Cambridge, MA, November 19, 2003 � Therion Biologics Corporation has initiated a multicenter randomized, double-blind Phase II trial to test the safety and efficacy of PROSTVAC^�-VF as an immunotherapy for prostate cancer. PROSTVAC-VF targets prostate specific antigen (PSA), a protein produced by prostate cancer cells. PROSTVAC-VF is designed to stimulate and sustain an immune response against prostate cancer tumor cells. The study builds on the clinical data of a previous Phase II trial which demonstrated that therapy with a PSA-based vaccine stabilized PSA levels in 52% of early metastatic prostate cancer patients with no significant adverse events attributed to the study vaccine reported. The current study plans are to enroll 120 patients with metastatic prostate cancer at approximately 40 participating treatment centers across the United States.

�This Phase II trial delivers on Therion�s commitment to advance the clinical development of immunotherapeutic vaccines towards commercialization,� said Mark Leuchtenberger, President and Chief Executive Officer of Therion Biologics Corporation. �This Phase II trial is designed to lay the foundation for moving our prostate cancer vaccine, PROSTVAC-VF, into pivotal Phase III trials. An additional Phase II trial for our pancreatic cancer vaccine, PANVAC-VF, is planned for early 2004.� PROSTVAC-VF consists of two injectable components, which are designed to �prime� or generate an initial immune response against cells displaying PSA, and to then �boost� or enhance and sustain that immune response. PROSTVAC also incorporates TRICOM, Therion�s immunological co-stimulatory system. The TRICOM component of the vaccine consists of three co-stimulatory molecules known to elicit strong cellular immune responses necessary for complete tumor destruction.

Study participants will be randomized in a double-blind fashion to receive either PROSTVAC-VF or placebo. Patients in the treatment group will receive an initial �priming� dose of PROSTVAC-VF in combination with GM-CSF, followed by six monthly �booster� vaccinations. Patients randomized to the control arm will receive inactive vaccine, with neither the specificity for PSA nor the co-stimulatory TRICOM molecules. The primary endpoint of the double-blind, randomized study is progression-free survival. The study will be led by Philip Kantoff, M.D., Director of the Lank Center for Genitourinary Oncology and Chief, Division of Solid Tumor Oncology at the Dana-Farber Cancer Institute.

�Prostate cancer is the second leading cause of cancer death in men and presents a serious treatment challenge when it becomes metastatic and spreads throughout the body,� said Dr. Kantoff. �Considering the lack of effective treatment options for this population, I am excited to lead this trial.�

About Therion Biologics Corporation
Therion Biologics Corporation develops therapeutic vaccines for cancer and preventive vaccines for AIDS. The Company has two lead programs involving multiple clinical trials: PANVAC-VF for pancreatic cancer and PROSTVAC^�-VF for prostate cancer. Therion is also applying its technology platform to develop vaccines to treat melanoma, breast cancer and other solid tumors. In addition to studies conducted under collaboration with the National Cancer Institute, the Company�s strategic partners include a global network of leading clinical institutions and Aventis Pasteur, Ltd., which is developing ALVAC-CEA/B7.1, a clinical candidate for the treatment of colon cancer. Therion is headquartered in Cambridge, Massachusetts.