Pharmacogenetic aspects of drug-induced torsade de pointes: potential tool for improving clinical drug development and prescribing

Contactless magnetocardiographic mapping in anesthetized Wistar rats: evidence of age-related changes of cardiac electrical activity

Magnetocardiography (MCG) is the recording of the magnetic field (MF) generated by cardiac electrophysiological activity. Because it is a contactless method, MCG is ideal for noninvasive cardiac mapping of small experimental animals. The aim of this study was to assess age-related changes of cardiac intervals and ventricular repolarization (VR) maps in intact rats by means of MCG mapping. Twenty-four adult Wistar rats (12 male and 12 female) were studied, under anesthesia, with the same unshielded 36-channel MCG instrumentation used for clinical recordings. Two sets of measurements were obtained from each animal: 1) at 5 mo of age (297.5 ± 21 g body wt) and 2) at 14 mo of age (516.8 ± 180 g body wt). RR and PR intervals, QRS segment, and QTpeak, QTend, JTpeak, JTend, and Tpeak-end were measured from MCG waveforms. MCG imaging was automatically obtained as MF maps and as inverse localization of cardiac sources with equivalent current dipole and effective magnetic dipole models. After 300 s of continuous recording were averaged, the signal-to-noise ratio was adequate for study of atrial and ventricular MF maps and for three-dimensional localization of the underlying cardiac sources. Clear-cut age-related differences in VR duration were demonstrated by significantly longer QTend, JTend, and Tpeak-end in older Wistar rats. Reproducible multisite noninvasive cardiac mapping of anesthetized rats is simpler with MCG methodology than with ECG recording. In addition, MCG mapping provides new information based on quantitative analysis of MF and equivalent sources. In this study, statistically significant age-dependent variations in VR intervals were found.

Assessing the proarrhythmic potential of drugs: current status of models and surrogate parameters of torsades de pointes arrhythmias

Torsades de pointes (TdP) is a potentially lethal cardiac arrhythmia that can occur as an unwanted adverse effect of various pharmacological therapies. Before a drug is approved for marketing, its effects on cardiac repolarisation are examined clinically and experimentally. This paper expresses the opinion that effects on repolarisation duration cannot directly be translated to risk of proarrhythmia. Current safety assessments of drugs only involve repolarisation assays, however the proarrhythmic profile can only be determined in the predisposed model. The availability of these proarrhythmic animal models is emphasised in the present paper. It is feasible for the pharmaceutical industry to establish one or more of these proarrhythmic animal models and large benefits are potentially available if pharmaceutical industries and patient-care authorities embraced these models. Furthermore, suggested surrogate parameters possessing predictive power of TdP arrhythmia are reviewed. As these parameters are not developed to finalisation, any meaningful study of the proarrhythmic potential of a new drug will include evaluation in an integrated model of TdP arrhythmia.

Therapeutic Drug Monitoring and Pharmacogenetic Tests as Tools in Pharmacovigilance

Therapeutic drug monitoring (TDM) and pharmacogenetic tests play a major role in minimising adverse drug reactions and enhancing optimal therapeutic response. The response to medication varies greatly between individuals, according to genetic constitution, age, sex, co-morbidities, environmental factors including diet and lifestyle (e.g. smoking and alcohol intake), and drug-related factors such as pharmacokinetic or pharmacodynamic drug-drug interactions. Most adverse drug reactions are type A reactions, i.e. plasma-level dependent, and represent one of the major causes of hospitalisation, in some cases leading to death. However, they may be avoidable to some extent if pharmacokinetic and pharmacogenetic factors are taken into consideration. This article provides a review of the literature and describes how to apply and interpret TDM and certain pharmacogenetic tests and is illustrated by case reports. An algorithm on the use of TDM and pharmacogenetic tests to help characterise adverse drug reactions is also presented. Although, in the scientific community, differences in drug response are increasingly recognised, there is an urgent need to translate this knowledge into clinical recommendations. Databases on drug-drug interactions and the impact of pharmacogenetic polymorphisms and adverse drug reaction information systems will be helpful to guide clinicians in individualised treatment choices.

Choice of cardiac tissue plays an important role in the evaluation of drug-induced prolongation of the QT interval in vitro in rabbit

INTRODUCTION

Regulatory guidelines (CPMP/986/96, ICHS7B) recommend the use of isolated cardiac tissues, including Purkinje fibers, papillary muscles and ventricular trabeculae, for detecting potential drug-induced long QT. However, the differential sensitivity of these tissues in experimental drug-induced long QT is relatively unknown. We investigated the electrophysiological characteristics of these tissue types in vitro together with their different responses to drugs that are known to induce prolongation of the QT interval in man.

METHODS

Electrophysiological parameters were measured in vitro using a micro-electrode technique. The isolated rabbit Purkinje fibers, papillary muscles or ventricular trabeculae were superperfused with Tyrode's solution and stimulated according to different stimulation protocols. The effects of dofetilide (1 x 10(-8) M), sertindole (1 x 10(-6) M), erythromycin (3 x 10(-4) M) and sparfloxacin (1 x 10(-4) M) were evaluated relative to solvent (n=8 to 12 for each group).

RESULTS

In isolated Purkinje fibers, action potential duration at 90% repolarization (APD(90) at 1 Hz) was markedly prolonged by 55% (erythromycin), 103% (dofetilide), 118% (sertindole) and 88% (sparfloxacin) from baseline. The prolongation of APD(90) caused by these 4 compounds was associated with a 28% to 78% incidence of early afterdepolarizations (EADs) at 0.2 Hz only in the Purkinje fiber. In contrast, APD(90) was altered by erythromycin, dofetilide, sertindole and sparfloxacin only by +15%, +6%, -7% or +15%, respectively, in isolated papillary muscles, and by 33%, +28%, +4% or +16%, respectively, in ventricular trabeculae. EADs were not induced by these four compounds in papillary muscles or in trabeculae. Reducing the stimulation rate to 0.2 Hz resulted in a 33% prolongation of APD(90) in Purkinje fibers, while APD(90) was shortened by 10% in papillary muscles and by 20% in ventricular trabeculae.

CONCLUSION

The present study demonstrates that the differential sensitivity of tissue types play an important role in the detection of drug-induced long APD and EADs. Indeed the Purkinje fiber was the only tissue type to display the well known phenomenon associated with I(kr) channel blockade (inverse-use dependence), when the stimulation rate was decreased below 1 Hz. Rabbit Purkinje fibers constitute the most sensitive tissue type for detecting drug-induced long action potential duration and EADs. As such the selection of tissue type needs to be taken into careful consideration in cardiac safety assessments when exploring drug induced long QT.

Use of preclinical assays to predict risk of drug-induced torsades de pointes

Numerous medications, including drugs prescribed for noncardiac indications, can induce electrophysiologic changes that trigger the rare, malignant polymorphic ventricular tachyarrhythmia known as torsades de pointes (TdP). Although the exact relationship between electrophysiologic events and the development of TdP is not defined, prolongation of the QT interval and inhibition of the rapidly activating potassium current I(Kr) by drugs may be associated with an increased risk of TdP. The ability of a drug to reduce I(Kr) and prolong the QT interval often is considered to predict the likelihood that the drug will cause TdP in humans. However, these surrogate measures of the drug-induced risk of causing TdP, and therefore of drug safety, now are recognized to be imperfect predictors of drug safety. New preclinical models should be used to assess drug risk, including preparations, conditions, and measurements used by basic research scientists to produce ventricular polymorphic arrhythmias in the laboratory. In this review, we discuss the task of assessing the arrhythmogenic potential of a drug. Assays of drug effect to induce early afterdepolarizations and ectopic beats and/or to increase the dispersion of ventricular repolarization when "repolarization reserve" is reduced appear to be the best predictors of the drug-induced risk of TdP. Current experimental models and protocols, especially those using conditions wherein the net repolarizing current is reduced, can detect the potential for a drug to induce TdP, even when the potential is extremely low.

Refining detection of drug-induced proarrhythmia: QT interval and TRIaD

QT interval prolongation is so frequently associated with torsades de pointes (TdP) that it has come to be recognized as a surrogate marker of this unique tachyarrhythmia. However, not only does TdP not always follow QT interval prolongation, but TdP can occur even in the absence of a prolonged QT interval. Worse still, even shortening of the QT interval may be associated with serious arrhythmias (particularly ventricular tachycardia [VT] and ventricular fibrillation [VF]). It appears increasingly probable that the distinction between various ventricular tachyarrhythmias may be arbitrary, and drug-induced TdP, polymorphic VT, VT, catecholaminergic polymorphic VT, and VF may represent discrete entities within a spectrum of drug-induced proarrhythmia. Although they are differentiated by the coupling interval and the duration of QT interval, they appear to share a common substrate: a set of disturbances of repolarization characterized by Triangulation, Reverse use dependency, electrical Instability of the action potential, and Dispersion (TRIaD). It is becoming increasingly evident that augmentation of TRIaD, rather than changes in the duration of QT interval, provides the proarrhythmic substrate. In contrast, when not associated with an increase of TRIaD, QT interval prolongation can be an antiarrhythmic property. Electrophysiologically, augmentation of TRIaD can be explained by inhibition of hERG (human ether-a-go-go related gene) channel. Because drug-induced disturbances in repolarization commonly result from inhibition of hERG channels or I(Kr), hERG blockade and the resulting prolongation of QT interval are important properties of a drug to be studied. However, these need only be a concern if associated with TRIaD. More significantly, TRIaD so often precedes prolongation of action potential duration or QT interval and ventricular tachyarrhythmias that it should be considered a marker of proarrhythmia until proven otherwise, even in the absence of QT interval prolongation. Detecting drug-induced augmentation of TRIaD may offer an additional, more sensitive, and accurate indicator of the broader proarrhythmic potential of a drug than may QT interval prolongation alone.

Pharmacogenetics in drug regulation: promise, potential and pitfalls

Pharmacogenetic factors operate at pharmacokinetic as well as pharmacodynamic levels-the two components of the dose-response curve of a drug. Polymorphisms in drug metabolizing enzymes, transporters and/or pharmacological targets of drugs may profoundly influence the dose-response relationship between individuals. For some drugs, although retrospective data from case studies suggests that these polymorphisms are frequently associated with adverse drug reactions or failure of efficacy, the clinical utility of such data remains unproven. There is, therefore, an urgent need for prospective data to determine whether pre-treatment genotyping can improve therapy. Various regulatory guidelines already recommend exploration of the role of genetic factors when investigating a drug for its pharmacokinetics, pharmacodynamics, dose-response relationship and drug interaction potential. Arising from the global heterogeneity in the frequency of variant alleles, regulatory guidelines also require the sponsors to provide additional information, usually pharmacogenetic bridging data, to determine whether data from one ethnic population can be extrapolated to another. At present, sponsors explore pharmacogenetic influences in early clinical pharmacokinetic studies but rarely do they carry the findings forward when designing dose-response studies or pivotal studies. When appropriate, regulatory authorities include genotype-specific recommendations in the prescribing information. Sometimes, this may include the need to adjust a dose in some genotypes under specific circumstances. Detailed references to pharmacogenetics in prescribing information and pharmacogenetically based prescribing in routine therapeutics will require robust prospective data from well-designed studies. With greater integration of pharmacogenetics in drug development, regulatory authorities expect to receive more detailed genetic data. This is likely to complicate the drug evaluation process as well as result in complex prescribing information. Genotype-specific dosing regimens will have to be more precise and marketing strategies more prudent. However, not all variations in drug responses are related to pharmacogenetic polymorphisms. Drug response can be modulated by a number of non-genetic factors, especially co-medications and presence of concurrent diseases. Inappropriate prescribing frequently compounds the complexity introduced by these two important non-genetic factors. Unless prescribers adhere to the prescribing information, much of the benefits of pharmacogenetics will be squandered. Discovering highly predictive genotype-phenotype associations during drug development and demonstrating their clinical validity and utility in well-designed prospective clinical trials will no doubt better define the role of pharmacogenetics in future clinical practice. In the meantime, prescribing should comply with the information provided while pharmacogenetic research is deservedly supported by all concerned but without unrealistic expectations.

Priorities and standards in pharmacogenetic research

The current enthusiasm for pharmacogenetics draws much of its inspiration from the relatively few examples of polymorphisms that have marked and seemingly clinically relevant effects on drug response. In this regard, pharmacogenetic research has paralleled the study of human disease, which has enjoyed success in identifying mutations underlying mendelian conditions. Progress in deciphering the genetics of complex diseases, involving the interaction of multiple genes with each other and with the environment has been considerably less successful. In most instances, drug responses will probably also prove to be complex, influenced by both the environment and multiple genetic factors. For pharmacogenetics to deliver on its potential, this complexity will need to be recognized and accommodated, both in basic research and in clinical application of pharmacogenetics. As the attention of researchers begins to shift toward more systematic pharmacogenetic investigations, we suggest some priorities and standards for pharmacogenetic research.

Cytochrome P450 gene-based drug prescribing and factors impacting translation into routine clinical practice

Pharmacogenetics represents a rapidly advancing, competitive field of investigation. Due to the potential for clinically recognizable interactions between a set of old polymorphic genes and a relatively new environmental insult (drugs), many human geneticists believe that variability in the drug-metabolizing enzyme systems will soon translate into clinical practice across entire populations. Despite this, the field has not yet received widespread clinical acceptance. This article will review the common cytochrome P450 gene polymorphisms and discuss the factors that may facilitate (or attenuate) their translation into clinical practice.

Pharmacogenomics and the drug discovery pipeline: when should it be implemented?

One of the key factors in developing improved medicines lies in understanding the molecular basis of the complex diseases we treat. Investigation of genetic associations with disease utilizing advances in linkage disequilibrium-based whole genome association strategies will provide novel targets for therapy and define relevant pathways contributing to disease pathogenesis. Genetic studies in conjunction with gene expression, proteomic, and metabonomic analyses provide a powerful tool to identify molecular subtypes of disease. Using these molecular data, pharmacogenomics has the potential to impact on the drug discovery and development process at many stages of the pipeline, contributing to both target identification and increased confidence in the therapeutic rationale. This is exemplified by the identified association of 5-lipoxygenase-activating protein (ALOX5AP/FLAP) with increased risk of myocardial infarction, and of the chemokine receptor 5 (CCR5) with HIV infection and therapy. Pharmacogenomics has already been used in oncology to demonstrate that molecular data facilitates assessment of disease heterogeneity, and thus identification of molecular markers of response to drugs such as imatinib mesylate (Gleevec) and trastuzumab (Herceptin). Knowledge of genetic variation in a target allows early assessment of the clinical significance of polymorphism through the appropriate design of preclinical studies and use of relevant animal models. A focussed pharmacogenomic strategy at the preclinical phase of drug development will produce data to inform the pharmacogenomic plan for exploratory and full development of compounds. Opportunities post-approval show the value of large well-characterized data sets for a systematic assessment of the contribution of genetic determinants to adverse drug reactions and efficacy. The availability of genomic samples in large phase IV trials also provides a valuable resource for further understanding the molecular basis of disease heterogeneity, providing data that feeds back into the drug discovery process in target identification and validation for the next generation of improved medicines.

Non-cardiac QTc-prolonging drugs and the risk of sudden cardiac death

AIMS

To assess the association between the use of non-cardiac QTc-prolonging drugs and the risk of sudden cardiac death.

METHODS AND RESULTS

A population-based case-control study was performed in the Integrated Primary Care Information (IPCI) project, a longitudinal observational database with complete medical records from more than 500,000 persons. All deaths between 1 January 1995 and 1 September 2003 were reviewed. Sudden cardiac death was classified based on the time between onset of cardiovascular symptoms and death. For each case, up to 10 random controls were matched for age, gender, date of sudden death, and general practice. The exposure of interest was the use of non-cardiac QTc-prolonging drugs. Exposure at the index date was categorized into three mutually exclusive groups of current use, past use, and non-use. The study population comprised 775 cases of sudden cardiac death and 6297 matched controls. Current use of any non-cardiac QTc-prolonging drug was associated with a significantly increased risk of sudden cardiac death (adjusted OR: 2.7; 95% CI: 1.6-4.7). The risk of death was highest in women and in recent starters.

CONCLUSION

The use of non-cardiac QTc-prolonging drugs in a general population is associated with an increased risk of sudden cardiac death.

Mechanistic basis of adverse drugreactions: the perils of inappropriate dose schedules

Adverse drug reactions (ADRs) have long been recognised as a significant cause of morbidity and mortality. They account for a substantial number of clinical consultations, hospital admissions and extended duration of in-patient stay as well as mortality. By far the most common ADRs are the concentration-dependent pharmacological reactions, the majority of which ought to be preventable. As a result of high concentrations of the parent drug and/or its metabolite(s), there is an augmentation of primary pharmacological activity and/or appearance of new and undesirable secondary pharmacological activity. Typically, these high concentrations result from administration of high doses in an attempt to maximise efficacy and/or modulation of the pharmacokinetics of a drug by either genetic or non-genetic factors. High plasma concentrations of parent drug may result from inherited impairment or drug-induced inhibition of its pharmacokinetic disposition. Conversely, inherited overcapacity or drug-induced induction of the metabolism of a drug may result in low concentrations of parent drug and frequently, rapid accumulation of its metabolites. Environmental, dietary and phytochemical factors may also influence the activity of drug metabolising enzymes. As with inherited polymorphisms of acetylation and cytochrome P450-based drug metabolising enzymes, polymorphisms of other conjugation reactions, such as glucuronidation, increasingly appear to be associated with drug toxicity. Diseases of organs involved in elimination of a drug also alter its pharmacokinetics, plasma concentration and, therefore, the profile of its concentration-dependent ADRs. Inherited mutations, concurrently administered drugs or presence of certain diseases may also alter the sensitivity of some pharmacological targets, accounting for a substantial number of ADRs and interactions. When there is enhanced pharmacodynamic sensitivity, plasma drug concentrations that are apparently within the normal 'non-toxic' range give rise to ADRs. Recent advances have also provided important insights into the wider scope of drug-drug interactions. Interactions that occur at P-glycoproteins, drug transporters and efflux pumps, at various transmembrane interfaces such as the gastrointestinal wall, renal tubules, hepatobiliary border and blood-brain barrier, are beginning to explain many non-metabolic interactions. These alter the systemic exposure to drugs and have so far, begun to explain unexpected neurotoxicity and hepatotoxicity. The function of these transporters is also genetically modulated. These advances, together with continued increased awareness and education of prescribers and pharmacists, offer great opportunities for substantially minimising concentration-related ADRs.

Short-term repeatability of electrocardiographic spatial T-wave axis and QT interval

Although ventricular repolarization abnormalities reflect arrhythmic susceptibility, few reliable tools exist to identify their presence. We investigated the repeatability of the spatial T-wave axis and QT interval from standard 12-lead electrocardiograms in 63 asymptomatic volunteers. Certified technicians used a standardized protocol to digitally record 2 electrocardiograms per participant at each of 2 visits separated by 1 to 2 weeks. Absolute paired differences within and between visits were 0.19 degrees and 0.90 degrees for the T-wave axis and 1.08 and 1.55 milliseconds for the QT interval, respectively. The intraclass correlation coefficients for the T-wave axis and QT interval were 0.87 and 0.86, respectively. The impact of repeated measurements on the precision of the QT-interval measurements was evaluated for a hypothetical clinical trial aimed at detecting a drug-induced QT prolongation. We conclude that the spatial T-wave axis is as repeatable a measure of ventricular repolarization as the QT interval.

Magnetocardiography for pharmacology safety studies requiring high patient throughput and reliability

Recent guideline drafts of the International Conference on Harmonisation of Technical Requirements for Registration of Pharmaceuticals for Human Use (ICH) underline the necessity to test nonantiarrhythmic drugs for their potential to prolong the QT or the corrected QT (QTc) interval. The implementation of these guidelines requires a large amount of ECG measurements on animals and humans in preclinical and clinical phases of the drug development process. We propose the use of magnetocardiography (MCG) as a complementary method with particular advantages in high-throughput studies, where signal quality and reliability are key factors. Our proposal is based on a review of recent MCG studies investigating the repolarization phase and results of methodological work assessing QT interval parameters from the MCG. The applicability of MCG for pre-clinical in-vivo studies is demonstrated by the ease of measurement in unrestrained non-anesthetized rabbits, guinea pigs, and hamsters..

Anti-HERG activity and the risk of drug-induced arrhythmias and sudden death

AIMS

Drug-induced QTc-prolongation, resulting from inhibition of HERG potassium channels may lead to serious ventricular arrhythmias and sudden death. We studied the quantitative anti-HERG activity of pro-arrhythmic drugs as a risk factor for this outcome in day-to-day practice.

METHODS AND RESULTS

All 284,426 case reports of suspected adverse drug reactions of drugs with known anti-HERG activity received by the International Drug Monitoring Program of the World Health Organization (WHO-UMC) up to the first quarter of 2003, were used to calculate reporting odds ratios (RORs). Cases were defined as reports of cardiac arrest, sudden death, torsade de pointes, ventricular fibrillation, and ventricular tachycardia (n = 5591), and compared with non-cases regarding the anti-HERG activity, defined as the effective therapeutic plasma concentration (ETCPunbound) divided by the HERG IC50 value, of suspected drugs. We identified a significant association of 1.93 (95% CI: 1.89-1.98) between the anti-HERG activity of drugs, measured as log10 (ETCPunbound/IC50), and reporting of serious ventricular arrhythmias and sudden death to the WHO-UMC database.

CONCLUSION

Anti-HERG activity is associated with the risk of reports of serious ventricular arrhythmias and sudden death in the WHO-UMC database. These findings are in support of the value of pre-clinical HERG testing to predict pro-arrhythmic effects of medicines.

Preclinical strategies to assess QT liability and torsadogenic potential of new drugs: The role of experimental models

The recognition of QT prolongation and torsade de pointes (TdP) in humans has resulted in the re-labeling of some drugs and the removal of others from the market in the past decade. Recent regulatory guidelines have recommended a battery of preclinical tests to assess a new drug for the QT liability in humans. The assessment includes the effect of a drug on: 1) the ionic current in stable cell lines expressing hERG channel; 2) action potential duration (APD) measured in isolated ventricular tissues; 3) the QTc interval and TdP in animals in vivo; and 4) APD, the QT interval, transmural dispersion of repolarization (TDR) and TdP potential in the isolated arterially-perfused ventricular wedge preparation. Because a noncardiac drug with an incidence of TdP even less than 0.1% can be potentially removed from the market, the experimental models used for preclinical testing have to be high sensitive and specific to the signals related to TdP. Among available experimental models, the rabbit left ventricle wedge preparation exhibits a high sensitivity and a high specificity in the identification of compounds positive and negative for QT prolongation and TdP. This is attributed to the fact that the preparation demonstrates strong signals related to QT prolongation in response to even a weaker QT prolonging agent. Signals specifically pertinent to the development of TdP, ie, early afterdepolarization (EAD) and an increase in TDR can be detected as well. The preclinical data obtained from the wedge preparation correlate well with clinical outcomes.

A mechanistic approach to assess the proarrhythmic risk of QT-prolonging drugs in preclinical pharmacologic studies

Drugs with diverse structures and from several therapeutic classes are reported to increase the risk that a patient will experience ventricular tachyarrhythmias (e.g., torsades de pointes [TdP]) during drug therapy. This review discusses the use of preclinical assays to assess the risk that a QT-prolonging drug will cause TdP. The mechanisms underlying the development of TdP and the factors that increase the risk of TdP are described and applied to the design of preclinical experimental models for detection of proarrhythmic drug actions. Recommended assays, conditions, and preparations for preclinical assessment of the drug-induced risk to TdP are given. No single preparation can simulate all conditions that cause TdP in patients. However, the assays described herein are capable of detecting the proarrhythmic effects of currently used drugs, even when these effects are reported to be extremely rare in clinical practice.