Editorial [Hot Topic: Cell-Based Screening (Guest Editor: Guido J.R. Zaman) ]

Although in vitro biochemical assays, such as enzyme activity and receptor binding, have been used extensively in the past to discover new drugs, there is a rapid increase in the use of assays based on living cells. Cell-based assays provide a target in a more physiologically relevant environment than biochemical assays. According to a worldwide survey involving more than 50 pharmaceutical and biotech companies, more than 50% of all primary screens are currently cell-based [1]. Due to the use of cryopreserved and division-arrested cells, cell-based screening also has become more efficient and more flexible [2]. This special issue of Combinatorial Chemistry & High Throughput Screening is devoted to cell-based screening. Review articles were collected on screening in live cells of all major drug target classes, i.e. receptors, ion channels and protein kinases. New technology developments are discussed, such as high content analysis, label-free assays and the use primary and embryonic stem cells. In the first article, Jorg Hüser and colleagues from Bayer HealthCare AG (Wuppertal, Germany) review the technological approaches used for ultra-High Throughput Screening (ultra-HTS). ‘Ultra’ means automated screening of more than 100,000 data points per day. To differentiate target hits from non-specifically acting compounds, integral reference signals are incorporated into the ultra-HTS assays. George Hanson and Bonnie Hanson from Invitrogen Discovery Sciences (Madison, WI) give an overview of fluorescent probes and types of fluorescent assays that are applied in cellular assays for a number of pharmaceutically relevant target classes, including protein kinases and ion channels. They also discuss cellular pathway analysis and the combining of multiple read-outs in one assay (multiplexing). With recent technological advances in fluorescent probes, the search for novel therapeutics targeting ion channels is accelerating. John Dunlop and colleagues of Wyeth Research (Princeton, NJ) review the various techniques used in screening ion channel targets. Real-life examples of the screening of a ligand-gated ion channel and a voltage-gated channel are presented to demonstrate the utility of fluorescence-based screening. ‘High content analysis’ refers to techniques involving the multiplexed analysis of fluorescent markers to measure multiple cellular responses to biological stimuli or drug treatment at the single-cell level. High content analysis is usually based on automated microscopy and provides multiparametric information on single cells within a population. Fabio Gasparri and colleagues from Nerviano Medical Sciences S.r.l. (Nerviano, Italy) review the concepts and techniques of high content analysis of protein kinases, with an emphasis on kinases implicated in oncology. The assay methods are illustrated with data from the author's laboratory of research on cell cycle kinase inhibitors. Morten Præstegaard and colleagues of Thermo Fisher Scientific Inc. (Soborg, Denmark) describe different strategies of multiplexing green fluorescent protein-based and immunofluorescence translocation assays. The authors differentiate between multiplexing of readouts in the same signal transduction pathway (vertical multiplexing) and of readouts across different signal transduction pathways (horizontal multiplexing). Examples are shown of multiplexing assays in the p38 MAPK pathway and of the activation and internalization of G protein-coupled receptors. Jim Inglese and colleagues of the Chemical Genomics Center of the National Institutes of Health (Bethesda, MD) present a case study of the screening of the glucocorticoid receptor with a translocation assay based on enzyme fragment complementation. A ‘quantitative’ screening approach was used, which means that compounds were screened at multiple concentrations in the primary run. Quantitative HTS increases the information content of HTS. Richard Eglen of PerkinElmer (Waltham, MA), Annette Gilchrist of Caden Biosciences (Madison, WI) and Terry Reisine review the disparities that have been found in the pharmacology of compounds acting on G protein-coupled receptors in recombinant cells used for screening and in natural tissues. The efficiency of identifying effective therapeutics may increase when natural tissues are used more in the drug discovery process. In the following article, Eglen et al. review the use of primary and embryonic stem cells for screening. Human stem cells offer unique opportunities in that they can be directed to specific phenotypes, providing a framework to identify tissue-selective agents. In the last article, Lisa Minor from Johnson & Johnson Pharmaceutical Research Institute (Spring House, PA) reviews the application of electrical impedance and refractive index to measure cell-based functional response. These label-free technologies are rapidly generating interest because they allow measuring phenotypic responses without the addition of exogenous labels or extraction of the cells.