November 22, 2002
ImmunoGen, Inc. and British Biotech plc Announce Completion of huN901-DM1 Phase I Study

CAMBRIDGE, Mass., Nov. 22 /PRNewswire-FirstCall/ -- ImmunoGen, Inc. (Nasdaq: IMGN) and British Biotech plc (LSE: BBG, Nasdaq: BBIOY) today announced completion of the Phase I portion of the Phase I/II clinical study of huN901-DM1 (BB-10901), a monoclonal antibody-based chemotherapy agent designed for the treatment of small-cell lung cancer (SCLC). The study, which is being conducted at two cancer research centers in the United States, is investigating once-weekly treatment with the agent.

Data from the Phase I portion of the study are to be presented today at an international medical conference in Frankfurt, Germany by Dr. Anthony Tolcher of the Cancer Therapy and Research Center (CTRC) in San Antonio, Texas. Dr. Tolcher, one of the study's Principal Investigators, will present the findings at the "Molecular Targets and Cancer Therapeutics" symposium, which has been organized jointly by the European Organization for Research and Treatment of Cancer (EORTC), the National Cancer Institute (NCI), and the American Association for Cancer Research (AACR). In addition to the CTRC, patients in the Phase I study were treated at the University of Texas MD Anderson Cancer Center in Houston.

Dr. Tolcher's poster presentation concludes that huN901-DM1 can be safely and feasibly administered for repetitive courses at doses comparable to those that produced antitumor activity in preclinical models. The Phase I study has determined the maximum tolerated dose (MTD) to be 60 mg/m2 under the dosing regimen studied. At this dose level and below, toxicities were modest (in particular, no hematological toxicity was observed) and preliminary evidence of anti-tumor activity was noted.

Progression to the Phase II stage of this study is now being discussed with regulators in light of the findings in the Phase I portion.

Mitchel Sayare, Ph.D., ImmunoGen Chairman and CEO, commented, "British Biotech continues to make solid progress in the development of huN901-DM1. Phase II evaluation will provide initial information on the efficacy of huN901-DM1 in the treatment of small-cell lung cancer."

huN901-DM1 is a conjugate of the cytotoxic maytansinoid drug, DM1, with the humanized monoclonal antibody, huN901. The drug is designed to selectively kill certain types of cancer cells including those found in small-cell lung cancer tumors. British Biotech acquired rights to develop and commercialize huN901-DM1 for Europe and Japan under an agreement with ImmunoGen in May 2000. ImmunoGen retained commercialization rights for the U.S. and the rest of the world. A second Phase I study, to assess the safety and determine the MTD of huN901-DM1 when administered on a more frequent basis, is being conducted in the United Kingdom.

About ImmunoGen, Inc.

ImmunoGen, Inc. develops targeted anticancer biopharmaceuticals. The Company's Tumor-Activated Prodrug (TAP) technology uses tumor-targeting antibodies to deliver a highly potent, cell-killing agent specifically to cancer cells. Two ImmunoGen-developed TAP products are in clinical trials: cantuzumab mertansine, which is licensed to GlaxoSmithKline, and huN901-DM1, which is licensed to British Biotech in certain territories. ImmunoGen helps fund its programs by licensing its TAP technology to other companies. Several companies are developing TAP products that use ImmunoGen's TAP technology with the partner's antibody: Boehringer Ingelheim (bivatuzumab mertansine), Millennium (MLN2704), and Genentech (Trastuzumab-DM1). ImmunoGen also has multitarget agreements with Genentech, Abgenix, and Millennium.

About British Biotech plc

British Biotech is a research and development stage pharmaceuticals company aiming to develop and commercialize specialist drugs for serious illnesses. It currently has four products active in patient trials, supplemented by focused drug discovery research programs. British Biotech's principal research activities, based on the proprietary position it has established in the discovery of inhibitors of human and bacterial metalloenzymes, are in the field of inflammatory disease, in collaboration with Serono; and the treatment of infectious diseases, in collaboration with GeneSoft.

For ImmunoGen, Inc.

This press release includes forward-looking statements based on management's current expectations. For these statements, ImmunoGen claims the protection of the safe harbor for forward-looking statements provided by the Private Securities Litigation Reform Act of 1995. Various factors could cause the Company' s actual results to differ materially from those discussed or implied in the forward-looking statements and you are cautioned not to place undue reliance on these forward-looking statements, which are current only as of the date of this release. Factors that could cause future results to differ materially from such expectations include, but are not limited to: the success of the Company's research and clinical development processes; the difficulties inherent in the development of pharmaceuticals, including uncertainties as to the timing, expense and results of preclinical studies and clinical trials; the Company's dependence upon existing and potential collaborative partners; uncertainty as to whether the Company's potential products or those of the Company's collaborators will succeed in entering human clinical trials and uncertainty as to the results of such trials; the risk that the Company may not be able to obtain regulatory approvals necessary to commercialize their product candidates; the potential development by competitors of competing products and technologies; uncertainty whether the Company's TAP technology will produce safe, effective and commercially viable products; and other factors more fully described in ImmunoGen's Annual Report on Form 10-K for the fiscal year ended June 30, 2002 and other current reports filed with the Securities and Exchange Commission.

For British Biotech plc

This news release contains forward-looking statements that reflect the Company's current expectations regarding future events. Forward-looking statements involve risks and uncertainties. Actual events could differ materially from those projected herein and depend on a number of factors including the success of the Company's research strategies, the applicability of the discoveries made therein, the successful and timely completion of clinical studies and the uncertainties related to the regulatory process.

SOURCE ImmunoGen, Inc.

CONTACT: Carol Hausner, Senior Director, Investor Relations and Corporate Communications of ImmunoGen, Inc., +1-617-995-2500; or Pete Holmberg of Rx Communications Group, LLC, +1-917-322-2164,; or Tony Weir, Finance Director of British Biotech plc +01865 781166; or Duke Coffey of GA Kraut, +1-212-696-5600; or John Olsen of Hogarth Partnership, +020 7357 9477



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