Clinical Pharmacokinetics of Contraceptive Steroids

Abstract

The present article should be read in conjunction with the original review published in the Journal in 1983. There is no new information of major significance about the pharmacokinetics of levonorgestrel, norethisterone (norethindrone) or ethinylestradiol, although it has been shown that the concentrations of these hormones secreted in breast milk are small and mothers taking combined oral contraceptive steroids may breast-feed safely. Both levonorgestrel and ethinylestradiol can be successfully administered from appropriate vaginal formulations, but no clear advantages over oral administration have been demonstrated. Several new progestogens have been investigated. Desogestrel is a prodrug for its active metabolite 3-keto-desogestrel, gestodene is itself an active progestogen and norgestimate is a prodrug acting by conversion to norgestrel and its metabolites. All 3 compounds have good bioavailability with wide intersubject variation. The newer progestogens, like norethisterone and levonorgestrel, are bound to sex hormone binding globulin (SHBG). This causes their plasma concentrations to increase with time, since SHBG is induced by ethinylestradiol even in doses of 30 micrograms daily. The binding capacity and affinity of SHBG do not increase in direct proportion to its concentration. Further drug interactions with oral contraceptive steroids have been described. Contraceptive steroids may inhibit hepatic microsomal enzyme metabolism and increase the plasma concentration and effect of some tricyclic antidepressants, the hydroxylated benzodiazepines, some beta-blocking drugs, methylxanthines, prednisolone and cyclosporin. There are no significant effects on vitamins. Oral contraceptive steroids induce glucuronidation and hence decrease plasma concentrations of some benzodiazepines, clofibric acid, paracetamol (acetaminophen) and possibly morphine. The plasma concentration of ethinylestradiol may be increased by competitive sulphation with paracetamol. Plasma concentrations of contraceptive steroids are decreased by griseofulvin, which induces their hepatic metabolism. The role of other antibiotics remains controversial but there is probably a group of susceptible women who have lower plasma contraceptive hormone concentrations and experience breakthrough bleeding or pregnancy when given broad spectrum antibiotics. This may relate to interruption of the enterohepatic recirculation of ethinylestradiol. Anticonvulsants, other than valproic acid, all induce contraceptive steroid metabolism and therefore lower plasma hormone concentrations, thus reducing contraceptive effectiveness.