Discrepancy in interactions and conformational dynamics of pregnane X receptor (PXR) bound to an agonist and a novel competitive antagonist.

Pregnane X receptor (PXR) is a nuclear receptor with an essential role in regulating drug metabolism genes. While the mechanism of action for ligand-mediated PXR agonism is well-examined, its ligand-mediated inhibition or antagonism is poorly understood. Here we employ microsecond timescale all-atom molecular dynamics (MD) simulations to investigate how our newly identified dual kinase and PXR inhibitor, compound 100, acts as a competitive PXR antagonist and not as a full agonist. We study the PXR ligand binding domain conformational changes associated with compound 100 and compare the results to the full agonist SR12813, in presence and absence of the coactivator. Furthermore, we complement our research by experimentally disclosing the effect of eight key-residue mutations on PXR activation. Finally, simulations of P2X4 inhibitor (BAY-1797) in complex with PXR, which shares an identical structural moiety with compound 100, provide further insights to ligand-induced PXR behaviour. Our MD data suggests ligand-specific influence on conformations of different PXR-LBD regions, including α6 region, αAF-2, α1-α2', β1'-α3 and β1-β1' loop. Our results provide important insights on conformational behaviour of PXR and offers guidance how to alleviate PXR agonism or to promote PXR antagonism.