Background and Purpose: Many structurally and therapeutically diverse drugs block the human ether-à-go-go related gene (HERG) potassium channel, predisposing patients to an increased risk of arrythmias and sudden cardiac death. Many drugs show state-dependence of block, exhibiting a greater preference for block of the inactivated state. Five key residues within the central pore cavity of HERG have been implicated in drug binding. However, whether drugs bind differently to these key residues to dictate preference for the inactivated over the open state is not known. Experimental Approach: We used the SyncroPatch 384PE, automated patch clamp platform to measure how drug block was impacted by point mutations at residues T623, S624, Y652, and F656 when introduced into wild-type HERG (preferentially occupies the inactivated state) and N588K-HERG (preferentially occupies the open state) channels at depolarised potentials. Key Results: Mutations to Y652 and F656, that abolished aromatic and hydrophobic characteristics respectively, reduced drug binding in both WT and N588K backgrounds. The S624A mutation attenuated block by cisapride, astemizole, and quinidine in the WT background but not in the N588K background. Conclusion and Implications: We suggest that relative movements between the polar S624 sidechain and the aromatic Y652 and F656 sidechains in WT (inactivated) compared to N588K (open) channels can explain preferential binding to the inactivated state. An improved understanding of the structural basis of where and how drugs bind to HERG channels should facilitate efforts to reduce inadvertent HERG drug block during the drug development process.

Background and Purpose: Hydroxychloroquine and chloroquine, alone or in combination with azithromycin, have been proposed as therapies for COVID-19. However, there is currently scant and inconsistent data regarding their proarrhythmic potential in these patients. Moreover, their risk profile in the setting of altered physiological states encountered in patients with COVID-19 (i.e. febrile state, electrolyte imbalances, and/or acidosis) is unknown. Experimental approach: Potency of hERG block was measured using high-throughput electrophysiology in the presence of variable environmental factors. These potencies informed simulations to predict population risk profiles. Effects on cardiac repolarisation were verified in human induced pluripotent stem cell-derived cardiomyocytes (hiPSC-CM) from three separate individuals. Key Results: Chloroquine and hydroxychloroquine blocked hERG with IC50 of 1.47±0.07 µM and 3.78±0.17 µM respectively, indicating proarrhythmic risk at concentrations effective against SARS-CoV-2 in vitro and proposed in COVID-19 clinical trials. Hypokalaemia and hypermagnesemia increased potency of chloroquine and hydroxychloroquine, indicating increased proarrhythmic risk. Acidosis significantly reduced potency of all drugs (i.e. reduced proarrhythmic risk), whereas increased temperature decreased potency of chloroquine and hydroxychloroquine but increased potency for azithromycin. In silico simulations across genetically diverse populations predicted that 17% of individuals exhibit action potential durations >500 ms at the highest proposed therapeutic levels, equating to significant QT prolongation. Conclusion and Implications: Significant proarrhythmic risk is predicted for hydroxychloroquine and chloroquine at doses proposed to treat COVID-19. Clinicians should carefully consider the risk of such treatments, and implement long term QT interval monitoring in trials, particularly in patients with electrolyte imbalances.