The concept of bioavailability is discussed with particular references to the sex steroids. Problems encountered in the measurement of bioavailability of these steroids and the various factors that may affect their bioavailability are briefly described. Information regarding the bioavailability of the estrogens and gestogens, some of which are prodrugs, used in oral contraception and hormone replacement therapy is summarized and the implications regarding the clinical use of these steroids are discussed.
PIP: This review examines the bioavailability of sex steroids used in oral contraceptives and hormone replacement therapy and the implications of the clinical use of these steroids. These steroids include the estrogens (estradiol, estrogen sulfates, ethinyl estradiol) and the gestogens (progesterone, norethisterone, levonorgestrel, desogestrel, gestodene, norgestimate, and medroxyprogesterone acetate). The naturally occurring sex steroids are estradiol, estrogen sulfates, and progesterone or their derivatives. The synthetic sex steroids are ethinyl estradiol, norethisterone, levonorgestrel, desogestrel, gestodene, norgestimate, and medroxyprogesterone acetate. Factors influencing bioavailability of these sex steroids revolve around drug formulation (dosage form, disintegration rate, and dissolution rate), drug characteristics (chemical properties and stability in the gastrointestinal tract), user's characteristics (gastrointestinal and hepatic functions), and possibly smoking, diet, and other drugs. A wide variation exists in the bioavailability values both within any study and between the different studies with the same steroid. Possible reasons for the variability include experimental error, a small number of subjects, the rate and extent of absorption of the compound, the compound's rate of metabolism and elimination (especially hepatic metabolism and elimination), differences in dose, and interaction between the estrogen and the gestogen. Some of the synthetic sex steroids are prodrugs.