Discovery of Novel HCN4 Blockers with Unique Blocking Kinetics and Binding Properties

The hyperpolarization-activated cyclic nucleotide-gated 4 (HCN4) channel underlies the pacemaker currents, called "If," in sinoatrial nodes (SANs), which regulate heart rhythm. Some HCN4 blockers such as ivabradine have been extensively studied for treating various heart diseases. Studies have shown that these blockers have diverse state dependencies and binding sites, suggesting the existence of potential chemical and functional diversity among HCN4 blockers. Here we report approaches for the identification of novel HCN4 blockers through a random screening campaign among 16,000 small-molecule compounds using an automated patch-clamp system. These molecules exhibited various blockade profiles, and their blocking kinetics and associating amino acids were determined by electrophysiological studies and site-directed mutagenesis analysis, respectively. The profiles of these blockers were distinct from those of the previously reported HCN channel blockers ivabradine and ZD7288. Notably, the mutagenesis analysis showed that blockers with potencies that were increased when the channel was open involved a C478 residue, located at the pore cavity region near the cellular surface of the plasma membrane, while those with potencies that were decreased when the channel was open involved residues Y506 and I510, located at the intracellular region of the pore gate. Thus, this study reported for the first time the discovery of novel HCN4 blockers by screening, and their profiling analysis using an automated patch-clamp system provided chemical tools that will be useful to obtain unique molecular insights into the drug-binding modes of HCN4 and may contribute to the expansion of therapeutic options in the future.