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You're getting the first look at the Kaplan-Meier overall survival curves from the phase III "PREVAIL" study of Medivation (MDVN) and Astellas' Xtandi (enzalutamide) in pre-chemo prostate cancer patients. An update from the PREVAIL study was released tonight in advance of a formal presentation on Thursday at a medical conference.

Investors have been eager to see the Xtandi overall survival curves ever since Medivation and Astellas announced positive results and an early stop to the phase III study last November. As you can clearly see, a survival benefit favoring Xtandi (the green line) over placebo (blue line) starts early in the study at about four months and continues until the very end when the curves finally cross with only four patients remaining alive. 

As previously disclosed, the overall survival hazard ratio is 0.706, translating into a 29% reduction in the risk of death favoring Xtandi over placebo. Highly statistically significant. At the median, the overall survival benefit favoring Xtandi is 2.2 months over placebo.

Xtandi is competing against Johnson & Johnson's (JNJ) - Get Johnson & Johnson Report Zytiga in the pre-chemo prostate cancer treatment market. The two drugs have not been studied directly against each other but J&J did conduct its own phase III study of Zytiga plus prednisone versus prednisone alone. Data from this study have been presented at various medical meetings already.

For comparison purposes, here's what the Zytiga overall survival curves look like from its respective phase III study:

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Notice: The Zytiga survival curve doesn't start to separate from the control arm until 18 months into the study. The hazard ratio for overall survival from the Zytiga study is 0.79, or a 21% reduction in the risk of death, favoring Zytiga over control. This survival result was not statistically significant because the study was stopped too early. 

The full presentation of the Xtandi PREVAIL data on Thursday will be made by Dr. Tomasz Beer of the Knight Cancer Institute at Oregon Health and Science University. Beer previewed his Xtandi presentation with the media earlier today, which is where these slides come from (with the exception of the abiraterone slide, which was shared with me by an investor who keeps better notes than I do.)

Medivation and Astellas did not comment on the new Xtandi data ahead of Thursday's presentation.

The co-primary endpoint of the Xtandi PREVAIL study was radiographic progression-free survival (rPFS.) Here's what those data look like:

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This is the first look at the rPFS curves although the rPFS hazard ratio of 0.186 favoring Xtandi was previously disclosed. Obviously, the separation here is early, wide and very statistically significant. 

For the first time, Xtandi tumor response rate data are being shown.

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Also for the first time, data showing treatment with Xtandi led to a significant delay in the median time to chemotherapy:

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Here's a slide on Xtandi safety from the PREVAIL study:

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Not noted on this slide is disclosure of two patients experiencing a seizure during the trial. According to Dr. Beer, one patient in each arm of the study reported a seizure (1 in the Xtandi arm, 1 in the placebo arm) and both patients entered the study with a history of seizures not known to investigators at that time. 

Beer characterized the seizure rate seen in the PREVAIL study as "extremely low" and "very reassuring."

Lastly, here is the conclusion slide from Beer's presentation of the Xtandi data earlier today:

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The Xtandi data are being presented Thursday at the 2014 Genitourinary Cancers Symposium, sponsored by the American Society of Clinical Oncology, the American Society for Radiation Oncology and the Society of Urologic Oncology. 

Based on the data from the PREVAIL study, Medivation and Astellas will be seeking regulatory approval to expand the Xtandi label to include treatment of prostate cancer patients who have not been treated with chemotherapy. [The drug is already approved in the post-chemo prostate cancer setting.]

Astellas reports results from its most recent quarter on Feb. 2. The current consensus for U.S. Xtandi sales in the fourth quarter is $111 million. Astellas and/or Medivation are also expected to provide Xtandi sales guidance for 2014. Current consensus stands at $636 million, according to J.P. Morgan. 

-- Reported by Adam Feuerstein in Boston.

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Adam Feuerstein writes regularly for TheStreet. In keeping with company editorial policy, he doesn't own or short individual stocks, although he owns stock in TheStreet. He also doesn't invest in hedge funds or other private investment partnerships. Feuerstein appreciates your feedback;

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