Search

The recognition of selective estrogen receptor modulation in the laboratory has resulted in the development of two selective estrogen receptor modulators (SERMs), tamoxifen and raloxifene, for clinical application in healthy women. SERMs are antiestrogenic in the breast but estrogen-like in the bones and reduce circulating cholesterol levels. SERMs also have different degrees of estrogenicity in the uterus. Tamoxifen is used specifically to reduce the incidence of breast cancer in premenopausal and postmenopausal women at risk for the disease. In contrast, raloxifene is used specifically to reduce the risk of osteoporosis in postmenopausal women at high risk for osteoporosis. The study of tamoxifen and raloxifene (STAR) trial is currently comparing the ability of these SERMs to reduce breast cancer incidence in high-risk postmenopausal women. There is intense interest in understanding the molecular mechanism(s) of action of SERMs at target sites in a woman's body. An understanding of the targeted actions of this novel drug group will potentially result in the introduction of new multifunctional medicines with applications as preventive agents or treatments of breast cancer and endometrial cancer, coronary heart disease, and osteoporosis.

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.

Background: The nonsteroidal anti-estrogen tamoxifen (TAM) is the front-line endocrine treatment for breast cancer, but disease recurrence is common. Treatment failure may occur because tumors become insensitive to TAM. Alternatively, resistance may occur because tumors become stimulated rather than inhibited by TAM. TAM-stimulated growth of MCF-7 human breast tumors has been observed in athymic mice after prolonged treatment with TAM., Purpose: Our purpose was to examine the mechanism of treatment failure by determining whether TAM-stimulated tumors acquire the ability to excrete TAM and its anti-estrogenic metabolites or to convert them to estrogenic compounds with weakened antiestrogenic activity., Methods: We used high-pressure liquid chromatography to quantitate TAM and its metabolites in serum and tumors from ovariectomized athymic mice and in MCF-7 cells grown in vitro. We treated tumor-bearing mice with subcutaneous sustained-release preparations of estradiol, TAM, or a nonisomerizable (fixed-ring) analogue and then assessed the activity of these compounds on TAM-inhibited parental MCF-7 tumors and on TAM-stimulated MCF-7 TAM tumors., Results: We found negligible differences in intratumoral TAM levels between TAM-inhibited parental MCF-7 tumors and TAM-stimulated MCF-7 TAM variants. We did not detect metabolite E (Met E), an estrogenic TAM metabolite, in serum or tumors. Using MCF-7 cells in vitro, we determined that the (Z) isomer of Met E, the form directly produced by TAM metabolism, must be present in the cell at a concentration of over 1000 ng/g to overcome growth inhibition by physiological levels of TAM and antiestrogenic metabolites, but the (E) isomer of Met E was effective at 10 ng/g. We reasoned that conversion of Met E from the (Z) (a weak estrogen) to (E) isomer (a potent estrogen) would be required if formation of Met E were responsible for TAM-stimulated growth. However, fixed-ring TAM, which can only form (Z) Met E, was shown to be as capable as TAM of initiating and maintaining anti-estrogen-stimulated growth of MCF-7 tumors in athymic mice., Conclusion: Metabolism and isomerization of TAM to estrogenic compounds is not the mechanism of TAM-stimulated growth in our model., Implication: Other potential mechanisms for TAM-stimulated growth, such as estrogen receptor mutation, must be investigated so that effective strategies can be devised to control breast cancer once therapy fails.

Background: P-glycoprotein mediates resistance to natural-product anti-neoplastic agents like vinblastine through an active transport process resulting in reduced intracellular concentration of these agents. The triphenylethylene antiestrogen tamoxifen and its major metabolite N-desmethyltamoxifen at concentrations of 4-6 microM enhance the intracellular concentration of natural-product antineoplastics and augment the cytotoxicity of such drugs three-fold to 10-fold in a variety of human and murine cell lines., Purpose: On the basis of these preclinical findings, we conducted a phase I clinical trial of high-dose, oral tamoxifen administered in conjunction with a 5-day continuous infusion of vinblastine., Methods: We studied 53 patients with advanced epithelial tumors. Tamoxifen was given orally as a loading dose on day 1, followed by two doses a day on days 2-13. Vinblastine was given as a 120-hour continuous infusion (1.5 mg/m2 per day) on days 9-13 of each tamoxifen course. The starting dose of tamoxifen was 40 mg/m2 administered twice a day following a loading dose of 150 mg/m2. The maximum dose was 260 mg/m2 twice a day following a loading dose of 680 mg/m2. Treatment cycles were repeated every 28 days., Results: The dose-limiting toxic effects of tamoxifen were neurologic and began within 3-5 days after the start of treatment. They consisted of tremor, hyperreflexia, dysmetria, unsteady gait, and dizziness. One patient experienced a grand mal seizure 24 hours after the last tamoxifen dose. Toxic effects were rapidly reversible. Asymptomatic prolongation of the QT interval on electrocardiogram occurred at doses of tamoxifen of 80 mg/m2 or higher given twice a day. No coagulation or ophthalmologic abnormalities occurred. Tamoxifen did not enhance the toxicity of vinblastine. Mean plasma concentrations of tamoxifen or N-desmethyltamoxifen at 260 mg/m2 tamoxifen given twice a day for 13 days were 6.04 and 6.56 microM, respectively. There was no relationship between plasma antiestrogen content and the development of neurotoxic effects., Conclusions: Tamoxifen at 150 mg/m2 given twice a day following a loading dose of 400 mg/m2 results in plasma levels of tamoxifen and N-desmethyltamoxifen of 4 and 6 microM, respectively, without dose-limiting toxicity. We recommend this dose for phase II trials of tamoxifen to modulate P-glycoprotein-mediated drug resistance., Implications: Our study demonstrates that high-dose tamoxifen can be safely administered and that plasma concentrations that may inhibit P-glycoprotein function can be achieved.

Background: The growth of estrogen receptor (ER)-positive breast cancer cells is hormonally regulated, but the majority of breast cancers are ER negative and unresponsive to hormonal therapy., Purpose and Methods: To test whether hormonal control over replication can be re-established in ER-negative cells, we transfected ER-negative MDA-MB-231 (clone 10A) cells with sense and antisense constitutive ER expression vectors containing the gene for either wild-type or mutant ER linked to the gene for neomycin resistance aminoglycoside phosphotransferase (neo). A Northern blot analysis was done on total RNA from eight of the 10 transfectant clones produced to detect messenger RNA coding for ER and neo, and a Western blot analysis was done on protein extracted from the cells of one mutant and two wild-type ER sense transfectant clones to determine the molecular weight of the ER in transfectants. Levels of ER in transfectants were measured both by enzyme immunoassay and by ligand-binding methods. To ascertain whether the ER in wild-type and mutant sense transfectants was functional, we tested the effects of 17 beta-estradiol (E2) and/or an antiestrogen, ICI 164,384, on 1) ER-activated gene regulation (by transient transfection of these cells a second time with a reporter plasmid containing an estrogen response element linked to the chloramphenicol acetyl transferase [CAT] gene), 2) induction of progesterone receptor, 3) DNA replication, and 4) cell cycle kinetics., Results: Messenger RNA coding for ER and for neo was detectable in both sense and antisense transfectant clones. Sense transfectants (both mutant and wild-type) expressed ER protein with a molecular weight similar to that found in ER-positive control cells. By the ligand-binding method high levels of ER were detected in both wild-type and mutant transfectants, although by the enzyme immunoassay method lower levels were detected in mutant transfectants. ER from both wild-type and mutant sense transfectants appeared functional, since E2 stimulated the expression of reporter-linked CAT and of progesterone receptor in these transfectants. E2 inhibited DNA replication in wild-type sense transfectants at a concentration of 10(-10) M and mutant sense transfectants at a concentration of 10(-8) M, and ICI 164,384 blocked this effect., Conclusion: ER-negative breast cancer cells stably transfected with either a mutant or wild-type ER gene regain hormonal responsiveness; however, E2 inhibits rather than stimulates cell growth., Implication: Reactivation of quiescent ER may provide a novel therapeutic approach for controlling ER-negative breast cancers.

Tamoxifen is used to treat selected patients at each stage of breast cancer. Although most clinical experience has been obtained with postmenopausal women, increasing numbers of premenopausal women will be treated with 5 or more years of adjuvant tamoxifen therapy following surgery. Indeed, the proposed use of tamoxifen to prevent breast cancer in high-risk women could result in its extended use during the childbearing years. We have monitored the changes in circulating hormone levels from the ovary and pituitary gland in premenopausal (41-47 years old) women with stage I or II breast cancer during adjuvant therapy with tamoxifen as the single agent for 4-72 months following a mastectomy. Each patient (total eight) continued to menstruate, and ovulation occurred. Circulating levels of luteinizing hormone and follicle-stimulating hormone (except in one woman) remained in the normal range (determined in 12 regularly menstruating women in a control group). The levels of estradiol, estrone, and progesterone were elevated onefold to threefold. Prolactin levels decreased by 30%-40%, but the levels of sex hormone binding-globulin were unaffected. These data demonstrate that premenopausal women taking tamoxifen are potentially at risk for pregnancy and must be counseled about barrier contraception. Furthermore, the impact of many years of ovarian stimulation by tamoxifen must be evaluated, especially in women with node-negative disease or in healthy women in whom tamoxifen is used to prevent breast cancer.

A sustained release of tamoxifen, which produced decreasing serum levels of this drug (24 to 4 ng/mL) over 6 months, suppressed mammary tumorigenesis in virgin or once pregnant C3H/OUJ female mice. Tamoxifen was consistently more effective than early ovariectomy, which only delayed tumorigenesis. Tamoxifen prevented the stimulatory action of cyclical (alternate-month) progesterone administration on mouse mammary tumorigenesis. However, when tamoxifen treatment (12 months) was stopped, progesterone treatment initiated tumorigenesis. In contrast, when long-term tamoxifen treatment was stopped in mice that had not undergone ovariectomy, and estrous cycle returned, the majority of these mice remained tumor free. A comparison of different durations (3, 6, and 12 months) of tamoxifen treatment of virgin mice, starting at approximately 4 months of age, showed an equivalent effect on mammary tumorigenesis. All virgin mice developed tumors by 18 months of age, whereas 80% of the tamoxifen-treated mice were tumor free. Nevertheless, cyclical progesterone administration caused rapid development of tumors after 3 months of tamoxifen treatment; only 15% of these mice were tumor free at 18 months. Cyclical progesterone administration caused an increase in tumorigenesis after 6 months of tamoxifen treatment; 50% of these mice were tumor free at 18 months of age. These data demonstrate the efficacy of tamoxifen to suppress mouse mammary tumorigenesis and demonstrate that continuous tamoxifen therapy is necessary to prevent the development of tumors by progesterone, a stimulatory hormone.

We conducted a 2-year, randomized, double-blind, placebo-controlled toxicity trial of therapy with tamoxifen (10 mg twice a day) in 140 postmenopausal women with a history of breast cancer and histologically negative axillary lymph nodes. These women had been treated with surgery with or without radiotherapy. At a 3-month evaluation, tamoxifen-treated women showed a significant decrease in fasting plasma levels of total cholesterol and low-density lipoprotein (LDL) cholesterol, which persisted at 6- and 12-month evaluations. During the first 12 months, plasma triglyceride levels increased; small but significant decreases in high-density lipoprotein cholesterol (HDL) were observed in tamoxifen-treated women, but ratios of total cholesterol to HDL cholesterol and of LDL to HDL cholesterol changed favorably. While data relating lipid/lipoprotein profiles and cardiovascular disease are limited in women, current evidence suggests that total cholesterol and possibly low-density lipoprotein cholesterol are risk factors. We conclude that during the first 12 months of treatment, tamoxifen exerts a favorable effect on the lipid profile in postmenopausal women with early stage breast cancer.