Validity and utility of blood tumor mutational burden (bTMB) is dependent on circulating tumor DNA (ctDNA) shed: SCRUM-Japan MONSTAR-SCREEN

Abstract

BackgroundThe tumor mutational burden (TMB) is a genomic biomarker associated with the benefits from immune checkpoint inhibitors (ICIs) cancer therapy. An elevated blood TMB (bTMB) in circulating tumor DNA (ctDNA) represents a compelling non-invasive diagnostic approach; however, the validity and utility of this emerging biomarker across cancer types have not been examined. Patient and methodsThe blood and tissue TMB was measured in a large pan-tumor clinical cohort and the MONSTAR-SCREEN observational study (UMIN000036749) using the FoundationOne Liquid CDx and FoundationOne CDx assays. A subset of the MONSTAR-SCREEN cohort was used to evaluate the association between the bTMB and the efficacy of ICIs therapy. ResultsThe majority of cancer types showed similar prevalence of TMB≥10 mutations/megabase and bTMB≥10. There was high concordance between bTMB and TMB in blood and tissue biopsy from the same patient when the plasma tumor fraction (TF) was at least 1% (Spearman's coefficient 0.74 and > 80% sensitivity to detect TMB-high). High microsatellite instability (MSI-H) was detected by ctDNA with 79% sensitivity when TF was at least 1%, but only 6% sensitivity when TF was <1%. Among patients with bTMB≥14 and elevated TF≥10% treated with ICIs, there was a trend toward a longer progression-free survival and overall survival compared with patients with bTMB<14 and elevated TF≥10% (HR, 0.62 [95%CI, 0.39–0.98]; p = 0.04 and HR, 0.60 [95%CI, 0.34-1.1]; p = 0.05). ConclusionsOur findings suggest that an elevated bTMB is correlated with elevated TMB and represents a pragmatic biomarker for assessing ICIs benefits. The utility of this biomarker is likely to be associated with high TF levels, informing future prospective investigations.