Synthesis and evaluation of geometric analogs of 1α,25-dihydroxyvitamin D2 as potential therapeutics

- Analogs of 1α,25-dihydroxyvitamin D2 were synthesized by an improved strategy.
- New (24Z) geometric isomers, PRI-1916 and PRI-1917, were obtained and identified.
- The structures were determined by 1H and 13C NMR, using COSY, HSQC and HMBC.
- Binding affinities were determined by a fluorescence polarization competition assay.
- New analogs were much more resistant to CYP24A1 than 1α,25-dihydroxyvitamin D2.

An improved convergent strategy was developed for the synthesis of the previously obtained side-chain extended and rigidified analogs of 1α,25-dihydroxyvitamin D2, PRI-1906 and PRI-1907. New (24Z) geometric isomers of the analogs, PRI-1916 and PRI-1917, were also obtained and identified. These side-chain isomers were separable by flash chromatography, as C-25 alcohols, from the synthetic precursors of PRI-1906 and PRI-1907, respectively. The structures of new analogs were determined by advanced techniques of 1H and 13C NMR, including COSY, HSQC and HMBC sequences. Binding affinities of the geometric analogs PRI-1906 and PRI-1916 and their respective C-26, C-27 homologs PRI-1907 and PRI-1917 for the full-length human vitamin D receptor were determined by a fluorescence polarization competition assay. The binding affinity of (24Z) methyl analog PRI-1906 was much higher than that of (24E) analog PRI-1906, while the affinity of (24Z) ethyl analog PRI-1917 was lower than that of the respective PRI-1907. Investigation of the metabolism of these compounds by human CYP24A1 revealed they are much more resistant to CYP24A1 than 1α,25-dihydroxyvitamin D2, indicating they could have longer-term biological effects on target tissues.

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