First Sila-Vitamin D Analogues: Design, Synthesis, Structural Analysis and Biological Activity

The incorporation of silicon bioisosteres into pharmacological structures has been used as a strategy to improve the therapeutic potential of drugs. However, no secosteroidal silicon-containing VDR ligands have been developed. Here we report the design, synthesis, and biological activity of six analogues of the natural hormone 1,25-dihydroxyvitamin D3 (1,25D₃), which incorporate a silicon atom as a side chain-C25 isostere. The analogues were synthesized by the Wittig-Horner approach starting from Inhoffen-Lythgoe diol. The crystal structures of the complexes formed by the sila-analogues with the ligand binding domain of VDR revealed additional interactions of the sila-containing side chains that stabilize the VDR active conformation. These sila-analogues show similar VDR binding and transcriptional activity in comparison with the natural hormone 1,25D₃, but with significantly less hypercalcemic activity. The new analogues, when combined with chemotherapy, significantly decrease cell proliferation.