The randomized , double-blind phase III AFFIRM trial demonstrated that enzalutamide , an oral androgen receptor inhibitor , significantly prolonged overall survival ( OS ) [ median 18.4 versus 13.6 months ( hazard ratio , HR ) 0.63 ( 95 % confidence interval , CI , 0.53-0 .75 ) ; P < 0.001 ] compared with placebo in patients with metastatic castration-resistant prostate cancer who received prior docetaxel chemotherapy .