PD-L1 as a biomarker of response to immune-checkpoint inhibitors

Immune-checkpoint inhibitors targeting PD-1 or PD-L1 have already substantially improved the outcomes of patients with many types of cancer, although only 20-40% of patients derive benefit from these new therapies. PD-L1, quantified using immunohistochemistry assays, is currently the most widely validated, used and accepted biomarker to guide the selection of patients to receive anti-PD-1 or anti-PD-L1 antibodies. However, many challenges remain in the clinical use of these assays, including the necessity of using different companion diagnostic assays for specific agents, high levels of inter-assay variability in terms of both performance and cut-off points, and a lack of prospective comparisons of how PD-L1(+) disease diagnosed using each assay relates to clinical outcomes. In this Review, we describe the current role of PD-L1 immunohistochemistry assays used to inform the selection of patients to receive anti-PD-1 or anti-PD-L1 antibodies, we discuss the various technical and clinical challenges associated with these assays, including regulatory issues, and we provide some perspective on how to optimize PD-L1 as a selection biomarker for the future treatment of patients with solid tumours. PD-L1 expression is currently the best available biomarker for the prediction of responsiveness to immune-checkpoint inhibitors. However, several immunohistochemical assays are now approved for clinical use in various settings, despite imperfect inter-assay concordance, with important implications for pathology services and, potentially, for clinical outcomes. In this Review, the authors compare the performance of the various FDA-approved PD-L1 assays, discuss the varying implications of PD-L1 expression across different tumour types and provide guidance on possible novel approaches that might optimize the clinical utility of PD-L1 as a biomarker.

Automated summary

PD-L1 expression is currently the best predictor of responsiveness to immune-checkpoint inhibitors, despite challenges in clinical use such as variability in different assays and cut-off points, as well as the lack of prospective comparisons with clinical outcomes. Nevertheless, several immunohistochemical assays have been approved for clinical use, with implications for pathology services and potential impacts on clinical outcomes. Comparison of FDA-approved PD-L1 assays, understanding of the varying implications of PD-L1 expression across different tumour types, and exploration of novel approaches to optimize its clinical utility are important topics for further research and practice.