Half-lives in plasma and bioavailability of ethinylestradiol in laboratory animals

The pharmacokinetics of 19-nor-17 alpha-pregna-1,3,5(10)-trien-20-yne-3,17-diol (ethinylestradiol, Progynon C) (EE2) has been studied after intravenous administration of 0.1 or 0.01 mg/kg and after intragastric administration of 1 mg/kg in female rats, rabbits, beagle dogs, rhesus monkeys and baboons. After intravenous administration disposition of unchanged drug in the plasma was biphasic with initial half-lives between 0.3 and 0.5 h and terminal half-lives between 2.3 and 3.0 h. Total plasma clearance was of the same magnitude as total plasma liver flow or even higher rat) indicating a rapid biotransformation of the estrogen in the liver. Systemic availability of intragastric EE2 amounted to 3% in the rat, 0.3% in the rabbit, 9% in the dog, 0.6% in rhesus monkeys and 2% in the baboon and was considerably lower than in humans (40%). Differences in the pharmacokinetics and in the systemic availability of EE2 between laboratory animals and man should be taken into account in the retrospective interpretation of pharmacological and toxicological data and in the design of new studies.