LBA1^ Background: Enzalutamide, an orally administered androgen receptor inhibitor, improved overall survival (OS) in men with mCRPC who had received prior docetaxel therapy (Scher et al, NEJM 367:13, 2012). This study examined whether enzalutamide could prolong OS and radiographic progression-free survival (rPFS) in asymptomatic or mildly symptomatic chemotherapy-naive men with mCRPC. Methods: In this randomized, double-blind, placebo-controlled, multinational phase 3 study (NCT01212991), chemotherapy-naive patients with mCRPC were stratified by site and randomized 1:1 to enzalutamide 160 mg/day or placebo. OS and rPFS were co-primary endpoints and analyzed for the intent-to-treat population. Planned sample size was 1,680 with 765 deaths to achieve 80% power to detect a target OS hazard ratio (HR) of 0.815 with a type I error rate of 0.049 and a single interim analysis at 516 (67%) deaths. The co-primary endpoint of rPFS had sufficient power to detect a target HR of 0.57 and a type I error rate of 0.001 ...