Introduction

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QT interval prolongation is a risk factor in a number of cardiovascular as well as noncardiovascular diseases. In the congenital long QT syndrome (LQTS), prolongation of the QT interval is associated with recurrent syncope and sudden cardiac death. Both result from potentially fatal arrhythmias, known as torsade de pointes (TdP). The occurrence of this particular form of ventricular tachyarrhythmia (VT) is not restricted to patients with LQTS. The most common cause of such proarrhythmia seems to be administration of antiarrhythmic drugs that prolong the action potential, i.e., the so-called class IA and class III antiarrhythmic agents (Haverkamp et al., 2000). The incidence of TdP in patients treated with quinidine (class IA) has been estimated to range between 1 and 4% (Selzer and Wray, 1964; Roden et al., 1986). For dl-sotalol, an incidence ranging between 1.8 and 4.8% has been reported (Lehmann et al., 1996; Haverkamp et al., 1997). A similar incidence has been seen for newer class III agents, e.g., dofetilide and ibutilide (Hohnloser and Singh, 1995; Chen et al., 1999). All these drugs have in common the fact that they block the rapidly activating component of the delayed rectifier potassium current I_Kr (Haverkamp et al., 2000). QT prolongation and TdP have been reported to occur secondary to treatment not only with antiarrhythmic drugs, but also with several drugs not generally thought to have significant effects on myocardial repolarization (e.g., antidepressants and other psychotropic drugs; antihistamines such as terfenadine and astemizole; some antibiotics, particularly macrolide antibiotics and some quinolones; and the promotility agent cisapride) (Haverkamp et al., 2000). Estimation of the true incidence of TdP during treatment with noncardiovascular drugs is difficult. For several noncardiovascular drugs that have been involved in the generation of TdP, only a few case reports are available. This is also true for sertindole, as well as for other new antipsychotics. For sertindole, most of the reported cases have occurred either in overdose or in combination with other drugs known to be associated with TdP. The occurrence of TdP secondary to a repolarization-prolonging drug is not a drug-specific but rather a patient-specific response. Patients with TdP typically show excessive, abnormal drug-induced prolongation of repolarization. Thus, the arrhythmia does not occur when drug-induced prolongation of repolarization is within the "normal" range. However, this does not necessarily mean that the individual patient will always show abnormal QT prolongation and TdP during exposure to repolarization-prolonging stimuli. This becomes obvious when considering the highly variable intervals between the initiation of drug therapy and the occurrence of TdP reported in the literature. The mechanisms responsible for these phenomena are not well understood. It has been suggested that a reduced "repolarization reserve", i.e., a lowered threshold for the development of abnormal QT prolongation and TdP upon challenge with a drug that lengthens myocardial repolarization may play a role (Roden, 1998). In addition, in some patients with TdP, the presence of a "forme fruste" of the LQTS has been reported (Abbott et al., 1999).

Sertindole (5-chloro-1-(4-fluorophenyl)-3-[1-[2-(2-imidazolidinon-1-yl)-ethyl]-4-piperidyl]-1H-indole) is a new indolylpiperidine antipsychotic agent, which has nanomolar affinities for dopamine D₂, serotonin 5-HT₂, and ₁-adrenergic receptors (Zimbroff et al., 1997; Arnt and Skarsfeldt, 1998). Together with risperidone, ziprazidone, quetiapine, pimozide, and olanzapine, sertindole belongs to the group of newer antipsychotic drugs that have been considered to exhibit both a greater antipsychotic efficacy than previously available agents and fewer extrapyramidal effects (Ereshefsky, 1996; Leucht et al., 1999). However, clinical (van Kammen et al., 1996) as well as experimental data (Drici et al., 1998) show that sertindole, as do other newer antipsychotics, may prolong myocardial repolarization and, in individual patients, cause ventricular proarrhythmia of the TdP type. This has raised concerns regarding the benefit/risk profile of sertindole and led to a temporary market suspension in 1998. H. Lundbeck A/S (Copenhagen, Denmark), the company producing sertindole, has consistently argued that there is no increased cardiac or all-cause mortality for patients treated with sertindole. These arguments are based on epidemiological studies, including comparative cohort studies, in more than 8500 patients after the initial approval. A recent comparative postmarketing surveillance study on sertindole and two other atypical antipsychotics, risperidone and olanzapine, showed no statistically significant differences in mortality rates between sertindole and comparator cohorts (Pezawas et al., 2000; Wilton et al., 2001). Due to the temporary suspension, the sertindole cohort was, however, too small to make firm conclusions on its proposed low proarrhythmic potential. The regulatory authorities in Europe are now reviewing available data to reach a final decision regarding its availability on the market.

To better understand the electrophysiologic effects of sertindole and its suggested low torsadogenic potential, we compared the electrophysiologic effects of sertindole to those of the antiarrhythmic agent dl-sotalol. As an experimental model we chose the isolated Langendorff-perfused rabbit heart. In this model, our group has previously demonstrated a high degree of proarrhythmia with drugs that are also clinically known to be torsadogenic, such as sotalol or erythromycin (Eckardt et al., 1998b).

	Materials and Methods

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Preparation of Hearts for Perfusion. The investigation conforms with the Guide for the Care and Use of Laboratory Animals published by the US National Institutes of Health (National Institutes of Health Publication 85-23, revised 1996). The rabbit heart model of TdP used in the present study has previously been described in detail (Eckardt et al., 1998b). In brief, male New Zealand White rabbits (n = 18) weighing 2.5 to 3.0 kg were anesthetized with sodium thiopental (100 mg, i.v.) and sacrificed by cervical dislocation. After midsternal incision, the hearts were removed and immediately placed in an ice-cold Krebs-Henseleit solution (1.80 mM CaCl₂, 4.70 mM KCl, 1.18 mM KH₂PO₄, 0.83 mM MgSO₄, 118 mM NaCl, 24.88 mM NaHCO₃, 2.0 mM sodium pyruvate, and 5.55 mM D-glucose). The aorta was cannulated, and the hearts were retrogradely perfused at constant flow (52 ml/min) with warm (36.8-37.2°C) Krebs-Henseleit solution. The perfusate was equilibrated with 95% O₂ and 5% CO₂ (pH 7.35; 37°C). The cannulated and perfused hearts were attached to a vertical Langendorff apparatus (Hugo Sachs Elektronik, Medical Research Instrumentation, March-Hugstetten, Germany). The atrioventricular (AV) node was ablated to slow the intrinsic heart rate. This resulted in complete AV dissociation with a ventricular escape below 60 beats per min.

Electrocardiographic and Electrophysiologic Measurements. ECG recording and QT measurements were performed as described previously (Eckardt et al., 1998b, 2000). A volume-conducted ECG was recorded by complete immersion of the heart into a bath of Krebs-Henseleit solution that had been thermally equilibrated with the myocardial perfusate. Signals from a simulated "Einthoven" configuration were amplified by a standard ECG amplifier (filter settings: 0.1-300 Hz).

Experimental Protocol. After placing the MAP catheters and achieving complete AV block, CL dependence was first investigated under baseline conditions. Thereafter, sertindole (H. Lundbeck A/S, Copenhagen, Denmark; 0.5, 1.0, and 1.5 µM), or dl-sotalol (Knoll, Ludwigshaven, Germany; 10 µM) was infused through a side port positioned just above the aorta by a peristaltic pump. The clinical therapeutic concentration of sertindole is 100 to 200 nM. The plasma protein binding of sertindole is 99%, and the concentrations studied were thus several multiples higher than the expected free plasma concentration in patients. The lower dose of 0.5 µM sertindole reflects the fact that sertindole, like other basic amines such as quinidine, accumulates in heart tissue with a factor of 5 to 10 to the total plasma concentration at steady-state conditions (Yata et al., 1990). For sertindole, an accumulation in myocardial tissue with a tissue to plasma ratio of 5 to 10 has been confirmed in Lundbeck toxicological studies with sertindole in rats and dogs (H. Lundbeck, A/S, Copenhagen, Denmark, personal communication). Sotalol, a class III antiarrhythmic drug, was used to create a model of drug-induced LQTS. It has been reported to induce TdP in several experimental studies (Eckardt et al., 1998a). The concentration of 10 µM was chosen as a high therapeutic concentration (considering the plasma protein binding) (Eckardt et al., 1998b).

Data Acquisition and Statistical Analysis. ECG, pressure, volume, and MAPs were recorded on a multichannel recorder. Data were digitized online at a rate of 1 kHz with 12-bit resolution and stored on a disk. All data are presented as mean ± S.D. The influence of each drug on ECG parameters and MAP duration, as well as dispersion of repolarization (estimated as the maximal difference of the simultaneously recorded MAP durations), were assessed using a paired and unpaired t test. Analysis of variance was used to investigate cycle length dependence. A value of p < 0.05 was the criterion for statistical significance.

	Results

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Dose-Dependent Effects of Sertindole and Sotalol on QT Interval and Action Potential Duration. All electrocardiographic parameters reached equilibrium within 10 min after AV block. MAP recordings and pacing thresholds (mean threshold 1.6 ± 1.4 mA) remained highly reproducible throughout the experimental protocol. In the presence of sertindole, there was a slight increase in pacing thresholds (mean threshold 1.9 ± 1.4 mA, p = N.S.). After an initial stabilization period of approximately 5 to 10 min, the MAP amplitude did not change by more than 20% for the subsequent investigation period.

View larger version (23K): [in this window] [in a new window]   Fig. 1.   Cycle length-dependent effects of sertindole (0.5, 1.0, and 1.5 µM) on QT interval as compared with control in isolated Langendorff-perfused rabbit hearts (n = 10). The inset shows an example of QT prolongation at 900 ms in the presence of increasing sertindole concentrations. , baseline; , 0.5 µM; , 1.0 µM; , 1.5 µM sertindole. , p < 0.05.

View larger version (36K): [in this window] [in a new window]   Fig. 2.   Cycle length-dependent effects of sertindole (0.5, 1.0, and 1.5 µM) on mean right ventricular epicardial and left endo- and epicardial MAP duration in isolated Langendorff-perfused rabbit hearts (n = 10). The insets show examples of MAP prolongation with increasing doses of sertindole at a cycle length of 900 ms. , baseline; , 0.5 µM; , 1.0 µM; , 1.5 µM sertindole.

View larger version (29K): [in this window] [in a new window]   Fig. 3.   Cycle length-dependent effect of dl-sotalol (10 µM) on QT interval (A) and mean APD90 (B) in isolated Langendorff-perfused rabbit hearts (n = 8). C, effect of 10 µM dl-sotalol on transmural dispersion of repolarization estimated as the maximum difference between endo- and epicardial APD90 in comparison with control. , baseline; , 10 µM dl-sotalol. , p < 0.05.

Induction of Arrhythmias. No sustained arrhythmias were observed in the presence of 0.5 and 1.0 µM sertindole in the AV-blocked normokalemic (5.8 mM KCl) rabbit hearts. In 5 of the 10 hearts, spontaneous ventricular premature beats were observed that often occurred in a bigeminal pattern (Fig. 4A). One of these hearts also demonstrated an episode of nonsustained monomorphic ventricular tachycardia. This episode did not show the typical characteristics of TdP (including the polymorphic nature of the arrhythmia with undulating QRS complexes). After increasing the sertindole concentration to 1.5 µM, 4 of 10 hearts (those demonstrating significant QRS prolongation; see above) developed nonsustained monomorphic ventricular tachycardia (mean cycle length 280 ms). Lowering potassium concentration to 1.5 mM resulted in the occurrence of nonsustained monomorphic VT in all hearts studied (Fig. 4B). However, only 2 of 10 animals showed arrhythmias of polymorphic character (Fig. 4C).

View larger version (45K): [in this window] [in a new window]   Fig. 4.   A, spontaneously occurring ventricular bigemini in the presence of 1 µM sertindole in a bradycardic AV-blocked Langendorff-perfused rabbit heart at normal potassium concentration of 5.88 mM. B, spontaneously occurring episodes of nonsustained monomorphic ventricular tachyarrhythmias in the presence of 1.5 µM sertindole in a bradycardic AV-blocked Langendorff-perfused rabbit heart (1.5 mM KCl). C, spontaneously occurring episode of a polymorphic nonsustained ventricular tachyarrhythmia in the presence of 1.5 µM sertindole in a bradycardic AV-blocked Langendorff-perfused rabbit heart (1.5 mM KCl). These short runs of VT were not associated with EADs on any of the simultaneously recorded MAPs.

View larger version (44K): [in this window] [in a new window]   Fig. 5.   A, polymorphic ventricular premature beats in the presence of dl-sotalol (10 µM) in an isolated Langendorff-perfused rabbit heart. Each trace shows ECG and MAPs recorded simultaneously from left endocardial (MAP7) and epicardial (MAP1/2 right ventricle; MAP3-6 left ventricle) together with ECG recordings. Note the significant increase in MAP dispersion in the presence of sotalol. B, polymorphic ventricular tachyarrhythmia displaying features of torsade de pointes in the presence of dl-sotalol (10 µM) and hypokalemia (1.5 mM) in an isolated Langendorff-perfused rabbit heart. EADs and triggered activity are associated with the occurrence of TdP.

Early Afterdepolarizations and Dispersion of Repolarization. In the presence of sotalol, EADs and triggered activity were a frequent finding (Fig. 5). Seven hearts (88%) showed MAP recordings with EADs in the presence of low potassium. EADs mainly occurred on endocardial recordings. The occurrence of EADs and triggered activity was associated with a significant CL-independent increase in dispersion of repolarization (Fig. 3C). The mean increase in transmural APD₉₀ dispersion measured 62 ms. For sotalol, the increase of APD₉₀ dispersion was more a transmural than an interventricular phenomenon. It was to a large extent based on the effect of sotalol on the endocardial APD₉₀. In contrast, with sertindole there was no significant effect on APD dispersion (Fig. 6). In the presence of sertindole, the increase in APD was homogeneous. Compared with sotalol, sertindole did not significantly increase transmural APD dispersion at any rate. Even a high concentration of 1.5 µM sertindole demonstrated no significant increase in APD₉₀ dispersion (Fig. 6D). This was paralleled by a more moderate effect on the endocardial MAP duration as compared with sotalol. In addition, few endocardial MAP recordings (2 in 10 hearts, 1.5 mM KCl) demonstrated humps in the late phase of repolarization that only resembled EADs but never did give rise to triggered activity or nonsustained polymorphic VT.

View larger version (26K): [in this window] [in a new window]   Fig. 6.   Cycle length-dependent effects of sertindole [0.5 µM (B), 1.0 µM (C), and 1.5 µM (D)] on transmural dispersion of repolarization estimated as the maximum difference between endo- and epicardial APD90 in comparison with control (A).

	Discussion

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Sertindole belongs to the new generation of antipsychotics that have led to significant improvements in the treatment of schizophrenia. However, clinical studies have shown that sertindole, as well as the other members of this group of drugs (e.g., risperidone, ziprazodine, haloperidol, olanzapine, and clozapine) may affect cardiac repolarization, i.e., induce prolongation of the QT interval (van Kammen et al., 1996). Although extremely rare, in individual patients proarrhythmia of the TdP type resulting from abnormal drug-induced prolongation of myocardial repolarization has been reported for antipsychotics (Kiriike et al., 1987; Hunt and Stern, 1995; Krahenbuhl et al., 1995; Jackson et al., 1997).

The principal finding of our study is that the pattern by which sertindole affects repolarization differs from that of the typical class III antiarrhythmic agent sotalol. This observation may lead to a better understanding of the low proarrhythmic potential of several drugs, such as sertindole, that prolong the QT interval but only rarely result in TdP. Although we found comparable QT prolongation between sertindole and dl-sotalol, sertindole displayed no significant inverse use dependence. EADs did not occur in the presence of sertindole, and compared with sotalol there was no increase in transmural or interventricular dispersion of repolarization. Thus, in contrast to the I_Kr blocker sotalol, channel block produced by sertindole seems to be less torsadogenic.

Effects of Sertindole on Myocardial Repolarization. The major mechanism by which sertindole induces prolongation of myocardial repolarization has been considered to be inhibition of I_Kr. In studies that assessed the effects of sertindole on human I_Kr the IC₅₀ varied between 12.6 and 64 nM, depending on the experimental conditions (Rampe et al., 1998; Maginn et al., 2000). In a comparative study, I_Kr (HERG) was also blocked by haloperidol, risperidone, olanzapine, ziprazidone, and pimozide with IC₅₀ values of 28, 163, 181, 152, and 49 nM, respectively (Crumb and Cavero, 1999). However, the IC₅₀ measured in vitro should not be considered as an absolute criterion for estimation of drug potency. Extrapolation of in vitro drug concentrations to in vivo conditions is extremely difficult. Many factors control drug concentration in vivo and produce both therapeutic and adverse effects. Verapamil is an example of a potent I_Kr blocker in vitro (IC₅₀, 143 nM) (Zhang et al., 1999). It may also prolong the QT interval at very high plasma concentrations, but so far no documented clinical report of verapamil-related TdP is available. The effects of sertindole on repolarization have previously been studied in isolated feline hearts (Drici et al., 1998). In this model, the potency of sertindole in lengthening QT interval was compared with that of haloperidol, risperidone, clozapine, and olanzapine (Drici et al., 1998). The hearts were infused with increasing concentrations for 40-min intervals at each concentration. All tested drugs caused a concentration-dependent increase in QT interval. Haloperidol and risperidone were significantly more potent than sertindole; olanzapine and clozapine were less potent. Noteworthy, the all-cause mortality rate for sertindole-treated patients was less than half that for risperidone-treated patients, and also considerably lower than that for olanzapine-treated patients (Mackay et al., 1998). The QT-prolonging potency of sertindole was equivalent to that of its metabolites Lu 28-092 and Lu 25-073 (Drici et al., 1998).

Low Torsadogenic Potential of Sertindole: Possible Causes. Lack of early afterdepolarizations and lack of an increase in dispersion of repolarization are two possible causes of the low torsadogenic potenial of sertindole. Slowing of myocardial repolarization and critical lengthening of APD appear to be necessary, but not sufficient to evoke EADs and triggered activity. For the occurrence of EADs, a net depolarizing current flowing during the repolarization phase of the action potential is necessary. Currents implicated in the generation of EADs include the sodium window current, the L-type calcium current, the Na/Ca exchange current, and repolarizing potassium currents (Eckardt et al., 1998a). For almost all agents that are clinically associated with abnormal QT prolongation and TdP, the generation of EADs and triggered activity under appropriate experimental conditions has been demonstrated. The premature beat that initiates TdP is generally thought to be due to an EAD-induced triggered response, arising from either Purkinje or subendocardial cells (M cell region); both are capable of developing EAD-induced triggered activity (Eckardt et al., 1998a).

	Conclusions

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Sertindole is a psychotropic agent that prolongs myocardial repolarization. This effect is due to a concentration-dependent inhibition of I_Kr. Compared with other agents that also inhibit this current, the torsadogenic potential of sertindole seems to be remarkably low. In the Langendorff-perfused rabbit heart model of TdP, sertindole did not display the proarrhythmic profile typical of other blockers of I_Kr such as sotalol. The mechanisms responsible for this behavior of sertindole are probably multifactorial. Next to a lack of reverse use dependence, we could demonstrate that even high doses of sertindole had no significant effect on dispersion of repolarization. It is possible that a different mode of interaction between sertindole and the channel and/or additional pharmacological effects of sertindole, e.g., its ability to inhibit I_Na and/or its ability to block ₁-receptors, may play a role. Further studies will be necessary to investigate whether similar effects are observed with other drugs that prolong repolarization but exhibit a low torsadogenic potential.