QT interval prolongation and the risk of torsades de pointes: essentials for clinicians

Abstract

Objective:

QT interval prolongation signifies an increased risk of the life-threatening arrhythmia torsades de pointes (TdP). The purpose of this paper is to review the diverse methods for assessing and monitoring the risk of TdP, discuss risk factors for TdP, and recommend interventions that may mitigate the risk of TdP.     

Microminipig, a non-rodent experimental animal optimized for life science research: in vivo proarrhythmia models of drug-induced long QT syndrome: development of chronic atrioventricular block model of microminipig

A new in vivo proarrhythmia model of drug-induced long QT syndrome was developed using the Microminipig, an incredibly small minipig established by Fuji Micra Inc. (Shizuoka). The atrioventricular (AV) node of the Microminipig of either sex weighing approximately 6 - 7 kg was ablated under halothane anesthesia, and proper care was taken for them. Proarrhythmic effects of drugs were assessed at >2 months after the onset of AV block using a Holter recording system. Oral administration of dl-sotalol (10 mg/kg) to the AV-block Microminipig prolonged the QT interval; moreover, it frequently induced dangerous ventricular premature beats, whereas no arrhythmia was detected after the vehicle administration (n = 4). Such dl-sotalol-induced ventricular arrhythmias were not detected in the intact Microminipig with sinus rhythm, although significant QT prolongation was observed (n = 4). Thus, the sensitivity and specificity of the AV-block Microminipig for detecting the drug-induced long QT syndrome can be considered to be comparable to previously established AV-block animal models of dogs and monkeys.     

K201, a multi-channel blocker, inhibits clofilium-induced torsades de pointes and attenuates an increase in repolarization

K201 (JTV519) is a 1,4-benzothiazepine derivative that exhibits a strong cardioprotective action and acts as a multiple-channel blocker, including as a K+ channel blocker. An experimental model of prolongation of the QT interval and torsades de pointes can be induced in rabbits by treatment with clofilium in the presence of the alpha1-adrenoreceptor agonist methoxamine. In this study we examined the effects of K201 with and without methoxamine on the QT and QTc intervals, and determined whether K201 inhibits clofilium-induced torsades de pointes in the presence of methoxamine (15 microg/kg/min) in rabbits (n=74). Administration of K201 (0, 40, 100, 200 and 400 microg/kg/min) with and without methoxamine prolonged the QT interval in a dose-dependent manner, and torsades de pointes did not occur in any animals. However, clofilium (50 microg/kg/min) with methoxamine induced torsades de pointes in all animals (6/6). Torsades de pointes occurred at rates of 100%, 67%, 40% and 0% at K201 concentrations of 0, 50, 200 and 400 microg/kg/min, respectively, in the clofilium-infused torsades de pointes model. Therefore, 400 microg/kg/min of K201 completely inhibited clofilium-induced torsades de pointes and attenuated the increase of repolarization caused by clofilium; the inhibitory effects of K201 may be related to its pharmacological properties as an alpha1-adrenoceptor blocker. Overall, our results show that K201 causes prolongation of the QT and QTc intervals, but does not induce torsades de pointes, with and without alpha1-adrenoceptor stimulation. Furthermore, K201 inhibits clofilium-induced torsades de pointes, despite QT prolongation, suggesting that QT prolongation alone is not a proarrhythmic signal.     

Mechanisms of zolpidem-induced long QT syndrome: acute inhibition of recombinant hERG K+ channels and action potential prolongation in human cardiomyocytes derived from induced pluripotent stem cells

Background and Purpose

Zolpidem, a short-acting hypnotic drug prescribed to treat insomnia, has been clinically associated with acquired long QT syndrome (LQTS) and torsade de pointes (TdP) tachyarrhythmia. LQTS is primarily attributed to reduction of cardiac human ether-a-go-go-related gene (hERG)/I_Kr currents. We hypothesized that zolpidem prolongs the cardiac action potential through inhibition of hERG K⁺ channels.

Experimental Approach

Two-electrode voltage clamp and whole-cell patch clamp electrophysiology was used to record hERG currents from Xenopus oocytes and from HEK 293 cells. In addition, hERG protein trafficking was evaluated in HEK 293 cells by Western blot analysis, and action potential duration (APD) was assessed in human-induced pluripotent stem cell (hiPSC)-derived cardiomyocytes.

Key Results

Zolpidem caused acute hERG channel blockade in oocytes (IC₅₀ = 61.5 μM) and in HEK 293 cells (IC₅₀ = 65.5 μM). Mutation of residues Y652 and F656 attenuated hERG inhibition, suggesting drug binding to a receptor site inside the channel pore. Channels were blocked in open and inactivated states in a voltage- and frequency-independent manner. Zolpidem accelerated hERG channel inactivation but did not affect I–V relationships of steady-state activation and inactivation. In contrast to the majority of hERG inhibitors, hERG cell surface trafficking was not impaired by zolpidem. Finally, acute zolpidem exposure resulted in APD prolongation in hiPSC-derived cardiomyocytes.

Conclusions and Implications

Zolpidem inhibits cardiac hERG K⁺ channels. Despite a relatively low affinity of zolpidem to hERG channels, APD prolongation may lead to acquired LQTS and TdP in cases of reduced repolarization reserve or zolpidem overdose.     

QT prolongation in anaesthetized guinea-pigs: an experimental approach for preliminary screening of torsadogenicity of drugs and drug candidates

Many non-cardiovascular drugs can prolong the QT interval of the electrocardiogram (ECG); this is an accessory property not necessary for their pharmacological action and generally linked to the block of the potassium HERG channels and delayed cardiac repolarization. The QT prolongation can lead to a dangerous tachyarrhythmia, called torsade de pointes, and potentially to fatal ventricular fibrillation. The experimental approaches, aimed at an early identification of this undesidered property, often require sophisticated and expensive equipment or the use of superior animal species (dog, primates) that cannot be employed easily for ethical and/or economic reasons.This work aimed to study drug-induced QT prolongation in anaesthetized guinea-pigs and to evaluate the reliability of such an experimental approach to obtain a satisfying predictive parameter of the torsadogenicity of drugs in humans. Seven drugs that were torsadogenic in humans (astemizole, cisapride, haloperidol, quinidine, sotalol, terfenadine and thioridazine) and two that were non-torsadogenic (chlorprotixene and diazepam) were administered i.v. to guinea-pigs under pentobarbital anaesthesia. The ECGs were recorded by four electrodes inserted in the subcutaneous layer of the limbs. Both RR and QT intervals were measured in Leads II and III and then the correct QT values were calculated by Bazett and Fridericia algorithms (QTcB and QTcF, respectively). All the drugs, with the exception of chlorprotixene and diazepam, produced a dose-dependent prolongation of the QT and RR intervals and a significant increase of QTcB and QTcF values. It can be concluded that this method represents a rapid and low-cost procedure to evaluate the cardiac safety pro fi le in the preliminary screening of a high number of drugs or drug candidates.     

In vivo effect of tramadol on locus coeruleus neurons is mediated by alpha2-adrenoceptors and modulated by serotonin

Tramadol is a centrally-acting analgesic endowed with opioid, noradrenergic and serotonergic properties. Various data suggest that, in addition to its analgesic effect, tramadol may have antidepressant and anxiolytic-like effects. This study investigates, through single-unit extracellular recording techniques, the in vivo effects of tramadol on locus coeruleus (LC) neurons and its possible effects on alpha(2)-adrenoceptors, opioid receptors and the 5-HT system. Tramadol produced a dose-dependent and complete inhibition of LC activity (ED(50)=2.1mg/kg). This inhibitory effect was prevented and reversed by the selective alpha(2)-adrenoceptor antagonist, idazoxan, but not by the opioid receptor antagonist, naloxone. The inhibition of the synthesis of 5-HT by p-chlorophenylalanine and the pre-administration of the 5-HT(1A) receptor agonist, 8-OH-DPAT at 40microg/kg, caused a significant potentiation of the tramadol effect decreasing the ED(50) by 53% and 67% respectively. Lower doses of 8-OH-DPAT, of 1 and 4microg/kg, did not significantly modify the tramadol effect. In summary, the results indicate that tramadol elicits an inhibitory effect on LC neurons in vivo through alpha(2)-adrenoceptors. Moreover, this effect is modulated by the 5-HT system and particularly by 5-HT(1A) receptors.     

In vitro models of proarrhythmia

Proarrhythmia models use electrophysiological markers to predict the risk of torsade de pointes (TdP) in patients. The set of variables used by each model to predict the torsadogenic propensity of a drug has been reported to correlate with clinical outcome; however, these reports should be interpreted cautiously as no model has been independently assessed. Each model is discussed along with its merits and shortcomings; none, as yet, having shown a predictive value that makes it clearly superior to the others. As predictive as these models may become, extrapolation of results directly to the clinic must be exercised with caution. The use of in silico models, from subcellular to whole system, is rapidly beginning to form the first line of screening activity in many drug discovery programmes, indicating that biological experimentation may become secondary to analysis by simulation. In vitro proarrhythmia models challenge current perceptions of appropriate surrogates for TdP in man and question existing non-clinical strategies for assessing proarrhythmic risk. The rapid emergence of such models, compounded by the lack of a clear understanding of the key proarrhythmic mechanisms has resulted in a regulatory reluctance to embrace such models. The wider acceptance of proarrhythmia models is likely to occur when there is a clear understanding and agreement on the key proarrhythmia mechanisms. With greater acceptance and ongoing improvements, these models have the potential to unravel the complex mechanisms underlying TdP.     

In vivo comparison of two 5-HT1A receptors agonists alnespirone (S-20499) and buspirone on locus coeruleus neuronal activity

The aim of the present study was to compare, in chloral-hydrate anaesthetized rats, the alpha(2)-adrenergic properties of the selective 5-HT(1A) receptor agonist, alnespirone (S-20499), with those of buspirone, a 5-HT(1A) receptor agonist exhibiting potent alpha(2)-adrenoceptor antagonist properties via its principal metabolite, 1-(2-pyrimidinyl)-piperazine. Both locus coeruleus spontaneous firing activity and noradrenaline release in the medial prefrontal cortex were potently inhibited by the alpha(2)-adrenoceptor agonist clonidine, at a dose of 40 microg/kg (i.p.). Such an inhibition was neither prevented nor reversed by alnespirone (10 mg/kg, i.p.), while buspirone, at the same dose, potently antagonized the locus coeruleus inhibitory effects of clonidine. These data demonstrate that, in contrast with some aryl-piperazine compounds (such as buspirone), alnespirone, either on its own or via a possible metabolite such as buspirone, is devoid in vivo of significant alpha(2)-adrenoceptor antagonist properties.     

Dexrazoxane protects the heart from acute doxorubicin-induced QT prolongation: a key role for IKs

Background and purpose:

The present study tested the hypothesis that selective caspase-3 (C-3) knock-out would regulate the contractile actions of noradrenaline (NA) in the dysfunction of adult rat ventricular myocytes (ARVMs) induced by sepsis. Here, we have studied the contractile response of ARVMs, transfected with C-3 small interfering RNA (C-3 siRNA), to NA.

Experimental approach:

Single ARVMs were isolated from the hearts of male Sprague-Dawley rats 3 days after induction of sepsis, and from sham-treated rats. The sham and septic ARVMs were treated with NA (10 µM) alone or after transfection with C-3 siRNA or non-silencing RNA (2 µM). Mechanical properties were measured digitally, and immunoblotting and immunocytochemical analyses were carried out.

Key results:

The NA-induced increase in peak shortening (PS) was less in septic ARVMs and transfection with C-3 siRNA produced a significant increase in this PS. Immunocytochemical and immunoblot analyses revealed that NA exacerbated sepsis-induced up-regulation of C-3. Transfection of septic ARVMs with C-3 siRNA exhibited a decreased expression of C-3 fluorescence after NA. In septic ARVMs, we also observed a down-regulation of contractile proteins (α-actin, myosin light chain-1 and tropomyosin) along with DNA damage. Transfection of septic ARVMs with C-3 siRNA produced an increase in the expression of contractile proteins, and a decrease in DNA damage.

Conclusions and implications:

These data suggest that C-3 knock-down improved the loss of contractile response to NA in septic ARVMs, suggesting that C-3 regulated contractile dysfunction induced by sepsis in ARVMs.     

Pharmacology of the short QT syndrome N588K-hERG K+ channel mutation: differential impact on selected class I and class III antiarrhythmic drugs

Background and purpose:

The short QT syndrome (SQTS) is associated with cardiac arrhythmias and sudden death. The SQT1 form of SQTS results from an inactivation-attenuated, gain-of-function mutation (N588K) to the human ether-à-go-go-related gene (hERG) potassium channel. Pharmacological blockade of this mutated hERG channel may have therapeutic value. However, hERG-blocking potencies of canonical inhibitors such as E-4031 and D-sotalol are significantly reduced for N588K-hERG. Here, five hERG-blocking drugs were compared to determine their relative potencies for inhibiting N588K channels, and two other inactivation-attenuated mutant channels were tested to investigate the association between impaired inactivation and altered drug potency.

Experimental approach:

Whole-cell patch clamp measurements of hERG current (I_hERG) mediated by wild-type and mutant (N588K, S631A and N588K/S631A) channels were made at 37 °C CHO cells.

Key results:

The N588K mutation attenuated I_hERG inhibition in the following order: E-4031>amiodarone>quinidine>propafenone>disopyramide. Comparing the three inactivation mutants, the two single mutations, although occurring in different modules of the channel, attenuated inactivation to a nearly identical degree, whereas the double mutant caused considerably greater attenuation, permitting the titration of inactivation. Attenuation of channel inhibition was similar between the single mutants for each drug, and was significantly greater with the double mutant.

Conclusions and implications:

The degree of drug inhibition of hERG channels may vary based on the level of channel inactivation. Drugs previously identified as useful for treating SQT1 have the least dependence on hERG inactivation. In addition, our findings indicate that amiodarone may warrant further investigation as a potential treatment for SQT1.     

A rabbit Langendorff heart proarrhythmia model: predictive value for clinical identification of Torsades de Pointes

BACKGROUND AND PURPOSE: The rabbit isolated Langendorff heart model (SCREENIT) was used to investigate the proarrhythmic potential of a range of marketed drugs or drugs intended for market. These data were used to validate the SCREENIT model against clinical outcomes. EXPERIMENTAL APPROACH: Fifty-five drugs, 3 replicates and 2 controls were tested in a blinded manner. Proarrhythmia variables included a 10% change in MAPD(60), triangulation, instability and reverse frequency-dependence of the MAP. Early after-depolarisations, ventricular tachycardia, TdP and ventricular fibrillation were noted. Data are reported at nominal concentrations relative to EFTPC(max). Proarrhythmic scores were assigned to each drug and each drug category. KEY RESULTS: Category 1 and 2 drugs have the highest number of proarrhythmia variables and overt proarrhythmia while Category 5 drugs have the lowest, at every margin. At 30-fold the EFTPC(max), the mean proarrhythmic scores are: Category 1, 101+/-24; Category 2, 101+/-14; Category 3, 72+/-20; Category 4, 59+/-16 and Category 5, 22+/-9 points. Only drugs in Category 5 have mean proarrhythmic scores, below 30-fold, that remain within the Safety Zone. CONCLUSIONS AND IMPLICATIONS: A 30-fold margin between effects and EFTPC(max) is sufficiently stringent to provide confidence to proceed with a new chemical entity, without incurring the risk of eliminating potentially beneficial drugs. The model is particularly useful where compounds have small margins between the hERG IC(50) and predicted EFTPC(max). These data suggest this is a robust and reliable assay that can add value to an integrated QT/TdP risk assessment.     

Relevance of anaesthesia for dofetilide-induced torsades de pointes in alpha1-adrenoceptor-stimulated rabbits

BACKGROUND AND PURPOSE: No information is available concerning the effects of anaesthetics in the most frequently used in vivo pro-arrhythmia model. Accordingly, in this study we examined the effect of pentobarbital, propofol or alpha-chloralose anaesthesia on the pro-arrhythmic activity of the class III anti-arrhythmic dofetilide in alpha(1)-adrenoceptor-stimulated rabbits. EXPERIMENTAL APPROACH: Rabbits anaesthetized intravenously with pentobarbital, propofol or alpha-chloralose were infused simultaneously with the alpha(1)-adrenoceptor agonist phenylephrine (15 microg kg(-1) min(-1), i.v.) and dofetilide (0.04 mg kg(-1) min(-1), i.v.). The electrocardiographic QT interval, the T (peak)-T (end) interval and certain QT variability parameters were measured. The heart rate variability and the baroreflex sensitivity were utilized to assess the vagal nerve activity. The spectral power of the systolic arterial pressure was calculated in the frequency range 0.15-0.5 Hz to assess the sympathetic activity. KEY RESULTS: Pentobarbital considerably reduced, whereas propofol did not significantly affect the incidence of dofetilide-induced torsades de pointes (TdP) as compared with the results with alpha-chloralose (40% (P=0.011) and 70% (P=0.211) vs 100%, respectively). In additional experiments, neither doubling of the rate of the dofetilide infusion nor tripling of the rate of phenylephrine infusion elevated the incidence of TdP to the level seen with alpha-chloralose. None of the repolarization-related parameters predicted TdP. The indices of the parasympathetic and sympathetic activity were significantly depressed in the alpha-chloralose and propofol anaesthesia groups. CONCLUSIONS AND IMPLICATIONS: In rabbits, anaesthetics may affect drug-induced TdP genesis differently, which must be considered when results of different studies are compared.     

Self-augmentation of the lengthening of repolarization is related to the shape of the cardiac action potential: implications for reverse rate dependency

Background and purpose:

The aims of the present work were to study the mechanism of the reverse rate dependency of different interventions prolonging cardiac action potential duration (APD).

Experimental approach:

The reverse rate-dependent lengthening effect of APD-prolonging interventions and the possible involvement of I_Kr (rapid component of the delayed rectifier potassium current) and I_K1 (inward rectifier potassium current) were studied by using the standard microelectrode and the whole-cell patch-clamp techniques in dog multicellular ventricular preparations and in myocytes isolated from undiseased human and dog hearts.

Key results:

All applied drugs – dofetilide (1 µmol·L⁻¹), BaCl₂ (10 µmol·L⁻¹), BAY-K-8644 (1 µmol·L⁻¹), veratrine (1 µg·mL⁻¹) – lengthened APD in a reverse rate-dependent manner regardless of their mode of action, suggesting that reverse rate dependency may not represent a specific mechanism of APD prolongation. The E-4031-sensitive current (I_Kr) and the Ba²⁺-sensitive current (I_K1) were recorded during repolarizing voltage ramps having various steepness and also during action potential waveforms with progressively prolonged APD. Gradually delaying repolarization results in smaller magnitude of I_Kr and I_K1 currents at an isochronal phase of the pulses. This represents a positive feedback mechanism, which appears to contribute to the reverse rate-dependent prolongation of action potentials.

Conclusions and implications:

Action potential configuration may influence the reverse rate-dependent APD prolongation due to the intrinsic properties of I_Kr and I_K1 currents. Drugs lengthening repolarization by decreasing repolarizing outward, or increasing depolarizing inward, currents are expected to cause reverse rate-dependent APD lengthening with high probability, regardless of which current they modify.