PD-L1-IN-4 (Compound X18) is an orally active PD-L1 inhibitor that exhibits remarkable inhibitory activity against the PD-1/PD-L1 interaction ( IC 50 = 1.3 nM) and enhances PD-L1 inhibitory effect on T cells ( EC 50 = 152.8 nM). PD-L1-IN-4 can be used for the research of cancer.