Effect of the progesterone antagonist RU486 on human myometrial spontaneous contractility and PGI2 release

We studied the effect of antiprogesterone RU 486 on spontaneous uterine contractility and PGI2 release with human myometrial strips superfused "in vitro". A decrease of PGI2 release into the superfusion medium was observed after 20 min superfusion. The inhibition was dose-dependent and reversible. After 20 min washing with tyrode medium without RU 486, the uterine strips recovered their initial rate of release. R5020, a progesterone agonist, did not affect PGI2 release nor dexamethasone and testosterone. Parallel to the decrease of PGI2 observed during RU 486 superfusion, the uterine spontaneous contraction frequency decreased, while the amplitude and duration of contractions increased. The alteration of uterine contractility was also rapid, dose-dependent and reversible. Modification of uterine strip spontaneous contractility, similar to those induced by RU 486, were also observed with superfusions of R5020 at concentrations as low as 10(-9)M, dexamethasone (10(-8)M), but not with superfusions of testosterone. These observations are not in favour of a progesterone-receptor mediated effect of RU 486 in our model. The mechanism of action may be related to the antiprogesterone specific structure i.e. the bulky substituent at the C-11 position. The RU 486 effect on uterine strip contractility, mimicked by other steroids, could point to a non-specific lipid/membrane interaction. However, the fact that testosterone did not affect motility, may indicate a possible specificity of steroids having a 3 oxo pregnene structure.

Pharmacokinetics of the antiprogesterone RU 486: no correlation to clinical performance of RU 486

The pharmacokinetics and metabolism of RU 486 were characterized in 17 women who received a single dose of 600 mg of RU 486 for termination of an early unwanted pregnancy. Based on the clinical outcome and serum chorionic gonadotropin values, the subjects were divided into two groups: those who aborted completely (i.e. responders: N = 13) and those who did not respond to RU 486 treatment (i.e. non-responders: N = 4). The serum levels of RU 486, the monodemethylated, didemethylated and hydroxylated metabolites of RU 486 were measured by HPLC preceded by column chromatography. There were no significant differences in the serum levels of RU 486 or its metabolites between the two groups. The serum concentrations of alpha 1-acid glycoprotein, the binding protein for RU 486, were quantitated by immunoturbidimetry. The alpha 1-acid glycoprotein concentrations were similar in responders and non-responders. The metabolism of RU 486 was also studied by fractionating extracts of serum pools of responders and non-responders on thin-layer chromatography, and subsequent RIA analysis of the eluates of the sliced thin-layer chromatography. Spots with similar distribution and percentages of cross-reactivity were found in both groups on the chromatography; the results were also similar to those from a serum pool to which synthetic RU 486 and its three metabolites had been added. Hence it is concluded that failure to abort in response to RU 486 therapy is not associated with altered pharmacokinetics or metabolism of RU 486.

Progesterone receptor antagonists and prostaglandins in human fertility regulation: a clinical review

Progesterone receptor antagonists have been developed by substitutions at the 11-beta and 17 side-chain positions of the progestagen norethisterone. The most studied progesterone receptor antagonists are mifepristone (Mifegyne; Roussel-UCLAF; RU486) and ZK98734 and ZK98299 (Schering AG). These compounds bind avidly to the progesterone receptor and glucocorticoid receptor but have essentially no binding to the mineralocortocoid, oestrogen or androgen receptors. Mifepristone also binds avidly to albumin, resulting in a half-life of approximately 24 h after oral administration. Progesterone receptor antagonists can induce menstruation by a direct action upon the endometrium. They have also been shown to exert weak progesterone agonist actions in certain circumstances and to modulate pituitary hormone secretion by antagonizing the feedback actions of progesterone. Moreover, they release prostaglandin F2 alpha and E2 from human endometrium or early pregnancy decidua and reduce the metabolism of these eicosanoids. Clinically, progesterone receptor antagonists have been used in trials of menstrual regulation, abortion and induction of labour, and during treatment of breast or ovarian cancer, some forms of hypertension and meningioma. Progesterone receptor antagonists have been administered to approximately 70,000 women in 18 countries as medical abortifacients. They have been proven, especially when combined with prostaglandin analogues, to be as effective as surgical methods of termination of pregnancy. Progesterone receptor antagonists have focussed international attention on menstrual regulation, abortion and the rights of women to regulate their fertility.

[Binding characteristics of RU 486 with glucocorticoid and progestin receptors]

RU 34886 (RU 486) has been proved to fully antagonize the actions of glucocorticoid and progestin at the receptor level. The binding characteristics of RU 486 with the thymic glucocorticoid receptor (GR) and the uterine progestin receptor (PR) were investigated in order to elucidate the mechanism of these antagonistic actions. The ability of RU 486 was studied to promote the "activation" and the "nuclear translocation" of GR and RP. Under heat activation, RU 486 dissociated faster from the activated GR than the non activated, and the binding complex of RU 486 and GR showed lower affinity for DNA-cellulose than the glucocorticoid agonist-GR complex. Nearly undetectable amounts of RU 486 were recovered in the nucleus. Conversely, the RU 486-PR complex showed no difference of dissociation rate between the activated and the non activated condition. Its affinity for DNA-cellulose was the same as that of the activated progestin agonist-PR complex. A large amount of this compound was demonstrated in the nucleus.