Key Inclusion Criteria:

          1. Male patients ≥18 years of age

          2. Histologically or cytologically confirmed adenocarcinoma of the prostate

          3. Progressive metastatic castrate-resistant prostate cancer (mCRPC):

               1. Serum testosterone levels less than 50 ng/dL (or ≤0.50 ng/mL or 1.73 nmol/L)
                  within 28 days prior to start of study drug

               2. Radiographic evidence of metastatic disease

               3. Disease progression on the prior systemic regimen

          4. Must have received at least 2 prior systemic therapies approved for mCRPC

          5. Eastern Cooperative Oncology Group (ECOG) performance status of 0 or 1

          6. Adequate bone marrow function

          7. Able to read, understand and provide written informed consent

        Key Exclusion Criteria:

          1. Previously treated or current brain metastases

          2. Untreated spinal cord compression. Participants must be neurologically stable off
             steroids for at least 4 weeks prior to first dose of study drug

          3. Concurrent treatment with anti-tumor necrosis factor (TNF) alpha therapies, systemic
             corticosteroids (prednisone dose >10 mg per day or equivalent), or other immune
             suppressive drugs within the 2 weeks prior to first dose of study drug

          4. History of or known or suspected autoimmune disease (exception(s): patients with
             vitiligo, resolved childhood atopic dermatitis, hypothyroidism, or hyperthyroidism
             that is clinically euthyroid at Screening are allowed)

          5. History of clinically significant cardiovascular disease such as symptomatic
             congestive heart failure (CHF), myocardial infarction within 6 months before first
             dose of study drug, history of thromboembolic event within 3 months before first dose
             of study drug

          6. Known active or chronic hepatitis B or hepatitis C as demonstrated by hepatitis B
             surface antigen (HBsAg) positivity and/or anti-hepatitis C virus (HCV) positivity,
             respectively, or known history of human immunodeficiency virus (HIV) seropositive
             status

          7. Clinically active liver disease, including liver cirrhosis that is Child-Pugh class B
             or C

          8. Second primary malignancy that has not been in remission for at least 3 years.