Table 1. . Study treatments.

Treatment	Dose	Frequency	Route of administration	Dosing time of each 3-week cycle
Pembrolizumab	200 mg	Q3W	IV infusion	Day 1 of each cycle
Placebo (normal saline)	NA	Q3W	IV infusion	Day 1 of each cycle
FP†
– Cisplatin‡	80 mg/m2¶	Q3W	IV infusion	Day 1 of each cycle
– 5-Fluorouracil	800 mg/m2/day¶	Continuous	IV infusion	Days 1–5 of each cycle
CAPOX†
– Oxaliplatin	130 mg/m2¶	Q3W	IV infusion	Day 1 of each cycle
– Capecitabine	1000 mg/m2¶	BID	Oral	Days 1–14 of each cycle
SOX§
– S-1	<1.25 m2; BSA: 40 mg¶ 1.25 to <1.5 m2; BSA: 50 mg¶ ≥1.5 m2; BSA: 60 mg¶	BID	Oral	Days 1–14 of each cycle
– Oxaliplatin	130 mg/m2¶	Q3W	IV infusion	Day 1 of each cycle
– Trastuzumab	8 mg/kg loading dose 6 mg/kg maintenance dose	Q3W	IV infusion	Day 1 of each cycle

^†Choice of chemotherapy backbone to be decided by the investigator before randomization.

^‡Duration of cisplatin treatment may be capped at 6 cycles per local country guidelines; however, treatment with 5-fluorouracil may continue per protocol.

^§SOX received only by patients in the Japan-specific cohort.

^¶Body surface area to be calculated per local guidance.

BID: Twice daily; CAPOX: Capecitabine + oxaliplatin; FP: 5-Fluorouracil + cisplatin; IV: Intravenous; Q3W: Every 3 weeks; SOX: S-1 + oxaliplatin.