The purpose of this study is to evaluate the effects of multiple doses of abiraterone acetate plus prednisone on the pharmacokinetics (study of what the body does to a drug) of single doses of dextromethorphan hydrobromide and theophylline in patients with castration resistant prostate cancer.

This is an open-label (identity of assigned study drug will be known) study of abiraterone acetate plus prednisone in male patients with metastatic castration-resistant prostate cancer. This study will consist of screening, treatment, and follow-up periods, and will have 2 study groups. Patients in Group A and B will receive daily abiraterone acetate (1000 mg) plus prednisone (5 mg) twice daily beginning on Cycle 1 Day 1 until disease progression. Patients in Group A will take dextromethorphan hydrobromide 30 mg administered orally once daily on Day -8 and Day 8 of Cycle 1. Patients in Group B will take theophylline 100 mg administered orally once daily on Day -8 and Day 8 of Cycle 1. Serial pharmacokinetic samples will be collected and safety will be monitored throughout the study.

Inclusion Criteria:

• Histologically or cytologically confirmed adenocarcinoma of the prostate without neuroendocrine differentiation or small cell histology
• Documented metastatic disease
• Documented prostate specific antigen (PSA) progression according to Prostate Cancer Working Group 2 criteria, with PSA value >=2 ng/mL despite medical or surgical castration, or prostate cancer progression documented by radiographic progression according to Response Evaluation Criteria In Solid Tumors criteria
• Surgically or medically castrated with testosterone levels of <50 ng/dL
• Eastern Cooperative Oncology Group (ECOG) Performance Status of <=2
• Group A only: genomic testing at screening indicating CYP2D6 extensive metabolizer status
• Protocol-defined laboratory values

Exclusion Criteria:

• Serious or uncontrolled co-existent non-malignant disease, including active and uncontrolled infection
• Group A only: genomic testing at screening indicating CYP2D6 non-extensive metabolizer status, or prior treatment with dextromethorphan-containing medication or any medication that is a strong inhibitor or inducer of CYP2D6 within 5 half-lives of that drug or 7 days, whichever is longer, prior to Cycle 1 Day -8
• Group B only: prior treatment with theophylline or any medication that is a strong inhibitor or inducer of CYP1A2 within 5 half-lives of that drug or 7 days, whichever is longer, prior to Cycle 1 Day -8
• Abnormal liver function
• Uncontrolled hypertension (repeated systolic blood pressure >=160 mmHg, or diastolic blood pressure >=95 mmHg)
• Active or symptomatic viral hepatitis or chronic liver disease
• Known brain metastasis
• History of pituitary or adrenal dysfunction
• Clinically significant heart disease as evidenced by myocardial infarction, or arterial thrombotic events in the past 6 months, severe or unstable angina, or New York Heart Association Class III or IV heart disease or cardiac ejection fraction measurement of <50% at baseline
• History of gastrointestinal disorders (medical disorders or extensive surgery) which may interfere with the absorption of the study drug
• Surgery or local prostatic intervention within 28 days of the first dose, and any clinically relevant sequelae from the surgery must have resolved prior to Cycle 1 Day 1
• Radiotherapy or immunotherapy within 28 days, or single fraction of palliative radiotherapy within 14 days of administration of Cycle 1 Day 1
• Any acute toxicities due to prior therapy that have not resolved
• Current enrollment in an investigational drug or device study or participation in such a study within 28 days of Cycle 1 Day 1
• Previous abiraterone acetate or other investigational CYP17 inhibitor (eg, TAK-700)