Assessment of the pro-arrhythmic potential of anti-arrhythmic drugs: an experimental approach

Transgenic Rabbit Models in Proarrhythmia Research

Drug-induced proarrhythmia constitutes a potentially lethal side effect of various drugs. Most often, this proarrhythmia is mechanistically linked to the drug's potential to interact with repolarizing cardiac ion channels causing a prolongation of the QT interval in the ECG. Despite sophisticated screening approaches during drug development, reliable prediction of proarrhythmia remains very challenging. Although drug-induced long-QT-related proarrhythmia is often favored by conditions or diseases that impair the individual's repolarization reserve, most cellular, tissue, and whole animal model systems used for drug safety screening are based on normal, healthy models. In recent years, several transgenic rabbit models for different types of long QT syndromes (LQTS) with differences in the extent of impairment in repolarization reserve have been generated. These might be useful for screening/prediction of a drug's potential for long-QT-related proarrhythmia, particularly as different repolarizing cardiac ion channels are impaired in the different models. In this review, we summarize the electrophysiological characteristics of the available transgenic LQTS rabbit models, and the pharmacological proof-of-principle studies that have been performed with these models—highlighting the advantages and disadvantages of LQTS models for proarrhythmia research. In the end, we give an outlook on potential future directions and novel models.

Rabbit models as tools for preclinical cardiac electrophysiological safety testing: Importance of repolarization reserve

It is essential to more reliably assess the pro-arrhythmic liability of compounds in development. Current guidelines for pre-clinical and clinical testing of drug candidates advocate the use of healthy animals/tissues and healthy individuals and focus on the test compound's ability to block the hERG current and prolong cardiac ventricular repolarization. Also, pre-clinical safety tests utilize several species commonly used in cardiac electrophysiological studies. In this review, important species differences in cardiac ventricular repolarizing ion currents are considered, followed by the discussion on electrical remodeling associated with chronic cardiovascular diseases that leads to altered ion channel and transporter expression and densities in pathological settings. We argue that the choice of species strongly influences experimental outcome and extrapolation of results to human clinical settings. We suggest that based on cardiac cellular electrophysiology, the rabbit is a useful species for pharmacological pro-arrhythmic investigations. In addition to healthy animals and tissues, the use of animal models (e.g. those with impaired repolarization reserve) is suggested that more closely resemble subsets of patients exhibiting increased vulnerability towards the development of ventricular arrhythmias and sudden cardiac death.

The electromechanical window is no better than QT prolongation to assess risk of Torsade de Pointes in the complete atrioventricular block model in dogs

BACKGROUND AND PURPOSE

The electromechanical window (EMW), the interval between the end of the T-wave and the end of the left ventricular pressure (LVP) curve, has recently been proposed as a predictor of risk of Torsade de Pointes (TdP) in healthy animals, whereby a negative EMW (mechanical relaxation earlier than repolarization) after drug administration indicates an increased TdP risk. The aims of this study were to assess (i) the effect of the ventricular remodelling in the canine chronic, complete atrioventricular block (CAVB) model on EMW; (ii) the effect of the I(Kr) -blocker dofetilide on EMW; and (iii) the correlation of EMW with TdP inducibility.

EXPERIMENTAL APPROACH

Our 11 year database of experiments of CAVB in dogs under general anaesthesia was reviewed and experiments included if ECG and LVP were recorded simultaneously at spontaneous rhythm. In total, 89 experiments in 44 dogs were appropriate and were analysed.

KEY RESULTS

During normally conducted sinus rhythm or acute atrioventricular block, EMW was positive. During CAVB, EMW was decreased to negative values. Dofetilide further reduced EMW before inducing repetitive TdP in 82% of the experiments. However, subclassification into inducible and non-inducible dogs revealed no difference in EMW. Analysis of the components of EMW revealed that the observed changes in EMW were solely caused by QT prolongation.

CONCLUSIONS AND IMPLICATIONS

In the canine CAVB model, ventricular remodelling and I(Kr) block by dofetilide are associated with negative EMW values, but this reflects QT prolongation, and implies that the EMW lacks specificity to predict dofetilide-induced TdP.

Vernakalant is devoid of proarrhythmic effects in the complete AV block dog model

The anesthetized chronic AV-blocked dog (cAVB) and methoxamine-sensitized rabbit model are widely used to determine pro-arrhythmic properties of drugs. In general, both models show similar results. However, conflicting data have also been reported; K201 and AZD1305 induced Torsade de Pointes (TdP) exclusively in cAVB dogs. Vernakalant, an antiarrhythmic drug that blocks several ion channels has been approved only in Europe. Its propensity to induce repolarization-dependent TdP arrhythmias has been evaluated solely in the methoxamine-sensitized rabbits. We therefore assessed the proarrhythmic potential of vernakalant in the cAVB dog model. Vernakalant was evaluated in 10 mongrel dogs (sinus rhythm (SR) 2mg/kg; chronic AV block (cAVB) 2+3mg/kg). The same dogs were challenged with dofetilide (25 μg/kg) to evaluate TdP inducibility. During the serial experiments the animals were paced from the right ventricular apex (60 beats/min). Short-term variability of repolarization (STV) was quantified for proarrhythmic risk. In SR (n=8) vernakalant prolonged QT (265 ± 11 to 311 ± 18 ms P<0.01(**)) but not PQ or QRS. In cAVB (n=8), 2mg/kg vernakalant prolonged QT (391 ± 43 to 519 ± 73 ms(**)) and QRS (103 ± 24 to 108 ± 23 ms(**)). After a 30 min lag-time, 3mg/kg vernakalant (n=4) increased QT to a lesser extent (413 ± 34 to 454 ± 27 ms(**)) while maintaining QRS prolongation (114 ± 18 to 122 ± 20 ms(**)). Neither dose increased STV or caused arrhythmias. Dofetilide prolonged QT (398 ± 51 to 615 ± 71 ms(**)), increased STV (1.0 ± 0.4 to 2.2 ± 1.0 ms P<0.05(⁎)) and induced TdP arrhythmias in 6/8(⁎) cAVB dogs. Vernakalant did not induce arrhythmias in the cAVB dog model. Higher dosages (3mg/kg) did not prolong repolarization further whereas negative inotropic effects were starting to become apparent precluding further increases in dose.

Assessing the proarrhythmic potential of drugs: current status of models and surrogate parameters of torsades de pointes arrhythmias

Torsades de pointes (TdP) is a potentially lethal cardiac arrhythmia that can occur as an unwanted adverse effect of various pharmacological therapies. Before a drug is approved for marketing, its effects on cardiac repolarisation are examined clinically and experimentally. This paper expresses the opinion that effects on repolarisation duration cannot directly be translated to risk of proarrhythmia. Current safety assessments of drugs only involve repolarisation assays, however the proarrhythmic profile can only be determined in the predisposed model. The availability of these proarrhythmic animal models is emphasised in the present paper. It is feasible for the pharmaceutical industry to establish one or more of these proarrhythmic animal models and large benefits are potentially available if pharmaceutical industries and patient-care authorities embraced these models. Furthermore, suggested surrogate parameters possessing predictive power of TdP arrhythmia are reviewed. As these parameters are not developed to finalisation, any meaningful study of the proarrhythmic potential of a new drug will include evaluation in an integrated model of TdP arrhythmia.

In vitro and in vivo models for testing arrhythmogenesis in drugs

The steadily increasing list of drugs associated with prolongation of the QT interval and torsades de pointes (TdP) constitute a medical problem of major concern. Hence, there is a need at an early stage to identify drug candidates with an inherent capacity to induce repolarization-related proarrhythmias, avoiding exposure of large populations to potentially harmful drugs. Furthermore, the availability of clinically relevant and predictive animal models should reduce the risk that effective and potentially life-saving drugs never reach the market. This review will discuss the pros and cons of some in vivo and in vitro animal models for assessing proarrhythmia liability.

Nonclinical proarrhythmia models: predicting Torsades de Pointes

Prolongation of the QT interval and the cardiac action potential have been linked to a potentially fatal but rare tachyarrhythmia known as Torsades de Pointes (TdP). Nonclinical assays, such as those investigating the effect on I(Kr) (the hERG channel current), prolongation of the action potential duration (APD) and the QT interval, in vivo, have been developed to predict the risk of QT interval prolongation and TdP in man. However, there seems to be a dissociation between the risk of QT interval prolongation and the torsadogenic risk. There is an increasing mass of evidence showing that an increase in the QT interval does not necessarily lead to TdP. Thus, it appears that while standard assays are very good, although perhaps not infallible, at predicting the risk of QT interval prolongation in man they do not predict the proarrhythmic risk. Recently there has been a plethora of publications suggesting that there are electrophysiological markers associated with drug-induced TdP other than hERG channel activity, APD and the QT interval, and these markers may be better predictors of TdP. In this review, three in vitro and, briefly, three in vivo models or methods are discussed. These proarrhythmia models use electrophysiological markers such as transmural dispersion of repolarization, action potential triangulation, instability, reverse use-dependence, and the incidence of early after-depolarizations to predict the risk of TdP. Most of the models presented have been published widely. The particular variable or set of variables used by each model to predict the torsadogenic propensity of a drug has been reported to correlate with clinical outcome. While each variable/model has been shown to discriminate between antiarrhythmic and nonarrhythmic drugs, these reports should be interpreted cautiously since none has been independently (externally) assessed. Each model is discussed along with its particular merits and shortcomings; none, as yet, having shown a predictive value that makes it clearly superior to the others. Proarrhythmia models, in particular in vitro models, challenge current perceptions of appropriate surrogates for TdP in man and question existing nonclinical 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. Regardless of regulatory acceptance, with further validation these models may still enhance pharmaceutical company decision-making to provide a rational basis for drug progression, particularly in areas of unmet medical need.

Electrophysiological safety of sertindole in dogs with normal and remodeled hearts

Inhibition of the potassium current IKr and QT prolongation are associated with drug-induced torsades de pointes arrhythmias (TdP) and sudden cardiac death. We investigated the cardiac electrophysiological effects of sertindole, an antipsychotic drug reported to prolong the QT interval in schizophrenic patients. In cell cultures, sertindole seemed to be a selective blocker of IHERG over other ion currents. For IHERG, the IC50 value was 64 +/- 7 nM, whereas ISCN5A, ICa,L, ICa,T, IKCNQ1/KCNE1, and IKv4.3 were blocked in the micromolar range. In canine ventricular myocytes, the IC50 value for IKr inhibition by sertindole was 107 +/- 21 nM. Action potentials in these cells prolonged in a reverse rate- and concentration-dependent manner at 10 to 300 nM sertindole. In vivo, sertindole was administered to anesthetized dogs at clinically relevant (0.05-0.20 mg/kg) and high doses (1.0-2.0 mg/kg) i.v. At 0.05 to 0.20 mg/kg sertindole (plasma concentrations 30-157 nM), QTc was prolonged by 1 to 5% in normal dogs and by 9 to 20% in dogs with remodeled hearts due to chronic atrioventricular block (CAVB). TdP was not induced at these doses in normal dogs or in CAVB dogs with reproducible induction of TdP by dofetilide in previous experiments. At 1.0 to 2.0 mg/kg sertindole (plasma concentrations 0.5-3.1 microM), QTc prolonged by 6 to 11% in normal dogs and by 22% in dofetilide-sensitive CAVB dogs. TdP occurred in three of five animals in the latter group. Thus, at high i.v. doses sertindole can pose a serious proarrhythmic risk when electrical remodeling of the ventricles is present. At clinically relevant doses, however, sertindole does not cause TdP in anesthetized dogs with normal or remodeled hearts.

Azimilide inhibits multiple cardiac potassium currents in human atrial myocytes

BACKGROUND

Studies in animal cell preparations suggest that azimilide may produce a more desirable rate-dependent profile of class III action as a result of its effects on both the slowly (I(Ks)) and rapidly (I(Kr)) activating components of potassium current (I(K)). However, relatively little is known about the effects of azimilide on K(+) currents in human atrial cells. The present study investigated the effect of azimilide on the inward rectifier potassium current (I(K1)), delayed rectifier potassium current (I(K)), ultrarapid delayed rectifier current (I(Kur)), and transient outward potassium current (I(to)) in isolated single human atrial myocytes.

METHODS

The tight-seal, whole-cell voltage clamp technique was used to investigate the acute effects of azimilide on K(+) currents in single human atrial myocytes. The cells were isolated enzymatically from atrial tissues that were obtained from patients undergoing open-heart surgeries, with the approval of the local Institutional Review Board.

RESULTS

The average cell capacitance of the human atrial myocytes was 77.5 +/- 2.8 pF (Mean +/- standard error of mean, total 28 cells from 17 patients). We found that 100 microM of azimilide in the extracellular solution significantly inhibited the inward rectifier potassium current (12.3 +/- 3.1 vs 6.7 +/- 2.0 pA/pF, n = 12, P < 0.05) at the testing potential of -100 mV. Superfusion with 100 microM of azimilide for 10 minutes inhibited I(K) by 51.7 +/- 5.1% (from 3.4 +/- 0.5 to 1.6 +/- 0.2 pA/pF, n = 9, P < 0.01) at the clamping membrane potential of +40 mV. Human atrial cell I(Kur) was inhibited with 100 microM of azimilide by 38.6 +/- 4.4% (from 3.9 +/- 0.5 to 2.3 +/- 0.2 pA/pF, n = 9, P < 0.01, test potential = 40 mV). We also found that the average peak current amplitude of I(to) in these cells was significantly inhibited with 100 microM of azimilide by 60.3 +/- 5.9% (from 10.3 +/- 1.5 to 3.6 +/- 0.3 pA/pF, n = 6, P < 0.01, test potential = 50 mV).

CONCLUSION

The present study provides direct evidence that azimilide inhibits multiple cellular transmembrane K(+) currents in freshly isolated human atrial myocytes. Inhibition of these K(+) currents by azimilide, especially of I(Ks) and I(Kur) is likely to be the electrophysiologic basis for the prolongation of the action potential duration in the human atria which mediates its known antifibrillatory effects in atrial fibrillation and flutter.

Electrophysiologic parameters and predisposing factors in the generation of drug-induced Torsade de Pointes arrhythmias

When a new (cardiovascular) drug shows signs of QT interval prolongation on the ECG (delay in repolarization time), the regulatory agencies demand screening of its possible proarrhythmic potential before approving it for clinical practice. In this review, identified predisposing factors have been related to specific electrophysiological parameters, allowing quantification of their contribution to Torsade de Pointes arrhythmias. In addition, arrhythmogenic mechanisms involved in the initiation and perpetuation of drug-induced Torsade de Pointes are discussed.