Colitis‐associated colorectal adenocarcinomas frequently express claudin 18 isoform 2: implications for claudin 18.2 monoclonal antibody therapy

Claudin 18 (CLDN18) is a member of the claudin family of cell surface proteins, which are widely expressed in epithelial cells and play a role in cell-cell adhesion. CLDN18 isoform 2 (CLDN18.2) is specifically expressed in gastric epithelial cells, and is frequently expressed at high levels in gastric adenocarcinoma. On the basis of this, zolbetuximab, a targeted monoclonal antibody, has been developed for patients with CLDN18.2-positive gastro-oesophageal adenocarcinoma. Colitis-associated colorectal adenocarcinomas (CACs) tend to lose intestinal markers and show aberrant gastric mucin expression. Furthermore, clinical trials of human epidermal growth factor receptor 2 (HER2) inhibitor therapy for colorectal carcinoma are ongoing. However, the expression profile of CLDN18.2 and HER2 has not been described in a series of human CACs.We performed immunohistochemistry for CLDN18 and HER2 on 56 consecutive CACs from 55 inflammatory bowel disease patients, and compared the expression profile with that of a control group of 56 sporadic colorectal adenocarcinomas (CRCs). CLDN18.1 expression and CLDN18.2 expression were validated by reverse transcription polymerase chain reaction (PCR) in paraffin-embedded CRC tissues. CLDN18 was positive in 27% (15/56) of CACs and in 5% (3/56) of sporadic CRCs (P = 0.004), and CLDN18-positive CACs were more likely to have lymph node metastasis than CLDN18-negative CACs (67% versus 36%; P = 0.017). CLDN18 expression was significantly associated with MUC5AC expression (P < 0.001) and loss of special AT-rich sequence-binding protein 2 expression (P = 0.005) in CACs. CLDN18.2 was expressed in CRCs that were immunoreactive for CLDN18. Only 4% of CACs were immunoreactive for HER2, and no differences were identified in sporadic CRCs.These findings suggest that certain CAC cases may be candidates for targeted zolbetuximab therapy.