PD-1/PD-L1-IN-43 (compound Z13) is a small-molecule inhibitors targeting the PD-1/PD-L1 interaction. PD-1/PD-L1-IN-43 exhibites potent in vivo antitumor efficacy against B16-F10 melanoma. PD-1/PD-L1-IN-43 inhibits tumor growth by blocking the interaction between PD-1 and PD-L1. PD-1/PD-L1-IN-43 can be used in anti-tumor studies.