Dendreon's ( DNDN) prostate cancer vaccine Provenge appears to cut the risk of death compared with a placebo, but the interim results were not robust enough to stop the ongoing clinical trial early.

Investors focused on the promising Provenge survival data Monday morning and looked past the disappointment that Dendreon would be unable to seek early approval for the experimental cancer vaccine.

Shares of Dendreon were up 65% to $8.58 in recent trading.

Monday's results came from an interim analysis of a phase III study of Provenge in patients with advanced prostate cancer. An independent data monitoring committee informed Dendreon that the study should continue to a final analysis planned for the middle of 2009.

At the same time, however, the data crunchers reported that prostate cancer patients treated with Provenge had a 20% reduction in the risk of death compared with patients treated with a placebo.

If Provenge can reduce the risk of death by 22% compared with placebo at the final analysis, the study will be successful, Dendreon said Monday.

In previous studies, Provenge has been shown to have a delayed positive impact on survival. And so far, patients in the ongoing Provenge study, dubbed IMPACT, are acting very much like patients in the old Provenge studies. These two factors prompted Dendreon CEO Mitch Gold to call today's results "encouraging."

"We're looking forward to seeing the final results next year," he said.

Provenge's ability to reduce the risk of death by 20% looks impressive at the interim analysis, but the result was neither statistically significant nor strong enough to stop the study early. Achieving a 22% reduction in the risk for death at the final analysis -- and making that result statistically significant -- could still be a struggle for Dendreon.

The survival benefit at the interim analysis announced Monday was actually lower than the company had predicted when it designed the IMPACT study. This means that for the final analysis of the IMPACT study to be successful and statistically significant, the net survival benefit for those Provenge patients remaining in the study will have to be greater than the Provenge patients who have already died.

This feat could prove challenging given changes made by Dendreon earlier this year to accelerate the timing of the final analysis of the IMPACT study.

At the time of publication, Feuerstein's Biotech Select model portfolio was long/short XXX.

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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