A Study of Abemaciclib (LY2835219) With Abiraterone in Men With Prostate Cancer That Has Spread to Other Parts of the Body and is Expected to Respond to Hormonal Treatment (Metastatic Hormone-Sensitive Prostate Cancer)

Inclusion Criteria

• Adenocarcinoma of the prostate (as the predominant histology)
• High-risk metastatic hormone-sensitive prostate cancer. High risk is defined as:
• Greater than or equal to (≥)4 bone metastases by bone scan and/or
• ≥1 visceral metastases by computed tomography or magnetic resonance imaging
• Must have initiated androgen deprivation therapy (ADT) with luteinizing hormone-releasing hormone (LHRH) agonist/antagonist or bilateral orchiectomy prior to randomization. Up to 3 months of ADT prior to randomization is permitted with or without first-generation anti-androgen.
• Adequate organ function
• Eastern Cooperative Oncology Group (ECOG) performance status of 0 or 1

Exclusion Criteria

• Prior treatment with abemaciclib or any other cyclin dependent kinase 4 and 6 (CDK4 & 6) inhibitor
• Development of metastatic prostate cancer in the context of castrate levels of testosterone
• Received any prior systemic therapy for metastatic prostate cancer (including investigational agents), except for ADT and first-generation anti-androgen
• Clinically significant cardiovascular disease as evidenced by myocardial infarction, arterial thrombotic events, or severe/unstable angina in the past 6 months, or New York Heart Association Class II to IV heart failure
• History of syncope of cardiovascular etiology, ventricular arrhythmia of pathological origin, or sudden cardiac arrest. Chronic and hemodynamically stable atrial arrhythmia well-controlled on medical therapy is permitted
• Uncontrolled hypertension
• Clinically active or chronic liver disease, moderate/severe hepatic impairment
• Known untreated central nervous system (CNS) metastasis. Participants with a history of treated brain metastases are eligible if stable for at least 8 weeks prior to randomization and off corticosteroid for at least 2 weeks prior to randomization