Zatebradine (UL-FS-49 (free base)) is a potent inhibitor of hyperpolarization-activated cyclic nucleotide-gated(HCN) channels withan IC 50 values 1.96 µM. Zatebradine blocks the slow inward current through human HCN1 , HCN2 , HCN3 and HCN4 channels , with IC 50 values of 1.83 µM, 2.21 µM, 1.90 µM and 1.88 µM, respectively.