Jet D.M. Beekman

Increased Short-Term Variability of Repolarization Predicts d-Sotalol–Induced Torsades de Pointes in Dogs

Background—Identification of patients at risk for drug-induced torsades de pointes arrhythmia (TdP) is difficult. Increased temporal lability of repolarization has been suggested as being valuable to predict proarrhythmia. The predictive value of different repolarization parameters, including beat-to-beat variability of repolarization (BVR), was compared in this serial investigation in dogs with chronic AV block. Methods and Results—In anesthetized dogs with electrically remodeled hearts, the dose-dependent difference in drug-induced TdP (d-sotalol, 2 and 4 mg/kg IV over 5 minutes, 25% and 75% TdP, respectively) could not be accounted for by prolongation of QTc (410±37 to 475±60 versus 415±47 to 484±52 ms, respectively). BVR was quantified by Poincaré plots at baseline and immediately before onset of d-sotalol–induced extrasystolic activity. TdP occurrence was associated with an increase in short-term variability (STV) of the left ventricular monophasic action potential duration (3.5±1.5 to 5.5±1.6 versus 3.0±0.7 to 8.6±3.8 ms, respectively), which was reversible when TdP was abolished by IK,ATP activation. The absence of TdP despite QTc prolongation after chronic amiodarone treatment could also be explained by an unchanged STV. In experiments with isolated ventricular myocytes, STV increased after IKr block and was highest in cells that subsequently showed early afterdepolarizations. Conclusions—Proarrhythmia is not related to differences in prolongation of repolarization but corresponds to BVR, here quantified as STV of the left ventricle. STV could be a new parameter to predict drug-induced TdP in patients.

Probing the Contribution of IKs to Canine Ventricular Repolarization: Key Role for β-Adrenergic Receptor Stimulation

Background—In large mammals and humans, the contribution of IKs to ventricular repolarization is still incompletely understood. Methods and Results—In vivo and cellular electrophysiological experiments were conducted to study IKs in canine ventricular repolarization. In conscious dogs, administration of the selective IKs blocker HMR 1556 (3, 10, or 30 mg/kg PO) caused substantial dose-dependent QT prolongations with broad-based T waves. In isolated ventricular myocytes under baseline conditions, however, IKs block (chromanols HMR 1556 and 293B) did not significantly prolong action potential duration (APD) at fast or slow steady-state pacing rates. This was because of the limited activation of IKs in the voltage and time domains of the AP, although at seconds-long depolarizations, the current was substantial. Isoproterenol increased and accelerated IKs activation to promote APD95 shortening. This shortening was importantly reversed by HMR 1556 and 293B. Quantitatively similar effects were obtained in ventricular-tissue preparations. Finally, when cellular repolarization was impaired by IKr block, IKs block exaggerated repolarization instability with further prolongation of APD. Conclusions—Ventricular repolarization in conscious dogs is importantly dependent on IKs. IKs function becomes prominent during &bgr;-adrenergic receptor stimulation, when it promotes AP shortening by increased activation, and during IKr block, when it limits repolarization instability by time-dependent activation. Unstimulated IKs does not contribute to cellular APD at baseline. These data highlight the importance of the synergism between an intact basal IKs and the sympathetic nervous system in vivo.

Late Na Current Inhibition by Ranolazine Reduces Torsades de Pointes in the Chronic Atrioventricular Block Dog Model

OBJECTIVES This study investigated whether ranolazine reduces dofetilide-induced torsades de pointes (TdP) in a model of long QT syndrome with down-regulated K(+) currents due to hypertrophic remodeling in the dog with chronic atrioventricular block (cAVB). BACKGROUND Ranolazine inhibits the late Na(+) current (I(NaL)) and is effective against arrhythmias in long QT3 syndromes despite its blocking properties of the rapid component of delayed rectifying potassium current. METHODS Ranolazine was administered to cAVB dogs before or after TdP induction with dofetilide and electrophysiological parameters were determined including beat-to-beat variability of repolarization (BVR). In single ventricular myocytes, effects of ranolazine were studied on I(NaL), action potential duration, and dofetilide-induced BVR and early afterdepolarizations. RESULTS After dofetilide, ranolazine reduced the number of TdP episodes from 10 +/- 3 to 3 +/- 1 (p < 0.05) and partially reversed the increase of BVR with no abbreviation of the dofetilide-induced QT prolongation. Likewise, pre-treatment with ranolazine, or using lidocaine as a specific Na(+) channel blocker, attenuated TdP, but failed to prevent dofetilide-induced increases in QT, BVR, and ectopic activity. In cAVB myocytes, ranolazine suppressed dofetilide-induced early afterdepolarizations in 25% of cells at 5 micromol/l, in 75% at 10 micromol/l, and in 100% at 15 micromol/l. At 5 micromol/l, ranolazine blocked 26 +/- 3% of tetrodotoxin-sensitive I(NaL), and 49 +/- 3% at 15 micromol/l. Despite a 54% reduction of I(NaL) amplitude in cAVB compared with control cells, I(NaL) inhibition by 5 micromol/l tetrodotoxin equally shortened relative action potential duration and completely abolished dofetilide-induced early afterdepolarizations. CONCLUSIONS Despite down-regulation of I(NaL) in remodeled cAVB hearts, ranolazine is antiarrhythmic against drug-induced TdP. The antiarrhythmic effects are reflected in concomitant changes of BVR.

The canine model with chronic, complete atrio-ventricular block

Proarrhythmic susceptibility to drug-induced Torsades de Pointes is restricted to individuals with a predisposed phenotype characterized by a reduced repolarization reserve. Additional factors are often involved in a further impairment of repolarization, possibly culminating with dangerous ventricular polymorphic tachyarrhythmias. Drugs that block repolarizing currents represent such an additional hit. The dog model with chronic, complete atrio-ventricular block has been used frequently for proarrhythmic drug screening. The ventricular remodeling seen after ablation of the AV node enhances the susceptibility for repolarization-dependent arrhythmias. In this review, we 1) describe the cellular and molecular basis of ventricular remodeling, 2) validate the CAVB dog as a drug screening model and 3) introduce a new surrogate predictive proarrhythmic parameter: beat-to-beat variability of repolarization.

Characterization of in vitro and in vivo lesions made by a novel multichannel ablation generator and a circumlinear decapolar ablation catheter.

Introduction: The aim of this study is to characterize ablation lesions using varying ratios of bipolar:unipolar energy and to show the feasibility of a circular decapolar pulmonary vein ablation catheter (PVAC™) to create transmural lesions in an in vivo porcine superior vena cava (SVC) model.

Comparison of the IKr blockers moxifloxacin, dofetilide and E‐4031 in five screening models of pro‐arrhythmia reveals lack of specificity of isolated cardiomyocytes

Drug development requires the testing of new chemical entities for adverse effects. For cardiac safety screening, improved assays are urgently needed. Isolated adult cardiomyocytes (CM) and human embryonic stem cell‐derived cardiomyocytes (hESC‐CM) could be used to identify pro‐arrhythmic compounds. In the present study, five assays were employed to investigate their sensitivity and specificity for evaluating the pro‐arrhythmic properties of IKr blockers, using moxifloxacin (safe compound) and dofetilide or E‐4031 (unsafe compounds).

Sudden cardiac death in dogs with remodeled hearts is associated with larger beat–to–beat variability of repolarization

AbstractIncreased proarrhythmia in dogs with chronic AV block (AVB) has been explained by ventricular remodeling causing a decrease in repolarization reserve. Beat–to–beat variability of repolarization (BVR) has been suggested to reflect repolarization reserve, in which high variability represents diminished reserve and larger propensity for repolarization–dependent ventricular arrhythmia. A subset of chronic AVB dogs (10%) suffers sudden cardiac death (SCD). With the assumption that repolarization defects constitute a potentially lethal proarrhythmic substrate, we hypothesized that BVR in SCD dogs are larger than in matched control chronic AVB dogs.From a population of 200 chronic AVB dogs, initially two groups were chosen retrospectively: 8 dogs that died suddenly (SCD) and 8 control dogs. Control dogs had a longer lifespan after AVB (10 to 18 weeks) than SCD dogs (5 to 10 weeks). All dogs had undergone electrophysiological testing under anesthesia where ECG, left and right ventricular endocardial monophasic action potentials (MAP) were recorded. BVR was assessed from 30 consecutive beats, illustrated by Poincaré plots and was the only parameter discriminating between SCD and control group. All other electrophysiological parameters (RR, QT and MAP durations) were comparable for the two groups. Extending the number of animals and groups confirmed a larger BVR in the SCD group (SCD: 5.1 ± 2.7; n = 11 versus control: 2.5 ± 0.4 ms; n = 61; P < 0.05) and showed reverse–use dependence of BVR. In comparison, dogs with acute AVB had low variability (1.3 ± 0.3 ms; n = 9; P < 0.05 versus chronic AVB).Cardiac electrical remodeling after AVB is associated with an increase in beat–to–beat variability of repolarization. Chronic AVB dogs displaying further elevated variability of repolarization are prone to arrhythmia–related SCD.

No proarrhythmic properties of the antibiotics Moxifloxacin or Azithromycin in anaesthetized dogs with chronic-AV block

The therapeutically available quinolone antibiotic moxifloxacin has been used as a positive control for prolonging the QT interval in both clinical and non‐clinical studies designed to assess the potential of new drugs to delay cardiac repolarization. Despite moxifloxacin prolonging QT, it has not been shown to cause torsades de pointes arrhythmias (TdP). Azithromycin is a macrolide antibiotic that has rarely been associated, clinically, with cases of proarrhythmia. As there is a lack of clinical data available, the cardiac safety of these drugs was assessed in a TdP‐susceptible animal model by evaluating their repolarization and proarrhythmia effects.

Beat-to-beat variability of repolarization as a new biomarker for proarrhythmia in vivo

Pharmacological safety evaluation of (pro) drugs includes cardiac safety assessment of proarrhythmic liability in healthy tissue with emphasis on the rapid component of the delayed rectifier (I(Kr)). The lack of (1) an arrhythmic end point, (2) tests in remodeled, predisposed tissue, and (3) testing chronic drug influence on channel trafficking impairs on the drawn conclusions of these assays regarding drug safety. Moreover, the currently used human ether-à-go-go-related gene assays, action potential duration, prolongation in multicellular preparations, or the QT interval have significant shortcomings in their prediction of an increased risk for drug-induced torsades de pointes arrhythmia. In this review, it will be proposed that beat-to-beat variability of repolarization quantified as short-term variability can (1) discriminate between safe and unsafe drugs even under predisposed and highly arrhythmogenic conditions despite accompanying QT prolongation and (2) identify the individual at risk for subsequent arrhythmic events.

Combined Na+/Ca2+ Exchanger and L-Type Calcium Channel Block as a Potential Strategy to Suppress Arrhythmias and Maintain Ventricular Function

Background—L-type calcium channel (LTCC) and Na+/Ca2+ exchanger (NCX) have been implicated in repolarization-dependent arrhythmias, but also modulate calcium and contractility. Although LTCC inhibition is negative inotropic, NCX inhibition has the opposite effect. Combined block may, therefore, offer an advantage for hemodynamics and antiarrhythmic efficiency, particularly in diseased hearts. In a model of proarrhythmia, the dog with chronic atrioventricular block, we investigated whether combined inhibition of NCX and LTCC with SEA-0400 is effective against dofetilide-induced torsade de pointes arrhythmias (TdP), while maintaining calcium homeostasis and hemodynamics. Methods and Results—Left ventricular pressure (LVP) and ECG were monitored during infusion of SEA-0400 and verapamil in anesthetized dogs. Different doses were tested against dofetilide-induced TdP in chronic atrioventricular block dogs. In ventricular myocytes, effects of SEA-0400 were tested on action potentials, calcium transients, and early afterdepolarizations. In cardiomyocytes, SEA-0400 (1 &mgr;mol/L) blocked 66±3% of outward NCX, 50±2% of inward NCX, and 33±9% of LTCC current. SEA-0400 had no effect on systolic calcium, but slowed relaxation, despite action potential shortening, and increased diastolic calcium. SEA-0400 stabilized dofetilide-induced lability of repolarization and suppressed early afterdepolarizations. In vivo, SEA-0400 (0.4 and 0.8 mg/kg) had no effect on left ventricular pressure and suppressed dofetilide-induced TdPs dose dependently. Verapamil (0.3 mg/kg) also inhibited TdP, but caused a 15±8% drop of left ventricular pressure. A lower dose of verapamil without effects on left ventricular pressure (0.06 mg/kg) was not antiarrhythmic. Conclusions—In chronic atrioventricular block dogs, SEA-0400 treatment is effective against TdP. Unlike specific inhibition of LTCC, combined NCX and LTCC inhibition has no negative effects on cardiac hemodynamics.