Dynamic Beat-to-Beat Modeling of the QT-RR Interval Relationship: Analysis of QT Prolongation during Alterations of Autonomic State versus Human Ether a-go-go-Related Gene Inhibition

A HESI consortium approach to assess the human predictive value of non-clinical repolarization assays

Drug-induced ventricular arrhythmia and Torsades de Pointes remain a serious public health issues in bringing safe new pharmaceuticals to the market place. Under the auspices of the International Life Science Institute (ILSI)-Health and Environmental Sciences Institute (HESI), a consortium involving representatives from pharmaceutical companies, regulatory agencies and opinion leaders from the scientific and medical research communities has been initiated. The objectives are (1) to assess the concordance between signals in non-clinical repolarization assays and clinical QT interval prolongation; (2) to investigate the mechanisms for any discrepancy identified between non-clinical and clinical results and to determine viable and successful alternative approaches to identify these compounds; and (3) to assess the proarrhythmic potential of such compounds. At present, the consortium is conducting a retrospective analysis of non-clinical and clinical data from both FDA and contributing companies' databases and supplementing with a literature review. The overall objectives of these initial efforts are to establish a quantitative integrated risk assessment for each compound; to define criteria for concordance and apply them to the database in order to identify non-concordant compounds.

Cardiac abnormalities in acute organophosphate poisoning

BACKGROUND

Potentially lethal cardiac complications can occur in patients with acute organophosphate poisoning (OPP) and may be overlooked.

PATIENTS AND METHODS

Thirty-six patients with acute OPP were studied. Clinical features and the nature of compound involved were recorded. The QT interval was plotted against heart rate to determine the risk for Torsades de Pointes using the Fossa nomogram. Echocardiography was undertaken in 29 patients. Twenty-four-hour Holter monitoring was performed on day 1 in five patients. Thirteen died. Necropsy was performed and hearts were studied both grossly and microscopically.

RESULTS

Gross examination of the heart in 13 cases revealed cardiac discoloration or blotchiness in 12, patchy pericarditis in six, auricular thrombus in six, right ventricular hypertrophy in four, and dilatation in three. On histopathology, all 13 cases had myocardial interstitial edema and vascular congestion, eight had patchy interstitial inflammation, two had patchy myocarditis, and six had a mural thrombus. Sinus tachycardia was the most common electrocardiographic abnormality. The others were corrected QT interval prolongation, ST-T changes, U waves, and ventricular premature contractions. Echocardiography in 29 patients showed minor abnormalities in 10. On Holter monitoring, episodic tachycardia and ST-T changes were observed in four, QT prolongation in three, and episodic bradycardia in two.

CONCLUSIONS

Patchy myocardial involvement as a result of direct cardiac toxicity could be one of the factors responsible for serious cardiac complications. As myocardial involvement is patchy, it may not be manifest clinically or on echocardiography. Continuous cardiac monitoring should be undertaken to detect dynamic cardiac changes.

Estimation of the upper limit of the reference value of the QT interval in rest electrocardiograms in healthy young Japanese men using the bootstrap method

BACKGROUND

A prolonged QT interval (QT) is associated with cardiac arrhythmia, and methods for identification of QT prolongation are required.

METHODS

The relationship between RR and QT was investigated in resting electrocardiograms of 1276 healthy young Japanese men using the bootstrap method.

RESULTS

The upper limit of QT (QT(upper limit)) was approximated well by the exponential equation: QT(upper limit) = 435 x RR(0.3409). We also defined an alternative upper limit of QTc(G upper limit) = 435 milliseconds, where QTc(G) was calculated by dividing QT by RR(0.3409). Thirty-two (2.51%) of the 1276 cases exceeded the criterion and were diagnosed as cases of QT prolongation.

CONCLUSION

Using this limit, we propose a criterion to discriminate cases with prolonged QT intervals. The accuracy of the estimation of the mean and the upper limit of the reference value of the QT was good within the range of the RR interval from 0.812 to 1.263 s (heart rates from 48 to 74 beats per minute). Our approach for estimation of the exponent of RR differs from the well-known exponential equations proposed by Fridericia, but the exponent of RR in our equation is very close to that of Fridericia.

Calculation of QT shift in non clinical safety pharmacology studies

INTRODUCTION

Drug-induced QT interval prolongation is a major concern in new drug candidate development. This study presents a method of assessment of drug-induced QT interval prolongation without need for QT correction in conscious Beagle dogs and Cynomolgus monkeys monitored by telemetry. Accuracy and reliability are analysed by comparison with a reference QT correction method (Holzgrefe) from experiments performed with reference substances terfenadine, thioridazine and sotalol.

METHODS

The QT shift method principle is assessment of any drug-induced QT interval shift directly from the individual QT/RR relationship. The individual QT/RR relationship is built from a treatment-free 24-hour recording period. QT and RR intervals are determined from a beat-to-beat analysis. A probabilistic method is used to define the individual QT/RR relationships. Checks were performed to compare results obtained with the QT shift method and the QT correction methods. The robustness of the QT shift method was tested under various conditions of drug-induced heart rate change (i.e. normal, bradycardia and tachycardia).

RESULTS

The extent of agreement with the used reference QT correction method, Holzgrefe formula, was excellent (3-4 ms) in both animal species under the various drug induced effects on heart rate. The statistical sensitivity threshold for detection of QT prolongation according to a standard safety pharmacology study design was 7-8 ms.

DISCUSSION

When combined with the probabilistic determination of individual QT/RR relationships, this simple method provides a direct assessment of a drug-induced effect on QT interval, without any curve fitting or application of correction formula. Despite noticeably different shapes in QT/RR relationships, the QT shift method is applicable to both Beagle dogs and Cynomolgus monkeys. It is likely that the QT shift method will be particularly helpful in problematic cases, enabling detection of drug-induced prolongation of less than 10 ms.

Electrophysiological changes of cardiac function during antidepressant treatment

Some antidepressant agents can cause electrophysiological changes of cardiac function leading to ventricular arrhythmias and sudden death. However, antidepressants have also protective effects on the heart through their capacity to modulate cardiac autonomic-mediated physiological responses. Heart rate variability and QTc length are two strictly linked parameters that allow us to appreciate the effects of different drugs on cardiac physiology. Heart rate variability reflects functioning of the autonomic nervous system and possibly also regulation by the limbic system. Autonomic regulation of cardiac activity influences also cardiac repolarization and QT length, both directly and via its effects on heart rate. In this review we present the methodologies adopted to study the effect of antidepressant drugs on QT length and heart rate variability and we summarize data on electrophysiological changes related to antidepressant treatment. Clinical implications for the choice of different antidepressants in different clinical populations are discussed.

International Life Sciences Institute (Health and Environmental Sciences Institute, HESI) initiative on moving towards better predictors of drug-induced torsades de pointes

Knowledge of the cardiac safety of emerging new drugs is an important aspect of assuring the expeditious advancement of the best candidates targeted at unmet medical needs while also assuring the safety of clinical trial subjects or patients. Present methodologies for assessing drug-induced torsades de pointes (TdP) are woefully inadequate in terms of their specificity to select pharmaceutical agents, which are human arrhythmia toxicants. Thus, the critical challenge in the pharmaceutical industry today is to identify experimental models, composite strategies, or biomarkers of cardiac risk that can distinguish a drug, which prolongs cardiac ventricular repolarization, but is not proarrhythmic, from one that prolongs the QT interval and leads to TdP. To that end, the HESI Proarrhythmia Models Project Committee recognized that there was little practical understanding of the relationship between drug effects on cardiac ventricular repolarization and the rare clinical event of TdP. It was on that basis that a workshop was convened in Virginia, USA at which four topics were introduced by invited subject matter experts in the following fields: Molecular and Cellular Biology Underlying TdP, Dynamics of Periodicity, Models of TdP Proarrhythmia, and Key Considerations for Demonstrating Utility of Pre-Clinical Models. Contained in this special issue of the British Journal of Pharmacology are reports from each of the presenters that set out the background and key areas of discussion in each of these topic areas. Based on this information, the scientific community is encouraged to consider the ideas advanced in this workshop and to contribute to these important areas of investigations over the next several years.

Cardioprotective effect of histamine H3-receptor activation: pivotal role of G beta gamma-dependent inhibition of voltage-operated Ca2+ channels

We previously showed that activation of G(i/o)-coupled histamine H(3)-receptors (H(3)R) is cardioprotective because it attenuates excessive norepinephrine release from cardiac sympathetic nerves. This action is characterized by a marked decrease in intraneuronal Ca(2+) ([Ca(2+)](i)), as G alpha(i) impairs the adenylyl cyclase-cAMP-protein kinase A (PKA) pathway, and this decreases Ca(2+) influx via voltage-operated Ca(2+) channels (VOCC). Yet, the G(i/o)-derived betagamma dimer could directly inhibit VOCC, and the subsequent reduction in Ca(2+) influx would be responsible for the H(3)R-mediated attenuation of transmitter exocytosis. In this study, we tested this hypothesis in nerve-growth factor-differentiated rat pheochromocytoma cells (PC12) stably transfected with H(3)R (PC12-H(3)) and with the G betagamma scavenger beta-adrenergic receptor kinase 1 (beta-ARK1)-(495-689)-polypeptide (PC12-H(3)/beta-ARK1). Thus, we evaluated the effects of H(3)R activation directly on the following: 1) Ca(2+) current (I(Ca)) using the whole-cell patch-clamp technique; and 2) K(+)-induced exocytosis of endogenous dopamine. H(3)R activation attenuated both peak I(Ca) and dopamine exocytosis in PC12-H(3) but not in PC12-H(3)/beta-ARK1 cells. Moreover, a membrane permeable phosducin-like G betagamma scavenger also prevented the antiexocytotic effect of H(3)R activation. In contrast, the H(3)R-induced attenuation of cAMP accumulation and dopamine exocytosis in response to forskolin were the same in both PC12-H(3) and PC12-H(3)/beta-ARK1 cells. Our findings reveal that although G alpha(i) participates in the H(3)-mediated antiexocytotic effect when the adenylyl cyclase-cAMP-PKA pathway is stimulated, a direct G betagamma-induced inhibition of VOCC, resulting in an attenuation of I(Ca), plays a pivotal role in the H(3)R-mediated decrease in [Ca(2+)](i) and associated cardioprotective antiexocytotic effects. The discovery of this H(3)R-signaling step may offer new therapeutic approaches to cardiovascular diseases characterized by hyperadrenergic activity.

Strategies to reduce the risk of drug-induced QT interval prolongation: a pharmaceutical company perspective

Drug-induced prolongation of the QT interval is having a significant impact on the ability of the pharmaceutical industry to develop new drugs. The development implications for a compound causing a significant effect in the 'Thorough QT/QTc Study' -- as defined in the clinical regulatory guidance (ICH E14) -- are substantial. In view of this, and the fact that QT interval prolongation is linked to direct inhibition of the hERG channel, in the early stages of drug discovery the focus is on testing for and screening out hERG activity. This has led to understanding of how to produce low potency hERG blockers whilst retaining desirable properties. Despite this, a number of factors mean that when an integrated risk assessment is generated towards the end of the discovery phase (by conducting at least an in vivo QT assessment) a QT interval prolongation risk is still often apparent; inhibition of hERG channel trafficking and partitioning into cardiac tissue are just two confounding factors. However, emerging information suggests that hERG safety margins have high predictive value and that when hERG and in vivo non-clinical data are combined, their predictive value to man, whilst not perfect, is >80%. Although understanding the anomalies is important and is being addressed, of greater importance is developing a better understanding of TdP, with the aim of being able to predict TdP rather than using an imperfect surrogate marker (QT interval prolongation). Without an understanding of how to predict TdP risk, high-benefit drugs for serious indications may never be marketed.

The impact of varying autonomic states on the dynamic beat-to-beat QT-RR and QT-TQ interval relationships

The beat-to-beat dynamicity of the QT-RR interval relationship is difficult to assess with the use of traditional correction factors (QTc) and changes in QTc do not accurately reflect or quantify arrhythmogenic risk. Further, the interpretation of arrhythmogenic risk is influenced by autonomic state. To visualize the QT-RR interval dynamics under varying conditions of autonomic state from impaired repolarization, we have developed a system to sequentially plot the beat-to-beat confluence of ECG data or 'clouds' obtained from conscious dogs and humans. To represent the non-uniformity of the clouds, a bootstrap sampling method that computes the mathematical centre of the uncorrected beat-to-beat QT value (QTbtb) and defines the upper and lower 95% confidence bounds is used. The same method can also be used to examine heterogeneity, hysteresis (both acceleration and deceleration) and restitution (beat-to-beat QT-TQ interval relationship). Impaired repolarization with the combination of E-4031 and L-768,673 (inhibitor of IKs current) increased heterogeneity of restitution at rest 55-91%; increased hysteresis during heart rate acceleration after isoproterenol challenge by approximately 40-60%; and dramatically diminished the minimum TQ boundary by 72% to only 28 ms. Impaired repolarization alters restitution during normal sinus rhythm and increases hysteresis/heterogeneity during heart rate acceleration following sympathetic stimulation. These findings are supported by similar clinical observations in LQT1 and LQT2 syndromes. Therefore, the assessment of the dynamic QT-RR and QT-TQ interval relationships through quantification of heterogeneity, hysteresis and restitution may allow a more accurate non-invasive evaluation of the conditions leading to cardiac arrhythmia.

Assessing risk of a prolonged QT interval-a survey of emergency physicians

Background

Although QT prolongation is associated with an increased risk of torsades de pointes (TdP), it is unclear how clinicians determine risk in individual patients with prolonged QT.

Aims

To investigate physicians’ interpretation of electrocardiogram (ECG) values in patients with a prolonged QT in reference to risk of TdP.

Methods

A survey was sent to Australasian emergency physicians (EPs) to investigate interpretation of ECG data in risk assessment for TdP. The survey contained three sections: demographic information, questions on heart rate correction and six sets of ECG data which the clinician ranked from low to high risk. Risk analysis for ECG values was performed by producing histograms of the distribution of responses for each of the six sets of ECG parameters. These distributions were compared to predicted distributions based on Bazett’s corrected QT>500 ms and the QT nomogram. The QT nomogram is a recently developed method for assessing whether QT-HR pairs are associated with increased risk of TdP by plotting them to determine if they are above an at risk line—the nomogram.

Results

Of 720 surveys sent out, 249 were returned (35%). A heart rate correction was used by 90% of respondents and the median “at risk” QTc judged by EPs was 450 ms [interquartile range (IQR): 440–500 ms]. Respondents were divided as to whether bradycardia increased the risk of TdP, with equal numbers responding “no change” and “more caution”. In four of the six sets of ECG parameters, EPs had a similar risk distribution to that predicted by Bazett. For one point predicted to be high risk by the QT nomogram, there was a uniform (undecided) risk distribution by EPs.

Conclusions

EPs mainly relied on Bazett’s correction as their method of TdP risk assessment, which may be problematic for bradycardic patients.

Dynamic QT/RR coupling in patients with pacemakers

The dynamic coupling between heart rate intervals (RR) and ventricular repolarization (QT) is analyzed. The analysis is based on measurements of 11 patients with pacemaker. In each measurement, there are at least 4 abrupt changes of RR preset by the pacemaker. With such a protocol, RR changes are important and well defined while disturbing factors and noise sources (such as those related with motion of patient) are minimized. The QT/RR coupling was described by 3 parameters (a1, b2, b3) transfer function (TRF) selected on the basis of a statistical analysis of performances of different TRF models. We found that our model is by far the best in its class: with more parameters (higher order models) the residuals remain almost the same while the extra parameters display variability much larger than that of our parameters. For all measurements, our TRF model describes more than 70% of QT variability. Within the patient set, we found interesting differences concerning dynamic non-linearity (response times longer with decreasing RR intervals than with increasing RR).

Analyses of dynamic beat-to-beat QT-TQ interval (ECG restitution) changes in humans under normal sinus rhythm and prior to an event of torsades de pointes during QT prolongation caused by sotalol

BACKGROUND

Restitution through intracardiac pacing has been used to assess arrhythmia vulnerability. We examined whether analyses of sequential beat-to-beat QT and TQ interval measures can be used to quantify ECG restitution changes under normal sinus rhythm.

METHODS

The QT, R-R and TQ intervals were examined 22.5 hour Holter monitoring before and after oral sotalol in normal male and female volunteers. Additionally, comparisons were made to those observed in the time-matched dataset prior to torsades de pointes in a heart diseased patient that received a single dose of sotalol.

RESULTS

Sotalol increased QT, R-R and TQ intervals 71, 101, and 125 ms after 160 mg (n = 38) and 194, 235, and 135 ms after 320 mg (n = 19) during maximum plasma concentrations, respectively. The percentage of beats with a QT/TQ ratio >1 was reduced 25% over the entire 22.5 hours after sotalol and the lower TQ interval boundary (5th quantile) was increased 22-30%. In contrast, 99% of the beats prior to torsades de pointes had a QT/TQ ratio > 1 and the median TQ interval was below the lower 98% confidence bounds of normals before and after sotalol.

CONCLUSIONS

ECG restitution changes are quantifiable under varying states (nocturnally, beta-adrenergic blockade, QT prolongation) in healthy subjects.

Activated Charcoal Decreases the Risk of QT Prolongation After Citalopram Overdose

STUDY OBJECTIVE

We determine whether single-dose activated charcoal (SDAC) administration after citalopram overdose reduces the proportion of patients developing abnormal QT prolongation.

METHODS

Data were collected retrospectively for citalopram overdose patients presenting to 8 emergency departments. Demographics, dose, coingested drugs, SDAC administration, and serial ECGs were extracted from medical records. The primary outcome was the proportion of patients who had an observed QT,RR combination at any time above an abnormal threshold, established as a predictor of torsade de pointes. We compared the proportion of patients with QT prolongation who received or did not receive SDAC. These data were analyzed within a Bayesian framework, using probabilities of abnormal QT,RR combinations with and without derived from a previous single-center study. WinBUGS was used to generate posterior estimates and credible intervals of the relative risk by combining the prior probabilities and the study data.

RESULTS

SDAC was administered on average 2.1 hours (range, 0.5 to 6.25 hours) after ingestion in 48 of 254 admissions, and abnormal QT,RR combinations occurred in 2 cases (4.2%), compared with 23 of 206 (11.2%) cases not receiving SDAC. There did not appear to be any clinically important difference in age, sex, dose, and cardiotoxic coingestants between the 2 groups. No cases of torsade de pointes occurred. The estimated relative risk of having an abnormal QT,RR combination for SDAC compared to no SDAC was 0.28 (0.06 to 0.70) (median with 2.5% and 97.5% credible limits). The probability that the relative risk was less than 1.0 was 0.99, which can be interpreted as very strong evidence in favor of a beneficial effect of SDAC. The absolute risk difference was estimated as 7.5% and the median number needed to treat as 13.3.

CONCLUSION

SDAC may be effective in reducing the risk of a prolonged QT in patients after citalopram overdose. Current trends toward nonuse of activated charcoal should be evaluated to determine whether patients poisoned by specific agents may benefit from activated charcoal administration.

Dynamic coupling between heart rate and ventricular repolarisation

A novel model for the coupling between ventricular repolarisation and heart rate (QT/RR) is presented. It is based upon a transfer function (TRF) formalism that describes the static and dynamic properties of this coupling, i.e., the behaviour after a sudden change in heart rate. Different TRF models were analysed by comparing their capability to describe experimental data collected from 19 healthy volunteers using several RR stimulation protocols: (i) rest with deep breathing at 0.1 Hz; (ii) tilt with controlled breathing at 0.1 and 0.33 Hz; and (iii) cycling. A search for the best TRF led to unambiguous identification of a three-parameter model as the most suitable descriptor of QT/RR coupling. Compared with established static models (linear or power-law), our model predictions are substantially closer to the experimental results, with errors approximately 50% smaller. The shape of the frequency and step responses of the TRF presented is essentially the same for all subjects and protocols. Moreover, each TRF may be uniquely identified by three parameters obtained from the step response, which are believed to be of physiological relevance: (i) gain for slow RR variability; (ii) gain for fast RR variability; and (iii) time during which QT attains 90% of its steady-state value. The TRF successfully describes the behaviour of the RR control following an abrupt change in RR interval, and its parameters may offer a tool for detecting pharmacologically induced changes, particularly those leading to increased arrhythmogenic risk.

Cardiovascular toxicity due to venlafaxine poisoning in adults: a review of 235 consecutive cases

AIMS

Venlafaxine may increase the risk of arrhythmia in certain patients. We sought to characterize the cardiovascular effects of venlafaxine overdose in adults.

METHODS

A retrospective casenote review of patients admitted to the Royal Infirmary of Edinburgh between January 2000 and June 2006. Haemodynamic and electrocardiographic data were examined in the whole group and a subset that ingested venlafaxine alone.

RESULTS

Two hundred and thirty-five patients (65 men) with median (interquartile range) age 34 years (27-43 years) had ingested venlafaxine 1500 mg (919-2800 mg). Tachycardia (40.0%), high blood pressure (28.4%) and mydriasis (36.6%) were common. Corrected QT >450 ms occurred in seven men (11.1%) and 17 women (10.5%) and transient arrhythmia in three patients. There was a positive correlation between stated quantity of venlafaxine ingested and heart rate [rho = 0.195, 95% confidence interval (CI) 0.054, 0.328] and QTc (rho = 0.314, 95% CI 0.089, 0.509).

CONCLUSIONS

Venlafaxine overdose is associated with sympathomimetic cardiovascular effects and prolonged QTc, irrespective of coingested drugs. These mechanisms might pose an increased risk of arrhythmia and require further exploration.

Analysis of the nonclinical telemetered ECG: Impact of logging rate and RR bin width in the dog and cynomolgus monkey

INTRODUCTION

Species-dependent ECG differences may affect QT interval analysis. Among these are the range of QT and RR values, heart rate variability, and respiratory sinus arrhythmia (SA). Importantly, the effects of data logging rates and RR bin width (BW) on QT analysis have not been characterized in dogs and nonhuman primates.

METHODS

Digital ECGs were collected for 18-21 h in telemetered dogs (n=7) and cynomolgus monkeys (n=7) employing epicardial ECG leads and analyzed by computerized algorithms. ECG intervals were determined employing beat-to-beat (1 ms BW) and 10 s logging rates (for 1, 10, 20, and 50 ms BW). Diurnal heart rate variability was defined as the beat-to-beat RR ratio (VR) where SA was defined as >+/-10% change in VR.

RESULTS

Canine beat-to-beat QT-RR data were curvilinear and plateaued, with multiple RR values associated to any given QT for RR>or=900 ms. Cynomolgus QT-RR data collected as beat-to-beat or at a 10 s logging rate were linear for all RR intervals. During the day, SA comprised 62.9+/-2.4% and 11.4+/-6.1% of total beats in the dog and cynomolgus, respectively, with no diurnal/nocturnal differences in either species. QT interval changes varied with BW such that 5 ms resolution was maintained for BW <or=11.5 and 41 ms in the dog and cynomolgus, respectively.

DISCUSSION

Differences in the QT interval, primarily due to SA, were responsible for skewed canine beat-to-beat QT-RR relationships. A 10 s logging rate eliminated this confounding factor, enabling the derivation of accurate QT rate-corrections. Thus, the uniform application of a 10 s logging rate in conjunction with a 10 ms RR BW ensured consistent and accurate QT analysis in both species. In conclusion, optimal BWs were defined which maintained beat-to-beat accuracy, while maximizing the number of QT-RR observations incorporated into QT-RR rate-correction models, a critical factor for robust probabilistic QT rate-corrections.

Points to consider emerging from a mini-workshop on cardiac safety: assessing torsades de pointes liability

A mini-workshop on cardiac safety focusing on assessing drug-induced Torsades de Pointes (TdP) liability was convened as part of the 6th Annual Meeting of the Safety Pharmacology Society. The purpose of this brief publication is to disseminate the salient points emanating from this workshop as a means of engaging the scientific community in the appropriate discussions needed to advance this important field of human safety. The recommendations in this publication extend those of the workshop on "Moving Towards Better Predictors of Drug-Induced Torsades de Pointes" held in November 2005 under the auspice of the International Life Sciences Institute, Health and Environmental Sciences Institute; they fall into four key areas: molecular and cellular biology underlying TdP, dynamics of periodicity, models of TdP proarrhythmia and key considerations for demonstrating utility of non-clinical models. The reader is encouraged to consider the recommendations emanating from the two workshops and align these with ongoing studies in their laboratories. The authors intend to convene a workshop in 2009/2010 to judge advancements in the field of study of drug-induced TdP and make recommendations for a focused validation of those methods holding the greatest promise of improving the predictivity of this unwanted human cardiac risk.

Pharmacokinetic-pharmacodynamic modelling of QT interval prolongation following citalopram overdoses

AIMS

To develop a pharmacokinetic-pharmacodynamic model describing the time-course of QT interval prolongation after citalopram overdose and to evaluate the effect of charcoal on the relative risk of developing abnormal QT and heart-rate combinations.

METHODS

Plasma concentrations and electrocardiograph (ECG) data from 52 patients after 62 citalopram overdose events were analysed in WinBUGS using a Bayesian approach. The reported doses ranged from 20 to 1700 mg and on 17 of the events a single dose of activated charcoal was administered. The developed pharmacokinetic-pharmacodynamic model was used for predicting the probability of having abnormal combinations of QT-RR, which was assumed to be related to an increased risk for torsade de pointes (TdP).

RESULTS

The absolute QT interval was related to the observed heart rate with an estimated individual heart-rate correction factor [alpha = 0.36, between-subject coefficient of variation (CV) = 29%]. The heart-rate corrected QT interval was linearly dependent on the predicted citalopram concentration (slope = 40 ms l mg(-1), between-subject CV = 70%) in a hypothetical effect-compartment (half-life of effect-delay = 1.4 h). The heart-rate corrected QT was predicted to be higher in women than in men and to increase with age. Administration of activated charcoal resulted in a pronounced reduction of the QT prolongation and was shown to reduce the risk of having abnormal combinations of QT-RR by approximately 60% for citalopram doses above 600 mg.

CONCLUSION

Citalopram caused a delayed lengthening of the QT interval. Administration of activated charcoal was shown to reduce the risk that the QT interval exceeds a previously defined threshold and therefore is expected to reduce the risk of TdP.

Sympathomimetic Infusion and Cardiac Repolarization: The Normative Effects of Epinephrine and Isoproterenol in Healthy Subjects

INTRODUCTION

Catecholamines are known to affect cardiac repolarization, and provocation with either isoproterenol or epinephrine has been proposed as a tool for uncovering latent repolarization abnormalities. This study systematically compares the effects of isoproterenol and epinephrine infusions on QT interval (QT), T waves and U waves in normal subjects.

METHODS AND RESULTS

Twenty-four normal subjects (29 +/- 8 years) were evaluated during graded infusions of up to 0.30 microg/kg/minute epinephrine and 5.0 microg/minute isoproterenol. Heart rates at peak doses were 81 +/- 13 bpm at 0.28 +/- 0.04 microg/kg/minute epinephrine and 104 +/- 5 bpm at 2.4 microg/minute isoproterenol. The longest absolute QT increase was 4 +/- 5 msec above baseline during isoproterenol (P < 0.001) and 12 +/- 23 msec during epinephrine (P = 0.07), while the longest corrected QT interval (QTc) increase was 67 +/- 28 msec (P < 0.0001) and 79 +/- 40 msec (P < 0.0001) above baseline during isoproterenol and epinephrine, respectively (P = 0.12 for difference). There was a 2-fold increase in U-wave amplitude during each intervention (P < 0.001). The specificity of paradoxical QT prolongation (>or=30 msec at 0.05 microg/kg/minute or >or=35 msec at 0.10 microg/kg/minute epinephrine) and an increase in QTc >or=600 msec at any dose epinephrine were 100%. However, the specificity of other proposed criteria that utilized QTc measurement (>or=30 msec at 0.10 microg/kg/minute or >or=65 msec at any dose) was poor whether all leads or only lead II were assessed.

CONCLUSION

Both epinephrine and isoproterenol are associated with QTc prolongation and amplification of the U wave in normal subjects. The specificity of proposed criteria for epinephrine provocation in diagnosis of the long-QT syndrome is variable; however, paradoxical QT prolongation at low-dose epinephrine or a QTc >or=600 msec is highly specific.

Novel probabilistic method for precisely correcting the QT interval for heart rate in telemetered dogs and cynomolgus monkeys

INTRODUCTION

QT intervals are not regulated on a beat-to-beat cadence, but are strongly influenced by the preceding heart rate history (hysteresis). ECG sampling, when performed over sufficiently long periods, results in the detection of ranges of different QT values for each discrete RR interval. Given the potential impact of QT hysteresis in QT interval rate-correction procedures, we hypothesized that, physiologically, the QT interval exists as a probabilistic variable where the exact value corresponding to any RR interval is precisely estimated from the associated QT population.

METHODS

Digital ECGs were collected for 18-21 h in telemetered dogs (n=7) and cynomolgus monkeys (n=7) employing epicardial ECG leads for accurate T(end) detection, and analyzed by computerized algorithms. Descriptive statistics were calculated for raw QT values in 10 ms RR increments. Individual rate-corrected QT (QTc) formulae were derived from the slopes of log-transformed QT-RR data where each QT point was the mean of >250 beats/RR increment. The aptness of this QTc model was assessed by residual analysis.

RESULTS

Beat-to-beat ECG analysis demonstrated that for all discrete cycle lengths, the associated raw QT intervals were normally distributed populations, spanning approximately 30-40 and 45-100 ms in the dog and cynomolgus monkey, respectively. In both species, QTc was stable (< or =5 ms variation) over all physiological RR intervals.

DISCUSSION

The probabilistic treatment of raw QT interval populations natively associated to any RR interval provides hysteresis-free raw QT estimates which can be accurately modeled, allowing the derivation of a precise QTc value. Previous unawareness of the probabilistic nature of the QT interval explains the historical failure of numerous QT rate-correction formulae to correctly solve this scientific issue. Importantly, QT distribution analysis has the potential to provide, for the first time, a universal and sensitive method for QT heart rate-correction, providing a robust method for nonclinical and clinical cardiac safety investigations of repolarization delay.

Application of pharmacokinetic-pharmacodynamic modelling in management of QT abnormalities after citalopram overdose

OBJECTIVE

To develop guidelines for the management of QT prolongation after citalopram overdose, including decontamination with single-dose activated charcoal (SDAC) and cardiac monitoring.

DESIGN

Simulation study using a previously developed pharmacokinetic-pharmacodynamic (PKPD) model which predicted the time-course of QT prolongation and the effect of citalopram dose and use of SDAC on QT prolongation.

MAIN MEASURES AND RESULTS

The previously developed PKPD model was used to address the following in patients following citalopram overdose: (1) Above what dose should patients be decontaminated? (2) Above what dose should patients have cardiac monitoring? (3) For what period of time should patients be monitored? The primary outcome was QT,RR combinations above an abnormal threshold as a surrogate predictor of torsades de pointes. Simulations were performed using MATLAB for an overdose patient with typical demographics: 30-year-old female with a heart rate of 79 bpm taking citalopram therapeutically. The simulations showed: (1) There was significant benefit associated with the administration of SDAC to patients following citalopram overdose ingesting >600 mg; (2) With citalopram overdoses >1,000mg it was advisable to give SDAC and cardiac monitor the patient; (3) The risk of developing future abnormal QT,RR combinations was less than 1% in patients with normal QT,RR combinations up to 13 h post-dose, so the minimum monitoring time for citalopram overdoses >1,000mg should be 13h. Recommended dose levels for intervention should be lowered in older patients and patients with tachycardia, while men are less sensitive to QT prolongation.

CONCLUSIONS

Guidelines for the management of QT prolongation after citalopram overdose were developed. We believe the model will help clinicians to decide which patients to decontaminate and monitor.

Effect of acute cocaine administration on the QTc interval of habitual users

The proarrhythmic effects of cocaine may be mediated in part by its effects on cardiac repolarization properties. This study evaluated the acute effects of smoking cocaine 25 mg on the electrocardiograms of 14 habitual cocaine users during a 12-minute observation period. After cocaine administration, heart rate increased by a mean of 22 beats/min (p <0.0001). One patient developed accelerated junctional rhythm, and 5 had nonspecific ST-T-wave abnormalities. The electrocardiograms revealed significant prolongation of the QTc interval (p <0.001) after cocaine administration. In addition, T-wave amplitude decreased and U-wave amplitude increased in response to cocaine use (p <0.05). QRS duration was unchanged by cocaine, whereas the PR interval shortened slightly. The repolarization changes observed after cocaine use were similar to those reported for other sympathomimetic agents and may be a contributing factor in the association between cocaine use and ventricular arrhythmias.

Moving towards better predictors of drug-induced Torsade de Pointes. 2-3 November 2005, Crystal City, Virginia, USA

The Health and Environmental Sciences Institute (HESI) convened a two-day workshop to explore the biological mechanisms responsible for inherited, acquired and induced long QT interval syndromes. Investigational areas included QT dynamics, cell biology, nonclinical models of Torsade de Pointes (TdP) arrhythmia as well as how to ascertain the predictability of such models for human outcome were discussed. Magisterial lectures from academia covered state-of-science knowledge on these domains, whereas industry and regulatory authority representatives dealt with the advantages, disadvantages and desirable requisites of nonclinical assays to assess drug-associated cardiac safety. Proposals for future research projects were not sufficiently factual to allow discrimination between pragmatic and ideal solutions. A key objective of the workshop was to foster initiatives addressing development of nonclinical proarrhythmic models for identifying without failure drug candidates with the potential to cause threshold QT interval increases of regulatory concern (5-10 ms) in healthy volunteers and a TdP event in 1/10(5)-10(7) patients. In the authors' opinion, fulfilling this goal (an FDA prerequisite for abrogating E14 'thorough QT study' requirement) pertains more to a Faustian quest than to a realistic, nonclinical safety pharmacology assignment. Indeed, biological assays are, by nature, characterised by an implicit degree of uncertainty, contradicting the precautionary principle of zero error expectancy. For the moment, relatively successful strategies should rely on expertly designed and executed S7B core assays, complemented as needed, by reliable proarrhythmia tests. These studies should be preceded, when available, by application of powerful in silico fingerprint technology mining databases containing pertinent public and proprietary (liberally released) cardiac safety information on reference, marketed, withdrawn and failed drugs.

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.

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

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

TRANSPORT OF ANTHELMINTIC BENZIMIDAZOLE DRUGS BY BREAST CANCER RESISTANCE PROTEIN (BCRP/ABCG2)

Methylcarbamate benzimidazoles [albendazole (ABZ), fenbendazole (FBZ), and their respective sulfoxide derivatives, albendazole sulfoxide (ABZSO) and oxfendazole (OXF)] are therapeutically important anthelmintic agents with low bioavailability. We studied their in vitro interaction with the apical ATP-binding cassette (ABC) drug efflux transporters, breast cancer resistance protein (BCRP/ABCG2), P-glycoprotein (ABCB1), and MRP2 (ABCC2) using MDCKII cells transduced with human MDR1, MRP2, and BCRP, and murine Bcrp1 cDNAs. These ABC drug efflux transporters extrude a wide range of xenotoxins from cells in intestine, liver, and other organs, thus affecting the bioavailability of many compounds. In transport experiments, ABZSO and OXF were transported efficiently by murine Bcrp1 and moderately by human BCRP, but not by MDR1 or MRP2. ABZ and FBZ were not found to be Bcrp1, MRP2, or P-glycoprotein substrates in vitro. OXF was found to be a good BCRP/Bcrp1 inhibitor, with somewhat higher potency in the MDCKII-BCRP cell line. The latter results were confirmed by flow cytometry experiments demonstrating inhibition by OXF of murine Bcrp1- and human BCRP-mediated mitoxantrone transport. Further studies of interactions between OXF and known BCRP/Bcrp1 substrates will be of interest. The use of efficacious BCRP/Bcrp1 inhibitors might increase the extent and duration of systemic exposure to ABZSO and OXF, with possible therapeutically beneficial effects in extra-intestinal infections.

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.