Evaluation of a High-Throughput Fluorescence Assay Method for hERG Potassium Channel Inhibition

The number of projects in drug development that fail in late phases because of cardiac side effects such as QT prolongation can impede drug discovery and development of projects. The molecular target responsible for QT prolongation by a wide range of pharmaceutical agents is the myocardial hERG potassium channel. It is therefore desirable to screen for compound interactions with the hERG channel at an early stage of drug development. Here, the authors report a cell-based fluorescence assay using membrane potential-sensitive fluorescent dyes and stably transfected hERG channels from CHO cells. The assay allows semiautomated screening of compounds for hERG activity on 384-well plates and is sufficiently rapid for testing a large number of compounds. The assay is robust as indicated by a Z′ factor larger than 0.6. The throughput is in the range of 10,000 data points per day, which is significantly higher than any other method presently available for hERG. The data obtained with the fluorescence assay were in qualitative agreement with those from patch-clamp electrophysiological analysis. There were no false-positive hits, and the rate of false-negative compounds is currently 12% but might be further reduced by testing compounds at higher concentration. Quantitative differences between fluorescence and electrophysiological methods may be due to the use- or voltage-dependentactivity of the antagonists.

SAR and biological evaluation of SEN12333/WAY-317538: Novel alpha 7 nicotinic acetylcholine receptor agonist

Alpha 7 nicotinic acetylcholine receptor (alpha(7) nAChR) agonists are promising therapeutic candidates for the treatment of cognitive impairment associated with a variety of disorders including Alzheimer's disease and schizophrenia. Alpha 7 nAChRs are expressed in brain regions associated with cognitive function, regulate cholinergic neurotransmission and have been shown to be down regulated in both schizophrenia and Alzheimer's disease. Herein we report a novel, potent small molecule agonist of the alpha 7 nAChR, SEN12333/WAY-317538. This compound is a selective agonist of the alpha(7) nAChR with excellent in vitro and in vivo profiles, excellent brain penetration and oral bioavailability, and demonstrates in vivo efficacy in multiple behavioural cognition models. The SAR and biological evaluation of this series of compounds are discussed.

Procognitive and neuroprotective activity of a novel alpha7 nicotinic acetylcholine receptor agonist for treatment of neurodegenerative and cognitive disorders

The alpha7 nicotinic acetylcholine receptor (nAChR) is a promising target for treatment of cognitive dysfunction associated with Alzheimer's disease and schizophrenia. Here, we report the pharmacological properties of 5-morpholin-4-yl-pentanoic acid (4-pyridin-3-yl-phenyl)-amide [SEN12333 (WAY-317538)], a novel selective agonist of alpha7 nAChR. SEN12333 shows high affinity for the rat alpha7 receptor expressed in GH4C1 cells (K(i) = 260 nM) and acts as full agonist in functional Ca(2+) flux studies (EC(50) = 1.6 microM). In whole-cell patch-clamp recordings, SEN12333 activated peak currents and maximal total charges similar to acetylcholine (EC(50) = 12 microM). The compound did not show agonist activity at other nicotinic receptors tested and acted as a weak antagonist at alpha3-containing receptors. SEN12333 treatment (3 mg/kg i.p.) improved episodic memory in a novel object recognition task in rats in conditions of spontaneous forgetting as well as cognitive disruptions induced via glutamatergic [5H-dibenzo[a,d]cyclohepten-5,10-imine (dizocilpine maleate); MK-801] or cholinergic (scopolamine) mechanisms. This improvement was blocked by the alpha7-selective antagonist methyllycaconitine, indicating that it is mediated by alpha7 activation. SEN12333 also prevented a scopolamine-induced deficit in a passive avoidance task. In models targeting other cognitive domains, including attention and perceptual processing, SEN12333 normalized the apomorphine-induced deficit of prepulse inhibition. Neuroprotection of SEN12333 was demonstrated in quisqualate-lesioned animals in which treatment with SEN12333 (3 mg/kg/day i.p.) resulted in a significant protection of choline acetyltransferase-positive neurons in the lesioned hemisphere. Cumulatively, our results demonstrate that the novel alpha7 nAChR agonist SEN12333 has procognitive and neuroprotective properties, further demonstrating utility of alpha7 agonists for treatment of neurodegenerative and cognitive disorders.

Functional properties of alpha7 nicotinic acetylcholine receptors co-expressed with RIC-3 in a stable recombinant CHO-K1 cell line

Heterologous functional expression of alpha7 nicotinic acetylcholine receptors (nAChRs) is difficult to achieve in mammalian cell lines, and the reasons have been associated with a lack of expression of the putative chaperone factor RIC-3. Here, we describe the generation and functional and pharmacological characterization of a Chinese hamster ovary (CHO)-K1 cell line co-expressing the human alpha7 nAChR and RIC-3. Stable recombinant cells expressing alpha7 nAChR on the plasma membrane were selected by binding of fluorochrome-conjugated alpha-bungarotoxin and fluorescence-activated cell sorting. The presence of functional alpha7 channels was demonstrated by whole cell patch clamp recordings. Nicotine and acetylcholine induced rapid desensitizing currents with 50% effective concentration values of 14 and 37 microM, respectively, with agonist-evoked currents detected in approximately 75% of the cell population. Surprisingly, when tested in a FLIPR (Molecular Devices, Sunnyvale, CA) Ca(2+) assay, activation of alpha7 nAChRs was measured only when nicotinic agonists were applied either in the presence of the positive allosteric modulator (PAM) PNU-120596 or after pretreatment of cells with the tyrosine kinase inhibitor genistein. No Ca(2+) influx was measured upon addition of agonists alone or together with allosteric potentiators such as 5-hydroxyindole that predominantly increase the apparent peak amplitude without robustly affecting the current desensitization rate, as exemplified by PNU-120596. These results show that functional alpha7 nAChRs can stably be expressed in the non-neuronal CHO-K1 cell line. This recombinant cell system is useful for characterization of alpha7 nAChRs and to study the mechanism of action of chemical modulators, in particular the detection of PAMs capable of slowing receptor desensitization kinetics.

Parallel synthesis of a series of potentially brain penetrant aminoalkyl benzoimidazoles

Alpha7 agonists were identified via GOLD (CCDC) docking in the putative agonist binding site of an alpha7 homology model and a series of aminoalkyl benzoimidazoles was synthesised to obtain potentially brain penetrant drugs. The array was prepared starting from the reaction of ortho-fluoronitrobenzenes with a selection of diamines, followed by reduction of the nitro group to obtain a series of monoalkylated phenylene diamines. N,N'-Carbonyldiimidazole (CDI) mediated acylation, followed by a parallel automated work-up procedure, afforded the monoacylated phenylenediamines which were cyclised under acidic conditions. Parallel work-up and purification afforded the array products in good yields and purities with a robust parallel methodology which will be useful for other libraries. Screening for alpha7 activity revealed compounds with agonist activity for the receptor.

In vitro screening strategies for nicotinic receptor ligands

A common historical strategy to the discovery of nicotinic receptor ligands has involved the use of radioligand-binding assays for ligand identification in combination with two-electrode voltage clamp in Xenopus oocytes for electrophysiological characterization. More recently, higher-throughput methodologies have replaced these approaches to accommodate screening of large compound libraries and to provide increased capacity for electrophysiological profiling in mammalian cell lines. We, and others, have implemented cell-based screening assays using the fluorometric imaging plate reader (FLIPR) for primary and lead optimization screening of nicotinic receptor agonists and positive allosteric modulators (PAMs). Using GH4C1 cells expressing the rat alpha7 nicotinic receptor, both acetylcholine and nicotine produced concentration-dependent elevations of intracellular calcium with EC(50) values of 5.5 and 1.6 microM, respectively. PAM activity was robustly detected using the FLIPR assay; for example, the known alpha7 receptor PAM 5-hydroxyindole failed to directly activate the receptor but produced a leftward shift of the nicotine concentration-response curve in combination with a potentiation of the maximum evoked response to nicotine. Electrophysiological confirmation of agonist activity was achieved using the Dynaflow rapid perfusion system and patch clamp in the same GH4C1 cell expression system. Estimated EC(50) values for acetylcholine-evoked currents in GH4C1/alpha7 cells were 55 and 576 microM for area-under-the-curve (AUC) and maximum peak height calculations, respectively. Similarly, PAM activity was confirmed using electrophysiological recordings while also allowing for the mechanistic discrimination of compounds, not possible using the FLIPR assay. Specifically, PAMs capable of slowing the rapid desensitization of alpha7 receptors to different extents were discernable in these studies. Further improvements in the capacity to screen compounds using electrophysiology has been achieved by implementation of high-throughput gigaohm quality recording systems such as the QPatch and PatchXpress where agonist EC(50) values are highly comparable to those obtained using conventional manual patch clamp.

The periplasmic Escherichia coli peptidylprolyl cis,trans-isomerase FkpA. I. Increased functional expression of antibody fragments with and without cis-prolines

The production of recombinant proteins in the periplasm of Escherichia coli can be limited by folding problems, leading to periplasmic aggregates. We used a selection system for periplasmic chaperones based on the coexpression of an E. coli library with a poorly expressing antibody single-chain Fv (scFv) fragment displayed on filamentous phage (Bothmann, H., and Plückthun, A. (1998) Nature Biotechnol. 16, 376-380). By selection for a functional antibody, the protein Skp had been enriched previously and shown to improve periplasmic expression of a wide range of scFv fragments. This selection strategy was now repeated with a library constructed from the genomic DNA of an skp-deficient strain, leading to enrichment of the periplasmic peptidylprolyl cis,trans-isomerase (PPIase) FkpA. Coexpression of FkpA increased the amount of fusion protein displayed on the phage and dramatically improved functional periplasmic expression even of scFv fragments not containing cis-prolines. In contrast, the coexpression of the periplasmic PPIases PpiA and SurA showed no increase in the functional scFv fragment level in the periplasm or displayed on phage. Together with the in vitro data in the accompanying paper (Ramm, K., and Plückthun, A. (2000) J. Biol. Chem. 275, 17106-17113), we conclude that the effect of FkpA is independent of its PPIase activity.

Selection for a periplasmic factor improving phage display and functional periplasmic expression

The efficiency of both phage display in Escherichia coli and periplasmic expression of recombinant proteins may be limited by the same periplasmic folding steps. To search for E. coli factors that improve the efficiency of both procedures, a library of E. coli proteins was coexpressed in a phagemid vector that contained a poorly folding single-chain Fv antibody (scFv) fragment fused to g3p. We enriched, by panning for antigen binding, those phagemids in which the amount of displayed scFv is highest. We thus identified the periplasmic protein Skp/OmpH/HlpA as improving phage display of a wide range of scFv fragments. This occurs as a result of an increase in the amount of hybrid protein displayed on the phage. Coexpression of skp also increases the functional yield of scFv fragments when expressed by secretion to the periplasm.