Inhibition of cardiac late sodium current (late I Na ) is a strategy to suppress arrhythmias and sodium-dependent calcium overload associated with myocardial ischemia and heart failure. Current inhibitors of late I Na are unselective and can be proarrhythmic. This study introduces GS967 (6-[4-(trifluoromethoxy), a potent and selective inhibitor of late I Na , and demonstrates its effectiveness to suppress ventricular arrhythmias. The effects of GS967 on rabbit ventricular myocyte ion channel currents and action potentials were determined. Anti-arrhythmic actions of GS967 were characterized in ex vivo and in vivo rabbit models of reduced repolarization reserve and ischemia. GS967 inhibited Anemonia sulcata toxin II (ATX-II)-induced late I Na in ventricular myocytes and isolated hearts with IC 50 values of 0.13 and 0.21 mM, respectively. Reduction of peak I Na by GS967 was minimal at a holding potential of 2120 mV but increased at 280 mV. GS967 did not prolong action potential duration or the QRS interval. GS967 prevented and reversed proarrhythmic effects (afterdepolarizations and torsades de pointes) of the late I Na enhancer ATX-II and the I Kr inhibitor E-4031 in isolated ventricular myocytes and hearts. GS967 significantly attenuated the proarrhythmic effects of methoxamine1clofilium and suppressed ischemiainduced arrhythmias. GS967 was more potent and effective to reduce late I Na and arrhythmias than either flecainide or ranolazine. Results of all studies and assays of binding and activity of GS967 at numerous receptors, transporters, and enzymes indicated that GS967 selectively inhibited late I Na . In summary, GS967 selectively suppressed late I Na and prevented and/or reduced the incidence of experimentally induced arrhythmias in rabbit myocytes and hearts.

Background: Among his major cardiac electrophysiological contributions, Miles Vaughan Williams (1918–2016) provided a classification of antiarrhythmic drugs that remains central to their clinical use. Methods: We survey implications of subsequent discoveries concerning sarcolemmal, sarcoplasmic reticular, and cytosolic biomolecules, developing an expanded but pragmatic classification that encompasses approved and potential antiarrhythmic drugs on this centenary of his birth. Results: We first consider the range of pharmacological targets, tracking these through to cellular electrophysiological effects. We retain the original Vaughan Williams Classes I through IV but subcategorize these divisions in light of more recent developments, including the existence of Na + current components (for Class I), advances in autonomic (often G protein–mediated) signaling (for Class II), K + channel subspecies (for Class III), and novel molecular targets related to Ca 2+ homeostasis (for Class IV). We introduce new classes based on additional targets, including channels involved in automaticity, mechanically sensitive ion channels, connexins controlling electrotonic cell coupling, and molecules underlying longer-term signaling processes affecting structural remodeling. Inclusion of this widened range of targets and their physiological sequelae provides a framework for a modernized classification of established antiarrhythmic drugs based on their pharmacological targets. The revised classification allows for the existence of multiple drug targets/actions and for adverse, sometimes actually proarrhythmic, effects. The new scheme also aids classification of novel drugs under investigation. Conclusions: We emerge with a modernized classification preserving the simplicity of the original Vaughan Williams framework while aiding our understanding and clinical management of cardiac arrhythmic events and facilitating future developments in this area.

The new anti-anginal drug ranolazine causes a slight (<10 milliseconds) prolongation of the QT interval, raising the concern that its use may be associated with an increased incidence of torsades de pointes ventricular tachyarrhythmias. The goal of this study was to show that ranolazine inhibits the late component of INa and attenuates prolongation of action potential duration when late INa is increased, both in the absence and presence of IK-blocking drugs. Currents and action potentials of guinea pig isolated ventricular myocytes were measured by whole-cell patch clamp. Sea anemone toxin (ATX)-II was used to increase late INa and mimic the effect of an SCN5A gene mutation. ATX-II (3-5 nmol/L) increased late INa by 5-fold; ranolazine attenuated this increase of late INa by up to 61 +/- 8%. ATX-II (10-20 nmol/L) increased action potential duration (APD) by > 1 seconds, and caused early afterdepolarizations; both actions were attenuated by ranolazine (0.1-30 micromol/L). Ranolazine (10 micromol/L) reduced by 89% the 13.6-fold increase in variability of APD caused by 10 nmol/L ATX-II. The effects of ATX-II (3 nmol/L) in combinations with either the IKr blocker E-4031 or the IKs blocker chromanol 293B to increase APD were attenuated 76 +/- 5% and 71 +/- 4%, respectively, by 10 micromol/L ranolazine. The results demonstrate that ranolazine reduces late INa and has an anti-arrhythmic effect when late INa is increased.

Prolongation of the QT interval of the ECG is associated with increased risk of torsades de pointes ventricular tachycardia. Ranolazine, a novel antianginal agent, is reported to decrease the delayed rectifier potassium current, I Kr , and to increase action potential duration (APD) and the QT interval. However, ranolazine is also reported to reduce late sodium current (late I Na) , a depolarizing current that contributes to prolongation of the plateau of the ventricular action potential. We hypothesized that ranolazine would decrease APD and the occurrence of arrhythmias when late I Na is increased. Therefore, we measured the effects of ranolazine alone and in the presence of anemone toxin (ATX)-II, whose action mimics the sodium channelopathy associated with long-QT3 syndrome, on epicardial monophasic action potentials and ECGs recorded from guinea pig isolated hearts. Ranolazine (0.1-50 M) prolonged monophasic APD at 90% repolarization (MAPD 90 ) by up to 22% but did not cause either early afterdepolarizations (EADs) or ventricular tachycardia (VT). ATX-II (1-20 nM) markedly increased APD and caused EADs and VT. Ranolazine (5-30 M) significantly attenuated increases in MAPD 90 and reduced episodes of EADs and VT produced by ATX-II. Ranolazine also attenuated the synergistic effect of MAPD 90 increase caused by combinations of ATX-II and blockers of I K [E-4031; 1-[2-(6-methyl-2-pyridyl)ethyl]-4-methylsulfonylaminobenzoyl)piperidine]. Thus, although ranolazine alone prolonged APD, it reduced APD and ventricular arrhythmias caused by agents that increased late I Na and decreased I K .Prolongation of the duration of the ventricular action potential (indicated on the surface electrocardiogram as an increase of the QT interval) due to inhibition of the rapid delayed-rectifier potassium current, I Kr , is caused by drugs from many therapeutic classes and is associated with an increased risk of ventricular tachyarrhythmias such as torsades de pointes (TdP) (Belardinelli et al., 2003). However, QT interval prolongation per se may not be proarrhythmic (Hondeghem et al., 2001). Ranolazine is a novel antianginal drug (Pepine and Wolff, 1999;Louis et al., 2002;Chaitman et al., 2004), and it prolongs the QT interval but is not known to increase the incidence of ventricular tachycardias (VTs) and may reduce the incidence of ischemia-related arrhythmias (Gralinski et al., 1996). At a cellular level, ranolazine has been found to reduce the peak outward current conducted by the delayed rectifier potassium channel I Kr with a potency (IC 50 value) of 11.5 M (Zygmunt et al., 2002) and to reduce the late inward sodium current (late I Na ) with an IC 50 of 5 to 21 M, depending on pacing frequency and membrane potential (Zygmunt et al., 2002;Song et al., 2004).Inhibitions of I Kr and late I Na by ranolazine would be expected to have opposite effects on the duration of the QT interval. Whereas inhibition of I Kr prolongs action potential duration (APD), inhibition of late I Na decreases APD. Thus, the effect of ranolazine o...

Background The reverse rate dependence (RRD) of actions of IKr-blocking drugs to increase action potential duration (APD) and beat-to-beat variability (BVR) of APD is proarrhythmic. Therefore we determined if inhibition of endogenous, physiological late Na+ current (late INa) attenuates the RRD and proarrhythmic effect of IKr inhibition. Methods and Results Duration of the monophasic action potential (MAPD) was measured from female rabbit hearts paced at cycle lengths from 400 to 2000 ms and BVR was calculated. In the absence of drug, MAPD90 and BVR increased as the cycle length was increased from 400 to 2000 ms (n=36 and 26, p<0.01). Both E-4031 (20 nmol/L) and d-sotalol (10 μmol/L) increased MAPD90 and BVR at all stimulation rates and the increase was greater at slower than at faster pacing rates (n=19 and 11, 12 and 7, respectively, p<0.01). TTX (1 μmol/L) significantly attenuated the RRD of MAPD90, reduced BVR, (p<0.01), and abolished torsade de pointes (TdP) in 5 of 6 hearts treated with either 20 nmol/L E-4031 or 10 μmol/L d-sotalol. Endogenous late INa in cardiomyocytes stimulated at cycle lengths from 500 to 4000 ms was greater at slower than at faster stimulation rates, and rapidly decreased during the first several beats at faster but not at slower rates (p<0.01, n=8). In a computational model, simulated RRD of APD caused by E-4031 and d-sotalol was attenuated when late INa was inhibited. Conclusions Endogenous late INa contributes to the RRD of IKr inhibitor-induced increases in APD and BVR and to bradycardia-related ventricular arrhythmias.

Assessment of the proarrhythmic risk associated with drugs that prolong the QT interval is difficult. We hypothesized that the proarrhythmic activities of drugs with very low to moderate risk of causing torsades de pointes would be well differentiated when the late sodium current (I NaL ) was greater than normal. The effects of selected QT-prolonging drugs on electrical activity of female rabbit isolated hearts were determined in the absence and presence of sea anemone toxin (ATX-II; an enhancer of I NaL ). I NaL recorded from ventricular myocytes isolated from female rabbit hearts was slightly increased by 1 and 3 nM ATX-II (n ϭ 13, P Ͻ 0.01). ATX-II (1 nM) prolonged the duration of the monophasic action potential (MAPD 90 ) of the isolated heart by 19 Ϯ 3% (P Ͻ 0.001, n ϭ 31) and shifted the concentration-response relationships for cisapride (1-30 nM), ziprasidone (0.01-3 M), quinidine (0.1-1 M), and moxifloxacin (0.01-1 M) to prolong MAPD 90 to the left by 2-to 12-fold. In contrast, the increases in MAPD 90 caused by 1 nM ATX-II and pentobarbital were only additive, and the increases in MAPD 90 caused by ATX-II and ranolazine [(Ϯ)-N-(2,6-dimethylphenyl)-(4[2-hydroxy-3-(2-methoxyphenoxy)propyl]-1-piperazine] were less than additive. Episodes of arrhythmic activity were commonly observed, and beat-to-beat variability of action potential duration was increased, during exposure of hearts to cisapride, ziprasidone, quinidine, and moxifloxacin but not during exposure of hearts to ranolazine or pentobarbital, in the presence of ATX-II. Thus, in the female rabbit heart, ATX-II potentiated the effects of QT-prolonging drugs to increase MAPD 90 and unmasked the proarrhythmic activities of these drugs at clinically relevant drug concentrations.

BACKGROUND-Quinidine is used to treat atrial fibrillation and ventricular arrhythmias. However, at low concentrations, it can induce torsade de pointes (TdP).

Reduction of repolarization reserve increases the risk of arrhythmia. We hypothesized that inhibition of K ϩ current (IK) to decrease repolarization reserve would unmask the proarrhythmic role of endogenous, physiological late Na ϩ current (late INa). Monophasic action potentials (MAP) and 12-lead electrocardiogram were recorded from female rabbit isolated hearts. To block I K and reduce repolarization reserve, E-4031, 4-aminopyridine, and BaCl 2 were used; to block endogenous late INa, tetrodotoxin (TTX) and ranolazine were used. E-4031 (1-60 nM) concentrationdependently prolonged MAP duration (MAPD 90) and increased duration of the T wave from T peak to Tend (Tpeak-Tend), transmural dispersion of repolarization (TDR), and beat-to-beat variability (BVR) of MAPD 90. E-4031 caused spontaneous and pause-triggered polymorphic ventricular tachycardia [torsade de pointes (TdP)]. In the presence of 60 nM E-4031, TTX (0.6 -3 M) and ranolazine (5-10 M) shortened MAPD 90, decreased TDR, BVR, and Tpeak-Tend (n ϭ 9 -20, P Ͻ 0.01), and abolished episodes of TdP. In hearts treated with BaCl 2 or 4-aminopyridine plus E-4031, TTX (0.6 -3 M) shortened MAPD 90 and decreased T peak-Tend. Ranolazine could not reverse the effect of E-4031 to inhibit human ether-a-go-go-related gene (HERG) K ϩ current; thus, the reversal by ranolazine of effects of E-4031 was likely due to inhibition of late I Na and not to antagonism of the HERG-blocking action of E-4031. We conclude that endogenous, physiological late I Na contributes to arrhythmogenesis in hearts with reduced repolarization reserve. Inhibition of this current partially reverses MAPD prolongation and abolishes arrhythmic activity caused by I K inhibitors. late sodium current; repolarization reserve; ventricular tachycardia; monophasic action potential; ion currents THE PLATEAU AND DURATION OF the cardiac action potential (AP) depends on a delicate balance of multiple ion channel currents, including inward (peak and late Na ϩ and Ca 2ϩ ) and outward [transient outward (I to ), inward-rectifier (I K1 ), and delayedrectifier (I K ) K ϩ ] currents. A decrease in outward or an increase in inward current prolong the duration of the AP (and its electrocardiographic surrogate, the QT interval) and are associated with an increased incidence of polymorphic ventricular tachycardia (torsade de pointes, TdP). (21) The concept of repolarization reserve suggests that there is normally a reserve of repolarizing current to protect against excessive QT interval prolongation and ventricular arrhythmia. (23) Reduction of repolarization reserve has been associated with an increased risk of arrhythmogenesis in patients with inherited long QT syndromes and/or structural heart disease, and in patients treated with QT-prolonging drugs (21-23, 31). Factors that either increase inward currents or decrease outward currents during repolarization of the cardiac AP are associated with reduced repolarization reserve and are arrhythmogenic.A rapid, inward Na ϩ current (I Na ) flowing through voltagegated Na ϩ ch...