Estetrol (E4), or oestetrol, is one of the four natural estrogenic

steroid hormone A steroid hormone is a steroid that acts as a hormone. Steroid hormones can be grouped into two classes: corticosteroids (typically made in the adrenal cortex, hence ''cortico-'') and sex steroids (typically made in the gonads or placenta). Wit ...

s found in humans, along with

estrone Estrone (E1), also spelled oestrone, is a steroid, a weak estrogen, and a minor female sex hormone. It is one of three major endogenous estrogens, the others being estradiol and estriol. Estrone, as well as the other estrogens, are synthesized ...

(E1),

estradiol Estradiol (E2), also called oestrogen, oestradiol, is an estrogen steroid hormone and the major female sex hormone. It is involved in the regulation of female reproductive cycles such as estrous and menstrual cycles. Estradiol is responsible ...

(E2), and

estriol Estriol (E3), also spelled oestriol, is a steroid, a weak estrogen, and a minor female sex hormone. It is one of three major endogenous estrogens, the others being estradiol and estrone. Levels of estriol in women who are not pregnant are almost ...

(E3). Estetrol is a major estrogen in the body. In contrast to estrone and estradiol, estetrol is a native estrogen of

fetal A fetus or foetus (; : fetuses, foetuses, rarely feti or foeti) is the unborn offspring of a viviparous animal that develops from an embryo. Following the embryonic stage, the fetal stage of development takes place. Prenatal development is a ...

life. Estetrol is produced exclusively by the

liver The liver is a major metabolic organ (anatomy), organ exclusively found in vertebrates, which performs many essential biological Function (biology), functions such as detoxification of the organism, and the Protein biosynthesis, synthesis of var ...

and is found in detectable levels only during pregnancy, with relatively high levels in the fetus and lower levels in the maternal circulation. In addition to its physiological role as a native hormone, estetrol can be used as a

medication Medication (also called medicament, medicine, pharmaceutical drug, medicinal product, medicinal drug or simply drug) is a drug used to medical diagnosis, diagnose, cure, treat, or preventive medicine, prevent disease. Drug therapy (pharmaco ...

, see estetrol (medication). Estetrol, in combination with

drospirenone Drospirenone is a progestin and antiandrogen medication which is used in birth control pills to prevent pregnancy and in menopausal hormone therapy, among other uses. It is available both alone under the brand name Slynd and in combination ...

, has recently been approved as a new estrogenic component of a combined oral contraceptive (COC) and estetrol alone is in clinical development for the treatment of menopausal symptoms as well as breast and prostate cancer.

Biological function

So far, the physiological function of estetrol remains unknown. The potential role of estetrol as a marker for fetal well-being has been studied quite extensively, but no correlation was found due to the large intra- and inter-individual variation in maternal estetrol plasma levels during pregnancy.

Biological activity

Estetrol has a moderate

affinity Affinity may refer to: Commerce, finance and law * Affinity (law), kinship by marriage * Affinity analysis, a market research and business management technique * Affinity Credit Union, a Saskatchewan-based credit union * Affinity Equity Pa ...

for estrogen receptors alpha ( ERα) and beta (

ERβ Estrogen receptor beta (ERβ) also known as NR3A2 (nuclear receptor subfamily 3, group A, member 2) is one of two main types of estrogen receptor—a nuclear receptor which is activated by the sex hormone estrogen. In humans ERβ is encoded by t ...

), with K_i values of 4.9 nM and 19 nM, respectively. As such, estetrol has 4- to 5-fold preference for ERα over ERβ. In different animal models, the potency of estetrol regarding its estrogenic effect observed ''in vivo'' is generally 10 to 20 times lower than the potency of ethinyl estradiol (EE) and is also lower than the potency of estradiol. Estetrol displays a highly selective binding to its primary targets ERα and

, which ensures that estetrol has a low risk of non-specific side effects.

Mode of action

Tissue-selective effect

Estetrol shows selective estrogenic, neutral or anti-estrogenic activities in certain cell types and tissues. In rodent models, estetrol has shown to elicit potent estrogenic activity on ovulation, brain, bone tissue, cardiovascular system, and uterus, associated with ovulation inhibition, prevention of bone demineralization, cardioprotective effects and maintenance of uterovaginal tissues, respectively. Data from preclinical studies also suggest that estetrol has ''anti-estrogenic like effects'' on the breast and a limited impact on normal or malignant breast tissue when used at therapeutic concentration. This property of estetrol is associated with antagonistic effects on breast cell proliferation, migration and invasion in the presence of estradiol. The molecular mechanisms of action driving its tissue-selective actions rely on a specific profile of ERα activation, uncoupling nuclear and membrane activation. In the liver, Estetrol has a neutral activity, which is reflected by a minimal impact on synthesis of hepatic coagulation factors, minimal impact on

sex hormone-binding globulin Sex hormone-binding globulin (SHBG) or sex steroid-binding globulin (SSBG) is a glycoprotein that binds to androgens and estrogens. When produced by the Sertoli cells in the seminiferous tubules of the testis, it is called androgen-binding prot ...

(SHBG) synthesis and limited impact on lipid parameters, including triglycerides. Estetrol can therefore be described as the first Native Estrogen with Selective Tissue activity (NEST).

Differences vs SERMs

The selective tissue activity of estetrol is different from the effects of selective estrogen receptor modulators (SERMs), like tamoxifen and raloxifene. Estetrol, like SERMs, has selective tissue activity. However, SERMs interact with the ligand binding domain of ERα in a manner that is distinct from that of estrogens, including estetrol. Estetrol recruits the same co-regulators as other estrogens, while SERMs recruit other co-regulators.

ERα activation

Estrogens can elicit their effects via nuclear ERα and/or membrane ERα signaling pathways. Estetrol presents a distinctive mode of action in terms of ERα activation. Like other estrogens, estetrol binds to, and activates the nuclear ERα to induce gene transcription. However, estetrol induces very limited activity via membrane ERα in several tissues (e.g. in the breast) and antagonizes this pathway in the presence of estradiol, thereby uniquely uncoupling nuclear and membrane activation.

Biochemistry

Biosynthesis

In the

, estetrol is synthesized from

(E2) and

(E3) by two fetal liver

enzyme An enzyme () is a protein that acts as a biological catalyst by accelerating chemical reactions. The molecules upon which enzymes may act are called substrate (chemistry), substrates, and the enzyme converts the substrates into different mol ...

s, 15α- and 16α-hydroxylase, through hydroxylation. Estetrol can be detected in maternal urine from the 9th week of gestation. After birth, the neonatal liver rapidly loses its capacity to synthesize estetrol. During the second trimester of pregnancy, high levels of estetrol can be found in maternal plasma, with steadily rising concentrations of unconjugated estetrol to about 1 ng/mL (>3 nM) towards the end of pregnancy. Fetal plasma levels have been reported to be over 10 times higher than maternal plasma levels at parturition.

Distribution

In terms of

plasma protein binding Plasma protein binding refers to the degree to which medications attach to blood proteins within the blood plasma. A drug's efficacy may be affected by the degree to which it binds. The less bound a drug is, the more efficiently it can traverse o ...

, estetrol displays moderate binding to

albumin Albumin is a family of globular proteins, the most common of which are the serum albumins. All of the proteins of the albumin family are water- soluble, moderately soluble in concentrated salt solutions, and experience heat denaturation. Alb ...

, and shows no binding to SHBG. The overall low plasma protein binding results in a ~50% free active fraction. This compares to a 1% active form for EE and ~2% for estradiol. Estetrol is equally distributed between red blood cells and plasma.

Metabolism

Cytochrome P450 (CYP) enzymes do not play a major role in the metabolism of estetrol. Instead, estetrol undergoes extensive phase 2 metabolism in the liver to form glucuronide and sulphate conjugates. The two main metabolites, estetrol-3-glucuronide and estetrol-16-glucuronide, have negligible estrogenic activity. ''(see Drospirenone/estetrol)''

Excretion

Estetrol is mainly excreted in

urine Urine is a liquid by-product of metabolism in humans and many other animals. In placental mammals, urine flows from the Kidney (vertebrates), kidneys through the ureters to the urinary bladder and exits the urethra through the penile meatus (mal ...

. Estetrol is an end-stage product of metabolism, which is not converted back into active metabolites like estriol, estradiol or estrone.

Chemistry

Estetrol, also known as 15α-hydroxyestriol or as estra-1,3,5(10)-triene-3,15α,16α,17β-tetrol, is an estrane

steroid A steroid is an organic compound with four fused compound, fused rings (designated A, B, C, and D) arranged in a specific molecular configuration. Steroids have two principal biological functions: as important components of cell membranes t ...

and

derivative In mathematics, the derivative is a fundamental tool that quantifies the sensitivity to change of a function's output with respect to its input. The derivative of a function of a single variable at a chosen input value, when it exists, is t ...

of estrin (estratriene). It is structurally different from the other estrogens because of the presence of four

hydroxyl group In chemistry, a hydroxy or hydroxyl group is a functional group with the chemical formula and composed of one oxygen atom covalently bonded to one hydrogen atom. In organic chemistry, alcohols and carboxylic acids contain one or more hydroxy ...

s, which explains the abbreviation E4.

Synthesis

Estetrol itself is a naturally-produced estrogen by the human fetal liver. However, for human use, estetrol is synthesized from estrone, which is obtained from phytosterols extracted from soybeans. The synthesis of estetrol results in very pure estetrol (>99.9%) without contaminants.

History

Estetrol was first described in 1965 by Egon Diczfalusy and coworkers at the

Karolinska Institute The Karolinska Institute (KI; ; sometimes known as the (Royal) Caroline Institute in English) is a research-led medical university in Solna within the Stockholm urban area of Sweden and one of the foremost medical research institutes globally ...

in Stockholm, Sweden, who identified and isolated this novel, native estrogen from late pregnancy urine and from the urine of newborn infants. Basic research on estetrol was conducted from 1965 to 1984. It was established that estetrol is exclusively synthesized in the human fetal liver. Since 1984, further research was virtually abandoned because estetrol was regarded as a weak and unimportant pregnancy estrogen. In 2001 Herjan Coelingh Bennink at Pantarhei Bioscience in the Netherlands re-started the investigation of estetrol as a potentially useful natural estrogen for human use, resulting in the introduction of E4 as the estrogenic component of a combined oral contraceptive in 2021.

References

{{Estrogen receptor modulators Animal reproductive system Estranes Estrogens Hormones of the hypothalamus-pituitary-gonad axis Hormones of the pregnant female Human female endocrine system Hydroxyarenes Polyols Sex hormones Orphan drugs Tetrols