Drug-induced long QT syndrome: hERG K+ channel block and disruption of protein trafficking by fluoxetine and norfluoxetine

BACKGROUND AND PURPOSE

Fluoxetine (Prozac) is a widely prescribed drug in adults and children, and it has an active metabolite, norfluoxetine, with a prolonged elimination time. Although uncommon, Prozac causes QT interval prolongation and arrhythmias; a patient who took an overdose of Prozac exhibited a prolonged QT interval (QTc 625 msec). We looked for possible mechanisms underlying this clinical finding by analysing the effects of fluoxetine and norfluoxetine on ion channels in vitro.

EXPERIMENTAL APPROACH

We studied the effects of fluoxetine and norfluoxetine on the electrophysiology and cellular trafficking of hERG K+ and SCN5A Na+ channels heterologously expressed in HEK293 cells.

KEY RESULTS

Voltage clamp analyses employing square pulse or ventricular action potential waveform protocols showed that fluoxetine and norfluoxetine caused direct, concentration-dependent, block of hERG current (IhERG). Biochemical studies showed that both compounds also caused concentration-dependent reductions in the trafficking of hERG channel protein into the cell surface membrane. Fluoxetine had no effect on SCN5A channel or HEK293 cell endogenous current. Mutations in the hERG channel drug binding domain reduced fluoxetine block of IhERG but did not alter fluoxetine's effect on hERG channel protein trafficking.

CONCLUSIONS AND IMPLICATIONS

Our findings show that both fluoxetine and norfluoxetine at similar concentrations selectively reduce IhERG by two mechanisms, (1) direct channel block, and (2) indirectly by disrupting channel protein trafficking. These two effects are not mediated by a single drug binding site. Our findings add complexity to understanding the mechanisms that cause drug-induced long QT syndrome.

Combined hERG channel inhibition and disruption of trafficking in drug-induced long QT syndrome by fluoxetine: a case-study in cardiac safety pharmacology

Drug-induced prolongation of the rate-corrected QT interval (QTCI) on the electrocardiogram occurs as an unwanted effect of diverse clinical and investigational drugs and carries a risk of potentially fatal cardiac arrhythmias. hERG (human ether-à-go-go-related gene) is the gene encoding the alpha-subunit of channels mediating the rapid delayed rectifier K+ current, which plays a vital role in repolarising the ventricles of the heart. Most QTCI prolonging drugs can inhibit the function of recombinant hERG K+ channels, consequently in vitro hERG assays are used widely as front-line screens in cardiac safety-testing of novel chemical entities. In this issue, Rajamani and colleagues report a case of QTCI prolongation with the antidepressant fluoxetine and correlate this with a dual effect of the drug and of its major metabolite norfluoxetine on hERG channels. Both compounds were found to produce an acute inhibition of the hERG channel by pharmacological blockade, but in addition they also were able to disrupt the normal trafficking of hERG protein to the cell membrane. Mutations to a key component of the drug binding site in the S6 region of the channel greatly attenuated channel block, but did not impair disruption of trafficking; this suggests that channel block and drug effects on trafficking were mediated by different mechanisms. These findings add to growing evidence for disruption of hERG channel trafficking as a mechanism for drug-induced long QT syndrome and raise questions as to possible limitations of acute screening methods in the assessment of QTcI prolonging liability of drugs in development.

Characterization of an LQT5-related mutation in KCNE1, Y81C: Implications for a role of KCNE1 cytoplasmic domain in IKs channel function

BACKGROUND

Y81C is a new long QT-5 (LQT5)-related KCNE1 mutation, which is located in the post-transmembrane domain (post-TMD) region in close proximity to three other LQT5 mutations (S74L, D76N, and W87R).

OBJECTIVE

We examine the effects of Y81C on the function and drug sensitivity of the slow delayed rectifier channel (I(Ks)) formed by KCNE1 with pore-forming KCNQ1 subunits. We also infer a structural basis for the detrimental effects of Y81C on I(Ks) function.

METHODS

Wild-type (WT) and mutant (harboring Y81C) I(Ks) channels are expressed in oocytes or COS-7 cells. Channel function and KCNQ1 protein expression/subcellular distribution are studied by techniques of electrophysiology, biochemistry, and immunocytochemistry. Ab initio structure predictions of KCNE1 cytoplasmic domain are performed by the Robetta server.

RESULTS

Relative to WT KCNE1, Y81C reduces I(Ks) current amplitude and shifts the voltage range of activation to a more positive range. Y81C does not reduce whole-cell KCNQ1 protein level or interfere with KCNQ1 trafficking to cell surface. Thus, its effects are mediated by altered KCNQ1/KCNE1 interactions in cell surface channels. Importantly, Y81C potentiates the effects of an I(Ks) activator. Preserving the aromatic or hydroxyl side chain at position 81 (Y81F or Y81T) does not prevent the detrimental effects of Y81C. Structure predictions suggest that the post-TMD region of KCNE1 may adopt a helical secondary structure.

CONCLUSION

We propose that the post-TMD region of KCNE1 interacts with the KCNQ1 channel to modulate I(Ks) current amplitude and gating kinetics. Other LQT5 mutations in this region share the Y81C phenotype and probably affect the I(Ks) channel function by a similar mechanism.

C-terminal HERG (LQT2) mutations disrupt IKr channel regulation through 14-3-3ϵ

Beta-adrenergic receptor-mediated cAMP or protein kinase A (PKA)-dependent modulation of cardiac potassium currents controls ventricular action potential duration (APD) at faster heart rates. HERG (KCNH2) gene mutations are associated with congenital long-QT syndrome (LQT2) and affect IKr activity, a key determinant in ventricular repolarization. Physical activity or emotional stress often triggers lethal arrhythmias in LQT2 patients. Beta-adrenergic stimulation of HERG channel activity is amplified and prolonged in vitro by the adaptor protein 14-3-3epsilon. In LQT2 families, we identified three novel heterozygous HERG mutations (G965X, R1014PfsX39, V1038AfsX21) in the C-terminus that led to protein truncation and loss of a PKA phosphorylation site required for binding of 14-3-3epsilon. When expressed in CHO cells, the mutants produced functional HERG channels with normal kinetic properties. We now provide evidence that HERG channel regulation by 14-3-3epsilon is of physiological significance in humans. Upon co-expression with 14-3-3epsilon, mutant channels still bound 14-3-3epsilon but did not respond with a hyperpolarizing shift in voltage dependence as seen in wild-type channels. Co-expression experiments of wild-type and mutant channels revealed dominant-negative behavior of all three HERG mutations. Simulations of the effects of sympathetic stimulation of HERG channel activity on the whole-cell action potential suggested a role in rate-dependent control of APD and an impaired ability of mutant cardiac myocytes to respond to a triggered event or an ectopic beat. In summary, the attenuated functional effects of 14-3-3epsilon on C-terminally truncated HERG channels demonstrate the physiological importance of coupling beta-adrenergic stimulation and HERG channel activity.

The cardiac hERG/IKr potassium channel as pharmacological target: structure, function, regulation, and clinical applications

Human ether-a-go-go-related gene (hERG) potassium channels conduct the rapid component of the delayed rectifier potassium current, IKr, which is crucial for repolarization of cardiac action potentials. Moderate hERG blockade may produce a beneficial class III antiarrhythmic effect. In contrast, a reduction in hERG currents due to either genetic defects or adverse drug effects can lead to hereditary or acquired long QT syndromes characterized by action potential prolongation, lengthening of the QT interval on the surface ECG, and an increased risk for "torsade de pointes" arrhythmias and sudden death. This undesirable side effect of non-antiarrhythmic compounds has prompted the withdrawal of several blockbuster drugs from the market. Studies on mechanisms of hERG channel inhibition provide significant insights into the molecular factors that determine state-, voltage-, and use-dependency of hERG current block. In addition, crucial properties of the high-affinity drug binding site in hERG and its interaction with drug molecules have been identified, providing the basis for more refined approaches in drug design, safety pharmacology and in silico modeling. Recently, mutations in hERG have been shown to cause current increase and hereditary short QT syndrome with a high risk for life-threatening arrhythmias. Finally, the discovery of adrenergic mechanisms of hERG channel regulation as well as the development of strategies to enhance hERG currents and to modify intracellular hERG protein processing may provide novel antiarrhythmic options in repolarization disorders. In conclusion, the increasing understanding of hERG channel function and molecular mechanisms of hERG current regulation could improve prevention and treatment of hERG-associated cardiac repolarization disorders.

hERG potassium channels and cardiac arrhythmia

hERG potassium channels are essential for normal electrical activity in the heart. Inherited mutations in the HERG gene cause long QT syndrome, a disorder that predisposes individuals to life-threatening arrhythmias. Arrhythmia can also be induced by a blockage of hERG channels by a surprisingly diverse group of drugs. This side effect is a common reason for drug failure in preclinical safety trials. Insights gained from the crystal structures of other potassium channels have helped our understanding of the block of hERG channels and the mechanisms of gating.

Mechanisms underlying arrhythmogenesis in long QT syndrome

Long QT syndrome is a disease of delayed ventricular repolarization. It manifests clinically as recurrent syncope and sudden cardiac death caused by an atypical form of polymorphic ventricular tachycardia known as torsades de pointes (TdP). Evidence obtained from the studies using the rabbit left and right ventricular wedge preparations indicates that the development of TdP is relying not only on the genesis of an R-on-T trigger, but also on the formation of a functional reentrant substrate. When ventricular endocardial or subendocardial repolarization is prolonged either because of gene mutations or by drugs that reduce the net repolarization current, cell membrane potential fluctuates during phase 2 of the action potential phase 2 because of reactivation of L-type calcium current, that is, the appearance of phase 2 early afterdepolarization (EAD). In the rabbit left ventricular wedge, QT prolongation and EAD due to pure IKr inhibition are accompanied by a disproportional increase in transmural dispersion of repolarization (TDR). Early afterdepolarization in endocardium or subendocardium is able to produce new action potentials in cells with a relatively short action potential duration (eg, ventricular epicardium) probably via an electrotonic effect when TDR is large enough. This, in turn, results in an R-on-T extrasystole that is capable of initiating TdP. Enhanced TDR is essential not only for the genesis of the first initiating beat of TdP by facilitating the propagation of EAD, but also for the maintenance of TdP by serving as a functional reentrant substrate.

Functional effects of a KCNQ1 mutation associated with the long QT syndrome

OBJECTIVE

Long QT syndrome (LQTS) is an inherited disorder of ventricular repolarization caused by mutations in cardiac ion channel genes, including KCNQ1. In this study the electrophysiological properties of a LQTS-associated mutation in KCNQ1 (Q357R) were characterized. This mutation is located near the C-terminus of S6, a region that is important for the gate structure.

METHODS AND RESULTS

Co-assembly of KCNE1 with the mutant Q357R elicited a current displaying slower activation compared to the wild-type KCNQ1/KCNE1 channels. The voltage dependence of activation of Q357R was shifted to more positive potentials. Moreover, a strong reduction in current density was observed that was partially attributed to the altered voltage dependence and kinetics of activation. The reduced current amplitude was also caused by intracellular retention of Q357R/KCNE1 channels as was shown by confocal microscopy. It indicated that the Q357R mutation disturbed protein expression by a trafficking or assembly problem of the Q357R/KCNE1 complex. To mimic the patient status KCNQ1, Q357R and KCNE1 were co-expressed, which revealed a dominant negative effect on current density and activation kinetics.

CONCLUSION

The effects of the Q357R mutation on the activation of the channel together with a reduced expression at the membrane would lead to a reduction in I(Ks) and thus in "repolarization reserve" under physiological circumstances. As such it explains the long QT syndrome observed in these patients.

Most LQT2 mutations reduce Kv11.1 (hERG) current by a class 2 (trafficking-deficient) mechanism

BACKGROUND

The KCNH2 or human ether-a-go-go related gene (hERG) encodes the Kv11.1 alpha-subunit of the rapidly activating delayed rectifier K+ current (IKr) in the heart. Type 2 congenital long-QT syndrome (LQT2) results from KCNH2 mutations that cause loss of Kv11.1 channel function. Several mechanisms have been identified, including disruption of Kv11.1 channel synthesis (class 1), protein trafficking (class 2), gating (class 3), or permeation (class 4). For a few class 2 LQT2-Kv11.1 channels, it is possible to increase surface membrane expression of Kv11.1 current (IKv11.1). We tested the hypotheses that (1) most LQT2 missense mutations generate trafficking-deficient Kv11.1 channels, and (2) their trafficking-deficient phenotype can be corrected.

METHODS AND RESULTS

Wild-type (WT)-Kv11.1 channels and 34 missense LQT2-Kv11.1 channels were expressed in HEK293 cells. With Western blot analyses, 28 LQT2-Kv11.1 channels had a trafficking-deficient (class 2) phenotype. For the majority of these mutations, the class 2 phenotype could be corrected when cells were incubated for 24 hours at reduced temperature (27 degrees C) or in the drugs E4031 or thapsigargin. Four of the 6 LQT2-Kv11.1 channels that had a wild-type-like trafficking phenotype did not cause loss of Kv11.1 function, which suggests that these channels are uncommon sequence variants.

CONCLUSIONS

This is the first study to identify a dominant mechanism, class 2, for the loss of Kv11.1 channel function in LQT2 and to report that the class 2 phenotype for many of these mutant channels can be corrected. This suggests that if therapeutic strategies to correct protein trafficking abnormalities can be developed, it may offer clinical benefits for LQT2 patients.

Long QT syndrome-associated I593R mutation in HERG potassium channel activates ER stress pathways

Hereditary long QT syndrome is a fatal arrhythmia associated with gene mutations in potassium and sodium channels that are expressed in ventricle. By employing heterologous expression and making comparisons to cells expressing wild-type human-ether-a-go-go-related protein (HERG), a potassium channel that contributes to I(Kr) current in ventricular cardiomyocytes, we demonstrate activation of an elevated endoplasmic reticulum (ER) stress response by the mutant I593R HERG potassium channel implicated in long QT syndrome type 2. I593R HERG is trafficking-impaired and forms Triton-insoluble aggregates. Expression of I593R HERG activates the unfolded protein response pathway and, separately, NF-kappaB signaling. ATF6, the activating transcription factor of the unfolded protein response pathway, is processed into the active transcription factor in the cells expressing I593R HERG. Consistent with ATF6 activation, the ER chaperones/calcium-binding proteins Grp78, Grp94, and calreticulin are elevated in I593R HERG-expressing cells. Coexpression of I593R HERG with wild-type HERG also results in ER stress pathway activation. By eliciting downstream links in signal transduction pathways associated with ER stress, expression of mutant trafficking impaired ion channels may contribute to disease etiology mechanisms that augment those associated with attenuated ion flux.

Pharmacogenetics and cardiac ion channels

Ion channels control electrical excitability in living cells. In mammalian heart, the opposing actions of Na(+) and Ca(2+) ion influx, and K(+) ion efflux, through cardiac ion channels determine the morphology and duration of action potentials in cardiac myocytes, thus controlling the heartbeat. The last decade has seen a leap in our understanding of the molecular genetic origins of inherited cardiac arrhythmia, largely through identification of mutations in cardiac ion channels and the proteins that regulate them. Further, recent advances have shown that 'acquired arrhythmias', which occur more commonly than inherited arrhythmias, arise due to a variety of environmental factors including side effects of therapeutic drugs and often have a significant genetic component. Here, we review the pharmacogenetics of cardiac ion channels-the interplay between genetic and pharmacological factors that underlie human cardiac arrhythmias.

Sudden infant death syndrome: how significant are the cardiac channelopathies?

Having an apparently healthy, thriving infant fail to reach his/her first birthday is profoundly tragic. This tragedy is compounded when the infant's death is unexpected and unexplained, signed out as sudden infant death syndrome (SIDS). Despite impressive success and welcome reductions in these tragic deaths due in large measure to "Back-to-Sleep" campaigns, the fundamental pathogenic mechanisms precipitating such deaths remain dimly exposed. Here, we review the causal link between SIDS and mutations involving the SCN5A-encoded cardiac sodium channel, provide new findings following extensive postmortem genetic testing of long QT syndrome (LQTS)-associated potassium channel genes in a population-based cohort of SIDS, and summarize the current understanding regarding the spectrum and prevalence of cardiac channelopathies in the pathogenesis of SIDS.

Sodium channels as macromolecular complexes: implications for inherited arrhythmia syndromes

Mutations in cardiac ion channels and their auxiliary subunits can lead to life-threatening cardiac arrhythmias. In recent years it has become apparent that ion channels are part of large, multi-protein complexes, comprising not only the ion channels and their auxiliary subunits, but also components of the cytoskeleton, regulatory kinases and phosphatases, trafficking proteins, extracellular matrix proteins, and possibly even other ion channels. Disruption of any member of a particular ion channel complex has the potential to disrupt the function of the associated channels, resulting in paroxysmal disease. Understanding the molecular composition of individual ion channel signaling complexes in heart may yield important insights into the molecular basis of cardiac arrhythmias and may suggest novel therapeutic approaches to treatment of these life-threatening conditions.

HERG mutation predicts short QT based on channel kinetics but causes long QT by heterotetrameric trafficking deficiency

OBJECTIVE

Mutations in the KCNH2 (hERG, human ether-à-go-go related gene) gene may cause a reduction of the delayed rectifier current I(Kr), thereby leading to the long QT syndrome (LQTS). The reduced I(Kr) delays the repolarisation of cardiac cells and renders patients vulnerable to ventricular arrhythmias and sudden death. We identified a novel mutation in a LQTS family and investigated its functional consequences using molecular and microscopic techniques.

METHODS AND RESULTS

Genetic screening in the LQTS family revealed a heterozygous frameshift mutation p.Pro872fs located in the C-terminus of the KCNH2 gene. The mutation leads to a premature truncation of the C-terminus of the hERG protein. p.Pro872fs channels lack 282 amino acids at the C-terminus and possess an extra 4-amino acid tail. Both the kinetic and biochemical properties of the p.Pro872fs and p.Pro872fs/WT channels were studied in HEK293 cells and resulted in a novel proof of concept for heterozygous LQTS mutations: homotetrameric p.Pro872fs channels displayed near-normal expression, trafficking, and channel kinetics. Unexpectedly, upon co-expression of p.Pro872fs and WT channels, the repolarising power (the proportion of hERG current contributing to the action potential as the percentage of the total current available) was substantially higher during action potential clamp experiments as compared to WT channels alone. This would lead to a shorter rather than a prolonged QT interval. However, at the same time, heterotetramerisation of p.Pro872fs and WT channels also caused a dominant negative effect on trafficking by an increase in ER retention of these heterotetrameric channels, which surpassed the former gain in repolarising power.

CONCLUSION

The LQTS phenotype in the studied family is caused by a mutation with novel properties. We demonstrate that a KCNH2 mutation that clinically leads to long QT syndrome causes at the cellular level both a "gain" and a "loss" of HERG channel function due to a kinetic increase in repolarising power and a decrease in trafficking efficiency of heteromultimeric channels.

The Effects of Sevoflurane and Propofol on QT Interval and Heterologously Expressed Human Ether-A-Go-Go Related Gene Currents in Xenopus Oocytes

Sevoflurane can induce prolongation of the cardiac QT interval by inhibiting the repolarization phase of the action potential. This may occur as a result of inhibition of the human ether-a-go-go related gene (HERG) channel. To clarify the mechanisms of anesthetics on HERG channels, we monitored the electrocardiogram and measured QT intervals in the guinea pig in the presence of sevoflurane and propofol. Sevoflurane (1%-4%) prolonged QTc dose-dependently (7.5%-21.2%), but propofol did not affect it. Furthermore, HERG channels were expressed in Xenopus oocytes and outward HERG currents were obtained on step depolarization from a holding potential of -70 mV. Repolarization to -70 mV from positive test potentials resulted in large outward tail currents. Sevoflurane (1%-4%), in a dose-dependent manner, inhibited the HERG outward tail currents (9.7%-26.6%), whereas steady-state currents were inhibited only at large concentrations. The time constant of the converging current was decreased in the presence of sevoflurane, but the inactivation and activation curves were not shifted. Propofol did not affect these currents within the clinically relevant concentration. In conclusion, compared with steady-state currents, sevoflurane was more potent in inhibiting the outward tail currents, suggesting that sevoflurane may modulate the HERG channel kinetics in its inactivated state.

Mechanisms of pharmacological rescue of trafficking-defective hERG mutant channels in human long QT syndrome

Long QT syndrome type 2 is caused by mutations in the human ether-a-go-go-related gene (hERG). We previously reported that the N470D mutation is retained in the endoplasmic reticulum (ER) but can be rescued to the plasma membrane by hERG channel blocker E-4031. The mechanisms of ER retention and how E-4031 rescues the N470D mutant are poorly understood. In this study, we investigated the interaction of hERG channels with the ER chaperone protein calnexin. Using coimmunoprecipitation, we showed that the immature forms of both wild type hERG and N470D associated with calnexin. The association required N-linked glycosylation of hERG channels. Pulse-chase analysis revealed that N470D had a prolonged association with calnexin compared with wild type hERG and E-4031 shortened the time course of calnexin association with N470D. To test whether the prolonged association of N470D with calnexin is due to defective folding of mutant channels, we studied hERG channel folding using the trypsin digestion method. We found that N470D and the immature form of wild type hERG were more sensitive to trypsin digestion than the mature form of wild type hERG. In the presence of E-4031, N470D became more resistant to trypsin even when its ER-to-Golgi transport was blocked by brefeldin A. These results suggest that defective folding of N470D contributes to its prolonged association with calnexin and ER retention and that E-4031 may restore proper folding of the N470D channel leading to its cell surface expression.

A novel splice mutation of HERG in a Chinese family with long QT syndrome

Congenital long QT syndrome (LQTS) is a genetically heterogeneous disease in which six ion-channel genes have been identified. The phenotype-genotype relationships of the HERG (human ether-a-go-go-related gene) mutations are not fully understood. The objective of this study is to identify the underlying genetic basis of a Chinese family with LQTS and to characterize the clinical manifestations properties of the mutation. Single strand conformation polymorphism (SSCP) analyses were conducted on DNA fragments amplified by polymerase chain reaction from five LQT-related genes. Aberrant conformers were analyzed by DNA sequencing. A novel splice mutation in C-terminus of HERG was identified in this Chinese LQTS family, leading to the deletion of 11-bp at the acceptor splice site of Exon9 [Exon9 IVS del (-12-->-2)]. The mutation might affect, through deficient splicing, the putative cyclic nucleotide binding domain (CNBD) of the HERG K(+) channel. This mutation resulted in a mildly affected phenotype. Only the proband had a history of syncopes, while the other three individuals with long QT interval had no symptoms. Two other mutation carriers displayed normal phenotype. No sudden death occurred in the family. The 4 affected individuals and the two silent mutation carriers were all heterozygous for the mutation. It is the first splice mutation of HERG reported in Chinese LQTS families. Clinical data suggest that the CNBD mutation may be less malignant than mutations occurring in the pore region and be partially dominant over wild-type function.

Functional assessment of compound mutations in the KCNQ1 and KCNH2 genes associated with long QT syndrome

BACKGROUND

Long QT syndrome (LQTS) is a cardiovascular disorder characterized by prolonged QTc time, syncope, or sudden death caused by torsades de pointes and ventricular fibrillation. We investigated the clinical and electrophysiologic phenotype of individual mutations and the compound mutations in a family in which different genotypes could be found.

OBJECTIVES

The purpose of this study was to determine the impact of genotype-based diagnostic assessment in LQTS.

METHODS

We used cascade screening and functional analyses to investigate the phenotype in a family with LQTS. The contributions of the compound mutations in the KCNQ1 and KCNH2 genes (KCNQ1 R591H, KCNH2 R328C) were analyzed by heterologous expression in Xenopus laevis oocytes using two-electrode voltage clamp and by confocal imaging.

RESULTS

KCNH2 R328C did not show any functional phenotype whereas KCNQ1 R591H resulted in severe reduction of current. Neither wild-type nor mutant channels affected each other functionally in coexpression experiments. Therefore, a direct interaction between KCNQ1 and KCNH2 was ruled out under these conditions.

CONCLUSION

Assessment of novel mutational findings in LQTS should include accurate genetic and functional analysis. Notably, appropriate studies are needed if two or more mutations in different genes are present in one proband. Our findings prompt reconsideration of the impact of compound mutations in LQTS families and reinforce the need for thorough functional evaluation of novel ion channel mutations before assignment of pathogenic status.

Pharmacokinetic-pharmacodynamic modeling in the data analysis and interpretation of drug-induced QT/QTc prolongation

In this review, factors affecting the QT interval and the methods that are currently in use in the analysis of drug effects on the QT interval duration are overviewed with the emphasis on (population) pharmacokinetic-pharmacodynamic (PK-PD) modeling. Among which the heart rate (HR) and the circadian rhythm are most important since they may interfere with the drug effect and need to be taken into account in the data analysis. The HR effect or the RR interval (the distance between 2 consecutive R peaks) effect is commonly eliminated before any further analysis, and many formulae have been suggested to correct QT intervals for changes in RR intervals. The most often used are Bazett and Fridericia formulae introduced in 1920. They are both based on the power function and differ in the exponent parameter. However, both assume the same exponent for different individuals. More recent findings do not confirm this assumption, and individualized correction is necessary to avoid under- or overcorrection that may lead to artificial observations of drug-induced QT interval prolongation. Despite the fact that circadian rhythm in QT and QTc intervals is a well-documented phenomenon, it is usually overlooked when drug effects are evaluated. This may result in a false-positive outcome of the analysis as the QTc peak due to the circadian rhythm may coincide with the peak of the drug plasma concentration. In view of these effects interfering with a potential drug effect on the QTc interval and having in mind low precision of QT interval measurements, a preferable way to evaluate the drug effect is to apply a population PK-PD modeling. In the literature, however, there are only a few publications in which population PK-PD modeling is applied to QT interval prolongation data, and they all refer to antiarrhythmic agents. In this review, after the most important sources of variability are outlined, a comprehensive population PK-PD model is presented that incorporates an individualized QT interval correction, a circadian rhythm in the individually corrected QT intervals, and a drug effect. The model application is illustrated using real data obtained with 2 compounds differing in their QT interval prolongation potential. The usefulness of combining data of several studies is stressed. Finally, the standard approach based on the raw observations and formal statistics, as described in the Preliminary Concept paper of the International Conference on Harmonization, is briefly compared with the method based on population PK-PD modeling, and the advantages of the latter are outlined.

An increase in late sodium current potentiates the proarrhythmic activities of low-risk QT-prolonging drugs in female rabbit hearts

Assessment of the proarrhythmic risk associated with drugs that prolong the QT interval is difficult. We hypothesized that the proarrhythmic activities of drugs with very low to moderate risk of causing torsades de pointes would be well differentiated when the late sodium current (I(NaL)) was greater than normal. The effects of selected QT-prolonging drugs on electrical activity of female rabbit isolated hearts were determined in the absence and presence of sea anemone toxin (ATX-II; an enhancer of I(NaL)). I(NaL) recorded from ventricular myocytes isolated from female rabbit hearts was slightly increased by 1 and 3 nM ATX-II (n = 13, P < 0.01). ATX-II (1 nM) prolonged the duration of the monophasic action potential (MAPD(90)) the isolated heart by of 19 +/- 3% (P < 0.001, n = 31) and shifted the concentration-response relationships for cisapride (1-30 nM), ziprasidone (0.01-3 microM), quinidine (0.1-1 microM), and moxifloxacin (0.01-1 microM) to prolong MAPD to the left by 2- to 12-fold. In contrast, the increases in MAPD(90) caused by 1 nM ATX-II and pentobarbital were only additive, and the increases in MAPD(90) caused by ATX-II and ranolazine [(+/-)-N-(2,6-dimethylphenyl)-(4[2-hydroxy-3-(2-methoxyphenoxy)propyl]-1-piperazine] were less than additive. Episodes of arrhythmic activity were commonly observed, and beat-to-beat variability of action potential duration was increased, during exposure of hearts to cisapride, ziprasidone, quinidine, and moxifloxacin but not during exposure of hearts to ranolazine or pentobarbital, in the presence of ATX-II. Thus, in the female rabbit heart, ATX-II potentiated the effects of QT-prolonging drugs to increase MAPD(90) and unmasked the proarrhythmic activities of these drugs at clinically relevant drug concentrations.

Genetics of acquired long QT syndrome

The QT interval is the electrocardiographic manifestation of ventricular repolarization, is variable under physiologic conditions, and is measurably prolonged by many drugs. Rarely, however, individuals with normal base-line intervals may display exaggerated QT interval prolongation, and the potentially fatal polymorphic ventricular tachycardia torsade de pointes, with drugs or other environmental stressors such as heart block or heart failure. This review summarizes the molecular and cellular mechanisms underlying this acquired or drug-induced form of long QT syndrome, describes approaches to the analysis of a role for DNA variants in the mediation of individual susceptibility, and proposes that these concepts may be generalizable to common acquired arrhythmias.

Long QT syndrome: from channels to cardiac arrhythmias

Long QT syndrome, a rare genetic disorder associated with life-threatening arrhythmias, has provided a wealth of information about fundamental mechanisms underlying human cardiac electrophysiology that has come about because of truly collaborative interactions between clinical and basic scientists. Our understanding of the mechanisms that control the critical plateau and repolarization phases of the human ventricular action potential has been raised to new levels through these studies, which have clarified the manner in which both potassium and sodium channels regulate this critical period of electrical activity.

QT Prolongation Modifies Dynamic Restitution and Hysteresis of the Beat-to-Beat QT-TQ Interval Relationship during Normal Sinus Rhythm under Varying States of Repolarization

The analysis of cardiac electrical restitution (the relationship between an action potential duration and its preceding diastolic interval) has been used to predict arrhythmia liability. However, the procedure to measure restitution is invasive and disrupts normal physiological autonomic balance. Dynamic analysis of sequential beat-to-beat ECG data was used to study restitution under normal sinus rhythm and to quantify changes in temporal hysteresis with heart rate acceleration/deceleration during QT prolongation. Congenital long QT (LQT) 1 and LQT2 syndromes during sympathetic stimulation were modeled because of their association with increased risk of ventricular arrhythmia. Temporal heterogeneity and hysteresis of restitution were examined in the conscious dog under varying conditions of delayed repolarization using either the selective inhibitors of the slowly activating delayed rectifier potassium current (R)-2-(4-trifluoromethyl)-N-[2-oxo-5-phenyl-1-(2,2,2-trifluoroethyl)-2,3-dihydro-1H-benzo[e][1,4]diazepin-3-yl]acetamide (L-768,673); the rapidly activating delayed rectifier potassium current (1-[2-(6-methyl-2-pyridyl)ethyl]-4-methyl-sulfonylaminobenzoyl)-piperidine (E-4031); or a combination of both at rest and during heart rate acceleration with sympathetic stimulation using isoproterenol challenges. Impaired repolarization with the combination of E-4031 and L-768,673 increased heterogeneity of restitution at rest 55 to 91%, increased hysteresis during heart rate acceleration after isoproterenol challenge by approximately 40 to 60%, and dramatically reduced the minimum TQ interval by 72% to only 28 ms. Impaired repolarization alters restitution during normal sinus rhythm and increases hysteresis/heterogeneity during heart rate acceleration following sympathetic stimulation. Thus, dynamic beat-to-beat measurements of restitution could lead to clinically applicable ECG obtained biomarkers for assessment of changes associated with arrhythmogenic risk.