Norethisterone (NET), also known as norethindrone, is a medication that is used in combination with estrogen or alone in hormonal contraceptives, menopausal hormone therapy, and in the treatment of gynecological disorders. It is a synthetic progestogen (or a progestin) of the 19-nortestosterone group and has similar effects to those of natural progesterone, including suppression of gonadotropins, ovulation inhibition, and endometrial transformation. In addition to its progestogenic activity, NET also has weak androgenic and estrogenic effects at high dosages. In addition to NET itself, several prodrugs of NET, such as norethisterone acetate (NETA), norethisterone enanthate (NETE), and others, have been marketed and have similar effects and uses.
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Medical uses
NET is used as a hormonal contraceptive in combination with an estrogen - usually ethinylestradiol (EE) - in combined oral contraceptive pills and alone in progestogen-only pills. NET has also been proven to be effective in inhibiting LH and FSH levels in men as well. Without these hormones, mature sperm cannot survive in the testes. This means that NET could also be an effective male hormonal contraceptive. Aside from its use as a contraceptive, NET can be used to treat premenstrual syndrome, dysmenorrhea, menorrhagia, irregular menstruation, menopausal symptoms (in combination with estrogen), or to postpone a period. It is also commonly used to help prevent uterine hemorrhage in complicated non-surgical or pre-surgical gynecologic cases and in treating non responsive cyclical mastalgia.
Another medical use of norethisterone is to alleviate endometriosis related pain. In fact, 50% of patients who received medical or surgical treatment for endometriosis-related pelvic pain have benefited from progestin therapy. This could be due to the fact that norethisterone induces endometrial proliferation during secretory phase, which has been shown to alleviate endometrial pain complaints. Another way in which norethisterone may be acting to reduce endometrial pain is via inhibition of ovulation. Endometriosis pain and discomfort is worse during ovulation.
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Contraindications
High-dose (10 mg/day) NET has been associated with hepatic veno-occlusive disease, and because of this adverse effect, NET should not be given to patients undergoing allogeneic bone marrow transplantation, as it has been associated with substantially lower one-year survival post-transplantation.
Side effects
At contraceptive and hormone replacement dosages (0.35 to 1 mg/day), NET has essentially progestogenic side effects only. In most clinical studies of NET for contraception or menopausal hormone therapy, the drug has been combined with an estrogen, and for this reason, it is difficult to determine which of the side effects were caused by NET and which of them were caused by estrogen in such research. However, NETE, an intramuscularly administered prodrug of NET which is used as a long-acting contraceptive, is used without an estrogen, and hence can be employed as a surrogate for NET in terms of understanding its effects and tolerability. In clinical studies, the most common side effect with NETE has been menstrual disturbances, including prolonged bleeding or spotting and amenorrhea. Other side effects have included periodic abdominal bloating and breast tenderness, both of which are thought to be due to water retention and can be relieved with diuretics. There has been no association with weight gain, and blood pressure, blood clotting, and glucose tolerance have all remained normal. However, a decrease in HDL cholesterol has been observed.
At high dosages (5 to 60 mg/day), for instance those used in the treatment of gynecological disorders, NET can cause hypogonadism due to its antigonadotropic effects and can have estrogenic and weak androgenic side effects.
Androgenic
Due to its weak androgenic activity, NET can produce androgenic side effects such as acne, hirsutism, and voice changes of slight severity in some women at high dosages (e.g., 10 to 40 mg/day). This is notably not the case with combined oral contraceptives that contain NET and EE, however. Such formulations contain low dosages of NET (0.35 to 1 mg/day) in combination with estrogen and are actually associated with improvement in acne symptoms. In accordance, they are in fact approved by the FDA for the treatment of acne in women in the United States. The improvement in acne symptoms is believed to be due to a 2- to 3-fold increase in sex hormone-binding globulin (SHBG) levels and a consequent decrease in free testosterone levels caused by EE, which results in an overall decrease in androgenic signaling in the body.
The sebaceous glands are highly androgen-sensitive and their size and activity are potential markers of androgenic effect. A high dosage of 20 mg/day NET or NETA has been found to significantly stimulate the sebaceous glands, whereas lower dosages of 5 mg/day and 2.5 mg/day NET and NETA, respectively, did not significantly stimulate sebum production and were consequently regarded as devoid of significant androgenicity. Conversely, dosages of NET of 0.5 to 3 mg/day have been found to dose-dependently decrease SHBG levels (and hence to suppress hepatic SHBG production), which is another highly sensitive marker of androgenicity.
A large clinical study of high to very high oral dosages of NET (10 to 40 mg/day) administered for prolonged periods of time (4 to 35 weeks) to prevent miscarriage in pregnant women found that 5.5% of the women experienced mild androgenic side effects such as mild voice changes (hoarseness), acne, and hirsutism and that 18.3% of female infants born to the mothers showed, in most cases only slight, virilization of the genitals. Maternal androgenic symptoms occurred most often in women who received a dosage of NET of 30 mg/day or more for a period of 15 weeks or longer. In the female infants who experienced virilization of the genitals, the sole manifestation in 86.7% of the cases was varied but almost always slight enlargement of the clitoris. In the remaining 13.3% of the affected cases, marked clitoral enlargement and partial fusion of the labioscrotal folds occurred. The dosages used in these cases were 20 to 40 mg/day.
In a letter to the editor on the topic of virilization caused by high dosages of NETA in women, a physician expressed that they had not observed the "slightest evidence of virilization" and that there had "certainly been no hirsutism nor any voice changes" in 55 women with advanced breast cancer that they had treated with 30 to 60 mg/day NET for up to six months.
High-dosage NET has been used to suppress menstruation in women with severe intellectual disability who were incapable of handling their own menses. A study of 118 nulliparous women treated with 5 mg/day NET for a period of 2 to 30 months found that the drug was effective in producing amenorrhea in 86% of the women, with breakthrough bleeding occurring in the remaining 14%. Side effects including weight gain, hirsutism, acne, headache, nausea, and vomiting all did not appear to increase in incidence and no "disturbing side effects" were noted in any of the women. Another study of 5 mg/day NET in 132 women also made no mention of androgenic side effects. These findings suggest little to no risk of androgenic side effects with NET at a dosage of 5 mg/day. A study of 194 women treated with 5 to 15 mg/day NETA for a median duration of 13 months of therapy to suppress symptoms of endometriosis observed no side effects in 55.2% of patients, weight gain in 16.1%, acne in 9.9%, mood lability in 8.9%, hot flashes in 8.3%, and voice deepening in two women (1.0%).
Estrogenic
NET is weakly estrogenic (via conversion into its metabolite EE), and for this reason, it has been found at high dosages to be associated with high rates of estrogenic side effects such as breast enlargement in women and gynecomastia in men, but also with improvement of menopausal symptoms in postmenopausal women. It has been suggested that very high dosages (e.g., 40 mg/day, which are sometimes used in clinical practice for various indications) of NETA (and by extension NET) may result in an increased risk of venous thromboembolism (VTE) analogously to high dosages (above 50 ?g/day) of EE, and that even dosages of NETA of 10 to 20 mg, which correspond to EE dosages of approximately 20 to 30 ?g/day, may in certain women be associated with increased risk.
Interactions
5?-Reductase plays an important role in the metabolism of NET, and 5?-reductase inhibitors such as finasteride and dutasteride can inhibit its metabolism. The ethynyl group on the carbon 17 of NET is preserved in approximately 90% of all of its metabolites, this is due to the steric hindrance at that carbon.
NET is partially metabolized via hydroxylation by CYP3A4, and inhibitors and inducers of CYP3A4 can significantly alter circulating levels of NET. For instance, the CYP3A4 inducers rifampicin and bosentan have been found to decrease NET exposure by 42% and 23%, respectively, and the CYP3A4 inducers carbamazepine and St. John's wort have also been found to accelerate NET clearance.
Pharmacology
Pharmacodynamics
NET is a potent progestogen and a weak androgen and estrogen. That is, it is a potent agonist of the progesterone receptor (PR) and a weak agonist of the androgen receptor (AR) and the estrogen receptor (ER). NET itself has insignificant affinity for the ER; its estrogenic activity is from an active metabolite that is formed in very small amounts, ethinylestradiol (EE), which is a very potent estrogen. NET and its metabolites have negligible affinity for the glucocorticoid receptor (GR) and mineralocorticoid receptor (MR) and hence have no glucocorticoid, antiglucocorticoid, mineralocorticoid, or antimineralocorticoid activity.
Progestogenic activity
NET displays 8 times higher progestational activity than progesterone. It is also a potent progestogen and binds to the PR with approximately 150% of the affinity of progesterone. In contrast, its parent compounds, testosterone, nandrolone (19-nortestosterone), and ethisterone (17?-ethynyltestosterone), have 2%, 22%, and 44% of the relative binding affinity of progesterone for the PR. Unlike NET, its major active metabolite 5?-dihydro-NET (5?-DHNET), which is formed by 5?-reductase, has been found to possess both progestogenic and marked antiprogestogenic activity, although its affinity for the PR is greatly reduced relative to NET at only 25% of that of progesterone. NET produces similar changes in the endometrium and vagina and is similarly thermogenic in women compared to progesterone, which is in accordance with its progestogenic activity.
Androgenic activity
NET has approximately 15% of the affinity of the anabolic-androgenic steroid (AAS) metribolone (R-1881) for the AR, and in accordance, is weakly androgenic. In contrast to NET, 5?-DHNET, the major metabolite of NET, shows higher affinity for the AR, with approximately 27% of the affinity of metribolone. However, although 5?-DHNET has higher affinity for the AR than NET, it has significantly diminished and in fact almost abolished androgenic potency in comparison to NET in rodent bioassays. Similar findings were observed for ethisterone (17?-ethynyltestosterone) and its 5?-reduced metabolite, whereas 5?-reduction enhanced both the AR affinity and androgenic potency of testosterone and nandrolone (19-nortestosterone) in rodent bioassays. As such, it appears that the ethynyl group of NET at the C17? position is responsible for its loss of androgenicity upon 5?-reduction.
NET (0.5 to 3 mg/day) has been found to dose-dependently decrease circulating SHBG levels, which is a common property of androgens and is due to AR-mediated suppression of hepatic SHBG production. The drug also has estrogenic activity, and estrogens are known to increase SHBG hepatic production and circulating levels, so it would appear that the androgenic activity of NET overpowers its estrogenic activity in this regard.
NET is bound to a considerable extent (36%) to SHBG in circulation. Although it has lower affinity for SHBG than endogenous androgens and estrogens, NET may displace testosterone from SHBG and thereby increase free testosterone levels, and this action may contribute to its weak androgenic effects.
Estrogenic activity
NET binds to the ERs, the ER? and the ER?, with 0.07% and 0.01% of the relative binding affinity of estradiol. Due to these very low relative affinities, it is essentially inactive itself as a ligand of the ERs at clinical concentrations. However, NET has been found to be a substrate for aromatase and is converted in the liver to a small extent (0.35%) to the highly potent estrogen EE, and for this reason, unlike most other progestins, NET has some estrogenic activity. However, with typical dosages of NET used in oral contraceptives (0.5 to 1 mg), the levels of EE produced are low, and it has been said that they are probably without clinical relevance. Conversely, doses of 5 and 10 mg of NET, which are used in the treatment of gynecological disorders, are converted at rates of 0.7% and 1.0% and produce levels of EE that correspond to those produced by 30 and 60 ?g dosages of EE, respectively. The levels of EE formed by 0.5 and 1 mg of NET have been calculated and estimated based on higher dosages as corresponding to 2 and 10 ?g dosages of EE, respectively.
Neurosteroid activity
Like progesterone and testosterone, NET is metabolized into 3,5-tetrahydro metabolites. Whether these metabolites of NET interact with the GABAA receptor similarly to the 3,5-tetrahydro metabolites of progesterone and testosterone like allopregnanolone and 3?-androstanediol, respectively, is a topic that does not appear to have been studied and hence requires clarification.
Steroidogenesis inhibition
NET is a substrate for and is known to be an inhibitor of 5?-reductase, with 4.4% and 20.1% inhibition at 0.1 and 1 ?M, respectively. However, therapeutic concentrations of NET are in the low nanomolar range, so this action may not be clinically relevant at typical dosages.
NET and its major active metabolite 5?-DHNET have been found to act as irreversible aromatase inhibitors (Ki = 1.7 ?M and 9.0 ?M, respectively). However, like the case of 5?-reductase, the concentrations required are probably too high to be clinically relevant at typical dosages. 5?-DHNET specifically has been assessed and found to be selective in its inhibition of aromatase, and does not affect cholesterol side-chain cleavage enzyme (P450scc), 17?-hydroxylase/17,20-lyase, 21-hydroxylase, or 11?-hydroxylase. Since it is not aromatized (and hence cannot be transformed into an estrogenic metabolite), unlike NET, 5?-DHNET has been proposed as a potential therapeutic agent in the treatment of ER-positive breast cancer.
Other activities
NET is a very weak inhibitor of CYP2C9 and CYP3A4 (IC50 = 46 ?M and 51 ?M, respectively), but these actions require very high concentrations of NET that are far above therapeutic circulating levels (which are in the nanomolar range) and hence are probably not clinically relevant.
NET and some of its 5?-reduced metabolites have been found to produce vasodilating effects in animals that are independent of sex steroid receptors and hence appear to be non-genomic in mechanism.
Pharmacokinetics
Absorption
The oral bioavailability of NET is between 47 to 73%, with a mean oral bioavailability of 64%. Micronization has been found to significantly improve the oral bioavailability of NET by increasing intestinal absorption and reducing intestinal metabolism. A single 2 mg oral dose of NET has been found to result in peak circulating levels of the drug of 12 ng/mL (40 nmol/L), whereas a single 1 mg oral dose of NET in combination with 2 mg estradiol resulted in peak levels of NET of 8.5 ng/mL (29 nmol/L) one-hour post-administration.
Distribution
NET is 61% bound to albumin and 36% bound to SHBG in the blood.
Metabolism
NET has a terminal half-life of 5.2 to 12.8 hours, with a mean terminal half-life of 8.0 hours. The metabolism of NET is very similar to that of testosterone (and nandrolone) and is mainly via reduction of the ?4 double bond to 5?- and 5?-dihydro-NET, which is followed by the reduction of the C3 keto group to the four isomers of 3,5-tetrahydro-NET. These transformations are catalyzed by 5?- and 5?-reductase and 3?- and 3?-hydroxysteroid dehydrogenase both in the liver and in extrahepatic tissues such as the pituitary gland, uterus, prostate gland, vagina, and breast. With the exception of 3?,5?- and 3?,5?-tetrahydro-NET, which have significant affinity for the ER and are estrogenic to some degree, the 3,5-tetrahydro metabolites of NET are inactive in terms of affinity for sex steroid receptors (specifically, the PR, AR, and ER). A small amount of NET is also converted by aromatase into EE. NET is metabolized in the liver via hydroxylation as well, mainly by CYP3A4. Some conjugation (including glucuronidation and sulfation) of NET and its metabolites occurs in spite of steric hindrance by the ethynyl group at C17?. The ethynyl group of NET is preserved in approximately 90% of all of its metabolites.
Elimination
NET is eliminated 33 to 81% in urine and 35 to 43% in feces.
Chemistry
NET, also known as 17?-ethynyl-19-nortestosterone or as 17?-ethynylestra-4-en-17?-ol-3-one, is a synthetic estrane steroid and a derivative of testosterone. It is specifically a derivative of testosterone in which an ethynyl group has been added at the C17? position and the methyl group at the C19 position has been removed; hence, it is a combined derivative of ethisterone (17?-ethynyltestosterone) and nandrolone (19-nortestosterone). These modifications result in gaining progestogenic and oral activity and reducing androgenic activity.
Derivatives
NET is the parent compound of a large group of progestins that includes most of the progestins known as the 19-nortestosterone derivatives. This group is divided by chemical structure into the estranes (derivatives of NET) and the gonanes (13?-ethylestranes; derivatives of levonorgestrel) and includes the following:
Several of these act as prodrugs of NET, including NETA, NETE, etynodiol diacetate, lynestrenol, and quingestanol acetate.
19-Nortestosterone (19-NT) progestins that are not derivatives of NET that have been marketed include the following: the C17? vinyl (ethenyl) derivatives norgesterone (17?-vinyl-?5(10)-19-NT) and norvinisterone (17?-vinyl-19-NT); the allyl derivatives allylestrenol (3-deketo-17?-allyl-19-NT) and altrenogest (17?-allyl-?9,11-19-NT); the C17? alkyl derivatives normethandrone (17?-methyl-19-NT) and norethandrolone (17?-ethyl-19-NT); the C17? cyanomethyl derivative dienogest (17?-cyanomethyl-?9-19-NT); and the C16? ethyl derivative oxendolone (16?-ethyl-19-NT).
Synthesis
Synthesis 1
Estradiol methyl ether (1, EME) is partially reduced to the 1,5-diene (2) as also occurs for the first step in the synthesis of nandrolone. Oppenauer oxidation then transforms the 17? alcohol into a ketone functionality (3). This is then reacted with metal acetylide into the corresponding 17?-ethynyl compound (4). Hydrolysis of the enolether under mild conditions leads directly to (5), which appears to be noretynodrel (although Lednicer states that it is "etynodrel" in his book (which may be a synonym etynodiol). Etynodrel is with a chlorine atom attached.) an orally active progestin. This is the progestogen component of the first oral contraceptive to be offered for sale (i.e., Enovid). Treatment of the ethynyl enol ether with strong acid leads to NET (6).
In practice, these and all other combined oral contraceptives are mixtures of 1-2% EE or mestranol and an oral progestin. It has been speculated that the discovery of the necessity of estrogen in addition to progestin for contraceptive efficacy is due to the presence of a small amount of unreduced EME (1) in early batches of 2. This when subjected to oxidation and ethynylation, would of course lead to mestranol (3). In any event, the need for the presence of estrogen in the mixture is now well established experimentally.
Synthesis 2
NET is made from estr-4-ene-3,17-dione (bolandione), which in turn is synthesized by partial reduction of the aromatic region of the 3-O-methyl ether of estrone with lithium in liquid ammonia, and simultaneously of the keto group at C17 to a hydroxy group, which is then oxidized back to a keto group by chromium trioxide in acetic acid. The congugated C4-C5 olefin and the carbonyl group at C3 is then transformed to dienol ethyl ether using ethyl orthoformate. The obtained product is ethynylated by acetylene in the presence of potassium tert-butoxide. After HCl hydrolysis of the formed O-potassium derivative, during which the enol ether is also hydrolyzed, and the remaining double bond is shifted, the desired NET is obtained.
History
NET was synthesized for the first time by chemists Luis Miramontes, Carl Djerassi, and George Rosenkranz at Syntex in Mexico City in 1951. It was derived from ethisterone, and was found to possess about 20-fold greater potency as a progestogen in comparison. NET was the first highly active oral progestogen to be synthesized, and was preceded (as a progestogen) by progesterone (1934), ethisterone (1938), 19-norprogesterone (1944), and 17?-methylprogesterone (1949) as well as by nandrolone (1950), whereas noretynodrel (1952) and norethandrolone (1953) followed the synthesis of NET. The drug was first introduced, alone as Norlutin, in the United States in 1957. NET was subsequently introduced in combination with mestranol as Ortho-Novum in the U.S. in 1963, and was the second progestin, after noretynodrel in 1960, to be used in an oral contraceptive. In 1964, additional contraceptive preparations containing NET in combination with mestranol or EE, such as Norlestrin and Norinyl, were marketed in the U.S.
Society and culture
Generic names
Norethisterone is the INN and BAN of NET while norethindrone is its USAN.
Brand names
NET is marketed under brand names including Primolut N, Norlutin, Micronor, Utovlan, and Noriday, among numerous others.
Availability
United States
NET was previously available alone in 5 mg tablets under the brand name Norlutin in the U.S., but this formulation has since been discontinued in this country. However, NETA remains available alone in 5 mg tablets under the brand names Aygestin and Norlutate in the U.S. It is one of the only non-contraceptive progestogen-only drug formulations that remains available in the U.S. The others include progesterone, medroxyprogesterone acetate, megestrol acetate, and hydroxyprogesterone caproate, as well as the atypical agent danazol.
Both NET and NETA are also available in the U.S. as contraceptives. NET is available both alone (brand names Camila, Errin, Heather, Micronor, Nor-QD, others) and in combination with EE (Ortho-Novum, others) or mestranol (Norinyl), while NETA is available only in combination with EE (Norlestrin, others). NETE is not available in the U.S. in any form.
References
Further reading
- Schoonen WG, Deckers GH, de Gooijer ME, de Ries R, Kloosterboer HJ (2000). "Hormonal properties of norethisterone, 7?-methyl-norethisterone and their derivatives". J. Steroid Biochem. Mol. Biol. 74 (4): 213-22. doi:10.1016/s0960-0760(00)00125-4. PMID 11162927.
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