A Place for High-Throughput Electrophysiology in Cardiac Safety: Screening hERG Cell Lines and Novel Compounds with the Ion Works HTTM System

Use of Patient Health Records to Quantify Drug-Related Pro-arrhythmic Risk

There is an increasing expectation that computational approaches may supplement existing human decision-making. Frontloading of models for cardiac safety prediction is no exception to this trend, and ongoing regulatory initiatives propose use of high-throughput in vitro data combined with computational models for calculating proarrhythmic risk. Evaluation of these models requires robust assessment of the outcomes. Using FDA Adverse Event Reporting System reports and electronic healthcare claims data from the Truven-MarketScan US claims database, we quantify the incidence rate of arrhythmia in patients and how this changes depending on patient characteristics. First, we propose that such datasets are a complementary resource for determining relative drug risk and assessing the performance of cardiac safety models for regulatory use. Second, the results suggest important determinants for appropriate stratification of patients and evaluation of additional drug risk in prescribing and clinical support algorithms and for precision health.

In vitro data and computational models can assist with calculating pro-arrhythmic risk

We use patient health records and FDA Adverse Event Reporting System reports

Use of such datasets helps assess relative drug risk and cardiac safety models

We quantify how patient characteristics can affect arrhythmia incidence

Cardiotoxicity screening: a review of rapid-throughput in vitro approaches

Cardiac toxicity represents one of the leading causes of drug failure along different stages of drug development. Multiple very successful pharmaceuticals had to be pulled from the market or labeled with strict usage warnings due to adverse cardiac effects. In order to protect clinical trial participants and patients, the International Conference on Harmonization published guidelines to recommend that all new drugs to be tested preclinically for hERG (Kv11.1) channel sensitivity before submitting for regulatory reviews. However, extensive studies have demonstrated that measurement of hERG activity has limitations due to the multiple molecular targets of drug compound through which it may mitigate or abolish a potential arrhythmia, and therefore, a model measuring multiple ion channel effects is likely to be more predictive. Several phenotypic rapid-throughput methods have been developed to predict the potential cardiac toxic compounds in the early stages of drug development using embryonic stem cells- or human induced pluripotent stem cell-derived cardiomyocytes. These rapid-throughput methods include microelectrode array-based field potential assay, impedance-based or Ca(2+) dynamics-based cardiomyocytes contractility assays. This review aims to discuss advantages and limitations of these phenotypic assays for cardiac toxicity assessment.

Industrializing electrophysiology: HT automated patch clamp on SyncroPatch® 96 using instant frozen cells

Patch-clamping is a powerful technique for investigating the ion channel function and regulation. However, its low throughput hampered profiling of large compound series in early drug development. Fortunately, automation has revolutionized the area of experimental electrophysiology over the past decade. Whereas the first automated patch-clamp instruments using the planar patch-clamp technology demonstrated rather a moderate throughput, few second-generation automated platforms recently launched by various companies have significantly increased ability to form a high number of high-resistance seals. Among them is SyncroPatch(®) 96 (Nanion Technologies GmbH, Munich, Germany), a fully automated giga-seal patch-clamp system with the highest throughput on the market. By recording from up to 96 cells simultaneously, the SyncroPatch(®) 96 allows to substantially increase throughput without compromising data quality. This chapter describes features of the innovative automated electrophysiology system and protocols used for a successful transfer of the established hERG assay to this high-throughput automated platform.

New screening system for selective blockers of voltage-gated K(+) channels using recombinant cell lines dying upon single action potential

To develop a simple screening system for blockers of voltage-gated Kv1.3 and Kv1.5 channels, new cell lines co-expressing mutated Nav1.5 (IFM/Q3), Kir2.1 (Kir), and Kv1.3 or Kv1.5 were introduced as IFM/Q3+Kir+Kv1.3 and IFM/Q3+Kir+Kv1.5, respectively. Electrical stimulation (ES) of a cell line, IFM/Q3+Kir, induced prolonged action potentials due to the slow inactivation of IFM/Q3 and subsequent cell death. Additional co-expression of Kv1.3 or Kv1.5 to IFM/Q3+Kir shortened the evoked action potentials and prevented cell death. In the presence of margatoxin, a selective Kv1.3-blocker, ES induced cell death in IFM/Q3+Kir+Kv1.3, but not in IFM/Q3+Kir+Kv1.5. In the presence of 4-aminopyridine, a non-selective Kv-channel blocker, ES application elicited cell death in both cell lines. The IC50s of acacetin, a Kv1.5-blocker, was 10.2 μM in IFM/Q3+Kir+Kv1.3 and almost identical to that in IFM/Q3+Kir+Kv1.5 (7.6 μM). The IC50s of citalopram, a 5-HT uptake-inhibitor, were 1.8 μM in IFM/Q3+Kir+Kv1.3 and 1.5 μM in IFM/Q3+Kir+Kv1.5, respectively. These IC50s were comparable to those determined electrophysiologically. In conclusion, acacetin and citalopram block both Kv1.3 and Kv1.5 without selectivity. The Kv1.3 or Kv1.5 channel inhibition assay using these new cell lines may be applicable to high-throughput screening because of its simplicity, accuracy, and high cost-performance.

Generation of an antibody toolbox to characterize hERG

The human ether-a-go-go related gene (hERG) potassium channel plays a major role in the repolarization of the cardiac action potential. Inhibition of the hERG function by mutations or a wide variety of pharmaceutical compounds cause long QT syndrome and lead to potentially lethal arrhythmias. For detailed insights into the structural and biochemical background of hERG function and drug binding, the purification of recombinant protein is essential. Because the hERG channel is a challenging protein to purify, fast and easy techniques to evaluate different expression, solubilization and purification conditions are of primary importance. Here, we describe the generation of a set of 12 monoclonal antibodies against hERG. Beside their suitability in western blot, immunoprecipitation and immunostaining, these antibodies were used to establish a sandwich ELISA for the detection and relative quantification of hERG in different expression systems. Furthermore, a Fab fragment was used in fluorescence size exclusion chromatography to determine the oligomeric state of hERG after solubilization. These new tools can be used for a fast and efficient screening of expression, solubilization and purification conditions.

Development and validation of an automated high-throughput system for zebrafish in vivo screenings

The zebrafish is a vertebrate model compatible with the paradigms of drug discovery. The small size and transparency of zebrafish embryos make them amenable for the automation necessary in high-throughput screenings. We have developed an automated high-throughput platform for in vivo chemical screenings on zebrafish embryos that includes automated methods for embryo dispensation, compound delivery, incubation, imaging and analysis of the results. At present, two different assays to detect cardiotoxic compounds and angiogenesis inhibitors can be automatically run in the platform, showing the versatility of the system. A validation of these two assays with known positive and negative compounds, as well as a screening for the detection of unknown anti-angiogenic compounds, have been successfully carried out in the system developed. We present a totally automated platform that allows for high-throughput screenings in a vertebrate organism.

Development of recombinant cell line co-expressing mutated Nav1.5, Kir2.1, and hERG for the safety assay of drug candidates

To provide a high-throughput screening method for human ether-a-go-go-gene-related gene (hERG) K(+) channel inhibition, a new recombinant cell line, in which single action potential (AP)-induced cell death was produced by gene transfection. Mutated human cardiac Na(+) channel Nav1.5 (IFM/Q3), which shows extremely slow inactivation, and wild-type inward rectifier K(+) channel, Kir2.1, were stably co-expressed in HEK293 cells (IFM/Q3+Kir2.1). In IFM/Q3+Kir2.1, application of single electrical stimulation (ES) elicited a long AP lasting more than 30 s and led cells to die by more than 70%, whereas HEK293 co-transfected with wild-type Nav1.5 and Kir2.1 fully survived. The additional expression of hERG K(+) channels in IFM/Q3+Kir2.1 shortened the duration of evoked AP and thereby markedly reduced the cell death. The treatment of the cells with hERG channel inhibitors such as nifekalant, E-4031, cisapride, terfenadine, and verapamil, recovered the prolonged AP and dose-dependently facilitated cell death upon ES. The EC(50) values to induce the cell death were 3 µM, 19 nM, 17 nM, 74 nM, and 3 µM, respectively, whereas 10 µM nifedipine did not induce cell death. Results indicate the high utility of this cell system for hERG K(+) channel safety assay.

The discovery of 2-fluoro-N-(3-fluoro-4-(5-((4-morpholinobutyl)amino)-1,3,4-oxadiazol-2-yl)phenyl)benzamide, a full agonist of the alpha-7 nicotinic acetylcholine receptor showing efficacy in the novel object recognition model of cognition enhancement

A series of α7 nicotinic acetylcholine receptor full agonists with a 1,3,4-oxadiazol-2-amine core has been discovered. Early lead 1 was found to have a limited therapeutic index with respect to its potential for cardiovascular side effects. Further optimisation of this series led to the identification of 22 a potent full agonist showing efficacy at a dose of 0.1mg/kg in the novel object recognition model of cognition enhancement. Comparison of 1 with 22 demonstrated the latter to have an improved oral pharmacokinetic profile and cardiovascular therapeutic index.

Identification of a series of 1,3,4-oxadiazol-2-amines as potent alpha-7 agonists with efficacy in the novel object recognition model of cognition

A series of α7 nicotinic acetylcholine receptor full-agonists with a 1,3,4-oxadiazol-2-amine core has been discovered. Systematic exploration of the structure-activity relationships for both α7 potency and selectivity with respect to interaction with the hERG channel are described. Further profiling led to the identification of compound 22, a potent full agonist showing efficacy in the novel object recognition model of cognition enhancement.

Toward a New Gold Standard for Early Safety: Automated Temperature-Controlled hERG Test on the PatchLiner

The Patchliner® temperature-controlled automated patch clamp system was evaluated for testing drug effects on potassium currents through human ether-à-go-go related gene (hERG) channels expressed in Chinese hamster ovary cells at 35-37°C. IC(50) values for a set of reference drugs were compared with those obtained using the conventional voltage clamp technique. The results showed good correlation between the data obtained using automated and conventional electrophysiology. Based on these results, the Patchliner(®) represents an innovative automated electrophysiology platform for conducting the hERG assay that substantially increases throughput and has the advantage of operating at physiological temperature. It allows fast, accurate, and direct assessment of channel function to identify potential proarrhythmic side effects and sets a new standard in ion channel research for drug safety testing.

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

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

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

Drug-induced torsades de pointes (TdP) arrhythmia is a major safety concern in the process of drug design and development. The incidence of TdP tends to be low, so early pre-clinical screens rely on surrogate markers of TdP to highlight potential problems with new drugs. hERG (human ether-à-go-go-related gene, alternative nomenclature KCNH2) is responsible for channels mediating the 'rapid' delayed rectifier K+ current (IKr) which plays an important role in ventricular repolarization. Pharmacological inhibition of native IKr and of recombinant hERG channels is a shared feature of diverse drugs associated with TdP. In vitro hERG assays therefore form a key element of an integrated assessment of TdP liability, with patch-clamp electrophysiology offering a 'gold standard'. However, whilst clearly necessary, hERG assays cannot be assumed automatically to provide sufficient information, when considered in isolation, to differentiate 'safe' from 'dangerous' drugs. Other relevant factors include therapeutic plasma concentration, drug metabolism and active metabolites, severity of target condition and drug effects on other cardiac ion channels that may mitigate or exacerbate effects of hERG blockade. Increased understanding of the nature of drug-hERG channel interactions may ultimately help eliminate potential hERG blockade early in the design and development process. Currently, for promising drug candidates integration of data from hERG assays with information from other pre-clinical safety screens remains essential.

High-throughput profiling of ion channel activity in primary human lymphocytes

We present a high-throughput method to quantify the functional activity of potassium (K (+)) ion channels in primary human lymphocytes. This method is rapid, automated, specific (here for the voltage-gated Kv1.3 ion channel), and capable of measuring, in parallel, the electrical currents of over 200 individual lymphocytes isolated from freshly drawn blood. The statistics afforded by high-throughput measurements allowed direct comparison of Kv1.3 activity in different subsets of lymphocytes, including CD4 (+) and CD8 (+) T cells, gammadelta T cells, and B cells. High-throughput measurements made it possible to quantify the heterogeneous, functional response of Kv1.3 ion channel activity upon stimulation of CD4 (+) and CD8 (+) T cells with mitogen. These experiments enabled elucidation of time-courses of functional Kv1.3 activity upon stimulation as well as studies of the effects of the concentration of mitogenic antibodies on Kv1.3 levels. The results presented here suggest that Kv1.3 ion channel activity can be used as a functional activation marker in T cells and that it correlates to cell size and levels of a surface antigen, CD25. Moreover, this work presents an enabling methodology that can be applied widely, allowing high-throughput screening of specific voltage-gated ion channels in a variety of primary cells.

High-throughput electrophysiology: an emerging paradigm for ion-channel screening and physiology

Ion channels represent highly attractive targets for drug discovery and are implicated in a diverse range of disorders, in particular in the central nervous and cardiovascular systems. Moreover, assessment of cardiac ion-channel activity of new chemical entities is now an integral component of drug discovery programmes to assess potential for cardiovascular side effects. Despite their attractiveness as drug discovery targets ion channels remain an under-exploited target class, which is in large part due to the labour-intensive and low-throughput nature of patch-clamp electrophysiology. This Review provides an update on the current state-of-the-art for the various automated electrophysiology platforms that are now available and critically evaluates their impact in terms of ion-channel screening, lead optimization and the assessment of cardiac ion-channel safety liability.

Validation of a medium-throughput electrophysiological assay for KCNQ2/3 channel enhancers using IonWorks HT

Enhancers of KCNQ channels are known to be effective in chronic pain models. To discover novel enhancers of KCNQ channels, the authors developed a medium-throughput electrophysiological assay by using the IonWorks platform. Screening of 20 CHO-K1 clones stably expressing KCNQ2/3 was performed on the IonWorks HT until the best clone (judged from seal rate, current level, and stability) was obtained. The KCNQ2/3 current amplitude in the cells was found to increase from 60 +/- 15 pA to 473 +/- 80 pA (at -10 mV), and the expression rate was increased by 56% when the cells were incubated at 27 degrees C overnight. The clone used for compound screening had a seal rate of greater than 90% and an overall success rate of greater than 70%. The voltage step protocol (hold cells at -80 mV and depolarize to -10 mV for 1 s) was designed to provide moderate current but still allow for pharmacological current enhancement. EC(50)s were generated from 8-point concentration-response curves with a control compound on each plate using compounds that were also tested with conventional patch clamp. The authors found that there was a very good correlation (R(2) > 0.9) between the 2 assays, thus demonstrating the highly predictive nature of the IonWorks assay.

Recording hERG potassium currents and assessing the effects of compounds using the whole-cell patch-clamp technique

The complex gating of the hERG channel makes it ideally suited to its principal role in controlling phase 3 repolarization of the cardiac ventricular action potential. Any abnormal delay in repolarization can lead to the re-activation of Ca(2)+ channels, giving rise to early after-depolarizations, and coupled with increased cardiac dispersion, typically associated with these delays, provides respectively both the trigger and substrate for the potentially life threatening arrhythmia Torsardes de Pointes (TdP). Owing to the fundamental role of hERG in controlling the duration of the cardiac action potential, it is not surprising that any drugs that potently and selectively block this channel are liable to have these effects. Consequently, much effort has been expended in developing standard voltage protocols to reliably assess the effects of compounds on hERG currents in vitro. This chapter describes how to record hERG currents in a recombinant cell line using the whole-cell patch-clamp technique. It also provides typical voltage protocols used for assessing the basic electrophysiological properties of these currents and for assessing the effects of compounds on hERG tail currents.

Population patch clamp electrophysiology: a breakthrough technology for ion channel screening

Population patch clamp (PPC) is a novel high throughput planar array electrophysiology technique that allows ionic currents to be recorded from populations of cells under voltage clamp. For the drug discovery pharmacologist, PPC promises greater speed and precision than existing methods for screening compounds at voltage-gated ion channel targets. Moreover, certain constitutively active or slow-ligand gated channels that have hitherto proved challenging to screen with planar array electrophysiology (e.g. SK/IK channels) are now more accessible. In this article we review early findings using PPC and provide a perspective on its likely impact on ion channel drug discovery. To support this, we include some new data on ion channel assay duplexing and on modulator assays, approaches that have thus far not been described.

Optimisation and validation of a medium-throughput electrophysiology-based hERG assay using IonWorks™ HT

INTRODUCTION

Regulatory and competitive pressure to reduce the QT interval prolongation risk of potential new drugs has led to focus on methods to test for inhibition of the human ether-a-go-go-related gene (hERG)-encoded K+ channel, the primary molecular target underlying this safety issue. Here we describe the validation of a method that combines medium-throughput with direct assessment of channel function.

METHODS

The electrophysiological and pharmacological properties of hERG were compared using two methods: conventional, low-throughput electrophysiology and planar-array-based, medium-throughput electrophysiology (IonWorks HT). A pharmacological comparison was also made between IonWorks HT and an indirect assay (Rb+ efflux).

RESULTS

Basic electrophysiological properties of hERG were similar whether recorded conventionally (HEK cells) or using IonWorks HT (CHO cells): for example, tail current V1/2 -12.1+/-5.0 mV (32) for conventional and -9.5+/-6.0 mV (46) for IonWorks HT (mean+/-S.D. (n)). A key finding was that as the number of cells per well was increased in IonWorks HT, the potency reported for a given compound decreased. Using the lowest possible cell concentration (250,000 cells/ml) and 89 compounds spanning a broad potency range, the pIC50 values from IonWorks HT (CHO-hERG) were found to correlate well with those obtained using conventional methodology (HEK-hERG)(r=0.90; p<0.001). Further validation using CHO-hERG cells with both methods confirmed the correlation (r=0.94; p<0.001). In contrast, a comparison of IonWorks HT and Rb+ efflux data with 649 compounds using CHO-hERG cells showed that the indirect assay consistently reported compounds as being, on average, 6-fold less potent, though the differences varied depending on chemical series.

DISCUSSION

The main finding of this work is that providing a relatively low cell concentration is used in IonWorks HT, the potency information generated correlates well with that determined using conventional electrophysiology. The effect on potency of increasing cell concentration may relate to a reduced free concentration of test compound owing to partitioning into cell membranes. In summary, the IonWorks HT hERG assay can generate pIC50 values based on a direct assessment of channel function in a timeframe short enough to influence chemical design.

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.

Population Patch Clamp Improves Data Consistency and Success Rates in the Measurement of Ionic Currents

Present whole-cell patch-clamp methodology has only moderate consistency and throughput, rendering impractical functional measurements on large numbers of ion channel ligands or on large numbers of unknown or mutant channel genes. In the population patch clamp (PPC) described herein, a single voltage-clamp amplifier sums the whole-cell currents from multiple cells at once, each sealed to a separate aperture in a planar substrate well. The resulting ensemble currents are more consistent from well to well, and the success rate for each recording attempt is >95%. The PPC was implemented by modifying the PatchPlate substrate and amplifiers in the IonWorks patch-clamp instrument. The increased data consistency and likelihood of a successful recording in each well, combined with 384-well measurements in parallel, allow the direct electrophysiological recording of thousands of ensemble ionic currents per day. Therapeutic groups in drug discovery programs require this order of throughput to screen directed compound libraries against ion channel targets. The potential for studying the function of large numbers of ion channel mutants may be realized with the technique. The procedure incorporates subtraction methods that correct for expected distortions and also reliably produces data that agree with previous patch-clamp studies.

QPatch: the past, present and future of automated patch clamp

The QPatch 16 significantly increases throughput for gigaseal patch clamp experiments, making direct measurements in ion channel drug discovery and safety testing feasible. Released to the market in the Autumn of 2004 by Sophion Bioscience, the QPatch originated from work done at NeuroSearch (Denmark) in the early days of automated patch clamp. Today, the QPatch provides many unique features. For example, only the QPatch includes an automated cell preparation station making several hours of unattended operation possible. The 16-channel electrode array, called the QPlate, includes glass-coated microfluidic channels for less compound absorption and, hence, more accurate IC(50) values. The microfluidic pathways also allow for very small amounts of compound used for each experiment ( approximately 5 microl per addition). Only the QPatch has four independent pipetting heads for more efficient liquid handling (especially for ligand-gated ion channel experiments). Patch clamp recordings with the QPatch match the high quality of conventional patch clamp and in some cases the results are even better. For example, only the QPatch includes 100% series resistance compensation for the elimination of false positives due to voltage errors. Finally, the modular QPatch 16 was designed with more channels in mind. The upgrade pathway to 48-channels (the QPatch HT) will be discussed.