Using automated patch clamp electrophysiology platforms in ion channel drug discovery: an industry perspective

INTRODUCTION

Automated patch clamp (APC) is now well established as a mature technology for ion channel drug discovery in academia, biotech and pharma companies, and in contract research organizations (CRO), for a variety of applications including channelopathy research, compound screening, target validation and cardiac safety testing.

AREAS COVERED

Ion channels are an important class of drugged and approved drug targets. The authors present a review of the current state of ion channel drug discovery along with new and exciting developments in ion channel research involving APC. This includes topics such as native and iPSC-derived cells in ion channel drug discovery, channelopathy research, organellar and biologics in ion channel drug discovery.

EXPERT OPINION

It is our belief that APC will continue to play a critical role in ion channel drug discovery, not only in 'classical' hit screening, target validation and cardiac safety testing, but extending these applications to include high throughput organellar recordings and optogenetics. In this way, with advancements in APC capabilities and applications, together with high resolution cryo-EM structures, ion channel drug discovery will be re-invigorated, leading to a growing list of ion channel ligands in clinical development.

Expanded clinical phenotype spectrum correlates with variant function in SCN2A-related disorders

SCN2A-related disorders secondary to altered function in the voltage-gated sodium channel NaV1.2 are rare with clinically heterogeneous expressions that include epilepsy, autism, and multiple severe to profound impairments and other conditions. To advance understanding of the clinical phenotypes and their relation to channel function, 81 patients (36, 44% female, median age 5.4 years) with 69 unique SCN2A variants were systematically phenotyped and their NaV1.2 channel function systematically assessed. Participants were recruited through the FamileSCN2A Foundation. Primary phenotype (epilepsy of neonatal-onset, N=27; infant onset, N=18; and later onset N=24; and autism without seizures, (N=12) was strongly correlated with a non-seizure severity index (p=0.002), which was based on presence of severe impairments in gross motor, fine motor, communication abilities, gastrostomy tube dependence, and diagnosis of cortical visual impairment and scoliosis. Non-seizure severity was greatest in the neonatal-onset group and least in the autism group (p=0.002). Children with the lowest severity indices were still severely impaired, as reflected by an average Vineland adaptive behavior composite score of 49.5 (>3 SD below the test's norm-referenced mean). Epileptic spasms were significantly more common in infant onset (67%) than in neonatal (22%) or later-onset (29%) epilepsy (p=0.007). Primary phenotype also strongly correlated with variant function (p<0.0001); gain of function and mixed function variants predominated in neonatal-onset epilepsy, shifting to moderate loss of function in infant-onset epilepsy, and severe and complete loss of function in later-onset epilepsy and autism groups. Exploratory cluster analysis identified five groups representing (1) primarily later-onset epilepsy with moderate loss of function variants and low severity indices, (2) mostly infant-onset epilepsy with moderate loss of function variants but higher severity indices, (3) late-onset and autism only with the lowest severity indices (mostly 0) and severe/complete loss of function variants. Two exclusively neonatal clusters were distinguished from each other largely on non-seizure severity scores and secondarily on variant function. The relation between primary phenotype and variant function emphasizes the role of developmental factors in the differential clinical expression of SCN2A variants based on their effects on NaV1.2 channel function. The non-seizure severity of SCN2A disorders depends on a combination of the age at seizure onset (primary phenotype) and variant function. As precision therapies for SCN2A-related disorders advance toward clinical trials, knowledge of the relationship between variant function and clinical disease expression will be valuable for identifying appropriate patients for these trials and in selecting efficient clinical outcomes.

Structure-function relationship of new peptides activating human Nav1.1

Nav1.1 is an important pharmacological target as this voltage-gated sodium channel is involved in neurological and cardiac syndromes. Channel activators are actively sought to try to compensate for haploinsufficiency in several of these pathologies. Herein we used a natural source of new peptide compounds active on ion channels and screened for drugs capable to inhibit channel inactivation as a way to compensate for decreased channel function. We discovered that JzTx-34 is highly active on Nav1.1 and subsequently performed a full structure-activity relationship investigation to identify its pharmacophore. These experiments will help interpret the mechanism of action of this and formerly identified peptides as well as the future identification of new peptides. We also reveal structural determinants that make natural ICK peptides active against Nav1.1 challenging to synthesize. Altogether, the knowledge gained by this study will help facilitate the discovery and development of new compounds active on this critical ion channel target.

An efficient and scalable data analysis solution for automated electrophysiology platforms

Ion channels are drug targets for neurologic, cardiac, and immunologic diseases. Many disease-associated mutations and drugs modulate voltage-gated ion channel activation and inactivation, suggesting that characterizing state-dependent effects of test compounds at an early stage of drug development can be of great benefit. Historically, the effects of compounds on ion channel biophysical properties and voltage-dependent activation/inactivation could only be assessed by using low-throughput, manual patch clamp recording techniques. In recent years, automated patch clamp (APC) platforms have drastically increased in throughput. In contrast to their broad utilization in compound screening, APC platforms have rarely been used for mechanism of action studies, in large part due to the lack of sophisticated, scalable analysis methods for processing the large amount of data generated by APC platforms. In the current study, we developed a highly efficient and scalable software workflow to overcome this challenge. This method, to our knowledge the first of its kind, enables automated curve fitting and complex analysis of compound effects. Using voltage-gated sodium channels as an example, we were able to immediately assess the effects of test compounds on a spectrum of biophysical properties, including peak current, voltage-dependent steady state activation/inactivation, and time constants of activation and fast inactivation. Overall, this automated data analysis method provides a novel solution for in-depth analysis of large-scale APC data, and thus will significantly impact ion channel research and drug discovery.

Identification of positive modulators of TRPM5 channel from a high-throughput screen using a fluorescent membrane potential assay

Transient Receptor Potential Melastatin 5 (TRPM5) is an intracellular calcium-activated cation-selective ion channel expressed in a variety of cell types. Dysfunction of this channel has recently been implied in a range of disease states including diabetes, enteric infections, inflammatory responses, parasitic infection and other pathologies. However, to date, agonists and positive modulators of this channel with sufficient selectivity to enable target validation studies have not been described, limiting the evaluation of TRPM5 biology and its potential as a drug target. We developed a high-throughput assay using a fluorescent membrane potential dye and a medium- and high-throughput electrophysiology assay using QPatch HTX and SyncroPatch 384PE. By employing these assays, we conducted a primary screening campaign and identified hit compounds as TRPM5 channel positive modulators. An initial selectivity profile confirmed hit selectivity to TRPM5 and is presented here. These small molecule TRPM5 compounds have a high potential both as early tool compounds to enable pharmacological studies of TRPM5 and as starting points for the development of potent, selective TRPM5 openers or positive modulators as novel drugs targeting several pathological states.

Reliable identification of cardiac conduction abnormalities in drug discovery using automated patch clamp II: Best practices for Nav1.5 peak current in a high throughput screening environment

INTRODUCTION

For reliable identification of cardiac safety risk, compounds should be screened for activity on cardiac ion channels in addition to hERG, including Na_V1.5 and Ca_V1.2. We identified different parameters that might affect IC₅₀s of compounds on Na_V1.5 peak and late currents recorded using automated patch clamp (APC) and suggest outlines for best practices.

METHODS

APC instruments SyncroPatch 384 and Patchliner were used to record Na_V1.5 peak and late current. Up to 24 CiPA compounds were used to investigate effects of voltage protocol, holding potential (-80 mV or - 95 mV) and temperature (23 ± 1 °C or 36 ± 1 °C) on IC₅₀ values on hNa_V1.5 overexpressed in HEK or CHO cells either as frozen cells or running cultures.

RESULTS

The IC₅₀s of 18 compounds on the Na_V1.5 peak current recorded on the SyncroPatch 384 using the CiPA step-ramp protocol correlated well with the literature. The use of frozen or cultured cells did not affect IC₅₀s but voltage protocol and holding potential did cause differences in IC₅₀ values. Temperature can affect V_half of inactivation and also compound potency. A compound incubation time of 5-6 min was sufficient for most compounds, however slow acting compounds such as terfenadine required longer to reach maximum effect.

DISCUSSION

We conclude that holding potential, voltage protocol and temperature can affect IC₅₀ values and recommend the use of the CiPA step-ramp protocol at physiological temperature to record Na_V1.5 peak and late currents for cardiac safety. Further recommendations include: a minimum compound incubation time of 5 min, a replicate number of 4 and the use of positive and negative controls for reliable IC₅₀s.

Use of automated patch clamp in cardiac safety assessment: Past, present & future perspectives

There is no doubt that automated patch clamp (APC) technology has revolutionized research in biomedical science. High throughput ion channel screening is now an integral part of the development and safety profiling of the majority of new chemical entities currently developed to address unmet medical needs. The increased throughput it provides has significantly improved the ability to overcome the time-consuming, low throughput bottlenecks resulting from the more conventional manual patch clamp method, considered the 'gold standard', for studying ion channel function and pharmacology. While systems offering the luxury of automation have only been commercially available for two decades, the road leading to this new technology is long and rich in seminal, hands-on, studies dating back as far as the 18th century. So where does this technology currently stand, and what will it look like in the future? In the current article, we review the scientific history leading to the development of APC systems, examine key drivers in the rapid development of this technology (such as failed ion channel programmes and the issue of drug-induced hERG inhibition and QT interval prolongation), highlight key capabilities and finally provide some perspective on the current and future impact of the technology on cardiac safety assessment and biomedical science.

Use of automated patch clamp in cardiac safety assessment: past, present and future perspectives

There is no doubt that automated patch clamp (APC) technology has revolutionized research in biomedical science. High throughput ion channel screening is now an integral part of the development and safety profiling of the majority of new chemical entities currently developed to address unmet medical needs. The increased throughput it provides has significantly improved the ability to overcome the time-consuming, low throughput bottlenecks resulting from the more conventional manual patch clamp method, considered the 'gold standard', for studying ion channel function and pharmacology. While systems offering the luxury of automation have only been commercially available for two decades, the road leading to this new technology is long and rich in seminal, hands-on, studies dating back as far as the 18th century. So where does this technology currently stand, and what will it look like in the future? In the current article, we review the scientific history leading to the development of APC systems, examine key drivers in the rapid development of this technology (such as failed ion channel programmes and the issue of drug-induced hERG inhibition and QT interval prolongation), highlight key capabilities and finally provide some perspective on the current and future impact of the technology on cardiac safety assessment and biomedical science.

Perforated Whole-Cell Recordings in Automated Patch Clamp Electrophysiology

The automated patch clamp (APC) technology is used for increasing the data throughput of electrophysiological measurements, especially in safety pharmacology and drug discovery. Typically, electrical access to the cells are obtained using standard whole-cell formation by rupturing the membrane, thereby causing a rapid washout of cytosolic components. In contrast the perforated whole-cell configuration provides electrical access to the cell interior while limiting intracellular wash-out. This method allows for recordings of ion channels that are gated by intracellular modulators (e.g., ATP, cyclic nucleotides, or Ca²⁺), prevents channel current "run down," and maintains a physiological membrane potential for action potential recordings. Here we present some practical approaches to the use of perforated patch clamp for APC recordings. Our findings from these high-throughput, data-rich measurements (e.g., defining optimized concentrations and practical recommendations for four different perforating agents) can be more broadly applied to perforated patch clamp experiments in general (automated and manual), improving success rates, experimental conditions, and applications.

Optogenetics and Optical Tools in Automated Patch Clamping

Automated patch clamping (APC) has been used for almost two decades to increase the throughput of electrophysiological measurements, especially in preclinical safety screening of drug compounds. Typically, cells are suctioned onto holes in planar surfaces and a stronger subsequent suction allows access to a whole cell configuration for electrical measurement of ion channel activity. The development of optogenetic tools over a wide range of wavelengths (UV to IR) provides powerful tools for improving spatiotemporal control of in vivo and in vitro experiments and is emerging as a powerful means of investigating cell networks (neuronal), single cell transduction, and subcellular pathways.Combining APC and optogenetic tools paves the way for improved investigation and control of cell kinetics and provides the opportunity for collecting robust data for new and exciting applications and therapeutic areas. Here, we present an APC optogenetics capability on the Qube Opto 384 system including experiments on light activated ion channels and photoactivated ligands.

Counteracting desensitization of human α7-nicotinic acetylcholine receptors with bispyridinium compounds as an approach against organophosphorus poisoning

Irreversible inhibition of acetylcholinesterase (AChE) resulting in accumulation of acetylcholine and overstimulation of muscarinic and nicotinic receptors accounts for the acute toxicity of organophosphorus compounds (OP). Accordingly, the mainstay pharmacotherapy against poisoning by OP comprises the competitive muscarinic acetylcholine receptor antagonist atropine to treat muscarinic effects and, in addition, oximes to reactivate inhibited AChE. A therapeutic gap still remains in the treatment of desensitized nicotinic acetylcholine receptors following OP exposure. Hereby, nicotinic effects result in paralysis of the central and peripheral respiratory system if untreated. Thus, these receptors pose an essential target for therapeutic indication to address these life-threatening nicotinic symptoms of the cholinergic crisis. Identification of ligands regulating dynamic transitions between functional states by binding to modulatory sites appears to be a promising strategy for therapeutic intervention. In this patch clamp study, the ability of differently substituted bispyridinium non-oximes to "resensitize" i.e. to recover the activity of desensitized human homomeric α7-type nAChRs stably transfected in CHO cells was investigated and compared to the already described α7-specific positive allosteric modulator PNU-120596. The structures of these bispyridinium analogues were based on the lead structure of the tert-butyl-substituted bispyridinium propane MB327, which has been shown to have a positive therapeutic effect due to a non-competitive antagonistic action at muscle-type nAChRs in vivo and has been found to have a positive allosteric activity at neuronal receptors in vitro. Prior to test compounds, desensitization of hα7-nAChRs was verified by applying an excess of nicotine revealing activation at low, and desensitization at high concentrations. Thereby, desensitization could be reduced by modulation with PNU-120596. Desensitization was further verified by dose-response profiles of agonists, carbamoylcholine and epibatidine in the absence and presence of PNU-120596. Although less pronounced than PNU-120596 and the lead structure MB327, bispyridinium compounds, particularly those substituted at position 3 and 4, resensitized the nicotine desensitized hα7-nAChRs in a concentration-dependent manner and prolonged the mean channel open time. In summary, identification of more potent compounds able to restore nAChR function in OP intoxication is needed for development of a putative efficient antidote.

Effect of antiarrhythmic drugs on small conductance calcium - activated potassium channels

Atrial fibrillation (AF) is the most common type of arrhythmia. Current pharmacological treatment for AF is moderately effective and/or increases the risk of serious ventricular adverse effects. To avoid ventricular adverse effects, a new target has been considered, the small conductance calcium-activated K⁺ channels (K_Ca2.X, SK channels). In the heart, K_Ca2.X channels are functionally more important in atria compared to ventricles, and pharmacological inhibition of the channel confers atrial selective prolongation of the cardiac action potential and converts AF to sinus rhythm in animal models of AF. Whether antiarrhythmic drugs (AADs) recommended for treating AF target K_Ca2.X channels is unknown. To this end, we tested a large number of AADs on the human K_Ca2.2 and K_Ca2.3 channels to assess their effect on this new target using automated whole-cell patch clamp. Of the AADs recommended for treatment of AF only dofetilide and propafenone inhibited hK_Ca2.X channels, with no subtype selectivity. The calculated IC₅₀ were 90±10µmol/l vs 60±10µmol/l for dofetilide and 42±4µmol/l vs 80±20µmol/l for propafenone (hK_Ca2.3 vs hK_Ca2.2). Whether this inhibition has clinical importance for their antiarrhythmic effect is unlikely, as the calculated IC₅₀ values are very high compared to the effective free therapeutic plasma concentration of the drugs when used for AF treatment, 40,000-fold for dofetilide and 140-fold higher for propafenone.

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.

Novel screening techniques for ion channel targeting drugs

Ion channels are integral membrane proteins that regulate the flux of ions across the cell membrane. They are involved in nearly all physiological processes, and malfunction of ion channels has been linked to many diseases. Until recently, high-throughput screening of ion channels was limited to indirect, e.g. fluorescence-based, readout technologies. In the past years, direct label-free biophysical readout technologies by means of electrophysiology have been developed. Planar patch-clamp electrophysiology provides a direct functional label-free readout of ion channel function in medium to high throughput. Further electrophysiology features, including temperature control and higher-throughput instruments, are continually being developed. Electrophysiological screening in a 384-well format has recently become possible. Advances in chip and microfluidic design, as well as in cell preparation and handling, have allowed challenging cell types to be studied by automated patch clamp. Assays measuring action potentials in stem cell-derived cardiomyocytes, relevant for cardiac safety screening, and neuronal cells, as well as a large number of different ion channels, including fast ligand-gated ion channels, have successfully been established by automated patch clamp. Impedance and multi-electrode array measurements are particularly suitable for studying cardiomyocytes and neuronal cells within their physiological network, and to address more complex physiological questions. This article discusses recent advances in electrophysiological technologies available for screening ion channel function and regulation.

The application of the Escherichia coli giant spheroplast for drug screening with automated planar patch clamp system

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Human Kv2.1 was expressed in the inner membrane of E. coli using prokaryotic codon.

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Bacterial spheroplasts large enough for the automated patch clamp were prepared by microfluidic chips.

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Kv2.1 current was recorded from the giant spheroplast by the automated patch clamp.

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E. coli spheroplasts were used for dose–response assay of potassium channel inhibitors.

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Our system will become the simple and sensitive drug assay method anyone can use.

Kv2.1, the voltage-gated ion channel, is ubiquitously expressed in variety of tissues and dysfunction of this ion channel is responsible for multiple diseases. Electrophysiological properties of ion channels are so far characterized with eukaryotic cells using the manual patch clamp which requires skilful operators and expensive equipments. In this research, we created a simple and sensitive drug screen method using bacterial giant spheroplasts and the automated patch clamp which does not require special skills. We expressed a eukaryotic voltage-gated ion channel Kv2.1 in Escherichia coli using prokaryotic codon, and prepared giant spheroplasts large enough for the patch clamp. Human Kv2.1 currents were successfully recorded from giant spheroplasts with the automated system, and Kv2.1-expressed E. coli spheroplasts could steadily reacted to the dose–response assay with TEA and 4-AP. Collectively, our results indicate for the first time that the bacterial giant spheroplast can be applied for practical pharmaceutical assay using the automated patch clamp.

In vitro chronic effects on hERG channel caused by the marine biotoxin azaspiracid-2

Azaspiracids (AZAs) are marine biotoxins produced by the dinoflagellate Azadinium spinosum that accumulate in many shellfish species. Azaspiracid poisoning caused by AZA-contaminated seafood consumption is primarily manifested by diarrhea in humans. To protect human health, AZA-1, AZA-2 and AZA-3 content in seafood has been regulated by food safety authorities in many countries. Recently AZAs have been reported as a low/moderate hERG channel blockers. Furthermore AZA-2 has been related to arrhythmia appearance in rats, suggesting potential heart toxicity. In this study AZA-2 in vitro effects on hERG channel after chronic exposure are analyzed to further explore potential cardiotoxicity. The amount of hERG channel in the plasma membrane, hERG channel trafficking and hERG currents were evaluated up to 12 h of toxin exposure. In these conditions AZA-2 caused an increase of hERG levels in the plasma membrane, probably related to hERG retrograde trafficking impairment. Although this alteration did not translate into an increase of hERG channel-related current, more studies will be necessary to understand its mechanism and to know what consequences could have in vivo. These findings suggest that azaspiracids might have chronic cardiotoxicity related to hERG channel trafficking and they should not be overlooked when evaluating the threat to human health.

Minimized cell usage for stem cell-derived and primary cells on an automated patch clamp system

INTRODUCTION

Chip-based automated patch clamp systems are widely used in drug development and safety pharmacology, allowing for high quality, high throughput screening at standardized experimental conditions. The merits of automation generally come at the cost of large amounts of cells needed, since cells are not targeted individually, but randomly positioned onto the chip aperture from cells in suspension. While cell usage is of little concern when using standard cell lines such as CHO or HEK cells, it becomes a crucial constraint with cells of limited availability, such as primary or otherwise rare and expensive cells, like induced pluripotent stem (IPS) cell-derived cardiomyocytes or neurons.

METHODS

We established application protocols for CHO cells, IPS cell-derived neurons (iCell® Neurons, Cellular Dynamics International), cardiomyocytes (Cor.4U®, Axiogenesis) and pancreatic islet cells, minimizing cell usage for automated patch clamp recordings on Nanion's Patchliner. Use of 5 μl cell suspension per well for densities between 55,000 cells/ml and 400,000 cells/ml depending on cell type resulted in good cell capture.

RESULTS

We present a new cell application procedure optimized for the Patchliner achieving>80% success rates for using as little as 300 to 2000 cells per well depending on cell type. We demonstrate that this protocol works for standard cell lines, as well as for stem cell-derived neurons and cardiomyocytes, and for primary pancreatic islet cells. We present recordings for these cell types, demonstrating that high data quality is not compromised by altered cell application.

DISCUSSION

Our new cell application procedure achieves high success rates with unprecedentedly low cell numbers. Compared to other standard automated patch clamp systems we reduced the average amount of cells needed by more than 150 times. Reduced cell usage crucially improves cost efficiency for expensive cells and opens up automated patch clamp for primary cells of limited availability.