Biomarkers for immune checkpoint inhibitors in solid tumors

The use of immune checkpoint inhibitors in solid organ malignancies has become widespread in the last decade. Accumulating evidence shows broad survival benefit as compared to traditional chemotherapies. At the same time, a need has emerged to stratify these drugs in various patient populations and histologies. Consequently, various immune biomarkers have been proposed to help in selecting patients for these therapies. Here, we review the evidence pertaining to biomarkers including programmed death-ligand 1, defective mismatch repair, tumor mutational burden, tumor-infiltrating lymphocytes, gene expression profiles, circulating blood cells, circulating DNA and the gut microbiome. The value of PD-L1 testing in certain malignancies, such as lung and urothelial cancer is highlighted as well as emerging data from trials such as GARNET and CheckMate142.