Inclusion Criteria:

          -  Histologically or cytologically confirmed prostate cancer

          -  Evidence of castrate testosterone level < 50 ng/dL (or surgical castration)

          -  For Phase I portion of the study: evidence of disease progression:

               -  2 or more new lesions on bone scan or

               -  Progressive disease on computed tomography (CT)/magnetic resonance imaging (MRI)
                  according to Response Evaluation Criteria in Solid Tumors (RECIST) 1.1 criteria
                  or

               -  Rising PSA: PSA evidence for progressive prostate cancer consists of a minimum
                  PSA level of at least 2 ng/ml, which has subsequently risen on at least 2
                  successive occasions, at least 2 weeks apart

          -  For Phase II portion of the study:

               -  Subjects must be on enzalutamide for metastatic CRPC and within the first 12
                  weeks of enzalutamide at 160mg/day

               -  Record of subject's enzalutamide start date and baseline PSA (within 28 days of
                  starting) before starting enzalutamide available

          -  Subjects must have documented clinically stable disease or better during the screening
             period of the study as defined by all of the following:

               -  PSA =<1.25 times the PSA at start of enzalutamide

               -  Lack of radiographic progression as defined by Response Evaluation Criteria in
                  Solid Tumors (RECIST) 1.1 and Prostate Cancer Working Group Criteria

               -  Clinically stable as confirmed by treating physician

          -  Any prior therapy for castrate disease is acceptable except prior specific cytochrome
             P450 family 17 (CYP17) antagonists (e.g. abiraterone acetate, orteronel) or prior
             second generation AR antagonists (e.g. enzalutamide or ARN509) which are excluded
             other than enzalutamide as specified for phase II portion; a minimum washout of 28
             days for any other anticancer therapy prior to first dose of study drug is required
             (only applicable for phase I)

          -  Any other radiotherapy or radionuclide require 28-day washout prior to first dose of
             study drug

          -  Denosumab or zoledronic acid are allowed

          -  Eastern Cooperative Oncology Group (ECOG) performance status =< 2

          -  Total bilirubin =< 1.5 x the upper limit of normal

          -  Aspartate aminotransferase (AST) (serum glutamic oxaloacetic transaminase
             [SGOT])/alanine aminotransferase (ALT) (serum glutamate pyruvate transaminase [SGPT])
             =< 2.5 x institutional upper limit of normal

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

        Exclusion Criteria:

          -  Therapy with other hormonal therapy, including any dose of megestrol acetate (Megace),
             finasteride (Proscar), dutasteride (Avodart), or any herbal product known to decrease
             PSA levels (e.g., saw palmetto and PC-SPES), or any systemic corticosteroid within 2
             weeks prior to first dose of study drug

          -  Inability to swallow capsules or known gastrointestinal malabsorption

          -  History of other malignancies, with the exception of: adequately treated non-melanoma
             skin cancer, adequately treated superficial bladder cancer, stage 1 or 2 solid tumor
             malignancies who are without evidence of disease, or other solid tumors curatively
             treated with no evidence of disease for >= 5 years from enrollment

          -  Blood pressure that is not controlled despite > 2 oral agents (systolic blood pressure
             [SBP] > 160 and diastolic blood pressure [DBP] > 90 documented during the screening
             period with no subsequent blood pressure readings < 160/100)

          -  History of seizure disorder or active use of anticonvulsants

          -  Corrected QT interval (QTc) on electrocardiogram (EKG) > 450 msec

          -  Serious intercurrent infections or non-malignant medical illnesses that are
             uncontrolled

          -  Active psychiatric illness/social situations that would limit compliance with protocol
             requirements

          -  New York Heart Association (NYHA) class II, NYHA class III, or IV congestive heart
             failure (any symptomatic heart failure)

          -  Concurrent therapy with strong inhibitors or inducers of cytochrome P450 family 3,
             subfamily A, polypeptide 4 (CYP3A4) or cytochrome P450 family 2, subfamily C,
             polypeptide 8 (CYP2C8) due to concerning possible drug-drug interactions