A Study of Pembrolizumab (MK-3475) in Combination With Etoposide/Platinum (Cisplatin or Carboplatin) for Participants With Extensive Stage Small Cell Lung Cancer (MK-3475-604/KEYNOTE-604)

Inclusion Criteria

• Has a documented new diagnosis of SCLC by histology or cytology from brushing, washing, or needle aspiration of a defined lesion. Participants who do not have histology samples (defined as core or excisional biopsy, or resections) will need to undergo a new biopsy to provide a tissue sample.
• Has extensive-stage disease defined as Stage IV (T any, N any, M 1a/b) by the American Joint Committee on Cancer (AJCC), Seventh Edition
• Has ≥1 lesion that meets the criteria for measurable, as defined by RECIST 1.1, and is appropriate for selection as a target lesion, as determined by local site investigator/radiology assessment
• Has provided archival tumor tissue sample or newly obtained core or excisional biopsy of a tumor lesion not previously irradiated
• Has Eastern Cooperative Oncology Group (ECOG) Performance Status of 0 or 1
• Has a life expectancy of ≥3 months
• Has adequate organ function
• Female and male participants of childbearing potential must be willing to use an adequate method of contraception, starting with the first dose of study treatment through 120 days after the last dose of study treatment and up to 180 days after last dose of chemotherapeutic agents

Exclusion Criteria

• Has received prior systemic therapy for the treatment of SCLC
• Is currently participating and receiving study treatment or has participated in a study of an investigational agent and received study treatment or used an investigational device within 4 weeks of the first dose of treatment for another health-related problem
• Is expected to require any other form of antineoplastic therapy for SCLC, including radiation therapy, while on study. (Prophylactic cranial irradiation will be possible for those participants with stable disease or better at the completion of the 4 cycles of chemotherapy with or without pembrolizumab.)
• Has known central nervous system (ie, brain and/or spinal cord) metastases and/or carcinomatous meningitis. Participants with brain metastases may participate only if they satisfy all of the following:
• Has completed treatment (eg, whole brain radiation treatment [WBRT], stereotactic radiosurgery, or equivalent) ≥14 days prior to the first dose of study treatment,
• Has no evidence of new or enlarging brain metastases confirmed by post-treatment repeat brain imaging performed ≥3 weeks after pre-treatment brain imaging, and
• Is neurologically stable without the need for steroids for ≥7 days before first dose of study treatment.
• Has had major surgery within 3 weeks prior to receiving the first dose of study treatment or has not recovered adequately from toxicity and/or complications from an intervention prior to receiving the first dose of study treatment.
• Has a history of (non-infectious) pneumonitis that required steroids or has current pneumonitis
• Has a known history of interstitial lung disease
• Has a known additional malignancy that is progressing or requires active treatment. Exceptions include early stage cancers (carcinoma in situ or Stage 1) treated with curative intent, basal cell carcinoma of the skin, squamous cell carcinoma of the skin, in situ cervical cancer, or in situ breast cancer that has undergone potentially curative therapy.
• Has active autoimmune disease that has required systemic treatment in the past 2 years. Replacement therapy is not considered a form of systemic treatment.
• Has a known history of, or active, neurologic paraneoplastic syndrome
• Has clinically active diverticulitis, intra-abdominal abscess, gastrointestinal obstruction, and/or abdominal carcinomatosis
• Has a history of a severe hypersensitivity reaction to treatment with another monoclonal antibody
• Is taking chronic systemic steroids (in doses exceeding 10 mg daily of prednisone equivalent) within 7 days prior to the first dose of study treatment
• Has a diagnosis of immunodeficiency or is receiving any form of immunosuppressive therapy within 7 days prior to the first dose of study treatme