Discovery of a novel, potent and selective small-molecule inhibitor of PD-1/PD-L1 interaction with robust in vivo anti-tumour efficacy.

<bold>Background and Purpose: </bold>PD-1/PD-L1 antibodies have achieved great success in clinical treatment. However, monoclonal antibody drugs also have challenges, such as high manufacturing costs, poor diffusion, low oral bioavailability and limited penetration into tumour tissue. The development of small-molecule inhibitors of PD-1/PD-L1 interaction represents a promising perspective to overcome the above challenges in cancer immunotherapy.<bold>Experimental Approach: </bold>We explored structural activity relationships and used biochemical assays to generate a lead compound (ZE132). CD8+ T-cells killing assay and Ifng expression assay were used to verify the in vitro cellular activity of ZE132. Off-target study was performed to verify the selectivity. Syngeneic mouse models were used to verify the in vivo activity of ZE132 in tumour immune microenvironment (TIME). We also performed pharmacokinetics profiling in mice and The Cancer Genome Atlas database analysis.<bold>Key Results: </bold>ZE132 can effectively inhibit the PD-1/PD-L1 interactions in vitro, and it has a potent affinity to PD-L1. ZE132 shows robust anti-tumour effects in vivo, better than anti-PD-1 antibody. In the analysis of TIME, we found that ZE132 treatment promotes cytotoxic T-cell tumour infiltration and induces IL-2 expression. In addition, ZE132 elicits strong inhibitory effects on the mRNA expression of TGF-β, which may serve as a potential biomarker to predict responsiveness to PD-1/PD-L1 immunotherapies.<bold>Conclusion and Implications: </bold>We identified a new lead compound ZE132 targeting PD-1/PD-L1 interactions, not only showing favourable drug-like properties in vitro and in vivo but also showing the advantage of overcoming the barrier of TIME compared to anti-PD-1 antibody. [ABSTRACT FROM AUTHOR]