Cell-based versus isolated target screening: how lucky do you feel?

"3D, human renal proximal tubule (RPTEC-TERT1) organoids 'tubuloids' for translatable evaluation of nephrotoxins in high-throughput"

The importance of human cell-based in vitro tools to drug development that are robust, accurate, and predictive cannot be understated. There has been significant effort in recent years to develop such platforms, with increased interest in 3D models that can recapitulate key aspects of biology that 2D models might not be able to deliver. We describe the development of a 3D human cell-based in vitro assay for the investigation of nephrotoxicity, using RPTEC-TERT1 cells. These RPTEC-TERT1 proximal tubule organoids 'tubuloids' demonstrate marked differences in physiologically relevant morphology compared to 2D monolayer cells, increased sensitivity to nephrotoxins observable via secreted protein, and with a higher degree of similarity to native human kidney tissue. Finally, tubuloids incubated with nephrotoxins demonstrate altered Na+/K+-ATPase signal intensity, a potential avenue for a high-throughput, translatable nephrotoxicity assay.

Personalized Medicine Using Cutting Edge Technologies for Genetic Epilepsies

Epilepsy is the most common chronic neurologic disorder in the world, affecting 1-2% of the population. Besides, 30% of epilepsy patients are drug-resistant. Genomic mutations seem to play a key role in its etiology and knowledge of strong effect mutations in protein structures might improve prediction and the development of efficacious drugs to treat epilepsy. Several genetic association studies have been undertaken to examine the effect of a range of candidate genes for resistance. Although, few studies have explored the effect of the mutations into protein structure and biophysics in the epilepsy field. Much work remains to be done, but the plans made for exciting developments will hold therapeutic potential for patients with drug-resistance. In summary, we provide a critical review of the perspectives for the development of individualized medicine for epilepsy based on genetic polymorphisms/mutations in light of core elements such as transcriptomics, structural biology, disease model, pharmacogenomics and pharmacokinetics in a manner to improve the success of trial designs of antiepileptic drugs.

High-Throughput Screening: today's biochemical and cell-based approaches

High-throughput screening (HTS) provides starting chemical matter in the adventure of developing a new drug. In this review, we survey several HTS methods used today for hit identification, organized in two main flavors: biochemical and cell-based assays. Biochemical assays discussed include fluorescence polarization and anisotropy, FRET, TR-FRET, and fluorescence lifetime analysis. Binding-based methods are also surveyed, including NMR, SPR, mass spectrometry, and DSF. On the other hand, cell-based assays discussed include viability, reporter gene, second messenger, and high-throughput microscopy assays. We devote some emphasis to high-content screening, which is becoming very popular. An advisable stage after hit discovery using phenotypic screens is target deconvolution, and we provide an overview of current chemical proteomics, in silico, and chemical genetics tools. Emphasis is made on recent CRISPR/dCas-based screens. Lastly, we illustrate some of the considerations that inform the choice of HTS methods and point to some areas with potential interest for future research.

Design and synthesis of 1,3-benzothiazinone derivatives as potential anti-inflammatory agents

A series of 1,3-benzothiazinone derivatives were designed and synthesized for pharmacological assessments. Among the synthesized 19 compounds, some compounds showed high activities on inhibiting LPS-induced nitrite oxide and TNF-α production, down-regulating COX-2 and increasing IL-10 production in RAW264.7 cells. All the compounds had no obvious cytotoxicity in in vitro assay. LD₅₀ value of compound 25 was greater than 2000 mg/kg, which was safer than meloxicam. Compound 25 significantly inhibited phosphorylation of NF-κB and STAT3 in LPS-induced RAW264.7 cells. Inhibition of synthesized compounds on COX activity was weaker than meloxicam. Compound 25 displayed lower gastrointestinal toxicity than meloxicam. Besides, compound 25 decreased the swelling in carrageenan-induced paw edema models of inflammation and reduced PGE₂ level significantly. In summary, 1,3-benzothiazinone derivatives are unique scaffolds with anti-inflammatory activity and low toxicity.

Identification of New Activators of Mitochondrial Fusion Reveals a Link between Mitochondrial Morphology and Pyrimidine Metabolism

Mitochondria are dynamic organelles that produce most of the cellular ATP, and are involved in many other cellular functions such as Ca²⁺ signaling, differentiation, apoptosis, cell cycle, and cell growth. One key process of mitochondrial dynamics is mitochondrial fusion, which is catalyzed by mitofusins (MFN1 and MFN2) and OPA1. The outer mitochondrial membrane protein MFN2 plays a relevant role in the maintenance of mitochondrial metabolism, insulin signaling, and mutations that cause neurodegenerative disorders. Therefore, modulation of proteins involved in mitochondrial dynamics has emerged as a potential pharmacological strategy. Here, we report the identification of small molecules by high-throughput screen that promote mitochondrial elongation in an MFN1/MFN2-dependent manner. Detailed analysis of their mode of action reveals a previously unknown connection between pyrimidine metabolism and mitochondrial dynamics. Our data indicate a link between pyrimidine biosynthesis and mitochondrial dynamics, which maintains cell survival under stress conditions characterized by loss of pyrimidine synthesis.

Protein-Structure Assisted Optimization of 4,5-Dihydroxypyrimidine-6-Carboxamide Inhibitors of Influenza Virus Endonuclease

Influenza is a serious hazard to human health that causes hundreds of thousands of deaths annually. Though vaccines and current therapeutics can blunt some of the perilous impact of this viral infection, new treatments are needed due to the constantly evolving nature of this virus. Recently, our growing understanding of an essential influenza viral protein, PA, has led to the development of focused libraries of new small molecules that specifically target the active site of the PA influenza endonuclease, which we report here. Our overarching approach has been to proactively develop lead inhibitors that are less likely to rapidly develop clinical resistance by optimizing inhibitors that retain activity against induced resistant mutants. Here, we report details behind the discovery of new potent inhibitors of wild type and resistant mutant endonucleases along with their high-resolution co-crystal structure-activity relationships. These results add to our understanding of nuclease protein targets and potentially serve as starting points for a new therapeutic approach to the treatment of influenza.

Utilizing Zebrafish Visual Behaviors in Drug Screening for Retinal Degeneration

Zebrafish are a popular vertebrate model in drug discovery. They produce a large number of small and rapidly-developing embryos. These embryos display rich visual-behaviors that can be used to screen drugs for treating retinal degeneration (RD). RD comprises blinding diseases such as Retinitis Pigmentosa, which affects 1 in 4000 people. This disease has no definitive cure, emphasizing an urgency to identify new drugs. In this review, we will discuss advantages, challenges, and research developments in using zebrafish behaviors to screen drugs in vivo. We will specifically discuss a visual-motor response that can potentially expedite discovery of new RD drugs.

Adjunct Automation to the Cellmate™ Cell Culture Robot

The design and implementation of an automated tissue culture flask labeling, delivery, scraping, and retrieval system centered around the CellmateTm cell culture robot is presented. Three new custom subsystems named FlaskMaster, FlaskLabeler, and FlaskScraper were created and integrated into the Cellmate robotic system. Flask-Master is a centralized multi-axis robot with a surrounding enclosure that houses racks of stacked tissue culture flasks (size T-175). The FlaskMaster system interfaces externally with the Cellmate's conveyor system to deliver and retrieve flasks. FlaskLabeler is a print and apply system that labels flasks prior to transfer between FlaskMaster and the Cellmate. FlaskScraper is a scraping rod and blade assembly that was specifically designed for automated scraping of T-175 flasks. The combination of these systems allows for large-scale, unattended cell seeding and harvesting of cells from flasks. The addition of these new capabilities significantly reduces the manpower needed to operate the Cellmate. Moreover, these changes greatly improve the performance and capability of the stand-alone Cellmate system.

A Simple Technique for Reducing Edge Effect in Cell-Based Assays

Several factors are known to increase the noise and variability of cell-based assays used for high-throughput screening. In particular, edge effects can result in an unacceptably high plate rejection rate in screening runs. In an effort to minimize these variations, the authors analyzed a number of factors that could contribute to edge effects in cell-based assays. They found that pre-incubation of newly seeded plates in ambient conditions (air at room temperature) resulted in even distribution of the cells in each well. In contrast, when newly seeded plates were placed directly in the CO2 incubator, an uneven distribution of cells occurred in wells around the plate periphery, resulting in increased edge effect. Here, the authors show that the simple, inexpensive approach of incubating newly seeded plates at room temperature before placing them in a 37° C CO2 incubator yields a significant reduction in edge effect. ( Journal of Biomolecular Screening 2003:566-570)

High-throughput platforms for the screening of new therapeutic targets for neurodegenerative diseases

Despite the recent progress in the understanding of neurodegenerative disorders, a lack of solid fundamental knowledge on the etiology of many of the major neurodegenerative diseases has made it difficult to obtain effective therapies to treat these conditions. Scientists have been looking to carry out more-human-relevant studies, with strong statistical power, to overcome the limitations of preclinical animal models that have contributed to the failure of numerous therapeutics in clinical trials. Here, we identify currently existing platforms to mimic central nervous system tissues, healthy and diseased, mainly focusing on cell-based platforms and discussing their strengths and limitations in the context of the high-throughput screening of new therapeutic targets and drugs.

A novel live cell assay to measure diacylglycerol lipase α activity

Diacylglycerol lipase α (DAGLα) hydrolyses DAG to generate the principal endocannabinoid (eCB) 2-arachidonoylglycerol (2-AG) in the central nervous system. DAGLα dependent cannabinoid (CB) signalling has been implicated in numerous processes including axonal growth and guidance, adult neurogenesis and retrograde signalling at the synapse. Recent studies have implicated DAGLα as an emerging drug target for several conditions including pain and obesity. Activity assays are critical to the drug discovery process; however, measurement of diacylglycerol lipase (DAGL) activity using its native substrate generally involves low-throughput MS techniques. Some relatively high-throughput membrane based assays utilizing surrogate substrates have been reported, but these do not take into account the rate-limiting effects often associated with the ability of a drug to cross the cell membrane. In the present study, we report the development of a live cell assay to measure DAGLα activity. Two previously reported DAGLα surrogate substrates, p-nitrophenyl butyrate (PNPB) and 6,8-difluoro-4-methylumbelliferyl octanoate (DiFMUO), were evaluated for their ability to detect DAGLα activity in live cell assays using a human cell line stably expressing the human DAGLα transgene. Following optimization, the small molecule chromogenic substrate PNPB proved to be superior by providing lower background activity along with a larger signal window between transfected and parental cells when compared with the fluorogenic substrate DiFMUO. The assay was further validated using established DAGL inhibitors. In summary, the live cell DAGLα assay reported here offers an economical and convenient format to screen for novel inhibitors as part of drug discovery programmes and compliments previously reported high-throughput membrane based DAGL assays.

Elucidating the signal responses of multi-parametric surface plasmon resonance living cell sensing: a comparison between optical modeling and drug-MDCKII cell interaction measurements

In vitro cell-based assays are widely used during the drug discovery and development process to test the biological activity of new drugs. Most of the commonly used cell-based assays, however, lack the ability to measure in real-time or under dynamic conditions (e.g. constant flow). In this study a multi-parameter surface plasmon resonance approach in combination with living cell sensing has been utilized for monitoring drug-cell interactions in real-time, under constant flow and without labels. The multi-parameter surface plasmon resonance approach, i.e. surface plasmon resonance angle versus intensity plots, provided fully specific signal patterns for various cell behaviors when stimulating cells with drugs that use para- and transcellular absorption routes. Simulated full surface plasmon resonance angular spectra of cell monolayers were compared with actual surface plasmon resonance measurements performed with MDCKII cell monolayers in order to better understand the origin of the surface plasmon resonance signal responses during drug stimulation of cells. The comparison of the simulated and measured surface plasmon resonance responses allowed to better understand and provide plausible explanations for the type of cellular changes, e.g. morphological or mass redistribution in cells, that were induced in the MDCKII cell monolayers during drug stimulation, and consequently to differentiate between the type and modes of drug actions. The multi-parameter surface plasmon resonance approach presented in this study lays the foundation for developing new types of cell-based tools for life science research, which should contribute to an improved mechanistic understanding of the type and contribution of different drug transport routes on drug absorption.

Principles of early drug discovery

Developing a new drug from original idea to the launch of a finished product is a complex process which can take 12-15 years and cost in excess of $1 billion. The idea for a target can come from a variety of sources including academic and clinical research and from the commercial sector. It may take many years to build up a body of supporting evidence before selecting a target for a costly drug discovery programme. Once a target has been chosen, the pharmaceutical industry and more recently some academic centres have streamlined a number of early processes to identify molecules which possess suitable characteristics to make acceptable drugs. This review will look at key preclinical stages of the drug discovery process, from initial target identification and validation, through assay development, high throughput screening, hit identification, lead optimization and finally the selection of a candidate molecule for clinical development.

Meganuclease-driven targeted integration in CHO-K1 cells for the fast generation of HTS-compatible cell-based assays

The development of cell-based assays for high-throughput screening (HTS) approaches often requires the generation of stable transformant cell lines. However, these cell lines are essentially created by random integration of a gene of interest (GOI) with no control over the level and stability of gene expression. The authors developed a targeted integration system in Chinese hamster ovary (CHO) cells, called the cellular genome positioning system (cGPS), based on the stimulation of homologous gene targeting by meganucleases. Five different GOIs were knocked in at the same locus in cGPS CHO-K1 cells. Further characterization revealed that the cGPS CHO-K1 system is more rapid (2-week protocol), efficient (all selected clones expressed the GOI), reproducible (GOI expression level variation of 12%), and stable over time (no change in GOI expression after 23 weeks of culture) than classical random integration. Moreover, in all cGPS CHO-K1 targeted clones, the recombinant protein was biologically active and its properties similar to the endogenous protein. This fast and robust method opens the door for creating large collections of cell lines of better quality and expressing therapeutically relevant GOIs at physiological levels, thereby enhancing the potential scope of HTS.

HTS-Compatible Patient-Derived Cell-Based Assay to Identify Small Molecule Modulators of Aberrant Splicing in Myotonic Dystrophy Type 1

Myotonic dystrophy type 1 (DM1) is a genetic disorder characterized by muscle wasting, myotonia, cataracts, cardiac arrhythmia, hyperinsulinism and intellectual deficits, and is caused by expansion of a CTG repeat in the 3’UTR of the Dystrophia Myotonica-Protein Kinase (DMPK) gene. The DMPK transcripts containing expanded CUG repeats accumulate in nuclear foci and ultimately cause mis-splicing of secondary genes through the dysregulation of RNA-binding proteins including Muscleblind 1 (MBNL1) and CUG binding protein 1 (CUGBP1). Correction of mis-splicing of genes such as the Skeletal muscle-specific chloride channel 1 (CLCN1), Cardiac troponin T (TNNT2), Insulin receptor (INSR) and Sarcoplasmic/endoplasmic reticulum Ca²⁺ATPase 1 (SERCA1) may alleviate some of the symptoms of DM1; hence identification of small molecule modulators is an important step towards a therapy for DM1 patients. Here we describe the generation of immortalized myoblast cell lines derived from healthy (DMPK CTG₅) and DM1 patient (DMPK CTG₁₀₀₀) fibroblasts by constitutive overexpression of human telomerase reverse transcriptase (hTERT) and inducible overexpression of the Myoblast determination factor (MYOD). MBNL1-containing nuclear foci, mis-splicing events and defective myotube differentiation defects characteristic of DM1 were observed in these cells. A CLCN1 luciferase minigene construct (CLCN1-luc) was stably introduced to monitor intron 2 retention in the DM1 cellular context (a reported splicing defect in DM1). The assay was validated by performing a high-throughput screen (HTS) of ~13,000 low molecular weight compounds against the CLCN1-luc DM1 myoblast cell line, providing an ideal system for conducting HTS to better understand and treat DM1.

Metabolic discrimination of select list agents by monitoring cellular responses in a multianalyte microphysiometer

Harnessing the potential of cells as complex biosensors promises the potential to create sensitive and selective detectors for discrimination of biodefense agents. Here we present toxin detection and suggest discrimination using cells in a multianalyte microphysiometer (MMP) that is capable of simultaneously measuring flux changes in four extracellular analytes (acidification rate, glucose uptake, oxygen uptake, and lactate production) in real-time. Differential short-term cellular responses were observed between botulinum neurotoxin A and ricin toxin with neuroblastoma cells, alamethicin and anthrax protective antigen with RAW macrophages, and cholera toxin, muscarine, 2,4-dinitro-phenol, and NaF with CHO cells. These results and the post exposure dynamics and metabolic recovery observed in each case suggest the usefulness of cell-based detectors to discriminate between specific analytes and classes of compounds in a complex matrix, and furthermore to make metabolic inferences on the cellular effects of the agents. This may be particularly valuable for classifying unknown toxins.

Target assessment for antiparasitic drug discovery

Drug discovery is a high-risk, expensive and lengthy process taking at least 12 years and costing upwards of US$500 million per drug to reach the clinic. For neglected diseases, the drug discovery process is driven by medical need and guided by pre-defined target product profiles. Assessment and prioritisation of the most promising targets for entry into screening programmes is crucial for maximising the chances of success. Here, we describe criteria used in our drug discovery unit for target assessment and introduce the 'traffic-light' system as a prioritisation and management tool. We hope this brief review will stimulate basic scientists to acquire additional information necessary for drug discovery.

A high-content glucocorticoid receptor translocation assay for compound mechanism-of-action evaluation

Ligand-induced cytoplasm to nucleus translocation is a critical event in the nuclear receptor (NR) signal transduction cascade. The development of green fluorescent proteins and their color variants fused with NRs, along with the recent developments in automated cellular imaging technologies, has provided unique tools to monitor and quantify the NR translocation events. These technology developments have important implications in the mechanistic evaluation of NR signaling and provide a powerful tool for drug discovery. The unique challenges for developing a robust NR translocation assay include cytotoxicity accompanied with chronic overexpression of NRs, basal translocation induced by serum present in culture medium, and interference from endogenous NRs, as well as subcellular dynamics. The authors have developed a robust assay system for the glucocorticoid receptor (GR) that was applied to a panel of nuclear receptor ligands. Using a high-content imaging system, ligand-induced, dose-dependent GR nuclear translocation was quantified and a correlation with other conventional assays established.

Utility of Large-Scale Transiently Transfected Cells for Cell-Based High-Throughput Screens to Identify Transient Receptor Potential Channel A1 (TRPA1) Antagonists

Despite increasing use of cell-based assays in high-throughput screening (HTS) and lead optimization, one challenge is the adequate supply of high-quality cells expressing the target of interest. To this end, cell lines stably expressing targets are often established, maintained, and scaled up by cell culture. These steps require large investments of time and resources. Moreover, significant variability invariably occurs in cell yield, viability, expression levels, and target activities. In particular, stable expression of targets such as transient receptor potential A1 (TRPA1) causes toxicity, cell line degeneration, and loss of functional activity. Therefore, in an effort to identify TRPA1 antagonists, the authors used large-scale transiently transfected (LSTT) cells, enabling rapid establishment of assays suitable for HTS. LSTT cells, which could- be stored frozen for a long period of time (e.g., at least 42 weeks), retained TRPA1 protein expression and could be easily revived to produce robust and consistent signals in calcium influx and electrophysiological assays. Using cells from a single transfection, a chemical library of 700,000 compounds was screened, and TRPA1 antagonists were identified. The use of LSTT circumvented issues associated with stable TRPA1 expression, increased flexibility and consistency, and greatly reduced labor and cost. This approach will also be applicable to other pharmaceutical targets.

Multiplexing Fluo-4 NW and a GeneBLAzer® Transcriptional Assay for High-Throughput Screening of G-Protein-Coupled Receptors

Activation of G-protein-coupled receptors (GPCRs) leads to a cascade of signaling events, including calcium mobilization and downstream transcriptional activation of various proteins. Two commonly used methods of high-throughput screening for GPCRs include calcium-sensitive dyes, such as Fluo-4 NW, and reporter gene assays, such as beta-lactamase. To determine whether the advantages of each assay format could be combined by multiplexing, Jurkat and CHO-K1 cell lines over-expressing the M1 muscarinic receptor and beta-lactamase under control of an NFAT response element were tested in a multiplexed format. The Jurkat cell line was further screened with a subset of the LOPAC1280 library. The multiplexing assay was compatible with both the CHO-K1 and Jurkat cell lines. For the screen, there was 100% correlation of on-target hits in the multiplexed format, and several false positives with each assay format were identified. Therefore, not only can the assays be multiplexed, but by multiplexing, the false positives associated with each assay format also could be easily identified. In addition to enhanced reliability, this method saves time and money because only half the amount of compounds, cells, and consumables are needed to screen a cell line in a multiplexed mode versus separate screening by both methods.

G-protein-coupled receptors in drug discovery: nanosizing using cell-free technologies and molecular biology approaches

Signal transduction by G-protein-coupled receptors (GPCRs) underpins a multitude of physiological processes. Ligand recognition by the receptor leads to activation of a generic molecular switch involving heterotrimeric G-proteins and guanine nucleotides. Signal transduction has been studied extensively with both cell-based systems and assays comprising isolated signaling components. Interest and commercial investment in GPCRs in areas such as drug targets, orphan receptors, high throughput screening, biosensors, and so on will focus greater attention on assay development to allow for miniaturization, ultra-high throughput and, eventually, microarray/biochip assay formats. Although cell-based assays are adequate for many GPCRs, it is likely that these formats will limit the development of higher density GPCR assay platforms mandatory for other applications. Stable, robust, cell-free signaling assemblies comprising receptor and appropriate molecular switching components will form the basis of future GPCR assay platforms adaptable for such applications as microarrays. The authors review current cell-free GPCR assay technologies and molecular biological approaches for construction of novel, functional GPCR assays.

Analysis of protein phosphorylation and cellular signaling events by flow cytometry: techniques and clinical applications

Analysis of protein phosphorylation with flow cytometric techniques has emerged as a powerful tool in the field of immunological signaling, allowing cellular subsets in complex populations to be analyzed accurately and rapidly. In this review, we examine the development of phospho-epitope, or phospho-specific, flow cytometry and the premises upon which the technique is based. Phospho-specific flow cytometry is compared to traditional biochemical methods, and its advantages, such as single cell analysis, multiparameter data acquisition, rapid protocols, and the ability to analyze rare cell subsets, are detailed. We also discuss the many technical considerations that must be addressed when developing new antibodies or analyzing new epitopes including antigen accessibility, stability of the phospho-epitope, fluorophore selection, surface phenotype integrity, and antibody suitability for staining epitopes inside fixed and permeabilized cells. The methods that have been used to date are described in light of these technical considerations. The importance of developing bioinformatic platforms in parallel with these techniques is emphasized due to the large, multiparameter datasets that are rapidly accumulated and which require more efficient data viewing and complex clustering methods than currently available for flow cytometric data. Finally, we discuss the potential clinical applications of phospho-specific flow cytometry in analyzing immune cell development and antigen-specific immune responses, as well as pharmacodynamic profiling of disease states or drug efficacy and specificity against particular signaling proteins.

High-content assays for ligand regulation of G-protein-coupled receptors

High-content assays rely on the imaging of cellular events. They can be used to monitor the activation of G-protein-coupled receptors (or other receptors), their internalization into the cell, or alterations in their amount. In addition, multiplexed assays can provide further information about the characteristics of the receptor. Recent improvements in throughput using high-content screening platforms means that such assays are now an integral element of functional analysis in the drug discovery process.

Principles: extending the utility of [35S]GTP gamma S binding assays

Ligand regulation of the binding of [35S]GTPgammaS is one of the most widely used methods to measure receptor activation of heterotrimeric G proteins. However, until recently this method was largely restricted to receptors that interact with members of the family of pertussis-toxin-sensitive G proteins. Here, the reasons for this restriction are discussed and recent approaches that have extended the utility of this method such that it is now suitable for analysis of the activation of any heterotrimeric G protein are reviewed.

Novel detection strategies for drug discovery

The Human Genome Project is expected to increase the number of potential drug targets from the current figure of 500 to approximately 3,000-4,000. This increased number of targets, and increasing knowledge of signaling-pathway networks and their complexities, sets new demands for efficiency on HTS assay technologies. Assessment of the total efficacy of a given drug candidate requires not only the classical assays, but also a wide variety of assays related to signaling cascades and cellular functions. Discrete functional assays traditionally involved Ca(2+) flux, kinases and cAMP, but today extend to the whole signaling network, from ligand binding to expression. This review discusses emerging novel non-radioisotopic assays, such as ligand-stimulated GTP-binding, the inositol triphosphate assay, cellular receptor trafficking, and protein-protein interactions.

Cellular platforms for HTS: three case studies

The field of cell-based screening is expanding rapidly as innovations in target selection and instrumentation increase the number of targets that can be efficiently screened in cellular formats. Cell-based screens can be configured to provide a broad range of data on chemical compound activity, mechanism of action and drugability. However, the decision to pursue a cell-based approach should not be made lightly, as cell-based assays can be challenging to implement in the high-throughput screening (HTS) laboratory. In this review, we describe three case studies in which targets were successfully interrogated in cell-based HTS, and highlight the necessary steps to ensure the validity of these screens.

Functional cell-based uHTS in chemical genomic drug discovery

The availability of genomic information significantly increases the number of potential targets available for drug discovery, although the function of many targets and their relationship to disease is unknown. In a chemical genomic research approach, ultra-high throughput screening (uHTS) of genomic targets takes place early in the drug discovery process, before target validation. Target-selective modulators then provide drug leads and pharmacological research tools to validate target function. Effective implementation of a chemical genomic strategy requires assays that can perform uHTS for large numbers of genomic targets. Cell-based functional assays are capable of the uHTS throughput required for chemical genomic research, and their functional nature provides distinct advantages over ligand-binding assays in the identification of target-selective modulators.

Application of beta-galactosidase enzyme complementation technology as a high throughput screening format for antagonists of the epidermal growth factor receptor

We have applied enzyme complementation technology to develop a screen for antagonists of the epidermal growth factor (EGF) receptor. Chimeric proteins containing two weakly complementing deletion mutants of Escherichia coli beta-galactosidase (beta-gal), each fused to the EGF receptor extracellular and transmembrane domains, have been stably expressed in C2C12 cells. In this cell line, formation of active beta-gal is dependent on agonist-stimulated dimerization of the EGF receptor. We have developed a homogenous 384-well assay protocol and have applied this to characterize the pharmacology of the receptor and to develop a high throughput screen (HTS) for EGF receptor antagonists. The assay is tolerant to DMSO concentrations of up to 2% and, across 21 passages in culture, exhibits an EC(50) for EGF of 5.4 +/- 3.6 ng/ml (n = 11) and a Z' of 0.55 +/- 0.13 (n = 11). A random set of 1,280 compounds was screened in duplicate at 11 microM to examine the robustness of enzyme complementation technology and to characterize the false-positive hit rate in the assay. Using a cutoff of 40% inhibition of EGF-promoted beta-gal activity, the hit rate on day 1 was 2.5% and on day 2 was 1.9%. After retesting the active compounds, the hit rate was reduced to 0.4%, of which one of the compounds was identified as a beta-gal inhibitor and the remainder appeared to be nonspecific inhibitors in the assay. This technology is amenable to automated screen workstations, there are highly sensitive chemiluminescent and fluorescent beta-gal assay reagents amenable to detection in miniaturized plate formats, and the assay benefits from a low false-positive hit rate. Enzyme complementation technology may have wide application within the HTS environment for the detection of modulators of receptor activation or inhibitors of protein-protein interactions in mammalian cells.