Abstract
The pregnane X receptor (PXR) is an important transcriptional regulator of the expression of xenobiotic metabolism and transporter genes. The receptor is promiscuous, binding many structural classes of molecules that act as agonists at the ligand-binding domain, triggering up-regulation of genes, increasing the metabolism and excretion of therapeutic agents, and causing drug-drug interactions. It has been suggested that human PXR antagonists represent a means to counteract such interactions. Several azoles have been hypothesized to bind the activation function-2 (AF-2) surface on the exterior of PXR when agonists are concurrently bound in the ligand-binding domain. In the present study, we have derived novel computational models for PXR agonists using different series of imidazoles, steroids, and a set of diverse molecules with experimental PXR agonist binding data. We have additionally defined a novel pharmacophore for the steroidal agonist site. All agonist pharmacophores showed that hydrophobic features are predominant. In contrast, a qualitative comparison with the corresponding PXR antagonist pharmacophore models using azoles and biphenyls showed that they are smaller and hydrophobic with increased emphasis on hydrogen bonding features. Azole antagonists were docked into a proposed hydrophobic binding pocket on the outer surface at the AF-2 site and fitted comfortably, making interactions with key amino acids involved in charge clamping. Combining computational and experimental data for different classes of molecules provided strong evidence for agonists and antagonists binding distinct regions on PXR. These observations bear significant implications for future discovery of molecules that are more selective and potent antagonists.
Footnotes
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M.D.K. is supported by grant K08-GM074238 from the National Institutes of Health and a Competitive Medical Research Fund grant from the University of Pittsburgh Medical Center. M.I. received support from a University of Pittsburgh Pathology Postdoctoral Research Training Program. S.M. was supported by the Damon Runyan Cancer Research Foundation (C1:15-02) N.A, V. K., and W.J.W. were supported by the United States Environmental Protection Agency-funded Environmental Bioinformatics and Computational Toxicology Center (ebCTC), under STAR Grant number GAD R 832721-010.
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S.E., C.C., and S.M. contributed equally to this work.
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Current affiliation: Pharmacokinetics, Dynamics, and Metabolism, Pfizer, Groton, CT.
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Current affiliation: Sun Pharmaceutical Industries Inc., Bryan, Ohio.
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Article, publication date, and citation information can be found at http://molpharm.aspetjournals.org.
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doi:10.1124/mol.107.038398.
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ABBREVIATIONS: PXR, pregnane X receptor; LBD, ligand-binding domain; AF-2, activation function-2; LXR, liver X receptor; T-0901317, N-(2,2,2-trifluoroethyl)-N-{4-[2,2,2-trifluoro-1-hydroxy-1-(trifluoromethyl)ethyl]phenyl}benzenesulfonamide; ET-743, trabectedin; SMRT, silencing mediator for retinoid and thyroid receptors; QSAR, quantitative structure-activity relationship; PLS, partial least-squares; L-742694, 2-((3,5-bis(trifluoromethyl)benzyl)oxy)-3-phenyl-4-((3-oxo-1,2,4-triazol-5-yl)methyl)morpholine; CoMFA, comparative molecular field analysis; DMSO, dimethyl sulfoxide; MTT, 3-(4,5-dimethylthiazol-2-yl)-2,5-diphenyltetrazolium bromide; PRESS, predictive error sum of squares; CB, chlorobiphenyl; SR12813, [2-(3,5-di-tert-butyl-4-hydroxy-phenyl)-1-(diethoxy-phosphoryl)-vinyl]-phosphonic acid diethlyl ester.
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The online version of this article (available at http://molpharm.aspetjournals.org) contains supplemental material. - Received May 22, 2007.
- Accepted June 18, 2007.
- The American Society for Pharmacology and Experimental Therapeutics
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