Background: Response to antiPD-1 monotherapy is poor in patients (pts) with advanced microsatellite stable (MSS)/mismatch-repair proficient (pMMR) CRC. Combination of chemotherapy with antiPD-1 pembro may potentiate the antitumor immune response and provide greater antitumor activity than either agent alone. Combination of pembro and mFOLFOX7 or FOLFIRI in the ongoing phase 1b multicohort KEYNOTE-651 (NCT03374254) study in metastatic MSS/pMMR CRC showed antitumor activity. We present results with approximately 20 mo of additional follow-up. Methods: Pts had metastatic MSS/pMMR CRC and were previously untreated (cohort B) or received 1 prior line including a fluoropyrimidine + oxaliplatin (cohort D). Pts received pembro 200 mg Q3W + mFOLFOX7 Q2W (cohort B) or pembro 200 mg Q3W + FOLFIRI Q2W (cohort D). Primary end points were safety (DLT) and RP2D. Secondary end point was ORR per RECIST v1.1 by investigator review. DOR, DCR, and PFS per RECIST v1.1, and OS were exploratory end points. Results: Median study follow-up (range) at data cutoff (Oct 15, 2021) was 30.2 mo (25.0-43.6) for cohort B (n = 31) and 33.5 mo (25.2-43.2) for cohort D (n = 32). Treatment was discontinued in 29 pts (94%) in cohort B and 28 pts (88%) in cohort D, mostly because of PD (61% and 66%, respectively). At prior analysis (Feb 10, 2020), RP2D was confirmed as the starting dose in both cohorts; no new DLT had occurred. In cohort B, gr 3/4 TRAEs occurred in 18 pts (58%), most commonly neutropenia and decreased neutrophil count (both n = 5; 16%); 19 pts (61%) discontinued drug because of a TRAE. In cohort D, gr 3/4 TRAEs occurred in 17 pts (53%), most commonly, neutropenia (n = 7; 22%), diarrhea and fatigue (both n = 4; 13%); 3 pts (9%) discontinued drug because of a TRAE. There were no gr 5 TRAEs in either cohort. Efficacy by cohort and KRAS mutation status are below (Table). PD-L1 data and DNA/RNA-based biomarker data including GEP, consensus signatures, TMB, and LoF mutations will be included in the presentation. Conclusions: After 2.5 y of follow-up, the combination of pembro with either mFOLFOX7 or FOLFIRI continued to demonstrate a manageable safety profile with no new safety signals. Efficacy data from these single-arm cohorts appear comparable to historical data for current SOC. Clinical trial information: NCT03374254.

"+" indicates there is no PD at last assessment.