Torsadegenic action of the antipsychotic drug sulpiride assessed using in vivo canine models

The canine chronic atrioventricular block model in cardiovascular preclinical drug research

Ventricular cardiac arrhythmia is a life threating condition arising from abnormal functioning of many factors in concert. Animal models mirroring human electrophysiology are essential to predict and understand the rare pro- and anti-arrhythmic effects of drugs. This is very well accomplished by the canine chronic atrioventricular block (CAVB) model. Here we summarize canine models for cardiovascular research, and describe the development of the CAVB model from its beginning. Understanding of the structural, contractile and electrical remodelling processes following atrioventricular (AV) block provides insight in the many factors contributing to drug-induced arrhythmia. We also review all safety pharmacology studies, efficacy and mechanistic studies on anti-arrhythmic drugs in CAVB dogs. Finally, we compare pros and cons with other in vivo preclinical animal models. In view of the tremendous amount of data obtained over the last 100 years from the CAVB dog model, it can be considered as man's best friend in preclinical drug research. LINKED ARTICLES: This article is part of a themed issue on Preclinical Models for Cardiovascular disease research (BJP 75th Anniversary). To view the other articles in this section visit http://onlinelibrary.wiley.com/doi/10.1111/bph.v179.5/issuetoc.

Acute exposure to N-Ethylpentylone induces developmental toxicity and dopaminergic receptor-regulated aberrances in zebrafish larvae

N-Ethylpentylone (NEP) is one of the most recent novel stimulants, and there is limited understanding of its toxicity. Here we employed zebrafish model for analyzing the effects of NEP on early embryos and cardiovascular and nervous systems at late developmental stages. We first observed multi-malformations in early embryos and larvae after NEP administration, together with significant deregulations of brain and heart development-associated genes (neurog1, her6, elavl3, nkx2.5, nppa, nppb, tnnt2a) at transcriptional level. Low-dosed NEP treatment induced an anxiety-like phenotype in zebrafish larvae, while higher doses of NEP exerted an inhibitory effect on locomotion and heart rate. Besides, the expression of th (tyrosine hydroxylase) and th2 (tyrosine hydroxylase 2), identifying dopamine (DA) release, were significantly increased during one-hour free swimming after effective low-dosed NEP administration, along with the upregulation of gene fosab and fosb related to stress and anxiety response. D1R antagonist SCH23390 and D2R antagonist sulpiride partially alleviated the aberrances of locomotion and heart rate, indicating dopaminergic receptors were involved in the bidirectional dosage-dependent pattern of NEP-induced performance. Meanwhile, sulpiride offset the upregulated expression of th, th2 and fosab in the group of 1.5 μM NEP, which highlighted the significant role of D2R in NEP-induced locomotive effects. This study systematically described the developmental, neuronal and cardiac toxicity of NEP in zebrafish, and identified the dopaminergic receptors as one of the downstream effectors of NEP administration.

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.

BPC 157 counteracts QTc prolongation induced by haloperidol, fluphenazine, clozapine, olanzapine, quetiapine, sulpiride, and metoclopramide in rats

AIM

Commonly, neuroleptics and prokinetics induce a prolonged QTc interval. In this study, stable gastric pentadecapeptide BPC 157 counteracts the prolongation of the QTc interval in Wistar rats that underwent daily administration of dopamine neuroleptics or prokinetics. Previously, in rats and mice, BPC 157 counteracted neuroleptic-induced catalepsy and gastric ulcers.

MAIN METHODS

To counteract neuroleptic- or prokinetic-induced prolongation of the QTc interval, rats were given a BPC 157 regimen once daily over seven days (10μg, 10ng/kg ip) immediately after each administrations of haloperidol (0.625, 6.25, 12.5, and 25.0mg/kg ip), fluphenazine (0.5, 5.0mg/kg ip), clozapine (1.0, 10.0mg/kg ip), quetiapine (1.0, 10.0mg/kg ip), sulpiride (1.6, 16.0mg/kg ip), metoclopramide (2.5, 25.0mg/kg ip) or (1.0, 10.0mg/kg ip). Controls simultaneously received saline (5ml/kg ip). To assess the ECG presentation before and after neuroleptic/prokinetic medication, the assessment was at 1, 2, 3, 4, 5, 10, 15, 20 and 30min (first administration) as well as at 30min, 60min and 24h (first administration and subsequent administrations) and the ECG recording started prior to drug administration.

KEY FINDINGS

Since very early, a prolonged QTc interval has been continually noted with haloperidol, fluphenazine, clozapine, olanzapine, quetiapine, sulpiride, and metoclopramide in rats as a central common effect not seen with domperidone. Consistent counteraction appears with the stable gastric pentadecapeptide BPC 157. Thus, BPC 157 rapidly and permanently counteracts the QTc prolongation induced by neuroleptics and prokinetics.

SIGNIFICANCE

Pentadecapeptide BPC 157 is suited for counteracting a prolonged QT interval.

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.

Sudden death associated with QT interval prolongation and KCNQ1 gene mutation in a family of English Springer Spaniels

Background

A 5‐year‐old, healthy English Springer Spaniel died suddenly 4 months after delivering a litter of 7 puppies. Within 4 months of the dam's death, 3 offspring also died suddenly.

Hypothesis

Abnormal cardiac repolarization, caused by an inherited long QT syndrome, is thought to be responsible for arrhythmias leading to sudden death in this family.

Animals

Four remaining dogs from the affected litter and 11 related dogs.

Methods

Physical examination and resting ECG were done on the littermates and 9 related dogs. Additional tests on some or all littermates included echocardiogram with Doppler, Holter monitoring, and routine serum biochemistry. Blood for DNA sequencing was obtained from all 15 dogs.

Results

Three of 4 littermates examined, but no other dogs, had prolonged QT intervals with unique T‐wave morphology. DNA sequencing of the KCNQ1 gene identified a heterozygous single base pair mutation, unique to these 3 dogs, which changes a conserved amino acid from threonine to lysine and is predicted to change protein structure.

Conclusions and Clinical Importance

This family represents the first documentation in dogs of spontaneous familial QT prolongation, which was associated with a KCNQ1 gene mutation and sudden death. Although the final rhythm could not be documented in these dogs, their phenotypic manifestations of QT interval prolongation and abnormal ECG restitution suggested increased risk for sudden arrhythmic death. The KCNQ1 gene mutation identified is speculated to impair the cardiac repolarizing current I_Ks, similar to KCNQ1 mutations causing long QT syndrome 1 in humans.

Response of i(kr) and HERG currents to the antipsychotics tiapride and sulpiride

The human ether-a-go-go-related gene (hERG) channel is important for repolarization in human myocardium and is a common target for drugs that prolong the QT interval. We studied the effects of two antipsychotics, tiapride and sulpiride, on hERG channels expressed in Xenopus oocytes and also on delayed rectifier K(+) currents in guinea pig cardiomyocytes. Neither the amplitude of the hERG outward currents measured at the end of the voltage pulse, nor the amplitude of hERG tail currents, showed any concentration-dependent changes with either tiapride or sulpiride (3~300 µM). However, our findings did show that tiapride increased the potential for half-maximal activation (V(1/2)) of HERG at 10~300 µM, whereas sulpiride increased the maximum conductance (G(max)) at 3, 10 and 100 µM. In guinea pig ventricular myocytes, bath applications of 100 and 500 µM tiapride at 36℃ blocked rapidly activating delayed rectifier K(+) current (I(Kr)) by 40.3% and 70.0%, respectively. Also, sulpiride at 100 and 500 µM blocked I(Kr) by 38.9% and 76.5%, respectively. However, neither tiapride nor sulpiride significantly affected the slowly activating delayed rectifier K(+) current (I(Ks)) at the same concentrations. Our findings suggest that the concentrations of the antipsychotics required to evoke a 50% inhibition of I(Kr) are well above the reported therapeutic plasma concentrations of free and total compound.

Drug-induced block of cardiac HERG potassium channels and development of torsade de pointes arrhythmias: the case of antipsychotics

The prolongation of the cardiac repolarization process, a result of the blocking of the Human Ether-a-go-go Related Gene potassium channel, is an undesired accessory property shared by many pharmacological classes of non-cardiovascular drugs. Often the delayed cardiac repolarization process can be identified by a prolongation of the QT interval of the electrocardiograph. In these conditions, premature action potentials can trigger a dangerous polymorphic ventricular tachyarrhythmia, known as torsade de pointes, which occasionally can result in lethal ventricular fibrillation. In this work, brief descriptions of the electrophysiological basis of torsade de pointes and of the several pharmacological classes of torsadogenic drugs are given. Attention is focused on antipsychotics, with a deeper overview on the experimental and clinical reports about their torsadogenic properties.

Long QT syndrome and torsades de pointes induced by acute sulpiride poisoning

Sulpiride, a selective dopamine D2 antagonist and a substituted benzamide derivative, is considered a safe antipsychotic and antidepressant agent with few adverse effects on the cardiovascular system. Sulpiride-induced torsades de pointes is rare. We report a case of long QT syndrome and torsades de pointes induced by ingestion of 1.5 g of sulpiride. Ventricular arrhythmia was initially treated with amiodarone, without success. Eventually, lidocaine and magnesium sulfate successfully terminated the ventricular arrhythmia. The patient was discharged uneventfully after 3 days of hospitalization. This case illustrates the fact that acute sulpiride poisoning may lead to life-threatening ventricular arrhythmia. Early recognition followed by effective therapy is crucial. Intensive cardiac monitoring is recommended for sulpiride poisoning.

Cellular mechanism of the QT prolongation induced by sulpiride

In this study, the authors investigated the electrophysiological effect of sulpiride on cardiac repolarization using conventional microelectrode recording techniques in isolated canine Purkinje fibers and a whole-cell patch clamp technique in transiently transfected cells with the hERG, KCNQ1/KCNE1, KCNJ2, and SCN5A cDNA and in rat cardiac myocytes for I(Ca). In studies of action potential duration, 10 microM, 100 microM, 300 microM, and 1 mM sulpiride prolonged action potential duration in a concentration-dependent manner. In studies of cardiac ion channels, sulpiride did not significantly affect I(Na), I(Ca), I(Ks), I(K1), except for I(Kr). Sulpiride dose-dependently decreased the hERG tail current. It is considered that the prolonged action potential duration by sulpiride was mainly the result of inhibition of the hERG channel. The data suggest that the clinical use of sulpiride is reasonable within therapeutic plasma concentrations, but all patients taking this drug should be cautiously monitored for clinical signs of long-QT syndrome and severe arrhythmia.

Sensitive and reliable proarrhythmia in vivo animal models for predicting drug-induced torsades de pointes in patients with remodelled hearts

As an increasing number of non-cardiac drugs have been reported to cause QT interval prolongation and torsades de pointes (TdP), we extensively studied the utility of atrioventricular (AV) block animals as a model to predict their torsadogenic action in human. The present review highlights such in vivo proarrhythmia models. In the case of the canine model, test substances were administered p.o. at conscious state >4 weeks after the induction of AV block, with subsequent Holter ECG monitoring to evaluate drug effects. Control AV block dogs (no pharmacological treatment) survive for several years without TdP attack. For pharmacologically treated dogs, drugs were identified as high, low or no risk. High-risk drugs induced TdP at 1-3 times the therapeutic dose. Low-risk drugs did not induce TdP at this dose range, but induced it at higher doses. No-risk drugs never induced TdP at any dose tested. Electrophysiological, anatomical histological and biochemical adaptations against persistent bradycardia-induced chronic heart failure were observed in AV block dogs. Recently, we have developed another highly sensitive proarrhythmia model using a chronic AV block cynomolgus monkey, which possesses essentially the same pathophysiological adaptations and drug responses as those demonstrated in the canine model. As a common remodelling process leading to a diminished repolarization reserve may present in patients who experience drug-induced TdP and in the AV block animals, the in vivo proarrhythmia models described in this review may be useful for predicting the risk of pharmacologically induced TdP in humans.

Cardiovascular profile of the canine torsades de pointes arrhythmia model assessed by echocardiographic and haemodynamic methods

Chronic atrioventricular block dogs have been established as an in vivo model of drug-induced torsades de pointes arrhythmias. We compared the cardiovascular profile of the canine model with that of sham-operated animals using echocardiographic and haemodynamic methods. In the echocardiographic study, the larger diameters of the left atria, inferior vena cava and left ventricle in end-diastole in addition to greater fractional shortening, end-diastolic volume, stroke volume and ejection fraction were more often detected in the chronic atrioventricular block dogs than in the sham-operated animals. During haemodynamic examination, lower cardiac output and higher pulmonary capillary wedge pressure were detected in chronic atrioventricular block dogs more than in sham-operated animals; however, these changes were within the physiological limits, and the results suggest that the chronic atrioventricular block dogs have a pathophysiological profile of chronic compensated heart failure.

A quantitative analysis of the effect of cycle length on arrhythmogenicity in hypokalaemic Langendorff-perfused murine hearts

The clinically established proarrhythmic effect of bradycardia and antiarrhythmic effect of lidocaine (10 μM) were reproduced in hypokalaemic (3.0 mM K⁺) Langendorff-perfused murine hearts paced over a range (80–180 ms) of baseline cycle lengths (BCLs). Action potential durations (at 90% repolarization, APD₉₀s), transmural conduction times and ventricular effective refractory periods (VERPs) were then determined from monophasic action potential records obtained during a programmed electrical stimulation procedure in which extrasystolic stimuli were interposed following regular stimuli at successively decreasing coupling intervals. A novel graphical analysis of epicardial and endocardial, local and transmural relationships between APD₉₀, corrected for transmural conduction time where appropriate, and VERP yielded predictions in precise agreement with the arrhythmogenic findings obtained over the entire range of BCLs studied. Thus, in normokalaemic (5.2 mM K⁺) hearts a statistical analysis confirmed that all four relationships were described by straight lines of gradients not significantly (P > 0.05) different from unity that passed through the origin and thus subtended constant critical angles, θ with the abscissa (45.8° ± 0.9°, 46.6° ± 0.5°, 47.6° ± 0.5° and 44.9° ± 0.8°, respectively). Hypokalaemia shifted all points to the left of these reference lines, significantly (P < 0.05) increasing θ at BCLs of 80–120 ms where arrhythmic activity was not observed (∼63°, ∼54°, ∼55° and ∼58°, respectively) and further significantly (P < 0.05) increasing θ at BCLs of 140–180 ms where arrhythmic activity was observed (∼68°, ∼60°, ∼61° and ∼65°, respectively). In contrast, the antiarrhythmic effect of lidocaine treatment was accompanied by a significant (P < 0.05) disruption of this linear relationship and decreases in θ in both normokalaemic (∼40°, ∼33°, ∼39° and ∼41°, respectively) and hypokalaemic (∼40°, ∼44°, ∼50° and ∼48°, respectively) hearts. This extended a previous approach that had correlated alterations in transmural repolarization gradients with arrhythmogenicity in murine models of the congenital long QT syndrome type 3 and hypokalaemia at a single BCL. Thus, the analysis in terms of APD₉₀ and VERP provided a more sensitive indication of the effect of lidocaine than one only considering transmural repolarization gradients and may be particularly applicable in physiological and pharmacological situations in which these parameters diverge.

Sulpiride induced torsade de pointes

Antipsychotic agents are known to be associated with a long QT interval and torsade de pointes. We report a 69 year old female who suffered from a syncopal attack at a psychiatric hospital and was referred to our center. Torsade de pointes with a long QT interval (QTc=680 ms) was observed on the 12 lead ECG in the emergency department and intensive care unit. A careful drug history revealed that sulpiride was the culprit agent. After stopping the medication, the QT interval returned to normal (420 ms). The patients taking sulpiride should be closely monitored, especially when it is used in combination with other antidepressant agents.

Two cases of amisulpride overdose: a cause for prolonged QT syndrome

Two cases of deliberate self-poisoning with 5 g and 3.6 g of amisulpride, respectively, are reported. In both cases, QT prolongation and hypocalcaemia were noted. The QT prolongation appeared to respond to administration of i.v. calcium gluconate.

Sudden cardiac death in Andersen-Tawil syndrome

Andersen-Tawil syndrome (ATS) is an autosomal dominant or sporadic disorder characterized by periodic paralysis, dysmorphic features, and ventricular arrhythmias. Although ventricular tachycardia burden is quite high sudden cardiac death in ATS is rare. We describe a case with sudden cardiac death due to electrical storm a few days after ICD implantation in KCNJ2 mutation-negative ATS.

Analysis of proarrhythmic potential of antipsychotics risperidone and olanzapine in anesthetized dogs

In vitro electrophysiological studies have shown that second-generation antipsychotic drugs risperidone and olanzapine inhibit rapidly activating delayed rectifier K(+) currents and prolong action potential duration of the isolated ventricular myocardium. In this study, we analyzed in vivo cardiohemodynamic and electrophysiological profiles of risperidone and olanzapine using the halothane-anesthetized canine model to clarify their proarrhythmic potential. A clinically relevant dose of risperidone (0.03 mg/kg, i.v.) did not affect the ventricular repolarization process, whereas the supra-therapeutic doses (0.3 and 3 mg/kg, i.v.) prolonged the duration of monophasic action potential of the ventricle. Furthermore, the terminal repolarization period, an index of extent of electrical vulnerability, was prolonged after the supra-therapeutic doses. In contrast, therapeutic to supra-therapeutic doses of olanzapine (0.03-3 mg/kg, i.v.) hardly affected the ventricular repolarization process. Therefore, more caution has to be paid on the use of risperidone than olanzapine for patients with risks of the elevated plasma concentration.

Long-term bradycardia caused by atrioventricular block can remodel the canine heart to detect the histamine H1 blocker terfenadine-induced torsades de pointes arrhythmias

Although a second-generation histamine H(1) blocker terfenadine induced torsades de pointes (TdP) arrhythmias in patients via the blockade of a rapid component of delayed rectifier K(+) current (I(Kr)), such action of terfenadine has not been detected in previous animal models. We analysed the potential of the canine persistent atrioventricular block heart, a new in vivo proarrhythmia model, to detect a torsadogenic effect of terfenadine of an oral dose of 3 or 30 mg kg(-1). The doses can provide therapeutic to supra-therapeutic plasma concentrations as an anti-histamine. In 2 weeks of bradycardiac heart model, there were no significant changes in any of the electrocardiogram parameters after the administration of both doses of terfenadine. In 4-6 weeks of bradycardiac heart model, the low dose of terfenadine hardly affected any of the electrocardiogram parameters except that it induced TdP in one out of six animals. The high dose significantly decreased the atrial rate and ventricular rate, prolonged the QT interval, and induced TdP in five out of six animals. Moreover, temporal variability of repolarization increased after the high-dose administration. These results suggest that long-term bradycardia caused by atrioventricular block can remodel the canine heart to detect terfenadine-induced TdP.

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.

The antipsychotic and antiemetic drug prochlorperazine delays the ventricular repolarization of the in situ canine heart

Electropharmacological effect of the antipsychotic and antiemetic drug prochlorperazine was assessed using the halothane-anesthetized in vivo canine model (n = 5). Up to 10 times higher than the clinically relevant doses of prochlorperazine (< or = 3 mg/kg, i.v.) did not induce cardiohemodynamic collapse in the model. Meanwhile, clinically relevant to supratherapeutic doses (0.3 - 3 mg/kg, i.v.) prolonged the ventricular repolarization period in a dose-related and reverse-use dependent manner that could become proarrhythmic substrates. Thus, caution has to be paid on the use of prochlorperazine particularly for patients with risks of the elevated plasma drug concentration, compromised cardiac repolarization, and/or frequent ventricular premature beats.

In vivo experimental approach for the risk assessment of fluoroquinolone antibacterial agents-induced long QT syndrome

The proarrhythmic effects of fluoroquinolone antibacterial agents, sitafloxacin, gatifloxacin and moxifloxacin, were compared using three in vivo models. In the halothane-anesthetized dogs (n=5), intravenous 10-min infusion of gatifloxacin and moxifloxacin (1-3 mg/kg) prolonged the ventricular effective refractory period and the repolarization period to a similar extent, whereas sitafloxacin (1-3 mg/kg) prolonged the former only. No significant change was detected in other cardiovascular parameters. In the chronic complete atrioventricular block dogs (n=4), oral administration of 100 mg/kg of gatifloxacin (2 of 4) and moxifloxacin (3 of 4) induced torsades de pointes, which was not observed by sitafloxacin. In the alpha-chloralose-anesthetized rabbits (n=5), intravenous 20-min infusion of 60 mg/kg of gatifloxacin induced torsades de pointes (1 of 5) in the presence of methoxamine infusion, which was not observed by sitafloxacin or moxifloxacin. Thus, the halothane-anesthetized model is suitable for assessing QT prolongation, whereas the chronic complete atrioventricular block model is sensitive for detecting torsadogenic action of drugs. The alpha-chloralose-anesthetized model is the simplest and least expensive method, but its sensitivity to detect proarrhythmic action may be less great.

Electropharmacological effects of a spironolactone derivative, potassium canrenoate, assessed in the halothane-anesthetized canine model

While aldosterone receptor blockers improve survival of patients with congestive heart failure, spironolactone and its derivatives were recently shown to block ether-a-go-go-related gene (HERG) channels and native IKs and IKr currents in guinea pig ventricular myocytes. In this study, we examined in vivo electropharmacological effects of an active derivative of spironolactone, potassium canrenoate, using a halothane-anesthetized canine model. Potassium canrenoate was intravenously administered in three doses of 1, 10, and 100 mg/kg per 10 min with a pause of 20 min between doses (n = 5). The low dose hardly affected any of the cardiovascular parameters. The middle dose, a clinically recommended daily maximum i.v. dose, slightly inhibited the intraventricular conduction. The high dose decreased the heart rate, ventricular contraction and blood pressure, delayed the atrioventricular and intraventricular conduction, and prolonged the ventricular repolarization and refractory period. Increment in the refractoriness by the high dose was greater than that in the repolarization, resulting in the reduction of ventricular electrical vulnerability. This unique electrophysiological profile of potassium canrenoate may in part contribute to the favorable clinical results, whereas caution has to be paid on the cardiohemodynamic actions, particularly for patients with risk of elevated plasma drug concentration.

Electropharmacological and Proarrhythmic Effects of a Class III Antiarrhythmic Drug Nifekalant Hydrochloride Assessed Using the In Vivo Canine Models

Cardiovascular effects of Nifekalant were examined using halothane-anesthetized dogs, and its proarrhythmic potential was estimated with chronic complete atrioventricular block dogs. Nifekalant was intravenously administered to the halothane-anesthetized dogs in three doses of 0.03, 0.3, and 3 mg/kg/10 minutes with a pause of 20 minutes (n = 6). The low dose hardly affected any of the cardiovascular parameters. The middle dose, a clinically recommended antiarrhythmic dose, decreased the total peripheral resistance, increased the cardiac output, and prolonged the ventricular repolarization phase and effective refractory period. The high dose increased the left ventricular contraction, transiently decreased the mean blood pressure, and enhanced the atrioventricular conduction, besides potentiation of the changes induced by the middle dose. Increment in the repolarization phase by the high dose was greater than that in the refractoriness, leading to increase of ventricular electrical vulnerability. To the atrioventricular block animals, clinically relevant antiarrhythmic dose of 3 mg/kg p.o. of Nifekalant and its 10-times-higher dose were administered. The high dose prolonged QT interval leading to torsades de pointes in all animals (n = 5), which was not detected by the clinical dose (n = 5). These results suggest that antiarrhythmic dose of Nifekalant can be used safely; however, caution should be paid for patients complicating bradycardia and/or a risk of elevated plasma drug concentration.

Drug-induced torsades de pointes and implications for drug development

Torsades de pointes is a potentially lethal arrhythmia that occasionally appears as an adverse effect of pharmacotherapy. Recently developed understanding of the underlying electrophysiology allows better estimation of the drug-induced risks and explains the failures of older approaches through the surface ECG. This article expresses a consensus reached by an independent academic task force on the physiologic understanding of drug-induced repolarization changes, their preclinical and clinical evaluation, and the risk-to-benefit interpretation of drug-induced torsades de pointes. The consensus of the task force includes suggestions on how to evaluate the risk of torsades within drug development programs. Individual sections of the text discuss the techniques and limitations of methods directed at drug-related ion channel phenomena, investigations aimed at action potentials changes, preclinical studies of phenomena seen only in the whole (or nearly whole) heart, and interpretation of human ECGs obtained in clinical studies. The final section of the text discusses drug-induced torsades within the larger evaluation of drug-related risks and benefits.

Effects of mexiletine on the canine model of sparfloxacin-induced long QT syndrome

Potential utility of mexiletine for the treatment of sparfloxacin-induced long QT syndrome was assessed using the in vivo halothane-anesthetized canine model. At 30 min after the administration of a supratherapeutic dose of sparfloxacin (30 mg/kg, i.v.), the mean blood pressure and heart rate decreased, whereas repolarization process and effective refractory period of the ventricular muscle were significantly prolonged. Additional administration of a clinically recommended dose of mexiletine (3 mg/kg, i.v.) at this time point increased the mean blood pressure, suppressed ventricular contraction, delayed atrioventricular as well as intraventricular conduction, and shortened repolarization process and effective refractory period. The extent of abbreviation of the repolarization was more prominent than that of the refractoriness, indicating that mexiletine could decrease the electrical vulnerability of the heart during sparfloxacin overdose. Thus, mexiletine may become a promising pharmacological strategy against the drug-induced long QT syndrome.

[Effects of clinically available drugs on the repolarization process of the heart assessed by the in vivo canine models]

The proarrhythmic effects of class III antiarrhythmic agents and non-cardiovascular drugs, which have been shown to prolong QT interval, were assessed using two types of in vivo canine models. First, electrophysiological effects of dofetilide, nifekalant, amiodarone, cisapride, astemizole, sulpiride, haloperidol, and sparfloxacin were assessed using halothane-anesthetized dogs. Each drug prolonged the monophasic action potential (MAP) duration and effective refractory period (ERP) at clinically recommended daily doses. The extent of increase was greater in the refractoriness than in the repolarization only for amiodarone, indicating abbreviation of the terminal repolarization period. The reverse was true for the other drugs. Next, torsadogenic action of sematilide, nifekalant, amiodarone, cisapride, terfenadine, sulpiride, and sparfloxacin was assessed using chronic complete atrioventricular block dogs with Holter ECG monitoring in the conscious state. Oral administration of 1-10 times higher doses than the clinically relevant doses of the drugs induced polymorphic ventricular tachycardia torsades de pointes (TdP), except for amiodarone. These results indicate that the prolongation and backward shift of the terminal repolarization period may be closely related to the drug-induced TdP and suggest that these in vivo models can be used to screen proarrhythmic potential of new drugs.

Famotidine does not induce long QT syndrome: experimental evidence from in vitro and in vivo test systems

The effects of famotidine on the cardiac repolarization process were assessed using four different levels of test systems described in the draft stage guideline ICH S7B. A supratherapeutic concentration of famotidine (10(-5) M), which is >8 times higher than C(max) obtained after its therapeutic dose, neither inhibited human ether-a-go-go-related gene (HERG) K(+) current expressed in human embryonic kidney 293 (HEK293) cells nor affected any of the action potential parameters of guinea pig papillary muscles. Therapeutic (0.3 mg/kg, i.v.) to supratherapeutic doses (3-10 mg/kg, i.v.) of famotidine did not affect the repolarization process of the halothane-anesthetized canine model, while only supratherapeutic doses exerted the positive chronotropic, inotropic and dromotropic effects without affecting the mean blood pressure. Moreover, supratherapeutic doses of famotidine (1-10 mg/kg, i.v.) neither induced torsades de pointes nor prolonged QT interval in the canine chronic atrioventricular conduction block model. These results suggest that famotidine possesses no cardiovascular effects at a therapeutic dose, while it may exert cardiostimulatory actions after drug overdoses that might potentiate the proarrhythmic potential of co-administered cardiotonic agents by increasing the intracellular Ca(2+) concentration.