About two-thirds of patients treated with cabozantinib had improvements on their bone scans, with 12 percent seeing complete resolution of uptake on bone scan. Bone scans assess the degree to which cancer is in the bone; improvements on these scans suggest a response to the drug.

"The effects of cabozantinib on bone scans are unprecedented in the treatment of prostate cancer," says lead study author David C. Smith, M.D., professor of internal medicine and urology at the University of Michigan Medical School.

Cabozantinib is designed to target two important pathways linked to the growth and spread of prostate cancer. The drug had the most effect on tumors that had spread to the bone, which is the major site where prostate cancer spreads. These tumors are typically very challenging to treat once they become resistant to hormone-based therapies.

In addition to the improvements on bone scans, 67 percent of patients with bone pain reported an improvement in pain control and 56 percent decreased or eliminated narcotic painkillers after treatment with cabozantinib. Results of the study appear in the Journal of Clinical Oncology.

The trial enrolled 171 men with castration-resistant prostate cancer, meaning their tumors no longer responded to hormone-based therapies. The study began as a randomized trial in which all patients received cabozantinib for 12 weeks, after which patients were randomized to receive continued cabozantinib or placebo. The randomization was stopped early because of the dramatic effects on bone scan, and because patients receiving placebo saw their cancer progress much more quickly than those that remained on drug.

Among the 31 patients who were randomized, cancer progressed after a median 23.9 weeks for patients taking cabozantinib, compared with 5.9 weeks for patients on placebo.

"Discontinuing randomization is not common. Stabilization of disease in advanced prostate cancer is rarely due to the natural history of the disease and is in this case due to drug effect," Smith says.

"While these initial results are promising, we are still uncertain how cabozantinib will impact the gold standard of survival," he adds.

Phase III studies have begun at some institutions, and U-M researchers are conducting a phase II study to better understand the effect cabozantinib has on bone. This drug is not offered routinely in clinical care at this time. For information about prostate cancer treatment options or clinical trials currently open at the U-M Comprehensive Cancer Center, call the Cancer AnswerLine at 800-865-1125.

Prostate cancer statistics: 241,740 Americans will be diagnosed with prostate cancer this year and 28,170 will die from the disease, according to the American Cancer Society