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

"Appraisal of state-of-the-art" The 2021 Distinguished Service Award of the Safety Pharmacology Society: Reflecting on the past to tackle challenges ahead

This appraisal of state-of-the-art manuscript highlights and expands upon the thoughts conveyed in the lecture of Dr. Jean-Pierre Valentin, recipient of the 2021 Distinguished Service Award of the Safety Pharmacology Society, given on the 2nd December 2021. The article reflects on the strengths, weaknesses, opportunities, and threats that surrounded the evolution of safety and secondary pharmacology over the last 3 decades with a particular emphasis on pharmaceutical drug development delivery, scientific and technological innovation, complexities of regulatory framework and people leadership and development. The article further built on learnings from past experiences to tackle constantly emerging issues and evolving landscape whilst being cognizant of the challenges facing these disciplines in the broader drug development and societal context.

From squid giant axon to automated patch-clamp: electrophysiology in venom and antivenom research

Ion channels play a crucial role in diverse physiological processes, including neurotransmission and muscle contraction. Venomous creatures exploit the vital function of ion channels by producing toxins in their venoms that specifically target these ion channels to facilitate prey capture upon a bite or a sting. Envenoming can therefore lead to ion channel dysregulation, which for humans can result in severe medical complications that often necessitate interventions such as antivenom administration. Conversely, the discovery of highly potent and selective venom toxins with the capability of distinguishing between different isoforms and subtypes of ion channels has led to the development of beneficial therapeutics that are now in the clinic. This review encompasses the historical evolution of electrophysiology methodologies, highlighting their contributions to venom and antivenom research, including venom-based drug discovery and evaluation of antivenom efficacy. By discussing the applications and advancements in patch-clamp techniques, this review underscores the profound impact of electrophysiology in unravelling the intricate interplay between ion channels and venom toxins, ultimately leading to the development of drugs for envenoming and ion channel-related pathologies.

A graphite furnace-atomic absorption spectrometry-based rubidium efflux assay for screening activators of the Kv 7.2/3 channel

For the characterization of K_v 7.2/3 channel activators, several analytical methods are available that vary in effort and cost. In addition to the technically elaborate patch-clamp method, which serves as a reference method, there exist several medium to high-throughput screening methods including a rubidium efflux flame-atomic absorption spectrometry (F-AAS) assay and a commercial thallium uptake fluorescence-based assay. In this study, the general suitability of a graphite furnace atomic absorption spectrometry (GF-AAS)-based rubidium efflux assay as a screening method for K_v 7.2/3 channel activators was demonstrated. With flupirtine serving as a reference compound, 16 newly synthesizedcompounds and the known K_v 7.2/3 activator retigabine were first classified as either active or inactive by using the GF-AAS-based rubidium (Rb) efflux assay. Then, the results were compared with a thallium (Tl) uptake fluorescence-based fluorometric imaging plate reader (FLIPR) potassium assay. Overall, 16 of 17 compounds were classified by the GF-AAS-based assay in agreement with their channel-activating properties determined by the more expensive Tl uptake, fluorescence-based assay. Thus, the performance of the GF-AAS-based Rb assay for primary drug screening of K_v 7.2/3-activating compounds was clearly demonstrated, as documented by the calculated Z'-factor of the GF-AAS-based method. Moreover, method development included optimization of the coating of the microtiter plates and the washing procedure, which extended the range of this assay to poorly adherent cells such as the HEK293 cells used in this study.

Intracellular Binding of Terfenadine Competes with Its Access to Pancreatic ß-cell ATP-Sensitive K+ Channels and Human ether-à-go-go-Related Gene Channels

Most blockers of both hERG (human ether-à-go-go-related gene) channels and pancreatic ß-cell ATP-sensitive K⁺ (K_ATP) channels access their binding sites from the cytoplasmic side of the plasma membrane. It is unknown whether binding to intracellular components competes with binding of these substances to K⁺ channels. The whole-cell configuration of the patch-clamp technique, a laser-scanning confocal microscope, and fluorescence correlation spectroscopy (FCS) were used to study hERG channels expressed in HEK (human embryonic kidney) 293 cells and K_ATP channels from the clonal insulinoma cell line RINm5F. When applied via the pipette solution in the whole-cell configuration, terfenadine blocked both hERG and K_ATP currents with much lower potency than after application via the bath solution, which was not due to P-glycoprotein-mediated efflux of terfenadine. Such a difference was not observed with dofetilide and tolbutamide. 37-68% of hERG/EGFP (enhanced green-fluorescent protein) fusion proteins expressed in HEK 293 cells were slowly diffusible as determined by laser-scanning microscopy in the whole-cell configuration and by FCS in intact cells. Bath application of a green-fluorescent sulphonylurea derivative (Bodipy-glibenclamide) induced a diffuse fluorescence in the cytosol of RINm5F cells under whole-cell patch-clamp conditions. These observations demonstrate the presence of intracellular binding sites for hERG and K_ATP channel blockers not dialyzable by the patch-pipette solution. Intracellular binding of terfenadine was not influenced by a mutated hERG (Y652A) channel. In conclusion, substances with high lipophilicity are not freely diffusible inside the cell but steep concentration gradients might exist within the cell and in the sub-membrane space.

Physicochemical QSAR analysis of hERG inhibition revisited: towards a quantitative potency prediction

In an earlier study (Didziapetris R & Lanevskij K (2016). J Comput Aided Mol Des. 30:1175-1188) we collected a database of publicly available hERG inhibition data for almost 6700 drug-like molecules and built a probabilistic Gradient Boosting classifier with a minimal set of physicochemical descriptors (log P, pK_a, molecular size and topology parameters). This approach favored interpretability over statistical performance but still achieved an overall classification accuracy of 75%. In the current follow-up work we expanded the database (provided in Supplementary Information) to almost 9400 molecules and performed temporal validation of the model on a set of novel chemicals from recently published lead optimization projects. Validation results showed almost no performance degradation compared to the original study. Additionally, we rebuilt the model using AFT (Accelerated Failure Time) learning objective in XGBoost, which accepts both quantitative and censored data often reported in protein inhibition studies. The new model achieved a similar level of accuracy of discerning hERG blockers from non-blockers at 10 µM threshold, which can be conceived as close to the performance ceiling for methods aiming to describe only non-specific ligand interactions with hERG. Yet, this model outputs quantitative potency values (IC₅₀) and is not tied to a particular classification cut-off. pIC₅₀ from patch-clamp measurements can be predicted with R² ≈ 0.4 and MAE < 0.5, which enables ligand ranking according to their expected potency levels. The employed approach can be valuable for quantitative modeling of various ADME and drug safety endpoints with a high prevalence of censored data.

Use of Solvent Mapping for Characterizing the Binding Site and for Predicting the Inhibition of the Human Ether-á-Go-Go-Related K+ Channel

Molecular dynamics was used to optimize the droperidol-hERG complex obtained from docking. To accommodate the inhibitor, residues T623, S624, V625, G648, Y652, and F656 did not move significantly during the simulation, while F627 moved significantly. Binding sites in cryo-EM structures and in structures obtained from molecular dynamics simulations were characterized using solvent mapping and Atlas ligands, which were negative images of the binding site, were generated. Atlas ligands were found to be useful for identifying human ether-á-go-go-related potassium channel (hERG) inhibitors by aligning compounds to them or by guiding the docking of compounds in the binding site. A molecular dynamics optimized structure of hERG led to improved predictions using either compound alignment to the Atlas ligand or docking. The structure was also found to be suitable to define a strategy for lowering inhibition based on the proposed binding mode of compounds in the channel.

Advances in our understanding of nematode ion channels as potential anthelmintic targets

Ion channels are specialized multimeric proteins that underlie cell excitability. These channels integrate with a variety of neuromuscular and biological functions. In nematodes, the physiological behaviors including locomotion, navigation, feeding and reproduction, are regulated by these protein entities. Majority of the antinematodal chemotherapeutics target the ion channels to disrupt essential biological functions. Here, we have summarized current advances in our understanding of nematode ion channel pharmacology. We review cys-loop ligand gated ion channels (LGICs), including nicotinic acetylcholine receptors (nAChRs), acetylcholine-chloride gated ion channels (ACCs), glutamate-gated chloride channels (GluCls), and GABA (γ-aminobutyric acid) receptors, and other ionotropic receptors (transient receptor potential (TRP) channels and potassium ion channels). We have provided an update on the pharmacological properties of these channels from various nematodes. This article catalogs the differences in ion channel composition and resulting pharmacology in the phylum Nematoda. This diversity in ion channel subunit repertoire and pharmacology emphasizes the importance of pursuing species-specific drug target research. In this review, we have provided an overview of recent advances in techniques and functional assays available for screening ion channel properties and their application.

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.

Machine Learning Strategies When Transitioning between Biological Assays

Machine learning is widely used in drug development to predict activity in biological assays based on chemical structure. However, the process of transitioning from one experimental setup to another for the same biological endpoint has not been extensively studied. In a retrospective study, we here explore different modeling strategies of how to combine data from the old and new assays when training conformal prediction models using data from hERG and Na_V assays. We suggest to continuously monitor the validity and efficiency of models as more data is accumulated from the new assay and select a modeling strategy based on these metrics. In order to maximize the utility of data from the old assay, we propose a strategy that augments the proper training set of an inductive conformal predictor by adding data from the old assay but only having data from the new assay in the calibration set, which results in valid (well-calibrated) models with improved efficiency compared to other strategies. We study the results for varying sizes of new and old assays, allowing for discussion of different practical scenarios. We also conclude that our proposed assay transition strategy is more beneficial, and the value of data from the new assay is higher, for the harder case of regression compared to classification problems.

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.

Identification and Profiling of a Novel Diazaspiro[3.4]octane Chemical Series Active against Multiple Stages of the Human Malaria Parasite Plasmodium falciparum and Optimization Efforts

A novel diazaspiro[3.4]octane series was identified from a Plasmodium falciparum whole-cell high-throughput screening campaign. Hits displayed activity against multiple stages of the parasite lifecycle, which together with a novel sp³-rich scaffold provided an attractive starting point for a hit-to-lead medicinal chemistry optimization and biological profiling program. Structure-activity-relationship studies led to the identification of compounds that showed low nanomolar asexual blood-stage activity (<50 nM) together with strong gametocyte sterilizing properties that translated to transmission-blocking activity in the standard membrane feeding assay. Mechanistic studies through resistance selection with one of the analogues followed by whole-genome sequencing implicated the P. falciparum cyclic amine resistance locus in the mode of resistance.

Cross-site and cross-platform variability of automated patch clamp assessments of drug effects on human cardiac currents in recombinant cells

Automated patch clamp (APC) instruments enable efficient evaluation of electrophysiologic effects of drugs on human cardiac currents in heterologous expression systems. Differences in experimental protocols, instruments, and dissimilar site procedures affect the variability of IC50 values characterizing drug block potency. This impacts the utility of APC platforms for assessing a drug's cardiac safety margin. We determined variability of APC data from multiple sites that measured blocking potency of 12 blinded drugs (with different levels of proarrhythmic risk) against four human cardiac currents (hERG [IKr], hCav1.2 [L-Type ICa], peak hNav1.5, [Peak INa], late hNav1.5 [Late INa]) with recommended protocols (to minimize variance) using five APC platforms across 17 sites. IC50 variability (25/75 percentiles) differed for drugs and currents (e.g., 10.4-fold for dofetilide block of hERG current and 4-fold for mexiletine block of hNav1.5 current). Within-platform variance predominated for 4 of 12 hERG blocking drugs and 4 of 6 hNav1.5 blocking drugs. hERG and hNav1.5 block. Bland-Altman plots depicted varying agreement across APC platforms. A follow-up survey suggested multiple sources of experimental variability that could be further minimized by stricter adherence to standard protocols. Adoption of best practices would ensure less variable APC datasets and improved safety margins and proarrhythmic risk assessments.

A Computational Pipeline to Predict Cardiotoxicity: From the Atom to the Rhythm

RATIONALE

Drug-induced proarrhythmia is so tightly associated with prolongation of the QT interval that QT prolongation is an accepted surrogate marker for arrhythmia. But QT interval is too sensitive a marker and not selective, resulting in many useful drugs eliminated in drug discovery.

OBJECTIVE

To predict the impact of a drug from the drug chemistry on the cardiac rhythm.

METHODS AND RESULTS

In a new linkage, we connected atomistic scale information to protein, cell, and tissue scales by predicting drug-binding affinities and rates from simulation of ion channel and drug structure interactions and then used these values to model drug effects on the hERG channel. Model components were integrated into predictive models at the cell and tissue scales to expose fundamental arrhythmia vulnerability mechanisms and complex interactions underlying emergent behaviors. Human clinical data were used for model framework validation and showed excellent agreement, demonstrating feasibility of a new approach for cardiotoxicity prediction.

CONCLUSIONS

We present a multiscale model framework to predict electrotoxicity in the heart from the atom to the rhythm. Novel mechanistic insights emerged at all scales of the system, from the specific nature of proarrhythmic drug interaction with the hERG channel, to the fundamental cellular and tissue-level arrhythmia mechanisms. Applications of machine learning indicate necessary and sufficient parameters that predict arrhythmia vulnerability. We expect that the model framework may be expanded to make an impact in drug discovery, drug safety screening for a variety of compounds and targets, and in a variety of regulatory processes.

On the potential of in vitro organ-chip models to define temporal pharmacokinetic-pharmacodynamic relationships

Functional human-on-a-chip systems hold great promise to enable quantitative translation to in vivo outcomes. Here, we explored this concept using a pumpless heart only and heart:liver system to evaluate the temporal pharmacokinetic/pharmacodynamic (PKPD) relationship for terfenadine. There was a time dependent drug-induced increase in field potential duration in the cardiac compartment in response to terfenadine and that response was modulated using a metabolically competent liver module that converted terfenadine to fexofenadine. Using this data, a mathematical model was developed to predict the effect of terfenadine in preclinical species. Developing confidence that microphysiological models could have a transformative effect on drug discovery, we also tested a previously discovered proprietary AstraZeneca small molecule and correctly determined the cardiotoxic response to its metabolite in the heart:liver system. Overall our findings serve as a guiding principle to future investigations of temporal concentration response relationships in these innovative in vitro models, especially, if validated across multiple time frames, with additional pharmacological mechanisms and molecules representing a broad chemical diversity.

Lowering Lipophilicity by Adding Carbon: One-Carbon Bridges of Morpholines and Piperazines

In this article, we report our investigation of a phenomenon by which bridging morpholines across the ring with one-carbon tethers leads to a counterintuitive reduction in lipophilicity. This effect was also found to occur in piperazines and piperidines and lowered the measured log D_7.4 of the bridged molecules by as much as -0.8 relative to their unbridged counterparts. As lowering lipophilicity without introducing additional heteroatoms can be desirable, we believe this potentially provides a useful tactic to improve the drug-like properties of molecules containing morpholine-, piperazine-, and piperidine-like motifs.

Biaryl sulfonamide motifs up- or down-regulate ion channel activity by activating voltage sensors

Voltage-gated ion channels are key molecules for the generation of cellular electrical excitability. Many pharmaceutical drugs target these channels by blocking their ion-conducting pore, but in many cases, channel-opening compounds would be more beneficial. Here, to search for new channel-opening compounds, we screen 18,000 compounds with high-throughput patch-clamp technology and find several potassium-channel openers that share a distinct biaryl-sulfonamide motif. Our data suggest that the negatively charged variants of these compounds bind to the top of the voltage-sensor domain, between transmembrane segments 3 and 4, to open the channel. Although we show here that biaryl-sulfonamide compounds open a potassium channel, they have also been reported to block sodium and calcium channels. However, because they inactivate voltage-gated sodium channels by promoting activation of one voltage sensor, we suggest that, despite different effects on the channel gates, the biaryl-sulfonamide motif is a general ion-channel activator motif. Because these compounds block action potential-generating sodium and calcium channels and open an action potential-dampening potassium channel, they should have a high propensity to reduce excitability. This opens up the possibility to build new excitability-reducing pharmaceutical drugs from the biaryl-sulfonamide scaffold.

Innovative organotypic in vitro models for safety assessment: aligning with regulatory requirements and understanding models of the heart, skin, and liver as paradigms

The development of improved, innovative models for the detection of toxicity of drugs, chemicals, or chemicals in cosmetics is crucial to efficiently bring new products safely to market in a cost-effective and timely manner. In addition, improvement in models to detect toxicity may reduce the incidence of unexpected post-marketing toxicity and reduce or eliminate the need for animal testing. The safety of novel products of the pharmaceutical, chemical, or cosmetics industry must be assured; therefore, toxicological properties need to be assessed. Accepted methods for gathering the information required by law for approval of substances are often animal methods. To reduce, refine, and replace animal testing, innovative organotypic in vitro models have emerged. Such models appear at different levels of complexity ranging from simpler, self-organized three-dimensional (3D) cell cultures up to more advanced scaffold-based co-cultures consisting of multiple cell types. This review provides an overview of recent developments in the field of toxicity testing with in vitro models for three major organ types: heart, skin, and liver. This review also examines regulatory aspects of such models in Europe and the UK, and summarizes best practices to facilitate the acceptance and appropriate use of advanced in vitro models.

Using automated patch clamp electrophysiology platforms in pain-related ion channel research: insights from industry and academia

Automated patch clamp (APC) technology was first developed at the turn of the millennium. The increased throughput it afforded promised a new paradigm in ion channel recordings, offering the potential to overcome the time-consuming, low-throughput bottleneck, arising from manual patch clamp investigations. This has relevance to the fast-paced development of novel therapies for chronic pain. This review highlights the advances in technology, using select examples that have facilitated APC usage in both industry and academia. It covers both first generation and the latest developments in second-generation platforms. In addition, it also provides an overview of the pain research field and how APC platforms have furthered our understanding of ion channel research and the development of pharmacological tools and therapeutics. APC platforms have much to offer to the ion channel research community, and this review highlights areas of best practice for both academia and industry. The impact of APC platforms and the prospects of ion channel research and improved therapeutics for chronic pain will be evaluated.

LINKED ARTICLES

This article is part of a themed section on Recent Advances in Targeting Ion Channels to Treat Chronic Pain. To view the other articles in this section visit http://onlinelibrary.wiley.com/doi/10.1111/bph.v175.12/issuetoc.

Novel Bacterial Topoisomerase Inhibitors with Potent Broad-Spectrum Activity against Drug-Resistant Bacteria

The novel bacterial topoisomerase inhibitor class is an investigational type of antibacterial inhibitor of DNA gyrase and topoisomerase IV that does not have cross-resistance with the quinolones. Here, we report the evaluation of the in vitro properties of a new series of this type of small molecule. Exemplar compounds selectively and potently inhibited the catalytic activities of Escherichia coli DNA gyrase and topoisomerase IV but did not block the DNA breakage-reunion step. Compounds showed broad-spectrum inhibitory activity against a wide range of Gram-positive and Gram-negative pathogens, including biodefence microorganisms and Mycobacterium tuberculosis No cross-resistance with fluoroquinolone-resistant Staphylococcus aureus and E. coli isolates was observed. Measured MIC₉₀ values were 4 and 8 μg/ml against a panel of contemporary multidrug-resistant isolates of Acinetobacter baumannii and E. coli, respectively. In addition, representative compounds exhibited greater antibacterial potency than the quinolones against obligate anaerobic species. Spontaneous mutation rates were low, with frequencies of resistance typically <10^-8 against E. coli and A. baumannii at concentrations equivalent to 4-fold the MIC. Compound-resistant E. coli mutants that were isolated following serial passage were characterized by whole-genome sequencing and carried a single Arg38Leu amino acid substitution in the GyrA subunit of DNA gyrase. Preliminary in vitro safety data indicate that the series shows a promising therapeutic index and potential for low human ether-a-go-go-related gene (hERG) inhibition (50% inhibitory concentration [IC₅₀], >100 μM). In summary, the compounds' distinct mechanism of action relative to the fluoroquinolones, whole-cell potency, low potential for resistance development, and favorable in vitro safety profile warrant their continued investigation as potential broad-spectrum antibacterial agents.

Identification of a Potential Antimalarial Drug Candidate from a Series of 2-Aminopyrazines by Optimization of Aqueous Solubility and Potency across the Parasite Life Cycle

Introduction of water-solubilizing groups on the 5-phenyl ring of a 2-aminopyrazine series led to the identification of highly potent compounds against the blood life-cycle stage of the human malaria parasite Plasmodium falciparum. Several compounds displayed high in vivo efficacy in two different mouse models for malaria, P. berghei-infected mice and P. falciparum-infected NOD-scid IL-2Rγ^null mice. One of the frontrunners, compound 3, was identified to also have good pharmacokinetics and additionally very potent activity against the liver and gametocyte parasite life-cycle stages.

Screening out irrelevant cell-based models of disease

The common and persistent failures to translate promising preclinical drug candidates into clinical success highlight the limited effectiveness of disease models currently used in drug discovery. An apparent reluctance to explore and adopt alternative cell- and tissue-based model systems, coupled with a detachment from clinical practice during assay validation, contributes to ineffective translational research. To help address these issues and stimulate debate, here we propose a set of principles to facilitate the definition and development of disease-relevant assays, and we discuss new opportunities for exploiting the latest advances in cell-based assay technologies in drug discovery, including induced pluripotent stem cells, three-dimensional (3D) co-culture and organ-on-a-chip systems, complemented by advances in single-cell imaging and gene editing technologies. Funding to support precompetitive, multidisciplinary collaborations to develop novel preclinical models and cell-based screening technologies could have a key role in improving their clinical relevance, and ultimately increase clinical success rates.

Discovery of Novel 3-Quinoline Carboxamides as Potent, Selective, and Orally Bioavailable Inhibitors of Ataxia Telangiectasia Mutated (ATM) Kinase

A novel series of 3-quinoline carboxamides has been discovered and optimized as selective inhibitors of the ataxia telangiectasia mutated (ATM) kinase. From a modestly potent HTS hit (4), we identified molecules such as 6-[6-(methoxymethyl)-3-pyridinyl]-4-{[(1R)-1-(tetrahydro-2H-pyran-4-yl)ethyl]amino}-3-quinolinecarboxamide (72) and 7-fluoro-6-[6-(methoxymethyl)pyridin-3-yl]-4-{[(1S)-1-(1-methyl-1H-pyrazol-3-yl)ethyl]amino}quinoline-3-carboxamide (74) as potent and highly selective ATM inhibitors with overall ADME properties suitable for oral administration. 72 and 74 constitute excellent oral tools to probe ATM inhibition in vivo. Efficacy in combination with the DSB-inducing agent irinotecan was observed in a disease relevant model.

Design, synthesis, and biological evaluation of a novel series of peripheral-selective noradrenaline reuptake inhibitors-Part 2

Peripherally selective inhibition of noradrenaline reuptake is a novel mechanism for the treatment of stress urinary incontinence to overcome adverse effects associated with central action. Herein, we describe our medicinal chemistry approach to discover peripheral-selective noradrenaline reuptake inhibitors to avert the risk of P-gp-mediated DDI at the blood-brain barrier. We observed that steric shielding of the hydrogen-bond acceptors and donors (HBA and HBD) of compound 1 reduced the multidrug resistance protein 1 (MDR1) efflux ratio; however, the resulting compound 6, a methoxyacetamide derivative, was mainly metabolized by CYP2D6 and CYP2C19 in the in vitro phenotyping study, implying the risk of PK variability based on the genetic polymorphism of the CYPs. Replacement of the hydrogen atom with a deuterium atom in a strategic, metabolically hot spot led to compound 13, which was mainly metabolized by CYP3A4. To our knowledge, this study represents the first report of the effect of deuterium replacement for a major metabolic enzyme. The compound 13, N-{[(6S,7R)-7-(4-chloro-3-fluorophenyl)-1,4-oxazepan-6-yl]methyl}-2-[(2H(3))methyloxy]acetamide hydrochloride, which exhibited peripheral NET selective inhibition at tested doses in rats, increased urethral resistance in a dose-dependent manner.

Discovery of a Potent, Selective, Orally Bioavailable, and Efficacious Novel 2-(Pyrazol-4-ylamino)-pyrimidine Inhibitor of the Insulin-like Growth Factor-1 Receptor (IGF-1R)

Optimization of cellular lipophilic ligand efficiency (LLE) in a series of 2-anilino-pyrimidine IGF-1R kinase inhibitors led to the identification of novel 2-(pyrazol-4-ylamino)-pyrimidines with improved physicochemical properties. Replacement of the imidazo[1,2-a]pyridine group of the previously reported inhibitor 3 with the related pyrazolo[1,5-a]pyridine improved IGF-1R cellular potency. Substitution of the amino-pyrazole group was key to obtaining excellent kinase selectivity and pharmacokinetic parameters suitable for oral dosing, which led to the discovery of (2R)-1-[4-(4-{[5-chloro-4-(pyrazolo[1,5-a]pyridin-3-yl)-2-pyrimidinyl]amino}-3,5-dimethyl-1H-pyrazol-1-yl)-1-piperidinyl]-2-hydroxy-1-propanone (AZD9362, 28), a novel, efficacious inhibitor of IGF-1R.

Novel diversity-oriented synthesis-derived respiratory syncytial virus inhibitors identified via a high throughput replicon-based screen

Respiratory syncytial virus (RSV) infections affect millions of children and adults every year. Despite the significant disease burden, there are currently no safe and effective vaccines or therapeutics. We employed a replicon-based high throughput screen combined with live-virus triaging assays to identify three novel diversity-oriented synthesis-derived scaffolds with activity against RSV. One of these small molecules is shown to target the RSV polymerase (L protein) to inhibit viral replication and transcription; the mechanisms of action of the other small molecules are currently unknown. The compounds described herein may provide attractive inhibitors for lead optimization campaigns.

Tetrahydroisoquinoline Phenols: Selective Estrogen Receptor Downregulator Antagonists with Oral Bioavailability in Rat

A series of tetrahydroisoquinoline phenols was modified to give an estrogen receptor downregulator-antagonist profile. Optimization around the core, alkyl side chain, and pendant aryl ring resulted in compounds with subnanomolar levels of potency. The phenol functionality was shown to be required to achieve highly potent compounds, but unusually this was compatible with obtaining high oral bioavailabilities in rat.

Discovery of AZD8165 – a clinical candidate from a novel series of neutral thrombin inhibitors

A novel series of neutral thrombin inhibitors has been developed using a selection process based on docking experiments and property calculations and predictions.

Investigation of miscellaneous hERG inhibition in large diverse compound collection using automated patch-clamp assay

AIM

hERG potassium channels display miscellaneous interactions with diverse chemical scaffolds. In this study we assessed the hERG inhibition in a large compound library of diverse chemical entities and provided data for better understanding of the mechanisms underlying promiscuity of hERG inhibition.

METHODS

Approximately 300 000 compounds contained in Molecular Library Small Molecular Repository (MLSMR) library were tested. Compound profiling was conducted on hERG-CHO cells using the automated patch-clamp platform-IonWorks Quattro(™).

RESULTS

The compound library was tested at 1 and 10 μmol/L. IC50 values were predicted using a modified 4-parameter logistic model. Inhibitor hits were binned into three groups based on their potency: high (IC50<1 μmol/L), intermediate (1 μmol/L< IC50<10 μmol/L), and low (IC50>10 μmol/L) with hit rates of 1.64%, 9.17% and 16.63%, respectively. Six physiochemical properties of each compound were acquired and calculated using ACD software to evaluate the correlation between hERG inhibition and the properties: hERG inhibition was positively correlative to the physiochemical properties ALogP, molecular weight and RTB, and negatively correlative to TPSA.

CONCLUSION

Based on a large diverse compound collection, this study provides experimental evidence to understand the promiscuity of hERG inhibition. This study further demonstrates that hERG liability compounds tend to be more hydrophobic, high-molecular, flexible and polarizable.

High throughput screening technologies for ion channels

Ion channels are involved in a variety of fundamental physiological processes, and their malfunction causes numerous human diseases. Therefore, ion channels represent a class of attractive drug targets and a class of important off-targets for in vitro pharmacological profiling. In the past decades, the rapid progress in developing functional assays and instrumentation has enabled high throughput screening (HTS) campaigns on an expanding list of channel types. Chronologically, HTS methods for ion channels include the ligand binding assay, flux-based assay, fluorescence-based assay, and automated electrophysiological assay. In this review we summarize the current HTS technologies for different ion channel classes and their applications.

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.

A Novel Pyrazolopyridine with in Vivo Activity in Plasmodium berghei- and Plasmodium falciparum-Infected Mouse Models from Structure-Activity Relationship Studies around the Core of Recently Identified Antimalarial Imidazopyridazines

Toward improving pharmacokinetics, in vivo efficacy, and selectivity over hERG, structure-activity relationship studies around the central core of antimalarial imidazopyridazines were conducted. This study led to the identification of potent pyrazolopyridines, which showed good in vivo efficacy and pharmacokinetics profiles. The lead compounds also proved to be very potent in the parasite liver and gametocyte stages, which makes them of high interest.

A New Perspective in the Field of Cardiac Safety Testing through the Comprehensive In Vitro Proarrhythmia Assay Paradigm

For the past decade, cardiac safety screening to evaluate the propensity of drugs to produce QT interval prolongation and Torsades de Pointes (TdP) arrhythmia has been conducted according to ICH S7B and ICH E14 guidelines. Central to the existing approach are hERG channel assays and in vivo QT measurements. Although effective, the present paradigm carries a risk of unnecessary compound attrition and high cost, especially when considering costly thorough QT (TQT) studies conducted later in drug development. The C: omprehensive I: n Vitro P: roarrhythmia A: ssay (CiPA) initiative is a public-private collaboration with the aim of updating the existing cardiac safety testing paradigm to better evaluate arrhythmia risk and remove the need for TQT studies. It is hoped that CiPA will produce a standardized ion channel assay approach, incorporating defined tests against major cardiac ion channels, the results of which then inform evaluation of proarrhythmic actions in silico, using human ventricular action potential reconstructions. Results are then to be confirmed using human (stem cell-derived) cardiomyocytes. This perspective article reviews the rationale, progress of, and challenges for the CiPA initiative, if this new paradigm is to replace existing practice and, in time, lead to improved and widely accepted cardiac safety testing guidelines.

Pharmacological and electrophysiological characterization of AZSMO-23, an activator of the hERG K(+) channel

BACKGROUND AND PURPOSE

We aimed to characterize the pharmacology and electrophysiology of N-[3-(1H-benzimidazol-2-yl)-4-chloro-phenyl]pyridine-3-carboxamide (AZSMO-23), an activator of the human ether-a-go-go-related gene (hERG)-encoded K(+) channel (Kv 11.1).

EXPERIMENTAL APPROACH

Automated electrophysiology was used to study the pharmacology of AZSMO-23 on wild-type (WT), Y652A, F656T or G628C/S631C hERG, and on other cardiac ion channels. Its mechanism of action was characterized with conventional electrophysiology.

KEY RESULTS

AZSMO-23 activated WT hERG pre-pulse and tail current with EC50 values of 28.6 and 11.2 μM respectively. At 100 μM, pre-pulse current at +40 mV was increased by 952 ± 41% and tail current at -30 mV by 238 ± 13% compared with vehicle values. The primary mechanism for this effect was a 74.5 mV depolarizing shift in the voltage dependence of inactivation, without any shift in the voltage dependence of activation. Structure-activity relationships for this effect were remarkably subtle, with close analogues of AZSMO-23 acting as hERG inhibitors. AZSMO-23 blocked the mutant channel, hERG Y652A, but against another mutant channel, hERG F656T, its activator activity was enhanced. It inhibited activity of the G628C/S631C non-inactivating hERG mutant channel. AZSMO-23 was not hERG selective, as it blocked hKv 4.3-hKChIP2.2, hCav 3.2 and hKv 1.5 and activated hCav 1.2/β2/α2δ channels.

CONCLUSION AND IMPLICATIONS

The activity of AZSMO-23 and those of its close analogues suggest these compounds may be of value to elucidate the mechanism of type 2 hERG activators to better understand the pharmacology of this area from both a safety perspective and in relation to treatment of congenital long QT syndrome.

In silico prediction of hERG inhibition

The voltage-gated potassium channel encoded by hERG carries a delayed rectifying potassium current (IKr) underlying repolarization of the cardiac action potential. Pharmacological blockade of the hERG channel results in slowed repolarization and therefore prolongation of action potential duration and an increase in the QT interval as measured on an electrocardiogram. Those are possible to cause sudden death, leading to the withdrawals of many drugs, which is the reason for hERG screening. Computational in silico prediction models provide a rapid, economic way to screen compounds during early drug discovery. In this review, hERG prediction models are classified as 2D and 3D quantitative structure–activity relationship models, pharmacophore models, classification models, and structure based models (using homology models of hERG).

Triaminopyrimidine is a fast-killing and long-acting antimalarial clinical candidate

The widespread emergence of Plasmodium falciparum (Pf) strains resistant to frontline agents has fuelled the search for fast-acting agents with novel mechanism of action. Here, we report the discovery and optimization of novel antimalarial compounds, the triaminopyrimidines (TAPs), which emerged from a phenotypic screen against the blood stages of Pf. The clinical candidate (compound 12) is efficacious in a mouse model of Pf malaria with an ED₉₉ <30 mg kg⁻¹ and displays good in vivo safety margins in guinea pigs and rats. With a predicted half-life of 36 h in humans, a single dose of 260 mg might be sufficient to maintain therapeutic blood concentration for 4–5 days. Whole-genome sequencing of resistant mutants implicates the vacuolar ATP synthase as a genetic determinant of resistance to TAPs. Our studies highlight the potential of TAPs for single-dose treatment of Pf malaria in combination with other agents in clinical development.

The emergence of resistant Plasmodium strains fuels the search for new antimalarials. Here, the authors present a new class of potent antimalarial compounds, the triaminopyrimidines, that display low toxicity and long half-life in animal models.

Global analysis reveals families of chemical motifs enriched for HERG inhibitors

Promiscuous inhibition of the human ether-à-go-go-related gene (hERG) potassium channel by drugs poses a major risk for life threatening arrhythmia and costly drug withdrawals. Current knowledge of this phenomenon is derived from a limited number of known drugs and tool compounds. However, in a diverse, naïve chemical library, it remains unclear which and to what degree chemical motifs or scaffolds might be enriched for hERG inhibition. Here we report electrophysiology measurements of hERG inhibition and computational analyses of >300,000 diverse small molecules. We identify chemical ‘communities’ with high hERG liability, containing both canonical scaffolds and structurally distinctive molecules. These data enable the development of more effective classifiers to computationally assess hERG risk. The resultant predictive models now accurately classify naïve compound libraries for tendency of hERG inhibition. Together these results provide a more complete reference map of characteristic chemical motifs for hERG liability and advance a systematic approach to rank chemical collections for cardiotoxicity risk.