Synthetic Modulators of the Vitamin D Receptor: From Structural Innovation to Disease-Specific Applications.

Vitamin D signaling via the vitamin D receptor (VDR) regulates calcium-phosphate homeostasis and extensive gene programs controlling cell proliferation, differentiation, immune tone, and metabolism. However, systemic use of the natural agonist 1α,25-dihydroxyvitamin D3 (calcitriol) for extraskeletal indications is limited by dose-limiting hypercalcemia. This review summarizes VDR biology and the structural basis of ligand action, emphasizing how ligand-induced repositioning of helix 12 and altered coregulator recruitment can be exploited to engineer selective VDR modulators. We highlight medicinal chemistry strategies spanning secosteroidal analogs with side-chain or ring modifications and emerging non-seco scaffolds and discuss clinically established agents (e.g., calcipotriol and paricalcitol) alongside experimental "super-agonists", partial agonists, and antagonists designed to widen the therapeutic window. Finally, we discuss current evidence for VDR targeting across cancer, metabolic disease, fibrosis, and immune-inflammatory disorders, including mechanisms of resistance such as dysregulated vitamin D metabolism and epigenetic repression. Structural and epigenomic insights are positioning next-generation VDR ligands as tissue- and pathway-biased therapeutics that may enable safer, mechanism-guided translation beyond bone and mineral indications.