacturing practices, the experimental use of animals and the use and disposal of hazardous substances, including radioactive compounds and infectious disease agents, used in connection with our research work. If we fail to comply with the laws and regulations pertaining to our business, we may be subject to sanctions, including the temporary or permanent suspension of operations, product recalls, marketing restrictions and civil and criminal penalties.

Our and our collaborative partners' product candidates will remain subject to ongoing regulatory review even if they receive marketing approval. If we or our collaborative partners fail to comply with continuing regulations, we could lose these approvals and the sale of our products could be suspended.

        Even if we or our collaborative partners receive regulatory approval to market a particular product candidate, the approval could be conditioned on us or our collaborative partners conducting costly post-approval studies or could limit the indicated uses included in product labeling. Moreover, the product may later cause adverse effects that limit or prevent its widespread use, force us or our

26

Table of Contents

collaborative partners to withdraw it from the market or impede or delay our or our collaborative partners' ability to obtain regulatory approvals in additional countries. In addition, the manufacturer of the product and its facilities will continue to be subject to FDA review and periodic inspections to ensure adherence to applicable regulations. After receiving marketing approval, the manufacturing, labeling, packaging, adverse event reporting, storage, advertising, promotion and record-keeping related to the product remain subject to extensive regulatory requirements. We or our collaborative partners may be slow to adapt, or we or our collaborative partners may never adapt, to changes in existing regulatory requirements or adoption of new regulatory requirements.

        If we or our collaborative partners fail to comply with the regulatory requirements of the FDA and other applicable U.S. and foreign regulatory authorities, or if previously unknown problems with our or our partners' products, manufacturers or manufacturing processes are discovered, we could be subject to administrative or judicially imposed sanctions, including:

•

restrictions on the products, manufacturers or manufacturing processes;



•

warning letters;



•

civil or criminal penalties;



•

fines;



•

injunctions;



•

product seizures or detentions;



•

import bans;



•

voluntary or mandatory product recalls and publicity requirements;



•

suspension or withdrawal of regulatory approvals;



•

total or partial suspension of production; and



•

refusal to approve pending applications for marketing approval of new drugs or supplements to approved applications.

        Any one of these could have a material adverse effect on our business or financial condition.

If our collaborative partners fail to perform their obligations under our agreements with them, or determine not to continue with clinical trials for particular product candidates, our business could be severely impacted.

        Our strategy for the development and commercialization of our product candidates depends, in large part, upon the formation and maintenance of collaborative arrangements. Collaborations provide an opportunity for us to:

•

generate cash flow and revenue;



•

fund some of the costs associated with our internal research and development, preclinical testing, clinical trials and manufacturing;



•

seek and obtain regulatory approvals faster than we could on our own;



•

successfully commercialize existing and future product candidates; and



•

secure access to targets which, due to intellectual property restrictions, would otherwise be unavailable to our technology.

        If we fail to secure or maintain successful collaborative arrangements, the development and marketing of compounds that use our technology may be delayed, scaled back or otherwise may not

27

Table of Contents

occur. In addition, we may be unable to negotiate other collaborative arrangements or, if necessary, modify our existing arrangements on acceptable terms. We cannot control the amount and timing of resources our collaborative partners may devote to our product candidates. Our collaborative partners may separately pursue competing product candidates, therapeutic approaches or technologies to develop treatments for the diseases targeted by us or our collaborative efforts, or may decide, for reasons not known to us, to discontinue development of product candidates under our agreements with them. Any of our collaborative partners may slow or discontinue the development of a product candidate covered by a collaborative arrangement for reasons that can include, but are not limited to:

•

a change in the collaborative partner's strategic focus as a result of merger, management changes, adverse business events, or other causes;



•

a change in the priority of the product candidate relative to other programs in the collaborator's pipeline;



•

a reassessment of the patent situation related to the compound or its target;



•

a change in the anticipated competition for the product candidate;



•

preclinical studies and clinical trial results; and



•

a reduction in the financial resources the collaborator can or is willing to apply to the development of new compounds.

        Even if our collaborative partners continue their collaborative arrangements with us, they may nevertheless determine not to actively pursue the development or commercialization of any resulting products. Also, our collaborative partners may fail to perform their obligations under the collaborative agreements or may be slow in performing their obligations. Our collaborative partners can terminate our collaborative agreements under certain conditions. The decision to advance a product that is covered by a collaborative agreement through clinical trials and ultimately to commercialization is in the discretion of our collaborative partners. If any collaborative partner were to terminate or breach our agreements, fail to complete its obligations to us in a timely manner, or decide to discontinue its development of a product candidate, our anticipated revenue from the agreement and from the development and commercialization of the products would be severely limited. If we are not able to establish additional collaborations or any or all of our existing collaborations are terminated and we are not able to enter into alternative collaborations on acceptable terms, or at all, our continued development, manufacture and commercialization of our product candidates could be delayed or scaled back as we may not have the funds or capability to continue these activities. If our collaborators fail to successfully develop and commercialize TAP compounds, our business prospects would be severely harmed.

We depend on a small number of collaborators for a substantial portion of our revenue. The loss of, or a material reduction in activity by, any one of these collaborators could result in a substantial decline in our revenue.

        We have and will continue to have collaborations with a limited number of companies. As a result, our financial performance depends on the efforts and overall success of these companies. Also, the failure of any one of our collaborative partners to perform its obligations under its agreement with us, including making any royalty, milestone or other payments to us, could have an adverse effect on our financial condition. Further, any material reduction by any one of our collaborative partners in its level of commitment of resources, funding, personnel, and interest in continued development under its agreement with us could have an adverse effect on our financial condition. To date, we have recorded $13.5 million in milestone payments with the advancement of T-DM1. Our agreement with Genentech, a member of the Roche Group, entitles us to receive up to $44 million in milestone payments and also royalties on commercial sales, if any. Failure of Genentech and Roche to continue to advance T-DM1

28

Table of Contents

would have an adverse effect on our financial outlook. Also, if consolidation trends in the healthcare industry continue, the number of our potential collaborators could decrease, which could have an adverse impact on our development efforts. If a present collaborator or future collaborator of ours were to be involved in a business combination, the collaborator's continued pursuit and emphasis on our product development program could be delayed, diminished or terminated.

If our collaborative partners' requirements for clinical materials to be manufactured by us are significantly lower than we have estimated, our financial results and condition could be adversely affected.

        We procure certain components of finished conjugate, including DM1, DM4, and linker, on behalf of our collaborators. In order to meet our commitments to our collaborative partners, we are required to enter into agreements with third parties to produce these components well in advance of our production of clinical materials on behalf of our collaborative partners. If our collaborative partners do not require as much clinical material as we have contracted to produce, we may not be able to recover our investment in these components and we may suffer significant losses. Collaborators have discontinued development of product candidates in the past and in the periods subsequent to these discontinuations, we had significantly reduced demand for conjugated material which adversely impacted our financial results.

        In addition, we operate a conjugate manufacturing facility. A portion of the cost of operating this facility, including the cost of manufacturing personnel, is reimbursed by our collaborators based on the number of batches of preclinical and clinical materials produced on their behalf. If we produce fewer batches of clinical materials for our collaborators, a smaller amount of the cost of operating the conjugate manufacturing facility will be charged to our collaborative partners and our financial condition could be adversely affected.

If our antibody requirements for clinical materials to be manufactured are significantly higher than we estimated, the inability to procure additional antibody in a timely manner could impair our ability to initiate or advance our clinical trials.

        We rely on third-party suppliers to manufacture antibodies used in our own proprietary compounds. Due to the specific nature of the antibody and availability of production capacity, there is significant lead time required by these suppliers to provide us with the needed materials. If our antibody requirements for clinical materials to be manufactured are significantly higher than we estimated, we may not be able to readily procure additional antibody which would impair our ability to advance our clinical trials currently in process or initiate additional trials. There can be no assurance that we will not have supply problems that could delay or stop our clinical trials or otherwise could have a material adverse effect on our business.

We currently rely on one third-party manufacturer with commercial production experience to produce our cell-killing agents, DM1 and DM4.

        We rely on third-party suppliers to manufacture materials used to make TAP compounds. Our cell-killing agents DM1 and DM4, collectively DMx, are manufactured from a precursor, ansamitocin P3. As part of preparing to produce TAP compounds for later-stage clinical trials and commercialization, we have transitioned from our original supplier of ansamitocin P3, as well as our single supplier that converts ansamitocin P3 to DMx, to one larger company with more commercial production experience. Any delay or interruption in our supply of DMx could lead to a delay or interruption in our manufacturing operations, preclinical studies and clinical trials of our product candidates and our collaborators' product candidates, which could negatively affect our business.

29

Table of Contents

We may be unable to establish the manufacturing capabilities necessary to develop and commercialize our and our collaborative partners' potential products.

        Currently, we have only one conjugate manufacturing facility that we use to manufacture conjugated compounds for us and our collaborative partners for preclinical studies and early-stage clinical testing. Two of our partners have contracted for separate, large-scale manufacturing capacity to make materials to support potential future commercialization of their TAP compounds. We do not currently have the manufacturing capacity needed to make our product candidates for commercial sale. In addition, our manufacturing capacity may be insufficient to complete all clinical trials contemplated by us and our collaborative partners over time. We intend to rely in part on third-party contract manufacturers to produce sufficiently large quantities of drug materials that are and will be needed for later-stage clinical trials and commercialization of our potential products. We are currently in the process of developing relationships with third-party manufacturers that we believe will be necessary to continue the development of our product candidates. Third-party manufacturers may not be able to meet our needs with respect to timing, quantity or quality of materials. If we are unable to contract for a sufficient supply of needed materials on acceptable terms, or if we should encounter delays or difficulties in our relationships with manufacturers, our clinical trials may be delayed, thereby delaying the submission of product candidates for regulatory approval and the market introduction and subsequent commercialization of our potential products. Any such delays may lower our revenues and potential profitability.

        In addition to the outsourcing of manufacturing, we may develop our manufacturing capacity in part by expanding our current facilities. This activity would require substantial additional funds and we would need to hire and train significant numbers of employees to staff these facilities. We may not be able to develop manufacturing facilities that are sufficient to produce drug materials for later-stage clinical trials or commercial use. We and any third-party manufacturers that we may use must continually adhere to current good manufacturing practice, or cGMP, regulations enforced by the FDA through its facilities inspection program. If our facilities or the facilities of third-party manufacturers cannot pass a pre-approval plant inspection, the FDA will not grant approval to our product candidates. In complying with these cGMP regulations and foreign regulatory requirements, we and any of our third-party manufacturers will be obligated to expend time, money and effort on production, record-keeping and quality control to assure that our potential products meet applicable specifications and other requirements. If we or any third-party manufacturer with whom we may contract fail to maintain regulatory compliance, we or the third party may be subject to fines and/or manufacturing operations may be suspended.

We have only one conjugate manufacturing facility and any prolonged and significant disruption at that facility could impair our ability to manufacture our and our collaborative partners' product candidates for clinical testing.

        Currently, in certain cases, we are contractually obligated to manufacture Phase I and non-pivotal Phase II clinical products for companies licensing our TAP technology. We manufacture this material, as well as material for our own product candidates, in our conjugate manufacturing facility. We have only one such manufacturing facility in which we can manufacture clinical products. Our current manufacturing facility contains highly specialized equipment and utilizes complicated production processes developed over a number of years that would be difficult, time-consuming and costly to duplicate. Any prolonged disruption in the operations of our manufacturing facility would have a significant negative impact on our ability to manufacture products for clinical testing on our own and would cause us to seek additional third-party manufacturing contracts, thereby increasing our development costs. Even though we carry business interruption insurance policies, we may suffer losses as a result of business interruptions that exceed the coverage available or any losses may be excluded under our insurance policies. Certain events, such as natural disasters, fire, political disturbances,

30

Table of Contents

sabotage or business accidents, which could impact our current or future facilities, could have a significant negative impact on our operations by disrupting our product development efforts until such time as we are able to repair our facility or put in place third-party contract manufacturers to assume this manufacturing role.

Unfavorable pricing regulations, third-party reimbursement practices or healthcare reform initiatives applicable to our product candidates could limit our potential product revenue.

        Antibody-based anticancer products are often much more costly to produce than traditional chemotherapeutics and tend to have significantly higher prices. Factors that help justify the price include the high mortality associated with many types of cancer and the need for more and better treatment options.

        Regulations governing drug pricing and reimbursement vary widely from country to country. Some countries require approval of the sales price of a drug before it can be marketed. Some countries restrict the physicians that can authorize the use of more expensive medications. Some countries establish treatment guidelines to help limit the use of more expensive therapeutics and the pool of patients that receive them. In some countries, including the U.S., third-party payers frequently seek discounts from list prices and are increasingly challenging the prices charged for medical products. Because our product candidates are in the development stage, we do not know the level of reimbursement, if any, we will receive for any products that we are able to successfully develop. If the reimbursement for any of our product candidates is inadequate in light of our development and other costs, our ability to achieve profitability would be affected.

        We believe that the efforts of governments and third-party payors to contain or reduce the cost of healthcare will continue to affect the business and financial condition of pharmaceutical and biopharmaceutical companies. A number of legislative and regulatory proposals to change the healthcare system in the U.S. and other major healthcare markets have been proposed and adopted in recent years. For example, the U.S. Congress enacted a limited prescription drug benefit for Medicare recipients as part of the Medicare Prescription Drug, Improvement and Modernization Act of 2003. While the program established by this statute may increase demand for any products that we are able to successfully develop, if we participate in this program, our prices will be negotiated with drug procurement organizations for Medicare beneficiaries and are likely to be lower than prices we might otherwise obtain. Non-Medicare third-party drug procurement organizations may also base the price they are willing to pay on the rate paid by drug procurement organizations for Medicare beneficiaries. Also, in March 2010, President Obama signed one of the most significant health care reform measures in decades, the Patient Protection and Affordable Care Act, as amended by the Health Care and Education Affordability Reconciliation Act, collectively referred to herein as the PPACA. The PPACA will significantly impact the pharmaceutical industry. The PPACA will require discounts under the Medicare drug benefit program and increased rebates on drugs covered by Medicaid. In addition, the PPACA imposes an annual fee, which will increase annually, on sales by branded pharmaceutical manufacturers starting in 2011. The financial impact of these discounts, increased rebates and fees and the other provisions of the PPACA on our business is unclear and there can be no assurance that our business will not be materially adversely affected by the PPACA. In addition, ongoing initiatives in the U.S. have increased and will continue to increase pressure on drug pricing. The announcement or adoption of any such initiative could have an adverse effect on potential revenues from any product candidate that we may successfully develop.

31

Table of Contents

We may be unable to establish sales and marketing capabilities necessary to successfully commercialize our potential products.

        We currently have no direct sales or marketing capabilities. We anticipate relying on third parties to market and sell most of our primary product candidates or we may outlicense these products prior to the time when these capabilities are needed. If we decide to market our potential products through a direct sales force, we would need either to hire a sales force with expertise in pharmaceutical sales or to contract with a third party to provide a sales force which meets our needs. We may be unable to establish marketing, sales and distribution capabilities necessary to commercialize and gain market acceptance for our potential products and be competitive. In addition, co-promotion or other marketing arrangements with third parties to commercialize potential products could significantly limit the revenues we derive from these potential products, and these third parties may fail to commercialize our compounds successfully.

If our product candidates or those of our collaborative partners do not gain market acceptance, our business will suffer.

        Even if clinical trials demonstrate the safety and efficacy of our and our collaborative partners' product candidates and the necessary regulatory approvals are obtained, our and our collaborative partners' product candidates may not gain market acceptance among physicians, patients, healthcare payors and other members of the medical community. The degree of market acceptance of any product candidates that we or our collaborative partners develop will depend on a number of factors, including:

•

their degree of clinical efficacy and safety;



•

their advantage over alternative treatment methods;



•

our/the marketer's and our collaborative partners' ability to gain acceptable reimbursement and the reimbursement policies of government and third-party payors; and



•

the quality of the distribution capabilities for product candidates, both ours and our collaborative partners.

        Physicians may not prescribe any of our future products until such time as clinical data or other factors demonstrate the safety and efficacy of those products as compared to conventional drug and other treatments. Even if the clinical safety and efficacy of therapies using our products is established, physicians may elect not to recommend the therapies for any number of other reasons, including whether the mode of administration of our products is effective for certain conditions, and whether the physicians are already using competing products that satisfy their treatment objectives. Physicians, patients, third-party payors and the medical community may not accept and use any product candidates that we, or our collaborative partners, develop. If our products do not achieve significant market acceptance and use, we will not be able to recover the significant investment we have made in developing such products and our business will be severely harmed.

We may be unable to compete successfully.

        The markets in which we compete are well established and intensely competitive. We may be unable to compete successfully against our current and future competitors. Our failure to compete successfully may result in pricing reductions, reduced gross margins and failure to achieve market acceptance for our potential products. Our competitors include research institutions, pharmaceutical companies and biotechnology companies, such as Pfizer, Seattle Genetics and Bristol-Meyers Squibb. Many of these organizations have substantially more experience and more capital, research and development, regulatory, manufacturing, human and other resources than we do. As a result, they may:

•

develop products that are safer or more effective than our product candidates;

32

Table of Contents

•

obtain FDA and other regulatory approvals or reach the market with their products more rapidly than we can, reducing the potential sales of our product candidates;



•

devote greater resources to market or sell their products;



•

adapt more quickly to new technologies and scientific advances;



•

initiate or withstand substantial price competition more successfully than we can;



•

have greater success in recruiting skilled scientific workers from the limited pool of available talent;



•

more effectively negotiate third-party licensing and collaboration arrangements; and



•

take advantage of acquisition or other opportunities more readily than we can.

        A number of pharmaceutical and biotechnology companies are currently developing products targeting the same types of cancer that we target, and some of our competitors' products have entered clinical trials or already are commercially available.

        Our product candidates, if approved and commercialized, will also compete against well-established, existing, therapeutic products that are currently reimbursed by government healthcare programs, private health insurers and health maintenance organizations. In addition, if our product candidates are approved and commercialized, we may face competition from generic products if the product candidate is a small molecule drug, or biosimilars if the product candidate is a biologic. The route to market for generic versions of small molecule drugs was established with the passage of the Hatch-Waxman Amendments in 1984 and for biosimilars with the passage of the PPACA in March 2010. The PPACA establishes a pathway for the FDA approval of follow-on biologics and provides twelve years 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.

        We face and will continue to face intense competition from other companies for collaborative arrangements with pharmaceutical and biotechnology companies, for relationships with academic and research institutions and for licenses to proprietary technology. In addition, we anticipate that we will face increased competition in the future as new companies enter our markets and as scientific developments surrounding antibody-based therapeutics for cancer continue to accelerate. While we will seek to expand our technological capabilities to remain competitive, research and development by others may render our technology or product candidates obsolete or noncompetitive or result in treatments or cures superior to any therapy developed by us.

If we are unable to protect our intellectual property rights adequately, the value of our technology and our product candidates could be diminished.

        Our success depends in part on obtaining, maintaining and enforcing our patents and other proprietary rights and our ability to avoid infringing the proprietary rights of others. Patent law relating to the scope of claims in the biotechnology field in which we operate is still evolving, is surrounded by a great deal of uncertainty and involves complex legal, scientific and factual questions. To date, no consistent policy has emerged regarding the breadth of claims allowed in biotechnology patents. Accordingly, our pending patent applications may not result in issued patents. Although we own numerous patents, the issuance of a patent is not conclusive as to its validity or enforceability. Through litigation, a third party may challenge the validity or enforceability of a patent after its issuance.

33

Table of Contents

        Also, patents and applications owned or licensed by us may become the subject of interference, opposition, nullity, or other proceedings in a court or patent office in the United States or in a foreign jurisdiction to determine validity, enforceability or priority of invention, which could result in substantial cost to us. An adverse decision in such a proceeding may result in our loss of rights under a patent or patent application. It is unclear how much protection, if any, will be given to our patents if we attempt to enforce them or if they are challenged in court or in other proceedings. A competitor may successfully invalidate our patents or a challenge could result in limitations of the patents' coverage. In addition, the cost of litigation or interference proceedings to uphold the validity of patents can be substantial. If we are unsuccessful in these proceedings, third parties may be able to use our patented technology without paying us licensing fees or royalties. Moreover, competitors may infringe our patents or successfully avoid them through design innovation. To prevent infringement or unauthorized use, we may need to file infringement claims, which are expensive and time-consuming. In an infringement proceeding, a court may decide that a patent of ours is not valid. Even if the validity of our patents were upheld, a court may refuse to stop the other party from using the technology at issue on the ground that its activities are not covered by our patents.

        In recent years, policymakers have also proposed reforming U.S. patent laws and regulations. For example, in March 2010, the Senate Judiciary Committee released the Patent Reform Act of 2010. Although it has not yet been approved by both houses, in general, the proposed legislation attempts to address issues surrounding the enforceability of patents and the increase in patent litigation by, among other things, changing the way damages for patent infringement are determined, moving to a first-inventor-to-file system, establishing new procedures for challenging patents and establishing different methods for invalidating patents. While we cannot predict what form any new patent reform laws or regulations ultimately may take, final legislation could introduce new substantive rules and procedures for challenging patents, and certain reforms that make it easier for competitors to challenge our patents could have a material adverse effect on our business and prospects.

        Policing unauthorized use of our intellectual property is difficult, and we may not be able to prevent misappropriation of our proprietary rights, particularly in countries where the laws may not protect such rights as fully as in the United States.

        In addition to our patent rights, we also rely on unpatented technology, trade secrets, know-how and confidential information. Third parties may independently develop substantially equivalent information and techniques or otherwise gain access to or disclose our technology. We may not be able to effectively protect our rights in unpatented technology, trade secrets, know-how and confidential information. We require each of our employees, consultants and corporate partners to execute a confidentiality agreement at the commencement of an employment, consulting or collaborative relationship with us. Further, we require that all employees enter into assignment of invention agreements as a condition of employment. However, these agreements may not provide effective protection of our information or, in the event of unauthorized use or disclosure, they may not provide adequate remedies.

Any inability to license from third parties their proprietary technologies or processes which we use in connection with the development and manufacture of our product candidates may impair our business.

        Other companies, universities and research institutions have or may obtain patents that could limit our ability to use, manufacture, market or sell our product candidates or impair our competitive position. As a result, we would have to obtain licenses from other parties before we could continue using, manufacturing, marketing or selling our potential products. Any necessary licenses may not be available on commercially acceptable terms, if at all. If we do not obtain required licenses, we may not be able to market our potential products at all or we may encounter significant delays in product development while we redesign products or methods that are found to infringe on the patents held by others.

34

Table of Contents

We may incur substantial costs as a result of litigation or other proceedings relating to patent and other intellectual property rights held by third parties and we may be unable to protect our rights to, or to commercialize, our product candidates.

        Patent litigation is very common in the biotechnology and pharmaceutical industries. Third parties may assert patent or other intellectual property infringement claims against us with respect to our technologies, products or other matters. From time to time, we have received correspondence from third parties alleging that we infringe their intellectual property rights. Any claims that might be brought against us alleging infringement of patents may cause us to incur significant expenses and, if successfully asserted against us, may cause us to pay substantial damages and limit our ability to use the intellectual property subject to these claims. Even if we were to prevail, any litigation would be costly and time-consuming and could divert the attention of our management and key personnel from our business operations. Furthermore, as a result of a patent infringement suit, we may be forced to stop or delay developing, manufacturing or selling potential products that incorporate the challenged intellectual property unless we enter into royalty or license agreements. There may be third-party patents, patent applications and other intellectual property relevant to our potential products that may block or compete with our products or processes. In addition, we sometimes undertake research and development with respect to potential products even when we are aware of third-party patents that may be relevant to our potential products, on the basis that such patents may be challenged or licensed by us. If our subsequent challenge to such patents were not to prevail, we may not be able to commercialize our potential products after having already incurred significant expenditures unless we are able to license the intellectual property on commercially reasonable terms. We may not be able to obtain royalty or license agreements on terms acceptable to us, if at all. Even if we were able to obtain licenses to such technology, some licenses may be non-exclusive, thereby giving our competitors access to the same technologies licensed to us. Ultimately, we may be unable to commercialize some of our potential products or may have to cease some of our business operations, which could severely harm our business.

We use hazardous materials in our business, and any claims relating to improper handling, storage or disposal of these materials could harm our business.

        Our research and development and manufacturing activities involve the controlled use of hazardous materials, chemicals, biological materials and radioactive compounds. We are subject to federal, state and local laws and regulations governing the use, manufacture, storage, handling and disposal of these materials and certain waste products. Although we believe that our safety procedures for handling and disposing of these materials comply with the standards prescribed by applicable laws and regulations, we cannot completely eliminate the risk of accidental contamination or injury from these materials. In the event of such an accident, we could be held liable for any resulting damages, and any liability could exceed our resources. We may be required to incur significant costs to comply with these laws in the future. Failure to comply with these laws could result in fines and the revocation of permits, which could prevent us from conducting our business.

We face product liability risks and may not be able to obtain adequate insurance.

        While we secure waivers from all participants in our clinical trials, the use of our product candidates during testing or after approval entails an inherent risk of adverse effects, which could expose us to product liability claims. Regardless of their merit or eventual outcome, product liability claims may result in:

•

decreased demand for our product;



•

injury to our reputation and significant negative media attention;



•

withdrawal of clinical trial volunteers;

35

Table of Contents

•

costs of litigation;



•

distraction of management; and



•

substantial monetary awards to plaintiffs.

        We may not have sufficient resources to satisfy any liability resulting from these claims. We currently have $5 million of product liability insurance for products which are in clinical testing. This coverage may not be adequate in scope to protect us in the event of a successful product liability claim. Further, we may not be able to maintain our current insurance or obtain general product liability insurance on reasonable terms and at an acceptable cost if we or our collaborative partners begin commercial production of our proposed product candidates. This insurance, even if we can obtain and maintain it, may not be sufficient to provide us with adequate coverage against potential liabilities.

We depend on our key personnel and we must continue to attract and retain key employees and consultants.

        We depend on our key scientific and management personnel. Our ability to pursue the development of our current and future product candidates depends largely on retaining the services of our existing personnel and hiring additional qualified scientific personnel to perform research and development. We will also need to hire personnel with expertise in clinical testing, government regulation, manufacturing, marketing and finance. Attracting and retaining qualified personnel will be critical to our success. We may not be able to attract and retain personnel on acceptable terms given the competition for such personnel among biotechnology, pharmaceutical and healthcare companies, universities and non-profit research institutions. Failure to retain our existing key management and scientific personnel or to attract additional highly qualified personnel could delay the development of our product candidates and harm our business.

Our stock price can fluctuate significantly and results announced by us and our collaborators can cause our stock price to decline.

        Our stock price can fluctuate significantly due to business developments announced by us and by our collaborators, as a result of market trends and daily trading volume. The business developments that could impact our stock price include disclosures related to clinical findings with compounds that make use of our TAP technology, new collaborations and clinical advancement or discontinuation of product candidates that make use of our TAP technology. Our stock price can also fluctuate significantly with the level of overall investment interest in small-cap biotechnology stocks.

        Our operating results have fluctuated in the past and are likely to continue to do so in the future. Our revenue is unpredictable and may fluctuate due to the timing of non-recurring licensing fees, decisions of our collaborative partners with respect to our agreements with them, reimbursement for manufacturing services, the achievement of milestones and our receipt of the related milestone payments under new and existing licensing and collaboration agreements. Revenue historically recognized under our prior collaboration agreements may not be an indicator of revenue from any future collaborations. In addition, our expenses are unpredictable and may fluctuate from quarter to quarter due to the timing of expenses, which may include obligations to manufacture or supply product or payments owed by us under licensing or collaboration agreements. It is possible that our quarterly and/or annual operating results will not meet the expectations of securities analysts or investors, causing the market price of our common stock to decline. We believe that quarter-to-quarter and year-to-year comparisons of our operating results are not good indicators of our future performance and should not be relied upon to predict the future performance of our stock price.

36

Table of Contents

The potential sale of additional shares of our common stock may cause our stock price to decline.

        On April 9, 2010, we filed a Registration Statement on Form S-3 with the Securities and Exchange Commission. The Securities and Exchange Commission declared the Registration Statement effective on April 22, 2010. Subject to our ongoing obligations under the Securities Act of 1933, as amended, and the Securities Exchange Act of 1934, as amended, the Registration Statement permits us to offer and sell up to an aggregate of $125 million of our common stock. Pursuant to the shelf registration statement, on May 6, 2010, we sold 10,350,000 shares of our common stock at $8.00 per share in a public offering resulting in gross proceeds of $82.8 million. Additionally, pursuant to a shelf registration statement filed on July 11, 2007, on June 23, 2009, we sold 5,750,000 shares of our common stock at $7.00 per share in a public offering resulting in gross proceeds of $40.3 million and, on June 20, 2008, a private investor purchased 7,812,500 shares of our common stock at $3.20 per share resulting in gross proceeds of $25 million. The potential sale of additional shares of our common stock may be dilutive to our shares outstanding and may cause our stock price to decrease.

We do not intend to pay cash dividends on our common stock.

        We have not paid cash dividends since our inception and do not intend to pay cash dividends in the foreseeable future. Therefore, shareholders will have to rely on appreciation in our stock price, if any, in order to achieve a gain on an investment.

37

Table of Contents

A WARNING ABOUT FORWARD-LOOKING STATEMENTS

        This report includes forward-looking statements within the meaning of the Private Securities Litigation Reform Act of 1995. These statements relate to analyses and other information which are based on forecasts of future results and estimates of amounts that are not yet determinable. These statements also relate to our future prospects, developments and business strategies.

        These forward-looking statements are identified by their use of terms and phrases, such as "anticipate," "believe," "could," "estimate," "expect," "intend," "may," "plan," "predict," "project," "will" and other similar terms and phrases, including references to assumptions. These statements are contained in the "Business," "Risk Factors" and "Management's Discussion and Analysis of Financial Condition and Results of Operations" sections, as well as other sections of this Annual Report on Form 10-K.

        Forward-looking statements in this report include, but are not limited to:

•

successfully finding and managing the relationships with collaborative partners;



•

the uncertainty as to whether our TAP compounds or those of our collaborators will succeed in entering human clinical trials and uncertainty as to the results of such trials;



•

the risk that we and/or our collaborators may not be able to obtain regulatory approvals necessary to commercialize product candidates;



•

the potential development of competing products and technologies; uncertainty whether our TAP technology will produce safe, effective and commercially viable products;



•

our ability to successfully protect our intellectual property;



•

our reliance on third-party manufacturers to achieve supplies of our maytansinoid cell-killing agents, DM1 and DM4;



•

the risk that we may be unable to establish the manufacturing capabilities necessary to develop and commercialize our potential products;



•

the adequacy of our liquidity and capital resources;



•

government regulation of our activities, facilities, products and personnel; the dependence on key personnel;



•

uncertainties as to the extent of reimbursement for the costs of our potential products and related treatments by government and private health insurers and other organizations; the potential adverse impact of government-directed health care reform; and



•

the risk of product liability claims; and economic conditions, both generally and those specifically related to the biotechnology industry.

        These forward-looking statements involve known and unknown risks, uncertainties and other factors that may cause actual results to be materially different from those contemplated by our forward-looking statements. These known and unknown risks, uncertainties and other factors are described in detail in the "Risk Factors" section and in other sections of this Annual Report on Form 10-K. We disclaim any intention or obligation to update or revise any forward-looking statements, whether as a result of new information, future events or otherwise.

Item 1B.    Unresolved Staff Comments

        None.

38

Table of Contents

Item 2.    Properties

        We lease approximately 89,000 square feet of laboratory and office space in a building located at 830 Winter Street, Waltham, MA. The initial term of the 830 Winter Street lease expires on March 31, 2020, with an option for us to extend the lease for two additional five-year terms. In December 2009, we entered into a sublease for 14,100 square feet of our office and laboratory space at 830 Winter Street, Waltham, MA through January 2015. We also lease approximately 43,850 square feet of space in Norwood, MA, which serves as our conjugate manufacturing facility and office space. The Norwood lease expires on June 30, 2011, with an option for us to extend the lease for an additional five-year term.

        We also lease approximately 15,000 square feet of laboratory and office space in a building located at 148 Sidney Street, Cambridge, MA, which we vacated in March 2008 when we relocated all our Cambridge operations to Waltham, MA. The 148 Sidney Street lease term was to expire on October 30, 2010. In May 2008, we entered into a sub-sublease for this entire space for the remainder of the lease term. Both the lease and sub-sublease for 148 Sidney Street were terminated as of July 31, 2010.

Item 3.    Legal Proceedings

        From time to time we may be a party to various legal proceedings arising in the ordinary course of our business. We are not currently subject to any material legal proceedings.

Item 3.1.    Executive Officers of the Registrant

        ImmunoGen's executive officers are appointed by the Board of Directors at the first meeting of the Board following the annual meeting of shareholders or at other Board meetings as appropriate, and hold office until the first Board meeting following the next annual meeting of shareholders and until a successor is chosen, subject to prior death, resignation or removal. Information regarding our executive officers is presented below.

        Daniel M. Junius, age 58, joined ImmunoGen in 2005, and has served as our President and Chief Executive Officer since January 2009. Prior to that he served as our President and Chief Operating Officer and Acting Chief Financial Officer from July 2008 to December 2008, as our Executive Vice President and Chief Financial Officer from 2006 to July 2008, and as our Senior Vice President and Chief Financial Officer from 2005 to 2006. Prior to joining ImmunoGen in 2005, he served as Executive Vice President and Chief Financial Officer of New England Business Service, Inc. (NEBS), a supplier of business products and services to small businesses, from 2002 to 2004, and as Senior Vice President and Chief Financial Officer of NEBS from 1998 to 2002. Mr. Junius holds a Masters of Management from Northwestern University's Kellogg School of Management.

        John M. Lambert, Ph.D., age 59, joined ImmunoGen in 1987, and has served as our Executive Vice President, Research and Development and Chief Scientific Officer since July 2008. Prior to that he served as our Senior Vice President, Research and Development and Chief Scientific Officer from early 2008 to July 2008, as our Senior Vice President, Pharmaceutical Development, from 2000 to early 2008, as our Vice President, Research and Development, from 1994 to 2000, and as our Senior Director of Research from 1987 to 1994. Prior to joining ImmunoGen, Dr. Lambert was an assistant professor at Harvard Medical School working at the Dana-Farber Cancer Institute. Dr. Lambert holds a Ph.D. in Biochemistry from University of Cambridge in England, and completed his postdoctoral work at the University of California at Davis and at Glasgow University in Scotland.

        James J. O'Leary, MD, age 46, joined ImmunoGen in 2008, and has served as our Vice President and Chief Medical Officer since that date. Prior to joining ImmunoGen, Dr. O'Leary served as Senior Medical Director Clinical Oncology of Bayer Corporation, a pharmaceutical company, from 2006 to 2008. Prior to that he served as Medical Director Clinical Oncology of Pfizer Global Research and

39

Table of Contents

Development, a pharmaceutical company, from 2003 to 2006, and as Assistant Medical Director Clinical Oncology of Pfizer from 2000 to 2003. Prior to that he served as a Medical Reviewer, Division of Oncology Drug Products at the U.S. Food and Drug Administration from 1998 to 2000. Dr. O'Leary has a Doctor of Medicine degree from the State University of New York–Health Science Center at Brooklyn.

        Gregory D. Perry, age 50, joined ImmunoGen in January 2009, and has served as our Senior Vice President and Chief Financial Officer since that date. Prior to joining ImmunoGen, he served as Chief Financial Officer of Elixir Pharmaceuticals, Inc., a pharmaceutical company, from 2007 to 2008. Prior to that he served as Chief Financial Officer for Domantis Ltd., a biopharmaceutical company, in 2006, as Senior Vice President, Finance and Chief Financial Officer of Transkaryotic Therapies, Inc., a biopharmaceutical company, from 2003 to 2005.

        Peter J. Williams, age 56, joined ImmunoGen in August 2009, and has served as our Vice President, Business Development since that date. Prior to joining ImmunoGen, he served as a Senior Director of Business Development at Alnylam Pharmaceuticals, Inc., a biopharmaceutical company, from 2006 to August 2009. Prior to that he served as Vice President of Business Development of Link Medicine Corporation, a drug development company, from 2005 to 2006. Prior to that he acted as an independent business development consultant from 2003 to 2006. Prior to that he served as a Senior Director of Business Development at Millennium Pharmaceuticals, Inc., a biopharmaceutical company, from 1998 to 2003.

Item 4.    Reserved

40

Table of Contents

PART II

Item 5.    Market for Registrant's Common Equity, Related Stockholder Matters and Issuer Purchases of Equity Securities

Market Price of Our Common Stock and Related Stockholder Matters

        Our common stock is quoted on the NASDAQ Global Market under the symbol "IMGN." The table below sets forth the high and low closing price per share of our common stock as reported by NASDAQ:

        As of August 11, 2010, the closing price per share of our common stock was $8.93, as reported by NASDAQ, and we had approximately 503 holders of record of our common stock and, according to our estimates, approximately 18,342 beneficial owners of our common stock.

        We have not paid any cash dividends on our common stock since our inception and do not intend to pay any cash dividends in the foreseeable future.

41

Table of Contents

Stock Price Performance Graph

        The graph and table below compare the annual percentage change in our cumulative total shareholder return on our common stock for the period from June 30, 2005 through June 30, 2010 (as measured by dividing (i) the sum of (A) the cumulative amount of dividends for the measurement period, assuming dividend reinvestment, and (B) the difference between our share price at the end and the beginning of the measurement period; by (ii) the share price at the beginning of the measurement period) with the total cumulative return of the NASDAQ Stock Market Index (U.S.) and the NASDAQ Pharmaceutical Stocks Total Return Index during such period. We have not paid any dividends on our common stock, and no dividends are included in the representation of our performance. The stock price performance on the graph below is not necessarily indicative of future price performance. This graph is not "soliciting material," is not deemed filed with the Commission and is not to be incorporated by reference in any of our filings under the Securities Act of 1933, as amended, or the Securities Exchange Act of 1934, as amended, whether made before or after the date hereof and irrespective of any general incorporation language in any such filing. Information used on the graph for the NASDAQ Pharmaceutical Stocks Total Return Index and the NASDAQ Stock Market Index (U.S.) was prepared by the Center for Research in Security Prices, a source believed to be reliable, but we are not responsible for any errors or omissions in such information.

*

This index represents a group of peer issuers compiled by the Center for Research in Security Prices.

        The above graph and table assume $100 invested on June 30, 2005 with all dividends reinvested, in each of our common stock, the NASDAQ Stock Market Index (U.S.) and the NASDAQ Pharmaceutical Stocks Total Return Index. Upon written request by any shareholder, we will promptly provide a list of the companies comprising the NASDAQ Pharmaceutical Stocks Total Return Index.

42

Table of Contents

Recent Sales of Unregistered Securities; Uses of Proceeds from Registered Securities; Issuer Repurchases of Equity Securities

        None.

Item 6.    Selected Financial Data

        The following table (in thousands, except per share data) sets forth our consolidated financial data for each of our five fiscal years through our fiscal year ended June 30, 2010. The information set forth below should be read in conjunction with "Management's Discussion and Analysis of Financial Condition and Results of Operations" and the consolidated financial statements and related notes included elsewhere in this Annual Report on Form 10-K.

Item 7.    Management's Discussion and Analysis of Financial Condition and Results of Operations

Overview

        Since our inception, we have been principally engaged in the development of novel, targeted therapeutics for the treatment of cancer using our expertise in cancer biology, monoclonal antibodies, highly potent cytotoxic, or cell-killing, agents, and the design of linkers that enable these agents to be stably attached to the antibodies while in the blood stream and released in their fully active form after delivery to a cancer cell. An anticancer compound made using our Targeted Antibody Payload, or TAP, technology consists of a monoclonal antibody that binds specifically to an antigen target found on cancer cells with multiple copies of one of our proprietary cell-killing agents attached using one of our engineered linkers. Its antibody component enables a TAP compound to bind specifically to cancer cells that express a particular target antigen, the highly potent cytotoxic agent serves to kill the cancer cell, and the engineered linker controls the release and activation of the cytotoxic agent inside the cancer cell. Our TAP technology is designed to enable the creation of highly effective, well-tolerated anticancer products. All of our and our collaborative partners' TAP compounds currently in preclinical and clinical testing contain either DM1 or DM4 as the cytotoxic agent. Both DM1 and DM4, collectively DMx, are our proprietary derivatives of a naturally occurring substance called maytansine. We also use our expertise in antibodies and cancer biology to develop "naked," or non-conjugated, antibody anticancer product candidates.

43

Table of Contents

        We have entered into collaborative agreements that enable companies to use our TAP technology to develop commercial product candidates to specified targets. We have also used our proprietary TAP technology in conjunction with our in-house antibody expertise to develop our own anticancer product candidates. Under the terms of our collaborative agreements, we are generally entitled to upfront fees, milestone payments, and royalties on any commercial product sales. In addition, under certain agreements we are entitled to research and development funding based on activities performed at our collaborative partner's request. We are reimbursed for our direct and a portion of overhead costs to manufacture preclinical and clinical materials and, under certain collaborative agreements, the reimbursement includes a profit margin. Currently, our collaborative partners include Amgen, Bayer Schering Pharma, Biogen Idec, Biotest, Genentech (a member of the Roche Group) and sanofi-aventis. We expect that substantially all of our revenue for the foreseeable future will result from payments under our collaborative arrangements. Details for some of our major and recent collaborative agreements can be found in this 10-K under Item 1, Business.

        To date, we have not generated revenues from commercial product sales and we expect to incur significant operating losses for the foreseeable future. As of June 30, 2010, we had approximately $110.3 million in cash and marketable securities compared to $71.1 million in cash and marketable securities as of June 30, 2009.

        We anticipate that future cash expenditures will be partially offset by collaboration-derived proceeds, including milestone payments, royalties and upfront fees. Accordingly, period-to-period operational results may fluctuate dramatically based upon the timing of receipt of the proceeds. We believe that our established collaborative agreements, while subject to specified milestone achievements, will provide funding to assist us in meeting obligations under our collaborative agreements while also assisting in providing funding for the development of internal product candidates and technologies. However, we can give no assurances that such collaborative agreement funding will, in fact, be realized in the time frames we expect, or at all. Should we or our partners not meet some or all of the terms and conditions of our various collaboration agreements, we may be required to secure alternative financing arrangements and/or defer or limit some or all of our research, development and/or clinical projects. However, we cannot provide assurance that any such opportunities presented by additional strategic partners or alternative financing arrangements will be entirely available to us, if at all.

Critical Accounting Policies

        We prepare our consolidated financial statements in accordance with accounting principles generally accepted in the U.S. The preparation of these financial statements requires us to make estimates and judgments that affect the reported amounts of assets, liabilities, revenues and expenses and related disclosure of contingent assets and liabilities. On an on-going basis, we evaluate our estimates, including those related to our collaborative agreements and inventory. We base our estimates on historical experience and various other assumptions that we believe to be reasonable under the circumstances. Actual results may differ from these estimates.

        We believe the following critical accounting policies affect our more significant judgments and estimates used in the preparation of our consolidated financial statements.

Revenue Recognition

        We enter into licensing and development agreements with collaborative partners for the development of monoclonal antibody-based anticancer therapeutics. We follow the provisions of FASB's Accounting Standards Codification (ASC) Topic 605, Revenue Recognition. Accordingly, we recognize revenue related to research activities as they are performed, as long as there is persuasive evidence of an arrangement, the fee is fixed or determinable, and collection of the related receivable is probable. The terms of our agreements contain multiple elements which typically include non-refundable license

44

Table of Contents

fees, payments based upon the achievement of certain milestones and royalties on product sales. We evaluate such arrangements to determine if the deliverables are separable into units of accounting and then apply applicable revenue recognition criteria to each unit of accounting. Agreements containing multiple elements are divided into separate units of accounting if certain criteria are met, including whether the delivered element has stand-alone value to the collaborator and whether there is objective and reliable evidence of the fair value of the undelivered obligation(s). The consideration received is allocated among the separate units either on the basis of each unit's fair value or using the residual method, and the applicable revenue recognition criteria are applied to each of the separate units.

        In October 2009 a new accounting standard for the recognition of revenue arrangements with multiple deliverables was issued. This standard provides accounting principles and application guidance on whether multiple deliverables exist, how the arrangement should be separated, and how the consideration should be allocated. This new approach is effective prospectively for revenue arrangements entered into or materially modified in fiscal years beginning on or after June 15, 2010 (our fiscal year 2011). While we do not expect the adoption of this standard to have a material impact on our financial position and results of operations, this standard may impact us in the event we complete future transactions or modify existing collaborative relationships. Refer to Note B, Recent Accounting Pronouncements, to our Consolidated Financial Statements for additional discussion of this standard and its impact on us.

        At June 30, 2010, we had the following three types of collaborative contracts with the parties identified below:

•

Exclusive license to use our TAP technology and/or certain other intellectual property to develop compounds to a single target antigen:

Amgen (multiple single-target licenses)

Bayer Schering Pharma (single-target license)

Biogen Idec (single-target license)

Biotest (single-target license)

Genentech (multiple single-target licenses)

sanofi-aventis (license to multiple individual targets)

•

Option agreement for a defined period of time to secure licenses to use our TAP technology to develop anticancer compounds to a limited number of targets on established terms (broad option agreement):

Amgen
sanofi-aventis

•

Non-exclusive license to our humanization technology:

sanofi-aventis

        Generally, the foregoing collaboration agreements provide that we will (i) at the collaborator's request, manufacture and provide to them preclinical and clinical materials which are reimbursed at our cost, or, in some cases, cost plus a margin, (ii) earn payments upon the collaborators' achievements of certain milestones and (iii) earn royalty payments, generally until the later of the last applicable patent expiration or 10 to 12 years after product launch. Royalty rates may vary over the royalty term depending on our intellectual property rights. We provide technical training and share any process improvements and know-how with our collaborators during the research term of the collaboration agreements.

45

Table of Contents

        Generally, upfront payments on single-target licenses are deferred over the period of our substantial involvement during development. The determination of the length of this period is subject to judgment and estimation and can have an impact on the amount of revenue recognized in a given period. Our employees are available to assist our collaborators during the development of their products. We estimate this development phase to begin at the inception of the collaboration agreement and conclude at the end of non-pivotal Phase II testing. We believe this period of involvement is, depending on the nature of the license, on average six and one-half years. Quarterly, we reassess our periods of substantial involvement over which we amortize our upfront license fees and we make adjustments as appropriate. In the event that a single-target license were to be terminated, we would recognize as revenue any portion of the upfront fee that had not previously been recorded as revenue, but was classified as deferred revenue at the date of such termination.

        We defer upfront payments received from our broad option agreements over the period during which the collaborator may elect to receive a license. These periods are specific to each collaboration agreement, but are between three and 12 years. If a collaborator selects an option to acquire a license under these agreements, any option fee is deferred and is recorded over the life of the option, generally 12 to 18 months. If a collaborator exercises an option and we grant a single-target license to the collaborator, we defer the license fee and account for the fee as we would an upfront payment on a single-target license, as discussed above. Upon exercise of an option to acquire a license, we would recognize any remaining deferred option fee over the period of our substantial involvement under the license acquired. In the event a broad option agreement were to be terminated, we would recognize as revenue any portion of the upfront fee that had not previously been recorded as revenue, but was classified as deferred revenue at the date of such termination. In the event a collaborator elects to discontinue development of a specific product candidate under a single- target license, but retains its right to use our technology to develop an alternative product candidate to the same target or a target substitute, we would cease amortization of any remaining portion of the upfront fee until there is substantial preclinical activity on another product candidate and our remaining period of substantial involvement can be estimated.

        When milestone fees are specifically tied to a separate earnings process and are deemed to be substantive and at risk, revenue is recognized when such milestones are achieved. In addition, we recognize research and development support revenue from certain collaboration and development agreements based upon the level of research services performed during the period of the research agreement. Deferred revenue substantially represents amounts received under collaborative agreements and not yet earned pursuant to these policies. Where we have no continuing involvement, we will record non-refundable license fees as revenue upon receipt and will record revenue upon achievement of milestones by our collaborative partners.

        We produce preclinical and clinical materials for our collaborators. We are reimbursed for our direct costs and a portion of our overhead costs to produce clinical materials. We recognize revenue on preclinical and clinical materials when the materials have passed all quality testing required for collaborator acceptance and title and risk of loss have transferred to the collaborator.

        We also produce research material for potential collaborators under material transfer agreements. Additionally, we perform research activities, including developing antibody-specific conjugation processes, on behalf of our collaborators and potential collaborators during the early evaluation and preclinical testing stages of drug development. Generally, we are reimbursed for certain of our direct and overhead costs of producing these materials or providing these services. We record the amounts received for the preclinical materials produced or services performed as a component of research and development support. We also develop conjugation processes for materials for later stage testing and commercialization for certain collaborators. We are reimbursed for certain of our direct and overhead costs and may receive milestone payments for developing these processes which are recorded as a component of research and development support.

46

Table of Contents

Inventory

        We review our estimates of the net realizable value of our inventory at each reporting period. Our estimate of the net realizable value of our inventory is subject to judgment and estimation. The actual net realizable value of our inventory could vary significantly from our estimates. We consider quantities of raw materials in excess of twelve-month projected usage that are not supported by firm, fixed collaborator orders and projections at the time of the assessment to be excess. During fiscal 2008, we obtained additional amounts of DMx from a new supplier. Due to the need to evaluate the process which was developed to prepare such material from this new supplier across multiple batches, we had committed to a level of production which yielded more material than would be required by our collaborators over the next twelve months. As a result, during the year ended June 30, 2008, we recorded a $2.1 million charge to research and development expense related to raw material inventory identified as excess. We also recorded $1.6 million to write down the raw material inventory purchased during the fiscal year 2008 to its net realizable value, which is also included in research and development expense for the year ended June 30, 2008. During fiscal 2010, we obtained additional amounts of DMx from our supplier which yielded more material than would be required by our collaborators over the next twelve months and as a result, we recorded $900,000 of charges to research and development expense related to raw material inventory identified as excess. We also recorded $28,000 to write down certain raw material inventory to its net realizable value, which is also included in research and development expense for the year ended June 30, 2010. No similar costs were recorded during the year ended June 30, 2009. Our collaborators' estimates of their clinical material requirements are based upon expectations of their clinical trials, including the timing, size, dosing schedule and the maximum tolerated dose likely to be reached for the compound being evaluated. Our collaborators' actual requirements for clinical materials may vary significantly from their projections. Significant differences between our collaborators' actual manufacturing orders and their projections could result in our actual twelve-month usage of raw materials varying significantly from our estimated usage at an earlier reporting period. Reductions in collaborators' projections could indicate that we have additional excess raw material inventory and we would then evaluate the need to record further write-downs, which would be included as charges to research and development expense.

Stock-based Compensation

        As of June 30, 2010, the Company is authorized to grant future awards under one share-based compensation plan, which is the ImmunoGen, Inc. 2006 Employee, Director and Consultant Equity Incentive Plan. The stock-based awards are accounted for under ASC Topic 718, "Compensation—Stock Compensation," using the modified prospective transition method. Under this methodology, the estimated fair value of awards is charged to the statement of operations over the requisite service period, which is the vesting period. Such amounts have been reduced by our estimate of forfeitures of all unvested awards.

        The fair value of each stock option is estimated on the date of grant using the Black-Scholes option-pricing model. Expected volatility is based exclusively on historical volatility data of our stock. The expected term of stock options granted is based exclusively on historical data and represents the period of time that stock options granted are expected to be outstanding. The expected term is calculated for and applied to one group of stock options as we do not expect substantially different exercise or post-vesting termination behavior amongst our employee population. The risk-free rate of the stock options is based on the U.S. Treasury rate in effect at the time of grant for the expected term of the stock options. Estimated forfeitures are based on historical data as well as current trends. Stock compensation cost incurred during the years ended June 30, 2010, 2009 and 2008 was $4.2 million, $4.0 million and $2.9 million, respectively. During fiscal year 2009, we recorded approximately $843,000 of stock compensation cost related to the modification of certain outstanding common stock options in accordance with the former Chief Executive Officer's succession plan.

47

Table of Contents

Investment in Marketable Securities

        We invest in marketable securities of highly rated financial institutions and investment-grade debt instruments and limit the amount of credit exposure with any one entity. We have classified our marketable securities as "available-for-sale" and, accordingly, carry such securities at aggregate fair value. In accounting for investments, we evaluate if a decline in the fair value of a marketable security below our cost basis is other-than-temporary, and if so, we record an impairment charge related to any credit loss in our consolidated statement of operations. The factors that we consider in our evaluation include the fair market value of the security, the duration and magnitude of the security's decline, and our intent to sell or whether we would more likely than not to be required to sell the security before the expected recovery of the amortized cost basis. The determination of whether a loss is other than temporary is highly judgmental and can have a material impact on our results. During the fiscal years ended June 30, 2009 and 2008, we recorded approximately $516,000 and $535,000, respectively, in other-than-temporary impairment charges. No similar charges were recorded in the fiscal year ended June 30, 2010. We adopted certain provisions of ASC Topic 320, "Investments—Debt and Equity Securities," on April 1, 2009. As a result of the adoption of these provisions, during fiscal year 2009 we reclassified $54,000 of previously recognized other-than-temporary impairment charges to other comprehensive loss as a cumulative effect adjustment.

Fair Value of Financial Instruments

        As of July 1, 2008, we partially adopted the provisions of ASC Topic 820, "Fair Value Measurements and Disclosures," for financial assets and liabilities recognized at fair value on a recurring basis. ASC Topic 820 defines fair value, establishes a framework for measuring fair value in accordance with accounting principles generally accepted in the U.S., and expands disclosures about fair value measurements. Certain provisions of ASC Topic 820 related to other non-financial assets and liabilities were adopted by us on July 1, 2009 and did not have a material impact on our financial position or results of operations upon adoption; however, this standard may impact us in subsequent periods and require additional disclosures.

        Fair value is defined under ASC Topic 820 as the exchange price that would be received for an asset or paid to transfer a liability (an exit price) in the principal or most advantageous market for the asset or liability in an orderly transaction between market participants on the measurement date. Valuation techniques used to measure fair value must maximize the use of observable inputs and minimize the use of unobservable inputs. The standard describes a fair value hierarchy to measure fair value which is based on three levels of inputs, of which the first two are considered observable and the last unobservable. This standard is applicable to all assets and liabilities (i.e. financial and non-financial) and requires enhanced disclosures, including interim and annual disclosure of the input and valuation techniques (or changes in techniques) used to measure fair value and the defining of the major security types comprising debt and equity securities held based upon the nature and risk of the security. The fair value of our investments is generally determined from market prices based upon either quoted prices from active markets or other significant observable market transactions at fair value.

48

Table of Contents

Results of Operations

Revenues

        Our total revenues for the year ended June 30, 2010 were $13.9 million compared with $28.0 million and $40.2 million for the years ended June 30, 2009 and 2008, respectively. The $14.1 million decrease in revenues in fiscal year 2010 from fiscal year 2009 is primarily attributable to lower revenues from research and development support, license and milestone fees and clinical materials reimbursement, as discussed below. The $12.2 million decrease in revenues in fiscal year 2009 from fiscal year 2008 is primarily attributable to lower revenues from research and development support and clinical materials reimbursement, partially offset by higher revenues from license and milestone fees, as discussed below.

        Research and development support was $5.4 million for the year ended June 30, 2010, $7.6 million for the year ended June 30, 2009, and $15.0 million for the year ended June 30, 2008. These amounts primarily represent research funding earned based on actual resources utilized under our agreements with our collaborators as shown in the table below. The decreased research and development support fees in fiscal years 2010 and 2009 compared to fiscal year 2008 is primarily due to a reduction in the amount earned from sanofi-aventis with the conclusion of its committed funding obligations in the first half of fiscal 2009. Also included in research and development support revenue are development fees charged for reimbursement of our direct and overhead costs incurred in producing and delivering research-grade materials to our collaborators and for developing antibody-specific conjugation processes on behalf of our collaborators and potential collaborators during the early evaluation and preclinical testing stages of drug development. The amount of development fees we earn is directly related to the number of our collaborators and potential collaborators, the stage of development of our collaborators' product candidates and the resources our collaborators allocate to the development effort. As such, the amount of development fees may vary widely from quarter to quarter and year to year. Total revenue recognized from research and development support from each of our collaborative partners in the years ended June 30, 2010, 2009 and 2008 is included in the following table (in thousands):

        Revenue from license and milestone fees for the year ended June 30, 2010 decreased approximately $9.4 million to $5.7 million from $15.1 million in the year ended June 30, 2009. Revenue from license and milestone fees for the year ended June 30, 2008 was $13.2 million. Included in license and milestone fees for the year ended June 30, 2010 were $1 million and $500,000 of preclinical milestones earned pursuant to our agreements with Bayer and sanofi-aventis, respectively, as well as a $1 million milestone related to the initiation of Phase I clinical testing of SAR650984 by sanofi-aventis. Included in license and milestone fees for the year ended June 30, 2009 was a $6.5 million milestone related to the initiation of Phase III clinical testing of trastuzumab-DM1, or T-DM1, by Genentech, a $4 million milestone related to the initiation of Phase II clinical testing of AVE1642 by sanofi-aventis

49

Table of Contents

and a $500,000 milestone related to the initiation of Phase I clinical testing of BT-062 by Biotest. Also during the year ended June 30, 2009, Millennium Pharmaceuticals and Boehringer Ingelheim agreed to terminate their licenses with us that were no longer being used to develop products and as a result, we recognized as license and milestone fees $361,000 and $486,000, respectively, of upfront fees previously deferred. Included in license and milestone fees for the year ended June 30, 2008 was $5 million related to the achievement of a milestone under the Genentech agreement from the initiation of Phase II clinical testing of T-DM1, $1.5 million related to the achievement of a milestone under the Biogen Idec agreement from the submission of the IND application for BIIB015, $2 million related to the achievement of milestones under the sanofi-aventis agreement from the initiation of clinical testing of SAR3419 and certain preclinical milestones. Total revenue recognized from license and milestone fees from each of our collaborative partners in the years ended June 30, 2010, 2009 and 2008 is included in the following table (in thousands):

        Deferred revenue of $11.7 million at June 30, 2010 represents payments received from our collaborators pursuant to our license and supply agreements with them which we have yet to earn pursuant to our revenue recognition policy.

        Clinical materials reimbursement decreased by approximately $2.4 million to $2.9 million in the year ended June 30, 2010 compared to $5.3 million in the year ended June 30, 2009. We earned clinical materials reimbursement of $12.1 million during the year ended June 30, 2008. During the years ended June 30, 2010, 2009 and 2008, we shipped clinical materials in support of a number of clinical trials including, for certain of these years, those of T-DM1, AVE9633, SAR3419, SAR566658, BIIB015, BT-062, as well as preclinical materials in support of the development efforts of certain other collaborators and DMx shipments to certain collaborators in support of development and manufacturing efforts. The decrease in clinical materials reimbursement in fiscal year 2010 as compared to fiscal year 2009 is primarily related to less clinical material shipped in support of two of our collaborators' trials due to various factors, including the dosage schedule and speed of enrollment within the trials. The decrease in clinical materials reimbursement in fiscal year 2009 as compared to fiscal year 2008 is primarily related to $5.0 million in revenue recognized from supplying DMx to a collaborator in fiscal year 2008, along with the transfer of T-DM1 to commercial-scale production at a third-party contract manufacturing organization in fiscal year 2009. During fiscal year 2009, sanofi-aventis discontinued clinical trial activity for AVE9633. We are reimbursed for certain of our direct and overhead costs to produce clinical materials plus, for certain programs, a profit margin. The amount of clinical materials reimbursement we earn, and the related cost of clinical materials charged to research and development expense, is directly related to the number of clinical trials our collaborators are preparing or have underway, the speed of enrollment in those trials, the dosage schedule of each clinical trial and the time period, if any, during which patients in the trial receive clinical benefit from

50

Table of Contents

the clinical materials, and the supply of clinical- grade material to our collaborators for process development and analytical purposes. As such, the amount of clinical materials reimbursement revenue and the related cost of clinical materials charged to research and development expense may vary significantly from quarter to quarter and year to year.

Research and Development Expenses

        Our net research and development expenses relate to (i) research to evaluate new targets and to develop and evaluate new antibodies, linkers and cytotoxic agents, (ii) preclinical testing of our own and, in certain instances, our collaborators' product candidates, and the cost of our own clinical trials, (iii) development related to clinical and commercial manufacturing processes and (iv) manufacturing operations. Our research and development efforts have been primarily focused in the following areas:

•

activities pursuant to our discovery, development and commercialization agreement with sanofi-aventis;



•

activities pursuant to our development and license agreements with various other collaborators;



•

activities related to the preclinical and clinical development of lorvotuzumab mertansine, IMGN388 and IMGN529;



•

process development related to production of the lorvotuzumab antibody and lorvotuzumab mertansine conjugate for clinical materials;



•

process development related to production of IMGN388 conjugate for clinical materials;



•

process improvements related to the production of DM1, DM4 and strain development of their precursor, ansamitocin P3;



•

funded development activities with contract manufacturers for the antibody component of IMGN529, the lorvotuzumab antibody, and DM1, DM4 and their precursor, ansamitocin P3;



•

production costs for the supply of the lorvotuzumab antibody;



•

production costs for the supply of DMx for our and our partners' preclinical and clinical activities;



•

operation and maintenance of our conjugate manufacturing facility, including production of our own and our collaborators' clinical materials;



•

process improvements to our TAP technology;



•

evaluation of potential antigen targets;



•

evaluation of internally developed and/or in-licensed product candidates and technologies; and



•

development and evaluation of additional cytotoxic agents and linkers.

        Research and development expense for the year ended June 30, 2010 increased $4.4 million to $50.3 million from $45.9 million for the year ended June 30, 2009. Research and development expense was $60.0 million for the year ended June 30, 2008. Research and development salaries and related expenses increased by $1.8 million in the year ended June 30, 2010 compared to the year ended June 30, 2009 and increased by $1.4 million in the year ended June 30, 2009 compared to the year ended June 30, 2008. Included in salaries and related expenses for the year ended June 30, 2010 is $2.7 million of stock compensation costs compared to $1.7 million and $1.6 million of stock compensation costs for fiscal years 2009 and 2008, respectively. The higher stock compensation costs in fiscal 2010 are driven by higher stock prices and increases in the number of annual options granted. Clinical trial costs increased $1.1 million during fiscal year 2010 compared to fiscal year 2009 and increased $523,000 in fiscal year 2009 compared to fiscal year 2008. Additionally, overhead utilization

51

Table of Contents

from the manufacture of clinical materials on behalf of our collaborators decreased $1.8 million during fiscal year 2010 compared to fiscal year 2009 and decreased by $1.3 million in fiscal year 2009 compared to fiscal year 2008.

        Included in research and development expenses for the year ended June 30, 2010 is $900,000 related to raw material inventory identified as excess, as well as $28,000 to write down certain raw material inventory to its net realizable value. Included in research and development expenses for the year ended June 30, 2008 is a $2.1 million charge related to raw material inventory identified as excess and a $1.6 million charge to write down raw material purchased during that year to its net realizable value. No similar costs were recorded during fiscal year 2009. Reserve requirements for excess quantities of DMx are principally determined based on our collaborators' forecasted demand compared to our inventory position. The DMx purchased during fiscal year 2008 was produced by a supplier in conjunction with process scale-up, resulting in the purchase of quantities of material exceeding anticipated collaborator demand in the next twelve months. Due to lead times required to secure material, process development and the changing requirements of our collaborators, expenses to recognize excess quantities have fluctuated from period to period and we expect that these period fluctuations will continue in the future. See "Inventory" within our Critical Accounting Policies above for further discussion of our inventory reserve policy.

        We are unable to accurately estimate which potential product candidates, if any, will eventually move into our internal preclinical research program. We are unable to reliably estimate the costs to develop these products as a result of the uncertainties related to discovery research efforts as well as preclinical and clinical testing. Our decision to move a product candidate into the clinical development phase is predicated upon the results of preclinical tests. We cannot accurately predict which, if any, of the discovery stage product candidates will advance from preclinical testing and move into our internal clinical development program. The clinical trial and regulatory approval processes for our product candidates that have advanced or that we intend to advance to clinical testing are lengthy, expensive and uncertain in both timing and outcome. As a result, the pace and timing of the clinical development of our product candidates is highly uncertain and may not ever result in approved products. Completion dates and development costs will vary significantly for each product candidate and are difficult to predict. A variety of factors, many of which are outside our control, could cause or contribute to the prevention or delay of the successful completion of our clinical trials, or delay or prevent our obtaining necessary regulatory approvals. The costs to take a product through clinical trials are dependent upon, among other factors, the clinical indications, the timing, size and design of each clinical trial, the number of patients enrolled in each trial, and the speed at which patients are enrolled and treated. Product candidates may be found to be ineffective or to cause unacceptable side effects during clinical trials, may take longer to progress through clinical trials than anticipated, may fail to receive necessary regulatory approvals or may prove impractical to manufacture in commercial quantities at reasonable cost or with acceptable quality.

        The lengthy process of securing FDA approvals for new drugs requires the expenditure of substantial resources. Any failure by us to obtain, or any delay in obtaining, regulatory approvals would materially adversely affect our product development efforts and our business overall. Accordingly, we cannot currently estimate, with any degree of certainty, the amount of time or money that we will be required to expend in the future on our product candidates prior to their regulatory approval, if such approval is ever granted. As a result of these uncertainties surrounding the timing and outcome of our clinical trials, we are currently unable to estimate when, if ever, our product candidates that have advanced into clinical testing will generate revenues and cash flows.

        We do not track our research and development costs by project. Since we use our research and development resources across multiple research and development projects, we manage our research and

52

Table of Contents

development expenses within each of the categories listed in the following table and described in more detail below (in thousands):

        Research—Research includes expenses associated with activities to evaluate new targets and to develop and evaluate new antibodies, linkers and cytotoxic agents for our products and in support of our collaborators. Such expenses primarily include personnel, fees to in-license certain technology, facilities and lab supplies. Research expenses increased $235,000 to $14.2 million in fiscal year 2010 from fiscal year 2009 and decreased $1.3 million to $14.0 million in fiscal year 2009 from fiscal year 2008. The increase in fiscal year 2010 was principally the result of an increase in stock compensation costs. The decrease in research expense in fiscal year 2009 was principally the result of a decrease in salaries and related expenses due to a reorganization of departments in March 2008 and July 2008, resulting in lower personnel costs included in research expense for the period.

        Preclinical and Clinical Testing—Preclinical and clinical testing includes expenses related to preclinical testing of our own and, in certain instances, our collaborators' product candidates, regulatory activities, and the cost of our own clinical trials. Such expenses include personnel, patient enrollment at our clinical testing sites, consultant fees, contract services, and facility expenses. Preclinical and clinical testing expenses increased $3.1 million to $12.9 million in fiscal year 2010 from fiscal 2009 and $1.5 million to $9.8 million in fiscal year 2009 from fiscal year 2008. The increase in fiscal year 2010 is primarily the result of an increase in clinical trial costs, an increase in consulting fees for regulatory assistance and preclinical studies conducted, and an increase in salaries and related expenses due to the addition of two executive officers and higher salary levels. The increase in fiscal year 2009 was primarily due to an increase in salaries and related expenses due to a reorganization of departments in March 2008 and July 2008, as well as an increase in clinical trials costs.

        Process and Product Development—Process and product development expenses include costs for development of clinical and commercial manufacturing processes for our own and collaborator compounds. Such expenses include the costs of personnel, contract services and facility expenses. Total development expenses decreased $78,000 to $6.0 million in fiscal year 2010 from fiscal year 2009 and expenses increased $306,000 to $6.0 million in fiscal year 2009 from fiscal year 2008. The increase in fiscal year 2009 was primarily the result of the reorganization of departments in July 2008.

        Manufacturing Operations—Manufacturing operations expense includes costs to manufacture preclinical and clinical materials for our own and our collaborators' product candidates, quality control and quality assurance activities and costs to support the operation and maintenance of our conjugate manufacturing facility. Such expenses include personnel, raw materials for our and our collaborators' preclinical studies and clinical trials, development costs with contract manufacturing organizations, manufacturing supplies, and facilities expense. Manufacturing operations expense increased $1.1 million to $17.2 million in fiscal year 2010 from fiscal year 2009 and decreased $14.6 million to $16.1 million in fiscal year 2009 from fiscal year 2008. The increase in fiscal year 2010 was primarily the result of (i) a decrease in overhead utilization from the manufacture of clinical materials on behalf of our collaborators; (ii) an increase in antibody supply and development expenses; and (iii) an increase in stock compensation costs. Partially offsetting these increases, contract service expense decreased and

53

Table of Contents

cost of clinical materials reimbursed decreased. The decrease in fiscal year 2009 was primarily the result of (i) a decrease in supply of DMx and clinical materials to our collaborators; (ii) a decrease in antibody supply and development expenses; (iii) a decrease in contract service expenses; and (iv) a significant decrease in charges incurred related to the write down of raw material inventory. Partially offsetting these decreases, salaries and related expenses increased, depreciation and amortization increased, and overhead utilization from the manufacture of clinical materials on behalf of our collaborators decreased.

        Antibody expense in anticipation of potential future clinical trials, as well as our ongoing trials, was $1.1 million in fiscal year 2010, $503,000 in fiscal year 2009, and $7.4 million in fiscal year 2008. The process of antibody production is lengthy as is the lead time to establish a satisfactory production process at a vendor. Accordingly, costs incurred related to antibody production and development have fluctuated from period to period and we expect these cost fluctuations to continue in the future.

General and Administrative Expenses

        General and administrative expenses for the year ended June 30, 2010 increased $998,000 to $14.9 million from $13.9 million for the year ended June 30, 2009. General and administrative expenses for the year ended June 30, 2008 were $14.3 million. The increase in fiscal year 2010 as compared to fiscal year 2009 was primarily due to an increase in patent expenses, an increase in consulting fees, an increase in directors' fees and an increase in other general corporate expenses, partially offset by a decrease in salaries and related expenses. During fiscal year 2009, the Company recognized a total of $1.6 million in stock compensation expense and other compensation costs related to the former Chief Executive Officer's succession plan and a termination of an executive. The decrease in fiscal year 2009 as compared to fiscal year 2008 was primarily the result of a decrease in rent expense and move-related expenses, partially offset by greater salaries and related expenses. During fiscal year 2008, the Company recognized $1.5 million of expense related to the rental of laboratory and office space in Waltham prior to occupying the space in late March 2008, as well as $799,000 of move-related expenses, classifying such as general and administrative expenses.

Other Income (Expense), net

        Other income (expense), net for the years ended June 30, 2010, 2009, and 2008 is included in the following table (in thousands):

Investment Income, net

        Investment income for the years ended June 30, 2010, 2009 and 2008 was $176,000, $583,000 and $2.2 million, respectively. The decrease in investment income in fiscal year 2010 and fiscal year 2009 from fiscal year 2008 is primarily the result of lower yields on investments reflecting lower market rates and lower average investable balances.

54

Table of Contents

Net Realized (Losses) Gains on Investments

        Net realized (losses) gains on investments were ($33,000) and $39,000, for the years ended June 30, 2009 and 2008, respectively. There were no gains or losses recognized during the year ended June 30, 2010.

Other than Temporary Impairment

        During the years ended June 30, 2009 and 2008, we recognized $516,000 and $535,000, respectively, in charges for the impairment of available-for-sale securities that were determined to be other-than-temporary following a decline in value. No similar charges were recognized during the year ended June 30, 2010.

Other (Expense) Income, net

        Other (expense) income, net for the years ended June 30, 2010, 2009 and 2008 was $(118,000), ($255,000) and $463,000, respectively. During the years ended June 30, 2010, 2009 and 2008, we recorded net (losses) gains on forward contracts of $(219,000), $(234,000) and $699,000, respectively. We incurred $104,000, $(29,000) and $(243,000) in foreign currency translation income (expenses) related to obligations with non-U.S. dollar-based suppliers during the years ended June 30, 2010, 2009 and 2008, respectively.

Liquidity and Capital Resources

Cash Flows

        We require cash to fund our operating expenses, including the advancement of our own clinical programs, and to make capital expenditures. Historically, we have funded our cash requirements primarily through equity financings in public markets and payments from our collaborators, including equity investments, license fees, milestones and research funding. As of June 30, 2010, we had approximately $110.3 million in cash, cash equivalents and marketable securities. Net cash used for operations was $40.6 million, $13.3 million and $20.1 million during the years ended June 30, 2010, 2009 and 2008, respectively. The principal use of cash in operating activities for all periods presented was to fund our net loss.

        Net cash (used for) provided by investing activities was $(882,000), $12.0 million and $15.2 million for the years ended June 30, 2010, 2009 and 2008, respectively, and substantially represents cash inflows from the sales and maturities of marketable securities partially offset by capital expenditures. Capital expenditures were $1.5 million, $1.9 million and $18.0 million for the fiscal years ended June 30, 2010, 2009 and 2008, respectively. The significant capital expenditures for fiscal 2008 were primarily leasehold improvements made to our Waltham, MA facility related to the construction allowance received from the landlord to build out laboratory and office space to our specifications, as well as expansion and improvements of our manufacturing plant in Norwood, MA. The leasehold improvements made to the Waltham, MA facility were paid by the landlord, with such reimbursement recorded as a benefit to cash

55

Table of Contents

used in operations. Capital expenditures for the years ended June 30, 2010 and 2009 consisted primarily of laboratory equipment and computer software applications.

        Net cash provided by financing activities was $81.0 million, $39.4 million and $26.0 million for the years ended June 30, 2010, 2009 and 2008, respectively, which includes the proceeds from the exercise of 634,000, 416,000 and 619,000 stock options, respectively. Also, in May 2010, pursuant to a public offering, we issued and sold 10,350,000 shares of our common stock resulting in net proceeds of $77.5 million. The shares of common stock offered were registered under our existing shelf registration statement on Form S-3 which was filed with the Securities and Exchange Commission in April 2010. In June 2009, pursuant to a public offering, we issued and sold 5,750,000 shares of our common stock resulting in net proceeds of $38 million. In June 2008, pursuant to a securities purchase agreement with a private investor, we issued and sold 7,812,500 shares of our common stock resulting in net proceeds of $24.9 million.

        We anticipate that our current capital resources and expected future collaborator payments will enable us to meet our operational expenses and capital expenditures for fiscal year 2011 and approximately the first half of fiscal 2012. However, we cannot provide assurance that such collaborative agreement funding will, in fact, be received. Should we or our partners not meet some or all of the terms and conditions of our various collaboration agreements, we may be required to pursue additional strategic partners, secure alternative financing arrangements, and/or defer or limit some or all of our research, development and/or clinical projects.

        As mentioned above, on April 9, 2010, we filed a Registration Statement on Form S-3 (Registration No. 333-144488) with the Securities and Exchange Commission. The Securities and Exchange Commission declared the Registration Statement effective on April 22, 2010. Subject to our ongoing obligations under the Securities Act of 1933, as amended, and the Securities Exchange Act of 1934, as amended, the Registration Statement permits us to offer and sell up to an aggregate of $125 million of our common stock, $82.8 million of which we sold in the transaction discussed above.

Contractual Obligations

        Below is a table that presents our contractual obligations and commercial commitments as of June 30, 2010 (in thousands):

⁽¹⁾

Lease agreement was signed on July 27, 2007. In December 2009, we entered into a sublease for 14,100 square feet of our office and laboratory space at 830 Winter Street, Waltham, MA through January 2015. We will receive approximately $2.8 million in minimum rental payments over the remaining term of the sublease, which is not included in the table above.

⁽²⁾

The Company entered into a sub-sublease in May 2008 for the entire space at 148 Sidney Street, Cambridge, MA through October 2010, the remainder of the sublease. Both the sublease and sub-sublease for the 148 Sidney Street facility were terminated as of July 31, 2010. Included in the table above is $233,000 in minimum rental payments owed by the Company through October 2010, however, only approximately $58,000 was paid by the Company for the month of July 2010. The Company was reimbursed this amount pursuant to the sub-sublease.

56

Table of Contents

        In addition to the above table, we are contractually obligated to make potential future success-based regulatory milestone payments in conjunction with certain collaborative agreements. These payments are contingent upon the occurrence of certain future events and, given the nature of these events, it is unclear when, if ever, we may be required to pay such amounts. As a result, these contingent payments have not been included in the table above or recorded in our consolidated financial statements. Further, the timing of any future payment is not reasonably estimable. As of June 30, 2010, the maximum amount that may be payable in the future under such arrangements is approximately $43.2 million.

Recent Accounting Pronouncements

        In October 2009, the FASB issued Accounting Standards Update (ASU) No. 2009-13, Multiple-Deliverable Revenue Arrangements (ASU No. 2009-13). ASU No. 2009-13, which amends existing revenue recognition accounting pronouncements and provides accounting principles and application guidance on whether multiple deliverables exist, how the arrangement should be separated, and the consideration allocated. This guidance eliminates the requirement to establish the fair value of undelivered products and services and instead provides for separate revenue recognition based upon management's estimate of the selling price for an undelivered item when there is no other means to determine the fair value of that undelivered item. Previous accounting principles required that the fair value of the undelivered item be the price of the item either sold in a separate transaction between unrelated third parties or the price charged for each item when the item is sold separately by the vendor. This was difficult to determine when the product was not individually sold because of its unique features. If the fair value of all of the elements in the arrangement was not determinable, then revenue was deferred until all of the items were delivered or fair value was determined. This new approach is effective prospectively for revenue arrangements entered into or materially modified in fiscal years beginning on or after June 15, 2010 (our fiscal year 2011). While we do not expect the adoption of this standard to have a material impact on our financial position and results of operations, this standard may impact us in the event we complete future transactions or modify existing collaborative relationships.

        The provisions of ASC Topic 810, "Consolidation", related to the changes to how a reporting entity determines when an entity that is insufficiently capitalized or is not controlled through voting (or similar rights) should be consolidated will be effective for fiscal years beginning after November 15, 2009 (our fiscal year 2011). Early application is not permitted. We do not expect the adoption of these provisions to have a significant impact on our financial position or results of operations.

        In March 2010, the FASB issued guidance related to the milestone method of revenue recognition, which will codify a method of revenue recognition that has been common practice. Under this method, contingent consideration from research and development activities that is earned upon the achievement of a substantive milestone is recognized in its entirety in the period in which the milestone is achieved. This guidance is effective for annual periods beginning on or after June 15, 2010 but may be adopted as of the beginning of an annual period. Because our current revenue recognition policy for milestone payments is consistent with the FASB's guidance, we do not expect the adoption of this standard will have a material effect on our consolidated financial position, results of operations and cash flows.

Off-Balance Sheet Arrangements

        None.

57

Table of Contents

Item 7A.    Quantitative and Qualitative Disclosure About Market Risk

        We maintain an investment portfolio in accordance with our investment policy. The primary objectives of our investment policy are to preserve principal, maintain proper liquidity to meet operating needs and maximize yields. Although our investments are subject to credit risk, our investment policy specifies credit quality standards for our investments and limits the amount of credit exposure from any single issue, issuer or type of investment. Our investments are also subject to interest rate risk and will decrease in value if market interest rates increase. However, due to the conservative nature of our investments and relatively short duration, interest rate risk is mitigated. We do not own derivative financial instruments in our investment portfolio. Accordingly, we do not believe that there is any material market risk exposure with respect to derivative or other financial instruments that would require disclosure under this item.

        Our foreign currency hedging program uses forward contracts to manage the foreign currency exposures that exist as part of our ongoing business operations. The contracts are denominated in Euros and have maturities of less than one year. Our foreign currency risk management strategy is principally designed to mitigate the future potential financial impact of changes in the value of transactions, anticipated transactions and balances denominated in foreign currency, resulting from changes in foreign currency exchange rates.

        Our market risks associated with changes in foreign currency exchange rates are concentrated primarily in a portfolio of short duration foreign currency forward contracts. Generally, these contracts provide that we receive certain foreign currencies and pay U.S. dollars at specified exchange rates at specified future dates. Although we are exposed to credit and market risk in the event of future nonperformance by a counterparty, management has no reason to believe that such an event will occur.

58

Table of Contents

Item 8.    Financial Statements and Supplementary Data

IMMUNOGEN, INC.

INDEX TO CONSOLIDATED FINANCIAL STATEMENTS

59

Table of Contents

Report of Independent Registered Public Accounting Firm

The Board of Directors and Shareholders of ImmunoGen, Inc.

        We have audited the accompanying consolidated balance sheets of ImmunoGen, Inc. as of June 30, 2010 and 2009, and the related consolidated statements of operations, shareholders' equity, and cash flows for each of the three years in the period ended June 30, 2010. Our audits also included the financial statement schedule listed in the Index at Item 15. These financial statements and schedule are the responsibility of the Company's management. Our responsibility is to express an opinion on these financial statements and schedule based on our audits.

        We conducted our audits in accordance with the standards of the Public Company Accounting Oversight Board (United States). Those standards require that we plan and perform the audit to obtain reasonable assurance about whether the financial statements are free of material misstatement. An audit includes examining, on a test basis, evidence supporting the amounts and disclosures in the financial statements. An audit also includes assessing the accounting principles used and significant estimates made by management, as well as evaluating the overall financial statement presentation. We believe that our audits provide a reasonable basis for our opinion.

        In our opinion, the financial statements referred to above present fairly, in all material respects, the consolidated financial position of ImmunoGen, Inc. at June 30, 2010 and 2009, and the consolidated results of its operations and its cash flows for each of the three years in the period ended June 30, 2010, in conformity with U.S. generally accepted accounting principles. Also, in our opinion, the related financial statement schedule, when considered in relation to the basic financial statements taken as a whole, presents fairly in all material respects the information set forth therein.

        We also have audited, in accordance with the standards of the Public Company Accounting Oversight Board (United States), the effectiveness of ImmunoGen, Inc.'s internal control over financial reporting as of June 30, 2010, based on criteria established in Internal Control—Integrated Framework issued by the Committee of Sponsoring Organizations of the Treadway Commission and our report dated August 27, 2010 expressed an unqualified opinion thereon.

Boston, Massachusetts
August 27, 2010

60

Table of Contents

IMMUNOGEN, INC.

CONSOLIDATED BALANCE SHEETS

AS OF JUNE 30, 2010 AND JUNE 30, 2009

In thousands, except per share amounts

The accompanying notes are an integral part of the consolidated financial statements.

61

Table of Contents

IMMUNOGEN, INC.

CONSOLIDATED STATEMENTS OF OPERATIONS

In thousands, except per share amounts

The accompanying notes are an integral part of the consolidated financial statements.

62

Table of Contents

IMMUNOGEN, INC.

CONSOLIDATED STATEMENTS OF SHAREHOLDERS' EQUITY

In thousands