BMS-1166 is a potent PD-1/PD-L1 immune checkpoint inhibitor. BMS-1166 induces dimerization of PD-L1 and blocks its interaction with PD-1, with an IC50 of 1.4 nM. BMS-1166 antagonizes the inhibitory effect of PD-1/PD-L1 immune checkpoint on T cell activation.