Efficacy and Safety of Pembrolizumab (MK-3475) Plus Enzalutamide Plus Androgen Deprivation Therapy (ADT) Versus Placebo Plus Enzalutamide Plus ADT in Participants With Metastatic Hormone-Sensitive Prostate Cancer (mHSPC) (MK-3475-991/KEYNOTE-991)

Open to males ages 18 years and up

• 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 Androgen Deprivation Therapy (ADT) with a luteinizing-hormone releasing hormone (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

• 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-programmed cell death-ligand 2 (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