Hot or Not: Tumor Mutational Burden (TMB) As a Biomarker of Immunotherapy Response in Genitourinary Cancers

Pembrolizumab was recently approved for treatment of cancers with high tumor mutational burden (TMB). We conduct a focused literature review of TMB as a predictive biomarker. TMB quantifies the sum of nonsynonymous coding mutations (typically single nucleotide substitutions and short insertion-deletions) per megabase of sequenced DNA. As a proxy for expression of immunogenic neoantigens, TMB may be an effective predictive biomarker for response to immune checkpoint inhibitors. However, like other biomarkers in this setting, TMB has many limitations; the effect of this FDA approval in the current management of genitourinary cancers is likely limited to select situations.