The news sent shares of Medivation up more than 11 percent to $61.75 in after-hours trading.

Medivation and partner Astellas Pharma Inc reported in November that the trial had been stopped early after it became clear that the drug, MDV3100, improved median overall survival by 4.8 months compared with a placebo.

Researchers said the trial of 1,199 patients with advanced prostate cancer showed that serious side effects occurred in 28.4 percent of MDV3100 patients, compared with 33.6 percent of the placebo group.

Five patients, or 0.6 percent of the treated group, experienced seizures, said Dr. Howard Scher of Memorial Sloan-Kettering Cancer Center in New York, the trial's lead investigator. An earlier trial of MDV3100 found that 1.5 percent of 198 treated patients had seizures, and some observers were concerned about the drug's safety profile.

The results will likely position MDV3100 as the first treatment for use after patients have stopped responding to chemotherapy, according to Scher. The drug is also being tested in patients with earlier-stage prostate cancer.

The safety profile of the drug "looked good with no major red flags," said ISI Group analyst Mark Schoenebaum, adding that the seizure rate "could pose some concern as the drug is developed in earlier stages of disease."

The full trial results will be presented this week in San Francisco at a meeting sponsored by the American Society of Clinical Oncology.

Dr. Nicholas Vogelzang, an oncologist at the Comprehensive Cancer Centers of Nevada and chair of ASCO's communications committee, described the Medivation trial results as "unprecedented," adding "this is going to definitely change the way we take care of patients every day in the office." He was not involved in the study.

MDV3100 is one of a new class of drugs, known as androgen inhibitors, designed to interfere with the ability of testosterone to bind to prostate cancer cells.

Prostate cancer kills about 250,000 men a year globally and is the second most common cause of cancer death in men in the United States, after lung cancer.

Since November, Medivation shares have soared on hopes that MDV3100 will be a commercial success.

IN TALKS OVER FDA SUBMISSION

Other serious side effects from MDV3100 included fatigue, cardiac disorders, liver function test abnormalities and myocardial infarction, said Medivation Chief Executive David Hung, noting that all were more prevalent in the placebo group than in the treatment group.

"The number of seizures is too small to make any conclusions as to whether or not there is even a causal relationship," he said.

Hung said Medivation is currently in discussions with the U.S. Food and Drug Administration regarding a new drug application.

Wall Street analysts, on average, have forecast MDV3100 sales of $846 million by 2016, according to Thomson Reuters.

New details of the trial included data showing that 54 percent of MDV3100 patients experienced a 50 percent or more decline in levels of prostate specific antigen, or PSA, while just 1.5 percent of placebo patients saw that much of a drop.

The median time to PSA progression was 8.3 months for the MDV3100 group and 3 months for the placebo patients.

Shrinkage of soft tissue tumors was seen in 28.9 percent of MDV3100 patients, compared with 3.8 percent of placebo patients.

This week's ASCO symposium will also feature full results from a trial of Alpharadin, a prostate cancer drug being developed by Germany's Bayer and Norwegian biotech Algeta.

The drug, which has been granted a fast-track review by U.S. regulators, is designed to deliver minute, highly-charged doses of radiation to secondary tumors in the bone.

Full results of a trial involving patients with advanced prostate cancer that had spread to the bone showed that Alpharadin delayed the time to a first skeletal-related event, including fractures, to 13.6 months, compared with 8.4 months for standard care.

Serious side effects, including anemia, were largely balanced between both arms of the trial.

Wall Street analysts have forecast Alpharadin sales of $485 million by 2016.

(Reporting by Deena Beasley; Editing by Steve Orlofsky, Gary Hill)

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