Patient must meet all eligibility criteria to be enrolled in the study. Below is the portion of the study requirements: histologically or cytologically confirmed adenocarcinoma of the prostate; metastatic disease radiographically documented by CT/MRI or bone scan. No abnormal laboratory test findings. Patient must be on continuous hormonal ablation with surgical or medical castration with baseline testosterone less than 50ng/dL. Prior treatment with Sipuleucel-T, Radium-223, abiraterone, ketoconazole, and/or Tak-700 within 2 weeks prior to the trial is allowed. Chemotherapy more than 6 months prior to this trial is allowed. Tumor biopsy of an accessible lesion at baseline and on treatment if the lesion can be biopsied is required. Prior treatment with enzalutamide, ARN-509, or galeterone is not allowed.