RU 58668: further in vitro and in vivo pharmacological data related to its antitumoral activity.

Previous studies with the pure antiestrogen RU 58668 showed that this compound proved to be highly antiproliferative in vitro, and to be the only antiestrogenic compound so far known to induce long-term regression of MCF-7 tumours implanted into nude mice. In order to obtain more insight into the therapeutic potential of this molecule, we performed a new set of experiments in vitro and in vivo in comparison with tamoxifen and/or ICI 182,780. In vitro, 1 nM RU 58668 induced an accumulation of MCF-7 cells in G0/G1 phases of the cell cycle within 48 h and, in contrast to trans-4-hydroxy-tamoxifen, blocked the invasiveness of ras-transfected MCF-7 cells into the chick embryo heart during the three weeks of co-culture. An in vivo dose-effect relationship study showed that RU 58668 induced a regression of MCF-7 tumour with as low a dose as 10 mg/kg/week, and that such an effect can not be obtained either with a sublethal dose of adriamycin or with ICI 182,780, (2-250 mg/kg/week). This reduction in the tumour volumes accords with histological modifications of the tumours, which showed a decrease in the ratio of epithelial cells over the tumoral mass, and with a concomitant decrease in their regrowth potential when reimplanted into naive nude mice. Taken together, these results suggest a promising usefulness for RU 58668 in the treatment of metastatic breast cancer in women.

Exploration of the therapeutic potential of the antiestrogen RU 58668 in breast cancer treatment.

The recently described pure antiestrogen RU 58668 displayed potent antiproliferative activities in vitro on several ER+ human mammary cell lines, stimulated either by estradiol or by endogenous or exogenous growth factors. Moreover, when administered to nude mice it proved to be the only antiestrogen to induce regression (at least 10 weeks) of estradiol-stimulated MCF-7 tumors, whereas tamoxifen only stabilized the tumor volume for 4 to 8 weeks. So the first purpose of this work was to study the effect of RU 58668 for 6 months and to evaluate its activity on tumors which escaped from the tamoxifen treatment. On the other hand, we looked for its effect on models more related to frequently described clinical observations, such as the overexpression of an oncogene or the implication of autocrine or paracrine growth factors. Long-term studies of RU 58668 on the estradiol-stimulated MCF-7 model showed that this compound induced a shrinking of tumor volumes for at least 25 weeks (3 to 6 times longer than the stabilization induced by tamoxifen) and was able to reduce the volume of tumors which escaped from, or even were stimulated by, tamoxifen. On models of spontaneously growing tumors, where the overexpression of an oncogene or the production of growth factors was involved, RU 58668 induced the same tumor shrinking that was previously observed on estradiol- or tamoxifen-stimulated models. Finally, when MCF-7 cells were injected in the uteri, a spontaneous tumor take was observed (in about 80-90% of the animals), leading to a more than twofold increase in uterus weight. This growth is largely stimulated by estradiol and tamoxifen. On this model, histological examination showed that only 30% of the animals receiving RU 58668 displayed tumoral microfoci. These studies suggest that RU 58668 may be used for the treatment of ER+ patients which are primarily resistant to or which escaped from tamoxifen treatment. Its preventive activity on tumor take also suggests its use as an adjuvant to prevent the development of metastases.

RU 58,668, a new pure antiestrogen inducing a regression of human mammary carcinoma implanted in nude mice.

RU 58,668, a new steroidal antiestrogen, has been synthesized. Its in vitro and in vivo pharmacological activities have been compared to those of tamoxifen and ICI 182,780. In vitro, it displayed affinities for human and murine estrogen receptors equivalent to those of 4-hydroxy-tamoxifen, along with moderate affinities for progestin and glucocorticoid receptors. RU 58,668 proved to be a potent antiproliferative agent on MCF-7 cells stimulated by estradiol, or by exogenous or endogenous growth factors (IC50, 0.01 nM). It also inhibited the growth of the insulin-stimulated T47D cell line. In vivo, RU 58,668 displayed a total anti-uterotrophic activity in mice or rats without exhibiting any agonistic effect. Moreover, RU 58,668 was the only antiestrogenic compound tested so far to be able to induce a long term regression of human mammary MCF-7 tumors implanted in nude mice, suggesting its potential use for the treatment of advanced breast cancer.