CLINICAL TRIAL / NCT04191096

Inclusion Criteria:

         - Male participants with histologically- or cytologically-confirmed adenocarcinoma of the prostate without small cell histology

         - Has metastatic disease assessed by investigator and verified by BICR by either ≥2 bone lesions on bone scan and/or visceral disease by computed tomography/magnetic resonance imaging (CT/MRI)

         - Willing to maintain continuous ADT with a LHRH agonists or antagonists during study treatment or have a history of bilateral orchiectomy

         - Has an Eastern Cooperative Oncology Group (ECOG) performance status of 0 or 1 assessed within 10 days of randomization

         - Participants receiving bone resorptive therapy (including, but not limited to, bisphosphonate or denosumab) must have been on stable doses prior to randomization

         - Has adequate organ function

         - Has provided newly obtained core or excisional biopsy (obtained within 12 months of screening) from soft tissue not previously irradiated (samples from tumors progressing in a prior site of radiation are allowed). Participants with bone only or bone predominant disease may provide a bone biopsy sample

         - Male participants must agree to the following during the intervention period and for at least 120 days after the last dose of study intervention: Refrain from donating sperm PLUS either be abstinent from heterosexual intercourse and agree to remain abstinent OR agree to use contraception, unless confirmed to be azoospermic

         - Male participants must agree to use male condom when engaging in any activity that allows for passage of ejaculate to another person of any sex

        Exclusion Criteria:

         - Has a known additional malignancy that is progressing or has required active treatment in the last 3 years

         - Has an active autoimmune disease that has required systemic treatment in past 2 years

         - Has a diagnosis of immunodeficiency or is receiving chronic systemic steroid therapy

         - Has undergone major surgery including local prostate intervention (excluding prostate biopsy) within 28 days prior to randomization and not recovered adequately from the toxicities and/or complications

         - Has a gastrointestinal disorder affecting absorption or is unable to swallow tablets/capsules

         - Has an active infection (including tuberculosis) requiring systemic therapy

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

         - Has known active human immunodeficiency virus (HIV), hepatitis B virus (HBV) or hepatitis C virus (HCV) infection

         - Has known or suspected central nervous system (CNS) metastases and/or carcinomatous meningitis

         - Has a history of seizure or any condition that may predispose to seizure

         - Has a history of loss of consciousness within 12 months of screening

         - Has had myocardial infarction or uncontrolled angina within 6 months prior to randomization, or has New York Heart Association class III or IV congestive heart failure or a history of New York Heart Association class III or IV congestive heart failure

         - Has hypotension (systolic blood pressure <86 millimeters of mercury [mmHg]) or uncontrolled hypertension (systolic blood pressure >170 mmHg or diastolic blood pressure >105 mmHg) at the screening visit

         - Has a history of clinically significant ventricular arrhythmias

         - Has hypersensitivity to pembrolizumab and/or enzalutamide and/or any of their excipients

         - Has received prior ADT as neoadjuvant/adjuvant therapy for non-metastatic prostate cancer for >39 months in duration or within 9 months prior to randomization or with evidence of disease progression while receiving ADT

         - Has had prior treatment with a next generation hormonal agent (eg, abiraterone, enzalutamide, apalutamide, darolutamide)

         - Has received prior therapy with an anti-programmed cell death-1 (anti-PD-1), anti-programmed cell death-ligand 1 (anti-PD-L1), or anti PD-L2 agent or with an agent directed to another stimulatory or coinhibitory T-cell receptor

         - Has received a live vaccine within 30 days prior to randomization

         - Has a "superscan" bone scan

         - Has had an allogenic tissue/solid organ transplant

         - 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 study treatment

         - Has received any prior pharmacotherapy, radiation therapy or surgery for metastatic prostate cancer with the following exceptions:

             1. Up to 3 months of ADT or orchiectomy with or without concurrent first-generation antiandrogens, if patient was not treated with docetaxel

             2. May have 1 course of palliative radiation or surgical therapy to treat symptoms resulting from metastatic disease if it was administered at least 4 weeks prior to randomization

             3. For participants with low volume metastatic disease, may have 1 course of definitive radiotherapy if it was administered at least 4 weeks prior to randomization

             4. Up to 6 cycles of docetaxel therapy with final treatment administration completed within 2 months of randomization and no evidence of disease progression. In these participants up to 6 months of ADT permitted