Study of Pembrolizumab (MK-3475) Plus Chemotherapy vs. Placebo Plus Chemotherapy for Previously Untreated Locally Recurrent Inoperable or Metastatic Triple Negative Breast Cancer (MK-3475-355/KEYNOTE-355)

Inclusion Criteria:

• Has locally recurrent inoperable breast cancer not previously treated withchemotherapy and which cannot be treated with curative intent OR has metastaticbreast cancer not previously treated with chemotherapy.

• Has centrally confirmed TNBC, as defined by the most recent American Society ofClinical Oncology/college of American Pathologists (ASCO/CAP) guidelines.

• Has completed treatment for Stage I-III breast cancer, if indicated, and ≥6 monthselapsed between the completion of treatment with curative intent (e.g., date ofprimary breast tumor surgery or date of last adjuvant chemotherapy administration,whichever occurred last) and first documented local or distant disease recurrence.

• Has been treated with (neo)adjuvant anthracycline, if they received systemictreatment in the (neo)adjuvant setting, unless anthracycline was contraindicated ornot considered the best treatment option for the participant in the opinion of thetreating physician.

• Has measurable disease based on Response Evaluation Criteria in Solid Tumors version 1.1 (RECIST 1.1) as determined by local radiology review.

• Has provided recently or newly obtained core or excisional biopsy from a locallyrecurrent inoperable or metastatic tumor lesion for central determination of TNBCstatus and PD-L1 expression, unless contraindicated due to site inaccessibilityand/or participant safety concerns.

• Has an Eastern Cooperative Oncology Group (ECOG) performance status of 0 or 1, asassessed within 10 days prior to the start of study drug.

• Has a life expectancy ≥12 weeks from randomization.

• Demonstrates adequate organ function, within 10 days prior to the start of studydrug.

• Female participants are eligible to participate if they are not pregnant orbreastfeeding AND they are not a woman of childbearing potential (WOCBP) OR is aWOCBP using a contraceptive method that is highly effective or is abstinent fromheterosexual intercourse during the intervention period and for at least the timeneeded to eliminate each study intervention after the last dose of studyintervention AND has a negative highly-sensitive pregnancy test ([urine or serum] asrequired by local regulations) within 24 hours (urine) or 72 hours (serum) beforethe first dose of study intervention.

• Male participants are eligible to participate if they agree to refrain from donatingsperm during the intervention period and for at least the time needed to eliminateeach study intervention after the last dose of study intervention PLUS be abstinentfrom heterosexual intercourse OR must agree to use contraception unless confirmed tobe azoospermic.

Exclusion Criteria:

• Is currently participating in a clinical study and receiving an investigationalagent and/or using an investigational device, or has participated in a clinicalstudy and received an investigational agent and/or used an investigational devicewithin 4 weeks prior to randomization.

• Has not recovered (e.g., to ≤ Grade 1 or to baseline) from AEs due to a previouslyadministered therapy.

• Has neuropathy ≥ Grade 2.

• Has an active autoimmune disease that has required systemic treatment in the past 2years (e.g., with use of disease modifying agents, corticosteroids, orimmunosuppressive drugs).

• Has a diagnosis of immunodeficiency or is receiving systemic steroid therapy or anyother form of immunosuppressive therapy within 7 days prior to randomization.

• Has a known additional malignancy that progressed or required active treatmentwithin the last 5 years. Exceptions include basal cell carcinoma of the skin,squamous cell carcinoma of the skin that has undergone potentially curative therapy,and in situ cervical cancer.

• Has known active central nervous system (CNS) metastases and/or carcinomatousmeningitis. Participants with previously treated brain metastases may participateprovided they have stable brain metastases and did not receive chemotherapy formetastatic breast cancer.

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

• Has active, or a history of, interstitial lung disease.

• Has a known history of active tuberculosis (TB).

• Has an active infection requiring systemic therapy.

• Has a history of Class II-IV congestive heart failure or myocardial infarctionwithin 6 months of randomization.

• Has a known psychiatric or substance abuse disorders that would interfere withcooperation with the requirements of the study.

• Is pregnant or breastfeeding, or expecting to conceive or father children within theprojected duration of the study, starting with the screening visit through 120 days (or longer as specified by local institutional guidelines) after the last dose ofstudy drug.

• Has received prior therapy with an anti-programmed cell death 1 (anti-PD-1),anti-PD-L1, or anti-PD-L2 agent or with an agent directed to another co-inhibitory Tcell receptor (such as cytotoxic T-lymphocyte-associated protein 4 [CTLA-4], OX-40,CD137) or has previously participated in Merck pembrolizumab (MK-3475) clinicalstudies.

• Has a known history of human immunodeficiency virus (HIV).

• Has known active hepatitis B or hepatitis C.

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

• Has a known history of hypersensitivity or allergy to pembrolizumab and any of itscomponents and/or to any of the study chemotherapies (e.g., nab-paclitaxel,paclitaxel, gemcitabine, or carboplatin) and any of their components.

• Is receiving any medication prohibited in combination with study chemotherapies asdescribed in the respective product labels, unless medication was stopped within 7days prior to randomization.