Background Claudin 18.2 (CLDN18.2) is a tight junction protein retained in malignant transformation of gastric cancer (GC) and is a promising therapeutic target. Despite the clinical benefit of zolbetuximab in CLDN18.2-positive tumors, the dynamic expression of CLDN18.2 during chemoimmunotherapy and its implications in the tumor microenvironment (TME) remain poorly understood. Methods In a prospective single-arm phase II trial, we evaluated serial tumor biopsies from patients with advanced GC receiving first-line chemotherapy (capecitabine/oxaliplatin) with sequential pembrolizumab. CLDN18.2 expression was assessed by immunohistochemistry, and integrated molecular analyses—including whole transcriptome, whole exome, and single-cell RNA sequencing—were performed to explore TME alterations and molecular correlates. Results Among 57 patients, 40.4% were CLDN18.2-positive at baseline. CLDN18.2 positivity was associated with diffuse-type histology, higher programmed death-ligand 1 (PD-L1) combined positive scores, and an immune-inflamed, stroma-rich TME characterized by enhanced T cell infiltration, transforming growth factor-β signaling, and matrix remodeling. Gene set enrichment analyses revealed immune activation and stromal remodeling in CLDN18.2-positive tumors. Single-cell analysis showed increased regulatory T cells and galectin-3-CD44 signaling in CLDN18.2-positive tumors. After one cycle of chemotherapy, CLDN18.2 expression was lost in 40% of initially positive tumors, while 10% of initially negative tumors gained expression—particularly in fibrotic TMEs. Survival outcomes were not significantly different between CLDN18.2-positive and CLDN18.2-negative groups; however, patients negative for both CLDN18.2 and PD-L1 showed poorer prognosis. Conclusions CLDN18.2 expression is dynamically regulated during chemoimmunotherapy and is associated with a distinct immunosuppressive and fibrotic TME. These findings highlight the importance of repeated biomarker assessment and suggest potential combinatorial therapeutic strategies targeting both epithelial and stromal compartments in GC. Trial registration number NCT04249739 .
Lim et al. (Mon,) studied this question.