Cambridge, MA, October 18, 1999 - Therion Biologics Corporation announced today that it has initiated a Phase II clinical trial in collaboration with the National Cancer Institute (NCI) to evaluate two novel therapeutic vaccines for treating melanoma. The trial is designed to evaluate the efficacy of recombinant viral vectors that express tyrosinase, a melanoma-associated antigen that has exhibited promising immunogenic properties in preclinical studies. Like Therion's other therapeutic products, the new tyrosinase vaccines are designed to stimulate potent, tumor-specific immune responses that result in the destruction of cancer cells.

"Recent clinical research has shown that tyrosinase is a particularly promising melanoma antigen due to its ability to stimulate a comprehensive cellular immune response," said Dennis L. Panicali, Ph.D., President and Chief Executive Officer of Therion. "Because this potentially offers an improved therapeutic approach for specifically attacking melanoma cells, we look forward to evaluating these tyrosinase vaccines in their first human clinical study."

Dr. Panicali continued, "This new trial is part of our strategy to rapidly develop and carefully evaluate in patients a number of promising product candidates for treatment of major cancers. Including our recent work in melanoma, Therion and NCI have successfully advanced seven therapeutic vaccines from preclinical research to Phase I and Phase II trials. We look forward to continued clinical growth through this collaboration."

The latest Phase II study, which will be conducted in adult patients with metastatic melanoma, is designed to evaluate clinical responses to multiple immunizations with two potential products, recombinant fowlpox and vaccinia viral vectors encoding tyrosinase. Tyrosinase has been shown to be commonly expressed in metastatic melanoma tumors. Therion's tyrosinase vaccines will be tested alone and in conjunction with interleukin-2 (IL-2), a known immunostimulatory molecule, to evaluate IL-2's potential to enhance the immune response. The study is expected to enroll up to 73 patients.

The trial is being conducted as part of an existing, five-year Cooperative Research and Development Agreement (CRADA) with the NCI to advance the development of immunotherapeutic vaccines for cancer. Suzanne L. Topalian, M.D., Senior Investigator, Surgery Branch of the NCI, will serve as principal investigator for the study; Steven A. Rosenberg, M.D., Chief of the NCI Surgery Branch, will serve as associate investigator.

According to the American Cancer Society, the overall incidence rate for melanoma is increasing faster than that of any other cancer. Melanoma is also the most serious form of skin cancer with 41,600 new cases and 7,300 deaths in the United States in 1998. By the year 2000, an American's lifetime risk of developing melanoma will be one in 75.

Therion Biologics Corporation is engaged in the development of therapeutic vaccines for cancer and preventive vaccines for AIDS. Currently, Therion has seven products in Phase I and Phase II clinical development for the treatment of major cancers, including prostate and colorectal cancers and melanoma. The Company also has a major corporate alliance with Pasteur M�rieux Connaught for the development of therapeutic colorectal cancer and melanoma vaccines. Therion is headquartered in Cambridge, Massachusetts.