Inclusion Criteria:

          -  Have metastatic castration-resistant prostate cancer

          -  Must have had prior abiraterone treatment

          -  Life expectancy of greater than 3 months

          -  ECOG performance status 0 or 2

          -  Age ≥18 years

          -  Have measurable disease

          -  Patients acceptance to have a tumor biopsy of an accessible lesion at baseline and on
             treatment if the lesion can be biopsied with acceptable clinical risk (as judged by
             the investigator).

          -  Ability to take oral medication

          -  Patients must have adequate organ and marrow function defined by study-specified
             laboratory tests

          -  Must use acceptable form of birth control while on study

          -  Ability to understand and willingness to sign a written informed consent document

        Exclusion Criteria:

          -  Known history or evidence of brain metastases

          -  Prior chemotherapy for metastatic disease in castration-resistant prostate cancer

          -  Had surgery within 4 weeks prior to the first dose of study drug

          -  Had radiation, biological, or other investigational cancer therapy within 2 weeks
             prior to the first dose of study drug

          -  Had second-line hormonal therapy within 2 weeks prior to the first dose of study drug

          -  Systemic steroids within 1 weeks prior to the first dose of study drug

          -  Had prior enzalutamide, ARN-509, or galeterone therapy

          -  Have moderate or severe cardiovascular disease

          -  Have a history of a seizure

          -  Have uncontrolled intercurrent illness, including but not limited to ongoing or active
             infection, systematic congestive heart failure, unstable angina pectoris, cardiac
             arrhythmia or psychiatric condition that would limit compliance with study
             requirements

          -  Have a history of any autoimmune disease:inflammatory bowel disease, (including
             ulcerative colitis and Crohn's Disease), rheumatoid arthritis, systemic progressive
             sclerosis (scleroderma), systemic lupus erythmatosus (SLE) autoimmune vasculitis
             (e.g., Wegener's Granulomatosis), CNS or motor neuropathy considered to be of
             autoimmune origin (e.g., Guillian-Barre Syndrome, Myasthenia Gravis, Multiple
             Sclerosis)

          -  Have known history of infection with HIV, hepatitis B, or hepatitis C