A Study of Abiraterone Acetate Plus Prednisone With or Without Abemaciclib (LY2835219) in Participants With Prostate Cancer

This study is being done to see how safe and effective abemaciclib is when given together with abiraterone acetate plus prednisone in participants with metastatic castration resistant prostate cancer. Prednisolone may be used instead of prednisone per local regulation.

Inclusion Criteria: * Histologically confirmed adenocarcinoma of the prostate. * Metastatic prostate cancer documented by positive bone scan and/or measurable soft tissue metastatic lesions by CT or magnetic resonance imaging (MRI). * Progressive disease at study entry demonstrated during continuous androgen-deprivation therapy (ADT)/post orchiectomy defined as one or more of the following: * PSA progression * Radiographic progression per Response Evaluation Criteria in Solid Tumors (RECIST)1.1 for soft tissue and/or per Prostate Cancer Working Group 3 (PCWG3) for bone, with or without PSA progression * Have adequate organ function. * Eastern Cooperative Oncology Group (ECOG) performance status (PS) 0-1. Exclusion Criteria: * Prior therapy with cytochrome P450 (CYP)17 inhibitors. * Prior treatment with abemaciclib or any cyclin-dependent kinase (CDK) 4 \& 6 inhibitors. * Prior cytotoxic chemotherapy for metastatic castration resistant prostate cancer (participants treated with docetaxel in the metastatic hormone-sensitive prostate cancer \[mHSPC\] are eligible). Prior radiopharmaceuticals for prostate cancer, or prior enzalutamide, apalutamide, darolutamide or sipuleucel-T. Participants who had prior radiation or surgery to all target lesions. * Currently enrolled in a clinical study involving an investigational product. * Gastrointestinal disorder affecting the absorption or ability to swallow large pills. * Clinically significant heart disease, active or chronic liver disease, moderate/severe hepatic impairment (Child-Pugh Class B and C).