Drugs effects on ventricular repolarization: A critical evaluation of the strengths and weaknesses of current methodologies and regulatory practices

Idiopathic short-coupled ventricular tachyarrhythmias: Systematic review and validation of electrocardiographic indices

INTRODUCTION

Idiopathic short-coupled ventricular tachyarrhythmias make up a considerable proportion of ventricular tachyarrhythmias in structurally normal hearts and are the cause of 5-10% of unexpected sudden cardiac deaths. There is disparity in the literature regarding their description and a lack of formal diagnostic criteria to define them.

OBJECTIVE

To validate ECG indices for the diagnosis of these ventricular tachyarrythmias and to subsequently unify their differing descriptions in the literature under a new terminology: Idiopathic Short-Coupled Ventricular Tachyarrhythmias.

METHODS

We conducted a systematic review of all published studies describing short-coupled torsades de pointes, idiopathic ventricular fibrillation and polymorphic ventricular tachycardia. Published tracings were analysed using a standard set of criteria to define the different ECG intervals. Previously proposed diagnostic indices were validated using a control group of previously published long-coupled torsades de pointes cases.

RESULTS

Validation of the ECG indices revealed that a coupling interval < 400 ms was the most reliable measurement (sensitivity 100%, specificity 97%), followed by a coupling interval/QT < 1 (sensitivity 96%, specificity 100%).

CONCLUSION

Idiopathic short-coupled ventricular tachyarrhythmias encompass all previous descriptions of this tachyarrhythmia including idiopathic ventricular fibrillation, short-coupled torsades de pointes, Purkinje-related torsades de pointes and idiopathic polymorphic ventricular tachycardia. This arrhythmia can be diagnosed by newly proposed criteria with high sensitivity and specificity.

Sensitivity and Specificity of the In Vitro Guinea Pig Papillary Muscle Action Potential Duration for the Assessment of Drug-Induced Torsades De Pointes Liability in Humans

The ICH S7B document, which provides guidance for the preclinical cardiovascular evaluation of pharmaceutical new chemical entities (NCE), is essentially focused on drug-induced QT lengthening, a biomarker for the proarrhythmic adverse drug reaction, torsades de pointes (TdP). In 2005, this guidance recommended the IKr assay and the in vivo QT telemetry study as mandatory assays for detecting potential torsades de pointes liability and relegated the cardiac action potential (AP) assay as a follow-up study. The IKr assay has become a mandatory screening tool in the early development and safety assessment process. Using only the IKr assay as a go/no go decision arbiter is regrettable since, due to the low specificity of the model (positives that are false for proarrhythmia liability, e.g. verapamil), promising, safe NCEs may be inadvertently discarded. Inclusion of additional medium throughput assays should be performed early to confirm or balance the putatively unfavourable IKr result with positive discovery model output (Pugsley et al., J Pharmacol Toxicol Methods 60:24-27, 2009). In the present chapter, the predictive value of in vitro guinea pig papillary muscle action potential assay will be discussed in terms of sensitivity and specificity and compared to currently available preclinical models such as IKr/hERG assay, dog Purkinje fibre action potential and in vivo QT measurements in dog and cynomolgus monkey.

A Historical View and Vision into the Future of the Field of Safety Pharmacology

Professor Gerhard Zbinden recognized in the 1970s that the standards of the day for testing new candidate drugs in preclinical toxicity studies failed to identify acute pharmacodynamic adverse events that had the potential to harm participants in clinical trials. From his vision emerged the field of safety pharmacology, formally defined in the International Conference on Harmonization (ICH) S7A guidelines as "those studies that investigate the potential undesirable pharmacodynamic effects of a substance on physiological functions in relation to exposure in the therapeutic range and above." Initially, evaluations of small-molecule pharmacodynamic safety utilized efficacy models and were an ancillary responsibility of discovery scientists. However, over time, the relationship of these studies to overall safety was reflected by the regulatory agencies who, in directing the practice of safety pharmacology through guidance documents, prompted transition of responsibility to drug safety departments (e.g., toxicology). Events that have further shaped the field over the past 15 years include the ICH S7B guidance, evolution of molecular technologies leading to identification of new therapeutic targets with uncertain toxicities, introduction of data collection using more sophisticated and refined technologies, and utilization of transgenic animal models probing critical scientific questions regarding novel targets of toxicity. The collapse of the worldwide economy in the latter half of the first decade of the twenty-first century, continuing high rates of compound attrition during clinical development and post-approval and sharply increasing costs of drug development have led to significant strategy changes, contraction of the size of pharmaceutical organizations, and refocusing of therapeutic areas of investigation. With these changes has come movement away from dedicated internal safety pharmacology capability to utilization of capabilities within external contract research organizations. This movement has created the opportunity for the safety pharmacology discipline to come "full circle" and return to the drug discovery arena (target identification through clinical candidate selection) to contribute to the mitigation of the high rate of candidate drug failure through better compound selection decision making. Finally, the changing focus of science and losses in didactic training of scientists in whole animal physiology and pharmacology have revealed a serious gap in the future availability of qualified individuals to apply the principles of safety pharmacology in support of drug discovery and development. This is a significant deficiency that at present is only partially met with academic and professional society programs advancing a minimal level of training. In summary, with the exception that the future availability of suitably trained scientists is a critical need for the field that remains to be effectively addressed, the prospects for the future of safety pharmacology are hopeful and promising, and challenging for those individuals who want to assume this responsibility. What began in the early part of the new millennium as a relatively simple model of testing to assure the safety of Phase I clinical subjects and patients from acute deleterious effects on life-supporting organ systems has grown with experience and time to a science that mobilizes the principles of cellular and molecular biology and attempts to predict acute adverse events and those associated with long-term treatment. These challenges call for scientists with a broad range of in-depth scientific knowledge and an ability to adapt to a dynamic and forever changing industry. Identifying individuals who will serve today and training those who will serve in the future will fall to all of us who are committed to this important field of science.

Electrocardiographic safety of cangrelor, a new intravenous antiplatelet agent: a randomized, double-blind, placebo- and moxifloxacin-controlled thorough QT study

Cangrelor is an intravenous P2Y12 inhibitor under investigation as an antiplatelet drug in the setting of acute coronary syndromes. To determine the electrophysiologic safety of parenteral cangrelor, cardiac repolarization effects were measured in 67 healthy volunteers (aged 18-45 years) in a randomized crossover design, including 4 treatment sequences of therapeutic cangrelor, supratherapeutic cangrelor, placebo, and moxifloxacin (positive control). Triplicate electrocardiogram measurements and pharmacokinetic samples were collected at baseline and 9 time points postdose on day 1. For both cangrelor and moxifloxacin, time-matched, placebo-adjusted change in QT from baseline was evaluated using an individual (QTcI) heart rate correction. After cangrelor dosing, change in QTcI was <5 ms at all times points and all corresponding upper 2-sided 90% confidence intervals (CIs) were <10 ms. Although moxifloxacin failed to show a lower CI >5 ms, expected time trends and lower CI >4.0 ms demonstrate assay sensitivity. QTcI was not affected by plasma concentrations of cangrelor metabolites, and cangrelor had no other adverse effects on electrocardiographic parameters. Clinically, cangrelor exposure was well tolerated. Thus, this thorough QT study demonstrated that therapeutic and supratherapeutic cangrelor doses do not adversely affect cardiac repolarization in normal volunteers (clinicaltrials.gov; identifier NCT00699504).

Direct effects of arsenic trioxide on action potentials in isolated cardiac tissues: importance of the choice of species, type of cardiac tissue and perfusion time

INTRODUCTION

The aim of the present study was to evaluate direct/acute effects of arsenic trioxide on action potentials (APs) in isolated cardiac tissues, and to investigate if the choice of species and tissue and the duration of the perfusion play a role in arsenic-induced acute/direct prolongation of AP/QT.

METHODS AND RESULTS

Direct electrophysiological effects of arsenic trioxide were measured in cardiac tissues isolated from four different species using micro-electrode recording. Arsenic (after 30 to 95 min perfusion at 10 μM) significantly prolonged APD(90), increased triangulation of the AP and elicited early afterdepolarizations (EADs) only in isolated guinea-pig and dog Purkinje fibers but not in rabbit and porcine (minipig) Purkinje fibers. Arsenic induced a prolongation of the APD(90) and increases in triangulation and the occurrence of EADs was not observed in papillary muscles of guinea-pigs and rabbits. Arsenic at 4 increasing concentrations from 0.1 μM to 10 μM at the standard perfusion-time of 15 min per concentration, and after a continuous 90-min perfusion at 1 μM and 1 Hz did not induce these direct effects on APD(90), triangulation and EADs in isolated guinea-pig Purkinje fibers, but it at 1 µM elicited EADs in 2 out of 7 preparations after 90 min at 0.2 Hz.

DISCUSSION

The present study demonstrates that the choice of species and cardiac tissue as well as perfusion-time play important roles in arsenic-induced direct/acute effects on APD(90) and induction of EADs in vitro.

Regulatory acceptability of the minipig in the development of pharmaceuticals, chemicals and other products

As part of the RETHINK European FP6 Project an overview of the acceptability and usefulness of minipigs has been carried out in the regulatory arenas of human and veterinary pharmaceuticals, food additives, cosmetics, biocides and agrochemicals, chemicals and medical devices. The safety of new pharmaceuticals for human use should be tested in non-rodents, but the regulatory world is not too prescriptive regarding the choice of species. The choice is most often dogs through long tradition. When dogs are not appropriate, in many cases non-human primates are chosen as an alternative. From information in the public domain as well as literature from the EMA and FDA, it is clear that minipigs have already been identified as suitable to take the role of non-rodent species in toxicity testing of pharmaceutical products. In the field of foodstuffs, the pig is used more extensively because of the apparent similarity in the omnivorous food pattern and digestive tract between humans and pigs. The extensive use of pigs in this field provides historical data. In the field of medical devices the ISO Guidelines indicate that the pig is regarded as a suitable animal model because of its haematological and cardiovascular similarities to man. The pig is also mentioned as suitable for testing local effects after implantation. Political and societal support for using nonhuman primates is decreasing, and it is an appropriate time to consider the role of the minipig. We have reviewed the costs of testing in minipigs, and these are not significantly higher than the costs for a study in dogs. Economical reasons should therefore not be used to argue against the use of minipigs instead of dogs or monkeys. For most purposes, minipigs may be considered an acceptable choice as non-rodent species, provided adequate justification for this choice is made.

hERG-related drug toxicity and models for predicting hERG liability and QT prolongation

BACKGROUND

hERG K(+) channels have been recognized as a primary antitarget in safety pharmacology. Their blockade, caused by several drugs with different therapeutic indications, may lead to QT prolongation and, eventually, to potentially fatal arrhythmia, namely torsade de pointes. Therefore, a number of preclinical models have been developed to predict hERG liability early in the drug development process.

OBJECTIVE

The aim of this review is to outline the present state of the art on drug-induced hERG blockade, providing insights on the predictive value of in vitro and in silico models for hERG liability.

METHODS

On the basis of latest reports, high-throughput preclinical models have been discussed outlining advantages and limitations.

CONCLUSION

Although no single model has an absolute value, an integrated risk assessment is recommended to predict the pro-arrhythmic risk of a given drug. This prediction requires expertise from different areas and should encompass emerging issues such as interference with hERG trafficking and QT shortening.

Absence of effect of telbivudine on cardiac repolarization: results of a thorough QT/QTc study in healthy participants

The effect of telbivudine on cardiac repolarization was evaluated in healthy participants at clinical and supratherapeutic doses. Sixty-two participants were enrolled, stratified by sex, and randomized according to a crossover design among 4 treatment sequences: placebo, a single moxifloxacin 400-mg dose as positive calibrator, and telbivudine 600 and 1800 mg/d for 7 days. Intensive time-matched electrocardiogram measurements and pharmacokinetic sampling were performed at baseline and on day 7 over 24 hours. For telbivudine and moxifloxacin, time-matched, placebo-adjusted change from baseline in QT was calculated and corrected using Fridericia's formula (QTcF). While moxifloxacin produced the expected significant changes in QTcF, none of the changes in QTcF for either doses of telbivudine exceeded 5 ms, and none of the associated upper 1-sided 95% confidence intervals (CI) exceeded the limit of 10 ms. There was no increase in QTcF with increasing plasma telbivudine. The supratherapeutic dose of telbivudine was well tolerated with a safety profile similar to the clinical dose despite a 3-fold increase in plasma exposure. This study therefore met the criteria for a negative thorough QT study. Telbivudine had no adverse effect on cardiac repolarization in healthy participants, even at supratherapeutic exposure, suggesting a broad safety margin.

Zebrafish: an emerging technology for in vivo pharmacological assessment to identify potential safety liabilities in early drug discovery

The zebrafish is a well-established model organism used in developmental biology. In the last decade, this technology has been extended to the generation of high-value knowledge on safety risks of novel drugs. Indeed, the larval zebrafish appear to combine advantages of whole organism phenotypic assays and those (rapid production of results with minimal resource engagement) of in vitro high-throughput screening techniques. Thus, if appropriately evaluated, it can offer undeniable advantages in drug discovery for identification of target and off-target effects. Here, we review some applications of zebrafish to identify potential safety liabilities, particularly before lead/candidate selection. For instance, zebrafish cardiovascular system can be used to reveal decreases in heart rate and atrial-ventricular dissociation, which may signal human ether-a-go-go-related gene (hERG) channel blockade. Another main area of interest is the CNS, where zebrafish behavioural assays have been and are further being developed into screening platforms for assessment of locomotor activity, convulsant and proconvulsant liability, cognitive impairment, drug dependence potential and impaired visual and auditory functions. Zebrafish also offer interesting possibilities for evaluating effects on bone density and gastrointestinal function. Furthermore, available knowledge of the renal system in larval zebrafish can allow identification of potential safety issues of drug candidates on this often neglected area in early development platforms. Although additional validation is certainly needed, the zebrafish is emerging as a versatile in vivo animal model to identify off-target effects that need investigation and further clarification early in the drug discovery process to reduce the current, high degree of attrition in development.

Predicting QT prolongation in humans during early drug development using hERG inhibition and an anaesthetized guinea-pig model

Background and purpose:

Drug-induced prolongation of the QT interval can lead to torsade de pointes, a life-threatening ventricular arrhythmia. Finding appropriate assays from among the plethora of options available to predict reliably this serious adverse effect in humans remains a challenging issue for the discovery and development of drugs. The purpose of the present study was to develop and verify a reliable and relatively simple approach for assessing, during preclinical development, the propensity of drugs to prolong the QT interval in humans.

Experimental approach:

Sixteen marketed drugs from various pharmacological classes with a known incidence—or lack thereof—of QT prolongation in humans were examined in hERG (human ether a-go-go-related gene) patch-clamp assay and an anaesthetized guinea-pig assay for QT prolongation using specific protocols. Drug concentrations in perfusates from hERG assays and plasma samples from guinea-pigs were determined using liquid chromatography-mass spectrometry.

Key results:

Various pharmacological agents that inhibit hERG currents prolong the QT interval in anaesthetized guinea-pigs in a manner similar to that seen in humans and at comparable drug exposures. Several compounds not associated with QT prolongation in humans failed to prolong the QT interval in this model.

Conclusions and implications:

Analysis of hERG inhibitory potency in conjunction with drug exposures and QT interval measurements in anaesthetized guinea-pigs can reliably predict, during preclinical drug development, the risk of human QT prolongation. A strategy is proposed for mitigating the risk of QT prolongation of new chemical entities during early lead optimization.

The hERG potassium channel and hERG screening for drug-induced torsades de pointes

Drug-induced torsades de pointes (TdP) arrhythmia is a major safety concern in the process of drug design and development. The incidence of TdP tends to be low, so early pre-clinical screens rely on surrogate markers of TdP to highlight potential problems with new drugs. hERG (human ether-à-go-go-related gene, alternative nomenclature KCNH2) is responsible for channels mediating the 'rapid' delayed rectifier K+ current (IKr) which plays an important role in ventricular repolarization. Pharmacological inhibition of native IKr and of recombinant hERG channels is a shared feature of diverse drugs associated with TdP. In vitro hERG assays therefore form a key element of an integrated assessment of TdP liability, with patch-clamp electrophysiology offering a 'gold standard'. However, whilst clearly necessary, hERG assays cannot be assumed automatically to provide sufficient information, when considered in isolation, to differentiate 'safe' from 'dangerous' drugs. Other relevant factors include therapeutic plasma concentration, drug metabolism and active metabolites, severity of target condition and drug effects on other cardiac ion channels that may mitigate or exacerbate effects of hERG blockade. Increased understanding of the nature of drug-hERG channel interactions may ultimately help eliminate potential hERG blockade early in the design and development process. Currently, for promising drug candidates integration of data from hERG assays with information from other pre-clinical safety screens remains essential.

Zebrafish based assays for the assessment of cardiac, visual and gut function--potential safety screens for early drug discovery

INTRODUCTION

Safety pharmacology is integral to the non-clinical safety assessment of new chemical entities prior to first administration to humans. The zebrafish is a well established model organism that has been shown to be relevant to the study of human diseases. The potential role of zebrafish in safety pharmacology was evaluated using reference compounds in three models assessing cardiac, visual and intestinal function.

METHODS

Compound toxicity was first established in zebrafish to determine the non toxic concentration of a blinded set of 16 compounds. In the cardiac assay, zebrafish larvae at 3 days post fertilisation (d.p.f.) were exposed to compounds for 3 h before measurement of the atrial and ventricular rates. To investigate visual function, the optomotor response was assessed in 8 d.p.f. larvae following a 5 day compound exposure. In the intestinal function assay, the number of gut contractions was measured in 7 d.p.f. larvae after a 1 h compound exposure. Finally, compound uptake was determined for 9 of the 16 compounds to measure the concentration of compound absorbed by the zebrafish larvae.

RESULTS

Seven compounds out of nine produced an expected effect that was statistically significant in the cardiac and visual functions assays. In the gut contraction assay, six out of ten compounds showed a statistically significant effect that was also the expected result whilst two displayed anticipated but non-significant effects. The compound uptake method was used to determine larval tissue concentrations and allowed the identification of false negatives when compound was poorly absorbed into the zebrafish.

DISCUSSION

Overall, results generated in three zebrafish larvae assays demonstrated a good correlation between the effects of compounds in zebrafish and the data available from other in vivo models or known clinical adverse effects. These results suggest that for the cardiac, intestinal and visual function, zebrafish assays have the potential to predict adverse drug effects and supports their possible role in early safety assessment of novel compounds.

High-throughput electrophysiology: an emerging paradigm for ion-channel screening and physiology

Ion channels represent highly attractive targets for drug discovery and are implicated in a diverse range of disorders, in particular in the central nervous and cardiovascular systems. Moreover, assessment of cardiac ion-channel activity of new chemical entities is now an integral component of drug discovery programmes to assess potential for cardiovascular side effects. Despite their attractiveness as drug discovery targets ion channels remain an under-exploited target class, which is in large part due to the labour-intensive and low-throughput nature of patch-clamp electrophysiology. This Review provides an update on the current state-of-the-art for the various automated electrophysiology platforms that are now available and critically evaluates their impact in terms of ion-channel screening, lead optimization and the assessment of cardiac ion-channel safety liability.

Assessment of QT liabilities in drug development

Since the publication, in 1997, of the CPMP (Committee for Proprietary Medicinal Products) Points to Consider document on "The assessment of potential for QT prolongation by non-cardiovascular medicinal products," both regulatory bodies and the pharmaceutical industry have paid increasing attention to the conduct of careful preclinical studies on the subject. Regulatory attention has focused on the drafting of Safety Pharmacology guidelines through the ICH (International Conference on Harmonization) process, which resulted in approval by the ICH and acceptance by the three main regions (USA, Europe, and Japan) of the ICH S7A guideline. The guideline does not deal only with cardiovascular studies and does not provide guidance on QT investigations. This part has been deferred to a second guideline (ICH S7B). Nevertheless, pharmaceutical companies have implemented screening strategies aimed at selecting compounds that do not present QT liabilities. These strategies can differ according to the pharmaceutical class, while experimental models differ according to the stage of development of the compound. Several in vitro models are employed in discovery (radioligand binding, high-throughput patch clamp, efflux, and fluorescence assays). These models, coupled with in silico methods, allow companies to screen a high number of compounds. Other in vitro models, applied later in the R&D process (action potential duration, APD, in Purkinje fibers or papillary muscle and the isolated heart) are useful in better describing the activity of compounds on cardiac ion channels. The most robust and accepted in vivo test is represented by telemetry studies in conscious non-rodents.

Age-dependent effects on cisapride-induced QTc prolongation in the isolated guinea pig heart

INTRODUCTION

The isolated guinea pig heart preparation has been suggested as a suitable small animal model for investigating potential for QTc prolongation. The purpose of this study was to investigate the effect of age on electrophysiological parameters measured in the isolated guinea pig heart preparation. In addition, the effect of a compound known to prolong the QT interval (cisapride) was investigated in both young and adult guinea pigs.

METHODS

Male guinea pigs were divided into 2 groups (n=6). One group of guinea pigs was between 3 and 4 weeks old (young) and the other group was between 16 and 17 weeks old (adult). Concentrations (0, 1, 5, and 50 ng/mL; 2, 11, and 110 nM) of cisapride were perfused for 15 min from low to high concentration. Measurements of PR, QRS, RR and QT intervals were typically made on 5 consecutive electrocardiogram complexes during the last minute of each concentration. The QT interval was corrected for changes in heart rate using the cube root formula of Fridericia (QTcF).

RESULTS

Adult guinea pigs had significantly longer RR and QTcF intervals when compared to young animals. Cisapride prolonged QTcF in both young and adult animals at the same concentrations (5 ng/mL and 50 ng/mL). The maximal change in QTcF at 50 ng/mL was similar in young (44+/-3 ms) and adult animals (40+/-1 ms).

DISCUSSION

In summary, the present study demonstrated that there was an increase in the RR and QTcF intervals with age in isolated guinea pig hearts. However, this age difference does not appear to impact the sensitivity of the assay to drug-induced QTcF prolongation.

Predictive value of in vitro safety studies

The predictive value of in vitro safety studies is discussed for three important areas of pharmaceutical safety evaluations. In genetic toxicology, currently assays are sensitive for the prediction of cancer, but their overall predictive value is strongly diminished because of their low specificity. In the area of safety pharmacology blockage of hERG channel in vitro has recently been introduced to predict cardiac repolarization delay (QT interval prolongation) in patients. There is a plethora of in vitro methods to predict and characterize liver toxicity. However, little data is available that demonstrate a reliable prediction for hepatotoxicity in vivo over a wide range of chemical structures. In all three areas, further improvements are needed. 'Omics' technologies and new cell lines derived from stem cells are expected to strongly contribute to establish new and more predictive in vitro assays.

In vivo recording of adult zebrafish electrocardiogram and assessment of drug-induced QT prolongation

In the last decade the zebrafish has become a major model organism for the study of development and organogenesis. To maximize the experimental utility of this organism, it will be important to establish methods for adult phenotyping. We previously proposed that the embryonic zebrafish may be useful in high-throughput screening for drug-induced cardiotoxicity. We now describe a method for the reproducible recording of the adult zebrafish ECG and illustrate its application in the investigation of QT-prolonging drugs. Zebrafish ECGs were obtained by inserting two needle electrodes through the ventral epidermis. Fish were perfused orally, and motion artifacts were eliminated with a paralytic dose of μ-conotoxin GIIIB. Test compounds were delivered via the perfusion system. Without a means of hydration and oxygenation, the fish succumb rapidly. The use of a perfusion system allowed stable recording for >6 h. Baseline conduction intervals were as follows: PR, 66 ms (SD 14); QRS, 34 ms (SD 11); QT, 242 ms (SD 54); and R-R, 398 ms (SD 77). The known QT-prolonging agents astemizole, haloperidol, pimozide, and terfenadine caused corrected QT increases of 18% (SD 9), 16% (SD 11), 17% (SD 9), and 11% (SD 6), respectively. The control drugs clonidine, penicillin and propranolol did not prolong the corrected QT interval. In conclusion, perfusion and muscular paralysis allows stable, low-noise recording of zebrafish ECGs. Agents known to cause QT prolongation in humans caused QT prolongation in fish in each case. The development of rigorous tools for the phenotyping of adult zebrafish will complement the high-throughput assays currently under development for embryonic and larval fish.

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.

Actions of flecainide on susceptibility to phase-2 ventricular arrhythmias during infarct evolution in rat isolated perfused hearts

The mechanism of flecainide-induced unexpected death remains uncertain. Phase-2 ventricular arrhythmias occur during infarct evolution. We examined whether flecainide (0.74 and 1.48 microM, representing the peak unbound plasma and total blood concentrations, respectively, at 'therapeutic' dosage) has proarrhythmic activity on phase-2 arrhythmia susceptibility during infarct evolution. To achieve this, we used the Langendorff-perfused rat heart preparation (n=8 per group) in which baseline phase-2 arrhythmia susceptibility is low. Left main coronary occlusion evoked phase-1 (acute ischaemia-induced) ventricular arrhythmias including fibrillation (VF) in all hearts. By 90 min, hearts were relatively arrhythmia-free. Randomized and blinded switch of perfusion to flecainide at 90 min caused no increase over baseline in the incidence of VF, tachycardia (VT) or premature beats (VPB) during the following 150 min of ischaemia, or during reperfusion (begun 240 min after the onset of ischaemia). In separate hearts, catecholamines (313 nM norepinephrine and 75 nM epinephrine) were co-perfused with flecainide from 90 min of ischaemia. Catecholamine perfusion increased heart rate, coronary flow and QT interval, and shortened PR interval (all P<0.05), actions that were not altered by flecainide. Catecholamine perfusion caused a weak nonsignificant increase in phase-2 VPB, VT and VF incidence, but there was no proarrhythmic interaction with flecainide. In conclusion, the present findings suggest that the increased risk of death associated with clinical use of flecainide is not due to facilitation of phase-2 ventricular arrhythmias.

A novel approach to data processing of the QT interval response in the conscious telemetered beagle dog

INTRODUCTION

Drug-induced QT interval prolongation may lead to ventricular arrhythmias. The aim of the study was to optimize QT interval data processing to quantify drug-induced QT interval prolongation in the telemetry instrumented conscious dog model.

METHODS

The test substances cisapride, dofetilide, haloperidol, and terfenadine and corresponding vehicles were given to male and female beagle dogs during two consecutive 90-min intravenous infusions. Cardiovascular parameters were recorded for 24 h and exposure to the drugs was measured. The delayed response in the QT interval after an abrupt change in heart rate was investigated. Eight mathematical models to describe the QT interval-heart rate relationship were compared and different sets of covariates were used to quantify the drug-induced effect on the QT interval.

RESULTS

After an abrupt decrease in heart rate, a 75% adaptation of the QT interval was reached after 54+/-9 s. A linear model was preferred to correct the drug-induced effect on the QT interval for heart rate, vehicle effect, serial correlation, plasma concentration and time of day. All test substances significantly prolonged the QT interval.

DISCUSSION

To optimize the processing of QT interval data, the delay in QT interval response after an abrupt change in heart rate should be considered. The QT interval-heart rate relationship and vehicle response were individual-specific and corrections were therefore made individually. When estimating the drug-induced effect on the QT interval it is considered advantageous to use plasma concentration as a covariate, as well as adjusting for vehicle effect and serial correlation in measurements. The conscious dog model detected significant increases in the QT interval for all test substances investigated.

Non-clinical evaluation of ventricular repolarization (ICH S7B): results of an interim survey of international pharmaceutical companies

INTRODUCTION

The propensity of drugs to cause a potentially fatal arrhythmia, torsades des pointes (TdP), is a significant public health issue. The draft International Conference on Harmonization (ICH) S7B guidelines describe a battery of non-clinical studies to evaluate a drug's potential to prolong ventricular repolarization (VR); an accepted surrogate/risk factor for TdP. A worldwide survey of pharmaceutical industry practices, related to ICH S7B was conducted. The findings were presented at the 4th Annual Meeting of the Safety Pharmacology Society (SPS) meeting (Cincinnati, OH, Sept., 2004).

METHODS

The survey was distributed by the SPS to 119 pharmaceutical companies; 29 surveys were returned. Survey topics included: (a) General Strategy and Testing for Risk of QT prolongation, (b) Study Timing and Relationship to Development, and (c) Application of GLPs.

RESULTS

Respondents indicate that the basis for assessing prolongation of VR was to: remove compounds with an arrhythmic risk to humans (86%), to satisfy regulatory expectations (79%), or to avoid obstacles in the clinical development of a compound (86%). Development of a compound based on prolongation of VR was halted for 52% of respondents (compared to 45% in a 2003 survey). Models used to evaluate changes in VR included: human ether a go-go related gene (hERG) (93%), action potential duration (APD) (68%; compared to 80% in the 2003 survey), and in vivo QT (100%). The distribution of assays being requested by regulatory agencies includes the hERG (83%), APD (28%), and in vivo QT (79%). In spite of uncertainty regarding the final requirements of ICH S7B, organizations continue to implement (36%) and validate (60%) their electrophysiology laboratories.

SUMMARY

The survey results indicate that pharmaceutical companies are testing for VR prolongation of drug candidates and that institutions have established in-house or outsourced capabilities to evaluate this potential risk, even in the absence of final guidelines.

Methodology used in safety pharmacology: appraisal of the state-of-the-art, the regulatory issues and new directions

This is the second focused issue of the Journal of Pharmacological and Toxicological Methods that is devoted exclusively to methodology, opinions and regulatory issues related to Safety Pharmacology. This issue of the Journal highlights a record number of articles and abstracts from the 4th Annual Safety Pharmacology Society (SPS) meeting that was held in Covington, Kentucky September 27-29, 2004. The focus of this issue of the Journal is to present a description of currently used fundamental pharmacological methods as outlined in the regulatory guidance documents for pre-clinical safety testing of drugs (ICH S7A and S7B). This issue features articles that discuss methods related to in vivo and in vitro CNS, respiratory and cardiovascular core battery studies. Additionally, articles are included that discuss species differences and recording issues in EKG measurement; automated (HTS) electrophysiology methods for assessing drug-induced hERG blockade (i.e., PatchXpress 7000A and hERG-lite); methods that describe and validate the use of the guinea pig as a safety pharmacology screening model; methods used to predict QTc prolongation and mathematical models that describe beat-to-beat instability of QT duration; supplemental studies that assess and discuss emerging renal biomarkers and renal function and food effects on gastrointestinal transit time; pro-arrhythmia models, cardiac function assessment using 3-D echocardiography and a description of a novel open field motor activity system to assess the possible CNS effects of drugs. Thus, this issue of the Journal of Pharmacological and Toxicological Methods should be a primary resource aid to individuals in academia and industry that are interested in understanding Safety Pharmacology methods. It provides a comprehensive overview of current and advanced methods used in Safety Pharmacology studies today.

Drugs, QTc Interval Prolongation and Final ICH E14 Guideline

Regulatory concerns on the ability of an ever-increasing number of non-antiarrhythmic drugs to delay ventricular repolarisation, prolong the corrected QT (QTc) interval and induce potentially fatal ventricular tachyarrhythmias have culminated in the adoption of two, internationally harmonised, regulatory guidelines. On 12 May 2005, the International Conference on Harmonisation (ICH) reached an important milestone when it adopted the final texts for clinical (ICH topic E14) and non-clinical (ICH topic S7B) strategies by which drugs should be investigated for their potential to induce these effects during their development.ICH E14 provides recommendations to sponsors concerning the design, conduct, analysis and interpretation of clinical studies to assess the potential of a drug to delay cardiac repolarisation. Specifically, it calls for a clinical 'thorough QT/QTc study' (typically conducted in healthy volunteers), which is intended to determine whether a drug has a threshold pharmacological effect on cardiac repolarisation, as detected by QT/QTc interval prolongation. The E14 recommendations are generally applicable not only to new drugs that have systemic bioavailability but also to approved drugs when a new dose, route of administration or target population that may result in an increased risk is explored. The guideline provides for exceptions when this study may not be required.Recognising the fractious relationship between ICH E14 and ICH S7B, and the persistence of a number of issues that may require clarity and/or the emergence of other new scientific issues in the future, the ICH Steering Committee has formed an Implementation Working Group that is charged with providing clarity on aspects of the guideline that are ambiguous and responding to issues on which the sponsors are uncertain. This paper provides a commentary on some of the challenges that are likely to be faced by the sponsors of drugs during the next few years of application of these two guidelines. The adoption of these guidelines has left a number of questions unanswered and raised some new ones. When in doubt, the sponsor should seek formal regulatory clarity before making key decisions that may impact further development, assessment and approval of a new chemical entity. Although the goal of developing drugs with much lower torsadogenic potential and without inappropriate restriction in the use (or even rejection) of potentially beneficial drugs is within sight, it is questionable whether the risk of drug-induced pro-arrhythmia will be eliminated completely.