Exelixis clashes with FDA over prostate cancer study, shares tank

Exelixis ($EXEL) upset investors with news late yesterday that the developer couldn't see eye to eye with U.S. regulators on the design of a key late-stage trial for its lead cancer drug in patients with aggressive prostate cancer. The news touched off an after-hours sell-off of the South San Francisco-based firm's stock, which dropped more than 30% during that time, Bloomberg reported.

Even though the FDA and the company could not come to terms on a special protocol assessment (SPA) for the study, Exelixis plans to advance the "306" trial anyway and follow regular rules for the trial with pain reduction as a primary measurement of the experimental drug cabozantinib's efficacy against aggressive prostate cancer. That trial will begin late this year, and next year the firm plans to launch a second late-stage trial of the drug in prostate cancer patients that will compare survival rates of patients on the treatment with those who get the steroid prednisone.

Clashing with the FDA on study protocols isn't popular in the drug development game, and questions now loom about the value of the data Exelixis will produce with its late-stage studies.

"This is a major negative development that is likely to trigger a precipitous sell-off and one that's all the more surprising given management's unwavering confidence that an SPA would be granted in the [near term], to the point where they were preparing study sites," Cory Kasimov, a biotech analyst for JP Morgan, wrote in a note to investors this morning.

Cabozantinib is arguably the most important drug in the pipeline at Exelixis, which has yet to bring a drug to market. After upbeat survival data reported last week from a late-stage study of the drug in patients with medullary thyroid cancer, the developer plans to file an app for approval with the FDA in early 2012. But yesterday's news might cast a shadow, at least for now, over the development of the drug for the big prostate cancer drug market.

- here's the release
- read the Bloomberg article
- and Xconomy's coverage

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