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)

The purpose of this study is to learn whether adding abemaciclib to abiraterone plus prednisone prolongs the time before prostate cancer gets worse. Participation may last approximately 60 months.

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