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Pharmacokinetics of ethinyl estradiol and mestranol.

American journal of obstetrics and gynecology (1990-12-01)
J W Goldzieher, S A Brody
ABSTRACT

Pharmacokinetally, a 50 micrograms oral dose of mestranol (which itself is inactive) is bioequivalent to a 35 micrograms dose of ethinyl estradiol. Physiologically, mestranol ranges from 50% to 100% of the activity of ethinyl estradiol, depending on the endpoint chosen. Compounds such as these, which are metabolized with a first-pass effect and are enterohepatically recirculated, demonstrate large interindividual and intraindividual variability in their pharmacokinetics. Thus a given dose of ethinyl estradiol in one person may produce an effect equivalent to a substantially larger (or smaller) dose in another person. This wide variability confounds efforts to establish tight dose-response relationships, a point rarely considered in clinical or epidemiologic studies of these compounds. The circulating levels of ethinyl estradiol sulfates may be higher than those of free ethinyl estradiol itself. It has been thought that these sulfates represent a "reservoir" of ethinyl estradiol. Our studies show that this idea is untenable because the half-life of the sulfates is not long enough for such an effect. Differences in the pharmacokinetics of ethinyl estradiol and mestranol have been observed in studies of various populations. The reality of these group differences is affirmed by analyses of urinary metabolite patterns.

MATERIALS
Product Number
Brand
Product Description

Supelco
Ethinyl Estradiol, Pharmaceutical Secondary Standard; Certified Reference Material
Ethinylestradiol, European Pharmacopoeia (EP) Reference Standard
Supelco
17α-Ethynylestradiol solution, 1.0 mg/mL in methanol, ampule of 1 mL, certified reference material, Cerilliant®
USP
Ethinyl estradiol, United States Pharmacopeia (USP) Reference Standard
Mestranol, European Pharmacopoeia (EP) Reference Standard
Sigma-Aldrich
17α-Ethynylestradiol, ≥98%
Sigma-Aldrich
Mestranol, 99%
Ethinylestradiol for system suitability, European Pharmacopoeia (EP) Reference Standard