AbstractInhibitors of PD-1 signaling have revolutionized cancer therapy. PD-1 and PD-L1 antibodies have been approved for the treatment of cancer. To date, therapeutic PD-1 inhibitors have not been compared in a functional assay. We used an efficient T cell reporter platform to evaluate the efficacy of five clinically used PD-1 inhibitors to block PD-1 signaling. The half maximal effective concentrations (EC₅₀) for nivolumab and pembrolizumab were 76.17 ng/ml (95% CI 64.95–89.34 ng/ml) and 39.90 ng/ml (34.01–46.80 ng/ml), respectively. The EC₅₀ values of the PD-L1 inhibitors were 6.46 ng/ml (5.48–7.61 ng/ml), 6.15 ng/ml (5.24–7.21 ng/ml) and 7.64 ng/ml (6.52–8.96 ng/ml) for atezolizumab, avelumab, and durvalumab, respectively. In conclusion, a functional assay evaluating antibodies targeting PD-1 inhibition in vitro revealed that pembrolizumab is a slightly more effective PD-1 blocker than nivolumab, and that PD-L1 antibodies are superior to PD-1 antibodies in reverting PD-1 signaling.