A Study to Assess the Benefit of Treatment Beyond Progression With Enzalutamide in Men Who Are Starting Treatment With Docetaxel After Worsening of Their Prostate Cancer When Taking Enzalutamide Alone

The study was conducted in consecutive periods of open label treatment with enzalutamide followed by randomized double-blind treatment with continued enzalutamide or placebo, in combination with docetaxel and prednisolone. Open Label (Period 1) Participants received open label treatment (OL) with enzalutamide. At week 13, all participants were assessed by prostate-specific antigen (PSA) and imaging. Participants with no confirmed PSA response or evidence of radiographic progression were ineligible for participation in Period 2 and typically had safety follow up; however, Period 1 treatment continued for some participants as long as the investigator considered it to be of clinical benefit (stopping on initiation of any new antineoplastic therapy). Participants with confirmed PSA response continued Period 1 until disease progression. Enrollment to Period 2 ceased after approximately 274 participants had been enrolled or 182 primary endpoint events had been reached, whichever occurred first. Participants who were not randomized into period 2 at this time continued to receive open label treatment in an extension period. Randomization (Double Blind \[DB\]) (Period 2) Participants with confirmed disease progression on enzalutamide alone who continued to meet all eligibility criteria proceeded to randomization. Treatment allocation was in a 1:1 ratio, stratified by disease progression in Period 1 to the following treatments: * Enzalutamide with docetaxel and prednisolone * Placebo with docetaxel and prednisolone Any ongoing participants in Period 2 at the point of unblinding in the enzalutamide+docetaxel arm that were still receiving and benefitting from enzalutamide treatment, had the option to continue treatment via an extension period.