A new drug demonstrated dramatic and rapid effects on prostate cancer that had spread to the bone. About two-thirds of patients treated with cabozantinib had improvements on their bone scans, with 12% seeing complete resolution on uptake on bone scans.

Prostate cancer will be diagnosed in 241,740 Americans and 28,170 will die of the disease, according to the American Cancer Society (ACS). Bone scans assess the degree to which cancer is in the bone; improvements on these scans suggest a response to the drug. 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.

“The effects of cabozantinib on bone scans are unprecedented in the treatment of prostate cancer,” says lead study author David C. Smith, MD, of the University of Michigan Medical School in Ann Arbor, Michigan.

This phase II trial enrolled 171 men with castration-resistant prostate cancer, meaning their tumors no longer responded to hormone-based therapies. The trial 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 the patients receiving placebo saw their cancer progress much more quickly than those that remained on the 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 taking 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 said. “While these initial results are promising, we are still uncertain how cabozantinib will impact the gold standard of survival.”

Phase III studies have begun at some institutions, and University of Michigan researchers are conducting a phase II to better understand the effect of cabozantinib on bone. The drug is not routinely offered in clinical care at this time.

The results of this study were published in Journal of Clinical Oncology (2012; doi:10.1200/JCO.2012.45.0494).