of estradiol and preventing tumor development in a 7,12-dimethylbenz(a)anthracene (dmba) mammary tumor model. as such, it is anticipated that estetrol may cause minimal proliferation of breast tissue and that it may be useful in the treatment of breast cancer. estetrol has relatively minimal effects on liver function. in contrast to estradiol and ethinylestradiol, estetrol does not stimulate the hepatic production of shbg in vitro. in addition, it has been found to produce minimal changes in liver protein synthesis in women relative to ethinylestradiol, including production of sex hormone-binding globulin (shbg), corticosteroid-binding globulin (cbg), angiotensinogen (agt), ceruloplasmin, cholesterol, triglycerides, estrogen-sensitive coagulation proteins, insulin-like growth factor 1 (igf-1), and insulin-like growth factor-binding proteins (igfbps). in a clinical study, 10 mg/day estetrol showed only 15 to 20% of the increase of 20 μg/day ethinylestradiol on shbg and agt levels (both dosages being oral contraceptive dosages). for comparison, it has been reported that a dosage of estradiol that is of similar potency to ethinylestradiol in terms of fsh suppression and hot flash relief possesses about 25% of the potency of ethinylestradiol on shbg increase and about 35% of the potency of ethinylestradiol on agt increase. estetrol has shown only minor effects on hemostatic biomarkers, including both on coagulation and fibrinolysis. due to its minimal influence on liver function, estetrol is expected to have a lower risk of venous thromboembolism (vte), a serious but rare adverse effect of all known estrogens, and other undesirable side effects. also, oral estrogens like ethinylestradiol are associated with a reduction in lean body mass due to suppression of hepatic igf-1 production, and this may not be expected with estetrol. estetrol has potent estrogenic effects in the brain in terms of relief of hot flashes, antigonadotropic effects, and ovulation inhibition. however, animal studies investigating the effects of estetrol on levels of allopregnanolone and β-endorphin in various brain areas have shown weak estrogenic effects when given alone and antiestrogenic effects in the presence of estradiol, suggesting that estetrol may have serm-like effects in some regions of the brain by mediating weak estrogenic effects on the levels of allopregnanolone and β-endorphin when administered alone, or by causing antiestrogenic effects in the presence of estradiol in-vivo. estetrol has mixed effects in the vascular system similarly. it has been found to have estrogenic effects on atheroma prevention in arteries (and hence might be expected to have beneficial effects on atherosclerosis), but has antiestrogenic effects against estradiol-induced endothelial nitric oxide synthase activation and acceleration of endothelial healing. relative oral potencies of estrogens estrogenhfveucafshlhhdl-cshbgcbgagtliver estradiol1.01.01.01.01.01.01.01.01.01.0 estrone ? ? ?0.30.3 ? ? ? ? ? estriol0.30.30.10.30.30.2 ? ? ?0.67 estrone sulfate ?0.90.90.8–0.90.90.50.90.5–0.71.4–1.50.56–1.7 conjugated estrogens1.21.52.01.1–1.31.01.53.0–3.21.3–1.55.01.3–4.5 equilin sulfate ? ?1.0 ? ?6.07.56.07.5 ? ethinylestradiol12015040060–150100400500–600500–6003502.9–5.0 diethylstilbestrol ? ? ?2.9–3.4 ? ?26–2825–37205.7–7.5 sources and footnotes notes: values are ratios, with estradiol as standard (i.e., 1.0). abbreviations: hf = clinical relief of hot flashes. ve = increased proliferation of vaginal epithelium. uca = decrease in uca. fsh = suppression of fsh levels. lh = suppression of lh levels. hdl-c, shbg, cbg, and agt = increase in the serum levels of these liver proteins. liver = ratio of liver estrogenic effects to general/systemic estrogenic effects (hot flashes/gonadotropins). sources: see template. antigonadotropic effects administration of single doses of estetrol to postmenopausal women strongly suppressed secretion of luteinizing hormone (lh) and follicle-stimulating hormone (fsh), demonstrating potent antigonadotropic effects. levels of lh were not suppressed by a dose of 0.1 or 1 mg, were slightly suppressed by a dose of 10 mg, and were profoundly suppressed by a dose of 100 mg (by a maximum of 48% 4-hours post-dose). a profound and sustained inhibition of fsh levels (by a maximum of 41% 48-hours post-dose), lasting up to a week, was found with a 100 mg dose of estetrol (other doses were not assessed). the antigonadotropic effects of estetrol result in inhibition of ovulation and hence are involved in its hormonal contraceptive effects in women. in addition, the antigonadotropic effects of estetrol cause suppression of gonadal sex hormone production. in healthy men, 40 mg/day estetrol suppressed total testosterone levels by 60% and estradiol levels by 62% when measured at day 28 of administration. in another study of healthy men, testosterone levels were suppressed with estetrol therapy by 28% at 20 mg/day, by 60% at 40 mg/day, and by 76% at 60 mg/day after 4 weeks. suppression of testosterone levels is the primary basis for the use of estetrol in the treatment of prostate cancer. pharmacokinetics estetrol levels following a single dose of different doses of oral estetrol (e4) in postmenopausal women the oral bioavailability of estetrol in rats was 70% relative to subcutaneous injection. the high oral bioavailability of estetrol is in contrast to estradiol, estrone, and estriol (all very low, in the range of 5% and below), but is more similar to ethinylestradiol (38–48%). estetrol shows a high and linear dose–response relationship across oral doses of 0.1 to 100 mg in humans, and shows low interindividual variability. upon oral administration, estetrol is very rapidly absorbed, with maximal levels in blood occurring within 15 to 80 minutes. at a dosage of 20 mg/day estetrol, peak levels of estetrol of 3,490 pg/ml and trough levels of 2,005 pg/ml have been reported. the high water solubility of estetrol makes it optimal for passage of the blood–brain barrier, and the drug may be expected to have effects in the central nervous system. in accordance, estetrol shows clear central effects such as alleviation of hot flashes and antigonadotropic effects in humans. in terms of plasma protein binding, estetrol is bound moderately to albumin, and is not bound to shbg. this is in contrast to estradiol, which binds to shbg with high affinity, but is similar to estriol and ethinylestradiol, which have only very low affinity for shbg. due to its lack of affinity for shbg, the plasma distribution or availability for target tissues of estetrol is not limited or otherwise influenced by shbg. estetrol is metabolized slowly and minimally, and is not transformed into other estrogens such as estradiol, estrone, or estriol. this is related to the fact that estetrol is already an end-stage product of phase i estrogen metabolism in humans. the medication is conjugated via glucuronidation and to a lesser extent via sulfation. the biological half-life of estetrol is about 28 hours, with a range of 18 to 60 hours. the blood half-lives of estradiol and estriol, at about 1 to 2 hours and 20 minutes, respectively, are far shorter than that of estetrol, whereas the biological half-life of ethinylestradiol, at approximately 20 hours, is more similar to that of estetrol. enterohepatic recirculation may occur with estetrol, similarly to other steroidal estrogens, although it has also been reported that estetrol does not seem to enter the enterohepatic circulation. estetrol is excreted mostly or completely in urine, virtually entirely in the form of conjugates (unconjugated accounting for 0.2–0.7%). in one study, a median of 79.7% (range 61.1–99.0%) was recovered from urine; this was primarily as estetrol glucuronide (median 60.7%, range 47.6–77.2%), and, to a lesser extent, as estetrol sulfate (median 17.6%, range 13.2–22.1%). chemistry