Revealing the structural basis of action of hERG potassium channel activators and blockers

Scientific Review of the Proarrhythmic Risks of Oligonucleotide Therapeutics: Are Dedicated ICH S7B/E14 Studies Needed for Low-Risk Modalities?

Oligonucleotide therapeutics (ONTs) represent a new modality with unique pharmacological and chemical properties that modulate gene expression with a high degree of target specificity mediated by complementary Watson-Crick base pair hybridization. To date, the proarrhythmic assessment of ONTs has been influenced by International Conference on Harmonization (ICH) E14 and S7B guidance. To document current hERG/QTc evaluation practices, we reviewed US Food and Drug Administration (FDA) and the European Medicines Agency (EMA) Approval Packages (source: PharmaPendium.com) and collated preclinical and clinical studies for 17 marketed ONTs. In addition, clinical QTc data from 12 investigational ONTs were obtained from the literature. Of the marketed ONTs, eight were tested in the hERG assay with no inhibitory effect identified at the top concentration (range: 34-3,000 μM) tested. Fourteen of the ONTs were evaluated in nonhuman primate cardiovascular studies with 11 of them in dedicated telemetry studies. No effect on QTc intervals were observed (at high exposure multiples) in all studies. Clinically, four ONTs were evaluated in TQT studies; an additional six ONTs were assessed by concentration-QTc interval analysis, and six by routine safety electrocardiogram monitoring. None of the clinical studies identified a QTc prolongation risk; the same was true for the 12 investigational ONTs. A search of the FDA Adverse Event Database indicated no association between approved ONTs and proarrhythmias. Overall, the collective weight of evidence from 29 ONTs demonstrate no clinical proarrhythmic risk based on data obtained from ICH S7B/E14 studies. Thus, new ONTs may benefit from reduced testing strategies because they have no proarrhythmic risk, a similar cardiac safety profile as monoclonal antibodies, proteins, and peptides.

Opioids-induced inhibition of HERG ion channels and sudden cardiac death, a systematic review of current literature

BACKGROUND

It is estimated that over 60 million individuals regularly use opioids globally, with opioid use disorder increasing substantially in the past decade. Several reports have linked sudden cardiac death, QTc prolongation, and other adverse cardiovascular outcomes with opioid use through their inhibitory effect on the human ether-a-go-go-related gene (HERG) ion channel. Therefore, understanding this underlying mechanism may be critical for risk prevention and management in prescribing opioids and treating patients with opioid dependency.

AIM

The present systematic review summarizes the current literature on the impact of opioids-induced inhibition of HERG channel function and its relationship with sudden cardiac death, QTc prolongation, and other cardiovascular adverse effects.

METHODS

A systematic review was conducted of the databases PubMed, EMBASE, Cochrane, and ClinicalTrials.gov of primary studies that reported the effects of opioids on HERG channel function and associated cardiovascular outcomes.

RESULTS

The search identified 1,546 studies, of which 12 were finally included for data extraction. Based on the current literature, methadone, oliceridine, l-α-acetylmethadol (LAAM), and fentanyl were found to inhibit the HERG channel function and were associated with QTc prolongation. However, other opioids such as morphine, codeine, tramadol, and buprenorphine were not associated with inhibition of HERG channels or QTc prolongation. Additional cardiac outcomes associated with opioid related HERG channels dysfunction included sudden cardiac death and Torsade de Pointes.

CONCLUSION

Our findings suggest that certain opioid consumption may result in the inhibition of HERG channels, subsequently prolonging the QTc interval and increasing patient susceptibility to sudden cardiac death.

Benchmarking of Small Molecule Feature Representations for hERG, Nav1.5, and Cav1.2 Cardiotoxicity Prediction

In the field of drug discovery, there is a substantial challenge in seeking out chemical structures that possess desirable pharmacological, toxicological, and pharmacokinetic properties. Complications arise when drugs interfere with the functioning of cardiac ion channels, leading to serious cardiovascular consequences. The discontinuation and removal of numerous approved drugs from the market or at late development stages in the pipeline due to such inhibitory effects further highlight the urgency of addressing this issue. Consequently, the early prediction of potential blockers targeting cardiac ion channels during the drug discovery process is of paramount importance. This study introduces a deep learning framework that computationally determines the cardiotoxicity associated with the voltage-gated potassium channel (hERG), the voltage-gated calcium channel (Cav1.2), and the voltage-gated sodium channel (Nav1.5) for drug candidates. The predictive capabilities of three feature representations─molecular fingerprints, descriptors, and graph-based numerical representations─are rigorously benchmarked. Additionally, a novel training and evaluation data set framework is presented, enabling predictive model training of drug off-target cardiotoxicity using a comprehensive and large curated data set covering these three cardiac ion channels. To facilitate these predictions, a robust and comprehensive small molecule cardiotoxicity prediction tool named CToxPred has been developed. It is made available as open source under the permissive MIT license at https://github.com/issararab/CToxPred.

An LQT2-related mutation in the voltage-sensing domain is involved in switching the gating polarity of hERG

BACKGROUND

Cyclic Nucleotide-Binding Domain (CNBD)-family channels display distinct voltage-sensing properties despite sharing sequence and structural similarity. For example, the human Ether-a-go-go Related Gene (hERG) channel and the Hyperpolarization-activated Cyclic Nucleotide-gated (HCN) channel share high amino acid sequence similarity and identical domain structures. hERG conducts outward current and is activated by positive membrane potentials (depolarization), whereas HCN conducts inward current and is activated by negative membrane potentials (hyperpolarization). The structural basis for the "opposite" voltage-sensing properties of hERG and HCN remains unknown.

RESULTS

We found the voltage-sensing domain (VSD) involves in modulating the gating polarity of hERG. We identified that a long-QT syndrome type 2-related mutation within the VSD, K525N, mediated an inwardly rectifying non-deactivating current, perturbing the channel closure, but sparing the open state and inactivated state. K525N rescued the current of a non-functional mutation in the pore helix region (F627Y) of hERG. K525N&F627Y switched hERG into a hyperpolarization-activated channel. The reactivated inward current induced by hyperpolarization mediated by K525N&F627Y can be inhibited by E-4031 and dofetilide quite well. Moreover, we report an extracellular interaction between the S1 helix and the S5-P region is crucial for modulating the gating polarity. The alanine substitution of several residues in this region (F431A, C566A, I607A, and Y611A) impaired the inward current of K525N&F627Y.

CONCLUSIONS

Our data provide evidence that a potential cooperation mechanism in the extracellular vestibule of the VSD and the PD would determine the gating polarity in hERG.

Facilitation of hERG Activation by Its Blocker: A Mechanism to Reduce Drug-Induced Proarrhythmic Risk

Modulation of the human Ether-à-go-go-Related Gene (hERG) channel, a crucial voltage-gated potassium channel in the repolarization of action potentials in ventricular myocytes of the heart, has significant implications on cardiac electrophysiology and can be either antiarrhythmic or proarrhythmic. For example, hERG channel blockade is a leading cause of long QT syndrome and potentially life-threatening arrhythmias, such as torsades de pointes. Conversely, hERG channel blockade is the mechanism of action of Class III antiarrhythmic agents in terminating ventricular tachycardia and fibrillation. In recent years, it has been recognized that less proarrhythmic hERG blockers with clinical potential or Class III antiarrhythmic agents exhibit, in addition to their hERG-blocking activity, a second action that facilitates the voltage-dependent activation of the hERG channel. This facilitation is believed to reduce the proarrhythmic potential by supporting the final repolarizing of action potentials. This review covers the pharmacological characteristics of hERG blockers/facilitators, the molecular mechanisms underlying facilitation, and their clinical significance, as well as unresolved issues and requirements for research in the fields of ion channel pharmacology and drug-induced arrhythmias.

Inhibitory Perturbations of Fluvastatin on Afterhyperpolarization Current, Erg-mediated K+ Current, and Hyperpolarization-activated Cation Current in Both Pituitary GH3 Cells and Primary Embryonic Mouse Cortical Neurons

Fluvastatin (FLV), the first synthetically derived 3-hydroxy-3-methylglutaryl coenzyme A reductase inhibitor, is a potent inhibitor of cholesterol biosynthesis. While its primary mechanism of action is to reduce cholesterol levels, there is some evidence suggesting that it may also have effects on K+ channels. However, the overall effects of fluvastatin on ionic currents are not yet well understood. The whole-cell clamp recordings were applied to evaluate the ionic currents and action potentials of cells. Here, we have demonstrated that FLV can effectively inhibit the amplitude of erg-mediated K+ current (IK(erg)) in pituitary tumor (GH3) cells, with an IC50 of approximately 3.2 µM. In the presence of FLV, the midpoint in the activation curve of IK(erg) was distinctly shifted to a less negative potential by 10 mV, with minimal modification of the gating charge. However, the magnitude of hyperpolarization-activated cation current (Ih) elicited by long-lasting membrane hyperpolarization was progressively decreased, with an IC50 value of 8.7 µM, upon exposure to FLV. More interestingly, we also found that FLV (5 µM) could regulate the action potential and afterhyperpolarization properties in primary embryonic mouse cortical neurons. Our study presents compelling evidence indicating that FLV has the potential to impact both the amplitude and gating of the ion channels IK(erg) and Ih. We also provide credible evidence suggesting that this drug has the potential to modify the properties of action potentials and the afterhyperpolarization current in electrically excitable cells. However, the assumption that these findings translate to similar in-vivo results remains unclear.

Structural modeling of the hERG potassium channel and associated drug interactions

The voltage-gated potassium channel, KV11.1, encoded by the human Ether-à-go-go-Related Gene (hERG), is expressed in cardiac myocytes, where it is crucial for the membrane repolarization of the action potential. Gating of the hERG channel is characterized by rapid, voltage-dependent, C-type inactivation, which blocks ion conduction and is suggested to involve constriction of the selectivity filter. Mutations S620T and S641A/T within the selectivity filter region of hERG have been shown to alter the voltage dependence of channel inactivation. Because hERG channel blockade is implicated in drug-induced arrhythmias associated with both the open and inactivated states, we used Rosetta to simulate the effects of hERG S620T and S641A/T mutations to elucidate conformational changes associated with hERG channel inactivation and differences in drug binding between the two states. Rosetta modeling of the S641A fast-inactivating mutation revealed a lateral shift of the F627 side chain in the selectivity filter into the central channel axis along the ion conduction pathway and the formation of four lateral fenestrations in the pore. Rosetta modeling of the non-inactivating mutations S620T and S641T suggested a potential molecular mechanism preventing F627 side chain from shifting into the ion conduction pathway during the proposed inactivation process. Furthermore, we used Rosetta docking to explore the binding mechanism of highly selective and potent hERG blockers - dofetilide, terfenadine, and E4031. Our structural modeling correlates well with much, but not all, existing experimental evidence involving interactions of hERG blockers with key residues in hERG pore and reveals potential molecular mechanisms of ligand interactions with hERG in an inactivated state.

Applications of the Novel Quantitative Pharmacophore Activity Relationship Method QPhAR in Virtual Screening and Lead-Optimisation

Pharmacophores are an established concept for the modelling of ligand–receptor interactions based on the abstract representations of stereoelectronic molecular features. They became widely popular as filters for the fast virtual screening of large compound libraries. A lot of effort has been put into the development of sophisticated algorithms and strategies to increase the computational efficiency of the screening process. However, hardly any focus has been put on the development of automated procedures that optimise pharmacophores towards higher discriminatory power, which still has to be done manually by a human expert. In the age of machine learning, the researcher has become the decision-maker at the top level, outsourcing analysis tasks and recurrent work to advanced algorithms and automation workflows. Here, we propose an algorithm for the automated selection of features driving pharmacophore model quality using SAR information extracted from validated QPhAR models. By integrating the developed method into an end-to-end workflow, we present a fully automated method that is able to derive best-quality pharmacophores from a given input dataset. Finally, we show how the QPhAR-generated models can be used to guide the researcher with insights regarding (un-)favourable interactions for compounds of interest.

Pore opening, not voltage sensor movement, underpins the voltage-dependence of facilitation by a hERG blocker

A drug that blocks the cardiac myocyte voltage-gated K+ channels encoded by the human Ether-à-go-go-Related Gene (hERG) carries a potential risk of long QT syndrome and life-threatening cardiac arrhythmia, including Torsade de Points Interestingly, certain hERG blockers can also facilitate hERG activation to increase hERG currents, which may reduce proarrhythmic potential. However, the molecular mechanism involved in the facilitation effect of hERG blockers remains unclear. The hallmark feature of the facilitation effect by hERG blockers is that a depolarizing preconditioning pulse shifts voltage-dependence of hERG activation to more negative voltages. Here we utilize a D540K hERG mutant to study the mechanism of the facilitation effect. D540K hERG is activated by not only depolarization but also hyperpolarization. This unusual gating property enables tests of the mechanism by which voltage induces facilitation of hERG by blockers. With D540K hERG, we find that nifekalant, a hERG blocker and Class III antiarrhythmic agent, blocks and facilitates not only current activation by depolarization but also current activation by hyperpolarization, suggesting a shared gating process upon depolarization and hyperpolarization. Moreover, in response to hyperpolarizing conditioning pulses, nifekalant facilitates D540K hERG currents but not wild-type currents. Our results indicate that induction of facilitation is coupled to pore opening, not voltage per se We propose that gated access to the hERG central cavity underlies the voltage-dependence of induction of facilitation. This study identifies hERG channel pore gate opening as the conformational change facilitated by nifekalant, a clinically important antiarrhythmic agent. Significance Statement Nifekalant is a clinically important antiarrhythmic agent and a hERG blocker which can also facilitate voltage-dependent activation of hERG channels after a preconditioning pulse. Here we show that the mechanism of action of the preconditioning pulse is to open a conductance gate to enable drug access to a facilitation site. Moreover, we find that facilitation increases hERG currents by altering pore dynamics, rather than acting through voltage sensors.

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.

Assessment of proarrhythmogenic risk for chloroquine and hydroxychloroquine using the CiPA concept

Chloroquine and hydroxychloroquine have been proposed recently as therapy for SARS-CoV-2-infected patients, but during 3 months of extensive use concerns were raised related to their clinical effectiveness and arrhythmogenic risk. Therefore, we estimated for these compounds several proarrhythmogenic risk predictors according to the Comprehensive in vitro Proarrhythmia Assay (CiPA) paradigm. Experiments were performed with either CytoPatch™2 automated or manual patch-clamp setups on HEK293T cells stably or transiently transfected with hERG1, hNav1.5, hKir2.1, hKv7.1+hMinK, and on Pluricyte® cardiomyocytes (Ncardia), using physiological solutions. Dose-response plots of hERG1 inhibition fitted with Hill functions yielded IC50 values in the low micromolar range for both compounds. We found hyperpolarizing shifts of tens of mV, larger for chloroquine, in the voltage-dependent activation but not inactivation, as well as a voltage-dependent block of hERG current, larger at positive potentials. We also found inhibitory effects on peak and late I_Na and on I_K1, with IC50 of tens of μM and larger for chloroquine. The two compounds, tested on Pluricyte® cardiomyocytes using the β-escin-perforated method, inhibited I_Kr, I_CaL, I_{Na peak}, but had no effect on I_f. In current-clamp they caused action potential prolongation. Our data and those from literature for I_to were used to compute proarrhythmogenic risk predictors B_net (Mistry HB, 2018) and Q_net (Dutta S et al., 2017), with hERG1 blocking/unblocking rates estimated from time constants of fractional block. Although the two antimalarials are successfully used in autoimmune diseases, and chloroquine may be effective in atrial fibrillation, assays place these drugs in the intermediate proarrhythmogenic risk group.

Intracellular uptake of agents that block the hERG channel can confound the assessment of QT interval prolongation and arrhythmic risk

BACKGROUND

Oliceridine is a biased ligand at the μ-opioid receptor recently approved for the treatment of acute pain. In a thorough QT study, corrected QT (QTc) prolongation displayed peaks at 2.5 and 60 minutes after a supratherapeutic dose. The mean plasma concentration peaked at 5 minutes, declining rapidly thereafter.

OBJECTIVE

The purpose of this study was to examine the basis for the delayed effect of oliceridine to prolong the QTc interval.

METHODS

Repolarization parameters and tissue accumulation of oliceridine were evaluated in rabbit left ventricular wedge preparations over a period of 5 hours. The effects of oliceridine on ion channel currents were evaluated in human embryonic kidney and Chinese hamster ovary cells. Quinidine was used as a control.

RESULTS

Oliceridine and quinidine produced a progressive prolongation of the QTc interval and action potential duration over a period of 5 hours, paralleling slow progressive tissue uptake of the drugs. Oliceridine caused modest prolongation of these parameters, whereas quinidine produced a prominent prolongation of action potential duration and QTc interval as well as development of early afterdepolarization (after 2 hours), resulting in a high torsades de pointes score. The 50% inhibitory concentration values for the oliceridine inhibition of the rapidly activating delayed rectifier current (human ether a-go-go current) and late sodium channel current were 2.2 and 3.45 μM when assessed after traditional acute exposure but much lower after 3 hours of drug exposure.

CONCLUSION

Our findings suggest that a gradual increase of intracellular access of drugs to the hERG channels as a result of their intracellular uptake and accumulation can significantly delay effects on repolarization, thus confounding the assessment of QT interval prolongation and arrhythmic risk when studied acutely. The multi-ion channel effects of oliceridine, late sodium channel current inhibition in particular, point to a low risk of devloping torsades de pointes.

The Prostacyclin Analogue, Treprostinil, Used in the Treatment of Pulmonary Arterial Hypertension, is a Potent Antagonist of TREK-1 and TREK-2 Potassium Channels

Pulmonary arterial hypertension (PAH) is an aggressive vascular remodeling disease that carries a high morbidity and mortality rate. Treprostinil (Remodulin) is a stable prostacyclin analogue with potent vasodilatory and anti-proliferative activity, approved by the FDA and WHO as a treatment for PAH. A limitation of this therapy is the severe subcutaneous site pain and other forms of pain experienced by some patients, which can lead to significant non-compliance. TWIK-related potassium channels (TREK-1 and TREK-2) are highly expressed in sensory neurons, where they play a role in regulating sensory neuron excitability. Downregulation, inhibition or mutation of these channels leads to enhanced pain sensitivity. Using whole-cell patch-clamp electrophysiological recordings, we show, for the first time, that treprostinil is a potent antagonist of human TREK-1 and TREK-2 channels but not of TASK-1 channels. An increase in TASK-1 channel current was observed with prolonged incubation, consistent with its therapeutic role in PAH. To investigate treprostinil-induced inhibition of TREK, site-directed mutagenesis of a number of amino acids, identified as important for the action of other regulatory compounds, was carried out. We found that a gain of function mutation of TREK-1 (Y284A) attenuated treprostinil inhibition, while a selective activator of TREK channels, BL-1249, overcame the inhibitory effect of treprostinil. Our data suggests that subcutaneous site pain experienced during treprostinil therapy may result from inhibition of TREK channels near the injection site and that pre-activation of these channels prior to treatment has the potential to alleviate this nociceptive activity.

Molecular determinants of pro-arrhythmia proclivity of d- and l-sotalol via a multi-scale modeling pipeline

Drug isomers may differ in their proarrhythmia risk. An interesting example is the drug sotalol, an antiarrhythmic drug comprising d- and l- enantiomers that both block the hERG cardiac potassium channel and confer differing degrees of proarrhythmic risk. We developed a multi-scale in silico pipeline focusing on hERG channel – drug interactions and used it to probe and predict the mechanisms of pro-arrhythmia risks of the two enantiomers of sotalol. Molecular dynamics (MD) simulations predicted comparable hERG channel binding affinities for d- and l-sotalol, which were validated with electrophysiology experiments. MD derived thermodynamic and kinetic parameters were used to build multi-scale functional computational models of cardiac electrophysiology at the cell and tissue scales. Functional models were used to predict inactivated state binding affinities to recapitulate electrocardiogram (ECG) QT interval prolongation observed in clinical data. Our study demonstrates how modeling and simulation can be applied to predict drug effects from the atom to the rhythm for dl-sotalol and also increased proarrhythmia proclivity of d- vs. l-sotalol when accounting for stereospecific beta-adrenergic receptor blocking.

Cardiac hERG K+ Channel as Safety and Pharmacological Target

The human ether-á-go-go related gene (hERG, KCNH2) encodes the pore-forming subunit of the potassium channel responsible for a fast component of the cardiac delayed rectifier potassium current (I_Kr). Outward I_Kr is an important determinant of cardiac action potential (AP) repolarization and effectively controls the duration of the QT interval in humans. Dysfunction of hERG channel can cause severe ventricular arrhythmias and thus modulators of the channel, including hERG inhibitors and activators, continue to attract intense pharmacological interest. Certain inhibitors of hERG channel prolong the action potential duration (APD) and effective refractory period (ERP) to suppress premature ventricular contraction and are used as class III antiarrhythmic agents. However, a reduction of the hERG/I_Kr current has been recognized as a predominant mechanism responsible for the drug-induced delayed repolarization known as acquired long QT syndromes (LQTS), which is linked to an increased risk for "torsades de pointes" (TdP) ventricular arrhythmias and sudden cardiac death. Many drugs of different classes and structures have been identified to carry TdP risk. Hence, assessing hERG/I_Kr blockade of new drug candidates is mandatory in the drug development process according to the regulatory agencies. In contrast, several hERG channel activators have been shown to enhance I_Kr and shorten the APD and thus might have potential antiarrhythmic effects against pathological LQTS. However, these activators may also be proarrhythmic due to excessive shortening of APD and the ERP.

Towards the Development of AgoKirs: New Pharmacological Activators to Study Kir2.x Channel and Target Cardiac Disease

Inward rectifier potassium ion channels (I_K1-channels) of the K_ir2.x family are responsible for maintaining a stable negative resting membrane potential in excitable cells, but also play a role in processes of non-excitable tissues, such as bone development. I_K1-channel loss-of-function, either congenital or acquired, has been associated with cardiac disease. Currently, basic research and specific treatment are hindered by the absence of specific and efficient K_ir2.x channel activators. However, twelve different compounds, including approved drugs, show off-target I_K1 activation. Therefore, these compounds contain valuable information towards the development of agonists of K_ir channels, AgoKirs. We reviewed the mechanism of I_K1 channel activation of these compounds, which can be classified as direct or indirect activators. Subsequently, we examined the most viable starting points for rationalized drug development and possible safety concerns with emphasis on cardiac and skeletal muscle adverse effects of AgoKirs. Finally, the potential value of AgoKirs is discussed in view of the current clinical applications of potentiators and activators in cystic fibrosis therapy.

Revealing Molecular Determinants of hERG Blocker and Activator Binding

The Kv11.1 potassium channel, encoded by the human ether-a-go-go-related gene (hERG), plays an essential role in the cardiac action potential. hERG blockade by small molecules can induce "torsade de pointes" arrhythmias and sudden death; as such, it is an important off-target to avoid during drug discovery. Recently, a cryo-EM structure of the open channel state of hERG was reported, opening the door to in silico docking analyses and interpretation of hERG structure-activity relationships, with a view to avoiding blocking activity. Despite this, docking directly to this cryo-EM structure has been reported to yield binding modes that are unable to explain known mutagenesis data. In this work, we use molecular dynamics simulations to sample a range of channel conformations and run ensemble docking campaigns at the known hERG binding site below the selectivity filter, composed of the central cavity and the four deep hydrophobic pockets. We identify a hERG conformational state allowing discrimination of blockers vs nonblockers from docking; furthermore, the binding pocket agrees with mutagenesis data, and blocker binding modes fit the hERG blocker pharmacophore. We then use the same protocol to identify a binding pocket in the hERG channel pore for hERG activators, again agreeing with the reported mutagenesis. Our approach may be useful in drug discovery campaigns to prioritize candidate compounds based on hERG liability via virtual docking screens.

LUF7244, an allosteric modulator/activator of Kv 11.1 channels, counteracts dofetilide-induced torsades de pointes arrhythmia in the chronic atrioventricular block dog model

BACKGROUND AND PURPOSE

K_v 11.1 (hERG) channel blockade is an adverse effect of many drugs and lead compounds, associated with lethal cardiac arrhythmias. LUF7244 is a negative allosteric modulator/activator of K_v 11.1 channels that inhibits early afterdepolarizations in vitro. We tested LUF7244 for antiarrhythmic efficacy and potential proarrhythmia in a dog model.

EXPERIMENTAL APPROACH

LUF7244 was tested in vitro for (a) increasing human I_Kv11.1 and canine I_Kr and (b) decreasing dofetilide-induced action potential lengthening and early afterdepolarizations in cardiomyocytes derived from human induced pluripotent stem cells and canine isolated ventricular cardiomyocytes. In vivo, LUF7244 was given intravenously to anaesthetized dogs in sinus rhythm or with chronic atrioventricular block.

KEY RESULTS

LUF7244 (0.5-10 μM) concentration dependently increased I_Kv11.1 by inhibiting inactivation. In vitro, LUF7244 (10 μM) had no effects on I_KIR2.1 , I_Nav1.5 , I_Ca-L , and I_Ks , doubled I_Kr , shortened human and canine action potential duration by approximately 50%, and inhibited dofetilide-induced early afterdepolarizations. LUF7244 (2.5 mg·kg^-1 ·15 min^-1 ) in dogs with sinus rhythm was not proarrhythmic and shortened, non-significantly, repolarization parameters (QTc: -6.8%). In dogs with chronic atrioventricular block, LUF7244 prevented dofetilide-induced torsades de pointes arrhythmias in 5/7 animals without normalization of the QTc. Peak LUF7244 plasma levels were 1.75 ± 0.80 during sinus rhythm and 2.34 ± 1.57 μM after chronic atrioventricular block.

CONCLUSIONS AND IMPLICATIONS

LUF7244 counteracted dofetilide-induced early afterdepolarizations in vitro and torsades de pointes in vivo. Allosteric modulators/activators of K_v 11.1 channels might neutralize adverse cardiac effects of existing drugs and newly developed compounds that display QTc lengthening.

The Pore-Lipid Interface: Role of Amino-Acid Determinants of Lipophilic Access by Ivabradine to the hERG1 Pore Domain

Abnormal cardiac electrical activity is a common side effect caused by unintended block of the promiscuous drug target human ether-à-go-go-related gene (hERG1), the pore-forming domain of the delayed rectifier K⁺ channel in the heart. hERG1 block leads to a prolongation of the QT interval, a phase of the cardiac cycle that underlies myocyte repolarization detectable on the electrocardiogram. Even newly released drugs such as heart-rate lowering agent ivabradine block the rapid delayed rectifier current I_Kr, prolong action potential duration, and induce potentially lethal arrhythmia known as torsades de pointes. In this study, we describe a critical drug-binding pocket located at the lateral pore surface facing the cellular membrane. Mutations of the conserved M651 residue alter ivabradine-induced block but not by the common hERG1 blocker dofetilide. As revealed by molecular dynamics simulations, binding of ivabradine to a lipophilic pore access site is coupled to a state-dependent reorientation of aromatic residues F557 and F656 in the S5 and S6 helices. We show that the M651 mutation impedes state-dependent dynamics of F557 and F656 aromatic cassettes at the protein-lipid interface, which has a potential to disrupt drug-induced block of the channel. This fundamentally new mechanism coupling the channel dynamics and small-molecule access from the membrane into the hERG1 intracavitary site provides a simple rationale for the well established state-dependence of drug blockade. SIGNIFICANCE STATEMENT: The drug interference with the function of the cardiac hERG channels represents one of the major sources of drug-induced heart disturbances. We found a novel and a critical drug-binding pocket adjacent to a lipid-facing surface of the hERG1 channel, which furthers our molecular understanding of drug-induced QT syndrome.

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.

Dehydroevodiamine and hortiamine, alkaloids from the traditional Chinese herbal drug Evodia rutaecarpa, are IKr blockers with proarrhythmic effects in vitro and in vivo

Evodiae fructus is a widely used herbal drug in traditional Chinese medicine. Evodia extract was found to inhibit hERG channels. The aim of the current study was to identify hERG inhibitors in Evodia extract and to investigate their potential proarrhythmic effects. Dehydroevodiamine (DHE) and hortiamine were identified as I_Kr (rapid delayed rectifier current) inhibitors in Evodia extract by HPLC-microfractionation and subsequent patch clamp studies on human embryonic kidney cells. DHE and hortiamine inhibited I_Kr with IC₅₀s of 253.2±26.3nM and 144.8±35.1nM, respectively. In dog ventricular cardiomyocytes, DHE dose-dependently prolonged the action potential duration (APD). Early afterdepolarizations (EADs) were seen in 14, 67, 100, and 67% of cells after 0.01, 0.1, 1 and 10μM DHE, respectively. The proarrhythmic potential of DHE was evaluated in 8 anesthetized rabbits and in 8 chronic atrioventricular block (cAVB) dogs. In rabbits, DHE increased the QT interval significantly by 12±10% (0.05mg/kg/5min) and 60±26% (0.5mg/kg/5min), and induced Torsade de Pointes arrhythmias (TdP, 0.5mg/kg/5min) in 2 rabbits. In cAVB dogs, 0.33mg/kg/5min DHE increased QT duration by 48±10% (P<0.05*) and induced TdP in 2/4 dogs. A higher dose did not induce TdP. In human induced pluripotent stem cell-derived cardiomyocytes (hiPSC-CMs), methanolic extracts of Evodia, DHE and hortiamine dose-dependently prolonged APD. At 3μM DHE and hortiamine induced EADs. hERG inhibition at submicromolar concentrations, APD prolongation and EADs in hiPSC-CMs and dose-dependent proarrhythmic effects of DHE at micromolar plasma concentrations in cAVB dogs should increase awareness regarding proarrhythmic effects of widely used Evodia extracts.

Risk of Cardiac Events Associated With Antidepressant Therapy in Patients With Long QT Syndrome

Patients with long QT syndrome (LQTS) are at a high risk of cardiac events. Many patients with LQTS are treated with antidepressant drugs (ADs). We investigated the LQTS genotype-specific risk of recurrent cardiac arrhythmic events (CAEs) associated with AD therapy. The study included 59 LQT1 and 72 LQT2 patients from the Rochester-based LQTS Registry with corrected QT (QT_c) prolongation and a history of AD therapy. Using multivariate Anderson-Gill models, we estimated the LQTS genotype-specific risk of recurrent CAEs (ventricular tachyarrhythmias, aborted cardiac arrest, or sudden cardiac death) associated with time-dependent ADs. Specifically, we examined the risk associated with all ADs, selective serotonin reuptake inhibitor (SSRI), and ADs classified on the CredibleMeds list (www.CredibleMeds.org) as "Conditional" or "Known risk of Torsades de pointes (TdP)." After adjusting for baseline QT_c duration, sex, and time-dependent beta-blocker usage, there was an increased risk of recurrent CAEs associated with ADs in LQT1 patients (hazard ratio = 3.67, 95% confidence interval 1.98-6.82, p < 0.001) but not in LQT2 patients (hazard ratio = 0.89, 95% confidence interval 0.49-1.64, p = 0.716; LQT1 vs LQT2 interaction, p < 0.001). Similarly, LQT1 patients who were on SSRIs or ADs with "Known risk of TdP" had a higher risk of recurrent CAEs than those patients off all ADs, whereas there was no association in LQT2 patients. ADs with "Conditional risk of TdP" were not associated with the risk of recurrent CAEs in any of the groups. In conclusion, the risk of recurrent CAEs associated with time-dependent ADs is higher in LQT1 patients but not in LQT2 patients. Results suggest a LQTS genotype-specific effect of ADs on the risk of arrhythmic events.

Correlation between human ether-a-go-go-related gene channel inhibition and action potential prolongation

BACKGROUND AND PURPOSE

Human ether-a-go-go-related gene (hERG; Kv 11.1) channel inhibition is a widely accepted predictor of cardiac arrhythmia. hERG channel inhibition alone is often insufficient to predict pro-arrhythmic drug effects. This study used a library of dofetilide derivatives to investigate the relationship between standard measures of hERG current block in an expression system and changes in action potential duration (APD) in human-induced pluripotent stem cell-derived cardiomyocytes (hiPSC-CMs). The interference from accompanying block of Cav 1.2 and Nav 1.5 channels was investigated along with an in silico AP model.

EXPERIMENTAL APPROACH

Drug-induced changes in APD were assessed in hiPSC-CMs using voltage-sensitive dyes. The IC50 values for dofetilide and 13 derivatives on hERG current were estimated in an HEK293 expression system. The relative potency of each drug on APD was estimated by calculating the dose (D150 ) required to prolong the APD at 90% (APD90 ) repolarization by 50%.

KEY RESULTS

The D150 in hiPSC-CMs was linearly correlated with IC50 of hERG current. In silico simulations supported this finding. Three derivatives inhibited hERG without prolonging APD, and these compounds also inhibited Cav 1.2 and/or Nav 1.5 in a channel state-dependent manner. Adding Cav 1.2 and Nav 1.2 block to the in silico model recapitulated the direction but not the extent of the APD change.

CONCLUSIONS AND IMPLICATIONS

Potency of hERG current inhibition correlates linearly with an index of APD in hiPSC-CMs. The compounds that do not correlate have additional effects including concomitant block of Cav 1.2 and/or Nav 1.5 channels. In silico simulations of hiPSC-CMs APs confirm the principle of the multiple ion channel effects.

Natural products modulating the hERG channel: heartaches and hope

This review covers natural products modulating the hERG potassium channel. Risk assessment strategies, structural features of blockers, and the duality target/antitarget are discussed.

Covering: 1996–December 2016

The human Ether-à-go-go Related Gene (hERG) channel is a voltage-gated potassium channel playing an essential role in the normal electrical activity in the heart. It is involved in the repolarization and termination of action potentials in excitable cardiac cells. Mutations in the hERG gene and hERG channel blockage by small molecules are associated with increased risk of fatal arrhythmias. Several drugs have been withdrawn from the market due to hERG channel-related cardiotoxicity. Moreover, as a result of its notorious ligand promiscuity, this ion channel has emerged as an important antitarget in early drug discovery and development. Surprisingly, the hERG channel blocking profile of natural compounds present in frequently consumed botanicals (i.e. dietary supplements, spices, and herbal medicinal products) is not routinely assessed. This comprehensive review will address these issues and provide a critical compilation of hERG channel data for isolated natural products and extracts over the past two decades (1996–2016). In addition, the review will provide (i) a solid basis for the molecular understanding of the physiological functions of the hERG channel, (ii) the translational potential of in vitro/in vivo results to cardiotoxicity in humans, (iii) approaches for the identification of hERG channel blockers from natural sources, (iv) future perspectives for cardiac safety guidelines and their applications within phytopharmaceuticals and dietary supplements, and (v) novel applications of hERG channel modulation (e.g. as a drug target).

Contributions of the membrane dipole potential to the function of voltage-gated cation channels and modulation by small molecule potentiators

The membrane dipole potential (Ψ_d) constitutes one of three electrical potentials generated by cell membranes. Ψ_d arises from the unfavorable parallel alignment of phospholipid and water dipoles, and varies in magnitude both longitudinally and laterally across the bilayer according to membrane composition and phospholipid packing density. In this work, we propose that dynamic counter-balancing between Ψ_d and the transmembrane potential (ΔΨ_m) governs the conformational state transitions of voltage-gated ion channels. Ψ_d consists of 1) static outer, and dynamic inner leaflet components (Ψ_d(extra) and Ψ_d(intra), respectively); and 2) a transmembrane component (ΔΨ_{d(inner-outer)}), ariing from differences in intra- and extracellular leaflet composition. Ψ_d(intra), which transitions between high and low energy states (Ψ_{d(intra, high)} and Ψ_{d(intra, low)}) as a function of channel conformation, is transduced by the pore domain. ΔΨ_{d(inner-outer)} is transduced by the voltage-sensing (VS) domain in summation with ΔΨ_m. Potentiation of voltage-gated ion channels is of interest for the treatment of cardiac, neuronal, and other disorders arising from inherited/acquired ion channel dysfunction. Potentiators are widely believed to alter the rates and voltage-dependencies of channel gating transitions by binding to pockets in the membrane-facing and other regions of ion channel targets. Here, we propose that potentiators alter Ψ_d(intra) and/or Ψ_d(extra), thereby increasing or decreasing the energy barriers governing channel gating transitions. We used quantum mechanical and molecular dynamics (MD) simulations to predict the overall Ψ_d-modulating effects of a series of published positive hERG potentiators partitioned into model DOPC bilayers. Our findings suggest a strong correlation between the magnitude of Ψ_d-lowering and positive hERG potentiation across the series.

Cardiac electrical defects in progeroid mice and Hutchinson-Gilford progeria syndrome patients with nuclear lamina alterations

Hutchinson-Gilford progeria syndrome (HGPS) is a rare genetic disease caused by defective prelamin A processing, leading to nuclear lamina alterations, severe cardiovascular pathology, and premature death. Prelamin A alterations also occur in physiological aging. It remains unknown how defective prelamin A processing affects the cardiac rhythm. We show age-dependent cardiac repolarization abnormalities in HGPS patients that are also present in the Zmpste24^-/- mouse model of HGPS. Challenge of Zmpste24^-/- mice with the β-adrenergic agonist isoproterenol did not trigger ventricular arrhythmia but caused bradycardia-related premature ventricular complexes and slow-rate polymorphic ventricular rhythms during recovery. Patch-clamping in Zmpste24^-/- cardiomyocytes revealed prolonged calcium-transient duration and reduced sarcoplasmic reticulum calcium loading and release, consistent with the absence of isoproterenol-induced ventricular arrhythmia. Zmpste24^-/- progeroid mice also developed severe fibrosis-unrelated bradycardia and PQ interval and QRS complex prolongation. These conduction defects were accompanied by overt mislocalization of the gap junction protein connexin43 (Cx43). Remarkably, Cx43 mislocalization was also evident in autopsied left ventricle tissue from HGPS patients, suggesting intercellular connectivity alterations at late stages of the disease. The similarities between HGPS patients and progeroid mice reported here strongly suggest that defective cardiac repolarization and cardiomyocyte connectivity are important abnormalities in the HGPS pathogenesis that increase the risk of arrhythmia and premature death.

Enhanced Human-Induced Pluripotent Stem Cell Derived Cardiomyocyte Maturation Using a Dual Microgradient Substrate

Human-induced pluripotent stem cell-derived cardiomyocytes (hiPSC-CM) raise many possibilities for cardiac research but they exhibit an immature phenotype, which influences experimental outcomes. The aim of our research is to investigate the effects of a topographical gradient substrate on the morphology and function of commercially available hiPSC-CM. The lateral dimensions the microgrooves on the substrate varied from 8 to 100 μm space between the 8 μm grooves on one axis and from ∼5 nm to ∼1 μm in depth on the other axis. Cells were seeded homogeneously across the substrate and according to the manufacturers protocols. At days 4 and 10, measures of eccentricity, elongation, orientation, sarcomere length (SL), and contractility of the hiPSC-CM were taken. Only the deepest and widest region (8-30 μm wide and 0.85-1 μm deep) showed a significantly higher percentage of hiPSC-CM with an increased eccentricity (31.3 ± 6.4%), elongation (10.4 ± 4.3%), and orientation (<10°) (32.1 ± 2.7%) when compared with the control (flat substrate) (15.8 ± 5.0%, 3.4 ± 2.7%, and 10.6 ± 1.1%, respectively). Additionally, during stimulus-induced contraction, the relaxation phase of the twitch was prolonged (400 ms) compared to nonelongated cells (200 ms). These findings support the potential use of dual microgradient substrates to investigate substrate topographies that stimulate migration and/or maturation of hiPSC-CM.

Role of the pH in state-dependent blockade of hERG currents

Mutations that reduce inactivation of the voltage-gated Kv11.1 potassium channel (hERG) reduce binding for a number of blockers. State specific block of the inactivated state of hERG block may increase risks of drug-induced Torsade de pointes. In this study, molecular simulations of dofetilide binding to the previously developed and experimentally validated models of the hERG channel in open and open-inactivated states were combined with voltage-clamp experiments to unravel the mechanism(s) of state-dependent blockade. The computations of the free energy profiles associated with the drug block to its binding pocket in the intra-cavitary site display startling differences in the open and open-inactivated states of the channel. It was also found that drug ionization may play a crucial role in preferential targeting to the open-inactivated state of the pore domain. pH-dependent hERG blockade by dofetilie was studied with patch-clamp recordings. The results show that low pH increases the extent and speed of drug-induced block. Both experimental and computational findings indicate that binding to the open-inactivated state is of key importance to our understanding of the dofetilide’s mode of action.

A new hERG allosteric modulator rescues genetic and drug-induced long-QT syndrome phenotypes in cardiomyocytes from isogenic pairs of patient induced pluripotent stem cells

Long-QT syndrome (LQTS) is an arrhythmogenic disorder characterised by prolongation of the QT interval in the electrocardiogram, which can lead to sudden cardiac death. Pharmacological treatments are far from optimal for congenital forms of LQTS, while the acquired form, often triggered by drugs that (sometimes inadvertently) target the cardiac hERG channel, is still a challenge in drug development because of cardiotoxicity. Current experimental models in vitro fall short in predicting proarrhythmic properties of new drugs in humans. Here, we leveraged a series of isogenically matched, diseased and genetically engineered, human induced pluripotent stem cell-derived cardiomyocytes (hiPSC-CMs) from patients to test a novel hERG allosteric modulator for treating congenital LQTS, drug-induced LQTS or a combination of the two. By slowing IK r deactivation and positively shifting IK r inactivation, the small molecule LUF7346 effectively rescued all of these conditions, demonstrating in a human system that allosteric modulation of hERG may be useful as an approach to treat inherited and drug-induced LQTS Furthermore, our study provides experimental support of the value of isogenic pairs of patient hiPSC-CMs as platforms for testing drug sensitivities and performing safety pharmacology.

Enhancement of hERG channel activity by scFv antibody fragments targeted to the PAS domain

The human human ether-à-go-go-related gene (hERG) potassium channel plays a critical role in the repolarization of the cardiac action potential. Changes in hERG channel function underlie long QT syndrome (LQTS) and are associated with cardiac arrhythmias and sudden death. A striking feature of this channel and KCNH channels in general is the presence of an N-terminal Per-Arnt-Sim (PAS) domain. In other proteins, PAS domains bind ligands and modulate effector domains. However, the PAS domains of KCNH channels are orphan receptors. We have uncovered a family of positive modulators of hERG that specifically bind to the PAS domain. We generated two single-chain variable fragments (scFvs) that recognize different epitopes on the PAS domain. Both antibodies increase the rate of deactivation but have different effects on channel activation and inactivation. Importantly, we show that both antibodies, on binding to the PAS domain, increase the total amount of current that permeates the channel during a ventricular action potential and significantly reduce the action potential duration recorded in human cardiomyocytes. Overall, these molecules constitute a previously unidentified class of positive modulators and establish that allosteric modulation of hERG channel function through ligand binding to the PAS domain can be attained.

Rescue of protein expression defects may not be enough to abolish the pro-arrhythmic phenotype of long QT type 2 mutations

KEY POINTS

Most missense long QT syndrome type 2 (LQTS2) mutations result in Kv11.1 channels that show reduced levels of membrane expression. Pharmacological chaperones that rescue mutant channel expression could have therapeutic potential to reduce the risk of LQTS2-associated arrhythmias and sudden cardiac death, but only if the mutant Kv11.1 channels function normally (i.e. like WT channels) after membrane expression is restored. Fewer than half of mutant channels exhibit relatively normal function after rescue by low temperature. The remaining rescued missense mutant Kv11.1 channels have perturbed gating and/or ion selectivity characteristics. Co-expression of WT subunits with gating defective missense mutations ameliorates but does not eliminate the functional abnormalities observed for most mutant channels. For patients with mutations that affect gating in addition to expression, it may be necessary to use a combination therapy to restore both normal function and normal expression of the channel protein.

ABSTRACT

In the heart, Kv11.1 channels pass the rapid delayed rectifier current (IKr ) which plays critical roles in repolarization of the cardiac action potential and in the suppression of arrhythmias caused by premature stimuli. Over 500 inherited mutations in Kv11.1 are known to cause long QT syndrome type 2 (LQTS2), a cardiac electrical disorder associated with an increased risk of life threatening arrhythmias. Most missense mutations in Kv11.1 reduce the amount of channel protein expressed at the membrane and, as a consequence, there has been considerable interest in developing pharmacological agents to rescue the expression of these channels. However, pharmacological chaperones will only have clinical utility if the mutant Kv11.1 channels function normally after membrane expression is restored. The aim of this study was to characterize the gating phenotype for a subset of LQTS2 mutations to assess what proportion of mutations may be suitable for rescue. As an initial screen we used reduced temperature to rescue expression defects of mutant channels expressed in Xenopus laevis oocytes. Over half (∼56%) of Kv11.1 mutants exhibited functional gating defects that either dramatically reduced the amount of current contributing to cardiac action potential repolarization and/or reduced the amount of protective current elicited in response to premature depolarizations. Our data demonstrate that if pharmacological rescue of protein expression defects is going to have clinical utility in the treatment of LQTS2 then it will be important to assess the gating phenotype of LQTS2 mutations before attempting rescue.

Synthesis and biological evaluation of negative allosteric modulators of the Kv11.1(hERG) channel

We synthesized and evaluated a series of compounds for their allosteric modulation at the Kv11.1 (hERG) channel. Most compounds were negative allosteric modulators of [(3)H]dofetilide binding to the channel, in particular 7f, 7h-j and 7p. Compounds 7f and 7p were the most potent negative allosteric modulators amongst all ligands, significantly increasing the dissociation rate of dofetilide in the radioligand kinetic binding assay, while remarkably reducing the affinities of dofetilide and astemizole in a competitive displacement assay. Additionally, both 7f and 7p displayed peculiar displacement characteristics with Hill coefficients significantly distinct from unity as shown by e.g., dofetilide, further indicative of their allosteric effects on dofetilide binding. Our findings in this investigation yielded several promising negative allosteric modulators for future functional and clinical research with respect to their antiarrhythmic propensities, either alone or in combination with known Kv11.1 blockers.

Probing binding sites and mechanisms of action of an I(Ks) activator by computations and experiments

The slow delayed rectifier (IKs) channel is composed of the KCNQ1 channel and KCNE1 auxiliary subunit, and functions to repolarize action potentials in the human heart. IKs activators may provide therapeutic efficacy for treating long QT syndromes. Here, we show that a new KCNQ1 activator, ML277, can enhance IKs amplitude in adult guinea pig and canine ventricular myocytes. We probe its binding site and mechanism of action by computational analysis based on our recently reported KCNQ1 and KCNQ1/KCNE1 3D models, followed by experimental validation. Results from a pocket analysis and docking exercise suggest that ML277 binds to a side pocket in KCNQ1 and the KCNE1-free side pocket of KCNQ1/KCNE1. Molecular-dynamics (MD) simulations based on the most favorable channel/ML277 docking configurations reveal a well-defined ML277 binding space surrounded by the S2-S3 loop and S4-S5 helix on the intracellular side, and by S4-S6 transmembrane helices on the lateral sides. A detailed analysis of MD trajectories suggests two mechanisms of ML277 action. First, ML277 restricts the conformational dynamics of the KCNQ1 pore, optimizing K(+) ion coordination in the selectivity filter and increasing current amplitudes. Second, ML277 binding induces global motions in the channel, including regions critical for KCNQ1 gating transitions. We conclude that ML277 activates IKs by binding to an intersubunit space and allosterically influencing pore conductance and gating transitions. KCNE1 association protects KCNQ1 from an arrhythmogenic (constitutive current-inducing) effect of ML277, but does not preclude its current-enhancing effect.

Breathing Stimulant Compounds Inhibit TASK-3 Potassium Channel Function Likely by Binding at a Common Site in the Channel Pore

Compounds PKTHPP (1-{1-[6-(biphenyl-4-ylcarbonyl)-5,6,7,8-tetrahydropyrido[4,3-d]-pyrimidin-4-yl]piperidin-4-yl}propan-1-one), A1899 (2''-[(4-methoxybenzoylamino)methyl]biphenyl-2-carboxylic acid 2,4-difluorobenzylamide), and doxapram inhibit TASK-1 (KCNK3) and TASK-3 (KCNK9) tandem pore (K2P) potassium channel function and stimulate breathing. To better understand the molecular mechanism(s) of action of these drugs, we undertook studies to identify amino acid residues in the TASK-3 protein that mediate this inhibition. Guided by homology modeling and molecular docking, we hypothesized that PKTHPP and A1899 bind in the TASK-3 intracellular pore. To test our hypothesis, we mutated each residue in or near the predicted PKTHPP and A1899 binding site (residues 118-128 and 228-248), individually, to a negatively charged aspartate. We quantified each mutation's effect on TASK-3 potassium channel concentration response to PKTHPP. Studies were conducted on TASK-3 transiently expressed in Fischer rat thyroid epithelial monolayers; channel function was measured in an Ussing chamber. TASK-3 pore mutations at residues 122 (L122D, E, or K) and 236 (G236D) caused the IC50 of PKTHPP to increase more than 1000-fold. TASK-3 mutants L122D, G236D, L239D, and V242D were resistant to block by PKTHPP, A1899, and doxapram. Our data are consistent with a model in which breathing stimulant compounds PKTHPP, A1899, and doxapram inhibit TASK-3 function by binding at a common site within the channel intracellular pore region, although binding outside the channel pore cannot yet be excluded.

Computational investigations of hERG channel blockers: New insights and current predictive models

Identification of potential human Ether-a-go-go Related-Gene (hERG) potassium channel blockers is an essential part of the drug development and drug safety process in pharmaceutical industries or academic drug discovery centers, as they may lead to drug-induced QT prolongation, arrhythmia and Torsade de Pointes. Recent reports also suggest starting to address such issues at the hit selection stage. In order to prioritize molecules during the early drug discovery phase and to reduce the risk of drug attrition due to cardiotoxicity during pre-clinical and clinical stages, computational approaches have been developed to predict the potential hERG blockage of new drug candidates. In this review, we will describe the current in silico methods developed and applied to predict and to understand the mechanism of actions of hERG blockers, including ligand-based and structure-based approaches. We then discuss ongoing research on other ion channels and hERG polymorphism susceptible to be involved in LQTS and how systemic approaches can help in the drug safety decision.

ERK and RSK are necessary for TRH-induced inhibition of r-ERG potassium currents in rat pituitary GH3 cells

The transduction pathway mediating the inhibitory effect that TRH exerts on r-ERG channels has been thoroughly studied in GH3 rat pituitary cells but some elements have yet to be discovered, including those involved in a phosphorylation event(s). Using a quantitative phosphoproteomic approach we studied the changes in phosphorylation caused by treatment with 1μM TRH for 5min in GH3 cells. The activating residues of Erk2 and Erk1 undergo phosphorylation increases of 5.26 and 4.87 fold, respectively, in agreement with previous reports of ERK activation by TRH in GH3 cells. Thus, we studied the possible involvement of ERK pathway in the signal transduction from TRH receptor to r-ERG channels. The MEK inhibitor U0126 at 0.5μM caused no major blockade of the basal r-ERG current, but impaired the TRH inhibitory effect on r-ERG. Indeed, the TRH effect on r-ERG was also reduced when GH3 cells were transfected with siRNAs against either Erk1 or Erk2. Using antibodies, we found that TRH treatment also causes activating phosphorylation of Rsk. The TRH effect on r-ERG current was also impaired when cells were transfected with any of two different siRNAs mixtures against Rsk1. However, treatment of GH3 cells with 20nM EGF for 5min, which causes ERK and RSK activation, had no effect on the r-ERG currents. Therefore, we conclude that in the native GH3 cell system, ERK and RSK are involved in the pathway linking TRH receptor to r-ERG channel inhibition, but additional components must participate to cause such inhibition.

Pharmacological and electrophysiological characterization of AZSMO-23, an activator of the hERG K(+) channel

BACKGROUND AND PURPOSE

We aimed to characterize the pharmacology and electrophysiology of N-[3-(1H-benzimidazol-2-yl)-4-chloro-phenyl]pyridine-3-carboxamide (AZSMO-23), an activator of the human ether-a-go-go-related gene (hERG)-encoded K(+) channel (Kv 11.1).

EXPERIMENTAL APPROACH

Automated electrophysiology was used to study the pharmacology of AZSMO-23 on wild-type (WT), Y652A, F656T or G628C/S631C hERG, and on other cardiac ion channels. Its mechanism of action was characterized with conventional electrophysiology.

KEY RESULTS

AZSMO-23 activated WT hERG pre-pulse and tail current with EC50 values of 28.6 and 11.2 μM respectively. At 100 μM, pre-pulse current at +40 mV was increased by 952 ± 41% and tail current at -30 mV by 238 ± 13% compared with vehicle values. The primary mechanism for this effect was a 74.5 mV depolarizing shift in the voltage dependence of inactivation, without any shift in the voltage dependence of activation. Structure-activity relationships for this effect were remarkably subtle, with close analogues of AZSMO-23 acting as hERG inhibitors. AZSMO-23 blocked the mutant channel, hERG Y652A, but against another mutant channel, hERG F656T, its activator activity was enhanced. It inhibited activity of the G628C/S631C non-inactivating hERG mutant channel. AZSMO-23 was not hERG selective, as it blocked hKv 4.3-hKChIP2.2, hCav 3.2 and hKv 1.5 and activated hCav 1.2/β2/α2δ channels.

CONCLUSION AND IMPLICATIONS

The activity of AZSMO-23 and those of its close analogues suggest these compounds may be of value to elucidate the mechanism of type 2 hERG activators to better understand the pharmacology of this area from both a safety perspective and in relation to treatment of congenital long QT syndrome.

Evaluation of the cardiotoxicity of mitragynine and its analogues using human induced pluripotent stem cell-derived cardiomyocytes

Introduction

Mitragynine is a major bioactive compound of Kratom, which is derived from the leave extracts of Mitragyna speciosa Korth or Mitragyna speciosa (M. speciosa), a medicinal plant from South East Asia used legally in many countries as stimulant with opioid-like effects for the treatment of chronic pain and opioid-withdrawal symptoms. Fatal incidents with Mitragynine have been associated with cardiac arrest. In this study, we determined the cardiotoxicity of Mitragynine and other chemical constituents isolated using human induced pluripotent stem cell-derived cardiomyocytes (hiPSC-CMs).

Methods and Results

The rapid delayed rectifier potassium current (I_Kr), L-type Ca²⁺ current (I_Ca,L) and action potential duration (APD) were measured by whole cell patch-clamp. The expression of KCNH2 and cytotoxicity was determined by real-time PCR and Caspase activity measurements. After significant I_Kr suppression by Mitragynine (10 µM) was confirmed in hERG-HEK cells, we systematically examined the effects of Mitragynine and other chemical constituents in hiPSC-CMs. Mitragynine, Paynantheine, Speciogynine and Speciociliatine, dosage-dependently (0.1∼100 µM) suppressed I_Kr in hiPSC-CMs by 67% ∼84% with IC₅₀ ranged from 0.91 to 2.47 µM. Moreover, Mitragynine (10 µM) significantly prolonged APD at 50 and 90% repolarization (APD50 and APD90) (439.0±11.6 vs. 585.2±45.5 ms and 536.0±22.6 vs. 705.9±46.1 ms, respectively) and induced arrhythmia, without altering the L-type Ca²⁺ current. Neither the expression,and intracellular distribution of KCNH2/Kv11.1, nor the Caspase 3 activity were significantly affected by Mitragynine.

Conclusions

Our study indicates that Mitragynine and its analogues may potentiate Torsade de Pointes through inhibition of I_Kr in human cardiomyocytes.

Translational toxicology and rescue strategies of the hERG channel dysfunction: biochemical and molecular mechanistic aspects

The human ether-à-go-go related gene (hERG) potassium channel is an obligatory anti-target for drug development on account of its essential role in cardiac repolarization and its close association with arrhythmia. Diverse drugs have been removed from the market owing to their inhibitory activity on the hERG channel and their contribution to acquired long QT syndrome (LQTS). Moreover, mutations that cause hERG channel dysfunction may induce congenital LQTS. Recently, an increasing number of biochemical and molecular mechanisms underlying hERG-associated LQTS have been reported. In fact, numerous potential biochemical and molecular rescue strategies are hidden within the biogenesis and regulating network. So far, rescue strategies of hERG channel dysfunction and LQTS mainly include activators, blockers, and molecules that interfere with specific links and other mechanisms. The aim of this review is to discuss the rescue strategies based on hERG channel toxicology from the biochemical and molecular perspectives.

The Therapeutic Potential of hERG1 K+ Channels for Treating Cancer and Cardiac Arrhythmias

hERG potassium channels present pharmacologists and medicinal chemists with a dilemma. On the one hand hERG is a major reason for drugs being withdrawn from the market because of drug induced long QT syndrome and the associated risk of inducing sudden cardiac death, and yet hERG blockers are still widely used in the clinic to treat cardiac arrhythmias. Moreover, in the last decade overwhelming evidence has been provided that hERG channels are aberrantly expressed in cancer cells and that they contribute to tumour cell proliferation, resistance to apoptosis, and neoangiogenesis. Here we provide an overview of the properties of hERG channels and their role in excitable cells of the heart and nervous system as well as in cancer. We consider the therapeutic potential of hERG, not only with regard to the negative impact due to drug induced long QT syndrome, but also its future potential as a treatment in the fight against cancer.