BOSTON ( TheStreet) -- Welcome to this week's Biotech Stock Mailbag. @biotechbaumer asks, "Do you know timeframe when FDA expected to make decision on SPA for $EXEL to use bone mets as primary endpoint in CRPC?"

I'll translate that tweet: He's asking if there is a time frame for when the Food and Drug Administration and Exelixis ( EXEL) are expected to reach agreement on a special protocol assessment (SPA) nailing down the design of the first pivotal, phase III study of Exelixis' prostate cancer drug cabozantinib.

Exelixis has said publicly that it expects to begin the phase III study of cabozantinib ("cabo") before the end of the year. In order to meet that timeline, the company must reach agreement with FDA on the SPA. It's nearly November, so clearly, the SPA agreement, if one is agreed to, will be coming soon.

Thursday night, Exelixis said it expects that FDA feedback "shortly."

Exelixis is proposing a phase III study for cabo that will use a composite primary endpoint of bone lesion clearance and pain resolution. Patients enrolled in this study will have advanced, metastatic prostate cancer that no longer responds to the standard chemo regimen of Taxotere or newer drugs like Johnson & Johnson's Zytiga ( JNJ) and Sanofi's ( SNY) Jevtana. These are very sick prostate cancer patients with significant prostate cancer-related pain who have few or no treatment options left.

This cabo study design is a point of controversy for investors (and an overhang on the Exelixis' stock price) because Exelixis is choosing not to use overall survival as the primary endpoint. All recently approved prostate cancer drugs -- Zytiga, Jevtana and Dendreon's Provenge -- used a survival benefit as the basis for approval. Still, experimental prostate cancer drugs from Medivation ( MDVN) and Algeta both employed overall survival in their respective phase III studies.

Exelixis believes demonstrating cabo's ability to shrink or eliminate the spread of prostate cancer to bone, which causes patients a lot of pain, is a clinically significant benefit and enough to convince FDA to approve the drug. To support their position, Exelixis points to the FDA's own guidance stating prostate cancer drugs can be approved on the basis of pain resolution. Mitoxantrone was approved for prostate cancer using a pain reduction endpoint in 1996.

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