Development of the predictor HERG fluorescence polarization assay using a membrane protein enrichment approach.

The life-threatening consequences of acquired, or drug-induced, long QT syndrome due to block of the human ether-a-go-go-related gene (hERG) channel are well appreciated and have been the cause of several drugs being removed from the market in recent years because of patient death. In the last decade, the propensity for block of the hERG channel by a diverse and expanding set of compounds has led to the requirement that all new drugs be tested for hERG channel block in a functional patch-clamp assay. Because of the need to identify potential hERG blockers early in the discovery process, radiometric hERG binding assays are preferred over patch-clamp assays for compound triage, because of relative advantages in speed and cost. Even so, these radiometric binding assays are laborious and require dedicated instrumentation and infrastructure to cope with the regulatory and safety issues associated with the use of radiation. To overcome these limitations, we developed a homogeneous, fluorescence polarization-based assay to identify and characterize the affinity of small molecules for the hERG channel and have demonstrated tight correlation with data obtained from either radioligand binding or patch-clamp assays. Key to the development of this assay was a cell line that expressed highly elevated levels of hERG protein, which was generated by coupling expression of the hERG channel to that of a selectable cell surface marker. A high-expressing clone was isolated by flow cytometry and used to generate membrane preparations that contained >50-fold the typical density of hERG channels measured by [(3)H]astemizole binding. This strategy enabled the Predictor (Invitrogen, Carlsbad, CA) hERG fluorescence polarization assay and should be useful in the development of other fluorescence polarization-based assays that use membrane proteins.

High-throughput screening assays for the assessment of CYP2C9*1, CYP2C9*2, and CYP2C9*3 metabolism using fluorogenic Vivid substrates.

CYP2C9 is a genetically polymorphic human cytochrome P450 isozyme involved in the oxidative metabolism of many drugs, including nonsteroidal anti-inflammatory compounds. Individuals genotyped heterozygous or homozygous for CYP2C9 allelic variants have demonstrated altered metabolism of some drugs primarily metabolized by CYP2C9. The ability to expand screening of CYP2C9 allelic variants to a larger set of drugs and pharmaceutical agents would contribute to a better understanding of the significance of CYP2C9 polymorphisms in the population and to predictions of possible outcomes. The authors report the development of an in vitro fluorescence-based assay employing recombinant CYP2C9 variants (CYP2C9*1, CYP2C9*2, and CYP2C9*3) and fluorogenic Vivid(R) CYP2C9 substrates to explore the effects of CYP2C9 polymorphisms on drug metabolism, using drugs primarily metabolized by CYP2C9. Several chemically diverse fluorogenic substrates (Vivid(R) CYP2C9 blue, green, and red substrates) were used as prototypic probes to obtain in vitro CYP2C9 metabolic rates and kinetic parameters, such as apparent K(m), V(max), and V(max)/K(m) ratios for each allelic variant. In addition, a diverse panel of drugs was screened as assay modifiers with CYP2C9*1, CYP2C9*2, CYP2C9*3, and the fluorogenic Vivid(R) CYP2C9 substrates. The inhibitory potential of this large group of chemically diverse drugs and compounds has been assessed on the basis of their ability to compete with Vivid(R) CYP2C9 substrates in fluorescent reporter assays, thus providing a sensitive and quick assessment of polymorphism-dependent changes in CYP2C9 metabolism.

Generation of site-specific retargeting platform cell lines for drug discovery using phiC31 and R4 integrases.

One of the challenges in developing cell lines for high-throughput screening in drug discovery is the labor- and time-intensive process required to create stable clonal cell lines that express specific reporters or drug targets. The authors report here the generation of a site-specific retargeting platform in 3 different cell lines: adherent HEK293, suspension CHO-S, and a human embryonic cell line (BGO1V). These platform cell lines were generated by using a combination of 2 site-specific integrases to develop a system that allows one to efficiently target a gene of interest to a specific locus and generates rapid production of homogeneous cell pools that stably express the gene of interest. The phiC31 integrase was used to create a platform line by placing a target site for the R4 integrase into a pseudo attP site, and then the R4 integrase was used to place a gene of interest into specific R4 target site. The authors demonstrate the successful and rapid retargeting of a G-protein-coupled receptor (cholecystokinin receptor A, CCKAR), an ion channel (the transient receptor potential cation channel, subfamily M, member 8, TRPM8), and a GFP-c-Jun(1-79) fusion protein into the specific loci in these cell lines and show that these retargeted cell lines exhibit functional and pharmacological responses consistent with those reported in the literature.