Mechanism of antiarrhythmic effects of flecainide in catecholaminergic polymorphic ventricular tachycardia

RYR2 Channel Inhibition Is the Principal Mechanism of Flecainide Action in CPVT

RATIONALE

The class Ic antiarrhythmic drug flecainide prevents ventricular tachyarrhythmia in patients with catecholaminergic polymorphic ventricular tachycardia (CPVT), a disease caused by hyperactive RyR2 (cardiac ryanodine receptor) mediated calcium (Ca) release. Although flecainide inhibits single RyR2 channels in vitro, reports have claimed that RyR2 inhibition by flecainide is not relevant for its mechanism of antiarrhythmic action and concluded that sodium channel block alone is responsible for flecainide's efficacy in CPVT.

OBJECTIVE

To determine whether RyR2 block independently contributes to flecainide's efficacy for suppressing spontaneous sarcoplasmic reticulum Ca release and for preventing ventricular tachycardia in vivo.

METHODS AND RESULTS

We synthesized N-methylated flecainide analogues (QX-flecainide and N-methyl flecainide) and showed that N-methylation reduces flecainide's inhibitory potency on RyR2 channels incorporated into artificial lipid bilayers. N-methylation did not alter flecainide's inhibitory activity on human cardiac sodium channels expressed in HEK293T cells. Antiarrhythmic efficacy was tested utilizing a Casq2 (cardiac calsequestrin) knockout (Casq2-/-) CPVT mouse model. In membrane-permeabilized Casq2-/- cardiomyocytes-lacking intact sarcolemma and devoid of sodium channel contribution-flecainide, but not its analogues, suppressed RyR2-mediated Ca release at clinically relevant concentrations. In voltage-clamped, intact Casq2-/- cardiomyocytes pretreated with tetrodotoxin to inhibit sodium channels and isolate the effect of flecainide on RyR2, flecainide significantly reduced the frequency of spontaneous sarcoplasmic reticulum Ca release, while QX-flecainide and N-methyl flecainide did not. In vivo, flecainide effectively suppressed catecholamine-induced ventricular tachyarrhythmias in Casq2-/- mice, whereas N-methyl flecainide had no significant effect on arrhythmia burden, despite comparable sodium channel block.

CONCLUSIONS

Flecainide remains an effective inhibitor of RyR2-mediated arrhythmogenic Ca release even when cardiac sodium channels are blocked. In mice with CPVT, sodium channel block alone did not prevent ventricular tachycardia. Hence, RyR2 channel inhibition likely constitutes the principal mechanism of antiarrhythmic action of flecainide in CPVT.

Computational Modeling of Electrophysiology and Pharmacotherapy of Atrial Fibrillation: Recent Advances and Future Challenges

The pathophysiology of atrial fibrillation (AF) is broad, with components related to the unique and diverse cellular electrophysiology of atrial myocytes, structural complexity, and heterogeneity of atrial tissue, and pronounced disease-associated remodeling of both cells and tissue. A major challenge for rational design of AF therapy, particularly pharmacotherapy, is integrating these multiscale characteristics to identify approaches that are both efficacious and independent of ventricular contraindications. Computational modeling has long been touted as a basis for achieving such integration in a rapid, economical, and scalable manner. However, computational pipelines for AF-specific drug screening are in their infancy, and while the field is progressing quite rapidly, major challenges remain before computational approaches can fill the role of workhorse in rational design of AF pharmacotherapies. In this review, we briefly detail the unique aspects of AF pathophysiology that determine requirements for compounds targeting AF rhythm control, with emphasis on delimiting mechanisms that promote AF triggers from those providing substrate or supporting reentry. We then describe modeling approaches that have been used to assess the outcomes of drugs acting on established AF targets, as well as on novel promising targets including the ultra-rapidly activating delayed rectifier potassium current, the acetylcholine-activated potassium current and the small conductance calcium-activated potassium channel. Finally, we describe how heterogeneity and variability are being incorporated into AF-specific models, and how these approaches are yielding novel insights into the basic physiology of disease, as well as aiding identification of the important molecular players in the complex AF etiology.

Patient-Specific Drug Screening Using a Human Induced Pluripotent Stem Cell Model of Catecholaminergic Polymorphic Ventricular Tachycardia Type 2

BACKGROUND

Catecholaminergic polymorphic ventricular tachycardia type 2 (CPVT2) results from autosomal recessive CASQ2 mutations, causing abnormal Ca²⁺-handling and malignant ventricular arrhythmias. We aimed to establish a patient-specific human induced pluripotent stem cell (hiPSC) model of CPVT2 and to use the generated hiPSC-derived cardiomyocytes to gain insights into patient-specific disease mechanism and pharmacotherapy.

METHODS AND RESULTS

hiPSC cardiomyocytes were derived from a CPVT2 patient (D307H-CASQ2 mutation) and from healthy controls. Laser-confocal Ca²⁺ and voltage imaging showed significant Ca²⁺-transient irregularities, marked arrhythmogenicity manifested by early afterdepolarizations and triggered arrhythmias, and reduced threshold for store overload-induced Ca²⁺-release events in the CPVT2-hiPSC cardiomyocytes when compared with healthy control cells. Pharmacological studies revealed the prevention of adrenergic-induced arrhythmias by β-blockers (propranolol and carvedilol), flecainide, and the neuronal sodium-channel blocker riluzole; a direct antiarrhythmic action of carvedilol (independent of its α/β-adrenergic blocking activity), flecainide, and riluzole; and suppression of abnormal Ca²⁺ cycling by the ryanodine stabilizer JTV-519 and carvedilol. Mechanistic insights were gained on the different antiarrhythmic actions of the aforementioned drugs, with carvedilol and JTV-519 (but not flecainide or riluzole) acting primarily through sarcoplasmic reticulum stabilization. Finally, comparable outcomes were found between flecainide and labetalol antiarrhythmic effects in vitro and the clinical results in the same patient.

CONCLUSIONS

These results demonstrate the ability of hiPSCs cardiomyocytes to recapitulate CPVT2 disease phenotype and drug response in the culture dish, to provide novel insights into disease and drug therapy mechanisms, and potentially to tailor patient-specific drug therapy.

Xanthohumol Modulates Calcium Signaling in Rat Ventricular Myocytes: Possible Antiarrhythmic Properties

Cardiac arrhythmia is a major cause of mortality in cardiovascular pathologies. A host of drugs targeted to sarcolemmal Na⁺, Ca²⁺, and K⁺ channels has had limited success clinically. Recently, Ca²⁺ signaling has been target of pharmacotherapy based on finding that leaky ryanodine receptors elevate local Ca²⁺ concentrations causing membrane depolarizations that trigger arrhythmias. In this study, we report that xanthohumol, an antioxidant extracted from hops showing therapeutic effects in other pathologies, suppresses aberrant ryanodine receptor Ca²⁺ release. The effects of xanthohumol (5-1000 nM) on Ca²⁺ signaling pathways were probed in isolated rat ventricular myocytes incubated with Fluo-4 AM using the perforated patch-clamp technique. We found that 5-50 nM xanthohumol reduced the frequency of spontaneously occurring Ca²⁺ sparks (>threefold) and Ca²⁺ waves in control myocytes and in cells subjected to Ca²⁺ overload caused by the following: 1) exposure to low K⁺ solutions, 2) periods of high frequency electrical stimulation, 3) exposures to isoproterenol, or 4) caffeine. At room temperatures, 50-100 nM xanthohumol reduced the rate of relaxation of electrically- or caffeine-triggered Ca²⁺transients, without suppressing I_Ca, but this effect was small and reversed by isoproterenol at physiologic temperatures. Xanthohumol also suppressed the Ca²⁺ content of the SR and its rate of recirculation. The stabilizing effects of xanthohumol on the frequency of spontaneously triggered Ca²⁺ sparks and waves combined with its antioxidant properties, and lack of significant effects on Na⁺ and Ca²⁺ channels, may provide this compound with clinically desirable antiarrhythmic properties.

In silico prediction of drug therapy in catecholaminergic polymorphic ventricular tachycardia

Key points

The mechanism of therapeutic efficacy of flecainide for catecholaminergic polymorphic ventricular tachycardia (CPVT) is unclear.

Model predictions suggest that Na⁺ channel effects are insufficient to explain flecainide efficacy in CPVT.

This study represents a first step toward predicting therapeutic mechanisms of drug efficacy in the setting of CPVT and then using these mechanisms to guide modelling and simulation to predict alternative drug therapies.

Abstract

Catecholaminergic polymorphic ventricular tachycardia (CPVT) is an inherited arrhythmia syndrome characterized by fatal ventricular arrhythmias in structurally normal hearts during β‐adrenergic stimulation. Current treatment strategies include β‐blockade, flecainide and ICD implementation – none of which is fully effective and each comes with associated risk. Recently, flecainide has gained considerable interest in CPVT treatment, but its mechanism of action for therapeutic efficacy is unclear. In this study, we performed in silico mutagenesis to construct a CPVT model and then used a computational modelling and simulation approach to make predictions of drug mechanisms and efficacy in the setting of CPVT. Experiments were carried out to validate model results. Our simulations revealed that Na⁺ channel effects are insufficient to explain flecainide efficacy in CPVT. The pure Na⁺ channel blocker lidocaine and the antianginal ranolazine were additionally tested and also found to be ineffective. When we tested lower dose combination therapy with flecainide, β‐blockade and CaMKII inhibition, our model predicted superior therapeutic efficacy than with flecainide monotherapy. Simulations indicate a polytherapeutic approach may mitigate side‐effects and proarrhythmic potential plaguing CPVT pharmacological management today. Importantly, our prediction of a novel polytherapy for CPVT was confirmed experimentally. Our simulations suggest that flecainide therapeutic efficacy in CPVT is unlikely to derive from primary interactions with the Na⁺ channel, and benefit may be gained from an alternative multi‐drug regimen.

The mechanism of therapeutic efficacy of flecainide for catecholaminergic polymorphic ventricular tachycardia (CPVT) is unclear.

Model predictions suggest that Na⁺ channel effects are insufficient to explain flecainide efficacy in CPVT.

This study represents a first step toward predicting therapeutic mechanisms of drug efficacy in the setting of CPVT and then using these mechanisms to guide modelling and simulation to predict alternative drug therapies.

High-dose flecainide with low-dose β-blocker therapy in catecholaminergic polymorphic ventricular tachycardia: A case report and review of the literature

Background

Catecholaminergic polymorphic ventricular tachycardia (CPVT) is characterized by recurrent syncopes and sudden cardiac death triggered by sympathetic activation in young individuals without structural heart disease and a normal baseline electrocardiogram. There is reason to question whether the current expert consensus treatment recommendation, maximal tolerated β-blockade alone or in combination with low-dose flecainide, is the optimal antiarrhythmic treatment strategy in CPVT, as high doses of β-blockers may eventually lead to adverse side effects and β-blocker discontinuation. Indeed, β-blocker non-compliance accounts for around 5% of sudden cardiac deaths in CPVT patients.

Case report

Differing from the current recommendation, we present the first report of a CPVT patient successfully treated with high-dose flecainide and minimal β-blockade. This combination resulted in complete suppression of ventricular arrhythmias during exercise stress tests and Holter monitoring and was well tolerated without any side effects. We review the current literature on β-blocker non-compliance-related sudden cardiac death in CPVT, summarize the in vitro and in vivo data on flecainide therapy in CPVT, and discuss the rationale of our antiarrhythmic approach.<Learning objective: This case illustrates typical features of CPVT including the therapeutic management of a young CPVT patient with poor β-blocker tolerance at normal dosages. In this setting, high-dose flecainide combined with minimal β-blockade may (1) result in complete antiarrhythmic response and may (2) improve the antiarrhythmic drug-compliance thereby reducing the risk of non-compliance-related sudden cardiac death.>.

New antiarrhythmic targets to control intracellular calcium handling

Sudden cardiac death due to ventricular arrhythmias is a major problem. Drug therapies to prevent SCD do not provide satisfying results, leading to the demand for new antiarrhythmic strategies. New targets include Ca²⁺/Calmodulin-dependent protein kinase II (CaMKII), the Na/Ca exchanger (NCX), the Ryanodine receptor (RyR, and its associated protein FKBP12.6 (Calstabin)) and the late component of the sodium current (I_Na-Late), all related to intracellular calcium (Ca²⁺) handling. In this review, drugs interfering with these targets (SEA-0400, K201, KN-93, W7, ranolazine, sophocarpine, and GS-967) are evaluated and their future as clinical compounds is considered. These new targets prove to be interesting; however more insight into long-term drug effects is necessary before clinical applicability becomes reality.

Flecainide and antiarrhythmic effects in a mouse model of catecholaminergic polymorphic ventricular tachycardia

Recent studies have shown that flecainide may be an effective therapy to prevent life-threatening arrhythmias in patients with catecholaminergic polymorphic ventricular tachycardia. Several hypotheses have been advanced to explain the antiarrhythmic mechanism of flecainide, including Na(+) channel blockade and a direct inhibitory action on the ryanodine receptor. In this article, we review the current literature on the topic and summarize the elements of the existing debate.

Ryanodine receptor inhibition potentiates the activity of Na channel blockers against spontaneous calcium elevations and delayed afterdepolarizations in Langendorff-perfused rabbit ventricles

BACKGROUND

Na channel blockers are effective in suppressing delayed afterdepolarizations (DADs) in isolated Purkinje fibers. However, in isolated mouse ventricular myocytes lacking calsequestrin, only those Na channel blockers that also inhibit type 2 ryanodine receptor channels were effective against spontaneous Ca elevation (SCaE) and DADs.

OBJECTIVE

To test the hypothesis that combined Na channel and type 2 ryanodine receptor channel blocker ((R)-propafenone) is more effective than a Na channel blocker (lidocaine) in suppressing SCaE and DADs in the intact rabbit ventricles.

METHODS

We compared (R)-propafenone (3 μmol/L) with lidocaine (50 μmol/L) on SCaE and DADs by using epicardial optical mapping of intracellular calcium (Ca(i)) and membrane voltage in Langendorff-perfused rabbit hearts. SCaE and DADs were induced by rapid pacing trains and isoproterenol (0.3 μmol/L) infusion. One arbitrary unit is equivalent to the Ca transient amplitude of paced beats.

RESULTS

SCaEs were observed at the cessation of rapid pacing in all hearts at baseline. (R)-Propafenone nearly completely inhibited DADs and SCaE (0.04 arbitrary units [95% confidence interval 0.02-0.06] vs 0.23 arbitrary units [95% confidence interval 0.18-0.28] at baseline; n = 6 hearts; P <.001). Lidocaine also significantly reduced the SCaE but was significantly (P <.05) less effective than (R)-propafenone. Both drugs increased the rise time of action potential upstroke and reduced conduction velocity to a similar extent, suggesting a significant inhibition of I(Na).

CONCLUSIONS

Both Na channel blockers significantly reduced tachycardia-induced SCaEs in the rabbit ventricles, but (R)-propafenone was significantly more effective than lidocaine. These data suggest that type 2 ryanodine receptor inhibition potentiates the activity of Na channel blockers against SCaE and DADs in the intact hearts.