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

Multitargeting in cardioprotection: An example of biaromatic compounds

A multitarget drug design approach is actively developing in modern medicinal chemistry and pharmacology, especially with regard to multifactorial diseases such as cardiovascular diseases, cancer, and neurodegenerative diseases. A detailed study of many well-known drugs developed within the single-target approach also often reveals additional mechanisms of their real pharmacological action. One of the multitarget drug design approaches can be the identification of the basic pharmacophore models corresponding to a wide range of the required target ligands. Among such models in the group of cardioprotectors is the linked biaromatic system. This review develops the concept of a "basic pharmacophore" using the biaromatic pharmacophore of cardioprotectors as an example. It presents an analysis of possible biological targets for compounds corresponding to the biaromatic pharmacophore and an analysis of the spectrum of biological targets for the five most known and most studied cardioprotective drugs corresponding to this model, and their involvement in the biological effects of these drugs.

Advances in basic and translational research in atrial fibrillation

Atrial fibrillation (AF) is a very common cardiac arrhythmia with an estimated prevalence of 33.5 million patients globally. It is associated with an increased risk of death, stroke and peripheral embolism. Although genetic studies have identified a growing number of genes associated with AF, the definitive impact of these genetic findings is yet to be established. Several mechanisms, including electrical, structural and neural remodelling of atrial tissue, have been proposed to contribute to the development of AF. Despite over a century of exploration, the molecular and cellular mechanisms underlying AF have not been fully established. Current antiarrhythmic drugs are associated with a significant rate of adverse events and management of AF using ablation is not optimal, especially in cases of persistent AF. This review discusses recent advances in our understanding and management of AF, including new concepts of epidemiology, genetics and pathophysiological mechanisms. We review the current status of antiarrhythmic drug therapy for AF, new potential agents, as well as mechanism-based AF ablation. This article is part of the theme issue 'The heartbeat: its molecular basis and physiological mechanisms'.

Late sodium current and calcium homeostasis in arrhythmogenesis

The cardiac late sodium current (INa,late) is the small sustained component of the sodium current active during the plateau phase of the action potential. Several studies demonstrated that augmentation of the current can lead to cardiac arrhythmias; therefore, INa,late is considered as a promising antiarrhythmic target. Fundamentally, enlarged INa,late increases Na+ influx into the cell, which, in turn, is converted to elevated intracellular Ca2+ concentration through the Na+/Ca2+ exchanger. The excessive Ca2+ load is known to be proarrhythmic. This review describes the behavior of the voltage-gated Na+ channels generating INa,late in health and disease and aims to discuss the physiology and pathophysiology of Na+ and Ca2+ homeostasis in context with the enhanced INa,late demonstrating also the currently accessible antiarrhythmic choices.

Modulation of the effects of class Ib antiarrhythmics on cardiac NaV1.5-encoded channels by accessory NaVβ subunits

Native myocardial voltage-gated sodium (Na_V) channels function in macromolecular complexes comprising a pore-forming (α) subunit and multiple accessory proteins. Here, we investigated the impact of accessory Na_Vβ1 and Na_Vβ3 subunits on the functional effects of 2 well-known class Ib antiarrhythmics, lidocaine and ranolazine, on the predominant Na_V channel α subunit, Na_V1.5, expressed in the mammalian heart. We showed that both drugs stabilized the activated conformation of the voltage sensor of domain-III (DIII-VSD) in Na_V1.5. In the presence of Na_Vβ1, the effect of lidocaine on the DIII-VSD was enhanced, whereas the effect of ranolazine was abolished. Mutating the main class Ib drug-binding site, F1760, affected but did not abolish the modulation of drug block by Na_Vβ1/β3. Recordings from adult mouse ventricular myocytes demonstrated that loss of Scn1b (Na_Vβ1) differentially affected the potencies of lidocaine and ranolazine. In vivo experiments revealed distinct ECG responses to i.p. injection of ranolazine or lidocaine in WT and Scn1b-null animals, suggesting that Na_Vβ1 modulated drug responses at the whole-heart level. In the human heart, we found that SCN1B transcript expression was 3 times higher in the atria than ventricles, differences that could, in combination with inherited or acquired cardiovascular disease, dramatically affect patient response to class Ib antiarrhythmic therapies.

Effects of Ranolazine and its Combination with Amiodarone on Rapid Pacing-induced Reentrant Atrial Tachycardia in Rabbits

Ranolazine (RAN) has previously been shown to lower the onset of cholinergic atrial fibrillation in intact animals; however, its efficacy in the setting of atrial tachycardia (AT) is unknown. The purpose of this study was to investigate the effects of RAN alone or in combination with amiodarone (AMIO) on rapid pacing-evoked right AT in rabbit hearts. Right atrial monophasic action potentials (MAPs) were recorded in 11 anesthetized rabbits, using combination MAP pacing catheters. Vulnerability to AT was tested by employing consecutive trains of rapid burst pacing prior to and after 2.4 mg/kg of RAN alone delivered intravenously and then in combination with 3 mg/kg of AMIO as a 15-minute infusion. Primary endpoints were postdrug AT reproducibility as well as cycle length (CL) and tachycardia duration. MAP duration at 75% repolarization and the effective refractory period (ERP) were assessed during programmed pacing to calculate the atrial postrepolarization refractoriness (aPRR = ERP – MAPD_75%). AT was elicited in eight out of 11 rabbits; only these animals were included for further investigation. RAN did not abolish the inducibility of AT in any experiment; however, it prolonged its CL (baseline vs. RAN: 120 ± 16 ms vs. 138 ± 18 ms; p = 0.053). Supplemental AMIO further increased the AT CL (baseline vs. RAN + AMIO: 120 ± 16 ms vs. 152 ± 23 ms; p = 0.006), without affecting arrhythmia reinducibility. Slowing of the tachycardia after RAN or RAN + AMIO was associated with spontaneous termination of the arrhythmia. RAN prolonged the aPRR significantly, while AMIO in addition to RAN potentiated this effect. Neither RAN alone nor its combination with AMIO abolished the elicitation of AT in this model. However, both agents synergistically prolonged the aPRR, resulting in the slowing of AT and promoting spontaneous termination of the arrhythmia.

Inhibition of voltage-gated Na+ currents by eleclazine in rat atrial and ventricular myocytes

Background

Atrial-ventricular differences in voltage-gated Na⁺ currents might be exploited for atrial-selective antiarrhythmic drug action for the suppression of atrial fibrillation without risk of ventricular tachyarrhythmia. Eleclazine (GS-6615) is a putative antiarrhythmic drug with properties similar to the prototypical atrial-selective Na⁺ channel blocker ranolazine that has been shown to be safe and well tolerated in patients.

Objective

The present study investigated atrial-ventricular differences in the biophysical properties and inhibition by eleclazine of voltage-gated Na⁺ currents.

Methods

The fast and late components of whole-cell voltage-gated Na⁺ currents (respectively, I_Na and I_NaL) were recorded at room temperature (∼22°C) from rat isolated atrial and ventricular myocytes.

Results

Atrial I_Na activated at command potentials ∼5.5 mV more negative and inactivated at conditioning potentials ∼7 mV more negative than ventricular I_Na. There was no difference between atrial and ventricular myocytes in the eleclazine inhibition of I_NaL activated by 3 nM ATX-II (IC₅₀s ∼200 nM). Eleclazine (10 μM) inhibited I_Na in atrial and ventricular myocytes in a use-dependent manner consistent with preferential activated state block. Eleclazine produced voltage-dependent instantaneous inhibition in atrial and ventricular myocytes; it caused a negative shift in voltage of half-maximal inactivation and slowed the recovery of I_Na from inactivation in both cell types.

Conclusions

Differences exist between rat atrial and ventricular myocytes in the biophysical properties of I_Na. The more negative voltage dependence of I_Na activation/inactivation in atrial myocytes underlies differences between the 2 cell types in the voltage dependence of instantaneous inhibition by eleclazine. Eleclazine warrants further investigation as an atrial-selective antiarrhythmic drug.

Late Sodium Current Inhibitors as Potential Antiarrhythmic Agents

Based on recent findings, an increased late sodium current (INa,late) plays an important pathophysiological role in cardiac diseases, including rhythm disorders. The article first describes what is INa,late and how it functions under physiological circumstances. Next, it shows the wide range of cellular mechanisms that can contribute to an increased INa,late in heart diseases, and also discusses how the upregulated INa,late can play a role in the generation of cardiac arrhythmias. The last part of the article is about INa,late inhibiting drugs as potential antiarrhythmic agents, based on experimental and preclinical data as well as in the light of clinical trials.

Mechanisms by Which Ranolazine Terminates Paroxysmal but Not Persistent Atrial Fibrillation

BACKGROUND

Ranolazine inhibits Na⁺ current (I_Na), but whether it can convert atrial fibrillation (AF) to sinus rhythm remains unclear. We investigated antiarrhythmic mechanisms of ranolazine in sheep models of paroxysmal (PxAF) and persistent AF (PsAF).

METHODS

PxAF was maintained during acute stretch (N=8), and PsAF was induced by long-term atrial tachypacing (N=9). Isolated, Langendorff-perfused sheep hearts were optically mapped.

RESULTS

In PxAF ranolazine (10 μmol/L) reduced dominant frequency from 8.3±0.4 to 6.2±0.5 Hz (P<0.01) before converting to sinus rhythm, decreased singularity point density from 0.070±0.007 to 0.039±0.005 cm^-2 s^-1 (P<0.001) in left atrial epicardium (LA_epi), and prolonged AF cycle length (AFCL); rotor duration, tip trajectory, and variance of AFCL were unaltered. In PsAF, ranolazine reduced dominant frequency (8.3±0.5 to 6.5±0.4 Hz; P<0.01), prolonged AFCL, increased the variance of AFCL, had no effect on singularity point density (0.048±0.011 to 0.042±0.016 cm^-2 s^-1; P=ns) and failed to convert AF to sinus rhythm. Doubling the ranolazine concentration (20 μmol/L) or supplementing with dofetilide (1 μmol/L) failed to convert PsAF to sinus rhythm. In computer simulations of rotors, reducing I_Na decreased dominant frequency, increased tip meandering and produced vortex shedding on wave interaction with unexcitable regions.

CONCLUSIONS

PxAF and PsAF respond differently to ranolazine. Cardioversion in the former can be attributed partly to decreased dominant frequency and singularity point density, and prolongation of AFCL. In the latter, increased dispersion of AFCL and likely vortex shedding contributes to rotor formation, compensating for any rotor loss, and may underlie the inefficacy of ranolazine to terminate PsAF.

Amiodarone plus Ranolazine for Conversion of Post-Cardiac Surgery Atrial Fibrillation: Enhanced Effectiveness in Reduced Versus Preserved Ejection Fraction Patients

PURPOSE

Ranolazine (RAN) added to amiodarone (AMIO) has been shown to accelerate termination of postoperative atrial fibrillation (POAF) following coronary artery bypass surgery in patients without heart failure (HF). This study aimed to investigate if treatment efficacy with AMIO or AMIO + RAN differs between patients with concomitant HF with reduced or preserved ejection fraction (HFrEF or HFpEF).

METHODS

Patients with POAF and HFrEF (n = 511, 446 males; 65 ± 9 years) and with HFpEF (n = 301, 257 males; 66 ± 10 years) were enrolled. Onset of AF occurred 2.15 ± 1.0 days after cardiac surgery, and patients within each group were randomly assigned to receive either AMIO monotherapy (300 mg in 30 min + 1125 mg in 36 h iv) or AMIO+RAN combination (500 mg po + 375 mg, after 6 h and 375 mg twice daily thereafter). Primary endpoint was the time to conversion of POAF within 36 h after initiation of treatment.

RESULTS

AMIO restored sinus rhythm earlier in HFrEF vs. in HFpEF patients (24.3 ± 4.6 vs. 26.8 ± 2.8 h, p < 0.0001). AMIO + RAN converted POAF faster than AMIO alone in both HFrEF and HFpEF groups, with conversion times 10.4 ± 4.5 h in HFrEF and 12.2 ± 1.1 h in HFpEF patients (p < 0.0001). Left atrial diameter was significantly greater in HFrEF vs. HFpEF patients (48.2 ± 2.6 vs. 35.2 ± 2.9 mm, p < 0.0001). No serious adverse drug effects were observed during AF or after restoration to sinus rhythm in any of the patients enrolled.

CONCLUSION

AMIO alone or in combination with RAN converted POAF faster in patients with reduced EF than in those with preserved EF. Thus, AMIO + RAN seems to be a valuable alternative treatment for terminating POAF in HFrEF patients.

Effects of the Inhibition of Late Sodium Current by GS967 on Stretch-Induced Changes in Cardiac Electrophysiology

PURPOSE

Mechanical stretch increases sodium and calcium entry into myocytes and activates the late sodium current. GS967, a triazolopyridine derivative, is a sodium channel blocker with preferential effects on the late sodium current. The present study evaluates whether GS967 inhibits or modulates the arrhythmogenic electrophysiological effects of myocardial stretch.

METHODS

Atrial and ventricular refractoriness and ventricular fibrillation modifications induced by acute stretch were studied in Langendorff-perfused rabbit hearts (n = 28) using epicardial multiple electrodes and high-resolution mapping techniques under control conditions and during the perfusion of GS967 at different concentrations (0.03, 0.1, and 0.3 μM).

RESULTS

On comparing ventricular refractoriness, conduction velocity and wavelength obtained before stretch had no significant changes under each GS967 concentration while atrial refractoriness increased under GS967 0.3 μM. Under GS967, the stretch-induced changes were attenuated, and no significant differences were observed between before and during stretch. GS967 0.3 μM diminished the normal stretch-induced changes resulting in longer (less shortened) atrial refractoriness (138 ± 26 ms vs 95 ± 9 ms; p < 0.01), ventricular refractoriness (155 ± 18 ms vs 124 ± 16 ms; p < 0.01) and increments in spectral concentration (23 ± 5% vs 17 ± 2%; p < 0.01), the fifth percentile of ventricular activation intervals (46 ± 8 ms vs 31 ± 3 ms; p < 0.05), and wavelength of ventricular fibrillation (2.5 ±0.5 cm vs 1.7 ± 0.3 cm; p < 0.05) during stretch. The stretch-induced increments in dominant frequency during ventricular fibrillation (control = 38%, 0.03 μM = 33%, 0.1 μM = 33%, 0.3 μM = 14%; p < 0.01) and the stretch-induced increments in arrhythmia complexity index (control = 62%, 0.03μM = 41%, 0.1 μM = 32%, 0.3 μM = 16%; p < 0.05) progressively decreased on increasing the GS967 concentration.

CONCLUSIONS

GS967 attenuates stretch-induced changes in cardiac electrophysiology.

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.

Mechanisms and Drug Development in Atrial Fibrillation

Atrial fibrillation is a highly prevalent cardiac arrhythmia and the most important cause of embolic stroke. Although genetic studies have identified an increasing assembly of AF-related genes, the impact of these genetic discoveries is yet to be realized. In addition, despite more than a century of research and speculation, the molecular and cellular mechanisms underlying AF have not been established, and therapy for AF, particularly persistent AF, remains suboptimal. Current antiarrhythmic drugs are associated with a significant rate of adverse events, particularly proarrhythmia, which may explain why many highly symptomatic AF patients are not receiving any rhythm control therapy. This review focuses on recent advances in AF research, including its epidemiology, genetics, and pathophysiological mechanisms. We then discuss the status of antiarrhythmic drug therapy for AF today, reviewing molecular mechanisms, and the possible clinical use of some of the new atrial-selective antifibrillatory agents, as well as drugs that target atrial remodeling, inflammation and fibrosis, which are being tested as upstream therapies to prevent AF perpetuation. Altogether, the objective is to highlight the magnitude and endemic dimension of AF, which requires a significant effort to develop new and effective antiarrhythmic drugs, but also improve AF prevention and treatment of risk factors that are associated with AF complications.

Synergistic Anti-arrhythmic Effects in Human Atria with Combined Use of Sodium Blockers and Acacetin

Atrial fibrillation (AF) is the most common cardiac arrhythmia. Developing effective and safe anti-AF drugs remains an unmet challenge. Simultaneous block of both atrial-specific ultra-rapid delayed rectifier potassium (K⁺) current (I_Kur) and the Na⁺ current (I_Na) has been hypothesized to be anti-AF, without inducing significant QT prolongation and ventricular side effects. However, the antiarrhythmic advantage of simultaneously blocking these two channels vs. individual block in the setting of AF-induced electrical remodeling remains to be documented. Furthermore, many I_Kur blockers such as acacetin and AVE0118, partially inhibit other K⁺ currents in the atria. Whether this multi-K⁺-block produces greater anti-AF effects compared with selective I_Kur-block has not been fully understood. The aim of this study was to use computer models to (i) assess the impact of multi-K⁺-block as exhibited by many I_Kur blokers, and (ii) evaluate the antiarrhythmic effect of blocking I_Kur and I_Na, either alone or in combination, on atrial and ventricular electrical excitation and recovery in the setting of AF-induced electrical-remodeling. Contemporary mathematical models of human atrial and ventricular cells were modified to incorporate dose-dependent actions of acacetin (a multichannel blocker primarily inhibiting I_Kur while less potently blocking I_to, I_Kr, and I_Ks). Rate- and atrial-selective inhibition of I_Na was also incorporated into the models. These single myocyte models were then incorporated into multicellular two-dimensional (2D) and three-dimensional (3D) anatomical models of the human atria. As expected, application of I_Kur blocker produced pronounced action potential duration (APD) prolongation in atrial myocytes. Furthermore, combined multiple K⁺-channel block that mimicked the effects of acacetin exhibited synergistic APD prolongations. Synergistically anti-AF effects following inhibition of I_Na and combined I_Kur/K⁺-channels were also observed. The attainable maximal AF-selectivity of I_Na inhibition was greatly augmented by blocking I_Kur or multiple K⁺-currents in the atrial myocytes. This enhanced anti-arrhythmic effects of combined block of Na⁺- and K⁺-channels were also seen in 2D and 3D simulations; specially, there was an enhanced efficacy in terminating re-entrant excitation waves, exerting improved antiarrhythmic effects in the human atria as compared to a single-channel block. However, in the human ventricular myocytes and tissue, cellular repolarization and computed QT intervals were modestly affected in the presence of actions of acacetin and I_Na blockers (either alone or in combination). In conclusion, this study demonstrates synergistic antiarrhythmic benefits of combined block of I_Kur and I_Na, as well as those of I_Na and combined multi K⁺-current block of acacetin, without significant alterations of ventricular repolarization and QT intervals. This approach may be a valuable strategy for the treatment of AF.

Atrial-ventricular differences in rabbit cardiac voltage-gated Na+ currents: Basis for atrial-selective block by ranolazine

Background

Class 1 antiarrhythmic drugs are highly effective in restoring and maintaining sinus rhythm in atrial fibrillation patients but carry a risk of ventricular tachyarrhythmia. The antianginal agent ranolazine is a prototypic atrial-selective voltage-gated Na⁺ channel blocker but the mechanisms underlying its atrial-selective action remain unclear.

Objective

The present study examined the mechanisms underlying the atrial-selective action of ranolazine.

Methods

Whole-cell voltage-gated Na⁺ currents (I_Na) were recorded at room temperature (∼22°C) from rabbit isolated left atrial and right ventricular myocytes.

Results

I_Na conductance density was ∼1.8-fold greater in atrial than in ventricular cells. Atrial I_Na was activated at command potentials ∼7 mV more negative and inactivated at conditioning potentials ∼11 mV more negative than ventricular I_Na. The onset of inactivation of I_Na was faster in atrial cells than in ventricular myocytes. Ranolazine (30 μM) inhibited I_Na in atrial and ventricular myocytes in a use-dependent manner consistent with preferential activated/inactivated state block. Ranolazine caused a significantly greater negative shift in voltage of half-maximal inactivation in atrial cells than in ventricular cells, the recovery from inactivation of I_Na was slowed by ranolazine to a greater extent in atrial myocytes than in ventricular cells, and ranolazine produced an instantaneous block that showed marked voltage dependence in atrial cells.

Conclusion

Differences exist between rabbit atrial and ventricular myocytes in the biophysical properties of I_Na. The more negative voltage dependence of I_Na activation and inactivation, together with trapping of the drug in the inactivated channel, underlies an atrial-selective action of ranolazine.

Ranolazine for Congenital Long-QT Syndrome Type III: Experimental and Long-Term Clinical Data

BACKGROUND

The basic defect in long-QT syndrome type III (LQT3) is an excessive inflow of sodium current during phase 3 of the action potential caused by mutations in the SCN5A gene. Most sodium channel blockers reduce the early (peak) and late components of the sodium current (I_Na and I_NaL), but ranolazine preferentially reduces I_NaL. We, therefore, evaluated the effects of ranolazine in LQT3 caused by the D1790G mutation in SCN5A.

METHODS AND RESULTS

We performed an experimental study of ranolazine in TSA201 cells expressing the D1790G mutation. We then performed a long-term clinical evaluation of ranolazine in LQT3 patients carrying the D1790G mutation. In the experimental study, I_NaL was significantly higher in D1790G than in wild-type channels expressed in the TSA201 cells. Ranolazine exerted a concentration-dependent block of I_NaL of the SCN5A-D1790G channel without reducing peak I_Na significantly. In the clinical study, among 8 patients with LQT3 and confirmed D1790G mutation, ranolazine had no effects on the sinus rate or QRS width but shortened the QTc from 509±41 to 451±26 ms, a mean decrease of 56±52 ms (10.6%; P=0.012). The QT-shortening effect of ranolazine remained effective throughout the entire study period of 22.8±12.8 months. Ranolazine reduced the QTc at all heart rates but less so during extreme nocturnal bradycardia. A type I Brugada ECG was never noticed.

CONCLUSIONS

Ranolazine blocks I_NaL in experimental models of LQT3 harboring the SCN5A-D1790G mutation and shortened the QT interval of LQT3 patients.

CLINICAL TRIAL REGISTRATION

URL: https://clinicaltrials.gov; Unique identifier: NCT01728025.

Risk of QT prolongation and torsade de pointes associated with exposure to hydroxyzine: re-evaluation of an established drug

Several noncardiac drugs have been linked to cardiac safety concerns, highlighting the importance of post‐marketing surveillance and continued evaluation of the benefit‐risk of long‐established drugs. Here, we examine the risk of QT prolongation and/or torsade de pointes (TdP) associated with the use of hydroxyzine, a first generation sedating antihistamine. We have used a combined methodological approach to re‐evaluate the cardiac safety profile of hydroxyzine, including: (1) a full review of the sponsor pharmacovigilance safety database to examine real‐world data on the risk of QT prolongation and/or TdP associated with hydroxyzine use and (2) nonclinical electrophysiological studies to examine concentration‐dependent effects of hydroxyzine on a range of human cardiac ion channels. Based on a review of pharmacovigilance data between 14th December 1955 and 1st August 2016, we identified 59 reports of QT prolongation and/or TdP potentially linked to hydroxyzine use. Aside from intentional overdose, all cases involved underlying medical conditions or concomitant medications that constituted at least 1 additional risk factor for such events. The combination of cardiovascular disorders plus concomitant treatment of drugs known to induce arrhythmia was identified as the greatest combined risk factor. Parallel patch‐clamp studies demonstrated hydroxyzine concentration‐dependent inhibition of several human cardiac ion channels, including the ether‐a‐go‐go‐related gene (hERG) potassium ion channels. Results from this analysis support the listing of hydroxyzine as a drug with “conditional risk of TdP” and are in line with recommendations to limit hydroxyzine use in patients with known underlying risk factors for QT prolongation and/or TdP.

Biophysical and molecular comparison of sodium current in cells isolated from canine atria and pulmonary vein

The collar of the pulmonary vein (PV) is the focal point for the initiation of atrial arrhythmias, but the mechanisms underlying how PV cells differ from neighboring left atrial tissue are unclear. We examined the biophysical and molecular properties of I_Na in cells isolated from the canine pulmonary sleeve and compared the properties to left atrial tissue. PV and left atrial myocytes were isolated and patch clamp techniques were used to record I_Na. Action potential recordings from either tissue type were made using high-resistance electrodes. mRNA was determined using quantitative RT-PCR and proteins were determined by Western blot. Analysis of the action potential characteristics showed that PV tissue had a lower Vmax compared with left atrial tissue. Fast I_Na showed that current density was slightly lower in PV cells compared with LA cells (-96 ± 18.7 pA/pF vs. -120 ± 6.7 pA/pF, respectively, p < 0.05). The recovery from inactivation of I_Na in PV cells was slightly slower but no marked difference in steady-state inactivation was noted. Analysis of late I_Na during a 225-ms pulse showed that late I_Na was significantly smaller in PV cells compared to LA cells at all measured time points into the pulse. These results suggest PV cells have lower density of both peak and late I_Na. Molecular analysis of Nav1.5 and the four beta subunits showed lower levels of Nav1.5 as well as Navβ1 subunits, confirming the biophysical findings. These data show that a lower density of I_Na may lead to depression of excitability and predispose the PV collar to re-entrant circuits under pathophysiological conditions.

The Past, Present, and Potential Future of Sodium Channel Block as an Atrial Fibrillation Suppressing Strategy

Despite major advances in arrhythmia therapy, atrial fibrillation (AF) remains a challenge. A significant limitation in AF management is the lack of safe and effective drugs to restore and/or maintain sinus rhythm. The rational design of a new generation of AF-selective Na(+) channel blockers (NCBs) is emerging as a promising AF-suppressing strategy. Recent theoretical and experimental advances have generated insights into the mechanisms underlying AF maintenance and termination by antiarrhythmic drugs. Our understanding of antiarrhythmic drug-induced proarrhythmia has also grown in sophistication. These discoveries have created new possibilities in therapeutic targeting and renewed interest in improved NCB antiarrhythmic drugs. Recently described differences in atrial versus ventricular electrophysiology can be exploited in the prospective design of atrial-selective NCBs. Furthermore, state-dependent block has been shown to be an important modulator of NCB rate selectivity. Together, differential atrial-ventricular electrophysiological actions and state-dependent block form the backbone for the rational design of an AF-selective NCB. Synergistic combinations incorporating both NCB and block of K(+) currents may allow for further enhancement of AF selectivity. Future work on translating these basic research advances into the development of an optimized AF-selective NCB has the potential to provide safer and more effective pharmacotherapeutic options for AF, thereby fulfilling a major unmet clinical need.

Ranolazine vs phenytoin: greater effect of ranolazine on the transient Na(+) current than on the persistent Na(+) current in central neurons

Voltage-gated Na(+) channels (NaV) are involved in pathologies and are important targets of drugs (NaV-blockers), e.g. some anti-epileptic drugs (AEDs). Besides the fast inactivating transient Na(+) current (INaT), they generate a slowly inactivating "persistent" current (INaP). Ranolazine, a NaV-blocker approved for treatment of angina pectoris, is considered a preferential inhibitor of INaP and has been proposed as a novel AED. Although it is thought that classic NaV-blockers used as AEDs target mainly INaT, they can also reduce INaP. It is important to disclose specific features of novel NaV-blockers, which could be necessary for their effect as AEDs in drug resistant patients. We have compared the action of ranolazine and of the classic AED phenytoin in transfected cells expressing the neuronal NaV1.1 Na(+) channel and in neurons of neocortical slices. Our results show that the relative block of INaT versus INaP of ranolazine and phenytoin is variable and depends on Na(+) current activation conditions. Strikingly, ranolazine blocks with less efficacy INaP and more efficacy INaT than phenytoin in conditions mimicking pathological states (i.e. high frequency firing and long lasting depolarizations). The effects are consistent with binding of ranolazine to both open/pre-open and inactivated states; larger INaT block at high stimulation frequencies is caused by the induction of a slow inactivated state. Thus, contrary than expected, ranolazine is not a better INaP blocker than phenytoin in central neurons, and phenytoin is not a better INaT blocker than ranolazine. Nevertheless, they show a complementary action and could differentially target specific pathological dysfunctions.

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.

In silico assessment of drug safety in human heart applied to late sodium current blockers

Drug-induced action potential (AP) prolongation leading to Torsade de Pointes is a major concern for the development of anti-arrhythmic drugs. Nevertheless the development of improved anti-arrhythmic agents, some of which may block different channels, remains an important opportunity. Partial block of the late sodium current (I(NaL)) has emerged as a novel anti-arrhythmic mechanism. It can be effective in the settings of free radical challenge or hypoxia. In addition, this approach can attenuate pro-arrhythmic effects of blocking the rapid delayed rectifying K(+) current (I(Kr)). The main goal of our computational work was to develop an in-silico tool for preclinical anti-arrhythmic drug safety assessment, by illustrating the impact of I(Kr)/I(NaL) ratio of steady-state block of drug candidates on "torsadogenic" biomarkers. The O'Hara et al. AP model for human ventricular myocytes was used. Biomarkers for arrhythmic risk, i.e., AP duration, triangulation, reverse rate-dependence, transmural dispersion of repolarization and electrocardiogram QT intervals, were calculated using single myocyte and one-dimensional strand simulations. Predetermined amounts of block of I(NaL) and I(Kr) were evaluated. "Safety plots" were developed to illustrate the value of the specific biomarker for selected combinations of IC(50)s for I(Kr) and I(NaL) of potential drugs. The reference biomarkers at baseline changed depending on the "drug" specificity for these two ion channel targets. Ranolazine and GS967 (a novel potent inhibitor of I(NaL)) yielded a biomarker data set that is considered safe by standard regulatory criteria. This novel in-silico approach is useful for evaluating pro-arrhythmic potential of drugs and drug candidates in the human ventricle.

Atria are More Sensitive Than Ventricles to GS-458967-Induced Inhibition of Late Sodium Current

INTRODUCTION

The differential response of atrial and ventricular cells to late sodium channel current (late INa) inhibition has not been thoroughly investigated. The aim of the present study was to compare the atrioventricular differences in electrophysiological actions of GS-458967, a potent late INa blocker.

METHODS AND MATERIALS

Canine coronary-perfused atrial and ventricular preparations and isolated ventricular myocytes were used. Transmembrane action potentials were recorded using standard microelectrode recording techniques.

RESULTS

In coronary-perfused preparations paced at a cycle length (CL) of 500 ms, GS-458967 (100-300 nmol/L) significantly abbreviated action potential duration at 50% to 90% (APD50-90) in atria but not in the ventricles. GS-458967 (≥100 nmol/L) prolonged the effective refractory period (ERP) in atria due to the development of postrepolarization refractoriness (PRR) but did not alter ERP in the ventricles. The maximum rate of rise in the action potential upstroke (Vmax) was significantly reduced at concentrations ≥100 nmol/L in atria but not in the ventricles (CL = 300 ms). At slower pacing rates (CL = 2000 ms) and higher concentrations, GS-458967 (100-1000 nmol/L) still failed to abbreviate ventricular APD. However, when APD was prolonged by the rapidly activating delayed rectifier potassium channel blocker E-4031 (1 µmol/L), addition of 1 μmol/L GS-458967 abbreviated APD in the ventricles at slow rates. In contrast, GS-458967 (300 nmol/L) consistently abbreviated APD in untreated isolated ventricular myocytes.

CONCLUSION

In canine coronary-perfused preparations, GS-458967 abbreviates APD, induces PRR, and reduces Vmax in atria but has no significant effect on these parameters in the ventricles, indicating an atrial-selective effect of GS-458967 on both peak and late INa-mediated parameters. In multicellular preparations, GS-458967 abbreviated ventricular APD only under long QT conditions, suggesting a pathology-specific action of GS-458967 in canine ventricular myocardium.

Ranolazine effectively suppresses atrial fibrillation in the setting of heart failure

BACKGROUND

There is a critical need for safer and more effective pharmacological management of atrial fibrillation (AF) in the setting of heart failure (HF).

METHODS AND RESULTS

This study investigates the electrophysiological, antiarrhythmic, and proarrhythmic effects of a clinically relevant concentration of ranolazine (5 μmol/L) in coronary-perfused right atrial and left ventricular preparations isolated from the hearts of HF dogs. HF was induced by ventricular tachypacing (2-6 weeks at 200-240 beats per minute; n=17). Transmembrane action potentials were recorded using standard microelectrode techniques. In atria, ranolazine slightly prolonged action potential duration but significantly depressed sodium channel current-dependent parameters causing a reduction of maximum rate of rise of the action potential upstroke, a prolongation of the effective refractory period secondary to the development of postrepolarization refractoriness, and an increase in diastolic threshold of excitation and atrial conduction time. Ranolazine did not significantly alter these parameters or promote arrhythmias in the ventricles. Ranolazine produced greater inhibition of peak sodium channel current in atrial cells isolated from HF versus normal dogs. A single premature beat reproducibly induced self-terminating AF in 10 of 17 atria. Ranolazine (5 μmol/L) suppressed induction of AF in 7 of 10 (70%) atria. In the remaining 3 atria, ranolazine reduced frequency and duration of AF.

CONCLUSIONS

Our results demonstrate more potent suppression of AF by ranolazine in the setting of HF than previously demonstrated in nonfailing hearts and absence of ventricular proarrhythmia. The data suggest that ranolazine may be of benefit as an alternative to amiodarone and dofetilide in the management of AF in patients with HF.

The Effects of Ranolazine on Paroxysmal Atrial Fibrillation in Patients with Coronary Artery Disease: A Preliminary Observational Study

The impact of ranolazine, an anti-ishemic agent with antiarrhythmic properties, on paroxysmal atrial fibrillation (PAF) in patients with coronary artery disease (CAD) remains unclear. Pacing devices can be useful tools for disclosing even asymptomatic PAF. Purpose of this study is to assess the effect of ranolazine on atrial fibrillation (AF), in patients with CAD, PAF and a dual-chamber pacemaker. We studied 74 patients with CAD, PAF, and sick sinus syndrome or atrio-ventricular block, treated with pacemakers capable to detect PAF episodes. The total time in AF, AF burden, and the number of PAF episodes within the last 6 months before enrolment in the study, mean AF duration per episode, and the QTc interval were initially assessed. Subsequently, patients were randomized into additional treatment with ranolazine (375 mg twice daily) or placebo. Following six months of treatment, all parameters were reassessed and compared to those before treatment. Ranolazine was associated with shorter total AF duration (81.56±45.24 hours versus 68.71±34.84 hours, p=0.002), decreased AF burden (1.89±1.05% versus 1.59±0.81%, p=0.002), and shortened mean AF duration (1.15±0.41 hours versus 0.92±0.35 hours, p=0.01). In the placebo group no such differences were observed. In both groups, no significant differences in the number of PAF episodes and QTc duration were observed. We conclude that in patients with CAD and PAF, ranolazine reduces the total time in AF, AF burden, and mean AF duration. These findings may imply additional antiarrhythmic properties of ranolazine on atrial myocardium and might indicate the necessity of its use in ischemic patients with PAF.

The role of late I Na in development of cardiac arrhythmias

Late I Na is an integral part of the sodium current, which persists long after the fast-inactivating component. The magnitude of the late I Na is relatively small in all species and in all types of cardiomyocytes as compared with the amplitude of the fast sodium current, but it contributes significantly to the shape and duration of the action potential. This late component had been shown to increase in several acquired or congenital conditions, including hypoxia, oxidative stress, and heart failure, or due to mutations in SCN5A, which encodes the α-subunit of the sodium channel, as well as in channel-interacting proteins, including multiple β subunits and anchoring proteins. Patients with enhanced late I Na exhibit the type-3 long QT syndrome (LQT3) characterized by high propensity for the life-threatening ventricular arrhythmias, such as Torsade de Pointes (TdP), as well as for atrial fibrillation. There are several distinct mechanisms of arrhythmogenesis due to abnormal late I Na, including abnormal automaticity, early and delayed after depolarization-induced triggered activity, and dramatic increase of ventricular dispersion of repolarization. Many local anesthetic and antiarrhythmic agents have a higher potency to block late I Na as compared with fast I Na. Several novel compounds, including ranolazine, GS-458967, and F15845, appear to be the most selective inhibitors of cardiac late I Na reported to date. Selective inhibition of late I Na is expected to be an effective strategy for correcting these acquired and congenital channelopathies.

Ranolazine reduces neuronal excitability by interacting with inactivated states of brain sodium channels

Ranolazine is an approved drug for chronic stable angina that acts by suppressing a noninactivating current conducted by the cardiac sodium channel [persistent sodium ion current (INa)]. Ranolazine has also been shown to inhibit the increased persistent INa carried by NaV1.1 channels encoding epilepsy- and migraine-associated mutations. Here, we investigate the antiepileptic properties of ranolazine exhibited through the reduction of hippocampal neuronal excitability. At therapeutically relevant concentrations, ranolazine reduced action potential firing frequency of hippocampal neurons in response to repetitive depolarizing current injections. Similarly, using a single current injection paradigm, ranolazine required a long depolarization (4 seconds) to produce significant inhibition of excitability, which was similar to that observed for the anticonvulsants phenytoin (slowly binds to the fast-inactivated state) and lacosamide (binds to the slow-inactivated state). Ranolazine enhanced the development of fast and slow inactivation assessed with conditioning prepulses of 100, 1000, or 10,000 milliseconds. Recovery of channels from inactivated states was also slowed in the presence of ranolazine. Interestingly, the use-dependent inhibition of hippocampal neurons was dependent on the duration of the voltage step, suggesting ranolazine does not selectively affect the open state and may also interact with inactivated states. NEURON (Yale University, New Haven, CT) computational simulations predict equal inhibition of action potential generation for binding to either fast-inactivated or slow-inactivated states. Binding of ranolazine to either preopen or open states did not affect the excitability of the simulation. Ranolazine was able to significantly reduce the epileptiform activity of the neuronal cultures, suggesting possible antiepileptic activity.

Electrophysiologic characteristics and pharmacologic response of human cardiomyocytes isolated from a patient with hypertrophic cardiomyopathy

BACKGROUND

Hypertrophic cardiomyopathy (HCM) is the most common monogenic cardiac disorder encountered in the clinic. Data relative to the electrophysiologic characteristics and pharmacologic responsiveness of human tissues and cells isolated from patients with HCM are rare. As a consequence, cellular mechanisms underlying arrhythmogenicity are poorly understood.

METHODS

Cardiomyocytes were enzymatically dissociated from a septal myectomy surgically removed from a patient with obstructive HCM. Sharp microelectrodes and patch-clamp techniques were used to evaluate action potential and sodium channel current (INa ) characteristics.

RESULTS

Action potential morphology recorded was typical of an M cell, but with a longer than normal duration (APD) and a relatively steep APD-rate relationship. APD at all rates was significantly reduced following exposure to ranolazine (10 μM). Whole cell patch-clamp recording yielded robust peak INa and large late INa (1.1% of peak INa vs 0.1-0.2% in healthy controls). A large window current was observed as well. Ranolazine (10 μM) shifted steady-state V0.5 of inactivation by -8 mV, reduced late INa by 82%, and significantly diminished the window current.

CONCLUSION

Our results indicate the presence of cells with M-cell characteristics in the septum of the human heart, as has previously been described in the canine heart. They also point to an ameliorative effect of ranolazine to reduce augmented late INa and thus to reduce the prolonged APD in the setting of HCM. These results suggest a potential therapeutic role for ranolazine in HCM.

Late sodium current inhibition in acquired and inherited ventricular (dys)function and arrhythmias

The late sodium current has been increasingly recognized for its mechanistic role in various cardiovascular pathologies, including angina pectoris, myocardial ischemia, atrial fibrillation, heart failure and congenital long QT syndrome. Although relatively small in magnitude, the late sodium current (I(NaL)) represents a functionally relevant contributor to cardiomyocyte (electro)physiology. Many aspects of I(NaL) itself are as yet still unresolved, including its distribution and function in different cell types throughout the heart, and its regulation by sodium channel accessory proteins and intracellular signalling pathways. Its complexity is further increased by a close interrelationship with the peak sodium current and other ion currents, hindering the development of inhibitors with selective and specific properties. Thus, increased knowledge of the intricacies of the complex nature of I(NaL) during distinct cardiovascular conditions and its potential as a pharmacological target is essential. Here, we provide an overview of the functional and electrophysiological effects of late sodium current inhibition on the level of the ventricular myocyte, and its potential cardioprotective and anti-arrhythmic efficacy in the setting of acquired and inherited ventricular dysfunction and arrhythmias.

hERG channel function: beyond long QT

To date, research on the human ether-a-go-go related gene (hERG) has focused on this potassium channel's role in cardiac repolarization and Long QT Syndrome (LQTS). However, growing evidence implicates hERG in a diversity of physiologic and pathological processes. Here we discuss these other functions of hERG, particularly their impact on diseases beyond cardiac arrhythmia.

Mechanisms of ranolazine's dual protection against atrial and ventricular fibrillation

Coronary artery disease and heart failure carry concurrent risk for atrial fibrillation and life-threatening ventricular arrhythmias. We review evidence indicating that at therapeutic concentrations, ranolazine has potential for dual suppression of these arrhythmias. Mechanisms and clinical implications are discussed.

Role of late sodium channel current block in the management of atrial fibrillation

The anti-arrhythmic efficacy of the late sodium channel current (late I(Na)) inhibition has been convincingly demonstrated in the ventricles, particularly under conditions of prolonged ventricular repolarization. The value of late I(Na) block in the setting of atrial fibrillation (AF) remains poorly investigated. All sodium channel blockers inhibit both peak and late I(Na) and are generally more potent in inhibiting late vs. early I(Na). Selective late I(Na) block does not prolong the effective refractory period (ERP), a feature common to practically all anti-AF agents. Although the late I(Na) blocker ranolazine has been shown to be effective in suppression of AF, it is noteworthy that at concentrations at which it blocks late I(Na) in the ventricles, it also potently blocks peak I(Na) in the atria, thus causing rate-dependent prolongation of ERP due to development of post-repolarization refractoriness. Late I(Na) inhibition in atria is thought to suppress intracellular calcium (Ca(i))-mediated triggered activity, secondary to a reduction in intracellular sodium (Na(i)). However, agents that block late I(Na) (ranolazine, amiodarone, vernakalant, etc) are also potent atrial-selective peak I(Na) blockers, so that the reduction of Na(i) loading in atrial cells by these agents can be in large part due to the block of peak I(Na). The impact of late I(Na) inhibition is reduced by the abbreviation of the action potential that occurs in AF patients secondary to electrical remodeling. It stands to reason that selective late I(Na) block may contribute more to inhibition of Ca(i)-mediated triggered activity responsible for initiation of AF in clinical pathologies associated with a prolonged atrial APD (such as long QT syndrome). Additional studies are clearly needed to test this hypothesis.

Atrial-selective prolongation of refractory period with AVE0118 is due principally to inhibition of sodium channel activity

The action of AVE0118 to prolong effective refractory period (ERP) in atria but not in ventricles is thought to be due to its inhibition of IKur. However, in nonremodeled atria, AVE0118 prolongs ERP but not action potential duration (APD70-90), which can be explained with the inhibition of sodium but not potassium channel current. ERP, APD, and the maximum rate of increase of the AP upstroke (Vmax) were measured in the canine-isolated coronary-perfused right atrial and in superfused ventricular tissue preparations. Whole-cell patch-clamp techniques were used to measure sodium channel current in HEK293 cells stably expressing SCN5A. AVE0118 (5-10 μM) prolonged ERP (P < 0.001) but not APD70 and decreased Vmax (by 15%, 10 μM, P < 0.05; n = 10 for each). Ventricular ERP, APD90, and Vmax were not changed significantly by 10 μM AVE0118 (all P = ns; n = 7). AVE0118 effectively suppressed acetylcholine-mediated persistent atrial fibrillation. AVE0118 (10 μM) reduced peak current amplitude of SCN5A-WT current by 36.5% ± 6.6% (P < 0.01; n = 7) and shifted half-inactivation voltage (V0.5) of the steady-state inactivation curve from -89.9 ± 0.5 to -96.0 ± 0.9 mV (P < 0.01; n = 7). Our data suggest that AVE0118-induced prolongation of atrial, but not ventricular ERP, is due largely to atrial-selective depression of sodium channel current, which likely contributes to the effectiveness of AVE0118 to suppress atrial fibrillation.

Comparison of effectiveness of ranolazine plus amiodarone versus amiodarone alone for conversion of recent-onset atrial fibrillation

Ranolazine, an antianginal agent with antiarrhythmic properties, prevents atrial fibrillation (AF) in patients with acute coronary syndrome. In experimental models, the combination of ranolazine and amiodarone has marked synergistic effects that potently suppress AF. Currently, the clinical effect of the ranolazine-amiodarone combination for the conversion of AF is unknown. This prospective randomized pilot study compared the safety and efficacy of ranolazine plus amiodarone versus amiodarone alone for the conversion of recent-onset AF. We enrolled 51 consecutive patients with AF (<48-hour duration) eligible for pharmacologic cardioversion. Patients (33 men, 63 ± 8 years of age) were randomized to intravenous amiodarone for 24 hours (group A, n = 26) or to intravenous amiodarone plus oral ranolazine 1,500 mg at time of randomization (group A + R, n = 25). The 2 groups were well balanced with respect to clinical characteristics and left atrial diameter. Conversion within 24 hours (primary end point) was achieved in 22 patients (88%) in group A + R versus 17 patients (65%) in group A (p = 0.056). Time to conversion was shorter in group A + R than in group A (9.8 ± 4.1 vs 14.6 ± 5.3 hours, p = 0.002). According to Cox regression analysis, left atrial diameter and A + R treatment were the only independent predictors of time to conversion (hazard ratio 5.35, 95% confidence interval 2.37 to 12.11, p <0.001; hazard ratio 0.81, 95% confidence interval 0.74 to 0.88, p <0.001, respectively). There were no proarrhythmic events in either group. In conclusion, addition of ranolazine to standard amiodarone therapy is equally safe and appears to be more effective compared to amiodarone alone for conversion of recent-onset AF.

Ranolazine-Induced Postrepolarization Refractoriness Suppresses Induction of Atrial Flutter and Fibrillation in Anesthetized Rabbits

Ranolazine (Ran) is a novel anti-ischemic agent with electrophysiologic properties mainly attributed to the inhibition of late Na+ current and atrial-selective early Na+ current. However, there are only limited data regarding its efficacy and mechanism of action against atrial flutter (Afl) and atrial fibrillation (AF) in intact animals. Therefore, we aimed to investigate the electrophysiologic mechanism of Ran in a rabbit model of inducible atrial tachyarrhythmias elicited by acetylcholine (ACh). Arrhythmias were produced in 19 rabbits by rapid atrial burst pacing during control, after intravenous ACh and after Ran + ACh administration. Recording of right atrial monophasic action potentials (MAPs) and programmed stimulation were utilized to determine the duration of atrial repolarization at various cycle lengths and voltage levels of action potential, including 75% of total MAP duration (MAPD75), effective refractory period (ERP), and postrepolarization refractoriness (PRR = ERP − MAPD75) prior to and after Ran. Control stimulation yielded no arrhythmias or maximal nonsustained runs of Afl/AF. Upon ACh, 17 of 19 rabbits exhibited sustained Afl and AF as well as mixed forms of Afl/AF, while 2 animals revealed none or short runs of nonsustained arrhythmias and were excluded from the study. High-frequency burst pacing during the first 30 minutes after Ran + ACh failed to induce any arrhythmia in 13 of 17 rabbits (76%), while 2 animals displayed sustained Afl/AF and 2 other animals nonsustained Afl/AF. At basic stimulation cycle length of 250 milliseconds, Ran prolonged baseline atrial ERP (80 ± 8 vs 120 ± 9 milliseconds, P < .001) much more than MAPD75 (65 ± 7 vs 85 ± 7 milliseconds, P < .001), leading to atrial PRR which was more pronounced after Ran compared with control measurements (35 ± 11 vs 15 ± 10 milliseconds, P < .001). This in vivo study demonstrates that Ran exerts antiarrhythmic activity by suppressing inducibility of ACh-mediated Afl/AF in intact rabbits. Its action may predominantly be related to a significant increase in atrial PRR, resulting in depressed electrical excitability and impediment of arrhythmia initiation.

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.

New pharmacological approaches to atrial fibrillation

Atrial fibrillation (AF) is the most common cardiac arrhythmia facing physicians, afflicting 13% of men and 11% of women over 85 years of age. Epidemiological studies estimate that there are ≥ 11 million AF sufferers in the seven major economies and that its prevalence will increase two- to threefold over the next 50 years. Current strategies for treating AF involve either sinus rhythm (SR) maintenance or heart rate control, combined with anticoagulation therapy. Although SR control is the preferred and most effective treatment of AF, none of the SR control drugs currently available are able to maintain rhythm without significant side effects. In this article we discuss some of the recent advancements in developing new antiarrhythmic drugs for AF.