Inclusion Criteria:

          -  Each subject must sign an informed consent form (ICF) indicating that he understands
             the purpose of and procedures required for the study and is willing to participate in
             the study. Consent is to be obtained prior to the initiation of any study related
             tests or procedures that are not part of standard of care for the subject's disease.

          -  Histologically confirmed adenocarcinoma of the prostate with metastatic disease.

          -  Progression on abiraterone defined by a rise in PSA at 2 time points at least 1 week
             apart.

          -  Male ≥18 years of age.

          -  Prior orchiectomy or serum testosterone levels < 50 ng/dL determined within 4 weeks
             prior to start of study drug

          -  Adequate baseline organ function as defined below:

               -  Hemoglobin > 9 with or without transfusion

               -  Platelets > 75 with or without transfusion

               -  Neutrophil: Absolute neutrophil > 1.0

               -  T bilirubin < 1.5 x Upper limit normal (ULN)

               -  Aspartate aminotransferase (AST)/Alanine Aminotransferase (ALT) < 2.5 x ULN

               -  Creatinine < 1.5 x ULN

          -  Eastern Cooperative Oncology Group (ECOG) performance status of 0 or 1 within 2 weeks
             of initiation of study drug administration.

          -  Ongoing androgen depletion therapy with a gonadotropin releasing hormone (GnRH) analog
             or inhibitor, or orchiectomy (ie, surgical or medical castration). Note: subjects who
             have not undergone orchiectomy must continue GnRH analog therapy for the duration of
             this protocol.

          -  For subjects previously treated with 1st generation anti androgens (ie, flutamide,
             nilutamide, or bicalutamide), discontinuation of flutamide or nilutamide therapy must
             occur > 4 weeks (> 6 weeks for bicalutamide) prior to start of study drug with no
             evidence of an anti androgen withdrawal response (ie, no decline in serum PSA; within
             6 weeks of last dose for bicalutamide and 4 weeks of last dose for all other drugs as
             above).

          -  For subjects previously treated with chemotherapy, targeted therapy, immunotherapy, or
             treatment with an investigational anticancer agent, discontinuation must have occurred
             ≥ 2 weeks, or after at least 4 half lives, whichever is longer, prior to study drug
             administration. For enzalutamide and apalutamide, the washout period will be at least
             3 weeks prior to start of study drug with no evidence of an anti androgen withdrawal
             response (ie, no decline in serum PSA within 4 weeks of last dose.)

          -  For subjects previously treated with other agents approved for the treatment of
             prostate cancer (5 α reductase inhibitors, estrogens, others), discontinuation of
             therapy must have occurred ≥ 4 weeks prior to start of study drug. This does not apply
             to abiraterone.

          -  Palliative radiotherapy (to bone or soft tissue lesions) must be completed > 2 weeks
             prior to start of study drug.

          -  For subjects receiving bone-loss prevention treatment (eg, bisphosphonates or
             denosumab), the subject must be on stable dose ≥ 4 weeks prior to start of study drug.

          -  QT interval corrected using Fridericia's method (QTcF) less than 460 msec (see
             Appendix B for Fridericia's criteria).

          -  A man who is sexually active with a woman of childbearing potential must agree to use
             an adequate method of contraception to avoid conception during the study and for 120
             days after receiving the last dose of study drug. All men must also not donate sperm
             during the study and for 120 days after receiving the last dose of study drug.

          -  Subject must be willing and able to adhere to the prohibitions and restrictions
             specified in this protocol.

        Exclusion Criteria:

          -  Previously documented or current brain metastases.

          -  Untreated spinal cord compression.

          -  Known positive test result for human immunodeficiency virus.

          -  History of clinically significant cardiovascular disease including, but not limited
             to:

               -  Myocardial infarction or unstable angina within the 6 months prior to the first
                  dose of study drug.

               -  Clinically significant cardiac arrhythmia.

               -  Uncontrolled (persistent) hypertension: systolic blood pressure > 180 mHg;
                  diastolic blood pressure >100 mmHg.

               -  Congestive heart failure (New York Heart Association class III IV).

          -  Known active or chronic hepatitis B or hepatitis C as demonstrated by hepatitis B
             surface antigen positivity and/or anti hepatitis C virus positivity, respectively.
             Subjects with clinically active or chronic liver disease, including liver cirrhosis of
             Child Pugh class C, are also excluded.

          -  History of a different malignancy except for the following circumstances: (a)
             individuals with a history of other malignancies are eligible if they have been
             disease free for at least 3 years and are deemed by the investigator to be at low risk
             for recurrence of that malignancy, (b) individuals with a history of treatment for the
             following cancers are eligible: non muscle invasive bladder cancer, basal cell, or
             squamous cell carcinoma of the skin and resected melanoma in situ.

          -  Any serious underlying medical or psychiatric condition (eg, alcohol or drug abuse),
             dementia or altered mental status or any issue that would impair the ability of the
             subject to receive or tolerate the planned treatment, to understand informed consent
             or that in the opinion of the investigator would contraindicate the subject's
             participation in the study or that would confound the results of the study.

          -  Evidence of active viral, bacterial, or systemic fungal infection requiring systemic
             treatment within 7 days prior to the first dose of study drug. Subjects requiring any
             systemic antiviral, antifungal, or antibacterial therapy for active infection must
             have completed treatment no less than 7 days prior to the first dose of study drug.

          -  Enrollment in another therapeutic study.

          -  Major surgery (eg, requiring general anesthesia) within 3 weeks before screening, or
             has not fully recovered from prior surgery (ie, unhealed wound), or surgery planned
             during the time the subject is expected to participate in the study. Note: subjects
             with planned surgical procedures to be conducted under local anesthesia may
             participate.