Synthesis of Gemini analogs of 19-norcalcitriol and their platinum(II) complexes

Hormonally active vitamin D₃ metabolite, calcitriol, plays an important role in calcium-phosphate homeostasis, immune system actions and cell differentiation. Although anticancer activity of calcitriol is well documented and thousands of its analogs have been synthesized, none has been approved as a potential drug against cancer. Therefore, we attempted to introduce the cytotoxic effect to the calcitriol molecule by its linking to cisplatin. Herein, we present the synthesis of vitamin D compounds, designed on the basis of molecular modeling and docking experiments to the vitamin D receptor, and characterized by the presence of significantly different two side chains attached to C-20. In this study, a new synthetic approach to Gemini analogs was developed. Preparation of the target 19-norcalcitriol compounds involved separate syntheses of several building blocks (the A-ring, C/D-rings and side-chain fragments). The convergent synthetic strategy was used to combine these components by the different coupling processes, the crucial one being Wittig-Horner reaction of the Grundmann ketone analog with the known 2-methylene A-ring phosphine oxide. Due to the nature of the constructed steroidal side chains (bidentate ligands), which allowed coordination of metal ions, the first conjugate-type platinum(II) complexes of the vitamin D analogs were also successfully prepared and characterized. The target vitamin D compounds, displaying significant affinity for a vitamin D receptor, were assessed in vitro for their anti-proliferative activities towards several cell lines.