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Proarrhythmic Effects of Intravenous Quinidine, Amiodarone, d -Sotalol, and Almokalant in the Anesthetized Rabbit Model of Torsade de Pointes

Farkas, András ; Leprán, István ; Papp, Julius Gy.

Journal of Cardiovascular Pharmacology: February 2002 - Volume 39 - Issue 2 - p 287-297
Original Articles
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The proarrhythmic effects of four antiarrhythmic agents were examined during α 1 -adrenoceptor stimulation in chloralose-anesthetized rabbits. Each dose of almokalant (26, 88, and 260 μg/kg), d -sotalol, quinidine, or amiodarone (each 3, 10, and 30 mg/kg) was infused i.v. over 5 min and there was a 20-min interval between each infusion. d -sotalol and almokalant evoked torsade de pointes (TdP) and other arrhythmics, frequently. The incidences of TdP were 0, 50, and 40% after administering the first, second, and third doses of the nonselective I Kr inhibitor d -sotalol, respectively. Similarly, these values were 20, 40, and 33% after administering the first, second, and third doses, respectively, of the selective I Kr inhibitor almokalant. Quinidine elicited only a few arrhythmics, but not TdP. Quinidine, d -sotalol, and almokalant evoked conduction blocks in a dose-related manner (p < 0.05) and prolonged QT and QT c intervals (p < 0.05). Amiodarone neither prolonged QT and QT c nor evoked ventricular tachyarrhythmias, blocks, or other proarrhythmias. In conclusion, these results show no direct correlation between the occurrence of TdP and the infusion rate or dose of anti-arrhythmics. Furthermore, the lack of TdP with quinidine warns of false-negative results in the applied model.

Department of Pharmacology and Pharmacotherapy, Faculty of Medicine, University of Szeged, Szeged, Hungary

Received March 14, 2001; revision accepted August 27, 2001.

Address correspondence and reprint requests to Dr. I. Leprán at Department of Pharmacology and Pharmacotherapy, Faculty of Medicine, University of Szeged, H-6701 Szeged, P.O. Box 427, Hungary. E-mail: lepran@phcol.szote.u-szeged.hu.

Torsade de pointes (TdP) can be evoked by various drugs and toxins in humans (1–3), especially in the presence of predisposing factors such as bradycardia, electrolyte abnormalities (i.e., hypokalemia and hypomagnesemia), prolonged repolarization, depressed left ventricular function, or a history of life-threatening arrhythmias (3). The underlying mechanisms of this arrhythmia are still not completely known, but early after depolarizations and dispersion of ventricular repolarization are suspected to be the main electrophysiologic substrates for initiation of TdP (3).

Drugs that delay repolarization have a tendency to cause TdP. Quinidine is by far the most frequently reported drug associated with this arrhythmia (1). Even though most studies say that i.v. amiodarone does not widen QT or cause TdP in most patients, there is still disagreement about this (4–6). Therefore i.v. amiodarone was included in the study. Almokalant and d -sotalol, which differ from quinidine and amiodarone by virtue of their greater selectivity for cardiac potassium channels, can also evoke TdP in humans (7,8).

Several animal models have been developed with TdP as the endpoint (9–11). The method developed by Carlsson et al. (12) of the acquired long QT syndrome uses anesthetized rabbits, and TdP is evoked by coadministration of a test agent with the selective α 1 -agonist, methoxamine. Almokalant and d -sotalol are highly proarrhythmic and prone to evoke TdP in rabbits (13–15).

Although this model has been used to examine the proarrhythmic activity of novel antiarrhythmic agents (14,16,17), the model has not been characterized sufficiently for its clinical relevance to be certain. Information on the effects of long-established drugs with clinically well-characterized proarrhythmic activity is sparse. Therefore, in the current study we examined the effects of quinidine and amiodarone, two widely used anti-arrhythmic drugs, and compared them with d -sotalol and almokalant in anesthetized rabbits during α 1 -adrenergic receptor stimulation by phenylephrine.

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METHODS

Animals

The experiments were performed on male New Zealand white rabbits weighing 2.1–2.9 kg. The animals were handled according to a protocol reviewed and approved by the Ethical Committee for the Protection of Animals in Research of the University of Szeged, Szeged, Hungary.

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Animal preparation

The proarrhythmic activity of antiarrhythmic compounds was examined using the method of Carlsson et al. (12,13) with minor modification. After sedation with pentobarbital sodium (5 mg/kg, i.v.), animals were anesthetized with α-chloralose (100 mg/kg i.v. into the marginal vein of the right ear, in 10 ml/kg infusion volume, at a rate of 1 ml/min). A catheter was introduced into the right carotid artery to measure blood pressure (Blood Pressure Monitor BP-1, World Precision Instruments, Sarasota, FL, U.S.A.). The catheter was filled with isotonic saline containing heparin (500 IU/ml), but the animals were not pretreated with heparin i.v. Two other catheters were introduced into the right jugular vein and the marginal vein of the left ear for infusion of drugs. After tracheal cannulation, the animals were mechanically ventilated with room air at 7 ml/kg per stroke and 40 strokes/min (Harvard rodent ventilator, model 683, Harvard Apparatus, South Natick, MA, U.S.A.). The electrocardiogram (lead I, II, III) was registered during the experiments by a thermographic recorder (ESC 110 4 CH, Multiline KFT, Esztergom, Hungary) using subcutaneous needle electrodes.

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Experimental protocol

The drug administration protocol is shown in Figure 1. Ten minutes after preparation, continuous phenylephrine infusion at a rate of 15 μg/kg/min via the right jugular vein was begun and was continued for 85 min (total infusion volume, 2 ml). Phenylephrine, an α 1 -adrenoceptor agonist (18), was used, as it facilitates almokalant to evoke TdP (15) to an extent almost identical to that seen by Carlsson et al. (13) with methoxamine. Thus, methoxamine and phenylephrine are pharmacologically equivalent in terms of sensitizing rabbit heart to TdP.

FIG. 1.

FIG. 1.

The animals were divided randomly into five groups, and 10 min after the beginning of phenylephrine infusion, increasing doses of almokalant (26, 88, 260 μg/kg), d -sotalol, quinidine, amiodarone (3, 10, 30 mg/kg), or isotonic saline were administered via the marginal vein of the left ear of the animals. Each dose was given over a period of 5 min and there was a 20-min interval between each dose of antiarrhythmic. The doses of almokalant, d -sotalol, and quinidine were chosen to span the antiarrhythmic dose range in rabbits used by our group and others (15,19,20). Because no in vivo experimental data have been published about the antiarrhythmic activity of i.v. amiodarone in rabbits so far, pilot experiments were performed in phenylephrine-primed anesthetized rabbits to choose the maximum dose of the drug that exerts tolerable hemodynamic effects. According to these experiments, 30 mg/kg of i.v. amiodarone possesses marked bradycardic and blood pressure–lowering effect (data not shown); therefore this dose was taken as the highest dose in the current study.

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Arrhythmia diagnosis and electrocardiographic analysis

From the electrocardiogram, the incidence, the time to onset, and the duration of ventricular arrhythmias were obtained. Ventricular premature beats, bigeminy, salvo, and ventricular fibrillation were defined in accordance with the definitions of the Lambeth Convention (21). When continuous ventricular fibrillation lasted > 120 s, then the experiment was terminated and ventricular fibrillation was defined as sustained ventricular fibrillation. TdP was defined as an arrhythmia in which five or more repetitive extrasystoles (ventricular premature beats) were coupled and for which the QRS complex showed a cyclic variation in size and shape. Every run of four ventricular premature beats with twisting QRS morphology and every run of four or more ventricular premature beats without the torsade-like twisting QRS morphology was differentiated from TdP and was defined as ventricular tachycardia. Blocks in the conduction system were also monitored. Conduction disturbances included atrioventricular blocks and intraventricular conduction defects (right or left bundle branch block).

Blood pressure, PP, RR, and QT intervals were also measured at predetermined intervals and at the first incidence of TdP. At least four electrocardiographic complexes were measured and averaged at each time point. Only sinus-origin R waves were used for measuring RR intervals. Because 2:1 atrioventricular block occurred frequently in some drug-treated groups, sinus rate (60/PP) and ventricular rate (60/RR) were calculated subsequently instead of calculating a single heart rate value. QT interval was defined as the time between the first deviations from the isoelectric line during the PR interval until the end of TU wave. T (or U) wave frequently overlaps the P wave of the following sinus-origin beat caused by relative high heart rate of the rabbit or excessive QT prolongation. In this case, the end of the TU wave was extrapolated from the curve of the TU wave to the isoelectric line under the P wave. Rate-corrected QT interval (QT c ) was calculated subsequently by using the equation of Carlsson et al. (13): QT c (Carlsson) = QT−0.175(RR−300) and the Bazett formula: QT c (Bazett) = QT/square root RR.

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Drugs

The following drugs were used: d -sotalol (Bristol-Myers Squibb, Wallingford, CT, U.S.A.), almokalant (AstraZeneca, Mölndal, Sweden), quinidine (Alkaloida, Tiszavasvári, Hungary), amiodarone (Cordarone injection, Sanofi Pharma, Montpellier, France), phenylephrine ( l -Phenylephrine HCl, Koch-Light Laboratories Ltd., Colnbrook-Bucks, U.K.), heparin-sodium (Richter Gedeon, Budapest, Hungary), pentobarbital-sodium (Nembutal, Phylaxia-Sanofi, Budapest, Hungary), and α-chloralose (Fluka Chemie AG, Buchs, Switzerland). Almokalant was prepared as a concentrated stock solution (100 μmol/ml) by AstraZeneca. The stock solution was diluted further with isotonic saline (saline). d -Sotalol and quinidine were also dissolved in saline. The three doses of each anti-arrhythmic agent (except amiodarone) were administered in a total volume of 2 ml. The same volume of saline (2 ml) was injected in three portions at comparable rates to the anti-arrhythmic agents to the animals in the control group. Amiodarone was prepared by diluting amiodarone injection (150 mg amiodarone in 3 ml) with saline, and the three doses of amiodarone were infused in a total volume of 3 ml. Each solution was prepared on the day of the experiment and all doses in the text refer to bases of the compounds.

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Statistical evaluation

The percentage incidence of arrhythmias was calculated and compared by using the Fisher Exact probability test. Continuous data were expressed as mean ± SEM. Comparisons within group were made using the Wilcoxon signed rank test. Comparisons between groups were made using the Kruskal-Wallis test. Differences were considered statistically significant when p < 0.05.

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RESULTS

Effects of phenylephrine before antiarrhythmic drug administrations

After 10 min of phenylephrine infusion (n = 45), mean arterial blood pressure increased from 85 ± 2 mm Hg to 123 ± 1 mm Hg (p < 0.05) and heart rate (sinus and ventricular rates) decreased from 290 ± 5 beats/min to 227 ± 6 beats/min (p < 0.05). QT intervals increased from 157 ± 2 ms 188 ± 4 ms (p < 0.05); QT c (Carlsson) increased from 173 ± 2 ms to 193 ± 4 ms (p < 0.05); and QT c (Bazett) increased from 343 ± 3 ms to 362 ± 6 ms (p < 0.05). All animals survived the infusion of phenylephrine. Premature ventricular beats, bigeminy, or salvo appeared in 60% and ventricular tachycardia occurred in 11%, whereas TdP, ventricular fibrillation, and conduction blocks were absent during this period.

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Arrhythmia incidences and onset times

The ventricular premature beats, bigeminies, or salvos elicited by phenylephrine persisted during the superimposition of saline infusion; there were no episodes of more serious arrhythmias and no deaths in this control group (Table 1). Two animals died of sustained ventricular fibrillation caused by the administration of antiarrhythmic drugs. One of them died after administration of 88 μg/kg of almokalant and the other one died after administration of 30 mg/kg d -sotalol (Table 1). Amiodarone and quinidine did not evoke TdP. Amiodarone did not evoke arrhythmias in animals that had not developed arrhythmias during infusion with phenylephrine alone, whereas the other three drugs evoked arrhythmias in some animals that had hitherto been arrhythmia-free (data not shown). Quinidine (30 mg/kg) induced ventricular tachycardia in two of seven animals. However, it did not cause TdP. Ventricular tachycardia was also induced by 10 and 30 mg/kg of d -sotalol and each dose of almokalant (Table 1). In contrast to amiodarone and quinidine, d -sotalol and almokalant additionally evoked TdP (Fig. 2). The second and third doses of d -sotalol (10 and 30 mg/kg) significantly increased the cumulative incidence of less complex arrhythmias (i.e., ventricular premature beats, bigeminy, and salvo) and the incidence of ventricular tachycardia and evoked TdP in 60% of animals (p < 0.05 versus saline controls) (Table 1). Likewise, the second and third doses of almokalant significantly increased the cumulative incidence of less complex arrhythmias and the incidence of ventricular tachycardia, and all three doses induced TdP (Table 1). Quinidine, d -sotalol, and almokalant evoked conduction blocks in a dose-related manner, whereas blocks never occurred after amiodarone and saline administration (Table 1).

TABLE 1

TABLE 1

FIG. 2.

FIG. 2.

There was no direct correlation between the occurrence of TdP and the infusion rate or the dose of d -sotalol and almokalant, because the percentage incidences of this arrhythmia were greatest after the administration of the second doses of the drugs (Table 1).

There was a significant difference between d -sotalol and almokalant in terms of ventricular tachyarrhythmia, e.g., ventricular tachycardia and TdP onset. Almokalant induced ventricular tachyarrhythmias shortly after the start of the administration of its second dose (log 10 onset time, 2.206 ± 0.089 s, n = 6). In contrast, a delay of several minutes occurred before the onset of ventricular tachyarrhythmias after the start of the administration of the second dose of d -sotalol (log 10 onset time, 2.816 ± 0.084 s, n = 6; p < 0.05 versus almokalant).

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Blood pressure and heart rate

When saline infusion was begun (control group) during continuous phenylephrine infusion, the blood pressure and the heart rate (sinus and ventricular) did not change further (Table 2).

TABLE 2

TABLE 2

Addition of d -sotalol reduced blood pressure and ventricular rate dose relatedly (Table 2). There was dissociation between sinus and ventricular rate after administration of 30 mg/kg of d -sotalol caused by frequent occurrence of 2:1 atrioventricular block (Table 2).

Similarly to d -sotalol, almokalant reduced blood pressure and there was dissociation between sinus and ventricular rate after administration of each dose of the drug caused by frequent 2:1 atrioventricular blocks (Table 2). Five minutes after administration of 88 and 260 μg/kg of almokalant, sinus rate increased significantly compared with saline control (232 ± 9 beats/min versus 196 ± 9 beats/min, p < 0.05 and 240 ± 12 beats/min versus 188 ± 14 beats/min, p < 0.05, respectively). These differences remained significant till the end of the 20-min drug-free intervals (Table 2). After the administration of the second dose of almokalant, sinus rate was also significantly different from those after the administration of the second dose of d -sotalol (Table 2). This difference remained significant between d -sotalol and almokalant nearly until the end of the experiment. Conversely, there was no significant difference between the effect of almokalant and d -sotalol on the blood pressure and ventricular rate during the whole experiment.

Quinidine treatment decreased the blood pressure markedly. The pressure drop after each dose of quinidine was significantly greater than those at the same time points in all other groups (Table 2). The first dose of quinidine (3 mg/kg) elevated heart rate, whereas the third dose (30 mg/kg) caused 2:1 (and total) atrioventricular blocks, allowing dissociation of sinus and ventricular rates (Table 2). After administration of 30 mg/kg of quinidine, ventricular rate decreased and sinus rate increased significantly compared with control (Table 2).

Amiodarone lowered blood pressure in a dose-related manner. Ten minutes after administration of 10 mg/kg of amiodarone, heart rate (sinus and ventricular rate) decreased significantly compared with saline control (170 ± 9/min versus 197 ± 9/min, p < 0.05). From this time point, heart rate decreased constantly in amiodarone-treated animals and remained significantly different from control till the end of the experiment (Table 2).

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QT prolongation

Saline infusion had no significant effect on the QT and QT c widening effect of the maintained infusion of phenylephrine (Table 3). Amiodarone did not add to the QT and QT c widening caused by phenylephrine (Table 3). However, d -sotalol and almokalant increased the QT and QT c in a dose-related manner (Table 3) and there was no difference between the time course of QT and QT c prolonging effects of the two drugs. Interestingly, only the third dose of quinidine (30 mg/kg) prolonged QT, whereas the rate-corrected QT intervals were already widened by the first dose of the drug (3 mg/kg) (Table 3). The second dose of d -sotalol and almokalant (10 mg/kg and 88 μg/kg, respectively) widened QT and QT c to such an extent that these were significantly different from those after administration of the second dose of quinidine and amiodarone (10 mg/kg)(Table 3).

TABLE 3

TABLE 3

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Variables measured before torsades de pointes occurrence

Because there was no statistical difference between the d -sotalol- and almokalant-treated animals in terms of blood pressure, ventricular rate, QT, and QT c, these data from the animals with TdP induced by either d -sotalol or almokalant were summarized. The values measured before the first TdP at the last sinus-origin beats were compared with those measured before phenylephrine was begun. In animals with TdP (n = 10), mean arterial blood pressure was elevated (120 ± 3 mm Hg versus 85 ± 2 mm Hg at baseline, p < 0.05) and ventricular rate was reduced significantly (183 ± 13 beats/min versus 286 ± 9 beats/min, p < 0.05) at the first incidence of TdP. Furthermore, QT and the rate-corrected QT intervals were prolonged markedly at this time (QT, 262 ± 8 ms versus 162 ± 5 ms at baseline, p < 0.05; QT c -Carlsson, 255 ± 8 ms versus 178 ± 4 ms, p < 0.05; QT c -Bazett, 453 ± 15 versus 353 ± 8 ms, p < 0.05).

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DISCUSSION

The present study has demonstrated that in anesthetized rabbits, of four antiarrhythmic agents administered i.v., only d -sotalol and almokalant induced TdP, whereas quinidine and amiodarone did not. Moreover, the present results showed no direct correlation between the occurrence of TdP and the infusion rate or the dose of antiarrhythmics.

In our investigation we modified the protocol of Carlsson et al. (12,13). Instead of giving repolarization-prolonging drugs in continuous infusions, we applied stepwise elevation of doses with an interval between each dose. This was for the following reasons: According to Carlsson et al. (13), the infusion rate of drugs is an important predisposing factor for TdP. With our protocol, three different infusion rates were tested in one animal, which decreased the total number of animals required. Not only the dose dependence, but also the time dependence of the occurrence of arrhythmias, can be compared between different treatment groups with this protocol as there is an interval between administrations of increasing doses of drugs.

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No torsade de pointes with quinidine

In our experiments quinidine did not evoke TdP, though this drug is by far the most frequently reported drug associated with this arrhythmia (1). In patients with TdP caused by quinidine, the plasma level of the drug is usually within or below the therapeutic range (22). Quinidine's clinical proarrhythmic profile (especially the dose dependence) is not mimicked in any of the presently available animal proarrhythmia models. In anesthetized dogs, Inoue and Sugimoto (23) used toxic dose of quinidine (30 mg/kg over 5 min, i.v.) to evoke TdP. Despite the observation that this dose prolonged QT in their study, TdP never occurred spontaneously and required additional programmed electrical stimulation to evoke it. Likewise, in conscious hypokalemic dogs with chronic complete atrioventricular block, TdP did not occur spontaneously during or after a continuous infusion of quinidine (∼20 mg/kg over 3 h), though QT interval was widened significantly by this dose (24); and an additional propranolol infusion was necessary to allow the generation of TdP in that study. In methoxamine-primed anesthetized rabbits, continuous quinidine infusion (1.25 mg/kg/min for 60 min) did not evoke TdP despite increasing QT, QT c, and QT dispersion significantly, but conversely, the drug evoked conduction blocks frequently (25). Thus, the lack of quinidine-induced TdP in the current study is in good accordance with the results of the previous in vivo animal studies.

In our investigations, in contrast to d -sotalol and almokalant, only the highest dose of quinidine prolonged QT intervals whereas QT c was already prolonged by the lower doses. Moreover, the second dose of quinidine was less potent in prolonging either QT or QT c intervals than the middle dose of almokalant and d -sotalol. This smaller QT- and QT c -prolonging potency might be a reason for the low proarrhythmic activity of the lower doses of quinidine in the current study. However, TdP was also absent even when marked QT and QT c prolongations were achieved by the highest dose of quinidine. This observation accords well with that of Carlsson et al. (13) and suggests that QT and QT c prolongation are not the only contributing factors to TdP generation in the anesthetized rabbit model.

The high heart rate of the rabbit compared with that of humans could also contribute to the blunted proarrhythmic activity of quinidine compared with its effects in humans. At high frequencies, quinidine's sodium channel inhibiting property is increased (use-dependent block) (26), which may prevent TdP at these high heart rates of the rabbit (27). In addition, after the administration of the first dose of quinidine, its antimuscarinic action on atrial muscarinic receptors (28) or its blood pressure–lowering effect might prevent phenylephrine-induced reflex bradycardia, which could predispose to TdP. However, this lack of reflex bradycardia became irrelevant especially after the administration of the third dose of quinidine, as this dose decreased the ventricular rate markedly caused by frequent occurrence of 2:1 and total atrioventricular blocks. However, this relatively low ventricular rate, which coincided with marked QT and QT c prolongation, was still insufficient to allow generation of TdP. Likewise, quinidine and terfenadine reduced heart rate and prolonged QT and QT c markedly without evoking TdP in α 1 -adrenoceptor-stimulated rabbits (25). These findings suggest that low ventricular (or heart) rate and prolonged QT and QT c intervals are not the only contributing factors to TdP generation in the rabbit model of the acquired long QT syndrome.

In the present investigation, quinidine dose-dependently lowered blood pressure and the pressure drops were significantly greater than those with any other drug. In methoxamine-sensitized anesthetized rabbits, terfenadine and quinidine reduced blood pressure to a very low level and did not evoke TdP despite increasing QT, QT c, and QT dispersion and reducing heart rate (25). These results suggest that marked blood pressure reduction may prevent TdP generation in the anesthetized rabbit model.

Quinidine inhibits α 1 -adrenoceptors competitively at therapeutic blood levels (29), thereby preventing the complex sensitizing effect of α 1 -adrenoceptor stimulation in this animal model. A similar effect was seen in a recent study in which cisapride, a potent inhibitor of the rapid component of delayed rectifier potassium current (I Kr ), was found to have very low proarrhythmic potential in the rabbit model of acquired long QT syndrome (30). This was attributed to the drug's high α 1 -adrenoceptor blocking potency.

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No torsade de pointes with amiodarone

Intravenous amiodarone did not induce ventricular tachyarrhythmias and did not significantly prolong QT and QT c in our experiments. Lack of i.v. amiodarone-induced QT and QT c prolongation in the current study is consistent with the findings of previous in vivo and in vitro rabbit studies. In anesthetized rabbits, 10 mg/kg i.v. bolus amiodarone failed to exert any effect electrocardiographically (31,32). Likewise, perfusion of isolated rabbit hearts with a buffer solution containing 1 μg/ml amiodarone had no effect on electrocardiographic intervals (33).

In anesthetized rabbits, the time of maximal uptake of i.v. bolus amiodarone by the myocardium has been estimated as between 5–15 min (31). Therefore, low accumulation of amiodarone in our experiments cannot be responsible for the low proarrhythmic activity. Amiodarone dose-relatedly lowered blood pressure, and the highest dose of the drug (30 mg/kg) induced marked blood pressure drop and bradycardia. This indicates that higher doses of the drug could not have been administered to the animals without inducing catastrophic hemodynamic effects. This suggests that despite the lack of effect on QT/QT c interval, amiodarone was not underdosed in the current study.

Acute and chronic electrophysiologic effects of amiodarone differ substantially (34,35). Although long-term administration of the drug prolongs repolarization markedly, it induces TdP only very rarely in humans (36). Though it is debatable whether acute amiodarone prolongs QT interval (5), TdP may be evoked by this administration of the drug in humans (4–6). However, proarrhythmic events are rare in patients treated with i.v. amiodarone (5), similarly to that seen in patients treated with long-term amiodarone therapy (36).

Intravenously administered amiodarone inhibits the delayed rectifier outward potassium current (I K ), the inward sodium current (I Na ), and the inward l -type calcium current (I Ca-L ) (34,37). The simultaneous inhibition of I K and I Na or I K and I Ca-L, achieved by combination of selective inhibitors, has been shown to have low proarrhythmic activity in the rabbit model of Carlsson et al. (16,17,27,38), probably as a result of the early after depolarization–suppressing or repolarization prolongation–limiting effect of I Ca-L and I Na inhibition, respectively. Intravenous amiodarone inhibits α-adrenoceptor in a noncompetitive manner (39), which could also contribute to the low proarrhythmic activity of the drug in this model.

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Torsade de pointes with d -sotalol and almokalant

In the current study, only d -sotalol and almokalant induced TdP. Moreover, these drugs induced large number of ventricular tachycardias and noncomplex arrhythmias, e.g., ventricular premature beat, bigeminy, and salvo. In fact, TdP was always preceded by frequent occurrence of noncomplex arrhythmias and sometimes, ventricular tachycardia. Conversely, similarly to quinidine, d -sotalol and almokalant also induced conduction blocks in a dose-related manner. This agrees with the results of Lu et al. (25), who showed that the selective I Kr blocker dofetilide and the nonselective I Kr blockers clofilium, quinidine, and terfenadine evoke conduction blocks frequently in α 1 -adrenergically stimulated rabbits. The primary targets of I Kr blockers are the cells of the conductive system and the M cells (3). Extreme repolarization and refractory period prolongation of these cells may lead to blocks, especially at the relatively short cycle length of the rabbit. The initiating mechanism of TdP is probably related to ventricular premature beats, whereas the maintenance mechanism is related to reentry (3). Thus, frequent noncomplex arrhythmias and conduction blocks may play a role in the generation of TdP and other reentrant arrhythmias, e.g., ventricular tachycardia and ventricular fibrillation in the rabbit model of acquired long QT syndrome.

In the current study, TdPs induced by either d -sotalol or almokalant were always preceded by markedly prolonged QT and QT c intervals, elevated blood pressure, and relatively slow ventricular rate. In contrast, neither of these factors coincided, nor did TdP develop in amiodarone- and quinidine-treated animals. These findings suggest that the coincidence of markedly prolonged QT and QT c intervals, elevated blood pressure, and slow ventricular rate may be a prerequisite for TdP generation in the presently used animal model.

In the present investigation, almokalant and d -sotalol showed different proarrhythmic profiles in terms of ventricular tachyarrhythmia onset. These arrhythmias occurred before, during, or after the administration of the middle dose of almokalant compared with those after the administration of the middle dose of d -sotalol. This earlier onset of ventricular tachyarrhythmias coincided with elevated sinus rate in the almokalant group, whereas the effects of d -sotalol and almokalant were not different on the QT and QT c intervals, ventricular rate, and blood pressure. This suggests that not only QT and QT c prolongation, relatively low ventricular rate, and elevated blood pressure play a role in arrhythmia genesis in the rabbit model of TdP. Similarly to our findings, the proarrhythmic effects of these two relatively selective potassium channel blockers were different in a dog torsade model (40). Although both drugs widened QT in that study (40) also, only almokalant evoked TdP spontaneously, whereas programmed electrical stimulation was necessary to initiate this arrhythmia in d -sotalol-treated animals. In that study, almokalant increased the interventricular dispersion of repolarization and the number of early after depolarizations to a greater degree than d -sotalol.

Almokalant is a selective I Kr inhibitor (41), whereas d -sotalol inhibits I Kr and other potassium currents, i.e., transient outward current (I to ) and inward rectifier current (I K1 ) (8). This difference in potassium channel selectivity, or d -sotalol's residual β-adrenoceptor-blocking property (42) or possible differences between the time course of the development of repolarization dispersion between the drugs, might play a role in this different proarrhythmic profile of the two drugs.

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Infusion rate of antiarrhythmics and occurrence of torsade de pointes

In the current study there was no direct correlation between the occurrence of TdP and the infusion rate or the dose of d -sotalol and almokalant, because the percentage incidences of this arrhythmia were greatest after the administration of the middle doses of the drugs. Carlsson et al. (13) reported that the occurrence of TdP is dependent on infusion rate. They suggested that this was fundamental to the occurrence of TdP. The present data demonstrate that dependence on infusion rate may be particular to the continuous infusion (13,15) and not to short-term infusions.

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Selective and nonselective I Kr blockers evoke torsade de pointes in the rabbit

Recently Lu et al. (25) concluded that nonselective I Kr blockers do not induce TdP in the rabbit model of long QT syndrome, as quinidine and terfenadine did not evoke this arrhythmia in their studies, whereas dofetilide and clofilium did so. However, clofilium is not a selective I Kr blocker, as it affects I Na, I Ca-L, I to, I Kr, the slow component of the delayed rectifier potassium currents (I Ks ), and the ATP-dependent potassium current (I KATP ) in a similar concentration range (43–46). Similarly to clofilium, other nonselective I Kr blockers, e.g., the combined I Kr and I Ca-L blocker BRL-32872 (16), the combined I Kr and I Ks blocker azimilide (47), the combined I Kr and I K1 blocker sematilide (12,47), the combined I Na enhancer and I Kr blocker ibutilide (14,17), and the combined β-adrenoceptor and multiple potassium channel inhibitor d, l -sotalol (47), have hitherto been shown to induce TdP in the rabbit model of Carlsson et al. In our study, almokalant and d -sotalol evoked TdP. Almokalant is a selective I Kr inhibitor (41), whereas d -sotalol inhibits I Kr, I to, and I K1(8). These results and the previously published studies with several nonselective I Kr blockers show that I Kr selectivity is not an exclusive factor in TdP generation in the rabbit model of acquired long QT syndrome.

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CONCLUSIONS

Our results suggest that the incidence of TdP may not depend on the infusion rate or the dose of antiarrhythmics when graded doses are applied with an interval. Furthermore, TdP generation may be a multifactorial process in the rabbit model of TdP, and the contributing factors may be slightly different from those in humans. Thus, drugs that have different pharmacodynamic actions at high heart rates (seen in the rabbit) compared with effects at lower heart rates (seen in humans), drugs that decrease blood pressure markedly, or drugs that possess α 1 -adrenoceptor inhibitory effects could elicit a false-negative outcome (i.e., low proarrhythmic activity) in the rabbit model of TdP.

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Acknowledgments:

The authors thank Dr. Leif Carlsson (Astra-Zeneca, Sweden) for the generous gift of almokalant; Mrs. Mária Györfi-Kosztka and Mrs. Zsuzsa Ábrahám for their skillful technical assistance; and Dr. Susan J. Coker (Department of Pharmacology and Therapeutics, The University of Liverpool, Liverpool, U.K.) and Dr. Michael J. Curtis (The Rayne Institute, St. Thomas' Hospital, London, U.K.) for their helpful comments in the preparation of the manuscript.

This work was supported by the Hungarian National Research Fund (OTKA T 022300), Ministry of Health (ETT 06127, 06521, 10/2000), and the Ministry of Education (FKFP 0263/1999).

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Keywords:

α1 -Adrenoceptor stimulation; Almokalant; Amiodarone; Quinidine; d -sotalol; Torsade de pointes

© 2002 Lippincott Williams & Wilkins, Inc.