Dendreon added more evidence Monday to its case for Provenge, the Seattle company's experimental prostate-cancer vaccine. In a report detailing...

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Dendreon added more evidence Monday to its case for Provenge, the Seattle company’s experimental prostate-cancer vaccine.

In a report detailing the combined results of two previously announced clinical trials, the company said that men who received its drug showed three-year survival rates more than twice those of patients receiving a placebo.

Of 147 men given Provenge in the late-stage clinical trials, 33 percent were alive three years later, compared with 15 percent of men on a placebo.

The final results of the combined trials were released at a cancer conference in Paris, quantifying what Dendreon had previously described as a “substantially greater” number of Provenge patients alive at the three-year follow-up.

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“This pretty much nails it down,” said Paul Latta, an analyst with McAdams Wright Ragen in Seattle. “It basically offers a substantial body of proof of the concept.”

Investors, however, pushed Dendreon’s stock down 43 cents, or 6.5 percent, to close at $6.16, on more than twice the usual volume. The stock has traded between $4.31 and $13.36 in the past year.

The skepticism could be because Dendreon is “rewriting the book” on immunotherapies such as Provenge, said David Miller, president of Biotech Stock Research in Seattle.

“No drug of this type has ever worked before,” said Miller, in Paris for the announcement.

The treatment is designed to stimulate a patient’s immune system to attack cancer. It has shown minimal side effects of fever and chills lasting a day or two. That compares to the nausea, hair loss and anemia that accompany the standard late-stage therapy, chemotherapy.

Dr. Mitchell Gold, Dendreon president and CEO, said during a conference call with analysts Monday that Provenge is “a better tolerated and less active form of therapy.”

Dr. Celestia Higano, a University of Washington professor who presented the data, said the treatment could be a “useful alternative for men prior to initiating chemotherapy.”

Dendreon plans to seek approval for the treatment from the U.S. Food and Drug Administration next year; a decision could come a year later.

More than 1 million men have prostate cancer in the United States. It kills 30,000 annually.

McAdams Wright Ragen, which maintains a market in Dendreon stock, projects Provenge sales of $225 million in 2007 if the drug gets FDA approval.

Benjamin J. Romano: 206-464-2149 or