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What: Shares of Exelixis (Nasdaq: EXEL) are soaring by as much 26% today after the company announced positive preliminary results from a phase 3 clinical trial for cabozantinib.

So what: The drug is intended for patients with advanced medullary thyroid cancer, and the trial met its primary endpoint of improving progression-free survival compared with placebo. It substantially exceeded the threshold of a 75% increase in PFS assumed at the onset of the trial.

Now what: Cabozantinib significantly improved median PFS by 7.2 months relative to the placebo. Exelixis CEO Michael Morrissey called the results "an important advance for MTC patients and for Exelixis." The company will consult with the Food and Drug Administration and is hoping to begin a rolling submission of a new drug application. Exelixis hopes that the filing will be completed by the first half of 2012.

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Fool contributor Evan Niu holds no position in any company mentioned. Click here to see his holdings and a short bio. The Motley Fool owns shares of Exelixis. Motley Fool newsletter services have recommended buying shares of Exelixis. Try any of our Foolish newsletter services free for 30 days. We Fools may not all hold the same opinions, but we all believe that considering a diverse range of insights makes us better investors. The Motley Fool has a disclosure policy.