Abemaciclib Plus Darolutamide in Prostate Cancer That Has Spread After Initial Treatment

Inclusion Criteria: * Histologically confirmed adenocarcinoma of the prostate. * Metastatic castration-resistant prostate cancer evidenced by: * Prostate-specific antigen (PSA) or radiographic progression despite castrate levels of testosterone * At least 1 bone metastasis on bone scan and/or 1 soft tissue metastasis on computed tomography/magnetic resonance imaging (CT/MRI) * Participants who have not undergone bilateral orchiectomy must continue luteinizing-hormone-releasing hormone (LHRH) agonists/antagonists throughout the study. * Have adequate organ function. * Eastern Cooperative Oncology Group (ECOG) performance status (PS) 0-1. Exclusion Criteria: * Prior treatment with cyclin-dependent kinase 4 and 6 (CDK4 and CDK6) inhibitors or darolutamide. * Prior systemic therapy for metastatic castration-resistant prostate cancer(mCRPC) with cytotoxic chemotherapy, PARP inhibitors, novel hormonal agents (NHAs) (enzalutamide, apalutamide, and abiraterone), and radiopharmaceuticals. * Serious preexisting medical condition(s) that, in the judgment of the investigator, would preclude participation in this study. * Clinically significant heart disease as evidenced by myocardial infarction, arterial thrombotic events, severe or unstable angina, or congestive heart failure (New York Heart Association Class III or IV) within 6 months of assignment to treatment.