This study evaluates how once daily enzalutamide affects the metabolism of caffeine and dextromethorphan in men with prostate cancer by measuring concentrations of these drugs and their metabolites in plasma.

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.

Inclusion Criteria:

• Subject is a male aged 18 years old or older (at screening) with histologically confirmed prostate cancer (all stages) for whom androgen deprivation therapy is indicated (except when indicated in a neoadjuvant/adjuvant setting). Subjects may be on ongoing androgen deprivation therapy with a gonadotropin releasing hormone (GnRH) analogue or have undergone prior bilateral orchiectomy at screening.
• Subject has progressive disease by prostate-specific antigen (PSA) or imaging.
• Subject has received no more than 2 prior chemotherapy regimens.
• Subject has an Eastern Cooperative Oncology Group (ECOG) performance status of 0-2.
• Male subject must use a condom if having sex with a pregnant woman.
• Male subject and their female spouse/partners who are of childbearing potential must use 2 acceptable methods of birth control starting at screening and continuing throughout the study period and for 3 months after final study drug administration.
• Subject has an estimated life expectancy of at least 6 months.

Exclusion Criteria:

• Subject has confirmed CYP2D6 poor metabolizer, or CYP2D6 ultrarapid metabolizer status based on genotyping analysis.
• Subject has known metastases in the liver or any hepatic disorder that could affect drug metabolism deemed clinically significant by the investigator after discussion with the sponsor.
• Subject has undergone major surgery within 4 weeks prior to day 1.
• Subject received treatment with chemotherapy within 4 weeks prior to enrollment (day 1 visit) or plans to initiate treatment with chemotherapy during the study.
• Subject uses concomitant medications that are potent inducers and/or inhibitors of CYP1A2, CYP2C8, CYP2D6, or CYP3A4.