Polyestradiol phosphate

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Polyestradiol phosphate
Polyestradiol phosphate.svg
Estradiol phosphate molecule ball.png
Skeletal structure of polyestradiol phosphate (top) and ball-and-stick model of estradiol phosphate (one monomer of polyestradiol phosphate) (bottom)
Clinical data
Trade namesEstradurin, Estradurine
Other namesPEP; Estradiol phosphate polymer; Estradiol 17β-phosphate polymer; Estradiol polymer with phosphoric acid; Leo-114
AHFS/Drugs.comInternational Drug Names
  • Contraindicated[1]
Routes of
Intramuscular injection[1][2]
Drug classEstrogen; Estrogen ester
ATC code
Legal status
Legal status
  • In general: ℞ (Prescription only)
Pharmacokinetic data
BioavailabilityIM: High
Protein bindingEstradiol: ~98% (to albumin and SHBG)[3][4][1]
MetabolismMainly in the liver, to a lesser extent in the kidneys, gonads, and muscle (by phosphatases)[1]
MetabolitesEstradiol, phosphoric acid, and metabolites of estradiol[5][6]
Elimination half-lifePEP: 70 days (10 weeks)[7]
Estradiol: 1–2 hours[8]
ExcretionUrine (as conjugates)[1]
CAS Number
PubChem SID
  • None
Chemical and physical data
(n = variable; n = 13)
Molar massPolymer: Variable
Repeat unit: 334.347 g/mol
Melting point195 to 202 °C (383 to 396 °F)

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.[1][9][2][10] 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.[1][11] It is given by injection into muscle once every four weeks.[1][2][12]

Common side effects of PEP include headache, breast tenderness, breast development, feminization, sexual dysfunction, infertility, and vaginal bleeding.[1][2] PEP is a synthetic estrogen and hence is an agonist of the estrogen receptor, the biological target of estrogens like estradiol.[2][6][5] It is an estrogen ester in the form of a polymer and is an extremely long-lasting prodrug of estradiol in the body.[2][12][5][6] The time it takes for 50% of a dose of PEP to be eliminated from the body is more than two months.[7] Because PEP works by being converted into estradiol, it is considered to be a natural and bioidentical form of estrogen.[5][2] The safety profile of parenteral estradiol esters like PEP is greatly improved relative to synthetic oral estrogens like ethinylestradiol and diethylstilbestrol.[2]

PEP was discovered around 1953 and was introduced for medical use in the United States in 1957.[12][13][14] 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.[15] However, it is no longer available in the United States.[13][16]

Medical uses[edit]

PEP is used as an intramuscular injection for estrogen therapy of prostate cancer in men.[1][2] It is also used to treat breast cancer in women who are at least 5 years postmenopausal.[1][17][18] In addition, PEP is used in hormone replacement therapy for low estrogen levels due to hypogonadism or menopause in women.[1] It is also used in feminizing hormone therapy for transgender women.[11][19] PEP is a form of high-dose estrogen therapy.[2] After an injection, it very slowly releases the active agent estradiol over at least several months.[20][7]

PEP has been compared to combined androgen blockade (CAB; castration plus flutamide) for the treatment of prostate cancer in a large randomized clinical trial of 915 patients.[21][22] At 18.5 months, there was no difference in survival or cardiovascular toxicity between the two treatment modalities.[21][22] These findings suggest that parenteral forms of estradiol may have similar effectiveness and safety relative to androgen deprivation therapy (ADT) in the treatment of prostate cancer.[21][22] In addition, estrogens may have significant advantages relative to ADT in terms of bone loss and fractures, hot flashes, sexual function, and quality of life, as well as considerable cost savings with parenteral forms of estradiol compared to GnRH analogue therapy.[21][22] On the other hand, breast tenderness and gynecomastia occur at very high rates with estrogens, whereas incidences are low with castration and CAB.[23] However, gynecomastia with estrogens is generally only mild-to-moderate in severity and is usually only modestly discomforting.[2] In addition, gynecomastia caused by estrogens can be prevented with prophylactic irradiation of the breasts or can be remediated with mastectomy.[2]

PEP has been studied for the treatment of prostate cancer at dosages of 160 mg/month (three studies) and 240 mg/month (four studies).[24] At a dosage of 160 mg/month, PEP incompletely suppresses testosterone levels, failing to reach the castrate range, and is significantly inferior to orchiectomy in slowing disease progression.[24][2] Conversely, PEP at a dosage of 240 mg/month results in greater testosterone suppression, into the castrate range similarly to orchiectomy, and is equivalent to orchiectomy in effectiveness.[24][2]

For prostate cancer in men, PEP is usually given at a dosage of 80 to 320 mg every 4 weeks for the first 2 to 3 months to rapidly build up estradiol levels.[1] Thereafter, to maintain estradiol levels, the dosage is adjusted down usually to 40 to 160 mg every 4 weeks based on clinical findings and laboratory parameters.[1] For breast cancer and low estrogen levels in women, the dosage is 40 to 80 mg every 4 weeks.[1] For transgender women, the dosage is 80 to 160 mg every 4 weeks.[11][19][25][5]

Estrogen dosages for prostate cancer
Route/form Estrogen Dosage
Oral Estradiol 1–2 mg 3x/day
Conjugated estrogens 1.25–2.5 mg 3x/day
Ethinylestradiol 0.15–3 mg/day
Ethinylestradiol sulfonate 1–2 mg 1x/week
Diethylstilbestrol 1–3 mg/day
Dienestrol 5 mg/day
Hexestrol 5 mg/day
Fosfestrol 100–480 mg 1–3x/day
Chlorotrianisene 12–48 mg/day
Quadrosilan 900 mg/day
Estramustine phosphate 140–1400 mg/day
Transdermal patch Estradiol 2–6x 100 μg/day
Scrotal: 1x 100 μg/day
IM or SC injection Estradiol benzoate 1.66 mg 3x/week
Estradiol dipropionate 5 mg 1x/week
Estradiol valerate 10–40 mg 1x/1–2 weeks
Estradiol undecylate 100 mg 1x/4 weeks
Polyestradiol phosphate Alone: 160–320 mg 1x/4 weeks
With oral EE: 40–80 mg 1x/4 weeks
Estrone 2–4 mg 2–3x/week
IV injection Fosfestrol 300–1200 mg 1–7x/week
Estramustine phosphate 240–450 mg/day
Note: Dosages are not necessarily equivalent. Sources: See template.
Estrogen dosages for breast cancer
Route/form Estrogen Dosage
Oral Estradiol 10 mg 3x/day
AI-resistant: 2 mg 1–3x/day
Estradiol valerate AI-resistant: 2 mg 1–3x/day
Conjugated estrogens 10 mg 3x/day
Ethinylestradiol 0.5–1 mg 3x/day
Diethylstilbestrol 5 mg 3x/day
Dienestrol 5 mg 3x/day
Dimestrol 30 mg/day
Chlorotrianisene 24 mg/day
IM or SC injection Estradiol benzoate 5 mg 2–3x/week
Estradiol dipropionate 5 mg 2–3x/week
Estradiol valerate 30 mg 1x/2 weeks
Polyestradiol phosphate 40–80 mg 1x/4 weeks
Estrone 5 mg ≥3x/week
Notes: (1) Only in women who are at least 5 years postmenopausal. (2) Dosages are not necessarily equivalent. Sources: See template.

Available forms[edit]

PEP is provided in the form of powder or an aqueous solution in vials and ampoules alone or in combination with mepivacaine and/or nicotinamide (vitamin B3) for administration via intramuscular injection.[1][26][27] Mepivacaine is a local anaesthetic and is used to avoid a burning sensation during injection of PEP.[1] Each vial/ampoule of Estradurin contains 80 mg PEP, 5 mg mepivacaine hydrochloride, 40 mg nicotinamide, and 2 mL water.[27]

Available forms of estradiol
Route Ingredient Form Dose Major brand names
Oral Estradiol Tablet 0.1, 0.2, 0.5, 1, 2, or 4 mg per tablet Estrace, Ovocyclin
Estradiol acetatea Tablet 0.45, 0.9, or 1.8 mg per tablet Femtrace
Estradiol valerate Tablet 0.5, 1, 2, or 4 mg per tablet Progynova
Sublingual Estradiola Tablet 0.125, 0.25, or 1 mg per tablet Diogynets, Estradiol Membrettes
Intranasal Estradiola Nasal spray 150 µg per spray (60 sprays per bottle) Aerodiol
Transdermal Estradiol Patch 14, 25, 37.5, 50, 60, 75, or 100 µg E2 per day for 3–4 or 7 days Climara, Estraderm, Vivelle
Gel dispenser 0.06% (0.87 or 1.25 g gel or 0.52 or 0.75 mg E2 per activation) Elestrin, EstroGel
Gel packet 0.1% (0.25, 0.5, or 1 g gel or 0.25, 0.5, or 1.0 mg E2 per packet) DiviGel, Sandrena
Emulsion 0.25% (1.74 g emulsion or 4.35 mg E2 or 25 µg/day E2 per pouch) Estrasorb
Spray 1.53 mg per spray Evamist
Vaginal Estradiol Tablet 10 or 25 µg per tablet Vagifem
Cream 0.01% (0.1 mg E2 per 1 g cream) Estrace
Suppositorya 4 or 40 μg per suppository Ovocyclin
Insert 4 or 10 µg per insert (daily for 2 weeks then twice weekly) Imvexxy
Ring 2 mg per ring (7.5 µg/day E2 for 3 months) Estring
Estradiol acetate Ring 12.4 or 24.8 mg per ring (50 or 100 µg/day E2 for 3 months) Femring
Estradiol benzoatea Tablet 0.125 mg Malun 25
Injection (IM or SC) Estradiol Aqueous suspensiona 0.22, 0.25, 0.44, 0.5, 1.0, 1.1, or 2.0 mg/mL Aquadiol, Diogyn, Progynon
Microspheres 1 mg/mL Juvenum E
Estradiol benzoate Oil solution 0.167, 0.2, 0.333, 1, 1.67, 2, 5, 10, 20, or 25 mg/mL Progynon-B
Aqueous suspensiona 5 mg/mL Agofollin Depot, Ovocyclin M
Estradiol cypionate Oil solution 1, 3, or 5 mg/mL Depo-Estradiol
Aqueous suspension 5 mg/0.5 mL (available only with a progestin) Cyclofem, Lunelle
Estradiol dipropionatea Oil solution 0.1, 0.2, 0.5, 1, 2.5, or 5 mg/mL Di-Ovocylin, Progynon-DP
Estradiol enanthate Oil solution 5 or 10 mg/mL (available only with a progestin) Perlutal, Topasel
Estradiol undecylatea Oil solution 100 mg/mL Delestrec, Progynon Depot 100
Estradiol valerate Oil solution 5, 10, 20, or 40 mg/mL Delestrogen, Progynon Depot
Polyestradiol phosphatea Aqueous solution 40 or 80 mg per vial/ampoule Estradurin
Implant Estradiola Pellet 20, 25, 50, or 100 mg per pellet (usually every 6 months) Estradiol Implants, Meno-Implant
Abbreviations: E2 = Estradiol. Footnotes: a = Discontinued or mostly discontinued. Notes: (1): This table mostly does not include combination products, for instance estradiol formulated in combination with a progestogen or androgen. (2): This table does not include compounded estradiol products; only approved pharmaceutical preparations are included. (3): The availability of pharmaceutical estradiol products differs by country (see Estradiol (medication) § Availability). (4): Some of these formulations and doses have been marketed previously but may no longer be available. Sources: See template.


The contraindications of PEP are largely the same as those of estradiol and include:[1][28][29][30][31]

Side effects[edit]

Systematic studies of the side effects of PEP are lacking.[1] However, its side effects are assumed to be identical to those of estradiol and other estradiol esters.[1] The side effects of PEP are partially dependent on sex.[1] Common or frequent (>10%) side effects are considered to include headache, abdominal pain, nausea, rash, pruritus, loss of libido, erectile dysfunction, breast tenderness, gynecomastia, feminization, demasculinization, infertility, and vaginal bleeding or spotting.[1][32] Side effects that occur occasionally or uncommonly (0.1–1%) include sodium and water retention, edema, hypersensitivity, breast tension, depression, dizziness, visual disturbances, palpitations, dyspepsia, erythema nodosum, urticaria, and chest pain.[1] All other side effects of PEP are considered to be rare.[1]

The rare (<0.1%) side effects of PEP are considered to include weight gain, impaired glucose tolerance, mood changes (elation or depression), nervousness, tiredness, headache, migraine, intolerance of contact lenses, hypertension, thrombosis, thrombophlebitis, thromboembolism, heart failure, myocardial infarction, vomiting, bloating, cholestatic jaundice, cholelithiasis, transient increases in transaminases and bilirubin, erythema multiforme, hyperpigmentation, muscle cramps, dysmenorrhea, vaginal discharge, premenstrual-like symptoms, breast enlargement, testicular atrophy, allergic reactions (e.g., urticaria, bronchial asthma, anaphylactic shock) due to mepivacaine, and injection site reactions (e.g., pain, sterile abscesses, inflammatory infiltrates).[1]

As thromboembolic and other cardiovascular complications are associated mainly with synthetic oral estrogens like ethinylestradiol and diethylstilbestrol, they occur much less often with parenteral bioidentical forms of estrogen like PEP.[1][2]

Cardiovascular effects[edit]

PEP produces minimal undesirable effects on coagulation factors and is thought to increase the risk of blood clots little or not at all.[33][34] This is in spite of the fact that estradiol levels can reach high concentrations of as much as 700 pg/mL with high-dose (320 mg/month) PEP therapy.[35] It is also in contrast to oral synthetic estrogens such as diethylstilbestrol and ethinylestradiol, which produce marked increases in coagulation factors and high rates of blood clots at the high doses used to achieve castrate levels of testosterone in prostate cancer.[33][34][6] The difference between the two types of therapies is due to the bioidentical and parenteral nature of PEP and its minimal influence on liver protein synthesis.[33][34][6] PEP might actually reduce the risk of blood clots, due to decreases in levels of certain procoagulatory proteins.[33][34] Although PEP does not increase the hepatic production or levels of procoagulatory factors, it has been found to significantly decrease levels of the anticoagulatory antithrombin III, which may indicate a potential risk of thromboembolic and cardiovascular complications.[2] On the other hand, PEP significantly increases levels of HDL cholesterol and significantly decreases levels of LDL cholesterol, changes which are thought to protect against coronary artery disease.[2] It appears that PEP may have beneficial effects on cardiovascular health at lower dosages (e.g., 160 mg/day) due to its beneficial effects on HDL and LDL cholesterol levels, but these are overshadowed at higher dosages (e.g., 240 mg/day) due to unfavorable dose-dependent effects on hemostasis, namely antithrombin III levels.[2]

Small early pilot studies of PEP for prostate cancer in men found no cardiovascular toxicity with the therapy.[33] A dosage of PEP of 160 mg/month specifically does not appear to increase the risk of cardiovascular complications.[2] In fact, potential beneficial effects on cardiovascular mortality have been observed at this dosage.[2] However, PEP at a higher dosage of 240 mg/month has subsequently been found in large studies to significantly increase cardiovascular morbidity relative to GnRH modulators and orchiectomy in men treated with it for prostate cancer.[33][34][2] The increase in cardiovascular morbidity with PEP therapy is due to an increase in non-fatal cardiovascular events, including ischemic heart disease and heart decompensation, specifically heart failure.[34][36][37] Conversely, PEP has not been found to significantly increase cardiovascular mortality relative to GnRH modulators and orchiectomy.[33][34] Moreover, numerically more patients with preexisting cardiovascular disease were randomized to the PEP group in one large study (17.1% vs. 14.5%; significance not reported), and this may have contributed to the increased incidence of cardiovascular morbidity observed with PEP.[34] In any case, some studies have found that the increased cardiovascular morbidity with PEP is confined mainly to the first one or two years of therapy, whereas one study found consistently increased cardiovascular morbidity across three years of therapy.[33] A longitudinal risk analysis that projected over 10 years suggested that the cardiovascular risks of PEP may be reversed with long-term treatment and that the therapy may eventually result in significantly decreased cardiovascular risk relative to GnRH modulators and orchiectomy, although this has not been confirmed.[33]

The cardiovascular toxicity of PEP is far less than that of oral synthetic estrogens like diethylstilbestrol and ethinylestradiol, which increase the risk of venous and arterial thromboembolism, consequently increase the risk of transient ischemic attack, cerebrovascular accident (stroke), and myocardial infarction (heart attack), and result in substantial increases in cardiovascular mortality.[33][34] It is thought that the relatively minimal cardiovascular toxicity of parenteral forms of estradiol, like PEP and high-dose transdermal estradiol patches,[38] is due to their absence of effect on hepatic coagulation factors.[33][34]


Acute toxicity studies have not indicated a risk of acute side effects with overdose of PEP.[1] The median lethal dose (LD50) of PEP in mice is approximately 700 mg/kg.[39] PEP has been administered in total amounts of 2,000 to 3,000 mg over several months in patients with cancer without toxicity observed.[39] The most likely sign of overdose is reversible feminization, namely gynecomastia.[1] Other symptoms of estrogen overdosage may include nausea, vomiting, bloating, increased weight, water retention, breast tenderness, vaginal discharge, heavy legs, and leg cramps.[28] These side effects can be diminished by reducing the estrogen dosage.[28] There is no specific antidote for overdose of PEP.[1] Treatment of PEP overdose should be based on symptoms.[1]


Known potential interactions of PEP are mostly the same as those of estradiol and include:[1]

Interactions with PEP may be less than with oral estrogens due to the lack of the first-pass through the liver.[1]



Estradiol, the active form of PEP.

PEP is an estradiol ester in the form of a polymer and is an extremely long-lasting prodrug of estradiol.[2][7][5][6] As such, it is an estrogen, or an agonist of the estrogen receptors.[2][6][5] PEP has antigonadotropic and functional antiandrogenic effects due to its estrogenic activity.[35] A single repeat unit of PEP, corresponding to estradiol phosphate (minus OH2), is of about 23% higher molecular weight than estradiol due to the presence of its C17β phosphate ester.[42][15] Because PEP is a prodrug of estradiol, it is considered to be a natural and bioidentical form of estrogen.[5]

PEP is a strong inhibitor of several enzymes, including acid phosphatase, alkaline phosphatase, and hyaluronidase, in vitro.[43][44][45] In light of the fact that phosphatases, which cleave PEP into estradiol and phosphoric acid, are present in most tissues in the body, it has been said that the long elimination half-life and slow release of PEP are somewhat surprising.[46] It is thought that PEP may inhibit its own metabolism.[46]

Potencies and durations of natural estrogens by intramuscular injection
Estrogen Form Major brand names EPD CIC-D Duration
Estradiol Aqueous solution ? <1 day
Oil solution Estradiol 40–60 mg 1–2 mg ≈ 1–2 days
Aqueous suspension Aquadiol, Diogyn, Progynon, Mego-E ? 3.5 mg 0.5–2 mg ≈ 2–7 days; 3.5 mg ≈ >5 days
Microspheres Juvenum-E, Juvenum ? 1 mg ≈ 30 days
Estradiol benzoate Oil solution Progynon-B 25–35 mg 1.66 mg ≈ 2–3 days; 5 mg ≈ 3–6 days
Aqueous suspension Agofollin-Depot, Ovocyclin M 20 mg 10 mg ≈ 16–21 days
Emulsion Menformon-Emulsion, Di-Pro-Emulsion ? 10 mg ≈ 14–21 days
Estradiol dipropionate Oil solution Agofollin, Di-Ovocylin, Progynon DP 25–30 mg 5 mg ≈ 5–8 days
Estradiol valerate Oil solution Delestrogen, Progynon Depot, Mesigyna 20–30 mg 5 mg 5 mg ≈ 7–8 days; 10 mg ≈ 10–14 days;
40 mg ≈ 14–21 days; 100 mg ≈ 21–28 days
Estradiol benzoate butyrate Oil solution Redimen, Soluna, Unijab ? 10 mg 10 mg ≈ 21 days
Estradiol cypionate Oil solution Depo-Estradiol, Depofemin 20–30 mg 5 mg ≈ 11–14 days
Aqueous suspension Cyclofem, Lunelle ? 5 mg 5 mg ≈ 14–24 days
Estradiol enanthate Oil solution Perlutal, Topasel, Yectames ? 5–10 mg 10 mg ≈ 20–30 days
Estradiol dienanthate Oil solution Climacteron, Lactimex, Lactostat ? 7.5 mg ≈ >40 days
Estradiol undecylate Oil solution Delestrec, Progynon Depot 100 ? 10–20 mg ≈ 40–60 days;
25–50 mg ≈ 60–120 days
Polyestradiol phosphate Aqueous solution Estradurin 40–60 mg 40 mg ≈ 30 days; 80 mg ≈ 60 days;
160 mg ≈ 120 days
Estrone Oil solution Estrone, Kestrin, Theelin ? 1–2 mg ≈ 2–3 days
Aqueous suspension Estrone Aq. Susp., Kestrone, Theelin Aq. ? 0.1–2 mg ≈ 2–7 days
Estriol Oil solution ? 1–2 mg ≈ 1–4 days
Polyestriol phosphate Aqueous solution Gynäsan, Klimadurin, Triodurin ? 50 mg ≈ 30 days; 80 mg ≈ 60 days
Notes: All aqueous suspensions are of microcrystalline particle size. Estradiol production during the menstrual cycle is 30–640 µg/day (6.4–8.6 mg total per month or cycle). The vaginal epithelium maturation dosage of estradiol benzoate or estradiol valerate has been reported as 5 to 7 mg/week. An effective ovulation-inhibiting dose of estradiol undecylate is 20–30 mg/month. Sources: See template.

Antigonadotropic effects[edit]

PEP has antigonadotropic effects due to its estrogenic activity.[36] It has been found to suppress testosterone levels in men by 55%, 75%, and 85% at intramuscular dosages of 80, 160, and 240 mg every 4 weeks, respectively.[46] A single intramuscular injection of 320 mg PEP in men has been found to suppress testosterone levels to within the castrate range (< 50 ng/dL) within 3 weeks.[7] This was associated with circulating estradiol levels of just over 200 pg/mL.[35] The suppression of testosterone levels that can be achieved with PEP is equal to that with orchiectomy.[47] However, to achieve such concentrations of testosterone, which are about 15 ng/dL on average, higher concentrations of estradiol of around 500 pg/mL were necessary.[35][47][48] This was associated with a dosage of intramuscular 320 mg PEP every four weeks and occurred by 90 days of treatment.[35] However, 240 mg PEP every four weeks has also been reported to eventually suppress testosterone levels in the castrate range.[49][50]

Mechanism of action in prostate cancer[edit]

The growth of prostate cancer is generally stimulated by dihydrotestosterone (DHT), and unless the cancer is castration-resistant, it can be treated by depriving it of androgens. Estradiol produces its therapeutic benefits mainly via exertion of negative feedback on the hypothalamic–pituitary–gonadal axis.[36][46][7] This blocks the secretion of luteinizing hormone, which in turn reduces testosterone production in the Leydig cells of the testes.[36][46][7] Estradiol also decreases the free percentage of testosterone by increasing sex hormone-binding globulin (SHBG) levels.[7] In addition, it exhibits direct cytotoxicity on prostate cancer cells.[51][20]

Differences from other estrogens[edit]

SHBG levels with 1) i.m. injection of 320 mg PEP every 4 weeks; 2) i.m. injection of 320 mg PEP every 4 weeks plus 150 µg/day oral ethinylestradiol; 3) orchiectomy.[35]
Effects of estrogens on coagulation factors
Factors Oral estrogensa Parenteral estrogensb
Factor VII Increases No change
Factor VIII activity Increases No change
Antithrombin III activity Decreases Decreases
Prothrombin fragment 1+2 Increases No change
Activated protein C resistance Increases No change
Fibrinogen Decreases No change
Footnotes: a = E.g., DES, EE. b = PEP. Sources: [34]

Estrogens have effects on liver protein synthesis, including on the synthesis of plasma proteins, coagulation factors, lipoproteins, and triglycerides.[47] These effects can result in an increased risk of thromboembolic and cardiovascular complications, which in turn can result in increased mortality.[47] Studies have found a markedly increased 5-year risk of cardiovascular mortality of 14 to 26% in men treated with oral synthetic estrogens like ethinylestradiol and diethylstilbestrol for prostate cancer.[47] However, whereas oral synthetic estrogens have a strong influence on liver protein synthesis, the effects of parenteral bioidentical estrogens like PEP on liver protein synthesis are comparatively very weak or even completely abolished.[47] This is because the first-pass through the liver with oral administration is avoided and because bioidentical estrogens are efficiently inactivated in the liver.[47] In accordance, PEP has minimal effect on the liver at a dosage of up to at least 240 mg/month.[52]

A study found that whereas 320 mg/month intramuscular PEP increased SHBG levels to 166% in men with prostate cancer, the combination of 80 mg/month intramuscular polyestradiol phosphate and 150 µg/day oral ethinylestradiol increased levels of SHBG to 617%, an almost 8-fold difference in increase and almost 4-fold difference in absolute levels between the two treatment regimens.[35][7][53] In addition, whereas there were no cardiovascular complications in the PEP-only group, there was a 25% incidence of cardiovascular complications over the course of a year in the group that was also treated with ethinylestradiol.[7] Another study found no change in levels of coagulation factor VII, a protein of particular importance in the cardiovascular side effects of estrogens, with 240 mg/month intramuscular PEP.[52] These findings demonstrate the enormous impact of synthetic oral estrogens like ethinylestradiol on liver protein production relative to parenteral bioidentical forms of estrogen like PEP.[7]

Originally, PEP was typically used at a dosage of 80 mg per month in combination with 150 μg per day oral ethinylestradiol in the treatment of prostate cancer.[46][54] This combination was found to produce a considerable incidence of cardiovascular toxicity,[47] and this toxicity was inappropriately attributed to PEP in some publications.[55] Subsequent research has shown that the toxicity is not due to PEP but rather to the ethinylestradiol component.[56][46][47]

A study found that therapy with intramuscular PEP resulting in estradiol levels of around 400 pg/mL in men with prostate cancer did not affect growth hormone or insulin-like growth factor 1 levels, whereas the addition of oral ethinylestradiol significantly increased growth hormone levels and decreased insulin-like growth factor 1 levels.[57][58]


PEP has a very long duration and is given by intramuscular injection once every 4 weeks.[35] In men, an initial intramuscular injection of PEP results in a rapid rise in estradiol levels measured at 24 hours followed by a slow and gradual further increase in levels up until at least day 28 (the time of the next injection).[35] Subsequent injections result in a progressive and considerable accumulation in estradiol levels up to at least 6 months.[35] The mean elimination half-life of PEP has been found to be 70 days (10 weeks) with a single 320 mg intramuscular dose of the medication.[7] The tmax (time to maximal concentrations) for estradiol was about 16 days.[7] PEP has a duration of approximately 1 month with a single dose of 40 mg, 2 months with 80 mg, and 4 months with 160 mg.[59]</ref>[60][61][62][39]

PEP reaches the bloodstream within hours after an injection (90% after 24 hours), where it circulates, and is accumulated in the reticuloendothelial system.[51] Estradiol is then cleaved from the polymer by phosphatases, although very slowly.[63] Levels of estradiol in men with intramuscular injections of PEP 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.[35] With monthly injections, steady-state estradiol concentrations are reached after 6 to 12 months.[51] Estradiol is metabolized primarily in the liver by CYP3A4 and other cytochrome P450 enzymes, and is metabolized to a lesser extent in extrahepatic tissues.[20][1] The metabolites are mainly excreted in urine via the kidneys.[1]

Early studies found that a dosage of 80 mg PEP every 4 weeks rapidly produced relatively high mean estradiol levels of about 400 to 800 pg/mL.[64] These levels are similar to those of 100 mg estradiol undecylate every month, which has been found to produce estradiol levels of around 500 to 600 pg/mL.[65][66] As a result, it has previously been said that 80 mg PEP per month and 100 mg estradiol undecylate per month are roughly equivalent.[67][68][7] However, subsequent studies showed that this dosage of PEP actually achieves much lower estradiol levels than originally demonstrated.[35]


PEP is a synthetic estrane steroid and the C17β phosphoric acid (phosphate) ester of estradiol (estradiol 17β-phosphate) in the form of a polymer.[69][15][46][70] It is also known as estradiol polymer with phosphoric acid or as estradiol 17β-phosphate polymer, as well as estra-1,3,5(10)-triene-3,17β-diol 17β-phosphate polymer.[15][69][46][70] It has been determined via ultracentrifugation that the mean molecular weight of PEP corresponds to a chain length of approximately 13 repeat units of estradiol 17β-phosphate.[46] PEP is closely related to polyestriol phosphate (Gynäsan, Klimadurin, Triodurin) and polytestosterone phloretin phosphate (never commercialized), which are estriol and testosterone esters in the forms of polymers, respectively.[59]A. Labhart (6 December 2012). Clinical Endocrinology: Theory and Practice. Springer Science & Business Media. pp. 551–. ISBN 978-3-642-96158-8. The polymer of estradiol or estriol and phosphoric acid has an excellent depot action when given intramuscularly (polyestriol phosphate or polyestradiol phosphate) (Table 16). Phosphoric acid combines with the estrogen molecule at C3 and C17 to form a macromolecule. The compound is stored in the liver and spleen where the estrogen is steadily released by splitting off of the phosphate portion due to the action of alkaline phosphatase. [...] Conjugated estrogens and polyestriol and estradiol phosphate can also be given intravenously in an aqueous solution. Intravenous administration of ovarian hormones offers no advantages, however, and therefore has no practical significance. [...] The following duarations of action have been obtained with a single administration (WlED, 1954; LAURITZEN, 1968): [...] 50 mg polyestradiol phosphate ~ 1 month; 50 mg polyestriol phosphate ~ 1 month; 80 mg polyestriol phosphate ~ 2 months.</ref>[71] It is also related to polydiethylstilbestrol phosphate (never commercialized), a diethylstilbestrol ester in the form of a polymer.[72]

Structural properties of selected estradiol esters
Estrogen Structure Ester(s) Relative
mol. weight
E2 contentb
Position(s) Moiet(ies) Type Lengtha
1.00 1.00 4.0
Estradiol acetate
Estradiol 3-acetate.svg
C3 Ethanoic acid Straight-chain fatty acid 2 1.15 0.87 4.2
Estradiol benzoate
Estradiol benzoate.svg
C3 Benzenecarboxylic acid Aromatic fatty acid – (~4–5) 1.38 0.72 4.7
Estradiol dipropionate
Estradiol dipropionate.svg
C3, C17β Propanoic acid (×2) Straight-chain fatty acid 3 (×2) 1.41 0.71 4.9
Estradiol valerate
Estradiol valerate.svg
C17β Pentanoic acid Straight-chain fatty acid 5 1.31 0.76 5.6–6.3
Estradiol benzoate butyrate
Estradiolbutyratebenzoate structure.png
C3, C17β Benzoic acid, butyric acid Mixed fatty acid – (~6, 2) 1.64 0.61 6.3
Estradiol cypionate
Estradiol 17 beta-cypionate.svg
C17β Cyclopentylpropanoic acid Aromatic fatty acid – (~6) 1.46 0.69 6.9
Estradiol enanthate
Estradiol enanthate.png
C17β Heptanoic acid Straight-chain fatty acid 7 1.41 0.71 6.7–7.3
Estradiol dienanthate
Estradiol dienanthate.svg
C3, C17β Heptanoic acid (×2) Straight-chain fatty acid 7 (×2) 1.82 0.55 8.1–10.4
Estradiol undecylate
Estradiol undecylate.svg
C17β Undecanoic acid Straight-chain fatty acid 11 1.62 0.62 9.2–9.8
Estradiol stearate
Estradiol stearate structure.svg
C17β Octadecanoic acid Straight-chain fatty acid 18 1.98 0.51 12.2–12.4
Estradiol distearate
Estradiol distearate.svg
C3, C17β Octadecanoic acid (×2) Straight-chain fatty acid 18 (×2) 2.96 0.34 20.2
Estradiol sulfate
Estradiol sulfate.svg
C3 Sulfuric acid Water-soluble conjugate 1.29 0.77 0.3–3.8
Estradiol glucuronide
Estradiol sulfate.svg
C17β Glucuronic acid Water-soluble conjugate 1.65 0.61 2.1–2.7
Estramustine phosphated
Estramustine phosphate.svg
C3, C17β Normustine, phosphoric acid Water-soluble conjugate 1.91 0.52 2.9–5.0
Polyestradiol phosphatee
Polyestradiol phosphate.svg
C3–C17β Phosphoric acid Water-soluble conjugate 1.23f 0.81f 2.9g
Footnotes: a = Length of ester in carbon atoms for straight-chain fatty acids or approximate length of ester in carbon atoms for aromatic fatty acids. b = Relative estradiol content by weight (i.e., relative estrogenic exposure). c = Experimental or predicted octanol/water partition coefficient (i.e., lipophilicity/hydrophobicity). Retrieved from PubChem, ChemSpider, and DrugBank. d = Also known as estradiol normustine phosphate. e = Polymer of estradiol phosphate (~13 repeat units). f = Relative molecular weight or estradiol content per repeat unit. g = logP of repeat unit (i.e., estradiol phosphate). Sources: See individual articles.


PEP is of very low solubility in water, acetone, chloroform, dioxane, and ethanol, but dissolves readily in bases, especially in aqueous pyridine.[51]


Like polyphosphates of polyphenols, PEP can be prepared from the monomer (in this case estradiol) and phosphoryl chloride. The latter reacts with both the phenolic hydroxyl group in position 3 and the aliphatic one in position 17β. The molecular mass of the resulting polymer can be controlled by interrupting the reaction after a given time: the longer the reaction is allowed to continue, the higher the mass.[63][73]


Pharmacological experiments on estradiol phosphates conducted around 1950 gave rise to the hypothesis that estradiol 3,17β-diphosphate acted as an inhibitor of kidney alkaline phosphatase.[63] When the same scientists wanted to synthesize simple phosphates of phloretin, a compound found in apple tree leaves,[74] they accidentally created a polymer instead.[73] This was later shown to exhibit the same anti-phosphatase properties as estradiol diphosphate, and so it was hypothesized that the original finding was due to contamination with estradiol phosphate polymers.[63] Consequently, these polymers were studied in more detail, which resulted in the development of PEP as early as 1953[12] and its subsequent introduction for medical use in 1957 in the United States.[13][14][75]

Society and culture[edit]

Generic names[edit]

Polyestradiol phosphate is the generic name of the drug and its INN and BAN.[15][69][42] It is also known by its developmental code name Leo-114.[15][42]

Brand names[edit]

PEP is marketed exclusively under the brand name Estradurin or Estradurine.[15][42]


Availability of polyestradiol phosphate in countries throughout the world as of March 2018. Blue is currently marketed, green is formerly marketed.

PEP has been marketed in the United States and widely throughout Europe, including in Austria, Belgium, the Czech Republic, Denmark, Finland, Germany, Italy, the Netherlands, Norway, Russia, Spain, Sweden, Switzerland, Ukraine, and the United Kingdom.[15][26][1][76][77][26][14] It is no longer available in the United States and possibly certain other countries however,[13][16] but is still known to be marketed in Austria, Belgium, Denmark, Finland, the Netherlands, Norway, Sweden, and Switzerland.[42][76][77][1]


PEP has been studied as a means of hormonal breast enhancement in women.[78]

A combination of PEP and medroxyprogesterone acetate was studied in women as a long-lasting combined injectable contraceptive for use by intramuscular injection once every three months.[79][80][81]

See also[edit]


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