Exelixis Inc.'s stock plummeted 40.2 percent Tuesday following the South San Francisco biotech's announcement that it was unable to secure a special protocol assessment (SPA) from the FDA for its pivotal Phase III XL184-306 trial of cabozantinib (cabo) in end-stage castrate resistant prostate cancer (CRPC).Exelixis said it has stopped pursuing the SPA and plans to initiate the '306 trial by the end of 2011 with pain as the primary efficacy endpoint, to be followed in the first half of 2012 by the Phase III XL184-307 trial to evaluate overall survival time.