Characterization of CLDN18 expression in a Western biliary tract cancer population.

4089 Background: Patients with advanced biliary tract cancer (BTC) have poor survival despite recent advances in chemoimmunotherapy. Claudin 18 (CLDN18) directed therapy has shown benefit in combination with FOLFOX or CAPOX in gastric cancer and may also have therapeutic utility in advanced BTC. However, there are limited data on CLDN18 RNA and protein expression in BTC, especially in Western populations. Methods: Exome-capture based RNA sequencing was performed on BTC tissue samples through the MI-ONCOSEQ study at the University of Michigan. Fragments per Kilobase of transcript per Million mapped reads (FPKM) was used to normalize raw read counts. Immunohistochemical staining was completed on a BTC tissue microarray (n = 28), and Western blotting was done on human BTC cell lines (SNU-1079, RBE, and SSP-25) using CLDN18 recombinant rabbit monoclonal antibody (34H14L15, Invitrogen). Statistical significance was defined as p < 0.05. Results: We identified transcriptomic data from 148 consecutive BTC cases with median age 61 (range 17-81) years and 75 (50.7%) were female. Of these patients, 45 (30.4%) expressed CLDN18 mRNA with FPKM > 1. A lower proportion of intrahepatic cholangiocarcinoma (CCA) patients (n = 23/110; 20.9%) expressed CLDN18 mRNA relative to extrahepatic CCA (n = 13/25; 54.2%) and gallbladder cancer (n = 8/11; 72.7%) (p < 0.006). CLDN18 mRNA expression was not associated with stage at diagnosis, but was higher in metastatic versus primary sites (35.6 vs 16.1%, p < 0.05). Overall survival was not associated with CLDN18 gene expression in univariate analysis (hazard ratio 1.02, p > 0.9). CLDN18 protein expression was observed in 8/28 (28.6%) patients using a cutoff defined in prior gastric cancer clinical trials (2+ to 3+ staining intensity in ≥ 75% of tumor cells). In human BTC cell lines, SSP-25 expressed CLDN18, but the SNU-1079 and RBE lines did not. Conclusions: CLDN18 is expressed in a modest subset of Western patients with advanced BTC and CLDN18-directed therapy may be effective in this disease, particularly in ECC and gallbladder cancer given their higher frequency of expression. CLDN18 positive human BTC cell lines represent a promising preclinical model system for further investigation of this therapeutic strategy.