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Free Content Transactivation Assays that Identify Indirect and Direct Activators of Human Pregnane X Receptor (PXR, NR1I2) and Constitutive Androstane Receptor (CAR, NR1I3)

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Background: Nuclear Receptors (NRs), including PXR and CAR, are presumed to be ligand-dependent transcription factors, but ligand binding is not an absolute requirement for activation. Indeed, many compounds activate PXR and CAR by indirect mechanisms. Detecting these indirect activators of specific nuclear receptors in vitro has been difficult. As NR activation of either or both PXR and CAR can lead to drug-drug interactions and adverse drug effects, false negatives obtained with screening tools incapable of detecting indirect activators could present liabilities.

Objective: The aim of this study was to establish assays that identify indirect activators of human PXR and CAR.

Methods: Commercially available human PXR and CAR transactivation assays were used for analyses.

Results: We show that transactivation assays containing full-length nuclear receptors with native promoters can identify indirect activators of human CAR and PXRwhen compared to those of commercially available assays containing only the LBD of PXR and CAR. Of these two assay systems, only human PXR and CAR1 assays with full-length receptors and native promoters are capable of detecting indirect and ligand activators. With this capability, several kinase inhibitors were identified that activate PXR and CAR by indirect mechanisms. Furthermore by using both the LBD and full-length receptors, phenobarbital and midostaurin were found to be direct and indirect activators of PXR while human CAR activation by phenobarbital occurs by indirect mechanisms only.

Conclusion: Cell based transactivation assays employing the full-length receptors and native promoters identify both direct and indirect activators of either or both human PXR and CAR.
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Keywords: CAR; DPX2 cells; Ligand binding assays; PXR; drug-drug interactions; full length nuclear receptor; indirect activators; nuclear receptors; phenobarbital; phosphorylation

Document Type: Research Article

Publication date: June 1, 2017

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  • Drug Metabolism Letters publishes short papers on major advances in all areas of drug metabolism and disposition. The emphasis will be on publishing quality papers very rapidly. Letters will be processed rapidly by taking full advantage of the Internet technology for both the submission and review of manuscripts. The journal covers the following areas:

    In vitro systems including CYP-450; enzyme induction and inhibition; drug-drug interactions and enzyme kinetics; pharmacokinetics, toxicokinetics, species scaling and extrapolations; P-glycoprotein and transport carriers; target organ toxicity and interindividual variability; drug metabolism and disposition studies; extrahepatic metabolism; phase I and phase II metabolism; recent developments for the identification of drug metabolites, reactive intermediate and glutathione conjugates.
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