Background: Although high instability of microsatellite lesion (MSI-H) is an established biomarker for response to immune checkpoint inhibitors, the relationship between the number of MSI markers and the efficacy of immune checkpoint inhibitors in MSI-H tumors has not been well elucidated. NO LIMIT (WJOG13320G/CA209-7W7) is a phase II trial of nivolumab plus low-dose ipilimumab (NIVO+IPI) for MSI-H advanced gastric or esophagogastric junction cancer (GC) in the first-line setting.¹ Here, we investigated the relationship between the number of positive MSI markers and the efficacy of NIVO+IPI in NO LIMIT study. Methods: Eligible patients were unresectable advanced, recurrent, or metastatic GC with confirmed as MSI-H with the MSI-IVD Kit (FALCO), where two or more of the five MSI markers are defined as positive. Nivolumab (240 mg) biweekly and ipilimumab (1 mg/kg) every six weeks were given until disease progression or unacceptable toxicity. The primary endpoint was the objective response rate (ORR) assessed by a blinded independent central review. Secondary endpoints included progression-free survival (PFS), overall survival (OS), safety, and the association between number of positive MSI markers and clinical efficacy (ORR, PFS, OS). The analysis was performed 18 weeks after the first dose in the last patient, with a median follow-up of 9 months (range, 4.0-18.0). Results: The breakdown of MSI markers in the 29 patients enrolled in NO LIMIT was as follows: BAT-26 (26/26, 100%), NR-21 (25/29, 86.2%), BAT-25 (25/29, 86.2%), MONO-27 (29/29, 100%), NR-24 (27/29, 89.4%). Since most of the cases were positive for all five of them (the number of cases with 2, 3, 4, and 5 positive markers was 2, 2, 1, and 24, respectively), we dichotomized cases into group A (all five positive) and group B (the others). Response rates in groups A vs. B were 62.5% (15/24) vs. 60.0% (3/5), respectively. There was no difference in PFS or OS between the two groups. Conclusions: The number of MSI markers is not related to the efficacy of NIVO+IPI in NO LIMIT study. Further follow-up is needed to confirm the relationship between survival. 1. Muro K, et al. ESMO Congress 2023. Clinical trial information: jRCT2080225304.