Claudin-18.2 immunohistochemical evaluation in gastric and gastroesophageal junction adenocarcinomas to direct targeted therapy: a practical approach

Claudin-18.2 (CLDN18.2) expression evaluated by immunohistochemistry is a new biomarker for gastric and gastroesophageal junction adenocarcinomas that will soon have market authorization for implementation into routine clinical practice. Despite successful testing in the setting of clinical trials, no specific practical testing guidelines have been proposed. Several pre-analytical and analytical variables may interfere with adequate CLDN18.2 staining interpretation, thus this article provides practical guidance on CLDN18.2 testing and scoring in gastric/gastroesophageal junction adenocarcinomas in order to identify patients who may respond to targeted therapy with monoclonal antibodies directed against CLDN18.2. Based on available data, moderate to strong (2+/3+) membrane staining in ≥75% of adenocarcinoma cells is the proposed cut-off for clinical use of the monoclonal antibody anti-CLDN18.2 (zolbetuximab).