A Study to Evaluate How Daily Dosing With Enzalutamide Affects the Metabolism of Caffeine and Dextromethorphan in Men With Prostate Cancer

This is an open-label, fixed-sequence, crossover drug-drug interaction study in subjects with prostate cancer. Screening takes place between Day -28 and Day -7, and subjects are admitted to the clinic on Day -1 where they remain until Day 4. On Day 1, they receive a single oral cocktail containing caffeine and dextromethorphan plus a dose of enzalutamide placebo in order to assess possible effects of excipients of the enzalutamide formulation. On Days 1 to 3 blood samples for pharmacokinetic (PK) assessment are collected. From Days 4 to 54, (or Day 55 if the subject rolls over into the extension study) the subjects take a daily oral dose of enzalutamide. On Day 28, the subjects return to the clinic where a plasma sample is taken to determine enzalutamide PK exposure. From Days 52 to 55 they are re-admitted to the clinic where a plasma sample is collected on Day 52 for enzalutamide PK exposure.On Day 53, subjects receive a single oral cocktail of caffeine and dextromethorphan concomitantly with enzalutamide. PK samples are collected from Days 53 to 55. From Day 55 onwards, subjects experiencing clinical benefit may roll over into an extension study. Only subjects who enroll into the extension study continue to receive enzalutamide otherwise daily dosing with enzalutamide is discontinued on Day 54. An End of Study Visit (ESV) takes place approximately 30 days (±7 days) after the last dose of enzalutamide.