Inclusion Criteria:

          -  Participant must have metastatic prostate cancer for which castration (medical or
             surgical) is no longer effective (castration-resistant).

          -  Participant must have disease spread to soft tissue that is measurable.

          -  Participant must have documented evidence of progressive disease by PSA test or
             imaging.

          -  Participant must have previously received at least one of the following treatment:
             abiraterone acetate, apalutamide, darolutamide or enzalutamide.

          -  Participant must have previously received chemotherapy with docetaxel and cabazitaxel.

          -  Participant must be willing and amenable to undergo a biopsy of tumor tissue (or able
             to provide adequate archived tumor tissue sample) and to provide blood for research.

          -  Participant must have good physical functioning ability and adequate organ function.

        Exclusion Criteria:

          -  Participant must not have received more than 3 therapy regimens for metastatic
             castration-resistant prostate cancer (NOTE: GnRHa, first-generation antiandrogens
             (flutamide, nilutamide, or bicalutamide), diethylstilbestrol (DES) (or other
             estrogens), corticosteroids, ketoconazole, and bone loss-prevention will not count as
             systemic therapy regimens.

          -  Participants must not have previously received abemaciclib or any cyclin-dependent
             kinase (CDK)4 and/or CDK6 inhibitors.

          -  Participants must not have serious and/or uncontrolled preexisting medical
             condition(s) including but not limited to severe renal impairment, severe hepatic
             impairment, interstitial lung disease (ILD)/pneumonitis, severe dyspnea at rest or
             requiring oxygen therapy or other serious preexisting medical condition(s) that, in
             the judgment of the investigator, would preclude participation in this study.

          -  Participants must not have, or suspected to have, brain metastasis.

          -  Participants must not have untreated spinal cord compression, evidence of spinal
             metastases with risk of spinal compression or structurally unstable bone lesions
             suggesting impending fracture.