2-Substituted-16-ene-22-thia-1α, 25-dihydroxy-26,27-dimethyl-19-norvitamin D3 analogs:: Synthesis, biological evaluation, and crystal structure

Recently, we have found that 16-ene-22-thia-26,27-dimethyl-19-norvitamin D(3) analogs 1a (n = 2, 3) are 20 times more active than the natural hormone 1a, 25-dihydroxyvitamin D3 in terms of transcriptional activity. To further investigate the effects of the A-ring modi. cation of 1a, b on the biological activity pro. le, novel 22-thia-19-norvitamin D analogs 2-11 bearing a hydroxyethoxy-, hydroxyethylidene- or methyl group at C-2 in combination with 20S- and 20R-isomers were prepared and tested for their in vitro biological activities. All of the synthesized analogs showed 0.5-140% of the activity of the natural hormone in binding to the vitamin D receptor (VDR). When compared with the transcriptional activity of C-2 or C- 20 isomeric pairs of the 22-thia analogs, the 20S- isomers 2-11a were more potent than the 20R- isomers 2, 3, 8-11b, and the 2 beta-hydroxyethoxy, 2E-hydroxyethylidene, and 2 alpha-methyl-2 beta-hydroxy-22-thia isomers showed higher potency than their corresponding counterparts. In particular, 3a exhibited an extremely higher level of potency (210-fold) than the natural hormone. To elucidate the action mode of superagonist 3a at the molecular level, we determined the crystal structures of the rat VDR-ligand-binding domain complexed with 3a or 3b in the presence of peptide containing a nuclear box motif (LxxLL) at 1.9-2.0 angstrom resolution. The crystal structures demonstrated that the 1 alpha-OH, 3 beta-OH, and 25-OH groups of the natural hormone and 3a were anchored by the same amino acid residues in the ligand-binding pocket, and the terminal OH moiety of the substituent at C-2 formed hydrogen bonds with Arg270 and a water molecule to create a tight water molecule network. Moreover, the methyl groups at C-26a and C-27a make additional contact with hydrophobic residues such as Leu223, Ala227, Val230, and Ala299. These hydrophilic and hydrophobic interactions in 3a may underlie the induction of superagonistic activity. (c) 2008 Elsevier Ltd. All rights reserved.