Inclusion Criteria:

          -  Adults (Age ≥ 18 years)

          -  Eastern Cooperative Oncology Group (ECOG) performance status 0-1

          -  Life expectancy of at least 12 weeks

          -  Histologically or cytologically confirmed adenocarcinoma of the prostate

          -  Progressive disease in the setting of medical or surgical castration (i.e. CRPC)

          -  Documented metastatic disease

          -  Must have undergone bilateral orchiectomy (surgical castration) or be willing to
             continue gonadotropin-releasing hormone (GnRH) analog or antagonist (medical
             castration)

          -  Serum testosterone <50 ng/dL

          -  Receipt of prior line of second generation androgen inhibitor

          -  Demonstrate adequate organ function as defined below:

               -  Absolute Neutrophil Count (ANC) ≥ 1,000/μL

               -  Platelet Count ≥ 100,000/μL

               -  Hemoglobin (Hgb) ≥ 8 g/dL

               -  Serum creatinine ≤ 2 × upper limit of normal (ULN) OR

               -  Creatinine clearance (CrCl) ≥ 40 mL/min as estimated by the Cockcroft and Gault
                  formula1 in subjects with creatinine > 2 X ULN

               -  Bilirubin ≤ 1.5 × ULN unless evidence of Gilbert's disease in which case < 3 x
                  ULN

               -  Aspartate aminotransferase (AST) ≤ 2.5 × ULN without liver metastases; must be ≤
                  5 × ULN with liver metastases

               -  Alanine aminotransferase (ALT) ≤ 2.5 × ULN without liver metastases; must be ≤ 5
                  × ULN with liver metastases

        Exclusion Criteria:

          -  Known symptomatic brain metastases (NOTE: patients with treated epidural disease are
             allowed)

          -  Treatment with any of the following for prostate cancer within the indicated timeframe
             prior to day 1 of treatment:

               1. First generation: AR antagonists (e.g., bicalutamide, nilutamide, flutamide)
                  within 4 weeks

               2. 5 alpha reductase inhibitors, ketoconazole, estrogens (including
                  diethylstilbesterol [DES]), or progesterones within 2 weeks

               3. Chemotherapy within 3 weeks

               4. Biologic therapy within 4 weeks

               5. Investigational therapy within 3 weeks (or within a time interval less than at
                  least 5 half-lives of the investigational agent [if known], whichever is longer).

               6. Immunotherapy within 4 weeks

               7. Prior radionuclide therapy within 4 weeks