Prior treatment for systemic prostate cancer. Prior treatment with enzalutamide. Treatment with androgen receptor antagonists (bicalutamide, flutamide, nilutamide,), ketoconazole, abiraterone, finasteride, dutasteride, estrogens, or chemotherapy in an adjuvant setting within 6 months of enrollment (Day 1 visit) or plans to initiate treatment with any of these treatments.