1 alpha,25-dihydroxy-16-ene-23-yne-vitamin D-3 and 1 alpha,25-dihydroxy-16-ene-23-yne-20-epi-vitamin D-3: Analogs of 1 alpha,25-dihydroxyvitamin D-3 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 th e side chain and the A-ring. The C-24 oxidation pathway, the previously wel l established main side chain modification pathway, is initiated by hydroxy lation at C-24 of the side chain. The C-3 epimerization pathway, the newly discovered A-ring modification pathway, is initiated by epimerization of th e hydroxyl group at C-3 of the A-ring. The end products of the metabolism o f 1 alpha ,25(OH)(2)D-3 through the C-24 oxidation and the C-3 epimerizatio n 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 show n 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-2 3-yne-20-epi-vitamin D-3 [1 alpha ,25(OH)(2)-16-ene-23-yne-20-epi-D-3], hav e been shown to resist their metabolism through the C-24 oxidation pathway. However, the possibility of the metabolism of these two analogs through th e C-3 epimerization pathway has not been studied. Therefore, in our present study, we investigated the metabolism of these two analogs in rat osteosar coma cells (UMR 106) which are known to express the C-3 epimerization pathw ay. 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 met abolized through the C-3 epimerization pathway in UMR 106 cells. The identi ty of the C-3 epimer of 1 alpha ,25(OH)(2)-16-ene-23-yne-D-3 [1 alpha ,25(O H)(2)-16-ene-23-yne-3-epi-D-3] was confirmed by GC/MS analysis and its comi gration with synthetic 1 alpha ,25(OH)(2)-16-ene-23-yne-3-epi-D-3 on both s traight 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-yn e-20-epi-3-epi-D-3] was confirmed by GC/MS and H-1 NMR analysis. Thus, we i ndicate 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)-1 6-ene-23-yne-D-3. Thus, we indicate for the first time that certain structu ral modifications of the side chain such as 20-epi modification can alter s ignificantly the rate of C-3 epimerization of vitamin D compounds. (C) 2000 Academic Press.