Pharmacodynamics of estradiol

Estradiol is an estrogen, or an agonist of the nuclear estrogen receptors (ERs), the estrogen receptor alpha (ERα) and the estrogen receptor beta (ERβ).^[1][2][6] In one study, the EC₅₀Tooltip half-maximal effective concentration value of estradiol for the human ERα was 50 pM (0.05 nM) and for the human ERβ was 200 pM (0.2 nM).^[2][7] Estradiol is also an agonist of the membrane estrogen receptors (mERs), including the G protein-coupled estrogen receptor (GPER) (3–6 nM),^[8] G_q-coupled membrane estrogen receptor (G_q-mER), ER-X, and ERx.^[9][10] It is far more potent as an estrogen than are other natural and bioidentical estrogens like estrone and estriol.^[1] Given by subcutaneous injection in mice, estradiol is about 10-fold more potent than estrone and about 100-fold more potent than estriol.^[11] In addition, much of the estrogenic potency of estrone in vivo is actually due to conversion into estradiol.^[1]

Estradiol has little to no affinity for other steroid hormone receptors, including the androgen, progesterone, glucocorticoid, and mineralocorticoid receptors.^[12][13][14] It has weak affinity for the androgen receptor, with about 8% of relative binding affinity of testosterone according to one study,^[15] and shows agonistic activity at this receptor.^[16] However, estrogens circulate in the picomolar (10⁻¹² M) range while androgens circulate in the nanomolar (10⁻⁹ M) to micromolar (10⁻⁶ M) range,^[17][18] and in accordance with this, estradiol is active as an estrogen in target tissues at approximately 1,000-fold lower concentrations than is testosterone.^[19] In addition, while estradiol did show activation of the androgen receptor in vitro at very high concentrations, its efficacy as an androgen receptor agonist was of such low potency that it was not possible to calculate an EC₅₀Tooltip half-maximal effective concentration value for the activity.^[16] As such, the weak activity of estradiol at the androgen receptor is unlikely to be of biological significance at normal physiological concentrations.^[15][16]

The affinities of estradiol for the ERs are high (around 0.1 nM), and there is a relatively low quantity of about 10,000 to 20,000 ERs in the cytoplasm per cell in estrogen target tissues.^[20] Estradiol stays bound to the ERs for about 24 hours, which is longer than that of other estrogens such as estriol (6 hours).^[1] A prolonged duration of binding to the ERs (e.g., 9 to 12 hours for endometrial effects), as with estradiol, is necessary for full estrogenic responses in various tissues.^[1] The ERs downregulate with exposure to estradiol, and in accordance, the expression of the ERs is dependent on estradiol concentrations.^[21][22] Constant levels of estradiol may result in downregulation of the ERs and relatively diminished responses to estradiol, although this has not been assessed clinically.^[21] Once bound to estradiol, the ERs are ubiquitinated and degraded by proteasomes, which is a major mechanism of ER downregulation.^[22] The unbound ERα has an intracellular half-life of up to 5 days, but this shortens to 3–4 hours once bound to a ligand such as estradiol.^[23][22] Estrogen deprivation can easily increase sensitivity to estrogens like estradiol by 10,000-fold or more, demonstrating a profound capacity of the ERs for upregulation and downregulation.^[24] This increase in sensitivity is mediated by a 100-fold increase in ERs, as well as other mechanisms such as changes in coactivator sensitivity and degree of phosphorylation of transactivation factors.^[24] Progestogens like progesterone and androgens like testosterone downregulate the ERs in certain tissues such as the endometrium and breasts, among others.^[25][26] While progestogens may reduce the expression of ERs and progesterone receptors (PR) in the breasts of primates, the estrogen-induced proliferation of the mammary epithelium is not inhibited, but rather enhanced by progestogens.^[1]

Estradiol is a steroid and a lipophilic compound.^[1][27] As a result, it readily enters cells via simple passive diffusion through the lipid bilayer of the cell membrane.^[27] This is in contrast to hydrophilic estrogen conjugates such as estrone sulfate and estradiol glucuronide, which require active transport via specific membrane transport proteins to enter cells.^[28][29][30] The ERs are nuclear receptors that are mostly present in the cell nucleus.^[27] Upon binding of estradiol to an ER, the receptor dimerizes (combines) with another estradiol-bound ER.^[1][27] These ER dimers can be ERα–ERα or ERβ–ERβ homodimers or ERα–ERβ heterodimers.^[1] Once in the dimerized state, the estradiol-bound ER–ER complex binds to short estrogen response elements (EREs) (of the minimal nucleotide sequence 5'-GGTCANNNTGACC-3', where N is any nucleotide) in the promoter regions of estrogen-responsive genes on chromosomes, in turn modulating their expression.^[1][27][31] Some prominent examples ERE-containing and hence estrogen-modulated genes in humans include the genes encoding the proteins oxytocin, c-fos, c-myc, and transforming growth factor alpha (TGFα).^[32]

The ERs are expressed widely throughout the body, including in the breasts, uterus, vagina, prostate gland, fat, skin, bone, liver, pituitary gland, hypothalamus, and elsewhere throughout the brain.^[42] Through activation of the ERs (as well as the mERs), estradiol has many effects, including the following:

• Promotes growth, function, and maintenance of the breasts, uterus, and vagina during puberty and thereafter^[42][43]
• Mediates deposition of subcutaneous fat in a feminine pattern, especially in the breasts, hips, buttocks, and thighs^[44]
• Maintains skin health, integrity, appearance, and hydration and slows the rate of aging of the skin^[45]
• Produces the growth spurt and epiphyseal closure in both sexes during puberty, mediates widening of the hips in females during puberty, and maintains bone mineral density in both sexes throughout life^[46][47]
• Modulates hepatic protein synthesis, such as the production of sex hormone-binding globulin (SHBG) and numerous other proteins, with consequent effects on the cardiovascular system and various other systems^[3]
• Exerts negative feedback on the hypothalamic–pituitary–gonadal axis (HPG axis) by suppressing the secretion of the gonadotropins FSH and LH from the pituitary gland, thereby inhibiting gonadal sex hormone production as well as ovulation and fertility^[48][3][49]
• Regulates the vasomotor system and body temperature via the hypothalamus, thereby preventing hot flashes^[50][51]
• Modulates brain function, with effects on mood, emotionality, and sexuality, as well as cognition and memory^[52]
• Influences the risk and/or progression of hormone-sensitive cancers including breast cancer, prostate cancer, and endometrial cancer^[53][3]
• Supports an anti-ferroptotic hydropersulfide system and suppresses the transcription of pro-ferroptotic ether-lipid pathway proteins thereby protecting from ferroptotic cell death. ^[54]

Estrogen has also been found to increase the secretion of oxytocin and to increase the expression of its receptor, the oxytocin receptor, in the brain.^[18] In women, a single dose of estradiol has been found to be sufficient to increase circulating oxytocin concentrations.^[55]

Estradiol has relatively low oral bioavailability of about 5%.^[1] In addition, there is considerable interindividual variability in levels of estradiol achieved with oral estradiol.^[1] In contrast to estradiol, the synthetic estrogen ethinylestradiol has about 45% oral bioavailability, around 80- to 200-fold greater systemic oral estrogenic potency, roughly 500- to 1,500-fold greater hepatic oral estrogenic potency, and less interindividual variability in circulating estrogen levels achieved.^{[78][1][157][158][159][160][161]} An oral dose of ethinylestradiol that is approximately 100-fold lower than that of estradiol achieves similar maximal circulating estrogen concentrations (e.g., 50 pg/mL ethinylestradiol with a single 20 μg dose of ethinylestradiol relative to 40 pg/mL estradiol with a single 2 mg dose of micronized estradiol or estradiol valerate).^[1] These differences are due to the introduction of an ethynyl group at the C17α position in ethinylestradiol (also known as 17α-ethynylestradiol), which results in steric hindrance and greatly diminishes the first-pass metabolism of ethinylestradiol relative to estradiol with oral administration.^[1] Estradiol and ethinylestradiol have similar affinities for and efficacies as agonists of the ERs,^[1][2] and the systemic estrogenic potency of estradiol and ethinylestradiol is similar when they are administered by the intravenous route.^[162]

Synthetic estrogens like ethinylestradiol and diethylstilbestrol and the natural but animal-derived conjugated estrogens have disproportionate effects on liver protein synthesis relative to their effects in other tissues when compared to estradiol.^[1] At doses via the oral route with comparable systemic estrogenic potency, conjugated estrogens have about 1.3 to 4.5 times the hepatotropic potency (i.e., potency in modulating liver protein synthesis) of estradiol, ethinylestradiol has about 2.9 to 5.0 times the hepatotropic potency of estradiol, and diethylstilbestrol shows about 5.7 to 7.5 times the hepatotropic potency of estradiol (all measured via a small selection of estrogen-modulated hepatic proteins that included HDL cholesterol, SHBGTooltip sex hormone-binding globulin, CBGTooltip corticosteroid-binding globulin, and angiotensinogen).^[1] The greater hepatotropic potency of these estrogens relative to estradiol is related to susceptibility to hepatic metabolism.^[1] Whereas estradiol is metabolized and thereby inactivated rapidly upon entry into the liver, other estrogens like ethinylestradiol and diethylstilbestrol are resistant to hepatic metabolism and persist in the liver for a longer amount of time.^[1] This is reflected in the biological half-lives of these estrogens; the blood half-life of estradiol is about 1 to 2 hours, while the half-lives of ethinylestradiol and diethylstilbestrol are approximately 20 hours and 24 hours, respectively.^{[163][164][161]} In accordance with its long half-life, ethinylestradiol passes through the liver many times prior to its elimination.^[165] Because humans are not adapted to efficiently metabolize conjugated estrogens (which are equine (horse) estrogens) and synthetic estrogens like ethinylestradiol and diethylstilbestrol, these estrogens are not properly inactivated in the liver, with markedly disproportionate hepatic estrogenic effects resulting.^[1]

In addition to differences in hepatotropic potency between estradiol and other estrogens, there are differences in hepatotropic potency between different routes of administration of estradiol.^[1] Due to the first pass through the liver, oral estradiol results in disproportionate and unphysiological hepatic estradiol levels that are 4- to 5-fold higher than in the circulation.^[166][1] Conversely, parenteral routes of estradiol, such as transdermal, vaginal, and injection, bypass the first pass through the liver and produce levels of estradiol in the circulation and liver that are comparable.^[166][1] As an example of the reduced hepatic impact of parenteral estradiol relative to oral estradiol, a study found that 1 mg/day oral estradiol significantly increased SHBG levels by 45%, while 50 μg/day transdermal estradiol increased SHBG levels non-significantly by only 12% (with these dosages being roughly equivalent in systemic estrogenic potency).^{[167][168][169]} As such, not only do oral non-bioidentical estrogens like ethinylestradiol and conjugated estrogens have substantially greater potency in the liver than does oral estradiol, oral estradiol has considerably greater potency in the liver than does parenteral estradiol.^[1] Thus, the hepatotropic effects of oral non-bioidentical estrogens like ethinylestradiol are massive in comparison to parenteral estradiol (see the graph above/to the right), which in contrast to these estrogens has very weak or even absent effects on liver protein synthesis at normal therapeutic dosages.^{[1][91][78][3]} Whereas high-dosage 320 mg/month intramuscular polyestradiol phosphate increased SHBG levels to 166% in men with prostate cancer, the combination of 80 mg/month intramuscular polyestradiol phosphate and high-dosage 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.^[4][91][170]

The effects of estrogens on liver protein synthesis, such as on the synthesis of coagulation factors, lipoproteins, and triglycerides, can cause an increased risk of thromboembolic and cardiovascular complications, which in turn can result in increased mortality.^[78] The risk of thromboembolic and cardiovascular complications is significantly increased in postmenopausal women taking oral conjugated estrogens as a component of menopausal hormone therapy.^{[1][171][172]} Both oral estradiol and oral esterified estrogens have been found to have a significantly lower risk of thromboembolic and cardiovascular complications than oral conjugated estrogens, and transdermal estradiol appears to have no such risks at all.^{[1][173][171][172]} Widely employed in the past, oral synthetic estrogens like ethinylestradiol and diethylstilbestrol are no longer used in menopausal hormone therapy due to their high risks of thromboembolic and cardiovascular complications.^[174] Studies have found a markedly increased 5-year risk of cardiovascular mortality of 14 to 26% in men treated with high-dosage oral synthetic estrogens like ethinylestradiol and diethylstilbestrol for prostate cancer.^[78] With diethylstilbestrol, there is an up to 35% incidence of cardiovascular toxicity and an up to 15% incidence of venous thromboembolism.^[175] In a small study comparing high-dosage 320 mg/month intramuscular polyestradiol phosphate versus the combination of 80 mg/month polyestradiol phosphate with high-dosage 150 μg/day oral ethinylestradiol for prostate cancer, there was a 25% incidence of cardiovascular complications over the course of a year in the group that was also treated with ethinylestradiol, whereas there were no cardiovascular complications in the polyestradiol phosphate-only group.^[91] In accordance, 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 polyestradiol phosphate.^[176] In spite of the markedly reduced impact of parenteral estradiol on the liver compared to other estrogens however, high dosages of parenteral estradiol, producing high levels of circulating estradiol, can still result in important and undesirable changes in liver protein synthesis as with other estrogens.^[49] A high dosage of 320 mg/month polyestradiol phosphate has been found to result in significantly increased cardiovascular morbidity (due to non-fatal ischemic heart events and heart decompensation) in men with prostate cancer in two large studies, though cardiovascular mortality was notably not increased.^[49][177]

In addition to the liver, ethinylestradiol shows disproportionate estrogenic effects in the uterus.^[1][52][178] This is due to its inability to be inactivated by uterine 17β-hydroxysteroid dehydrogenase (17β-HSD).^[1][52][178] Because of its disproportionate effects in the uterus, ethinylestradiol is associated with a significantly lower incidence of vaginal bleeding and spotting than is estradiol, particularly in combination with progestogens (which induce 17β-HSD expression and hence estradiol metabolism in the uterus),^[1] and is an important contributing factor in why ethinylestradiol, among other reasons and in spite of its inferior safety profile, has been widely used in oral contraceptives instead of estradiol.^[142][141] Although ethinylestradiol has increased effects in the uterus relative to estradiol, it is similarly not associated with an increase in the risk of endometrial hyperplasia and endometrial cancer when used in combination with a progestogen, but instead with a significant decrease.^[1][179]

• Pharmacokinetics of estradiol
• Pharmacodynamics of progesterone
• Pharmacokinetics of progesterone