The structurally related estrogens, estrone (E1), estriol (E3), and estetrol (E4) have one, three, and four hydroxyl groups, respectively. Hemihydrate
A hemihydrate form of estradiol, estradiol hemihydrate, is widely used medically under a large number of brand names similarly to estradiol. In terms of activity and bioequivalence, estradiol and its hemihydrate are identical, with the only disparities being an approximate 3% difference in potency by weight (due to the presence of water molecules in the hemihydrate form of the substance) and a slower rate of release with certain formulations of the hemihydrate. This is because estradiol hemihydrate is more hydrated than anhydrous estradiol, and for this reason, is more insoluble in water in comparison, which results in slower absorption rates with specific formulations of the drug such as vaginal tablets. Estradiol hemihydrate has also been shown to result in less systemic absorption as a vaginal tablet formulation relative to other topical estradiol formulations such as vaginal creams. Estradiol hemihydrate is used in place of estradiol in some estradiol products. Derivatives
A variety of C17β and/or C3 ester prodrugs of estradiol, such as estradiol acetate, estradiol benzoate, estradiol cypionate, estradiol dipropionate, estradiol enantate, estradiol undecylate, estradiol valerate, and polyestradiol phosphate (an estradiol ester in polymeric form), among many others, have been developed and introduced for medical use as estrogens. Estramustine phosphate is also an estradiol ester, but with a nitrogen mustard moiety attached, and is used as a cytostatic antineoplastic agent in the treatment of prostate cancer.
Estradiol is highly selective for these ERs and mERs, and does not interact importantly with other steroid hormone receptors. It is far more potent as an estrogen than are other bioidentical estrogens like estrone and estriol. Given by subcutaneous injection in mice, estradiol is about 10-fold more potent than estrone and about 100-fold more potent than estriol.The ERs are expressed widely throughout the body, including in the breasts, uterus, vagina, fat, skin, bone, liver, pituitary gland, hypothalamus, and other parts of the brain. In accordance, estradiol has numerous effects throughout the body. Among other effects, estradiol produces breast development, feminization, changes in the female reproductive system, changes in liver protein synthesis, and changes in brain function. The effects of estradiol can influence health in both positive and negative ways. In addition to the aforementioned effects, estradiol has antigonadotropic effects due to its estrogenic activity, and can inhibit ovulation and suppress gonadal sex hormone production. At sufficiently high dosages, estradiol is a powerful antigonadotropin, capable of suppressing testosterone levels into the castrate/female range in men.There are differences between estradiol and other estrogens, such as non-bioidentical estrogens like natural conjugated estrogens and synthetic estrogens like ethinylestradiol and diethylstilbestrol, with implications for pharmacodynamics and pharmacokinetics as well as efficacy, tolerability, and safety. Pharmacokinetics
Estradiol can be taken by a variety of different routes of administration. These include oral, buccal, sublingual, intranasal, transdermal (gels, creams, patches), vaginal (tablets, creams, rings, suppositories), rectal, by intramuscular or subcutaneous injection (in oil or aqueous), and as a subcutaneous implant.