Pharmacology of the class III antiarrhythmic agent sematilide in patients with arrhythmias

Halothane sensitizes the canine heart to pharmacological IKr blockade

The effects of halothane and pentobarbital on the cardiovascular system were compared using the in vivo canine models. The ventricular repolarization process was longer under the halothane-anesthesia than pentobarbital-anesthesia. Intravenous administration of a selective blocker of rapidly activating delayed rectifier K+ currents (I(Kr)) sematilide prolonged the ventricular repolarization period without affecting the intraventricular conduction under both anesthesia; however, the potency was about 1.5-folds greater under the halothane-anesthesia than pentobarbital-anesthesia. These results suggest that halothane can more effectively sensitize the heart to pharmacological I(Kr) blockade, resulting in the excessive QT interval prolongation. Thus, the halothane-anesthetized canine model can be useful for predicting the in vivo I(Kr) blocking property of new drugs.

Comparison of the in vivo electrophysiological and proarrhythmic effects of amiodarone with those of a selective class III drug, sematilide, using a canine chronic atrioventricular block model

Amiodarone effectively blocks both the sodium and calcium channels and beta-adrenoceptors, in addition to blocking several potassium currents including IKr, IKs, Ito, IK1, IKACh and IKNa. The incidence of clinical torsade de pointes (TdP) associated with amiodarone has been reported to be low and the present study compared the proarrhythmic potential of amiodarone with that of a selective IKr channel blocker, sematilide, using a canine chronic atrioventrucular block model. Amiodarone or sematilide (3 and 30 mg/kg; n=4 for each group) was administered orally without anesthesia under continuous ECG monitoring. Both drugs prolonged the QT interval, although the onset was faster for sematilide. The high dose of sematilide induced TdP in 3 of 4 animals, which caused their death, but neither the low dose of sematilide nor the 2 dosages of amiodarone induced lethal ventricular arrhythmias. These results suggest that IKr channel inhibition by amiodarone with its additional ion channel blocking action may contribute to the prevention of TdP.

Effects of a typical I(Kr) channel blocker sematilide on the relationship between ventricular repolarization, refractoriness and onset of torsades de pointes

The effects of a typical I(Kr) channel blocker sematilide on the relationship between ventricular repolarization, refractoriness and onset of torsades de pointes (TdP) were studied using the canine isolated, blood-perfused ventricular septum preparation with monophasic action potential (MAP) recording. Intracoronary infusion of sematilide (10-100 microg/min) prolonged the repolarization phase and effective refractory period, the extent of which was greater in the former than in the latter, resulting in prolongation of terminal repolarization process. Prolonging the basic pacing cycle length from 400 to 600 ms and/or increasing the drug doses enhanced each of these actions. Reverse use-dependence was obvious in the drug-induced prolongation of MAP duration, but it was less clear in the effective refractory period. More importantly, during sematilide infusion, in preparations paced at longer basic cycle length of 600 - 2000 ms, TdP-like polymorphic ventricular tachycardia was repeatedly induced by an extra-stimulus applied on the terminal repolarization phase, which indicates the appearance of electrically vulnerable period. Prolonging the basic pacing cycle length and/or increasing the drug doses prolonged this electrically vulnerable period in parallel with the terminal repolarization phase. These results suggest that prolongation of the terminal repolarization process by sematilide would enhance the chance of conduction slowing at less complete repolarization levels, which may be associated with a high incidence of TdP induction.

Simultaneous assessment of the hemodynamic, cardiomechanical, and electrophysiological effects of terfenadine on the in vivo canine model

Prolongation of the QT interval, sometimes leading to torsades de pointes, has been clinically reported during terfenadine treatment. However, information regarding the cardiovascular profile of terfenadine is still limited, particularly in in vivo animal models. In the current study, we examined the cardiovascular effects of terfenadine using halothane-anesthetized, closed-chest in vivo canine models (n = 6) to better simulate the clinical situation. Intravenous infusion of 0.3 mg/kg of terfenadine over 10 min, which would attain the antihistaminic plasma concentration, reduced the heart rate and left ventricular contractility and prolonged the repolarization period as well as the ventricular effective refractory period. An additional infusion of a ten times higher dose of terfenadine over 10 min caused hypotension and increased left ventricular preload and atrioventricular conduction time, in addition to potentiating the changes observed by the lower dose. A reverse use-dependent prolongation of the repolarization period was observed after the higher dose infusion. Moreover, early afterdepolarization-like potential was detected in four out of six experiments. Since each suppressive effect can become deleterious during terfenadine overdose, caution must be taken for those patients with potential cardiac dysfunction and with the risk of elevated plasma drug concentrations.

HERG, a primary human ventricular target of the nonsedating antihistamine terfenadine

BACKGROUND

Administration of the antihistamine terfenadine (Seldane) to patients may result in acquired long QT syndrome and ventricular arrhythmias. One human cardiac target is Kv1.5, which expresses the ultrarapid outward K+ current (Ikur) in atrium but may play only a minor role in ventricle. Another possible target is HERG, the human ether-a-go-go-related gene that expresses a delayed rectifier current (IKr) in human ventricle and produces hereditary long QT syndrome when defective.

METHODS AND RESULTS

We therefore heterologously expressed Kv1.5 and HERG in Xenopus oocytes to compare the sensitivity of each to terfenadine. We found that HERG was 10 times more sensitive than Kv1.5 to terfenadine block. The apparent Kd values for HERG and Kv1.5 currents were 350 nmol/L and 2.7 mumol/L, respectively. These Kd values compare well with values reported for terfenadine block of IKr and IKur currents in human atrial myocytes. The Kd value for HERG block is relevant to the toxicity of the antihistamine, since the clinical terfenadine concentrations in human plasma may reach the 100 nmol/L range.

CONCLUSIONS

Terfenadine carboxylate, the major metabolite of terfenadine, does not block either HERG or Kv1.5, which agrees with the hypothesis that the buildup of parent terfenadine is the likely explanation for its cardiotoxicity. We propose that the blocking of HERG by terfenadine explains the acquired long QT syndrome. HERG is likely to be the primary target for the known cardiotoxic effects of other, related antihistamines.

Pharmacokinetics of sematilide in renal failure

A randomized, two-period, two-treatment study was conducted to investigate the effect of renal impairment on the pharmacokinetics of the Class III antiarrhythmic sematilide HCl. The pharmacokinetic-pharmacologic effect relationship and tolerability of sematilide HCl were also studied. The study included 22 subjects: 6 healthy volunteers and 16 patients with various degrees of renal impairment, including functionally anephric patients on intermittent hemodialysis. Separated by a 14-day washout period, the subjects received a constant rate intravenous infusion of 40 mg sematilide HCl over 30 minutes and a tablet containing 100 mg of the drug. The functionally anephric patients were studied during and off dialysis after intravenous and oral administration of the drug, respectively. Blood and urine samples were collected at defined times up to 48 hours and 72 hours, respectively, after administration. Sematilide concentrations in plasma, urine, and dialysate were measured by a validated high-performance liquid chromatography (HPLC) method with ultraviolet detection. The pharmacokinetic data analysis used a compartment model independent approach. The heart rate-corrected Lead II QT interval was recorded as a pharmacologic endpoint. Subjective symptoms, cardiovascular parameters, routine serum chemistry, and hematology and urinalysis parameters were measured to assess tolerability. Mean renal clearance after intravenous and oral administration was reduced in patients with severe renal impairment. Statistically significant linear correlations existed between total clearance of sematilide and creatinine clearance for all subjects who could be evaluated after both intravenous and oral administration. Steady-state volume of distribution, absolute bioavailability, and nonrenal clearance of sematilide were independent of renal function. The mean dialysis clearance was 98 mL/min, indicating effective removal of the drug by hemodialysis. In accord with the drug's Class III pharmacologic activity, the heart rate corrected Lead II QT intervals were prolonged in all subjects after intravenous and oral administration of the drug. The pharmacologic effect to plasma concentration relationship in renal patients and in healthy subjects was comparable. Based on the experimentally determined linear relationship between total clearance of sematilide and creatinine clearance, modified dose regimens for sematilide HCl in patients with renal impairment and functionally anephric patients off hemodialysis were developed.

Comparison of direct negative chronotropic and positive inotropic effects of sematilide to those of E-4031 and MS-551 and the reverse frequency-dependent prolongation of cardiac refractoriness of sematilide

Direct cardiac effects of sematilide, a new class III antiarrhythmic drug, were compared with those of E-4031 and MS-551 in canine isolated blood-perfused heart preparations. Doses of sematilide, E-4031, and MS-551 causing a 10% decrease in the spontaneous sinoatrial beating rate were 58 +/- 15, 9 +/- 5, and 84 +/- 10 micrograms (n = 5); those causing a 10% increase in developed tension of the papillary muscle were 485 +/- 49, 17 +/- 2, and 267 +/- 50 micrograms (n = 6); and those causing a 10% prolongation of effective refractory period (ERP) of the atrioventricular node were 68 +/- 10, 11 +/- 2, and 53 +/- 15 micrograms (n = 5), respectively. There were few effects on atrio-His or His-ventricular intervals. Also, in in situ open-chest dog hearts, the percent increases in ERP of the atrioventricular conduction system caused by 1 mg/kg of sematilide were 21 +/- 3, 16 +/- 2 and 9 +/- 1% at cycle lengths of 800, 600, and 400 ms, respectively (p < 0.01; n = 8). These results indicate that (a) sematilide, as well as E-4031 and MS-551, has direct negative chronotropic and positive inotropic effects and prolongs cardiac refractoriness without affecting conduction velocities; (b) quantitatively, the cardiac effects of sematilide were almost identical to those of MS-551 and five to ten times less potent than those of E-4031; (c) and prolongation of ERP of the atrioventricular conduction system by sematilide occurred in a reverse frequency-dependent manner.

Hemodynamic effects of intravenous sematilide in patients with congestive heart failure: a class III antiarrhythmic agent without cardiodepressant effects

OBJECTIVES

This study sought to evaluate the hemodynamic effects of intravenous sematilide hydrochloride, a selective class III antiarrhythmic agent, in patients with heart failure and left ventricular systolic dysfunction.

BACKGROUND

Class I antiarrhythmic agents, which primarily slow conduction, can depress ventricular function, particularly in patients with heart failure. In contrast, pure class III agents, which selectively prolong repolarization, do not adversely affect hemodynamic variables in animal models, but there are no data evaluating their hemodynamic effects in humans.

METHODS

In 39 patients with congestive heart failure and a left ventricular ejection fraction < 40%, hemodynamic and electrocardiographic measurements were obtained at baseline, after a loading dose and during a maintenance infusion of intravenous sematilide using either a low (0.75 then 0.3 mg/min) or high dose (1.5 then 0.6 mg/min) regimen. The study had an 80% power to detect clinically meaningful differences in hemodynamic variables.

RESULTS

Both low (n = 20) and high (n = 19) dose sematilide infusions produced dose-dependent increases in QT interval (5 +/- 8% [mean +/- SD] and 18 +/- 10%, respectively) and corrected QT interval (4 +/- 8% and 14 +/- 10%), and high dose sematilide decreased heart rate by 7 +/- 10% (all p < 0.025 vs. baseline). Neither dose regimen had a statistically significant effect on any other hemodynamic variable, including mean arterial, right atrial, pulmonary artery and pulmonary capillary wedge pressures; cardiac index, stroke volume, systemic and pulmonary vascular resistances; and left ventricular stroke work index. Sematilide showed no adverse hemodynamic effects in patients with left ventricular ejection fraction < or = 25% or > 25% and in patients with cardiac index < 2 or > or = 2 liters/min per m2. Sustained polymorphic ventricular tachycardia (n = 1) and excessive QT prolongation (n = 4) were seen during the high dose.

CONCLUSIONS

Sematilide, in the doses administered, prolonged repolarization but did not alter hemodynamic variables in patients with heart failure. These data suggest that class III antiarrhythmic agents, which selectively prolong repolarization, are not cardiodepressant but may be proarrhythmic in humans, especially at high doses.

Two components of the delayed rectifier K+ current in ventricular myocytes of the guinea pig type. Theoretical formulation and their role in repolarization

Two distinct delayed rectifier K+ currents, IKr and IKs, were found recently in ventricular cells. We formulated these currents theoretically and investigated their roles in action potential repolarization and the restitution of action potential duration (APD). The Luo-Rudy (L-R) model of the ventricular action potential was used in the simulations. The single delayed rectifier K+ current in the model was replaced by IKr and IKs. Our results show that IKs is the major outward current during the plateau repolarization. A specific block of either IKr or IKs can effectively prolong APD to the same degree. Therefore, either channel provides a target for class III antiarrhythmic drugs. In the simulated guinea pig ventricular cell, complete block of IKr does not result in early afterdepolarizations (EADs). In contrast, > 80% block of IKs results in abnormal repolarization and EADs. This behavior reflects the high IKs-to-IKr density ratio (approximately 8:1) in this cell and can be reversed (ie, IKr block can cause EADs) by reducing the ratio of IKs to IKr. The computed APD restitution curve is consistent with the experimental behavior, displaying fast APD variation at short diastolic intervals (DIs) and downward shift at longer DIs with the decrease of basic drive cycle length (BCL). Examining the ionic currents and their underlying kinetic processes, we found that activation of both IKr and IKs is the primary determinant of the APD restitution at shorter DIs, with Ca2+ current through L-type channels (ICa) playing a minor role. The rate of APD change depends on the relative densities of IKr and IKs; it increases when the IKr-to-IKs density ratio is large. The BCL-dependent shift of restitution at longer DIs is primarily attributed to long-lasting changes in [Ca2+]i. This in turn causes different degrees of Ca(2+)-dependent inactivation of ICa and different degrees of Ca(2+)-dependent conductance of IKs at very long DIs (> 5 s) for different BCLs. This BCL dependence of ICa and IKs that is secondary to long-lasting changes in [Ca2+]i is responsible for APD changes at long DIs and can be viewed as a "memory property" of cardiac cells.

Ibutilide, a methanesulfonanilide antiarrhythmic, is a potent blocker of the rapidly activating delayed rectifier K+ current (IKr) in AT-1 cells. Concentration-, time-, voltage-, and use-dependent effects

BACKGROUND

Ibutilide is an action potential-prolonging antiarrhythmic currently in clinical trials. The drug shares structural similarities with E-4031 and dofetilide, specific blockers of the rapidly activating delayed rectifier K+ current (IKr). However, previous in vitro studies in guinea pig myocytes have indicated that ibutilide does not block IKr but rather increases a slow inward sodium current.

METHODS AND RESULTS

In this study, we compared the effects of ibutilide with those of dofetilide on outward current in mouse atrial tumor myocytes (AT-1 cells), a preparation in which, unlike guinea pig, a typical IKr is the major delayed rectifier and can be readily recorded in isolation from other currents. In AT-1 cells, ibutilide and dofetilide were both potent IKr blockers, with EC50 values of 20 (n = 12) and 12 (n = 8) nmol/L, respectively, at +20 mV. The time and voltage dependence of IKr inhibition by the two compounds were virtually identical. The following characteristics were most consistent with open channel block: (1) block increased with depolarizing pulses; (2) block increased with longer pulses; (3) currents deactivated more slowly in the presence of drug, resulting in a "crossover" typical of open channel block; and (4) with repetitive pulsing after drug wash-in, use-dependent block was observed.

CONCLUSIONS

These data suggest that the clinical actions of ibutilide are mediated at least in part by block of IKr; an effect on inward currents is not excluded. AT-1 cells are a useful model system for the study of drug block of this important repolarizing current.

Torsade de pointes

The polymorphic ventricular tachycardia torsade de pointes can occur in the congenital long QT syndromes or as a consequence of therapy with QT-prolonging drugs. The latter can include not only antiarrhythmic drugs such as quinidine, but also a number of drugs which are not usually considered to have major cardiovascular effects: these include nonsedating antihistamines, such as terfenadine; antibiotics such as erythromycin; and neuroleptics such as thioridazine. The electrocardiographic hallmark of both the congenital and acquired forms of the long QT syndrome is marked QT(U) lability, particularly as a function of heart rate. The underlying mechanism is thought to be triggered activity arising as a consequence of early afterdepolarizations. An understanding of the basic mechanism has led to an understanding of the effective forms of therapy, which include maneuvers to include the heart rate (pacing, isoproterenol) as well as maneuvers which may not necessarily alter the QT interval but may prevent the arrhythmia (magnesium, beta blockers). Intensive study of the clinical features and basic mechanisms underlying torsade de pointes has led to the definition of a new mechanism for cardiac arrhythmias; understanding such mechanisms may ultimately lead to the development of safer antiarrhythmic therapy.