Case Report: A case of zolbetuximab-based chemotherapy for Claudin 18.2-positive gastric phenotype adenocarcinoma arising in Meckel's diverticulum.

A 74-year-old man presented with abdominal bloating and was diagnosed with unresectable advanced adenocarcinoma from a Meckel's diverticulum with distant lung metastases and peritoneal dissemination. On immunohistochemical analysis, the tumor was positive for MUC5AC and MUC6 and negative for MUC2 and CDX2, indicating a gastric phenotype. Histological examination of the adjacent non-cancerous tissues confirmed the presence of ectopic gastric epithelium, resulting in a final diagnosis of adenocarcinoma arising from ectopic gastric tissue within a Meckel's diverticulum. Biomarker profiling demonstrated Claudin 18.2 (CLDN18.2) positivity, HER2 negativity, and mismatch repair proficiency. Treatment was started with zolbetuximab plus 5-fluorouracil/leucovorin/oxaliplatin (FOLFOX), which is typically used for CLDN18.2-positive gastric cancer, resulting in a partial response. This is the first case of zolbetuximab-based therapy applied to adenocarcinoma from a Meckel's diverticulum with a gastric phenotype due to a biomarker profile of gastric cancer. This case highlights the importance of identifying both the biomarker profile and tissue of origin of the tumor.