Pembrolizumab/Vibostolimab (MK-7684A) or Atezolizumab in Combination With Chemotherapy in First Line Treatment of Extensive-Stage Small Cell Lung Cancer (MK-7684A-008/KEYVIBE-008)

This study will evaluate the combination of a fixed dose pembrolizumab/vibostolimab co-formulation (MK-7684A) with etoposide/platinum chemotherapy followed by MK-7684A compared to the combination of atezolizumab with etoposide/platinum chemotherapy followed by atezolizumab in the first-line treatment of Extensive-Stage Small Cell Lung Cancer (ES-SCLC). The primary hypothesis is, with respect to overall survival, MK-7684A in combination with the background therapy of etoposide/platinum followed by MK-7684A, is superior to atezolizumab in combination with the background therapy of etoposide/platinum followed by atezolizumab.

With Amendment 4, the experimental Pembrolizumab/Vibostolimab arm (MK-7684A) was discontinued and all ongoing participants were offered an option to move to the comparator Atezolizumab monotherapy arm for the remainder of the study. There will be no further analyses of efficacy, and no European Organization for Research and Treatment of Cancer (EORTC)-Quality of Life Questionnaire-Core 30 (QLQ-C30) and Lung Cancer 13 module (EORTC QLQ-LC13) outcome measures will be reported. Effective as of Amendment 5, participants with access to approved standard of care (SOC) should be considered for discontinuation from the study. Those benefiting from atezolizumab, but unable to access it as SOC outside the study, may continue on study and receive treatment with atezolizumab until discontinuation criteria are met.

Inclusion Criteria: The main inclusion criteria include but are not limited to the following: * Has histologically or cytologically confirmed diagnosis of extensive-stage small cell lung cancer (ES-SCLC) in need of first-line therapy * Has ES-SCLC defined as Stage IV (T any, N any, M1a/b/c) by the American Joint Committee on Cancer, Eighth Edition or T3-T4 due to multiple lung nodules that are too extensive or have tumor/nodal volume that is too large to be encompassed in a tolerable radiation plan * Males agree to use contraception, refrain from donating sperm, and abstain from heterosexual intercourse * Females are not pregnant or breastfeeding, is not a woman of childbearing potential (WOCBP) or is a WOCBP who uses a highly effective contraceptive method, or is abstinent from heterosexual intercourse * Has measurable disease per Response Evaluation Criteria In Solid Tumors (RECIST) 1.1 * Has a predicted life expectancy of \>3 months Exclusion Criteria: The main exclusion criteria include but are not limited to the following: * Is considered a poor medical risk due to a serious, uncontrolled medical disorder or non-malignant systemic disease * Has received prior treatment for Small Cell Lung Cancer (SCLC) * Is expected to require any other form of antineoplastic therapy for SCLC while on study * Has received a live or live-attenuated vaccine within 30 days before the first dose of study intervention * Has received an investigational agent or has used an investigational device within 4 weeks prior to study intervention administration * Has a diagnosis of immunodeficiency or is receiving chronic systemic steroid therapy or any other form of immunosuppressive therapy within 7 days prior the first dose of study medication * Has a known additional malignancy that is progressing or has required active treatment within the past 3 years * Has known active central nervous system (CNS) metastases and/or carcinomatous meningitis * Has a history of severe hypersensitivity reaction (≥Grade 3) to any study intervention and/or any of its excipients * Has an active autoimmune disease that has required systemic treatment in past 2 years * Has a history of (noninfectious) pneumonitis/interstitial lung disease that required steroids or has current pneumonitis/interstitial lung disease * Has a known history of, or active, neurologic paraneoplastic syndrome * Has an active infection requiring systemic therapy * Has a known history of human immunodeficiency virus (HIV) infection * Has a known history of Hepatitis B or known active Hepatitis C virus infection * Has had an allogenic tissue/solid organ transplant * Has had major surgery within prior 3 weeks or has not recovered adequately from toxicity and/or complications from an intervention prior to receiving the first dose of study intervention * Has symptomatic ascites or pleural effusion