Novel anti-arrhythmic drugs for atrial fibrillation management

Atria-selective antiarrhythmic drugs in need of alliance partners

Atria-selective antiarrhythmic drugs in need of alliance partners. Guideline-based treatment of atrial fibrillation (AF) comprises prevention of thromboembolism and stroke, as well as antiarrhythmic therapy by drugs, electrical rhythm conversion, ablation and surgical procedures. Conventional antiarrhythmic drugs are burdened with unwanted side effects including a propensity of triggering life-threatening ventricular fibrillation. In order to solve this therapeutic dilemma, 'atria-selective' antiarrhythmic drugs have been developed for the treatment of supraventricular arrhythmias. These drugs are designed to aim at atrial targets, taking advantage of differences in atrial and ventricular ion channel expression and function. However it is not clear, whether such drugs are sufficiently antiarrhythmic or whether they are in need of an alliance partner for clinical efficacy. Atria-selective Na⁺ channel blockers display fast dissociation kinetics and high binding affinity to inactivated channels. Compounds targeting atria-selective K⁺ channels include blockers of ultra rapid delayed rectifier (Kv1.5) or acetylcholine-activated inward rectifier K⁺ channels (Kir3.x), inward rectifying K⁺ channels (Kir2.x), Ca²⁺-activated K⁺ channels of small conductance (SK), weakly rectifying two-pore domain K⁺ channels (K_2P), and transient receptor potential channels (TRP). Despite good antiarrhythmic data from in-vitro and animal model experiments, clinical efficacy of atria-selective antiarrhythmic drugs remains to be demonstrated. In the present review we will briefly summarize the novel compounds and their proposed antiarrhythmic action. In addition, we will discuss the evidence for putative improvement of antiarrhythmic efficacy and potency by addressing multiple pathophysiologically relevant targets as possible alliance partners.

Atrial-selective K+ channel blockers: potential antiarrhythmic drugs in atrial fibrillation?

In the wake of demographic change in Western countries, atrial fibrillation has reached an epidemiological scale, yet current strategies for drug treatment of the arrhythmia lack sufficient efficacy and safety. In search of novel medications, atrial-selective drugs that specifically target atrial over other cardiac functions have been developed. Here, I will address drugs acting on potassium (K⁺) channels that are either predominantly expressed in atria or possess electrophysiological properties distinct in atria from ventricles. These channels include the ultra-rapidly activating, delayed outward-rectifying Kv1.5 channel conducting I_Kur, the acetylcholine-activated inward-rectifying Kir3.1/Kir3.4 channel conducting I_K,ACh, the Ca²⁺-activated K⁺ channels of small conductance (SK) conducting I_SK, and the two-pore domain K⁺ (K2P) channels (tandem of P domains, weak inward-rectifying K⁺ channels (TWIK-1), TWIK-related acid-sensitive K⁺ channels (TASK-1 and TASK-3)) that are responsible for voltage-independent background currents I_TWIK-1, I_TASK-1, and I_TASK-3. Direct drug effects on these channels are described and their putative value in treatment of atrial fibrillation is discussed. Although many potential drug targets have emerged in the process of unravelling details of the pathophysiological mechanisms responsible for atrial fibrillation, we do not know whether novel antiarrhythmic drugs will be more successful when modulating many targets or a single specific one. The answer to this riddle can only be solved in a clinical context.

Atrial fibrillation: Therapeutic potential of atrial K+ channel blockers

Despite the epidemiological scale of atrial fibrillation, current treatment strategies are of limited efficacy and safety. Ideally, novel drugs should specifically correct the pathophysiological mechanisms responsible for atrial fibrillation with no other cardiac or extracardiac actions. Atrial-selective drugs are directed toward cellular targets with sufficiently different characteristics in atria and ventricles to modify only atrial function. Several potassium (K⁺) channels with either predominant expression in atria or distinct electrophysiological properties in atria and ventricles can serve as atrial-selective drug targets. These channels include the ultra-rapidly activating, delayed outward-rectifying Kv1.5 channel conducting I_Kur, the acetylcholine-activated inward-rectifying Kir3.1/Kir3.4 channel conducting I_K,ACh, the Ca²⁺-activated K⁺ channels of small conductance (SK) conducting I_SK, and the two pore domain K⁺ (K2P) channels TWIK-1, TASK-1 and TASK-3 that are responsible for voltage-independent background currents I_TWIK-1, I_TASK-1, and I_TASK-3. Here, we briefly review the characteristics of these K⁺ channels and their roles in atrial fibrillation. The antiarrhythmic potential of drugs targeting the described channels is discussed as well as their putative value in treatment of atrial fibrillation.

Atrial selectivity of antiarrhythmic drugs

New antiarrhythmic drugs for treatment of atrial fibrillation should ideally be atrial selective in order to avoid pro-arrhythmic effects in the ventricles. Currently recognized atrial selective targets include atrial Nav1.5 channels, Kv1.5 channels and constitutively active Kir3.1/3.4 channels, each of which confers atrial selectivity by different mechanisms. Na(+) channel blockers with potential- and frequency-dependent action preferentially suppress atrial fibrillation because of the high excitation rate and less negative atrial resting potential, which promote drug binding in atria. Kv1.5 channels are truly atrial selective because they do not conduct repolarizing current IKur in ventricles. Constitutively active IK,ACh is predominantly observed in remodelled atria from patients in permanent atrial fibrillation (AF). A lot of effort has been invested to detect compounds which will selectively block Kir3.1/Kir3.4 in their remodelled constitutively active form. Novel drugs which have been and are being developed aim at atrial-selective targets. Vernakalant and ranolazine which mainly block atrial Na(+) channels are clinically effective. Newly designed selective IKur blockers and IK,ACh blockers are effective in animal models; however, clinical benefit in converting AF into sinus rhythm (SR) or reducing AF burden remains to be demonstrated. In conclusion, atrial-selective antiarrhythmic agents have a lot of potential, but a long way to go.

Markov models of use-dependence and reverse use-dependence during the mouse cardiac action potential

The fast component of the cardiac transient outward current, I_Ktof, is blocked by a number of drugs. The major molecular bases of I_Ktof are Kv4.2/Kv4.3 voltage-gated potassium channels. Drugs with similar potencies but different blocking mechanisms have differing effects on action potential duration (APD). We used in silico analysis to determine the effect of I_Ktof-blocking drugs with different blocking mechanisms on mouse ventricular myocytes. We used our existing mouse model of the action potential, and developed 4 new Markov formulations for I_Ktof, I_Ktos, I_Kur, I_Ks. We compared effects of theoretical I_Ktof-specific channel blockers: (1) a closed state, and (2) an open channel blocker. At concentrations lower or close to IC₅₀, the drug which bound to the open state always had a much greater effect on APD than the drug which bound to the closed state. At concentrations much higher than IC₅₀, both mechanisms had similar effects at very low pacing rates. However, an open state binding drug had a greater effect on APD at faster pacing rates, particularly around 10 Hz. In summary, our data indicate that drug effects on APD are strongly dependent not only on IC₅₀, but also on the drug binding state.

Dronedarone: current evidence for its safety and efficacy in the management of atrial fibrillation

Atrial fibrillation (AF) is the most common sustained arrhythmia. Management of AF includes rate control, rhythm control if necessary, prevention of thromboembolic events, and treatment of the underlying disease. Rate control is usually achieved by pharmacological suppression of calcium currents or by applying β-blockers or digitalis compounds. In contrast, the number of compounds available for rhythm control is still limited. Class Ic agents increase mortality in patients with structural heart disease, and amiodarone harbors an extensive side effect profile despite its efficacy in maintaining sinus rhythm. Furthermore, rhythm control by these compounds has not been shown to reduce patient mortality. Dronedarone is a new anti-arrhythmic drug that has been developed to provide rhythm and rate control in AF patients with fewer side effects compared with amiodarone. This review primarily focuses on clinical trials evaluating efficacy and safety of the novel drug. Conclusions from these studies are critically reviewed, and recommendations for clinical practice are discussed. Dronedarone significantly reduced the incidence of hospitalization due to cardiovascular events or death in high-risk patients with atrial fibrillation (ATHENA trial). However, dronedarone was less efficient than amiodarone in maintaining normal sinus rhythm (DIONYSOS trial) and is contraindicated in severe or deteriorating heart failure (ANDROMEDA trial). In summary, dronedarone represents a valuable addition to the limited spectrum of antiarrhythmic drugs and is currently recommended in patients with paroxysmal and persistent AF to achieve rate and rhythm control, excluding cases of severe or unstable congestive heart failure.

Novel approaches for pharmacological management of atrial fibrillation

In the light of the progressively increasing prevalence of atrial fibrillation (AF), medical awareness of the need to develop improved therapeutic approaches for the arrhythmia has also risen over the last decade. AF reduces quality of life and is associated with increased morbidity and mortality. Despite several setbacks as a result of negative results from rhythm control trials, the potential advantages of sinus-rhythm (SR) maintenance have motivated continued efforts to design novel pharmacological options aiming to terminate AF and prevent its recurrence, with a hope that optimized medical therapy will improve outcomes in AF patients. Pathophysiologically, AF is associated with electrical and structural changes in the atria, which increase the propensity to arrhythmia perpetuation but may eventually allow for new modalities for therapeutic intervention. Antiarrhythmic drug therapy has traditionally targeted ionic currents that modulate excitability and/or repolarization of cardiac myocytes. Despite efficacious suppression of ventricular and supraventricular arrhythmias, traditional antiarrhythmic drugs present problematic risks of pro-arrhythmia, potentially leading to excess mortality in the case of Na+-channel blockers or IKr (IKr=the rapid component of the delayed rectifier potassium current) blockers. New anti-AF agents in development do not fit well into the classical Singh and Vaughan-Williams formulation, and are broadly divided into 'atrial-selective compounds' and 'multiple-channel blockers'. The prototypic multiple-channel blocker amiodarone is the most efficient presently available compound for SR maintenance, but the drug has extra-cardiac adverse effects and complex pharmacokinetics that limit widespread application. The other available drugs are not nearly as efficient for SR maintenance and have a greater risk of proarrhythmia than amiodarone. Two new antiarrhythmic drugs are on the cusp of introduction into clinical practice. Vernakalant affects several atrially expressed ion channels and has rapid unbinding Na+-channel blocking action along with promising efficacy for AF conversion to SR. Dronedarone is an amiodarone derivative with an electrophysiological profile similar to its predecessor but lacking most amiodarone-associated adverse effects. Furthermore, dronedarone has shown benefits for important clinical endpoints, including cardiovascular mortality in specific AF populations, the first AF-suppressing drug to do so in prospective randomized clinical trials. Agents that modulate non-ionic current targets (termed 'upstream' therapies) may help to modify the substrate for AF maintenance. Among these, drugs such as angiotensin II type 1 (AT1) receptor antagonists, immunosuppressive agents or HMG-CoA reductase inhibitors (statins) deserve mention. Finally, drugs that block atrial-selective ion-channel targets such as the ultra-rapid delayed rectifier current (IKur) and the acetylcholine-regulated K+-current (IKACh) are presently in development. The introduction of novel antiarrhythmic agents for the management of AF may eventually improve patient outcomes. The potential value of a variety of other novel therapeutic options is currently under active investigation.

Characterization of human cardiac Kv1.5 inhibition by the novel atrial-selective antiarrhythmic compound AVE1231

OBJECTIVE

Atrial-selective drug therapy represents a novel therapeutic approach for atrial fibrillation management. The aim of the present study was to investigate the mechanism of hKv1.5 channel inhibition by the atrial-selective compound AVE1231.

METHODS

Ionic currents were recorded from CHO cells transfected with KCNA5 cDNA with whole-cell patch-clamp technique. The effect of AVE1231 on human atrial cell action potentials was explored with a computer model.

RESULTS

KCNA5 expression resulted in typical K currents that activated and inactivated voltage dependently. Ascending concentrations of AVE1231 (0.1-100 microM) led to concentration- and voltage-dependent current inhibition (IC50 at +40 mV: 2.0 +/- 0.5 microM, Hill coefficient 0.69 +/- 0.12). Acceleration of hKv1.5 current inactivation occurred with increasing AVE1231 concentrations, indicating channel inhibition in the open state (eg, taufast at +40 mV: 318 +/- 92 milliseconds under control; 14 +/- 1 milliseconds with 3 microM, P < 0.05). Using 1/taufast as an approximation of the time course of drug-channel interaction, association rate (K+1) and dissociation rate (K-1) constants were 8.18 x 10 M/s and 45.95 seconds, respectively (KD = 5.62 microM). The onset of current inhibition occurred more rapidly with higher concentrations along with a prominent tail current crossover phenomenon after AVE1231 application. Drug inhibition remained effective through a range of stimulation frequencies. Computer modeling suggested more pronounced prolongation of action potential duration under conditions of atrial remodeling.

CONCLUSION

AVE1231 is an inhibitor of hKv1.5 currents with predominant action on channels in their open state; thus, it may be suitable for the treatment of AF.

Pathology-specific effects of the IKur/Ito/IK,ACh blocker AVE0118 on ion channels in human chronic atrial fibrillation

BACKGROUND AND PURPOSE

This study was designed to establish the pathology-specific inhibitory effects of the IKur/Ito/IK,ACh blocker AVE0118 on atrium-selective channels and its corresponding effects on action potential shape and effective refractory period in patients with chronic AF (cAF).

EXPERIMENTAL APPROACH

Outward K+-currents of right atrial myocytes and action potentials of atrial trabeculae were measured with whole-cell voltage clamp and microelectrode techniques, respectively. Outward currents were dissected by curve fitting.

KEY RESULTS

Four components of outward K+-currents and AF-specific alterations in their properties were identified. Ito was smaller in cAF than in SR, and AVE0118 (10 microM) apparently accelerated its inactivation in both groups without reducing its amplitude. Amplitudes of rapidly and slowly inactivating components of IKur were lower in cAF than in SR. The former was abolished by AVE0118 in both groups, the latter was partially blocked in SR, but not in cAF, even though its inactivation was apparently accelerated in cAF. The large non-inactivating current component was similar in magnitude in both groups, but decreased by AVE0118 only in SR. AVE0118 strongly suppressed AF-related constitutively active IK,ACh and prolonged atrial action potential and effective refractory period exclusively in cAF.

CONCLUSIONS AND IMPLICATIONS

In atrial myocytes of cAF patients, we detected reduced function of distinct IKur components that possessed decreased component-specific sensitivity to AVE0118 most likely as a consequence of AF-induced electrical remodelling. Inhibition of profibrillatory constitutively active IK,ACh may lead to pathology-specific efficacy of AVE0118 that is likely to contribute to its ability to convert AF into SR.

New concepts in understanding and modulating atrial repolarisation in patients with atrial fibrillation

Atrial fibrillation is the most frequent cardiac arrhythmia in clinical practice. Although much has been learned, the underlying mechanisms are incompletely understood. Clinically used antiarrhythmic drugs are limited in their efficacy to terminate atrial fibrillation or to maintain sinus rhythm and were associated with substantial toxicity including life-threatening ventricular arrhythmias. Novel therapeutic approaches suggest targeting of atrium-selective ion channels and pathology-specific alterations in atrial repolarisation and arrhythmogenesis as promising drug targets for patients with atrial fibrillation. This article focuses on novel aspects of altered atrial repolarisation and discusses atrium-selective (I(Kur), I(K,ACh)) and pathology-specific (I(K,ACh)) ion channels as potential targets for safe and effective treatment of atrial fibrillation.

Is there a functional correlate of Kv1.5 in the ventricle of canine heart and what would it mean for the use of I(Kur) blockers?

The cardiac ultrarapid outward current I(Kur), encoded by KCNA5, is of special pharmacological interest, because it is considered to be atrium-specific. I(Kur) has therefore become a target in the therapy of atrial tachyarrhythmias. However, the concept of atrium specificity is only valid if a functional I(Kur) current is in fact absent from the ventricle. However, new work has detected a I(Kur)-like current in canine ventricular myocytes, sensitive to 4-aminopyridine and suppressed by the I(Kur) blocker DPO-1, findings that support the existence of a functional ventricular I(Kur). These indications are, however, indirect and more effort is needed to clarify unequivocally the putative role of an expectedly small I(Kur) component in the ventricle.