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

Individualized QT interval (QTi) is a powerful diagnostic tool in long QT syndrome: results from a large validation study

Aims

Diagnosis of Long QT syndrome (LQTS) is based on prolongation of the QT interval corrected for heart rate (QTc) on surface ECG and genotyping. However, up to 25% of genotype positive patients have a normal QTc interval. We recently showed that individualized QT interval (QTi) derived from 24 h holter data and defined as the QT value at the intersection of an RR interval of 1,000 ms with the linear regression line fitted through QT-RR data points of each individual patient was superior over QTc to predict mutation status in LQTS families. This study aimed to confirm the diagnostic value of QTi, fine-tune its cut-off value and evaluate intra-individual variability in patients with LQTS.

Methods

From the Telemetric and Holter ECG Warehouse, 201 recordings from control individuals and 393 recordings from 254 LQTS patients were analysed. Cut-off values were obtained from ROC curves and validated against an in house LQTS and control cohort.

Results

ROC curves indicated very good discrimination between controls and LQTS patients with QTi, both in females (AUC 0.96) and males (AUC 0.97). Using a gender dependent cut-off of 445 ms in females and 430 ms in males, a sensitivity of 88% and specificity of 96% were achieved, which was confirmed in the validation cohort. No significant intra-individual variability in QTi was observed in 76 LQTS patients for whom at least two holter recordings were available (483 ± 36 ms vs. 489 ± 42 ms, p = 0.11).

Conclusions

This study confirms our initial findings and supports the use of QTi in the evaluation of LQTS families. Using the novel gender dependent cut-off values, a high diagnostic accuracy was achieved.

Triple Combination with Direct Acting Antivirals in the Treatment of Hepatitis C Does not Prolong the QT Interval

AIMS

Antiviral drugs are considered as potentially cardiotoxic, due to prolongation of QT interval which may affect incidence of severe ventricular arrhythmias. The main aim of this retrospective study was to assess the influence of treatment by three antiviral drugs on QT interval and to find patients who are at an increased risk of developing malignant ventricular arrhythmias.

METHODS

The study included 23 patients (14 men, 9 women) who were treated with a combination of interferon alpha, ribavirin, and an NS3/4A protease inhibitor. The parameters from the 12 leads electrocardiograms were evaluated before treatment, and then 3 ± 1 and 6 ± 1 months after treatment.

RESULTS

Heart rate (HR) 69 ± 12 / min and corrected QT interval (QTc) 412 ± 35 ms were obtained before the treatment and there was not observed a significant prolongation of intervals after 3 months (HR 72 ± 11 / min, QTc 412 ± 33 ms) and after 6 months (HR 64 ± 12 / min, QTc 405 ± 28 ms) respectively. In total QTc interval was prolonged from the baseline in 53% and in 43% of the patients 3 months respectively 6 months after treatment. A QTc prolongation over of 450 ms and new treatment-related repolarization change was noted in 1 (4%) patient.

CONCLUSION

The study demonstrates that a combination therapy of 3 antiviral drugs does not significantly prolong the QTc interval and does not cause severe pathological changes on the ECG. Patients undergoing this treatment are not at risk of developing heart disease as an undesirable side effect.

Improving corrected QT; Why individual correction is not enough

The use of QT-prolongation as a biomarker for arrhythmia risk requires that researchers correct the QT-interval (QT) to control for the influence of heart rate (HR). QT correction methods can vary but most used are the universal correction methods, such as Bazett's or Van de Water's, which use a single correction formula to correct QT-intervals in all the subjects of a study. Such methods fail to account for differences in the QT/HR relationship between subjects or over time, instead relying on the assumption that this relationship is consistent. To address these changes in rate relationships, we test the effectiveness of linear and non-linear individual correction methods. We hypothesize that individual correction methods that account for additional influences on the rate relationship will result in more effective and consistent correction. To increase the scope of this study we use bootstrap sampling on ECG recordings from non-human primates and beagle canines dosed with vehicle control. We then compare linear and non-linear individual correction methods through their ability to reduce HR correlation and standard deviation of corrected QT values. From these results, we conclude that individual correction methods based on post-treatment data are most effective with the linear methods being the best option for most cases in both primates and canines. We also conclude that the non-linear methods are more effective in canines than primates and that accounting for light status can improve correction while examining the data from the light periods separately. Individual correction requires careful consideration of inter-subject and intra-subject variabilities.

Influence of heart rate correction formulas on QTc interval stability

Monitoring of QTc interval is mandated in different clinical conditions. Nevertheless, intra-subject variability of QTc intervals reduces the clinical utility of QTc monitoring strategies. Since this variability is partly related to QT heart rate correction, 10 different heart rate corrections (Bazett, Fridericia, Dmitrienko, Framingham, Schlamowitz, Hodges, Ashman, Rautaharju, Sarma, and Rabkin) were applied to 452,440 ECG measurements made in 539 healthy volunteers (259 females, mean age 33.3 ± 8.4 years). For each correction formula, the short term (5-min time-points) and long-term (day-time hours) variability of rate corrected QT values (QTc) was investigated together with the comparisons of the QTc values with individually corrected QTcI values obtained by subject-specific modelling of the QT/RR relationship and hysteresis. The results showed that (a) both in terms of short-term and long-term QTc variability, Bazett correction led to QTc values that were more variable than the results of other corrections (p < 0.00001 for all), (b) the QTc variability by Fridericia and Framingham corrections were not systematically different from each other but were lower than the results of other corrections (p-value between 0.033 and < 0.00001), and (c) on average, Bazett QTc values departed from QTcI intervals more than the QTc values of other corrections. The study concludes that (a) previous suggestions that Bazett correction should no longer be used in clinical practice are fully justified, (b) replacing Bazett correction with Fridericia and/or Framingham corrections would improve clinical QTc monitoring, (c) heart rate stability is needed for valid QTc assessment, and (d) development of further QTc corrections for day-to-day use is not warranted.

Implications of Individual QT/RR Profiles-Part 1: Inaccuracies and Problems of Population-Specific QT/Heart Rate Corrections

Introduction

Universal QT correction formulas are potentially problematic in corrected QT (QTc) interval comparisons at different heart rates. Instead of individual-specific corrections, population-specific corrections are occasionally used based on QT/RR data pooled from all study subjects.

Objective

To investigate the performance of individual-specific and population-specific corrections, a statistical modeling study was performed using QT/RR data of 523 healthy subjects.

Methods

In each subject, full drug-free QT/RR profiles were available, characterized using non-linear regression models. In each subject, 50 baseline QT/RR readings represented baseline data of standard QT studies. Using these data, linear and log-linear heart rate corrections were optimized for each subject and for different groups of ten and 50 subjects. These corrections were applied in random combinations of heart rate changes between − 10 and + 25 beats per minute (bpm) and known QTc interval changes between − 25 and + 25 ms.

Results

Both the subject-specific and population-specific corrections based on the 50 baseline QT/RR readings tended to underestimate/overestimate the QTc interval changes when heart rate was increasing/decreasing, respectively. The result spread was much wider with population-specific corrections, making the estimates of QTc interval changes practically unpredictable.

Conclusion

Subject-specific heart rate corrections based on limited baseline drug-free data may lead to inconsistent results and, in the presence of underlying heart rate changes, may potentially underestimate or overestimate QTc interval changes. The population-specific corrections lead to results that are much more influenced by the combination of individual QT/RR patterns than by the actual QTc interval changes. Subject-specific heart rate corrections based on full profiles derived from drug-free baseline recordings with wide QT/RR distribution should be used when studying drugs expected to cause heart rate changes.

Electronic supplementary material

The online version of this article (10.1007/s40264-018-0736-1) contains supplementary material, which is available to authorized users.

Assessment of the effect of erdafitinib on cardiac safety: analysis of ECGs and exposure-QTc in patients with advanced or refractory solid tumors

PURPOSE

To characterize the effect of erdafitinib on electrocardiogram (ECG) parameters and the relationship between erdafitinib plasma concentrations and QTc interval changes in patients with advanced or refractory solid tumors.

METHODS

Triplicate ECGs and continuous 12-lead Holter data were collected in the dose escalation part (Part 1) of the first-in-human study, with doses ranging from 0.5 to 12 mg. Triplicate ECG monitoring continued in Parts 2-4 where 2 dose regimens selected from Part 1 were expanded in prespecified tumor types. Analyses of ECG data included central tendency analyses, identification of categorical outliers and morphological assessment. A concentration-QTc analysis was conducted using a linear mixed-effect model based on extracted time matching Holter data.

RESULTS

Central tendency, categorical outlier, and ECG morphologic analyses from 187 patients revealed no clinically significant effect of erdafitinib on heart rate, atrioventricular conduction or cardiac depolarization (PR and QRS), and no effect on cardiac repolarization (QTc). Concentration-QTc analysis from 62 patients indicated that the slopes of relationship between total and free erdafitinib plasma concentrations and QTcI (mean exponent of 0.395) were estimated as - 0.00269 ms/(ng/mL) and - 1.138 ms/(ng/mL), respectively. The predicted change in QTcI at the observed geometric mean of total and free concentration at the highest therapeutic erdafitinib dose (9 mg daily) was < 10 ms at the upper bound of the two-sided 90% confidence interval.

CONCLUSIONS

ECG data and the concentration-QTc relationships demonstrate that erdafitinib does not prolong QTc interval and has no effects on cardiac repolarization or other ECG parameters. Clinical trial registration numbers NCT01703481, EudraCT: 2012-000697-34.

Comparison of Electrocardiographic Biomarkers for Differentiating Drug-Induced Single vs. Multiple Cardiac Ion Channel Block

Since introduction of the International Conference on Harmonization proarrhythmia guidelines in 2005, no new marketed drugs have been associated with unacceptable risk of Torsade de Pointes. Although cardiac safety improved, these guidelines had the unintended consequence of eliminating potentially beneficial drugs from pipelines early in development. More recently, it has been shown that a corrected QT (QTc) prolonging drug may be safe if it impacts multiple ion channels vs. only human ether‐a‐go‐go related gene (hERG) and that this effect can be discriminated using QT subintervals. We compared the predictive power of four electrocardiogram (ECG) repolarization metrics to discriminate single vs. multichannel block: (i) traditional 10‐second signal averaged triplicates, and (ii) three metrics that used increasing density of automatically measured beat‐to‐beat (btb) intervals. Predictive power was evaluated using logistic regression and quantified with receiver operating characteristic (ROC) area under the curve (AUC). Compared with the traditional 10‐second signal averaged triplicates, the reduction in classification error ranged from 2−6 with increasing density of btb measurements.

Prevalence and prognostic significance of long QT interval among patients with chest pain: selecting an optimum QT rate correction formula

BACKGROUND

Little is known about the prevalence and prognostic significance of long QT interval among patients with chest pain during the acute phase of suspected cardiovascular injury.

OBJECTIVES

Our aim was to investigate the prevalence and prognostic significance of long QT interval among patients presenting to the emergency department (ED) with chest pain using an optimum QT rate correction formula.

METHODS

We performed secondary analysis on data obtained from the IMMEDIATE AIM trial (N, 145). Data included 24-hour 12-lead Holter electrocardiographic recordings that were stored for offline computer analysis. The QT interval was measured automatically and rate corrected using seven QTc formulas including subject specific correction. The formula with the closer to zero absolute mean QTc/RR correlation was considered the most accurate.

RESULTS

Linear and logarithmic subject specific QT rate correction outperformed other QTc formulas and resulted in the closest to zero absolute mean QTc/RR correlations (mean±SD: 0.003±0.002 and 0.017±0.016, respectively). These two formulas produced adequate correction in 100% of study participants. Other formulas (Bazett's, Fridericia's, Framingham's, and study specific) resulted in inadequate correction in 47.6 to 95.2% of study participants. Using the optimum QTc formula, linear subject specific, the prevalence of long QTc interval was 14.5%. The QTc interval did not predict mortality or hospital admission at short and long term follow-up. Only the QT/RR slope predicted mortality at 7year follow-up (odds ratio, 2.01; 95% CI, 1.02-3.96; p<0.05).

CONCLUSIONS

Adequate QT rate correction can only be performed using subject specific correction. Long QT interval is not uncommon among patients presenting to the ED with chest pain.

Validation of QT Interval Correction Methods When a Drug Changes Heart Rate

The QT interval is correlated with heart rate; therefore, the QT interval is usually corrected by heart rate when drug-induced QT effect is studied. Currently, there are many correction methods that use either fixed or data-driven approaches. The effectiveness of correction methods depends on many factors and varies from study to study. Statistical validation and comparisons need to be performed to determine the most appropriate correction method for each study. We examined different validation methods and explored a new approach to use when the testing drug changes heart rate.

Proarrhythmic safety of repeat doses of mirabegron in healthy subjects: a randomized, double-blind, placebo-, and active-controlled thorough QT study

Potential effects of the selective β(3)-adrenoceptor agonist mirabegron on cardiac repolarization were studied in healthy subjects. The four-arm, parallel, two-way crossover study was double-blind and placebo- and active (moxifloxacin)-controlled. After 2 baseline ECG days, subjects were randomized to one of eight treatment sequences (22 females and 22 males per sequence) of placebo crossed over with once-daily (10 days) 50, 100, or 200 mg mirabegron or a single 400-mg moxifloxacin dose on day 10. In each period, continuous ECGs were recorded at two baselines and on the last drug administration day. The lower one-sided 95% confidence interval for moxifloxacin effect on QTcI was >5 ms, demonstrating assay sensitivity. According to ICH E14 criteria, mirabegron did not cause QTcI prolongation at the 50-mg therapeutic and 100-mg supratherapeutic doses in either sex. Mirabegron prolonged QTcI interval at the 200-mg supratherapeutic dose (upper one-sided 95% CI >10 ms) in females, but not in males.

The DPP-4 inhibitor linagliptin does not prolong the QT interval at therapeutic and supratherapeutic doses

AIM

To evaluate the potential effects of therapeutic and supratherapeutic doses of linagliptin (BI 1356) on the QT/QT(c) interval in healthy subjects.

METHODS

The study was a randomized, double-blind, placebo-controlled, four-period crossover study using single oral doses of linagliptin (5 mg and 100 mg), moxifloxacin (400 mg) and placebo. Electrocardiogram (ECG) profiles using triplicates of 12-lead 10-s ECGs were digitally recorded pre-dose and after drug administration. The mean change from baseline (MCfB) of the individually heart rate corrected QT interval (QT(c) I) between 1 and 4 h postdrug administration was the primary end point. Blood samples to measure plasma concentrations of linagliptin and its main metabolite were also obtained.

RESULTS

Forty-four Caucasian subjects (26 male) entered the study and 43 subjects completed the study as planned in the protocol. Linagliptin was not associated with an increase in the baseline-adjusted mean QT(c) I, at any time point. The placebo-corrected MCfB of QT(c) I was -1.1 (90% CI -2.7, 0.5) ms and -2.5 (-4.1, -0.9) ms for linagliptin 5 mg and 100 mg, respectively, thus within the non-inferiority margin of 10 ms according to ICH E14. Linagliptin was well tolerated; the assessment of ECGs and other safety parameters gave no clinically relevant findings at either dose tested. Maximum plasma concentrations after administration of 100-mg linagliptin were ∼24-fold higher than those observed previously for chronic treatment with the therapeutic 5-mg dose. Assay sensitivity was confirmed by a placebo-corrected MCfB of QT(c) I with moxifloxacin of 6.9 (90% CI 5.4, 8.5) ms.

CONCLUSIONS

Therapeutic and significantly supratherapeutic exposure to linagliptin is not associated with QT interval prolongation.

Pharmacokinetic-pharmacodynamic modelling of the effect of Moxifloxacin on QTc prolongation in telemetered cynomolgus monkeys

INTRODUCTION

Delayed ventricular repolarisation is manifested electrocardiographically in a prolongation of the QT interval. Such prolongation can lead to potentially fatal Torsades de Pointes. Moxifloxacin is a fluoroquinolone antibiotic which has been associated with QT prolongation and, as a result, is recommended by the regulatory authorities as a positive control in thorough QT studies performed to evaluate the potential of new chemical entities to induce QT prolongation in humans. The sensitivity of the cynomolgus monkey as a quantitative preclinical predictor of the PK-QTc relationship is discussed.

METHODS

Cardiovascular monitoring was performed in the telemetered cynomolgus monkey for 22 h following oral administration of Moxifloxacin (10, 30 and 90 mg/kg) or placebo. QTc was derived using an individual animal correction factor (ICAF): RR-I = QT-I--(RR-550)* (IACF). A PKPD analysis was performed to quantify the increase in placebo-adjusted QTc) elicited by administration of Moxifloxacin. In addition, the rate of onset of hERG channel blockade of Moxifloxacin was compared to Dofetilide by whole cell patch clamp technique in HEK-293 cells stably expressing the hERG channels.

RESULTS

Moxifloxacin induced a dose dependent increase in QTc). A maximum increase of 28 ms was observed following administration of 90 mg/kg Moxifloxacin. The corresponding maximum free systemic exposure was 18μM. Interrogation of the PK-QTc relationship indicated a direct relationship between the systemic exposure of Moxifloxacin and increased QTc. A linear PKPD model was found to describe this relationship whereby a 1.5 ms increase in QTc was observed for every 1 μM increase in free systemic exposure.

DISCUSSION

The exposure dependent increases in QTc observed following oral administration of Moxifloxacin to the cynomolgus monkey are in close agreement with those previously reported in human subjects. A direct effect linear relationship was found to be conserved in both species. As a result of the quantitative agreement in both species, the utility of the telemetered cynomolgus monkey as a preclinical predictor of QTc) prolongation is exemplified. Furthermore, the rate of onset of hERG channel blockade observed in patch clamp offers a mechanistic insight into the relative rates of channel blockade observed in vivo with both Moxifloxacin and Dofetilide.

Statistical models for heart rate correction of the QT interval

The analysis of QT interval data is now an essential part of the assessment of drug safety. As the QT interval is inversely associated with heart rate, an appropriate correction must be applied in order to evaluate QT data in clinical trials. The aim is to characterize changes in QT interval at a standard heart rate, taking into account the correlation between these two variables to adjust for heart rate changes during the course of the trial. It has been shown that the relationship between the RR interval (=1/heart rate) and the QT interval is highly variable between individuals but stable over time within each individual.Many mathematical models have been developed to describe the QT-RR relationship. However, there has been less emphasis on the derivation of suitable statistical models that account for the multilevel structure of the ECG data.An important example is the interpretation of the so-called population-specific heart rate corrections, which are based on data pooled from different subjects. Often, simple regression techniques are used to quantify the population correction, disregarding the subject level and leading to biased parameter estimates. Instead, population-based corrections that account for individual intercepts should be used, in order to distinguish within-subject-effects from between-subject effects. Therefore, population-specific corrections cannot be derived solely from the cross-sectional data. The impact of the different statistical models is illustrated by data from the baseline periods of six clinical QT studies.

Correction for QT/RR hysteresis in the assessment of drug-induced QTc changes--cardiac safety of gadobutrol

BACKGROUND

The so-called thorough QT/QTc (TQT) studies required for every new pharmaceutical compound are negative if upper single-sided 95% confidence interval (CI) of placebo and baseline corrected QTc prolongation is <10 ms. This tight requirement has many methodological implications. If the investigated drug has a fast action and ECGs cannot be obtained at stable heart rates, QT/RR hysteresis correction is needed.

METHODS

This was used in a TQT study of gadobutrol. The TQT study was a randomized double-blind five-times crossover study of three doses of gadobutrol (0.1, 0.3, and 0.5 mmol/kg) that was placebo and positive effect controlled (moxifloxacin 400 mg). The study enrolled 50 healthy subjects with data of all periods. QT/RR hysteresis was assessed from prestudy exercise test ECGs. Among others, comparisons were made between population heart rate correction without hysteresis considerations and combined population heart rate and hysteresis correction.

RESULTS

The highest heart rate increase (placebo and baseline controlled) of 13.1 beats per minute (90% CI 9.9-16.4) occurred 1 minute after the administration of the highest dose of gadobutrol. Without hysteresis consideration, the highest DeltaDeltaQTc were 9.91 ms (90% CI 8.01-11.81) while with hysteresis correction, these values were 7.62 ms (90% CI 6.37-8.87), thus turning a marginally positive TQT study into a negative finding.

CONCLUSION

Hence, omitting hysteresis correction from episodes of fast heart rate changes may lead to incorrect conclusions. Despite substantial rate acceleration, accurate hysteresis correction confirms that gadobutrol does not have any effects on cardiac repolarization that would be within the limits of regulatory relevance.

The effect of different stressors on the QT interval and the T wave

Prolonged QT interval is associated with the generation of life-threatening arrhythmias and sudden death. However, neither the relation between QT duration and heart rate, nor the association between mental stress and QT time has been clarified. Aim: The relationship between QT duration and smoking, cardiovascular reactivity, and mental stress as well as newer methods of QT correction were studied. Methods: In six laboratory experiments 166 volunteers were studied. Smoking, treadmill exercise, mental arithmetic and videogame were applied as stressors. Besides fixed formulae, study and subject-specific QT correction methods were also used. Results: 1. Bazett formula is not appropriate to compare QT durations. 2. Acute smoking has no effect on QT time. 3. QT changes are related to cardiovascular reactivity. 4. Mental stress may induce QT prolongation. 5. Bifid T waves often develop during mental and isometric stress. Conclusions: New methods for QT correction are more reliable than preformed formulae. QT prolongation and bifid T waves may be the links between mental stress and life threatening arrhythmias.

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.

Bias and variance evaluation of QT interval correction methods

There is an increasing regulatory emphasis on assessing drug-induced QT interval prolongation. Since QT interval is correlated with heart rate (HR), assessment of drug-induced QT interval prolongation should be made at a standardized HR, resulting in the need to correct QT interval (QTc) for HR. This study investigates the statistical properties of QTc intervals using individual based correction (IBC), population based correction (PBC), or fixed correction (FC) methods under both the linear and log-linear regression models for the QT-RR relationship where RR is the time elapsing between two consecutive heart beats (inversely related to HR through RR = 60/HR). This study shows that QTc intervals using PBC and FC methods are conditionally biased. The QTc interval using the IBC method is conditionally unbiased under the linear regression model, but is conditionally biased under the log-linear regression model. It also shows that under both the linear and log-linear regression models, the conditional variances of the QTc intervals using the three correction methods satisfy the order FC < or = PBC < or = IBC. Suggestions for analyzing QT intervals based on these findings are discussed.

Isolated perfused and paced guinea pig heart to test for drug-induced changes of the QT interval

INTRODUCTION

One of the biomarkers for assessing the risk of a cardiac adverse event is drug-induced prolongation of the QT interval. A model is needed for evaluating the potential liability of test compounds on QT interval in vitro. Since QT intervals can be generated from paced or spontaneously beating hearts, data so generated can also be used for validating QT(c) correction equations.

METHODS

Isolated guinea pig hearts were perfused in Locke's solution according to the Langendorff method. QT intervals were routinely measured from Lead II ECG waveforms.

RESULTS

Compounds known to inhibit HERG channel, such as dofetilide, prolonged the QT interval in this model. (+/-)Bay K8644, a calcium channel activator, prolonged the QT interval, while verapamil, a calcium channel blocker, shortened it. Procainamide, a sodium channel blocker, also prolonged the QT interval. Many of the compounds, which prolonged the QT interval, also prolonged PR interval, suggesting dual inhibition of the Ikr channel, the rapid component of delayed rectifier potassium channel, and the calcium channel. The QT/RR intervals exhibited a curvilinear relationship, which could be corrected into nearly straight horizontal lines by using correction equations derived from linear, parabolic, and hyperbolic models. However, these correction equations yielded different results on the QT prolongation produced by sotalol, which also slowed down the heart rate. With the data set obtained in this investigation, correction equations derived from linear and parabolic models worked better than the equations derived from the hyperbolic model. The exponential model did not fit at all.

CONCLUSION

QT intervals obtained under paced conditions provide the most direct and reliable QT information for a drug. The isolated perfused and paced guinea pig heart is a convenient model for studying the effect of compounds on QT interval in vitro.

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.

Application of a probabilistic method for the determination of drug-induced QT prolongation in telemetered cynomolgus monkeys: effects of moxifloxacin

INTRODUCTION

Moxifloxacin is the most widely used positive reference agent in clinical cardiac repolarization safety studies, but it has not been characterized in the cynomolgus monkey. This important experimental animal species exhibits pronounced heart rate variability, complicating the temporal evaluation of QT interval data.

METHODS

Digitized epicardial ECGs and aortic blood pressures were collected for 20 h in telemetered cynomolgus monkeys (n=6) following the administration of either vehicle or moxifloxacin (10 or 50 mg/kg, p.o.). Moxifloxacin plasma concentrations were determined 4 h postdose. ECG intervals were analyzed by computerized algorithms. Individual probabilistic QT rate-corrections (QTc) were derived from the slopes of predose log-transformed QT-RR data where each QT value was the mean of >250 beats/RR increment. The resulting QTc was used to determine the repolarization effects of moxifloxacin, expressed as the placebo-adjusted change in QTc (DeltaQTc), and as the integrated response from 0 to 12 h (AUC(0-->12)) postdose.

RESULTS

No DeltaQTc effect was produced by 10 mg/kg moxifloxacin. However, moxifloxacin (50 mg/kg; 5.86+/-0.5 microg/mL C(max)) significantly prolonged the RR interval by 50 to 112 ms from 3.5 to 7.5 h postdose and DeltaQTc by >or=7.2 ms from 1.83 to 9.17 h, with a maximal DeltaQTc effect of +26.4 ms. No notable effects on either systemic blood pressure or body temperature occurred with either dose.

DISCUSSION

Probabilistic QT rate-corrections appear to have eliminated the confounding effects of heart rate, provided for a stable QTc baseline, and enabled the demonstration of an exposure-dependent QTc prolongation by moxifloxacin. The duration and magnitude of the QTc effect paralleled moxifloxacin pharmacokinetics, and C(max) values were similar to those achieved clinically in thorough QT/QTc studies. Thus, novel probabilistic QT rate-corrections may offer highly robust assessments of repolarization risk in both nonclinical and clinical investigations.

Bazett and Fridericia QT correction formulas interfere with measurement of drug-induced changes in QT interval

BACKGROUND

The QT interval on the ECG is prolonged by more than 50 marketed drugs, an effect that has been associated with syncope and/or sudden cardiac death due to an arrhythmia. Because changes in heart rate also change the QT interval, it has become standard practice to use a correction formula, such as the Bazett formula, to normalize the QT interval to a heart rate of 60 bpm, that is, the rate-corrected QT or QTc. Numerous other formulas have been devised to make this correction, including the Fridericia, Hodges, and Framingham formulas.

OBJECTIVES

The purpose of this study was to investigate how the Bazett formula and three other formulas influence assessment of the QT-prolonging effect of the potassium channel-blocking drug ibutilide.

METHODS

Using a standardized physical activity protocol, the QT interval was assessed over a broad range of heart rates before and after an infusion of ibutilide (4.75 microg/kg) that produced a stable 15- to 20-ms QT prolongation in consenting normal subjects (9 men and 9 women). The QT interval was measured digitally over a range of heart rates from 60 to 120 bpm, and then four correction formulas (Bazett, Fridericia, Framingham, or Hodges) were applied. The uncorrected change in QT interval due to ibutilide was compared with the change using each of the formulas by repeated measures analysis of variance.

RESULTS

At heart rates from 60 to 120 bpm, the Bazett and Fridericia correction formulas overestimated the change in QT in both men and women (P <.001). However, the Framingham and Hodges formulas did not alter the accuracy of the assessment of QT interval change.

CONCLUSION

Rate correction of QT intervals using the standard Bazett and Fridericia formulas can introduce significant errors in the assessment of drug effects on the QT interval. This has implications for the clinical assessment of drug effects and for the safety assessment of new drugs under development.

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

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

Electrocardiogram, hemodynamics, and core body temperatures of the normal freely moving laboratory beagle dog by remote radiotelemetry

INTRODUCTION

The objectives of this study were to provide baseline normative values for circadian changes in the time-series data collected over the course of a normal day in laboratory-housed dogs and to assess the relative efficiency of standard correction formulas to correct for the variations in QT intervals and heart rate functions.

METHODS

One hundred and twenty-three beagle dogs (65 M, 58 F) were equipped with radiotelemetry transmitters and continuously monitored, while freely moving in their home cages. Electrocardiograms (ECGs), hemodynamic parameters (diastolic, systolic, and mean arterial pressures) as well as core body temperatures were recorded for 22 h.

RESULTS AND DISCUSSION

Blood pressures and core body temperatures demonstrated only very slight variations in their respective values over the 22-h monitoring period. ECGs were measured by a computerized waveform analysis program and quantitative elements reported as RR, PR, QRS, and QT intervals. Little circadian rhythmicity was demonstrated in the ECG intervals. Standard study-specific correction formulas appeared to satisfactorily normalize (i.e., compensate for) the relationship between heart rate and QT intervals in these beagle dogs but elevated the values of the QTc as compared to the uncorrected QT intervals. In sharp contrast, a subject-specific correction method based on analysis of covariance produced a more linear function between heart rates and QT intervals and, more importantly, provided QTc values within the normal range of actual, recorded QT interval data.

Clinical trial design to evaluate the effects of drugs on cardiac repolarization: Current state of the art

Prolongation of the QT interval associated with the potentially fatal arrhythmia known as torsades de pointes has been a common cause of the withdrawal of several promising drugs from the market. Many antihistamines, antibiotics, antimalarials, antidepressants, neuroleptics, antipsychotics, and imidazole antifungal agents have been shown to produce torsades, and all by the same mechanism. Advances in basic science and preclinical testing have begun to provide a scientific basis for distinguishing arrhythmogenicity from drug-induced QT effects. Many new techniques have been developed, and many others currently are being developed to facilitate the design of clinical trials to evaluate the effects of drugs on cardiac repolarization. The improvements in clinical trial design may help identify drugs that could induce torsades, halting futile research, potentially saving lives, and saving hundreds of millions of dollars in drug development. In the absence of any completely reliable surrogate measure for the arrhythmogenic potential of a drug, regulators have determined that QT interval prolongation should be intensively investigated in every drug that is developed. This article presents the basic mechanics of QT interval assessment and describes new developments that may make this measure a more accurate predictor of the effects of drugs on cardiac repolarization. It is absolutely essential that trial designs incorporate many ECG recordings, consistent QT interval measurement, and appropriate control or correction of the QT interval for heart rate in order to provide reproducible, scientifically meaningful results.

Pharmacokinetic-pharmacodynamic modeling in the data analysis and interpretation of drug-induced QT/QTc prolongation

In this review, factors affecting the QT interval and the methods that are currently in use in the analysis of drug effects on the QT interval duration are overviewed with the emphasis on (population) pharmacokinetic-pharmacodynamic (PK-PD) modeling. Among which the heart rate (HR) and the circadian rhythm are most important since they may interfere with the drug effect and need to be taken into account in the data analysis. The HR effect or the RR interval (the distance between 2 consecutive R peaks) effect is commonly eliminated before any further analysis, and many formulae have been suggested to correct QT intervals for changes in RR intervals. The most often used are Bazett and Fridericia formulae introduced in 1920. They are both based on the power function and differ in the exponent parameter. However, both assume the same exponent for different individuals. More recent findings do not confirm this assumption, and individualized correction is necessary to avoid under- or overcorrection that may lead to artificial observations of drug-induced QT interval prolongation. Despite the fact that circadian rhythm in QT and QTc intervals is a well-documented phenomenon, it is usually overlooked when drug effects are evaluated. This may result in a false-positive outcome of the analysis as the QTc peak due to the circadian rhythm may coincide with the peak of the drug plasma concentration. In view of these effects interfering with a potential drug effect on the QTc interval and having in mind low precision of QT interval measurements, a preferable way to evaluate the drug effect is to apply a population PK-PD modeling. In the literature, however, there are only a few publications in which population PK-PD modeling is applied to QT interval prolongation data, and they all refer to antiarrhythmic agents. In this review, after the most important sources of variability are outlined, a comprehensive population PK-PD model is presented that incorporates an individualized QT interval correction, a circadian rhythm in the individually corrected QT intervals, and a drug effect. The model application is illustrated using real data obtained with 2 compounds differing in their QT interval prolongation potential. The usefulness of combining data of several studies is stressed. Finally, the standard approach based on the raw observations and formal statistics, as described in the Preliminary Concept paper of the International Conference on Harmonization, is briefly compared with the method based on population PK-PD modeling, and the advantages of the latter are outlined.

Electrocardiogram, hemodynamics, and core body temperatures of the normal freely moving cynomolgus monkey by remote radiotelemetry

INTRODUCTION

The objectives of this study were to provide baseline normative values for circadian changes in the time-series data collected over the course of a normal day in laboratory-housed monkeys, and to assess the relative efficiency of standard correction formulas to correct for the variations in QT interval durations and heart rate functions.

METHODS

Ninety-nine cynomolgus monkeys (58 M, 41 F) were equipped with radiotelemetry transmitters and continuously monitored, while freely moving in their home cages. Electrocardiograms (ECGs), hemodynamic parameters (diastolic, systolic, and mean arterial pressures) as well as core body temperatures were recorded for 22 h from each of 99 monkeys. ECGs were measured by a computerized waveform analysis program and reported as RR, PR, QRS, and QT intervals.

RESULTS

Blood pressures and core body temperatures demonstrated a normal circadian variation in their respective values over the 22 h monitoring period. Standard study-specific correction formulas failed to satisfactorily normalize the relationship between heart rate and QT intervals in the cynomolgus monkeys. In contrast, a subject-specific correction method based on analysis of covariance produced a linear function between heart rates and QT intervals and provided QTc values within the normal range of actual, recorded data.

DISCUSSION

We believe these procedures represent the contemporary industry's preferred practice for measuring such parameters under the ICH guidelines, and are amenable to routine use in a variety of other relevant safety/efficacy studies.

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).