A common polymorphism associated with antibiotic-induced cardiac arrhythmia

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.

[Genetic background predisposing the drug-induced long QT syndrome]

Molecular and cellular mechanisms underlying the QT prolongation have been elucidated largely because of the recent understanding of the generation of the congenital forms of QT prolongation; i.e., the long QT syndrome. To date, at least 7 different genes that modulate cardiac ion channels were identitied to be associated with the syndrome. In the clinical setting, the drug-induced long QT syndrome is more frequently seen and therefore important. We found several mutations as well as an SNP specific among the Japanese population in probands referred to as the secondary long QT patients, including the drug-induced cases. These findings raised the potential that there are also predisposing risk factors at patient's side.

Pore- and state-dependent cadmium block of I(Ks) channels formed with MinK-55C and wild-type KCNQ1 subunits

Human MinK and KCNQ1 subunits assemble to form I(Ks) channels. When MinK position 55 is mutated to cysteine (MinK-55C), I(Ks) channels can be blocked by external cadmium (Cd(2+)). We have supported a pore-associated location for MinK-55C because Cd(2+) block is sensitive to voltage, permeant ions on the opposite side of the membrane (trans-ions), and external tetraethylammonium (TEA), an I(Ks) pore-blocker. Two recent reports argue that MinK-55C is distant from the pore: one finds TEA does not affect Cd(2+) block if channels are formed with a KCNQ1 mutant (K318I, V319Y) that increases TEA affinity; the second proposes that Cd(2+) binds between MinK-55C and a cysteine in KCNQ1 that is posited to lie toward the channel periphery. Here, these discrepancies are considered. First, Cd(2+) block of MinK-55C channels formed with wild-type KCNQ1 is shown to depend not only on voltage and trans-ions but state (showing decreased on-rate with increased open time and blocker trapping on channel closure). Conversely, MinK-55C channels with K318I, V319Y KCNQ1 are found to demonstrate Cd(2+) block that is independent of voltage, trans-ions and state (and to have a lower unitary conductance): thus, the KCNQ1 mutations alter the process under study, yielding Cd(2+) inhibition that is pore-independent and, perforce, TEA-insensitive. Second, MinK-55C channels are found to remain sensitive to Cd(2+) despite mutation of any single native cysteine in KCNQ1 or all nine simultaneously; this suggests no KCNQ1 cysteine binds Cd(2+) and can serve to localize MinK-55C. Despite many concerns that are enumerated, we remain obliged to conclude that Cd(2+) enters and leaves the pore to reach MinK-55C, placing that residue in or near the pore.

The long QT interval is not only inherited but is also linked to cardiac hypertrophy

This review focuses on the molecular determinants of the duration of the QT interval as measured on by electrocardiography in normal subjects and during cardiac hypertrophy and failure. (a) In control conditions, on a single cell, the shape and duration of the action potential is the result of a balance between different ion currents which in turn were determined by the number of functional channels. On multicellular preparations the QT duration also represents the repolarization time; nevertheless it is modified by the transmural gradients. On body-surface electrocardiography the duration of the QT interval depends also of an additional factor: the spatial three-dimensional projection of the electrical waves vectors, which makes any determination of the epicardial dispersion by measuring QT interval dispersion questionable. (b) The enhanced action potential duration is well documented in cardiac hypertrophy and heart failure and is usually caused by a reduction in outward current densities in most of the species except mice. Among these currents I(tO) is the most frequently altered, especially in humans. Such an altered current density is caused by a diminished expression of the genes encoding either the ion channel subunits or regulatory proteins, such as KChIP2. In addition, hypertrophy modifies or even reverses the transmural gradient. In human and rats hypertensive cardiopathy is associated with a prolongation of the QT interval duration. The reduction in I(tO) is likely to be adaptive; it participates in the slowing of the cardiac cycle and reflects the fetal genetic reprogramming. Recent data also suggest that a reduction in the transient outward K(+) current density triggers protein synthesis through an activation of the calcineurin pathways. Thus a prolongation of the QT interval is not only inherited or drug-induced; it is also an essential component of the adaptive process in chronic mechanical overload. It is fundamentally incorrect to measure QT dispersion on a surface electrocardiography, but the mean QT interval may provide information concerning the progression of the disease, just as, and with the same restrictions, in the case of the quantification of V(max).

[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.

Novel gene hKCNE4 slows the activation of the KCNQ1 channel

The KCNE genes encode small, single transmembrane domain peptides that associate with pore-forming potassium channel subunits to form mixed complexes with unique characteristics. We have identified a novel member of the human KCNE gene family, hKCNE4. The hKCNE4 gene encodes 170 amino acid protein and is localized to chromosome 2q35-36. The protein sequence shows 90% homology to mouse KCNE4 and 38% identity to human KCNE1. Northern blot analysis revealed that hKCNE4 is expressed strongly in heart, skeletal muscle, and kidney, less in placenta, lung, and liver, and weakly in brain and blood cells. Electrophysiological study showed that hKCNE4 modulates the activation of the KCNQ1 channel.

A potassium channel-MiRP complex controls neurosensory function in Caenorhabditis elegans

MinK-related peptides (MiRPs) are single transmembrane proteins that associate with mammalian voltage-gated K(+) subunits. Here we report the cloning and functional characterization of a MiRP beta-subunit, MPS-1, and of a voltage-gated pore-forming potassium subunit, KVS-1, from the nematode Caenorhabditis elegans. mps-1 is expressed in chemosensory and mechanosensory neurons and co-localizes with kvs-1 in a subset of these. Inactivation of either mps-1 or kvs-1 by RNA interference (RNAi) causes partially overlapping neuronal defects and results in broad-spectrum neuronal dysfunction, including defective chemotaxis, disrupted mechanotransduction, and impaired locomotion. Inactivation of one subunit by RNAi dramatically suppresses the expression of the partner subunit only in cells where the two proteins co-localize. Co-expression of MPS-1 and KVS-1 in mammalian cells gives rise to a potassium current distinct from the KVS-1 current. Taken together these data indicate that potassium currents constitute a basic determinant for C. elegans neuronal function and unravel a unifying principle of evolutionary significance: that potassium channels in various organisms use MiRPs to generate uniqueness of function with rich variation in the details.

RNA interference reveals that endogenous Xenopus MinK-related peptides govern mammalian K+ channel function in oocyte expression studies

The physiological properties of most ion channels are defined experimentally by functional expression of their pore-forming alpha subunits in Xenopus laevis oocytes. Here, we cloned a family of Xenopus KCNE genes that encode MinK-related peptide K(+) channel beta subunits (xMiRPs) and demonstrated their constitutive expression in oocytes. Electrophysiological analysis of xMiRP2 revealed that when overexpressed this gene modulates human cardiac K(+) channel alpha subunits HERG (human ether-a-go-go-related gene) and KCNQ1 by suppressing HERG currents and removing the voltage dependence of KCNQ1 activation. The ability of endogenous levels of xMiRP2 to contribute to the biophysical attributes of overexpressed mammalian K(+) channels in oocyte studies was assessed next. Injection of an xMiRP2 sequence-specific short interfering RNA (siRNA) oligo reduced endogenous xMiRP2 expression 5-fold, whereas a control siRNA oligo had no effect, indicating the effectiveness of the RNA interference technique in Xenopus oocytes. The functional effects of endogenous xMiRP2 silencing were tested using electrophysiological analysis of heterologously expressed HERG channels. The RNA interference-mediated reduction of endogenous xMiRP2 expression increased macroscopic HERG current as much as 10-fold depending on HERG cRNA concentration. The functional effects of human MiRP1 (hMiRP1)/HERG interaction were also affected by endogenous xMiRP2. At high HERG channel density, at which the effects of endogenous xMiRP2 are minimal, hMiRP1 reduced HERG current. At low HERG current density, hMiRP1 paradoxically up-regulated HERG current, a result consistent with hMiRP1 rescuing HERG from suppression by endogenous xMiRP2. Thus, endogenous Xenopus MiRP subunits contribute to the base-line properties of K(+) channels like HERG in oocyte expression studies, which could explain expression level- and expression system-dependent variation in K(+) channel function.

Unraveling monogenic channelopathies and their implications for complex polygenic disease

Ion channels are a large family of >400 related proteins representing >1% of our genetic endowment; however, ion-channel diseases reflect a relatively new category of inborn error. They were first recognized in 1989, with the discovery of cystic fibrosis transmembrane conductance regulator, and rapidly advanced as positional and functional studies converged in the dissection of components of the action potential of excitable tissues. Although it remains true that diseases of excitable tissue still most clearly illustrate this family of disease, ion-channel disorders now cover the gamut of medical disciplines, causing significant pathology in virtually every organ system, producing a surprising range of often unanticipated symptoms, and providing valuable targets for pharmacological intervention. Many of the features shared among the monogenic ion-channel diseases provide a general framework for formulating a foundation for considering their intrinsically promising role in polygenic disease. Since an increasingly important approach to the identification of genes underlying polygenic disease is to identify "functional candidates" within a critical region and to test their disease association, it becomes increasingly important to appreciate how these ion-channel mechanisms can be implicated in pathophysiology.

Role of estrogen receptor-alpha in pharmacogenetics of estrogen action

PURPOSE OF REVIEW

To summarize recent data regarding the role of estrogen receptor-alpha polymorphisms in determining the response to estrogen therapy or the risk of clinical cardiovascular events.

RECENT FINDINGS

Recent clinical trials of hormone replacement therapy for cardiovascular disease have yielded surprisingly negative results, shifting clinical opinions from a position of presumed cardiovascular benefit to one of confirmed harm. Understanding why hormone replacement therapy has beneficial effects on intermediate risk markers for cardiovascular disease, but produces an increase in cardiovascular events, is an important public health question with the potential to elucidate fundamentally important aspects on atherogenesis, cardiovascular disease, and the biology of estrogen action. One question concerning the cardiovascular effects of hormone replacement therapy is whether genetic factors can substantially modify individual responses to estrogen treatment. New clinical trial evidence is emerging that links the presence of particular variants in the estrogen receptor to the response of HDL and other intermediate endpoints to hormone replacement therapy.

SUMMARY

One or more common variants in estrogen receptor-alpha are associated with a differential response to hormone replacement therapy in several domains of estrogen action. However, the effect of these variants on the risk of clinical cardiovascular events in the setting of hormone replacement therapy is not yet known. Additional research focusing on the clinical impact of common variants in estrogen receptor-alpha, estrogen receptor-beta and the progesterone receptor promise to improve clinical decision-making concerning the use of hormone replacement therapy and other novel estrogen agonists.

Drugs that induce repolarization abnormalities cause bradycardia in zebrafish

BACKGROUND

Drug-induced QT prolongation and torsades de pointes remain significant and often unpredictable clinical problems. Current in vitro preclinical assays are limited by biological simplicity, and in vivo models suffer from expense and low throughput.

METHODS AND RESULTS

During a screen for the effects of 100 small molecules on the heart rate of the zebrafish, Danio rerio, we found that drugs that cause QT prolongation in humans consistently caused bradycardia and AV block in the zebrafish. Of 23 such drugs tested, 18 were positive in this initial screen. Poor absorption explained 4 of 5 false-negative results, as demonstrated by microinjection. Overall, 22 of 23 compounds that cause repolarization abnormalities were positive in this assay. Antisense "knockdown" of the zebrafish KCNH2 ortholog yielded bradycardia in a dose dependent manner confirming the effects of reduction of repolarizing potassium current in this model. Classical drug-drug interactions between erythromycin and cisapride, as well as cimetidine and terfenadine, were also reproduced.

CONCLUSION

This simple high-throughput assay is a promising addition to the repertoire of preclinical tests for drug-induced repolarization abnormalities. The genetic tractability of the zebrafish will allow the exploration of heritable modifiers of such drug effects.

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.

Implications of pharmacogenetics for individualizing drug treatment and for study design

Adverse drug reactions and ineffective drug treatment are responsible for a large health care burden. Considerable variability in drug response makes the prediction of the individual reaction difficult. Pharmacogenetics can help to individualize drug treatment in accordance with the genetic make-up of the patient. Drug response is best understood as a complex interplay between pharmacokinetics, pharmacodynamics, and other disease-associated factors. There are a large number of genetic variants in the enzymes of phase I and phase II drug metabolism, in drug transporters, and drug targets, all of which account for differences in drug response. The polymorphisms in the cytochrome P450 enzyme system have been investigated most extensively. Genotype-based dose adjustment which should ensure "bioequivalent" drug concentrations in all patients has been derived from pharmacokinetic parameters, but this approach will have to be verified in prospective studies. Drug transport has recently been recognized as a further crucial determinant in pharmacokinetics. The effect of genetics on disease susceptibility and drug treatment has been studied quite extensively; however, hardly any of this progress is at present reflected in routine health care. The integration of pharmacogenetic factors in clinical trials requires novel considerations for study design and data interpretation. It is to be hoped that the new science bioinformatics will (a) help us identify the contribution of genetics to disease and treatment response and will (b) create data-processing devices which help the physician in the face of the enormously expanding scientific knowledge in selecting the best individually adapted treatment for the patient.

Cardiovascular pharmacogenomics: current status, future prospects

Pharmacogenomics is a field dedicated to exploring the contribution of genetics to interindividual variability in drug response. A goal of cardiovascular pharmacogenomics is to guide cardiovascular drug development and selection so as to optimize therapeutic benefit and minimize the potential for toxicity. Genetic-based differences in drug metabolism have long been recognized but just now are on the verge of wider clinical application. Differences in efficacy of cardiovascular drugs (independent of drug concentration) based on common genetic variations (polymorphisms) only recently have begun to be explored, but the potential for clinical application appears promising. Examples are presented of important pharmacodynamic effects of genetic variants on several drugs, including those in antiarrhythmic, reninangiotensin, beta-blocker, lipid-lowering, and antithrombotic classes. Principles of pharmacogenomics applied to drug metabolism are discussed that are relevant to drug development and clinical use, and examples are given for CYP450 phase I enzymes, phase II enzymes, and drug transporters. Challenges in establishing true pharmacogenetic associations are discussed, and current and future clinical potential is summarized. Rapid research progress and initial clinical applications with pharmacogenomics are foreseen in the near future.

Antiarrhythmic drugs: new agents and evolving concepts

Properties of several new antiarrhythmic drugs are summarised in this review article. Recent concepts concerning their safety and efficacy of antiarrhythmics are discussed. A brief perspective on possible future strategies for pharmacotherapy of arrhythmias is provided.

A common SCN5A polymorphism modulates the biophysical effects of an SCN5A mutation

Our understanding of the genetic basis of disease has expanded with the identification of rare DNA sequence variations ("mutations") that evoke inherited syndromes such as cystic fibrosis, congenital epilepsy, and cardiac arrhythmias. Common sequence variants ("polymorphisms") have also been implicated as risk factors in multiple diseases. Mutations in SCN5A, the cardiac Na(+) channel gene, that cause a reduction in Na(+) current may evoke severe, life-threatening disturbances in cardiac rhythm (i.e., Brugada syndrome), isolated cardiac conduction disease, or combinations of these disorders. Conduction disease is manifest clinically as heart rate slowing (bradycardia), syncope, or "lightheadedness". Recent electrophysiologic studies reveal that mutations in particular families exhibiting cardiac conduction disease cause marked effects on several competing voltage-dependent gating processes, but nonetheless cause a mild "net" reduction in Na(+) current. Here we show that a common SCN5A polymorphism (H558R) in the Na(+) channel I-II interdomain cytoplasmic linker, present in 20% of the population, can mitigate the in vitro effects of a nearby mutation (T512I) on Na(+) channel function. The mutation and the polymorphism were both found in the same allele of a child with isolated conduction disease, suggesting a direct functional association between a polymorphism and a mutation in the same gene.

Should class III drugs be initiated in hospital to prevent drug-induced torsade de pointes arrhythmias?

OBJECTIVES

In the US, the FDA requires in-hospital institution of class III drugs. This study retrospectively assessed whether these criteria, which differ markedly from the Dutch exclusion criteria, could predict sotalol-induced torsade de pointes arrhythmias (TdP).

METHOD

Oral sotalol effect in a control group (50 patients, 62±12 years, 23 men, 27 women) was compared with five patients developing TdP (75±5years, all women), using known and new (JTU area measured in lead V2) risk parameters. Paroxysmal atrial fibrillation was the most common indication for sotalol treatment.

RESULTS

At baseline the strict US regulations would have identified four of five TdP patients on the basis of individual K⁺ levels, creatinine clearance and QT_c. However, 7 of 49 controls would also have been excluded, although they did not develop documented TdP in the >2 years follow-up. Sotalol slightly increased QT_c (361±34 to 387±33ms) in controls, due to heart rate reduction. In the TdP group, sotalol dramatically increased QT_c (467±33 to 626±52 ms, +35%, p<0.05) accompanied by deep negative TU waves and an increased JTU area and all could be identified as risk patients.

CONCLUSION

Patients developing TdP on oral sotalol can be identified using the FDA risk criteria and hospitalisation may therefore be appropriate. A European prospective study is required to investigate the costs, sensitivity and specificity of this strategy.

Molecular determinants of repolarization time

This review analyzes recent data concerning the molecular determinants of repolarization time (RT) in normal and disease conditions. Considerations concerning the prognostic significance of RT were excluded. On a single normal cell, the duration of the action potential is the result of a balance between different ion currents. In vivo or on a multicellular preparation, the QT duration is modified by different transmural gradients, including the endo/epicardial gradient and the apex/base gradient. Spatial heterogeneity of the RT is not reflected by the range of the body surface QT dispersion. Inherited long QT syndrome is due to a gain or loss of function mutations located on the sodium current, the rapidly activating component of the delayed rectifier (I(Kr)) and the slowly activating component of the delayed rectifier. So far, no mutations have been detected on the transient outward K(+) current (I(tO)). Drug-induced long QT is caused by drugs that act as potassium blockers, which interact on specific domains of K(+) channel subunits, mainly on I(Kr). Several drugs may reveal 'forme frustes' of an inherited long QT. A prolonged RT is a well documented finding in cardiac hypertrophy and heart failure and is mostly caused by the noninduction and corresponding decreased density of the K(+) channel responsible for I(tO). Hypertrophy can even reverse the trans-mural gradient. In humans and rats, isolated pressure overload prolongs the QT interval. The reduction in I(tO) is likely to participate in the slowing of the cardiac cycle and reflects the re-expression of the fetal programme.

Antipsychotic drugs and QT interval prolongation

The QTc prolongation by antipsychotic drugs is of major concern, especially in light of the data indicating an increased risk of sudden death in psychiatric patients taking these drugs. Sudden death in psychiatric patients could be partially attributed to drug-induced torsades de pointes and for this reason careful evaluation of QTc prolonging properties of antipsychotic drugs is needed. Antipsychotic drugs prolong QT interval usually by blocking the potassium IKr current. Improved understanding of ion channel structure and kinetics and its role in repolarization has tremendous impact on understanding of the mechanisms of drug-induced QT prolongation and torsades de pointes. Proarrhythmia caused by a QT-prolonging drug occurs infrequently, and usually multiple factors need to operate to precipitate such an event including a combination of two or more drugs affecting the same pathway, hypokalemia, and possibly genetic predisposition. Currently prescribed antipsychotics might cause QT prolongation ranging from 4-6 ms for haloperidol and olanzapine to 35 ms for thioridazine. The response of a patient to a drug is very individual and therefore an individualized system of drug administration and monitoring needs to be developed which takes into account baseline QTc duration and its changes after a drug was introduced. A systematic approach while stratifying psychiatric patients as those with short QTc (QTc < or = 0.41 sec), borderline QTc (QTC = 0.42-0.44 sec), and prolonged QTc (0.45 sec) is being proposed to improve the safety of administering antipsychotic drugs and to decrease the risk of drug-related sudden death in psychiatric patients.

Torsade de pointes

Torsade de pointes ventricular tachyarrhythmia in the long QT syndrome is a prime example of how molecular biology, ion channel, and cellular and organ physiology, coupled with clinical observations, promise to be the future paradigm for advancement of medical knowledge. Both the congenital and the acquired long QT syndrome are caused by abnormalities (intrinsic, acquired, or both) of the ionic currents underlying ventricular repolarization. In this review, the continually unraveling molecular biology of congenital long QT syndrome is discussed. The various pharmacologic agents associated with the acquired long QT syndrome are listed. Although it is difficult to predict which patients are at risk for torsade de pointes, careful assessment of the risk to benefit ratio is important before prescribing drugs known to cause QT prolongation. The in vivo electrophysiologic mechanism of torsade de pointes in the long QT syndrome is described, using as a paradigm the anthopleurin-A canine model, a surrogate for LQT3. The characteristic association of torsade de pointes with T-wave alternans and short-long cardiac sequences is discussed, with emphasis on electrophysiologic mechanisms. Finally, the expanding knowledge of genetic mutations other than long QT syndrome associated with polymorphic ventricular tachyarrhythmia is emphasized.

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.

The antihistamine fexofenadine does not affect I(Kr) currents in a case report of drug-induced cardiac arrhythmia

1. The human HERG gene encodes the cardiac repolarizing K(+) current I(Kr) and is genetically inactivated in inherited long QT syndrome 2 (LQTS2). The antihistamine terfenadine blocks HERG channels, and can cause QT prolongation and torsades de pointes, whereas its carboxylate fexofenadine lacks HERG blocking activity. 2. In the present study the ability of fexofenadine to block the K897T HERG channel variant was investigated. The underlying single nucleotide polymorphism (SNP) A2960C was identified in a patient reported to develop fexofenadine-associated LQTS. 3. K897T HERG channels produced wild-type-like currents in Xenopus oocytes. Even at a concentration of 100 micro M, fexofenadine did not inhibit wild-type or K897T HERG channels. Coexpression of wild-type and K897T HERG with the ss-subunit MiRP1, slightly changed current kinetics but did not change sensitivity to terfenadine and fexofenadine. 4. Western blot analysis and immunostaining of transiently transfected COS-7 cells demonstrated that overall expression level, glycosylation pattern and subcellular localization of K897T HERG is indistinguishable from wild-type HERG protein, and not altered in the presence of 1 micro M fexofenadine. 5. We provide the first functional characterization of the K897T HERG variant. We demonstrated that K897T HERG is similar to wild-type HERG, and is insensitive to fexofenadine. Although the polymorphism changes PKA and PKC phosphorylation sites, regulation of K897T HERG by these kinases is not altered. 6. Our results strongly indicate that QT lengthening and cardiac arrhythmia in the reported case of drug-induced LQT are not due to the K897T exchange or to an inhibitory effect of fexofenadine on cardiac I(Kr) currents. British Journal of

Safety of non-antiarrhythmic drugs that prolong the QT interval or induce torsade de pointes: an overview

The long and growing list of non-antiarrhythmic drugs associated with prolongation of the QT interval of the electrocardiogram has generated concern not only for regulatory interventions leading to drug withdrawal, but also for the unjustified view that QT prolongation is usually an intrinsic effect of a whole therapeutic class [e.g. histamine H(1) receptor antagonists (antihistamines)], whereas, in many cases, it is displayed only by some compounds within a given class of non-antiarrhythmic drugs because of an effect on cardiac repolarisation. We provide an overview of the different classes of non-antiarrhythmic drugs reported to prolong the QT interval (e.g. antihistamines, antipsychotics, antidepressants and macrolides) and discusses the clinical relevance of the QT prolonging effect. Drug-induced torsade de pointes are sometimes considered idiosyncratic, totally unpredictable adverse drug reactions, whereas a number of risk factors for their occurrence is now recognised. Widespread knowledge of these risk factors and implementation of a comprehensive list of QT prolonging drugs becomes an important issue. Risk factors include congenital long QT syndrome, clinically significant bradycardia or heart disease, electrolyte imbalance (especially hypokalaemia, hypomagnesaemia, hypocalcaemia), impaired hepatic/renal function, concomitant treatment with other drugs with known potential for pharmacokinetic/pharmacodynamic interactions (e.g. azole antifungals, macrolide antibacterials and class I or III antiarrhythmic agents). This review provides insight into the strategies that should be followed during a drug development program when a drug is suspected to affect the QT interval. The factors limiting the predictive value of preclinical and clinical studies are also outlined. The sensitivity of preclinical tests (i.e. their ability to label as positive those drugs with a real risk of inducing QT pronglation in humans) is sufficiently good, but their specificity (i.e. their ability to label as negative those drugs carrying no risk) is not well established. Verapamil is a notable example of a false positive: it blocks human ether-a-go-go-related (HERG) K(+) channels, but is reported to have little potential to trigger torsade de pointes. Although inhibition of HERG K(+) channels has been proposed as a primary test for screening purposes, it is important to remember that several ion currents are involved in the generation of the cardiac potential and that metabolites must be specifically tested in this in vitro test. At the present state of knowledge, no preclinical model has an absolute predictive value or can be considered as a gold standard. Therefore, the use of several models facilitates decision making and is recommended by most experts in the field.

Genotyping of drug targets: a method to predict adverse drug reactions?

In the last decades, advances in molecular biology have led to modern pharmacogenetics, which started as a science that focused on investigating drug metabolising enzymes and genetic determinants of pharmacokinetic variability. As more evidence has become available on the structure of drug targets and the genes coding for them, increasing attention has been directed towards pharmacodynamic explanations of variability in therapeutic response as well as in the risk for adverse drug reactions. Traditionally, genetic drug safety research has focused on variations in single genes whose functions are known to be related to given adverse drug reactions. A few such examples, malignant hyperthermia, the long QT syndrome, venous thromboembolic disease, tardive dyskinesia, and drug addiction, are presented in this article. In the future, results from the Human Genome Project together with tools such as DNA microarray technology, high-output screening systems and advanced bioinformatics, will permit a more thorough elucidation than is currently possible of the genetic components of adverse drug reactions. By screening for a large number of single nucleotide polymorphisms (SNPs), SNP patterns associated with adverse drug reactions can be discovered even though the functions of the SNPs as such are completely unknown. On the basis of these findings, it can be expected that pharmacogenetic research will identify situations where a drug should be avoided in certain individuals in order to reduce the risk for adverse drug reactions. If so, it will be feasible to use molecular diagnostics to select drugs that are safe for the individual patient.

Antipsychotic-related QTc prolongation, torsade de pointes and sudden death

Sudden unexpected deaths have been reported with antipsychotic use since the early 1960s. In some cases the antipsychotic may be unrelated to death, but in others it appears to be a causal factor. Antipsychotics can cause sudden death by several mechanisms, but particular interest has centred on torsade de pointes (TdP), a polymorphic ventricular arrhythmia that can progress to ventricular fibrillation and sudden death. The QTc interval is a heart rate-corrected value that represents the time between the onset of electrical depolarisation of the ventricles and the end of repolarisation. Prolongation of the QTc interval is a surrogate marker for the ability of a drug to cause TdP. In individual patients an absolute QTc interval of >500 msec or an increase of 60 msec from baseline is regarded as indicating an increased risk of TdP. However, TdP can occur with lower QTc values or changes. Concern about a relationship between QTc prolongation, TdP and sudden death applies to a wide range of drugs and has led to the withdrawal or restricted labelling of several. Among antipsychotics available in the UK, sertindole was voluntarily suspended, droperidol was withdrawn, and restricted labelling introduced for thioridazine and pimozide. The degree of QTc prolongation is dose dependent and varies between antipsychotics reflecting their different capacity to block cardiac ion channels. Significant prolongation is not a class effect. Among currently available agents, thioridazine and ziprasidone are associated with the greatest QTc prolongation. Virtually all drugs known to cause TdP block the rapidly activating component of the delayed rectifier potassium current (I(kr)). Arrhythmias are more likely to occur if drug-induced QTc prolongation coexists with other risk factors, such as individual susceptibility, presence of congenital long QT syndromes, heart failure, bradycardia, electrolyte imbalance, overdose of a QTc prolonging drug, female sex, restraint, old age, hepatic or renal impairment, and slow metaboliser status. Pharmacodynamic and pharmacokinetic interactions can also increase the risk of arrhythmias. Further research is needed to quantify the risk of sudden death with antipsychotics. The risk should be viewed in the context of the overall risks and benefits of antipsychotic treatment. It seems prudent, where possible, to select antipsychotics that are not associated with marked QTc prolongation. If use of a QTc-prolonging drug is warranted, then measures to reduce the risk should be adopted.

Drug-induced long-QT syndrome associated with a subclinical SCN5A mutation

BACKGROUND

Subclinical mutations in genes associated with the congenital long-QT syndromes (LQTS) have been suggested as a risk factor for drug-induced LQTS and accompanying life-threatening arrhythmias. Recent studies have identified genetic variants of the cardiac K+ channel genes predisposing affected individuals to acquired LQTS. We have identified a novel Na+ channel mutation in an individual who exhibited drug-induced LQTS.

METHODS AND RESULTS

An elderly Japanese woman with documented QT prolongation and torsade de pointes during treatment with the prokinetic drug cisapride underwent mutational analysis of LQTS-related genes. A novel missense mutation (L1825P) was identified within the C-terminus region of the cardiac Na+ channel (SCN5A). The L1825P channel heterologously expressed in tsA-201 cells showed Na+ current with slow decay and a prominent tetrodotoxin-sensitive noninactivating component, similar to the gain-of-function phenotype most commonly observed for SCN5A-associated congenital LQTS (LQT3). In addition, L1825P exhibited loss of function Na+ channel features characteristic of Brugada syndrome. Peak Na+ current density observed in cells expressing L1825P was significantly diminished, and the voltage dependence of activation and inactivation was shifted toward more positive and negative potentials, respectively.

CONCLUSIONS

This study demonstrates that subclinical mutations in the LQTS-related gene SCN5A may predispose certain individuals to drug-induced cardiac arrhythmias.

Troubleshooting problems with in vitro screening of drugs for QT interval prolongation using HERG K+ channels expressed in mammalian cell lines and Xenopus oocytes

The majority of drugs associated with QT interval prolongation share an ability to inhibit ionic currents passed by HERG potassium channels. One method of screening new chemical entities (NCEs) for QT prolonging potential is therefore to use heterologous systems expressing HERG channels. Such systems are also of value in the understanding of the function, kinetics, sorting, pharmacological sensitivities, and important molecular determinants of the HERG potassium channel. The methods for incorporating the HERG potassium channel into cells and measuring the consequent current are a mixture of techniques that are standard (for heterologous expression of most ion channels) and individualised to HERG. This review presents a selection of the most commonly used methods for examining heterologous HERG currents, as well as introducing some of the technical problems that may be encountered and their solutions. In mammalian cell lines, problems such as fragile membranes, high leak currents, inability to form a gigaseal, diminished HERG current, endogenous transient outward current, altered kinetics, and even occasional run down can interfere with measurements. In Xenopus oocytes, endogenous chloride currents, insufficient superfusate flow, diminished HERG current and HERG current 'run up' may create difficulties.

A single transmembrane site in the KCNE-encoded proteins controls the specificity of KvLQT1 channel gating

KCNEs are a family of genes encoding small integral membrane proteins whose role in governing voltage-gated potassium channel gating is emerging. Whether each member of this homologous family interacts with channel proteins in the same manner is unknown; however, it is clear that the functional effect of each KCNE on channel gating is different. The specificity of KCNE1 (minK) and KCNE3 control of activation of the potassium channel KvLQT1 maps to a triplet of amino acids within the KCNE transmembrane domain by chimera analysis. We now define the structural determinants of functional specificity within this triplet. The central amino acid of the triplet (Thr-58 of minK and Val-72 of KCNE3) is essential for the specific control of voltage-dependent channel activation characteristics of both minK and KCNE3. Using site-directed mutations that substitute minK and KCNE3 residues, we determined that a hydroxylated central amino acid is necessary for the slow sigmoidal activation produced by minK. The precise spacing of the hydroxyl group was required for minK-like activation. An aliphatic amino acid substituted at position 58 of minK is capable of reproducing KCNE3-like kinetics and voltage-independent constitutive current activation. The bulk of the central residue is another critical parameter, indicating precise positioning of this portion of the KCNE proteins within the channel complex. An intermediate phenotype produced by several smaller aliphatic-substituted mutants yields conditional voltage independence that is distinct from the voltage-dependent gating process, suggesting that KCNE3 traps the channel in a stable open state. From these results, we propose a model of KCNE-potassium channel interaction where the functional consequence depends on the precise contact at a single amino acid.

Blockade of human cardiac potassium channel human ether-a-go-go-related gene (HERG) by macrolide antibiotics

Several macrolides have been reported to cause QT prolongation and ventricular arrhythmias such as torsades de pointes. To clarify the underlying ionic mechanisms, we examined the effects of six macrolides on the human ether-a-go-go-related gene (HERG)-encoded potassium current stably expressed in human embryonic kidney-293 cells. All six drugs showed a concentration-dependent inhibition of the current with the following IC(50) values: clarithromycin, 32.9 microM; roxithromycin, 36.5 microM; erythromycin, 72.2 microM; josamycin, 102.4 microM; erythromycylamine, 273.9 microM; and oleandomycin, 339.6 microM. A metabolite of erythromycin, des-methyl erythromycin, was also found to inhibit HERG current with an IC(50) of 147.1 microM. These findings imply that the blockade of HERG may be a common feature of macrolides and may contribute to the QT prolongation observed clinically with some of these compounds. Mechanistic studies showed that inhibition of HERG current by clarithromycin did not require activation of the channel and was both voltage- and time-dependent. The blocking time course could be described by a first-order reaction between the drug and the channel. Both binding and unbinding processes appeared to speed up as the membrane was more depolarized, indicating that the drug-channel interaction may be affected by electrostatic responses.

Expression and coassociation of ERG1, KCNQ1, and KCNE1 potassium channel proteins in horse heart

In dogs and in humans, potassium channels formed by ether-a-go-go-related gene 1 protein ERG1 (KCNH2) and KCNQ1 alpha-subunits, in association with KCNE beta-subunits, play a role in normal repolarization and may contribute to abnormal repolarization associated with long QT syndrome (LQTS). The molecular basis of repolarization in horse heart is unknown, although horses exhibit common cardiac arrhythmias and may receive drugs that induce LQTS. In horse heart, we have used immunoblotting and immunostaining to demonstrate the expression of ERG1, KCNQ1, KCNE1, and KCNE3 proteins and RT-PCR to detect KCNE2 message. Peptide N-glycosidase F-sensitive forms of horse ERG1 (145 kDa) and KCNQ1 (75 kDa) were detected. Both ERG1 and KCNQ1 coimmunoprecipitated with KCNE1. Cardiac action potential duration was prolonged by antagonists of either ERG1 (MK-499, cisapride) or KCNQ1/KCNE1 (chromanol 293B). Patch-clamp analysis confirmed the presence of a slow delayed rectifier current. These data suggest that repolarizing currents in horses are similar to those of other species, and that horses are therefore at risk for acquired LQTS. The data also provide unique evidence for coassociation between ERG1 and KCNE1 in cardiac tissue.

Cardiac ion channels

The normal electrophysiologic behavior of the heart is determined by ordered propagation of excitatory stimuli that result in rapid depolarization and slow repolarization, thereby generating action potentials in individual myocytes. Abnormalities of impulse generation, propagation, or the duration and configuration of individual cardiac action potentials form the basis of disorders of cardiac rhythm, a continuing major public health problem for which available drugs are incompletetly effective and often dangerous. The integrated activity of specific ionic currents generates action potentials, and the genes whose expression results in the molecular components underlying individual ion currents in heart have been cloned. This review discusses these new tools and how their application to the problem of arrhythmias is generating new mechanistic insights to identify patients at risk for this condition and developing improved antiarrhythmic therapies.

Three-dimensional quantitative structure-activity relationship for inhibition of human ether-a-go-go-related gene potassium channel

The protein product of the human ether-a-go-go gene (hERG) is a potassium channel that when inhibited by some drugs may lead to cardiac arrhythmia. Previously, a three-dimensional quantitative structure-activity relationship (3D-QSAR) pharmacophore model was constructed using Catalyst with in vitro inhibition data for antipsychotic agents. The rationale of the current study was to use a combination of in vitro and in silico technologies to further test the pharmacophore model and qualitatively predict whether molecules are likely to inhibit this potassium channel. These predictions were assessed with the experimental data using the Spearman's rho rank correlation. The antipsychotic-based hERG inhibitor model produced a statistically significant Spearman's rho of 0.71 for 11 molecules. In addition, 15 molecules from the literature were used as a further test set and were also well ranked by the same model with a statistically significant Spearman's rho value of 0.76. A Catalyst General hERG pharmacophore model was generated with these literature molecules, which contained four hydrophobic features and one positive ionizable feature. Linear regression of log-transformed observed versus predicted IC(50) values for this training set resulted in an r(2) value of 0.90. The model based on literature data was evaluated with the in vitro data generated for the original 22 molecules (including the antipsychotics) and illustrated a significant Spearman's rho of 0.77. Thus, the Catalyst 3D-QSAR approach provides useful qualitative predictions for test set molecules. The model based on literature data therefore provides a potentially valuable tool for discovery chemistry as future molecules may be synthesized that are less likely to inhibit hERG based on information provided by a pharmacophore for the inhibition of this potassium channel.

Allelic variants in long-QT disease genes in patients with drug-associated torsades de pointes

BACKGROUND

DNA variants appearing to predispose to drug-associated "acquired" long-QT syndrome (aLQTS) have been reported in congenital long-QT disease genes. However, the incidence of these genetic risk factors has not been systematically evaluated in a large set of patients with aLQTS. We have previously identified functionally important DNA variants in genes encoding K+ channel ancillary subunits in 11% of an aLQTS cohort.

METHODS AND RESULTS

The coding regions of the genes encoding the pore-forming channel proteins KvLQT1, HERG, and SCN5A were screened in (1) the same aLQTS cohort (n=92) and (2) controls, drawn from patients tolerating QT-prolonging drugs (n=67) and cross sections of the Middle Tennessee (n=71) and US populations (n=90). The frequency of three common nonsynonymous coding region polymorphisms was no different between aLQTS and control subjects, as follows: 24% versus 19% for H558R (SCN5A), 3% versus 3% for R34C (SCN5A), and 14% versus 14% for K897T (HERG). Missense mutations (absent in controls) were identified in 5 of 92 patients. KvLQT1 and HERG mutations (one each) reduced K+ currents in vitro, consistent with the idea that they augment risk for aLQTS. However, three SCN5A variants did not alter I(Na), which argues that they played no role in the aLQTS phenotype.

CONCLUSIONS

DNA variants in the coding regions of congenital long-QT disease genes predisposing to aLQTS can be identified in approximately 10% to 15% of affected subjects, predominantly in genes encoding ancillary subunits.

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).

Disease-associated mutations in KCNE potassium channel subunits (MiRPs) reveal promiscuous disruption of multiple currents and conservation of mechanism

KCNE genes encode single transmembrane-domain subunits, the MinK-related peptides (MiRPs), which assemble with pore-forming alpha subunits to establish the attributes of potassium channels in vivo. To investigate whether MinK, MiRP1, and MiRP2 operate similarly with their known native alpha subunit partners (KCNQ1, HERG, and Kv3.4, respectively) two conserved residues associated with human disease and influential in channel function were evaluated. As MiRPs assemble with a variety of alpha subunits in experimental cells and may do so in vivo, each peptide was also assessed with the other two alpha subunits. Inherited mutation of aspartate to asparagine (D --> N) to yield D76N-MinK is linked to cardiac arrhythmia and deafness; the analogs D82N-MiRP1 and D90N-MiRP2 were studied. Mutation of arginine to histidine (R --> H) to yield R83H-MiRP2 is associated with periodic paralysis; the analogs K69H-MinK and K75H-MiRP1 were also studied. Macroscopic and single-channel currents showed that D --> N mutations suppressed a subset of functions whereas R/K --> H changes altered the activity of MinK, MiRP1, and MiRP2 with all three alpha subunits. The findings indicate that the KCNE peptides interact similarly with different alpha subunits and suggest a hypothesis: that clinical manifestations of inherited KCNE point mutations result from disruption of multiple native currents via promiscuous interactions.

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.

The genetic basis of variability in drug responses

It is almost axiomatic that patients vary widely in their beneficial responses to drug therapy, and serious and apparently unpredictable adverse drug reactions continue to be a major public health problem. Here, we discuss the concept that genetic variants might determine much of this variability in drug response, and propose an algorithm to enable further evaluation of the benefits and pitfalls of this enticing possibility.

Molecular mechanisms underlying the long QT syndrome

Recent studies of the molecular basis of the long QT syndrome (LQTS) have advanced our understanding of the mechanisms responsible for the abnormal prolongation of ventricular repolarization and revealed associations between LQTS and other primary electrical diseases of the heart such as Brugada syndrome. The role of DNA single nucleotide polymorphisms in acquired LQTS and differences between the Romano-Ward and Jervell-Lange-Nielsen forms of congenital LQTS are gradually coming into focus. In this brief review, our goal is to summarize the molecular mechanisms proposed to underlie the susceptibility to arrhythmias in LQTS and discuss the direction of current and future research.

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.

Evaluation of drug-induced QT interval prolongation: implications for drug approval and labelling

Assessment of proarrhythmic toxicity of newly developed drugs attracts significant attention from drug developers and regulatory agencies. Although no guidelines exist for such assessment, the present experience allows several key suggestions to be made and an appropriate technology to be proposed. Several different in vitro and in vitro preclinical models exist that, in many instances, correctly predict the clinical outcome. However, the correspondence between different preclinical models is not absolute. None of the available models has been demonstrated to be more predictive and/or superior to others. Generally, compounds that do not generate any adverse preclinical signal are less likely to lead to cardiac toxicity in humans. Nevertheless, differences in likelihood offer no guarantee compared with entities with a preclinical signal. Thus, the preclinical investigations lead to probabilistic answers with the possibility of both false positive and false negative findings. Clinical assessment of drug-induced QT interval prolongation is crucially dependent on the quality of electrocardiographic data and the appropriateness of electrocardiographic analyses. An integral part of this is a precise heart rate correction of QT interval, which has been shown to require the assessment of QT/RR relationship in each study individual. The numbers of electrocardiograms required for such an assessment are larger than usually obtained in pharmacokinetic studies. Thus, cardiac safety considerations need to be an integral part of early phase I/II studies. Once proarrhythmic safety has been established in phase I/II studies, large phase III studies and postmarketing surveillance can be limited to less strict designs. The incidence of torsade de pointes tachycardia varies from 1 to 5% with clearly proarrhythmic drugs (e.g. quinidine) to 1 in hundreds of thousands with drugs that are still considered unsafe (e.g. terfenadine, cisapride). Thus, not recording any torsade de pointes tachycardia during large phase III studies offers no guarantee, and the clinical premarketing evaluation has to rely on the assessment of QT interval changes. However, since QT interval prolongation is only an indirect surrogate of predisposition to the induction of torsade de pointes tachycardia, any conclusion that a drug is safe should be reserved until postmarketing surveillance data are reviewed. The area of drug-related cardiac proarrhythmic toxicity is fast evolving. The academic perspective includes identification of markers more focused compared with simple QT interval measurement, as well as identification of individuals with an increased risk of torsade de pointes. The regulatory perspective includes careful adaptation of new research findings.