1α,25-dihydroxy-16-ene-23-yne-vitamin D3 and 1α,25-dihydroxy-16-ene-23-yne-20-epi-vitamin D3:: Analogs of 1α,25-dihydroxyvitamin D3 that resist metabolism through the C-24 oxidation pathway are metabolized through the C-3 epimerization pathway

The secosteroid hormone 1 alpha ,25-dihydroxyvitamin D-3 [1 alpha ,25(OH)(2)D-3] is metabolized in its target tissues through modifications of both the side chain and the A-ring. The C-24 oxidation pathway, the previously well established main side chain modification pathway, is initiated by hydroxylation at C-24 of the side chain. The C-3 epimerization pathway, the newly discovered A-ring modification pathway, is initiated by epimerization of the hydroxyl group at C-3 of the A-ring. The end products of the metabolism of 1 alpha ,25(OH)(2)D-3 through the C-24 oxidation and the C-3 epimerization pathways are calcitroic acid and 1 alpha ,25-dihydroxy-3-epi-vitamin-D-3 respectively. During the past two decades, numerous noncalcemic analogs of 1 alpha ,25(OH)(2)D-3 were synthesized. Several of the analogs have altered side chain structures and as a result some of these analogs have been shown to resist their metabolism through side chain modifications. For example, two of the analogs, namely, 1 alpha ,25-dihydroxy-16-ene-23-yne-vitamin D-3 [1 alpha ,25(OH)(2)-16-ene-23-yne-D-3] and 1 alpha ,25-dihydroxy-16-ene-23-yne-20-epi-vitamin D-3 [1 alpha ,25(OH)(2)-16-ene-23-yne-20-epi-D-3], have been shown to resist their metabolism through the C-24 oxidation pathway. However, the possibility of the metabolism of these two analogs through the C-3 epimerization pathway has not been studied. Therefore, in our present study, we investigated the metabolism of these two analogs in rat osteosarcoma cells (UMR 106) which are known to express the C-3 epimerization pathway. The results of our study indicate that both analogs [1 alpha ,25(OH)(2)-16-ene-23-yne-D-3 and 1 alpha ,25(OH)(2)-16-ene-23-yne-20-epi-D-3] are metabolized through the C-3 epimerization pathway in UMR 106 cells. The identity of the C-3 epimer of 1 alpha ,25(OH)(2)-16-ene-23-yne-D-3 [1 alpha ,25(OH)(2)-16-ene-23-yne-3-epi-D-3] was confirmed by GC/MS analysis and its comigration with synthetic 1 alpha ,25(OH)(2)-16-ene-23-yne-3-epi-D-3 on both straight and reverse-phase HPLC systems. The identity of the C-3 epimer of 1 alpha ,25(OH)(2)-16-ene-23-yne-20-epi-D-3 [1 alpha ,25(OH)(2)-16-ene-23-yne-20-epi-3-epi-D-3] was confirmed by GC/MS and H-1 NMR analysis. Thus, we indicate that vitamin D analogs which resist their metabolism through the C-24 oxidation pathway, have the potential to be metabolized through the C-3 epimerizatiom pathway. In our present study, we also noted that the rate of C-3 epimerization of 1 alpha ,25(OH)(2)-16-ene-23-yne-20-epi-D-3 is about 10 times greater than the rate of C-3 epimerization of 1 alpha ,25(OH)(2)-16-ene-23-yne-D-3. Thus, we indicate for the first time that certain structural modifications of the side chain such as 20-epi modification can alter significantly the rate of C-3 epimerization of vitamin D compounds. (C) 2000 Academic Press.