Radioimmunoassay of unconjugated plasma ethynylestradiol in women given a single oral dose of ethynylestradiol or mestranol.

ABSTRACT

PIP: 25 healthy women of reproductive age who had not previously used oral contraceptive steroids, were each given a single tablet containing 50, 80, or 100 mcg of mestranol or 50 or 80 mcg of ethinyl estradiol. Blood samples were obtained before taking the tablets and at intervals of 1, 2, 4, and 24 hours afterward. Anti-ethinyl-estradiol antibody in 1 to 100,000 initial dilution was used. Details of techniques employed are given. With ethinyl estradiol, the 1-hour sampling yielded the maximum plasma levels. At 24 hours, the plasma level was not detectable in 4 of 5 subjects given 50 mcg or in 1 of 5 given 80 mcg. With mestranol, the disappearance curve was more variable with the peak levels usually at 2 hours but occasionally at 4 hours. At all 3 dose levels of mestranol, measurable serum ethinyl estradiol levels were found at 24 hours. These levels were reached more slowly and were lower than when ethinyl estradiol was given. In contrast to natural estrogens ethinyl, estrogens are bound to plasma proteins chiefly by nonspecific binding and are therefore less likely to affect the metabolism of the ethinyl estrogens than are the endogenous steroids. Also, significant amounts of ethinyl estradiol are was given. In contrast to natural estrogens ethinyl, estrogens are bound di-ethynylated in vitro. The pharmacokinetics of ethinyl estrogens differ from those of natural estrogens. This complicates interpretation of plasma or urinary estrone and estradiol measurements.