Suppression of erythromycin-induced early afterdepolarizations and torsade de pointes ventricular tachycardia by mexiletine

QT Dispersion and Drug-Induced Torsade de Pointes

Background

Amiodarone causes less drug-induced torsade de pointes (TdP) compared to other class III antiarrhythmics. Two theories proposed for this finding include that amiodarone has less repolarization heterogeneity, and/or decreases early after depolarization (EADs). Corrected QT (QTc) dispersion as measured on a surface electrocardiogram (ECG) represents spatial heterogeneity of ventricular repolarization.

Objective

The purpose of this study was to analyze the difference in QT dispersion between amiodarone and other class III antiarrhythmics and to determine the etiology of TdP.

Methods

This was a retrospective, observational study at Montefiore Medical Center between January 2005 and January 2015. Inclusion criteria were adults >18 years on amiodarone, dofetilide, or sotalol with prolonged QT interval on 12-lead ECG. ECGs were reviewed by three blinded observers. QTc was calculated using the Bazett and Framingham formulas. QTc dispersion was calculated by subtracting the shortest from the longest QTc. Analysis of variance (ANOVA) was applied for comparison between antiarrhythmic groups with Bonferroni correction for multiple comparisons.

Results

A total of 447 ECGs were reviewed and 77 ECGs met inclusion criteria. The average QT dispersion for amiodarone, dofetilide, and sotalol was 0.050, 0.037, and 0.034, respectively (p=0.006) and the average QTc dispersion by Bazett was 0.053, 0.038, and 0.037 (p=0.008) and by Framingham was 0.049, 0.036, and 0.035 (p=0.009), respectively.

Conclusion

Our results show that given the increase in QT dispersion seen with amiodarone, heterogeneous ventricular repolarization as measured by QTc dispersion likely does not account for the lower incidence of drug-induced TdP seen with amiodarone. The ability of amiodarone to decrease EADs via sodium-channel blockade is more likely the explanation for its lower incidence of drug-induced TdP.

Nonclinical cardiovascular safety of pitolisant: comparing International Conference on Harmonization S7B and Comprehensive in vitro Pro-arrhythmia Assay initiative studies

BACKGROUND AND PURPOSE

We evaluated the concordance of results from two sets of nonclinical cardiovascular safety studies on pitolisant.

EXPERIMENTAL APPROACH

Nonclinical studies envisaged both in the International Conference on Harmonization (ICH) S7B guideline and Comprehensive in vitro Pro-arrhythmia Assay (CiPA) initiative were undertaken. The CiPA initiative included in vitro ion channels, stem cell-derived human ventricular myocytes, and in silico modelling to simulate human ventricular electrophysiology. ICH S7B-recommended assays included in vitro hERG (KV 11.1) channels, in vivo dog studies with follow-up investigations in rabbit Purkinje fibres and the in vivo Carlsson rabbit pro-arrhythmia model.

KEY RESULTS

Both sets of nonclinical data consistently excluded pitolisant from having clinically relevant QT-liability or pro-arrhythmic potential. CiPA studies revealed pitolisant to have modest calcium channel blocking and late INa reducing activities at high concentrations, which resulted in pitolisant reducing dofetilide-induced early after-depolarizations (EADs) in the ICH S7B studies. Studies in stem cell-derived human cardiomyocytes with dofetilide or E-4031 given alone and in combination with pitolisant confirmed these properties. In silico modelling confirmed that the ion channel effects measured are consistent with results from both the stem cell-derived cardiomyocytes and rabbit Purkinje fibres and categorized pitolisant as a drug with low torsadogenic potential. Results from the two sets of nonclinical studies correlated well with those from two clinical QT studies.

CONCLUSIONS AND IMPLICATIONS

Our findings support the CiPA initiative but suggest that sponsors should consider investigating drug effects on EADs and the use of pro-arrhythmia models when the results from CiPA studies are ambiguous.

Drug-induced proarrhythmia: risk factors and electrophysiological mechanisms

Drug-induced ventricular tachyarrhythmias can be caused by cardiovascular drugs, noncardiovascular drugs, and even nonprescription agents. They can result in arrhythmic emergencies and sudden cardiac death. If a new arrhythmia or aggravation of an existing arrhythmia develops during therapy with a drug at a concentration usually considered not to be toxic, the situation can be defined as proarrhythmia. Various cardiovascular and noncardiovascular drugs can increase the occurrence of polymorphic ventricular tachycardia of the 'torsade de pointes' type. Antiarrhythmic drugs, antimicrobial agents, and antipsychotic and antidepressant drugs are the most important groups. Age, female sex, and structural heart disease are important risk factors for the occurrence of torsade de pointes. Genetic predisposition and individual pharmacodynamic and pharmacokinetic sensitivity also have important roles in the generation of arrhythmias. An increase in spatial or temporal dispersion of repolarization and a triangular action-potential configuration have been identified as crucial predictors of proarrhythmia in experimental models. These studies emphasized that sole consideration of the QT interval is not sufficient to assess the proarrhythmic risk. In this Review, we focus on important triggers of proarrhythmia and the underlying electrophysiological mechanisms that can enhance or prevent the development of torsade de pointes.

Toxicity of macrolide antibiotics on isolated heart mitochondria: a justification for their cardiotoxic adverse effect

1. Macrolides belong to the polyketide class of natural products. These products are a group of drugs (typically antibiotics) which their activity stems from the presence of a macrolide ring. Antibiotic macrolides are used to treat infections caused by Gram-positive bacteria and Haemophilus influenzae infections such as respiratory tract and soft-tissue infections. Macrolides, mainly erythromycin and clarithromycin, rarely show QT prolongation, as their infamous adverse reaction which can lead to torsades de pointes. Electrophysiological studies showed that macrolides prolonging the QT interval inhibit the rapid component of the delayed rectifier K(+) current (IKr) through the block of potassium channels encoded by the human ether-a-go-go-related gene (HERG). Other studies suggest that increased ROS generation alters the kinetics of hERG K(+) conductance. 2. In our study, rat cardiomyocytes were isolated with collagen perfusion technique. Finally, mitochondria isolated from cardiomyocytes were exposed to erythromycin, azithromycin and clarithromycin for their probable toxicity effects. 3. Our results demonstrated that macrolides induced reactive oxygen species formation, mitochondrial membrane permeabilization and mitochondrial swelling and finally cytochrome c release in cardiomyocyte mitochondria. 4. These findings suggested that the toxicity of heart mitochondria is a starting point for cardiotoxic effects of macrolides including QT prolongation, torsades de pointes and arrhythmia.

Optimal antiarrhythmic drug therapy for electrical storm

Electrical storm, defined as 3 or more separate episodes of ventricular tachycardia or ventricular fibrillation within 24 hours, carries significant morbidity and mortality. These unstable ventricular arrhythmias have been described with a variety of conditions including ischemic heart disease, structural heart disease, and genetic conditions. While implantable cardioverter defibrillator implantation and ablation may be indicated and required, antiarrhythmic medication remains an important adjunctive therapy for these persons.

Torsades de pointes induced by antibiotics

BACKGROUND

Several frequently used antibiotics are associated with an arrhythmia called "torsades de pointes" (TdP). This potentially fatal arrhythmia is considered unpredictable.

METHODS

In order to investigate the prevalence of risk factors for TdP prior to initiation of antibiotic therapy, we conducted a literature search for all published reports on TdP induced by antibiotics and we asked pharmaceutical companies for additional unpublished reports.

RESULTS

We studied 61 reports on 78 patients with TdP induced by antibiotics. Female gender was the most common risk factor for TdP: 66.7% (n=52) of all patients were women. Advanced heart disease and concomitant use of a QT interval-prolonging agent or an inhibitor of liver drug metabolism were also frequently present (59% and 48.7%, respectively). Most patients had at least one and 58 patients (74.3%) had two risk factors or more for TdP prior to initiation of antibiotic therapy.

CONCLUSION

Contrary to common belief, TdP induced by antibiotics may be predictable by simple history-taking and by obtaining a baseline electrocardiogram. We wish to draw attention to risk factors for TdP prior to the initiation of antibiotic therapy.

Electrophysiologic properties and antiarrhythmic actions of a novel antianginal agent

Ranolazine is a novel antianginal agent capable of producing anti-ischemic effects at plasma concentrations of 2 to 6 microM without a significant reduction of heart rate or blood pressure. This review summarizes the electrophysiologic properties of ranolazine. Ranolazine significantly blocks I(Kr) (IC(50) = 12 microM), late I(Na), late I(Ca), peak I(Ca), I(Na-Ca) (IC(50) = 5.9, 50, 296, and 91 microM, respectively) and I(Ks) (17% at 30 microM), but causes little or no inhibition of I(to) or I(K1). In left ventricular tissue and wedge preparations, ranolazine produces a concentration-dependent prolongation of action potential duration (APD) in epicardium, but abbreviation of APD of M cells, leading to either no change or a reduction in transmural dispersion of repolarization (TDR). The result is a modest prolongation of the QT interval. Prolongation of APD and QT by ranolazine is fundamentally different from that of other drugs that block I(Kr) and induce torsade de pointes in that APD prolongation is rate-independent (ie, does not display reverse rate-dependent prolongation of APD) and is not associated with early after depolarizations, triggered activity, increased spatial dispersion of repolarization, or polymorphic ventricular tachycardia. Torsade de pointes arrhythmias were not observed spontaneously nor could they be induced with programmed electrical stimulation in the presence of ranolazine at concentrations as high as 100 microM. Indeed, ranolazine was found to possess significant antiarrhythmic activity, acting to suppress the arrhythmogenic effects of other QT-prolonging drugs. Ranolazine produces ion channel effects similar to those observed after chronic exposure to amiodarone (reduced late I(Na), I(Kr), I(Ks), and I(Ca)). Ranolazine's actions to reduce TDR and suppress early after depolarization suggest that in addition to its anti-anginal actions, the drug possesses antiarrhythmic activity.

Effects of mexiletine on the canine model of sparfloxacin-induced long QT syndrome

Potential utility of mexiletine for the treatment of sparfloxacin-induced long QT syndrome was assessed using the in vivo halothane-anesthetized canine model. At 30 min after the administration of a supratherapeutic dose of sparfloxacin (30 mg/kg, i.v.), the mean blood pressure and heart rate decreased, whereas repolarization process and effective refractory period of the ventricular muscle were significantly prolonged. Additional administration of a clinically recommended dose of mexiletine (3 mg/kg, i.v.) at this time point increased the mean blood pressure, suppressed ventricular contraction, delayed atrioventricular as well as intraventricular conduction, and shortened repolarization process and effective refractory period. The extent of abbreviation of the repolarization was more prominent than that of the refractoriness, indicating that mexiletine could decrease the electrical vulnerability of the heart during sparfloxacin overdose. Thus, mexiletine may become a promising pharmacological strategy against the drug-induced long QT syndrome.

Torsade de pointes due to noncardiac drugs: most patients have easily identifiable risk factors

Numerous medications, including drugs prescribed for noncardiac indications, can lead to QT prolongation and trigger torsade de pointes. Although this complication occurs only rarely, it may have lethal consequences. It is therefore important to know if patients with torsade de pointes associated with noncardiac drugs have risk factors that are easy to identify. We reviewed reports of drug-induced torsade de pointes and analyzed each case of torsade de pointes associated with a noncardiac drug for the presence of risk factors for the long QT syndrome that can be easily identified from the medical history or clinical evaluation (female gender, heart disease, electrolyte disturbances, excessive dosing, drug interactions, and history of familial long QT syndrome). We identified 249 patients with torsade de pointes caused by noncardiac drugs. The most commonly identified risk factor was female gender (71%). Other risk factors were frequently present (18%-41%). Virtually all patients had at least 1 of these risk factors, and 71% of patients had 2 or more risk factors. Our study suggests that almost all patients with torsade de pointes secondary to noncardiac drugs have risk factors that can be easily identified from the medical history before the initiation of therapy with the culprit drug.

Divergent proarrhythmic potential of macrolide antibiotics despite similar QT prolongation: fast phase 3 repolarization prevents early afterdepolarizations and torsade de pointes

Macrolide antibiotics are known to have a different proarrhythmic potential in the presence of comparable QT prolongation in the surface ECG. Because the extent of QT prolongation has been used as a surrogate marker for cardiotoxicity, we aimed to study the different electrophysiological effects of the macrolide antibiotics erythromycin, clarithromycin, and azithromycin in a previously developed experimental model of proarrhythmia. In 37 Langendorff-perfused rabbit hearts, erythromycin (150-300 microM, n = 13) clarithromycin (150-300 microM, n = 13), and azithromycin (150-300 microM, n = 11) led to similar increases in QT interval and monophasic action potential (MAP) duration. In bradycardic (atrioventricular-blocked) hearts, eight simultaneously recorded epi- and endocardial MAPs demonstrated increased dispersion of repolarization in the presence of all three antibiotics. Erythromycin and clarithromycin led to early afterdepolarizations (EADs) and torsade de pointes (TdP) after lowering of potassium concentration. In the presence of azithromycin, no EAD or TdP occurred. Erythromycin and clarithromycin changed the MAP configuration to a triangular pattern, whereas azithromycin caused a rectangular pattern of MAP prolongation. In 13 additional hearts, 150 microM azithromycin was administered after previous treatment with 300 microM erythromycin and suppressed TdP provoked by erythromycin. In conclusion, macrolide antibiotics lead to similar prolongation of repolarization but show a different proarrhythmic potential (erythromycin > clarithromycin > azithromycin). In the presence of azithromycin, neither EAD nor TdP occur. This effect may be related to a rectangular pattern of action potential prolongation, whereas erythromycin and clarithromycin cause triangular action potential prolongation and induce TdP.

Limited induction of torsade de pointes by terikalant and erythromycin in an in vivo model

The proarrhythmic activities of the selective I(Kr) blocker erythromycin and the less selective K(+) channel blockers, terikalant and clofilium, have been compared in an alpha(1)-adrenoceptor-stimulated, anaesthetized rabbit model. Terikalant (2.5, 7.5 and 25 nmol kg(-1) min(-1); n = 10), erythromycin (133, 400 and 1330 nmol kg(-1) min(-1); n = 8), clofilium (20, 60 and 200 nmol kg(-1) min(-1); n=10) or vehicle (n = 8) was infused intravenously over 19 min and there was a 15-min interval between each infusion [corrected]. QT and QTc intervals, and epicardial monophasic action potential duration were prolonged significantly (and to a similar extent) only by clofilium and terikalant. The total incidences of torsade de pointes were 60%*, 20%, 0% and 0% in clofilium-, terikalant-, erythromycin- and vehicle-treated animals, respectively (*P < 0.05 compared to vehicle control). In conclusion, terikalant exerted mild proarrhythmic activity though it prolonged repolarisation markedly. Despite being given in high doses, erythromycin neither prolonged repolarisation nor induced proarrhythmia.

Clinical and therapeutic aspects of congenital and acquired long QT syndrome

The long QT syndrome is characterized by prolongation of the corrected QT (QTc) interval on the surface electrocardiogram. It is associated with precipitation of a polymorphic ventricular tachycardia, torsade de pointes, which may cause sudden death. The syndrome is a disorder of cardiac repolarization caused by the alterations in the transmembrane potassium and sodium currents. Six genetic loci for the congenital forms of the syndrome have been identified; sporadic cases occur because of spontaneous mutations. Acquired causes of the long QT syndrome include drugs, electrolyte imbalance, toxins, marked bradycardia, subarachnoid hemorrhage, stroke, myocardial ischemia, protein-sparing fasting, autonomic neuropathy, and human immunodeficiency virus disease. Clinical symptoms are the result of the precipitation of torsade de pointes and range from such minor symptoms as dizziness to syncope and sudden death. Short-term treatment is aimed at preventing the recurrences of torsade de pointes and includes intravenous magnesium and potassium administration, temporary cardiac pacing, and correction of electrolyte imbalance; rarely, intravenous isoproterenol is indicated. Long-term management includes use of beta-blockers, permanent pacemaker placement, and cardioverter-defibrillator implantation. Asymptomatic patients are treated if under the age of 40 years at the time of diagnosis.

Long QT syndrome: diagnosis and management

BACKGROUND

Long QT syndrome (LQT) is characterized by prolongation of the QT interval, causing torsade de pointes and sudden cardiac death. The LQT is a disorder of cardiac repolarization caused by alterations in the transmembrane potassium and sodium currents. Congenital LQT is a disease of transmembrane ion-channel proteins. Six genetic loci of the disease have been identified. Sporadic cases of the disease occur as a result of spontaneous mutations. The acquired causes of LQT include drugs, electrolyte imbalance, marked bradycardia, cocaine, organophosphorus compounds, subarachnoid hemorrhage, myocardial ischemia, protein sparing fasting, autonomic neuropathy, and human immunodeficiency virus disease.

METHODS

Data on the diagnosis and management of LQT were thoroughly reviewed.

RESULTS AND CONCLUSIONS

The diagnosis of LQT primarily rests on clinical and electrocardiographic features and family history. The clinical presentations range from dizziness to syncope and sudden death. Genetic screening is available primarily as a research tool. Short-term treatment of LQT is aimed at preventing the recurrences of torsades and includes intravenous magnesium and potassium administration, temporary cardiac pacing, withdrawal of the offending agent, correction of electrolyte imbalance, and, rarely, intravenous isoproterenol administration. The long-term treatment is aimed at reducing the QT-interval duration and preventing the torsades and sudden death and includes use of oral beta-adrenergic blockers, implantation of permanent pacemaker/cardioverter-defibrillator, and left thoracic sympathectomy. Sodium channel blockers are promising agents under investigation. Electrocardiograms are recorded for screening of family members. The data favor treating asymptomatic patients, if <40 years old at the time of diagnosis, with beta-adrenergic blockers.