Orosomucoid in human pregnancy serum diminishes bioavailability of the progesterone antagonist RU 486 in rats.

Bioavailability of the progesterone antagonist RU 486 was examined in vivo by measurement of first-pass uptake by hepatic and uterine tissues in rats. Presence of human pregnancy serum or alpha 1-acid glycoprotein (orosomucoid) in injection vehicles diminished uptake of this steroid, which suggests limited bioavailability. These data indicate that elevated human serum orosomucoid levels may account for some failures of RU 486 in early pregnancy termination.