Antiarrhythmics--from cell to clinic: past, present, and future

Cardiac transmembrane ion channels and action potentials: cellular physiology and arrhythmogenic behavior

Cardiac arrhythmias are among the leading causes of mortality. They often arise from alterations in the electrophysiological properties of cardiac cells and their underlying ionic mechanisms. It is therefore critical to further unravel the pathophysiology of the ionic basis of human cardiac electrophysiology in health and disease. In the first part of this review, current knowledge on the differences in ion channel expression and properties of the ionic processes that determine the morphology and properties of cardiac action potentials and calcium dynamics from cardiomyocytes in different regions of the heart are described. Then the cellular mechanisms promoting arrhythmias in congenital or acquired conditions of ion channel function (electrical remodeling) are discussed. The focus is on human-relevant findings obtained with clinical, experimental, and computational studies, given that interspecies differences make the extrapolation from animal experiments to human clinical settings difficult. Deepening the understanding of the diverse pathophysiology of human cellular electrophysiology will help in developing novel and effective antiarrhythmic strategies for specific subpopulations and disease conditions.

Critical Volume of Human Myocardium Necessary to Maintain Ventricular Fibrillation

BACKGROUND

Abnormal QT intervals, long QT or short QT, have been epidemiologically linked with sudden cardiac death because of ventricular fibrillation (VF). Consequently, Food and Drug Administration recommends testing all pharmacological agents for QT toxicity as a risk factor for cardiac toxicity. Such tests assess QT/QTc interval, which represents ventricular depolarization and repolarization. However, the current QT toxicity analysis does not account for the well-known anisotropy in cardiac tissue conductivity. Mines demonstrated in 1913 that cardiac wavelength (λ) determines inducibility of reentrant arrhythmia, where both repolarization time or action potential duration and conduction velocity determine λ=action potential duration×conduction velocity. We aimed to determine the role of anisotropic wavelength in inducibility of VF in explanted human left ventricular preparations. We tested the hypothesis that 3-dimensional cardiac wavelength, which takes into account anisotropic cardiac tissue conductivity, can accurately predict VF sustainability.

METHODS

We conducted panoramic optical mapping of coronary perfused human left ventricular wedge preparations subjected to pharmacologically induced shortening and prolongation of action potential duration, by I_K,ATP agonist pinacidil and antagonist glybenclamide, respectively. This measured action potential duration, conduction velocity, and thus determined pacing cycle length-dependent wavelengths in longitudinal (λ_L), transverse (λ_TV), and transmural (λ_TM) directions using S1S1 pacing protocol, from which wavelength volume (V_λ) was determined, as V_λ=λ_L×λ_TV×λ_TM, and compared with tissue volume_. We tested a hypothesis that tissue volume/V_λ ratio can predict VF sustainability.

RESULTS

At baseline, at pacing rate of 240 beats per minute, the wavelengths were λ_L=9.6±0.6 cm, λ_TV=4.2±0.3 cm, and λ_TM=5.8±0.2 cm, respectively (n=7), and thus V_λ=246.4±42.1 cm³. Administration of pinacidil at escalating concentrations progressively decreased V_λ, and VF became sustained, when tissue volume/V_λ was above safety factor κ=4.4±0.6 (n=9) during rapid pacing. Treatment with glybenclamide decreased V_T/V_λ below κ at any pacing rate and prevented VF sustainability.

CONCLUSIONS

Sustained VF was only sustained in ventricular volume exceeding critical V_λ=λ_L×λ_TV×λ_TM.

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.

Prophylactic Antiarrhythmic Drug Therapy in Atrial Fibrillation

In patients with recurrent atrial fibrillation (AF), the hallmark of treatment has been the use of antiarrhythmic drugs (AADs). Goals of therapy include reduction in the frequency and duration of episodes of arrhythmia as well an emerging goal of reducing mortality and hospitalizations associated with AF. Safety and efficacy are important factors when choosing an antiarrhythmic drug for the treatment of AF, hence, if AAD are required for maintenance of sinus rhythm, their safety profi le, together with individual patient characteristics, should be of utmost concern. In the next paragraphs we would like to review some aspects (electrophysiologic effects, metabolism, side effects, current evidence and indication) of the most commonly used AAD for the management of patients with AF, following the Vaughan-Williams classification. However, this system is mainly based on ventricular activity, therefore, and due to its relatively atrial selective actions, some agents will not readily fit in the Vaughan Williams AAD classification. For that reason, in the final part of the manuscript, new promising agents will be reviewed separately.

Antiarrhythmic drugs for the maintenance of sinus rhythm: risks and benefits

Atrial fibrillation (AF) is the most common arrhythmia seen in clinical practice, and its complications impose a significant economic burden. The development of more effective agents to manage patients with AF is essential. While clinical trials show no major differences in outcomes between rate and rhythm control strategies, some patients with AF require treatment with antiarrhythmic drugs (AADs) to maintain sinus rhythm, reduce symptoms, improve exercise tolerance, and improve quality of life. Currently available AADs, while effective, have limitations including limited efficacy, adverse events, toxicity, and proarrhythmic potential. The 6 most commonly used AADs (amiodarone, disopyramide, dofetilide [USA but not Europe], flecainide, propafenone, sotalol) have proarrhythmic effects (fewer with amiodarone). Amiodarone is the most effective AAD, but its safety profile limits its usefulness. Recent advances in AAD therapy include dronedarone and vernakalant. Dronedarone, approved by the United States Food and Drug Administration and the European Medicines Authority and others, has been proven efficacious in maintaining sinus rhythm and reducing the incidence of hospitalization due to cardiovascular events or death in patients with AF. The intravenous formulation of vernakalant is approved in the European Union, Iceland, and Norway. Oral vernakalant is currently undergoing evaluation for preventing AF recurrence and appears to be effective with an acceptable safety profile. Treatment should be individualized to the patient with consideration of pharmacologic risks and benefits according to AF management guidelines. Accumulating efficacy and safety data for new and emerging AADs holds promise for improved AF management and outcomes.

Pilsicainide

Pilsicainide is a class Ic antiarrhythmic agent used for the treatment of supraventricular and ventricular tachycardia. The pharmacodynamic effects of pilsicainide are achieved via selective sodium channel blockade. In randomized, multicentre trials in patients with atrial fibrillation, restoration of sinus rhythm was achieved in significantly more patients treated with a single oral dose of pilsicainide than those who received placebo, and in a numerically higher proportion of oral pilsicainide than intravenous disopyramide recipients. In another well designed trial in patients with persistent atrial fibrillation, the proportion of patients whose arrhythmia converted to normal sinus rhythm was significantly higher among those randomized to receive 2 weeks of oral pilsicainide therapy than among patients who received placebo. Over a 1-year period, both pilsicainide and cibenzoline (another class Ic agent) were effective in preventing recurrence of atrial fibrillation in a substantial proportion of patients in a single-centre crossover trial. There were no between-group differences in the subgroup of patients with shorter-duration atrial fibrillation, but actuarial results over 1 year significantly favoured cibenzoline over pilsicainide in patients with longer-duration atrial fibrillation. Both oral and intravenous pilsicainide have demonstrated efficacy in ventricular tachyarrhythmias, including ventricular extrasystole. Clinical trial and postmarketing surveillance data indicate that pilsicainide is generally well tolerated in most patients.

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.

A review of the investigational antiarrhythmic agent dronedarone

Arrhythmias are a major cause of morbidity and mortality, and atrial fibrillation is the most widespread disorder of cardiac rhythm. Amiodarone is an effective antiarrhythmic agent that has been in clinical use for about 20 years. It is effective for multiple types of arrhythmias, including atrial fibrillation, and has a low incidence of cardiac adverse events, including Torsade de Pointes. It has many noncardiac adverse effects that are serious and limit its long-term use. Dronedarone is an investigational antiarrhythmic agent that is designed to have similar cardiac effects to amiodarone but with fewer adverse effects. This review presents some of the animal and human studies that evaluate the effects of dronedarone.

Simultaneous determination of ten antiarrhythic drugs and a metabolite in human plasma by liquid chromatography—tandem mass spectrometry

A simple, accurate and selective LC-MS/MS method was developed and validated for simultaneous quantification of ten antiarrhythic drugs (diltiazem, amiodarone, mexiletine, propranolol, sotalol, verapamil, bisoprolol, metoprolol, atenolol, carvedilol) and a metabolite (norverapamil) in human plasma. Plasma samples were simply pretreated with acetonitrile for deproteinization. Chromatographic separation was performed on a Capcell C(18) column (50mmx2.0mm, 5microm) using a gradient mixture of acetonitrile and water (both containing 0.02% formic acid) as a mobile phase at flow rate of 0.3ml/min. The analytes were protonated in the positive electrospray ionization (ESI) interface and detected in multiple reaction monitoring (MRM) mode. Calibration curves were linear over wide ranges from sub- to over-therapeutic concentration in plasma for all analytes. Intra- and inter-batch precision of analysis was <12.0%, accuracy ranged from 90% to 110%, average recovery from 85.0% to 99.7%. The validated method was successfully applied to therapeutic drug monitoring (TDM) of antiarrhythic drugs in routine clinical practice.

Short QT syndrome

The idiopathic short QT syndrome (SQTS) is a recently identified condition characterized by abbreviated QT intervals (typically 300 ms or less) and in affected families is associated with an increased incidence of atrial and ventricular arrhythmias and sudden cardiac death. Genetic analysis has, to date, identified three distinct forms of the condition, involving gain-of-function mutations to three different cardiac potassium channel genes: KCNH2 (SQT1), KCNQ1 (SQT2) and KCNJ2 (SQT3). This article reviews recent advances in understanding this syndrome, discussing the basis of QT interval shortening, possible mechanisms for the associated arrhythmogenic risk in SQT1, current approaches to treatment of the SQTS (focusing on SQT1) and avenues for future investigation.

Recent advances in understanding sex differences in cardiac repolarization

A number of gender differences exist in the human electrocardiogram (ECG): the P-wave and P-R intervals are slightly longer in men than in women, whilst women have higher resting heart rates than do men, but a longer rate-corrected QT (QT(C)) interval. Women with the LQT1 and LQT2 variants of congenital long-QT syndrome (LQTS) are at greater risk of adverse cardiac events. Similarly, many drugs associated with acquired LQTS have a greater risk of inducing torsades de pointes (TdP) arrhythmia in women than in men. There are also male:female differences in Brugada syndrome, early repolarisation syndrome and sudden cardiac death. The differences in the ECG between men and women, and in particular those relating to the QT interval, have been explored experimentally and provide evidence of differences in the processes underlying ventricular repolarization. The data available from rabbit, canine, rat, mouse and guinea pig models are reviewed and highlight involvement of male:female differences in Ca and K currents, although the possible involvement of rapid and persistent Na current and Na-Ca exchange currents cannot yet be excluded. The mechanisms underlying observed differences remain to be elucidated fully, but are likely to involve the influence of gonadal steroids. With respect to the QT interval and risk of TdP, a range of evidence implicates a protective role of testosterone in male hearts, possibly by both genomic and non-genomic pathways. Evidence regarding oestrogen and progesterone is less unequivocal, although the interplay between these two hormones may influence both repolarization and pro-arrhythmic risk.

Acquired QT interval prolongation and HERG: implications for drug discovery and development

Putative interactions between the Human Ether-a-go-go Related Gene (HERG), QT interval prolongation and Torsades de Pointes (TdP) are now integral components of any discussion on drug safety. HERG encodes for the inwardly rectifying potassium channel (I(Kr)), which is essential to the maintenance of normal cardiac function. HERG channel mutations are responsible for one form of familial long QT syndrome, a potentially deadly inherited cardiac disorder associated with TdP. Moreover, drug-induced (acquired) QT interval prolongation has been associated with an increase in the incidence of sudden unexplained deaths, with HERG inhibition implicated as the underlying cause. Subsequently, a number of non-cardiovascular drugs which induce QT interval prolongation and/or TdP have been withdrawn. However, a definitive link between HERG, QT interval prolongation and arrhythmogenesis has not been established. Nevertheless, this area is subject to ever increasing regulatory scrutiny. Here we review the relationship between HERG, long QT syndrome and TdP, together with a summary of the associated regulatory issues, and developments in pre-clinical screening.

Mechanisms of genetic arrhythmias: from DNA to ECG

A precise balance of ionic currents underlies normal cardiac excitation and relaxation. Disruption of this equilibrium by genetic defects, polymorphisms, therapeutic intervention, and structural abnormalities can cause arrhythmogenic phenotypes leading to syncope, seizures, and sudden cardiac death. Congenital defects result in an unpredictable expression of phenotypes with variable penetrance, even within single families. Additionally, phenotypically opposite and overlapping cardiac arrhythmogenic syndromes can even stem from the same mutation. Accordingly, the relationship between genetic mutations and clinical syndromes is becoming increasingly complex.

Rate-dependent effect of verapamil on atrial refractoriness

OBJECTIVES

The purpose of this study was to determine whether verapamil has rate-dependent effects on the atrial effective refractory period (AERP).

BACKGROUND

Block of calcium current (I(Ca)) and rapid component of the delayed rectifier potassium current (I(Kr)) by verapamil is frequency-dependent. This may result in variable effects of verapamil on the AERP, depending on the rate.

METHODS

The subjects of this study were 30 adults with a mean age of 45 +/- 13 years who did not have structural heart disease. In 20 subjects, the AERP was measured at basic drive cycle lengths (BDCLs) of 650 to 250 ms, in 50 ms decrements, before and after infusion of 0.1 mg/kg verapamil. The effective refractory periods (ERPs) were measured in the setting of autonomic blockade in 10 subjects and without autonomic blockade in 10 subjects. Ten subjects served as a control group and received a saline infusion instead of verapamil.

RESULTS

Verapamil significantly prolonged the AERP at BDCLs of 650 to 500 ms (p < 0.01 or p < 0.05) and significantly shortened the ERP at BDCLs of 300 and 250 ms (p < 0.01). In the control group, there were no significant differences between the baseline and post-saline measurements of ERP.

CONCLUSIONS

Verapamil prolongs AERP at slow rates and shortens AERP at rapid rates. These findings are consistent with a predominant effect on I(Ca) at rapid rates and a predominant effect on I(Kr) at slow rates.

New approaches to antiarrhythmic therapy, part II: emerging therapeutic applications of the cell biology of cardiac arrhythmias

Cardiac arrhythmias complicate many diseases affecting the heart and circulation, and they incorporate a multiplicity of underlying mechanisms. The evolution of scientific knowledge has made the complex changes produced by cardiovascular disease sufficiently understood at the organ, cellular, and molecular levels such that there is a diversity of therapeutic targets for pharmacological therapy and/or prevention. Moreover, the approach of rational drug design in mechanism-specific and disease-specific fashions facilitates the targeting of therapy using the methods of molecular, structural, and translational biology. Additional approaches, using similar drug design strategies but based on gene therapy and transcriptional and translational modification, are on the horizon. Hence, there is reason to be optimistic regarding the design, testing, and clinical availability of novel antiarrhythmic therapies.

Effects of the class III antiarrhythmic agent dofetilide (UK-68,798) on L-type calcium current from rabbit ventricular myocytes

The methanesulphonanilide agent dofetilide (UK-68,798) exerts Class III antiarrhythmic effects by inhibiting the cardiac rapid delayed rectifier potassium current (I(Kr)) encoded by HERG. The aim of the present study was to determine whether dofetilide also exhibits Class IV (L-type calcium-channel blocking) effects. L-type calcium current (I(Ca,L)) was measured from rabbit isolated ventricular myocytes, using the whole-cell patch-clamp technique under selective recording conditions. Positive control experiments demonstrated inhibition of I(Ca,L) elicited by pulses to + 10 mV by both nifedipine and externally applied Ni2+ ions. Three concentrations of dofetilide were tested: 100 nM, 1 microM and 10 microM. I(Ca,L) magnitude was not significantly reduced by any of the concentrations tested (P > 0.05; n = minimum of seven cells per drug concentration). The inactivation time-course of I(Ca,L) was also unaffected by 10 microM dofetilide. Heterologously expressed HERG current (I(HERG)) recorded from Chinese Hamster Ovary cells was extensively inhibited by 100 nm and 1 microM dofetilide, with inhibition at 1 microM not significantly different from 100% (P > 0.1). It is concluded that dofetilide produced no I(Ca,L) blocking effects at concentrations up to and exceeding that required for maximal I(HERG) inhibition. The findings support the notion that dofetilide is a highly selective Class III antiarrhythmic agent, devoid of Class IV antiarrhythmic activity.

Inhibition of HERG potassium channel current by the class 1a antiarrhythmic agent disopyramide

The Class 1a antiarrhythmic drug disopyramide (DISO) is associated with 'acquired' prolongation of the QT interval of the electrocardiogram (ECG). This potentially proarrhythmic effect is likely to reflect drug actions on ion channels involved in ventricular action potential repolarisation. In this study, we examined the effects of DISO on potassium channels encoded by HERG, as this K channel type has been implicated in both congenital and acquired long-QT syndromes (LQTS). Chinese hamster ovary cells were transiently transfected with HERG cDNA for subsequent whole cell patch clamp recording. HERG tail currents recorded at -40 mV following test pulses to +30 mV were inhibited in a dose-dependent fashion by DISO concentrations within the clinical range (IC50 = 7.23 +/- 0.72 microM; mean +/- SEM). Experiments with 10 microM DISO indicated that the degree of HERG blockade showed some voltage dependence. Further data obtained using an 'envelope of tails' protocol (pulse potential +40 mV) were consistent with a significant role for open-channel blockade at lower drug concentrations. At higher concentrations it is possible that blockade may have involved drug binding to both resting and open channels. Inhibition of the inactivation-deficient mutant HERG-S631A was comparable to that seen for wild-type HERG. Therefore, channel inactivation was not obligatory for DISO to exert its effect. Native delayed rectifier tail currents from rabbit isolated ventricular myocytes were also inhibited by DISO. We conclude (a) that DISO inhibits HERG encoded potassium channels at clinically relevant concentrations and (b) that this action may constitute the molecular basis for acquired LQTS associated with this drug.