Received for publication June 3, 2008.
Revised June 24, 2008.
Accepted for publication June 24, 2008.

COMPUTATIONAL DISCOVERY OF NOVEL LOW MICROMOLAR HUMAN PREGNANE X RECEPTOR ANTAGONISTS

Abstract

To date there have been very few antagonists identified for the human pregnane X receptor (PXR). These molecules may be of utility for modulating the effects of therapeutic drugs which are potent agonists for this receptor (e.g. some anticancer compounds and macrolide antibiotics), with subsequent effects on transcriptional regulation of xenobiotic metabolism and transporter genes. A recent novel pharmacophore for PXR antagonists was developed using three azoles and consisted of two hydrogen bond acceptor regions and two hydrophobic features. This pharmacophore also suggested an overall small binding site that was identified on the outer surface of the receptor at the AF-2 site and validated by docking studies. Using computational approaches to search libraries of known drugs or commercially available molecules is preferred over random screening. We have now described several new smaller antagonists of PXR discovered with the antagonist pharmacophore with in vitro activity in the low micromolar range (SPB03255 (IC₅₀ 6.3 µM) and SPB00574 (IC₅₀ 24.8 µM)). We have also used our computational pharmacophore and docking tools to suggest that most of the known PXR antagonists such as coumestrol and sulforaphane, could also interact on the outer surface of PXR at the AF-2 domain. The involvement of this domain was also suggested by further site-directed mutagenesis work. We have additionally described an FDA approved prodrug, leflunomide (IC₅₀ 6.8 µM) which appears to be a PXR antagonist in vitro. These observations are important for predicting whether further molecules may interact with PXR as antagonists in vivo with potential therapeutic applications.

Key words: Transcriptional coactivators, Structure-activity relationships and modeling, Mutagenesis/Chimeric approaches, Cytochrome P450, Regulation - transcriptional, Regulation - xenobiotic, Antifungal drugs, Resistance, Transcription targets