PD-1/PD-L1-IN 6 (compound A13) is a potent PD-1/PD-L1 interaction inhibitor, with an IC 50 of 132.8 nM. PD-1/PD-L1-IN 6 exhibits outstanding immunoregulatory activity. PD-1/PD-L1-IN 6 significantly elevates interferon-γ secretion in a Hep3B/OS-8/hPD-L1 and CD3 T cell co-culture model, without significant toxic effect. PD-1/PD-L1-IN 6 restores the immune response in a T cell-tumor co-culture model.