A vaccine to treat prostate cancer came one step closer to approval last week, when an advisory panel to the US Food and Drug Administration said the product was safe and effective.

The panel recommended the FDA approve Provenge (sipuleucel-T), made by Dendreon Corp., for men whose prostate cancer has spread to other parts of the body and no longer responds to hormone therapy. The FDA isn't required to follow the advice of its panels, but often does. A decision on whether to approve Provenge is expected in May.

If approved, Provenge will become the first vaccine designed to treat existing cancer, rather than prevent the disease from occurring. To make the vaccine, a patient's own cells are combined with a protein that revs up the immune system and causes it to attack the tumor.

In a clinical trial of 127 men, Provenge seemed to help vaccinated men live about 4.5 months longer than those given an inactive placebo treatment. However, there was no difference in how long it took for the men's cancers to begin growing again. A second study also found no difference in how long it took the cancers to resume growing and so was stopped early.

Nevertheless, the panel voted 13-4 in favor of saying Provenge is effective. It voted 17-0 in favor of saying the vaccine is safe. Side effects during the clinical trials were mostly mild and included chills, fatigue, fever, and back pain. However, slightly more than 5% of men given Provenge suffered a stroke, compared to none of those on placebo.

If approved, Provenge would provide another treatment option for men who currently have few options. Once prostate cancer has become resistant to hormone therapy, it is considered incurable. The chemotherapy drug docetaxel can extend survival by several months and relieve cancer pain. Bisphosphonate drugs can reduce pain from cancer that has spread to the bone and can slow the growth of the cancer.

Because Provenge has not yet been approved by the FDA, it is only available through clinical trials. Dendreon is conducting a phase 3 trial called IMPACT to test Provenge further in men with metastatic prostate cancer that has worsened while on hormone therapy.