The estrogenic and antiestrogenic activities of droloxifene in human breast cancers

Comparison of the effects of EM-652 (SCH57068), tamoxifen, toremifene, droloxifene, idoxifene, GW-5638 and raloxifene on the growth of human ZR-75-1 breast tumors in nude mice

EM-652 exerts pure antiestrogenic activity in the mammary gland and endometrium, while tamoxifen, the antiestrogen most widely used for the treatment of breast cancer, exerts mixed antiestrogenic-estrogenic activity in these tissues. Our objective was to compare the agonistic and antagonistic effects of EM-652 with tamoxifen and 5 other antiestrogens on the growth of ZR-75-1 human breast xenografts in ovariectomized nude mice. During the 23 weeks of treatment at a daily oral dose of 50 microg, EM-652 was the only compound that decreased tumor size relative to pretreatment values, whereas the 6 other antiestrogens only decreased to various extents the progression rate stimulated by estrone. Under estrone stimulation, all groups of animals had more than 60% of their tumors in the progression category except for the EM-652-treated group, where only 7% of the tumors progressed. In the absence of estrone stimulation, progression was seen in 60%, 33%, 21% and 12% of tumors in the tamoxifen-, idoxifene-, toremifene- and raloxifene-treated groups, respectively, while only 4% of tumors progressed in the EM-652-treated group. The agonistic and antagonistic actions of each antiestrogen were also measured on endometrial epithelial cell thickness. Our present findings indicate that EM-652, in addition to being the most potent antiestrogen on human breast tumor growth, has no agonistic effect in breast and endometrial tissues. Since previous data have shown benefits of EM-652 on bone density and lipid profile, this compound could be an ideal candidate for chemoprevention of breast and uterine cancers, while protecting against osteoporosis and cardiovascular disease.

Selective estrogen receptor modulators: structure, function, and clinical use

The sex hormone estrogen is important for many physiologic processes. Prolonged stimulation of breast ductal epithelium by estrogen, however, can contribute to the development and progression of breast cancer, and treatments designed to block estrogen's effects are important options in the clinic. Tamoxifen and other similar drugs are effective in breast cancer prevention and treatment by inhibiting the proliferative effects of estrogen that are mediated through the estrogen receptor (ER). However, these drugs also have many estrogenic effects depending on the tissue and gene, and they are more appropriately called selective estrogen receptor modulators (SERMs). SERMs bind ER, alter receptor conformation, and facilitate binding of coregulatory proteins that activate or repress transcriptional activation of estrogen target genes. Theoretically, SERMs could be synthesized that would exhibit nearly complete agonist activity on the one hand or pure antiestrogenic activity on the other. Depending on their functional activities, SERMs could then be developed for a variety of clinical uses, including prevention and treatment of osteoporosis, treatment and prevention of estrogen-regulated malignancies, and even for hormone replacement therapy. Tamoxifen is effective in patients with ER-positive metastatic breast cancer and in the adjuvant setting. The promising role for tamoxifen in ductal carcinoma-in-situ or for breast cancer prevention is evolving, and its use can be considered in certain patient groups. Other SERMs are in development, with the goal of reducing toxicity and/or improving efficacy, and future agents have the potential of providing a new paradigm for maintaining the health of women.

The evolving role of specific estrogen receptor modulators (SERMs)

Estrogens are the most effective therapy for women with postmenopausal problems. However, relatively few women use estrogen and then often for a limited time because of the fear of its carcinogenic effects on the uterus and breast; in addition, estrogen is not advised for women who have had breast cancer. Selective estrogen receptor modulators (SERMs) are agents with antagonist action on the uterus and breast and agonist action on the bones, cardiovascular system, and brain. Unlike estrogens, however, existing SERMs do not help alleviate the vasomotor and urogenital problems associated with menopause. A comprehensive review of the literature published from January 1995 to June 1999 was conducted. Reports were identified using Medline and Cancer Lit. The effect of menopausal problems on the health of women and the socioeconomic effects of menopause are discussed. All currently available and investigational SERMs are reviewed and discussed, including their mechanism of action, metabolism, dose scheduling, antitumor activity, and potential role in maintaining the health of menopausal women and in preventing breast cancer.

Endocrine therapy in metastatic breast cancer

Endocrine therapy represents a mainstay of effective, minimally toxic, palliative treatment for metastatic breast cancer. Research focusing on the mechanism of action of endocrine agents will provide new insights leading to new hormonal approaches in breast cancer treatment. Development of new agents, especially the 'pure' antiestrogens, is of great interest. Combining endocrine therapy with biologic agents, especially antiproliferative compounds, may lead to more effective treatment in the adjuvant as well as the advanced setting. Tables 4 and 5 summarize response rates to the different groups of endocrine agents used in metastatic breast cancer and doses of commonly used agents, respectively. At present, tamoxifen is the drug of choice as first-line endocrine therapy for metastatic breast cancer with no or minimal symptoms in premenopausal or postmenopausal women. Second-line therapy usually consists of megace. Aromatase inhibitors may be used as second- or third-line therapy in postmenopausal women. In premenopausal women, LHRH analogues are a reasonable choice. The other hormonal agents may be beneficial as salvage therapy. More effective endocrine approaches are under development.

Mechanisms of action of endocrine treatment in breast cancer

Endocrine treatment plays an important role in the therapy of breast cancer. While the basic mechanisms are understood, additional mechanisms may be of importance to their action and they may also contribute to the mechanism(s) of acquired resistance. Currently, several novel drugs are entering into clinical trials. Observations of the absence or presence of cross resistance to novel 'pure' steroidal antiestrogens and the non-steroidal tamoxifen may add important information to our understanding of the mechanisms of action of both classes of drugs. Similarly, exploration of different aromatase inhibitors in sequence or concert, as well as the combining of different endocrine treatment options may be warranted. Additionally, alterations in different biochemical parameters such as growth factors should not only be carefully explored in relation to treatment options but should also be followed during the course of treatment to asess alterations over time and in relation to the development of drug resistance.

Binding sites of droloxifene in the cytosol of 7,12-dimethylbenz[a]anthracene-induced rat mammary tumor cells

The binding sites, other than the estrogen receptor (ER), of the antiestrogens droloxifene (DROL, (E)-a-[p-[2-(dimethylamino)ethoxy]-phenyl]-a'-ethyl-3-stilbenol) and tamoxifen (TAM), and estradiol-17 beta (E2) in the cytosol of 7,12-dimethylbenz[a]anthracene-induced rat mammary ER-positive tumor cells were studied using a high-performance liquid chromatography (HPLC) gel filtration assay. The cytosol was incubated with 3H-labeled drug with or without unlabeled drug, and separated by HPLC gel filtration. 3H-E2 produced two major peaks of radioactivity at fractions No. 40 and No. 70. The peak at fraction No. 70 was identified as the ER in an ER-enzyme-immuno assay. This peak was dose-dependently inhibited by unlabeled DROL or TAM, DROL being a more potent inhibitor than TAM. The peak at fraction No. 40 was also inhibited by co-incubation with unlabeled DROL or TAM. 3H-DROL or 3H-TAM provided only one peak at fraction No. 43. This peak was thought to be an antiestrogen binding site (AEBS), because it was inhibited by unlabeled antiestrogen but not by E2. The results suggest that the antiestrogens DROL and TAM have a higher affinity for the AEBS than for the ER in the absence of E2, while in the presence of E2 both have an affinity for the ER and inhibit E2 binding to the ER.