Pharmacokinetics and pharmacodynamics of transdermal dosage forms of 17β-estradiol: Comparison with conventional oral estrogens used for hormone replacement

This open-label, multiple-crossover study compared the pharmacokinetics and pharmacodynamics of transdermal 17β-estradiol and two oral forms of estrogen replacement therapy in postmenopausal women. The transdermal systems delivered either 0.025, 0.05, or 0.1 mg/day; oral dosages were 2 mg of micronized 17β-estradiol or 1.25 mg of conjugated equine estrogens. Transdermal estradiol provided serum and urinary levels of estradiol conjugates typical of the early follicular phase of the premenopausal woman and an estradiol/estrone ratio that approximated 1. The increments of both serum and urinary estradiol showed dose proportionality. Serum levels of estradiol obtained 24 hours after oral administration of estrogens were in a range similar to the steady-state levels obtained with transdermal estradiol delivery. Oral estrogens, however, induced an excessive rise in estrone to levels far beyond those observed in premenopausal women. Continuous application of transdermal estradiol over 3 weeks did not result in any accumulation of estradiol or estradiol conjugates. After only three doses of oral estrogens, there were signs of retention of estrogens. Suppression of gonadotropins by oral and transdermal administration of estrogens was in a similar range. This observation supports the conclusions that levels of circulating estradiol are relevant to efficacy, and that excessively high levels of estrone after oral administration of estrogens merely represents a nonphysiologic precursor or metabolite pattern.