The new drug, which will be called Xtandi, was developed by Medivation, a small San Francisco pharmaceutical company, in partnership with the Japanese firm Astellas Pharma.
In clinical trials, men who received the drug, which was previously known as MDV3100, lived a median of 18.4 months, nearly five months longer than the median of 13.6 months for those who received a placebo.
While the approval was not a surprise, its timing was. The F.D.A. approved the drug after only a three-month review, three months ahead of the late November deadline. This is fairly rare, although a number of other cancer drugs have been approved at least a month ahead of deadline in recent years.
"The need for additional treatment options for advanced prostate cancer continues to be important," Dr. Richard Pazdur, the director of the agency's cancer drug office, said in a statement.
Xtandi is one of several new prostate cancer drugs that have come to market in the last two years after a long fallow period. While the new drugs have been good for men with the disease, they could add billions of dollars to the nation's medical bills.
The price of Xtandi was not immediately available, but Geoffrey Porges, an analyst at Sanford C. Bernstein Company, estimated it would be about $6,000 a month.
Before 2004, the only drug shown to prolong the survival of men with advanced prostate cancer was the chemotherapy drug docetaxel. Now there are four others on the market — Sanofi's Jevtana, Dendreon's Provenge, Johnson Johnson's Zytiga and Xtandi, which is known generically as enzalutamide.
Xtandi is expected to compete most directly with Zytiga. Both are pills, work in similar ways and are approved for the same patient population — men whose cancer has spread elsewhere in the body or recurred despite treatment aimed at suppressing production of the hormone testosterone, which fuels prostate cancer growth.
Both drugs are approved for men who have already tried docetaxel, though both Medivation and Johnson Johnson hope to eventually win approval for their drugs to be used before docetaxel, a potentially much larger market. Many patients would prefer to use the pills before having to try chemotherapy.
Zytiga prolonged median survival by 3.9 months, as initially reported, though Johnson Johnson later updated that figure to 4.6 months. Zytiga, which was approved in April 2011, had worldwide sales of $432 million in the first six months of this year.
Xtandi and Zytiga have not been compared head-to-head in a clinical trial. But some analysts say Xtandi would have an edge because it does not have to be given with a steroid to minimize side effects, as Zytiga does.
Xtandi has its own side effect, however, with about 1 percent of men taking it in the clinical trial suffering seizures.
There are expected to be about 241,000 new cases of prostate cancer this year in the United States and about 28,000 deaths.
Many men are treated with drugs such as Lupron that, in effect, induce a chemical castration, suppressing production of testosterone. But the cancers can eventually become resistant to castration therapy.
Xtandi works by blocking the action of testosterone, rather than by turning off its production.
It is the first product to reach the market for Medivation. It previously developed an old Russian antihistamine as a potential treatment for Alzheimer's disease, signing a big partnership with Pfizer. But that drug failed in late-stage clinical trials.
Medivation shares were up 5 percent to $102.36 when trading resumed late Friday following a suspension because of the news. The shares have roughly quintupled since Medivation announced the results of its clinical trial last November.
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