Claudin 18.2 Immunohistochemistry Expression in Gastric Cancer: A Systematic Review.

Claudin 18.2 is a transmembrane protein, part of the tight-junction complex, selectively expressed in gastric epithelium. It is showing promising results as a target in advanced gastric cancer in phase 3 clinical trials using a monoclonal antibody against claudin 18.2. A systematic review on expression of claudin 18.2 in gastric cancer was performed using the PubMed database. The following search expression was used: ("Stomach Neoplasms" [Mesh]) AND (("claudin-18[TIAB]") OR ("CLDN18[TIAB]")). A total of n=99 articles were retrieved. Of those, 17 preclinical studies about claudin 18.2 expression by immunohistochemistry were selected. The results of those studies showed great variability in the criteria used for defining the thresholds for positivity of the stain. The proportion of claudin 18.2 positive cases varied between 24% and 83%. In works using a positivity threshold set at >40% or >70% of cells with membranous/cytoplasmic staining at 2+/3+ intensity, the average rate of positive cases was 50% or 30%, respectively (similar with clones 43-14A and EPR19202). Positivity of claudin 18.2 was associated with advanced stage, diffuse phenotype and PD-L1 and EBV positivity in some of the studies. Variability in criteria used to define claudin 18.2 positivity, as well as methodological differences, could explain the variation in the proportion of positive cases described, as well as the inconsistency of the association with clinical, molecular, and survival variables. The upcoming anticlaudin 18.2 therapy in advanced gastric cancer should prompt pathology laboratories to adjust their staining protocols and evaluation criteria in their series of patients, to further establish the association of claudin expression with clinical and molecular variables.