Novel approaches for pharmacological management of atrial fibrillation

[Dronedarone]

Dronedarone is a new antiarrhythmic agent, approved in January 2010 for the therapy of atrial fibrillation and for rate control in tachyarrhythmic atrial fibrillation. It was developed with the goal of replicating the high antiarrhythmic potency of amiodarone but with fewer side effects. An essential difference compared to amiodarone is the elimination of the iodine substituents in order to avoid the thyroid side effects and the addition of a methane-sulfonamyl group group in order to reduce lipophilicity. In the meantime, the Guidelines of the European Society of Cardiology list dronedarone in the group of antiarrhythmically effective substances along with the class 1c antiarrhythmia agents and amiodarone that can be administered before catheter ablation. Dronedarone is a well-tolerated antiarrhythmic agent, which due to its side effect profile permits its application in an outpatient setting in patients with atrial fibrillation.

Modulation of drug block of the cardiac potassium channel KCNA5 by the drug transporters OCTN1 and MDR1

BACKGROUND AND PURPOSE

A common site for drug binding on voltage-gated ion channels is at the interior face of the channel pore. In this study, we tested the hypothesis that the extent of drug block of the human cardiac KCNA5 (K(v) 1.5) channel underlying the atrial-specific, ultra-rapidly activating, delayed K(+) current (I(Kur) ) is modulated by the drug uptake and efflux transporters encoded by organic cation transporter 1 (OCTN1) and multiple drug-resistant gene 1 (MDR1) and expressed in human heart.

EXPERIMENTAL APPROACH

Drug block of KCNA5 was assessed in Chinese hamster ovary cells transiently transfected with KCNA5 alone or in combination with the OCTN1 or MDR1 transporter construct, as well as in an MDR1 stably expressed cell line.

KEY RESULTS

Co-expression of OCTN1 significantly facilitated block by quinidine (10 µM), verapamil (20 µM), propafenone (5 µM) and clofilium (30 µM). Further evidence of drug transport modulating drug block was the finding that with OCTN1, block developed faster and only partially washed-out, and that block potentiation was prevented by cimetidine, an inhibitor of OCTN1. MDR1 expression attenuated KCNA5 block by erythromycin (an MDR1 substrate). Block was restored by reversin-205 (10 µM, an MDR1 inhibitor). MDR1 did not affect KCNA5 inhibition by KN-93 (1 µM), a blocker acting on the outer mouth of the channel pore.

CONCLUSIONS AND IMPLICATIONS

The extent of drug block of KCNA5 can be modulated by drug uptake and efflux transporters. These data provide further support for the idea that modifying intracellular drug concentrations could modulate the effects of blocking ion channels in patients.

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.

New developments in atrial antiarrhythmic drug therapy

Atrial fibrillation (AF) is a growing clinical problem associated with increased morbidity and mortality. Currently available antiarrhythmic drugs (AADs), although highly effective in acute cardioversion of paroxysmal AF, are generally only moderately successful in long-term maintenance of sinus rhythm. The use of AADs is often associated with an increased risk of ventricular proarrhythmia, extracardiac toxicity, and exacerbation of concomitant diseases such as heart failure. AF is commonly associated with intracardiac and extracardiac disease, which can modulate the efficacy and safety of AAD therapy. In light of the multifactorial intracardiac and extracardiac causes of AF generation, current development of anti-AF agents is focused on modulation of ion channel activity as well as on upstream therapies that reduce structural substrates. The available data indicate that multiple ion channel blockers exhibiting potent inhibition of peak I(Na) with relatively rapid unbinding kinetics, as well as inhibition of late I(Na) and I(Kr), may be preferable for the management of AF when considering both safety and efficacy.

The impact of new and emerging clinical data on treatment strategies for atrial fibrillation

INTRODUCTION

The Atrial Fibrillation (AF) Exchange Group, an international multidisciplinary group concerned with the management of AF, was convened to review recent advances in the field and the potential impact on treatment strategies.

METHODS

Issues discussed included epidemiology and the impact of the rising incidence of AF on health care systems, developments in pharmacological and surgical interventions in the management of arrhythmias and thromboprophylaxis, the potential to affect treatment strategies, and barriers to implementing them.

RESULTS

The incidence of AF and the associated burden on health care systems are increasing with aging populations, prevalence of comorbidities and more effective treatment of cardiovascular diseases. Advances in available medical treatments, in particular dronedarone and dabigatran, with other products in development, offer the possibility of changes in treatment paradigms and a greater emphasis on reducing hospitalizations and improvement in long-term outcomes instead of a symptom/safety-driven approach in which the priority is symptom suppression without provoking drug toxicity. Developments in catheter ablation techniques may mean that, in experienced centers, ablation may be offered as first-line treatment in selected patient populations. Barriers to optimal treatment include underdiagnosis, lack of recognition as a serious condition and as a risk factor for stroke, limited access to care, inadequate implementation of guidelines, and poor adherence to treatment.

CONCLUSIONS

The focus of the management of AF may be changing as a consequence of new treatments based on the outcome improvements they offer. However, the benefits will not be fully realized if guidelines and guidance are not observed in routine clinical practice.

Cardiac adrenergic control and atrial fibrillation

Atrial fibrillation (AF) is the most common cardiac arrhythmia, and it causes substantial mortality. The autonomic nervous system, and particularly the adrenergic/cholinergic balance, has a profound influence on the occurrence of AF. Adrenergic stimulation from catecholamines can cause AF in patients. In human atrium, catecholamines can affect each of the electrophysiological mechanisms of AF initiation and/or maintenance. Catecholamines may produce membrane potential oscillations characteristic of afterdepolarisations, by increasing Ca(2+) current, [Ca(2+)](i) and consequent Na(+)-Ca(2+) exchange, and may also enhance automaticity. Catecholamines might affect reentry, by altering excitability or conduction, rather than action potential terminal repolarisation or refractory period. However, which arrhythmia mechanisms predominate is unclear, and likely depends on cardiac pathology and adrenergic tone. Heart failure (HF), a major cause of AF, causes adrenergic activation and adaptational changes, remodelling, of atrial electrophysiology, Ca(2+) homeostasis, and adrenergic responses. Chronic AF also remodels these, but differently to HF. Myocardial infarction and AF cause neural remodelling that also may promote AF. beta-Adrenoceptor antagonists (beta-blockers) are used in the treatment of AF, mainly to control the ventricular rate, by slowing atrioventricular conduction. beta-Blockers also reduce the incidence of AF, particularly in HF or after cardiac surgery, when adrenergic tone is high. Furthermore, the chronic treatment of patients with beta-blockers remodels the atria, with a potentially antiarrhythmic increase in the refractory period. Therefore, the suppression of AF by beta-blocker treatment may involve an attenuation of arrhythmic activity that is caused by increased [Ca(2+)](i), coupled with effects of adaptation to the treatment. An improved understanding of the involvement of the adrenergic system and its control in basic mechanisms of AF under differing cardiac pathologies might lead to better treatments.

Atrial Ca2+ signaling in atrial fibrillation as an antiarrhythmic drug target

Atrial fibrillation (AF) is the most frequent arrhythmia and is associated with increased morbidity and mortality. Current drugs for AF treatment have moderate efficacy and increase the risk of life-threatening antiarrhythmias, making novel drug development crucial. Newer antiarrhythmic drugs like dronedarone and possibly vernakalant are efficient and may have less proarrhythmic potential. Emerging evidence suggests that abnormal intracellular Ca(2+) signaling is the key contributor to focal firing, substrate evolution, and atrial remodeling during AF. Accordingly, identification of the underlying atrial Ca(2+)-handling abnormalities is expected to discover novel mechanistically based therapeutic targets. This article reviews the molecular mechanisms of altered Ca(2+) signaling in AF and discusses the potential value of novel approaches targeting atrial Ca(2+)-handling abnormalities.