Synthesis and Evaluation of Aromatic A-Ring 23-Oxavitamin D₃ Analogues as Hedgehog Pathway Inhibitors

The Hedgehog (Hh) signaling pathway plays a crucial role in the initiation and progression of tumors, and Hh inhibitors have been used as potential chemotherapeutic agents for the treatment of basal cell carcinomas (BCCs). Vitamin D₃ (VD₃) and its derivatives have been identified as potent Hh inhibitors. However, the selectivity of VD₃ derivatives to vitamin D receptor (VDR) and the Hh signaling pathway still needs optimization. In this study, a series of aromatic A-ring mimics VD₃ analogues that contain a C-23 oxygen atom or incorporate C-25 hydroxyl on side chains were designed and synthesized. These compounds were tested in various cell lines for anti-Hh activity, with analogues 3j and 4i identified as potent inhibitors. Mechanism studies showed their anti-Hh effects are mainly due to targeting Smoothened (Smo) without binding to the cyclopamine site. Structure-activity relationship (SAR) studies revealed that VD₃-based inhibitors enhance anti-Hh activity by adding a hydroxyl group at C25 while reducing VDR activity by incorporating an oxygen atom into the side chain.