Methods of assessment and clinical relevance of QT dynamics

The dependence on heart rate of the QT interval has been investigated for many years and several mathematical formulae have been proposed to describe the QT interval/heart rate (or QT interval/RR interval) relationship. While the most popular is Bazett's formula, it overcorrects the QT interval at high heart rates and under-corrects it at slow heart rates. This formulae and many others similar ones, do not accurately describe the natural behaviour of the QT interval. The QT interval/RR interval relationship is generally described as QT dynamics. In recent years, several methods of its assessment have been proposed, the most popular of which is linear regression. An increased steepness of the linear QT/RR slope correlates with the risk of arrhythmic death following myocardial infarction. It has also been demonstrated that the QT interval adapts to heart rate changes with a delay (QT hysteresis) and that QT dynamics parameters vary over time. New methods of QT dynamics assessment that take into account these phenomena have been proposed. Using these methods, changes in QT dynamics have been observed in patients with advanced heart failure, and during morning hours in patients with ischemic heart disease and history of cardiac arrest. The assessment of QT dynamics is a new and promising tool for identifying patients at increased risk of arrhythmic events and for studying the effect of drugs on ventricular repolarisation.     

Pharmacogenetic issues in thorough QT trials

Drug-induced QT prolongation (DI-LQT), through its associated arrhythmias, is a leading cause of drugs being withdrawn from the market. As a consequence, the US FDA and other regulatory agencies are mandating that all new drugs go through a so-called 'Thorough QT' (TQT) study to evaluate the potential for 'QT liability', specifically the potential for a drug to cause a discernible increase in the QT interval. Several genetic factors that modulate the risk of DI-LQT have been discovered. These are genes responsible for the congenital long QT syndrome, drug metabolism genes (mainly CYP2D6 and CYP3A4), and genes in other regulatory pathways. Here, we briefly review the links between genetic variants and drug-induced QT risk, and propose approaches to consider for using pharmacogenetics in planning and analyzing TQT studies.     

The QT Interval and Selection of Alpha-Blockers for Benign Prostatic Hyperplasia

The QT interval is the electrocardiographic manifestation of ventricular depolarization and repolarization. Drug-induced long QT syndrome is characterized by acquired, corrected QT (QTc) interval prolongation that is associated with increased risk of torsade de pointes. Every physician must recognize if the drugs he or she prescribes prolongs the QTc interval, especially if the drug is prescribed for a chronic condition in older patients who are on polypharmacy. The evolution of alpha-blockers for the treatment of benign prostatic hyperplasia has allowed the development of drugs that are easier to administer and better tolerated. Because alpha-blockers generally have equivalent efficacy, this class of drugs is typically differentiated by safety and side effects. Studies suggest that alpha-blockers may vary in regard to their effect on the QT interval, and, therefore, on their predisposition to cause potentially life-threatening ventricular arrhythmias.     

Issues in QT interval measurement

The QT interval, apart from clinical implications is crucial for safety assessment of new drugs under development. A QTc prolongation of even 10 msec in a study group is a warning signal for a new drug. There are various issues involved in the measurement of the QT interval especially regarding the ending of the T wave and different morphological pattern of T-U complex. The other issue is significant spontaneous variability in the QT interval, resulting in spurious QT prolongation and unnecessary concern.To minimize all these confounding factors, all clinical trials for assessing QT interval prolongation should be randomized and double blinded with appropriate control groups including placebo. ECG measurements should be done by trained readers with electronic calipers at ECG core Lab. ECGs should be compared with multiple baseline values with multiple, time-matched on-treatment values.     

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

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

Congenital Short QT Syndrome

The Short QT Syndrome is a recently described new genetic disorder, characterized by abnormally short QT interval, paroxysmal atrial fibrillation and life threatening ventricular arrhythmias. This autosomal dominant syndrome can afflict infants, children, or young adults; often a remarkable family background of cardiac sudden death is elucidated. At electrophysiological study, short atrial and ventricular refractory periods are found, with atrial fibrillation and polymorphic ventricular tachycardia easily induced by programmed electrical stimulation. Gain of function mutations in three genes encoding K⁺ channels have been identified, explaining the abbreviated repolarization seen in this condition: KCNH2 for I_kr (SQT1), KCNQ1 for I_ks (SQT2) and KCNJ2 for I_k1 (SQT3). The currently suggested therapeutic strategy is an ICD implantation, although many concerns exist for asymptomatic patients, especially in pediatric age. Pharmacological treatment is still under evaluation; quinidine has shown to prolong QT and reduce the inducibility of ventricular arrhythmias, but awaits additional confirmatory clinical data.     

A chronobiological study on the relation between heart rate and QT interval duration in newborn infants

The relation between heart rate and QT interval is the result of the autonomic nervous system control on cardiac function in healthy adults; accordingly, chronobiological studies have shown that adult subjects have circadian rhythms of heart rate (expressed as R-R interval) and QT interval in phase. We have employed chronobiological methods to study heart rate and QT interval relation in 10 newborn infants, who are known to have an immature cardiac control. Findings from this study indicate that not all the newborns show circadian rhythms of heart rate and QT interval and that when both rhythms are present they do not correlate like in the adults. Likely, this lack of relationship between heart rate and QT interval in newborns is due to different maturational stages of the newborns studied. As a practical implication, in newborn infants, mathematical correction of QT interval by heart rate is not a reliable method.     

Predicting the cardiac toxicity of drugs using a novel multiscale exposure–response simulator

A common but serious side effect of many drugs is torsades de pointes, a rhythm disorder that can have fatal consequences. Torsadogenic risk has traditionally been associated with blockage of a specific potassium channel and an increased recovery period in the electrocardiogram. However, the mechanisms that trigger torsades de pointes remain incompletely understood. Here we establish a computational model to explore how drug-induced effects propagate from the single channel, via the single cell, to the whole heart level. Our mechanistic exposure–response simulator translates block-concentration characteristics for arbitrary drugs into three-dimensional excitation profiles and electrocardiogram recordings to rapidly assess torsadogenic risk. For the drug of dofetilide, we show that this risk is highly dose-dependent: at a concentration of 1x, QT prolongation is 55% but the heart maintains its regular sinus rhythm; at 5.7x, QT prolongation is 102% and the heart spontaneously transitions into torsades de points; at 30x, QT prolongation is 132% and the heart adapts a quasi-depolarized state with numerous rapidly flickering local excitations. Our simulations suggest that neither potassium channel blockage nor QT interval prolongation alone trigger torsades de pointes. The underlying mechanism predicted by our model is early afterdepolarization, which translates into pronounced U waves in the electrocardiogram, a signature that is correctly predicted by our model. Beyond the risk assessment of existing drugs, our exposure–response simulator can become a powerful tool to optimize the co-administration of drugs and, ultimately, guide the design of new drugs toward reducing life threatening drug-induced rhythm disorders in the heart.     

Pharmacometabolomic Approach to Predict QT Prolongation in Guinea Pigs

Drug-induced torsades de pointes (TdP), a life-threatening arrhythmia associated with prolongation of the QT interval, has been a significant reason for withdrawal of several medicines from the market. Prolongation of the QT interval is considered as the best biomarker for predicting the torsadogenic risk of a new chemical entity. Because of the difficulty assessing the risk for TdP during drug development, we evaluated the metabolic phenotype for predicting QT prolongation induced by sparfloxacin, and elucidated the metabolic pathway related to the QT prolongation. We performed electrocardiography analysis and liquid chromatography–mass spectroscopy-based metabolic profiling of plasma samples obtained from 15 guinea pigs after administration of sparfloxacin at doses of 33.3, 100, and 300 mg/kg. Principal component analysis and partial least squares modelling were conducted to select the metabolites that substantially contributed to the prediction of QT prolongation. QTc increased significantly with increasing dose (r = 0.93). From the PLS analysis, the key metabolites that showed the highest variable importance in the projection values (>1.5) were selected, identified, and used to determine the metabolic network. In particular, cytidine-5′-diphosphate (CDP), deoxycorticosterone, L-aspartic acid and stearic acid were found to be final metabolomic phenotypes for the prediction of QT prolongation. Metabolomic phenotypes for predicting drug-induced QT prolongation of sparfloxacin were developed and can be applied to cardiac toxicity screening of other drugs. In addition, this integrative pharmacometabolomic approach would serve as a good tool for predicting pharmacodynamic or toxicological effects caused by changes in dose.     

Preclinical cardio-safety assessment of torsadogenic risk and alternative methods to animal experimentation: The inseparable twins

The last decade has been marked by the withdrawal from the market of several medicines whose use in patients has been associated with the development of torsade de pointes (TdP), a potentially life-threatening polymorphic tachycardia. In a few cases, TdP can degenerate into ventricular fibrillation and lead to sudden death, thus constituting a real problem of public health. The recently finalized ICH S7B guideline defines the prolongation of the QT interval on the electrocardiogram as the best biomarker for predicting the torsadogenic risk of a given compound. However, a growing body of evidence suggests that drugs’ torsadogenic potential may not necessarily be proportional to their ability to prolong the QT interval. It is a dynamic combination of multiple predisposing factors and components rather than a single particular event that can trigger this particular tachycardia. Following recommendations of the guideline, pharmaceutical companies have intensively implemented methodologies to assess the possible risk of QT prolongation and TdP in humans. The main problem in cardiac safety pharmacology is how best to combine the capabilities of different methodologies with their strengths and limitations in order to detect the potential of one molecular entity to induce a lethal arrhythmia of very low clinical incidence. This publication will review the current methodologies, focusing on the alternative methods to animal experimentation, including an overview of cardiac modeling.     

The electrocardiogram of the Beagle dog: reference values and effect of sex, genetic strain, body position and heart rate

The aim of the study was to establish a database for electrocardiographic parameters of Beagle dogs used for toxicological studies and to evaluate the influence of supplier, sex, heart rate (HR) and body position for electrocardiogram (ECG) recording on ECG parameters. Peripheral ECG leads were recorded from 934 female and 946 male dogs from Marshall Farms and 27 females and 30 males from Harlan, either standing on a table or restrained in a hammock. HR, RR, PQ and QT intervals, P and QRS duration and P-wave amplitude were measured. There were no major differences between sexes for ECG parameters. The axis of the heart was shifted to the left when the animals were restrained in a hammock compared to when they were standing on a table. The PQ interval was higher (about 9%) in Harlan than in Marshall dogs. HR was negatively correlated with QT (coefficient of linear correlation: r=-0.61 to -0.74), which emphasizes the need for a formula correcting QT interval for HR when interpreting changes in QT interval. HR was also negatively correlated with PQ intervals (r=-0.26 to -0.11), whereas a positive correlation was found between HR and the amplitude of the P wave (r=0.21-0.34). The level of the respiratory sinus arrhythmia (SA) was quantified by calculating the ratio of maximum to minimum RR interval measured over a 10 s period. This ratio was negatively correlated with HR (r =-0.49 to -0.33). Therefore, at high HRs, SA was less marked than at low HRs, but it did not completely disappear. Analysis of beat-to-beat variation indicated that QT and PQ intervals and the amplitude of P wave fluctuated over time and the degree of this variability was positively correlated with the level of SA. In conclusion, we have established reference values for the duration and/or amplitude of some ECG parameters both in terms of means and variability over the recording period, and we have evaluated the influence of body position, genetic strain and HR on the ECG parameters. These data can be used as baseline for the interpretation of the ECG of Beagle dogs.     

Quantitative relationship between plasma potassium levels and QT interval in beagle dogs

The aim of the current study was to establish the quantitative relationship between plasma potassium concentrations and the QT interval of the electrocardiogram in dogs. Furosemide, a potent diuretic, was given at increasing doses (5-60 mg/kg) to five male and five female beagle dogs. Electrocardiogram (ECG) was recorded three times each day, simultaneous to blood sampling for measurement of plasma potassium. Furosemide treatment produced a clear hypokalaemia, which was associated with an increase in QT and corrected QT intervals (QTc) duration. On average, the slopes of the negative linear correlation between potassium plasma levels and QT or QTc were steeper in females than in males. These results show that a decrease in potassium plasma level may explain a concomitant increase in QT duration in a toxicity study in dogs, in particular if potassium values are decreased below 3.3 mmol/L. Correction of QT interval for K+ plasma level has, therefore, been established separately for males and females. A global formula correcting QT for K+ and heart rate simultaneously was established. Hypokalaemia was also associated with changes in the morphology of the T wave recorded in CV5RL, in particular, with a flattening and/or a notching of the wave (appearance of a second peak), biphasic aspect or inversion of polarity. These changes are probably related to an increased heterogeneity of repolarization between different populations of cardiomyocytes. In conclusion, hypokalaemia is quantitatively associated with an increase in QT and QTc duration in dogs. The relationship is apparently stronger for females than for males. A formula may be used to correct QT for potassium plasma level.     

A 24-HOUR AMBULATORY ECG MONITORING IN ASSESSMENT OF QT INTERVAL DURATION AND DISPERSION IN ROWERS WITH PHYSIOLOGICAL MYOCARDIAL HYPERTROPHY

Myocardial hypertrophy (MH) due to cardiac pathology is characterized by an increase in QT interval duration and dispersion, while the findings for exercise-induced myocardial hypertrophy are contradictory. The majority of published research findings have not explored this relationship, but there have only been a few conducted studies using 24-hour ECG monitoring. The aim of the study was to determine the QT interval duration and dispersion in short-term and 24-hour ECG in endurance athletes with myocardial hypertrophy and without it. Methods: A total of 26 well-trained rowers underwent a resting 12-lead ECG, 24-hour ECG monitoring and echocardiography. Results: Athletes with MH (n = 7) at rest did not show any increase in QTc interval duration and dispersion, or mean and maximal QTc duration in Holter monitoring compared to athletes without MH (n = 19). Left ventricular mass was not significantly correlated with any QTc characteristics. Furthermore, athletes with MH had significantly longer mean QT (P = 0.01) and maximal QT (P = 0.018) intervals in Holter monitoring and higher 24-hour heart rate variability indexes due to stronger vagal effects. Conclusions: The present study demonstrated that athlete''s heart syndrome with myocardial hypertrophy as a benign phenomenon does not lead to an increase in QT interval duration, or increases in maximal and mean duration in a 24-hour ECG. An increase in QT interval duration in athletes may have an autonomic nature.     

Cardiac-specific overexpression of SCN5A gene leads to shorter P wave duration and PR interval in transgenic mice

The Cardiac sodium channel gene SCN5A plays a critical role in cardiac electrophysiology and its mutations, either gain- or loss-of-functions, are associated with lethal arrhythmias. In this study, we investigated the effect of overexpression of SCN5A on the cardiac phenotype in a transgenic mouse model (TG-WT L10). Compared to NTG mice, heart rate, QRS duration, and QT intervals remained unchanged in TG-WT mice. Moreover, no spontaneous ventricular arrhythmias were detected in TG-WT hearts. Despite these results, a mild, irregular cardiac phenotype was observed in TG-WT mice. The P wave and PR interval were significantly shorter in TG-WT compared with NTG mice (P, 8.8+/-0.8 ms vs. 12.6+/-0.9 ms; PR, 12.5+/-2 ms vs. 33.5+/-0.7 ms). Furthermore, spontaneous premature atrial contractions were often detected in TG-WT mice. These results suggest that the expression level of the SCN5A gene is a determinant for the length of the P wave duration and PR interval on electrocardiograms (ECG).     

A common variant in SLC8A1 is associated with the duration of the electrocardiographic QT interval

Prolongation of the electrocardiographic QT interval, a measure of cardiac repolarization, predisposes one to ventricular arrhythmias and sudden cardiac death. Since NOS1AP, a regulator of neuronal nitric oxide synthase, was discovered in a genome-wide association study (GWAS) as a novel target that modulates cardiac repolarization, several loci have been linked to the QT interval in studies (QTGEN and QTSCD) of European descendents. However, there has been no GWAS of the QT interval in Asian populations. We conducted a GWAS with regard to the QT interval in Korea Association Resource (KARE [n = 6,805]) cohorts. Replication studies in independent populations of Korean (n = 4,686) and Japanese (n = 2,687) groups validated the association between a SNP, rs13017846, which maps to near SLC8A1 (sodium/calcium exchanger 1 precursor, overall p = 8.0 × 10(-14)), and the QT interval. The minor allele frequency (MAF) of rs13017846 varies widely between ethnicities-0.053 in Europeans (HapMap CEU [Utah residents with ancestry from northern and western Europe from the Centre d'étude du Polymorphisme Humain collection] samples) versus 0.080 in Africans (HapMap YRI [Yoruba in Ibadan, Nigeria] samples)-whereas a MAF of 0.500 has been reported in Asians (HapMap HCB [Han Chinese in Beijing, China] and JPT [Japanese in Tokyo, Japan] samples). This might explain why this locus has not been identified in Europeans in previous studies.     

The antipsychotic drugs sertindole and pimozide block erg3, a human brain K(+) channel

The antipsychotic drugs sertindole and pimozide are known to prolong the QT interval on the electrocardiogram via a high affinity block of the cardiac K(+) channel known as HERG (human ether-a-go-go-related gene; erg1). We wished to test whether these drugs also displayed high affinity for the related neuronal K(+) channel erg3. The cDNA encoding erg3 channel was cloned from a human brain library. Northern analysis confirmed that the channel was localized to brain relative to other tissues including heart, liver and lung. Within the brain, erg3 was expressed in higher amounts in the frontal lobe and cerebellum relative to the temporal, parietal and occipital lobes. Transient expression of erg3 in Chinese hamster ovary cells produced outwardly directed K(+) currents that activated at approximately -50 mV and produced a large transient component at positive membrane potentials. Inward tail currents measured at -100 mV were blocked in a dose-dependent fashion by sertindole resulting in an IC(50) value of 43 nM. Significant inhibition was observed at concentrations as low as 3 nM. Block of erg3 by sertindole also displayed a positive voltage-dependence. Pimozide blocked erg3 channel currents with an IC(50) of 103 nM and significant inhibition was noted at concentrations of 10 nM and higher. We conclude that erg3 can be blocked by certain antipsychotic drugs like sertindole and pimozide. Inhibition of erg3 or related K(+) channels in the brain may contribute to the efficacy/side effect profiles of some antipsychotic drugs.     

Review of T-wave morphology-based biomarkers of ventricular repolarisation using the surface electrocardiogram

Many drugs fail in clinical trials due to adverse effects on cardiac electrical function, as measured by an increase in the QT interval in the surface electrocardiogram (ECG). However, there are several limitations associated with the QT interval, including poor sensitivity and specificity in predicting drug-induced arrhythmia. This is a growing concern for both regulatory and pharmaceutical agencies, as it translates into significant socio-economic costs. As a result, there has been a growing interest in identifying alternative biomarkers of drug-induced arrhythmia. Studies of the electrophysiological mechanisms underlying drug-induced arrhythmia have identified the morphology of the T-wave as a potential indicator of proarrhythmic activity. A plethora of new T-wave morphology based biomarkers have been proposed recently. This article presents a comprehensive review of the recently published biomarkers of drug-induced arrhythmia based on T-wave morphology.     

Computational prediction of drug response in short QT syndrome type 1 based on measurements of compound effect in stem cell-derived cardiomyocytes

Short QT (SQT) syndrome is a genetic cardiac disorder characterized by an abbreviated QT interval of the patient’s electrocardiogram. The syndrome is associated with increased risk of arrhythmia and sudden cardiac death and can arise from a number of ion channel mutations. Cardiomyocytes derived from induced pluripotent stem cells generated from SQT patients (SQT hiPSC-CMs) provide promising platforms for testing pharmacological treatments directly in human cardiac cells exhibiting mutations specific for the syndrome. However, a difficulty is posed by the relative immaturity of hiPSC-CMs, with the possibility that drug effects observed in SQT hiPSC-CMs could be very different from the corresponding drug effect in vivo. In this paper, we apply a multistep computational procedure for translating measured drug effects from these cells to human QT response. This process first detects drug effects on individual ion channels based on measurements of SQT hiPSC-CMs and then uses these results to estimate the drug effects on ventricular action potentials and QT intervals of adult SQT patients. We find that the procedure is able to identify IC₅₀ values in line with measured values for the four drugs quinidine, ivabradine, ajmaline and mexiletine. In addition, the predicted effect of quinidine on the adult QT interval is in good agreement with measured effects of quinidine for adult patients. Consequently, the computational procedure appears to be a useful tool for helping predicting adult drug responses from pure in vitro measurements of patient derived cell lines.     

Genetic determinants of QT interval variation and sudden cardiac death

Electrocardiographic QT interval prolongation or shortening is a risk factor for sudden cardiac death. The study of Mendelian syndromes in families with extreme long and short QT interval duration and ventricular arrhythmias has led to the identification of genes encoding ion channel proteins important in myocardial repolarization. Rare mutations in such ion channel genes do not individually contribute substantially to the population burden of ventricular arrhythmias and sudden cardiac death. Only now are studies systematically testing the relationship between common variants in these genes--or elsewhere in the genome--and QT interval variation and sudden cardiac death. Identification of genetic variation underlying myocardial repolarization could have important implications for the prevention of both sporadic and drug-induced arrhythmias.