April 29 (Bloomberg) -- Dendreon Corp. gained 94 percent in market value, wiping out yesterday’s 45 percent plunge that caused a trading halt before the biotechnology company said its lead drug candidate worked against prostate cancer.

The shares climbed to $22.94 at 4:30 p.m. New York time, from $11.81 yesterday, when the Nasdaq Stock Market halted trading in the company at 1:27 p.m. The pause ended after U.S. exchanges officially closed. Almost 5 million shares changed hands after hours, according to data on Nasdaq’s Web site.

The drug, Provenge, extended the lives of men with advanced prostate cancer by four months in a study that Seattle-based Dendreon presented yesterday to a meeting of the American Urological Association in Chicago. Provenge, if approved, would be the first drug designed to train the body’s immune system to attack cancer cells like a virus. The medicine stands to generate $2 billion a year in revenue, said Joel Sendek, an analyst for Lazard Capital Markets Ltd. in New York.

“I feel sorry for whoever sold the stock at $11,” Paul Latta, an analyst at McAdams Wright Ragen Inc. in Seattle, in an interview. “There were a lot of people on pins and needles, and once the shares started to go down there may have been some panic.”

Nasdaq investigated 2 minutes’ worth of transactions in Dendreon yesterday and let them stand after initially suspecting they were the result of a brokerage error, the exchange said in a statement.

Wayne Lee, a Nasdaq spokesman, and Brendan Intindola, a spokesman for the Financial Industry Regulatory Authority, declined to comment today.

Investment Losses

Dennis Thorfeldt, a retired computer systems analyst in Wheaton, Illinois, was among investors hurt yesterday by the swing in Dendreon stock price. Thorfeldt, 74, said he had purchased shares during the past week paying at least $17.70 after learning about Provenge from members of his cancer- patients support group.

Thorfeldt said he sought to protect the investment by entering a so-called stop-loss orders that would automatically sell some shares if the stock dropped under a specific price. All of his holdings were sold during the downdraft yesterday, the majority of the shares for $9.31, he said.

“I was shocked by the low price,” Thorfeldt said in a telephone interview. “I strongly suspect that there was some kind of market manipulation. I have no idea how it would be done, but I strongly suspect that’s the reason that I got sucked into this.”

Survival Data

Dendreon released survival data from the Provenge trial, called IMPACT, at 2 p.m. New York time yesterday. Additional details on side effects, presented at 3:20 p.m. by a trial investigator at the meeting in Chicago, weren’t released to the public by the company until a 5:30 p.m. Webcast.

Officials from Dendreon, the American Urological Association and researchers said they were following usual procedures for releasing data from medical meetings and avoiding conflicts.

“We handled this the same way we handle any other presentation,” Wendy Waldsachs Isset, a spokeswoman for the American Urological Association, said today. AUA issues press releases at the start of presentations at the meeting, and companies are free to do the same, she said.

David Penson, an associate professor of urology at the University of Southern California in Los Angeles, presented the data in Chicago. He said he was a last-minute choice to present the Provenge data after the AUA told Dendreon another researcher, Paul Schellhammer, couldn’t speak about the trial because he has financial ties to the company.

‘Favorable Safety’

Company spokeswoman, Katherine Stueland, said today that Dendreon scheduled its Webcast to present all the data “as soon as official AUA activities were over.”

Findings from the trial showed “Provenge exhibited a favorable safety profile,” the company said in a 2 p.m. statement yesterday. Side effects seen more commonly with Provenge than with placebo were chills, fever, and headache, according to the statement.

During the Webcast at 5:30 p.m., when detailed data from the slides was released to the public, Dendreon said the percentages of patients who experienced serious adverse events were about the same for both treatment groups: 24 percent for Provenge and 23.8 percent for placebo.

“I haven’t been able to find anything negative in any of the data,” said Reni Benjamin, an analyst with Rodman & Renshaw in New York, in an interview. “We all go to the meetings because the slides are rarely available unless you’re there.”

To contact the reporters on this story: Lisa Rapaport in New York at lrapaport1@bloomberg.net; Edgar Ortega in New York at ebarrales@bloomberg.net

Last Updated: April 29, 2009 16:35 EDT