Biological Evaluation And Synthesis Of Calcitroic Acid

We describe the synthesis and broad profiling of calcitroic acid (CTA) as vitamin D receptor (VDR) ligand. The x-ray co-crystal structure of the Danio Rerio VDR ligand binding domain in complex with CTA and peptide MED1 confirmed an agonistic conformation of the receptor. CTA adopted a similar conformation as 1,25(OH)(2)D-3 in the binding pocket. A hydrogen bond with His333 and a water molecule were observed in the binding pocket, which was accommodated due to the shorter CTA side chain. In contrast, 1,25(OH)(2)D-3 interacted with His423 and His333 due to its longer side chain. In vitro, the EC50 values of CTA and CTA-ME for VDR-mediated transcription were 2.89 mu M and 0.66 mu M, respectively, confirming both compounds as VDR agonists. CTA was further evaluated for interaction with fourteen nuclear receptors demonstrating selective activation of VDR. VDR mediated gene regulation by CTA in intestinal cells was observed for the VDR target gene CYP24A1. CTA at 10 mu M upregulated CYP24A1 with similar efficacy as 1,25(OH)(2)D-3 at 20 nM and 100-fold stronger compared to lithocholic acid at 10 mu M. CTA reduced the transcription of iNOS and IL-1 beta in interferon gamma and lipopolysaccharide stimulated mouse macrophages resulting in a reduction of nitric oxide production and secretion of IL-1 beta. These observed anti-inflammatory properties of 20 mu M CTA were similar to 20 nM 1,25(OH)(2)D-3.