Enzalutamide in combination fails to improve progression-free survival in chemotherapy-naive mCRPC patients in a Phase 4 trial.

Pfizer and Astellas Pharma announced that a Phase 4 clinical trial(PLATO) investigating longer-term use of Xtandi®  (enzalutamide) as a combination treatment for patients with metastatic castration-resistant prostate cancer (mCRPC), did not reach its primary goal of improved progression-free survival. The trial is concluding, but data will continue to be evaluated, according to the companies. 
Top-line results failed to show that continued treatment with Xtandi (enzalutamide) in combination with the chemotherapy drugs Zytiga® (abiraterone acetate) and prednisone, improved progression-free survival (PFS) in chemotherapy-naive mCRPC patients whose PSA levels had progressed despite previous enzalutamide therapy.

Xtandi is an oral, once-daily androgen receptor inhibitor that blocks, using  three different steps, male hormones from signaling within tumor cells to prevent their growth. The U.S. Food and Drug Administration (FDA) approved Xtandi as a treatment for mCRPC patients in 2012, based on results from previous clinical trials showing a significant overall survival benefit with this drug.

The Phase 4 PLATO trial (NCT01995513) is a global, double-blind, placebo-controlled study, designed to assess the safety and efficacy of continued treatment with Xtandi plus Zytiga and prednisone following confirmed PSA progression.

The study enrolled 509 patients with mCRPC who had never received chemotherapy treatment before, and was divided in two parts. In part one, patients received Xtandi (160mg/day) until an increase in their PSA levels was confirmed.

In the second part of the trial, patients were randomly assigned to either continue Xtandi treatment, now combined with Zytiga (1,000 mg/day orally) and prednisone (5 mg administered orally twice daily), or treatment with only Zytiga and prednisone. Its primary goal was progression-free survival, defined by either radiographic progression, unequivocal clinical progression, or death.

Eligible patients were then randomized to one of the two treatments and assessed for the primary endpoint of the study, PFS, defined by either: 1) radiographic progression or 2) unequivocal clinical progression or 3) death during the study.

Period 1 patients were treated with XTANDI 160mg/day orally and period 2 patients were treated with blinded XTANDI 160 mg/day orally in combination with abiraterone at a dose of 1,000 mg/day administered orally and prednisone at a dose of 5 mg administered orally twice daily, versus placebo plus the same doses of abiraterone acetate and prednisone.

Metastatic castration-resistant prostate cancer (CRPC) refers to prostate cancer that has spread to parts of the body other than the prostate, and continues to spread despite treatment.1 Up to 40 percent of men diagnosed with prostate cancer who undergo therapy develop metastatic, or advanced, prostate cancer.2 

References: 
1 Cancer.net -  Treatment of Metastatic Castration-Resistant Prostate Cancer. http://www.cancer.net/research-and-advocacy/asco-care-and-treatment-recommendations-patients/treatment-metastatic-castration-resistant-prostate-cancer. Accessed 12-19-2016.

2 "Current and emerging treatments in the management of castration-resistant prostate cancer." David Shapiro and Basir Tareen. Expert Rev Anticancer Ther. 2012;12(7):951-964.
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