Claudin 18 (CLDN18) gene expression and related molecular profile in gastric cancer (GC).

4048 Background: Claudins are transmembrane proteins which maintain the tight junction between cells. The stomach specific isoform, CLDN18 isoform 2 (CLDN18.2), is emerging as a promising treatment target because of high expression in GC cells, including targeting via adoptive T-cell strategies. We characterized the molecular features associated with CLDN18 isoform 1 and 2 ( CLDN18.1/ 18.2) gene expression in GC. Methods: Tumor profiling was performed from 1967 samples by NextGen Sequencing on DNA (592 genes or WES) and RNA (WTS) at Caris Life Sciences (Phoenix, AZ). EBER (Epstein Barr Virus) was tested by CISH. Top quartile transcripts per million for CLDN18.1/18.2 were considered high while bottom quartile low expression. X2, Fisher-exact, and Mann Whitney tests were used and significance adjusted for multiple testing by Benjamini-Hochberg (q <.05). Cell infiltration in the tumor microenvironment (TME) was estimated by quanTIseq. Gene expression profiles were analyzed for a transcriptional signature predictive of response to immunotherapy (T cell-inflamed signature, TIS). Results: CLDN18.2 expression was detected in 97% of the samples and CLDN18.1 in 63%. Primary tumors had significantly higher expression levels of both CLDN18.1/18.2 (Fold Change 18.1: 0.50; 18.2: 0.65), compared to metastatic tumors ( p <.05), thus we focused on the comparison of CLDN18 high and low in primary tumors. CLDN18.2 high expression group had higher CLDN18: ARHGAP26 fusion positive rate (low vs high: 0.91% vs 5.5%, q <.0001), and a trending association with CDH1 mutation (11.7% vs 20.7%, p <.01, q >.05) and EBER (2.15% vs 6.31%, p <.05, q >.05). There were more prevalent ARHGAP26 fusions in the CLDN18.1/18.2 high group (18.1: 9.5% vs 3.86%, p =.001; 18.2: 10.1% vs 0.9%, q <.0001), with the most common fusion between CLDN18 exon 5 and ARHGAP26 exon 12. CLDN18.2 high expression demonstrated an inverse trending correlation with PD-L1 (24.9% vs 18.3%, p <.05; q >.05) and TMB-H (19.6% vs 12.2%, p <.05; q >.05). Similarly, CLDN18.1/18.2 displayed an inverse relationship with M1 Macrophages, NK cells, CD4+ T cells, myeloid dendritic cells in the TME ( q <.05); with higher CLDN18 expression associated with fewer immune cells and a colder TME, especially in isoform 2. The TIS score was significantly higher in the CLDN18.2 high expression group ( q <.05), but lower in CLDN18.1 high expression group ( q <.0001), respectively. Conclusions: This is one of the most comprehensive dedicated analyses on CLDN18 related to tumor molecular features, TME and immunotherapy response in GC. Tumors expressing high CLDN18, especially 18.2, displayed distinct genomic and transcriptomic alterations in immune biomarkers and immune cell infiltration in the TME. Anti-CLDN18.2 monoclonal antibodies are being tested in GC and CLDN18 is a target for ADC and CAR-T therapies. Our data suggest that expression may play a role in guiding patient selection and treatment combinations.