PD-L1-IN-4 (Compound X18) is an orally active PD-L1 inhibitor that demonstrates notable inhibitory activity on the PD-1/PD-L1 interaction (IC50= 1.3 nM) and enhances the inhibitory effect of PD-L1 on T cells (EC50= 152.8 nM). PD-L1-IN-4 can be utilized in cancer research.