(conjugated estrogens)
Endogenous estrogens are largely responsible for the development and maintenance of the female reproductive system and secondary sexual characteristics. Although circulating estrogens exist in a dynamic equilibrium of metabolic interconversions, estradiol is the principal intracellular human estrogen and is substantially more potent than its metabolites, estrone and estriol, at the receptor level.
The primary source of estrogen in normally cycling adult women is the ovarian follicle, which secretes 70 to 500 mcg of estradiol daily, depending on the phase of the menstrual cycle. After menopause, most endogenous estrogen is produced by conversion of androstenedione, which is secreted by the adrenal cortex, to estrone in the peripheral tissues. Thus, estrone and the sulfate-conjugated form, estrone sulfate, are the most abundant circulating estrogens in postmenopausal women.
Estrogens act through binding to nuclear receptors in estrogen-responsive tissues. To date, two estrogen receptors have been identified. These vary in proportion from tissue to tissue.
Circulating estrogens modulate the pituitary secretion of the gonadotropins, luteinizing hormone (LH) and FSH, through a negative feedback mechanism. Estrogens act to reduce the elevated levels of these gonadotropins seen in postmenopausal women.
Absorption
Conjugated estrogens are water soluble and are well-absorbed through the skin, mucous membranes, and the gastrointestinal (GI) tract. The vaginal delivery of estrogens circumvents first-pass metabolism.
A bioavailability study was conducted in 24 postmenopausal women with atrophic vaginitis. The mean (SD) pharmacokinetic parameters for unconjugated estrone, unconjugated estradiol, total estrone, total estradiol and total equilin following 7 once-daily doses of PREMARIN vaginal cream 0.5 g is shown in Table 2.
Pharmacokinetic Profiles of Unconjugated Estrogens | |||
PK Parameters | Cmax | Tmax | AUCss |
Estrone | 42.0 ± 13.9 | 7.4 ± 6.2 | 826 ± 295 |
Baseline-adjusted estrone | 21.9 ± 13.1 | 7.4 ± 6.2 | 365 ± 255 |
Estradiol | 12.8 ± 16.6 | 8.5 ± 6.2 | 231 ± 285 |
Baseline-adjusted estradiol | 9.14 ± 14.7 | 8.5 ± 6.2 | 161 ± 252 |
Pharmacokinetic Profiles of CE | |||
PK Parameters | Cmax | Tmax | AUCss |
Total estrone | 0.60 ± 0.32 | 6.0 ± 4.0 | 9.75 ± 4.99 |
Baseline-adjusted total estrone | 0.40 ± 0.28 | 6.0 ± 4.0 | 5.79 ± 3.7 |
Total estradiol | 0.04 ± 0.04 | 7.7 ± 5.9 | 0.70 ± 0.42 |
Baseline-adjusted total estradiol | 0.04 ± 0.04 | 7.7 ± 6.0 | 0.49 ± 0.38 |
Total equilin | 0.12 ± 0.15 | 6.1 ± 4.7 | 3.09 ± 1.37 |
Distribution
The distribution of exogenous estrogens is similar to that of endogenous estrogens. Estrogens are widely distributed in the body and are generally found in higher concentration in the sex hormone target organs. Estrogens circulate in the blood largely bound to SHBG and albumin.
Metabolism
Exogenous estrogens are metabolized in the same manner as endogenous estrogens. Circulating estrogens exist in a dynamic equilibrium of metabolic interconversions. These transformations take place mainly in the liver. Estradiol is converted reversibly to estrone, and both can be converted to estriol, which is a major urinary metabolite. Estrogens also undergo enterohepatic recirculation via sulfate and glucuronide conjugation in the liver, biliary secretion of conjugates into the intestine, and hydrolysis in the intestine followed by reabsorption. In postmenopausal women, a significant proportion of the circulating estrogens exist as sulfate conjugates, especially estrone sulfate, which serves as a circulating reservoir for the formation of more active estrogens.
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