Extracellular potassium modulation of drug block of IKr. Implications for torsade de pointes and reverse use-dependence

QT Prolongation and Fatal Arrhythmias: A Review of Clinical Implications and Effects of Drugs

A long QT interval due to prolonged repolarization may be associated with a polymorphic ventricular tachycardia known as torsades de pointes. During marked prolongation of the action potential (long QT) early after depolarizations may occur, which when propagated may trigger an arrhythmia. The duration of QTc interval is the major determinant of the risk of drug-induced torsades. Congenital long QT syndrome, female gender, hypokalemia and use of sympathomimetics increase the risk of torsades, and potentiate the QT prolonging effects of drugs. Antiarrhythmics that block the potassium channel prolong the QT and increase the risk for torsades (amiodarone, sotalol, quinidine, procainamide, ibutilide, disopyramide). Additionally, some macrolide and fluoroquinolone antibiotics, antipsychotic and antidepressant drugs, serotonin agonists of the triptan class, cisapride, dolasetron and others have been reported to be associated with QT prolongation or cases of torsades. Drug-induced effects on the QT interval with the associated possibility of inducing fatal arrhythmias have become a new challenge for the practitioner, the drug development process and the regulatory agencies.

Does QT Widening in the Langendorff-perfused Rat Heart Represent the Effect of Repolarization Delay or Conduction Slowing?

It has been suggested that class I antiarrhythmic drugs and ischemia can widen the QT interval in the Langendorff-perfused rat heart preparation as a consequence of slowed ventricular conduction. If this were so, it would undermine the clinical relevance of the preparation and its effectiveness as an antiarrhythmic bioassay. To test this, the authors determined whether three different class I drugs could prolong QT in the preparation and whether this effect was augmented by ischemia and elevation of the potassium (K+) content of the perfusion solution. Baseline drug-free QT intervals correlated inversely with the K+ content (3 microM vs. 5 mM). QT intervals widened during the first 3-5 minutes of ischemia (P < 0.05), then returned gradually to baseline. Lidocaine (3.88 microM and 12.93 microM) had no effect on the QT interval before or during ischemia, whereas quinidine (7.90 microM but not 0.79 microM) and flecainide (1.48 microM but not 0.74 microM) caused QT widening before and during ischemia (P < 0.05). Elevating perfusion solution K+ content from 3 microM to 5 mM reduced the QT-widening effects of quinidine and flecainide (P < 0.05). Because lidocaine, a relatively selective sodium (Na+) channel blocker, failed to widen QT interval whereas quinidine and flecainide (combined Na+ and K+ channel blockers) did so, and because K+ elevation reduced rather than potentiated the drug-induced QT widening, it is unlikely that Na+ channel blockade and conduction slowing play any role in ischemia- or class I drug-induced QT widening in this model.

Torsade de pointes due to noncardiac drugs: most patients have easily identifiable risk factors

Numerous medications, including drugs prescribed for noncardiac indications, can lead to QT prolongation and trigger torsade de pointes. Although this complication occurs only rarely, it may have lethal consequences. It is therefore important to know if patients with torsade de pointes associated with noncardiac drugs have risk factors that are easy to identify. We reviewed reports of drug-induced torsade de pointes and analyzed each case of torsade de pointes associated with a noncardiac drug for the presence of risk factors for the long QT syndrome that can be easily identified from the medical history or clinical evaluation (female gender, heart disease, electrolyte disturbances, excessive dosing, drug interactions, and history of familial long QT syndrome). We identified 249 patients with torsade de pointes caused by noncardiac drugs. The most commonly identified risk factor was female gender (71%). Other risk factors were frequently present (18%-41%). Virtually all patients had at least 1 of these risk factors, and 71% of patients had 2 or more risk factors. Our study suggests that almost all patients with torsade de pointes secondary to noncardiac drugs have risk factors that can be easily identified from the medical history before the initiation of therapy with the culprit drug.

Structural and functional basis for the long QT syndrome: relevance to veterinary patients

Long QT syndrome (LQTS) is a condition characterized by prolongation of ventricular repolarization and is manifested clinically by lengthening of the QT interval on the surface ECG. Whereas inherited forms of LQTS associated with mutations in the genes that encode ion channel proteins are identified only in humans, the acquired form of LQTS occurs in humans and companion animal species. Often, acquired LQTS is associated with drug-induced block of the cardiac K+ current designated I(Kr). However, not all drugs that induce potentially fatal ventricular arrhythmias antagonize I(Kr), and not all drugs that block I(Kr), are associated with ventricular arrhythmias. In clinical practice, the extent of QT interval prolongation and risk of ventricular arrhythmia associated with antagonism of I(Kr) are modulated by pharmacokinetic and pharmacodynamic variables. Veterinarians can influence some of the potential risk factors (eg, drug dosage, route of drug administration, presence or absence of concurrent drug therapy, and patient electrolyte status) but not all (eg, patient gender/genetic background). Veterinarians need to be aware of the potential for acquired LQTS during therapy with drugs identified as blockers of HERG channels and I(Kr).

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.

[Electropharmacological assessment of the risk of drug-induced long-QT syndrome using native cardiac cells and cultured cells expressing HERG channels]

In order to prevent the drug-induced long-QT syndrome it is important to assess the risk in the early phase of drug development. Most of the drugs, which clinically prolong the QT interval and induce torsades de pointes (Tdp), are known to inhibit the rapid component of the delayed rectifier K(+) current (I(Kr)) in cardiac cells. It is acknowledged that HERG (human ether-a-go-go-related gene) encodes the channel pore protein underlying I(Kr). The most sensitive method to evaluate the risk would be electropharmacological assessment using patch clamp techniques. When enzymatically-dissociated native cardiac cells are used, overlapping contamination of the slow component of the delayed rectifier K(+) current (I(Ks)) makes it difficult to analyze the drug effect on I(Kr) accurately. Therefore, heterologous expression systems of HERG channel are usually used to evaluate the inhibitory effect of drugs on I(Kr). Since the Xenopus oocyte system expressing HERG channels appears to be less sensitive to drug inhibition, use of a mammalian cell expression system may be desirable for the screening. A detailed analysis using various pulse protocols may be needed for the careful assessment of the HERG channel inhibition. In addition, many factors that may affect the susceptibility of patients to QT prolongation must be also taken into consideration.

Patch-clamp studies of human cardiac ion channels in the evaluation of cardiac electrophysiological effects of compounds

Drugs prolonging the QT interval appear to consistently inhibit the outward, rapid delayed rectifier K+ current (IKr) conveyed by the HERG channel. Hence, for determining whether a new drug candidate blocks the latter channel, this unit presents a basic electrophysiology protocol to conduct patch clamp studies in single cell preparations expressing heterologously cloned HERG channels. An additional protocol details the isolation of myocytes from specimens of human atria which are used in the study of native cardiac ion currents (INa, ICa, Ito, Isus, IK1). The results of these tests are useful for determining whether drug candidates have the desired cardiac safety profile for human use.

Long QT syndrome caused by noncardiac drugs

Numerous medications not intended for cardiac use (including antibiotics, histamine blockers, and antipsychotic medications) incidentally block potassium channels in myocardial cells, prolong the QT interval, and may trigger malignant arrhythmias. Although the odds for a given patient for developing arrhythmias are small, the number of patients receiving such drugs is enormous. Most patients developing proarrhythmia have additional risk factors that could be easily identified from their medical history. The list of risk factors includes female gender, organic heart disease, hypokalemia, and a history of long QT or drug-induced arrhythmias. Patients without risk factors are at very low risk. For these patients, it is neither practical nor necessary to record an electrocardiogram before therapy is initiated and the most important preventive measure is to avoid concurrent administration of 2 or more drugs that prolong the QT interval or administration of a medication that impairs the metabolism of a QT-prolonging drug. We performed a computerized literature search using the key words "long QT," "torsade," "drug-adverse effects," and "drug-ventricular arrhythmias," searching for published reports of drug-induced torsade de pointes. The references in each of these reports also were reviewed to identify additional publications. In addition, we reviewed the published reviews and the Internet sites dealing with drug-induced arrhythmias. All the original articles quoted in these reviews and Web sites were examined critically.

Desbutylhalofantrine: evaluation of QT prolongation and other cardiovascular effects after intravenous administration in vivo

Desbutylhalofantrine (Hfm) is an active and equipotent metabolite of halofantrine (Hf). Both compounds are effective in the treatment of sensitive and multidrug-resistant and In vitro data and interpretation of some clinical studies of Hf have suggested that, unlike Hf, Hfm may be devoid of adverse cardiac effects. The aim of these investigations was to provide the first in vivo examination of the intrinsic capacity of Hfm to affect repolarization in the heart, using an anesthetized rabbit model. Using a dose-rising regimen, Hfm was administered IV at doses of 1, 1, 2, 4, and 8 mg/kg and the baseline rate-corrected QT interval (QTc) value of 377 +/- 13 ms rose to 394 +/- 16, 396 +/- 12, 429 +/- 18, 433 +/- 16, and 489 +/- 15 ms, respectively. There were no significant changes in blood pressure, heart rate, or PR or QRS intervals. The Hfm plasma concentrations were quantitated after high-performance liquid chromatographic analysis, the results indicating a significant correlation between Hfm plasma concentration and QT(c) prolongation. The study also identified a concentration-dependent hemolysis of erythrocytes after administration of Hfm. The conclusions from this study are that IV administration of Hfm does cause a significant prolongation of the QT(c) interval in a rabbit model.

Kv1.4 channel block by quinidine: evidence for a drug-induced allosteric effect

We studied quinidine block of Kv1.4DeltaN, a K(+) channel lacking N-type inactivation, expressed in Xenopus ooctyes. Initially, quinidine intracellularly blocked the open channel so rapidly it overlapped with activation. This rapid open channel block was reduced (non-additively) by interventions that slow C-type inactivation: [K(+)](o) elevation and an extracellular lysine to tyrosine mutation (K532Y). These manipulations reduced the affinity of rapid open channel block ~10-fold, but left the effective electrical distance unchanged at ~0.15. Following rapid open channel block, there were time-dependent quinidine effects: the rate of inactivation during a single depolarisation was increased, and repetitive pulsing showed use dependence. The rate of recovery from the time-dependent aspect of quinidine block was similar to recovery from normal C-type inactivation. Manipulations that prevented the channel from entering the C-type inactivated state (i.e. high [K(+)](o) or the K532Y mutation) prevented the development of the time-dependent quinidine-induced inactivation. The concentration dependence of the rapid block and the time-dependent quinidine-induced inactivation were similar, but the time-dependent component was strongly voltage sensitive, with an effective electrical distance of 2. Clearly, this cannot reflect the permeation of quinidine through the electric field, but must be the result of some other voltage-sensitive change in the channel. We propose that quinidine promotes the entry of the channel into a C-type inactivated state in a time- and voltage-dependent manner. We developed a mathematical model based on these results to test the hypothesis that, following rapid open channel block, quinidine promotes development of the C-type inactivated state through a voltage-dependent conformational change.

QTc-prolonging drugs and hospitalizations for cardiac arrhythmias

Cardiac arrhythmia as an adverse effect of noncardiac drugs has been an issue of growing importance during the past few years. In this population-based study, we evaluated the risk for serious cardiac arrhythmias during the use of several noncardiac QTc-prolonging drugs in day-to-day practice, and subsequently focused on several specific groups of patients who could be extremely vulnerable for drug-induced arrhythmias. We performed a case-control study in which patients (cases), hospitalized for nonatrial cardiac arrhythmias from 1987 to 1998, were compared with their matched controls regarding current use of QTc-prolonging drugs. Odds ratios (OR) and 95% confidence intervals (CI) were calculated using multivariate conditional logistic regression, adjusting for potential confounding factors. Data were obtained from the PHARMO record linkage system. We identified 501 cases, 39 of whom used QTc-prolonging drugs. A statistically nonsignificant increased risk for arrhythmias (OR 1.2, 95% CI 0.8 to 1.9) was observed in patients who received QTc-prolonging drugs. A clearly increased risk of arrhythmias was, however, found in patients with a history of asthma (OR 9.9, 95% CI 1.0 to 100) and in patients using potassium-lowering drugs (OR 5.3, 95% CI 1.1 to. 25.9). Our data do not suggest that there is a strong overall association between the use of QTc-prolonging drugs and hospitalization for cardiac arrhythmias in the population at large. However, there appears to be clinically relevant associations of patients with a history of asthma and patients taking potassium-lowering drugs. The use of QTc-prolonging drugs should therefore be either avoided or monitored closely in these specific patients.

Probable case of torsades de pointes induced by fluconazole

A 25-year-old woman who was hospitalized for worsening endocarditis had a prolonged QT interval at baseline and developed monomorphic ventricular arrhythmias, which were managed successfully with pacing and antiarrhythmic therapy. Several days later, the patient started receiving high-dose fluconazole for fungemia and subsequently experienced episodes of torsades de pointes, a polymorphic ventricular arrhythmia associated with a prolonged QT interval or prominent U wave on the electrocardiogram. The arrhythmia developed in the presence of known risk factors. Clinicians should be aware of these risk factors and other relevant structural similarities with drugs that cause torsades de pointes so that they can recognize patients who may be at risk for fluconazole-associated arrhythmia.

Divergent proarrhythmic potential of macrolide antibiotics despite similar QT prolongation: fast phase 3 repolarization prevents early afterdepolarizations and torsade de pointes

Macrolide antibiotics are known to have a different proarrhythmic potential in the presence of comparable QT prolongation in the surface ECG. Because the extent of QT prolongation has been used as a surrogate marker for cardiotoxicity, we aimed to study the different electrophysiological effects of the macrolide antibiotics erythromycin, clarithromycin, and azithromycin in a previously developed experimental model of proarrhythmia. In 37 Langendorff-perfused rabbit hearts, erythromycin (150-300 microM, n = 13) clarithromycin (150-300 microM, n = 13), and azithromycin (150-300 microM, n = 11) led to similar increases in QT interval and monophasic action potential (MAP) duration. In bradycardic (atrioventricular-blocked) hearts, eight simultaneously recorded epi- and endocardial MAPs demonstrated increased dispersion of repolarization in the presence of all three antibiotics. Erythromycin and clarithromycin led to early afterdepolarizations (EADs) and torsade de pointes (TdP) after lowering of potassium concentration. In the presence of azithromycin, no EAD or TdP occurred. Erythromycin and clarithromycin changed the MAP configuration to a triangular pattern, whereas azithromycin caused a rectangular pattern of MAP prolongation. In 13 additional hearts, 150 microM azithromycin was administered after previous treatment with 300 microM erythromycin and suppressed TdP provoked by erythromycin. In conclusion, macrolide antibiotics lead to similar prolongation of repolarization but show a different proarrhythmic potential (erythromycin > clarithromycin > azithromycin). In the presence of azithromycin, neither EAD nor TdP occur. This effect may be related to a rectangular pattern of action potential prolongation, whereas erythromycin and clarithromycin cause triangular action potential prolongation and induce TdP.

Cytosolic Ca2+ triggers early afterdepolarizations and Torsade de Pointes in rabbit hearts with type 2 long QT syndrome

The role of intracellular Ca2+ (Ca2+i) in triggering early afterdepolarizations (EADs), the origins of EADs and the mechanisms underlying Torsade de Pointes (TdP) were investigated in a model of long QT syndrome (Type 2). Perfused rabbit hearts were stained with RH327 and Rhod-2/AM to simultaneously map membrane potential (V(m)) and Ca2+i with two photodiode arrays. The I(Kr) blocker E4031 (0.5 microM) together with 50 % reduction of [K+]o and [Mg2+]o elicited long action potentials (APs), V(m) oscillations on AP plateaux (EADs) then ventricular tachycardia (VT). Cryoablation of both ventricular chambers eliminated Purkinje fibres as sources of EADs. E4031 prolonged APs (0.28 to 2.3 s), reversed repolarization sequences (baseapex) and enhanced repolarization gradients (30 to 230 ms, n = 12) indicating a heterogeneous distribution of I(Kr). At low [K+]o and [Mg2+]o, E4031 elicited spontaneous Ca2+iand V(m) spikes or EADs (3.5 +/- 1.9 Hz) during the AP plateau (n = 6). EADs fired 'out-of-phase' from several sites, propagated, collided then evolved to TdP. Phase maps (Ca2+ivs. V(m)) had counterclockwise trajectories shaped like a 'boomerang' during an AP and like ellipses during EADs, with V(m) preceding Ca2+iby 9.2 +/- 1.4 (n = 6) and 7.2 +/- 0.6 ms (n = 5/6), respectively. After cryoablation, EADs from surviving epicardium (~1 mm) fired at the same frequency (3.4 +/- 0.35 Hz, n = 6) as controls. At the origins of EADs, Ca2+ipreceded V(m) and phase maps traced clockwise ellipses. Away from EAD origins, V(m) coincided with or preceded Ca2+i. In conclusion, overload elicits EADs originating from either ventricular or Purkinje fibres and 'out-of-phase' EAD activity from multiple sites generates TdP, evident in pseudo-ECGs.

Limited induction of torsade de pointes by terikalant and erythromycin in an in vivo model

The proarrhythmic activities of the selective I(Kr) blocker erythromycin and the less selective K(+) channel blockers, terikalant and clofilium, have been compared in an alpha(1)-adrenoceptor-stimulated, anaesthetized rabbit model. Terikalant (2.5, 7.5 and 25 nmol kg(-1) min(-1); n = 10), erythromycin (133, 400 and 1330 nmol kg(-1) min(-1); n = 8), clofilium (20, 60 and 200 nmol kg(-1) min(-1); n=10) or vehicle (n = 8) was infused intravenously over 19 min and there was a 15-min interval between each infusion [corrected]. QT and QTc intervals, and epicardial monophasic action potential duration were prolonged significantly (and to a similar extent) only by clofilium and terikalant. The total incidences of torsade de pointes were 60%*, 20%, 0% and 0% in clofilium-, terikalant-, erythromycin- and vehicle-treated animals, respectively (*P < 0.05 compared to vehicle control). In conclusion, terikalant exerted mild proarrhythmic activity though it prolonged repolarisation markedly. Despite being given in high doses, erythromycin neither prolonged repolarisation nor induced proarrhythmia.

A method for QT correction based on beat-to-beat analysis of the QT/RR interval relationship in conscious telemetred beagle dogs

INTRODUCTION

Drug-induced QT prolongation is a major clinical risk factor for arrhythmia induction, particularly torsades de pointes. QT interval is rate dependent, and many formulae exist that attempt to correct QT for changes in heart rate. Most correction factors are acknowledged to overcorrect at high heart rates, undercorrect at low heart rates, and tend to be species specific. Data collected from computerised data acquisition systems are normally reported as means over a given logging period, and so extremes of heart rate are averaged out. Therefore, the aim of this study was to develop a technique for assessing drug-induced changes in the QT/RR relationship, which is simple, suitable for small group sizes, and better able to determine rate-dependent effects of drugs.

METHODS

Telemetred beagle dogs (n=4) instrumented for the measurement of electrocardiogram (ECG) were monitored for four separate 20-h periods to define the control QT/RR relationship. Data were binned by RR interval, in 10 ms bins, to produce a control curve. Each dog was treated with vehicle and sotalol (4, 8, 32 mg/kg) in a crossover design to determine whether drug-induced changes in the QT/RR relationship could be detected using the data binning technique.

RESULTS

The control QT/RR relationship was curvilinear with a steep section for RR intervals below 580 ms, and was much less steep after this point. Sotalol produced QT prolongation and bradycardia-Fridericia's correction (QTf) reduced the magnitude of this prolongation. The data analysed by the binning technique showed a larger prolongation in QT than was suggested by QTf, and an inverse frequency-dependent response.

DISCUSSION

Beat-to-beat analysis and binning allows accurate determination of the QT/RR relationship and assessment of QT prolongation without recourse to mathematical modelling. It also highlights the importance of assessing QT effects in well-trained animals over a range of heart rates.

Serum quinidine concentrations and effect on QT dispersion and interval

OBJECTIVE

To establish a relationship between serum quinidine concentrations (SQCs) and QT interval dispersion, compared with corresponding QT intervals, in order to identify a reason why many reports describe torsade de pointes as occurring at subtherapeutic concentrations.

DESIGN

Retrospective study.

SETTING

University teaching hospital.

PARTICIPANTS

Eleven patients with atrial arrhythmias managed with quinidine therapy.

MAIN OUTCOME MEASURES

Patients with subtherapeutic (<2 microg/mL) and therapeutic (2-5 microg/mL) SQCs with corresponding 12-lead electrocardiograms (ECGs) (25 mm/sec) and baseline ECG were evaluated for QT interval dispersion, calculated as the maximum minus the minimum QT interval on the 12-lead ECG.

RESULTS

Mean +/- SD subtherapeutic and therapeutic SQCs were 1.48 +/- 0.39 microg/mL and 3.78 +/- 0.88 microg/mL (p < 0.001). Baseline values for QT/QTc intervals were 376.4 +/- 59.2/429.5 +/- 57.3 msec. At subtherapeutic and therapeutic SQCs, mean QT/QTc intervals were 403.6 +/- 59.9/450.5 +/- 38.5 msec and 439.1 +/- 48.9/472.4 +/- 44.6 msec, respectively. Mean QT dispersion was 47 +/- 16.2 msec at baseline, 98.2 +/- 27.5 msec at subtherapeutic SQC, and 70.9 +/- 33.9 msec at therapeutic SQCs (p = 0.001 for overall analysis; p < 0.001 for baseline vs. subtherapeutic concentrations; p = NS for therapeutic vs. subtherapeutic in post hoc comparison).

CONCLUSIONS

Despite QT interval lengthening with increasing SQCs, QT dispersion was numerically greatest at subtherapeutic SQCs. Further study is required to determine the value of QT dispersion as a tool for identifying proarrhythmic risk with drugs that prolong the QT interval.

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.

Importance of potassium in cardiovascular disease

The pivotal role of potassium (K+) in cardiovascular disease and the importance of preserving potassium balance have become clinical hot points, particularly as relates to new and emerging cardioprotective and renoprotective therapies that promote potassium retention. Although clinicians may be aware of the critical nature of this relationship, quite frequently there is some uncertainty as to the best way to monitor potassium levels in the face of a host of pathologic states and/or accompanying drug therapies that affect serum levels and/or total body potassium balance. Moreover, guidelines for monitoring of serum potassium levels are at best tentative and oftentimes are translated according to the level of concern of the respective physician. To address these uncertainties, an expert group was convened that included representatives from multiple disciplines. They attempted to reach consensus on the importance of K+ in hypertension, stroke, and arrhythmias as well as practical issues on maintaining K+ balance and avoiding K+ depletion. Because of the complexity of this topic, issues of hyperkalemia will be addressed in a forthcoming manuscript.

A comparison of currents carried by HERG, with and without coexpression of MiRP1, and the native rapid delayed rectifier current. Is MiRP1 the missing link?

Although it has been suggested that coexpression of minK related peptide (MiRP1) is required for reconstitution of native rapid delayed-rectifier current (I(Kr)) by human ether-a-go-go related gene (HERG), currents resulting from HERG (I(HERG)) and HERG plus MiRP1 expression have not been directly compared with native I(Kr). We compared the pharmacological and selected biophysical properties of I(HERG) with and without MiRP1 coexpression in Chinese hamster ovary (CHO) cells with those of guinea-pig I(Kr) under comparable conditions. Comparisons were also made with HERG expressed in Xenopus oocytes. MiRP1 coexpression significantly accelerated I(HERG) deactivation at potentials negative to the reversal potential, but did not affect more physiologically relevant deactivation of outward I(HERG), which remained slower than that of I(Kr). MiRP1 shifted I(HERG) activation voltage dependence in the hyperpolarizing direction, whereas I(Kr) activated at voltages more positive than I(HERG). There were major discrepancies between the sensitivity to quinidine, E-4031 and dofetilide of I(HERG) in Xenopus oocytes compared to I(Kr), which were not substantially affected by coexpression with MiRP1. On the other hand, the pharmacological sensitivity of I(HERG) in CHO cells was indistinguishable from that of I(Kr) and was unaffected by MiRP1 coexpression. We conclude that the properties of I(HERG) in CHO cells are similar in many ways to those of native I(Kr) under the same recording conditions, and that the discrepancies that remain are not reduced by coexpression with MiRP1. These results suggest that the physiological role of MiRP1 may not be to act as an essential consituent of the HERG channel complex carrying native I(Kr).

Development of drugs that alter ventricular repolarization

Many drugs are found to alter ventricular repolarization, as manifest by T-wave and U-wave changes on the surface electrocardiogram. These changes have frequently been associated with malignant ventricular arrhythmias. There is no perfectly sensitive and specific way of anticipating such arrhythmias, but some clinical and preclinical screening methods are better than others. The author reviews some of these methods, commenting on some of the regulatory implications.

The binding site for channel blockers that rescue misprocessed human long QT syndrome type 2 ether-a-gogo-related gene (HERG) mutations

Mutations in the human ether-a-gogo-related gene (HERG) K(+) channel gene cause chromosome 7-linked long QT syndrome type 2 (LQT2), which is characterized by a prolonged QT interval in the electrocardiogram and an increased susceptibility to life-threatening cardiac arrhythmias. LQT2 mutations produce loss-of-function phenotypes and reduce I(Kr) currents either by the heteromeric assembly of non- or malfunctioning channel subunits with wild type subunits at the cell surface or by retention of misprocessed mutant HERG channels in the endoplasmic reticulum. Misprocessed mutations often encode for channel proteins that are functional upon incorporation into the plasma membrane. As a result the pharmacological correction of folding defects and restoration of protein function are of considerable interest. Here we report that the trafficking-deficient pore mutation HERG G601S was rescued by a series of HERG channel blockers that increased cell surface expression. Rescue by these pharmacological chaperones varied directly with their blocking potency. We used structure-activity relationships and site-directed mutagenesis to define the binding site of the pharmacological chaperones. We found that binding occurred in the inner cavity and correlated with hydrophobicity and cationic charge. Rescue was domain-restricted because the trafficking of two misprocessed mutations in the C terminus, HERG F805C and HERG R823W, was not restored by channel blockers. Our findings represent a first step toward the design of pharmacological chaperones that will rescue HERG K(+) channels without block.

Is gender a risk factor for adverse drug reactions? The example of drug-induced long QT syndrome

Drug-induced torsade de pointes is a rare life-threatening adverse drug reaction (ADR) which is strongly influenced by gender. Drugs that prolong cardiac repolarisation include antiarrhythmics, gastrokinetics, antipsychotics, antihistamines and antibacterials. Such drugs share the potential to block cardiac voltage-gated potassium channels, particularly the rapid component (I(Kr)) of the delayed rectifier potassium current (I(K)). By doing so, such drugs usually, but not always, prolong the QT interval. Even if the electrocardiographic signs are subdued, the underlying blockade of I(Kr) current may precipitate the occurrence of arrhythmia. Women are perceived to be more prone to ADRs than men. Such a propensity may result from gender-associated differences in drug exposure, in the number of drugs prescribed (polypharmacy), in drug pharmacology, as well as from possible differences in the way the adverse event is perceived. A prolonged QT interval on the electrocardiogram (time that elapses from the onset of the cardiac ventricular depolarisation to the completion of its repolarisation) is associated with the occurrence of torsade de pointes and related ventricular arrhythmias. The QT interval is influenced by heart rate, autonomic nervous system, electrolyte disturbances and above all, drugs that block potassium channels. Two-thirds of the cases of drug-induced torsade de pointes occur in women. Therefore, this adverse effect represents a perfect example of gender differences impairing women's health. Clinical and experimental studies show that female gender is associated with a longer corrected QT interval at baseline and a greater response to drugs that block I(Kr), both of which facilitate the emergence of arrhythmia. This results most likely from a specific regulation of ionic channel expression (potassium, calcium, etc) by sex steroids, even though nongenomic effects may play a role as well. Estrogens facilitate bradycardia-induced prolongation of the QT interval and the emergence of arrhythmia whereas androgens shorten the QT interval and blunt the QT response to drugs. Hence, underlying genetic defects of potassium channels that may be asymptomatic in normal conditions, may precipitate drug-induced arrhythmia in women more frequently than in men. Even in the presence of a drug that mildly blocks I(Kr) and seldom prolongs the QT interval, women are still more prone to drug-induced torsade de pointes, due to their reduced cardiac 'repolarisation reserve'. This is an important aspect of I(Kr) blockade to be aware of during the development of new drugs.

A Benefit-Risk Assessment of Class III Antiarrhythmic Agents

With beta-blockers as the exception, increasing doubt is emerging on the value of antiarrhythmic drug therapy following a series of trials that have either shown no mortality benefit or even an excess mortality. Vaughan Williams class I drugs are generally avoided in patients with structural heart disease, and class IV drugs are avoided in heart failure. Unfortunately, arrhythmias are a growing problem due to an increase in the incidence of atrial fibrillation and sudden death. The population is becoming older and more patients survive for a longer time period with congestive heart failure, which again increases the frequency of both supraventricular as well as ventricular arrhythmias. Class III antiarrhythmic drugs act by blocking repolarising currents and thereby prolong the effective refractory period of the myocardium. This is believed to facilitate termination of re-entry tachyarrhythmias. This class of drugs is developed for treatment of both supraventricular and ventricular arrhythmias. Amiodarone, sotalol, dofetilide, and ibutilide are examples of class III drugs that are currently available. Amiodarone and sotalol have other antiarrhythmic properties in addition to pure class III action, which differentiates them from the others. However, all have potential serious adverse events. Proarrhythmia, especially torsade de pointes, is a common problem making the benefit-risk ratio of these drugs a key question. Class III drugs have been evaluated in different settings: primary and secondary prevention of ventricular arrhythmias and in treatment of atrial fibrillation or flutter. Based on existing evidence there is no routine indication for antiarrhythmic drug therapy other than beta-blockers in patients at high risk of sudden death. Subgroup analyses of trials with amiodarone and dofetilide suggest that patients with atrial fibrillation may have a mortality reduction with these drugs. However, this needs to be tested in a prospective trial. Similarly, subgroups that will benefit from prophylactic treatment with class III antiarrhythmic drugs may be found based on QT-intervals or - in the future - from genetic testing. Class III drugs are effective in converting atrial fibrillation to sinus rhythm and for the maintenance of sinus rhythm after conversion. This is currently by far the most important indication for this class of drugs. As defined by recent guidelines, amiodarone and dofetilide have their place as second-line therapy except for patients with heart failure where they are first line therapy being the only drugs where the safety has been documented for this group of high risk patients.

Retrospective analysis of the effects of cisapride on the QT interval and QT dispersion in chronic hemodialysis patients

Cisapride is contraindicated in patients with end-stage renal disease (ESRD) and gastrointestinal motility disorders. Ventricular arrhythmias have been associated with both cisapride and hemodialysis (HD). However, reports conflict regarding the safety of cisapride in HD patients. We undertook this study to characterize the effects of cisapride on QT intervals and QT dispersion (QTD) in HD patients. Baseline and steady-state electrocardiograms (ECGs) were retrospectively selected for calendar year 1999 for each patient administered cisapride if ECGs showed sinus rhythm, potassium level was 3.5 mEq/dL or greater, and there was no pharmacokinetic drug interaction. QT intervals were measured by two investigators, and QTDs were calculated (maximum [QT(max)] - minimum QT interval [QT(min)]). Averages between investigator measures (+/- SD), presented for each value, were evaluated using Wilcoxon's signed-rank test. Thirty-one HD patients were administered cisapride. Seventeen patients failed to meet entry criteria, and no patient had a pharmacokinetic drug interaction. In included patients (6 men, 8 women), heart rates were 86.71 +/- 20.87 beats/min at baseline and 86.57 +/- 14.23 beats/min during treatment (P = not significant). Serum potassium levels were 4.97 +/- 1.2 mEq/dL at baseline and 4.94 +/- 0.76 mEq/dL during treatment (P = not significant). Average baseline QT(max) and QT(min) were 391.07 +/- 42.43 and 330.71 +/- 40.94 milliseconds, respectively. Treatment QT(max) and QT(min) were 391.43 +/- 38.2 and 343.93 +/- 35.69 milliseconds, respectively (P = not significant for both). QTD was 60.36 +/- 17.59 milliseconds at baseline and 47.5 +/- 19.59 milliseconds during treatment (P = 0.074). Mean corrected QT (QTc) intervals increased from 426.57 +/- 26.62 to 431.71 +/- 29.98 milliseconds (P = 0.55) from baseline to treatment. No ventricular arrhythmia was observed during at least 160 days (range, 2 to 830 days) of cisapride exposure. Two patients died during this study, both of other causes 4 days after discontinuing cisapride therapy. Cisapride did not significantly increase mean QTc interval, QT(max), or QTD in patients with ESRD managed by HD when potassium levels were stable and pharmacokinetic drug interactions were avoided.

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.

Response of the QT interval to mental and physical stress in types LQT1 and LQT2 of the long QT syndrome

OBJECTIVE

To study and compare the effects of mental and physical stress on long QT syndrome (LQTS) patients.

DESIGN

Case-control study.

MAIN OUTCOME MEASURES

QT intervals were measured from lead V3. Serum potassium and plasma catecholamine concentrations were also monitored.

PATIENTS

16 patients with type 1 LQTS (LQT1), 14 with type 2 LQTS (LQT2), both groups asymptomatic, and 14 healthy control subjects.

INTERVENTIONS

Three types of mental stress tests and a submaximal exercise stress test.

RESULTS

Heart rate responses to mental stress and exercise were similar in all groups. During mental stress, the mean QT interval shortened to a similar extent in controls (-29 ms), LQT1 patients (-34 ms), and LQT2 patients (-30 ms). During exercise, the corresponding QT adaptation to exercise stress was more pronounced (p < 0.01) in healthy controls (-47 ms) than in LQT1 (-38 ms) or LQT2 patients (-38 ms). During exercise changes in serum potassium concentrations were correlated to changes in QT intervals in controls, but not in LQTS patients. LQT1 and LQT2 patients did not differ in serum potassium, catecholamine or heart rate responses to mental or physical stress.

CONCLUSIONS

QT adaptation to mental and exercise stress in healthy people and in patients with LQTS is different. In healthy people QT adaptation is more sensitive to physical than to mental stress while no such diverging pattern was seen in asymptomatic LQTS patients.

Response of the QT interval to mental and physical stress in types LQT1 and LQT2 of the long QT syndrome

OBJECTIVE

To study and compare the effects of mental and physical stress on long QT syndrome (LQTS) patients.

DESIGN

Case–control study.

MAIN OUTCOME MEASURES

QT intervals were measured from lead V3. Serum potassium and plasma catecholamine concentrations were also monitored.

PATIENTS

16 patients with type 1 LQTS (LQT1), 14 with type 2 LQTS (LQT2), both groups asymptomatic, and 14 healthy control subjects.

INTERVENTIONS

Three types of mental stress tests and a submaximal exercise stress test.

RESULTS

Heart rate responses to mental stress and exercise were similar in all groups. During mental stress, the mean QT interval shortened to a similar extent in controls (–29 ms), LQT1 patients (–34 ms), and LQT2 patients (–30 ms). During exercise, the corresponding QT adaptation to exercise stress was more pronounced (p < 0.01) in healthy controls (–47 ms) than in LQT1 (–38 ms) or LQT2 patients (–38 ms). During exercise changes in serum potassium concentrations were correlated to changes in QT intervals in controls, but not in LQTS patients. LQT1 and LQT2 patients did not differ in serum potassium, catecholamine or heart rate responses to mental or physical stress.

CONCLUSIONS

QT adaptation to mental and exercise stress in healthy people and in patients with LQTS is different. In healthy people QT adaptation is more sensitive to physical than to mental stress while no such diverging pattern was seen in asymptomatic LQTS patients.

Tedisamil and dofetilide-induced torsades de pointes, rate and potassium dependence

1. Tedisamil is a bradycardiac agent that prolongs the QT interval of the ECG and prevents cardiac arrhythmias. Given this profile, tedisamil might be expected to have proarrhythmic actions similar to Class III antiarrhythmic drugs. To address this question, the actions of dofetilide and tedisamil were examined in rabbit isolated hearts in which bradycardia was induced by AV ablation. 2. The QT interval was prolonged in a reverse rate-dependent fashion by dofetilide (3 and 30 nM) and tedisamil (0.3 and 3 microM). 3. Torsades de pointes was observed in 1/7 hearts treated with 3 nM dofetilide and 0/7 hearts treated with 0.3 microM tedisamil. The incidence of torsades de pointes was increased to 5/7 in hearts treated with 30 nM dofetilide and to 7/7 in hearts treated with 3 microM tedisamil (both P < 0.05 vs control). 4. The actions of 30 nM dofetilide and 3 microM tedisamil were also examined in hearts paced at 50, 100, 200 and 50 beats min(-1) successively. Both drugs caused torsades de pointes in 5/5 hearts paced at 50 beats min(-1); however, the incidence was reduced to 0/5 during pacing at 200 beats min(-1). Thus, drug-induced proarrhythmia was bradycardia-dependent. 5. Drug-induced prolongation of the interval between the peak and end of the T-wave (QTa-e) was reverse rate-dependent and was associated with the occurrence of torsades de pointes (r = 0.91, P < 0.01). 6. The results suggest that tedisamil, like dofetilide, presents a risk for development of torsades de pointes.

Effects of EGIS-7229 (S 21407), a novel class III antiarrhythmic drug, on myocardial refractoriness to electrical stimulation in vivo and in vitro

The I(Kr) blocker EGIS-7229 (S-21407), displays class Ib and class IV effects that may alter its pharmacologic profile compared with those of pure I(Kr) blockers. Therefore, the concentration- and frequency-dependent effects of EGIS-7229, and of the I(Kr) blockers d,l-sotalol and dofetilide, on the effective refractory period (ERP) were measured in isolated right ventricular papillary muscle of the rabbit in vitro. The effects of these drugs on right ventricular fibrillation threshold (RVFT) at increasing intravenous doses were also determined in anesthetized cats. Dofetilide and d,l-sotalol increased ERP in a concentration-dependent manner (dofetilide: 3-100 nM; d,l-sotalol: 3-100 microM) with strong reverse frequency dependence at high concentrations. EGIS-7229 concentration dependently lengthened ERP at 1-30 microM. Its effect on ERP was clearly reverse frequency dependent at 3 microM, but this feature of the drug diminished at 10 microM and was not apparent at 30 microM. The effect of EGIS-7229 (30 microM) on ERP was devoid of reverse frequency dependence as it was more effective (31%) than dofetilide (16 %) at high-pacing rate (3 Hz), whereas it was less effective (50%) than dofetilide (70%) at slow-pacing rate (1 Hz). Reverse frequency-dependent ERP effect of dofetilide (100 nM) was similarly abolished by the addition of lidocaine (30 microM). EGIS-7229 (1-8 mg/kg iv), d,l-sotalol (1-8 mg/kg iv), and dofetilide (10-80 microg/kg iv) caused a dose-dependent increase in RVFT. The minimum effective dose of d,l-sotalol and EGIS-7229 was 1 and 2 mg/kg, respectively, whereas that of dofetilide was 10 microg/kg. EGIS-7229 induced a smaller peak effect in RVFT than sotalol or dofetilide. In conclusion, EGIS-7229 markedly increased refractoriness to electrical stimulation in vitro and in vivo. Compared with pure I(Kr) blockers, the benefits of EGIS-7229 seem to be a greater lengthening of effective refractory period at rapid stimulation rates, suggesting a strong antiarrhythmic action, and a smaller effect at slow stimulation rates, suggesting low potential to induce early afterdepolarizations.

Potentiation of halofantrine-induced QTc prolongation by mefloquine: correlation with blood concentrations of halofantrine

1. The antimalarial drug halofantrine can prolong the QT interval and this may be enhanced by prior use of mefloquine. This possible interaction has been investigated by examining the effects of halofantrine and mefloquine alone and in combination. 2. In anaesthetized rabbits (n=6 per group), halofantrine given as bolus doses of 1, 3, 10, and 30 mg kg(-1) at 25 min intervals dose-dependently prolonged the rate-corrected QT (QTc) interval from 313+/-12 ms pre-drug to 410+/-18 ms after the highest dose. Similar doses of mefloquine did not alter QTc intervals significantly. The highest dose of mefloquine (30 mg kg(-1)) caused cardiac contractile failure. 3. Pretreatment with 3 mg kg(-1) mefloquine 25 min before the first dose of halofantrine potentiated the effects of all doses of halofantrine on QTc intervals. 4. The blood concentrations of halofantrine were two to six times higher in the group pretreated with mefloquine compared to the halofantrine alone group; e.g. 1.03+/-0.17 and 0.16+/-0.02 microM respectively after 1 mg kg(-1) halofantrine. There was a significant correlation between blood halofantrine concentrations and QTc intervals (r=0.673). Even after making allowance for overestimation of the potency of halofantrine that may result from the hypokalaemia that is prevalent in anaesthetized rabbits, these effects occurred with concentrations of halofantrine that are found in clinical use. 5. These data indicate clearly that while mefloquine does not alter QTc intervals itself, it does enhance the effects of halofantrine by increasing the circulating concentration of halofantrine.

Dofetilide: a new pure class III antiarrhythmic agent

BACKGROUND

Although there are a variety of antiarrhythmic agents used for the treatment of atrial fibrillation of flutter, each drug has drawbacks, and room exists for new pharmacologic agents. Dofetilide, a pure class III agent, has recently been approved by the Food and Drug Administration for therapy of these arrhythmias and is reviewed.

METHODS

Data for dofetilide, published in full or in abstract form, were reviewed, concentrating on the properties related to its efficacy for the therapy of supraventricular arrhythmias.

RESULTS

Results from animal and human studies indicate that dofetilide, a renally excreted drug, has pure class III properties related to blockade of the delayed rectifier potassium current. It is effective for the therapy of atrial arrhythmias, particularly atrial fibrillation and flutter, and has no demonstrable negative inotropic effect. Despite an incidence of torsades de pointes of approximately 2% in patients with impaired ventricular function, dofetilide exhibited no association with an increased mortality rate when studied in a large series of patients with a reduced ejection fraction.

CONCLUSIONS

Dofetilide's electrophysiologic and clinical profiles suggest that it will be safe and clinically useful for the termination and prevention of atrial fibrillation or flutter, even in patients with impaired ventricular function.

Prevention of ventricular arrhythmias in the congenital long QT syndrome

Life-long therapy is necessary for patients with symptomatic long QT syndrome to prevent arrhythmic death. The merits and limitations of the different therapeutic modalities are discussed. beta-blockers remain the mainstay of therapy, but this medication may not be sufficient for cardiac arrest survivors and for those with the LQT3 genotype. "Genotype-specific" therapy, like potassium-channel openers for patients with inadequate potassium outflow (LQT1 and LQT2 genotypes) or sodium-channel blockers for patients with excessive sodium inflow (LQT3), significantly shortens the QT interval, but the effects of these drugs on arrhythmia prevention is less well established. Cardiac pacemakers may be especially beneficial for patients with LQT2 or LQT3 and for those with pause-dependent torsade de pointes. More important is to recognize that device programming for preventing tachyarrhythmias in patients with long QT differs from the standard pacemaker programming. Finally, implantable defibrillators with dual-chamber pacing capability are indicated for patients at high risk for arrhythmic death, including all cardiac arrest survivors.

RSD1019 suppresses ischaemia-induced monophasic action potential shortening and arrhythmias in anaesthetized rabbits

The electrophysiological actions of lidocaine, tedisamil and RSD1019 were assessed on normal and ischaemic cardiac tissue using monophasic action potentials (MAPs) recorded from the epicardium of anaesthetized rabbits. Drug effects on ischaemia-induced arrhythmias were assessed simultaneously in the same rabbits. Lidocaine, infused at 2.5, 5 and 10 micromol kg(-1) min(-1) i.v., accelerated and worsened the electrophysiological derangement caused by ischaemia, had profibrillatory actions and reduced the time to the occurrence of ventricular fibrillation (VF) relative to controls. Tedisamil, infused at 0.063, 0.125 and 0.25 micromol kg(-1) min(-1) i.v., prolonged MAP duration at 90% repolarization (MAPD(90%)) before induction of ischaemia in a dose-related manner; however, this effect was not maintained 5 min after induction of ischaemia. Tedisamil had no significant antiarrhythmic actions over the dose-range tested. RSD1019, infused at 2, 4 and 8 micromol kg(-1) min(-1) i.v., produced a small increase in MAPD(90%) before induction of ischaemia and only at the highest dose tested. In contrast to tedisamil, RSD1019 suppressed ischaemia-induced MAP shortening assessed 5 min after induction of ischaemia. This effect was dose-related. RSD1019 completely prevented ischaemia-induced tachyarrhythmias at the mid and highest infusion levels tested. The results of this study illustrate a pathologically targeted approach for preventing ischaemia-induced arrhythmias. Suppression of ischaemia-induced MAP shortening, demonstrated herein for RSD1019, represents a novel antifibrillatory approach.

Hypokalemia-induced long QT syndrome with an underlying novel missense mutation in S4-S5 linker of KCNQ1

Congenital long QT syndrome (LQTS) is caused by mutations in at least five genes coding for cardiac potassium or sodium channels that regulate the duration of ventricular action potentials. Acquired LQTS often is associated with drugs or metabolic abnormalities. A 47-year-old woman who presented with marked QT prolongation (QTc = 620 msec(1/2)) and repeated episodes of torsades de pointes associated with hypokalemia (2.6 mEq/L) was screened for mutations in LQTS genes using polymerase chain reaction/single-strand conformation polymorphism (PCR/SSCP). We identified a novel missense mutation in the intracellular linker of S4-S5 domains of KCNQ1, resulting in an amino acid substitution of cysteine for arginine at position 259 (R259C). Whole cell, patch clamp experiments were conducted on COS7 cells transfected with wild-type and/or R259C KCNQ1 with or without KCNE1. Functional analyses of the mutant KCNQ1 subunit on COS7 cells revealed its functional channels in the homozygous state, producing a significantly smaller current than the KCNQ1 channels and a less severe dominant-negative effect on I(Ks). The novel KCNQ1 mutation R259C is the molecular basis for I(Ks) dysfunction underlying an apparently sporadic case of hypokalemia-induced LQTS, consistent with a mild mutation likely to disclose the clinical manifestation of LQTS in a context of severe hypokalemia. Our findings suggest that gene carriers with such mild mutations might not be so rare as commonly expected in patients with acquired LQTS, and stress the importance of mutational analysis for detecting either "silent" forms of congenital LQTS or de novo mutations.

Clinical overview of the novel inotropic agent toborinone

Toborinone (OPC-18790, Otsuka Pharmaceutical Co. Ltd, 2(1H) -quinolone,6-[3-[ [3,4-dimethoxyphenyl)methyl] amino]-2-hydroxy prop oxyl]-,(.+-.)-) is a novel iv. inotropic agent. Positive inotropic effects are produced by PDE inhibition with the resulting increase in cAMP and intracellular calcium levels. Unlike other inotropic agents that increase cAMP, there is an absence of positive chronotropic effects, which are attributed to prolongation of the action potential due to blockade of delayed rectifier currents. There is also marked venous and arterial vasodilating properties. The absence of heart rate increases results in decreased myocardial oxygen consumption compared with conventional inotropes. Studies in human heart failure patients have been consistent with previous work in animal studies, confirming the effects of toborinone as being positive inotropy (relatively weak), marked arterial and venous vasodilatation and absence of increase in myocardial oxygen consumption. Data regarding safety in larger clinical trials, particularly regarding arrhythmias, is at present unavailable. This information will determine whether this agent becomes an accepted iv. therapeutic option for congestive heart failure.

Effects of antiarrhythmic drugs on QT interval dispersion--relationship to antiarrhythmic action and proarrhythmia

Class IA, IC, and III antiarrhythmic drugs prolong ventricular repolarization (VR) which is manifest as QT interval prolongation on the surface electrocardiogram. These drugs may prolong VR in a spatially heterogeneous manner which results in increased dispersion of VR. This may be manifest as increased QT interval dispersion. Antiarrhythmic drug-induced decreases in QT interval dispersion are associated with antiarrhythmic efficacy in patients with the long QT syndrome and in patients with sustained ventricular tachycardia. Antiarrhythmic drug-induced increases in QT interval dispersion are associated with ventricular proarrhythmia secondary to torsades de points ventricular tachycardia. A number of factors may modulate the effects of antiarrhythmic drugs on dispersion of VR, including the disease state, transient ischemia, electrolyte abnormalities, changes in autonomic tone, and hemodynamic stress.

Proarrhythmia of azimilide and other class III antiarrhythmic agents in the adrenergically stimulated rabbit

The ventricular proarrhythmic actions of five class III antiarrhythmic agents were compared in the Carlsson rabbit model. In adrenergically stimulated anesthetized rabbits, azimilide, clofilium, dofetilide, sematilide, and d,l-sotalol caused premature ventricular contractions and nonsustained and sustained ventricular tachyarrhythmias (NSVT and SVT) at pharmacologically equivalent intravenous doses that increased QTc intervals 20% (ED20). There were no significant differences between agents in the percentage of rabbits with serious arrhyhthmias at the ED20 doses of 5.2, 0.033, 0.015, 0.66, and 2.8 mg/kg i.v., respectively. Proarrhythmia was dose-dependent. Linear regression analysis of arrhythmia score versus log dose estimated the NSVT doses as 6.2, 0.055, 0.0089, 1.5, and 5.7, respectively. Analysis of arrhythmia states during a 10-min window after infusion when QTc prolongation was 20% showed that the compounds differed significantly in the proportion of time treated rabbits spent in SVT and combined NSVT and SVT. Rabbits treated with azimilide spent significantly less time in SVT and combined NSVT and SVT, followed in order of increasing time by d,l-sotalol, sematilide, clofilium, and dofetilide.

Influence of extracellular K+ concentrations on quinidine-induced K+ current inhibition in rat ventricular myocytes

Hypokalaemia is one of the important risk factors for development of torsades de pointes. We recently reported that hypokalaemia increased the electrocardiographic QT interval in rats treated with quinidine, but did not alter the arrhythmogenic potency of quinidine. In this study, we have investigated the influence of extracellular potassium concentration ([K+]o) on the inhibition of several types of cardiac potassium currents by quinidine. Such types of currents include the delayed rectifier potassium current (I(K)), the transient outward current (Ito), and the inward rectifier potassium current (I(K1)), as measured in isolated rat ventricular cells using patch-clamp techniques. Concentration-dependent effects of quinidine on I(K), Ito, and I(K1) were evaluated under both normal ([K+]o = 5.4 mM) and hypokalaemic ([K+]o = 3.5 mM) conditions. In contrast to both I(K) and Ito, which were barely influenced by changes in [K+]o, I(K1) was significantly inhibited by hypokalaemia. Furthermore, while quinidine suppressed both I(K) and Ito in a concentration-dependent manner, the inhibitory potency of quinidine on these currents was not influenced by changes in [K+]o. The respective normal and hypokalaemic IC50 values for quinidine were 11.4 and 10.0 microM (I(K)), and 17.6 and 17.3 microM (Ito). Although higher concentrations of quinidine were required to inhibit I(K1), the inhibitory potency of quinidine was also found to be insensitive to changes in [K+]o. Thus, in rats, the inhibitory potency of quinidine for the K+ current-types I(K), Ito and I(K1) is barely influenced by changes in [K+]o. These findings are consistent with our previous report showing that the QT-prolonging potency of quinidine was not altered under hypokalaemic conditions. However, whilst hypokalaemia does not affect I(K) or Ito, it can inhibit I(K1) and can result in QT prolongation in-vivo.

Inherited long QT syndromes: a paradigm for understanding arrhythmogenesis

The inherited long QT syndrome (LQTS) is a familial disease characterized by QT interval changes that often are labile, syncope, and sudden death due to arrhythmias, predominantly in young people. Multiple mutations in five genes encoding structural subunits of cardiac ion channels now have been identified in families with LQTS. Correlations are being described between genotype and specific clinical features in LQTS. However, increasing screening of affected families and sporadic cases has identified incomplete penetrance with highly variable clinical manifestations, even among individuals carrying the same mutations. The identification of LQTS disease genes represents a crucial first step in developing an understanding of the molecular basis for normal cardiac repolarization. This information will be important not only for identifying new therapies in LQTS, but also in further understanding arrhythmias, and their potential therapies, in situations such as heart failure, cardiac hypertrophy, myocardial infarction, or sudden infant death syndrome, where abnormal repolarization has been linked to sudden death. LQTS thus presents a new paradigm to cardiac electrophysiology, in which new molecular information is being brought to bear both on clinical management of patients and on development of a new framework to study the fundamental causes of arrhythmias and new approaches to therapy.

Long QT syndromes and torsade de pointes

In the long QT syndromes (LQTS), malfunction of ion channels impairs ventricular repolarisation and triggers a characteristic ventricular tachyarrhythmia: torsade de pointes. Symptoms in the LQTS (syncope or cardiac arrest) are caused by this arrhythmia. In congenital LQTS, mutations in the genes encoding for ion channels cause this channel malfunction. Six genotypes (LQT1 to LQT6) have been identified, and attempts are being made to correlate different mutations with clinical signs and specific therapy. In acquired LQTS, channel malfunction is caused by metabolic abnormalities or drugs. The list of drugs that may impair ion-channel function expands continuously. Moreover, attributes that increase the risk for drug-induced torsade (eg, female sex, recent heart-rate slowing, or hypokalaemia) and electrocardiographic "warning signs" are recognised. Recent data suggest that patients with an acquired LQTS have some underlying predisposition to proarrhythmia. Mutations causing "silent" forms of congenital LQTS, in which the patient remains free of arrhythmias until exposed to drugs that further impair repolarisation, are now recognised.

Potassium concentrations and ventricular ectopy: a prospective, observational study in post-cardiac surgery patients

OBJECTIVE

To determine whether a correlation exists between concentrations of intracellular and extracellular potassium and to determine the frequency of ventricular ectopy in patients after cardiac operations.

DESIGN

Prospective, observational clinical evaluation.

SETTING

Surgical-respiratory intensive care unit of a university-affiliated tertiary care center.

PATIENTS

Continuous 24-hr electrocardiographic monitoring was performed, and serum (extracellular) and erythrocyte (intracellular) potassium concentrations ([K+]e and [K+]i) were determined, before cardiopulmonary bypass, immediately postoperatively, and at 2, 4, 12, and 20 hrs after elective coronary bypass grafting in 31 patients.

INTERVENTIONS

None. Potassium replacement was left to the discretion of the attending physicians.

MEASUREMENTS AND MAIN RESULTS

Although the mean [K+]e varied significantly during the postoperative 24-hr period (p<.0001), the [K+]i did not (p = .953). No significant correlations were found between premature ventricular beats and [K+]i, [K+]e, or [K+]i/[K+]e (all p>.05). However, among the few patients who had one or more episodes of ventricular tachycardia (VT) within 30 mins of a study K+ sample, the mean [K+]e was significantly lower during the episode(s) of VT compared with the mean [K+]e in the absence of VT (p<.01).

CONCLUSIONS

Although it is clear that over the clinically acceptable range of [K+]e and [K+]i concentrations seen in this population, there is no correlation between potassium concentrations and the occurrence of premature ventricular beats, the infrequent association of more serious ventricular ectopy, VT, with lower [K+]e concentrations supports the practice of using serum potassium to guide potassium replacement in patients after cardiac operations.

Potassium channel openers antagonize the effects of class III antiarrhythmic agents in canine Purkinje fiber action potentials. Implications for prevention of proarrhythmia induced by class III agents

We studied the effects of potassium channel openers (PCOs) on frequency dependent prolongations of action potential duration (APD), triggered activities and oscillatory action potentials (OSC) induced by E-4031 and dofetilide. The action potentials of canine Purkinje fibers were recorded by a glass microelectrode technique. The effects of E-4031 (10(-6)M) as well as that of additional nicorandil (2 x 10(-5) M) on the APD were examined. When abnormal automaticity was observed under perfusion of E-4031 (10(-5) M) or dofetilide (10(-5) M), action potentials were recorded continuously to estimate the sequential effects of additional perfusion of nicorandil (6 x 10(-5) M) or Y-26763 (10(-5) M) on triggered activities and OSC. APD prolongation by E-4031 at slower stimulation rates (cycle lengths > or = 1,000 msec) was suppressed by nicorandil in a dose dependent manner. Both nicorandil and Y-26763 abolished the train of early afterdepolarization (EAD) due to E-4031 or dofetilide with a shifting of the resting membrane potential to a more negative level. PCOs also normalized dofetilide induced abnormal automaticities (EAD, OSC). The antagonistic actions of PCOs on changes in action potential induced by class III antiarrhythmic agents may prevent the development of proarrhythmias produced by these agents.