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

Comparison of guinea-pig ventricular myocytes and dog Purkinje fibres for in vitro assessment of drug-induced delayed repolarization

INTRODUCTION

QT interval prolongation and Torsade de Pointes (TdP) arrhythmias are recognised as a potential risk with many drugs, most of which delay cardiac repolarization by inhibiting the rapidly activating K(+) current (I(Kr)). The objective of this study was to compare the effects of compounds on cardiac action potentials recorded from guinea-pig ventricular myocytes and dog Purkinje fibres.

METHODS AND RESULTS

Effects of dofetilide, sotalol, cisapride, terfenadine, haloperidol and sparfloxacin, compounds known to cause QT prolongation (positive controls), and nifedipine and verapamil, not associated with QT prolongation (negative controls) were studied on intracellular action potentials recorded from guinea-pig isolated ventricular myocytes (VM) and dog isolated Purkinje fibres (PF). Prolongation of action potential duration (APD) by sotalol, dofetilide and sparfloxacin was concentration-dependent and of greater magnitude in dog PF compared to guinea-pig VM. The maximum prolongation of APD in guinea-pig VM at 0.5 and 1 Hz was approximately 25% and this was associated with complete inhibition of I(Kr) by dofetilide. Effects on APD of cisapride and haloperidol in both preparations, and terfenadine in guinea-pig VM, were biphasic, consistent with inhibition of multiple ion channels. There was no effect of terfenadine on APD in dog PF. Haloperidol increased APD by more than 25% in guinea-pig VM, consistent with effects on additional repolarizing currents. The negative controls shortened APD to a greater extent in guinea-pig VM compared to dog PF. In general, the positive control drugs increased action potential triangulation (APD(40-90)) to a greater extent than APD(90).

CONCLUSION

Guinea-pig isolated VM may be more sensitive for detecting APD prolongation with compounds inhibiting multiple ion channels and action potential triangulation (APD(40-90)). Effects on repolarizing currents other than I(Kr) were also distinguished in guinea-pig VM.

Optimization of a pyrazoloquinolinone class of Chk1 kinase inhibitors

The development of 2,5-dihydro-4H-pyrazolo[4,3-c]quinolin-4-ones as inhibitors of Chk1 kinase is described. Introduction of a fused ring at the C7/C8 positions of the pyrazoloquinolinone provided an increase in potency while guidance from overlapping inhibitor bound Chk1 X-ray crystal structures contributed to the discovery of a potent and solubilizing propyl amine moiety in compound 52 (Chk1 IC(50)=3.1 nM).

Molecularly Targeted Oncology Therapeutics and Prolongation of the QT Interval

Investigation and utilization of molecularly targeted agents has induced a number of drug adverse effects that are not typically associated with conventional chemotherapy. QT interval prolongation, a cardiac toxicity that increases the risk of fatal arrhythmia, is associated with several novel oncology therapies. Classes of molecularly targeted agents with described QT effects include histone deacetylase inhibitors, multitargeted tyrosine kinase inhibitors, vascular disruption agents, farnesyl protein transferase inhibitors, Src/Abl kinase inhibitors, and protein kinase C inhibitors. Concurrently, guidelines for monitoring the QT-prolonging effects of drugs under development have become increasingly rigorous. Although these guidelines apply to anticancer agents, they were not specifically designed for the oncology patient population. This article will review the pathophysiology of QT prolongation, methods of preclinical QT assessment, and current guidelines for QT evaluation in early phase trials. Additionally, molecularly targeted agents with QT effects will be summarized, and mechanisms of addressing this toxicity in the context of oncology drug development will be explored.

Verapamil blocks HERG channel by the helix residue Y652 and F656 in the S6 transmembrane domain

AIM

The objectives of this study were to investigate the inhibitory action of verapamil on wild-type(WT) and mutation HERG K+ channel current (I(HERG)), and to determine whether mutations in the S6 region are important for the inhibition of I(HERG) by verapamil.

METHODS

HERG channels (WT, Y652A, and F656A) were expressed in oocytes of Xenopus laevis and studied using the 2-electrode voltage- clamp technique.

RESULTS

WT HERG is blocked in a concentration-dependent manner by verapamil (half-maximal inhibition concentration [IC(50)]=5.1 micromol/L), and the steady state activation and inactivation parameters are shifted to more negative values. However, mutation to Ala of Y652 and F656 located on the S6 domain produced 16-fold and 20-fold increases in IC(50) for IHERG blockade, respectively. Simultaneously, the steady state activation and inactivation parameters for Y652A are also shifted to more negative values in the presence of the blockers.

CONCLUSION

Verapamil preferentially binds to and blocks open HERG channels. Tyr-652 and Phe-656, 2 aromatic amino-acid residues in the inner (S6) helix, are critical in the verapamil-binding site.

Functional consequences of methionine oxidation of hERG potassium channels

Reactive species oxidatively modify numerous proteins including ion channels. Oxidative sensitivity of ion channels is often conferred by amino acids containing sulfur atoms, such as cysteine and methionine. Functional consequences of oxidative modification of methionine in human ether à go-go related gene 1 (hERG1), which encodes cardiac I(Kr) channels, are unknown. Here we used chloramine-T (ChT), which preferentially oxidizes methionine, to examine the functional consequences of methionine oxidation of hERG channels stably expressed in a human embryonic kidney cell line (HEK 293) and native hERG channels in a human neuroblastoma cell line (SH-SY5Y). ChT (300 microM) significantly decreased whole-cell hERG current in both HEK 293 and SH-SY5Y cells. In HEK 293 cells, the effects of ChT on hERG current were time- and concentration-dependent, and were markedly attenuated in the presence of enzyme methionine sulfoxide reductase A that specifically repairs oxidized methionine. After treatment with ChT, the channel deactivation upon repolarization to -60 or -100 mV was significantly accelerated. The effect of ChT on channel activation kinetics was voltage-dependent; activation slowed during depolarization to +30 mV but accelerated during depolarization to 0 or -10mV. In contrast, the reversal potential, inactivation kinetics, and voltage-dependence of steady-state inactivation remained unaltered. Our results demonstrate that the redox status of methionine is an important modulator of hERG channel.

Understanding hERG inhibition with QSAR models based on a one-dimensional molecular representation

Blockage of the potassium channel encoded by the human ether-a-go-go related gene (hERG) is well understood to be the root cause of the cardio-toxicity of numerous approved and investigational drugs. As such, a cascade of in vitro and in vivo assays have been developed to filter compounds with hERG inhibitory activity. Quantitative structure activity relationship (QSAR) models are used at the very earliest part of this cascade to eliminate compounds that are likely to have this undesirable activity prior to synthesis. Here a new QSAR technique based on the one-dimensional representation is described in the context of the development of a model to predict hERG inhibition. The model is shown to perform close to the limits of the quality of the data used for model building. In order to make optimal use of the available data, a general robust mathematical scheme was developed and is described to simultaneously incorporate quantitative data, such as IC50 = 50 nM, and qualitative data, such as inactive or IC50 > 30 microM into QSAR models without discarding any experimental information.

The safety profile of moxifloxacin and other fluoroquinolones in special patient populations

BACKGROUND

Fluoroquinolones, including ciprofloxacin, levofloxacin, gemifloxacin, and moxifloxacin, represent a major advance in the development of antimicrobial agents. They offer significant activity against Gram-negative pathogens, while more advanced generation fluoroquinolones including levofloxacin, gemifloxacin, and moxifloxacin are significantly active against Gram-positive (e.g., Streptococcus pneumoniae for some members of the class), typical, atypical, and anaerobic pathogens. Fluoroquinolones have a pharmacokinetic/pharmacodynamic profile that exhibits concentration-dependent killing and good oral absorption, allowing for once-daily dosing.

OBJECTIVE

Review of data from fluoroquinolone studies, with an emphasis on the associated rare, but potentially clinically important, adverse events in specific patient populations. Review of clinical efficacy is included where relevant to the topic under discussion.

METHODS

A literature search was conducted using terms including fluoroquinolones, moxifloxacin, ciprofloxacin, levofloxacin, gatifloxacin, gemifloxacin, safety, adverse events, drug interactions, and pharmacokinetic parameters to identify literature providing information regarding the safety profile of specified fluoroquinolones in special patient populations (i.e., the elderly, patients with liver disease, kidney disease, glycemic disorder, those at risk for cardiovascular events). Although specific date criteria were not applied to the search, preference was given to more recent publications. Online databases searched include MEDLINE and EMBASE and relevant textbooks were utilized as well.

FINDINGS

Fluoroquinolones, when used either as monotherapy or as combination therapy depending on their individual indications, attain adequate concentrations for treating infections in different target sites, including epithelial lining fluid, alveolar macrophages, skin, and gastrointestinal tissues. Overall, fluoroquinolones have predictable and mild-to-moderate adverse-event profiles and are generally well tolerated. Findings of this review are limited by the availability of publications and case reports.

CONCLUSIONS

Fluoroquinolones, are associated with rare, but clinically important, adverse events in special patient populations (including the elderly; those with hepatic, renal, or glycemic disorders; and those at risk for cardiovascular events). Recognition of differences in the clinical efficacy and safety profiles of fluoroquinolones in special patient populations should lead to better antimicrobial agent selection.

Hydroxypropyl β-Cyclodextrins: A Misleading Vehicle for the In Vitro hERG Current Assay

Delayed cardiac repolarization and fatal proarrhythmia have been linked to block of the repolarizing current, Ikr or hERG (human ether-a-go-go related gene) current. Thus, determining the potency of hERG block is critical in evaluating cardiac safety during preclinical development. Hydroxypropyl beta-cyclodextrins (HbetaC) are cyclic oligosaccharides used to enhance drug solubility. To evaluate the utility of HbetaC to enhance drug solubility in hERG screening assays, we studied the effect of HbetaC on hERG current and the sensitivity of the hERG assay to 3 structurally different hERG blocking drugs using whole-cell voltage clamp technique and HEK-293 cells expressing the hERG channel. HbetaC inhibited hERG activation and tail current and accelerated current deactivation in a concentration-dependent manner. HbetaC (6%) reduced the apparent potency of block by terfenadine (IC50 12000 nM vs 45 nM), cisapride (IC50 281 nM vs 28 nM), and E-4031 (163 nM vs 26 nM). Reduced potency of block was consistent with loss of activity as a result of complexation with HbetaC by terfenadine and cisapride (demonstrated in solubility studies) and interactions with HbetaC by E-4031 (demonstrated in absorbance studies). These results demonstrate that HbetaC is an unsuitable agent for enhancing compound solubility in the in vitro hERG current assay and may mask drug effects, allowing potentially dangerous drugs to advance into clinical development.

Early evaluation of compound QT prolongation effects: A predictive 384-well fluorescence polarization binding assay for measuring hERG blockade

INTRODUCTION

A large number of drugs from a variety of pharmacological classes have been demonstrated to cause adverse effects on cardiac rhythm, including the life-threatening arrhythmia Torsades de Pointes. These side effects are often associated with prolongation of the QT interval and are mediated via blockade of the human ether-a-go-go related gene (hERG) encoded potassium channel. In order to manage this risk in the pharmaceutical industry it is desirable to evaluate QT prolongation as early as possible in the drug discovery process.

METHODS

Here we describe the development of a 384-well fluorescence polarization (FP) binding assay compatible with high-throughput assessment of compound blockade of the hERG channel during the lead optimisation process. To characterise the fluorescent ligand that was developed, competition binding studies, kinetic studies and electrophysiology studies were performed. Furthermore, to validate the assay as a key screening method a series of competition binding studies were performed and correlated with functional data obtained via patch-clamp.

RESULTS

Evaluation of the assay indicates that high quality data is obtained (Z'>0.6), that the K(i) values determined are equivalent to more traditional radiometric methods and that it is predictive for functional hERG blockade as assessed by patch clamp.

DISCUSSION

Whilst FP assays, utilizing a variety of fluors, have become well established for the evaluation of G-protein-coupled receptor (GPCRs) and kinase ligand interactions, this technique has not been applied widely to the study of ion channels. Therefore, this represents a novel assay format that is amenable to the evaluation of thousands of compounds per day. Whilst other assay formats have proven predictive or high throughput, this assay represents one of few that combines both attributes, moreover it represents the most cost effective assay, making it truly amenable to early assessment of hERG blockade.

A quantitative assessment of hERG liability as a function of lipophilicity

The impact of lipophilicity as a factor contributing to hERG potency is assessed for a large dataset of compounds of differing ionisation type. This dataset is derived from compounds tested in the IonWorks-based in vitro electrophysiology hERG assay at AstraZeneca. Using logistic regression, a quantification of the risk associated with increasing lipophilicity is presented. The anticipated differences between acidic, basic and neutral compounds are apparent in the data but lipophilicity is shown to be a stronger driver for hERG potency than might have been expected. Simple rules defining target lipophilicity values for minimizing hERG liability are derived.

Intravenous conivaptan: effects on the QTc interval and other electrocardiographic parameters in healthy volunteers

Prolongation of the QT interval is clinically important because it may be associated with torsade de pointes, a potentially fatal arrhythmia. The objective of this study was to define the effects on electrocardiogram (ECG) of intravenous conivaptan, the first arginine vasopressin V1A/V2-receptor antagonist indicated for the treatment of euvolemic hyponatremia, on hospitalized patients without congestive heart failure. After a placebo run-in period, participants in this randomized, single-blind, placebo- and positive-controlled, parallel-group study received an intravenous 20-mg loading dose of conivaptan (day 1), followed by a 40-mg/d continuous infusion (days 1-4); a 20-mg loading dose of conivaptan (day 1), followed by an 80-mg/d continuous infusion (days 1-4); or moxifloxacin 400 mg (positive control) or placebo from day 1 to day 4. The primary ECG endpoint was QTc interval duration, which was determined by the individually corrected QT interval for each subset; secondary endpoints included QT intervals corrected with Bazett's formula and Fridericia's formula. No clinically notable changes in ECG parameters were associated with conivaptan, suggesting that conivaptan did not affect cardiac repolarization or cardiac conduction.

Arrhythmias associated with fluoroquinolone therapy

Fluoroquinolones are widely used and well tolerated antibacterial agents. However, prolongation of the QT interval is an adverse effect associated with the use of fluoroquinolones. According to the available case reports and clinical studies, moxifloxacin carries the greatest risk of QT prolongation from all available quinolones in clinical practice and it should be used with caution in patients with predisposing factors for Torsades de pointes (Tdp). Although gemifloxacin, levofloxacin and ofloxacin are associated with a lower risk of QT prolongation compared with moxifloxacin, they should also be used with caution in patients with risk factors for QT prolongation. Ciprofloxacin appears to be associated with the lowest risk for QT prolongation and the lowest rate of Tdp. The overall risk of Tdp is small with the use of fluoroquinolones. Clinicians can minimise that risk by avoiding prescriptions of multiple medications associated with QT interval prolongation, especially in high-risk patients.

Mechanism of hERG K+ channel blockade by the fluoroquinolone antibiotic moxifloxacin

The fluoroquinolone antibiotic moxifloxacin has been associated with the acquired long QT syndrome and is used as a positive control in the evaluation of the QT-interval prolonging potential of new drugs. In common with other QT-prolonging agents, moxifloxacin is known to inhibit the hERG potassium K+ channel, but at present there is little mechanistic information available on this action. This study was conducted in order to characterise the inhibition of hERG current (I(hERG)) by moxifloxacin, and to determine the role in drug binding of the S6 aromatic amino-acid residues Tyr652 and Phe656. hERG currents were studied using whole-cell patch clamp (at room temperature and at 35-37 degrees C) in an HEK293 cell line stably expressing hERG channels. Moxifloxacin reversibly inhibited currents in a dose-dependent manner. We investigated the effects of different voltage commands to elicit hERG currents on moxifloxacin potency. Using a 'step-ramp' protocol, the IC50 was 65 microM at room temperature and 29 microM at 35 degrees C. When a ventricular action potential waveform was used to elicit currents, the IC50 was 114 microM. Block of hERG by moxifloxacin was found to be voltage-dependent, occurred rapidly and was independent of stimulation frequency. Mutagenesis of the S6 helix residue Phe656 to Ala failed to eliminate or reduce the moxifloxacin-mediated block whereas mutation of Tyr652 to Ala reduced moxifloxacin block by approximately 66%. Our data demonstrate that moxifloxacin blocks the hERG channel with a preference for the activated channel state. The Tyr652 but not Phe656 S6 residue is involved in moxifloxacin block of hERG, concordant with an interaction in the channel inner cavity.

Telithromycin-induced digoxin toxicity and electrocardiographic changes

A 58-year-old woman who had been taking digoxin 0.25 mg/day for more than 35 years for heart palpitations after mitral valve repair was prescribed a 5-day course of telithromycin for acute bronchitis. On the sixth day of therapy, she came to the emergency department complaining of general malaise and having experienced three episodes of syncope over the previous 2 days. Laboratory analysis revealed elevated digoxin plasma levels, and electrocardiography showed several nonspecific repolarization anomalies. Telithromycin is known to increase digoxin plasma levels; however, the clinical significance of this interaction is not known. To our knowledge, this is the first report of elevated plasma digoxin levels associated with signs and symptoms of toxicity. This drug interaction-determined as probable according to the Naranjo adverse drug reaction probability scale-may be mediated by P-glycoprotein. By inhibiting the transport of digoxin by P-glycoprotein, telithromycin may have decreased digoxin elimination in the intestinal lumen and its renal tubular excretion, resulting in elevated plasma levels and drug toxicity. Clinicians should be aware of possible digoxin toxicity after concomitant administration with telithromycin, especially in patients who are at risk, such as those with electrolyte abnormalities and decreased renal function.

Antimicrobial-associated QT interval prolongation: pointes of interest

Until recently, cardiac toxicity manifesting in the form of arrhythmias related to QT interval prolongation was uncommonly appreciated within the antimicrobial class of drugs, but it was well described among antiarrhythmic agents. Antimicrobials that are associated with QT prolongation include the macrolides/ketolides, certain fluoroquinolones and antimalarials, pentamidine, and the azole antifungals. Although, in most cases, mild delays in ventricular repolarization caused by these drugs are clinically unnoticeable, they may serve to amplify the risk for torsades de pointes (TdP) when prescribed in the setting of other risk factors. Conditions or variables that influence proarrhythmic risk include sex, age, electrolyte derangements, structural heart disease, pharmacokinetic/pharmacodynamic interactions, and genetic predisposition. It is important that clinicians be knowledgeable about drugs with QT liability, as well as the risk factors that increase the probability of TdP. Additionally, because TdP remains a difficult-to-measure adverse event, we must rely upon multiple data sources to determine the risk versus the benefit for newly approved drugs.

The D3-dopaminergic agonist 7-hydroxy-dipropylaminotetralin (7-OH-DPAT) increases cardiac action potential duration and blocks human ether-a-go-go-related gene K+ channel

The D3-dopaminergic agonist (+/-) 7-hydroxy-dipropylaminotetralin (7-OH-DPAT) prolonged cycle length and action potential duration, depolarized maximum diastolic potential, and reduced the upstroke velocity of the action potential of rabbit sinoatrial node cells. These effects were not mediated by D3-dopaminergic receptors. In cat Purkinje fibers, the drug increased action potential duration. In voltage-clamped cat ventricular myocytes, 7-OH-DPAT blocked the rapid component of the delayed rectifier potassium current, IKr. This effect was corroborated in experiments studying the effect of the drug on human Ether-a-go-go-related Gene channels expressed in Xenopus oocytes and in HEK293 cells. We conclude that the direct electrophysiologic effects of 7-OH-DPAT on cardiac tissues are caused by the blockade of the rapid component of the delayed rectifier potassium current, IKr.

Mallotoxin is a novel human ether-a-go-go-related gene (hERG) potassium channel activator

Human ether-a-go-go-related gene (hERG) encodes a rapidly activating delayed rectifier potassium channel that plays important roles in cardiac action potential repolarization. Although many drugs and compounds block hERG channels, activators of the channel have only recently been described. Three structurally diverse synthetic compounds have been reported to activate hERG channels by altering deactivation or inactivation or by unidentified mechanisms. Here, we describe a novel, naturally occurring hERG channel activator, mallotoxin (MTX). The effects of MTX on hERG channels were investigated using the patch-clamp technique. MTX increased both step and tail hERG currents with EC(50) values of 0.34 and 0.52 microM, respectively. MTX leftward shifted the voltage dependence of hERG channel activation to less depolarized voltages ( approximately 24 mV at 2.5 microM). In addition, MTX increased hERG deactivation time constants. MTX did not change the half-maximal inactivation voltage of the hERG channel, but it reduced the slope of the voltage-dependent inactivation curve. All of these factors contribute to the enhanced activity of hERG channels. During a voltage-clamp protocol using prerecorded cardiac action potentials, 2.5 microM MTX increased the total potassium ions passed through hERG channels by approximately 5-fold. In conclusion, MTX activates hERG channels through distinct mechanisms and with significantly higher potency than previously reported hERG channel activators.

K201, a multi-channel blocker, inhibits clofilium-induced torsades de pointes and attenuates an increase in repolarization

K201 (JTV519) is a 1,4-benzothiazepine derivative that exhibits a strong cardioprotective action and acts as a multiple-channel blocker, including as a K+ channel blocker. An experimental model of prolongation of the QT interval and torsades de pointes can be induced in rabbits by treatment with clofilium in the presence of the alpha1-adrenoreceptor agonist methoxamine. In this study we examined the effects of K201 with and without methoxamine on the QT and QTc intervals, and determined whether K201 inhibits clofilium-induced torsades de pointes in the presence of methoxamine (15 microg/kg/min) in rabbits (n=74). Administration of K201 (0, 40, 100, 200 and 400 microg/kg/min) with and without methoxamine prolonged the QT interval in a dose-dependent manner, and torsades de pointes did not occur in any animals. However, clofilium (50 microg/kg/min) with methoxamine induced torsades de pointes in all animals (6/6). Torsades de pointes occurred at rates of 100%, 67%, 40% and 0% at K201 concentrations of 0, 50, 200 and 400 microg/kg/min, respectively, in the clofilium-infused torsades de pointes model. Therefore, 400 microg/kg/min of K201 completely inhibited clofilium-induced torsades de pointes and attenuated the increase of repolarization caused by clofilium; the inhibitory effects of K201 may be related to its pharmacological properties as an alpha1-adrenoceptor blocker. Overall, our results show that K201 causes prolongation of the QT and QTc intervals, but does not induce torsades de pointes, with and without alpha1-adrenoceptor stimulation. Furthermore, K201 inhibits clofilium-induced torsades de pointes, despite QT prolongation, suggesting that QT prolongation alone is not a proarrhythmic signal.

Pharmacokinetic-pharmacodynamic modelling of QT interval prolongation following citalopram overdoses

AIMS

To develop a pharmacokinetic-pharmacodynamic model describing the time-course of QT interval prolongation after citalopram overdose and to evaluate the effect of charcoal on the relative risk of developing abnormal QT and heart-rate combinations.

METHODS

Plasma concentrations and electrocardiograph (ECG) data from 52 patients after 62 citalopram overdose events were analysed in WinBUGS using a Bayesian approach. The reported doses ranged from 20 to 1700 mg and on 17 of the events a single dose of activated charcoal was administered. The developed pharmacokinetic-pharmacodynamic model was used for predicting the probability of having abnormal combinations of QT-RR, which was assumed to be related to an increased risk for torsade de pointes (TdP).

RESULTS

The absolute QT interval was related to the observed heart rate with an estimated individual heart-rate correction factor [alpha = 0.36, between-subject coefficient of variation (CV) = 29%]. The heart-rate corrected QT interval was linearly dependent on the predicted citalopram concentration (slope = 40 ms l mg(-1), between-subject CV = 70%) in a hypothetical effect-compartment (half-life of effect-delay = 1.4 h). The heart-rate corrected QT was predicted to be higher in women than in men and to increase with age. Administration of activated charcoal resulted in a pronounced reduction of the QT prolongation and was shown to reduce the risk of having abnormal combinations of QT-RR by approximately 60% for citalopram doses above 600 mg.

CONCLUSION

Citalopram caused a delayed lengthening of the QT interval. Administration of activated charcoal was shown to reduce the risk that the QT interval exceeds a previously defined threshold and therefore is expected to reduce the risk of TdP.

Empirical-Bayes adjustment improved conventional estimates in postmarketing drug-safety studies

OBJECTIVES

Empirical-Bayes adjustment was used to investigate the effects of antibacterials on risk of arrhythmia in a large cohort study based upon automated record linkage.

STUDY DESIGN AND SETTING

The Prescription Drug Database of the Varese Province, Italy, was used to form a population-based cohort of patients who received at least one prescription of antibacterials for systemic use. All cohort members were followed-up from the beginning of each antibacterial therapeutic cycle up to 60 days afterward, identifying the first prescription of an antiarrhythmic drug and/or the first admission for arrhythmia. Standardized incidence ratios for eight classes of antibacterials, as well as for the specific antibacterial drugs, were estimated using both conventional methods and empirical-Bayes (EB) adjustment.

RESULTS

Using EB adjustment, overall precision was greatly enhanced compared to the conventional analysis. Statistical evidence of drug-induced dysrhythmic events was found for 17 drugs mostly belonging to Macrolides and Quinolones. Strongest proarrhythmic effects were found for Ciprofloxacin (standardized incidence ratio [SIR]: 1.98; 95% CI: 1.67, 2.34) and Erythromycin (SIR: 1.91; 95% CI: 1.20, 2.88).

CONCLUSION

Empirical-Bayes may mitigate some problems inherent the conventional analysis. Macrolides and Quinolones were associated with the highest number of positive signals of dysrhythmic events. This requires further investigation by other types of epidemiologic studies.

Torsades de pointes induced by antibiotics

BACKGROUND

Several frequently used antibiotics are associated with an arrhythmia called "torsades de pointes" (TdP). This potentially fatal arrhythmia is considered unpredictable.

METHODS

In order to investigate the prevalence of risk factors for TdP prior to initiation of antibiotic therapy, we conducted a literature search for all published reports on TdP induced by antibiotics and we asked pharmaceutical companies for additional unpublished reports.

RESULTS

We studied 61 reports on 78 patients with TdP induced by antibiotics. Female gender was the most common risk factor for TdP: 66.7% (n=52) of all patients were women. Advanced heart disease and concomitant use of a QT interval-prolonging agent or an inhibitor of liver drug metabolism were also frequently present (59% and 48.7%, respectively). Most patients had at least one and 58 patients (74.3%) had two risk factors or more for TdP prior to initiation of antibiotic therapy.

CONCLUSION

Contrary to common belief, TdP induced by antibiotics may be predictable by simple history-taking and by obtaining a baseline electrocardiogram. We wish to draw attention to risk factors for TdP prior to the initiation of antibiotic therapy.

Screening the receptorome

The term 'receptorome' is now being used to describe receptors, ion channels and transporters in the human genome that are potential drug targets. These proteins comprise a considerable fraction of the human genome, and include the G protein-coupled receptors, which are the targets for many medications. In this review, we summarize recent advances in the field, including the concept that the ultimate goal of drug discovery may not be the development of highly selective single-target drugs, the idea that potential side-effects can also be the goal of multi-target drug screening, and a discussion of the application of computational screening and public domain databases available to interested investigators.

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

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

Design of novel, potent, and selective human beta-tryptase inhibitors based on alpha-keto-[1,2,4]-oxadiazoles

A series of novel alpha-keto-[1,2,4]-oxadiazoles has been synthesized as human tryptase inhibitors for evaluation as a new class of anti-asthmatic agent. The inhibitor design is focused on using a prime-side hydrophobic pocket and the S2 pocket of beta-tryptase to achieve inhibition potency and selectivity over other serine proteases.

Exploring the effect of transient exposure on the risk of acute events by means of time-window designs: an application to fluoroquinolone antibacterials and arrhythmia

PURPOSE

To compare different strategies in allocating time-window at risk and in choosing optimal comparator in the setting of proarrhythmic effects of antibacterial fluoroquinolones.

DESIGN

A population-based cohort study.

SETTING

Resident population of a Northern Italian Province.

PARTICIPANTS

The study cohort comprised 75 226 patients who received at least one prescription for fluoroquinolone from 1997 to 1999. Cohort members who experienced at least an antiarrhythmic drug prescription and/or hospitalisation with diagnosis of arrhythmia during the follow-up were identified.

ANALYSIS

Trends in incidence rates of arrhythmia according to different strategies in allocating time-windows at risk were calculated. Standardised incidence ratios were obtained by comparing arrhythmic events experienced by the study cohort during the time-windows at risk with those expected from both external (unexposed target population) and internal (study cohort during reference time-windows) comparators.

RESULTS

The rate of arrhythmia was 21.0 per 100 000 person-weeks during the in-treatment period, remained stable during the following week, and progressively decreased until to converge to 7.7 per 100 000 person-weeks from 16th weeks after stop of exposure. Standardised incidence ratios were 2.35 (95%CI: 1.82, 2.99) and 1.86 (1.44, 2.37) according to external and internal comparators respectively. Evidence of linear increasing of proarrhythmic effect with the length of the antibacterial therapy appeared from every approaches, but it was stronger by using internal comparator.

CONCLUSIONS

Time-windows designs should be performed by allocating the time-windows at risk and comparing outcome events observed in the cohort during these periods with those experienced by the same cohort during an adequate reference period.

Risque cardiovasculaire des antipsychotiques, une surveillance nécessaire de l’intervalle QT

Torsades de pointes (TdP), a form of ventricular arrhythmia that can cause ventricular fibrillation and sudden death, may occur during prolongation of the QT interval. QT prolongation has recently been reported with antipsychotic drugs. Physicians should be able to obtain a corrected measurement of the QT interval. QT is measured from the beginning of the QRS complex to the end of the T wave (QTm). This value must then be corrected to take heartbeat into account. The most common formula in current use is Bazett's. In practice, a QTc interval value greater than 500 ms indicates an increased risk of TdP. Safe combinations of antipsychotic drugs have been recommended by the French drug agency (Agence française de sécurité sanitaire des produits de santé). Many other drugs, including psychotropic drugs such as tricyclic antidepressants, can prolong the QT interval. Combinations of these medications with one another, with antipsychotic medications, or with other concomitant factors, such as hypokalemia, also increase the risk. TdP is most often diagnosed only after observing QT prolongation. This underlines the need to monitor QT intervals attentively to prevent the risk of cardiac arrhythmia in patients treated with antipsychotic drugs.

Pharmacological and pharmacokinetic aspects of functional gastrointestinal disorders

Medications are commonly used for the treatment of patients with functional gastrointestinal disorders. The general goal of this report is to review the pharmacokinetics and pharmacology of medications used in functional gastrointestinal disorders. Methods included literature review, consensus evaluation of the evidence for each topic assigned originally to 1 or 2 authors, and broader review at a harmonization session as part of the Rome III process. This report reviews the animal models that have been validated for the study of effects of pharmacologic agents on sensation and motility; the preclinical pharmacology, pharmacokinetics, and toxicology usually required for introduction of novel therapeutic agents; the biomarkers validated for studies of sensation and motility end points with experimental medications in humans; the pharmacogenomics applied to these medications and disorders; and the pharmacology of agents that are applied or have potential for treatment of functional gastrointestinal disorders, including psychopharmacologic agents. Clinician and basic investigators involved in the treatment or investigation of functional gastrointestinal disorders or disease models need to have a comprehensive understanding of a vast range of medications. It is anticipated that the interaction between investigators of basic science, basic and applied pharmacology, and clinical trials will lead to better treatment of these disorders.

Kinesin spindle protein (KSP) inhibitors. Part 3: Synthesis and evaluation of phenolic 2,4-diaryl-2,5-dihydropyrroles with reduced hERG binding and employment of a phosphate prodrug strategy for aqueous solubility

2,4-Diaryl-2,5-dihydropyrroles have been discovered to be novel, potent and water-soluble inhibitors of KSP, an emerging therapeutic target for the treatment of cancer. A potential concern for these basic KSP inhibitors (1 and 2) was hERG binding that can be minimized by incorporation of a potency-enhancing C2 phenol combined with neutral N1 side chains. Aqueous solubility was restored to these, and other, non-basic inhibitors, through a phosphate prodrug strategy.

An Accurate and Interpretable Bayesian Classification Model for Prediction of hERG Liability

Drug-induced QT interval prolongation has been identified as a critical side-effect of non-cardiovascular therapeutic agents and has resulted in the withdrawal of many drugs from the market. As almost all cases of drug-induced QT prolongation can be traced to the blockade of a voltage-dependent potassium ion channel encoded by the hERG (the human ether-à-go-go-related gene), early identification of potential hERG channel blockers will decrease the risk of cardiotoxicity-induced attritions in the later and more expensive development stage. Presented herein is a naive Bayes classifier to categorize hERG blockers into active and inactive classes, by using a universal, generic molecular descriptor system.1 The naive Bayes classifier was built from a training set containing 1979 corporate compounds, and exhibited an ROC accuracy of 0.87. The model was validated on an external test set of 66 drugs, of which 58 were correctly classified. The cumulative probabilities reflected the confidence of prediction and were proven useful for the identification of hERG blockers. Relative performance was compared for two classifiers constructed from either an atom-type-based molecular descriptor or the long range functional class fingerprint descriptor FCFP_6. The combination of an atom-typing descriptor and the naive Bayes classification technique enables the interpretation of the resulting model, which offers extra information for the design of compounds free of undesirable hERG activity.

A novel approach to data processing of the QT interval response in the conscious telemetered beagle dog

INTRODUCTION

Drug-induced QT interval prolongation may lead to ventricular arrhythmias. The aim of the study was to optimize QT interval data processing to quantify drug-induced QT interval prolongation in the telemetry instrumented conscious dog model.

METHODS

The test substances cisapride, dofetilide, haloperidol, and terfenadine and corresponding vehicles were given to male and female beagle dogs during two consecutive 90-min intravenous infusions. Cardiovascular parameters were recorded for 24 h and exposure to the drugs was measured. The delayed response in the QT interval after an abrupt change in heart rate was investigated. Eight mathematical models to describe the QT interval-heart rate relationship were compared and different sets of covariates were used to quantify the drug-induced effect on the QT interval.

RESULTS

After an abrupt decrease in heart rate, a 75% adaptation of the QT interval was reached after 54+/-9 s. A linear model was preferred to correct the drug-induced effect on the QT interval for heart rate, vehicle effect, serial correlation, plasma concentration and time of day. All test substances significantly prolonged the QT interval.

DISCUSSION

To optimize the processing of QT interval data, the delay in QT interval response after an abrupt change in heart rate should be considered. The QT interval-heart rate relationship and vehicle response were individual-specific and corrections were therefore made individually. When estimating the drug-induced effect on the QT interval it is considered advantageous to use plasma concentration as a covariate, as well as adjusting for vehicle effect and serial correlation in measurements. The conscious dog model detected significant increases in the QT interval for all test substances investigated.

Evaluation of the Rubidium Efflux Assay for Preclinical Identification of hERG Blockade

Inhibition of the delayed-rectifier potassium channel current, human ether-a-go-go (hERG), by pharmaceutical agents can lead to acquired long QT syndrome and the generation of potentially lethal arrhythmias and sudden death. There remains an unmet need for higher-throughput assays to screen compounds in preclinical development for the potential to block hERG and cause QT prolongation. We evaluated the rubidium efflux assay for its ability to determine block of the hERG potassium channel. hERG-transfected human embryonic kidney-293 cells were cultured on 96-well assay plates and loaded with rubidium ion by incubating in media in which potassium was replaced by 5.4 mM Rb+. Cells were exposed to test compounds and then depolarized with a K+ channel opening buffer containing 50 mM K+. The supernatant was removed, and cells were lysed using 0.1% Triton X-100. Concentration-response curves were generated for test agents by determining the Rb+ efflux using a flame atomic absorption spectrometer. Multiple trials with cisapride yielded 50% inhibitory concentration values between 308.1 +/- 11 nM to 456.3 +/- 24 nM for inhibition of Rb+ efflux and a Z factor of 0.80 +/- 0.07 (n = 5 plates, 12 wells per plate). The values for inhibition of the hERG channel exhibited a rightward shift in potency as compared to those measured using electrophysiological techniques. In addition, we evaluated 19 blinded compounds at 10 microM in the Rb+ efflux assay, and compared results to those using patch clamp electrophysiology and the dofetilide displacement binding assay. The dofetilide displacement binding assay yielded a good correlation with electrophysiological measurements of hERG block. The rubidium efflux assay lacked sensitivity to consistently identify significant channel blockade. In conclusion, the rubidium efflux assay provides a higher-throughput means to identify potent hERG channel blocking agents, but lacks the sensitivity required to accurately determine the potency of blockade.

Assessment of the effect of a single oral dose of telithromycin on sotalol-induced qt interval prolongation in healthy women

AIMS

Telithromycin belongs to ketolides, a new class of macrolide antibiotics. Macrolides are known to have the potential to prolong QT interval duration. Previous studies have shown that telithromycin did not induce significant QT interval prolongation in healthy subjects compared with placebo. The main objective of this study was to demonstrate the absence of amplification of QT interval prolongation induced by sotalol, when telithromycin and sotalol were co-administered. The secondary objective was to correlate the QT interval changes induced by the study drugs to plasma concentrations during the elimination phase.

METHODS

Twenty-four women received sotalol (160 mg) together with placebo or telithromycin (800 mg) in a two-period, double-blind, randomized study. Electrocardiograms were recorded at rest. Comparison of maximal corrected QT interval (QTc(max)) with sotalol in the presence or absence of telithromycin was performed. The relation between sotalol concentration and QTc was studied using linear regression.

RESULTS

Mean difference (95% CI) between QTc(max) with sotalol-placebo and QTc(max) with sotalol-telithromycin was -15.5 ms (-27.7 to -3.2 ms). QTc(max) interval prolongation was lower (P < 0.05) with sotalol-telithromycin than with sotalol-placebo, in relation to decreased sotalol plasma concentrations. Regression analysis showed that the relationship between sotalol plasma concentration and QTc interval duration was not modified by telithromycin co-administration.

CONCLUSION

Our results do not support a potential synergistic effect on QT interval prolongation between sotalol and telithromycin. The decrease of mean QTc interval in subjects taking telithromycin and sotalol may be explained by a decrease of sotalol concentration.

Ensemble of Linear Models for Predicting Drug Properties

We propose a new classification method for the prediction of drug properties, called random feature subset boosting for linear discriminant analysis (LDA). The main novelty of this method is the ability to overcome the problems with constructing ensembles of linear discriminant models based on generalized eigenvectors of covariance matrices. Such linear models are popular in building classification-based structure-activity relationships. The introduction of ensembles of LDA models allows for an analysis of more complex problems than by using single LDA, for example, those involving multiple mechanisms of action. Using four data sets, we show experimentally that the method is competitive with other recently studied chemoinformatic methods, including support vector machines and models based on decision trees. We present an easy scheme for interpreting the model despite its apparent sophistication. We also outline theoretical evidence as to why, contrary to the conventional AdaBoost ensemble algorithm, this method is able to increase the accuracy of LDA models.

Effects of Dofetilide and EGIS-7229, an Antiarrhythmic Agent Possessing Class III, IV, and IB Activities, on Myocardial Refractoriness in Hyperkalemia, Hypokalemia, and During β-Adrenergic Activation in the Rabbit Papillary Muscle In Vitro

Lengthening of the effective refractory period (ERP) by EGIS-7229, a class III/Ib/IV drug, and by dofetilide, a selective I(Kr) blocker, was compared in normokalemia (NK), hypokalemia (LK), and hyperkalemia (HK) in right ventricular papillary muscles of rabbits paced at 0.5, 1, and 2 Hz, in vitro, and also during beta-adrenergic activation. In NK, EGIS-7229 (3 and 10 microM) and dofetilide (30 and 100 nM) similarly lengthened ERP in a steeply reverse frequency-dependent manner. The two compounds produced smaller ERP prolongations at 0.5 Hz in HK and LK, so rate-dependence of ERP changes decreased. EGIS-7229 lengthened ERP more at 2 Hz than at 0.5 Hz at 10 microM in LK, that is, the effect of EGIS-7229 turned into positive frequency-dependence from 3 to 10 microM. Furthermore, EGIS-7229 lengthened ERP at 10 microM more than dofetilide at 100 nM at 2 Hz stimulation rate (P<0.05). Isoproterenol (30 nM) eliminated the effect of dofetilide on ERP, while EGIS-7229 prolonged ERP during beta-adrenergic activation. In conclusion, efficacy of EGIS-7229 was superior to that of dofetilide in LK and during beta-adrenergic stimulation, suggesting improved antiarrhythmic action for EGIS-7229 under certain conditions in the patient.

Structural determinants of potassium channel blockade and drug-induced arrhythmias

Cardiac K+ channels play an important role in the regulation of the shape and duration of the action potential. They have been recognized as targets for the actions of neurotransmitters, hormones, and anti-arrhythmic drugs that prolong the action potential duration (APD) and increase refractoriness. However, pharmacological therapy, often for the purpose of treating syndromes unrelated to cardiac disease, can also increase the vul- nerability of some patients to life-threatening rhythm disturbances. This may be due to an underlying propensity stemming from inherited mutations or polymorphisms, or structural abnormalities that provide a substrate allowing for the initiation of arrhythmic triggers. A number of pharmacological agents that have proved useful in the treatment of allergic reactions, gastrointestinal disorders, and psychotic disorders, among others, have been shown to reduce repolarizing K+ currents and prolong the Q-T interval on the electrocardiogram. Understanding the structural determinants of K+ channel blockade might provide new insights into the mechanism and rate-dependent effects of drugs on cellular physiology. Drug-induced disruption of cellular repolarization underlies electrocardiographic abnormalities that are diagnostic indicators of arrhythmia susceptibility.

Block of hERG channel by ziprasidone: biophysical properties and molecular determinants

Ziprasidone, an antipsychotic agent, delays cardiac repolarization and, thus, prolongs the QT interval of the cardiac ECG. In this study, we examined the biophysical properties and the molecular determinants of the ziprasidone block of wild-type hERG potassium channels stably expressed in HEK-293 cells or wild-type and mutant hERG channels expressed in Xenopus oocytes. In stably transfected HEK-293 cells, ziprasidone blocked wild-type hERG current in a voltage- and concentration-dependent manner (IC(50)=120nM, 0mV, 37 degrees C). Ziprasidone showed minimal tonic block of hERG current estimated during a depolarizing voltage (-20 or +30mV) or evaluated by the envelope of tails test (+30mV). Rate of the block onset was rapid, but not significantly affected by test potentials ranging from -20 to +30mV (time constant (tau)=114+/-14ms at +30mV). The time constant of the slow component of hERG current deactivation (at -50mV) was significantly increased by ziprasidone (tau=1776+/-90 versus 1008+/-71ms, P<0.01). Time course of channel inactivation was slowed by ziprasidone in a voltage-dependent manner. The V(1/2) values for steady-state activation and inactivation of hERG channel in HEK-293 cells were not significantly altered by ziprasidone. In Xenopus oocytes, ziprasidone exhibited less potent block of wild-type hERG current (IC(50)=2.8microM, 0mV, 23 degrees C). Mutation of the aromatic residues (Tyr-652 or Phe-656) located in the S6 domain of hERG dramatically reduced the potency of channel block by ziprasidone (IC(50)>0.4 and 1mM at 0mV for Y652A and F656A, respectively). In conclusion, ziprasidone preferentially binds to and blocks open hERG channels. Tyr-652 and Phe-656 are two critical residues in the ziprasidone-binding site.

Utilizing diagnostic investigations in the poisoned patient

Numerous diagnostic tests may be useful to clinicians caring for poisoned patients. Clinicians should not order a broad range of tests indiscriminately,but rather thoughtfully consider appropriate tests. The results'of the tests should be reviewed in the context of the clinical scenario.

A Randomized Trial Comparing the Cardiac Rhythm Safety of Moxifloxacin vs Levofloxacin in Elderly Patients Hospitalized With Community-Acquired Pneumonia

CONTEXT

Antimicrobial cardiac safety is of particular concern during the treatment of community-acquired pneumonia (CAP) in elderly patients, due to the presence of comorbid conditions and the use of multiple medications that may individually or synergistically affect cardiac repolarization.

STUDY OBJECTIVE

To assess the cardiac rhythm safety of moxifloxacin vs levofloxacin in elderly patients hospitalized with CAP.

DESIGN AND SETTING

Prospective, randomized, double-blind trial conducted at 47 hospitals in the United States.

PATIENTS

Subjects > or = 65 years old with clinical signs/symptoms of CAP requiring initial parenteral therapy, including those with comorbidities. The safety population included 394 patients: 51.3% male; 85.3% white; mean age, 77.8 years. Two-thirds of the patients were > 75 years old, and 74.1% had a history of cardiac disease.

INTERVENTIONS

Patients received IV/oral moxifloxacin (400 mg qd) or IV/oral levofloxacin (500 mg qd) for 7 to 14 days. Safety evaluations included 72 h of digital continuous 12-lead Holter monitoring, 12-lead ECGs at baseline and at maximum serum concentration on day 3, and adverse events.

MAIN OUTCOME MEASURES

The primary safety end point was a composite of ventricular arrhythmia events based on Holter monitoring.

RESULTS

Holter monitor data were available for 387 patients (192 receiving moxifloxacin and 195 receiving levofloxacin). Sixteen moxifloxacin-treated patients (8.3%) and 10 levofloxacin-treated patients (5.1%) had a primary composite cardiac event (p = 0.29); most events were nonsustained ventricular tachycardia (VT) [14 patients receiving moxifloxacin, 7.3%; and 10 patients receiving levofloxacin, 5.1%]. One moxifloxacin-treated patient had sustained monomorphic VT (> 30 s), and one levofloxacin-treated patient had torsade de pointes. Mean +/- SD QTc (Fridericia formula) change on day 3 was + 6.4 +/- 23.2 ms for moxifloxacin and - 2.5 +/- 22.9 ms for levofloxacin (p = 0.04). No deaths clearly related to study drugs occurred during the observation period.

CONCLUSIONS

IV/oral moxifloxacin, although known to cause QTc interval prolongation, has a comparable cardiac rhythm safety profile to IV/oral levofloxacin in high-risk elderly patients with CAP.

Drug-induced q-T prolongation

Drug therapy may induce Q-T prolongation by alteration of potassium ion currents in cardiac cells, resulting in abnormal repolarization. Q-T prolongation, whether congenital or acquired, has been associated with the development of the malignant dysrhythmia Torsade de Pointes (TdP), which may result in sudden death. Re-cent regulatory actions and drug withdrawals due to Q-T prolongation or TdP have focused attention on this issue. Although our understanding of the pathophysiology continues to evolve, both patient and medication factors contribute to the individual risk of drug-induced Q-T prolongation or TdP. The clinician should be aware of these issues when prescribing new drugs and should weigh the risks and benefits carefully when prescribing drugs known to prolong the Q-T interval.

Pharmacokinetic-pharmacodynamic modeling of drug-induced effect on the QT interval in conscious telemetered dogs

INTRODUCTION

To assure drug safety, the investigation of the relationship between plasma concentration and drug-induced prolongation of the QT interval of the ECG is a challenge in drug discovery. For this purpose, dofetilide was utilized to demonstrate the benefits of characterizing the complete time course of concentrations and effect in conscious beagle dogs in the assessment of drug safety.

METHOD

On two separate occasions, four male and two female beagle dogs were given vehicle or the test substance, dofetilide (0.25 mumol/kg), over a 3-h intravenous infusion. Cardiovascular parameters, including QT intervals, were recorded for 24-h using radiotelemetry. The QT interval was corrected individually for heart rate, vehicle treatment, and serial correlation (QT(c)). Exposure (plasma concentration) to dofetilide was measured and described by a two-compartment model. The individual concentration-time course of dofetilide was linked to the QT(c) interval via an effect compartment and a pharmacodynamic E(max) model, to account for the observed hysteresis.

RESULTS

Dofetilide induced a concentration-dependent increase in the QT(c) interval, with an EC(50) of 9 nM (3-30 nM, 95% C.I.) and an E(max) of 59+/-9 ms. A hysteresis loop was observed by plotting plasma concentrations vs. QT interval in time order, indicating a delay in onset of effect. It was found to have an equilibrium half-life of 11+/-8 min. Based on the parameters potency and E(max), a representation was made of the drug-induced changes to the QT interval.

DISCUSSION

An effect compartment model was found to accurately mimic the QT interval prolongation following administration of the test substance, dofetilide. The assessment of the individual concentration-effect relationship and confounding factors such as hysteresis might provide a better prediction of the safety profiles of new drug candidates.

Identifying modulators of hERG channel activity using the PatchXpress planar patch clamp

The authors used the PatchXpress 7000A system to measure compound activity at the hERG channel using procedures that mimicked the "gold-standard" conventional whole-cell patch clamp. A set of 70 compounds, including hERG antagonists with potencies spanning 3 orders of magnitude, were tested on hERG302-HEK cells using protocols aimed at either identifying compound activity at a single concentration or obtaining compound potency from a cumulative concentration dependence paradigm. After exposure to compounds and subsequent washout of the wells to determine reversibility of the block, blockade by a reference compound served as a quality control. Electrical parameters and voltage dependence were similar to those obtained using a conventional whole-cell patch clamp. Rank order of compound potency was also comparable to that determined by conventional methods. One exception was flunarizine, a particularly lipophilic compound. The PatchXpress accurately identified the activity of 29 moderately potent antagonists, which only weakly displace radiolabeled astemizole and are false negatives in the binding assay. Finally, no false hits were observed from a collection of relatively inactive compounds. High-quality data acquisition by PatchXpress should help accelerate secondary screening for ion channel modulators and the drug discovery process.

Effects of moxifloxacin on QT interval in conscious dogs

Moxifloxacin has been shown to induce QT prolongation in both clinical and preclinical models. However, the ability to observe this effect at clinically relevant concentration in normal conscious dogs has not been reported. The purpose of this study was to investigate the effects of moxifloxacin on the QT interval in conscious, healthy dogs. Four male mongrel dogs were chronically instrumented for the measurement of arterial blood pressure, left ventricular blood pressure, cardiac output, electrocardiograms (ECGs), and body temperature. Animals were administered a 1-h i.v. infusion of moxifloxacin once per day via a catheter in the cephalic vein. Each dog received all doses (0, 1, 10, 25 and 50 mg/kg) in an escalating fashion. Moxifloxacin caused a statistically significant increase in arterial blood pressure at 50 mg/kg. A dose-response effect on QT and QTc prolongation was observed. A statistically significant prolongation in the QT interval was observed at 10, 25 and 50 mg/kg and a prolongation of QTc was observed at 25 and 50 mg/kg. These effects occurred at clinically relevant plasma concentrations. This study demonstrate that a study design with four dogs was sensitive enough to measure moxifloxacin-induced QT prolongation at clinically relevant plasma concentrations.