Effect of disopyramide on potassium currents in rabbit ventricular myocytes

Investigating the relevance of CYP2J2 inhibition for drugs known to cause intermediate to high risk torsades de pointes

Cardiac cytochrome P450 2J2 (CYP2J2) metabolizes endogenous polyunsaturated fatty acid, arachidonic acid (AA), to bioactive regioisomeric epoxyeicosatrienoic acid (EET) metabolites. This endogenous metabolic pathway has been postulated to play a homeostatic role in cardiac electrophysiology. However, it is unknown if drugs that cause intermediate to high risk torsades de pointes (TdP) exhibit inhibitory effects against CYP2J2 metabolism of AA to EETs. In this study, we demonstrated that 11 out of 16 drugs screened with intermediate to high risk of TdP as defined by the Comprehensive in vitro Proarrhythmia Assay (CiPA) initiative are concurrently reversible inhibitors of CYP2J2 metabolism of AA, with unbound inhibitory constant (K_i,AA,u) values ranging widely from 0.132 to 19.9 µM. To understand the physiological relevancy of K_i,AA,u, the in vivo unbound drug concentration within human heart tissue (C_u,heart) was calculated via experimental determination of in vitro unbound partition coefficient (K_puu) for 10 CYP2J2 inhibitors using AC16 human ventricular cardiomyocytes as well as literature-derived values of fraction unbound in plasma (f_u,p) and plasma drug concentrations in clinical scenarios leading to TdP. Notably, all CYP2J2 inhibitors screened belonging to the high TdP risk category, namely vandetanib and bepridil, exhibited highest K_puu values of 18.2 ± 1.39 and 7.48 ± 1.16 respectively although no clear relationship between C_u,heart and risk of TdP could eventually be determined. R values based on basic models of reversible inhibition as per FDA guidelines were calculated using unbound plasma drug concentrations (C_u,plasma) and adapted using C_u,heart which suggested that 4 out of 10 CYP2J2 inhibitors with intermediate to high risk of TdP demonstrate greatest potential for clinically relevant in vivo cardiac drug-AA interactions. Our results shed novel insights on the relevance of CYP2J2 inhibition in drugs with risk of TdP. Further studies ascertaining the role of CYP2J2 metabolism of AA in cardiac electrophysiology, characterizing inherent cardiac ion channel activities of drugs with risk of TdP as well as in vivo evidence of drug-AA interactions will be required prior to determining if CYP2J2 inhibition could be an alternative mechanism contributing to drug-induced TdP.

Ionic Mechanisms of Disopyramide Prolonging Action Potential Duration in Human-Induced Pluripotent Stem Cell-Derived Cardiomyocytes From a Patient With Short QT Syndrome Type 1

Short QT syndrome (SQTS) is associated with tachyarrhythmias and sudden cardiac death. So far, only quinidine has been demonstrated to be effective in patients with SQTS type 1(SQTS1). The aim of this study was to investigate the mechanisms of disopyramide underlying its antiarrhythmic effects in SQTS1 with the N588K mutation in HERG channel. Human-induced pluripotent stem cell–derived cardiomyocytes (hiPSC-CMs) from a patient with SQTS1 and a healthy donor, patch clamp, and calcium imaging measurements were employed to assess the drug effects. Disopyramide prolonged the action potential duration (APD) in hiPSC-CMs from a SQTS1-patient (SQTS1-hiPSC-CMs). In spontaneously beating SQTS1-hiPSC-CMs challenged by carbachol plus epinephrine, disopyramide reduced the arrhythmic events. Disopyramide enhanced the inward L-type calcium channel current (I_Ca-L), the late sodium channel current (late I_Na) and the Na/Ca exchanger current (I_NCX), but it reduced the outward small-conductance calcium-activated potassium channel current (I_SK), leading to APD-prolongation. Disopyramide displayed no effects on the rapidly and slowly activating delayed rectifier and ATP-sensitive potassium channel currents. In hiPSC-CMs from the healthy donor, disopyramide reduced peak I_Na, I_Ca-L, I_Kr, and I_SK but enhanced late I_Na and I_NCX. The results demonstrated that disopyramide may be effective for preventing tachyarrhythmias in SQTS1-patients carrying the N588K mutation in HERG channel by APD-prolongation via enhancing I_Ca-L, late I_Na, I_NCX, and reducing I_SK.

Drug-induced fatal arrhythmias: Acquired long QT and Brugada syndromes

Since the early 1990s, the concept of primary "inherited" arrhythmia syndromes or ion channelopathies has evolved rapidly as a result of revolutionary progresses made in molecular genetics. Alterations in genes coding for membrane proteins such as ion channels or their associated proteins responsible for the generation of cardiac action potentials (AP) have been shown to cause specific malfunctions which eventually lead to cardiac arrhythmias. These arrhythmic disorders include congenital long QT syndrome, Brugada syndrome, catecholaminergic polymorphic ventricular tachycardia, short QT syndrome, progressive cardiac conduction disease, etc. Among these, long QT and Brugada syndromes are the most extensively studied, and drugs cause a phenocopy of these two diseases. To date, more than 10 different genes have been reported to be responsible for each syndrome. More recently, it was recognized that long QT syndrome can be latent, even in the presence of an unequivocally pathogenic mutation (silent mutation carrier). Co-existence of other pathological conditions in these silent mutation carriers may trigger a malignant form of ventricular arrhythmia, the so called torsade de pointes (TdP) that is most commonly brought about by drugs. In analogy to the drug-induced long QT syndrome, Brugada type 1 ECG can also be induced or unmasked by a wide variety of drugs and pathological conditions; so physicians may encounter patients with a latent form of Brugada syndrome. Of particular note, Brugada syndrome is frequently associated with atrial fibrillation whose therapeutic agents such as Vaughan Williams class IC drugs can unmask the dormant and asymptomatic Brugada syndrome. This review describes two types of drug-induced arrhythmias: the long QT and Brugada syndromes.

Analysis of the contribution of I(to) to repolarization in canine ventricular myocardium

BACKGROUND AND PURPOSE

The contribution of the transient outward potassium current (I(to)) to ventricular repolarization is controversial as it depends on the experimental conditions, the region of myocardium and the species studied. The aim of the present study was therefore to characterize I(to) and estimate its contribution to repolarization reserve in canine ventricular myocardium.

EXPERIMENTAL APPROACH

Ion currents were recorded using conventional whole-cell voltage clamp and action potential voltage clamp techniques in canine isolated ventricular cells. Action potentials were recorded from canine ventricular preparations using microelectrodes. The contribution of I(to) to repolarization was studied using 100 µM chromanol 293B in the presence of 0.5 µM HMR 1556, which fully blocks I(Ks).

KEY RESULTS

The high concentration of chromanol 293B used effectively suppressed I(to) without affecting other repolarizing K(+) currents (I(K1), I(Kr), I(p)). Action potential clamp experiments revealed a slowly inactivating and a 'late' chromanol-sensitive current component occurring during the action potential plateau. Action potentials were significantly lengthened by chromanol 293B in the presence of HMR 1556. This lengthening effect induced by I(to) inhibition was found to be reverse rate-dependent. It was significantly augmented after additional attenuation of repolarization reserve by 0.1 µM dofetilide and this caused the occurrence of early afterdepolarizations. The results were confirmed by computer simulation.

CONCLUSIONS AND IMPLICATIONS

The results indicate that I(to) is involved in regulating repolarization in canine ventricular myocardium and that it contributes significantly to the repolarization reserve. Therefore, blockade of I(to) may enhance pro-arrhythmic risk.

Molecular determinants of hERG potassium channel inhibition by disopyramide

The Class Ia antiarrhythmic drug disopyramide (DISO) causes QT interval prolongation that is potentially dangerous in acquired Long QT Syndrome but beneficial in short QT syndrome, through inhibition of the hERG-encoded channels responsible for rapid delayed rectifier K(+) current (I(Kr)). In this study, alanine mutants of hERG S6 and pore helix residues and MthK-based homology modelling and ligand docking were used to investigate molecular determinants of DISO binding to hERG. Whole-cell hERG current (I(hERG)) recordings were made at 37°C from HEK-293 cells expressing WT or mutant hERG channels. WT outward I(hERG) tails were inhibited with an IC(50) of 7.3μM, whilst inward I(hERG) tails in a high [K(+)](e) of 94mM were blocked with an IC(50) of 25.7μM. The IC(50) for the Y652A mutation was ~55-fold that of WT I(hERG); this mutation also abolished a leftward shift in voltage-dependent I(hERG) activation present for WT hERG. The IC(50) for F656A I(hERG) was ~51 fold its corresponding WT control. In contrast to previously studied methanesulphonanilide hERG inhibitors, neither the G648A S6 nor the T623A and S624A pore helical mutations modified DISO IC(50). Computational docking with the hERG model showed that DISO did not exhibit a single unique binding pose; instead several low energy binding poses at the lower end of the pore cavity favoured interactions with Y652 and F656. In the WT hERG model DISO did not interact directly with residues at the base of the pore helix, consistent with the minimal effect of mutation of these residues on drug block.

Electrophysiological effects of ivabradine in dog and human cardiac preparations: potential antiarrhythmic actions

Ivabradine is a novel antianginal agent which inhibits the pacemaker current. The effects of ivabradine on maximum rate of depolarization (V(max)), repolarization and spontaneous depolarization have not yet been reported in human isolated cardiac preparations. The same applies to large animals close to human in heart size and spontaneous frequency. Using microelectrode technique action potential characteristics and by applying patch-clamp technique ionic currents were studied. Ivabradine exerted concentration-dependent (0.1-10 μM) decrease in the amplitude of spontaneous diastolic depolarization and reduction in spontaneous rate of firing of action potentials and produced a concentration- and frequency-dependent V(max) block in dog Purkinje fibers while action potential duration measured at 50% of repolarization was shortened. In the presence of ivabradine, at 400 ms cycle length, V(max) block developed with an onset kinetic rate constant of 13.9 ± 3.2 beat(-1) in dog ventricular muscle. In addition to a fast recovery of V(max) from inactivation (τ=41-46 ms) observed in control, a second slow component for recovery of V(max) was expressed (offset kinetics of V(max) block) having a time constant of 8.76 ± 1.34 s. In dog after attenuation of the repolarization reserve ivabradine moderately but significantly lengthened the repolarization. In human, significant prolongation of repolarization was only observed at 10 μM ivabradine. Ivabradine in addition to the Class V antiarrhythmic effect also has Class I/C and Class III antiarrhythmic properties, which can be advantageous in the treatment of patients with ischemic heart disease liable to disturbances of cardiac rhythm.

Kv1.5 open channel block by the antiarrhythmic drug disopyramide: molecular determinants of block

Kv1.5 is considered to be a potential molecular target for treatment of atrial fibrillation or flutter. Disopyramide is widely used in the treatment of atrial flutter and/or atrial fibrillation. The present study was undertaken to characterize the effects of disopyramide on currents mediated by Kv1.5 channels and to determine the putative binding site involved in the inhibitory effects of disopyramide. Experiments were carried out on wild-type and site directed mutated hKv1.5 channels expressed on HEK 293 cells using the patch-clamp technique. Disopyramide acting from the cytoplasmic side of the membrane produced blocking effects on Kv1.5 that exhibited several features typical of an open channel blocker. Ala-scanning mutagenesis of the Kv1.5 pore domain combined with macroscopic current analysis suggested that disopyramide interacted only with the Val512 residue that faces to the central cavity of the channel. Mutation of this key residue to Ala caused marked change in the IC(50) of disopyramide (22-fold). The single interaction between disopyramide and Val512 in the PVP region is able to change the mechanism of channel closure, reproducing the "foot-in-the-door" phenomenon.

A rabbit Langendorff heart proarrhythmia model: predictive value for clinical identification of Torsades de Pointes

BACKGROUND AND PURPOSE

The rabbit isolated Langendorff heart model (SCREENIT) was used to investigate the proarrhythmic potential of a range of marketed drugs or drugs intended for market. These data were used to validate the SCREENIT model against clinical outcomes.

EXPERIMENTAL APPROACH

Fifty-five drugs, 3 replicates and 2 controls were tested in a blinded manner. Proarrhythmia variables included a 10% change in MAPD(60), triangulation, instability and reverse frequency-dependence of the MAP. Early after-depolarisations, ventricular tachycardia, TdP and ventricular fibrillation were noted. Data are reported at nominal concentrations relative to EFTPC(max). Proarrhythmic scores were assigned to each drug and each drug category.

KEY RESULTS

Category 1 and 2 drugs have the highest number of proarrhythmia variables and overt proarrhythmia while Category 5 drugs have the lowest, at every margin. At 30-fold the EFTPC(max), the mean proarrhythmic scores are: Category 1, 101+/-24; Category 2, 101+/-14; Category 3, 72+/-20; Category 4, 59+/-16 and Category 5, 22+/-9 points. Only drugs in Category 5 have mean proarrhythmic scores, below 30-fold, that remain within the Safety Zone.

CONCLUSIONS AND IMPLICATIONS

A 30-fold margin between effects and EFTPC(max) is sufficiently stringent to provide confidence to proceed with a new chemical entity, without incurring the risk of eliminating potentially beneficial drugs. The model is particularly useful where compounds have small margins between the hERG IC(50) and predicted EFTPC(max). These data suggest this is a robust and reliable assay that can add value to an integrated QT/TdP risk assessment.

Inhibitory effect of erythromycin on potassium currents in rat ventricular myocytes in comparison with disopyramide

Disopyramide, a class Ia antiarrhythmic agent, has been reported to induce torsades de pointes (TdP) associated with excessive QT prolongation in electrocardiogram (ECG), especially when concomitantly administered with erythromycin, a macrolide antibiotic agent. In this study, we have evaluated the effects of erythromycin on action potential duration (APD) and potassium currents in rat ventricular myocytes in comparison with disopyramide. We have evaluated the relationship between in-vitro potassium current inhibition and in-vivo QT prolongation observed in a previous study. Action potentials and membrane potassium currents, including delayed rectifier current (I(K)) and transient outward current (I(to)), were recorded using a whole-cell patch clamp method in enzymatically-dissociated ventricular cells. Erythromycin and disopyramide prolonged APD in a concentration-dependent manner. Disopyramide (10-100 microM) and erythromycin (100 microM) led to increases in the APD at 90% repolarization level. Disopyramide reduced I(K) (IC50 = 37.2 +/- 0.17 microM) and I(to) (IC50 = 20.9 +/- 0.13 microM) while erythromycin reduced I(K) (IC50 = 60.1 +/- 0.29 microM) but not I(to). The observed prolongation of APD might be ascribed to the inhibition of potassium currents. Erythromycin produced the prolongation of APD and the inhibition of potassium currents with a lag time after addition of the drugs, which suggested that erythromycin might not reach potassium channels from outside the ventricular cells. The potency of disopyramide was almost equivalent under in-vitro and in-vivo conditions. However, potency of erythromycin in-vitro was far weaker than that in-vivo reported in a previous study, presumably due to a difference in the uptake of erythromycin into ventricular myocytes between in-vivo and in-vitro conditions. Therefore, when drug-induced risks of QT prolongation are to be evaluated, the difference of potencies between in-vitro and in-vivo should be taken into consideration.

Characterisation of recombinant HERG K+ channel blockade by the Class Ia antiarrhythmic drug procainamide

Class Ia antiarrhythmic drugs, including procainamide (PROC), are associated with cardiac sodium channel blockade, delayed ventricular repolarisation and with a risk of ventricular pro-arrhythmia. The HERG K(+) channel is frequently linked to drug-induced pro-arrhythmia. Therefore, in this study, interactions between PROC and HERG K(+) channels were investigated, with particular reference to potency and mechanism of drug action. Whole-cell patch-clamp recordings of HERG current (I(HERG)) were made at 37 degrees C from human embryonic kidney (HEK 293) cells stably expressing the HERG channel. Following activating pulses to +20 mV, I(HERG) tails were inhibited by PROC with an IC(50) value of approximately 139 microM. I(HERG) blockade was found to be both time- and voltage-dependent, demonstrating contingency upon HERG channel gating. However, I(HERG) inhibition by PROC was relieved by depolarisation to a highly positive membrane potential (+80 mV) that favoured HERG channel inactivation. These data suggest that PROC inhibits the HERG K(+) channel by a primarily 'open' or 'activated' channel state blocking mechanism and that avidity of drug-binding is decreased by extensive I(HERG) inactivation. The potency of I(HERG) blockade by PROC is much lower than for other Class Ia agents that have been studied previously under analogous conditions (quinidine and disopyramide), although the blocking mechanism appears similar. Thus, differences between the chemical structure of PROC and other Class Ia antiarrhythmic drugs may help provide insight into chemical determinants of blocking potency for agents that bind to open/activated HERG channels.

Comparison of the effect of class IA antiarrhythmic drugs on transmembrane potassium currents in rabbit ventricular myocytes

BACKGROUND

The blockade of cardiac transmembrane potassium channels, which is commonly seen with various antiarrhythmic drugs, plays an important role in their mechanism of action. We studied and compared the less-explored effects of three Class IA antiarrhythmics on the transient outward current (I(to)) and on the inward rectifier (I(kl)), ATP sensitive (I(KATP)), and delayed rectifier (I(K)) potassium currents in rabbit ventricular myocytes.

METHODS AND RESULTS

Transmembrane currents were measured by applying the whole-cell configuration of the patch-clamp technique at 37 degrees C in myocytes enzymatically isolated from rabbit ventricular preparations. Quinidine (10 microM), disopyramide (10 microM), and procainamide (50 microM) were studied at concentrations close to or exceeding the therapeutic plasma level. All studied drugs significantly decreased the amplitude of I(KATP) (activated by 50 microM pinacidil) and I(K) currents. None of them influenced significantly I(kl). The amplitude of I(to) was decreased by quinidine and disopyramide but was not considerably altered by procainamide. The fast inactivation of I(to) was not changed by procainamide and was significantly accelerated by quinidine and disopyramide.

CONCLUSION

Although quinidine, disopyramide, and procainamide are all classified as Class IA antiarrhythmics, these drugs had different effects on various potassium currents, which may partially explain their distinct effect on repolarization in various cardiac tissues and on cardiac arrhythmias in clinical settings.

Abnormal response to sodium channel blockers in patients with Brugada syndrome: augmented localised wall motion abnormalities in the right ventricular outflow tract region detected by electron beam computed tomography

OBJECTIVE

To investigate the relation between the wall motion abnormalities and sodium channel abnormalities in cases of the Brugada syndrome.

DESIGN

Consecutive prospective case-control study in a single hospital.

SETTING

Tertiary referral centre.

PATIENTS

13 consecutive patients with Brugada syndrome and 13 age and sex matched control subjects.

INTERVENTIONS

Each subject underwent electron beam computed tomography (EBT) and a 12 lead ECG before and after disopyramide injection.

MAIN OUTCOME MEASURES

QRS width and the magnitude of ST segment elevation in the 12 lead ECG; wall motion by EBT.

RESULTS

After disopyramide, EBT revealed deterioration of focal wall motion abnormalities in the right ventricular outflow tract region in eight of the 13 patients (62%). Prolongation of the QRS width after disopyramide injection in lead V2, which usually reflects the electrical activity in right ventricular outflow tract region, was greater in these eight patients (p < 0.01) than in the other five patients, in whom wall motion did not change after disopyramide. The degree of augmentation of ST segment elevation did not differ significantly between the two groups

CONCLUSIONS

The deterioration of wall motion abnormalities in the right ventricular outflow tract region after disopyramide suggests the presence of functional abnormalities of the sodium channel. Some patients with Brugada syndrome may have arrhythmogenic substrates with abnormal responses to sodium channel blockers.

Effects of disopyramide and mexiletine on the terminal repolarization process of the in situ heart assessed using the halothane-anesthetized in vivo canine model

This study was designed to assess the effects of typical class I drugs on the terminal repolarization process of the in situ heart, which is a useful marker of the potential of drug-induced long QT syndrome. Disopyramide (0.3 and 3.0 mg/kg per 10 min, n = 6) or mexiletine (0.3 and 3.0 mg/kg per 30s, n = 6) was intravenously administered to halothane-anesthetized beagle dogs under the monitoring of multiple cardiovascular parameters. Antiarrhythmic concentrations were obtained with the high dose of each drug. The low dose of disopyramide or mexiletine hardly affected any of the electrophysiological parameters assessed. The high dose of disopyramide prolonged the monophasic action potential duration (MAP90) and effective refractory period (ERP) to a similar extent, thus displacing the terminal repolarization period backward, which might provide a potential proarrhythmic substrate, particularly at a slow heart rate. On the other hand, the high dose of mexiletine shortened the MAP90, but prolonged the ERP, resulting in the disappearance of the terminal repolarization period, which could prevent premature excitation with its associated conduction slowing. These electrophysiological effects of disopyramide and mexiletine on the terminal repolarization phase may at least in part explain their clinically described antiarrhythmic and proarrhythmic properties.

Inhibition of the current of heterologously expressed HERG potassium channels by flecainide and comparison with quinidine, propafenone and lignocaine

1. The inhibition of the cardiac 'rapid' delayed rectifier current (I(Kr)) and its cloned equivalent HERG mediate QT interval prolonging effects of a wide range of clinically used drugs. In this study, we investigated the effects of the Class Ic antiarrhythmic agent flecainide (FLEC) on ionic current (I(HERG)) mediated by cloned HERG channels at 37 degrees C. We also compared the inhibitory potency of FLEC with other Class I agents: quinidine (QUIN, Class Ia); lignocaine (LIG, Class Ib) and propafenone (PROPAF, Class Ic). 2. Whole cell voltage clamp recordings of I(HERG) were made from an HEK293 cell line stably expressing HERG. FLEC inhibited I(HERG) 'tails' following test pulses to +30 mV with an IC(50) of 3.91+/-0.68 microM (mean+/-s.e.mean) and a Hill co-efficient close to 1 (0.76+/-0.09). 3. In experiments in which I(HERG) tails were monitored following voltage commands to a range of test potentials, I(HERG) inhibition by FLEC was observed to be voltage-dependent and to be associated with a approximately -5 mV shift of the activation curve for the current. Voltage-dependence of inhibition was greatest over the range of potentials corresponding to the steep portion of the I(HERG) activation curve. The time-course of I(HERG) tail deactivation was not significantly altered by FLEC. 4. In experiments in which 10 s depolarizing pulses were applied from -80 to 0 mV, the level of current inhibition by FLEC did not increase between 1 and 10 s. Some time-dependence of inhibition was observed during the first 200 - 300 ms of depolarization. This observation and the voltage-dependence of inhibition are collectively consistent with FLEC exerting a rapid open channel state inhibition of I(HERG). 5. Under similar recording conditions QUIN inhibited I(HERG) with an IC(50) of 0.41+/-0.04 microM and PROPAF inhibited I(HERG) with an IC(50) of 0.44+/-0.07 microM. Similar to FLEC, both QUIN and PROPAF showed voltage-dependence of inhibition and blockade developed rapidly during a sustained depolarization. 6. LIG showed little effect on I(HERG) at low micromolar concentrations, but could inhibit the current at higher concentrations; the observed IC(50) was 262.90+/-22.40 microM. 7. Our data are consistent with FLEC, PROPAF and QUIN exerting I(HERG) blockade at clinically relevant concentrations. The rank potency as HERG blockers of the Class I drugs tested in this study was QUIN=PROPAF>FLEC>>LIG.

Drug block of I(kr): model systems and relevance to human arrhythmias

The long QT-related arrhythmia torsades de pointes (TdP) can arise with mutations in HERG and during treatment with drugs that block cardiac I Kr, the current encoded by HERG. Multiple test systems have been used to assess drug block of I Kr. This study evaluated the I Kr blocking potency of a series of antiarrhythmics associated with a range of clinical risks of TdP in two such systems: mouse AT-1 cells (in which I Kr is the major repolarizing current) and Ltk cells transiently transfected with HERG (n = 4-10 cells per drug). For each compound, the concentration required to produce 50% block of I Kr or HERG tail currents (IC 50 ) was determined. There was an excellent correlation ( r = 0.98, p < 10 -5 ) between values obtained in the two systems. However, the relation between the liability of a drug to cause TdP appeared dissociated from I Kr blocking potency. Quinidine, dofetilide, ibutilide, procainamide, and disopyramide are all associated with TdP, but only the first three were potent blockers (IC 50 < or = 1 microM ), whereas procainamide and disopyramide were not (IC 50 > 50 microM ). Conversely, verapamil and amiodarone, drugs not associated with TdP, were also blockers (IC 50 < or = 1 microM ). We conclude that I Kr blocking potency can be readily assessed in either AT-1 cells or systems in which HERG is heterologously expressed. However, not all drugs causing TdP are potent I Kr blockers, and I Kr block is not necessarily associated with TdP. Other properties of these drugs, therefore, contribute to their propensity to cause TdP.

Inhibition of HERG potassium channel current by the class 1a antiarrhythmic agent disopyramide

The Class 1a antiarrhythmic drug disopyramide (DISO) is associated with 'acquired' prolongation of the QT interval of the electrocardiogram (ECG). This potentially proarrhythmic effect is likely to reflect drug actions on ion channels involved in ventricular action potential repolarisation. In this study, we examined the effects of DISO on potassium channels encoded by HERG, as this K channel type has been implicated in both congenital and acquired long-QT syndromes (LQTS). Chinese hamster ovary cells were transiently transfected with HERG cDNA for subsequent whole cell patch clamp recording. HERG tail currents recorded at -40 mV following test pulses to +30 mV were inhibited in a dose-dependent fashion by DISO concentrations within the clinical range (IC50 = 7.23 +/- 0.72 microM; mean +/- SEM). Experiments with 10 microM DISO indicated that the degree of HERG blockade showed some voltage dependence. Further data obtained using an 'envelope of tails' protocol (pulse potential +40 mV) were consistent with a significant role for open-channel blockade at lower drug concentrations. At higher concentrations it is possible that blockade may have involved drug binding to both resting and open channels. Inhibition of the inactivation-deficient mutant HERG-S631A was comparable to that seen for wild-type HERG. Therefore, channel inactivation was not obligatory for DISO to exert its effect. Native delayed rectifier tail currents from rabbit isolated ventricular myocytes were also inhibited by DISO. We conclude (a) that DISO inhibits HERG encoded potassium channels at clinically relevant concentrations and (b) that this action may constitute the molecular basis for acquired LQTS associated with this drug.

Comparative actions of cibenzoline and disopyramide on I(Kr) and I(Ks) currents in rat sino-atrial nodal cells

Modulation by class Ia antiarrhythmic drugs, cibenzoline and disopyramide, of the pacemaking activity and the underlying ionic currents in rat sino-atrial nodal cells was investigated using current-clamp and whole-cell patch-clamp techniques. Both drugs depressed the spontaneous activity and often caused sinus arrest. The negative chronotropic effect was significant at 10 microM cibenzoline and 30 microM disopyramide. The L-type Ca(2+) current (I(Ca)) and the hyperpolarization-activated inward current decreased by 69.7+/-3.2% and by 45.8+/-3.0% at 30 microM cibenzoline and by 51. 2+/-3.3% and by 48.3+/-2.7% at 100 microM disopyramide, respectively. The delayed rectifier K(+) current, which is composed of rapidly and slowly activated currents (I(Kr) and I(Ks)), also decreased. The IC(50) values of I(Kr) for cibenzoline and disopyramide were 8.8+/-1. 1 and 25.1+/-2.3 microM, respectively. In the presence of 5 microM E-4031 (1-[2-(6-methyl-2-pyridyl)ethyl]-4-(4-methylsulfonylaminobenzoyl) piperidine), the IC(50) values of I(Ks) for cibenzoline and disopyramide were 12.3+/-1.8 and 81.1+/-2.3 microM, respectively. The I(Ks) was completely blocked by 30 microM 293B (trans-6-cyano-4-(N-ethylsulphonyl-N-methtamino)-3-hydroxy-2 , 2-dimethyl-chromane). These results indicate that the ionic currents are more sensitive to cibenzoline than disopyramide in rat sino-atrial nodal cells, and that I(Ca) and I(Kr) make major contributions to pacemaking activity.

Familial and acquired long qt syndrome and the cardiac rapid delayed rectifier potassium current

1. Long QT syndrome (LQTS) is a cardiac disorder characterized by syncope, seizures and sudden death; it can be congenital, idiopathic, or iatrogenic. 2. Long QT syndrome is so-named because of the connection observed between the distinctive polymorphic ventricular tachycardia torsade de pointes and prolongation of the QT interval of the electrocardiogram, reflecting abnormally slowed ventricular action potential (AP) repolarization. Acquired LQTS has many similar clinical features to congenital LQTS, but typically affects older individuals and is often associated with specific pharmacological agents. 3. A growing number of drugs associated with QT prolongation and its concomitant risks of arrhythmia and sudden death have been shown to block the 'rapid' cardiac delayed rectifier potassium current (IKr) or cloned channels encoded by the human ether-a-go-go-related gene (HERG; the gene believed to encode native IKr). Because IKr plays an important role in ventricular AP repolarization, its inhibition would be expected to result in prolongation of both the AP and QT interval of the electrocardiogram. 4. The drugs that produce acquired LQTS are structurally heterogeneous, including anti-arrhythmics, such as quinidine, non-sedating antihistamines, such as terfenadine, and psychiatric drugs, such as haloperidol. In addition to heterogeneity in their structure, the electrophysiological characteristics of HERG/IKr inhibition differ between agents. 5. Here, clinical observations are associated with cellular data to correlate acquired LQTS with the IKr/HERG potassium (K+) channel. One strategy for developing improved compounds in those drug classes that are currently associated with LQTS could be to design drug structures that preserve clinical efficacy but are modified to avoid pharmacological interactions with IKr. Until such time, awareness of the QT-prolongation risk of particular agents is important for the clinician.

QT-prolonging class I drug, disopyramide, does not aggravate but suppresses adrenaline-induced arrhythmias. Comparison with cibenzoline and pilsicainide

We investigated the effects of class I antiarrhythmic drugs on corrected QT (QTc) interval and adrenaline-induced arrhythmias in halothane-anaesthetized, closed-chest dogs. For this purpose, we plotted a dose-response curve for adrenaline by calculating the arrhythmic ratio, which is the number of ventricular ectopic beats induced by adrenaline divided by the total heart rate, and observed the changes in the arrhythmic ratio-adrenaline dose relation before and after administration of class I drugs. Disopyramide and cibenzoline decreased the arrhythmic ratio induced by adrenaline. Disopyramide prolonged the QTc interval by 20% (P<0.01), but cibenzoline did not. Pilsicainide prolonged the QTc interval (12%), but this drug did not change the arrhythmic ratio. These results indicate that in contrast to the class III drugs which we have reported earlier, i.e. 1, 3-dimethyl-6-2-[N-(2-hydroxyethyl)-3-(4-nitrophenyl)-propylamino]eth ylamino-2,4 (1H,3H)-pyrimidinedione hydrochloride (MS-551), 1-(2-amino-4-methanesulfonamidophenoxy)2-[N-(3, 4-dimethoxyphenethyl)-N-methylamino]ethane hydrochloride (KCB-328) and E-1-[(5-(4-chlorophenyl)-2-furanyl)methylene]amino-3-[4-(4-methyl-1 -piperazinyl)butyl]-2,4-imidazolidinedione dihydrochloride (azimilide), class I drugs do not aggravate adrenaline-induced arrhythmias even though some drugs prolong the QTc interval.

Effect of GLG-V-13, a class III antiarrhythmic agent, on potassium currents in rabbit ventricular myocytes

The effects of a new Class III antiarrhythmic drug, GLG-V-13, on the 4-aminopyridine sensitive transient outward current, on the inward rectifier potassium current, on the ATP sensitive potassium current and on the rapid and slow components of the delayed rectifier potassium current were studied in single rabbit ventricular myocytes using the whole-cell voltage-clamp technique. GLG-V-13 blocked the rapid component of the delayed rectifier potassium current in a dose-dependent manner, with an estimated EC50 value of 0.36 microM. At high concentration, the slow component of the delayed rectifier potassium current was also depressed by the drug (40% effect at 10 microM concentration). The transient outward current, the inward rectifier potassium current and the ATP sensitive potassium current were not influenced by GLG-V-13, even at 10 microM concentration. Thus, GLG-V-13 blocks predominantly the rapid component of the delayed rectifier potassium current which may play a significant role in the prolongation of repolarization by the drug in ventricular tissue.

Cellular basis for the Brugada syndrome and other mechanisms of arrhythmogenesis associated with ST-segment elevation

BACKGROUND

The Brugada syndrome is characterized by marked ST-segment elevation in the right precordial ECG leads and is associated with a high incidence of sudden and unexpected arrhythmic death. Our study examines the cellular basis for this syndrome.

METHODS AND RESULTS

Using arterially perfused wedges of canine right ventricle (RV), we simultaneously recorded transmembrane action potentials from 2 epicardial and 1 endocardial sites, together with unipolar electrograms and a transmural ECG. Loss of the action potential dome in epicardium but not endocardium after exposure to pinacidil (2 to 5 micromol/L), a K(+) channel opener, or the combination of a Na(+) channel blocker (flecainide, 7 micromol/L) and acetylcholine (ACh, 2 to 3 micromol/L) resulted in an abbreviation of epicardial response and a transmural dispersion of repolarization, which caused an ST-segment elevation in the ECG. ACh facilitated loss of the action potential dome, whereas isoproterenol (0.1 to 1 micromol/L) restored the epicardial dome, thus reducing or eliminating the ST-segment elevation. Heterogeneous loss of the dome caused a marked dispersion of repolarization within the epicardium and transmurally, thus giving rise to phase 2 reentrant extrasystole, which precipitated ventricular tachycardia (VT) and ventricular fibrillation (VF). Transient outward current (I(to)) block with 4-aminopyridine (1 to 2 mmol/L) or quinidine (5 micromol/L) restored the dome, normalized the ST segment, and prevented VT/VF. Conclusions-Depression or loss of the action potential dome in RV epicardium creates a transmural voltage gradient that may be responsible for the ST-segment elevation observed in the Brugada syndrome and other syndromes exhibiting similar ECG manifestations. Our results also demonstrate that extrasystolic activity due to phase 2 reentry can arise in the intact wall of the canine RV and serve as the trigger for VT/VF. Our data point to I(to) block (4-aminopyridine, quinidine) as an effective pharmacological treatment.