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

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).