Biological and therapeutic impact of tumor mutational burden in microsatellite instability/mismatch repair-deficient (MSI/dMMR) metastatic colorectal cancer (mCRC) treated with immune checkpoint inhibitors (ICI).

272 Background: Microsatellite instability (MSI) is linked to hypermutability and response to ICI(s), especially in colorectal cancer. The predictive impact of tumor mutational burden (TMB) in ICI-treated MSI/dMMR metastatic colorectal cancer remains controversial, with some studies suggesting low TMB as a potential biomarker of resistance to ICI(s). We aimed to investigate the biological relevance of TMB in MSI/dMMR mCRC and its impact on survival outcomes in ICI(s)-treated patients (pts). Methods: MSI/dMMR mCRC pts treated with ICI(s) from the NIPICOL trial (NCT03350126) and the immunoMSI prospective monocentric cohort with available data from whole exome sequencing (WES) and RNA-sequencing (RNA-seq) were included. All cases were centrally assessed for MSI and dMMR status. TMB was calculated as described (Dupain et al., BMC, 2024). The search for a cutpoint able to optimize the difference in survival between 2 groups of patients with low and high-TMB was applied. The main endpoint was progression free survival (PFS) by iRECIST criteria. The cohort (NIPICOL, immunoMSI) was included as a covariate in the analysis of differential gene expression and pathway enrichment, with various tumor mutational burden (TMB) thresholds evaluated, specifically 10 and 30 mutations per megabase (mut/Mb). Results: A total of 99 pts were analyzed, with 4 (4%) excluded after central MSI/dMMR status review. Of these, 52 received anti-PD1 plus anti-CTLA4 therapy, while 43 received anti-PD1 alone. Median follow-up was 25 months. Median TMB was 51.8 mut/Mb (IQR: 35.9-74.9). TMB was not predictive of patient PFS, whatever the TMB threshold applied, including cutpoints already reported in the literature, the 2-year PFS rates being 80% versus 80% for TMB ≤ 10 and > 10 mut/Mb, and 79% versus 82% for TMB ≤ 30 and > 30 mut/Mb, respectively. Multivariate analysis including treatment type confirmed no association between TMB and PFS (HR 1.00, 95% CI [0.97–1.04]). TMB levels were not linked with specific signalling pathways related to immune checkpoints or antigen presentation as evaluated by RNA-seq with Gene Set Enrichment Analysis (GSEA) at a TMB threshold of 10 or 30 mut/Mb. However pathways involved in cell proliferation (e.g., “cell cycle”, “DNA replication”) were increased in TMB high (adjusted p-value <0.05). RNA-seq signature quantification and deconvolution analysis revealed similar cell populations (e.g., fibroblasts, B cells, T cells) between high and low TMB groups at a TMB threshold of 10 and 30 mut/Mb. Conclusions: TMB has no survival impact for patients with ICI(s)-treated dMMR/MSI mCRC and does not discriminate two biologically distinct subpopulations. Other biomarkers should be developed for personalized medicine amongst these patients.