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

Predictive models for hERG potassium channel blockers

We report here a general method for the prediction of hERG potassium channel blockers using computational models generated from correlation analyses of a large dataset and pharmacophore-based GRIND descriptors. These 3D-QSAR models are compared favorably with other traditional and chemometric based HQSAR methods.

2-(2-Furanyl)-7-phenyl[1,2,4]triazolo[1,5-c]pyrimidin-5-amine analogs as adenosine A2A antagonists: The successful reduction of hERG activity. Part 2

The structure-activity relationship (SAR) exploration using 2-(2-furanyl)-7-phenyl[1,2,4]triazolo-[1,5-c]pyrimidin-5-amine (1) as a template led to the identification of a novel class of potent and selective adenosine A2A receptor (AR) antagonists. However, these compounds were found to be associated with significant hERG activity. This report discusses the strategy and outcome of an expanded SAR focused on addressing the hERG liability. As a result, compounds 21 and 24 possess excellent in vitro profiles, highly promising in vivo profiles, and acceptable levels of hERG channel inhibition.

Prediction of the risk of Torsade de Pointes using the model of isolated canine Purkinje fibres

Torsade de Pointes (TdP) is a well-described major risk associated with various kinds of drugs. However, prediction of this risk is still uncertain both in preclinical and clinical trials. We tested 45 reference compounds on the model of isolated canine Purkinje fibres. Of them, 22 are clearly associated and/or labelled with a risk of TdP, and 13 others are drugs with published clinical evidence of QT prolongation, with only one or two exceptional cases of TdP. The 10 remaining drugs are without reports of TdP and QT prolongation. The relevance of different indicators such as APD(90) increase, reverse use dependency, action potential triangulation or effect on V(max) was evaluated by comparison with available clinical data. Finally, a complex algorithm called TDPscreen and based on two subalgorithms corresponding to particular electrophysiological patterns was defined. This latter algorithm enabled a clear separation of drugs into three groups: (A) drugs with numerous or several reports (>2 cases) of TdP, (B) drugs causing QT prolongation and/or TdP only, the latter at a very low frequency (< or =2 cases), (C) drugs without reports of TdP or QT prolongation. The use of such an algorithm combined with a database accrued from reference compounds with available clinical data is suggested as a basis for testing new candidate drugs in the early stages of development for proarrhythmic risk prediction.

Clinical Relevance of Ion Channels for Diagnosis and Therapy of Cancer

Ion channels have a critical role in cell proliferation and it is well documented that channel blockers can inhibit the growth of cancer cells. The concept of ion channels as therapeutic targets or prognostic biomarkers attracts increasing interest, but the lack of potent and selective channel modulators has hampered a critical verification for many years. Today, the knowledge of human ion channel genes is almost complete and molecular correlates for many native currents have already been identified. This information triggered a wave of experimental results, identifying individual ion channels with relevance for specific cancer types. The current pattern of cancer-related ion channels is not arbitrary, but can be reduced to few members from each ion channel family. This review aims to provide an overview of the molecularly identified ion channels that might be relevant for the most common human cancer types. Possible applications of these candidates for a targeted cancer therapy or for clinical diagnosis are discussed.

ICH S7B draft guideline on the non-clinical strategy for testing delayed cardiac repolarisation risk of drugs: a critical analysis

The International Conference on Harmonization (ICH) stems from the initiative of three major world partners (Japan, USA, European Community) who composed a mutually accepted body of regulations concerning the safety, quality and efficacy requirements that new medicines have to meet in order to receive market approval. Documents on non-clinical safety pharmacology already composed by this organisation include two guidelines: the S7A adopted in 2000 and, its companion, the S7B guideline, in a draft form since 2001. The S7A guideline deals with general principles and recommendations on safety pharmacology studies designed to protect healthy volunteers and patients from potential drug-induced adverse reactions. The S7B recommends a general non-clinical testing strategy for determining the propensity of non-cardiovascular pharmaceuticals to delay ventricular repolarisation, an effect that at times progresses into life-threatening ventricular arrhythmia. In the most recent version of this document (June 2004), the strategy proposes experimental assays and a critical examination of other pertinent information for applying an 'evidence of risk' label to a compound. Regrettably, the guideline fails to deal satisfactorily with a number of crucial issues such as scoring the evidence of risk and the clinical consequences of such scoring. However, in the latter case, the S7B relies on the new ICH guideline E14 which is currently in preparation. E14 is the clinical counterpart of the S7B guideline which states that non-clinical data are a poor predictor of drug-induced repolarisation delay in humans. The present contribution summarises and assesses salient aspects of the S7A guideline as its founding principles are also applicable to the S7B guideline. The differences in strategies proposed by the various existing drafts of the latter document are critically examined together with some unresolved, crucial problems. The need for extending the objective of the S7B document to characterise the full electrophysiological profile of new pharmaceuticals is argued as this approach would more extensively assess the non-clinical cardiac safety of a drug. Finally, in order to overcome present difficulties in arriving at the definitive version of the S7B guideline, the Expert Working Group could reflect on the introduction of the S7B guideline recommendations in the S7A document, as originally intended, or on postponing the adoption of an harmonized text until the availability of novel scientific data allows solving presently contentious aspects of this and the E14 guidelines.

Atypical antipsychotics: from potassium channels to torsade de pointes and sudden death

Syncope and sudden death are features of schizophrenia that can be attributed to ischaemic heart disease, the use of antipsychotics (because of proarrhythmia or other reasons such as pharyngeal dyskinesia) or the psychiatric disease itself. Cases have been described with most antipsychotics and have led to the withdrawal, temporary suspension from the market or restricted use of antipsychotics, such as sultopride, droperidol, sertindole or thioridazine. Reviewing the available data shows that all antipsychotics tested affect the cardiac potassium channel, with the concentration that produces 50% inhibition (IC50) ranging from 1 nmol/L (haloperidol) to 6 micromol/L (olanzapine). Experimental in vitro or in vivo electrophysiological studies have shown a dose-dependent increase in the duration of the action potential with various degrees of indicators of serious arrhythmogenicity. However, this does not always translate clinically into an increased duration of the QT interval or increased risk of torsade de pointes or sudden death in clinical trials or pharmacoepidemiological studies. In turn, QT prolongation in clinical trials does not always translate to an increased risk of torsade de pointes or sudden death. The reasons for these apparent discrepancies are unclear and could be related to insufficiently powered field studies, low plasma and tissue drug concentrations with reference to in vitro data or drug effects on other receptors or ion channels that have a protective effect. Alternatively, risks that were not apparent from preclinical or clinical data could be related to the use of the drug in high-risk patients, metabolic interactions or other factors that would only be encountered in large postmarketing populations. The assessment of cardiovascular safety, both preclinical and during premarketing clinical trials, needs to be supported by appropriately powered pharmacoepidemiology studies.

QT-screen: high-throughput cardiac safety pharmacology by extracellular electrophysiology on primary cardiac myocytes

Cardiac safety pharmacology focuses mostly on the drug-induced prolongation of the QT interval in the electrocardiogram. A prolonged QT interval is an important indicator for an increased risk of severe ventricular arrhythmia. Guidelines demand safety tests addressing QT prolongation in vitro and in vivo before a drug enters clinical trials. If safety risks will be detected not until an advanced stage of preclinical drug development, a considerable sum of money has already been invested into the drug development process. To prevent this, high-throughput systems have been developed to obtain information on the potential toxicity of a substance earlier. We will discuss in this publication that the QT-Screen system, which is based on primary cardiac myocytes, is able to provide a sufficient throughput for secondary screening. With this system, extracellular field potentials can be recorded from spontaneously beating cultures of mammalian or avian ventricular cardiac myocytes simultaneously on 96 channels. The system includes software-controlled and automated eight-channel liquid handling, data acquisition, and analysis. These features allow a user-friendly and unsupervised operation. The throughput is over 100 compounds in six replicates and with full dose-response relationships per day. This equals a maximum of approximately 6,000 data points per day at an average cost for consumables of 0.20 US pennies (U.S.) per data point. The system is intended for a non-good laboratory practice-compliant screening; however, it can be adapted to be used in a good laboratory practice environment.

The [3H]dofetilide binding assay is a predictive screening tool for hERG blockade and proarrhythmia: Comparison of intact cell and membrane preparations and effects of altering [K+]o

INTRODUCTION

The human ether-a-go-go-related gene (hERG) encodes a potassium channel responsible for the cardiac delayed rectifier current (IKr) involved in ventricular repolarization. Drugs that block hERG have been associated with QT interval prolongation and serious, sometimes fatal, cardiac arrhythmias (including torsade de pointes). While displacement of [3H]dofetilide, a potent methanesulfonanilide hERG blocker, from cells heterologously expressing hERG has been suggested as a screening assay, questions have been raised about its predictive value.

METHODS

To validate the utility of this assay as a screening tool, we performed a series of saturation and competition binding studies using [3H]dofetilide as ligand and either intact cells or membrane preparations from HEK 293 cells stably transfected with hERG K+ channels. The object of these experiments was to (1) compare binding Ki values for 22 hERG blockers using intact cells or membrane homogenates to determine whether maintaining cell integrity enhanced assay reliability; (2) evaluate the ability of different K+ concentrations (2, 5, 10, 20, and 60 mM) to modulate hERG binding; and (3) to establish the predictive value of the assay by comparing Ki values from binding studies at 5 and 60 mM [K+]o to functional IC50 values for hERG current block using 56 structurally diverse drugs.

RESULTS

We found (a) comparable Ki values in the intact cell and isolated membrane binding assays, although there were some differences in rank order; (b) increasing [K+]o lowered the Kd and increased the Bmax for [3H]dofetilide, particularly in the membrane assay; and (c) good correlation between binding Ki values and functional IC50 values for hERG current block.

DISCUSSION

In conclusion, increasing K+ concentrations results in an increase in both [3H]dofetilide affinity for hERG and available binding sites, particularly when using membrane homogenates. There are no meaningful differences between Ki values when comparing intact cell versus membrane assay, neither are there meaningful trends with increasing [K+]o within assays. There is good correlation between binding Ki values and functional (whole-cell patch clamp) IC50 values at both 5 and 60 mM K+ concentrations (R2 values of .824 and .863, respectively). The simplicity, predictability, and adaptability to high-throughput platforms make the [3H]dofetilide membrane binding assay a useful tool for screening and ranking compounds for their potential to block the hERG K+ channel.

QT prolongation through hERG K(+) channel blockade: current knowledge and strategies for the early prediction during drug development

Prolongation of the QT interval of the electrocardiogram is a typical effect of Class III antiarrhythmic drugs, achieved through blockade of potassium channels. In the past decade, evidence has accrued that several classes of drugs used for non-cardiovascular indications may prolong the QT interval with the same mechanism (namely, human ether-a-go-go-related gene (hERG) K(+) channel blockade). The great interest in QT prolongation is because of several reasons. First, drug-induced QT prolongation increases the likelihood of a polymorphous ventricular arrhythmia (namely, torsades de pointes, TdP), which may cause syncope and degenerate into ventricular fibrillation and sudden death. Second, the fact that several classes of drugs, such as antihistamines, fluoroquinolones, macrolides, and neuroleptics may cause the long QT syndrome (LQTS) raises the question whether this is a class effect (e.g., shared by all agents of a given pharmacological class) or a specific effect of single agents within a class. There is now consensus that, in most cases, only a few agents within a therapeutic class share the ability to significantly affect hERG K(+) channels. These compounds should be identified as early as possible during drug development. Third, QT prolongation and interaction with hERG K(+) channels have become surrogate markers of cardiotoxicity and have received increasing regulatory attention. This review briefly outlines the mechanisms leading to QT prolongation and the different strategies that can be followed to predict this unwanted effect. In particular, it will focus on the approaches recently proposed for the in silico screening of new compounds.

The electrocardiographic toxidrome: the ECG presentation of hydrofluoric acid ingestion

The clinician can approach the poisoned patient using the toxidrome system of toxin identification; this approach makes use of findings noted on the physical examination, highlighting the importance of abnormalities in blood pressure, heart rate, respiratory effort, body temperature, mental status, pupillary size, skin color, diaphoresis, and gastrointestinal sounds. Such a method provides structure and guidance to the clinical evaluation, providing the clinician with rapid diagnostic information and suggesting urgent management issues. A case of hydrofluoric acid poisoning is used as an example of this diagnostic approach. The patient demonstrated systemic toxicity accompanied by oral irritation and electrocardiographic abnormality (QRS complex widening and QT interval prolongation). The constellation of these findings suggested the possibility of a caustic agent (history and examination) with potential effect on potassium and calcium metabolism (electrocardiographic abnormalities). Such a constellation strongly suggested hydrofluoric acid as the culprit toxin.

Effect of trazodone on hERG channel current and QT-interval

Trazodone has been associated with prolonged QT-interval and increased risk of polymorphous ventricular tachycardias clinically and has demonstrated in vitro inhibition of hERG (human ether-á-go-go-related gene) channel current. This study attempts to put the effects of trazodone into perspective by comparing its hERG inhibition to that of three agents known to inhibit I(Kr), and comparing the effects of trazodone and cisapride on action potential duration and the QT-interval in the rabbit Langendorff heart preparation. Trazodone inhibited hERG channel current in a concentration-dependent manner with an IC50 of 0.69 microM. Like astemizole, terfenadine and cisapride, trazodone inhibits hERG channel current at clinically relevant concentrations. Like cisapride, trazodone increased both the QT-interval and APD90 in the Langendorff heart preparation in a reverse frequency-dependent manner at clinically relevant concentrations. These data strongly suggest that trazodone prolongs the QT-interval through inhibition of hERG channel current.

QT Correction Methods in Children and Adolescents

INTRODUCTION

Accurate determination of the QTc interval has become increasingly important in the assessment of a drug's ability to prolong cardiac repolarization. Previous work suggests the most appropriate correction formula for adults is QTc=QT/RR0.40, but little on correction methods for children and adolescents has been published.

METHODS AND RESULTS

In this study, ECG data were obtained from a meta-analysis of seven clinical trials for attention deficit/hyperactivity disorder (ADHD) involving 2,288 children and adolescents. The most appropriate formula for children and adolescents included in this database was found to be QTc=QT/RR0.38. Adjustments of the correction factor specifically for males and females of different ages also are reported.

CONCLUSION

QT correction methods developed for adults do not apply to children. As accurate QTc determination plays a larger role in assessing a drug's potential to retard repolarization, use of age- and gender-specific correction formulas becomes more important.

In silico antitarget screening

The need to early predict the possible failure of a drug candidate is becoming an absolute requirement in the drug discovery process. For this reason, from the initial phases of lead development, great attention is paid to the ADMET characteristics of the compounds. In this context, the recent discovery that hitting some well-identified macromolecular targets can induce undesired side effects has led drug designers to apply some classical in silico technologies to the goal of avoiding the interaction of lead candidates with such antitargets.:

Comparative Pharmacology of Guinea Pig Cardiac Myocyte and Cloned hERG (I Kr ) Channel

INTRODUCTION

This study used whole-cell, patch clamp techniques on isolated guinea pig ventricular myocytes and HEK293 cells expressing cloned human ether-a-go-go-related gene (hERG) to examine the action of drugs causing QT interval prolongation and torsades de pointes (TdP) in man. Similarities and important differences in drug actions on cardiac myocytes and cloned hERG I(Kr) channels were established. Qualitative actions of the drugs on cardiac myocytes corresponded with results obtained from Purkinje fibers and measurement of QT interval prolongation in animal and human telemetry studies.

METHODS AND RESULTS

Adult guinea pig ventricular myocytes were isolated by enzymatic digestion. Cells were continuously perfused with Tyrode's solution at 33-35 degrees C. Recordings were made using the whole-cell, patch clamp technique. Action potentials (APs) were elicited under current clamp. Voltage clamp was used to study the effect of drugs on I(Kr) (rapidly activating delayed rectifier potassium current), I(Na) (sodium current), and I(Ca) (L-type calcium current). Dofetilide increased the myocyte action potential duration (APD) in a concentration-dependent manner, with a pIC50 of 7.3. Dofetilide 1 microM elicited early afterdepolarizations (EADs) but had little affect on I(Ca) or I(Na). E-4031 increased APD in a concentration-dependent manner, with a pIC50 of 7.2. In contrast, 10 microM loratadine, desloratadine, and cetirizine had little effect on APD or I(Kr). Interestingly, cisapride displayed a biphasic effect on myocyte APD and inhibited I(Ca) at 1 microM. Even at this high concentration, cisapride did not elicit EADs. A number of AstraZeneca compounds were tested on cardiac myocytes, revealing a mixture of drug actions that were not observed in hERG currents in HEK293 cells. One compound, particularly AR-C0X, was a potent blocker of myocyte AP (pIC50 of 8.4). AR-C0X also elicited EADs in cardiac myocytes. The potencies of the same set of drugs on the cloned hERG channel also were assessed. The pIC50 values for dofetilide, E-4031, terfenadine, loratadine, desloratadine, and cetirizine were 6.8, 7.1, 7.3, 5.1, 5.2, and <4, respectively. Elevation of temperature from 22 to 35 degrees C significantly enhanced the current kinetics and amplitudes of hERG currents and resulted in approximately fivefold increase in E-4031 potency.

CONCLUSION

Our study demonstrates the advantages of cardiac myocytes over heterologously expressed hERG channels in predicting QT interval prolongation and TdP in man. The potencies of some drugs in cardiac myocytes were similar to hERG, but only myocytes were able to detect important changes in APD characteristics and display EADs predictive of arrhythmia development. We observed similar qualitative drug profiles in cardiac myocytes, dog Purkinje fibers, and animal and human telemetry studies. Therefore, isolated native cardiac myocytes are a better predictor of drug-induced QT prolongation and TdP than heterologously expressed hERG channels. Isolated cardiac myocytes, when used with high-throughput patch clamp instruments, may have an important role in screening potential cardiotoxic compounds in the early phase of drug discovery. This would significantly reduce the attrition rate of drugs entering preclinical and/or clinical development. The current kinetics and amplitudes of the cloned hERG channel were profoundly affected by temperature, significantly altering the potency of one drug (E-4031). This finding cautions against routine drug testing at room temperature compared to physiologic temperature when using the cloned hERG channel.

Multiple complications and withdrawal syndrome associated with quetiapine/venlafaxine intoxication

OBJECTIVE

To report a case of quetiapine/venlafaxine intoxication associated with multiple complications and to review their possible relationship with these 2 drugs.

CASE SUMMARY

A 53-year-old white man was admitted to the hospital for loss of consciousness secondary to voluntary intoxication with venlafaxine and quetiapine. Several complications were attributable to this intoxication including seizures, prolonged coma, respiratory depression, neuroleptic malignant syndrome, prolonged QRS and QTc intervals, and a possible venlafaxine withdrawal syndrome.

DISCUSSION

Quetiapine could be responsible for the neuroleptic malignant syndrome presented in this case. Moreover, venlafaxine intoxication, fever, autonomic instability, and myoclonus presented serotonin syndrome as a differential diagnosis. Potential causes of seizures and prolongation of the QRS and QTc intervals are reviewed. Finally, prolonged coma and late venlafaxine withdrawal are discussed with regard to the pharmacodynamics and pharmacokinetics of drug elimination in the context of intoxication.

CONCLUSIONS

Clinicians should be aware of possible complications following intoxication with atypical antipsychotics and anti-depressants, including protracted altered mental status.

Block of wild-type and inactivation-deficient human ether-a-go-go-related gene K+ channels by halofantrine

Halofantrine is an antimalarial drug developed as a treatment of P. falciparum resistant to chloroquine. However, halofantrine can also induce long QT syndrome (LQTS) and torsades de pointes, a potentially life-threatening ventricular arrhythmia. Drug-induced LQTS is usually caused by block of the human ether-a-go-go-related gene (HERG) channels that conduct the rapid delayed rectifier K(+) current, I(Kr), in the heart. Here we show that halofantrine preferentially blocks open and inactivated HERG channels heterologously expressed in Xenopus laevis oocytes. The half-maximal inhibitory concentration (IC(50)) for block of wild-type (WT) HERG was 1.0 microM. As we reported previously for other HERG channel blockers, the potency of halofantrine was reduced by mutation to Ala of aromatic residues (Y652, F656) located in the S6 domain, or a Val (V625) located in the pore helix. Halofantrine at a concentration 10 microM did not affect the transient outward potassium channel, Kv4.3, the slow delayed rectifier potassium channel, KvLQT1+minK and inward rectifier potassium channel, Kir2.1. An inactivation deficient mutant (G628C/S631C HERG) was only slightly less sensitive (IC(50)=2.0 microM). The rate of block onset by halofantrine at 0 mV was used to estimate the apparent association (k(on)) and dissociation (k(off)) rate constants for drug binding. For WT and G628C/S631C HERG, k(on) was similar (0.0114 and 0.0163 M(-1)/s(-1) respectively). In contrast, k(off) was significantly faster for G628C/S631C (0.357 s(-1)) than WT (0.155 s(-1)), and explains the observed decrease in drug potency for the inactivation-deficient mutant channel. We conclude that halofantrine requires channels to open before it can gain access to its binding site located in the central cavity of the HERG channel.

Torsade de pointes: the clinical considerations

Torsade de pointes is a form of polymorphic ventricular tachycardia occurring in a setting of prolonged QT interval on surface electrocardiogram. Congenital causes of prolonged QT interval occur in individuals with genetic mutations in genes that control expression of potassium and sodium channels and acquired causes are numerous, predominantly drugs causing prolonged QT interval by blockade of potassium channels. Among the drugs, antiarrhythmic agents most notably quinidine, sotalol, dofetilide and ibutilide have the potential to induce the fatal torsade de pointes. Many non-antiarrhythmic drugs can also cause torsade de pointes. Although it is important to distinguish between the congenital and the acquired forms of long QT syndrome as the later can often be reversed by correction of the underlying disorder or discontinuation of the offending drug, both forms are not mutually exclusive. Clinical considerations and management of torsade de pointes are described.

Drug therapy in the elderly

Drug dosage in the elderly requires an understanding of the age-dependent changes in drug disposition and sensitivity. The most important pharmacokinetic alteration is a decline in renal function, the elderly should therefore be treated as renally insufficient patients. Metabolic clearance is primarily reduced with drugs that display high hepatic extraction, whereas the metabolism of drugs with low hepatic extraction usually is not diminished. The reduction of metabolic clearance is especially pronounced in malnourished or frail patients. The water content of the aging body decreases, the fat content rises. Hence the distribution volume of hydrophilic drugs may be reduced in the elderly, resulting in increased plasma concentrations. In contrast, the distribution volume of liphophilic drugs is increased, their plasma concentrations may decrease. Intestinal absorption of most drugs is not altered in the elderly. Aside of these pharmacokinetic changes, one of the characteristics of old age is a progressive decline in counterregulatory (homeostatic) mechanisms. Therefore, drug effects are attenuated less, the responses are usually stronger than in younger subjects, the rate and intensity of adverse effects are higher. Examples of drug actions augmented is this manner are postural hypotension with agents that lower blood pressure, dehydration and electrolyte disturbances in response to diuretics, bleeding complications with oral anticoagulants, hypoglycemia with antidiabetics, and gastrointestinal irritation with non-steroidal anti-inflammatory drugs. The brain is an especially sensitive drug target in old age. Psychotropic drugs, anticonvulsants, and centrally acting antihypertensives may impede intellectual function and motor coordination. Hence drugs should be used restrictively in geriatric patients.

QT prolongation with antimicrobial agents: understanding the significance

Cardiac toxicity has been relatively uncommon within the antimicrobial class of drugs, but well described for antiarrhythmic agents and certain antihistamines. Macrolides, pentamidine and certain antimalarials were traditionally known to cause QT-interval prolongation, and now azole antifungals, fluoroquinolones and ketolides can be added to the list. Over time, advances in preclinical testing methods for QT-interval prolongation and a better understanding of its sequelae, most notably torsades de pointes (TdP), have occurred. This, combined with the fact that five drugs have been removed from the market over the last several years, in part because of QT-interval prolongation-related toxicity, has elevated the urgency surrounding early detection and characterisation methods for evaluating non-antiarrhythmic drug classes. With technological advances and accumulating literature regarding QT prolongation, it is currently difficult or overwhelming for the practising clinician to interpret these data for purposes of formulary review or for individual patient treatment decisions. Certain patients are susceptible to the effects of QT-prolonging drugs. For example, co-variates such as gender, age, electrolyte derangements, structural heart disease, end organ impairment and, perhaps most important, genetic predisposition, underlie most if not all cases of TdP. Between and within classes of drugs there are important differences that contribute to delayed repolarisation (e.g. intrinsic potency to inhibit certain cardiac ion currents or channels, and pharmacokinetics). To this end, a risk stratification scheme may be useful to rank and compare the potential for cardiotoxicity of each drug. It appears that in most published cases of antimicrobial-associated TdP, multiple risk factors are present. Macrolides in general are associated with a greater potential than other antimicrobials for causing TdP from both a pharmacodynamic and pharmacokinetic perspective. The azole antifungal agents also can be viewed as drugs that must be weighed carefully before use since they also have both pharmacodynamic and pharmacokinetic characteristics that may trigger TdP. The fluoroquinolones appear less likely to be associated with TdP from a pharmacokinetic perspective since they do not rely on cytochrome P450 (CYP) metabolism nor do they inhibit CYP enzyme isoforms, with the exception of grepafloxacin and ciprofloxacin. Nonetheless, patient selection must be carefully made for all of these drugs. For clinicians, certain responsibilities are assumed when prescribing antimicrobial therapy: (i) appropriate use to minimise resistance; and (ii) appropriate patient and drug selection to minimise adverse event potential. Incorporating information learned regarding QT interval-related adverse effects into the drug selection process may serve to minimise collateral iatrogenic toxicity.

HERG binding specificity and binding site structure: evidence from a fragment-based evolutionary computing SAR study

We describe the application of genetic programming, an evolutionary computing method, to predicting whether small molecules will block the HERG cardiac potassium channel. Models based on a molecular fragment-based descriptor set achieve an accuracy of 85-90% in predicting whether the IC(50) of a 'blind' set of compounds is <1 microM. Analysis of the models provides a 'meta-SAR', which predicts a pharmacophore of two hydrophobic features, one preferably aromatic and one preferably nitrogen-containing, with a protonatable nitrogen asymmetrically situated between them. Our experience of the approach suggests that it is robust, and requires limited scientist input to generate valuable predictive results and structural understanding of the target.

Acquired QT interval prolongation and HERG: implications for drug discovery and development

Putative interactions between the Human Ether-a-go-go Related Gene (HERG), QT interval prolongation and Torsades de Pointes (TdP) are now integral components of any discussion on drug safety. HERG encodes for the inwardly rectifying potassium channel (I(Kr)), which is essential to the maintenance of normal cardiac function. HERG channel mutations are responsible for one form of familial long QT syndrome, a potentially deadly inherited cardiac disorder associated with TdP. Moreover, drug-induced (acquired) QT interval prolongation has been associated with an increase in the incidence of sudden unexplained deaths, with HERG inhibition implicated as the underlying cause. Subsequently, a number of non-cardiovascular drugs which induce QT interval prolongation and/or TdP have been withdrawn. However, a definitive link between HERG, QT interval prolongation and arrhythmogenesis has not been established. Nevertheless, this area is subject to ever increasing regulatory scrutiny. Here we review the relationship between HERG, long QT syndrome and TdP, together with a summary of the associated regulatory issues, and developments in pre-clinical screening.

Voriconazole-Induced QT Interval Prolongation and Ventricular Tachycardia: A Non--Concentration-Dependent Adverse Effect

A 15-year-old patient with acute lymphoblastic leukemia and Fusarium infection was treated with voriconazole. She developed asymptomatic bradycardia, QT interval prolongation, and nonsustained, polymorphic ventricular tachycardia, which recurred upon rechallenge with the drug. Voriconazole levels and metabolism were within expected normal values. This non-concentration-dependent, voriconazole-associated ventricular tachycardia mandates cardiac rhythm monitoring during voriconazole treatment.

A model for identifying HERG K+ channel blockers

Acquired long QT syndrome (LQTS) occurs frequently as a side effect of blockade of cardiac HERG K(+) channels by commonly used medications. A large number of structurally diverse compounds have been shown to inhibit K(+) current through HERG. There is considerable interest in developing in silico tools to filter out potential HERG blockers early in the drug discovery process. We describe a binary classification model that combines a 2D topological similarity filter with a 3D pharmacophore ensemble procedure to discriminate between HERG actives and inactives with an overall accuracy of 82%, with false negative and false positive rates of 29% and 15%, respectively. This model should be generally applicable in virtual library counterscreening against HERG.

Prediction of Torsade-Causing Potential of Drugs by Support Vector Machine Approach No funding was used to assist in conducting the study and the authors do not have any conflicts of interest directly relevant to the contents of the manuscript

In an effort to facilitate drug discovery, computational methods for facilitating the prediction of various adverse drug reactions (ADRs) have been developed. So far, attention has not been sufficiently paid to the development of methods for the prediction of serious ADRs that occur less frequently. Some of these ADRs, such as torsade de pointes (TdP), are important issues in the approval of drugs for certain diseases. Thus there is a need to develop tools for facilitating the prediction of these ADRs. This work explores the use of a statistical learning method, support vector machine (SVM), for TdP prediction. TdP involves multiple mechanisms and SVM is a method suitable for such a problem. Our SVM classification system used a set of linear solvation energy relationship (LSER) descriptors and was optimized by leave-one-out cross validation procedure. Its prediction accuracy was evaluated by using an independent set of agents and by comparison with results obtained from other commonly used classification methods using the same dataset and optimization procedure. The accuracies for the SVM prediction of TdP-causing agents and non-TdP-causing agents are 97.4 and 84.6% respectively; one is substantially improved against and the other is comparable to the results obtained by other classification methods useful for multiple-mechanism prediction problems. This indicates the potential of SVM in facilitating the prediction of TdP-causing risk of small molecules and perhaps other ADRs that involve multiple mechanisms.

Micro-Electrode Arrays in Cardiac Safety Pharmacology

Drug-induced QT interval prolongation is now a major concern in safety pharmacology. Regulatory authorities such as the US FDA and the European Medicines Agency require in vitro testing of all drug candidates against the potential risk for QT interval prolongation prior to clinical trials. Common in vitro methods include organ models (Langendorff heart), conventional electrophysiology on cardiac myocytes, and heterologous expression systems of human ether-a-go-go-related gene (hERG) channels. A novel approach is to study electrophysiological properties of cultured cardiac myocytes by micro-electrode arrays (MEA). This technology utilises multi channel recording from an array of embedded substrate-integrated extracellular electrodes using cardiac tissue from the ventricles of embryonic chickens. The detected field potentials allow a partial reconstruction of the shape and time course of the underlying action potential. In particular, the duration of action potentials of ventricular myocytes is closely related to the QT interval on an ECG. This novel technique was used to study reference substances with a reported QT interval prolonging effect. These substances were E4031, amiodarone, quinidine and sotalol. These substances show a significant prolongation of the field potential. However, verapamil, a typical 'false positive' when using the hERG assay does not cause any field potential prolongation using the MEA assay. Whereas the heterologous hERG assay limits cardiac repolarisation to just one channel, the MEA assay reflects the full range of mechanisms involved in cardiac action potential regulation. In summary, screening compounds in cardiac myocytes with the MEA technology against QT interval prolongation can overcome the problem of a single cell assay to potentially report 'false positives'.

Optimization of the indolyl quinolinone class of KDR (VEGFR-2) kinase inhibitors

Modifications to the basic side-chain of early lead structures of the indolyl quinolinone class of KDR kinase inhibitors resulted in improved pharmacokinetic and ancillary profiles. Specifically, compounds bearing 5-amido- and 5-sulphonamido-indolyl substituents exhibited lower plasma clearance and weaker binding affinity for the I(Kr) potassium channel hERG.

Generating signals of drug-adverse effects from prescription databases and application to the risk of arrhythmia associated with antibacterials

BACKGROUND

Although it is well known that a variety of antibacterials may incidentally cause malignant arrhythmia, the list of drugs causing arrhythmia and the impact of these adverse effects are still uncertain. We investigated on this topic by using a large prescription database with different observational designs.

METHODS

Prescription data on all incident users of several antibacterial and antiarrhythmic drugs over the period July 1997 through December 1999 were retrieved from the Drug Prescription Database (DPD) of the Italian Province of Varese. The association between the use of antibacterial and antiarrhythmic drugs was investigated by applying prescription sequence symmetry, cohort and nested case-control designs.

RESULTS

Lower proarrhythmic effects were on an average obtained from prescription sequence symmetry approach with respect to both cohort and nested case-control. Evidence of association between exposure to drugs (erythromycin and ciprofloxacin) and the risk of arrhythmia was consistently found by the three approaches. No other signals were generated from the prescription sequence symmetry analysis. Two drugs (clarithromycin and levofloxacin) showed patterns compatible with an arrhythmic effect according to both cohort and nested case-control designs.

CONCLUSIONS

Prescription databases are useful tools to explore drug safety through both conventional and emerging observational designs. In spite of its appealing features, prescription sequence symmetry design shows lower sensitivity with respect to conventional designs. Evidence about the association between the use of certain macrolides and fluoroquinolones and the onset of arrhythmia is confirmed by this study.

Zebrafish embryos express an orthologue of HERG and are sensitive toward a range of QT-prolonging drugs inducing severe arrhythmia☆

A wide range of drugs has been shown to prolong the QT interval of the electrocardiogram by blocking the pore-forming subunit of the rapidly activating delayed rectifier K+ channel, HERG (ether-à-go-go-related gene), sometimes leading to life-threatening arrhythmia. In this paper we describe cloning, sequence, and expression of the zebrafish orthologue of HERG, Zerg. Further, we studied effects of Zerg inhibition in zebrafish embryos caused by drugs or by an antisense approach. Zerg is expressed specifically in both heart chambers of zebrafish embryos, is composed of six transmembrane domains, and shows an especially high degree of amino acid conservation in the S6 and pore domain (99% identity). Several QT-prolonging drugs added to the bathing medium elicited bradycardia and arrhythmia in zebrafish embryos. The arrhythmia induced ranged from an atrioventricular 2:1 block, the ventricle beating half as often as the atrium, to more severe irregular arrhythmia with higher concentrations of the drugs. These effects were highly specific, reproducible, and rapid, e.g., 10 microM astemizole caused a 2:1 heartbeat within a minute after addition of the compound in all the embryos studied. Morpholino antisense oligonucleotides targeting Zerg were injected into zebrafish embryos and elicited similar dose-sensitive and specific arrhythmia as the QT-prolonging drugs, suggesting an evolutionarily conserved role for Erg in regulating heartbeat rate and rhythm. Further, we identified a mutation in the Per-Arnt-Sim domain of the Zerg channel in the breakdance mutant, also characterized by a 2:1 atrioventricular block. In conclusion, the zebrafish could be a tractable model organism for the study of Erg function and modulation but might also have a value in the field of cardiovascular pharmacology, e.g., as an early preclinical model for testing drugs under development for potential QT prolongation.

New Generation Antipsychotic Drugs and QTc Interval Prolongation

BACKGROUND: Recent regulatory and clinical concerns have brought into sharp focus antipsychotic drug-induced QTc interval prolongation, torsades de pointes, and sudden cardiac death. Several new generation (atypical) antipsychotic drugs have either been withdrawn from clinical use or delayed in reaching the marketplace due to these concerns. Because torsades de pointes is rarely found, QTc interval prolongation serves as a surrogate marker for this potentially life-threatening arrhythmia. Current methods of calculating this electrocardiographic parameter have limitations. The primary care physician is a key member of the team managing a patient who requires administration of antipsychotic drugs. This article focuses on new generation antipsychotic drugs and principles useful to both the primary care physician and the psychiatrist. METHOD: PubMed was searched in September 2002 using the terms antipsychotic drug and QT interval. References were examined from review articles describing antipsychotic drugs and the QT interval. The author's files gathered over the past 20 years on the QT interval were also reviewed. RESULTS: Nine cases were available in which drug-induced QTc interval prolongation was associated with new generation antipsychotic drug administration. Eight cases were taken from the literature, and the author added one additional report. The newer agents involved were risperidone, quetiapine, and ziprasidone. In at least 8 cases, there was evidence of other risk factors associated with QTc interval prolongation. In one case frequently referenced in the literature, the authors misunderstood their own data showing that QTc interval prolongation did not relate to delayed ventricular repolarization. In another instance, 2 authors reported on the same patient, with important information missing from both articles. No evidence of torsades de pointes appeared in any of the 9 cases. CONCLUSIONS: No evidence is currently available in the literature implicating new generation antipsychotic drugs in the production of torsades de pointes. However, the absence of such evidence does not prove that newer antipsychotic drugs do not cause torsades de pointes. Among patients free of risk factors for QTc interval prolongation and torsades de pointes, current literature does not dictate any specific consultative or laboratory intervention before administering new generation antipsychotic drugs. When risk factors are present, evaluation and intervention specific to those risk factors should dictate the clinician's course of action. More specific guidelines for monitoring the QT interval and risk of torsades de pointes await improved methods of measuring the QTc interval relevant to each patient.

When drug therapy gets old: pharmacokinetics and pharmacodynamics in the elderly

The age-related changes in the functions and composition of the human body require adjustments of drug selection and dosage for old individuals. Drug excretion via the kidneys declines with age, the elderly should therefore be treated as renally insufficient patients. The metabolic clearance is primarily reduced with drugs that display high hepatic extraction ('blood flow-limited metabolism'), whereas the metabolism of drugs with low hepatic extraction ('capacity-limited metabolism') usually is not diminished. Reduction of metabolic drug elimination is more pronounced in malnourished or frail subjects. The water content of the aging body decreases, the fat content rises, hence the distribution volume of hydrophilic compounds is reduced in the elderly, whereas that of lipophilic drugs is increased. Intestinal absorption of most drugs is not altered in the elderly. Aside of these pharmacokinetic changes, one of the characteristics of old age is a progressive decline in counterregulatory (homeostatic) mechanisms. Therefore drug effects are mitigated less, the reactions are usually stronger than in younger subjects, the rate and intensity of adverse effects are higher. Examples of drug effects augmented is this manner are postural hypotension with agents that lower blood pressure, dehydration, hypovolemia, and electrolyte disturbances in response to diuretics, bleeding complications with oral anticoagulants, hypoglycemia with antidiabetics, and gastrointestinal irritation with non-steroidal anti-inflammatory drugs. The brain is an especially sensitive drug target in old age. Psychotropic drugs but also anticonvulsants and centrally acting antihypertensives may impede intellectual functions and motor coordination. The antimuscarinic effects of some antidepressants and neuroleptic drugs may be responsible for agitation, confusion, and delirium in elderly. Hence drugs should be used very restrictively in geriatric patients. If drug therapy is absolutely necessary, the dosage should be titrated to a clearly defined clinical or biochemical therapeutic goal starting from a low initial dose.

Clinical relevance and management of drug-related QT interval prolongation

Much attention recently has focused on drugs that prolong the QT interval, potentially leading to fatal cardiac dysrhythmias (e.g., torsade de pointes). We provide a detailed review of the published evidence that supports or does not support an association between drugs and their risk of QT prolongation. The mechanism of drug-induced QT prolongation is reviewed briefly, followed by an extensive evaluation of drugs associated with QT prolongation, torsade de pointes, or both. Drugs associated with QT prolongation are identified as having definite, probable, or proposed associations. The role of the clinician in the prevention and management of QT prolongation, drug-drug interactions that may occur with agents known to affect the QT interval, and the impact of this adverse effect on the regulatory process are addressed.

Droperidol, QT prolongation, and sudden death: what is the evidence?

STUDY OBJECTIVE

Droperidol is a butyrophenone commonly used as an antiemetic and antipsychotic in the United States since US Food and Drug Administration (FDA) approval in 1970. Its labeling has recently been revised, with a black box warning for cases of QT prolongation leading to torsades de pointes and death. A black box warning is applied when serious adverse drug reactions are uncovered for medications. We sought to examine the evidence of a causal association suggested by the black box warning to aid clinicians in their risk-benefit analyses regarding further use of droperidol.

METHODS

A literature search was undertaken to determine the evidence regarding the association between droperidol and QT prolongation or torsades de pointes. The evidence was then evaluated by using evidence-based medicine principles. In addition, a review of the FDA regulatory process is presented.

RESULTS

Three clinical studies, 1 published abstract, and 7 case reports were reviewed. Available postmarketing surveillance data (MedWatch reports) were also reviewed. Applying the criteria of evidence-based medicine and Hill's criteria, the evidence is not convincing for a causal relationship between therapeutic droperidol administration and life-threatening cardiac events.

CONCLUSION

The recent black box warning appears to have originated from postmarketing surveillance data rather than data reported in the peer-reviewed medical literature. Ongoing monitoring of drug safety and more definitive study appear appropriate.

Sertindole: cardiac electrophysiological profile

QT interval prolongation is the ECG correlate of prolongation of the cardiac action potential (AP). Abnormal or excessive QT interval prolongation may be associated with an increased risk of ventricular tachycardia. This association appears increasingly evident in congenital long QT syndrome and with certain classes of cardiovascular and non-cardiovascular therapeutics. Almost all drugs causing QT interval prolongation inhibit the rapid component of the delayed rectifier potassium current (I Kr ), an ion channel involved in the termination of the myocardial AP. Inhibition of I Kr leads to AP and QT interval prolongation. Drugs, which do not encounter a sufficient electrophysiological counterbalance to the inhibitory effect on I Kr , may thus impose a risk of ventricular tachyarrhythmia. Some non-cardiac drugs, including the antipsychotic sertindole, have inhibitory effects on I Kr but, in contrast to the drugs that are known to cause tachyarrhythmia, sertindole possesses an important electrophysiological counterbalancing profile. Sertindole inhibits &#102 1 -adrenoceptors and blocks both sodium and calcium channels. The balanced electrophysiological profile of sertindole may well explain the low proarrhythmic potential observed in animal proarrhythmia models against positive comparators. It also supports the lack of increased cardiac mortality observed in clinical trials with sertindole and in large epidemiological studies.