Comparison between the interaction of steroids with [35S]TBPS binding to cerebral cortical and to pituitary membranes: correlation with inhibition of prolactin release.

It has been shown previously that 5 alpha-pregnan-3 alpha-ol-20-one (5 alpha 3 alpha P) can inhibit prolactin release from anterior pituitary gland cells in culture through an interaction with a specific modulatory site on the GABAA receptor complex in anterior pituitary gland membranes. In the present work, this receptor site has been labelled with [35S]t-butylbicyclophosphorothionate ([35S]TBPS) to enable a study of the relative binding affinities (RBA) of different steroids for the GABAA receptor complex to be made. We have found a high correlation (r = +0.88) between the inhibition of [35S]TBPS binding to anterior pituitary membranes and the inhibition of [35S]TBPS binding to cerebral cortical membranes by nine different steroids. There was also a high correlation between the inhibition of prolactin release from anterior pituitary gland cells in culture by these steroids and the inhibition of [35S]TBPS binding to anterior pituitary membranes (r = +0.99) or to cortical membranes (r = +0.81). These observations suggest that the measurement of prolactin release from anterior pituitary gland cells in culture is a good indicator of the functional activity of drugs that bind to the allosteric modulatory TBPS-binding site on the GABAA-receptor complex.

Pharmacology of antiandrogens and value of combining androgen suppression with antiandrogen therapy.

Antiandrogens are compounds able to block the effect of androgens directly on their target cells by inhibiting their binding to the androgen receptor (AR). Two chemical classes of antiandrogens are presently on the market or in clinical trials: steroids (cyproterone, megestrol acetates), and nonsteroids (flutamide, nilutamide). Steroid antiandrogens interact not only with AR but also with progestin and glucocorticoid receptors and thus give rise to progestin and glucocorticoid effects. By contrast, nonsteroid antiandrogens interact only with AR and are thus devoid of other hormonal or antihormonal activities. Nilutamide does not need to be transformed into an active metabolite, unlike flutamide, and interacts with dog, rat, and human prostate AR in vitro. Its kinetics lead to a prolonged interaction with AR in vivo after administration to rats. In prostate cancer treatment, it is necessary to combine an antiandrogen to surgical or chemical (estrogens, LH-RH agonists) castration to obtain a complete suppression of androgens. The antiandrogen will block specifically, at the target site, the trophic effect of adrenal androgens left intact by castration, and the secretion of which can only be suppressed by treatments (adrenalectomy, aminoglutethimide, ketoconazole) that also suppress corticoid synthesis. We have shown that nilutamide counteracts the trophic effect, on the prostate of castrated rats, of adrenal androgens administered continuously (minipumps) at circulating levels similar to those recorded in castrated men. Nilutamide will also impede the flare-up effect of the testosterone increase induced by LH-RH agonists at the beginning of treatment. We have shown in the rat treated with buserelin that the increase in prostate weight observed during the initial days of treatment by the LH-RH agonist can be inhibited by a combined treatment with nilutamide. This combined treatment "nilutamide plus castration" has been tested in an experimental androgen-dependent cancer model, the Shionogi tumor. The administration of nilutamide to mice, castrated twenty-four hours before the inoculation of tumor cells, delayed the appearance of tumors and reduced their number. Finally, the absence of androgen effect and the antiandrogen activity of the product were also demonstrated in human tumor cells in culture (T-47 D cells) transfected with the MMTV androgen-dependent promoter coupled with the CAT reporter gene.

New analogues of mifepristone with more dissociated antiprogesterone activities.

Mifepristone (RU 486 or RU 38486) possesses strong antiprogesterone and antiglucocorticoid along with moderate antiandrogen properties, which would limit its use in some therapeutic applications. In a search for more dissociated derivatives, the hydroxy substituent and the propynyl group in position 17 of the RU 486 series was replaced by a spiroether group, which is known to induce specific affinity for the progestin receptor in steroid series. The substituents in the para position of the 11 beta-phenyl group, leading to the most potent derivatives in the RU 486 series, were retained. The new derivatives have been studied in vitro for their relative binding affinities (RBAs) for the steroid receptor and in vivo for their hormonal and antihormonal activities. The selected compounds, RU 46556 and RU 49295 display the following properties: in vitro, like RU 486, they show a strong RBA for the rabbit progestin receptor, but a much lower one for the rat thymus glucocorticoid receptor; in vivo they are about three times more active than RU 486 for inducing abortion in rats, but unlike the latter they are devoid of any antiglucocorticoid activity on the thymus weight in rats. These antiprogesterone effects have been confirmed on the deciduoma formation in rats and on the endometrial proliferation in rabbits. However, in contrast to RU 486 in the latter test, some progestomimetic activity has been observed. RU 46556 and RU 49295 are now under extensive pharmacological study.