Tumour Mutational Burden and Immune Checkpoint Inhibitor Response in Non-small Cell Lung Cancer: A Continuous Modelling Approach

Background: Tumour mutational burden (TMB) is an established biomarker for patients treated with immune checkpoint inhibitors (ICIs). The optimal TMB cut-off is uncertain. It is also uncertain whether there is a sharp TMB threshold or a more graduated change in clinical outcomes as TMB increases.

Objective: We aimed to determine the relationship between TMB and ICI treatment outcomes using alternative statistical approaches in patients with non-small cell lung cancer.

Methods: Tumour mutational burden was evaluated as a prognostic and predictive biomarker in advanced non-small cell lung cancer utilising data from two real-world cohorts of ICI use (n = 968) and three randomised controlled trials evaluating ICIs (n = 1588). The non-linear relationship between continuous TMB and response/survival/efficacy outcomes was evaluated using statistical methods that do not require specifying a TMB cut-off.

Results: Median TMB for all cohorts was seven mutations/megabase, excluding MYSTIC, where the median was 13 mutations/megabase. Progressively higher TMB was significantly associated with a progressively higher objective response rate and progression-free survival in ICI-treated patients in Memorial Sloan Kettering-Integrated Mutation Profiling of Actionable Cancer Targets [MSK-IMPACT] (objective response rate: p < 0.001, progression-free survival: p < 0.001), Strata Clinical Molecular Database [SCMD] (progression-free survival: p = 0.023) and OAK/POPLAR (objective response rate: p = 0.017, progression-free survival: p < 0.001) This relationship was not apparent for patients treated with chemotherapy. There was no obvious TMB threshold for ICI response. The relationship between TMB and overall survival was more complex and heterogeneous.

Conclusions: Using a single cut-off to analyse a continuous biomarker may hide important information. Methods that provide more nuance to the underlying relationship between TMB and outcomes enable readers to judge for themselves the value and limitations of TMB cut-offs proposed for clinical practice.