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.
Pharmacokinetics of estradiol
Polyestradiol phosphate is an ester prodrug of estradiol in the form of a polymer which is used via intramuscular injection primarily to treat prostate cancer, but also to treat breast cancer and menopausal symptoms. It has an extremely long duration of action, with a terminal half-life of about 70 days (10 weeks) following a single intramuscular administration of the medication. In addition, unlike most other estradiol esters, the estradiol levels achieved with polyestradiol phosphate are highly constant and uniform. Levels of estradiol in men with intramuscular injections of polyestradiol phosphate once every 4 weeks were about 350 pg/mL with 160 mg, 450 pg/mL with 240 mg, and almost 700 pg/mL with 320 mg, all measured after 6 months of treatment. Polyestradiol phosphate has been discontinued in many countries, and remains available today in only a few countries, mostly the Nordic countries of Europe.