Search

BackgroundThe QT interval is a phase of the cardiac cycle that corresponds to action potential duration (APD) including cellular repolarization (T-wave). In both clinical and experimental settings, prolongation of the QT interval of the electrocardiogram (ECG) and related proarrhythmia have been so strongly associated that a prolonged QT interval is largely accepted as surrogate marker for proarrhythmia. Accordingly, drugs that prolong the QT interval are not considered for further preclinical development resulting in removal of many promising drugs from development. While reduction of drug interactions with hERG is an important goal, there are promising means to mitigate hERG block. Here, we examine one possibility and test the hypothesis that selective inhibition of the cardiac late Na current (INaL) by the novel compound GS-458967 can suppress proarrhythmic markers.Methods and resultsNew experimental data has been used to calibrate INaL in the Soltis-Saucerman computationally based model of the rabbit ventricular action potential to study effects of GS-458967 on INaL during the rabbit ventricular AP. We have also carried out systematic in silico tests to determine if targeted block of INaL would suppress proarrhythmia markers in ventricular myocytes described by TRIaD: Triangulation, Reverse use dependence, beat-to-beat Instability of action potential duration, and temporal and spatial action potential duration Dispersion.ConclusionsOur computer modeling approach based on experimental data, yields results that suggest that selective inhibition of INaL modifies all TRIaD related parameters arising from acquired Long-QT Syndrome, and thereby reduced arrhythmia risk. This study reveals the potential for adjunctive pharmacotherapy via targeted block of INaL to mitigate proarrhythmia risk for drugs with significant but unintended off-target hERG blocking effects.

Background and Purpose: Enhanced late Na+ current (late INa ) in the myocardium is pro-arrhythmic. Inhibition of this current is a promising strategy to stabilize ventricular repolarization and suppress arrhythmias. Here, we describe GS-6615, a selective inhibitor of late INa , already in clinical development for the treatment of long QT syndrome 3 (LQT3).Experimental Approach: The effects of GS-6615 to inhibit late INa , versus other ion currents to shorten the ventricular action potential duration (APD), monophasic APD (MAPD) and QT interval, and decrease to the incidence of ventricular arrhythmias was determined in rabbit cardiac preparations. To mimic the electrical phenotype of LQT3, late INa was increased using the sea anemone toxin (ATX-II).Key Results: GS-6615 inhibited ATX-II enhanced late INa in ventricular myocytes (IC50  = 0.7 μM), shortened the ATX-II induced prolongation of APD, MAPD, QT interval, and decreased spatiotemporal dispersion of repolarization and ventricular arrhythmias. Inhibition by GS-6615 of ATX-II enhanced late INa was strongly correlated with shortening of myocyte APD and isolated heart MAPD (R2  = 0.94 and 0.98 respectively). In contrast to flecainide, GS-6615 had the minimal effects on peak INa . GS-6615 did not decrease the maximal upstroke velocity of the action potential (Vmax) nor widen QRS intervals.Conclusions and Implications: GS-6615 was a selective inhibitor of late INa , stabilizes the ventricular repolarization and suppresses arrhythmias in a model of LQT3. The concentrations at which the electrophysiological effects of GS-6615 were observed are comparable to plasma levels associated with QTc shortening in patients with LQT3, indicating that these effects are clinically relevant. [ABSTRACT FROM AUTHOR]

The physiological role of cardiac late Na+ current (INa) has not been well described. In this study, we tested the hypothesis that selective inhibition of physiological late INa abbreviates the normal action potential (AP) duration (APD) and counteracts the prolongation of APD and arrhythmic activities caused by inhibition of the delayed rectifier K+ current (IKr). The effects of GS-458967 (GS967) on the physiological late INa and APs in rabbit isolated ventricular myocytes and on the monophasic APs and arrhythmias in rabbit isolated perfused hearts were determined. In ventricular myocytes, GS967 and, for comparison, tetrodotoxin concentration dependently decreased the physiological late INa with IC50 values of 0.5 and 1.9 µM, respectively, and significantly shortened the APD measured at 90% repolarization (APD90). A strong correlation between inhibition of the physiological late INa and shortening of APD by GS967 or tetrodotoxin (R² of 0.96 and 0.97, respectively) was observed. Pretreatment of isolated myocytes or hearts with GS967 (1 µM) significantly shortened APD90 and monophasic APD90 and prevented the prolongation and associated arrhythmias caused by the IKr inhibitor E4031 (1 µM). In conclusion, selective inhibition of physiological late INa shortens the APD, stabilizes ventricular repolarization, and decreases the proarrhythmic potential of pharmacological agents that slow ventricular repolarization. Thus, selective inhibition of late INa may constitute a generalizable approach to stabilize ventricular repolarization and suppress arrhythmogenicity associated with conditions whereby AP or QT intervals are prolonged. [ABSTRACT FROM AUTHOR]

Background: Eleclazine (GS-6615) is a sodium channel blocker designed to improve the selectivity for cardiac late Na+ current (INa) over peak INa.Objectives: The goals of this study were to investigate the inhibition of late INa by eleclazine using a sample of long QT syndrome type 3 (LQT3) and overlap LQT3/Brugada syndrome mutant channels; to compare the apparent binding rates for eleclazine with those for other class 1 antiarrhythmic agents; and to investigate the binding site.Methods: Wild-type human cardiac voltage-gated sodium channel (hNaV1.5) and 21 previously reported variants were studied using patch clamp recordings from a heterologous expression system.Results: Eleclazine inhibited anemone toxin II-enhanced late INa from wild-type hNaV1.5 with a drug concentration that causes 50% block of 0.62 ± 0.12 μM (84-fold selectivity over peak INa). The drug concentration that causes 50% block of eleclazine to inhibit the enhanced late INa from LQT3 mutant channels ranged from 0.33 to 1.7 μM. At predicted therapeutic concentrations, eleclazine and ranolazine inhibited peak INa to a similar degree as assessed with 4 overlap LQT3/Brugada syndrome mutations. Eleclazine was found to interact with hNaV1.5 significantly faster than ranolazine and 6 other class 1 antiarrhythmic agents. Engineered mutations (F1760A/Y1767A) located within the local anesthetic binding site decreased the inhibition of late INa and peak INa by eleclazine.Conclusion: At predicted therapeutic concentrations, eleclazine elicits potent inhibition of late INa across a cohort of NaV1.5 mutant channels. These properties are consistent with a class 1b antiarrhythmic agent that associates with unusually rapid binding/unbinding rates. [ABSTRACT FROM AUTHOR]