Very early data are showing promising results for a drug treatment for castration-resistant prostate cancer (CRPC), with the agent demonstrating particular effectiveness against tumors that have metastasized to the bone.

The new therapeutic agent, cabozantinib, is an inhibitor of the MET and VEGF pathways, which contribute to tumor growth. In a phase II study, of men (median age 68 years) with metastatic CRPC (mCRPC) who were followed for a median of 4 months, nearly half (47%) of the 100 evaluable patients had undergone prior treatment with docetaxel. The majority of men (78%) had bone metastasis.

Among the 65 patients evaluable by bone scan, 56 (86%) experienced complete or partial resolution of bone lesions as early as week 6 of treatment with cabozantinib. In addition, 64% of the 28 men receiving narcotics for bone pain had improved pain relief, with narcotics reduced or halted in 46%. By week 12, the disease control rate was 71%.

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“[Cabozantinib] showed clinical activity regardless of prior [docetaxel] in patients with mCPRC, particularly those patients with bone disease, as reflected by high rates of [bone-scan] resolution and pain relief, in addition to improvements in [hemoglobin] and tumor regression,” wrote Maha Hussain, MD, and coinvestigators in their study abstract presented at the annual meeting of the American Society of Clinical Oncology (ASCO), held June 3-7, 2011, in Chicago, Illinois (

In a separate statement, Hussain, a professor of internal medicine and urology and associate director of clinical research at the University of Michigan Comprehensive Cancer Center in Ann Arbor, observed that cabozantinib was associated with “dramatic improvements in bone scans,” which are “unprecedented in this disease.”

The manufacturer of cabozantinib, Exelixis (South San Francisco, California), will expand on this early data with a randomized clinical trial currently enrolling patients at Hussain’s facility as well as other locations. For information, call the University of Michigan Cancer AnswerLine at 1-800-865-1125.