The HARMONY Trial: Combined Ranolazine and Dronedarone in the Management of Paroxysmal Atrial Fibrillation: Mechanistic and Therapeutic Synergism

Mechanisms underlying atrial-selective block of sodium channels by Wenxin Keli: Experimental and theoretical analysis

INTRODUCTION

Atrial-selective inhibition of cardiac sodium channel current (INa) and INa-dependent parameters has been shown to contribute to the safe and effective management of atrial fibrillation. The present study was designed to examine the basis for the atrial-selective actions of Wenxin Keli.

METHODS

Whole cell INa was recorded at room temperature in canine atrial and ventricular myocytes. Trains of 40 pulses were elicited over a range of pulse durations and interpulse intervals to determine tonic and use-dependent block. A Markovian model for INa that incorporates interaction of Wenxin Keli with different states of the channel was developed to examine the basis for atrial selectivity of the drug.

RESULTS

Our data indicate that Wenxin Keli does not bind significantly to either closed or open states of the sodium channel, but binds very rapidly to the inactivated state of the channel and dissociates rapidly from the closed state. Action potentials recorded from atrial and ventricular preparations in the presence of 5g/L Wenxin Keli were introduced into the computer model in current clamp mode to simulate the effects on maximum upstroke velocity (Vmax). The model predicted much greater inhibition of Vmax in atrial vs. ventricular cells at rapid stimulation rates.

CONCLUSION

Our findings suggest that atrial selectivity of Wenxin Keli to block INa is due to more negative steady-state inactivation, less negative resting membrane potential, and shorter diastolic intervals in atrial vs. ventricular cells at rapid activation rates. These actions of Wenxin Keli account for its relatively safe and effective suppression of atrial fibrillation.

The value of basic research insights into atrial fibrillation mechanisms as a guide to therapeutic innovation: a critical analysis

Atrial fibrillation (AF) is an extremely common clinical problem associated with increased morbidity and mortality. Current antiarrhythmic options include pharmacological, ablation, and surgical therapies, and have significantly improved clinical outcomes. However, their efficacy remains suboptimal, and their use is limited by a variety of potentially serious adverse effects. There is a clear need for improved therapeutic options. Several decades of research have substantially expanded our understanding of the basic mechanisms of AF. Ectopic firing and re-entrant activity have been identified as the predominant mechanisms for arrhythmia initiation and maintenance. However, it has become clear that the clinical factors predisposing to AF and the cellular and molecular mechanisms involved are extremely complex. Moreover, all AF-promoting and maintaining mechanisms are dynamically regulated and subject to remodelling caused by both AF and cardiovascular disease. Accordingly, the initial presentation and clinical progression of AF patients are enormously heterogeneous. An understanding of arrhythmia mechanisms is widely assumed to be the basis of therapeutic innovation, but while this assumption seems self-evident, we are not aware of any papers that have critically examined the practical contributions of basic research into AF mechanisms to arrhythmia management. Here, we review recent insights into the basic mechanisms of AF, critically analyse the role of basic research insights in the development of presently used anti-AF therapeutic options and assess the potential value of contemporary experimental discoveries for future therapeutic innovation. Finally, we highlight some of the important challenges to the translation of basic science findings to clinical application.

Potassium Channel Blockade Enhances Atrial Fibrillation–Selective Antiarrhythmic Effects of Optimized State-Dependent Sodium Channel Blockade

Background—

The development of effective and safe antiarrhythmic drugs for atrial fibrillation (AF) rhythm control is an unmet clinical need. Multichannel blockers are believed to have advantages over single-channel blockers for AF, but their development has been completely empirical to date. We tested the hypothesis that adding K + -channel blockade improves the atrium-selective electrophysiological profile and anti-AF effects of optimized Na + -channel blockers.

Methods and Results—

Realistic cardiomyocyte-, tissue-, and state-dependent Na + -channel block mathematical models, optical mapping, and action potential recording were used to study the effect of Na + -current ( I Na ) blockade with or without concomitant inhibition of the rapid or ultrarapid delayed-rectifier K + currents ( I Kr and I Kur , respectively). In the mathematical model, maximal AF selectivity was obtained with an inactivated-state Na + -channel blocker. Combining optimized Na + -channel blocker with I Kr block increased rate-dependent and atrium-selective peak I Na reduction, increased AF selectivity, and more effectively terminated AF compared with optimized Na + -channel blocker alone. Combining optimized Na + -channel blocker with I Kur block had similar effects but without I Kr block–induced ventricular action potential prolongation. Consistent with the mathematical model, in coronary-perfused canine hearts, the addition of dofetilide (selective I Kr blocker) to pilsicainide (selective I Na blocker) produced enhanced atrium-selective effects on maximal phase 0 upstroke and conduction velocity. Furthermore, pilsicainide plus dofetilide had higher AF termination efficacy than pilsicainide alone. Pilsicainide alone had no statistically significant effect on AF inducibility, whereas pilsicainide plus dofetilide rendered AF noninducible.

Conclusions—

K + -channel block potentiates the AF-selective anti-AF effects obtainable with optimized Na + -channel blockade. Combining optimized Na + -channel block with blockade of atrial K + currents is a potentially valuable AF-selective antiarrhythmic drug strategy.

Does the FDA Owe Us an Explanation?

The approval process for new medications sometimes appears to require pharmacokinetic data in some circumstances and clinical trial data in others. This editorial calls for more clarity and transparency in the process and uses three examples to raise the issue.