Trial Outcomes & Findings for Pembrolizumab (MK-3475) Plus Chemotherapy Versus Placebo Plus Chemotherapy in Participants Gastric or Gastroesophageal Junction (GEJ) Adenocarcinoma (MK-3475-859/KEYNOTE-859) (NCT NCT03675737)

Last Updated: 2025-07-24

Results Overview

OS was defined as the time from randomization to death due to any cause. Participants without documented death at the time of the analysis were censored at the date of the last follow-up. OS was estimated using the product-limit (Kaplan-Meier) method for censored data.

Recruitment status

COMPLETED

Study phase

PHASE3

Target enrollment

1579 participants

Primary outcome timeframe

Up to 45.9 months

Results posted on

2025-07-24

Participant Flow

Participant milestones

Participant milestones
Measure	Pembrolizumab + Chemotherapy (FP or CAPOX Regimen) Participants received pembrolizumab 200 mg intravenously (IV) on Day 1 of each 21-day cycle (Q3W) for up to 35 cycles (approximately 2 years) + physicians' choice of either cisplatin 80 mg/m\^2 IV on Day 1 Q3W and 5-fluorouracil (5FU) 800 mg/m\^2/day via continuous IV infusion on Days 1 to 5 Q3W (FP regimen) OR oxaliplatin 130 mg/m\^2 IV on Day 1 Q3W + capecitabine 1000 mg/m\^2 orally twice a day (BID) on Days 1 to 14 Q3W (CAPOX regimen). Participants who completed up to 35 administrations of pembrolizumab (approximately 2 years) or achieved a complete response (CR) but experienced progression of disease (PD), could have initiated a second course of pembrolizumab for up to 17 cycles (approximately 1 additional year).	Placebo + Chemotherapy (FP or CAPOX Regimen) Participants received placebo on Day 1 Q3W for up to 35 cycles (approximately 2 years) + physicians' choice of either cisplatin 80 mg/m\^2 IV on Day 1 Q3W and 5FU 800 mg/m\^2/day via continuous IV infusion on Days 1 to 5 Q3W (FP regimen) OR oxaliplatin 130 mg/m\^2 IV on Day 1 Q3W + capecitabine 1000 mg/m\^2 orally BID on Days 1 to 14 Q3W (CAPOX regimen).
Overall Study STARTED	790	789
Overall Study Treated	785	787
Overall Study COMPLETED	0	0
Overall Study NOT COMPLETED	790	789

Reasons for withdrawal

Reasons for withdrawal
Measure	Pembrolizumab + Chemotherapy (FP or CAPOX Regimen) Participants received pembrolizumab 200 mg intravenously (IV) on Day 1 of each 21-day cycle (Q3W) for up to 35 cycles (approximately 2 years) + physicians' choice of either cisplatin 80 mg/m\^2 IV on Day 1 Q3W and 5-fluorouracil (5FU) 800 mg/m\^2/day via continuous IV infusion on Days 1 to 5 Q3W (FP regimen) OR oxaliplatin 130 mg/m\^2 IV on Day 1 Q3W + capecitabine 1000 mg/m\^2 orally twice a day (BID) on Days 1 to 14 Q3W (CAPOX regimen). Participants who completed up to 35 administrations of pembrolizumab (approximately 2 years) or achieved a complete response (CR) but experienced progression of disease (PD), could have initiated a second course of pembrolizumab for up to 17 cycles (approximately 1 additional year).	Placebo + Chemotherapy (FP or CAPOX Regimen) Participants received placebo on Day 1 Q3W for up to 35 cycles (approximately 2 years) + physicians' choice of either cisplatin 80 mg/m\^2 IV on Day 1 Q3W and 5FU 800 mg/m\^2/day via continuous IV infusion on Days 1 to 5 Q3W (FP regimen) OR oxaliplatin 130 mg/m\^2 IV on Day 1 Q3W + capecitabine 1000 mg/m\^2 orally BID on Days 1 to 14 Q3W (CAPOX regimen).
Overall Study Death	593	660
Overall Study Lost to Follow-up	1	5
Overall Study Physician Decision	1	0
Overall Study Withdrawal by Subject	14	12
Overall Study Ongoing in Study	181	112

Baseline Characteristics

Pembrolizumab (MK-3475) Plus Chemotherapy Versus Placebo Plus Chemotherapy in Participants Gastric or Gastroesophageal Junction (GEJ) Adenocarcinoma (MK-3475-859/KEYNOTE-859)

Baseline characteristics by cohort

Baseline characteristics by cohort
Measure	Pembrolizumab + Chemotherapy (FP or CAPOX Regimen) n=790 Participants Participants received pembrolizumab 200 mg intravenously (IV) on Day 1 of each 21-day cycle (Q3W) for up to 35 cycles (approximately 2 years) + physicians' choice of either cisplatin 80 mg/m\^2 IV on Day 1 Q3W and 5-fluorouracil (5FU) 800 mg/m\^2/day via continuous IV infusion on Days 1 to 5 Q3W (FP regimen) OR oxaliplatin 130 mg/m\^2 IV on Day 1 Q3W + capecitabine 1000 mg/m\^2 orally twice a day (BID) on Days 1 to 14 Q3W (CAPOX regimen). Participants who completed up to 35 administrations of pembrolizumab (approximately 2 years) or achieved a complete response (CR) but experienced progression of disease (PD), could have initiated a second course of pembrolizumab for up to 17 cycles (approximately 1 additional year).	Placebo + Chemotherapy (FP or CAPOX Regimen) n=789 Participants Participants received placebo on Day 1 Q3W for up to 35 cycles (approximately 2 years) + physicians' choice of either cisplatin 80 mg/m\^2 IV on Day 1 Q3W and 5FU 800 mg/m\^2/day via continuous IV infusion on Days 1 to 5 Q3W (FP regimen) OR oxaliplatin 130 mg/m\^2 IV on Day 1 Q3W + capecitabine 1000 mg/m\^2 orally BID on Days 1 to 14 Q3W (CAPOX regimen).	Total n=1579 Participants Total of all reporting groups
Age, Continuous	59.3 Years STANDARD_DEVIATION 11.9 • n=5 Participants	60.0 Years STANDARD_DEVIATION 11.8 • n=7 Participants	59.6 Years STANDARD_DEVIATION 11.8 • n=5 Participants
Sex: Female, Male Female	263 Participants n=5 Participants	245 Participants n=7 Participants	508 Participants n=5 Participants
Sex: Female, Male Male	527 Participants n=5 Participants	544 Participants n=7 Participants	1071 Participants n=5 Participants
Ethnicity (NIH/OMB) Hispanic or Latino	175 Participants n=5 Participants	157 Participants n=7 Participants	332 Participants n=5 Participants
Ethnicity (NIH/OMB) Not Hispanic or Latino	590 Participants n=5 Participants	615 Participants n=7 Participants	1205 Participants n=5 Participants
Ethnicity (NIH/OMB) Unknown or Not Reported	25 Participants n=5 Participants	17 Participants n=7 Participants	42 Participants n=5 Participants
Race (NIH/OMB) American Indian or Alaska Native	31 Participants n=5 Participants	36 Participants n=7 Participants	67 Participants n=5 Participants
Race (NIH/OMB) Asian	270 Participants n=5 Participants	269 Participants n=7 Participants	539 Participants n=5 Participants
Race (NIH/OMB) Native Hawaiian or Other Pacific Islander	1 Participants n=5 Participants	2 Participants n=7 Participants	3 Participants n=5 Participants
Race (NIH/OMB) Black or African American	12 Participants n=5 Participants	9 Participants n=7 Participants	21 Participants n=5 Participants
Race (NIH/OMB) White	426 Participants n=5 Participants	435 Participants n=7 Participants	861 Participants n=5 Participants
Race (NIH/OMB) More than one race	43 Participants n=5 Participants	30 Participants n=7 Participants	73 Participants n=5 Participants
Race (NIH/OMB) Unknown or Not Reported	7 Participants n=5 Participants	8 Participants n=7 Participants	15 Participants n=5 Participants
Programmed Cell Death Ligand 1 (PD-L1) Combined Positive Score (CPS) <1 and CPS ≥1 Status CPS ≥1	618 Participants n=5 Participants	617 Participants n=7 Participants	1235 Participants n=5 Participants
Programmed Cell Death Ligand 1 (PD-L1) Combined Positive Score (CPS) <1 and CPS ≥1 Status CPS <1	172 Participants n=5 Participants	172 Participants n=7 Participants	344 Participants n=5 Participants
Programmed Cell Death Ligand 1 (PD-L1) Combined Positive Score (CPS) ≥10 Status CPS ≥10	279 Participants n=5 Participants	272 Participants n=7 Participants	551 Participants n=5 Participants
Programmed Cell Death Ligand 1 (PD-L1) Combined Positive Score (CPS) ≥10 Status CPS <10	509 Participants n=5 Participants	517 Participants n=7 Participants	1026 Participants n=5 Participants
Programmed Cell Death Ligand 1 (PD-L1) Combined Positive Score (CPS) ≥10 Status Unknown	2 Participants n=5 Participants	0 Participants n=7 Participants	2 Participants n=5 Participants
Chemotherapy Regimen FP Regimen	108 Participants n=5 Participants	108 Participants n=7 Participants	216 Participants n=5 Participants
Chemotherapy Regimen CAPOX Regimen	682 Participants n=5 Participants	681 Participants n=7 Participants	1363 Participants n=5 Participants
Geographic Region Western Europe/Israel/North America/Australia	201 Participants n=5 Participants	202 Participants n=7 Participants	403 Participants n=5 Participants
Geographic Region Asia	263 Participants n=5 Participants	262 Participants n=7 Participants	525 Participants n=5 Participants
Geographic Region Rest of World	326 Participants n=5 Participants	325 Participants n=7 Participants	651 Participants n=5 Participants

PRIMARY outcome

Timeframe: Up to 45.9 months

Population: The analysis population included all randomized participants.

Outcome measures

Outcome measures
Measure	Pembrolizumab + Chemotherapy (FP or CAPOX Regimen) n=790 Participants Participants received pembrolizumab 200 mg intravenously (IV) on Day 1 of each 21-day cycle (Q3W) for up to 35 cycles (approximately 2 years) + physicians' choice of either cisplatin 80 mg/m\^2 IV on Day 1 Q3W and 5-fluorouracil (5FU) 800 mg/m\^2/day via continuous IV infusion on Days 1 to 5 Q3W (FP regimen) OR oxaliplatin 130 mg/m\^2 IV on Day 1 Q3W + capecitabine 1000 mg/m\^2 orally twice a day (BID) on Days 1 to 14 Q3W (CAPOX regimen). Participants who completed up to 35 administrations of pembrolizumab (approximately 2 years) or achieved a complete response (CR) but experienced progression of disease (PD), could have initiated a second course of pembrolizumab for up to 17 cycles (approximately 1 additional year).	Placebo + Chemotherapy (FP or CAPOX Regimen) n=789 Participants Participants received placebo on Day 1 Q3W for up to 35 cycles (approximately 2 years) + physicians' choice of either cisplatin 80 mg/m\^2 IV on Day 1 Q3W and 5FU 800 mg/m\^2/day via continuous IV infusion on Days 1 to 5 Q3W (FP regimen) OR oxaliplatin 130 mg/m\^2 IV on Day 1 Q3W + capecitabine 1000 mg/m\^2 orally BID on Days 1 to 14 Q3W (CAPOX regimen).
Overall Survival (OS) in All Participants	12.9 Months Interval 11.9 to 14.0	11.5 Months Interval 10.6 to 12.1

PRIMARY outcome

Timeframe: Up to 45.9 months

Population: The analysis population included randomized participants with a PD-L1 CPS of ≥1.

Outcome measures

Outcome measures
Measure	Pembrolizumab + Chemotherapy (FP or CAPOX Regimen) n=618 Participants Participants received pembrolizumab 200 mg intravenously (IV) on Day 1 of each 21-day cycle (Q3W) for up to 35 cycles (approximately 2 years) + physicians' choice of either cisplatin 80 mg/m\^2 IV on Day 1 Q3W and 5-fluorouracil (5FU) 800 mg/m\^2/day via continuous IV infusion on Days 1 to 5 Q3W (FP regimen) OR oxaliplatin 130 mg/m\^2 IV on Day 1 Q3W + capecitabine 1000 mg/m\^2 orally twice a day (BID) on Days 1 to 14 Q3W (CAPOX regimen). Participants who completed up to 35 administrations of pembrolizumab (approximately 2 years) or achieved a complete response (CR) but experienced progression of disease (PD), could have initiated a second course of pembrolizumab for up to 17 cycles (approximately 1 additional year).	Placebo + Chemotherapy (FP or CAPOX Regimen) n=617 Participants Participants received placebo on Day 1 Q3W for up to 35 cycles (approximately 2 years) + physicians' choice of either cisplatin 80 mg/m\^2 IV on Day 1 Q3W and 5FU 800 mg/m\^2/day via continuous IV infusion on Days 1 to 5 Q3W (FP regimen) OR oxaliplatin 130 mg/m\^2 IV on Day 1 Q3W + capecitabine 1000 mg/m\^2 orally BID on Days 1 to 14 Q3W (CAPOX regimen).
Overall Survival (OS) In Participants With Programmed Cell Death Ligand 1 (PD-L1) Combined Positive Score (CPS) ≥1	13.0 Months Interval 11.6 to 14.2	11.4 Months Interval 10.5 to 12.0

PRIMARY outcome

Timeframe: Up to 45.9 months

Population: The analysis population included randomized participants with a PD-L1 CPS of ≥10.

Outcome measures

Outcome measures
Measure	Pembrolizumab + Chemotherapy (FP or CAPOX Regimen) n=279 Participants Participants received pembrolizumab 200 mg intravenously (IV) on Day 1 of each 21-day cycle (Q3W) for up to 35 cycles (approximately 2 years) + physicians' choice of either cisplatin 80 mg/m\^2 IV on Day 1 Q3W and 5-fluorouracil (5FU) 800 mg/m\^2/day via continuous IV infusion on Days 1 to 5 Q3W (FP regimen) OR oxaliplatin 130 mg/m\^2 IV on Day 1 Q3W + capecitabine 1000 mg/m\^2 orally twice a day (BID) on Days 1 to 14 Q3W (CAPOX regimen). Participants who completed up to 35 administrations of pembrolizumab (approximately 2 years) or achieved a complete response (CR) but experienced progression of disease (PD), could have initiated a second course of pembrolizumab for up to 17 cycles (approximately 1 additional year).	Placebo + Chemotherapy (FP or CAPOX Regimen) n=272 Participants Participants received placebo on Day 1 Q3W for up to 35 cycles (approximately 2 years) + physicians' choice of either cisplatin 80 mg/m\^2 IV on Day 1 Q3W and 5FU 800 mg/m\^2/day via continuous IV infusion on Days 1 to 5 Q3W (FP regimen) OR oxaliplatin 130 mg/m\^2 IV on Day 1 Q3W + capecitabine 1000 mg/m\^2 orally BID on Days 1 to 14 Q3W (CAPOX regimen).
Overall Survival (OS) In Participants With Programmed Cell Death Ligand 1 (PD-L1) Combined Positive Score (CPS) ≥10	15.7 Months Interval 13.8 to 19.3	11.8 Months Interval 10.3 to 12.7

SECONDARY outcome

Timeframe: Up to 49.5 months

Population: The analysis population included all randomized participants.

PFS was defined as the time from randomization to the first documented progressive disease (PD) or death due to any cause, whichever occurred first. Per RECIST 1.1 as assessed by BICR, PD was defined as ≥20% increase in the sum of diameters of target lesions. In addition to the relative increase of 20%, the sum must also have demonstrated an absolute increase of ≥5 mm. The appearance of ≥1 new lesions was also considered PD. PFS was estimated using the product-limit (Kaplan-Meier) method for censored data.

Outcome measures

Outcome measures
Measure	Pembrolizumab + Chemotherapy (FP or CAPOX Regimen) n=790 Participants Participants received pembrolizumab 200 mg intravenously (IV) on Day 1 of each 21-day cycle (Q3W) for up to 35 cycles (approximately 2 years) + physicians' choice of either cisplatin 80 mg/m\^2 IV on Day 1 Q3W and 5-fluorouracil (5FU) 800 mg/m\^2/day via continuous IV infusion on Days 1 to 5 Q3W (FP regimen) OR oxaliplatin 130 mg/m\^2 IV on Day 1 Q3W + capecitabine 1000 mg/m\^2 orally twice a day (BID) on Days 1 to 14 Q3W (CAPOX regimen). Participants who completed up to 35 administrations of pembrolizumab (approximately 2 years) or achieved a complete response (CR) but experienced progression of disease (PD), could have initiated a second course of pembrolizumab for up to 17 cycles (approximately 1 additional year).	Placebo + Chemotherapy (FP or CAPOX Regimen) n=789 Participants Participants received placebo on Day 1 Q3W for up to 35 cycles (approximately 2 years) + physicians' choice of either cisplatin 80 mg/m\^2 IV on Day 1 Q3W and 5FU 800 mg/m\^2/day via continuous IV infusion on Days 1 to 5 Q3W (FP regimen) OR oxaliplatin 130 mg/m\^2 IV on Day 1 Q3W + capecitabine 1000 mg/m\^2 orally BID on Days 1 to 14 Q3W (CAPOX regimen).
Progression Free Survival (PFS) Per Response Evaluation Criteria in Solid Tumors Version 1.1 (RECIST 1.1) Assessed by Blinded Independent Central Review (BICR) in All Participants	6.9 Months Interval 6.3 to 7.2	5.6 Months Interval 5.5 to 5.7

SECONDARY outcome

Timeframe: Up to 49.5 months

Population: The analysis population included randomized participants with a PD-L1 CPS ≥1.

Outcome measures

Outcome measures
Measure	Pembrolizumab + Chemotherapy (FP or CAPOX Regimen) n=618 Participants Participants received pembrolizumab 200 mg intravenously (IV) on Day 1 of each 21-day cycle (Q3W) for up to 35 cycles (approximately 2 years) + physicians' choice of either cisplatin 80 mg/m\^2 IV on Day 1 Q3W and 5-fluorouracil (5FU) 800 mg/m\^2/day via continuous IV infusion on Days 1 to 5 Q3W (FP regimen) OR oxaliplatin 130 mg/m\^2 IV on Day 1 Q3W + capecitabine 1000 mg/m\^2 orally twice a day (BID) on Days 1 to 14 Q3W (CAPOX regimen). Participants who completed up to 35 administrations of pembrolizumab (approximately 2 years) or achieved a complete response (CR) but experienced progression of disease (PD), could have initiated a second course of pembrolizumab for up to 17 cycles (approximately 1 additional year).	Placebo + Chemotherapy (FP or CAPOX Regimen) n=617 Participants Participants received placebo on Day 1 Q3W for up to 35 cycles (approximately 2 years) + physicians' choice of either cisplatin 80 mg/m\^2 IV on Day 1 Q3W and 5FU 800 mg/m\^2/day via continuous IV infusion on Days 1 to 5 Q3W (FP regimen) OR oxaliplatin 130 mg/m\^2 IV on Day 1 Q3W + capecitabine 1000 mg/m\^2 orally BID on Days 1 to 14 Q3W (CAPOX regimen).
Progression Free Survival (PFS) Per Response Evaluation Criteria in Solid Tumors Version 1.1 (RECIST 1.1) Assessed by Blinded Independent Central Review (BICR) in Participants With Programmed Cell Death Ligand 1 (PD-L1) Combined Positive Score (CPS) ≥1	6.9 Months Interval 6.0 to 7.2	5.6 Months Interval 5.4 to 5.7

SECONDARY outcome

Timeframe: Up to 49.5 months

Population: The analysis population included randomized participants with a PD-L1 CPS ≥10.

Outcome measures

Outcome measures
Measure	Pembrolizumab + Chemotherapy (FP or CAPOX Regimen) n=279 Participants Participants received pembrolizumab 200 mg intravenously (IV) on Day 1 of each 21-day cycle (Q3W) for up to 35 cycles (approximately 2 years) + physicians' choice of either cisplatin 80 mg/m\^2 IV on Day 1 Q3W and 5-fluorouracil (5FU) 800 mg/m\^2/day via continuous IV infusion on Days 1 to 5 Q3W (FP regimen) OR oxaliplatin 130 mg/m\^2 IV on Day 1 Q3W + capecitabine 1000 mg/m\^2 orally twice a day (BID) on Days 1 to 14 Q3W (CAPOX regimen). Participants who completed up to 35 administrations of pembrolizumab (approximately 2 years) or achieved a complete response (CR) but experienced progression of disease (PD), could have initiated a second course of pembrolizumab for up to 17 cycles (approximately 1 additional year).	Placebo + Chemotherapy (FP or CAPOX Regimen) n=272 Participants Participants received placebo on Day 1 Q3W for up to 35 cycles (approximately 2 years) + physicians' choice of either cisplatin 80 mg/m\^2 IV on Day 1 Q3W and 5FU 800 mg/m\^2/day via continuous IV infusion on Days 1 to 5 Q3W (FP regimen) OR oxaliplatin 130 mg/m\^2 IV on Day 1 Q3W + capecitabine 1000 mg/m\^2 orally BID on Days 1 to 14 Q3W (CAPOX regimen).
Progression Free Survival (PFS) Per Response Evaluation Criteria in Solid Tumors Version 1.1 (RECIST 1.1) Assessed by Blinded Independent Central Review (BICR) in Participants With Programmed Cell Death Ligand 1 (PD-L1) Combined Positive Score (CPS) ≥10	8.1 Months Interval 6.8 to 8.5	5.6 Months Interval 5.4 to 6.7

SECONDARY outcome

Timeframe: Up to 49.5 months

Population: The analysis population included all randomized participants.

ORR was defined as the percentage of the participants who had a confirmed Complete Response (CR: disappearance of all target lesions) or Partial Response (PR: ≥30% decrease in the sum of diameters (SOD) of target lesions, taking as reference the baseline SOD) according to RECIST 1.1 as assessed by BICR.

Outcome measures

Outcome measures
Measure	Pembrolizumab + Chemotherapy (FP or CAPOX Regimen) n=790 Participants Participants received pembrolizumab 200 mg intravenously (IV) on Day 1 of each 21-day cycle (Q3W) for up to 35 cycles (approximately 2 years) + physicians' choice of either cisplatin 80 mg/m\^2 IV on Day 1 Q3W and 5-fluorouracil (5FU) 800 mg/m\^2/day via continuous IV infusion on Days 1 to 5 Q3W (FP regimen) OR oxaliplatin 130 mg/m\^2 IV on Day 1 Q3W + capecitabine 1000 mg/m\^2 orally twice a day (BID) on Days 1 to 14 Q3W (CAPOX regimen). Participants who completed up to 35 administrations of pembrolizumab (approximately 2 years) or achieved a complete response (CR) but experienced progression of disease (PD), could have initiated a second course of pembrolizumab for up to 17 cycles (approximately 1 additional year).	Placebo + Chemotherapy (FP or CAPOX Regimen) n=789 Participants Participants received placebo on Day 1 Q3W for up to 35 cycles (approximately 2 years) + physicians' choice of either cisplatin 80 mg/m\^2 IV on Day 1 Q3W and 5FU 800 mg/m\^2/day via continuous IV infusion on Days 1 to 5 Q3W (FP regimen) OR oxaliplatin 130 mg/m\^2 IV on Day 1 Q3W + capecitabine 1000 mg/m\^2 orally BID on Days 1 to 14 Q3W (CAPOX regimen).
Objective Response Rate (ORR) Per Response Evaluation Criteria in Solid Tumors Version 1.1 (RECIST 1.1) Assessed by Blinded Independent Central Review (BICR) in All Participants	51.3 Percentage of Participants Interval 47.7 to 54.8	42.0 Percentage of Participants Interval 38.5 to 45.5

SECONDARY outcome

Timeframe: Up to 49.5 months

Population: The analysis population included randomized participants with a PD-L1 CPS ≥1.

Outcome measures

Outcome measures
Measure	Pembrolizumab + Chemotherapy (FP or CAPOX Regimen) n=618 Participants Participants received pembrolizumab 200 mg intravenously (IV) on Day 1 of each 21-day cycle (Q3W) for up to 35 cycles (approximately 2 years) + physicians' choice of either cisplatin 80 mg/m\^2 IV on Day 1 Q3W and 5-fluorouracil (5FU) 800 mg/m\^2/day via continuous IV infusion on Days 1 to 5 Q3W (FP regimen) OR oxaliplatin 130 mg/m\^2 IV on Day 1 Q3W + capecitabine 1000 mg/m\^2 orally twice a day (BID) on Days 1 to 14 Q3W (CAPOX regimen). Participants who completed up to 35 administrations of pembrolizumab (approximately 2 years) or achieved a complete response (CR) but experienced progression of disease (PD), could have initiated a second course of pembrolizumab for up to 17 cycles (approximately 1 additional year).	Placebo + Chemotherapy (FP or CAPOX Regimen) n=617 Participants Participants received placebo on Day 1 Q3W for up to 35 cycles (approximately 2 years) + physicians' choice of either cisplatin 80 mg/m\^2 IV on Day 1 Q3W and 5FU 800 mg/m\^2/day via continuous IV infusion on Days 1 to 5 Q3W (FP regimen) OR oxaliplatin 130 mg/m\^2 IV on Day 1 Q3W + capecitabine 1000 mg/m\^2 orally BID on Days 1 to 14 Q3W (CAPOX regimen).
Objective Response Rate (ORR) Per Response Evaluation Criteria in Solid Tumors Version 1.1 (RECIST 1.1) Assessed by Blinded Independent Central Review (BICR) in Participants With Programmed Cell Death Ligand 1 (PD-L1) Combined Positive Score (CPS) ≥1	52.1 Percentage of Participants Interval 48.1 to 56.1	42.6 Percentage of Participants Interval 38.7 to 46.6

SECONDARY outcome

Timeframe: Up to 49.5 months

Population: The analysis population included randomized participants with a PD-L1 CPS ≥10.

Outcome measures

Outcome measures
Measure	Pembrolizumab + Chemotherapy (FP or CAPOX Regimen) n=279 Participants Participants received pembrolizumab 200 mg intravenously (IV) on Day 1 of each 21-day cycle (Q3W) for up to 35 cycles (approximately 2 years) + physicians' choice of either cisplatin 80 mg/m\^2 IV on Day 1 Q3W and 5-fluorouracil (5FU) 800 mg/m\^2/day via continuous IV infusion on Days 1 to 5 Q3W (FP regimen) OR oxaliplatin 130 mg/m\^2 IV on Day 1 Q3W + capecitabine 1000 mg/m\^2 orally twice a day (BID) on Days 1 to 14 Q3W (CAPOX regimen). Participants who completed up to 35 administrations of pembrolizumab (approximately 2 years) or achieved a complete response (CR) but experienced progression of disease (PD), could have initiated a second course of pembrolizumab for up to 17 cycles (approximately 1 additional year).	Placebo + Chemotherapy (FP or CAPOX Regimen) n=272 Participants Participants received placebo on Day 1 Q3W for up to 35 cycles (approximately 2 years) + physicians' choice of either cisplatin 80 mg/m\^2 IV on Day 1 Q3W and 5FU 800 mg/m\^2/day via continuous IV infusion on Days 1 to 5 Q3W (FP regimen) OR oxaliplatin 130 mg/m\^2 IV on Day 1 Q3W + capecitabine 1000 mg/m\^2 orally BID on Days 1 to 14 Q3W (CAPOX regimen).
Objective Response Rate (ORR) Per Response Evaluation Criteria in Solid Tumors Version 1.1 (RECIST 1.1) Assessed by Blinded Independent Central Review (BICR) in Participants With Programmed Cell Death Ligand 1 (PD-L1) Combined Positive Score (CPS) ≥10	60.6 Percentage of Participants Interval 54.6 to 66.3	43.0 Percentage of Participants Interval 37.1 to 49.1

SECONDARY outcome

Timeframe: Up to 49.5 months

Population: The analysis population included all randomized participants who experienced a confirmed CR or PR.

DOR was defined as the time from first documented evidence of a confirmed Complete Response (CR) or Partial Response (PR) until progressive disease (PD) or death, whichever occurred first. Per RECIST 1.1 as assessed by BICR, CR was the disappearance of all target lesions and PR was ≥30% decrease in the sum of diameters (SOD) of target lesions, taking as reference the baseline SOD. PD was defined as ≥20% increase in the SOD of target lesions according to RECIST 1.1. In addition to the relative increase of 20%, the SOD must also have demonstrated an absolute increase of ≥5 mm. The appearance of ≥1 new lesions was also considered PD. DOR for participants who had not progressed or died at the time of analysis was censored at the date of the last tumor assessment. DOR was estimated using the product-limit (Kaplan-Meier) method for censored data.

Outcome measures

Outcome measures
Measure	Pembrolizumab + Chemotherapy (FP or CAPOX Regimen) n=405 Participants Participants received pembrolizumab 200 mg intravenously (IV) on Day 1 of each 21-day cycle (Q3W) for up to 35 cycles (approximately 2 years) + physicians' choice of either cisplatin 80 mg/m\^2 IV on Day 1 Q3W and 5-fluorouracil (5FU) 800 mg/m\^2/day via continuous IV infusion on Days 1 to 5 Q3W (FP regimen) OR oxaliplatin 130 mg/m\^2 IV on Day 1 Q3W + capecitabine 1000 mg/m\^2 orally twice a day (BID) on Days 1 to 14 Q3W (CAPOX regimen). Participants who completed up to 35 administrations of pembrolizumab (approximately 2 years) or achieved a complete response (CR) but experienced progression of disease (PD), could have initiated a second course of pembrolizumab for up to 17 cycles (approximately 1 additional year).	Placebo + Chemotherapy (FP or CAPOX Regimen) n=331 Participants Participants received placebo on Day 1 Q3W for up to 35 cycles (approximately 2 years) + physicians' choice of either cisplatin 80 mg/m\^2 IV on Day 1 Q3W and 5FU 800 mg/m\^2/day via continuous IV infusion on Days 1 to 5 Q3W (FP regimen) OR oxaliplatin 130 mg/m\^2 IV on Day 1 Q3W + capecitabine 1000 mg/m\^2 orally BID on Days 1 to 14 Q3W (CAPOX regimen).
Duration of Response (DOR) Per Response Evaluation Criteria in Solid Tumors Version 1.1 (RECIST 1.1) Assessed by Blinded Independent Central Review (BICR) in All Participants	8.0 Months Interval 7.0 to 9.7	5.7 Months Interval 5.5 to 6.9

SECONDARY outcome

Timeframe: Up to 49.5 months

Population: The analysis population included randomized participants, with a PD-L1 CPS ≥1, who experienced a confirmed CR or PR.

Outcome measures

Outcome measures
Measure	Pembrolizumab + Chemotherapy (FP or CAPOX Regimen) n=322 Participants Participants received pembrolizumab 200 mg intravenously (IV) on Day 1 of each 21-day cycle (Q3W) for up to 35 cycles (approximately 2 years) + physicians' choice of either cisplatin 80 mg/m\^2 IV on Day 1 Q3W and 5-fluorouracil (5FU) 800 mg/m\^2/day via continuous IV infusion on Days 1 to 5 Q3W (FP regimen) OR oxaliplatin 130 mg/m\^2 IV on Day 1 Q3W + capecitabine 1000 mg/m\^2 orally twice a day (BID) on Days 1 to 14 Q3W (CAPOX regimen). Participants who completed up to 35 administrations of pembrolizumab (approximately 2 years) or achieved a complete response (CR) but experienced progression of disease (PD), could have initiated a second course of pembrolizumab for up to 17 cycles (approximately 1 additional year).	Placebo + Chemotherapy (FP or CAPOX Regimen) n=263 Participants Participants received placebo on Day 1 Q3W for up to 35 cycles (approximately 2 years) + physicians' choice of either cisplatin 80 mg/m\^2 IV on Day 1 Q3W and 5FU 800 mg/m\^2/day via continuous IV infusion on Days 1 to 5 Q3W (FP regimen) OR oxaliplatin 130 mg/m\^2 IV on Day 1 Q3W + capecitabine 1000 mg/m\^2 orally BID on Days 1 to 14 Q3W (CAPOX regimen).
Duration of Response (DOR) Per Response Evaluation Criteria in Solid Tumors Version 1.1 (RECIST 1.1) Assessed by Blinded Independent Central Review (BICR) in Participants With Programmed Cell Death Ligand 1 (PD-L1) Combined Positive Score (CPS) ≥1	8.3 Months Interval 7.0 to 10.9	5.6 Months Interval 5.4 to 6.9

SECONDARY outcome

Timeframe: Up to 49.5 months

Population: The analysis population included randomized participants, with a PD-L1 CPS ≥10, who experienced a confirmed CR or PR.

Outcome measures

Outcome measures
Measure	Pembrolizumab + Chemotherapy (FP or CAPOX Regimen) n=169 Participants Participants received pembrolizumab 200 mg intravenously (IV) on Day 1 of each 21-day cycle (Q3W) for up to 35 cycles (approximately 2 years) + physicians' choice of either cisplatin 80 mg/m\^2 IV on Day 1 Q3W and 5-fluorouracil (5FU) 800 mg/m\^2/day via continuous IV infusion on Days 1 to 5 Q3W (FP regimen) OR oxaliplatin 130 mg/m\^2 IV on Day 1 Q3W + capecitabine 1000 mg/m\^2 orally twice a day (BID) on Days 1 to 14 Q3W (CAPOX regimen). Participants who completed up to 35 administrations of pembrolizumab (approximately 2 years) or achieved a complete response (CR) but experienced progression of disease (PD), could have initiated a second course of pembrolizumab for up to 17 cycles (approximately 1 additional year).	Placebo + Chemotherapy (FP or CAPOX Regimen) n=117 Participants Participants received placebo on Day 1 Q3W for up to 35 cycles (approximately 2 years) + physicians' choice of either cisplatin 80 mg/m\^2 IV on Day 1 Q3W and 5FU 800 mg/m\^2/day via continuous IV infusion on Days 1 to 5 Q3W (FP regimen) OR oxaliplatin 130 mg/m\^2 IV on Day 1 Q3W + capecitabine 1000 mg/m\^2 orally BID on Days 1 to 14 Q3W (CAPOX regimen).
Duration of Response (DOR) Per Response Evaluation Criteria in Solid Tumors Version 1.1 (RECIST 1.1) Assessed by Blinded Independent Central Review (BICR) in Participants With Programmed Cell Death Ligand 1 (PD-L1) Combined Positive Score (CPS) ≥10	10.9 Months Interval 8.0 to 13.8	5.8 Months Interval 5.3 to 7.0

SECONDARY outcome

Timeframe: Up to 36.7 months

Population: The analysis population included all participants who received at least one dose of study treatment.

An AE was defined as any untoward medical occurrence, in a clinical study participant, temporally associated with the use of study intervention, whether or not considered related to the study intervention. An AE could therefore have been any unfavorable and unintended sign (including an abnormal laboratory finding), symptom, or disease (new or exacerbated) temporally associated with the use of a study intervention. The number of participants with at least one AE is presented.

Outcome measures

Outcome measures
Measure	Pembrolizumab + Chemotherapy (FP or CAPOX Regimen) n=785 Participants Participants received pembrolizumab 200 mg intravenously (IV) on Day 1 of each 21-day cycle (Q3W) for up to 35 cycles (approximately 2 years) + physicians' choice of either cisplatin 80 mg/m\^2 IV on Day 1 Q3W and 5-fluorouracil (5FU) 800 mg/m\^2/day via continuous IV infusion on Days 1 to 5 Q3W (FP regimen) OR oxaliplatin 130 mg/m\^2 IV on Day 1 Q3W + capecitabine 1000 mg/m\^2 orally twice a day (BID) on Days 1 to 14 Q3W (CAPOX regimen). Participants who completed up to 35 administrations of pembrolizumab (approximately 2 years) or achieved a complete response (CR) but experienced progression of disease (PD), could have initiated a second course of pembrolizumab for up to 17 cycles (approximately 1 additional year).	Placebo + Chemotherapy (FP or CAPOX Regimen) n=787 Participants Participants received placebo on Day 1 Q3W for up to 35 cycles (approximately 2 years) + physicians' choice of either cisplatin 80 mg/m\^2 IV on Day 1 Q3W and 5FU 800 mg/m\^2/day via continuous IV infusion on Days 1 to 5 Q3W (FP regimen) OR oxaliplatin 130 mg/m\^2 IV on Day 1 Q3W + capecitabine 1000 mg/m\^2 orally BID on Days 1 to 14 Q3W (CAPOX regimen).
Number of Participants Who Experienced an Adverse Event (AE)	776 Participants	771 Participants

SECONDARY outcome

Timeframe: Up to 33.7 months

Population: The analysis population included all participants who received at least one dose of study treatment.

An AE was defined as any untoward medical occurrence in a clinical study participant, temporally associated with the use of study intervention, whether or not considered related to the study intervention. An AE could therefore have been any unfavorable and unintended sign (including an abnormal laboratory finding), symptom, or disease (new or exacerbated) temporally associated with the use of a study intervention. The number of participants who discontinued study treatment due to an AE is presented.

Outcome measures

Outcome measures
Measure	Pembrolizumab + Chemotherapy (FP or CAPOX Regimen) n=785 Participants Participants received pembrolizumab 200 mg intravenously (IV) on Day 1 of each 21-day cycle (Q3W) for up to 35 cycles (approximately 2 years) + physicians' choice of either cisplatin 80 mg/m\^2 IV on Day 1 Q3W and 5-fluorouracil (5FU) 800 mg/m\^2/day via continuous IV infusion on Days 1 to 5 Q3W (FP regimen) OR oxaliplatin 130 mg/m\^2 IV on Day 1 Q3W + capecitabine 1000 mg/m\^2 orally twice a day (BID) on Days 1 to 14 Q3W (CAPOX regimen). Participants who completed up to 35 administrations of pembrolizumab (approximately 2 years) or achieved a complete response (CR) but experienced progression of disease (PD), could have initiated a second course of pembrolizumab for up to 17 cycles (approximately 1 additional year).	Placebo + Chemotherapy (FP or CAPOX Regimen) n=787 Participants Participants received placebo on Day 1 Q3W for up to 35 cycles (approximately 2 years) + physicians' choice of either cisplatin 80 mg/m\^2 IV on Day 1 Q3W and 5FU 800 mg/m\^2/day via continuous IV infusion on Days 1 to 5 Q3W (FP regimen) OR oxaliplatin 130 mg/m\^2 IV on Day 1 Q3W + capecitabine 1000 mg/m\^2 orally BID on Days 1 to 14 Q3W (CAPOX regimen).
Number of Participants Who Discontinued Study Treatment Due To an Adverse Event (AE)	257 Participants	204 Participants

Adverse Events

Pembrolizumab + Chemotherapy (FP or CAPOX Regimen)

Serious events: 355 serious events

Other events: 755 other events

Deaths: 601 deaths

Placebo + Chemotherapy (FP or CAPOX Regimen)

Serious events: 316 serious events

Other events: 751 other events

Deaths: 666 deaths

Pembrolizumab Second Course

Serious events: 0 serious events

Other events: 3 other events

Deaths: 2 deaths

Serious adverse events

Other adverse events

Additional Information

Senior Vice President, Global Clinical Development

Merck Sharp & Dohme LLC

Phone: 1-800-672-6372

Email: [email protected]

Results disclosure agreements

Principal investigator is a sponsor employee In accordance with standard editorial and ethical practice, the Sponsor will generally support publication of multicenter studies only in their entirety and not as individual site data. In this case, a coordinating investigator will be designated by mutual agreement. If publication activity is not directed by the Sponsor, the investigator agrees to submit all manuscripts or abstracts to the Sponsor before submission.
Publication restrictions are in place

Restriction type: OTHER