Calcium regulating activity of 26,27-dimethyl analog of 24R,25-dihydroxyvitamin D3

To determine the possibility that methyl substitution in 26- and 27-positions of 24R,25-dihydroxyvitamin D₃ [24,25(OH)₂D₃] alters activities of the original compound, the effects of 24,25(OH)₂D₃ on calcium (Ca) regulating activity were compared with those of its methyl analog [24,25(OH)₂(CH₃)₂D₃] in addition to 1,25-dihydroxyvitamin D₃ [1,25(OH)₂D₃]. 24,25(OH)₂D₃ at 10^-6 M and 24,25(OH)₂(CH₃)₂D₃ at 10^-7 M and above significantly stimulated both bone resorption in neonatal mouse calvaria cultures and formation of osteoclast-like multinucleated cells (MNC) in mouse bone marrow cultures. A stimulative effect of 1,25(OH)₂D₃ on bone resorption and MNC formation was recognized in very low concentrations (10^-11 M and above). Although a potency of 24,25(OH)₂(CH₃)₂D₃ in stimulating bone calcium (Ca) mobilization and intestinal Ca transport was higher than that of 24,25(OH)₂D₃, the potencies of both compounds were similar to that of 1,25(OH)₂D₃ unlike in vitro experiments. As 1,24R,25-trihydroxy-26,27-dimethylvitamin D₃ showed almost the same effect as 24,25(OH)₂(CH₃)₂D₃, the dihydroxy form is suggested to be hydroxylated at 1 position and converted to trihydroxy form in vitamin D-deficient rats. From these results, methyl substitution in 26- and 27-position of 24,25(OH)₂D₃ was found to elevate Ca regulating activity of the original compound. In addition, it is suggested that the basis for a similarity in potency between 1,25(OH)₂D₃ and 24,25(OH)₂D₃ or its dimethyl analog in vitamin D-deficient rats is likely the result of 1 -hydroxylation.