The pharmacokinetics of tritiated ethinylestradiol in the rabbit

The disappearance of ethinylestradiol from the blood of rabbits has been studied, following the intravenous administration of this steroid. The disappearance followed two exponentials, the first having a half life ( t 1 2 ) of 5.5 min and the second, apparently terminal exponential was also rapid ( t 1 2 = 69 min ). The plasma clearance was 150 ml/min which suggests almost total clearance of this steroid during a single passage through the liver. Bile contained a significant concentration of EE conjugates and thus this steroid could undergo enterohepatic recirculations. A large oral dose of unlabelled EE, given prior to intravenous administra tion of tritiated EE, considerably altered the pharmacokinetics of the latter by saturating both phase one metabolism (changes of the steroid nucleus) and the secretion of conjugates into bile. It was not clear whether phase two metabolism (conjugation) was also saturated.