This is a study of pembrolizumab (MK-3475) as first-line treatment for participants with advanced gastric or gastroesophageal junction (GEJ) adenocarcinoma. Participants whose tumors express programmed death-ligand 1 (PD-L1) will be randomly assigned to one of the three treatment arms of the study: pembrolizumab as monotherapy [pembro mono], pembrolizumab plus standard of care (SOC) chemotherapy with cisplatin plus 5-fluorouracil (5-FU) or capecitabine [pembro combo], or placebo plus SOC chemotherapy with cisplatin plus 5-fluorouracil (5-FU) or capecitabine [SOC]. The primary study hypotheses are that pembrolizumab in combination with SOC chemotherapy is superior to SOC chemotherapy alone in terms of Progression-free Survival (PFS) and Overall Survival (OS) in participants with PD-L1 Combined Positive Score (CPS) ≥1, pembrolizumab in combination with SOC chemotherapy is superior to SOC chemotherapy alone in terms of OS in participants with PD-L1 CPS ≥10, pembrolizumab monotherapy is non-inferior to SOC chemotherapy alone in terms of OS in participants with PD-L1 CPS ≥1, and pembrolizumab monotherapy is superior to SOC chemotherapy alone in terms of OS in participants with PD-L1 CPS ≥1 and in participants with PD-L1 CPS ≥10.
A Randomized, Active-Controlled, Partially Blinded, Biomarker Select, Phase III Clinical Trial of Pembrolizumab as Monotherapy and in Combination With Cisplatin+5-Fluorouracil Versus Placebo+Cisplatin+5-Fluorouracil as First-Line Treatment in Subjects With Advanced Gastric or Gastroesophageal Junction (GEJ) Adenocarcinoma
As specified by the protocol, primary and secondary efficacy analyses will be evaluated in gastric cancer participants with PD-L1 CPS ≥1 (all participants) and PD-L1 CPS ≥10 (OS) by comparing the pembro mono arm or pembro combo arm separately to the SOC arm.