A new drug to treat prostate cancer shows early promise, particularly against tumors that have spread to the bone, a multi-site study shows.

The drug Cabozantinib is designed to target mainly 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.

“Not only did three-quarters of bone scans have partial or complete resolution, but this was accompanied by improvement in bone pain and decreased need for narcotic use,” says lead study author Maha Hussain, M.D., FACP, professor of internal medicine and urology and associate director of clinical research at the University of Michigan Comprehensive Cancer Center.

Hussain presented the findings at the American Society of Clinical Oncology annual meeting.

The trial enrolled 171 men with metastatic prostate cancer. In more than three-quarters of the men enrolled, cancer had spread to the bone.

Researchers found 76 percent of patients saw some or all of their tumor shrink on bone scans following treatment with Cabozantinib. In addition, among patients who were on narcotics due to bone pain, 67 percent reported less pain and 56 percent either stopped taking narcotics or reduced the dosage. In addition, more than two-thirds of patients had some tumor regressions in areas of spread outside the bone. The treatment effects lasted on average 29 weeks.

The study found moderate side effects from Cabozantinib, including fatigue, gastrointestinal symptoms and high blood pressure.

“What’s interesting about this drug is it brings to the table something we haven’t seen before. Dramatic improvements in bone scans are unprecedented in this disease. Despite measurable progress, current treatment options for advanced prostate cancer tend to be modest in effect, so adding to and improving these options is a high priority,” Hussain says.

Hussain cautions that this is very early data, but it opens a new door for further investigation. The manufacturer, Exelixis, has developed a randomized clinical trial that is currently open at the U-M Comprehensive Cancer Center and other locations. For information, call the U-M Cancer AnswerLine at 800-865-1125.

U-M researchers are also planning a clinical trial with this drug in patients with metastatic prostate cancer who have had no previous chemotherapy. Laboratory research at the University of Michigan will look to better understand Cabozantinib’s effects on the bone. Cabozantinib is not approved by the U.S. Food and Drug Administration.

Prostate cancer statistics: 217,730 Americans will be diagnosed with prostate cancer this year and 32,050 will die from the disease, according to the American Cancer Society

Additional authors: M. R. Smith, C. Sweeney, P. G. Corn, A. Elfiky, M. S. Gordon, N. B. Haas, A. L. Harzstark, R. Kurzrock, P. Lara, C. Lin, A. Sella, E. J. Small, A. I. Spira, U. N. Vaishampayan, N. J. Vogelzang, C. Scheffold, M. D. Ballinger, F. Schimmoller, D. C. Smith

Disclosure: None

Reference: American Society of Clinical Oncology Annual Meeting, June 3-7, 2011, Chicago, “Cabozantinib (XL184) in metastatic castration-resistant prostate cancer (mCRPC): Results from a phase II randomized discontinuation trial.” Abstract No. 4516

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