The hERG potassium channel and hERG screening for drug-induced torsades de pointes

Azithromycin, cardiovascular risks, QTc interval prolongation, torsade de pointes, and regulatory issues: A narrative review based on the study of case reports

Over the past year, three articles have appeared in the New England Journal of Medicine describing conflicting findings about azithromycin and cardiac safety, particular azithromycin-induced QTc interval prolongation and torsade de pointes. The FDA wants healthcare providers to consider azithromycin-induced fatal cardiac arrhythmias for patients already at risk for cardiac death and other potentially arrhythmogenic cardiovascular conditions. In a systematic review of case reports we sought to determine factors that link to azithromycin-induced/associated QTc interval prolongation and torsade de pointes. We found 12 cases: seven female and five male. Of the nine adults with reported azithromycin doses, concurrent QTc interval measurement, and without congenital long QT syndrome, we found no significant relationship between dose and QTc interval duration. Additional risk factors were female sex, older age, heart disease, QTc interval prolonging drugs and metabolic inhibitors, hypokalemia, and bradycardia. All 12 subjects had at least two additional risk factors. Elderly women with heart disease appear to be at particularly risk for drug-related QTc interval prolongation and torsade de pointes.

Topical antiangiogenic SRPK1 inhibitors reduce choroidal neovascularization in rodent models of exudative AMD

PURPOSE

Exudative AMD (wet AMD) is treated by monthly injection into the eye of anti-VEGF proteins. VEGF is alternatively spliced to produce numerous isoforms that differ in angiogenic activity. Serine-rich protein kinase-1 (SRPK1) has been identified as a regulator of pro-angiogenic VEGF splicing by phosphorylating serine-rich splicing factor-1 (SRSF1), which binds to VEGF pre-mRNA. We tested the hypothesis that topical (eye drop) SRPK1-selective inhibitors could be generated that reduce pro-angiogenic isoforms, and prevent choroidal neovascularization in vivo.

METHODS

Novel inhibitors were tested for SRPK inhibition in vitro, pro-angiogenic VEGF production in RPE cells by PCR and ELISA, and for inhibition of choroidal neovascularisation in mice and rats.

RESULTS

A novel disubstituted furan inhibitor was selective for the SRPK family of kinases and reduced expression of pro-angiogenic but not antiangiogenic VEGF isoforms. This inhibitor and previously identified SRPK inhibitors significantly reduced choroidal neovascularisation in vivo. Topical administration of SRPK inhibitors dose-dependently blocked CNV with an EC50 of 9 μM.

CONCLUSIONS

These results indicate that novel SRPK1 selective inhibitors could be a potentially novel topical (eye drop) therapeutic for wet AMD.

hERG drug response measured in droplet bilayers

We show measurements of the human cardiac potassium ion channel Kv11.1 (hERG) in droplet bilayers incorporated directly from commercial membrane preparations of HEK293 cells. Although we do not obtain ensemble conductance kinetics and rectification observed in patch clamp measurements of hERG, ensemble currents measured in our system showed inhibition dependent on astemizole and E-4031 concentration, with IC50 values similar to those found with patch clamp. The availability of engineered HEK cells expressing a variety of ion channels, combined with the simplicity of the inhibition measurement, suggest that droplet bilayers may have considerable technological potential for determination of ion channel drug potency.

Risperidone, QTc interval prolongation, and torsade de pointes: a systematic review of case reports

RATIONALE

A recent publication asserted that even low-dose risperidone may induce corrected QT (QTc) interval prolongation up to 500 ms without drug-induced IKr blockade. We seek to better understand the complexity of any link between risperidone-induced/associated QTc interval prolongation and torsade de pointes (TdP).

OBJECTIVES

The objective of this study is to systematically analyze all available case reports of risperidone, QTc interval prolongation, and/or TdP.

METHOD

We identify case reports using PubMed, Medline, EMBASE, and Cochrane.

RESULTS

Of the 15 cases found, nine were adult women (ages 31, 33, 34, 37, 47, "elderly", 77, 84, and 87 years) and one was a teenager. There were four men (ages 28, 29, 29, and 46 years) and one preadolescent boy. Besides risperidone administration or overdose, traditional risk factors for QTc interval prolongation and TdP included female sex (n = 10), older age (n = 4), heart disease (n = 3), hypokalemia (n = 2), bradycardia (n = 1), liver disease (n = 1), QTc interval prolonging drugs other than risperidone (n = 8), and metabolic inhibitors (n = 2). TdP occurred in four cases. Six patients died, and three deaths were probably related to TdP.

CONCLUSION

Risperidone (when properly prescribed in patients free of other risk factors for QTc interval prolongation and TdP) is a relatively safe drug. Conventional statistics can neither predict the individual patient who will experience TdP nor determine the relationship of drug dose to QTc interval prolongation and TdP. Narrative medicine using a case report format appears to be an alternative and valuable additional approach to advance our understanding of this relationship and to reduce risks.

Methadone, QTc interval prolongation and torsade de pointes: Case reports offer the best understanding of this problem

We reviewed the literature and found 31 adult cases and 1 newborn case of methadone-associated QTc interval prolongation and/or torsade de pointes (TdP). Parametric statistics may not be useful in studying this issue because methadone-associated TdP is a very rare event and, hence, "an extreme outlier" consistent with scalable randomness. We may have to rely upon narrative medicine in the form of case reports with all its limitations and hazards to provide our best understanding. We report risk factors for methadone-associated QTc interval prolongation and TdP based on review of published case reports. We believe both drug manufacturers and the FDA would better serve our patients and inform clinicians if they more readily reported drug-induced outliers such as methadone-associated TdP using a case report format.

Clarithromycin, QTc interval prolongation and torsades de pointes: the need to study case reports

BACKGROUND

The manufacturers of clarithromycin sought a drug similar in efficacy to erythromycin but with a superior side-effect profile. They generally achieved this outcome, but postmarketing findings identified a series of reports linking clarithromycin to QTc interval prolongation and torsades de pointes (TdP) ultimately leading to a Black Box Warning. We sought to clarify risk factors associated with TdP among case reports of patients receiving clarithromycin linked to QTc interval prolongation and TdP.

METHODS AND RESULTS

In a detailed literature search, we found 15 women, five men, and one boy meeting our search criteria. Among the 17 adults with reported clarithromycin dose and concurrent QTc interval measurement, we found no statistically significant relationship between clarithromycin dose and QTc interval duration. This did not change for the adults who developed TdP. Among adults, major risk factors were female sex (15), old age (11) and heart disease (17). A total of eight adult subjects had all three major risk factors and 14 of the 20 adults had at least two major risk factors. All adult subjects had at least two risk factors besides clarithromycin. A total of four of the 20 adults received cisapride and three received disopyramide. Three adults were considered to suffer from some aspect of the congenital long QT syndrome.

CONCLUSIONS

We believe that the risk factor description for this drug should be refined to emphasize the major risk factors of (1) female sex, (2) old age and (3) heart disease.

Novel hydroxyl tricyclics (e.g., GSK966587) as potent inhibitors of bacterial type IIA topoisomerases

During the course of our research to find novel mode of action antibacterials, we discovered a series of hydroxyl tricyclic compounds that showed good potency against Gram-positive and Gram-negative pathogens. These compounds inhibit bacterial type IIA topoisomerases. Herein we will discuss structure-activity relationships in this series and report advanced studies on compound 1 (GSK966587) which demonstrates good PK and in vivo efficacy properties. X-ray crystallographic studies were used to provide insight into the structural basis for the difference in antibacterial potency between enantiomers.

Computational assessment of drug-induced effects on the electrocardiogram: from ion channel to body surface potentials

Background and Purpose

Understanding drug effects on the heart is key to safety pharmacology assessment and anti-arrhythmic therapy development. Here our goal is to demonstrate the ability of computational models to simulate the effect of drug action on the electrical activity of the heart, at the level of the ion-channel, cell, heart and ECG body surface potential.

Experimental Approach

We use the state-of-the-art mathematical models governing the electrical activity of the heart. A drug model is introduced using an ion channel conductance block for the hERG and fast sodium channels, depending on the IC₅₀ value and the drug dose. We simulate the ECG measurements at the body surface and compare biomarkers under different drug actions.

Key Results

Introducing a 50% hERG-channel current block results in 8% prolongation of the APD₉₀ and 6% QT interval prolongation, hERG block does not affect the QRS interval. Introducing 50% fast sodium current block prolongs the QRS and the QT intervals by 12% and 5% respectively, and delays activation times, whereas APD₉₀ is not affected.

Conclusions and Implications

Both potassium and sodium blocks prolong the QT interval, but the underlying mechanism is different: for potassium it is due to APD prolongation; while for sodium it is due to a reduction of electrical wave velocity. This study shows the applicability of in silico models for the investigation of drug effects on the heart, from the ion channel to the ECG-based biomarkers.

Potassium channels in Drosophila: historical breakthroughs, significance, and perspectives

Drosophila has enabled important breakthroughs in K(+) channel research, including identification and fi rst cloning of a voltage-activated K(+) channel, Shaker, a founding member of the K(V)1 family. Drosophila has also helped in discovering other K(+) channels, such as Shab, Shaw, Shal, Eag, Sei, Elk, and also Slo, a Ca(2+) - and voltage-dependent K(+) channel. These findings have contributed significantly to our understanding of ion channels and their role in physiology. Drosophila continues to play an important role in ion channel studies, benefiting from an unparalleled arsenal of genetic tools and availability of tens of thousands of genetically modified strains. These tools allow deletion, expression, or misexpression of almost any gene in question with temporal and spatial control. The combination of these tools and resources with the use of forward genetic approach in Drosophila further enhances its strength as a model system. There are many areas in which Drosophila can further help our understanding of ion channels and their function. These include signaling pathways involved in regulating and modulating ion channels, basic information on channels and currents where very little is currently known, and the role of ion channels in physiology and pathology.

Effect of microculture on cell metabolism and biochemistry: do cells get stressed in microchannels?

Microfluidics is emerging as a promising platform for cell culture, enabling increased microenvironment control and potential for integrated analysis compared to conventional macroculture systems such as well plates and Petri dishes. To advance the use of microfluidic devices for cell culture, it is necessary to better understand how miniaturization affects cell behavior. In particular, microfluidic devices have significantly higher surface-area-to-volume ratios than conventional platforms, resulting in lower volumes of media per cell, which can lead to cell stress. We investigated cell stress under a variety of culture conditions using three cell lines: parental HEK (human embryonic kidney) cells and transfected HEK cells that stably express wild-type (WT) and mutant (G601S) human ether-a-go-go related gene (hERG) potassium channel protein. These three cell lines provide a unique model system through which to study cell-type-specific responses in microculture because mutant hERG is known to be sensitive to environmental conditions, making its expression a particularly sensitive readout through which to compare macro- and microculture. While expression of WT-hERG was similar in microchannel and well culture, the expression of mutant G601S-hERG was reduced in microchannels. Expression of the endoplasmic reticulum (ER) stress marker immunoglobulin binding protein (BiP) was upregulated in all three cell lines in microculture. Using BiP expression, glucose consumption, and lactate accumulation as readouts we developed methods for reducing ER stress including properly increasing the frequency of media replacement, reducing cell seeding density, and adjusting the serum concentration and buffering capacity of culture medium. Indeed, increasing the buffering capacity of culture medium or frequency of media replacement partially restored the expression of the G601S-hERG in microculture. This work illuminates how biochemical properties of cells differ in macro- and microculture and suggests strategies that can be used to modify cell culture protocols for future studies involving miniaturized culture platforms.

Action potential clamp and pharmacology of the variant 1 Short QT Syndrome T618I hERG K⁺ channel

BACKGROUND

The familial Short QT Syndrome (SQTS) is associated with an increased risk of cardiac arrhythmia and sudden death. Gain-of-function mutations in the hERG K(+) channel protein have been linked to variant 1 of the SQTS. A hERG channel pore (T618I) mutation has recently been identified in families with heritable SQTS. This study aimed to determine effects of the T618I-hERG mutation on (i) hERG current (I(hERG)) elicited by ventricular action potentials; (ii) the sensitivity of I(hERG) to inhibition by four clinically used antiarrhythmic drugs.

METHODS

Electrophysiological recordings of I(hERG) were made at 37°C from HEK 293 cells expressing wild-type (WT) or T618I hERG. Whole-cell patch clamp recording was performed using both conventional voltage clamp and ventricular action potential (AP) clamp methods.

RESULTS

Under conventional voltage-clamp, WT I(hERG) peaked at 0-+10 mV, whilst for T618I I(hERG) maximal current was right-ward shifted to ∼ +40 mV. Voltage-dependent activation and inactivation of T618I I(hERG) were positively shifted (respectively by +15 and ∼ +25 mV) compared to WT I(hERG). The I(hERG) 'window' was increased for T618I compared to WT hERG. Under ventricular AP clamp, maximal repolarising WT I(hERG) occurred at ∼ -30 mV, whilst for T618I hERG peak I(hERG) occurred earlier during AP repolarisation, at ∼ +5 mV. Under conventional voltage clamp, half-maximal inhibitory concentrations (IC(50)) for inhibition of I(hERG) tails by quinidine, disopyramide, D-sotalol and flecainide for T618I hERG ranged between 1.4 and 3.2 fold that for WT hERG. Under action potential voltage clamp, T618I IC(50)s ranged from 1.2 to 2.0 fold the corresponding IC(50) values for WT hERG.

CONCLUSIONS

The T618I mutation produces a more modest effect on repolarising I(hERG) than reported previously for the N588K-hERG variant 1 SQTS mutation. All drugs studied here appear substantially to retain their ability to inhibit I(hERG) in the setting of the SQTS-linked T618I mutation.

Identification and characterization of a compound that protects cardiac tissue from human Ether-à-go-go-related gene (hERG)-related drug-induced arrhythmias

The human Ether-à-go-go-related gene (hERG)-encoded K(+) current, I(Kr) is essential for cardiac repolarization but is also a source of cardiotoxicity because unintended hERG inhibition by diverse pharmaceuticals can cause arrhythmias and sudden cardiac death. We hypothesized that a small molecule that diminishes I(Kr) block by a known hERG antagonist would constitute a first step toward preventing hERG-related arrhythmias and facilitating drug discovery. Using a high-throughput assay, we screened a library of compounds for agents that increase the IC(70) of dofetilide, a well characterized hERG blocker. One compound, VU0405601, with the desired activity was further characterized. In isolated, Langendorff-perfused rabbit hearts, optical mapping revealed that dofetilide-induced arrhythmias were reduced after pretreatment with VU0405601. Patch clamp analysis in stable hERG-HEK cells showed effects on current amplitude, inactivation, and deactivation. VU0405601 increased the IC(50) of dofetilide from 38.7 to 76.3 nM. VU0405601 mitigates the effects of hERG blockers from the extracellular aspect primarily by reducing inactivation, whereas most clinically relevant hERG inhibitors act at an inner pore site. Structure-activity relationships surrounding VU0405601 identified a 3-pyridiyl and a naphthyridine ring system as key structural components important for preventing hERG inhibition by multiple inhibitors. These findings indicate that small molecules can be designed to reduce the sensitivity of hERG to inhibitors.

Induced pluripotent stem cells as a disease modeling and drug screening platform

Induced pluripotent stem cells (iPSCs) hold great hopes for therapeutic application in various diseases. Although ongoing research is dedicated to achieving clinical translation of iPSCs, further understanding of the mechanisms that underlie complex pathogenic conditions is required. Compared with other classical models for studying diseases, iPSCs provide considerable advantages. A newly emerging application of iPSCs is in vitro disease modeling, which can significantly improve the never-ending search for new pharmacological cures. Here, we will discuss current efforts to create iPSC-dependent patient-specific disease models. Furthermore, we will review the use of iPSCs for development and testing of new therapeutic agents and the implications for high-throughput drug screening.

High potency inhibition of hERG potassium channels by the sodium-calcium exchange inhibitor KB-R7943

BACKGROUND AND PURPOSE

KB-R7943 is an isothiourea derivative that is used widely as a pharmacological inhibitor of sodium-calcium exchange (NCX) in experiments on cardiac and other tissue types. This study investigated KB-R7943 inhibition of hERG (human ether-à-go-go-related gene) K(+) channels that underpin the cardiac rapid delayed rectifier potassium current, I(Kr) .

EXPERIMENTAL APPROACH

Whole-cell patch-clamp measurements were made of hERG current (I(hERG) ) carried by wild-type or mutant hERG channels and of native rabbit ventricular I(Kr) . Docking simulations utilized a hERG homology model built on a MthK-based template.

KEY RESULTS

KB-R7943 inhibited both I(hERG) and native I(Kr) rapidly on membrane depolarization with IC(50) values of ∼89 and ∼120 nM, respectively, for current tails at -40 mV following depolarizing voltage commands to +20 mV. Marked I(hERG) inhibition also occurred under ventricular action potential voltage clamp. I(hERG) inhibition by KB-R7943 exhibited both time- and voltage-dependence but showed no preference for inactivated over activated channels. Results of alanine mutagenesis and docking simulations indicate that KB-R7943 can bind to a pocket formed of the side chains of aromatic residues Y652 and F656, with the compound's nitrobenzyl group orientated towards the cytoplasmic side of the channel pore. The structurally related NCX inhibitor SN-6 also inhibited I(hERG) , but with a markedly reduced potency.

CONCLUSIONS AND IMPLICATIONS

KB-R7943 inhibits I(hERG) /I(Kr) with a potency that exceeds that reported previously for acute cardiac NCX inhibition. Our results also support the feasibility of benzyloxyphenyl-containing NCX inhibitors with reduced potential, in comparison with KB-R7943, to inhibit hERG.

A novel compound PTIQ protects the nigral dopaminergic neurones in an animal model of Parkinson's disease induced by MPTP

BACKGROUND AND PURPOSE

In Parkinson's disease, the dopaminergic neurones in the substantia nigra undergo degeneration. While the exact mechanism for the degeneration is not completely understood, neuronal apoptosis and neuroinflammation are thought to be key contributors. We have recently established that MMP-3 plays crucial roles in dopaminergic cell death and microglial activation.

EXPERIMENTAL APPROACH

We tested the effects of 7-hydroxy-6-methoxy-2-propionyl-1,2,3,4-tetrahydroisoquinoline (PTIQ) on expression of MMP-3 and inflammatory molecules and dopaminergic cell death in vitro and in an animal model of Parkinson's disease, and Parkinson's disease-related motor deficits. The pharmacokinetic profile of PTIQ was also evaluated.

KEY RESULTS

PTIQ effectively suppressed the production of MMP-3 induced in response to cellular stress in the dopaminergic CATH.a cell line and prevented the resulting cell death. In BV-2 microglial cells activated with lipopolysaccharide, PTIQ down-regulated expression of MMP-3 along with IL-1β, TNF-α and cyclooxygenase-2 and blocked nuclear translocation of NF-κB. In the mouse model of Parkinson's disease ,induced by 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine (MPTP), PTIQ attenuated the associated motor deficits, prevented neurodegeneration and suppressed microglial activation in the substantia nigra. Pharmacokinetic analysis showed it was relatively stable against liver microsomal enzymes, did not inhibit the cytochrome p450 isozymes or the hERG ion channel, exhibited no cytotoxicity on liver cells or lethality when administered at 1000 mg kg(-1) and entered the brain rather rapidly yielding a 28% brain:plasma ratio after i.p. injection.

CONCLUSIONS AND IMPLICATIONS

These results suggest PTIQ has potential as a candidate drug for disease-modifying therapy for Parkinson's disease.

hERG K+ Channels: Structure, Function, and Clinical Significance

The human ether-a-go-go related gene (hERG) encodes the pore-forming subunit of the rapid component of the delayed rectifier K+ channel, Kv11.1, which are expressed in the heart, various brain regions, smooth muscle cells, endocrine cells, and a wide range of tumor cell lines. However, it is the role that Kv11.1 channels play in the heart that has been best characterized, for two main reasons. First, it is the gene product involved in chromosome 7-associated long QT syndrome (LQTS), an inherited disorder associated with a markedly increased risk of ventricular arrhythmias and sudden cardiac death. Second, blockade of Kv11.1, by a wide range of prescription medications, causes drug-induced QT prolongation with an increase in risk of sudden cardiac arrest. In the first part of this review, the properties of Kv11.1 channels, including biogenesis, trafficking, gating, and pharmacology are discussed, while the second part focuses on the pathophysiology of Kv11.1 channels.

Cytocompatible performance of thermosensitive poly(N-isopropylacrylamide) nanoparticles

The development of biomedical materials with biocompatibility, especially cytocompatibility, is the frontal research field for material science, biology, medicine, pharmacology and related interdisciplines. We have successfully synthesized a new biomedical material, PNIPAM-g-P(NIPAMco-St) (PNNS) core-shell nanoparticles, and investigated its thermosensitive and fluorescent properties. In order to evaluate the cytocompatibility of the PNNS nanoparticles, the effect of the PNNS nanoparticles on the human ether-àgo-go-related gene (hERG) K(+) channel in HEK-293 cells was investigated for the first time with the inverted fluorescence microscope and the whole-cell patch-clamp technique. The PNNS nanoparticles can be adsorbed on the surface of the cell membrane of HEK-293 cells, and cannot change the structure of HEK-293 cells. The low concentration of the PNNS nanoparticles can slightly inhibit the stable and tail current of the hERG K(+) channel, left-shift the activation curve of the hERG K(+) channel and decrease the deactivation time constant (τ)of the hERG K(+) channel. However, in the presence of the high concentration of the PNNS nanoparticles, the changes mentioned above gradually return to the level in the absence of the PNNS nanoparticles. These results indicated that the PNNS nanoparticles can not damage the cells. Thus, the PNNS nanoparticles have a good cytocompatibility and might be applied as a biomedical material.

Inroads to predict in vivo toxicology-an introduction to the eTOX Project

There is a widespread awareness that the wealth of preclinical toxicity data that the pharmaceutical industry has generated in recent decades is not exploited as efficiently as it could be. Enhanced data availability for compound comparison (“read-across”), or for data mining to build predictive tools, should lead to a more efficient drug development process and contribute to the reduction of animal use (3Rs principle). In order to achieve these goals, a consortium approach, grouping numbers of relevant partners, is required. The eTOX (“electronic toxicity”) consortium represents such a project and is a public-private partnership within the framework of the European Innovative Medicines Initiative (IMI). The project aims at the development of in silico prediction systems for organ and in vivo toxicity. The backbone of the project will be a database consisting of preclinical toxicity data for drug compounds or candidates extracted from previously unpublished, legacy reports from thirteen European and European operation-based pharmaceutical companies. The database will be enhanced by incorporation of publically available, high quality toxicology data. Seven academic institutes and five small-to-medium size enterprises (SMEs) contribute with their expertise in data gathering, database curation, data mining, chemoinformatics and predictive systems development. The outcome of the project will be a predictive system contributing to early potential hazard identification and risk assessment during the drug development process. The concept and strategy of the eTOX project is described here, together with current achievements and future deliverables.

Model for long QT syndrome type 2 using human iPS cells demonstrates arrhythmogenic characteristics in cell culture

Long QT syndrome (LQTS) is caused by functional alterations in cardiac ion channels and is associated with prolonged cardiac repolarization time and increased risk of ventricular arrhythmias. Inherited type 2 LQTS (LQT2) and drug-induced LQTS both result from altered function of the hERG channel. We investigated whether the electrophysiological characteristics of LQT2 can be recapitulated in vitro using induced pluripotent stem cell (iPSC) technology. Spontaneously beating cardiomyocytes were differentiated from two iPSC lines derived from an individual with LQT2 carrying the R176W mutation in the KCNH2 (HERG) gene. The individual had been asymptomatic except for occasional palpitations, but his sister and father had died suddenly at an early age. Electrophysiological properties of LQT2-specific cardiomyocytes were studied using microelectrode array and patch-clamp, and were compared with those of cardiomyocytes derived from control cells. The action potential duration of LQT2-specific cardiomyocytes was significantly longer than that of control cardiomyocytes, and the rapid delayed potassium channel (I_Kr) density of the LQT2 cardiomyocytes was significantly reduced. Additionally, LQT2-derived cardiac cells were more sensitive than controls to potentially arrhythmogenic drugs, including sotalol, and demonstrated arrhythmogenic electrical activity. Consistent with clinical observations, the LQT2 cardiomyocytes demonstrated a more pronounced inverse correlation between the beating rate and repolarization time compared with control cells. Prolonged action potential is present in LQT2-specific cardiomyocytes derived from a mutation carrier and arrhythmias can be triggered by a commonly used drug. Thus, the iPSC-derived, disease-specific cardiomyocytes could serve as an important platform to study pathophysiological mechanisms and drug sensitivity in LQT2.

Inhibition of HERG potassium channels by celecoxib and its mechanism

BACKGROUND

Celecoxib (Celebrex), a widely prescribed selective inhibitor of cyclooxygenase-2, can modulate ion channels independently of cyclooxygenase inhibition. Clinically relevant concentrations of celecoxib can affect ionic currents and alter functioning of neurons and myocytes. In particular, inhibition of Kv2.1 channels by celecoxib leads to arrhythmic beating of Drosophila heart and of rat heart cells in culture. However, the spectrum of ion channels involved in human cardiac excitability differs from that in animal models, including mammalian models, making it difficult to evaluate the relevance of these observations to humans. Our aim was to examine the effects of celecoxib on hERG and other human channels critically involved in regulating human cardiac rhythm, and to explore the mechanisms of any observed effect on the hERG channels.

METHODS AND RESULTS

Celecoxib inhibited the hERG, SCN5A, KCNQ1 and KCNQ1/MinK channels expressed in HEK-293 cells with IC(50)s of 6.0 µM, 7.5 µM, 3.5 µM and 3.7 µM respectively, and the KCND3/KChiP2 channels expressed in CHO cells with an IC(50) of 10.6 µM. Analysis of celecoxib's effects on hERG channels suggested gating modification as the mechanism of drug action.

CONCLUSIONS

The above channels play a significant role in drug-induced long QT syndrome (LQTS) and short QT syndrome (SQTS). Regulatory guidelines require that all new drugs under development be tested for effects on the hERG channel prior to first administration in humans. Our observations raise the question of celecoxib's potential to induce cardiac arrhythmias or other channel related adverse effects, and make a case for examining such possibilities.

Novel cyclohexyl-amides as potent antibacterials targeting bacterial type IIA topoisomerases

As part of our wider efforts to exploit novel mode of action antibacterials, we have discovered a series of cyclohexyl-amide compounds that has good Gram positive and Gram negative potency. The mechanism of action is via inhibition of bacterial topoisomerases II and IV. We have investigated various subunits in this series and report advanced studies on compound 7 which demonstrates good PK and in vivo efficacy properties.

Novel amino-piperidines as potent antibacterials targeting bacterial type IIA topoisomerases

We have identified a series of amino-piperidine antibacterials with a good broad spectrum potency. We report the investigation of various subunits in this series and advanced studies on compound 8. Compound 8 possesses good pharmacokinetics, broad spectrum antibacterial activity and demonstrates oral efficacy in a rat lung infection model.

Effect of lead ion on the function of the human ether-à-go-go-related gene K+ channel

Lead (Pb) is a trace metal element in the human body. In order to understand the hazard mechanism of the elevated blood lead level on the human body, the effect of Pb(2+) on the human ether-à-go-go-related gene (hERG) K(+) channel in the HEK 293 cell was investigated for the first time using whole-cell patch clamp technique, molecular dynamics simulation, and quantum chemistry calculation methods. We found that Pb(2+) obviously inhibits the current of the hERG K(+) channel, and delays the "activation" and "deactivation" of the hERG K(+) channel, indicating that Pb(2+) evidently decreases the function of the K(+) channel in the cell. The effect is increased with increasing the concentration of Pb(2+). When the concentration of Pb(2+) is 400 μg L(-1), the function of the K(+) channel is entirely lost. The results from the molecular dynamics simulation and quantum chemistry calculation indicated that Pb(2+) can coordinate with the oxygen/sulfur atoms in the K(+) channel protein, leading to the decrease in the function of the K(+) channel. According to the experimental results, we suggested that once the K(+) channel in the human body was irreversibly inactivated by Pb(2+), it would affect the treatment and prognosis of Pb(2+) intoxication.

Molecular determinants of hERG potassium channel inhibition by disopyramide

The Class Ia antiarrhythmic drug disopyramide (DISO) causes QT interval prolongation that is potentially dangerous in acquired Long QT Syndrome but beneficial in short QT syndrome, through inhibition of the hERG-encoded channels responsible for rapid delayed rectifier K(+) current (I(Kr)). In this study, alanine mutants of hERG S6 and pore helix residues and MthK-based homology modelling and ligand docking were used to investigate molecular determinants of DISO binding to hERG. Whole-cell hERG current (I(hERG)) recordings were made at 37°C from HEK-293 cells expressing WT or mutant hERG channels. WT outward I(hERG) tails were inhibited with an IC(50) of 7.3μM, whilst inward I(hERG) tails in a high [K(+)](e) of 94mM were blocked with an IC(50) of 25.7μM. The IC(50) for the Y652A mutation was ~55-fold that of WT I(hERG); this mutation also abolished a leftward shift in voltage-dependent I(hERG) activation present for WT hERG. The IC(50) for F656A I(hERG) was ~51 fold its corresponding WT control. In contrast to previously studied methanesulphonanilide hERG inhibitors, neither the G648A S6 nor the T623A and S624A pore helical mutations modified DISO IC(50). Computational docking with the hERG model showed that DISO did not exhibit a single unique binding pose; instead several low energy binding poses at the lower end of the pore cavity favoured interactions with Y652 and F656. In the WT hERG model DISO did not interact directly with residues at the base of the pore helix, consistent with the minimal effect of mutation of these residues on drug block.

K(+) channels as therapeutic targets in oncology

Ion channels are involved in a variety of tumors. In particular, potassium channels are expressed abnormally in many cancer types, where their pharmacologic manipulation impairs tumor progression. Since this group of molecules has been successfully targeted for decades in other therapeutic areas, there is a significant body of knowledge on the pharmacology of potassium channels. Several groups of potassium channels with defined molecular identities have been proposed as candidates for therapeutic intervention. The strategies put forward range from classical small molecule blockade to gene therapy approaches, and include the use of potassium channels as targets for adjuvant therapy. We will discuss the reasons for these proposals and explore possible future developments.

The hERG K(+) channel S4 domain L532P mutation: characterization at 37°C

hERG (human Ether-à-go-go Related Gene) is responsible for ion channels mediating rapid delayed rectifier potassium current, I_Kr, which is key to cardiac action potential repolarization. Gain-of-function hERG mutations give rise to the SQT1 variant of the Short QT Syndrome (SQTS). Reggae mutant zebrafish, with a S4 zERG mutation (Leucine499Proline; L499P), display arrhythmic features analogous to those seen in the SQTS. The affected S4 domain ERG residue is highly conserved. This study was executed to determine how the homologous hERG mutation (L532P) influences channel function at 37 °C. Whole-cell measurements of current (I_hERG) were made from HEK 293 cells expressing WT or L532P hERG. The half maximal activation voltage (V_0.5) of L532P I_hERG was positively shifted by ~+36 mV compared to WT I_hERG; however at negative voltages a pronounced L532P I_hERG was observed. Both activation and deactivation time-courses were accelerated for L532P I_hERG. The inactivation V_0.5 for L532P I_hERG was shifted by ~+32 mV. Under action potential (AP) voltage-clamp, L532P I_hERG exhibited a dome-shaped current peaking at ~+16 mV, compared to ~−31 mV for WT-I_hERG. The L532P mutation produced an ~ 5-fold increase in the IC₅₀ for dronedarone inhibition of I_hERG. Homology modeling indicated that the L532 residue within the S4 helix lies closely apposed to the S5 region of an adjacent hERG subunit. Alterations to the S4 domain structure and, potentially, to interactions between adjacent hERG subunits are likely to account for the functional effects of this mutation.

Quantification of repolarization reserve to understand interpatient variability in the response to proarrhythmic drugs: a computational analysis

BACKGROUND

"Repolarization reserve" is frequently invoked to explain why potentially proarrhythmic drugs cause, across a population, a range of changes to cardiac action potentials (APs). However, the mechanisms underlying this interindividual variability are not understood quantitatively.

OBJECTIVE

The purpose of this study was to perform a novel analysis of mathematical models of ventricular myocytes to quantify repolarization reserve and gain insight into the factors responsible for variability in the response to proarrhythmic drugs.

METHODS/RESULTS

In several models of human or canine ventricular myocytes, variability was simulated by randomizing model parameters and running repeated simulations. With each randomly generated set of parameters, APs before and after simulated 75% block of the rapid delayed rectifier current (I(Kr)) were calculated. Multivariable regression was performed to determine how much each model parameter attenuated or exacerbated the AP prolongation caused by the I(Kr)-blocking drug. Simulations with a human ventricular myocyte model suggest that drug response is influenced most strongly by (1) the density of I(Kr), (2) the density of slow delayed rectifier current I(Ks), (3) the voltage dependence of I(Kr) inactivation, (4) the density of L-type Ca2+ current, and (5) the kinetics of I(Ks) activation. The analysis also identified mechanisms underlying nonintuitive behavior, such as ionic currents that prolong baseline APs but decrease drug-induced AP prolongation. Finally, the simulations provided quantitative insight into conditions that aggravate the drug response, such as silent ion channel mutations and heart failure.

CONCLUSION

These modeling results provide the first thorough quantification of repolarization reserve and improve our understanding of interindividual variability in adverse drug reactions.

Role of voltage-gated sodium, potassium and calcium channels in the development of cocaine-associated cardiac arrhythmias

Cocaine is a highly active stimulant that alters dopamine metabolism in the central nervous system resulting in a feeling of euphoria that with time can lead to addictive behaviours. Cocaine has numerous deleterious effects in humans including seizures, vasoconstriction, ischaemia, increased heart rate and blood pressure, cardiac arrhythmias and sudden death. The cardiotoxic effects of cocaine are indirectly mediated by an increase in sympathomimetic stimulation to the heart and coronary vasculature and by a direct effect on the ion channels responsible for maintaining the electrical excitability of the heart. The direct and indirect effects of cocaine work in tandem to disrupt the co-ordinated electrical activity of the heart and have been associated with life-threatening cardiac arrhythmias. This review focuses on the direct effects of cocaine on cardiac ion channels, with particular focus on sodium, potassium and calcium channels, and on the contributions of these channels to cocaine-induced arrhythmias. Companion articles in this edition of the journal examine the epidemiology of cocaine use (Wood & Dargan) and the treatment of cocaine-associated arrhythmias (Hoffmann).

Developing predictive CSF biomarkers-a challenge critical to success in Alzheimer's disease and neuropsychiatric translational medicine

The need to develop effective treatments for Alzheimer's disease has been confounded by repeated clinical failures where promising new chemical entities that have been extensively characterized in preclinical models of Alzheimer's disease have failed to show efficacy in the human disease state. This has been attributed to: the selection of drug targets that have yet to be shown as causal to the disease as distinct from being the result of the disease process, a lack of congruence in the animal models of Alzheimer's disease, wild-type and transgenic, to the human disease, and the enrollment of patients in proof of concept clinical trials who are at too advanced a stage of the disease to respond to any therapeutic. The development of validated biomarkers that can be used for disease diagnosis and progression is anticipated to improve patient enrollment in clinical trials, to develop new animal models and to identify new disease targets for drug discovery. The present review assesses the status of current efforts in developing CSF biomarkers for Alzheimer's disease and briefly discusses the status of CSF biomarker efforts in schizophrenia, depression, Parkinson's disease and multiple sclerosis.

Techniques for recording reconstituted ion channels

This review describes and discusses techniques useful for monitoring the activity of protein ion channels in vitro. In the first section the biological importance and the classification of ion channels are outlined in order to justify the strong motivation for dealing with this important class of membrane proteins. The expression, reconstitution and integration of recombinant proteins into lipid bilayers are crucial steps to obtain consistent data when working with ion channels. In the second section recording techniques used in research are presented. Since this review focuses on analytical systems bearing reconstituted ion channels the industrial most important patch-clamp techniques of cells are only briefly mentioned. In section three, artificial systems developed in the last decades are described while the emerging technologies using nanostructured supports or microfluidic systems are presented in section four. Finally, the remaining challenges of membrane protein analysis and its potential applications are briefly outlined.

Microfluidic cell culture and its application in high-throughput drug screening: cardiotoxicity assay for hERG channels

Evaluation of drug cardiotoxicity is essential to the safe development of novel pharmaceuticals. Assessing a compound's risk for prolongation of the surface electrocardiographic QT interval and hence risk for life-threatening arrhythmias is mandated before approval of nearly all new pharmaceuticals. QT prolongation has most commonly been associated with loss of current through hERG (human ether-a-go-go related gene) potassium ion channels due to direct block of the ion channel by drugs or occasionally by inhibition of the plasma membrane expression of the channel protein. To develop an efficient, reliable, and cost-effective hERG screening assay for detecting drug-mediated disruption of hERG membrane trafficking, the authors demonstrate the use of microfluidic-based systems to improve throughput and lower cost of current methods. They validate their microfluidics array platform in polystyrene (PS), cyclo-olefin polymer (COP), and polydimethylsiloxane (PDMS) microchannels for drug-induced disruption of hERG trafficking by culturing stably transfected HEK cells that overexpressed hERG (WT-hERG) and studying their morphology, proliferation rates, hERG protein expression, and response to drug treatment. Results show that WT-hERG cells readily proliferate in PS, COP, and PDMS microfluidic channels. The authors demonstrated that conventional Western blot analysis was possible using cell lysate extracted from a single microchannel. The Western blot analysis also provided important evidence that WT-hERG cells cultured in microchannels maintained regular (well plate-based) expression of hERG. The authors further show that experimental procedures can be streamlined by using direct in-channel immunofluorescence staining in conjunction with detection using an infrared scanner. Finally, treatment of WT-hERG cells with 5 different drugs suggests that PS (and COP) microchannels were more suitable than PDMS microchannels for drug screening applications, particularly for tests involving hydrophobic drug molecules.

Drug-induced long QT syndrome

The drug-induced long QT syndrome is a distinct clinical entity that has evolved from an electrophysiologic curiosity to a centerpiece in drug regulation and development. This evolution reflects an increasing recognition that a rare adverse drug effect can profoundly upset the balance between benefit and risk that goes into the prescription of a drug by an individual practitioner as well as the approval of a new drug entity by a regulatory agency. This review will outline how defining the central mechanism, block of the cardiac delayed-rectifier potassium current I(Kr), has contributed to defining risk in patients and in populations. Models for studying risk, and understanding the way in which clinical risk factors modulate cardiac repolarization at the molecular level are discussed. Finally, the role of genetic variants in modulating risk is described.

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.

State-dependent blockade of human ether-a-go-go-related gene (hERG) K(+) channels by changrolin in stably transfected HEK293 cells

AIM

To study the effect of changrolin on the K(+) channels encoded by the human ether-a-go-go-related gene (hERG).

METHODS

hERG channels were heterologously stably expressed in human embryonic kidney 293 cells, and the hERG K(+) currents were recorded using a standard whole-cell patch-clamp technique.

RESULTS

Changrolin inhibited hERG channels in a concentration-dependent and reversible manner (IC(50)=18.23 mumol/L, 95% CI: 9.27-35.9 mumol/L; Hill coefficient=-0.9446). In addition, changrolin shifted the activation curve of hERG channels by 14.3+/-1.5 mV to more negative potentials (P<0.01, n=9) but did not significantly affect the steady-state inactivation of hERG (n=5, P>0.05). The relative block of hERG channels by changrolin was close to zero at the time point of channel opening by the depolarizing voltage step and quickly increased afterwards. The maximal block was achieved in the inactivated state, with no further development of the open channel block. In the "envelope of tails" experiments, the time constants of activation were found to be 287.8+/-46.2 ms and 174.2+/-18.4 ms, respectively, for the absence and presence of 30 mumol/L changrolin (P<0.05, n=7). The onset of inactivation was accelerated significantly by changrolin between -40 mV and +60 mV (P<0.05, n=7).

CONCLUSION

The results demonstrate that changrolin is a potent hERG blocker that preferentially binds to hERG channels in the open and inactivated states.

Revealing the structural basis of action of hERG potassium channel activators and blockers

Human ether-á-go-go related gene (hERG) potassium (K(+)) channels play a critical role in cardiac action potential repolarization. This is due, in large part, to the unique gating properties of these channels, which are characterized by relatively slow activation and an unusually fast and voltage-dependent inactivation. A large number of structurally diverse compounds bind to hERG and carry an unacceptably high risk of causing arrhythmias. On the other hand, drugs that increase hERG current may, at least in principle, prove useful for treatment of long QT syndrome. A few blockers have been shown to increase hERG current at potentials close to the threshold for channel activation--a process referred to as facilitation. More recently, a novel group of hERG channel activators have been identified that slow deactivation and/or attenuate inactivation. Structural determinants for the action of two different types of activators have been identified. These compounds bind at sites that are distinct from each other and also separate from the binding site of high affinity blockers. They reveal not only novel ways of chemically manipulating hERG channel function, but also interactions between structural domains that are critical to normal activation and inactivation gating.

Two four-marker haplotypes on 7q36.1 region indicate that the potassium channel gene HERG1 (KCNH2, Kv11.1) is related to schizophrenia: a case control study

Background

The pathobiology of schizophrenia is still unclear. Its current treatment mainly depends on antipsychotic drugs. A leading adverse effect of these medications is the acquired long QT syndrome, which results from the blockade of cardiac HERG1 channels (human ether-a-go-go-related gene potassium channels 1) by antipsychotic agents. The HERG1 channel is encoded by HERG1 (KCNH2, Kv11.1) gene and is most highly expressed in heart and brain. Genetic variations in HERG1 predispose to acquired long QT syndrome. We hypothesized that the blockade of HERG1 channels by antipsychotics might also be significant for their therapeutic mode of action, indicating a novel mechanism in the pathogenesis of schizophrenia.

Methods

We genotyped four single nucleotide polymorphisms (SNPs) in 7q36.1 region (two SNPs, rs1805123 and rs3800779, located on HERG1, and two SNPs, rs885684 and rs956642, at the 3'-downstream intergenic region) and then performed single SNP and haplotype association analyses in 84 patients with schizophrenia and 74 healthy controls after the exclusion of individuals having prolonged or shortened QT interval on electrocardiogram.

Results

Our analyses revealed that both genotype and allele frequencies of rs3800779 (c.307+585G>T) were significantly different between populations (P = 0.023 and P = 0.018, respectively). We also identified that two previously undescribed four-marker haplotypes which are nearly allelic opposite of each other and located in chr7:150225599-150302147bp position encompassing HERG1 were either overrepresented (A-A-A-T, the at-risk haplotype, P = 0.0007) or underrepresented (C-A-C-G, the protective haplotype, P = 0.005) in patients compared to controls.

Conclusions

Our results indicate that the potassium channel gene HERG1 is related to schizophrenia. Our findings may also implicate the whole family of HERG channels (HERG1, HERG2 and HERG3) in the pathogenesis of psychosis and its treatment.