Shares of biotechnology company Cell Genesys (CEGE) jumped 20% Friday after the company reported clinical tests showed prostate cancer patients not only had an immune response to one of the company’s vaccines, GVAX, but they also lived twice as long as those who did not have an immune response.

GVAX is designed to fight cancer by boosting the immune system response, and it did just that in 34 of 80 patients in the trial. Those 34 patients lived an average of 43 months, compared to the average of 21 months that patients without the response lived. Apparent associations between the presence of antibody responses and survival were shown to be independent of both dose and duration of treatment.

“Since GVAX immunotherapy for prostate cancer is a multi-antigen product that can induce a broad immune response, we believe we have a unique opportunity to identify the widest possible array of specific antibody responses that may be associated with clinical benefit,” said Peter K. Working, Ph.D., senior vice president of research and development at Cell Genesys.

GVAX, which is administered via intradermal injections on an outpatient basis, is an immunotherapy comprised of whole tumor cells that have been modified to secrete an immune stimulatory factor. More than 600 patients have been treated with GVAX, and Cell Genesys is currently testing the drug on 626 more patients to compare its efficacy to that of chemotherapy and the corticosteroid drug prednisone.

Shares of the San Francisco, Calif.-based company closed at $2.36, up from $1.97 the day before.

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