Structure of the ligand-binding domain of rat VDR in complex with the nonsecosteroidal vitamin D3 analogue YR301

Vitamin D receptor (VDR) is a ligand-inducible hormone receptor that mediates 1alpha,25(OH)(2)D(3) action, regulating calcium and phosphate metabolism, induces potent cell differentiation activity and has immunosuppressive effects. Analogues of 1alpha,25(OH)(2)D(3) have been used clinically for some years. However, the risk of potential side effects limits the use of these substances. LG190178 is a novel nonsecosteroidal ligand for VDR. (2S)-3-[4-(3-{4-[(2R)-2-hydroxy-3,3-dimethylbutoxy]-3-methylphenyl}pentan-3-yl)-2-methylphenoxy] propane-1,2-diol (YR301) is the only one of the four evaluated stereoisomers of LG190178 to have strong activity. To understand the strong activity of YR301, the crystal structure of YR301 complexed with the rat VDR ligand-binding domain (VDR LBD) was solved at 2.0 A resolution and compared with the structure of the VDR LBD-1alpha,25(OH)(2)D(3) complex. YR301 and 1alpha,25(OH)(2)D(3) share the same position and the diethylmethyl group occupies a similar space to the C and D rings of 1alpha,25(OH)(2)D(3). YR301 has two characteristic hydroxyl groups which contribute to its potent activity. The first is 2'-OH, which forms hydrogen bonds to the NE2 atoms of both His301 and His393. The other is 2-OH, which interacts with Ser233 OG and Arg270 NH1. These two hydroxyl groups of YR301 correspond exactly to 25-OH and 1-OH, respectively, of 1alpha,25(OH)(2)D(3). The terminal hydroxyl group (3-OH) of YR301 is directly hydrogen bonded to Arg270 and also interacts indirectly with Tyr232 OH and the backbone NH of Asp144 via water molecules. Additional derivatization of the terminal hydroxyl group using the positions of the water molecules might be useful for the design of more potent compounds.

Metabolism of the vitamin D3 analogue EB1089 alters receptor complex formation and reduces promoter selectivity

1. 1alpha,25-dihydroxyvitamin3 (VD) is a nuclear hormone that has important cell regulatory functions but also a strong calcemic effect. EB1089 is a potent antiproliferative VD analogue, which has a modified side chain resulting in increased metabolic stability and a selective functional profile. Since EB1089 is considered for potential systemic application, it will be investigated to what extent its recently identified metabolites (hydroxylated at positions C26 and C26a) contribute to biological profile of the VD analogue. 2. Limited protease digestion analysis demonstrated that EB1089 is able to stabilize the high affinity ligand binding conformation of the VDR, starting at concentrations of 0.1 nM and affecting up to 80% of all receptor molecules. The metabolites EB1445 and EB1470 showed to be 100 fold less potent than EB1089, whereas the remaining three metabolites (EB1435, EB1436 and EB1446) showed a clearly reduced ability to stabilize the high affinity ligand binding conformation. Interestingly, at pharmacological concentrations all EB1089 metabolites stabilized a second, apparently lower affinity conformation to a much higher extent than EB1089. 3. In reporter gene assays all metabolites showed lower potency than EB1089. Moreover, the preference of EB1089 for activation of VDR binding to sites formed by inverted palindromic arrangements spaced by nine nucleotide (IP9-type VD response elements) appeared to be reduced (with EB1445 and EB1470) or completely lost (with EB1435, EB1436 and EB1446). The ranking of EB1089 and its metabolites that was obtained by limited protease digestion and reporter gene assays was confirmed by an analysis of their antiproliferative effect in breast cancer cells. . The potency and selectivity of the EB1089 metabolites in mediating gene regulatory effects was found to be drastically reduced in comparison to the parent compound suggesting that the contribution of the metabolites to the biological effect of EB1089 is minor. However, the compounds showed to be interesting tools for understanding the selective biological profile of EB1089.