|Systematic (IUPAC) name|
|ATC code||G03FA09 (WHO)|
|Synonyms||Noretynodrel; (17β)-17-ethynyl-17-hydroxyestr-5(10)-en-3-one; 17α-ethinyl-5(10)-19-nortestosterone; 5(10)-norethisterone; NSC-15432|
|Molar mass||298.419 g/mol|
|(what is this?)|
Noretynodrel (INN), or norethynodrel (USAN, BAN), also known as 17α-ethinyl-δ5(10)-19-nortestosterone or as 5(10)-norethisterone, is a steroidal progestin of the 19-nortestosterone group and the Δ5(10)-isomer of norethisterone. Noretynodrel was introduced in 1957 in Enovid, a combination formulation of noretynodrel and mestranol, for the treatment of gynecological and menstrual disorders. A few years later, in May 1960, Enovid was also approved as the first oral contraceptive.
Noretynodrel, unlike most progestins but similarly to etynodiol diacetate, has some estrogenic activity. It has little or no androgenic activity. The drug is a relatively weak progestogen, with only about one-tenth of the progestogenic activity of norethisterone, and in relation to this fact, is no longer used in oral contraceptives.
Noretynodrel is very closely related to norethisterone and tibolone, which are the Δ4-isomer and the 7α-methyl derivative of noretynodrel, respectively. Noretynodrel is metabolized in a very similar manner to tibolone, whereas the metabolism of norethisterone differs. Both noretynodrel and tibolone are transformed into 3α- and 3β-hydroxylated metabolites and a Δ4-isomer metabolite (in the case of noretynodrel, this being norethisterone), whereas norethisterone is not 3α- or 3β-hydroxylated (and of course does not form a Δ4-isomer metabolite). The major metabolites of noretynodrel are 3α-hydroxynoretynodrel and to a lesser extent 3β-hydroxynoretynodrel, formed by 3α- and 3β-hydroxysteroid dehydrogenases (AKR1C1–4), while the Δ4-isomer norethisterone is a minor metabolite formed in small amounts. Tibolone is considered to be a prodrug into both its 3α- and 3β-hydroxylated and Δ4-isomerized metabolites. Noretynodrel is also thought to be a prodrug, as it is rapidly metabolized and shows relatively weak affinity for the progesterone receptor (PR), although noretynodrel forms norethisterone in only minor quantities.
In terms of the PR, noretynodrel possesses only about 6 to 19% of the affinity of norethisterone for the PRA, whereas the affinity of the two drugs for the PRB is similar (noretynodrel possesses 94% of the affinity of norethisterone for the PRB). Tibolone and the Δ4-isomer metabolite of tibolone have similar affinity for the PRs as noretynodrel and norethisterone, respectively, whereas 3α- and 3β-hydroxylated metabolites of tibolone are virtually devoid of affinity for the PR. Since trestolone (7α-methyl-19-nortestosterone) is known to be a potent progestogen, 3α- and 3β-hydroxynoretynodrel likely are devoid of affinity for the PR similarly to the 3α- and 3β-hydroxylated metabolites of tibolone.
Relative to norethisterone, noretynodrel has 45 to 81% reduced affinity for the androgen receptor (AR). In contrast to norethisterone (which is mildly androgenic), noretynodrel is said to have no or only very weak androgenic activity. In accordance, noretynodrel has not been found to virilize female fetuses, in contrast to many other testosterone-derived progestins including ethisterone, norethisterone, and norethisterone acetate. However, according to Korn (1961), at least one case of pseudohermaphroditism has been observed in association with noretynodrel. The Δ4-isomer metabolite of tibolone shows dramatically and disproportionately increased affinity for the AR relative to norethisterone and noretynodrel (5.7- to 18.5-fold greater than that of norethisterone), indicating that the 7α-methyl group of tibolone markedly increases its androgenic activity and is responsible for the greater androgenic effects of tibolone relative to noretynodrel.
Whereas norethisterone has virtually no affinity for the estrogen receptors (ERs), noretynodrel shows some, albeit very weak affinity for both the ERα and the ERβ (in terms of relative binding affinity, 0.7% and 0.22% of that of estradiol, respectively). The estrogenic activity of 3α- and 3β-hydroxynoretynodrel has never been assessed. However, while tibolone shows similar affinity for the ERs as noretynodrel, the 3α- and 3β-hydroxylated metabolites of tibolone have several-fold increased affinity for the ERs. As such, the 3α- and 3β-hydroxylated metabolites of noretynodrel may also show increased estrogenic activity, and this may account for the known estrogenic effects of noretynodrel. The Δ4-isomer of tibolone, similarly to norethisterone, is virtually devoid of affinity for the ERs. Neither tibolone nor its metabolites are aromatized, whereas trestolone is readily aromatized similarly to testosterone and 19-nortestosterone, and for these reasons, it is unlikely that noretynodrel or its metabolites are aromatized either. As such, aromatization likely does not play a role in the estrogenic activity of tibolone or noretynodrel. However, controversy on this matter exists, and other researchers have suggested that tibolone and noretynodrel may be aromatized in small amounts to highly potent estrogens (ethinyl estradiol and its 7α-methyl derivative, respectively).
Noretynodrel was first synthesized by Frank B. Colton of G. D. Searle & Company in 1952, and this was preceded by the synthesis of norethisterone by Luis E. Miramontes and Carl Djerassi of Syntex in 1951. In 1957, both noretynodrel and norethisterone were approved in the United States for the treatment of menstrual disorders. In 1960, noretynodrel, in combination with mestranol (as Enovid), was introduced in the United States as the first oral contraceptive, and the combination of norethisterone and mestranol followed in 1963 as the second oral contraceptive to be introduced. In 1988, Enovid, along with other oral contraceptives containing high doses of estrogen, was discontinued.
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