Enzalutamide (Xtandi, MDV3100) is an androgen receptor antagonist drug that produces an up to an 89% decrease in prostate specific antigen serum levels after a month of use. It is more potent than Casodex. In August of 2012, the U.S. Food and Drug Administration approved enzalutamide for the treatment of castration-resistant prostate cancer in patients who failed docetaxel. This was based on the AFFIRM study results. AFFIRM showed that Enzalutamide significantly prolonged the survival of men with metastatic castration-resistant prostate cancer after chemotherapy(Scher et al). The PREVAIL study that will conclusively demonstrate that Xtandi is effective before docetaxel is still ongoing: A Multinational Phase 3, Randomized, Double-Blind, Placebo-Controlled Efficacy and Safety Study of Oral MDV3100 in Chemotherapy-Naive Patients With Progressive Metastatic Prostate Cancer Who Have Failed Androgen Deprivation Therapy.
There is currently limited evidence to support the use of Xtandi for patients who had not or cannot receive docetaxel, before the PREVAI results become available. Median time to radiographic progression in one phase II study by Scher, 2010, was 56 weeks for chemo-naive patients and 25 weeks for patients previously treated with chemotherapy.
Several studies, in addition to Prevail, are looking at Xtandi before docetaxel. There is a phase II trial began in March 2011 comparing MDV3100 with Casodex in patients patients who have progressed while on LHRH analogue therapy (e,g., leuprorelin) or surgical castration. This ASPIRE trial is an an open-label study intended to evaluate the effects of enzalutamide in about 150 men who are theoretically eligible to receive chemotherapy but have chosen not to do this. Another phase II study, STRIVE trial is a randomized, double-blind, multi-center clinical study of the efficacy and safety study of enzalutamide (160 mg/day) compared to bicalutamide (50 mg/day) in men with recurrent prostate cancer who have serologic and/or radiographic disease progression subsequent to primary androgen deprivation therapy (ADT). The TERRAIN trial, is a randomized, double-blind, multi-center, Phase II trial designed to test the efficacy and safety of enzalutamide compared to bicalutamide (Casodex) in men with metastatic prostate cancer already controlled by either bilateral orchiectomy (surgical castration) or ongoing androgen deprivation therapy with an LHRH agonist or an LHRH antagonist at a stable dose. There are also combination trials.
The evolution of Zytiga(arbiterone) suggests that Xtandin will show itself very effective in non-docetaxel treated patients once trials are completed, but study results to confirm this widely shared opinon are not yet in. It has the advantage of not requiring steroids which Zytiga does require.
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