It seems only fitting that Medivation (Nasdaq: MDVN) got its prostate cancer treatment, Xtandi, approved on a blue moon on Friday. It isn't every day that you see the Food and Drug Administration finish reviewing a marketing application three months early. The last time I remember that happening was two years ago with Sanofi's (NYSE: SNY) Jevtana, which curiously was fairly close to the last time we had a blue moon.

Timing aside, no one should be shocked that Medivation's Xtandi was approved. The drug, formerly known as MDV3100, looked good in clinical trials in patients that had already received chemotherapy. Patients taking Xtandi lived almost five months longer than those taking placebo.

Gaining FDA approval will allow Medivation and marketing partner Astellas Pharma to sell Xtandi, but the companies will be limited to marketing the drug in the post-chemotherapy setting since those are the patients that were enrolled in the trial used to support its approval.

The big money is in giving metastatic castration-resistant prostate cancer patients an alternative to chemotherapy, as Dendreon's (Nasdaq: DNDN) Provenge has done and Johnson & Johnson (NYSE: JNJ) will be able to do once the FDA signs off on its expanded indication for Zytiga toward the end of this year.

Xtandi is in a clinical trial in that setting, but it'll be awhile before data proves that the drug extends lives in the pre-chemo setting. Doctors can use it off-label to treat those patients, which might happen if Medivation can get the drug listed in the treatment compendiums, but most of the potential won't be reached until the FDA signs off on the earlier indication and until Medivation and Astellas can market it as a first-line therapy.

Medivation's early approval could be a good omen for Exelixis (Nasdaq: EXEL), which is waiting for approval for cabozantinib to treat medullary thyroid cancer. The FDA recently canceled the advisory committee meeting to review the drug, which -- if you're willing to read between the lines -- is probably a good sign that the FDA has made up its mind and that cabozantinib could also be approved early.

Of course, the next blue moon isn't until 2015, so maybe not.

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