Updated from 3:10 p.m. EDT

CHICAGO -- Dendreon's ( DNDN) cancer drug Provenge extended the survival of patients with advanced prostate cancer patients by more than four months, according to results from a closely watched clinical trial being presented this afternoon at a medical meeting here.

Provenge could become the first so-called cancer immunotherapy or cancer "vaccine" approved for use in the U.S. Dendreon plans to re-file Provenge with the U.S. Food and Drug Administration in the fourth quarter. The agency refused to approve Provenge in 2007.

Provenge is not a cure for advanced prostate cancer but the 4.1-month survival benefit nearly matches the result reported in a previous clinical trial of Provenge and exceeds the survival advantage provided by Taxotere, a chemotherapy drug marketed by Sanofi Aventis ( SNY).

Dr. David Penson, an investigator in the Provenge study from the University of Southern California, will be discussing detailed results from the phase III study of Provenge later this afternoon at the annual meeting of the American Urological Association. A media embargo on some of the data was lifted prior to the presentation at 3:20 pm EDT.

Dendreon shares have more than tripled since the company announced on April 14 that the Provenge study, known as IMPACT, was a success. The stock was up as much as 19% Tuesday before falling 45% to $11.81 right before trading was halted in advance of the Provenge data release.

The IMPACT study enrolled 512 men with advanced prostate cancer who no longer responded to hormone therapy. The primary endpoint of the study was to determine whether treatment with Provenge prolonged overall survival of patients compared to a placebo.

Provenge did just that. Median overall survival for patients treated with Provenge was 25.8 months compared to a 21.7-month median survival for patients in the control arm of the study.

After three years, 31.7% of Provenge-treated patients were alive compared to 23% for control patients -- a 38% improvement.

Expressed another way, Provenge reduced the risk of death by 22.5% compared to the control arm. The survival benefit was statistically significant with a "p value" of 0.032. This means the odds were less than 3.2% that the outcome resulted from random chance.

"The results from this landmark study confirm that PROVENGE prolongs survival with a favorable benefit to risk profile," said Dr. Paul Schellhammer, the principal investigator of the Provenge study and a professor of urology at Eastern Virginia Medical School.

"The treatment effect on overall survival was clinically meaningful,robust and consistent across subpopulations, and the effect on long-term survival was particularly impressive. If approved, PROVENGE has the potential to fill a large unmet medical need and create a new paradigm using immunotherapy for the treatment of men with advanced prostate cancer."

Provenge is a cancer immunotherapy designed to prime a patient's immune system to recognize a certain protein expressed on prostate cancer cells. When T-cells in the immune system encounter this protein, they recognize it as foreign and attack.

Previous phase III studies of Provenge showed the drug to prolong survival by 4.3 months over placebo, but the outcome was cast in doubt because the studies were not originally designed to assess the drug's ability to prolong survival.

In March 2007, an advisory committee of outside experts convened by the FDA recommended the approval of Provenge as a treatment for prostate cancer based on clinical data from those prior phase III studies. But in May of that year, the FDA rejected the panel's recommendation and refused to approve Provenge until additional survival data could be compiled from the IMPACT study.

Adam Feuerstein writes regularly for TheStreet.com. In keeping with TSC's editorial policy, he doesn't own or short individual stocks, although he owns stock in TheStreet.com. He also doesn't invest in hedge funds or other private investment partnerships. Feuerstein appreciates your feedback; click here to send him an email.

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