title: Human pregnane X receptor antagonists and agonists define molecular requirements for different binding sites.
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.
authors.affiliation: ACT LLC, 601 Runnymede Avenue, Jenkintown, PA 19046, USA. ekinssean@yahoo.com
authors.forename: Sean
authors.initials: S
authors.lastname: Ekins
authors.forename: Cheng
authors.initials: C
authors.lastname: Chang
authors.forename: Sridhar
authors.initials: S
authors.lastname: Mani
authors.forename: Matthew D
authors.initials: MD
authors.lastname: Krasowski
authors.forename: Erica J
authors.initials: EJ
authors.lastname: Reschly
authors.forename: Manisha
authors.initials: M
authors.lastname: Iyer
authors.forename: Vladyslav
authors.initials: V
authors.lastname: Kholodovych
authors.forename: Ni
authors.initials: N
authors.lastname: Ai
authors.forename: William J
authors.initials: WJ
authors.lastname: Welsh
authors.forename: Michael
authors.initials: M
authors.lastname: Sinz
authors.forename: Peter W
authors.initials: PW
authors.lastname: Swaan
authors.forename: Rachana
authors.initials: R
authors.lastname: Patel
authors.forename: Kenneth
authors.initials: K
authors.lastname: Bachmann
mesh_terms: D001665:Binding Sites; D006528:Carcinoma, Hepatocellular / Q000473:pathology; D045744:Cell Line, Tumor; D003198:Computer Simulation; D017930:Genes, Reporter; D006801:Humans; D006860:Hydrogen Bonding; D057927:Hydrophobic and Hydrophilic Interactions; D020128:Inhibitory Concentration 50; D008113:Liver Neoplasms / Q000473:pathology; D008156:Luciferases / Q000378:metabolism; D008956:Models, Chemical; D008958:Models, Molecular; D015394:Molecular Structure; D010957:Plasmids; D000077297:Pregnane X Receptor; D011485:Protein Binding; D017434:Protein Structure, Tertiary; D011987:Receptors, Steroid / Q000819:agonists* / Q000037:antagonists & inhibitors* / Q000737:chemistry / Q000378:metabolism; D015533:Transcriptional Activation
chemical_list: D000077297:Pregnane X Receptor; D011987:Receptors, Steroid; D008156:Luciferases