Synthesis, metabolism, and biological activity of 2-[3-(tetrazolyl)propyl]-1α,25-dihydroxy-19-norvitamin D3

- We synthesized four novel 2-[3-(tetrazolyl)propyl]-1α,25-dihydroxy-19-norvitamin D3.
- Only 2α-[3-(tetrazol-1-yl)propyl] analog showed weak binding affinity for hVDR.
- Metabolic stability of 2α-substituted analogs was higher than that of 2β-ones.

Recently, we found that 2α-[2-(tetrazol-2-yl)ethyl]-1α,25-dihydroxyvitamin D3 showed higher osteocalcin promoter transactivation activity in human osteosarcoma (HOS) cells and a greater therapeutic effect in ovariectomized (OVX) rats in vivo than those of active vitamin D3, 1α,25(OH)2D3. We were interested in introducing a heterocyclic ring to the C2 position of the seco-steroidal structure via an alkyl linker, and four novel C2-(3-tetrazolylpropyl) substituted 1α,25-dihydroxy-19-norvitamin D3 analogs, 2α-[3-(tetrazol-1-yl)propyl]-, 2β-[3-(tetrazol-1-yl)propyl]-, 2α-[3-(tetrazol-2-yl)propyl]-, and 2β-[3-(tetrazol-2-yl)propyl]-19-nor-1α,25(OH)2D3 were synthesized. Among them, 2α-[3-(tetrazol-1-yl)propyl]-19-nor-1α,25(OH)2D3 showed weak binding affinity for human vitamin D receptor (hVDR) (2.6% of 1α,25(OH)2D3 and ca. 15% of 19-nor-1α,25(OH)2D3) and weak VDR transactivation activity in HOS cells (EC50 7.3 nM, when 1α,25(OH)2D3 0.23 nM). Although the other three compounds could not act as VDR binders by evaluation of the competition assays, 2α-[3-(tetrazol-2-yl)propyl]-19-nor-1α,25(OH)2D3 showed weak transactivation activity (EC50 12.5 nM). Metabolic stability of the 2α-substituted compounds 2α-[3-(tetrazol-1-yl)propyl]- and 2α-[3-(tetrazol-2-yl)propyl]-19-nor-1α,25(OH)2D3 was higher than that of the 2β-substituted counterparts 2β-[3-(tetrazol-1-yl)propyl]- and 2β-[3-(tetrazol-2-yl)propyl]-19-nor-1α,25(OH)2D3 against human CYP24A1. Introduction of a tetrazole ring to the C2-position of the 19-norvitamin D3 skeleton with the propyl linker led to weak VDR agonistic activity with stability against CYP24A1 metabolism.

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