hERG agonists pose challenges to web-based machine learning methods for prediction of drug-hERG channel interaction

Pharmacological blockade of the IKr channel (hERG) by diverse drugs in clinical use is associated with the Long QT Syndrome that can lead to life threatening arrhythmia. Various computational tools including machine learning models (MLM) for the prediction of hERG inhibition have been developed to facilitate the throughput screening of drugs in development and optimise thus the prediction of hERG liabilities. The use of MLM relies on large libraries of training compounds for the quantitative structure-activity relationship (QSAR) modelling of hERG inhibition. The focus on inhibition omits potential effects of hERG channel agonist molecules and their associated QT shortening risk. It is instructive, therefore, to consider how known hERG agonists are handled by MLM. Here, two highly developed online computational tools for the prediction of hERG liability, Pred-hERG and HergSPred were probed for their ability to detect hERG activator drug molecules as hERG interactors. In total, 73 hERG blockers were tested with both computational tools giving overall good predictions for hERG blockers with reported IC50s below Pred-hERG and HergSPred cut-off threshold for hERG inhibition. However, for compounds with reported IC50s above this threshold such as disopyramide or sotalol discrepancies were observed. HergSPred identified all 20 hERG agonists selected as interacting with the hERG channel. Further studies are warranted to improve online MLM prediction of hERG related cardiotoxicity, by explicitly taking into account channel agonism as well as inhibition.

Inhibition of the hERG Potassium Channel by a Methanesulphonate-Free E-4031 Analogue

hERG (human Ether-à-go-go Related Gene)-encoded potassium channels underlie the cardiac rapid delayed rectifier (IKr) potassium current, which is a major target for antiarrhythmic agents and diverse non-cardiac drugs linked to the drug-induced form of long QT syndrome. E-4031 is a high potency hERG channel inhibitor from the methanesulphonanilide drug family. This study utilized a methanesulphonate-lacking E-4031 analogue, "E-4031-17", to evaluate the role of the methanesulphonamide group in E-4031 inhibition of hERG. Whole-cell patch-clamp measurements of the hERG current (IhERG) were made at physiological temperature from HEK 293 cells expressing wild-type (WT) and mutant hERG constructs. For E-4031, WT IhERG was inhibited by a half-maximal inhibitory concentration (IC50) of 15.8 nM, whilst the comparable value for E-4031-17 was 40.3 nM. Both compounds exhibited voltage- and time-dependent inhibition, but they differed in their response to successive applications of a long (10 s) depolarisation protocol, consistent with greater dissociation of E-4031-17 than the parent compound between applied commands. Voltage-dependent inactivation was left-ward voltage shifted for E-4031 but not for E-4031-17; however, inhibition by both compounds was strongly reduced by attenuated-inactivation mutations. Mutations of S6 and S5 aromatic residues (F656V, Y652A, F557L) greatly attenuated actions of both drugs. The S624A mutation also reduced IhERG inhibition by both molecules. Overall, these results demonstrate that the lack of a methanesulphonate in E-4031-17 is not an impediment to high potency inhibition of IhERG.

Pharmacological activation of the hERG K + channel for the management of the long QT syndrome: A review

In the human heart, the rapid delayed rectifier K⁺ current (I_Kr) contributes significantly to ventricular action potential (AP) repolarization and to set the duration of the QT interval of the surface electrocardiogram (ECG). The pore‐forming (α) subunit of the I_Kr channel is encoded by KCNH2 or human ether‐à‐go‐go‐related gene 1 (hERG1). Impairment of hERG function through either gene mutation (congenital) or pharmacological blockade by diverse drugs in clinical use (acquired) can cause a prolongation of the AP duration (APD) reflected onto the surface ECG as a prolonged QT interval or Long QT Syndrome (LQTS). LQTS can increase the risk of triggered activity of ventricular cardiomyocytes and associated life‐threatening arrhythmia. Current treatments all focus on reducing the incidence of arrhythmia or terminating it after its onset but there is to date no prophylactic treatment for the pharmacological management of LQTS. A new class of hERG modulators (agonists) have been suggested through direct interaction with the hERG channel to shorten the action potential duration (APD) and/or increase the postrepolarisation refractoriness period (PRRP) of ventricular cardiomyocytes protecting thereby against triggered activity and associated arrhythmia. Although promising drug candidates, there remain major obstacles to their clinical development. The aim of this review is to summarize the latest advances as well as the limitations of this proposed pharmacotherapy.

COVID-19 Management and Arrhythmia: Risks and Challenges for Clinicians Treating Patients Affected by SARS-CoV-2

The COVID-19 pandemic is an unprecedented challenge and will require novel therapeutic strategies. Affected patients are likely to be at risk of arrhythmia due to underlying comorbidities, polypharmacy and the disease process. Importantly, a number of the medications likely to receive significant use can themselves, particularly in combination, be pro-arrhythmic. Drug-induced prolongation of the QT interval is primarily caused by inhibition of the hERG potassium channel either directly and/or by impaired channel trafficking. Concurrent use of multiple hERG-blocking drugs may have a synergistic rather than additive effect which, in addition to any pre-existing polypharmacy, critical illness or electrolyte imbalance, may significantly increase the risk of arrhythmia and Torsades de Pointes. Knowledge of these risks will allow informed decisions regarding appropriate therapeutics and monitoring to keep our patients safe.

Structure-activity relationship and cardiac safety of 2-aryl-2-(pyridin-2-yl)acetamides as a new class of broad-spectrum anticonvulsants derived from Disopyramide

A series of 2-aryl-2-(pyridin-2-yl)acetamides were synthesized and screened for their anticonvulsant activity in animal models of epilepsy. The compounds were broadly active in the 'classical' maximal electroshock seizure (MES) and subcutaneous Metrazol (scMET) tests as well as in the 6 Hz and kindling models of pharmacoresistant seizures. Furthermore, the compounds showed good therapeutic indices between anticonvulsant activity and motor impairment. Structure-activity relationship (SAR) trends clearly showed the highest activity resides in unsubstituted phenyl derivatives or compounds having ortho- and meta- substituents on the phenyl ring. The 2-aryl-2-(pyridin-2-yl)acetamides were derived by redesign of the cardiotoxic sodium channel blocker Disopyramide (DISO). Our results show that the compounds preserve the capability of the parent compound to inhibit voltage gated sodium currents in patch-clamp experiments; however, in contrast to DISO, a representative compound from the series 1 displays high levels of cardiac safety in a panel of in vitro and in vivo experiments.

The macrolide drug erythromycin does not protect the hERG channel from inhibition by thioridazine and terfenadine

The macrolide antibiotic erythromycin has been associated with QT interval prolongation and inhibition of the hERG-encoded channels responsible for the rapid delayed rectifier K⁺ current I(_Kr ). It has been suggested that low concentrations of erythromycin may have a protective effect against hERG block and associated drug-induced arrhythmia by reducing the affinity of the pore-binding site for high potency hERG inhibitors. This study aimed to explore further the notion of a potentially protective effect of erythromycin. Whole-cell patch-clamp experiments were performed in which hERG-expressing mammalian (Human Embryonic Kidney; HEK) cells were preincubated with low to moderate concentrations of erythromycin (3 or 30 µM) prior to whole-cell patch clamp recordings of hERG current (I_hERG ) at 37°C. In contrast to a previous report, exposure to low concentrations of erythromycin did not reduce pharmacological sensitivity of hERG to the antipsychotic thioridazine and antihistamine terfenadine. The IC₅₀ value for I_hERG tail inhibition by terfenadine was decreased by ~32-fold in the presence of 3 µM erythromycin (p < .05 vs. no preincubation). Sensitivity to thioridazine remained unchanged (p > .05 vs. no preincubation). The effects of low concentrations of erythromycin were investigated for a series of pore blocking drugs, and the results obtained were consistent with additive and/or synergistic effects. Experiments with the externally acting blocker BeKm-1 on WT hERG and a pore mutant (F656V) were used to explore the location of the binding site for erythromycin. Our data are inconsistent with the use of erythromycin for the management of drug-induced QT prolongation.

Structural implications of hERG K+ channel block by a high-affinity minimally structured blocker

Cardiac potassium channels encoded by human ether-à-go-go–related gene (hERG) are major targets for structurally diverse drugs associated with acquired long QT syndrome. This study characterized hERG channel inhibition by a minimally structured high-affinity hERG inhibitor, Cavalli-2, composed of three phenyl groups linked by polymethylene spacers around a central amino group, chosen to probe the spatial arrangement of side chain groups in the high-affinity drug-binding site of the hERG pore. hERG current (I_hERG) recorded at physiological temperature from HEK293 cells was inhibited with an IC₅₀ of 35.6 nm with time and voltage dependence characteristic of blockade contingent upon channel gating. Potency of Cavalli-2 action was markedly reduced for attenuated inactivation mutants located near (S620T; 54-fold) and remote from (N588K; 15-fold) the channel pore. The S6 Y652A and F656A mutations decreased inhibitory potency 17- and 75-fold, respectively, whereas T623A and S624A at the base of the selectivity filter also decreased potency (16- and 7-fold, respectively). The S5 helix F557L mutation decreased potency 10-fold, and both F557L and Y652A mutations eliminated voltage dependence of inhibition. Computational docking using the recent cryo-EM structure of an open channel hERG construct could only partially recapitulate experimental data, and the high dependence of Cavalli-2 block on Phe-656 is not readily explainable in that structure. A small clockwise rotation of the inner (S6) helix of the hERG pore from its configuration in the cryo-EM structure may be required to optimize Phe-656 side chain orientations compatible with high-affinity block.

Interactions between amiodarone and the hERG potassium channel pore determined with mutagenesis and in silico docking

The antiarrhythmic drug amiodarone delays cardiac repolarisation through inhibition of hERG-encoded potassium channels responsible for the rapid delayed rectifier potassium current (I_Kr). This study aimed to elucidate molecular determinants of amiodarone binding to the hERG channel. Whole-cell patch-clamp recordings were made at 37 °C of ionic current (I_hERG) carried by wild-type (WT) or mutant hERG channels expressed in HEK293 cells. Alanine mutagenesis and ligand docking were used to investigate the roles of pore cavity amino-acid residues in amiodarone binding. Amiodarone inhibited WT outward I_hERG tails with a half-maximal inhibitory concentration (IC₅₀) of ∼45 nM, whilst inward I_hERG tails in a high K⁺ external solution ([K⁺]_e) of 94 mM were blocked with an IC₅₀ of 117.8 nM. Amiodarone’s inhibitory action was contingent upon channel gating. Alanine-mutagenesis identified multiple residues directly or indirectly involved in amiodarone binding. The IC₅₀ for the S6 aromatic Y652A mutation was increased to ∼20-fold that of WT I_hERG, similar to the pore helical mutant S624A (∼22-fold WT control). The IC₅₀ for F656A mutant I_hERG was ∼17-fold its corresponding WT control. Computational docking using a MthK-based hERG model differentiated residues likely to interact directly with drug and those whose Ala mutation may affect drug block allosterically. The requirements for amiodarone block of aromatic residues F656 and Y652 within the hERG pore cavity are smaller than for other high affinity I_hERG inhibitors, with relative importance to amiodarone binding of the residues investigated being S624A ∼ Y652A > F656A > V659A > G648A > T623A.

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 1.49μM 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.

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 (I_hERG) were made from hERG expressing cells at 37°C. I_hERG 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 I_hERG 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 I_hERG with similar potency to that reported for native I_f 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.

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 (I_Kr), 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 (I_hERG) is known to exhibit a paradoxical dependence on external potassium concentration ([K⁺]_e), but effects of acute [K⁺]_e changes on the response of I_hERG 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 I_hERG 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 I_hERG was decreased. Increasing [K⁺]_e produced a +10 mV shift in voltage‐dependent inactivation of WT I_hERG 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 I_hERG during the AP and suppressed the maximal I_hERG response to premature stimuli. Raising [K⁺]_e increased I_hERG early during the AP and augmented the I_hERG response to premature stimuli. Our results are suggestive that during hypokalemia not only is the contribution of I_Kr to ventricular repolarization reduced but its ability to protect against unwanted premature stimuli also becomes impaired.

hERG potassium channels are important for ventricular repolarization and for protecting the ventricles of the heart from unwanted premature stimuli. This study shows that, in addition to reducing the contribution of hERG channel current to ventricular repolarization, hypokalemia impairs the protective response of hERG to premature stimulation.

Modification by KCNE1 variants of the hERG potassium channel response to premature stimulation and to pharmacological inhibition

human Ether-à-go-go-Related Gene (hERG) encodes the pore-forming subunit of cardiac rapid delayed rectifier K(+) current (I Kr) channels, which play important roles in ventricular repolarization, in protecting the myocardium from unwanted premature stimuli, and in drug-induced Long QT Syndrome (LQTS). KCNE1, a small transmembrane protein, can coassemble with hERG. However, it is not known how KCNE1 variants influence the channel's response to premature stimuli or if they influence the sensitivity of hERG to pharmacological inhibition. Accordingly, whole-cell patch-clamp measurements of hERG current (I hERG) were made at 37°C from hERG channels coexpressed with either wild-type (WT) KCNE1 or with one of three KCNE1 variants (A8V, D76N, and D85N). Under both conventional voltage clamp and ventricular action potential (AP) clamp, the amplitude of I hERG was smaller for A8V, D76N, and D85N KCNE1 + hERG than for WT KCNE1 + hERG. Using paired AP commands, with the second AP waveform applied at varying time intervals following the first to mimic premature ventricular excitation, the response of I hERG carried by each KCNE1 variant was reduced compared to that with WT KCNE1 + hERG. The I hERG blocking potency of the antiarrhythmic drug quinidine was similar between WT KCNE1 and the three KCNE1 variants. However, the I hERG inhibitory potency of the antibiotic clarithromycin and of the prokinetic drug cisapride was altered by KCNE1 variants. These results demonstrate that naturally occurring KCNE1 variants can reduce the response of hERG channels to premature excitation and also alter the sensitivity of hERG channels to inhibition by some drugs linked to acquired LQTS.

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 (I(hERG)) elicited by ventricular action potentials; (ii) the sensitivity of I(hERG) to inhibition by four clinically used antiarrhythmic drugs.

METHODS

Electrophysiological recordings of I(hERG) 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 I(hERG) peaked at 0-+10 mV, whilst for T618I I(hERG) maximal current was right-ward shifted to ∼ +40 mV. Voltage-dependent activation and inactivation of T618I I(hERG) were positively shifted (respectively by +15 and ∼ +25 mV) compared to WT I(hERG). The I(hERG) 'window' was increased for T618I compared to WT hERG. Under ventricular AP clamp, maximal repolarising WT I(hERG) occurred at ∼ -30 mV, whilst for T618I hERG peak I(hERG) occurred earlier during AP repolarisation, at ∼ +5 mV. Under conventional voltage clamp, half-maximal inhibitory concentrations (IC(50)) for inhibition of I(hERG) 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 IC(50)s ranged from 1.2 to 2.0 fold the corresponding IC(50) values for WT hERG.

CONCLUSIONS

The T618I mutation produces a more modest effect on repolarising I(hERG) than reported previously for the N588K-hERG variant 1 SQTS mutation. All drugs studied here appear substantially to retain their ability to inhibit I(hERG) in the setting of the SQTS-linked T618I mutation.

Molecular determinants of hERG potassium channel inhibition by disopyramide

The Class Ia antiarrhythmic drug disopyramide (DISO) causes QT interval prolongation that is potentially dangerous in acquired Long QT Syndrome but beneficial in short QT syndrome, through inhibition of the hERG-encoded channels responsible for rapid delayed rectifier K(+) current (I(Kr)). In this study, alanine mutants of hERG S6 and pore helix residues and MthK-based homology modelling and ligand docking were used to investigate molecular determinants of DISO binding to hERG. Whole-cell hERG current (I(hERG)) recordings were made at 37°C from HEK-293 cells expressing WT or mutant hERG channels. WT outward I(hERG) tails were inhibited with an IC(50) of 7.3μM, whilst inward I(hERG) tails in a high [K(+)](e) of 94mM were blocked with an IC(50) of 25.7μM. The IC(50) for the Y652A mutation was ~55-fold that of WT I(hERG); this mutation also abolished a leftward shift in voltage-dependent I(hERG) activation present for WT hERG. The IC(50) for F656A I(hERG) was ~51 fold its corresponding WT control. In contrast to previously studied methanesulphonanilide hERG inhibitors, neither the G648A S6 nor the T623A and S624A pore helical mutations modified DISO IC(50). Computational docking with the hERG model showed that DISO did not exhibit a single unique binding pose; instead several low energy binding poses at the lower end of the pore cavity favoured interactions with Y652 and F656. In the WT hERG model DISO did not interact directly with residues at the base of the pore helix, consistent with the minimal effect of mutation of these residues on drug block.

Action potential clamp and chloroquine sensitivity of mutant Kir2.1 channels responsible for variant 3 short QT syndrome

Recently identified genetic forms of short QT syndrome (SQTS) are associated with an increased risk of arrhythmia and sudden death. The SQT3 variant is associated with an amino-acid substitution (D172N) in the KCNJ2-encoded Kir2.1 K⁺ channel. In this study, whole-cell action potential (AP) clamp recording from transiently transfected Chinese Hamster Ovary cells at 37 °C showed marked augmentation of outward Kir2.1 current through D172N channels, associated with right-ward voltage-shifts of peak repolarizing current during both ventricular and atrial AP commands. Peak outward current elicited by ventricular AP commands was inhibited by chloroquine with an IC₅₀ of 2.45 μM for wild-type (WT) Kir2.1, of 3.30 μM for D172N-Kir2.1 alone and of 3.11 μM for co-expressed WT and D172N (P > 0.05 for all). These findings establish chloroquine as an effective inhibitor of SQT3 mutant Kir2.1 channels.

Rabbit, a relevant model for the study of cardiac beta 3-adrenoceptors

The beta(3)-adrenoceptors (beta(3)-ARs) have been identified and characterized in the human heart. Specific beta(3)-AR stimulation, unlike beta(1)-AR or beta(2)-AR stimulation, decreases cardiac contractility, partly via the G(i)-NO pathway. However, the precise role of cardiac beta(3)-ARs is not yet completely understood. Indeed, under normal conditions, the beta(3)-AR response is present only to a very low degree in rats and mice. Therefore, we evaluated whether beta(3)-ARs were present and functional in rabbit ventricular cardiomyocytes, and whether the rabbit could serve as a relevant model for the study of cardiac beta(3)-ARs. We used RT-PCR and Western blot to measure the beta(3)-AR transcripts and protein levels in rabbit ventricular cardiomyocytes. We also analysed the effect of beta(3)-AR stimulation using isoproterenol in combination with nadolol or SR 58611A on cardiomyocyte shortening, Ca(2+) transient, L-type Ca(2+) current (I(Ca,L)), delayed rectifier potassium current (I(Ks)) and action potential duration (APD). For the first time, we show that beta(3)-ARs are expressed in rabbit ventricular cardiomyocytes. The mRNA and protein sequences present a high homology to those of rat and human beta(3)-ARs. Furthermore, beta(3)-AR stimulation decreases cardiomyocyte shortening, Ca(2+) transient and I(Ca,L) amplitudes, via a G(i)-NO pathway. Importantly, beta(3)-AR stimulation enhances I(Ks) amplitude and shortens the APD. Taken together, our results indicate that the rabbit provides a relevant model, easily used in laboratories, to study the roles of cardiac beta(3)-ARs in physiological conditions.

The hERG potassium channel and hERG screening for drug-induced torsades de pointes

Drug-induced torsades de pointes (TdP) arrhythmia is a major safety concern in the process of drug design and development. The incidence of TdP tends to be low, so early pre-clinical screens rely on surrogate markers of TdP to highlight potential problems with new drugs. hERG (human ether-à-go-go-related gene, alternative nomenclature KCNH2) is responsible for channels mediating the 'rapid' delayed rectifier K+ current (IKr) which plays an important role in ventricular repolarization. Pharmacological inhibition of native IKr and of recombinant hERG channels is a shared feature of diverse drugs associated with TdP. In vitro hERG assays therefore form a key element of an integrated assessment of TdP liability, with patch-clamp electrophysiology offering a 'gold standard'. However, whilst clearly necessary, hERG assays cannot be assumed automatically to provide sufficient information, when considered in isolation, to differentiate 'safe' from 'dangerous' drugs. Other relevant factors include therapeutic plasma concentration, drug metabolism and active metabolites, severity of target condition and drug effects on other cardiac ion channels that may mitigate or exacerbate effects of hERG blockade. Increased understanding of the nature of drug-hERG channel interactions may ultimately help eliminate potential hERG blockade early in the design and development process. Currently, for promising drug candidates integration of data from hERG assays with information from other pre-clinical safety screens remains essential.

IKs response to protein kinase A-dependent KCNQ1 phosphorylation requires direct interaction with microtubules

AIMS

KCNQ1 (alias KvLQT1 or Kv7.1) and KCNE1 (alias IsK or minK) co-assemble to form the voltage-activated K(+) channel responsible for I(Ks)-a major repolarizing current in the human heart-and their dysfunction promotes cardiac arrhythmias. The channel is a component of larger macromolecular complexes containing known and undefined regulatory proteins. Thus, identification of proteins that modulate its biosynthesis, localization, activity, and/or degradation is of great interest from both a physiological and pathological point of view.

METHODS AND RESULTS

Using a yeast two-hybrid screening, we detected a direct interaction between beta-tubulin and the KCNQ1 N-terminus. The interaction was confirmed by co-immunoprecipitation of beta-tubulin and KCNQ1 in transfected COS-7 cells and in guinea pig cardiomyocytes. Using immunocytochemistry, we also found that they co-localized in cardiomyocytes. We tested the effects of microtubule-disrupting and -stabilizing agents (colchicine and taxol, respectively) on the KCNQ1-KCNE1 channel activity in COS-7 cells by means of the permeabilized-patch configuration of the patch-clamp technique. None of these agents altered I(Ks). In addition, colchicine did not modify the current response to osmotic challenge. On the other hand, the I(Ks) response to protein kinase A (PKA)-mediated stimulation depended on microtubule polymerization in COS-7 cells and in cardiomyocytes. Strikingly, KCNQ1 channel and Yotiao phosphorylation by PKA-detected by phospho-specific antibodies-was maintained, as was the association of the two partners.

CONCLUSION

We propose that the KCNQ1-KCNE1 channel directly interacts with microtubules and that this interaction plays a major role in coupling PKA-dependent phosphorylation of KCNQ1 with I(Ks) activation.

Expression of human ERG K+ channels in the mouse heart exerts anti-arrhythmic activity

OBJECTIVE

The K(+) channel encoded by the human ether-a-go-go-related gene (HERG) is crucial for repolarization in the human heart. In order to investigate the impact of HERG current (I(Kr)) on the incidence of cardiac arrhythmias, we generated a transgenic mouse expressing HERG specifically in the heart.

METHODS AND RESULTS

ECG recordings at baseline showed no obvious difference between transgenic and wild-type (WT) mice with the exception of the T wave, which was more negative in transgenic mice than in WT mice. E4031 (20 mg/kg) prolonged the QTc interval and flattened the T wave in transgenic mice, but not in WT mice. Injection of BaCl(2) (25 mg/kg) induced short runs of ventricular tachycardia in 9/10 WT mice, but not in transgenic animals. Atrial pacing reproducibly induced atrial tachyarrhythmias in 11/15 WT mice. In contrast, atrial arrhythmia was inducible in only 2/11 transgenic mice. When pretreated with dofetilide (10 mg/kg), transgenic mice were as sensitive to experimental arrhythmias as WT mice. Microelectrode studies showed that atrial action potentials have a steeper slope of duration-rate adaptation in WT than in transgenic mice. Transgenic mice were also characterized by a post-repolarization refractoriness, which could result from the substantial amount of I(Kr) subsisting after repolarization as assessed with action potential-clamp experiments and simulations with a model of the transgenic mouse action potential.

CONCLUSION

HERG expression in the mouse heart can protect against experimental induction of arrhythmias. This is the first report of such a protective effect of HERG in vivo.

Long-Term Amiodarone Administration Remodels Expression of Ion Channel Transcripts in the Mouse Heart

Background— The basis for the unique effectiveness of long-term amiodarone treatment on cardiac arrhythmias is incompletely understood. The present study investigated the pharmacogenomic profile of amiodarone on genes encoding ion-channel subunits.

Methods and Results— Adult male mice were treated for 6 weeks with vehicle or oral amiodarone at 30, 90, or 180 mg · kg −1 · d −1 . Plasma and myocardial levels of amiodarone and N -desethylamiodarone increased dose-dependently, reaching therapeutic ranges observed in human. Plasma triiodothyronine levels decreased, whereas reverse triiodothyronine levels increased in amiodarone-treated animals. In ECG recordings, amiodarone dose-dependently prolonged the RR, PR, QRS, and corrected QT intervals. Specific microarrays containing probes for the complete ion-channel repertoire (IonChips) and real-time reverse transcription–polymerase chain reaction experiments demonstrated that amiodarone induced a dose-dependent remodeling in multiple ion-channel subunits. Genes encoding Na + (SCN4A, SCN5A, SCN1B), connexin (GJA1), Ca 2+ (CaCNA1C), and K + channels (KCNA5, KCNB1, KCND2) were downregulated. In patch-clamp experiments, lower expression of K + and Na + channel genes was associated with decreased I to,f , I K,slow , and I Na currents. Inversely, other K + channel α- and β-subunits, such as KCNA4, KCNK1, KCNAB1, and KCNE3, were upregulated.

Conclusions— Long-term amiodarone treatment induces a dose-dependent remodeling of ion-channel expression that is correlated with the cardiac electrophysiologic effects of the drug. This profile cannot be attributed solely to the amiodarone-induced cardiac hypothyroidism syndrome. Thus, in addition to the direct effect of the drug on membrane proteins, part of the therapeutic action of long-term amiodarone treatment is likely related to its effect on ion-channel transcripts.