Prevalence and clinicopathological features of microsatellite instability-high metastatic or recurrent gastric and esophagogastric junction cancer: WJOG13320GPS.

Background: Microsatellite instability (MSI)-high tumors represent a distinct, small-fraction subtype in esophagogastric junction cancer or gastric cancer (GC), yet their clinical significance remains poorly understood. This study aimed to investigate the prevalence and clinicopathological features of chemotherapy-naïve metastatic or recurrent MSI-high GC as a prescreening study for a phase II trial of nivolumab plus ipilimumab.

Methods: Key inclusion criteria included metastatic or recurrent adenocarcinoma of GC, ECOG performance status of 0 or 1, and no prior systemic therapy for metastatic or recurrent disease. MSI status was tested using multiplex PCR fragment analysis (MSI Testing Kit, FALCO). The primary endpoint was the prevalence of MSI-high GC.

Results: Between October 2020 and October 2022, 930 eligible patients from 75 centers in Japan were analyzed. The prevalence of MSI-high GC was 5.6% (95% CI 4.2-7.3). MSI-high GC was more frequently observed in females than males (9.6% vs 3.8%, p < 0.001), patients aged ≥ 70 years compared to those < 70 years (8.0% vs 2.8%, p < 0.001), in the lower stomach than other locations (10.5% vs 3.2%, p < 0.001), HER2-negative tumors than HER2-positive tumors (6.5% vs 1.8%, p = 0.02), and in patients without liver metastasis than those with liver metastasis (6.9% vs 2.2%, p = 0.004).

Conclusions: The prevalence of MSI-high tumors among chemotherapy-naïve patients with unresectable GC was 5.6%. These tumors were associated with female sex, older age, lower stomach, HER2-negative, and absence of liver metastasis. These findings would help assuming MSI-high tumors and may have significant implications for clinical practice and studies targeting this GC subtype.