Study of Pembrolizumab (MK-3475) Plus Enzalutamide Versus Placebo Plus Enzalutamide in Participants With Metastatic Castration-resistant Prostate Cancer (mCRPC) (MK-3475-641/KEYNOTE-641)

The main inclusion and exclusion criteria include but are not limited to the following:

Inclusion Criteria:

Has histologically- or cytologically-confirmed adenocarcinoma of the prostate without small cell histology
Has prostate cancer progression while on androgen deprivation therapy (or post bilateral orchiectomy) within 6 months prior to randomization
Has current evidence of metastatic disease documented by either bone lesions on bone scan and/or soft tissue disease by computed tomography/magnetic resonance imaging (CT/MRI)
Has met one of the following criteria with regard to abiraterone acetate exposure: (1) is abiraterone-naïve; (2) received prior abiraterone acetate for the treatment of mHSPC or mCRPC, for a minimum of 4 weeks and not progressed while on treatment; or (3) received prior abiraterone acetate for the treatment of mHSPC or mCRPC and progressed on treatment after a minimum of 8 weeks treatment (minimum 14 weeks for those with bone progression)
Has ongoing androgen deprivation with serum testosterone <50 ng/dL (<2.0 nM)
Participants receiving bone resorptive therapy (including, but not limited to, bisphosphonate or denosumab) must have been on stable doses prior to randomization
Participants must agree to the following during the study treatment period and for at least 90 days after the last dose of enzalutamide: Refrain from donating sperm, plus EITHER be abstinent OR must agree to use male condom
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
Has an Eastern Cooperative Oncology Group (ECOG) performance status of 0 or 1 assessed within 7 days of randomization

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 (noninfectious) pneumonitis/interstitial lung disease that required steroids or current pneumonitis/interstitial lung disease
Has known active human immunodeficiency virus (HIV), concurrent active hepatitis B virus (HBV) or known active hepatitis C virus (HCV) infection
Has known active central nervous system (CNS) metastases and/or carcinomatous meningitis
Has hypersensitivity to pembrolizumab and/or enzalutamide and/or any of their excipients
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 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 bradycardia (heart rate of <50 beats per minute) on the screening electrocardiogram (ECG)
Has history of prostate cancer progression on ketoconazole
Has had prior treatment with enzalutamide, apalutamide, darolutamide or cytochrome P450 (CYP) 17 inhibitor other than abiraterone acetate
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 prior treatment with radium or other therapeutic radiopharmaceuticals for prostate cancer
Has received prior treatment with docetaxel or another chemotherapy agent for mCRPC
Has had a prior anti-cancer monoclonal antibody (mAb) prior to randomization
Has used herbal products that may have hormonal anti-prostate cancer activity and/or are known to decrease PSA levels (eg, saw palmetto) prior to randomization
Has received a live or live attenuated vaccine within 30 days prior to randomization
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 treatment
Has a "superscan" bone scan
Is expecting to conceive or father children within the projected duration of the study, starting with the screening visit through 90 days after the last dose of enzalutamide
Has had an allogenic tissue/solid organ transplant