Action potential experiments complete hERG assay and QT-interval measurements in cardiac preclinical studies

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.

Prediction of drug pro-arrhythmic cardiotoxicity using a semi-physiologically based pharmacokinetic model linked to cardiac ionic currents inhibition

Drug-induced torsades de pointes (TdP) risks are responsible for the withdrawal of many drugs from the market. Nowadays, assessments of drug-induced TdP risks are mainly based on maximum effective free therapeutic plasma concentration (EFTPCmax) and cardiac ionic current inhibitions using the human ventricular myocytes model (Tor-ORd model). Myocytes are targets of drug-induced TdP. The TdP risks may be directly linked to myocyte drug concentrations. We aimed to develop a semi-physiologically based pharmacokinetic (Semi-PBPK) model linked to cardiac ionic current inhibition (pharmacodynamics, PD) (Semi-PBPK-PD) to simultaneously predict myocyte drug concentrations and their TdP risks in humans. Alterations in action potential duration (ΔAPD90) were simulated using the Tor-ORd model and ionic current inhibition parameters based on myocyte or plasma drug concentrations. The predicted ΔAPD90 values were translated into in vivo alterations in QT interval(ΔQTc) induced by moxifloxacin, dofetilide, or sotalol. Myocyte drug concentrations of moxifloxacin, dofetilide, and sotalol gave better predictions of ΔQTc than plasma. Following validating the developed semi-PBPK-PD model, TdP risks of 37 drugs were assessed using ΔAPD90 and early afterdepolarization occurrence, which were estimated based on 10 × EFTPCmax and 10 × EFTMCmax (maximum effective free therapeutic myocyte concentration). 10 × EFTMCmax gave more sensitive and accurate predictions of pro-arrhythmic cardiotoxicity and the predicted TdP risks were also closer to clinic practice than 10 × EFTPCmax. In conclusion, pharmacokinetics and TdP risks of 37 drugs were successfully predicted using the semi-PBPK-PD model. Myocyte drug concentrations gave better predictions of ΔQTc and TdP risks than plasma.

Anti-malarial drugs: Mechanisms underlying their proarrhythmic effects

Malaria remains the leading cause of parasitic death in the world. Artemisinin resistance is an emerging threat indicating an imminent need for novel combination therapy. Given the key role of mass drug administration, it is pivotal that the safety of anti-malarial drugs is investigated thoroughly prior to widespread use. Cardiotoxicity, most prominently arrhythmic risk, has been a concern for anti-malarial drugs. We clarify the likely underlying mechanisms by which anti-malarial drugs predispose to arrhythmias. These relate to disruption of (1) action potential upstroke due to effects on the sodium currents, (2) action potential repolarisation due to effects on the potassium currents, (3) cellular calcium homeostasis, (4) mitochondrial function and reactive oxygen species production and (5) cardiac fibrosis. Together, these alterations promote arrhythmic triggers and substrates. Understanding these mechanisms is essential to assess the safety of these drugs, stratify patients based on arrhythmic risk and guide future anti-malarial drug development.

Computational modeling of atrial fibrillation

With the aging society, the prevalence of atrial fibrillation (AF) continues to increase. Nevertheless, there are still limitations in antiarrhythmic drugs (AAD) or catheter interventions for AF. If it is possible to predict the outcome of AF management according to various AADs or ablation lesion sets through computational modeling, it will be of great clinical help. AF computational modeling has been utilized for in-silico arrhythmia research and enabled high-density entire chamber mapping, reproducible condition control, virtual intervention, not possible clinically or experimentally, in-depth mechanistic research. With the recent development of computer science and technology, more sophisticated and faster computational modeling has become available for clinical application. In particular, it can be applied to determine the extra-PV target of persistent AF catheter ablation or to select the AAD with the best effect. AF computational modeling combined with artificial intelligence is expected to contribute to precision medicine for more diverse uses in the future. Therefore, in this review, we will deal with the history, development, and various applications of computation modeling.

Computational Modeling for Antiarrhythmic Drugs for Atrial Fibrillation According to Genotype

Background: The efficacy of antiarrhythmic drugs (AAD) can vary in patients with atrial fibrillation (AF), and the PITX2 gene affects the responsiveness of AADs. We explored the virtual AAD (V-AAD) responses between wild-type and PITX2 ^+/--deficient AF conditions by realistic in silico AF modeling. Methods: We tested the V-AADs in AF modeling integrated with patients' 3D-computed tomography and 3D-electroanatomical mapping, acquired in 25 patients (68% male, 59.8 ± 9.8 years old, 32.0% paroxysmal type). The ion currents for the PITX2 ^+/- deficiency and each AAD (amiodarone, sotalol, dronedarone, flecainide, and propafenone) were defined based on previous publications. Results: We compared the wild-type and PITX2 ^+/- deficiency in terms of the action potential duration (APD₉₀), conduction velocity (CV), maximal slope of restitution (Smax), and wave-dynamic parameters, such as the dominant frequency (DF), phase singularities (PS), and AF termination rates according to the V-AADs. The PITX2 ^+/--deficient model exhibited a shorter APD₉₀ (p < 0.001), a lower Smax (p < 0.001), mean DF (p = 0.012), PS number (p < 0.001), and a longer AF cycle length (AFCL, p = 0.011). Five V-AADs changed the electrophysiology in a dose-dependent manner. AAD-induced AFCL lengthening (p < 0.001) and reductions in the CV (p = 0.033), peak DF (p < 0.001), and PS number (p < 0.001) were more significant in PITX2 ^+/--deficient than wild-type AF. PITX2 ^+/--deficient AF was easier to terminate with class IC AADs than the wild-type AF (p = 0.018). Conclusions: The computational modeling-guided AAD test was feasible for evaluating the efficacy of multiple AADs in patients with AF. AF wave-dynamic and electrophysiological characteristics are different among the PITX2-deficient and the wild-type genotype models.

Core battery safety pharmacology testing - An assessment of its utility in early drug development

Requirements for safety pharmacology testing have been in place since the issue of initial regulatory guidance over 20 years ago. An evaluation of such testing, supporting first clinical entry of 105 small molecule drug candidates over the last decade, showed that a "core battery" of in vitro electrophysiological (hERG), conscious non-rodent telemetry cardiovascular, rodent central nervous system (CNS) (modified Irwin's or functional observational battery [FOB] test) and respiratory function (plethysmography) studies was performed. Routine use of the latter 2 studies appears to have limited utility, with only 21% and 28% of studies, respectively, giving findings of which none were identified as of obvious concern to moving the affected drugs into the clinic. The use of a stand-alone hERG assay does not appear to be particular sensitive in predicting proarrythmic risk as a tool by itself. Telemetry study testing had utility especially for identifying effects on QTc interval (about 10% of studies), resulting on some occasions in a lower clinical starting dose and/or increased awareness for potential effects on the cardiovascular system in the Phase I study. Overall, this investigation provides information supporting an overhaul of the current "box ticking" core battery approach used for safety pharmacology testing. However, in order to achieve a more focused examination to investigate potential undesirable pharmacodynamic effects of a new candidate drug and also support 3Rs (Replacement, Reduction and Refinement) thinking in performing unnecessary studies, there will not only need to be a sea change by drug developers but also a change in current regulatory guidance.

Translating QT interval prolongation from conscious dogs to humans

AIM

In spite of screening procedures in early drug development, uncertainty remains about the propensity of new chemical entities (NCEs) to prolong the QT/QTc interval. The evaluation of proarrhythmic activity using a comprehensive in vitro proarrhythmia assay does not fully account for pharmacokinetic-pharmacodynamic (PKPD) differences in vivo. In the present study, we evaluated the correlation between drug-specific parameters describing QT interval prolongation in dogs and in humans.

METHODS

Using estimates of the drug-specific parameter, data on the slopes of the PKPD relationships of nine compounds with varying QT-prolonging effects (cisapride, sotalol, moxifloxacin, carabersat, GSK945237, SB237376 and GSK618334, and two anonymized NCEs) were analysed. Mean slope estimates varied between -0.98 ms μM-1 and 6.1 ms μM-1 in dogs and -10 ms μM-1 and 90 ms μM-1 in humans, indicating a wide range of effects on the QT interval. Linear regression techniques were then applied to characterize the correlation between the parameter estimates across species.

RESULTS

For compounds without a mixed ion channel block, a correlation was observed between the drug-specific parameter in dogs and humans (y = -1.709 + 11.6x; R2  = 0.989). These results show that per unit concentration, the drug effect on the QT interval in humans is 11.6-fold larger than in dogs.

CONCLUSIONS

Together with information about the expected therapeutic exposure, the evidence of a correlation between the compound-specific parameter in dogs and in humans represents an opportunity for translating preclinical safety data before progression into the clinic. Whereas further investigation is required to establish the generalizability of our findings, this approach can be used with clinical trial simulations to predict the probability of QT prolongation in humans.

Pharmacokinetic-pharmacodynamic modelling of drug-induced QTc interval prolongation in man: prediction from in vitro human ether-à-go-go-related gene binding and functional inhibition assays and conscious dog studies

BACKGROUND AND PURPOSE

Functional measures of human ether-à-go-go-related gene (hERG; Kv 11.1) channel inhibition have been prioritized as an in vitro screening tool for candidate molecules. However, it is unclear how these results can be translated to humans. Here, we explore how data on drug binding and functional inhibition in vitro relate to QT prolongation in vivo. Using cisapride, sotalol and moxifloxacin as paradigm compounds, we assessed the relationship between drug concentrations, binding, functional measures and in vivo effects in preclinical species and humans.

EXPERIMENTAL APPROACH

Pharmacokinetic-pharmacodynamic modelling was used to characterize the drug effects in hERG functional patch clamp, hERG radio-labelled dofetilide displacement experiments and QT interval in conscious dogs. Data were analysed in parallel to identify potential correlations between pharmacological activity in vitro and in vivo.

KEY RESULTS

An Emax model could not be used due to large variability in the functional patch clamp assay. Dofetilide displacement revealed that binding curves are unrelated to the in vivo potency estimates for QTc prolongation in dogs and humans. Mean in vitro potency estimates ranged from 99.9 nM for cisapride to 1030 μM for moxifloxacin.

CONCLUSIONS AND IMPLICATIONS

The lack of standardized protocols for in vitro assays leads to significant differences in experimental conditions, making the assessment of in vitro-in vivo correlations unreliable. Identification of an accurate safety window during the screening of candidate molecules requires a quantitative framework that disentangles system- from drug-specific properties under physiological conditions, enabling translation of the results to humans. Similar considerations will be relevant for the comprehensive in vitro pro-arrhythmia assay initiative.

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.

Establishing in vitro to clinical correlations in the evaluation of cardiovascular safety pharmacology

Preclinical studies are vital in establishing the efficacy and safety of a new chemical entity (NCE) in humans. To deliver meaningful information, experiments have to be well defined and provide outcome that is relevant and translatable to humans. This review briefly surveys the various preclinical experiments that are frequently conducted to assess drug effects on cardiac conductivity in early drug development. We examine the different approaches used to establish correlations between non-clinical and clinical settings and discuss their value in the evaluation of cardiovascular risk.

Scalable nano-bioprobes with sub-cellular resolution for cell detection

Here we present a carbon nanotube based device to noninvasively and quickly detect mobile single cells with the potential to maintain a high degree of spatial resolution. The device utilizes standard complementary metal oxide semiconductor (CMOS) technologies for fabrication, allowing it to be easily scalable (down to a few nanometers). Nanotubes are deposited using electrophoresis after fabrication in order to maintain CMOS compatibility. The devices are spaced by 6 μm which is the same size or smaller than a single cell. To demonstrate its capability to detect cells, we performed impedance spectroscopy on mobile human embryonic kidney (HEK) cells, neurons cells from mice, and yeast cells (S. pombe). Measurements were performed with and without cells and with and without nanotubes. Nanotubes were found to be crucial to successfully detect the presence of cells. The devices are also able to distinguish between cells with different characteristics.

The comparative sensitivity of three in vitro safety pharmacology models for the detection of lidocaine-induced cardiac effects

INTRODUCTION

In the current ICH S7B guideline, in vitro evaluation of proarrhythmic liability is limited to the risk of QT interval prolongation, whilst the effect of new chemical entities on cardiac conductivity is often overlooked. The aim of this work was to compare the effects of the sodium channel blocker, lidocaine in three in vitro safety pharmacology models: hNa(v)1.5 channel test, atrial action potential (AP) and Purkinje fiber AP and to identify the most sensitive model for detecting cardiac conduction slowing.

METHODS

Whole-cell patch-clamp methods were used to record the sodium current (I(Na)) encoded by hNa(v)1.5 in stably transfected HEK293 cells at ambient temperature. Transmembrane APs were recorded in rabbit Purkinje fibers and rabbit and guinea-pig left stimulated atria at physiological temperature. Parameters involved in depolarization or repolarization were reported.

RESULTS

Lidocaine (from 10 to 1000 μM) decreased the amplitude of I(Na) (IC(50): 256±37 μM) in a concentration-dependent manner. In the Purkinje fiber assay, lidocaine (10, 30 and 100 μM) had no effects on maximal upstroke velocity (Vmax), but shortened AP duration at 90% repolarization (APD(90)). At 30 and 100 μM, lidocaine also increased AP triangulation. In guinea-pig atria, lidocaine decreased Vmax starting from 30 μM and conduction velocity (CV) at 100 μM, but had no effects on other parameters. In rabbit atria, lidocaine decreased Vmax and CV at 100 μM without affecting APD(90). The effects of 100 μM lidocaine on Vmax and CV were more marked in rabbit than in guinea-pig atria.

CONCLUSION

Rabbit atria are more sensitive than rabbit Purkinje fibers or guinea-pig atria for detecting lidocaine-induced cardiac conduction slowing. These data suggest that isolated rabbit atria in addition to the hNa(v)1.5 channel assay could be relevant models to predict drug-induced conduction slowing.

Field and action potential recordings in heart slices: correlation with established in vitro and in vivo models

BACKGROUND AND PURPOSE

Action potential (AP) recordings in ex vivo heart preparations constitute an important component of the preclinical cardiac safety assessment according to the ICH S7B guideline. Most AP measurement models are sensitive, predictive and informative but suffer from a low throughput. Here, effects of selected anti-arrhythmics (flecainide, quinidine, atenolol, sotalol, dofetilide, nifedipine, verapamil) on field/action potentials (FP/AP) of guinea pig and rabbit ventricular slices are presented and compared with data from established in vitro and in vivo models.

EXPERIMENTAL APPROACH

Data from measurements of membrane currents (hERG, I(Na) ), AP/FP (guinea pig and rabbit ventricular slices), AP (rabbit Purkinje fibre), haemodynamic/ECG parameters (conscious, telemetered dog) were collected, compared and correlated to complementary published data (focused literature search).

KEY RESULTS

The selected anti-arrhythmics, flecainide, quinidine, atenolol, sotalol, dofetilide, nifedipine and verapamil, influenced the shape of AP/FP of guinea pig and rabbit ventricular slices in a manner similar to that observed for rabbit PF. The findings obtained from slice preparations are in line with measurements of membrane currents in vitro, papillary muscle AP in vitro and haemodynamic/ECG parameters from conscious dogs in vivo, and were also corroborated by published data.

CONCLUSION AND IMPLICATIONS

FP and AP recordings from heart slices correlated well with established in vitro and in vivo models in terms of pharmacology and predictability. Heart slice preparations yield similar results as papillary muscle but offer enhanced throughput for mechanistic investigations and may substantially reduce the use of laboratory animals.

Collation, assessment and analysis of literature in vitro data on hERG receptor blocking potency for subsequent modeling of drugs' cardiotoxic properties

The assessment of the torsadogenic potency of a new chemical entity is a crucial issue during lead optimization and the drug development process. It is required by the regulatory agencies during the registration process. In recent years, there has been a considerable interest in developing in silico models, which allow prediction of drug-hERG channel interaction at the early stage of a drug development process. The main mechanism underlying an acquired QT syndrome and a potentially fatal arrhythmia called torsades de pointes is the inhibition of potassium channel encoded by hERG (the human ether-a-go-go-related gene). The concentration producing half-maximal block of the hERG potassium current (IC(50)) is a surrogate marker for proarrhythmic properties of compounds and is considered a test for cardiac safety of drugs or drug candidates. The IC(50) values, obtained from data collected during electrophysiological studies, are highly dependent on experimental conditions (i.e. model, temperature, voltage protocol). For the in silico models' quality and performance, the data quality and consistency is a crucial issue. Therefore the main objective of our work was to collect and assess the hERG IC(50) data available in accessible scientific literature to provide a high-quality data set for further studies.

Blockade of HERG K+ channel by isoquinoline alkaloid neferine in the stable transfected HEK293 cells

We studied the effects of isoquinoline alkaloid neferine (Nef) extracted from the seed embryo of Nelumbo nucifera Gaertn on Human ether-à-go-go-related gene (HERG) channels stably expressed in human embryonic kidney (HEK293) cells using whole-cell patch clamp technique, western blot analysis and immunofluorescence experiment. Nef induced a concentration-dependent decrease in current amplitude according to the voltage steps and tail currents of HERG with an IC(50) of 7.419 microM (n(H) -0.5563). Nef shifted the activation curve in a significantly negative direction and accelerated recovery from inactivation and onset of inactivation, however, slowed deactivation. In addition, it had no significant influence on steady-state inactivation curve. Western blot and immunofluorescence results suggested Nef had no significant effect on the expression of HERG protein. In summary, Nef can block HERG K(+) channels that functions by changing the channel activation and inactivation kinetics. Nef has no effect on the generation and trafficking of HERG protein. A blocked-off HERG channel was one mechanism of the anti-arrhythmic effects by Nef.

Model systems for the discovery and development of antiarrhythmic drugs

Cardiovascular diseases are the leading cause of mortality worldwide and about 25% of cardiovascular deaths are due to disturbances in cardiac rhythm or "arrhythmias". Arrhythmias were traditionally treated with antiarrhythmic drugs, but increasing awareness of the risks of presently available antiarrhythmic agents has greatly limited their usefulness. Most common treatment algorithms still involve small molecule drugs, and antiarrhythmic agents with improved efficacy and safety are sorely needed. This paper reviews the model systems that are available for discovery and development of new antiarrhythmic drugs. We begin with a presentation of screening methods used to identify specific channel-interacting agents, with a particular emphasis on high-throughput screens. Traditional manual electrophysiological methods, automated electrophysiology, fluorescent dye methods, flux assays and radioligand binding assays are reviewed. We then discuss a variety of relevant arrhythmia models. Two models are widely used in testing for arrhythmogenic actions related to excess action potential prolongation, an important potential adverse effect of chemical entities affecting cardiac rhythm: the methoxamine-sensitized rabbit and the dog with chronic atrioventricular block. We then go on to review models used to assess potential antiarrhythmic actions. For ventricular arrhythmias, chemical induction methods, cardiac or neural electrical stimulation, ischaemic heart models and models of cardiac channelopathies can be used to identify effective antiarrhythmic agents. For atrial arrhythmias, potentially useful models include vagally-maintained atrial fibrillation, acute asphyxia with atrial burst-pacing, sterile pericarditis, Y-shaped atria surgical incisions, chronic atrial dilation models, atrial electrical remodelling due to sustained atrial tachycardia, heart failure-related atrial remodelling, and acute atrial ischaemia. It is hoped that the new technologies now available and the recently-developed models for arrhythmia-response assessment will permit the introduction of newer and more effective antiarrhythmic therapies in the near future.

Comparison of electropharmacological effects of bepridil and sotalol in halothane-anesthetized dogs

BACKGROUND

Bepridil is known to have a multiple ion channel-blocking property in the heart, which has been applied for the treatment of atrial fibrillation and drug-refractory ventricular tachyarrhythmias. In this study, the electro-pharmacological effects of bepridil were compared with those of dl-sotalol, a representative class III antiarrhythmic drug, using the halothane-anesthetized canine model.

METHODS AND RESULTS

Cardiovascular and electrophysiological variables were measured under the halothane anesthesia. Intravenous administration of bepridil (0.3 mg/kg, n=4) delayed the intraventricular conduction and prolonged the ventricular effective refractory period, whereas dl-sotalol (0.3 mg/kg, iv, n=4) inhibited atrioventricular conduction and prolonged the atrial and ventricular effective refractory period. The additional administration of 10 times the higher dose of bepridil or dl-sotalol (ie, 3 mg/kg, iv, n=4 for each group) decreased blood pressure, suppressed ventricular contraction and sinus automaticity, and prolonged the atrial and ventricular effective refractory period and monophasic action potential duration, in addition to the effects of the low dose.

CONCLUSIONS

The electropharmacological effects of bepridil and dl-sotalol were similar, although their potency for each cardiovascular variable varied significantly. These findings can be useful when selecting these drugs according to the pathophysiological condition of a patient.

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.