CHICAGO ( TheStreet) -- Exelixis' ( EXEL) experimental drug cabozantinib fully or partially cleared up bone lesions from prostate cancer in 76% of patients and reduced the pain that results when prostate cancer spreads to bones, according to updated results from a mid-stage study presented Monday. Based on these and other findings from the phase II study, Exelixis said it intends to start by the end of the year a pivotal, phase III study of cabozantinib using a combined endpoint of pain reduction and clearance of bone lesions in heavily pre-treated prostate cancer patients. The approval of new prostate cancer drugs have typically required that drugs show they can prolong the survival of patients, so Exelixis' trial design for cabozantinib has generated some controversy amongst investors. Exelixis CEO Michael Morrissey said the company intends to get clearance from the U.S. Food and Drug Administration before starting its phase III study. Potentially complicating matters for Exelixis was the announcement Monday from Bayer that its prostate cancer drug alpharadin extended survival in a phase III study that enrolled prostate cancer patients similar to those to be enrolled in the cabozantinib phase III study. The new data on cabozantinib were presented at the American Society of Clinical Oncology (ASCO) annual meeting. Exelixis shares were down $2, or 18%, to $2.06 in Monday trading on typical post-ASCO selling as well as on concerns about the design of future studies planned for cabozantinib. Among 108 prostate cancer patients with evidence of cancer spreading to their bones, 82 patients, or 76%, had either a partial or complete resolution of bone lesions. Last February, Exelixis reported an 85% rate of partial or complete resolution of bone lesions from 62 patients in the same study. Cancer that metastasizes, or spreads, to bones is a serious complication leading to fractures, increased pain and eventual death. While many cancer drugs can shrink or eliminate tumors in soft tissue, few if any have demonstrated an ability to clear up bone metastases. Patients treated with cabozantinib who had partial or complete resolution of bone lesions also experienced marked reductions in pain and were able to reduce or eliminate the need for narcotic painkillers more than patients without resolution of bone lesions, researchers reported Monday. These findings of additional clinical benefit from cabozantinib were based on post-hoc, or retrospective, analysis of the study data, however.