An analog of 1alpha,25-dihydroxy-19-norvitamin D3 with the 1alpha-hydroxy group fixed in the axial position lacks biological activity in vitro

The relationship between the A-ring chair conformation of vitamin D compounds and their ability to bind the vitamin D receptor (VDR) has long attracted the attention of many researchers. It was established that in the crystalline complexes of hVDRmt with the natural hormone, 1alpha,25-dihydroxyvitamin D(3) (1), and its side-chain analogs the vitamins exist in beta-chair form with an equatorial orientation of 1alpha-OH. However, with all these ligands the interconversion between both A-ring forms would be possible in solution. In an attempt to verify the conformation of vitamin D compounds required for binding the VDR we prepared analog 4, characterized by the presence of an axial 1alpha-hydroxy group. Since the additional ring connecting 3beta-oxygen and C-2 prevents A-ring conformational flexibility, the synthesized vitamin 4 can exist exclusively in the alpha-chair form. The geometrical isomer 5 with a free 3beta-OH group was also obtained. The analog 5 binds very poorly to VDR, whereas the vitamin 4 is practically devoid of binding ability. Both compounds also show very low HL-60-differentiating activity. When tested in vivo in mice the analogs 4 and 5 exhibit significant calcemic responses with analog 4 showing more activity than analog 5.