PD-1/PD-L1-IN-17 (Compound P20) is a potent inhibitor of PD-1 / PD-L1 with an IC 50 value of 26.8 nM. PD-1/PD-L1-IN-17 is a promising lead compound for the development of inhibitors of the PD-1/PD-L1 interaction. PD-1/PD-L1-IN-17 has the potential for the research of cancer diseases.