The use of interaction kinetics to distinguish potential antagonists from agonists

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.

Improvement in glucocorticoid receptor binding affinity concomitant to shift from antagonist to agonist activity in a series of 17 beta-carboxamide derivatives of dexamethasone

Modification of the 17 beta-side chain of the synthetic glucocorticoid agonist dexamethasone by periodic oxidation and subsequent coupling to various primary amines yield secondary 17 beta-carboxamide derivatives displaying antiglucocorticoid activity in vitro, but not in vivo. To obtain more potent antiglucocorticoids, new secondary and tertiary 17 beta-carboxamide derivatives were synthesized. Although they displayed an improved affinity for the glucocorticoid receptor in rat thymus cytosol and antiglucocorticoid activity in rat hepatoma (HTC) cells, these new compounds were again devoid of in vivo antiglucocorticoid activity in the rat. Moreover, the increase in receptor binding affinity was correlated for most compounds with the appearance of a partial agonist activity in HTC cells. The tertiary 17 beta-carboxamide derivative DX diMe displayed the highest affinity but was also a partial agonist in vivo. Kinetic studies with several tritiated 17 beta-carboxamide derivatives showed that they had association rate constants similar to that of dexamethasone, but different dissociation rate constants. The rapid dissociation of the compounds displaying antiglucocorticoid activity contrasted with the slow dissociation of DX diMe. Therefore, antiglucocorticoid activity in the 17 beta-carboxamide series is probably related to the formation of rapidly dissociating glucocorticoid receptor-ligand complexes that are unable to undergo the transformation step.

Contragestion and other clinical applications of RU 486, an antiprogesterone at the receptor

RU 486, a steroid with high affinity for the progesterone receptor, is the first available active antiprogesterone. It has been used successfully as a medical alternative for early pregnancy interruption, and it also has other potential applications in medicine and for biochemical and pathophysiological endocrine research.

Interaction of cyproterone acetate with rat prostatic androgen receptors

We have investigated the binding of cyproterone acetate (CA) to cytosolic androgen receptors (RC) and translocation of the RCCA complex into the nucleus. In a cell-free system (3H)CA binds to cytosolic androgen receptors with high affinity (KD = 11.6 nM) and limited capacity (180-200 femtomoles/mg protein). (3H)CA, however, dissociates very rapidly from the cytosolic and nuclear androgen receptors (Rn) at 0 degree C. Incubation of RC (3H)CA at 20 degrees C increased its ability to bind to nuclei. Translocation of RC (3H)CA to nuclei of intact cells was demonstrated after incubation of prostatic tissue with (3H)CA in tissue culture medium at 37 degrees C. In vivo administration of CA to castrated rats promoted RCCA translocation but did not induce androgen receptor replenishment. These data demonstrate that CA binds to and translocates androgen receptors to nuclei without concomitant receptor replenishment.

RU 38486: potent antiglucocorticoid activity correlated with strong binding to the cytosolic glucocorticoid receptor followed by an impaired activation

In order to explain the potent antiglucocorticoid activity of RU 38486 and the absence of agonist effect in spite of its very strong interaction with the cytoplasmic glucocorticoid receptor (GR), we investigated the compound's ability to promote GR "activation" and nuclear translocation. We have compared the dissociation-rates of the "non-activated" (molybdate stabilized) and of the "activated" (25 degrees C pre-heated) GR complexes formed either with [3H]RU 38486 or with different tritiated glucocorticoid agonists. While agonists dissociated more slowly from the "activated" than from the "non-activated" complex, RU 38486 dissociated much faster from the "activated" than from the "native" receptor. This difference of activation was confirmed in a DNA-cellulose binding assay. The affinity of the "activated" RU 38486-GR complex for DNA was much lower than that of the dexamethasone-GR complex. Finally, the in vitro nuclear uptake of [3H]RU 38486 was compared with that of [3H]dexamethasone after incubation with thymus minces at 25 or 37 degrees C. A very weak or nearly undetectable level of specific uptake of [3H]RU 38486 was observed in purified nuclei, whatever the concentration or the time of incubation used. These observations suggest that while glucocorticoid agonists form with the non-activated receptor a complex able to be activated into a more stable form (lower k-1), RU 38486 interacts strongly with the non-activated receptor (impeding the binding of DM) but the complex is "transformed" by heat to a less stable form (higher k-1), unable to translocate properly into the nucleus in order to trigger a glucocorticoid response.

Dependence of rate constants of the glucocorticoid hormone-receptor interaction on steroid structure

Making use of the methods developed earlier [P. Arányi and V. Quiroga, J. steroid Biochem. 13, 1167-1172 (1980)] rate constants of the steroid-glucocorticoid-receptor interactions were determined for a number of non-labeled analogues. Comparison of receptors from different species (chick thymus vs rat thymus) showed very similar specificities. Association rate constants varied over almost as large a range as did dissociation rate constants. Rate constants of the forward and reverse reactions are determined by the steroid structure apparently independently of each other.

Differences between oestrogen receptor activation by oestrogen and antioestrogen

Triphenylethylene antioestrogens such as tamoxifen, nafoxidine and Ci 628 specifically inhibit oestrogen action at the target cell level, probably by interacting with the oestrogen receptor (ER) and competitively displacing oestrogens from their binding sites. It is not clear, however, why these ligands are less biologically active than oestrogens when the bind to the ER, as no reliable difference has been found either in the binding affinity of these two series of ligands to the ER or in their ability to translocate the ER to the nucleus. In fact, these antioestrogens are transformed in vivo into hydroxylated metabolites which display a better antioestrogenic activity than the injected compound and at least the same high affinity as oestradiol for the ER. With the aim of finding an in vitro criterion to predict the agonistic or antagonistic properties of ER ligands, we have stabilized the ER in its 'native' or non-activated form by the use of molybdate and have compared the binding of oestradiol (E2) and of 4-hydroxytamoxifen (OHT), an active metabolite of tamoxifen, to the molybdate-treated and to the activated ER. We report here that molybdate prevented the DNA binding and the 4S to 5S transformation of the ER bound to both ligands, and that it increased the dissociation rate of oestrogens but not that of antioestrogens. Moreover, in the absence of molybdate, receptor activation by heating decreased the dissociation rate of E2 but not that of OHT. We conclude that a difference exists between the ER activation triggered by oestrogens and antioestrogens and propose that antioestrogens are acting as allosteric ligands of the ER.