Approach to evaluating tumor mutational burden in routine clinical practice

Immune checkpoint inhibition with monoclonal antibodies has emerged as a promising therapeutic approach but in most tumor types responses are unpredictable and observed in a minority of treated patients. Positive and negative predictive biomarkers for efficacy of these costly drugs are desperately needed. Immunohistochemistry (IHC) for programmed death ligand (PD-L1) expression in tumor and inflammatory infiltrate has emerged as one predictive biomarker of some value. However, multiple confounders including those inherent to any IHC and the unique complexities of the biology of the immune response have limited its utility. Tumor mutational burden (TMB) has emerged as a seemingly more promising predictive biomarker for immunotherapy with checkpoint inhibitors in several tumor types and is likely to be incorporated into future treatment algorithms for these agents. Given this, the need to define and standardize key parameters of the most promising biomarkers becomes essential to allow all stakeholders to make meaningful observations and inferences as to the efficacy of ostensibly similar agents and combinations in various settings. This review briefly summarizes approaches to measurement of TMB and ongoing efforts to achieve harmonization of this key biomarker.