The potassium/sodium hyperpolarization-activated cyclic nucleotide-gated channels (HCNs) are hyperpolarization-activated ion channels exhibiting weak selectivity for potassium over sodium ions which contribute to the native pacemaker currents in the heart (If) and in neurons (Ih). The If current is an important ionic current for regulating pacemaker activity in the sinoatrial (SA) node. The sinus node inhibitors (SNIs) cilobradine, ivabradine, and zatebradine all blocked the slow inward current through human HCN1, HCN2, HCN3, and HCN4 channels (Stieber et al. 2006). Ivabradine is indicated for the symptomatic management of stable heart-related chest pain and heart failure not fully managed by beta blockers (Giavarini & de Silva 2016, Gammone et al. 2020). Even as the close structural analogues of ivabradine cilobradine and zatebradine entered clinical trials (de Mendizábal et al. 2013), their development has been discontinued. They may be useful as starting points for novel compound optimization (Del Lungo et al. 2012) or further research (Romanelli et al. 2019, Lu et al. 2020).