Intratumoral Heterogeneity of Intermediate CLDN18.2 Expression in Gastric Cancer

Background CLDN18.2-targeted therapies show promise in HER2-negative, CLDN18.2-positive advanced gastric cancer (GC); however, cutoffs defining CLDN18.2 positivity vary across studies. Tumors with high expression (moderate to strong membranous expression in ≥ 75% of tumor cells) generally exhibit intratumoral homogeneity and a uniform staining pattern, but intratumoral heterogeneity and staining pattern of CLDN18.2 at lower expression thresholds remains unclear. This study investigated intratumoral heterogeneity of CLDN18.2 by comparing inter-block concordance of expression levels and staining patterns in GC. Methods Eighty-six resected GC specimens were retrospectively analyzed using CLDN18.2 immunohistochemistry on two tissue blocks per tumor. CLDN18.2 expression was categorized as high (≥ 75%), intermediate (< 75% and ≥ 40%), or negative (< 40%), and staining patterns as uniform or variable. Inter-block concordance of expression levels and staining patterns and their influence on concordance and outcomes were assessed. Results Overall inter-block concordance of expression levels was high (77%, κ = 0.89). Discordance was substantially higher in intermediate expression tumors (63.5%) than in high (15.5%) or negative (19%) expression tumors. Uniform staining was observed exclusively in high expression tumors, whereas all intermediate expression tumors displayed variable staining (100%). Among high expression tumors, inter-block concordance was substantially higher with uniform staining (98.0%) than with variable staining (71.5%). CLDN18.2 expression and staining pattern heterogeneity were not associated with worse survival. Conclusion Significant intratumoral heterogeneity of CLDN18.2 expression was observed in GC, particularly in intermediate expression tumors; however, this heterogeneity does not affect survival. These results highlight the importance of standardized assessment methods when applying lower thresholds for CLDN18.2-targeted therapies.