Cell Genesys and Medarex Announce Encouraging Follow-Up Results from a Phase 1 Combination Therapy Trial With GVAX Immunotherapy for Prostate Cancer and Ipilimumab (MDX-010) Antibody

Announced encouraging follow-up data from the ongoing Phase 1 clinical trial of Cell Genesys' GVAX immunotherapy for prostate cancer, administered in combination with Medarex's fully human anti-CTLA-4 antibody, ipilimumab (MDX-010).

Cell Genesys, Inc. and Medarex, Inc. today announced encouraging follow-up data from the ongoing Phase 1 clinical trial of Cell Genesys' GVAX immunotherapy for prostate cancer, administered in combination with Medarex's fully human anti-CTLA-4 antibody, ipilimumab (MDX-010). Medarex is jointly developing ipilimumab with Bristol- Myers Squibb Company. Twelve patients with advanced prostate cancer have completed treatment to date. Of the six patients treated in the two highest dose groups, antitumor activity has been observed in five patients, including prostate-specific antigen (PSA) declines of greater than 50% that were maintained in four of these patients for at least six months, with the longest response ongoing at more than 12 months. Moreover, clinical evidence of antitumor activity has been observed in three of these five PSA responders, including improvement of multiple lesions on bone scan, resolution of abdominal lymph node disease by CT scan, and improvement in pain due to bone metastases, respectively. Two additional patients have had stable disease on bone scan for at least three months. All five patients with PSA declines experienced either Grade 2 or 3 immune-mediated endocrine deficiencies similar in type to those previously reported with ipilimumab therapy and were successfully treated with standard hormone replacement therapy. Two patients requiring thyroid replacement therapy were successfully tapered off after recovery of thyroid function, with one patient subsequently maintaining a PSA response. One patient developed a Grade 3 dose-limiting pulmonary alveolitis. A maximum tolerated dose for the combination therapy has not yet been defined. Immunomonitoring studies showed that the combination therapy enhanced T cell and dendritic cell activity, which was more pronounced at the higher dose levels. These data were presented today by Winald Gerritsen, M.D., Ph.D., director of the University Hospital Vrije Universiteit Cancer Center in Amsterdam, at the American Society of Clinical Oncology (ASCO) Prostate Cancer Symposium being held in Orlando, FL.

This dose-escalation combination trial is currently expected to enroll a total of approximately 25 to 30 patients with metastatic, hormone-refractory prostate cancer (HRPC). The treatment dose for GVAX immunotherapy for prostate cancer used in this combination trial is the same dose currently being tested in Cell Genesys' VITAL-1 Phase 3 trial. The treatment dose for ipilimumab was escalated in sequential groups of three patients and has now reached dose levels associated with therapeutic activity. The primary endpoints of the study are safety and the determination of a maximum tolerated dose for the combination therapy. Efficacy endpoints include time to clinical disease progression, time to PSA progression and PSA response, immune response to GVAX, reduction in metabolic bone activity and survival. The study is being conducted under a research collaboration agreement between Cell Genesys, Inc. and Medarex, Inc. signed in 2003, which provides an equal sharing of expenses for the above clinical trial. In 2005 Medarex entered into a collaboration agreement for the development and commercialization of ipilimumab with Bristol-Myers Squibb Company.

"We continue to be encouraged by the interim results of this trial which now include evidence for the durability of the antitumor responses along with the frequency of such responses, observations which we believe have not been previously experienced for a combination of immunotherapies in advanced prostate cancer," stated Rob Dow, M.D., chief medical officer of Cell Genesys. "Moreover, we believe these findings provide further support for our ongoing Phase 3 trials of GVAX immunotherapy for prostate cancer and additional validation for the GVAX immunotherapy platform in general."

"We are pleased with the interim results and believe that the data continues to support the use of ipilimumab in combination with other treatment modalities, including GVAX immunotherapy," said Geoffrey M. Nichol, senior vice president, Product Development at Medarex.

About GVAX immunotherapy for prostate cancer

GVAX immunotherapy for prostate cancer is currently being studied as a single agent and in combination with docetaxel chemotherapy in two Phase 3 clinical trials expected to enroll approximately 1200 patients with metastatic HRPC. Cell Genesys received Special Protocol Assessments (SPA) from the Food and Drug Administration (FDA) for each of the Phase 3 studies and recently also received Fast Track designation for the product. Cell Genesys' ongoing Phase 3 program is supported by the median survival results from two, independent, multi-center Phase 2 clinical trials in approximately 115 patients that are not only consistent with each other, but also compare favorably to the previously published median survival of 18.9 months for metastatic HRPC patients treated with Taxotere(R) (docetaxel) chemotherapy plus prednisone, the current standard of care. GVAX immunotherapy for prostate cancer is comprised of two prostate cancer cell lines that have been modified to secrete GM-CSF (granulocyte-macrophage colony stimulating factor), an immune stimulatory hormone, and irradiated for safety. GVAX cancer immunotherapy for prostate cancer is being developed as a non patient- specific, "off-the-shelf" pharmaceutical product.

Clinical Trials Information for GVAX:

Patients seeking information about clinical trials of GVAX immunotherapy for prostate cancer can obtain information by calling 1.800.648.6747 ext. 3210, and also by checking www.clinicaltrials.gov (keyword: GVAX) and by visiting the company's website at www.cellgenesys.com.

About Ipilimumab

Ipilimumab (also known as MDX-010) is a fully human antibody against human CTLA-4, a molecule on T cells that is believed to be responsible for suppressing the immune response. Medarex and Bristol-Myers Squibb are investigating the potential of ipilimumab to enable the immune systems of cancer patients to help suppress tumor growth. Ipilimumab is currently in three separate registrational studies for metastatic melanoma as a second-line monotherapy treatment, as a first-line treatment in combination with dacarbazine, and as a second-line treatment in combination with a melanoma- peptide vaccine. Ipilimumab is also involved in multiple Phase II clinical trials to investigate the product's potential activity in other tumor types, as well as in combination studies with chemotherapy, immunotherapy and vaccines. Further information regarding the Medarex ipilimumab program can be found in Medarex's public disclosure filings with the U.S. Securities and Exchange Commission (SEC).

Clinical Trials Information for Ipilimumab:

For study enrollment information, in the United States and Canada contact 1-866-892-1BMS Ext. 342 or www.clinicaltrials.gov. Outside of the United States and Canada, call 1-941-906-4711 Ext. 342.

About Cell Genesys

Cell Genesys is focused on the development and commercialization of novel biological therapies for patients with cancer. The company is currently pursuing two clinical stage product platforms -- GVAX cancer immunotherapies and oncolytic virus therapies. Ongoing clinical trials include Phase 3 trials of GVAX immunotherapy for prostate cancer, Phase 2 trials of GVAX immunotherapy for pancreatic cancer and leukemia, and a Phase 1 trial of CG0070 oncolytic virus therapy for bladder cancer. Cell Genesys continues to hold an equity interest in its former subsidiary, Ceregene, Inc., which is developing gene therapies for neurodegenerative disorders. Cell Genesys is headquartered in South San Francisco, CA and has its principal manufacturing operation in Hayward, CA. For additional information, please visit the company's website at www.cellgenesys.com.

About Medarex

Medarex is a biopharmaceutical company focused on the discovery, development and potential commercialization of fully human antibody-based therapeutics to treat life-threatening and debilitating diseases, including cancer, inflammation, autoimmune disorders and infectious diseases. Medarex applies its UltiMAb(R) technology and product development and clinical manufacturing experience to generate, support and potentially commercialize a broad range of fully human antibody product candidates for itself and its partners. Over thirty of these therapeutic product candidates derived from Medarex technology are in human clinical testing or have had INDs submitted for such trials, with six of the most advanced product candidates currently in Phase III clinical trials. Medarex is committed to building value by developing a diverse pipeline of antibody products to address the world's unmet healthcare needs. For more information about Medarex, visit its website at www.medarex.com.

Cautionary Statements

For Cell Genesys: Statements made herein about the company, other than statements of historical fact, including statements about the company's progress, financial results, timing and results of clinical trials and preclinical programs, and the nature of product pipelines are forward-looking statements and are subject to a number of uncertainties that could cause actual results to differ materially from the statements made, including risks associated with the success of clinical trials and research and development programs, the regulatory approval process for clinical trials, competitive technologies and products, patents, continuation of corporate partnerships and the need for additional funding. For information about these and other risks which may affect Cell Genesys, please see the company's Annual Report on Form 10-K for the year ended December 31, 2005 filed on March 13, 2006 as well as Cell Genesys' reports on Form 10-Q and 8-K and other reports filed from time to time with the Securities and Exchange Commission. The company assumes no obligation to update the forward-looking information in this press release.

For Medarex: Except for the historical information presented herein, matters discussed herein may constitute forward-looking statements that are subject to certain risks and uncertainties that could cause actual results to differ materially from any future results, performance or achievements expressed or implied by such statements. Statements that are not historical facts, including statements preceded by, followed by, or that include the words "potential"; "believe"; "anticipate"; "intend"; "plan"; "expect"; "estimate"; "could"; "may"; or similar statements are forward-looking statements. Medarex disclaims, however, any intent or obligation to update these forward-looking statements. Risks and uncertainties include risks associated with product discovery and development, uncertainties related to the outcome of clinical trials, slower than expected rates of patient recruitment, unforeseen safety issues resulting from the administration of antibody products in patients, uncertainties related to product manufacturing as well as risks detailed from time to time in Medarex's public disclosure filings with the U.S. Securities and Exchange Commission (SEC), including its Annual Report on Form 10-K/A for the fiscal year ended December 31, 2005 and subsequent Quarterly Reports on Form 10Q/A and Form 10-Q. There can be no assurance that such development efforts will succeed or that other developed products will receive required regulatory clearance or that, even if such regulatory clearance were received, such products would ultimately achieve commercial success. Copies of Medarex's public disclosure filings are available from its investor relations department.

NOTE: Cell Genesys(R) and the Cell Genesys logo are registered trademarks of Cell Genesys, Inc. All rights are reserved.

Medarex(R), the Medarex logo and UltiMAb(R) are registered trademarks of Medarex, Inc. All rights are reserved.

SOURCE: Cell Genesys, Inc

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