Removal of the 20-methyl group from 2-methylene-19-nor-(20S)-1α,25-dihydroxyvitamin D3 (2MD) selectively eliminates bone calcium mobilization activity

The 18-nor (7), 21-nor (8) and 18,21-dinor (9) analogs of (20S)-1α,25-dihydroxy-2-methylene-19-norvitamin D3 (6, 2MD) were prepared by convergent syntheses. The known phosphine oxide 10 was coupled by the Wittig–Horner process with the corresponding C,D-fragments (13–15), obtained by a multi-step procedure from commercial vitamin D2. The goal of our studies was to examine the influence of removal of the methyl groups located at carbons 13 and 20 on the biological potency of 2MD in the hope of finding analogs with improved therapeutic profiles.Replacement of the 20-methyl with hydrogen in 2-methylene-19-nor-(20S)-1α,25-dihydroxyvitamin D3 (2MD) did not affect binding to the rat vitamin D receptor and had little effect on transcription activity and on HL-60 differentiation. However, the mobilization of calcium from bone was largely eliminated while intestinal calcium transport remained strong. Curiously, removal of both the C-13-methyl and 20-methyl restored slightly the bone calcium mobilizing activity. Thus, the 21-nor analog of 2MD may provide a potent analog with a greater margin of safety than 2MD.

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