AbstractThe development of small molecule inhibitors of programmed cell death‐1/programmed cell death‐ligand 1 (PD‐1/PD‐L1) has drawn research interest for the treatment of cancer. Recently, we reported the discovery of a novel dimeric core small molecule PD‐1/PD‐L1 inhibitor. In an effort to discover more potent inhibitors, we further explored the dimeric core scaffold. Our investigations of the structure‐activity‐relationship revealed that introduction of lipophilic substituents onto one of the di‐alkoxylated phenyl rings improved binding affinities to PD‐L1, and inhibitory activities of PD‐1/PD‐L1 in cellular assays. Furthermore, conversion of the ether linker part to an olefin linker not only improved binding affinity but also led to slow dissociation binding kinetics. We also explored more potent, as well as downsized, scaffolds. Compounds bearing a linear chain in place of one of the di‐alkoxylated phenyl rings exhibited good binding affinity with improved ligand efficiency (LE). Representative compounds demonstrated potent inhibitory activities of PD‐1/PD‐L1 in the submicromolar range in cellular assays as well as cellular function in the mixed lymphocyte reaction (MLR) assay with efficacy comparable to anti‐PD‐1 antibody. Our results provide applicable information for the design of more potent inhibitors targeting PD‐1/PD‐L1 pathway.