The evolving role of PD-L1 testing in patients with metastatic urothelial carcinoma

Immune checkpoint inhibitors targeting the programmed cell death 1 (PD-1)/programmed cell death ligand 1 (PD-L1) pathway improve clinical outcomes in patients with locally advanced/metastatic urothelial carcinoma (UC). PD-L1 complementary or companion diagnostic assays are now available for anti-PD-1 and anti-PD-L1 antibodies and these assays enable testing at diagnosis. The role of PD-L1 testing in UC is, however, the subject of much discussion within the medical community, particularly in light of recent restrictions on recruitment of PD-L1-low patients in clinical trials of atezolizumab and pembrolizumab as first-line therapy, and the European Medicines Agency and US Food and Drug Administration limiting use of these agents as first-line therapy in cisplatin-ineligible patients to those with high PD-L1 expression. We explore the evolving evidence for PD-L1 expression testing in UC and the role of PD-L1 expression in both tumor cells and tumor-infiltrating immune cells. We review clinical data on the prognostic and predictive value of PD-L1 expression in response to anti-PD-1/PD-L1 agents as first- and second-line therapy, considering issues such as the differences among complementary diagnostic assays in terms of the type of cells scored, antibodies used, and cutoff values. We consider how PD-L1 testing fits into decision-making and the potential of emerging biomarkers in UC. We conclude that, based on the scientific rationale for its use and evidence from clinical trials, PD-L1 testing provides enriched information on the patients most likely to benefit from immune checkpoint blockade and should be routinely offered to patients with metastatic UC.