Inclusion Criteria:

          -  Male participants who are at least 18 years of age on the day of signing informed
             consent with histologically confirmed diagnosis of prostate adenocarcinoma.

          -  A male participant must agree to use a contraception during the treatment period and
             for at least 4 months after the last dose of study treatment and refrain from donating
             sperm during this period.

          -  Documented progressive metastatic CRPC based on Prostate Cancer Working Group 3
             (PCWG3) criteria, which includes at least one of the following criteria: PSA
             progression, Objective radiographic progression in soft tissue, New bone lesions

          -  ECOG performance status of 0-1

          -  Have serum testosterone < 50 ng/dL. Subjects must continue primary androgen
             deprivation with an LHRH/GnRH analogue (agonist/antagonist) if they have not undergone
             orchiectomy.

          -  Have previously been treated with at least one of the following in any disease state:
             Androgen receptor signaling inhibitor (such as enzalutamide), CYP 17 inhibitor (such
             as abiraterone acetate). These drugs may have been initiated in the metastatic hormone
             sensitive or non-metastatic (M0) CRPC setting provided they meet criteria for
             progressive mCRPC at study entry.

          -  Age > 18 years

          -  Patients must have normal organ and marrow function as defined: Absolute neutrophil
             count >2,000 cells/mm3, Hemoglobin ≥9 g/dL, Platelet count >150,000 x 109/mcL, Serum
             creatinine <1.5 x upper limit of normal (ULN) or calculated creatinine clearance ≥ 50
             mL/min/1.73 m2 by Cockcroft-Gault, Serum total bilirubin <1.5 x ULN (unless due to
             Gilbert's syndrome in which case direct bilirubin must be normal), Serum AST and ALT
             <3 x ULN in absence of liver metastases; < 5x ULN if due to liver metastases (in both
             circumstances bilirubin must meet entry criteria), Serum internalized normalized ratio
             (INR) OR prothrombin time (PT) AND activated partial thromboplastin time (aPTT) must
             be <1.5 x ULN unless participant is receiving anticoagulant therapy as long as PT or
             aPTT is within therapeutic range of intended use of anticoagulants.

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

        Exclusion Criteria:

          -  Prior receipt of chemotherapy for castration-resistant prostate cancer. Prior receipt
             of docetaxel chemotherapy in the hormone sensitive setting or for localized disease is
             acceptable.

          -  Has received prior therapy with an anti-PD-1, anti-PD-L1, or anti PD L2 agent or with
             an agent directed to another stimulatory or co-inhibitory T-cell receptor (eg, CTLA-4,
             OX 40, CD137).

          -  Has active autoimmune disease that has required systemic treatment in the past 2 years
             (i.e. with use of disease modifying agents, corticosteroids or immunosuppressive
             drugs). Replacement therapy (eg., thyroxine, insulin, or physiologic corticosteroid
             replacement therapy for adrenal or pituitary insufficiency, etc.) is not considered a
             form of systemic treatment and is allowed.

          -  Prior bone-seeking beta-emitting radioisotopes (e.g. Sm-153, Sr-89) or investigational
             PSMA-targeted therapy; prior radium-223 is allowed provided last dose administered >12
             weeks prior to C1D1 on this study

          -  Has a history of a second malignancy, unless potentially curative treatment has been
             completed with no evidence of malignancy for 2 years.

          -  Known history of myelodysplastic syndrome

          -  Has a known history of Human Immunodeficiency Virus (HIV) infection

          -  Has a known history of Hepatitis B (defined as Hepatitis B surface antigen [HBsAg]
             reactive) or known active Hepatitis C virus (defined as HCV RNA [qualitative] is
             detected) infection.

          -  Has a diagnosis of immunodeficiency or is receiving chronic systemic steroid therapy
             (in dosing exceeding 10 mg daily of prednisone equivalent) or any other form of
             immunosuppressive therapy within 7 days prior to the first dose of study drug.

          -  Is currently participating in or has participated in a study of an investigational
             agent or has used an investigational device within 4 weeks prior to the first dose of
             study intervention.

          -  Has received prior systemic anti-cancer therapy including investigational agents
             within 4 weeks or <5 half-lives prior to enrollment.

          -  Has known active CNS metastases and/or carcinomatous meningitis. Participants with
             previously treated brain metastases may participate provided they are radiologically
             stable, i.e. without evidence of progression for at least 4 weeks by repeat imaging
             (note that the repeat imaging should be performed during study screening), clinically
             stable and without requirement of steroid treatment for at least 14 days prior to
             first dose of study intervention.

          -  Diagnosis of deep vein thrombosis and/or pulmonary embolus within 1 month of C1D1

          -  Has a history or current evidence of any condition, therapy, or laboratory abnormality
             that might confound the results of the study, interfere with the subject's
             participation for the full duration of the study, or is not in the best interest of
             the subject to participate, in the opinion of the treating investigator

          -  Has received radiotherapy within 4 weeks of start of study treatment. Participants
             must have recovered from all radiation-related toxicities, not require
             corticosteroids, and not have had radiation pneumonitis. A 1-week washout is permitted
             for palliative radiation (≤2 weeks of radiotherapy) to non-CNS disease.

          -  Patients on stable dose of bisphosphonates or denosumab, which have been started no
             less than 4 weeks prior to treatment start, may continue on this medication, however
             patients are not allowed to initiate bisphosphonate/Denosumab therapy during the
             DLT-assessment period of the study. These drugs may be added after week 12.

          -  Unless azoospermia is present (whether due to surgery or underlying medical
             condition), having partners of childbearing potential and not willing to use a method
             of birth control deemed acceptable by the principal investigator and chairperson
             during the study and for 4 months after last study drug administration.

          -  Is expecting to conceive or father children within the projected duration of the
             study, starting with the screening visit through 120 days after the last dose of trial
             treatment

          -  Has received a live vaccine within 30 days. Examples of live vaccines include, but are
             not limited to, the following: measles, mumps, rubella, varicella/zoster (chicken
             pox), yellow fever, rabies, Bacillus Calmette-Guérin (BCG), and typhoid vaccine.
             Seasonal influenza vaccines for injection are generally killed virus vaccines and are
             allowed; however, intranasal influenza vaccines (eg, FluMist®) are live attenuated
             vaccines and are not allowed.

          -  Has severe hypersensitivity (≥Grade 3) to pembrolizumab and/or any of its excipients.

          -  Has a history of (non-infectious) pneumonitis that required steroids or has current
             pneumonitis.

          -  Has an active infection requiring systemic therapy at the time of treatment initiation

          -  Has a known history of active TB (Bacillus Tuberculosis)

          -  Has known psychiatric or substance abuse disorders that would interfere with
             cooperation with the requirements of the trial

          -  Has had an allogenic tissue/solid organ transplant