Abstract 371: A single-center retrospective review assessing the use of tumor mutation burden as a predictor of therapeutic success in patients receiving immunotherapy
Authors: Nguyen Khai , Xu Bing , Elsey Rachel , Meissner Tobias , Williams Casey 
Published: July 2021

Abstract
Background
Cancer is a genetic disease in which cells may accumulate somatic mutations. The measurement of the frequency these somatic mutations occur is referred to as tumor mutational burden or TMB. A minority of these mutations may give rise to neoantigens that could in turn be recognized and targeted by the immune system. The more somatic mutations a tumor develops, the more likely these neoantigens are to form. Recent studies, namely KEYNOTE-158, found high TMB status to be associated with improved outcomes in patients receiving pembrolizumab monotherapy.
Materials and Methods
This was a single-center retrospective study that analyzed 73 patients who received cancer treatment at Avera Cancer Institute between June 2016 and July 2019. Patients were included/excluded from the final analysis based on having received at least one dose of pembrolizumab, ipilimumab, and/or nivolumab. Information was collected on patient characteristics, treatment, and clinical outcomes. The primary outcomes were complete and partial response rates and were assessed objectively based on a composite of radiographic reports, physician documentation, and treatment course. Secondary outcomes were clinical outcomes in patients with PD-1 and/or PD-L1 expression, clinical outcomes in patients receiving mono-immunotherapy vs combination therapy, and clinical outcomes based on the line of treatment in which immunotherapy was utilized.
Results
A total of 34 patients met the inclusion criteria of having a high TMB value (10 mutations/megabase) and having received one dose of pembrolizumab, ipilimumab, or nivolumab.
Primary Outcomes•16 patients experienced a partial response (AR = 0.47) and 6 of these patients experienced a complete response (AR = 0.17)
Secondary Outcomes•Absolute risk with prior TMB information (PR 0.43, CR: 0.17)•Absolute risks for monotherapy (PR: 0.375, CR: 0.125) vs combination therapy (PR: 0.556, CR: 0.375)•Average line of treatment for TF, PR, and CR were 2.24, 2, and 1.83 respectively.
Discussion
Studies like KEYNOTE-158 found a significant correlation between high TMB and improved clinical outcomes in patients receiving checkpoint inhibitors. However, in this cohort of patients high TMB status was not a good predictor of therapeutic success compared to the data presented in KEYNOTE-001, CHECKMATE-057, OAK, or PACIFIC. While there are limitations associated with the sample size, the data support that checkpoint therapy benefits patients when it is used earlier in their treatment course and combined with other treatments. Further exploration is required to determine the most appropriate cut point for calling a patient TMB high as well as how TMB and PD-L1 predict response to specific therapies.
Citation Format: Khai Nguyen, Bing Xu, Rachel Elsey, Tobias Meissner, Casey Williams. A single-center retrospective review assessing the use of tumor mutation burden as a predictor of therapeutic success in patients receiving immunotherapy [abstract]. In: Proceedings of the American Association for Cancer Research Annual Meeting 2021; 2021 Apr 10-15 and May 17-21. Philadelphia (PA): AACR; Cancer Res 2021;81(13_Suppl):Abstract nr 371.