New pharmacological strategies for the treatment of atrial fibrillation

Curcumin, a Multi-Ion Channel Blocker That Preferentially Blocks Late Na+ Current and Prevents I/R-Induced Arrhythmias

Increasing evidence shows that Curcumin (Cur) has a protective effect against cardiovascular diseases. However, the role of Cur in the electrophysiology of cardiomyocytes is currently not entirely understood. Therefore, the present study was conducted to investigate the effects of Cur on the action potential and transmembrane ion currents in rabbit ventricular myocytes to explore its antiarrhythmic property. The whole-cell patch clamp was used to record the action potential and ion currents, while the multichannel acquisition and analysis system was used to synchronously record the electrocardiogram and monophasic action potential. The results showed that 30 μmol/L Cur shortened the 50 and 90% repolarization of action potential by 17 and 7%, respectively. In addition, Cur concentration dependently inhibited the Late-sodium current (I _Na.L), Transient-sodium current (I _Na.T), L-type calcium current (I _Ca.L), and Rapidly delayed rectifying potassium current (I _Kr), with IC₅₀ values of 7.53, 398.88, 16.66, and 9.96 μmol/L, respectively. Importantly, the inhibitory effect of Cur on I _Na.L was 52.97-fold higher than that of I _Na.T. Moreover, Cur decreased ATX II-prolonged APD, suppressed the ATX II-induced early afterdepolarization (EAD) and Ca²⁺-induced delayed afterdepolarization (DAD) in ventricular myocytes, and reduced the occurrence and average duration of ventricular tachycardias and ventricular fibrillations induced by ischemia-reperfusion injury. In conclusion, Cur inhibited I _Na.L, I _Na.T, I _Ca.L, and I _Kr; shortened APD; significantly suppressed EAD and DAD-like arrhythmogenic activities at the cellular level; and exhibited antiarrhythmic effect at the organ level. It is first revealed that Cur is a multi-ion channel blocker that preferentially blocks I _Na.L and may have potential antiarrhythmic property.

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.

Vernakalant versus ibutilide for immediate conversion of recent-onset atrial fibrillation

BACKGROUND

The pharmacological cardioversion of recent-onset atrial fibrillation (AF) is a challenge for the clinician. The aim of the study was to compare the efficacy, the safety, and the overall cost of intravenous (iv) administration of vernakalant, which is a relatively new atrial-selective antiarrhythmic agent, versus ibutilide, in cardioversion of recent-onset AF.

METHODS

We enrolled in this study 78 patients (56 men, 22 women; mean age 63.72 ± 6.67 years) who presented with recent-onset AF. Cardioversion was attempted in 36 patients (group A: 24 men, 12 women; mean age 62.44 ± 7.24 years) by iv administration of vernakalant (3 mg/kg over 10 min and if needed after 15 min, a second dose 2 mg/kg over 10 min) while in 42 patients (group B: 32 men, 10 women; mean age 64.81 ± 6 years) iv ibutilide was administered (1 mg over 10 min and if needed after 10 min, a second dose 1 mg over 10 min).

RESULTS

AF was successfully converted in 52.78 % of (n =19) patients of group A vs 52.38 % of (n =22) patients of group B (p =0.58), with an average time of conversion 11.8 ± 4.3 min for group A patients vs 33.9 ± 20.25 min for group B patients (p <0.0001). The average length of hospital stay for patients of group A was 17.64 ± 15.96 hours vs 41.09 ± 17.6 hours for patients of Group B (p <0.0001). In one patient of group A, the administration of vernakalant was discontinued due to hypotension while two other patients reported dysgeusia during their hospitalization. In three patients of group B, the administration of ibutilide was discontinued due to development of nonsustained ventricular tachycardia, which resolved with discontinuation of the drug. The cost of administered drugs was estimated at 488.22 ± 170.34 € for patients of group A vs 142.43 ± 54.45 € for patients of group B (p <0.0001), however, hospitalization costs were significantly lower in patients of group A (258.5 8± 124.73 € over 414.43 ± 100.32; p =0.002).

CONCLUSION

There was no significant difference in the efficiency of converting recent-onset AF between vernakalant and ibutilide. Although vernakalant is an expensive drug, we recorded fewer side effects and more rapid restoration, which reduced the overall cost of hospitalization of these patients. HIPPOKRATIA 2017, 21(2): 67-73.

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.

Amiodarone Versus Propafenone to Treat Atrial Fibrillation after Coronary Artery Bypass Grafting: A Randomized Double Blind Controlled Trial

Background

Atrial fibrillation (AF) is one of the most common complications after cardiac surgery. Several therapeutic and preventive strategies have been introduced for postoperative AF, but the treatment and prophylaxis of AF remain controversial. We aimed to compare the efficacy of intravenous amiodarone and oral propafenone in the treatment of AF after coronary artery bypass grafting (CABG).

Methods

This was a randomized controlled trial performed in two hospitals in Shiraz, Iran from 2009 to 2012. We included all patients who underwent elective CABG and developed AF postoperatively. The patients were randomly assigned to receive propafenone or amiodarone. The duration of AF, the success rate of the treatment, the need for cardioversion, the frequency of repeated AF, and the need for repeating the treatment were compared.

Results

The duration of the first (p=0.361), second (p=0.832), and third (p=0.298) episodes of AF, the need for cardioversion (p=0.998), and the need to repeat the first and second doses of drugs (p=0.557, 0.699) were comparable between the study groups. Repeated AF was observed in 17 patients (30.9%) in the propafenone group and 23 patients (34.3%) in the amiodarone group (p=0.704).

Conclusion

Oral propafenone and intravenous amiodarone are equally effective in the treatment and conversion of recent-onset AF after CABG.

MK-0448, a Specific Kv1.5 Inhibitor

Background—

We evaluated the viability of I Kur as a target for maintenance of sinus rhythm in patients with a history of atrial fibrillation through the testing of MK-0448, a novel I Kur inhibitor.

Methods and Results—

In vitro MK-0448 studies demonstrated strong inhibition of I Kur with minimal off-target activity. In vivo MK-0448 studies in normal anesthetized dogs demonstrated significant prolongation of the atrial refractory period compared with vehicle controls without affecting the ventricular refractory period. In studies of a conscious dog heart failure model, sustained atrial fibrillation was terminated with bolus intravenous MK-0448 doses of 0.03 and 0.1 mg/kg. These data led to a 2-part first-in-human study: Part I evaluated safety and pharmacokinetics, and part II was an invasive electrophysiological study in healthy subjects. MK-0448 was well-tolerated with mild adverse experiences, most commonly irritation at the injection site. During the electrophysiological study, ascending doses of MK-0448 were administered, but no increases in atrial or ventricular refractoriness were detected, despite achieving plasma concentrations in excess of 2 μmol/L. Follow-up studies in normal anesthetized dogs designed to assess the influence of autonomic tone demonstrated that prolongation of atrial refractoriness with MK-0448 was markedly attenuated in the presence of vagal nerve simulation, suggesting that the effects of I Kur blockade on atrial repolarization may be negated by enhanced parasympathetic neural tone.

Conclusions—

The contribution of I Kur to human atrial electrophysiology is less prominent than in preclinical models and therefore is likely to be of limited therapeutic value for the prevention of atrial fibrillation.

In silico optimization of atrial fibrillation-selective sodium channel blocker pharmacodynamics

Atrial fibrillation (AF) is the most common type of clinical arrhythmia. Currently available anti-AF drugs are limited by only moderate efficacy and an unfavorable safety profile. Thus, there is a recognized need for improved antiarrhythmic agents with actions that are selective for the fibrillating atrium. State-dependent Na(+)-channel blockade potentially allows for the development of drugs with maximal actions on fibrillating atrial tissue and minimal actions on ventricular tissue at resting heart rates. In this study, we applied a mathematical model of state-dependent Na(+)-channel blocking (class I antiarrhythmic drug) action, along with mathematical models of canine atrial and ventricular cardiomyocyte action potentials, AF, and ventricular proarrhythmia, to determine the relationship between their pharmacodynamic properties and atrial-selectivity, AF-selectivity (atrial Na(+)-channel block at AF rates versus ventricular block at resting rates), AF-termination effectiveness, and ventricular proarrhythmic properties. We found that drugs that target inactivated channels are AF-selective, whereas drugs that target activated channels are not. The most AF-selective drugs were associated with minimal ventricular proarrhythmic potential and terminated AF in 33% of simulations; slightly fewer AF-selective agents achieved termination rates of 100% with low ventricular proarrhythmic potential. Our results define properties associated with AF-selective actions of class-I antiarrhythmic drugs and support the idea that it may be possible to develop class I antiarrhythmic agents with optimized pharmacodynamic properties for AF treatment.

Mechanisms of atrial-selective block of Na⁺ channels by ranolazine: II. Insights from a mathematical model

Block of Na(+) channel conductance by ranolazine displays marked atrial selectivity that is an order of magnitude higher that of other class I antiarrhythmic drugs. Here, we present a Markovian model of the Na(+) channel gating, which includes activation-inactivation coupling, aimed at elucidating the mechanisms underlying this potent atrial selectivity of ranolazine. The model incorporates experimentally observed differences between atrial and ventricular Na(+) channel gating, including a more negative position of the steady-state inactivation curve in atrial versus ventricular cells. The model assumes that ranolazine requires a hydrophilic access pathway to the channel binding site, which is modulated by both activation and inactivation gates of the channel. Kinetic rate constants were obtained using guarded receptor analysis of the use-dependent block of the fast Na(+) current (I(Na)). The model successfully reproduces all experimentally observed phenomena, including the shift of channel availability, the sensitivity of block to holding or diastolic potential, and the preferential block of slow versus fast I(Na.) Using atrial and ventricular action potential-shaped voltage pulses, the model confirms significantly greater use-dependent block of peak I(Na) in atrial versus ventricular cells. The model highlights the importance of action potential prolongation and of a steeper voltage dependence of the time constant of unbinding of ranolazine from the atrial Na(+) channel in the development of use-dependent I(Na) block. Our model predictions indicate that differences in channel gating properties as well as action potential morphology between atrial and ventricular cells contribute equally to the atrial selectivity of ranolazine. The model indicates that the steep voltage dependence of ranolazine interaction with the Na(+) channel at negative potentials underlies the mechanism of the predominant block of I(Na) in atrial cells by ranolazine.

Mechanisms of atrial-selective block of Na⁺ channels by ranolazine: I. Experimental analysis of the use-dependent block

Atrial-selective inhibition of cardiac Na(+) channel current (I(Na)) and I(Na)-dependent parameters has been shown to contribute to the safe and effective management of atrial fibrillation. The present study examined the basis for the atrial-selective actions of ranolazine. Whole cell I(Na) was recorded at 15°C in canine atrial and ventricular myocytes and in human embryonic kidney (HEK)-293 cells expressing SCN5A. Tonic block was negligible at holding potentials from -140 to -100 mV, suggesting minimal drug interactions with the closed state. Trains of 40 pulses were elicited over a range of holding potentials to determine use-dependent block. Guarded receptor formalism was used to analyze the development of block during pulse trains. Use-dependent block by ranolazine increased at more depolarized holding potentials, consistent with an interaction of the drug with either preopen or inactivated states, but was unaffected by longer pulse durations between 5 and 200 ms, suggesting a weak interaction with the inactivated state. Block was significantly increased at shorter diastolic intervals between 20 and 200 ms. Responses in atrial and ventricular myocytes and in HEK-293 cells displayed a similar pattern. Ranolazine is an open state blocker that unbinds from closed Na(+) channels unusually fast but is trapped in the inactivated state. Kinetic rates of ranolazine interactions with different states of atrial and ventricular Na(+) channels were similar. Our data suggest that the atrial selectivity of ranolazine is due to a more negative steady-state inactivation curve, less negative resting membrane potential, and shorter diastolic intervals in atrial cells compared with ventricular cells at rapid rates.

A randomized active-controlled study comparing the efficacy and safety of vernakalant to amiodarone in recent-onset atrial fibrillation

OBJECTIVES

This randomized double-blind study compared the efficacy and safety of intravenous vernakalant and amiodarone for the acute conversion of recent-onset atrial fibrillation (AF).

BACKGROUND

Intravenous vernakalant has effectively converted recent-onset AF and was well tolerated in placebo-controlled studies.

METHODS

A total of 254 adult patients with AF (3 to 48 h duration) eligible for cardioversion were enrolled in the study. Patients received either a 10-min infusion of vernakalant (3 mg/kg) followed by a 15-min observation period and a second 10-min infusion (2 mg/kg) if still in AF, plus a sham amiodarone infusion, or a 60-min infusion of amiodarone (5 mg/kg) followed by a maintenance infusion (50 mg) over an additional 60 min, plus a sham vernakalant infusion.

RESULTS

Conversion from AF to sinus rhythm within the first 90 min (primary end point) was achieved in 60 of 116 (51.7%) vernakalant patients compared with 6 of 116 (5.2%) amiodarone patients (p < 0.0001). Vernakalant resulted in rapid conversion (median time of 11 min in responders) and was associated with a higher rate of symptom relief compared with amiodarone (53.4% of vernakalant patients reported no AF symptoms at 90 min compared with 32.8% of amiodarone patients; p = 0.0012). Serious adverse events or events leading to discontinuation of study drug were uncommon. There were no cases of torsades de pointes, ventricular fibrillation, or polymorphic or sustained ventricular tachycardia.

CONCLUSIONS

Vernakalant demonstrated efficacy superior to amiodarone for acute conversion of recent-onset AF. Both vernakalant and amiodarone were safe and well tolerated in this study. (A Phase III Superiority Study of Vernakalant vs Amiodarone in Subjects With Recent Onset Atrial Fibrillation [AVRO]; NCT00668759).

AZD1305 exerts atrial predominant electrophysiological actions and is effective in suppressing atrial fibrillation and preventing its reinduction in the dog

Recent development of drugs for the treatment of atrial fibrillation (AF) has focused on atrial selective agents. We examined the atrioventricular differences in sodium channel block of the antiarrhythmic agent AZD1305 in atria and ventricles of anesthetized dogs in vivo, canine isolated arterially perfused preparations in vitro, and isolated myocytes using whole-cell patch-clamp techniques. AZD1305 did not change heart rate or blood pressure in vivo but prolonged action potential duration and increased effective refractory period, diastolic threshold of excitation, and conduction time preferentially in atria both in vitro and in vivo. AZD1305 reduced the maximum rate of rise of the action potential upstroke (V(max)) predominantly in atria (-51% +/- 10% in atria vs. -31% +/- 23% in ventricles; 3 microM; cycle length = 500 milliseconds). Fast sodium current (I(Na)) was blocked by AZD1305 to a greater degree in atrial versus ventricular myocytes (particularly tonic inhibition). In coronary-perfused right atria, AZD1305 very effectively prevented induction of persistent acetylcholine-mediated AF and, in a different set of atria, terminated persistent AF (in 5 of 5 and 7 of 8 atria, respectively). In conclusion, AZD1305 exerts atrial predominant sodium channel-blocking effects in vitro and in vivo and effectively suppresses AF.

New pharmacological targets and treatments for atrial fibrillation

Atrial fibrillation (AF) is an arrhythmia of growing clinical concern that is increasing in prevalence and is associated with significant morbidity and mortality. Pharmacological agents remain the first-line therapy for the AF patient, and the potential advantages of sinus rhythm maintenance motivate continued efforts to identify novel pharmacological means to restore and maintain sinus rhythm. Traditional antiarrhythmic agents only moderately suppress AF and present problematic concerns of proarrhythmia and extracardiac toxicity. Current investigational or recently approved strategies for improving efficacy and safety of anti-AF agents include (i) specific or predominant blockade of atrial ion channels; (ii) "upstream therapies" affecting non-ion channel targets that influence electrical and structural remodeling, inflammation and oxidative stress; (iii) amiodarone derivatives with an improved safety profile; (iv) intracellular calcium handling; and (v) therapies aiming at alleviating conduction disturbances (gap junction coupling enhancers). This review provides a succinct overview of some of these strategies.

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.

Atrial-selective sodium channel block as a novel strategy for the management of atrial fibrillation

Safe and effective pharmacologic management of atrial fibrillation (AF) is one of the greatest challenges facing an aging society. Currently available pharmacologic strategies for rhythm control of AF are associated with ventricular arrhythmias and in some cases multi-organ toxicity. Consequently, drug development has focused on atrial-selective agents such as IKur blockers. Recent studies suggest that IKur block alone may be ineffective for suppression of AF and may promote AF in healthy hearts. Recent experimental studies have demonstrated other important electrophysiologic differences between atrial and ventricular cells, particularly with respect to sodium channel function, and have identified sodium channel blockers that exploit these electrophysiologic distinctions. Atrial-selective sodium channel blockers, such as ranolazine and amiodarone, effectively suppress and/or prevent the induction of AF in experimental models, while producing little to no effect on ventricular myocardium. These findings suggest that atrial-selective sodium channel block may be a fruitful new strategy for the management of AF.

Cellular basis for atrial fibrillation in an experimental model of short QT1: implications for a pharmacological approach to therapy

BACKGROUND

Short QT (SQT) syndrome (SQT) 1 is an inherited sudden death syndrome often associated with atrial fibrillation (AF). We examined the cellular basis for AF in a newly developed experimental atrial model of SQT1.

METHODS

Action potentials (APs) were recorded from the pectinate muscle (PM) and crista terminalis (CT) regions of coronary-perfused canine right atrial preparations, together with a pseudoelectrocardiogram. The I(Kr) agonist PD-118057 (20 microM) was used to mimic the gain of function in I(Kr) known to underlie SQT1.

RESULTS

The I(Kr) agonist significantly abbreviated the AP duration (APD) of CT and PM and of the effective refractory period (ERP) measured in PM (n = 28). Spatial dispersion of repolarization (SDR), defined as inter-regional differences of APD, increased from 27 +/- 17 ms to 51 +/- 32 ms (P = .002; n = 28). AF could be induced by a single premature stimulus after but not before exposure to PD-118057 in 26/28 (93%) preparations. Quinidine (10 microM), which prolonged APD and ERP, but not lidocaine (20 microM) or E-4031 (5 microM), which prolonged only ERP or APD, respectively, was effective in preventing the PD-118057-mediated AF. In the presence of PD-118057, isoproterenol (100 nM) further abbreviated both APD and ERP and facilitated induction of sustained AF in five of six preparations.

CONCLUSIONS

The I(Kr) agonist recapitulates the electrophysiologic and arrhythmic manifestations of SQT1. Abbreviation of APD and ERP and amplification of SDR predispose to the development of AF by creating the substrate for reentry. Quinidine, but not E-4031 or lidocaine, was effective in preventing AF in this setting.

Atrial-selective sodium channel block for the treatment of atrial fibrillation

The pharmacological approach to therapy of atrial fibrillation (AF) is often associated with adverse effects resulting in the development of ventricular arrhythmias. As a consequence, much of the focus in recent years has been on development of atrial-selective agents. Atrial-selective sodium channel blockers have recently been shown to exist and be useful in the management of AF. This review summarizes the available data relative to current therapies, focusing on our understanding of the actions of atrial selective sodium channel blockers in suppressing and preventing the induction of AF and electrophysiological properties that confer atrial-selectivity to these antifibrillatory drugs.

Atrial-selective sodium channel block as a novel strategy for the management of atrial fibrillation

Pharmacological management of atrial fibrillation (AF) remains an important unmet medical need. Because available drugs for rhythm control of AF are often associated with a significant risk for development of ventricular arrhythmias or extracardiac toxicity, recent drug development has focused on agents that are atrial selective. Inhibition of the ultrarapid delayed rectifier potassium current (I(Kur)), a current exclusive to atria, is an example of an atrial-selective approach. Recent studies, however, have shown that loss-of-function mutations in KCNA5, the gene that encodes K(V)1.5, the alpha subunit of the I(Kur) channel, is associated with the development of AF and that inhibition of I(Kur) can promote the induction of AF in experimental models. Another potential atrial-selective approach has recently been identified. Experimental studies have demonstrated important atrioventricular differences in the biophysical properties of the sodium channel and have identified sodium channel blockers that can exploit electrophysiological distinctions between atria and ventricles. Atrial-selective/predominant sodium channel blockers such as ranolazine effectively suppress AF in experimental models involving canine-isolated right atrial preparations at concentrations that produce little to no effect on electrophysiological parameters in ventricular myocardium. Chronic administration of amiodarone was also found to exert atrial-selective depression of I(Na)-dependent parameters and thus to prevent the induction of AF. Ranolazine and amiodarone have in common the ability to rapidly dissociate from the sodium channel and to prolong the atrial action potential duration via inhibition of I(Kr). Our observations suggest that atrial-selective sodium channel block may be a fruitful strategy for the management of AF.