Dario Melgari

hERG potassium channel blockade by the HCN channel inhibitor bradycardic agent ivabradine.

Background Ivabradine is a specific bradycardic agent used in coronary artery disease and heart failure, lowering heart rate through inhibition of sinoatrial nodal HCN‐channels. This study investigated the propensity of ivabradine to interact with KCNH2‐encoded human Ether‐à‐go‐go–Related Gene (hERG) potassium channels, which strongly influence ventricular repolarization and susceptibility to torsades de pointes arrhythmia. Methods and Results Patch clamp recordings of hERG current (IhERG) were made from hERG expressing cells at 37°C. IhERG was inhibited with an IC50 of 2.07 μmol/L for the hERG 1a isoform and 3.31 μmol/L for coexpressed hERG 1a/1b. The voltage and time‐dependent characteristics of IhERG block were consistent with preferential gated‐state‐dependent channel block. Inhibition was partially attenuated by the N588K inactivation‐mutant and the S624A pore‐helix mutant and was strongly reduced by the Y652A and F656A S6 helix mutants. In docking simulations to a MthK‐based homology model of hERG, the 2 aromatic rings of the drug could form multiple π‐π interactions with the aromatic side chains of both Y652 and F656. In monophasic action potential (MAP) recordings from guinea‐pig Langendorff‐perfused hearts, ivabradine delayed ventricular repolarization and produced a steepening of the MAPD90 restitution curve. Conclusions Ivabradine prolongs ventricular repolarization and alters electrical restitution properties at concentrations relevant to the upper therapeutic range. In absolute terms ivabradine does not discriminate between hERG and HCN channels: it inhibits IhERG with similar potency to that reported for native If and HCN channels, with S6 binding determinants resembling those observed for HCN4. These findings may have important implications both clinically and for future bradycardic drug design.

Molecular basis of hERG potassium channel blockade by the class Ic antiarrhythmic flecainide

The class Ic antiarrhythmic drug flecainide inhibits KCNH2-encoded “hERG” potassium channels at clinically relevant concentrations. The aim of this study was to elucidate the underlying molecular basis of this action. Patch clamp recordings of hERG current (IhERG) were made from hERG expressing cells at 37 °C. Wild-type (WT) IhERG was inhibited with an IC50 of 1.49 μM and this was not significantly altered by reversing the direction of K+ flux or raising external [K+]. The use of charged and uncharged flecainide analogues showed that the charged form of the drug accesses the channel from the cell interior to produce block. Promotion of WT IhERG inactivation slowed recovery from inhibition, whilst the N588K and S631A attenuated-inactivation mutants exhibited IC50 values 4–5 fold that of WT IhERG. The use of pore-helix/selectivity filter (T623A, S624A V625A) and S6 helix (G648A, Y652A, F656A) mutations showed < 10-fold shifts in IC50 for all but V625A and F656A, which respectively exhibited IC50s 27-fold and 142-fold their WT controls. Docking simulations using a MthK-based homology model suggested an allosteric effect of V625A, since in low energy conformations flecainide lay too low in the pore to interact directly with that residue. On the other hand, the molecule could readily form π–π stacking interactions with aromatic residues and particularly with F656. We conclude that flecainide accesses the hERG channel from the cell interior on channel gating, binding low in the inner cavity, with the S6 F656 residue acting as a principal binding determinant.

Action Potential Clamp and Pharmacology of the Variant 1 Short QT Syndrome T618I hERG K+ Channel

Background The familial Short QT Syndrome (SQTS) is associated with an increased risk of cardiac arrhythmia and sudden death. Gain-of-function mutations in the hERG K+ channel protein have been linked to variant 1 of the SQTS. A hERG channel pore (T618I) mutation has recently been identified in families with heritable SQTS. This study aimed to determine effects of the T618I-hERG mutation on (i) hERG current (IhERG) elicited by ventricular action potentials; (ii) the sensitivity of IhERG to inhibition by four clinically used antiarrhythmic drugs. Methods Electrophysiological recordings of IhERG were made at 37°C from HEK 293 cells expressing wild-type (WT) or T618I hERG. Whole-cell patch clamp recording was performed using both conventional voltage clamp and ventricular action potential (AP) clamp methods. Results Under conventional voltage-clamp, WT IhERG peaked at 0-+10 mV, whilst for T618I IhERG maximal current was right-ward shifted to ∼ +40 mV. Voltage-dependent activation and inactivation of T618I IhERG were positively shifted (respectively by +15 and ∼ +25 mV) compared to WT IhERG. The IhERG ‘window’ was increased for T618I compared to WT hERG. Under ventricular AP clamp, maximal repolarising WT IhERG occurred at ∼ -30 mV, whilst for T618I hERG peak IhERG occurred earlier during AP repolarisation, at ∼ +5 mV. Under conventional voltage clamp, half-maximal inhibitory concentrations (IC50) for inhibition of IhERG tails by quinidine, disopyramide, D-sotalol and flecainide for T618I hERG ranged between 1.4 and 3.2 fold that for WT hERG. Under action potential voltage clamp, T618I IC50s ranged from 1.2 to 2.0 fold the corresponding IC50 values for WT hERG. Conclusions The T618I mutation produces a more modest effect on repolarising IhERG than reported previously for the N588K-hERG variant 1 SQTS mutation. All drugs studied here appear substantially to retain their ability to inhibit IhERG in the setting of the SQTS-linked T618I mutation.

hERG potassium channel inhibition by ivabradine may contribute to QT prolongation and risk of torsades de pointes

Deposited with reference to the publishers Green Open Access policy, available at https://uk.sagepub.com/en-gb/eur/the-green-route-%E2%80%93-open-access-archiving-policy

Suppression of the hERG potassium channel response to premature stimulation by reduction in extracellular potassium concentration

Potassium channels encoded by human ether‐à‐go‐go‐related gene (hERG) mediate the cardiac rapid delayed rectifier K+ current (IKr), which participates in ventricular repolarization and has a protective role against unwanted premature stimuli late in repolarization and early in diastole. Ionic current carried by hERG channels (IhERG) is known to exhibit a paradoxical dependence on external potassium concentration ([K+]e), but effects of acute [K+]e changes on the response of IhERG to premature stimulation have not been characterized. Whole‐cell patch‐clamp measurements of hERG current were made at 37°C from hERG channels expressed in HEK293 cells. Under conventional voltage‐clamp, both wild‐type (WT) and S624A pore‐mutant IhERG during depolarization to +20 mV and subsequent repolarization to −40 mV were decreased when superfusate [K+]e was decreased from 4 to 1 mmol/L. When [K+]e was increased from 4 to 10 mmol/L, pulse current was increased and tail IhERG was decreased. Increasing [K+]e produced a +10 mV shift in voltage‐dependent inactivation of WT IhERG and slowed inactivation time course, while lowering [K+]e from 4 to 1 mmol/L produced little change in inactivation voltage dependence, but accelerated inactivation time course. Under action potential (AP) voltage‐clamp, lowering [K+]e reduced the amplitude of IhERG during the AP and suppressed the maximal IhERG response to premature stimuli. Raising [K+]e increased IhERG early during the AP and augmented the IhERG response to premature stimuli. Our results are suggestive that during hypokalemia not only is the contribution of IKr to ventricular repolarization reduced but its ability to protect against unwanted premature stimuli also becomes impaired.

Letter by Melgari et al Regarding Article, “Ivabradine: Role in the Chronic Heart Failure Armamentarium”

We read with great interest Psotka and Teerlink’s1 recent review on the role of ivabradine in the treatment of chronic heart failure. A year after the US Food and Drug Administration granted approval for the use of ivabradine to decrease hospitalization from heart failure, the authors have evaluated the results from the major clinical trials on ivabradine. They conclude that, despite the overall superiority of β-blockers in managing heart failure, ivabradine can reduce heart failure hospitalization in selected patients with high resting heart rates (≥75 bpm) and a reduced ejection fraction who are already treated with a maximized guideline-based therapy. The authors also point …