In a study presented at the 2014 American Society of Clinical Oncologists meeting, researchers reported that the use of two hormone blocking agents—Xtandi® (enzalutamide) and Zytiga® (abiraterone)—appeared safe and reduced testosterone levels in metastatic prostate cancer patients who had been resistant to initial hormone therapy.

Cells in the prostate gland have testosterone receptors and when exposed to testosterone, are stimulated to grow. When such cells become cancerous, the growth of these cancer cells can be increased by exposure to testosterone.

Hormone therapy, also known as androgen-deprivation therapy, is designed to stop testosterone from being released or to prevent it from acting on the prostate cells. Hormone therapy may work for many years but eventually prostate cancer becomes resistant to it.

A team at the MD Anderson Cancer Center led this small, proof-of-concept, phase II study. It included 60 men with metastatic prostate cancer who had had, at least, one prior treatment of hormone therapy and up to four treatments. All patients were given daily doses of Xtandi and Zytiga, along with prednisone, a steroid.

According to researchers, the dual therapy dropped hormone levels in the blood and bone marrow to undetectable levels in 80% of the patients. Further, Dr. Eleni Efstathiou, who led the study, noted that there were no significant negative interactions between the two drugs. There was, however, an increase in toxicity. Seventy-three percent of patients experienced mild to severe fatigue. In previous studies of the drugs administered independently, only 36% of Xtandi patients and 40% of Zytiga patients experienced mild to severe fatigue. Serious adverse events occurred in 13% of patients.

Researchers concluded that the combination therapy could be developed safely. Larger phase III trials are currently underway and will likely shed more light on the toxicity issue.

Reference: Efstathiou E., et al. Enzalutamide in combination with abiraterone acetate in bone metastatic castration resistant prostate cancer. ASCO 2014; Abstract 5000.

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