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Background: Tamoxifen has been shown to promote the growth of human endometrial tumors implanted in athymic mice, and it has been associated with a twofold to threefold increase in endometrial cancer. Toremifene, a chlorinated derivative of tamoxifen, and ICI 182,780, a pure antiestrogen, are two new antiestrogens being developed for the treatment of breast cancer. The effects of these drugs on endometrial cancer are currently unknown. Our objective was to evaluate the effects of toremifene and ICI 182,780 on the growth of human endometrial cancer in athymic mice., Methods: Athymic, ovariectomized mice were implanted with human endometrial tumors and treated with estrogen, tamoxifen, or the new antiestrogens., Results: The effects of tamoxifen and toremifene on the growth of either tamoxifen-stimulated or tamoxifen-naive endometrial tumors in athymic mice were not substantially different. ICI 182,780 inhibited the growth of tamoxifen-stimulated endometrial cancer, in both the presence and the absence of estrogen., Conclusions: Toremifene and tamoxifen produce identical effects in our endometrial cancer models. Therefore, it is possible that toremifene, like tamoxifen, may be associated with an increased incidence of endometrial cancer. In contrast, ICI 182,780 inhibited tamoxifen-stimulated endometrial cancer, both in the presence and in the absence of estrogen, suggesting that this drug may be safe with regard to the endometrium, even if it is used following tamoxifen, and that it may not result in an increased incidence of endometrial cancer. Indeed, it is even possible that ICI 182,780 may prove useful as an adjuvant agent in early stage endometrial cancer.

It is clear that growth of the MCF-7 breast cancer cell line is stimulated by estrogen and when estrogen is removed, growth slows. We have observed a tumor derived from MCF-7 cells that grows in athymic mice in the absence of estrogen stimulation. We hypothesized that a mutation in the estrogen receptor (ER) could be responsible for this constitutive growth. Using single stranded conformational polymorphism (SSCP) and DNA sequencing analysis, we have identified an ER containing a point mutation at position 415 (gly to val) within the hormone binding domain. The functional activity of this mutant was assessed in vitro and in vivo. Using transient transfection into an ER negative breast cancer cell with an ERE luciferase reporter gene, we found that both the wild-type and mutant receptors have similar efficacy. Additionally, the estrogenic responses were blocked by antiestrogens in a concentration related manner. We also found that tumors with the mutant receptor show similar growth response in athymic mice as wild-type: stimulation with estradiol and inhibition with antiestrogens. We conclude that the point mutation at position 415 (gly to val) is not responsible for constitutive growth.

Tamoxifen is the endocrine therapy of choice for all stages of breast cancer. However, the drug cannot be considered to be a cure as drug resistance will eventually develop. The resistance can take two forms: either the loss of estrogen receptor or the selection of estrogen receptor positive disease that is tamoxifen stimulated for growth. Laboratory studies have demonstrated that tamoxifen-stimulated MCF-7 breast tumors can develop in athymic mice. A number of pure (nonestrogenic) antiestrogens have been discovered that can either be administered by injection (e.g., ICI 182,780) or orally (e.g., EM-800). In preliminary clinical studies, the compound ICI 182,780 (Faslodex) has been shown to be an effective second-line therapy after tamoxifen failure. The goal of future clinical studies is to evaluate the therapeutic efficacy and patient acceptability of aromatase inhibitors (postmenopausal estrogen withdrawal), and injectable or oral pure antiestrogens after the failure of long-term tamoxifen therapy. Clearly, the primary purpose for the treatment of advanced breast cancer is to control disease growth; nevertheless, an evaluation of the effect of new agents on bones and lipids is required before pure antiestrogens could be considered for adjuvant therapy.