Abstract 740: Standardization and validation of CLDN18 antibodies for CLDN18.2 testing in pancreatic cancer: performance across antibodies and immunohistochemistry platforms

Abstract Background: Pancreatic cancer, with a 5-year survival rate of just 10%, is one of the most lethal cancers. Claudin 18.2 (CLDN18.2) is a promising therapeutic target under investigation. This study aims to standardize and validate CLDN18.2 IHC testing in pancreatic cancer by: (1) extending a gold-standard CLDN18 IHC method from gastric to pancreatic cancer, (2) validating two CLDN18 antibodies across three IHC platforms, and (3) assessing a CLDN18.2-specific antibody in pancreatic cancer samples. Methods: The Ventana CLDN18 (43-14A) assay on the Benchmark Ultra platform served as reference standard. Results from CLDN18 antibodies—Abcam 43-14A and LSBio B16145—were evaluated on Benchmark Ultra, Dako AS Link48, and Leica BOND III. Sixty pancreatic cancer samples verified the Ventana assay with inter-rater agreement by two pathologists. Twenty samples reflecting the full CLDN18 expression level (0-100%), were used to validate the two CLDN18 antibodies on the three platforms. Concordance with the reference standard was calculated for system suitability and accuracy. Precision was evaluated with five samples. A sample was deemed CLDN18.2 positive if at least 75% of tumor cells showed ≥2+ membranous CLDN18 staining. In addition, ten samples were used to explore the performance of the CLDN18.2-specific Abcam EPR19202 antibody on the Benchmark Ultra platform using OptiView DAB IHC detection kits. Results: The verification of the Ventana CLDN18 IHC Assay demonstrated its reliability and reproducibility in determining CLDN18.2 positivity in human pancreatic cancer samples, with an interrater agreement of 92%. This validation study showed that the Abcam 43-14A antibody outperformed the LSBio antibody in concordance analysis with the reference standard. The Benchmark Ultra platform provided the best results with strong, specific membranous CLDN18 staining and minimal background. The Leica platform performed the poorest, with strong cytoplasmic staining and lower specificity. The Dako platform was intermediate, more specific than Leica but weaker than the Benchmark Ultra. The Abcam EPR19202 antibody performed inadequately with OptiView; staining was faint, diffuse, non-specific with significant background, and poor membranous delineation. Conclusions: This study reveals key variations in anti-CLDN18 antibodies and IHC platforms, crucial for assessing borderline cases at the cutoff used in clinical trials (≥75%, IHC 2+/3+). It underscores the need for selecting suitable antibodies and optimizing antibody-platform combinations for reliable results. The Abcam 43-14A antibody on the Benchmark Ultra platform performed comparably to the Ventana assay. By evaluating multiple combinations, the study helps standardize CLDN18.2 IHC testing, enhancing diagnostic accuracy and healthcare decision-making. Citation Format: Gudrun Baenfer, Ali Jason Saleh, Rolf Diezko, Riem Adjam, Hans-Ulrich Schildhaus, Matthias Kirchner, Josef Rüschoff, Michael Friedrich. Standardization and validation of CLDN18 antibodies for CLDN18.2 testing in pancreatic cancer: performance across antibodies and immunohistochemistry platforms [abstract]. In: Proceedings of the American Association for Cancer Research Annual Meeting 2025; Part 1 (Regular Abstracts); 2025 Apr 25-30; Chicago, IL. Philadelphia (PA): AACR; Cancer Res 2025;85(8_Suppl_1):Abstract nr 740.