Study of Pembrolizumab (MK-3475) Plus Docetaxel Versus Placebo Plus Docetaxel in Chemotherapy-naïve Metastatic Castration-resistant Prostate Cancer (mCRPC) (MK-3475-921/KEYNOTE-921)-China Extension

Inclusion Criteria:

• Has histologically- or cytologically-confirmed adenocarcinoma of the prostatewithout small cell histology

• Has prostate cancer progression while on androgen deprivation therapy (or postbilateral orchiectomy) within 6 months prior to screening

• Has current evidence of metastatic disease documented by either bone lesions on bonescan and/or soft tissue disease by computed tomography/magnetic resonance imaging (CT/MRI)

• Has received prior treatment with one (but not more than one) NHA (eg, abirateroneacetate, enzalutamide, apalutamide, or darolutamide) for metastatichormone-sensitive prostate cancer (mHSPC) or castration-resistant prostate cancer (CRPC) and either a) progressed through treatment OR b) has become intolerant of thedrug

• 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 forat least 120 days after the last dose of pembrolizumab or for at least 180 daysafter the last dose of docetaxel (whichever is longer): Refrain from donating spermPLUS Use contraception unless confirmed to be azoospermic (vasectomized or secondaryto medical cause)

• Participants must agree to use male condom when engaging in any activity that allowsfor passage of ejaculate to another person of any sex

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

• Has an Eastern Cooperative Oncology Group (ECOG) performance status of 0 or 1assessed within 7 days of randomization

Exclusion Criteria:

• Has a known additional malignancy that is progressing or has required activetreatment in the last 3 years

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

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

• Has undergone major surgery including local prostate intervention (excludingprostate biopsy) within 28 days prior to randomization and not recovered adequatelyfrom the toxicities and/or complications

• Has a gastrointestinal disorder affecting absorption or is unable to swallowtablets/capsules

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

• Has a history of (non-infectious) pneumonitis that required steroids or currentpneumonitis

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

• Has known active central nervous system (CNS) metastases and/or carcinomatousmeningitis

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

• Has symptomatic congestive heart failure (New York Heart Association Class III or IVheart disease)

• Has had a prior anti-cancer monoclonal antibody (mAb) prior to randomization or whohas not recovered (i.e., Grade ≤1 or at baseline) from AEs due to mAbs

• Has used herbal products that may have hormonal anti-prostate cancer activity and/orare known to decrease PSA levels (e.g. saw palmetto) prior to randomization

• Has received prior treatment with radium or other therapeutic radiopharmaceuticalsfor prostate cancer

• 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 anagent directed to another stimulatory or co-inhibitory T-cell receptor (e.g.,cytotoxic T-lymphocyte-associated protein 4 [CTLA-4], OX-40, CD137)

• Has received prior treatment with docetaxel or another chemotherapy agent for mCRPC

• Has hypersensitivity to docetaxel or polysorbate 80

• Is currently receiving either strong or moderate inhibitors of cytochrome P450 (CYP)3A4 that cannot be discontinued for the duration of the study

• Has received prior targeted small molecule therapy or abiraterone acetate,enzalutamide, apalutamide, or darolutamide within 4 weeks prior to the first dose ofstudy treatment, or has not recovered (i.e., Grade ≤1 or at baseline) from AEs dueto a previously administered agent

• Has received prior radiotherapy to within 2 weeks of start of study treatment.Participants must have recovered from all radiation-related toxicities, not requirecorticosteroids, and not have had radiation pneumonitis

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

• Has received treatment with 5α reductase inhibitors (eg, finasteride ordutasteride), estrogens, and/or cyproterone within 4 weeks prior to randomization

• Has received prior treatment with ketoconazole for prostate cancer

• Is currently participating in or has participated in a study of an investigationalagent or has used an investigational device within 4 weeks prior to the first doseof study treatment

• Has a "superscan" bone scan

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

• Has had an allogenic tissue/solid organ transplant