Updated from 9:18 a.m. EDT

Shares of Dendreon ( DNDN) soared Tuesday after the company said its prostate cancer vaccine Provenge significantly prolonged the overall survival of patients compared to a placebo.

The Seattle-based drugmaker said the phase III study of Provenge, known as IMPACT, met its primary endpoint with statistical significance. Details of the study are being withheld so that they can be presented at a medical conference April 28.

Dendreon shares were rising 175% to $20.10 in recent trading.

"Survival is the gold standard outcome for oncology clinical trials, and overall survival was the primary endpoint of the IMPACT trial. The positive results from this landmark study provide confirmatory evidence demonstrating that treatment with PROVENGE may prolong survival," said Dendreon CEO Mitch Gold, in a statement.

The company plans to submit the new Provenge data to the U.S. Food and Drug Administration in the fourth quarter. Assuming a six-month review, an approval decision could come in the second quarter of next year.

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.

On a conference call Tuesday morning, Gold said that the Provenge results from the IMPACT study were "unambiguous" and "consistent" with previous studies of the cancer vaccine.

The IMPACT study was designed to reduce the risk of death in prostate cancer patients by 22%. Previous studies of Provenge showed the drug to prolong survival by 4.5 months over placebo.

"We've been saying for years the drug worked, we just did not know how many trials it would take to prove it to the FDA. This morning, we received our answer and it is a great one for guys everywhere with prostate cancer," said David Miller of Biotech Monthly, a biotech research firm that has long championed Dendreon.

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.

In March 2007, and 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 a prior phase III study. 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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