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

QT PRODACT: In Vivo QT Assay in Anesthetized Dog for Detecting the Potential for QT Interval Prolongation by Human Pharmaceuticals

The purpose of this study was to assess the utility of the isoflurane-anesthetized dog model for detecting the potential for QT interval prolongation by human pharmaceuticals. The effects of 10 positive compounds with torsadogenic potential, 8 negative compounds with little torsadogenic potential, and dl-sotalol as a common positive compound were evaluated in 5 facilities in accordance with the common protocol approved by QT PRODACT. Each test compound was cumulatively infused into male beagle dogs anesthetized with isoflurane. Surface lead II ECG, blood pressure, and plasma concentrations for the positive compounds were measured. Repeated administration of the vehicle examined in each facility before the start of the experiments resulted in a slight, but not significant, change in corrected QT (QTc) interval, indicating that this model only shows slight experimental variation. Although an inter-facility variability in the extent of dl-sotalol-induced QT interval prolongation was observed, dl-sotalol significantly prolonged QTc interval in all facilities. All positive compounds significantly prolonged QTc interval at plasma levels up to 10 times those in patients who developed prolonged QTc interval or TdP, whereas no negative compounds did so. These data suggest that the in vivo QT assay using the anesthetized dog is a useful model for detecting the potential for QT interval prolongation by human pharmaceuticals.

QT PRODACT: Sensitivity and Specificity of the Canine Telemetry Assay for Detecting Drug-Induced QT Interval Prolongation

The purpose of this investigation was to define the sensitivity and specificity of the canine telemetry assay for detecting drug-induced QT interval prolongation. Data from twelve studies generated in the QT PRODACT project were used in this investigation. The study design was a 4x4 Latin square cross-over design and included the following drugs: MK-499, E-4031, terfenadine, haloperidol, cisapride, bepridil, propranolol, diphenhydramine, captopril, verapamil, amoxicillin, and ciprofloxacin. The estimated root squared error of the model, which estimated the slope of the QT-RR relationships for each animal, for all dogs during the pre-dosing period was 5.45%. Using the QT-RR model, the sensitivity and specificity in each cutoff value that judges QT prolongation were estimated based on the experiment errors and measurement errors in the 12 studies. When the cutoff value was 5%, the sensitivity in 10% prolongation was 0.978 and the specificity in 0% was 0.996. In conclusion, it was judged that a 5% cutoff value for changes in heart rate corrected QT interval using the canine telemetry assay is practical, and the sensitivity and specificity of the telemetry assay are very high when using the analytical method presented here. Based upon this information, the canine telemetry assay using the individual subject heart rate correction model is recommended as a sensitive test system for the in vivo assessment of risk for QT interval prolongation.

QT Correction Methods in Children and Adolescents

INTRODUCTION

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

METHODS AND RESULTS

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

CONCLUSION

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

Effects of the antiepileptic drugs lamotrigine, topiramate and gabapentin on hERG potassium currents

Drugs that inhibit the cardiac rapid delayed rectifier potassium ion current (I(Kr)) can be proarrhythmic and their clinical use has been associated with sudden unexpected death (SUD) due to cardiac arrhythmia. SUD is 20-40 times more common among people with epilepsy than in the general population and case-control studies have identified polytherapy with antiepileptic drugs (AEDs) as a risk factor. In a previous study, it was described that the old AEDs phenytoin and phenobarbital had the potential to inhibit the I(Kr) channel and it was suggested that this could contribute to the increased risk for SUD in patients with epilepsy. In this study, we have investigated the I(Kr) blocking potential of some more recently introduced AEDs, lamotrigine (LTG), topiramate (TPM) and gapapentin (GBP). The whole cell patch-clamp recording technique was used to study the effects on I(Kr) channels expressed by the human ether-a-go-go related gene (hERG) stably expressed in human embryo kidney (HEK) 293 cells. Tail currents, which are purely related to hERG currents, were blocked with IC50 and IC20 (the concentrations when 50% and 20% inhibition was obtained compared to control values) of 229 and 21 microM, respectively, for LTG. A 40% inhibition of tail currents was obtained at GBP concentrations of 100 mM and a 20% inhibition at 54 mM. A 35% inhibition of tail currents was obtained at TPM concentrations of 1000 microM and a 20% inhibition at 87 microM, respectively. Collective data show that drugs with the same margins (ratio hERG IC50/unbound therapeutic concentration) as LTG, may have arrhythmogenic potential. The risk for arrhythmia may be clinically significant in the presence of predisposing factors such as seizure-induced acidosis and in the case of concurrent treatment with other I(Kr) blocking drugs, or in case of pharmacokinetic drug-drug interactions resulting in excessively high concentrations of LTG.

Sample Size, Power Calculations, and Their Implications for the Cost of Thorough Studies of Drug Induced QT Interval Prolongation

Regulatory authorities require new drugs to be investigated using a so-called "thorough QT/QTc study" to identify compounds with a potential of influencing cardiac repolarization in man. Presently drafted regulatory consensus requires these studies to be powered for the statistical detection of QTc interval changes as small as 5 ms. Since this translates into a noticeable drug development burden, strategies need to be identified allowing the size and thus the cost of thorough QT/QTc studies to be minimized. This study investigated the influence of QT and RR interval data quality and the precision of heart rate correction on the sample sizes of thorough QT/QTc studies. In 57 healthy subjects (26 women, age range 19-42 years), a total of 4,195 drug-free digital electrocardiograms (ECG) were obtained (65-84 ECGs per subject). All ECG parameters were measured manually using the most accurate approach with reconciliation of measurement differences between different cardiologists and aligning the measurements of corresponding ECG patterns. From the data derived in this measurement process, seven different levels of QT/RR data quality were obtained, ranging from the simplest approach of measuring 3 beats in one ECG lead to the most exact approach. Each of these QT/RR data-sets was processed with eight different heart rate corrections ranging from Bazett and Fridericia corrections to the individual QT/RR regression modelling with optimization of QT/RR curvature. For each combination of data quality and heart rate correction, standard deviation of individual mean QTc values and mean of individual standard deviations of QTc values were calculated and used to derive the size of thorough QT/QTc studies with an 80% power to detect 5 ms QTc changes at the significance level of 0.05. Irrespective of data quality and heart rate corrections, the necessary sample sizes of studies based on between-subject comparisons (e.g., parallel studies) are very substantial requiring >140 subjects per group. However, the required study size may be substantially reduced in investigations based on within-subject comparisons (e.g., crossover studies or studies of several parallel groups each crossing over an active treatment with placebo). While simple measurement approaches with ad-hoc heart rate correction still lead to requirements of >150 subjects, the combination of best data quality with most accurate individualized heart rate correction decreases the variability of QTc measurements in each individual very substantially. In the data of this study, the average of standard deviations of QTc values calculated separately in each individual was only 5.2 ms. Such a variability in QTc data translates to only 18 subjects per study group (e.g., the size of a complete one-group crossover study) to detect 5 ms QTc change with an 80% power. Cost calculations show that by involving the most stringent ECG handling and measurement, the cost of a thorough QT/QTc study may be reduced to approximately 25%-30% of the cost imposed by the simple ECG reading (e.g., three complexes in one lead only).

Comparing methods of measurement for detecting drug-induced changes in the QT interval: implications for thoroughly conducted ECG studies

BACKGROUND

The aim of this study was to compare the reproducibility and sensitivity of four commonly used methods for QT interval assessment when applied to ECG data obtained after infusion of ibutilide.

METHODS

Four methods were compared: (1) 12-lead simultaneous ECG (12-SIM), (2) lead II ECG (LEAD II), both measured on a digitizing board, (3) 3-LEAD ECG using a manual tangential method, and (4) a computer-based, proprietary algorithm, 12SL trade mark ECG Analysis software (AUT). QT intervals were measured in 10 healthy volunteers at multiple time points during 24 hours at baseline and after single intravenous doses of ibutilide 0.25 and 0.5 mg. Changes in QT interval from baseline were calculated and compared across ECG methods, using Bland-Altman plots. Variability was studied using a mixed linear model.

RESULTS

Baseline QT values differed between methods (range 376-395 ms), mainly based on the number of leads incorporated into the measurement, with LEAD II and 3-LEAD providing the shortest intervals. The 3-LEAD generated the largest QT change from baseline, whereas LEAD II and 12-SIM generated essentially identical result within narrow limits of agreement (0.4 ms mean difference, 95% confidence interval +/- 20.5 ms). Variability with AUT (standard deviation 15.8 ms for within-subject values) was clearly larger than with 3-LEAD, LEAD II, and 12-SIM (9.6, 10.0, and 11.3 ms).

CONCLUSION

This study demonstrated significant differences among four commonly used methods for QT interval measurement after pharmacological prolongation of cardiac repolarization. Observed large differences in variability of measurements will have a substantial impact on the sample size required to detect QT prolongation in the range that is currently advised in regulatory guidance.

Ziprasidone: first year experience in a hospital setting

BACKGROUND

The antipsychotic drug ziprasidone, FDA-approved and introduced in the United States in February 2001 for the treatment of schizophrenia, appears to have similar efficacy but better tolerability than older antipsychotics and requires further evaluation under clinical conditions.

METHODS

We analyzed medical records of McLean Hospital inpatients treated with ziprasidone between March 2001 and February 2002, gathering data on DSM-IV diagnoses, presenting symptoms, dosing, concomitant psychotropic medications, clinical changes, adverse effects, and electrocardiographic (ECG) findings.

RESULTS

Ziprasidone was given to 151 inpatients (3.4% of admissions; 108 women, 43 men), aged 37.5 +/- 11.4 years, who presented with depression (n = 79), psychosis (n = 46), mania (n = 18), bipolar mixed-states (n = 4), or other conditions (n = 4). Daily doses averaged 49.8 +/- 34.1 mg initially and 83.2 +/- 46.3 mg at discharge; the greatest dose increases during hospitalization (by a mean of 61%) were in patients with schizoaffective disorder (n = 46; 30% of cases). In 41 cases (27%), ziprasidone was the only antipsychotic at discharge; in 61 (40%) it was used with other antipsychotics. Ziprasidone was discontinued during hospitalization in 49 cases (32.5%), due to lack of efficacy (n = 26; 17.2%), adverse effects (n = 13, 8.6%), or reasons not stated (n = 10, 6.6%). Of 70 patients for whom ECG data were obtained during treatment with ziprasidone, 8 (11%) had QTc intervals > 450 msec during treatment, but none of the 39 patients with ECGs both before and during ziprasidone treatment showed clinically meaningful increases in QTc intervals. Ziprasidone was discontinued in 4 patients (2.6%) due to concern about QTc intervals, but in no case was the QTc interval > or = 500 msec or associated with clinical cardiac toxicity. Improvements in CGI and GAF scores from admission to discharge were similar across diagnoses and unrelated to length of stay or ziprasidone dose.

CONCLUSIONS

Ziprasidone was well tolerated by hospitalized patients with various major psychiatric disorders and may be of value in conditions other than schizophrenia.

Practice Standards for Electrocardiographic Monitoring in Hospital Settings

The goals of electrocardiographic (ECG) monitoring in hospital settings have expanded from simple heart rate and basic rhythm determination to the diagnosis of complex arrhythmias, myocardial ischemia, and prolonged QT interval. Whereas computerized arrhythmia analysis is automatic in cardiac monitoring systems, computerized ST-segment ischemia analysis is available only in newer-generation monitors, and computerized QT-interval monitoring is currently unavailable. Even in hospitals with ST-monitoring capability, ischemia monitoring is vastly underutilized by healthcare professionals. Moreover, because no computerized analysis is available for QT monitoring, healthcare professionals must determine when it is appropriate to manually measure QT intervals (eg, when a patient is started on a potentially proarrhythmic drug). The purpose of the present review is to provide ‘best practices’ for hospital ECG monitoring. Randomized clinical trials in this area are almost nonexistent; therefore, expert opinions are based upon clinical experience and related research in the field of electrocardiography. This consensus document encompasses all areas of hospital cardiac monitoring in both children and adults. The emphasis is on information clinicians need to know to monitor patients safely and effectively. Recommendations are made with regard to indications, timeframes, and strategies to improve the diagnostic accuracy of cardiac arrhythmia, ischemia, and QT-interval monitoring. Currently available ECG lead systems are described, and recommendations related to staffing, training, and methods to improve quality are provided.

QT prolongation with antimicrobial agents: understanding the significance

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

Droperidol for perioperative sedation causes a transient prolongation of the QTc time in children under volatile anesthesia

BACKGROUND

Droperidol is useful for postoperative sedation in infants and children after cardiac surgery because it provides sedation and akinesia with minimal respiratory depression. However, droperidol has been associated with QT prolongation and ventricular arrhythmias. We investigated, if neuroleptanalgesic doses of droperidol led to QT prolongation and cardiac arrhythmias in children undergoing cardiac surgery.

METHODS

We retrospectively analysed electrocardiogram rhythm strips that were obtained before and in time increments after a 100 microg x kg(-1) intravenous bolus of droperidol in 20 children undergoing cardiac surgery. The longest QT interval was determined in each ECG and corrected for heart rate (QTc). All arrhythmias were recorded.

RESULTS

Droperidol led to a significant increase in QTc time that was still present at 15 min but had resolved within 30 min after the bolus. No associated arrhythmias were observed.

CONCLUSIONS

The statistically significant prolongation of QTc time after a sedative dose of droperidol is of concern because it may increase the risk for malignant cardiac arrhythmias. A large, prospective study is necessary to identify the true risk for arrhythmias after droperidol in this patient population, but our study suggests that any arrhythmogenic risk, if present, will be very transient, since the increase in QTc time was limited to a period of less than 30 min after the bolus.

Errors and misconceptions in ECG measurement used for the detection of drug induced QT interval prolongation

Drug-induced changes in cardiac repolarisation receive substantial attention by regulatory agencies. Since there are presently no established accurate possibilities of testing the propensity to torsade induction during clinical drug development, the regulators require drug-related QT interval changes to be thoroughly investigated with almost all new pharmacological agents. Small QT interval changes are easy to miss and the regulators therefore expect the relevant studies to be very precise. Such a precision is not easy to achieve and different strategies have been proposed. The purpose of this article is to review the most frequent misconceptions and errors in the electrocardiogram handling and measurements related to the detection to drug-induced QT interval changes. Specifically, the article discusses (a) the possibilities of automatic measurement by standard electrocardiographic equipment, (b) the danger of casual measurement by central laboratories handling the electrocardiograms, (c) the selection of recording leads for QT interval measurement, and (d) the problem of the so-called heart rate hysteresis of the QT interval adaptation. Suggestions are made for drug developers of what study design and quality control aspects are needed to avoid the most frequent imprecisions.

Absence of clinically important HERG channel blockade by three compounds that inhibit phosphodiesterase 5—sildenafil, tadalafil, and vardenafil

Compounds that inhibit phosphodiesterase 5 (PDE5) have been developed for the treatment of erectile dysfunction. Because men with erectile dysfunction frequently have comorbid cardiovascular disease, they may have limited cardiac repolarization reserve and be at risk of arrhythmia if treated with medications that prolong ventricular repolarization. The human ether-a-go-go related gene (HERG) channel is important for repolarization in human myocardium and is a common target for drugs that prolong the QT interval. We studied the ability of three compounds that inhibit PDE5--sildenafil, tadalafil, and vardenafil--to block the HERG channel. Using a whole cell variant of the patch-clamp method, the HERG current was measured in a stably transfected human embryonic kidney cell line expressing the HERG channel. The compounds produced dose-dependent reductions in HERG current amplitude over a concentration range of 0.1 to 100 microM. The IC50 values were 12.8 microM for vardenafil and 33.3 microM for sildenafil. Because the maximum soluble concentration of tadalafil (100 microM) produced only a 50.9% inhibition of the HERG current amplitude, the IC50 value for tadalafil could not be determined with the Hill equation. Tadalafil had the weakest capacity to block the HERG channel, producing a 50.9% blockade at the maximum soluble concentration (100 microM), compared with 86.2% for vardenafil (100 microM) and 75.2% for sildenafil (100 microM). In conclusion, the concentrations of the PDE5 inhibitors required to evoke a 50% inhibition of the HERG current were well above reported therapeutic plasma concentrations of free and total compound. None of the three compounds was a potent blocker of the HERG channel.

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

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

Relative Toxicity of Selective Serotonin Reuptake Inhibitors (SSRIs) in Overdose

BACKGROUND

Selective serotonin reuptake inhibitors (SSRIs) have increasingly replaced tricyclic antidepressants (TCAs) in the treatment of depression. They appear to be safer in overdose, but there is little information on their spectrum of toxicity in overdose, or relative toxicity of each agent.

OBJECTIVE

To determine the effect of SSRIs in overdose, as a group, and the relative toxicity of five different SSRIs.

METHODS

A review of consecutive SSRI poisoning admissions to a single toxicology unit. Outcomes examined were length of stay [LOS], intensive care [ICU] admission rate, coma, seizures, electrocardiographic [ECG] abnormalities, and presence of serotonin syndrome [SS]. Logistic regression was used to model the outcome QTc >440 msec.

RESULTS

There were 469 SSRI poisoning admissions analyzed after exclusions. The median LOS for all SSRI overdose admissions was 15.3 h (IQR: 10.5-21.3) and 30 of 469 (6.4%; 95% CI 4.3-9.0%) cases were admitted to ICU. The incidence of seizures was 1.9% and coma was 2.4%. Serotonin syndrome occurred in 14% of overdoses. Comparison of median QTc intervals of the five SSRIs was significantly different (p=0.0002); citalopram (450 IQR: 436-484) was individually different to fluoxetine (p=0.045), fluvoxamine (p=0.022), paroxetine (p=0.0002), and sertraline (p=0.001). The proportion of citalopram overdoses with a QTc >440 msec was 68%, differing significantly from sertraline (adjusted OR: 5.11 95% CI 2.32-11.27). Comparison of median QT intervals of the five SSRIs was statistically different (p=0.026); citalopram (400 IQR: 380-440) was individually different from sertraline (p=0.023).

CONCLUSIONS

This study shows SSRIs are relatively safe in overdose despite serotonin syndrome being common. The exception was citalopram, which was significantly associated with QTc prolongation. We believe that cardiac monitoring should be considered in citalopram overdose, particularly with large ingestions and patients with associated cardiac disease.

Determination of myocardium to plasma concentration ratios of five antipsychotic drugs: comparison with their ability to induce arrhythmia and sudden death in clinical practice

Reviewing available data shows that most of antipsychotic drugs are associated with arrhythmia and sudden death. Experimental studies have shown a HERG channel blockade, a dose-dependent increase in duration of action potential or of QT interval, with various degrees of indicators of serious arrhythmogenicity. However, it seems difficult to relate these in vitro and in vivo preclinical models to clinical findings, in part, because the relationship between concentrations used and in vivo tissue concentrations during treatment in man is not known. Consequently, we established the myocardium to plasma concentration ratios for a series of antipsychotic drugs by intraperitoneal administration of different level doses to the guinea pig. Then, we compared these values to their ability to induce arrhythmia or torsade de pointes in clinical practice. The myocardium to plasma concentration ratios were 2.2 for clozapine, 2.7 for olanzapine, 3.1 for sertindole, 4.5 for risperidone, and 6.4 for haloperidol. These data suggest that when the ratio is higher than 4, arrhythmia and sudden death may be expected. On the contrary, when the ratio is less than 3, little effect may be predicted. These results underscore the importance of interpreting HERG channel data and electrophysiological data in the context of other pharmacokinetic parameters such as myocardium to plasma distribution.

The utility of hERG and repolarization assays in evaluating delayed cardiac repolarization: influence of multi-channel block

Drug-induced delayed cardiac repolarization is a recognized risk factor for proarrhythmia and is associated with block of IKr (the potassium current encoded by the human ether-a- go-go-related gene [hERG]). To evaluate the utility of 2 in vitro assays widely used to assess delayed repolarization, we compared the effects of haloperidol and 9 structurally diverse drugs in a hERG and repolarization (canine Purkinje fiber action potential duration [APD]) assay over wide concentrations. Despite potent hERG current block (IC50 = 0.174 microM), haloperidol elicited a bell-shaped concentration-response relationship for APD prolongation, with lesser prolongation (and reduced plateau height) observed with concentrations eliciting maximal hERG block, consistent with multi-channel block at higher concentrations. Consistent with this hypothesis, APD prolongation with the specific IKr blocker dofetilide was a) reduced by concomitant administration of nifedipine (calcium current block) and b) reversed by lidocaine (late sodium current block). Additional studies demonstrated prominent (>50%) hERG inhibition with most (9/10) drugs despite wide APD changes (158% prolongation - 16% shortening), consistent with multi-channel block. The poor correlation between hERG and repolarization assays suggests that the hERG assay oversimplifies drug effects on the complex repolarization process for drugs demonstrating multi-channel block and that neither assay alone adequately predicts proarrhythmic risk.

Differences Between Study-Specific and Subject-Specific Heart Rate Corrections of the QT Interval in Investigations of Drug Induced QTc Prolongation

A computational study was designed to investigate the differences between the so-called study-specific and subject-specific heart rate corrections of QT interval. In 53 healthy subjects (25 women, mean age 26.7 +/- 8.7 years), serial 10-second electrocardiograms (ECG) were obtained during daytime hours. In each subject, 200 ECGs were selected representative of the individual QT/RR relationship. Of the population of 53 subjects, 30,000 different subgroups of 16 subjects were considered and their data used to model drug induced QT interval prolongation by 0, 5, 10, and 20 ms combined with drug induced heart rate acceleration and deceleration. In each modeled study, QTc changes were assessed by: (1) Six study-specific heart rate corrections designed by regression modeling of the baseline QT/RR data pooled from all subjects; (2) Six subject-specific heart rate corrections designed by the same regression modeling of the baseline QT/RR data in each subject separately; (3) subject optimized correction that selected the best fitting regression model for each individual; and (4) by Bazett and Fridericia corrections. In each modeled study, the errors of the correction approaches were estimated and statistically summarized over all modeled studies. The subject-specific corrections led to maximum errors in single milliseconds (error range of 2.4, 5.7, and 2.6 ms with linear, log/log linear, and exponential models, respectively) while the study-specific corrections led to substantially greater errors (error range of 17.8, 19.4, and 16.9 ms with linear, log/log linear, and exponential models, respectively). Both Bazett and Fridericia corrections led not only to substantial errors (error range of 28.3 and 16.9 ms) but also to regular bias with systematically false negative and false positive conclusions dependent on modeled heart rate acceleration and deceleration. Thus, subjects-specific corrections should be used in the intensive and definite studies aimed at providing the final answer on the ability of a drug to prolong the QT interval.

Electrocardiographic identification of drug-induced QT prolongation: assessment by different recording and measurement methods

BACKGROUND

Careful assessment of QT interval prolongation is required before novel drugs are approved by regulatory authorities. The choice of the most appropriate method of electrocardiogram (ECG) acquisition and QT/RR interval measurement in clinical trials requires better understanding of the differences among currently available approaches. This study compared standard and Holter-derived 12-lead ECGs for utility in detecting sotalol-induced QT/QTc and RR changes. Manual methods (digitizing pad and digital on-screen calipers) were compared for precision of QT and RR interval measurement.

METHODS AND RESULTS

Sixteen hundred pairs of serial 12-lead digital ECGs were recorded simultaneously by standard resting ECG device and by continuous 12-lead digital Holter over 3 days in 39 healthy male and female volunteers. No therapy was given on the 1st day followed by 160 mg and 320 mg of sotalol on the 2nd and 3rd day, respectively. Holter-derived and standard ECGs produced nearly identical sotalol-induced QT/QTc and RR changes from baseline, as did the manual digipad and on-screen caliper measurements. The variability of on-screen QT measurement in this study was greater than that of digipad.

CONCLUSIONS

Digital 12-lead Holter and standard 12-lead ECG recorders, as well as the manual digitizing pad and digital on-screen calipers, are of equal utility for the assessment of drug-induced change from baseline in QT and RR interval, although the variability of the on-screen method in this study was greater than of the digipad.

Origins, practices and future of safety pharmacology

The origins of safety pharmacology are grounded upon observations that organ functions (like organ structures) can be toxicological targets in humans exposed to novel therapeutic agents, and that drug effects on organ functions (unlike organ structures) are not readily detected by standard toxicological testing. Safety pharmacology is " em leader those studies that investigate the potential undesirable pharmacodynamic effects of a substance on physiological functions in relationship to exposure in the therapeutic range and above em leader " [International Conference on Harmonization (ICH) S7A guidelines; Safety Pharmacology Studies for Human Pharmaceuticals]. This publication provides a comprehensive review of the history of safety pharmacology, international regulatory guidelines that govern the practices of this important field, and the scientific challenges that are being faced by its rapid emergence in pharmaceutical development. The criticality of identifying undesired adverse effects of new drugs in nonclinical models, which reflect the overall human condition, is reflected in the importance of generating an integrated and accurate assessment of possible human risk. The conundrum posed by the challenge of formulating a reliable risk assessment is the importance of improving and enhancing the safe progression of new drugs to the marketplace, while preventing unnecessary delays (or discontinuances), based on nonclinical findings that are not relevant or interpretable in terms of clinical response or human risk.

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

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

Developing a strategy for the nonclinical assessment of proarrhythmic risk of pharmaceuticals due to prolonged ventricular repolarization

The aspects for developing a strategy for the preclinical testing of drug candidates for proarrhythmic potential are presented. The rationale for such a strategy reflects primarily the needs for efficient and scientifically based drug development and also attempts to anticipate the possible outcomes of the currently ongoing regulatory activity (ICH S7b and E14). Whereas a wealth of new data have emerged over the past few years, demonstrating the utility of test systems for detecting drug effects on myocardial repolarization, the current regulatory trend appears to not use such data for the clinical trial design or risk assessment. Nevertheless, certain types of preclinical tests are highly recommended for optimizing drug development, despite their still questionable regulatory acceptance. This includes (1) testing for blockade of I(Kr) or hERG-mediated potassium current in heterologous cell systems, (2) measurement of effects on the myocardial action potential in vitro; and (3) assessment of effects on the ECG in a well-conducted in vivo study. Due to their requirement for little compound, the first two in vitro tests lend themselves for early safety testing of drug candidates still in the lead optimization phase of drug discovery; together, they form a useful and predictive in vitro assessment. This strategy is not new but reflects what was initially suggested by the Committee for Proprietary Medicinal Products (CPMP) some years ago. However, the validation of such a strategy and its utility in drug development is now well established and recommended, independent from future regulatory requirements.

Drug-induced torsades de pointes and implications for drug development

Torsades de pointes is a potentially lethal arrhythmia that occasionally appears as an adverse effect of pharmacotherapy. Recently developed understanding of the underlying electrophysiology allows better estimation of the drug-induced risks and explains the failures of older approaches through the surface ECG. This article expresses a consensus reached by an independent academic task force on the physiologic understanding of drug-induced repolarization changes, their preclinical and clinical evaluation, and the risk-to-benefit interpretation of drug-induced torsades de pointes. The consensus of the task force includes suggestions on how to evaluate the risk of torsades within drug development programs. Individual sections of the text discuss the techniques and limitations of methods directed at drug-related ion channel phenomena, investigations aimed at action potentials changes, preclinical studies of phenomena seen only in the whole (or nearly whole) heart, and interpretation of human ECGs obtained in clinical studies. The final section of the text discusses drug-induced torsades within the larger evaluation of drug-related risks and benefits.

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

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

Micro-Electrode Arrays in Cardiac Safety Pharmacology

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

Electrophysiological effects of a single intravenous administration of ivabradine (S 16257) in adult patients with normal baseline electrophysiology

INTRODUCTION

Ivabradine is a heart rate-lowering agent that selectively inhibits the pacemaker current, I(f), in the sinoatrial node. The objective of this study was to evaluate the effects of a single intravenous administration of ivabradine on cardiac electrophysiological parameters in patients with normal baseline electrophysiology. The safety profile of ivabradine was also investigated.

STUDY DESIGN

This was an open-label, single-dose, non-controlled study conducted at one centre. Patients received a single dose of ivabradine (0.2 mg/kg) intravenously as a slow bolus over 15 seconds. Electrophysiological investigations, after catheter ablation for cardiac dysrhythmia, were performed at baseline and 30 minutes and 1 hour after drug administration. Electrode catheters were introduced and advanced to the right atrium, the bundle of His and the right ventricular apex of the heart. Electrophysiological parameters assessed included heart rate, QT interval, corrected QT interval (QTc), PR interval, sinoatrial conduction time, sinus node recovery time, and right atrial and ventricle refractory periods. Changes in electrophysiological parameters over time were assessed using one-way analysis of variance. In the case of a significant time effect, the Newman-Keuls procedure was used for comparison.

PATIENTS

A total of 14 patients, 12 male and 2 female, aged 18-75 years were included in the study. The arrhythmia requiring catheter ablation was atrioventricular (AV) excitation in seven patients, paroxysmal supraventricular tachycardia in five patients, atrial fibrillation and flutter in one patient, and cardiac dysrhythmia in one patient. All patients had normal electrophysiology at baseline.

RESULTS

Mean heart rate decreased significantly with ivabradine by 12.9 beats/min at 30 minutes and 14.1 beats/min at 1 hour. The mean QT interval increased but QTc showed no significant change from baseline. The PR and QRS intervals were unchanged. The right atrial and right ventricle refractory periods showed no significant change from baseline. The measured QT interval and the sinus node recovery time were increased. There were no clinically relevant changes in any other major electrophysiological parameters. Ivabradine was well tolerated and no serious adverse events occurred.

CONCLUSION

A single intravenous dose of ivabradine had a significant heart rate-lowering effect, observed at 30 minutes and 1 hour after administration. Ivabradine did not prolong QTc or modify conductivity and refractoriness of the atrium, AV node, His-Purkinje system and ventricles, or repolarisation duration. These results confirm the action of ivabradine as a specific heart rate-lowering agent.

Quetiapine poisoning

STUDY OBJECTIVE

We describe the effects of quetiapine in overdose.

METHODS

Quetiapine poisonings were identified from a prospective database of poisoning admissions to a regional toxicology service. Data extracted included details of ingestion, clinical features, investigations (including ECG), and other outcomes (length of stay and ICU admission rate).

RESULTS

There were 45 cases of quetiapine overdose, of which 18 patients with quetiapine assay results were included. Median length of stay was 35 hours (interquartile range [IQR] 14 to 42 hours) for the 18 patients, and 9 were admitted to the ICU. The median ingested dose was 3.5 g (IQR 1.7 to 6.2 g), and reported ingested dose was highly correlated with estimated peak drug concentration (r(2)=0.84; P<.0001), confirming patient-provided history of ingestion. Seizures occurred in 2 patients, delirium occurred in 3 patients, and mechanical ventilation was required in 4 patients. No arrhythmias or deaths occurred. Six of the 18 patients ingested quetiapine alone, with a median length of stay of 35 hours, and 3 were admitted to the ICU. In 1 patient who ingested 24 g, hypotension and seizures occurred. For 10 patients for whom ECGs were available and who had ingested no cardiotoxic drugs, tachycardia occurred in 8 patients. For these 10 patients, the mean corrected QT (QTc) interval was increased at 487 ms, but the mean uncorrected QT interval was 349 ms. Reported dose and peak quetiapine concentrations were significantly associated with ICU admission and length of stay more than 24 hours. A reported dose less than 3 g and a Glasgow Coma Scale score not less than 15 predicted patients not requiring ICU admission or length of stay more than 24 hours.

CONCLUSION

Quetiapine overdose causes central nervous system depression and sinus tachycardia. In large overdoses, patients may require intubation and ventilation for associated respiratory depression. Although a prolonged QTc occurs, its clinical significance is unclear because it is most likely caused by an overcorrection caused by the tachycardia. In our experience, a reported dose of less than 3 g for patients who are not drowsy (with a Glasgow Coma Scale score of 15) at least 4 hours after ingestion and who did not coingest another toxic agent defined a group not requiring ICU admission or inpatient admission greater than 24 hours.

Teratogenicity by the hERG potassium channel blocking drug almokalant: use of hypoxia marker gives evidence for a hypoxia-related mechanism mediated via embryonic arrhythmia

The rapid component of the delayed rectifying potassium ion current (IKr), plays an important role in cardiac repolarization. In rats, potent IKr channel blocking drugs cause similar stage-specific malformations (such as orofacial clefts and digital reductions) on gestational days (GDs) 10-14 as after periods of embryonic oxygen deprivation (hypoxia). The idea of a hypoxia-related teratogenic mechanism is supported by studies using rat embryos cultured in vitro. These studies show that the embryonic heart reacts with concentration-dependent bradycardia, arrhythmia, and cardiac arrest when exposed to IKr blockers on GDs 10-14. The main purpose of this study was to investigate whether previously shown teratogenic doses on GD 11 and 13 of the selective IKr blocker almokalant (ALM) induce hypoxia in rat embryos in vivo by using the hypoxia marker pimonidazole hydrochloride (PIM). Rats were orally dosed with almokalant or tap water on GD 11 (150 micromol/kg), 13 (50 micromol/kg), or 16 (800 micromol/kg), followed by PIM intravenously 30 min later. Two hours after the PIM dose, the embryonic heart activity was videotaped and analysed, and the embryos were fixed, sectioned, and immunostained. Computer-assisted image analysis showed a two- and threefold increase in hypoxia staining in embryos exposed to teratogenic doses of ALM on GDs 11 and 13. Embryonic arrhythmia was observed in almokalant groups on these GDs, but not in controls. In contrast, dosing on GD 16, with a much higher dose (800 micromol/kg), caused neither hypoxia nor any effects on heart rhythm. The results support the IKr-related arrhythmia-hypoxia hypothesis, by showing that the potent IKr-blocking drug, almokalant, (1) causes severe embryonic hypoxia and arrhythmia at stages (GDs 11 and 13) when developmental toxicity could be induced and IKr is functional and (2) does not cause hypoxia or affect heart rhythm at a developmental stage when IKr is suppressed (GD 16) and potent IKr blockers do not induce developmental toxicity.

Ziprasidone in the management of schizophrenia : the QT interval issue in context

Ziprasidone is a new atypical antipsychotic recently marketed in a number of countries. Its main advantage over other atypical and typical drugs is its low propensity for causing weight gain. However, ziprasidone has been shown to prolong to some extent the cardiac corrected QT (QTc) interval, a property shared by a number of other antipsychotics. Prolongation of the QTc interval is linked to the ventricular tachyarrhythmia torsade de pointes, which is occasionally fatal, although the precise association between QTc changes and risk of sudden cardiac death has not been determined. QTc prolongation is certainly linked in some way to an increased risk of sudden cardiac death, and this may explain the recent, somewhat preliminary, reports of increased risk associated with use of some antipsychotics. Ziprasidone prolongs QTc to a moderate degree, though to a greater extent than quetiapine, risperidone, olanzapine and haloperidol. There is also preliminary evidence that ziprasidone blocks the delayed potassium rectifier channel in cardiac cells. Because of this, and despite the fact that no increased risk of arrhythmia or sudden death has been demonstrated for ziprasidone, some caution is required. Ziprasidone should be avoided in patients with some types of cardiac disease and with uncontrolled electrolyte disturbance. Coprescription of ziprasidone with other drugs that prolong the QT interval should be avoided where possible. When cross-tapering with other antipsychotics, care should be taken to avoid high total load of antipsychotics, and cross-tapering with drugs known to prolong QT interval at normal clinical doses should be avoided. Under most clinical circumstances, however, ziprasidone may be safely used without ECG monitoring or other special precautions. Its effect on QT interval and possible effect on risk of arrhythmia should be balanced with the observation that the drug has a more favourable effect on bodyweight and glucose homeostasis (and so perhaps cardiac risk) than many other antipsychotics.

Bupropion overdose: QTc prolongation and its clinical significance

OBJECTIVE

To investigate the cardiotoxicity of bupropion hydrochloride in deliberate self-poisoning.

METHODS

A prospective study was conducted in a national poisons information center (PIC) of cases of adult deliberate self-poisoning with medical record follow-up of the patients. Fifty-nine cases of bupropion deliberate self-poisoning managed in the hospital, in which the New South Wales PIC was contacted for advice, were evaluated from November 2000 through July 2001. Clinical effects and electrocardiographic (ECG) parameters (QRS, QT, QTc) were the main outcome measures.

RESULTS

ECGs were available for 17 of the 59 patients for analysis, 9 patients (53%) were women, and median patient age was 28 years (interquartile range 22-37). The mean +/- SD ingested bupropion dose was 3.8 +/- 3.1 g. Tachycardia occurred in 13 patients (76%; 95% CI 50 to 93) and hypertension in 8 patients (47%). There were no reports of hypotension or arrhythmias. There was a significantly increased QTc of 461 +/- 34 msec in the patients with bupropion overdose compared with previously developed controls; 13 of the 17 cases had a QTc >440 msec (76%; 95% CI 50 to 93). The uncorrected QT interval did not differ from that of controls.

CONCLUSIONS

A moderately prolonged QTc (>440 msec) is common in bupropion overdose. However, this may not be a result of intrinsic cardiac toxicity, but overcorrection of the QTc due to the tachycardia that occurs. It is important that the QTc is interpreted with caution in overdoses of agents that cause significant tachycardia (>100 beats/min).

Clinical relevance and management of drug-related QT interval prolongation

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

Phenytoin and phenobarbital inhibit human HERG potassium channels

Drugs that inhibit the cardiac rapid delayed rectifier potassium ion current (IKr) channel can be proarrhythmic and their clinical use has been associated with sudden unexpected death (SUD). Since SUD is about 20 times more common among people with epilepsy than in the general population, and some data indicate that drug treatment may contribute, we tested the hypothesis that the classic antiepileptic drugs phenytoin (PHT), carbamazepine (CBZ), and phenobarbital (PB) have a potential to block IKr. The whole cell patch-clamp recording technique was used to study the effects on IKr channels expressed by the human ether-a-go-go related gene (HERG) stably expressed in Human Embryo Kidney (HEK) 293 cells. Tail currents, which are purely related to HERG, were blocked with an IC50 (the concentration when 50% inhibition was obtained compared to control values) of 240 microM for PHT and 3 mM for PB. A 20% inhibition of tail currents was obtained at CBZ concentrations of 250 and 500 microM. Collective data show that drugs with the same margins (ratio HERG IC50/unbound therapeutic concentration), as PHT and PB, may have arrhythmogenic potential, especially when used in predisposed patients and in the case of drug-drug interactions. SUD in epilepsy is generally a seizure-related phenomenon. However, our data suggest that PHT and PB may play a contributing role, perhaps by making some patients more vulnerable to the cardiovascular depression induced by seizures.

Antipsychotics and QT prolongation

OBJECTIVE

To evaluate literature relating to cardiac QT prolongation and the use of antipsychotic drugs.

METHOD

Literature searches of EMBASE, Medline, PsychLIT were performed in December 2001 and reference sections of retrieved papers scrutinized for further relevant reports.

RESULTS

The Cardiac QTc interval is difficult to measure precisely or accurately but appears to be a useful predictor of risk of dysrhythmia (specifically torsade de pointes) and sudden death. It is less clear that drug-induced QTc prolongation gives rise to similar risks but data are emerging, linking antipsychotic use to increased cardiac mortality. Many antipsychotics have been clearly associated with QTc prolongation. Methodological considerations arguably preclude assuming that any antipsychotic is free of the risk of QTc prolongation and dysrhythmia.

CONCLUSION

Available data do not allow assessment of relative or absolute risk of dysrhythmia or sudden death engendered by antipsychotics but caution is advised. Risk of dysrhythmia can very probably be reduced by careful prescribing of antipsychotics in low doses in simple drug regimens which avoid metabolic interactions. Electrocardiographic monitoring may also help to reduce risk but review by specialist cardiologist may be necessary.

Prolongation of the QT interval and cardiac arrhythmias associated with cisapride: limitations of the pharmacoepidemiological studies conducted and proposals for the future

Not all hazards can be identified from clinical studies prior to marketing of medicinal products. Pre-marketing large-scale trials for cisapride did not report any serious cardiac arrhythmias. After a long period of availability in several countries it was withdrawn in 2000 because of reports of serious, and in many cases fatal, cardiac events. Whilst spontaneous reporting systems for adverse drug reactions (ADRs) have limitations such as under-reporting, they are an effective system for signal generation, particularly of rare ADRs. Pharmacoepidemiological studies aim to identify and calculate the incidence of adverse reactions, with increased sensitivity to less common ADRs compared to randomised controlled trials, yet cohort sizes may be insufficient to detect very rare ADRs such as drug-induced Torsade de Pointes, with an estimated incidence of the order of 1 per 12,000 to 1 per 120,000 patients. Several pharmacoepidemiological studies investigated adverse events associated with cisapride, one of which specifically examined the association between serious cardiac arrhythmias and cisapride. These observational studies were conducted using large population databases, but each failed to identify sufficient cases to establish a causal relationship. Explanations include that the cohort sample sizes were too small, and either under-, or mis-reporting of events of interest may have occurred. To estimate the risk of very rare adverse events, pharmacoepidemiological studies require very large numbers. Furthermore, the events in question need to be clinically recognisable by doctors and adequately documented in patients' notes, computer records, or on study questionnaires. The establishment of a national registry for drug-induced QT prolongation to identify cases and correlate clinical information may help to better identify these rare ADRs earlier. Such proactive surveillance could avoid unnecessary delays for other drugs where QT prolongation and serious cardiac arrhythmias may be an issue.

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

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

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

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

Non-antiarrhythmic drugs prolonging the QT interval: considerable use in seven countries

AIMS

Many drugs belonging to different therapeutic classes appear to share a potentially fatal side-effect: ventricular tachyarrhythmias associated with QT prolongation. The aim of this study was to assess the relevance and the magnitude of the problem in seven countries by grouping all nonantiarrhythmic drugs according to the type of evidence on QT prolongation and analysing their sales data.

METHODS

We divided all nonantiarrhythmic QT-prolonging agents into the following categories (in increasing order of clinical relevance): group A, drugs with published clinical or preclinical evidence on QT prolongation or with relevant official warnings; group B, drugs with published clinical or preclinical evidence; group C, drugs with published clinical evidence; group D, drug with published clinical evidence on torsades de pointes or ventricular arrhythmias associated with QT prolongation; group E, drugs belonging to group D with official warnings. We retrieved 1998 sales data from community pharmacies in seven countries (Australia, Denmark, England, Germany, Greece, Italy and Sweden). Data for individual agents were expressed as defined daily doses per 1000 inhabitants per day (DDD/1000/day). Overall use in each country was calculated for each drug group. Groups D and E were considered as the most clinically relevant.

RESULTS

Among the 102 nonantiarrhythmic agents meeting at least one of the inclusion criteria, 33 drugs had sales data > or =1 DDD/1000/day and 71 drugs had a use > or =0.1 DDD/1000/day in at least one country. Among the 37 nonantiarrhythmic agents with published reports of ventricular arrhythmias associated with QT prolongation, 12 compounds had sales data > or =1 DDD/1000/day. Total consumption in each country ranged: from 51.9 to 94.7 DDD/1000/day for group A; from 51.6 to 92.7 DDD/1000/day for group B; from 37.1 to 76.6 DDD/1000/day for group C; from 12.9 to 29.1 DDD/1000/day for group D; and from 5.8 to 15.3 DDD/1000/day for group E.

CONCLUSIONS

In spite of wide variations in the sales of individual agents, the overall extent of use of nonantiarrhythmic QT-prolonging drugs was of the same order of magnitude in all countries. The significant use of drugs belonging to categories D and E should prompt careful risk/benefit assessment of each agent.

Serum quinidine concentrations and effect on QT dispersion and interval

OBJECTIVE

To establish a relationship between serum quinidine concentrations (SQCs) and QT interval dispersion, compared with corresponding QT intervals, in order to identify a reason why many reports describe torsade de pointes as occurring at subtherapeutic concentrations.

DESIGN

Retrospective study.

SETTING

University teaching hospital.

PARTICIPANTS

Eleven patients with atrial arrhythmias managed with quinidine therapy.

MAIN OUTCOME MEASURES

Patients with subtherapeutic (<2 microg/mL) and therapeutic (2-5 microg/mL) SQCs with corresponding 12-lead electrocardiograms (ECGs) (25 mm/sec) and baseline ECG were evaluated for QT interval dispersion, calculated as the maximum minus the minimum QT interval on the 12-lead ECG.

RESULTS

Mean +/- SD subtherapeutic and therapeutic SQCs were 1.48 +/- 0.39 microg/mL and 3.78 +/- 0.88 microg/mL (p < 0.001). Baseline values for QT/QTc intervals were 376.4 +/- 59.2/429.5 +/- 57.3 msec. At subtherapeutic and therapeutic SQCs, mean QT/QTc intervals were 403.6 +/- 59.9/450.5 +/- 38.5 msec and 439.1 +/- 48.9/472.4 +/- 44.6 msec, respectively. Mean QT dispersion was 47 +/- 16.2 msec at baseline, 98.2 +/- 27.5 msec at subtherapeutic SQC, and 70.9 +/- 33.9 msec at therapeutic SQCs (p = 0.001 for overall analysis; p < 0.001 for baseline vs. subtherapeutic concentrations; p = NS for therapeutic vs. subtherapeutic in post hoc comparison).

CONCLUSIONS

Despite QT interval lengthening with increasing SQCs, QT dispersion was numerically greatest at subtherapeutic SQCs. Further study is required to determine the value of QT dispersion as a tool for identifying proarrhythmic risk with drugs that prolong the QT interval.

How to correct the QT interval for the effects of heart rate in clinical studies

Much inter- and intra-subject variability in the QT interval in health and disease is accounted for by differences in heart rate, leading to difficulties when determining the effects of disease and drugs on the QT interval. Traditionally, heart rate correction formulae have been used to overcome this problem in man. However, the commonly used Bazett's heart rate correction formulae (QT=QT(C) radical RR interval) does not remove the effect of heart rate; indeed, it overcorrects at high heart rates. Fredericia's formula (QT=QT(C)x(3) radical RR interval) does remove the effects of heart rate; this is the preferable formula, if one is to be used. However, all formulae make assumptions about the nature of the QT-heart rate relationship, assumptions that may not apply to those with disease or on drugs. A more intellectually rigorous approach to QT interval-heart rate correction is to determine the QT-heart relationship for each individual, using data obtained from exercise tests or 24-h Holter tapes. The best mathematical relationship (linear, exponential, etc.) is obtained from analysis of this data, and is used to determine the QT interval at a heart rate of 60 bpm, the QT(60). The QT(60) measure makes no assumptions about the nature of the QT interval-heart rate relationship, removes the dependence of QT interval on heart rate, and maintains genuine biological differences in the QT interval. It should become the standard in QT interval-heart rate correction.

QT-RR relationship in healthy subjects exhibits substantial intersubject variability and high intrasubject stability

Recently, it was demonstrated that the QT-RR relationship pattern varies significantly among healthy individuals. We compared the intra- and interindividual variations of the QT-RR relationship. Twenty-four-hour 12-lead digital electrocardiograms (ECGs; SEER MC, GE Marquette; 10-s ECG recorded every 30 s) were obtained at baseline and after 24 h, 1 wk, and 1 mo in 75 healthy subjects (42 women, 33 men, age 27.9 +/- 9.6 vs. 26.8 +/- 7.5 yr, P = not significant). QT interval was measured automatically in each ECG by six different algorithms, and the mean of the six measurements was analyzed. In each recording of each individual, QT-RR relationship was assessed by 10 different regression models including linear (QT = beta + alpha x RR) and parabolic (QT = beta x RR(alpha)) models. Standard deviations (SDs) of regression parameters alpha and beta of consecutive recordings of each individual were compared with SD of the individual means. Intrasubject stability and interindividual variability were further tested by ANOVA. With all models, intraindividual SDs of the regression parameters were highly significantly smaller than SD of individual means (P < 10(-5)-10(-9)). The intrasubject stability was further confirmed by ANOVA (P < 10(-19)-10(-30)). The QT-RR relationship exhibits substantial intersubject variability as well as a high intrasubject stability. This has practical implications for a precise estimation of the heart rate-corrected QT interval in which optimized subject-specific rate correction formulas should be used.

Risperidone prolongs cardiac action potential through reduction of K+ currents in rabbit myocytes

Prolongation of QT interval by antipsychotic drugs is an unwanted side effect that may lead to ventricular arrhythmias. The antipsychotic agent risperidone has been shown to cause QT prolongation, especially in case of overdosage. We investigated risperidone effects on action potentials recorded from rabbit Purkinje fibers and ventricular myocardium and on potassium currents recorded from atrial and ventricular rabbit isolated myocytes. The results showed that (1) risperidone (0.1-3 microM) exerted potent lengthening effects on action potential duration in both tissues with higher potency in Purkinje fibers and caused the development of early afterdepolarizations at low stimulation rate; (2) risperidone (0.03-0.3 microM) reduced significantly the current density of the delayed rectifier current and at 30 microM decreased the transient outward and the inward rectifier currents. This study might explain QT prolongation observed in some patients treated with risperidone and gives enlightenment on the risk of cardiac adverse events.

Sertindole: cardiac electrophysiological profile

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

Safety and tolerability: how do newer generation "atypical" antipsychotics compare?

Previously, clinicians worked with antipsychotic drugs that almost invariably caused extrapyramidal side effects (EPS) at the dose at which they were clinically effective. By definition, all newer generation atypical antipsychotic agents are significantly better than conventional agents with regard to EPS; i.e., they are clinically effective at doses at which they do not cause EPS. This EPS advantage of atypical antipsychotics translates into several important clinical benefits, including better negative symptom efficacy, lesser dysphoria, less impaired cognition, and a lower risk of tardive dyskinesia; in fact, this "EPS advantage" is the principal basis of the many clinical advantages provided by the class of atypical antipsychotics. While all atypical agents share this "EPS advantage," there are important differences between these agents with regard to the ease and consistency with which this EPS advantage can be realized. Pharmacologically, different atypical antipsychotics differ; these differences translate into differences in their side effect profiles. Five atypical antipsychotics are currently available: clozapine, risperidone, olanzapine, quetiapine, and ziprasidone. Meaningful differences between these agents with regard to weight gain, sedation, anticholinergic side effects, cardiovascular issues, endocrine side effects, hepatic and sexual issues, will be considered and their clinical implications discussed.