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 .
• Be able and willing to undergo mandatory tumor biopsy of at least one metastatic site.
• 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, 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).