Inclusion Criteria:

          -  Signed informed consent

          -  Diagnosis of progressive metastatic castration-resistant prostate cancer (mCRPC)

          -  Previously failed at least one potent anti-androgen therapy

          -  Castrate levels of serum testosterone

          -  ECOG performance status 0-2

          -  Adequate bone marrow function

          -  Adequate kidney, heart and liver function

        Exclusion Criteria:

          -  Prior solid organ transplant

          -  Prior treatment with small molecules including chemotherapy, antibody, or other
             experimental anticancer therapeutic within 4 weeks of first dose of study treatment

          -  Prior radiation therapy within 4 weeks prior to initiation of study treatment
             (including radiofrequency ablation)

          -  Prior androgen antagonist therapy (enzalutamide, apalutamide, abiraterone acetate, or
             darolutamide) within 2 weeks

          -  Prior radium-223 therapy within 6 weeks

          -  Symptomatic, untreated or actively progressing central nervous system (CNS) metastasis

          -  Unstable or severe, uncontrolled medical condition (e.g., unstable cardiac function,
             unstable pulmonary condition including pneumonitis and/or interstitial lung disease,
             uncontrolled diabetes, active or uncontrolled infection requiring systemic therapy) or
             any important medical illness or abnormal laboratory finding that would, in the
             Investigator's judgement, increase the risk to the patient associated with
             participation in the study

          -  Concomitant medications that cause Torsades de Pointes that have not reached steady
             state before first dose of the study drug

          -  Concomitant medications that are strong inhibitors or inducers of CYP3A4 or an
             inhibitor of P-gp

          -  History of infection with human immunodeficiency virus (HIV)

          -  Active infection with hepatitis B, or hepatitis C virus