KEYNOTE-059 cohort 2: Safety and efficacy of pembrolizumab (pembro) plus 5-fluorouracil (5-FU) and cisplatin for first-line (1L) treatment of advanced gastric cancer.

4012Background: Preliminary analyses from the global, multicohort, phase 2 KEYNOTE-059 (NCT02335411) study suggested that safety of pembro + 5-FU + cisplatin is manageable as 1L therapy in pts with advanced gastric or gastroesophageal junction (G/GEJ) cancer (cohort 2). We present efficacy and updated safety data from KEYNOTE-059 cohort 2. Methods: Cohort 2 enrolled pts ≥18 y with HER2– recurrent or metastatic G/GEJ adenocarcinoma, measurable disease, no prior therapy for metastatic/advanced disease, and ECOG PS 0-1. Pts received pembro 200 mg on day 1 of each 21-day cycle + cisplatin 80 mg/m2 for 6 cycles + 5-FU 800 mg/m2 (or capecitabine 1000 mg/m2 in Japan) Q3W for up to 2 y or until disease progression, investigator/pt decision to withdrawal, or unacceptable toxicity. PD-L1+pts had expression in ≥1% tumor or stromal cells using IHC (22C3 antibody). End points were safety and tolerability (primary), ORR (RECIST v1.1, by central review), DOR, PFS, and OS (secondary). Results: Of 25 enrolled pts, 64% wer...