Circadian rhythm of the corrected QT interval: impact of different heart rate correction models

Circadian influences on sudden cardiac death and cardiac electrophysiology

Cardiologists have analyzed daily patterns in the incidence of sudden cardiac death to identify environmental, behavioral, and physiological factors that trigger fatal arrhythmias. Recent studies have indicated an overall increase in sudden cardiac arrest during daytime hours when the frequency of arrhythmogenic triggers is highest. The risk of fatal arrhythmias arises from the interaction between these triggers such as elevated sympathetic signaling, catecholamine levels, heart rate, afterload, and platelet aggregation and the susceptibility of the heart (myocardial substrate) to them. A healthy myocardial substrate has structural and functional properties that protect against arrhythmias. However, individuals with cardiovascular disease often exhibit structural and electrophysiological alterations in the myocardial substrate that predispose them to sustained lethal arrhythmias. This review focuses on how day-night and circadian rhythms, both extrinsic and intrinsic, influence the protective properties of the myocardial substrate. Specifically, it explores recent advances in the temporal regulation of ion channel gene transcription, drawing on data from comprehensive bioinformatics resources (CircaDB, CircaAge, and CircaMET) and recent RNA sequencing studies. We also examine potential mechanisms underlying the temporal regulation of mRNA expression and the challenges in linking rhythmic mRNA expression to corresponding changes in protein levels. As chronobiological research in cardiology progresses, we anticipate the development of novel therapeutic strategies to enhance the protective properties of the myocardial substrate to reduce the risk of fatal arrhythmias and sudden cardiac arrest.

Altered circadian rhythmicity of the QT interval predicts mortality in a large real-world academic hospital population

Objective and rationale

Small studies have shown that the QT interval follows a circadian rhythm. This finding has never been confirmed in a large real-world hospital population and the clinical meaning of disrupted rhythmicity remains unknown.

Methods

In this cohort study, all consecutive adult patients with at least one 12-lead ECG acquired between 1991 and 2021 were considered. Sinus rhythm ECGs without QRS conduction or ST-segment abnormalities obtained at the wards or outpatient clinic were included. The QT interval was corrected for age, sex and ventricular rate in a personalized manner. Subsequently, the added value of a 24-h sinusoid of time-of-day was evaluated. An individual 24-h QT interval amplitude was obtained from the model in a subset with patients that had at least 3 ECGs of which one during the night before their last ECG. The association of this individual QT interval with all-cause mortality was assessed using a left-truncated Cox regression model.

Results

The baseline QT correction model was fitted using 237,555 ECGs of 100,644 patients. The personalized corrected QT interval had no relationship with ventricular rate (r = -0.008). Adding the 24-h sinusoidal to the baseline model resulted in a significantly better fit (p < 0.0001). The mean circadian variation of the QT interval was 15 ms, with the maximum QT duration around midnight and an effect that is largest in young female patients. A non-linear relationship between peak-to-trough amplitude in QT interval rhythmicity and all-cause mortality was found, with both lower and higher values associated with increased risk.

Conclusions

Using heterogeneous, real-world hospital data of more than 100,000 patients, circadian rhythmicity proved to be an independent determinant of the QT interval. Both increased and diminished QT rhythmicity was shown to be a predictor of all-cause mortality. QT interval should be corrected for the time-of-day and altered circadian rhythmicity should trigger awareness of increased mortality risk (https://qt.ecgx.ai).

No Influence of Asundexian on Cardiac Repolarization

Inhibition of activated factor XI reduces thrombogenesis while maintaining physiological hemostasis, with the expectation of reduced bleeding risk compared with standard of care in the clinical setting. Asundexian (BAY 2433334), an activated factor XI inhibitor, is in clinical development for the prevention of thromboembolic events. The effect of asundexian and its plasma metabolite M10 on cardiac repolarization and potential interactions with the hNav1.5 sodium, hCav1.2 calcium, and human ether-à-go-go-related gene (hERG) potassium channels was investigated in vitro. Additionally, asundexian effects on cardiac parameters and electrocardiogram were examined in telemetered beagle dogs. A randomized, placebo-controlled, 4-way crossover, thorough QT study in healthy adults evaluated the influence of 50 and 150 mg of asundexian on the corrected QT interval, including 400 mg of moxifloxacin as positive control. Across all studies, asundexian and M10 were not associated with any effects on cardiac repolarization. The largest in vitro effects of asundexian (approximately 20% inhibition) were seen for hCav1.2 and hERG. Throughout the thorough QT study, the upper limits of the one-sided 95% confidence interval of placebo-corrected mean changes from baseline in Fridericia corrected QT for 50 and 150 mg of asundexian were below Δ = 10 milliseconds. Asundexian demonstrated favorable safety and tolerability profiles.

Serial electrocardiogram recordings revealed a high prevalence of QT interval prolongation in patients with tuberculosis receiving fluoroquinolones

BACKGROUND

Fluoroquinolones, crucial components of treatment regimens for drug-resistant tuberculosis (TB), are associated with QT interval prolongation and risks of fatal cardiac arrhythmias. However, few studies have explored dynamic changes in the QT interval in patients receiving QT-prolonging agents.

METHODS

This prospective cohort study recruited hospitalized patients with TB who received fluoroquinolones. The study investigated the variability of the QT interval by using serial electrocardiograms (ECGs) recorded four times daily. This study analyzed the accuracy of intermittent and single-lead ECG monitoring in detecting QT interval prolongation.

RESULTS

This study included 32 patients. The mean age was 68.6 ± 13.2 years. The results revealed mild-to-moderate and severe QT interval prolongation in 13 (41%) and 5 (16%) patients, respectively. The incremental yields in sensitivity of one to four daily ECG recordings were 61.0%, 26.1%, 5.6%, and 7.3% in detecting mild-to-moderate QT interval prolongation, and 66.7%, 20.0%, 6.7%, and 6.7% in detecting severe QT interval prolongation. The sensitivity levels of lead II and V5 ECGs in detecting mild-to-moderate and severe QT interval prolongation exceeded 80%, and their specificity levels exceeded 95%.

CONCLUSION

This study revealed a high prevalence of QT interval prolongation in older patients with TB who receive fluoroquinolones, particularly those with multiple cardiovascular risk factors. Sparsely intermittent ECG monitoring, the prevailing strategy in active drug safety monitoring programs, is inadequate owing to multifactorial and circadian QT interval variability. Additional studies performing serial ECG monitoring are warranted to enhance the understanding of dynamic QT interval changes in patients receiving QT-prolonging anti-TB agents.

Analysis of the effect of cortisone on the QT interval

BACKGROUND

Cardiac death caused by malignant arrhythmias is very prevalent. Prolongation of the QT interval is a relevant aspect in arrhythmia mechanisms. Prior studies have revealed that the QTc interval could be shortened by cortisone. Moreover, in an animal model of long QT syndrome, cortisone treatment shortens the ventricular action potential duration. The present study investigated the effect of methylprednisolone (MPS) on the QTc interval in cardiovascularly healthy humans.

METHODS

Patients who had just been diagnosed with multiple sclerosis receiving MPS therapy were analysed prospectively. Demographic data, laboratory values, anti-arrhythmic medication and baseline and follow-up ECGs were extracted from the patients' medical records.

RESULTS

Seventy-eight patients were included. The mean ± standard deviation age was 47 ± 15 years. The values of the electrolytes were normal. All patients were treated with MPS for 3 or 5 days. The heart rate increased at the beginning of MPS therapy and decreased during the subsequent period. ECG measurements showed that the QTc interval was prolonged at the beginning of MPS therapy and shortened over the course of treatment. The longest QTc intervals were obtained by calculation with Bazett's formula.

CONCLUSIONS

In humans, cortisone shortens the QTc interval over time. The analysis indicates a cumulative effect of cortisone that lasts longer. The results of our pilot study reveal that cortisone might be added to therapeutic strategies in patients with long QT syndromes. Further clinical studies have to be carried out to analyze potential clinical options.

A Systematic Review of Utilisation of Diurnal Timing Information in Clinical Trial Design for Long QT Syndrome

Diurnal oscillations in human cardiac electrophysiology are thought to be under the control of the endogenous circadian clock. The incidence of arrhythmic events in patients with Long QT syndrome (LQTS) varies diurnally. The diurnal variation in QT interval has previously been identified as a potential for error in clinical trials which utilise ECG measurement. We performed a systematic review of clinical trials for LQTS to identify practice around specification of timing information for point electrocardiogram (ECG) measurements, analysis of continual ECG recordings ≥24 h, and drug delivery. Despite guidelines having been issued around the analysis of 24-h ECG recordings, we identify a lack of usage of detailed time of day information in trial design for LQTS studies, which has the potential to affect the interpretation of the results of drug trials. We identify that, in contrast, clinical trials for QT prolonging drugs demonstrate increased incorporation of time of day information of both QT analysis and drug dosing. We provide a visual portal to allow trial designers and clinicians to better understand timing of common cardiac-targeting drugs, and to bear this concept in mind in the design of future clinical trials.

Practical Approaches to Antipsychotic-Associated Corrected QT Interval Prolongation in Patients With Serious Mental Illness: A Review of Cases

BACKGROUND

There is no consensus for assessment and management of patients with serious mental illness (SMI) who are at risk for cardiac morbidity and mortality due to antipsychotic-associated QTc prolongation.

OBJECTIVE

The objective of this review was to assess methods for risk scoring, QT correction calculation, and clinical management in SMI patients with antipsychotic-associated QTc prolongation.

METHODS

A search was performed in PubMed for case reports that described QTc prolongation in adult patients with schizophrenia or bipolar disorder prescribed an antipsychotic. Reports published in North America between 2000 and 2020 were eligible. The Mayo, Tisdale, and RISQ-PATH scoring tools were applied to cases to categorize risk level.

RESULTS

Seventeen cases were included. Most patients were prescribed a second-generation antipsychotic for schizophrenia, with baseline and maximum QTc values of 429 milliseconds and 545 milliseconds, respectively. The Mayo scoring tool identified 17 (100%) cases as "high risk," Tisdale identified 9 (53%) cases as "moderate risk" and 7 (41%) cases as "low risk," while RISQ-PATH identified 9 (53%) cases as "not low risk" and 8 (47%) cases as "low risk." Three cases reported the QT correction formula utilized (18%). The most common intervention to address antipsychotic-associated QTc prolongation was switching to a different antipsychotic (35%). Approximately one third of patients experienced Torsades de Pointes.

CONCLUSION

There is a lack of standardization for antipsychotic-associated QTc prolongation risk assessment and management in patients with SMI. This review provides real-world data representing actual clinical practice.

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.

Biological variations in hemodynamics and electrocardiogram rhythms among telemetered cynomolgus monkeys

INTRODUCTION

Telemetered cynomolgus monkeys are widely used in cardiovascular toxicology research. However, the biological variations in their hemodynamics and electrocardiogram rhythms have not been fully elucidated.

METHOD

To determine the potential effects of sex, handling stress, and circadian rhythm on the hemodynamics and electrocardiogram rhythms, data from 23 cynomolgus monkeys, implanted with DSI telemetry devices were examined.

RESULTS

Our data showed that males had a longer RR interval (RRi), slower heart rate (HR), shorter QT and corrected QT intervals (QTc), and lower blood pressure than females. During the night time, the animals showed a longer RRi, PRi, QTi, and QTc; slower HR, and lower blood pressure. Handling stress at 0.25- to 1-h post-treatment caused a decrease in RRi and increase in HR and QTi. For RRi, HR, and systolic, diastolic, and mean blood pressure, the coefficients of variation (CVs) between studies of individual animals were less than 30%; for other parameters, the CVs were less than 20%.

DISCUSSION

We demonstrated that sex, circadian rhythms, and handling stress all contributed towards variations in telemetry data, albeit to different extents. For each individual animal, the biological variation across different studies was relatively small and acceptable.

Randomized Controlled Trial of the Electrocardiographic Effects of Four Antimalarials for Pregnant Women with Uncomplicated Malaria on the Thailand-Myanmar Border

Quinoline antimalarials cause drug-induced electrocardiograph QT prolongation, a potential risk factor for torsade de pointes. The effects of currently used antimalarials on the electrocardiogram (ECG) were assessed in pregnant women with malaria.

Quinoline antimalarials cause drug-induced electrocardiographic QT prolongation, a potential risk factor for torsade de pointes. The effects of currently used antimalarials on the electrocardiogram (ECG) were assessed in pregnant women with malaria. Pregnant women with microscopy-confirmed parasitemia of any malaria species were enrolled in an open-label randomized controlled trial on the Thailand-Myanmar border from 2010 to 2016. Patients were randomized to the standard regimen of dihydroartemisinin-piperaquine (DP) or artesunate-mefloquine (ASMQ) or an extended regimen of artemether-lumefantrine (AL+). Recurrent Plasmodium vivax infections were treated with chloroquine. Standard 12-lead electrocardiograms were assessed on day 0, 4 to 6 h following the last dose, and day 7. QT was corrected for the heart rate by a linear mixed-effects model-derived population-based correction formula (QTcP = QT/RR^0.381). A total of 86 AL+, 82 ASMQ, 88 DP, and 21 chloroquine-treated episodes were included. No patients had an uncorrected QT interval nor QTcP of >480 ms at any time. QTcP corresponding to peak drug concentration was longer in the DP group (adjusted predicted mean difference, 17.84 ms; 95% confidence interval [CI], 11.58 to 24.10; P < 0.001) and chloroquine group (18.31 ms; 95% CI, 8.78 to 27.84; P < 0.001) than in the AL+ group, but not different in the ASMQ group (2.45 ms; 95% CI, −4.20 to 9.10; P = 0.47) by the multivariable linear mixed-effects model. There was no difference between DP and chloroquine (P = 0.91). QTc prolongation resulted mainly from widening of the JT interval. In pregnant women, none of the antimalarial drug treatments exceeded conventional thresholds for an increased risk of torsade de pointes.

How circadian variability of the heart rate and plasma electrolytes concentration influence the cardiac electrophysiology - model-based case study

The circadian rhythm of cardiac electrophysiology is dependent on many physiological and biochemical factors. Provided, that models describing the circadian patterns of cardiac activity and/or electrophysiology which have been verified to the acceptable level, modeling and simulation can give answers to many of heart chronotherapy questions. The aim of the study was to assess the performance of the circadian models implemented in Cardiac Safety Simulator v 2.2 (Certara, Sheffield, UK) (CSS), as well as investigate the influence ofcircadian rhythms on the simulation results in terms of cardiac safety. The simulations which were run in CSS accounted for inter-individual and intra-individual variability. Firstly, the diurnal variations in QT interval length in a healthy population were simulated accounting for heart rate (HR) circadian changes alone, or with concomitant diurnal variations of plasma ion concentrations. Next, tolterodine was chosen as an exemplary drug for PKPD modelling exercise to assess the role of circadian rhythmicity in the prediction of drug effects on QT interval. The results of the simulations were in line with clinical observations, what can serve as a verification of the circadian models implemented in CSS. Moreover, the results have suggested that the circadian variability of the electrolytes balance is the main factor influencing QT circadian pattern. The fluctuation of ion concentration increases the intra-subject variability of predicted drug-triggered QT corrected for HR (QTc) prolongation effect and, in case of modest drug effect on QTc interval length, allows to capture this effect.

Circadian variation of the QT interval and heart rate variability and their interrelationship

BACKGROUND

Whether the QT interval displays circadian rhythm after heart rate correction is unresolved and the relationship of QT interval to heart rate variability (HRV) is uncertain.

OBJECTIVES

To test the hypothesis that there is a circadian rhythm to QT interval and HRV and determine the relationship between QTc and HRV.

METHODS

The hourly average ECG data from 24-h ECGs were examined in individuals (50 without medications and 9 on beta blockers only) with no evidence of coronary artery disease or structural heart disease. The QT duration of normal QRS complexes from a series of 30-s windows was measured. The presence of circadian rhythm was tested by the data analytic approach of goodness of fit to a cosine function.

RESULTS

QT interval with and without heart rate correction showed a circadian rhythm for five heart rate adjustment formulae except for the Bazett formula. HRV also showed circadian rhythm but with different acrophages and nadirs depending on the HRV component. There were significant (p < 0.05) positive correlations of QTc with pNN50 rms-SD and SDNN and significant (p < 0.05) negative correlations with SDANN and Tri. The beta blocker group did not generally show circadian rhythm for QT interval or HRV.

CONCLUSION

QT, after heart rate adjustment, and HRV have circadian rhythmicity. There are significant correlations between QT interval and HRV indices. Circadian rhythm was blunted with beta blockers. The data are consistent with the concept of a predominance of parasympathetic activity to increase QTc and sympathetic activity to shorten QTc, even after 'correction' of the QT interval for heart rate.

Relationship of QTc Interval Prolongation with Acute Ischemic Stroke

Introduction:

Many electrocardiographic (ECG) changes have been observed after strokes. We analyzed the QTc interval prolongation following stroke.

Aim:

The study aimed to assess if the prolongation in QTc interval is related to the occurrence of acute ischemic stroke.

Methods:

This cross-sectional study was conducted from July to December 2018. We included 100 consecutive patients with first-ever ischemic stroke who were admitted to our emergency department, who were age-matched and gender-matched with a control group of 100 non-stroke patients that visited our outpatients department for diseases other than cerebrovascular or cardiovascular ones. A single 12-lead resting ECG examination was done in all patients at the time of their emergency department admission.

Results:

No significant difference between the two groups regarding the age distribution and mean age was found. 56.5% of the sample were males but the difference was not significant between both gender groups. The main presenting symptoms of stroke cases were right-sided weakness (47%), left-sided weakness (36%), and right-sided weakness and aphasia (10%). 34% of the cases had prolonged QTc interval while none of the controls had a prolonged interval (p-value<0.001). No significant difference was observed among stroke patients concerning gender (p-value=0.584).

Conclusion:

Our findings support many previous studies on the brain-heart interaction during acute ischemic strokes and reinforce previous conclusions that assessment of the QTc interval might aid to stratify morbidity and mortality risks in patients with acute ischemic stroke. To accomplish the acute stroke effects on QTc interval prolongation, we need further larger size analytic studies.

Diurnal Profile of the QTc Interval Following Moxifloxacin Administration

Understanding the physiological fluctuations in the corrected QT (QTc) interval is important to accurately interpret the variations in drug-induced prolongation. The present study aimed to define the time course of the effect of moxifloxacin on the QT interval to understand the duration of the responses to moxifloxacin. This retrospective analysis was performed on data taken from a thorough QT 4-way crossover study with 40 subjects. Each period consisted of a baseline electrocardiogram (ECG) day (day -1) and a treatment day (day 1). On both days, ECGs were recorded simultaneously using 2 different systems operating in parallel: a bedside ECG and a continuous Holter recording. The subjects were randomized to 1 of 4 treatments: 5 mg and 40 mg of intravenous amisulpride, a single oral dose of moxifloxacin (400 mg), or placebo. Standardized meals, identical in all 4 periods, with similar nutritional value were served. Bedside ECG results confirmed that the moxifloxacin peak effect was delayed in the fed state and showed that the Fridericia corrected QT prolongation induced by moxifloxacin persisted until the end of the 24-hour measurement period. The use of continuous Holter monitoring provided further insight, as it revealed that the moxifloxacin effect on QTc was influenced by diurnal and nocturnal environmental factors, and hysteresis effects were noticeable. The findings suggested that moxifloxacin prolongs QTc beyond its elimination from the blood circulation. This is of relevance to current concentration-effect modeling approaches, which presume the absence of hysteresis effects.

Limited Evidence for Risk Factors for Proarrhythmia and Sudden Cardiac Death in Patients Using Antidepressants: Dutch Consensus on ECG Monitoring

Currently, there is a lack of international and national guidelines or consensus documents with specific recommendations for electrocardiogram (ECG) screening and monitoring during antidepressant treatment. To make a proper estimation of the risk of cardiac arrhythmias and sudden (cardiac) death during antidepressant use, both the drug and patient-specific factors should be taken into account; however, solid evidence on how this should be done in clinical practice is lacking. Available recommendations on the management of QT(c) prolongation (with antidepressant treatment) emphasize that special attention should be given to high-risk patients; however, clinicians are in need of more concrete suggestions about how to select patients for ECG screening and monitoring. Based on a review of the literature, a Dutch multidisciplinary expert panel aimed to formulate specific guidelines to identify patients at risk for cardiac arrhythmias and sudden death by developing a consensus statement regarding ECG screening before, and monitoring during, antidepressant use. We first reviewed the literature to identify the relative risks of various risk factors on cardiac arrhythmia and sudden (cardiac) death during antidepressant use. These relative contributions of risk factors could not be determined since no systematic reviews or meta-analyses quantitatively addressed this topic. Because evidence was insufficient, additional expert opinion was used to formulate recommendations. This resulted in readily applicable recommendations for clinical practice for selection of high-risk patients for ECG screening and monitoring. ECG screening and monitoring is recommended before and following the start of QTc-prolonging antidepressants in the presence of vulnerability to QTc prolongation or two or more risk factors (age > 65 years, female sex, concomitant use of a QTc-prolonging drug or concomitant use of a drug that influences the metabolism of a QTc-prolonging drug, cardiac disease, excessive dosing and specific electrolyte disturbances).

Individual-Specific QT Interval Correction for Drugs With Substantial Heart Rate Effect Using Holter ECGs Extracted Over a Wide Range of Heart Rates

Although fixed QT correction methods are typically used to adjust for the effect of heart rate on the QT interval in thorough QT/QTc studies, individual-specific QT correction (QTcI = QT/RR^I ) is advisable for drugs that increase the heart rate by >5 to 10 beats/minute (bpm). QTcI is traditionally derived using resting drug-free electrocardiograms (ECGs) collected at prespecified times. However, the resting heart rate range in healthy individuals is narrow, and extrapolation of inferences from these data to higher heart rates could be inappropriate. Accordingly, the QTcI derived from triplicate ECGs extracted at prespecified times (the traditional [T] method, yielding QTcIT) was compared with QTcIs obtained using ECGs with a wider heart rate range (alternative Holter [H] method, yielding QTcIH) from 24-hour Holter recordings from 40 healthy individuals selected from a central ECG laboratory database. For QTcIH, 10-second ECGs were extracted at stable heart rates in the ranges of 51-60, 61-70, 71-80, and 81-90 bpm (9 ECGs in each bin = 36 ECGs). An independent set of 40 ECGs with heart rates from 51 to 90 bpm was extracted from each individual to validate the accuracy of QTcI by the 2 methods. For the validation set, the QTcIH was a better QT correction method (slope of QTc vs heart rate closer to zero) than QTcIT. The mean difference between QTcIT and QTcIH increased from 3.1 milliseconds at 65 bpm to 10.0 milliseconds at 90 bpm (P < 0.01). The QTcIT exceeded QTcIH at heart rates > 60 bpm. Employment of the QTcIH may be more appropriate for studies involving drugs that increase heart rate.

Cardiac Effects of Sulfonylurea-Related Hypoglycemia

OBJECTIVE

To determine the effect of sulfonylurea-related hypoglycemia on cardiac repolarization and ectopy in the setting of well-controlled type 2 diabetes.

RESEARCH DESIGN AND METHODS

Thirty subjects with sulfonylurea-treated type 2 diabetes underwent 48 h of concurrent continuous glucose monitoring and ambulatory electrocardiography. Ventricular repolarization (QTc) and QT dynamicity were analyzed during periods of hypoglycemia (<3.5 mmol/L for >20 min) and compared with periods of euglycemia and hyperglycemia combined. Cardiac ectopy rates during hypoglycemia were compared with ectopy rates when blood glucose was 4-10 mmol/L.

RESULTS

Mean HbA_1c was 6.9% (52 mmol/mol). Hypoglycemia was detected in 9 of 30 subjects (30%); episodes were typically nocturnal (67%) and asymptomatic (73%). Hypoglycemia-associated QTc prolongation was seen in five of nine subjects with a large variation in individual response. Higher QT dynamicity, a poor prognostic factor in cardiac disease, was seen in subjects who experienced hypoglycemia compared with subjects who did not (0.193 vs. 0.159 for the nocturnal period; P = 0.01). This finding persisted after the hypoglycemic event. The rates of ventricular and supraventricular ectopy demonstrated a nonsignificant trend toward an increase during hypoglycemia (median rate ratio 1.58 and 1.33, respectively). Similar, nonsignificant results were observed in a separate insulin-treated cohort.

CONCLUSIONS

Hypoglycemia, often unrecognized, is a frequent finding in well-controlled sulfonylurea-treated type 2 diabetes. It is associated with the novel finding of increased QT dynamicity and QTc prolongation in some individuals. Our findings suggest sulfonylurea-related hypoglycemia can have detrimental cardiovascular sequelae. Similar effects are also seen in the setting of insulin therapy.

Translating QT interval prolongation from conscious dogs to humans

AIM

In spite of screening procedures in early drug development, uncertainty remains about the propensity of new chemical entities (NCEs) to prolong the QT/QTc interval. The evaluation of proarrhythmic activity using a comprehensive in vitro proarrhythmia assay does not fully account for pharmacokinetic-pharmacodynamic (PKPD) differences in vivo. In the present study, we evaluated the correlation between drug-specific parameters describing QT interval prolongation in dogs and in humans.

METHODS

Using estimates of the drug-specific parameter, data on the slopes of the PKPD relationships of nine compounds with varying QT-prolonging effects (cisapride, sotalol, moxifloxacin, carabersat, GSK945237, SB237376 and GSK618334, and two anonymized NCEs) were analysed. Mean slope estimates varied between -0.98 ms μM-1 and 6.1 ms μM-1 in dogs and -10 ms μM-1 and 90 ms μM-1 in humans, indicating a wide range of effects on the QT interval. Linear regression techniques were then applied to characterize the correlation between the parameter estimates across species.

RESULTS

For compounds without a mixed ion channel block, a correlation was observed between the drug-specific parameter in dogs and humans (y = -1.709 + 11.6x; R2  = 0.989). These results show that per unit concentration, the drug effect on the QT interval in humans is 11.6-fold larger than in dogs.

CONCLUSIONS

Together with information about the expected therapeutic exposure, the evidence of a correlation between the compound-specific parameter in dogs and in humans represents an opportunity for translating preclinical safety data before progression into the clinic. Whereas further investigation is required to establish the generalizability of our findings, this approach can be used with clinical trial simulations to predict the probability of QT prolongation in humans.

Circadian rhythm in QT interval is preserved in mice deficient of potassium channel interacting protein 2

Potassium Channel Interacting Protein 2 (KChIP2) is suggested to be responsible for the circadian rhythm in repolarization duration, ventricular arrhythmias, and sudden cardiac death. We investigated the hypothesis that there is no circadian rhythm in QT interval in the absence of KChIP2. Implanted telemetric devices recorded electrocardiogram continuously for 5 days in conscious wild-type mice (WT, n = 9) and KChIP2^-/- mice (n = 9) in light:dark periods and in complete darkness. QT intervals were determined from all RR intervals and corrected for heart rate (QT₁₀₀ = QT/(RR/100)^1/2). Moreover, QT intervals were determined from complexes within the RR range of mean-RR ± 1% in the individual mouse (QT_mean-RR). We find that RR intervals are 125 ± 5 ms in WT and 123 ± 4 ms in KChIP2^-/- (p = 0.81), and QT intervals are 52 ± 1 and 52 ± 1 ms, respectively(p = 0.89). No ventricular arrhythmias or sudden cardiac deaths were observed. We find similar diurnal (light:dark) and circadian (darkness) rhythms of RR intervals in WT and KChIP2^-/- mice. Circadian rhythms in QT₁₀₀ intervals are present in both groups, but at physiological small amplitudes: 1.6 ± 0.2 and 1.0 ± 0.3 ms in WT and KChIP2^-/-, respectively (p = 0.15). A diurnal rhythm in QT₁₀₀ intervals was only found in WT mice. QT_mean-RR intervals display clear diurnal and circadian rhythms in both WT and KChIP2^-/-. The amplitude of the circadian rhythm in QT_mean-RR is 4.0 ± 0.3 and 3.1 ± 0.5 ms in WT and KChIP2^-/-, respectively (p = 0.16). In conclusion, KChIP2 expression does not appear to underlie the circadian rhythm in repolarization duration.

Levofloxacin-Induced QTc Prolongation Depends on the Time of Drug Administration

Understanding the factors influencing a drug's potential to prolong the QTc interval on an electrocardiogram is essential for the correct evaluation of its safety profile. To explore the effect of dosing time on drug‐induced QTc prolongation, a randomized, crossover, clinical trial was conducted in which 12 healthy male subjects received levofloxacin at 02:00, 06:00, 10:00, 14:00, 18:00, and 22:00. Using a pharmacokinetic‐pharmacodynamic (PK‐PD) modeling approach to account for variations in PKs, heart rate, and daily variation in baseline QT, we find that the concentration‐QT relationship shows a 24‐hour sinusoidal rhythm. Simulations show that the extent of levofloxacin‐induced QT prolongation depends on dosing time, with the largest effect at 14:00 (1.73 (95% prediction interval: 1.56–1.90) ms per mg/L) and the smallest effect at 06:00 (−0.04 (−0.19 to 0.12) ms per mg/L). These results suggest that a 24‐hour variation in the concentration‐QT relationship could be a potentially confounding factor in the assessment of drug‐induced QTc prolongation.

Cardiac effects of sertindole and quetiapine: analysis of ECGs from a randomized double-blind study in patients with schizophrenia

The QT interval is the most widely used surrogate marker for predicting TdP; however, several alternative surrogate markers, such as Tpeak-Tend (TpTe) and a quantitative T-wave morphology combination score (MCS) have emerged. This study investigated the cardiac effects of sertindole and quetiapine using the QTc interval and newer surrogate markers. Data were derived from a 12 week randomized double-blind study comparing flexible dosage of sertindole 12-20mg and quetiapine 400-600mg in patients with schizophrenia. ECGs were recorded digitally at baseline and after 3, 6 and 12 weeks. Between group effects were compared by using a mixed effect model, whereas assessment within group was compared by using a paired t-test. Treatment with sertindole was associated with QTcF and QTcB interval prolongation and an increase in MCS, T-wave asymmetry, T-wave flatness and TpTe. The mean increase in QTcF from baseline to last observation was 12.1ms for sertindole (p<0.001) and -0.5ms for quetiapine (p=0.8). Quetiapine caused no increase in MCS, T-wave asymmetry, T-wave flatness or TpTe compared to baseline. In the categorical analysis, there were 11 patients (9.6%) receiving quetiapine who experienced more than 20ms QTcF prolongation compared with 36 patients (33.3%) in the sertindole group. Sertindole (12-20mg) was associated with moderate QTc prolongation and worsening of T-wave morphology in a study population of patients with schizophrenia. Although, quetiapine (400-600mg) did not show worsening of repolarization measures some individual patients did experience significant worsening of repolarization. Clinical Trials NCT00654706.

The QTc effect of low-dose methadone for chronic pain: a prospective pilot study

OBJECTIVE

Methadone is associated with QT prolongation and serious cardiac complications, but this has been primarily demonstrated in opioid dependent patients receiving moderate to high doses. This study investigates the effect of low-dose methadone on the QTc interval in a chronic pain population.

DESIGN AND SUBJECTS

We conducted a prospective cohort study in a chronic pain clinic including 82 patients receiving methadone and 102 patients receiving non-methadone opioid therapy.

METHODS

We analyzed automated QTc calculations from 12-lead electrocardiograms at baseline and during the subsequent 6 months. The primary outcome of interest was the incidence of QTc greater than 470 milliseconds or an increase from baseline of greater than 60 milliseconds.

RESULTS

The methadone group did not manifest an overall higher frequency of QTc > 470 milliseconds (6% for the methadone group vs 5% for controls, P = 0.722) or an increase in the QTc of > 60 milliseconds (4% for the methadone group vs 4% for controls, P = 0.94). In the first month after initiating methadone, patients demonstrated an increase in QTc compared to controls (5% for the methadone group vs 0% for the controls, P = 0.073) but the difference disappeared in the third and sixth months.

CONCLUSION

Data from our chronic pain clinic support a potential association of QTc prolongation during the initiation of methadone, but this effect is small and short lived. We believe larger scale studies to further characterize the safety profile of low-dose methadone are warranted.

Prolonged QTc Interval Predicts Poststroke Paroxysmal Atrial Fibrillation

Background and Purpose—

Paroxysmal atrial fibrillation (PAF) is often difficult to detect in patients with acute ischemic stroke. We aimed to assess the predictive value of a prolonged QT interval corrected for heart rate (QTc) in PAF detection after acute ischemic stroke.

Methods—

We enrolled 972 patients with acute ischemic stroke consecutively extracted from our observational stroke registry system. Exclusion criteria were as follows: (1) AF on the initial 12-lead ECG (n=171); (2) previously diagnosed PAF (n=47); and (3) the use of a cardiac pacemaker (n=10). Of the 972 patients, 744 (mean age, 67.6 years; men, 62.6%) were eligible for analysis. The clinical characteristics and 12-lead ECG findings of the patients with and without PAF were compared, and multiple logistic regression analysis was performed to identify predictors of poststroke PAF.

Results—

The poststroke cardiac work-up yielded 69 (9.3%) de novo PAF cases among the 744 patients. The QTc interval was significantly longer in patients with PAF than in those without PAF (436 versus 417 ms; P <0.001). Each 10-ms increase in the QTc interval was associated with an increased risk of PAF after multivariate adjustments (odds ratio, 1.41; 95% confidence interval, 1.24–1.61; P <0.001). The optimal threshold value of QTc interval calculated by a receiver-operating characteristic curve was 438 ms, and the area under the curve was 0.73 in this data set.

Conclusions—

The QTc interval prolongation is potentially a strong and useful predictor for poststroke PAF.

Evaluating the Use of Linear Mixed-Effect Models for Inference of the Concentration-QTc Slope Estimate as a Surrogate for a Biological QTc Model

In concentration-QTc modeling, oscillatory functions have been used to characterize biological rhythms in QTc profiles. Fitting such functions is not always feasible because it requires sufficient electrocardiograph sampling. In this study, drug concentration and QTc data were simulated using a published biological QTc model (oscillatory functions). Then, linear mixed-effect models and the biological model were fitted and evaluated in terms of biases, precisions, and qualities of inferences. The simpler linear mixed-effect model with day and time as a factor variables provided similar accuracy of the concentration-QTc slope estimates to the complex biological model and was able to accurately predict the drug-induced QTc prolongation with less than 1 ms bias, despite its empirical nature to account for biological rhythm. The current study may guide a concentration-QTc modeling strategy that can be easily prespecified, does not suffer from poor convergence, and achieves little bias in drug-induced QTc estimates.

Sensitivity of pharmacokinetic-pharmacodynamic analysis for detecting small magnitudes of QTc prolongation in preclinical safety testing

INTRODUCTION

Preclinical concentration-effect (pharmacokinetic-pharmacodynamic, PKPD) modeling has successfully quantified QT effects of several drugs known for significant QT prolongation. This study investigated its sensitivity for detecting small magnitudes of QT-prolongation in a typical preclinical cardiovascular (CV) safety study in the conscious telemetered dog (crossover study in 4-8 animals receiving a vehicle and three dose levels). Results were compared with conventional statistical analysis (analysis of covariance, ANCOVA).

METHODS

A PKPD model predicting individual QTc was first developed from vehicle arms of 28 typical CV studies and one positive control study (sotalol). The model quantified between-animal, inter-occasion and within-animal variability and described QTc over 24h as a function of circadian variation and drug concentration. This "true" model was used to repeatedly (n = 500) simulate studies with typical drug-induced QTc prolongation (∆QTc) of 1 to 12 ms at high-dose peak concentrations. Simulated studies were re-analyzed by both PKPD analysis (with varying complexity) and ANCOVA. Sensitivity (power) was calculated as the percentage of studies in which a significant (α = 0.05) drug effect was found. One simulation scenario did not include a concentration-effect relationship and served to investigate false-positive rates. Exposure-effect relationships were derived from both PKPD analysis (linear concentration-effect) and ANCOVA (linear trend test for dose) and compared.

RESULTS

PKPD analysis/ANCOVA had a sensitivity of 80% to detect the effects of 7/13 ms (n = 4), 5/10 ms (n = 6) and 4.5/8 ms (n = 8), respectively. The false-positive rate was much higher using ANCOVA (40%) compared to PKPD analysis (1%). Typical drug effects were more precisely predicted using estimated concentration-effect slopes (± 1.5-2.8 ms) than dose-effect slopes (± 3.3-3.7 ms).

DISCUSSION

Preclinical PKPD analysis can increase the confidence in the quantification of small QTc effects and potentially allow reducing the number of animals while maintaining the required study sensitivity. This underscores the value of PKPD modeling in preclinical safety testing.

Timing of pre-dose baseline electrocardiograms in clinical trials: increased sampling over a prolonged baseline period worsens variance of QTc

BACKGROUND AND PURPOSE

The US Food and Drug Administration (US FDA) currently recommends recording of electrocardiograms (ECGs) prior to drug administration in thorough QT studies over an hour or more time to improve reliability of baseline ECG values. However, the baseline period is usually in the morning during a period of intense trial activity and rapid circadian change in QTc. The purpose of this study was to determine if the practice of recording an extended baseline does, in fact, decrease QTc variance at baseline.

METHODS

ECG data from three thorough QT studies (TQTS) in which three ECGs (commonly referred to as triplicates) were recorded at each of three pre-specified time points during the 60 to 90 minutes before drug administration were analyzed by determining the intra-subject and inter-subject standard deviation (SD) of QTcF (Fridericia-correct QT) for each of the three pre-drug time points and for the three time points combined.

RESULTS

QTcF was relatively normally distributed in each study. Intra-subject variability of QTcF was greater for the combined triplicate recordings than for the individual triplicates at baseline treatment time points in 39 of 42 cases (93%). This was the case in 48% of the comparisons in the inter-subject analysis.

CONCLUSIONS

The practice of recording three sets of triplicate ECGs over an hour or more before drug administration in a TQTS increases variability of baseline QTcF consistently in cross-over designed trials, and in roughly half of parallel comparisons. Higher variability suggests that the three-triplicate approach does not provide a more reliable baseline value. Less variability of QTcF can be obtained by simply recording one triplicate prior to drug administration. This principal may apply to other ECG and other physiological variables that have a monotonic circadian trend or that may be affected by intense trial activity during the pre-drug hour.

Moxifloxacin versus placebo modeling of the QT interval

The objectives were to develop a population model for placebo-corrected moxifloxacin QT interval in healthy subjects using non-linear mixed effects modeling and to examine effect of covariates on the observed QT. Based on the parameters of interest, optimizations of observation times and number of subjects were proposed. A pool of four thorough QT studies was used, representing 99 subjects receiving placebo and moxifloxacin. The data was modeled using Monolix. The placebo effect on QT was satisfactorily described using a 2-oscillator model. It reflected the circadian rhythm variability which is taken into account when assessing the time-matched mean difference on QT between treatment and baseline. Based on this model, the moxifloxacin effect on QT was satisfactorily described by the same equation with the adjunct of a direct and proportional drug concentration-effect. The Emax model provided the best description of the effect. The unique covariate was gender for both baseline QTc and individual heart rate correction factor. The present design included up to 16 observations for pharmacodynamics. Using this model, 9 observation times for pharmacodynamics provided satisfactory estimates for the parameters of interest (Emax). With 15% precision limit on Emax, 60 subjects was optimal. The simultaneous placebo-moxifloxacin QT model proposed is an interesting alternative to the ICH E14 guideline in assessing QT prolongation effect. This approach provides accurate information over a range of concentrations using different relationships (slope or Emax models) to quantify the drug-response relationship versus placebo. This allowed optimizing the observation times and number of subjects.

Assessing QT interval prolongation and its associated risks with antipsychotics

Several antipsychotics are associated with the ventricular tachycardia torsade de pointes (TdP), which may lead to sudden cardiac death (SCD), because of their inhibition of the cardiac delayed potassium rectifier channel. This inhibition extends the repolarization process of the ventricles of the heart, illustrated as a prolongation of the QT interval on a surface ECG. SCD in individuals receiving antipsychotics has an incidence of approximately 15 cases per 10,000 years of drug exposure but the exact association with TdP remains unknown because the diagnosis of TdP is uncertain. Most patients manifesting antipsychotic-associated TdP and subsequently SCD have well established risk factors for SCD, i.e. older age, female gender, hypokalaemia and cardiovascular disease. QT interval prolongation is the most widely used surrogate marker for assessing the risk of TdP but it is considered somewhat imprecise, partly because QT interval changes are subject to measurement error. In particular, drug-induced T-wave changes (e.g. flattening of the T-wave) may complicate the measurement of the QT interval. Furthermore, the QT interval depends on the heart rate and a corrected QT (QTc) interval is often used to compensate for this. Several correction formulas have been suggested, with Bazett's formula the most widely used. However, Bazett's formula overcorrects at a heart rate above 80 beats per minute and, therefore, Fridericia's formula is considered more appropriate to use in these cases. Several other surrogate markers for TdP have been developed but none of them is clinically implemented yet and QT interval prolongation is still considered the most valid surrogate marker. Although automated QT interval determination may offer some assistance, QT interval determination is best performed by a cardiologist skilled in its measurement. A QT interval >500 ms markedly increases the risk for TdP and SCD, and should lead to discontinuation of the offending drug and, if present, correction of underlying electrolyte disturbances, particularly serum potassium and magnesium derangements. Before prescribing antipsychotics that may increase the QTc interval, the clinician should ask about family and personal history of SCD, presyncope, syncope and cardiac arrhythmias, and recommend cardiology consultation if history is positive.

QT interval variability and adaptation to heart rate changes in patients with long QT syndrome

BACKGROUND

Increased QT variability (QTV) has been reported in conditions associated with ventricular arrhythmias. Data on QTV in patients with congenital long QT syndrome (LQTS) are limited.

METHODS

Ambulatory electrocardiogram recordings were analyzed in 23 genotyped LQTS patients and in 16 healthy subjects (C). Short-term QTV was compared between C and LQTS. The dependence of QT duration on heart rate was evaluated with three different linear models, based either on the RR interval preceding the QT interval (RR(0)), the RR interval preceding RR(0) (RR(-1)), or the average RR interval in the 60-second period before QT interval (mRR).

RESULTS

Short-term QTV was significantly higher in LQTS than in C subjects (14.94 +/- 9.33 vs 7.31 +/- 1.29 ms; P < 0.001). It was also higher in the non-LQT1 than in LQT1 patients (23.00 +/- 9.05 vs 8.74 +/- 1.56 ms; P < 0.001) and correlated positively with QTc in LQTS (r = 0.623, P < 0.002). In the C subjects, the linear model based on mRR predicted QT duration significantly better than models based on RR(0) and RR(-1). It also provided better fit than any nonlinear model based on RR(0). This was also true for LQT1 patients. For non-LQT1 patients, all models provided poor prediction of QT interval.

CONCLUSIONS

QTV is elevated in LQTS patients and is correlated with QTc in LQTS. Significant differences with respect to QTV exist among different genotypes. QT interval duration is strongly affected by noninstantaneous heart rate in both C and LQT1 subjects. These findings could improve formulas for QT interval correction and provide insight on cellular mechanisms of QT adaptation.

QT intervals and QT dispersion determined from a 12-lead 24-hour Holter recording in patients with coronary artery disease and patients with heart failure

BACKGROUND

QT dispersion is considered an index of spatial inhomogeneity of repolarization duration and increased dispersion of ventricular repolarization is supposed to increase the risk of ventricular arrhythmia. Circadian variation in QT dispersion was investigated.

METHODS

Three different modes of lead selection was used: all 12-leads (QTdisp 12), only precordial leads (QTdisp 6), and one pair of preselected leads (QTdisp 2) in a 24-hour Holter recording every fourth hour each comprising 10 consecutive measurements in 54 healthy subjects, 29 patients with coronary artery disease (CAD), and 29 patients with heart failure (HF).

RESULTS

A significant circadian variation was observed in healthy subjects when modes QTdisp 12 and QTdisp 6 were used (Mean +/- SD 35.58 +/- 16.48 ms; P < 0.0001; and 28.82 +/- 16.02 ms; P < 0.0001, respectively), and in patients with CAD (Mean +/- SD 37.86 +/- 17.87 ms; P < 0.01; and 28.72 +/- 17.06 ms; P < 0.0001, respectively), whereas no circadian variation was observed in QTdisp 2. No circadian variation was observed in patients with HF irrespectively of lead selection. Patients with CAD without myocardial infarction (MI) had a circadian variation in QTdisp 12 (Mean +/- SD 33.13 +/- 14.86 ms; P < 0.05), whereas no circadian variation was observed in patients with MI (Mean +/- SD 40.35 +/- 18.80 ms; P = NS).

CONCLUSIONS

Circadian variation of QT dispersion was detected in healthy subjects and in patients with uncomplicated CAD, but not in those who had suffered a previous MI and in patients with HF. The number of leads among which selection of the longest and shortest QT intervals took place was critical for the disclosure of circadian variation of QT dispersion.

A highly sensitive canine telemetry model for detection of QT interval prolongation: Studies with moxifloxacin, haloperidol and MK-499

INTRODUCTION

Preclinical evaluation of delayed ventricular repolarization manifests electrocardiographically as QT interval prolongation and is routinely used as an indicator of potential risk for pro-arrhythmia (potential to cause Torsades de Pointes) of novel human pharmaceuticals. In accordance with ICH S7A and S7B guidelines we evaluated the sensitivity and validity of the beagle dog telemetry (Integrated Telemetry Services (ITS)) model as a preclinical predictor of QT interval prolongation in humans.

METHODS

Cardiovascular monitoring was conducted for 2 h pre-dose and 24 h post-dosing with moxifloxacin (MOX), haloperidol (HAL), and MK-499, with a toxicokinetic (TK) evaluation in a separate group of dogs. In both cardiovascular and TK studies, MOX (0, 10, 30 and 100 mg/kg), HAL (0, 0.3, 1, 3 mg/kg) and MK-499 (0, 0.03, 0.3 and 3 mg/kg) were administered orally by gavage in 0.5% methylcellulose. Each dog received all 4 doses using a dose-escalation paradigm. Inherent variability of the model was assessed with administration of vehicle (0.5% methylcellulose) alone for 4 days.

RESULTS

Significant increases in QT(c) were evident with 10, 30 and 100 mg/kg of MOX (C(max)< or =40 microM), 0.3, 1 and 3 mg/kg of HAL (C(max)< or =0.36 microM) and 0.3 and 3 mg/kg of MK-499 (C(max)< or =825 nM) with peak increases of 45 (20%), 31 (13%), and 45 (19%) ms, respectively (p< or =0.05).

DISCUSSION

In conclusion, we have demonstrated that the ITS-telemetry beagle dog exhibits low inherent intra-animal variability and high sensitivity to detect small but significant increases in QT/QT(c) interval ( approximately 3-6%) with MOX, HAL and MK-499 in the same range of therapeutic plasma concentrations attained in humans. Therefore, this dog telemetry model should be considered an important preclinical predictor of QT prolongation of novel human pharmaceuticals.

Magnitude of error introduced by application of heart rate correction formulas to the canine QT interval

BACKGROUND

Accurate detection of drug-induced QT interval changes is often confounded by concurrent heart rate changes. Application of heart rate correction formulas has been the traditional approach to account for heart rate-induced QT interval changes, and thereby identify the direct effect of the test article on cardiac repolarization. Despite numerous recent studies identifying the imprecision of these formulas they continue to be applied.

METHODS

Using a chronic atrioventricular dissociated His-paced canine model, heart rate correction methods were evaluated for their ability to generate a corrected QT interval independent of original heart rate. Additionally, His bundle pacing at a heart rate of 60 beats/min allowed calculation of the magnitude of error introduced by application of heart rate correction formulas.

RESULTS

Of the fixed parameter heart rate correction formulas, only Van de Water was able to predict corrected QT values independent of the original heart rate. The magnitude of error discovered by application of heart rate correction formulas varied, but in many cases was very large. Bazett's formula was associated with a mean overcorrection of 67.9 ms; Fridericia's 28.7 ms. Van de Water was the best fixed parameter formula with a mean error of 10.8 ms. As expected, group and individual corrections derived from linear regression of the HR-QT data offered improvement over the traditional formulas. Both were able to predict QTc values independent of the heart rate. However, errors of the magnitude of 10 and 6 ms, respectively, were still introduced.

CONCLUSION

Van de Water and linear regression correction methods were superior to others in this study, but all methods generated QTc errors equal to or much greater than the magnitude of interest for drug safety evaluation.

The impact of posture on cardiac repolarization: more than heart rate?

INTRODUCTION

The effect of standing on heart rate and QT is well known but its impact on QTc is less clear.

METHODS

Serial supine and standing 12-lead ECGs (seven pairs each day) were recorded from 54 healthy volunteers each day of a three-day period. ECGs were captured digitally and over-read by a cardiologist.

RESULTS

A statistically significant shortening of RR (216 ms), QT (40 ms), and decreases in QTc-F (Fridericia) and QTc-LR (Framingham) were demonstrated on standing (8.3 and 6.9 ms, respectively). In contrast, QTc-B (Bazett's) significantly increased by 9.6 ms. Two subject-individualized correction methods were derived using each subject's supine measurements. Both showed significant decreases in QTc of approximately 13-14 ms upon standing. Using the bin analysis method, comparisons between positions using 25 ms interval RR bins revealed significant QT shortening of up to 15 ms upon standing.

CONCLUSION

We have demonstrated a postural effect on cardiac repolarization independent of heart rate using two individualized correction methods, as well as QTc-F and QTc-LR, and the bin method. Characterization of postural differences in QT/QTc (other than QTc-B) may provide a safe and inexpensive physiological control to validate the ECG methodology used in clinical trials to assess potential drug-induced QT interval changes.

Cardiovascular monkey telemetry: Sensitivity to detect QT interval prolongation

INTRODUCTION

Preclinical evaluation of delayed ventricular repolarization manifests electrocardiographically as QT interval prolongation and is routinely used as an indicator of potential risk for pro-arrhythmia (potential to cause Torsades de Pointes) of novel human pharmaceuticals. In accordance with ICH S7A and S7B guidelines we evaluated the sensitivity and validity of the monkey telemetry model as a preclinical predictor of QT interval prolongation in humans.

METHODS

Cardiovascular monitoring was conducted for 2 h pre-dose and 24 h post-dosing with Moxifloxacin (MOX), with a toxicokinetic (TK) evaluation in a separate group of monkeys. In both studies, MOX was administered orally by gavage in 0.5% methylcellulose at 0, 10, 30, 100, 175 mg/kg. Each monkey received all 5 doses using a dose-escalation paradigm. Inherent variability of the model was assessed with administration of vehicle alone for 4 days in all 4 monkeys (0.5% methylcellulose in deionized water).

RESULTS

MOX had no significant effect on mean arterial pressure, heart rate, PR or QRS intervals. MOX produced significant dose-related increases in QTc at doses of 30 (Cmax=5.5+/-0.6 microM), 100 (Cmax=16.5+/-1.6 microM), and 175 (Cmax=17.3+/-0.7 microM) mg/kg with peak increases of 22 (8%), 27 (10%), and 47 (18%) ms, respectively (p<or=0.05; compared to vehicle).

DISCUSSION

In conclusion, we have developed a reproducible, sensitive and reliable primate telemetry model in rhesus monkeys, which exhibits low inherent intra-animal variability and high sensitivity to detect small but significant increases in QT/QTc interval (approximately 4%) with MOX in the same range of therapeutic plasma concentrations attained in humans. Therefore, the primate telemetry model should be considered an important preclinical predictor of QT prolongation of novel human pharmaceuticals.

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

INTRODUCTION

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

METHODS

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

RESULTS

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

DISCUSSION

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

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.

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.

EFFECT OF AUTONOMIC NERVOUS FUNCTION ON QT INTERVAL IN DOGS

The effects of drugs on the QT interval should be evaluated precisely in the early stages of drug development because QT prolongation can trigger the so-called torsades de pointes, a life-threatening polymorphic ventricular tachycardia. It has been reported that the QT interval is affected by autonomic nervous tone besides the heart rate. In this study, we investigated the direct effect of autonomic nervous tone on the QT interval using the parameters of heart rate variability in dogs, when the RR interval was constant (400 or 700 msec). Our results showed that the QT interval at the high HF (high vagal tone) or low LF/HF ratio (low sympathetic tone) was longer than that at the low HF (low vagal tone) or high LF/HF ratio (high sympathetic tone), when the RR intervals were constant, and that the effect of vagal tone on the QT interval might be somewhat stronger than that of the sympathetic tone. The present observations would support the idea that sympathetic as well as parasympathetic tone regulates QT interval and that QT interval may be controlled physiologically by myocardial autonomic nerves via and not via a sinus node. Therefore, a more precise correction formula of QT interval could be established using autonomic parameters other than RR interval (heart rate), while the QT interval is widely known to be dependent on the RR interval or heart rate.