A New Class of Vitamin D Analogues that Induce Structural Rearrangement of the Ligand-Binding Pocket of the Receptor

To identify novel vitamin D receptor (VDR) ligands that induce a novel architecture within the ligand-binding pocket (LBP), we have investigated eight 22-butyl-1alpha,24-dihydroxyvitamin D(3) derivatives (3-10), all having a butyl group as the branched alkyl side chain. We found that the 22S-butyl-20-epi-25,26,27-trinorvitamin D derivative 5 was a potent VDR agonist, whereas the corresponding compound 4 with the natural configuration at C(20) was a potent VDR antagonist. Analogues with the full vitamin D(3) side chain were less potent agonist, and whether they were agonists or antagonists depended on the 24-configuration. X-ray crystal structures demonstrated that the VDR-LBD accommodating the potent agonist 5 has an architecture wherein the lower side and the helix 11 side of the LBP is simply expanded relative to the canonical active-VDR situation; in contrast, the potent antagonist 4 induces an extra cavity to accommodate the branched moiety. This is the first report of a VDR antagonist that generates a new cavity to alter the canonical pocket structure of the ligand occupied VDR.