Pharmacokinetics of estradiol
Estradiol can be administered in the form of a very long-lasting subcutaneous pellet implant. These implants are replaced once every 6 to 12 months, and can achieve high and very constant circulating levels of estradiol. They are surgically inserted by a trained physician in a medical office or clinic and can be placed into locations including the lower abdomen, lower back, buttocks, or hips. Subcutaneous implants containing 25, 50, and 100 mg estradiol for replacement usually once every 6 months are available as approved pharmaceutical medications. Up to 800 mg estradiol per implantation has been used. Pharmaceutical estradiol implants have been said to be used almost exclusively in the United Kingdom. An estradiol implant has not been approved by the FDA as a pharmaceutical medication in the United States, but hormone implants, including estradiol implants, are available as custom compounded products in this country.
Estrone (medication)
Estrone is an estrogen, specifically an agonist of the estrogen receptors ERα and ERβ. It is a far less potent estrogen than is estradiol, and as such is a relatively weak estrogen. Given by subcutaneous injection in mice, estradiol is about 10-fold more potent than estrone and about 100-fold more potent than estriol. According to one study, the relative binding affinities of estrone for the human ERα and ERβ were 4.0% and 3.5% of those estradiol, respectively, and the relative transactivational capacities of estrone at the ERα and ERβ were 2.6% and 4.3% of those of estradiol, respectively. In accordance, the estrogenic activity of estrone has been reported to be approximately 4% of that of estradiol. Because estrone can be transformed into estradiol, which is far more potent as an estrogen in comparison, most of the estrogenic potency of estrone "in vivo" is actually due to conversion into estradiol. As such, similarly to the case of estrone sulfate, estrone is considered to be a prodrug of estradiol. Some "in vitro" research has suggested that estrone might be able to partially antagonize the actions of estradiol, but this does not appear to be of clinical significance.