PD-1-IN-25 (compound 43) is a potent PD-1/PD-L1 interaction inhibitor with an IC50 value of 10.2 nM in the HTRF assay. PD-1-IN-25 can promote CD8+ T cell activation through inhibiting PD-1/PD-L1 cellular signaling. PD-1-IN-25 delays the tumor growth.