Each eligible subject will begin treatment with a 28-day enzalutamide 160 mg/day lead-in cycle. If a subject is able to tolerate the lead-in enzalutamide cycle without Grade 2 or Grade 2-4 drug-related toxicity, dose reduction, or missed doses due to toxicity, cycle 1 of combined enzalutamide and niraparib will commence. Enzalutamide will continue at 160 mg daily.