METABOLISM OF THE VITAMIN-D ANALOG EB-1089 - IDENTIFICATION OF IN-VIVO AND IN-VITRO LIVER METABOLITES AND THEIR BIOLOGICAL-ACTIVITIES

ien-1'-yl)-9,10-secopregna-5(Z),7(E),10(19)-triene (EB 1089) is a nove l analog of the vitamin D hormone, calcitriol that has been modified i n the side-chain resulting in an increased metabolic stability relativ e to other side-chain modified analogs (e.g. calcipotriol and 22-oxaca lcitriol). To further investigate the metabolism of EB 1089, we set ou t to study this metabolism both in the rat in vivo as well as in the p ostmitochondrial liver fractions from rat, man, and minipig in vitro. The same pattern of metabolism was observed in all biological systems employed, both in vivo and in vitro, namely 26- and 26a-hydroxylation of EB 1089. The same metabolites were produced using cultured cell sys tems (Shankar et al., see this issue). All the possible isomers of 26- and 26a-hydroxy EB 1089 were synthesised and these were compared to b iologically generated material using HPLC, NMR, and CC-MS techniques. The predominant natural isomer observed in vitro and in vivo in rats a s well as in vitro in humans was identified to be (25S),26R-hydroxy EB 1089. The biological activities of the EB 1089 metabolites on cell gr owth regulation were 10- to 100-fold lower than that of EB 1089. The e ffects of the metabolites on calcium metabolism in vivo were comparabl e to the effect of EB 1089; however, these effects were reduced for th e major metabolite in rat and man and for the isomers of 26a-hydroxy E B 1089. We conclude that EB 1089 is metabolised by a different route o f side chain metabolism than calcitriol and that this may explain its relative metabolic stability in pharmacokinetic experiments in vivo co mpared to that of other vitamin D analogs. (C) 1997 Elsevier Science I nc.