Flecainide-Associated Bradycardia-Dependent Torsade de Pointes: Another Potential Mechanism of Proarrhythmia

Kir2.1 mutations differentially increase the risk of flecainide proarrhythmia in Andersen Tawil Syndrome

Background

Flecainide and other class-Ic antiarrhythmic drugs (AADs) are widely used in Andersen-Tawil syndrome type 1 (ATS1) patients. However, class-Ic drugs might be proarrhythmic in some cases. We investigated the molecular mechanisms of class-I AADs proarrhythmia and whether they might increase the risk of death in ATS1 patients with structurally normal hearts.

Methods and Results

Of 53 ATS1 patients reviewed from the literature, 54% responded partially to flecainide, with ventricular arrhythmia (VA) reduction in only 23%. Of the latter patients, VA persisted in 20-50%. Flecainide was ineffective in 23%, and surprisingly, 13.5% suffered a non-fatal cardiac arrest. In five cardiac-specific ATS1 mouse models (Kir2.1Δ314-315, Kir2.1C122Y, Kir2.1G215D and Kir2.1R67W and Kir2.1S136F), flecainide or propafenone (40 mg/Kg i.p.) differentially prolonged the P wave, and the PR, QRS and QTc intervals compared to Kir2.1WT; Kir2.1S136F had milder effects. Flecainide increased VA inducibility in all mutant mice except Kir2.1S136F, which exhibited significant VA reduction. At baseline, Kir2.1G215D cardiomyocytes had the lowest inward rectifier K+ channel (IK1) reduction, followed by Kir2.1C122Y, Kir2.1R67W and Kir2.1S136F. Kir2.1C122Y cardiomyocytes had a significant decrease in sodium inward current (INa). Flecainide (10 μM) slightly increased IK1 density in Kir2.1WT and Kir2.1S136F, while it decreased both IK1 and INa in Kir2.1C122Y and Kir2.1R67W, despite normal trafficking of mutant channels. Optical mapping in ATS1 patient-specific iPSC-CM monolayers expressing Kir2.1C122Y, Kir2.1G215D and Kir2.1R67W showed an increase in rotor incidence at baseline and under flecainide, confirming the drugś proarrhythmic effect. Lastly, in-silico molecular docking predicts that the Kir2.1-Cys311 pharmacophore-binding site is altered in Kir2.1C122Y heterotetramers, reducing flecainide accessibility and leading to channel closure and arrhythmias.

Conclusions

Class-Ic AADs are only partially effective and might be proarrhythmic in some ATS1 patients. Kir2.1 mutations impacting the resting membrane potential and cellular excitability create a substrate for life-threatening arrhythmias, raising significant concern about using these drugs in some ATS1 patients.

Fast and furious: flecainide toxicity presenting as monomorphic ventricular tachycardia

A 63-year-old woman on flecainide, furosemide, and triamterene-hydrochlorothiazide presented with weakness and diarrhoea. She had profound hyponatraemia, hypokalaemia and a pre-renal acute kidney injury (AKI). Her ECG showed a regular wide complex tachycardia concerning for monomorphic ventricular tachycardia. She was haemodynamically stable and treated with aggressive electrolyte repletion and amiodarone. Flecainide toxicity can present as a variety of arrhythmias and early recognition is crucial. This case focuses on flecainide toxicity from multiple concomitant insults: diuretic use, diarrhoea, hypokalaemia, hyponatraemia and pre-renal AKI. We emphasise the importance of close outpatient monitoring of electrolytes in a patient on diuretics and flecainide to prevent life-threatening arrhythmias. We discourage use of multiple diuretics in patients taking flecainide.

Role of abnormal repolarization in the mechanism of cardiac arrhythmia

In cardiac patients, life-threatening tachyarrhythmia is often precipitated by abnormal changes in ventricular repolarization and refractoriness. Repolarization abnormalities typically evolve as a consequence of impaired function of outward K⁺ currents in cardiac myocytes, which may be caused by genetic defects or result from various acquired pathophysiological conditions, including electrical remodelling in cardiac disease, ion channel modulation by clinically used pharmacological agents, and systemic electrolyte disorders seen in heart failure, such as hypokalaemia. Cardiac electrical instability attributed to abnormal repolarization relies on the complex interplay between a provocative arrhythmic trigger and vulnerable arrhythmic substrate, with a central role played by the excessive prolongation of ventricular action potential duration, impaired intracellular Ca²⁺ handling, and slowed impulse conduction. This review outlines the electrical activity of ventricular myocytes in normal conditions and cardiac disease, describes classical electrophysiological mechanisms of cardiac arrhythmia, and provides an update on repolarization-related surrogates currently used to assess arrhythmic propensity, including spatial dispersion of repolarization, activation-repolarization coupling, electrical restitution, TRIaD (triangulation, reverse use dependence, instability, and dispersion), and the electromechanical window. This is followed by a discussion of the mechanisms that account for the dependence of arrhythmic vulnerability on the location of the ventricular pacing site. Finally, the review clarifies the electrophysiological basis for cardiac arrhythmia produced by hypokalaemia, and gives insight into the clinical importance and pathophysiology of drug-induced arrhythmia, with particular focus on class Ia (quinidine, procainamide) and Ic (flecainide) Na⁺ channel blockers, and class III antiarrhythmic agents that block the delayed rectifier K⁺ channel (dofetilide).

Assessments of the QT/QRS restitution in perfused guinea-pig heart can discriminate safe and arrhythmogenic drugs

INTRODUCTION

Drug-induced arrhythmia remains a matter of serious clinical concern, partly due to low prognostic value of currently available arrhythmic biomarkers.

METHODS

This study examined whether arrhythmogenic risks can be predicted through assessments of the rate adaptation of QT interval, ventricular effective refractory period (ERP), or the QT/QRS ratio, in perfused guinea-pig hearts.

RESULTS

When the maximum restitution slope was taken as a metric of proarrhythmia, neither QT interval nor ERP measurements at progressively increased pacing rates were found to fully discriminate arrhythmogenic drugs (dofetilide, quinidine, flecainide, and procainamide) from those recognized as safe antiarrhythmics (lidocaine and mexiletine). For example, the slope of QT restitution was increased by dofetilide and quinidine, but remained unchanged by flecainide, procainamide, lidocaine, and mexiletine. With ERP rate adaptation, even though the restitution slope was increased by dofetilide, all class I agents reduced the slope value independently of their safety profile. The QRS measurements revealed variable drug effects, ranging from significant use-dependent conduction slowing (flecainide, quinidine, and procainamide) to only modest increase in QRS (lidocaine and mexiletine), or no change at all (dofetilide). However, with the QT/QRS rate adaptation, the restitution slope was significantly increased by all agents which have been reported to produce proarrhythmic effects (dofetilide, quinidine, flecainide, and procainamide), but not changed by lidocaine and mexiletine.

DISCUSSION

These findings suggest that the slope of the QT/QRS rate adaptation can be considered as a novel electrophysiological biomarker in predicting potential arrhythmic risks associated with pharmacotherapy in cardiac patients.

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.

Late sodium current inhibition in acquired and inherited ventricular (dys)function and arrhythmias

The late sodium current has been increasingly recognized for its mechanistic role in various cardiovascular pathologies, including angina pectoris, myocardial ischemia, atrial fibrillation, heart failure and congenital long QT syndrome. Although relatively small in magnitude, the late sodium current (I(NaL)) represents a functionally relevant contributor to cardiomyocyte (electro)physiology. Many aspects of I(NaL) itself are as yet still unresolved, including its distribution and function in different cell types throughout the heart, and its regulation by sodium channel accessory proteins and intracellular signalling pathways. Its complexity is further increased by a close interrelationship with the peak sodium current and other ion currents, hindering the development of inhibitors with selective and specific properties. Thus, increased knowledge of the intricacies of the complex nature of I(NaL) during distinct cardiovascular conditions and its potential as a pharmacological target is essential. Here, we provide an overview of the functional and electrophysiological effects of late sodium current inhibition on the level of the ventricular myocyte, and its potential cardioprotective and anti-arrhythmic efficacy in the setting of acquired and inherited ventricular dysfunction and arrhythmias.