Can non-clinical repolarization assays predict the results of clinical thorough QT studies? Results from a research consortium.

BACKGROUND AND PURPOSE: Translation of non-clinical markers of delayed ventricular repolarization to clinical prolongation of the QT interval corrected for heart rate (QTc) (a biomarker for torsades de pointes proarrhythmia) remains an issue in drug discovery and regulatory evaluations. We retrospectively analysed 150 drug applications in a US Food and Drug Administration database to determine the utility of established non-clinical in vitro IKr current human ether-à-go-go-related gene (hERG), action potential duration (APD) and in vivo (QTc) repolarization assays to detect and predict clinical QTc prolongation. EXPERIMENTAL APPROACH: The predictive performance of three non-clinical assays was compared with clinical thorough QT study outcomes based on free clinical plasma drug concentrations using sensitivity and specificity, receiver operating characteristic (ROC) curves, positive (PPVs) and negative predictive values (NPVs) and likelihood ratios (LRs). KEY RESULTS: Non-clinical assays demonstrated robust specificity (high true negative rate) but poor sensitivity (low true positive rate) for clinical QTc prolongation at low-intermediate (1×-30×) clinical exposure multiples. The QTc assay provided the most robust PPVs and NPVs (ability to predict clinical QTc prolongation). ROC curves (overall test accuracy) and LRs (ability to influence post-test probabilities) demonstrated overall marginal performance for hERG and QTc assays (best at 30× exposures), while the APD assay demonstrated minimal value. CONCLUSIONS AND IMPLICATIONS: The predictive value of hERG, APD and QTc assays varies, with drug concentrations strongly affecting translational performance. While useful in guiding preclinical candidates without clinical QT prolongation, hERG and QTc repolarization assays provide greater value compared with the APD assay.

Halide ion effects on human Ether-à-go-go related gene potassium channel properties.

The human Ether-à-go-go related gene (hERG) potassium channel has been widely used to counter screen potential pharmaceuticals as a biomarker to predict clinical QT prolongation. Thus, higher throughput assays of hERG are valuable for early in vitro screening of drug candidates to minimize failure in later-stage drug development due to this potentially adverse cardiac risk. We have developed a novel method utilizing potassium fluoride to improve throughput of hERG counter screening with an automated patch clamp system, PatchXpress 7000A. In that method, ∼50% substitution of internal Cl(-) with F(-) greatly increases success rate without substantially altering the biophysical properties of the hERG channel or compromising data quality. However, effect of F(-) or other halide ions on hERG channel properties has not been studied in detail. In this study, we examined effects of complete replacement of Cl(-) in internal solution with halide ions, F(-), or Br(-). We found that (1) F(-) slightly shifts the voltage dependence of hERG channel activation to more positive voltages, while Br(-) shifts it to more negative voltages; (2) Br(-) shifts to more positive voltages both the inactivation-voltage relationship and the peak position of channel full activation of hERG; (3) F(-) slows hERG activation, while both F(-) and Br(-) make the channel close faster; (4) neither F(-) nor Br(-) have any effect on hERG inactivation kinetics. In conclusion, compared to Cl(-), F(-) has subtle effect on hERG activation, while Br(-) has distinct effects on certain, but not all biophysical properties of hERG channel.

Sequence and structure-specific elements of HERG mRNA determine channel synthesis and trafficking efficiency.

Human ether-á-gogo-related gene (HERG) encodes a potassium channel that is highly susceptible to deleterious mutations resulting in susceptibility to fatal cardiac arrhythmias. Most mutations adversely affect HERG channel assembly and trafficking. Why the channel is so vulnerable to missense mutations is not well understood. Since nothing is known of how mRNA structural elements factor in channel processing, we synthesized a codon-modified HERG cDNA (HERG-CM) where the codons were synonymously changed to reduce GC content, secondary structure, and rare codon usage. HERG-CM produced typical IKr-like currents; however, channel synthesis and processing were markedly different. Translation efficiency was reduced for HERG-CM, as determined by heterologous expression, in vitro translation, and polysomal profiling. Trafficking efficiency to the cell surface was greatly enhanced, as assayed by immunofluorescence, subcellular fractionation, and surface labeling. Chimeras of HERG-NT/CM indicated that trafficking efficiency was largely dependent on 5' sequences, while translation efficiency involved multiple areas. These results suggest that HERG translation and trafficking rates are independently governed by noncoding information in various regions of the mRNA molecule. Noncoding information embedded within the mRNA may play a role in the pathogenesis of hereditary arrhythmia syndromes and could provide an avenue for targeted therapeutics.

Klotho sensitivity of the hERG channel.

Klotho, a hormone and enzyme, is a powerful regulator of ageing and life span. Klotho deficiency leads to cardiac arrythmia and sudden cardiac death. We thus explored whether klotho modifies cardiac K(+)-channel hERG. Current was determined utilizing dual electrode voltage clamp and hERG protein abundance utilizing immunohistochemistry and chemiluminescence in Xenopus oocytes expressing hERG with or without klotho. Coexpression of klotho increased cell membrane hERG-protein abundance and hERG current at any given voltage without significantly modifying the voltage required to activate the channel. The effect of klotho coexpression was mimicked by recombinant klotho protein and reversed by ß-glucuronidase-inhibitor D-saccharic acid-1,4-lactone.

Interactions between hERG and KCNQ1 α-subunits are mediated by their COOH termini and modulated by cAMP.

KCNQ1 and hERG encode the voltage-gated potassium channel α-subunits of the cardiac repolarizing currents I(Ks) and I(Kr), respectively. These currents function in vivo with some redundancy to maintain appropriate action potential durations (APDs), and loss-of-function mutations in these channels manifest clinically as long QT syndrome, characterized by the prolongation of the QT interval, polymorphic ventricular tachycardia, and sudden cardiac death. Previous cellular electrophysiology experiments in transgenic rabbit cardiomyocytes and heterologous cell lines demonstrated functional downregulation of complementary repolarizing currents. Biochemical assays indicated direct, protein-protein interactions between KCNQ1 and hERG may underlie the interplay between I(Ks) and I(Kr). Our objective was to investigate hERG-KCNQ1 interactions in the intact cellular environment primarily through acceptor photobleach FRET (apFRET) experiments. We quantitatively assessed the extent of interactions based on fluorophore location and the potential regulation of interactions by physiologically relevant signals. apFRET experiments established specific hERG-KCNQ1 associations in both heterologous and primary cardiomyocytes. The largest FRET efficiency (E(f); 12.0 ± 5.2%) was seen between ion channels with GFP variants fused to the COOH termini. Acute treatment with forskolin + IBMX or a membrane-permeable cAMP analog significantly and specifically reduced the extent of hERG-KCNQ1 interactions (by 41 and 38%, respectively). Our results demonstrate direct interactions between KCNQ1 and hERG occur in both intact heterologous cells and primary cardiomyocytes and are mediated by their COOH termini. Furthermore, this interplay between channel proteins is regulated by intracellular cAMP.

Confirmation of a proarrhythmic risk underlying the clinical use of common Chinese herbal intravenous injections.

ETHNOPHARMACOLOGICAL RELEVANCE: The Chinese herbal intravenous injections (CHI) which are extracted from herb(s) are used clinically in China as putative therapies for a variety of diseases. AIM OF THE STUDY: The mechanism(s) which underline findings of severe adverse drug reactions (ADR) noted in more than a thousand published articles on CHIs, are still poorly understood. With 109 CHIs currently in clinical use, we investigated the proarrhythmic effects of three specific CHIs, Shuanghuanglian (SHL), Qingkailing (QKL) and Yinzhihuang (YZH), using in vivo and in vitro ion channel models. MATERIALS AND METHODS: In vivo and in vitro guinea pig electrocardiogram, intracellular action potential and patch clamp recording techniques were carried out. RESULTS: Both SHL and QKL (both in one, five and ten times clinically relevant doses (CRD) for in vivo and clinically relevant concentrations (CRC) for in vitro) prolonged P-R intervals in a dose or concentration-dependent manner and SHL also prolonged QTc. YZH (ten and 20 times CRD and CRC) prolonged P-R intervals without changing QTc. Intracellular action potential recordings from guinea pig papillary muscle indicated SHL and QKL abolished the firing of action potentials at ten and 30 times CRC respectively. SHL significantly suppressed L-type Ca(2+) current from left ventricular myocytes of guinea pig, hNav1.5 current and hERG current with 50% inhibiting concentrations (IC(50)) of 6.0, 3.0 and 10.7 times CRC, respectively. Also, QKL significantly suppressed L-type Ca(2+) and hNav1.5 currents with IC(50)s of 10.7 and 13.8 times CRC. YZH significantly suppressed L-type Ca(2+), hNav1.5 and hERG currents with IC(50)s of 12.1, 32.9 and 141.7 times CRC, respectively. CONCLUSIONS: The three CHIs studied caused bradyarrhythmia mainly by inhibiting Na(+) current and L-type Ca(2+) current.

Distal end of carboxyl terminus is not essential for the assembly of rat Eag1 potassium channels.

The assembly of four pore-forming α-subunits into tetramers is a prerequisite for the formation of functional K(+) channels. A short carboxyl assembly domain (CAD) in the distal end of the cytoplasmic carboxyl terminus has been implicated in the assembly of Eag α-subunits, a subfamily of the ether-à-go-go K(+) channel family. The precise role of CAD in the formation of Eag tetrameric channels, however, remains unclear. Moreover, it has not been determined whether other protein regions also contribute to the assembly of Eag subunits. We addressed these questions by studying the biophysical properties of a series of different rat Eag1 (rEag1) truncation mutants. Two truncation mutants without CAD (K848X and W823X) yielded functional phenotypes similar to those for wild-type (WT) rEag1 channels. Furthermore, nonfunctional rEag1 truncation mutants lacking the distal region of the carboxyl terminus displayed substantial dominant-negative effects on the functional expression of WT as well as K848X and W823X channels. Our co-immunoprecipitation studies further revealed that truncation mutants containing no CAD indeed displayed significant association with rEag1-WT subunits. Finally, surface biotinylation and protein glycosylation analyses demonstrated that progressive truncations of the carboxyl terminus resulted in aggravating disruptions of membrane trafficking and glycosylation of rEag1 proteins. Overall, our data suggest that the distal carboxyl terminus, including CAD, is dispensable for the assembly of rEag1 K(+) channels but may instead be essential for ensuring proper protein biosynthesis. We propose that the S6 segment and the proximal carboxyl terminus may constitute the principal subunit recognition site for the assembly of rEag1 channels.

IonFlux: a microfluidic patch clamp system evaluated with human Ether-à-go-go related gene channel physiology and pharmacology.

Ion channel assays are essential in drug discovery, not only for identifying promising new clinical compounds, but also for minimizing the likelihood of potential side effects. Both applications demand optimized throughput, cost, and predictive accuracy of measured membrane current changes evoked or modulated by drug candidates. Several competing electrophysiological technologies are available to address this demand, but important gaps remain. We describe the industrial application of a novel microfluidic-based technology that combines compounds, cells, and buffers on a single, standard well plate. Cell trapping, whole cell, and compound perfusion are accomplished in interconnecting microfluidic channels that are coupled to pneumatic valves, which emancipate the system from robotics, fluidic tubing, and associated maintenance. IonFlux™ is a state-of-the-art, compact system with temperature control and continuous voltage clamp for potential application in screening for voltage- and ligand-gated ion channel modulators. Here, ensemble recordings of the IonFlux system were validated with the human Ether-à-go-go related gene (hERG) channel (stably expressed in a Chinese hamster ovary cell line), which has established biophysical and pharmacological characteristics in other automated planar patch systems. We characterized the temperature dependence of channel activation and its reversal potential. Concentration response characteristics of known hERG blockers and control compounds obtained with the IonFlux system correlated with literature and internal data obtained on this cell line with the QPatch HT system. Based on the biophysical and pharmacological data, we conclude that the IonFlux system offers a novel, versatile, automated profiling, and screening system for ion channel targets with the benefit of temperature control.

A major role for HERG in determining frequency of reentry in neonatal rat ventricular myocyte monolayer.

RATIONALE: the rapid delayed rectifier potassium current, I(Kr), which flows through the human ether-a-go-go-related (hERG) channel, is a major determinant of the shape and duration of the human cardiac action potential (APD). However, it is unknown whether the time dependency of I(Kr) enables it to control APD, conduction velocity (CV), and wavelength (WL) at the exceedingly high activation frequencies that are relevant to cardiac reentry and fibrillation. OBJECTIVE: to test the hypothesis that upregulation of hERG increases functional reentry frequency and contributes to its stability. METHODS AND RESULTS: using optical mapping, we investigated the effects of I(Kr) upregulation on reentry frequency, APD, CV, and WL in neonatal rat ventricular myocyte (NRVM) monolayers infected with GFP (control), hERG (I(Kr)), or dominant negative mutant hERG G628S. Reentry frequency was higher in the I(Kr)-infected monolayers (21.12 ± 0.8 Hz; n=43 versus 9.21 ± 0.58 Hz; n=16; P<0.001) but slightly reduced in G628S-infected monolayers. APD(80) in the I(Kr)-infected monolayers was shorter (>50%) than control during pacing at 1 to 5 Hz. CV was similar in both groups at low frequency pacing. In contrast, during high-frequency reentry, the CV measured at varying distances from the center of rotation was significantly faster in I(Kr)-infected monolayers than controls. Simulations using a modified NRVM model predicted that rotor acceleration was attributable, in part, to a transient hyperpolarization immediately following the AP. The transient hyperpolarization was confirmed experimentally. CONCLUSIONS: hERG overexpression dramatically accelerates reentry frequency in NRVM monolayers. Both APD and WL shortening, together with transient hyperpolarization, underlies the increased rotor frequency and stability.

Aconitine blocks HERG and Kv1.5 potassium channels.

ETHNOPHARMACOLOGICAL RELEVANCE: Aconitum has been widely used to treat various diseases in China for a long time. However, improper use of this drug results in severe intoxication. Aconitine (ACO), a diterpenoid alkaloid from aconitum, mainly contributes to cardio-toxic effects of aconitum and has also been commonly known to induce arrhythmias in animal models. However, its pro-arrhythmic mechanisms are not clear. AIM OF THE STUDY: The effects of ACO on HERG and Kv1.5 channels were investigated. MATERIALS AND METHODS: HERG and Kv1.5 channels were expressed in Xenopus laevis oocytes, and the resulting currents were recorded using a two-microelectrode voltage clamp technique. RESULTS: In HERG channels, ACO exhibited a blockade in a voltage- and time-dependent manner. The blockade was enhanced by further activation of currents, which were consistent with an open-channel blockade. In Kv1.5 channels, ACO produced a voltage-, time-, and frequency-dependent inhibition. The blockade was enhanced by higher rates of stimulation, consistent with preferential binding of the drug to the open state. In addition, ACO blocked Kv1.5 and HERG channels in a concentration-dependent manner with an IC(50) of 0.796+/-0.123 and 1.801+/-0.332 microM, respectively. CONCLUSIONS: ACO blocks HERG and Kv1.5 potassium channels in the open state. Blockade of potassium channels, particular the HERG channel, may be one of the important mechanisms of how ACO induces arrhythmias.

Comparison of human Ether-à-go-go related gene screening assays based on IonWorks Quattro and thallium flux.

In vitro screens using cellular preparations expressing human Ether-à-go-go related gene (hERG) potassium channels have become an intrinsic tool for evaluating cardiac liability of compounds during early preclinical stage development. Although hERG channel blocking effects are most reliably evaluated using the low-throughput, manual patch clamp technique, methods and technologies that yield hERG activity data in multiwell format are required to address increased throughput requirements. In most cases, multiwell approaches to measuring hERG activity involve achieving a reasonable balance between throughput and data fidelity. Here we compared two functional multiwell hERG assays: a fluorescence-based fluorometric imaging plate reader (FLIPR(®)) screen measuring thallium (Tl(+)) influx through hERG channels and an automated patch clamp assay using an IonWorks Quattro(®). Mean Z' values for FLIPR-Tl(+) and IonWorks Quattro assays were similar, 0.57 ± 0.09 (±SD; n = 10) versus 0.63 ± 0.11 (n = 12), respectively. IC50 determinations for a set of 17 reference compounds were used to evaluate potency shifts relative to conventional voltage clamp data. The reference compound set included members that are known to exert severe potency shifts in multiwell assays. Mean potency shift values for FLIPR-Tl(+) and IonWorks Quattro assays were 117- and 8-fold, respectively. On the basis of reduced potency shifts and low data variability, we conclude that the IonWorks Quattro screen was a better predictor of hERG activity in conventional whole-cell patch clamp than the Tl(+) influx assay.

Predicting drug-induced slowing of conduction and pro-arrhythmia: identifying the 'bad' sodium current blockers.

BACKGROUND AND PURPOSE: The regulatory guidelines (ICHS7B) for the identification of only drug-induced long QT and pro-arrhythmias have certain limitations. EXPERIMENTAL APPROACH: Conduction time (CT) was measured in isolated Purkinje fibres, left ventricular perfused wedges and perfused hearts from rabbits, and sodium current was measured in Chinese hamster ovary cells, transfected with Na(v)1.5 channels. KEY RESULTS: A total of 355 compounds were screened for their effects on CT: 32% of these compounds slowed conduction, 65% had no effect and 3% accelerated conduction. Lidocaine and flecainide, which slow conduction, were tested in more detail as reference compounds. In isolated Purkinje fibres, flecainide largely slowed conduction and markedly increased triangulation, while lidocaine slightly slowed conduction and did not produce significant triangulation. Also in isolated left ventricular wedge preparations, flecainide largely slowed conduction in a rate-dependent manner, and elicited ventricular tachycardia (VT). Lidocaine slightly slowed conduction, reduced Tp-Te and did not induce VT. Similarly in isolated hearts, flecainide markedly slowed conduction, increased Tp-Te and elicited VT or ventricular fibrillation (VF). The slowing of conduction and induction of VT/VF with flecainide was much more evident in a condition of ischaemia/reperfusion. Lidocaine abolished ischaemia/reperfusion-induced VT/VF. Flecainide blocked sodium current (I(Na)) preferentially in the activated state (i.e. open channel) with slow binding and dissociation rates in a use-dependent manner, and lidocaine weakly blocked I(Na). CONCLUSION AND IMPLICATIONS: Slowing conduction by blocking I(Na) could be potentially pro-arrhythmic. It is possible to differentiate between compounds with 'good' (lidocaine-like) and 'bad' (flecainide-like) I(Na) blocking activities in these models.

Improved methodology for identifying the teratogenic potential in early drug development of hERG channel blocking drugs.

Drugs blocking the potassium current IKr of the heart (via hERG channel-inhibition) have the potential to cause hypoxia-related teratogenic effects. However, this activity may be missed in conventional teratology studies because repeat dosing may cause resorptions. The aim of the present study was to investigate an alternative protocol to reveal the teratogenic potential of IKr-blocking drugs. The IKr blocker astemizole, given as a single dose (80 mg/kg) on gestation day (GD) 13 to pregnant rats caused digital defects. In whole rat embryo culture (2h) on GD 13, astemizole caused a decrease in embryonic heart rate at 20 nM, and arrhythmias at 200-400 nM. Cetirizine, without IKr-blocking properties, did not affect the rat embryonic heart in vitro. The present study shows that single dose testing on sensitive days of development, together with whole embryo culture, can be a useful methodology to better characterize the teratogenic potential of IKr-blocking drugs.

Safety Pharmacology Society: 8th annual meeting.

Of the numerous topics discussed during the eighth annual meeting of the Safety Pharmacology (SP) Society the author identified the following key topics: i) the impact of hERG (human ether-à-go-go related gene) channel data on drug development, ii) safety evaluation of biological products, iii) opportunities and expectations from SP, iv) human stem cell derived cardiomyocytes for safety assessment, v) role of cellular calcium pathways in drug-induced arrhythmias, vi) collaboration initiatives for finding solutions to cardiac repolarisation and other recognised SP risks, vii) Pharma and FDA perspectives on ICH S7B guideline, viii) joint PhRMA-FDA dialogue on drug abuse potential assessment, ix) frontloading SP strategies for mitigating non-clinical and clinical attrition; and x) approaches to measure non-clinical assay predictability for human outcome. The establishment of consortia to accelerate the solution of critical SP issues and the implementation of Frontloading SP or Exploratory SP strategies for an early selection of safe clinical candidates are promising avenues for consolidating SP as an indispensable drug development discipline and for transforming established regulatory SP investigations into a risk-known exercise.

High throughput functional screening of an ion channel library for drug safety and efficacy.

Development of a large, representative library of ion channel-expressing cell lines is described. Validation on a full range of automated patch clamp and fluorescence illumination platforms is ongoing. Library "books" can be mixed and matched into channel panels according to tissue, therapeutic area and ion channel family. Unexpected results using a cardiac channel panel show that this panel may serve as a biomarker for cardiac risk assessment.

Collation, assessment and analysis of literature in vitro data on hERG receptor blocking potency for subsequent modeling of drugs' cardiotoxic properties.

The assessment of the torsadogenic potency of a new chemical entity is a crucial issue during lead optimization and the drug development process. It is required by the regulatory agencies during the registration process. In recent years, there has been a considerable interest in developing in silico models, which allow prediction of drug-hERG channel interaction at the early stage of a drug development process. The main mechanism underlying an acquired QT syndrome and a potentially fatal arrhythmia called torsades de pointes is the inhibition of potassium channel encoded by hERG (the human ether-a-go-go-related gene). The concentration producing half-maximal block of the hERG potassium current (IC(50)) is a surrogate marker for proarrhythmic properties of compounds and is considered a test for cardiac safety of drugs or drug candidates. The IC(50) values, obtained from data collected during electrophysiological studies, are highly dependent on experimental conditions (i.e. model, temperature, voltage protocol). For the in silico models' quality and performance, the data quality and consistency is a crucial issue. Therefore the main objective of our work was to collect and assess the hERG IC(50) data available in accessible scientific literature to provide a high-quality data set for further studies.

[Effect of sophocarpine on HERG K+ channels].

Human ether-a-go-go-related gene (HERG) encodes the rapid component of the cardiac delayed rectifier K+ current, which has an important effect on both proarrhythmia and antiarrhythmia. To investigate the effect of sophocarpine (SC) on HERG channel stably expressing in human embryonic kidney-293 (HEK293) cells, whole-cell patch-clamp technique was used to record HERG current and kinetic curves. As the result, it was found that SC inhibited HERG current in a concentration-dependent manner (10, 30, 100, and 300 micromol x L(-1)). At 0 mV, 10, 30, 100, and 300 micromol x L(-1) SC respectively inhibited IHERG by Istep ( 10.7 +/- 2.8)% , (11.3 +/- 5.5)% , (47.0 +/- 2.3)% and (53.7 +/- 2.5)% , and Itail (1.1 +/- 3.0)%, (17.1 +/- 3.3)%, (32.7 +/- 1.9)% (P < 0.05, n = 12) and (56.0 +/- 2.4)% (P < 0.05, n = 13). The time constants of inactivation, recovery from inactivation and onset of inactivation were accelerated. SC did not change other channel kinetics (activation and deactivation). It is concluded that SC inhibited the transfected HERG channels by influencing the inactivation state, which is the probable anti-arrhythmic mechanism.

Green tea flavonoid epigallocatechin-3-gallate (EGCG) inhibits cardiac hERG potassium channels.

The catechin EGCG is the main flavonoid compound of green tea and has received enormous pharmacological attention because of its putative beneficial health effects. This study investigated for the first time the effect of EGCG on hERG channels, the main pharmacological target of drugs that cause acquired long QT syndrome. Cloned hERG channels were expressed in Xenopus oocytes and in HEK293 cells. Heterologous hERG currents were inhibited by EGCG with an IC50 of 6.0 micromol/l in HEK293 cells and an IC50 of 20.5 micromol/l in Xenopus laevis oocytes. Onset of effect was slow and only little recovery from inhibition was observed upon washout. In X. laevis oocytes EGCG inhibited hERG channels in the open and inactivated states, but not in the closed states. The half-maximal activation voltage of hERG currents was shifted by EGCG towards more positive potentials. In conclusion, EGCG is a low-affinity inhibitor of hERG sharing major electrophysiological features with pharmaceutical hERG antagonists.

Comparison of the QT interval response during sinus and paced rhythm in conscious and anesthetized beagle dogs.

INTRODUCTION: The aim of the present study was to compare sensitivity in detecting the drug-induced QT interval prolongation in three dog models: conscious telemetered at sinus rhythm and conscious and anesthetized dogs during atrial pacing. The test substances used represent different chemical classes with different pharmacological and pharmacokinetic profiles. METHOD: Dofetilide and moxifloxacin were tested in all models, whereas cisapride and terfenadine were tested in the conscious telemetered and paced models. All substances were given as two consecutive 1.5-h intravenous infusions (infusions 1 and 2). The individual concentration-time courses of dofetilide, moxifloxacin, and cisapride were linked to the drug-induced effects on the QT interval and described with a pharmacokinetic-pharmacodynamic model to obtain an estimate of the unbound plasma concentrations at steady state that give a 10- and 20-ms drug-induced QT interval prolongation (CE10ms and CE20ms). RESULTS: In the conscious telemetered, conscious paced, and anesthetized dog models, the mean CE10ms values were 1.4, 4.0, and 2.5 nM for dofetilide and 1300, 1800, and 12,200 nM for moxifloxacin. For cisapride, the CE10ms values were 8.0 and 4.4 nM in the conscious telemetered and conscious paced dog models. The drug-induced QT interval prolongation during the last 30 min of infusions 1 and 2 was comparable in the conscious models, but smaller in the anesthetized dog model. Terfenadine displayed a marked delay in onset of response, which could only be detected by the extended ECG recording. DISCUSSION: All dog models investigated detected QT interval prolongation after administration of the investigated test substances with similar sensitivity, except for a lower sensitivity in the anesthetized dogs following moxifloxacin administration. The conscious telemetered dog model was favorable, mainly due to the extended continuous ECG recording, which facilitated detection and quantification of delayed temporal differences between systemic exposure and drug-induced QT interval prolongation.

Action potential experiments complete hERG assay and QT-interval measurements in cardiac preclinical studies.

INTRODUCTION: The ICHS7B guideline focused on hERG and QT assays, although other factors have also been linked with the induction of severe arrhythmias. Thus, the aim of the present study was to demonstrate that two in vitro action potential recordings constitute convincing models of predictive drug-induced Torsades de pointes (TdP) and re-entry arrhythmias. METHODS: The effects of D,L-sotalol, flecainide and quinidine were investigated on potassium (hERG) and sodium (Na(V)1.5) currents transfected in HEK-293 cells to determine the repercussion of the blockade of these currents on rabbit Purkinje fibre (PF) and atrial action potentials. Atrial conduction velocity was also investigated as a model of re-entry arrhythmias. RESULTS: hERG channels were blocked by D,L-sotalol, quinidine and flecainide (IC(50): 69, 0.33 and 0.74 micromol/L, respectively). D,L-sotalol (30 micromol/L) induced reverse-use dependent increases in action potential duration (APD(90): +31.7% and +81.2% at 1 and 0.2 Hz) and triangulation (APD(90-40): +34.7% and +73.6% at 1 and 0.2 Hz) in PF but not in atria. Quinidine (10 micromol/L) also increased APD(90) (+14.5% and +68.5% at 1 and 0.2 Hz) and APD(90-40) (+73.3% and +152.1% at 1 and 0.2 Hz) in PF. Flecainide (10 micromol/L) shortened APD(90) in PF (-26.0% and - 22.2% at 1 and 0.2 Hz). Quinidine and flecainide blocked Na(V)1.5 channels by 32.3% and 73.1%, respectively, and produced decreases in dV/dt(max) which were more marked in atria (-20.4% and -31.9%) compared to PF (-12.8% and 22.4%) at 1 Hz. Finally, quinidine and flecainide decreased atrial conduction speed by 14.6% and 30.8%, respectively. CONCLUSION: Results obtained with flecainide demonstrate that use of the hERG channel alone should not be considered as a useful single assay. Rabbit Purkinje fiber action potentials can be considered as a comparable model for detection of reverse-use dependent APD prolongation and triangulation whereas the rabbit atria can be considered as a useful model for detection of sodium channel blockade associated with decreases in dV/dt(max) and conduction velocity.