Polyestradiol phosphate

Polyestradiol phosphate (PEP), sold under the brand name Estradurin, is an estrogen medication which is used primarily in the treatment of prostate cancer in men. It is also used in women to treat breast cancer, as a component of hormone therapy to treat low estrogen levels and menopausal symptoms, and as a component of feminizing hormone therapy for transgender women. It is given by injection into muscle once every four weeks. Common side effects of PEP include headache, breast tenderness, breast development, feminization, sexual dysfunction, infertility, and vaginal bleeding. PEP is an estrogen and hence is an agonist of the estrogen receptor, the biological target of estrogens like estradiol. It is an estrogen ester in the form of a polymer and is an extremely long-lasting prodrug of estradiol in the body. The biological half-life of PEP is more than two months. Because PEP works by being converted into estradiol, it is considered to be a natural and bioidentical form of estrogen. The safety profile of parenteral estradiol esters like PEP is greatly improved relative to synthetic oral estrogens like ethinylestradiol and diethylstilbestrol. PEP was discovered around 1953 and was introduced for medical use in the United States in 1957. Along with estradiol undecylate and estradiol valerate, it has been frequently used in the United States and Europe as a parenteral form of estrogen to treat men with prostate cancer. However, it is no longer available in the United States. Estradiol (medication)#Medical uses PEP is used as an intramuscular injection for estrogen therapy of prostate cancer in men. It is also used to treat breast cancer in women who are at least 5 years postmenopausal. In addition, PEP is used in hormone replacement therapy for low estrogen levels due to hypogonadism or menopause in women. It is also used in feminizing hormone therapy for transgender women. PEP is a form of high-dose estrogen therapy. After an injection, it very slowly releases the active agent estradiol over at least several months.