GAITHERSBURG, MD, and CONCORD, CA, April 21, 2004 – MedImmune, Inc. (Nasdaq: MEDI) and Cerus Corporation (Nasdaq: CERS) today announced an agreement to develop and commercialize a novel therapeutic vaccine designed to treat cancers of the breast, prostate and colon, as well as metastatic melanomas.

“Cerus Corporation's therapeutic vaccine technology greatly complements MedImmune's existing program targeting the EphA2 protein in cancer,” commented Peter Kiener, D.Phil., vice president, research at MedImmune. “Because EphA2 is overexpressed by many types of human cancers, we believe Cerus' technology may be employed to develop a vaccine that can stimulate the immune system to attack cancerous cells expressing EphA2.”

Cerus' vaccine technology can be engineered to target specific cancer antigens, such as EphA2, and has been shown to induce potent and specific immune responses in laboratory tests. In preclinical efficacy studies, the resulting EphA2 vaccine induced a robust anti-tumor response and substantial prolongation of life in mice. MedImmune and Cerus recently announced the results of pre-clinical tests demonstrating that 80 percent of treated mice survived for more than 43 days following tumor implantation, while untreated controls had a median survival time of approximately 20 days. The findings were presented at the annual meeting of the American Association for Cancer Research.

“Targeting EphA2 with our vaccine platform demonstrates the tremendous potential for our therapeutic vaccine approach,” said Stephen T. Isaacs, president and chief executive officer, Cerus Corporation. “We are excited to enter into a collaboration with MedImmune, a leading biotechnology company with a track record of developing successful products and an important new cancer target in EphA2.”

Under the terms of the agreement, MedImmune is responsible for clinical testing, manufacturing and commercialization of any product resulting from the collaboration. Cerus will participate in the development of the therapeutic vaccine, receiving an up-front payment and development funding, as well as milestone payments and royalties on future product sales. Additional terms were not disclosed.