Microsatellite Instability, Epstein–Barr Virus, and Programmed Cell Death Ligand 1 as Predictive Markers for Immunotherapy in Gastric Cancer

SIMPLE SUMMARY: Immunotherapy is approved in selected cases of gastric cancer, and durable responses have been observed in exceptional responders. Several potential predictive biomarkers have been identified in gastric cancer, such as microsatellite instability-high (MSI-H), Epstein–Barr virus (EBV), and programmed death ligand 1 (PD-L1). We explored the real-world evidence of these biomarkers and their outcomes. When only combined positive score (CPS) ≥ 1 was used as the biomarker, the overall response rate (ORR) and progression-free survival (PFS) were not statistically significant. CPS ≥ 1 was commonly combined with MSI-H (75%) and Epstein–Barr encoding region (EBER) (80%). MSI-H and CPS ≥ 5 were prognostic biomarkers associated with better ORR and PFS. In patients with EBER, better ORR and PFS were observed only in patients with CPS ≥ 1. These results could transform clinical practice and can be used to formulate more precise treatment suggestions for patients with gastric cancer. ABSTRACT: Immunotherapy benefits selected cases of gastric cancer (GC), but the correlation between biomarkers and prognosis is still unclear. Fifty-two patients with GC who underwent immunotherapy were enrolled from June 2016 to December 2020. Their clinical features and biomarkers—microsatellite instability-high (MSI-H), programmed cell death ligand 1 (PD-L1) combined positive score (CPS), and Epstein–Barr encoding region (EBER)—were analyzed. Eight patients had MSI-H, five patients had EBER, 29 patients had CPS ≥ 1, and 20 patients had no biomarker. The overall response rates (ORRs) of the MSI-H, EBER, PD-L1 CPS ≥ 1, and all-negative group were 75%, 60%, 44.8%, and 15%, respectively. Compared with that of the all-negative group, progression-free survival (PFS) was better in the MSI-H (p = 0.018), CPS ≥ 5 (p = 0.012), and CPS ≥ 10 (p = 0.006) groups, but not in the EBER (p = 0.2) and CPS ≥ 1 groups (p = 0.35). Ten patients had combined biomarkers, CPS ≥ 1 with either MSI-H or EBER. The ORRs were 66.7% for CPS ≥ 1 and MSI-H and 75% for CPS ≥ 1 and EBER. PFS was better in patients with combined biomarkers (p = 0.01). MSI-H, EBER, and CPS are useful biomarkers for predicting the efficacy of immunotherapy.