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SAN FRANCISCO (TheStreet) -- Medivation (MDVN) and partner Astellas are stopping early a large phase III study of their prostate cancer drug Xtandi because patients are living significantly longer, the companies announced Tuesday.

With positive results from the phase III "Prevail" study, Medivation and Astellas will seek FDA and European approval to expand use of Xtandi to treat men with advanced prostate cancer who have not yet received chemotherapy. This so-called "pre-chemo" prostate cancer patient population has the potential to turn Xtandi into a blockbuster drug.

Medivation shares rise 9% to $54.40 in pre-market trading.

Based on an interim analysis of the Prevail study, prostate cancer patients treated with Xtandi had a 30% reduction in the risk of death compared to patients treated with a placebo. The survival benefit was statistically significant.

By comparison, Johnson & Johnson's (JNJ) - Get Johnson & Johnson Report competing prostate cancer drug Zytiga demonstrated a 21% reduction in the risk of death in a similar phase III study. 

Xtandi also demonstrated a statistically significant 81% reduction in the risk of tumor progression or death compared to placebo, Medivation and Astellas said. 

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An independent data monitoring committee advised the companies to stop the study early due to the benefit derived from treatment with Xtandi. All patients treated with a placebo will now be offered Xtandi. 

-- Reported by Adam Feuerstein in Boston.

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