Antiarrhythmic therapy--future trends and forecast for the 21st century

Effects of a new class III antiarrhythmic drug nibentan in a canine model of vagally mediated atrial fibrillation

Nibentan, a new class III antiarrhythmic drug, is highly effective in patients with atrial flutter and fibrillation. However, its mechanism of action remains unclear. The aim of this study was to investigate the effects of nibentan using a canine model of vagally sustained atrial fibrillation (AF). Nibentan was intravenously infused to anesthetized open-chest dogs during vagally induced AF. Cumulative doses of nibentan (0.063, 0.125, and 0.250 mg/kg) successfully terminated AF in 78, 88, and 100% as well as prevented AF reinduction in 11, 63, and 90% of cases, respectively. All doses of nibentan significantly and rate-independently increased atrial effective refractory period (AERP) with and without vagal stimulation. Activation mapping (224 epicardial electrodes) during AF showed that nibentan reduced the number of simultaneously occurring reentrant wavelets. Herewith the atrial excitation slowed down until conduction failure of reentrant wavelets led to arrhythmia termination. These changes in activation patterns can be accounted for by nibentan-induced increase of AERP (55 +/- 9%, 82 +/- 12%, and 90 +/- 6%; p < 0.01) and wavelength for reentry (47 +/- 7%, 68 +/- 12%, and 72 +/- 4%; p < 0.01) at rapid atrial rates in the presence of vagal stimulation. In conclusion, the high efficacy of nibentan against AF was associated with significant rate-independent increase in AERP and in wavelength, and might be in part explained by block of both delayed rectifier (I(K)) and muscarinic I(K,ACh) currents.

Antiarrhythmic agents for atrial fibrillation: focus on prolonging atrial repolarization

Atrial fibrillation (AF) has been the subject of considerable attention and intensive clinical research in recent years. Current opinion among physicians on the management of AF favors the restoration and maintenance of normal sinus rhythm. This has several potential benefits, including the alleviation of arrhythmia-associated symptoms, hemodynamic improvements, and possibly a reduced risk of thromboembolic events. After normal sinus rhythm has been restored, antiarrhythmic therapy is necessary to reduce the frequency of AF recurrence. In the selection of an antiarrhythmic agent, both efficacy and safety should be taken into consideration. Many antiarrhythmic agents have the capacity to provoke proarrhythmia, which may result in an increase in mortality. This is of particular concern with sodium-channel blockers in the context of patients with structural heart disease. Flecainide and propafenone are well tolerated and effective in maintaining sinus rhythm in patients without significant cardiac disease but with AF. Recent interest has focused on the use of class III antiarrhythmic agents, such as amiodarone, sotalol, dofetilide (recently approved), ibutilide (approved for chemical conversion of AF and atrial flutter), and azimilide (still to be approved) in patients with AF and structural heart disease. To date, amiodarone and sotalol still hold the greatest interest, and although controlled clinical trials with these agents have been few, a number are in progress and some have been recently completed. These agents are effective in maintaining normal sinus rhythm in patients with paroxysmal and persistent AF and are associated with a low incidence of proarrhythmia when used appropriately. Because of the relative paucity of placebo-controlled trials of antiarrhythmic agents in patients with AF, experience until recently has tended to dictate treatment decisions. Increasingly, selection of drug therapy is being based on a careful and individualized benefit-risk evaluation by means of controlled clinical trials, an approach that is likely to dominate the overall approach to the control of atrial fibrillation in the largest numbers of cases of the arrhythmia. Pharmacologic therapy is likely to be dominated by compounds that exert their predominant effect by prolonging atrial repolarization.

What should we expect from the next generation of antiarrhythmic drugs?

Five drugs currently constitute approximately 70% of the world market for antiarrhythmic medications. Since the publication of studies documenting that certain Class I drugs may increase mortality in high-risk postinfarction patients, basic science and clinical studies have focused on Class III antiarrhythmic drugs. However, drugs that prolong repolarization and cardiac refractoriness are sometimes associated with potentially lethal torsades de pointes. Amiodarone, a multichannel blocker, may be the exception to this observation, but it nevertheless fails to reduce total mortality compared with placebo in high-risk patients following myocardial infarction. However, Class III agents remain the focus of drug development efforts because they lack the negative hemodynamic effects, affect both atrial and ventricular tissue, and can be administered as either parenteral or oral preparations. Developers of newer antiarrhythmic agents have focused on identifying antiarrhythmic medications with the following characteristics: appropriate modification of the arrhythmia substrate, suppression of arrhythmia triggers, efficacy in pathologic tissues and states, positive rate dependency, appropriate pharmacokinetics, equally effective oral and parenteral formulations, similar efficacy in arrhythmias and their surrogates, few side effects, positive frequency blocking actions, and cardiac-selective ion channel blockade. New and investigational agents that more closely approach these goals include azimilide, dofetilide, dronedarone, ersentilide, ibutilide, tedisamil, and trecetilide. In the near future, medications will increasingly constitute only part of an antiarrhythmic strategy. Instead of monotherapy, they will often be used in conjunction with an implanted device. Combination therapy offers many potential advantages. Long-term goals of antiarrhythmic therapy include upstream approaches, such as identification of the biochemical intermediaries of the process and, eventually, of molecular and genetic lesions involved in arrhythmogenesis.