Programmed death-ligand 1 expression in gastric cancer by the combined positive score using the PD-L1 IHC 28-8 pharmDx test: a single institution's experience.

BACKGROUND: Programmed death-ligand 1 (PD-L1) Combined Positive Score (CPS) is a biomarker of nivolumab efficacy in advanced gastric cancer (AGC). Reported CPS ≥ 5 in AGC has been inconsistent across studies. We aimed to examine the distribution of CPS in real-world clinical practice using the Dako PD-L1 IHC 28-8 pharmDx (Dako 28-8) assay and to evaluate its clinical utility. METHODS: We retrospectively assessed CPS using Dako 28-8 in 138 patients with AGC treated at our institution between January 2022 and December 2022. RESULTS: The numbers of patients with CPS ≥ 5, 1 ≤ CPS < 5, and CPS < 1 were 65 (47.1%), 50 (36.2%), and 23 (16.7%), respectively. Among the patients evaluated for microsatellite instability (MSI) or tumor mutation burden (TMB), MSI-high was identified in 3/32 (9.4%), 2/31 (6.5%), and 1/20 (5.0%), while TMB-high was identified in 5/14 (35.7%), 4/20 (20.0%), and 2/11 (18.2%) in the CPS ≥ 5, 1 ≤ CPS < 5, and CPS < 1 groups, respectively. Nivolumab combination therapy was administered as first-line treatment in 14, 10, and 11 patients in the CPS ≥ 5, 1 ≤ CPS < 5, and CPS < 1 groups, respectively. Median Progression-free survival was 5.6 months in CPS ≥ 5, 5.9 months in 1 ≤ CPS < 5, and 6.5 months in CPS < 1. CONCLUSION: The frequency of CPS ≥5 in patients with AGC was approximately half of the total population. Because TMB-high tumors were identified in some patients with CPS < 5, further studies are needed to clarify the potential role of TMB in predicting ICI responsiveness in this population.