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.
Skeletal structure of polyestradiol phosphate (top) and ball-and-stick model of estradiol phosphate (one monomer of polyestradiol phosphate) (bottom) | |
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| Trade names | Estradurin, Estradurine |
| Other names | PEP; Polymeric estradiol phosphate; Polymeric estradiol 17β-phosphate; Estradiol phosphate polymer; Estradiol 17β-phosphate polymer; Estradiol polymer with phosphoric acid; Leo-114 |
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| Routes of administration | Intramuscular injection |
| Drug class | Estrogen; Estrogen ester |
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| Bioavailability | IM: High |
| Protein binding | Estradiol: ~98% (to albumin and SHBG) |
| Metabolism | Mainly in the liver, to a lesser extent in the kidneys, gonads, and muscle (by phosphatases) |
| Metabolites | Estradiol, phosphoric acid, and metabolites of estradiol |
| Elimination half-life | PEP: 70 days (10 weeks) Estradiol: 1–2 hours |
| Excretion | Urine (as conjugates) |
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| Formula | (C18H23O4P)n (n = variable; n = 13) |
| Molar mass | Polymer: Variable Repeat unit: 334.347 g/mol |
| Melting point | 195 to 202 °C (383 to 396 °F) |
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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.