Tumor mutational burden (TMB) in real-world patients with pancreatic ductal adenocarcinoma (PDAC): Differences in genomic alterations (GA) and predictive value for immune checkpoint inhibitor (ICI) effectiveness.
Authors: Mahipal Amit , Quintanilha Julia C. F. , Graf Ryon , Storandt Michael H. , Keller Rachel B , Li Gerald , Ross Jeffrey S. , Huang Richard S.P. , Schrock Alexa Betzig , Oxnard Geoffrey R. , Chakrabarti Sakti 
Published: June 2023

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Background: Currently, ICI therapy is recommended for patients with metastatic PDAC whose tumors exhibit dMMR, MSI-H, or high TMB (≥10 mutations/Mb). However, due to the low prevalence of high TMB in PDAC (~1%), few studies have evaluated the role of ICI therapy in this subpopulation. This study aimed to compare GA between PDAC patients with high TMB and low TMB ( < 10 mutations/Mb) and to evaluate the effectiveness of ICIs in real-world PDAC patients by TMB status. Methods: This study included PDAC patients who underwent genomic testing using Foundation Medicine tissue comprehensive genomic profiling (CGP) assays. GA were compared between tissue specimens with high and low TMB by chi-squared, adjusted for multiple comparisons. Patient clinical data was obtained by the US-wide de-identified Flatiron Health and Foundation Medicine real-world clinicogenomic pancreatic database (CGDB), originated from ~280 US cancer clinics between 01/2011 and 09/2022. Real-world overall survival (OS) and time to treatment discontinuation (TTD) were compared between patients receiving an ICI (high TMB versus low TMB) and between patients with high TMB (ICI versus other therapies) by Cox models. Results: We included 21,932 patients with PDAC with tissue CGP data available; 98.7% with low TMB and 1.3% with high TMB. Among actionable alterations, patients with high TMB had higher prevalence of mutations in BRCA2 (p < 0.0001), BRAF (p < 0.0001), PALB2 (p < 0.0001), and genes of the mismatch repair pathway ( MSH2, MSH6, MLH1, and PMS2, p < 0.0001), but lower prevalence of KRAS mutations (p < 0.0001). The most common KRAS mutation in both groups of patients was G12D. In CGDB, 51 patients received an ICI (10 with high TMB and 41 with low TMB) and 17 patients with high TMB received other therapies. Among patients receiving an ICI, those with high TMB had more favorable median OS compared to those with low TMB (25.7 versus 5.2 months, hazard ratio (HR) 0.27, 95% confidence interval (CI) 0.09 - 0.76, p = 0.01) and median TTD (20.7 versus 3.0 months, HR 0.33, 95% CI 0.13 - 0.82, p = 0.02). Among patients with high TMB, those receiving an ICI had more favorable OS compared to those receiving other therapies (25.7 versus 6.6 months, HR 0.31, 95% CI 0.10-0.96, p = 0.043), nominally favoring ICI use in this small cohort (n = 10 versus 17). Conclusions: There is no randomized clinical trial evaluating ICI versus other therapies in PDAC. To our knowledge, this is the largest cohort to date comparing ICI effectiveness in PDAC. We observed more favorable OS among PDAC patients with high TMB receiving ICI versus those with low TMB who received an ICI and more favorable to those with high TMB receiving other therapies. This study supports the FDA-approved use of ICIs in patients with PDAC and high TMB and demonstrates the importance of TMB assessment for patients with PDAC.