Possible mechanisms in the emergence of tamoxifen-resistant breast cancer

Tamoxifen (TAM), a non-steroidal antiestrogen, is the endocrine treatment of choice for all stages of breast cancer. However, despite a favorable initial response to therapy, most tumors will eventually exhibit TAM resistance resulting in disease recurrence. Several mechanisms of TAM resistance have been proposed, yet a single distinct mechanism has not been identified. We will systematically consider the following steps of the estrogen receptor (ER)-mediated signal transduction pathway to identify possible sites of alteration leading to tamoxifen-resistance: (1) ligand metabolism and availability, (2) loss or mutation of the ER, (3) defects in ER post-translational modification, and (4) alteration of the estrogen response element (ERE). In particular, the ERE will be discussed as a position in the signal transduction pathway with considerable potential, if altered, to confer TAM resistance.     

Synthesis and antiestrogenic activity of diaryl thioether derivatives

The reaction of 1,2-diarylethanol and mercapto side chain catalyzed by ZnI2 was used as a key step in the short (three to five steps) and efficient synthesis of 17 diaryl thioether derivatives. Several of these compounds contain a methyl butyl amide chain and an hydroxyaryl moiety, respectively, for antiestrogenic activity and binding affinity on estrogen receptor. No binding affinity for crude cytosolic preparation of the estrogen receptor was observed for compounds without phenolic group, while a low affinity (0.01-0.05%) was measured for mono- or diphenol derivatives. Like the pure steroidal antiestrogen EM-139, these novel nonsteroidal compounds did not exert any stimulatory effect on cell proliferation of (ER+) ZR-75-1 human breast cancer cells and partially reversed the amplitude of the stimulatory effect induced by estradiol on this (ER+) cell line. No proliferative or antiproliferative effect on (ER-) MDA-MB-231 human breast cancer cells was also observed for three of these compounds (39-41). Among the newly synthesized nonsteroidal compounds, the thioether derivative 41 (N-butyl-N-methyl-13,14-bis(4'-hydroxyphenyl)-12-thiatetradecanamide+ ++), with a long methylbutylalkanamide side chain and a diphenolic nucleus, was selected as the best antiestrogenic compound. However, this compound was 100-fold less antiestrogenic in (ER+) ZR-75-1 cells than the steroidal antiestrogen EM-139.     

Effect of twenty-four-week treatment with the antiestrogen EM-800 on estrogen-sensitive parameters in intact and ovariectomized mice

Treatment with the antiestrogen EM-800, at the daily oral dose of 3 microg, 10 microg, 30 microg, or 100 microg for 24 weeks, caused a marked inhibition of uterine and vaginal weight in both intact and ovariectomized mice. Maximal 64% and 41% inhibitions of uterine weight were achieved in intact and ovariectomized animals, respectively. Similar inhibitory effects of EM-800 were observed on vaginal weight with maximal inhibitions of 71% and 35%, in intact and ovariectomized animals, respectively. The pure antiestrogenic activity of EM-800 on the hypothalamo-pituitary-ovarian axis is illustrated by the 76-91% increases in ovarian weight observed in intact animals treated with the 10-100 microg doses of the antiestrogen. Serum 17beta-estradiol was 93% increased at the 100 microg daily dose of EM-800, whereas serum androstenedione, testosterone, and dihydrotestosterone were 141-713% increased over control at the same dose of the antiestrogen. Serum LH was increased by treatment with EM-800 in intact animals, whereas no effect was observed on the elevated gonadotropin levels in ovariectomized animals. At all doses used in intact animals, the antiestrogen caused a complete disappearance of the glandular elements of the mammary gland, the atrophy being comparable with that observed in ovariectomized mice. The mammary gland of EM-800-treated animals was exclusively composed of an atrophied ductal system lined by atrophied epithelial cells with an absence of lobulo-glandular elements. No effect of the compound was observed on the histology of the mammary gland in ovariectomized animals, thus showing the pure antiestrogenic effect of EM-800 on the mammary gland, as shown also for the uterus, vagina, and hypothalamo-pituitary axis. At histopathology, all doses of EM-800 in intact animals led to a moderate to severe uterine and vaginal atrophy. The uterine atrophy affected both the myometrium and the endometrium. Interestingly, the uterine atrophy achieved in intact animals treated with EM-800 was greater than that observed after ovariectomy alone, thus clearly demonstrating the pure antiestrogenic activity of EM-800. The present data show the highly potent and pure antiestrogenic activity of EM-800 on all parameters measured after 6 months of treatment in both intact and ovariectomized mice, a maximal effect being reached at the daily 10 microg dose of the antiestrogen in intact animals.     

Differential regulation by estrogens of growth and prolactin synthesis in pituitary cells suggests that only a small pool of estrogen receptors is required for growth

PR1 cells are a prolactin (PRL)-secreting cell line derived from a pituitary lactotroph tumor found in 17beta-estradiol-treated Fischer 344 rats. We examined the effect of estrogen on cell proliferation and PRL synthesis under various culture conditions. Estrogen, at extremely low concentrations, induces cell proliferation in this cell line, whereas antiestrogen inhibits proliferation. Interestingly, the proliferation response is much more sensitive than the PRL response because 0.01 pM estradiol or diethylstilbestrol induces half-maximal growth induction [ approximately 0.1% estrogen receptor (ER) occupancy is required], whereas 0.01 nM concentration is required for half-maximal PRL induction ( approximately 50% ER occupancy is required). The proliferation response is not as sensitive to antiestrogen as the PRL response, because 10 nM concentration of the pure antiestrogen ICI 182,780 could not inhibit 1 nM estradiol- or diethylstilbestrol-induced proliferation. The same concentration of ICI 182,780 decreased PRL secretion to 1% of estradiol- or diethylstilbestrol-induced prolactin secretion suggesting a possible dichotomy of ER control of proliferation and PRL synthesis. The Kd of ER binding in these cells is about 3 x 10(-11) M. These results with the PR1 cells extend previous studies in other estrogen- regulated systems and suggest that only a small pool of ER is required for cell proliferation in contrast with the regulation of expression of specific genes. They also raise questions as to how a dimeric receptor functions when only one ligand site is occupied or when both an estrogen and an antiestrogen occupy one dimer.     

Effect of the high-affinity estrogen receptor ligand ICI 182,780 on the rat tibia

We examined the specificity of the steroidal antiestrogen ICI 182,780 (ICI) on bone and reproductive tissues in adult and growing female rats. Using a 1.5-mg/kg dose (s.c.), we evaluated the effects of ICI on the bone, body weight, uterine weight, serum cholesterol, and serum estradiol in either adult and/or growing rats. ICI increased serum estradiol cholesterol in ovary-intact rats, had no effect on uterine weight in ovariectomized rats, and resulted in uterine atrophy in ovary-intact animals comparable with ovariectomy. In contrast, ICI had no effect on body weight. In bone, ICI significantly increased the rate of periosteal bone formation in long bones of growing and mature female rats. In contrast, ICI had no effect on longitudinal bone growth in rapidly growing rats. When ICI was administered to mature rats with or without ovaries, two-factor ANOVA revealed significant interaction (P < or = 0.05) between ovariectomy and ICI treatment for cancellous bone area and labeled bone perimeter. ICI increased skeletal indices of bone turnover in the cancellous bone of ovary-intact rats but reduced these indices of bone turnover in the cancellous bone of ovariectomized rats. The increase in bone turnover was associated with a reduction in cancellous bone area in the ovary-intact rats. A reduction in bone turnover was similarly associated with an increase in bone area in the ICI-treated ovariectomized rats. In summary, ICI exhibited complete estrogen antagonism in cortical and cancellous bone, partial agonism in cancellous bone, and no activity on tibial longitudinal growth rate of growing ovary-intact rats. The effects in adult rats were influenced by circulating levels of estradiol. ICI had no activity on body weight and complete antagonism on uterine weight. These results demonstrate that a ligand with high binding affinity to the estrogen receptor(s) can elicit an array of estrogen-mediated regulation of bone metabolism.     

The estrogen receptor from a tamoxifen stimulated MCF-7 tumor variant contains a point mutation in the ligand binding domain

The nonsteroidal antiestrogen tamoxifen (TAM) is the most commonly used endocrine treatment for all stages of breast cancer in both pre- and postmenopausal women. However, the development of resistance to the drug is common, as most patients treated with TAM eventually experience a recurrence of tumor growth. One of the potential mechanisms of treatment failure is the acquisition by the tumor of the ability to respond to TAM as a stimulatory rather than inhibitory ligand. We (Gottardis and Jordan, Cancer Res 48:5183-5187, 1988; Wolf et al., J Natl Cancer Inst 85:806-812, 1993) and others (Osborne et al., Eur J Cancer Clin Oncol 23: 1189-1196, 1987; Osborne et al., J Natl Cancer Inst 83: 1477-1482, 1991) have extensively described the reproducible development of TAM stimulated growth in a laboratory model system using MCF-7 human breast cancer cells grown as solid tumors in athymic mice. In this paper we report on the isolation of an estrogen receptor (ER) from a TAM stimulated tumor (MCF-7/MT2) which contains a point mutation that causes a tyrosine for aspartate substitution at amino acid 351 in the ligand binding domain. The mutant appears to the major form of ER expressed by this tumor. We also report that only wild type ER was detected in three other TAM stimulated MCF-7 tumor variants, suggesting that multiple mechanisms are possible for the development of TAM stimulated growth. The implications of these findings are discussed.     

Differential spatiotemporal regulation of lactoferrin and progesterone receptor genes in the mouse uterus by primary estrogen, catechol estrogen, and xenoestrogen

Many xenobiotics are considered reproductive toxins because of their ability to interact with the nuclear estrogen receptors (ERalpha and ERbeta). However, there is evidence that these xenobiotics can regulate gene expression in the reproductive targets by mechanisms that do not involve these ERs. To examine this further, we compared the effects of estrogenic (o,p'-DDT [1-(o-chlorophenyl)-1-(p-chlorophenyl)2,2,2-trichloroethane] and Kepone, chlordecone) and nonestrogenic (p,p'-DDD [1,1-dichloro-2,2-bis(p-chlorophenyl)ethane], a metabolite of p,p'-DDT) xenobiotics with those of 17beta-estradiol (E2) and 4-hydroxyestradiol-17beta (4-OH-E2), a catechol metabolite of E2, on uterine expression of lactoferrin (LF) and progesterone receptor (PR). These genes are estrogen responsive in the mouse uterus. Normally, LF is expressed in the uterine epithelium, whereas PR is expressed in both the epithelium and stroma in response to estrogenic stimulation. Ovariectomized mice were injected with xenobiotics (7.5 mg/kg), E2 (10 microg/kg), 4-OH-E2 (10 microg/kg), or the vehicle (oil, 0.1 ml/mouse), and uterine tissues were processed for Northern blot and in situ hybridization. The pure antiestrogen ICI-182780 (ICI; 1 or 20 mg/kg) was used to interfere with estrogenic responses that were associated with the ERs. The results of Northern and in situ hybridization demonstrated increased uterine levels of PR and LF messenger RNAs (mRNAs) by all of these xenobiotics, but quantitatively the responses were much lower than those induced by E2 or 4-OH-E2. The results further showed that the E2-inducible epithelial LF mRNA accumulation was markedly abrogated by pretreatment with ICI (20 mg/kg). In contrast, this treatment retained the epithelial expression of PR mRNA, but down-regulated the stromal expression. In contrast, ICI had negligible effects on LF and PR mRNA responses to 4-OH-E2, indicating that this catechol estrogen exerted its effects primarily via a mechanism(s) other than the ERs. The heightened accumulation of LF mRNA in the epithelium in response to Kepone and o,p'-DDT was also severely compromised by pretreatment with ICI, but this antiestrogen had little effect on responses to p,p'-DDD. Similar to E2, Kepone increased the expression of PR mRNA in both uterine epithelium and stroma. However, pretreatment with ICI decreased stromal cell expression, whereas epithelial cell expression remained unaltered or increased. These responses were not noted in mice treated with o,p'-DDT or p,p'-DDD. Collectively, the results demonstrate that catechol estrogens or xenobiotics can alter uterine expression of estrogen-responsive genes by mechanisms that are not totally mediated by the classical nuclear ERs, and these alterations are cell type specific. We conclude that an interaction of a compound with the nuclear ERalpha and/or ERbeta is not an absolute requirement for producing specific estrogen-like effects in the reproductive target tissues.     

Mediation by epidermal growth factor of the estradiol-induced increase in cyclic guanosine 3',5'-monophosphate content in the rat uterus

Estrogen treatment of immature or ovariectomized mature rats induces an increase in uterine cGMP content, with a peak 2-3 h after hormone administration. This response to estrogenic action also develops in vitro, in incubated uterine horns, thus excluding the intervention of another organ. Its function is still unknown. We show here that treatment of incubated uterine horns from immature or mature rats with 8 nM epidermal growth factor (EGF), exactly mimicked the effect of 1 nM estradiol on cGMP levels. The estradiol-induced increase in uterine cGMP was canceled in the presence of the phosphotyrosine kinase inhibitor genistein. Like the cGMP response to EGF, the estradiol-induced increase in uterine cGMP was completely suppressed in the presence of an antimouse EGF antibody. On the other hand, whereas the induction of cGMP accumulation by estradiol in vivo or in vitro was suppressed by prior treatment of the animals with the pure antiestrogen ICI 164,384, such pretreatment had no effect on the EGF-induced increase in uterine cGMP content. Together, these data support the concept that the uterine cGMP response to estrogens is entirely due to auto/paracrine mediation by the EGF-EGF receptor system. Considering reports from the literature showing that EGF can directly induce the phosphorylated active form of the estrogen receptor, we speculate that this might implicate its action on cGMP, with the latter then intervening as cofactor of the involved phosphokinase(s).     

Cloning of cDNA for a cell wall-bound acid invertase from tomato (Lycopersicon esculentum) and expression of soluble and cell wall-bound invertases in plants and wounded leaves of L. esculentum and L. peruvianum

Diethylstilbestrol (DES) is a well-characterized carcinogen in humans and animals although its mechanisms of carcinogenicity are not yet known. While the estrogenic activity of DES is important, there is evidence that oxidative metabolism also plays an important role for its toxicity. DES is oxidatively metabolized in vivo and in vitro to a number of compounds including diethylstilbestrol-4',4"-quinone (DQ), an unstable and reactive intermediate, and Z,Z-dienestrol (ZZ-DIEN). Estrogen receptor (ER) binding assays with mouse uterine cytosol indicate that DES, DQ and ZZ-DIEN have relative binding affinities of 286, 3.6 and 0.3, respectively, relative to estradiol as 100. In addition, DQ binds irreversibly and specifically to ER suggesting that DQ may be biologically active despite its rapid metabolism and lower binding affinity compared to DES. To test this, COS-1 cells were transfected with an estrogen responsive reporter construct containing of VitA2 estrogen response element (ERE) with or without an ER expression vector. In the presence of ER, treatments with DES, DQ and ZZ-DIEN resulted in 11, 10, and 2-fold induction of chloramphenicol acetyltransferase (CAT) activity, respectively. This induction was mediated by estrogen receptor since it was suppressed by pretreatment with a 10-fold excess of the pure antiestrogen ICI 182,780. These data indicate that DQ is a biologically active intermediate that is capable of transactivation of estrogen responsive genes through the ER. Furthermore, the data suggest that the ability of DQ to irreversibly bind ER may result in persistent stimulation of ER. This persistent stimulation may be related to the carcinogenicity of DES.     

Molecular structures and conformational studies of triarylcyclopropyl and related nonsteroidal antiestrogens

Molecular structures and conformational characteristics of a series of 1,1-dichloro-2,2,3-triarylcyclopropanes (DTACs), which were reported previously to be distinctly antiestrogenic and inhibitors of the estrogen-receptor-positive MCF-7 human breast cancer cells in culture, are reported. In addition, structural and conformational features of the DTACs were compared to the first-known nonsteroidal antiestrogen, MER25, and the clinically useful antiestrogen Tamoxifen. The molecular structures of four DTAC compounds were determined by X-ray diffraction. Crystallographic structures show that the DTAC molecules have nearly the same relative conformation for the three aryl rings which is designated as a "nonpropeller" conformation in contrast to the observed "propeller" conformation for the three rings in all known triarylethylenes. Systematic conformational searches were performed to find the conformational preferences of DTACs, MER25, and Tamoxifen using idealized model compounds built from their respective crystal structure. Energy-minimization and conformational-search studies demonstrated that all DTAC molecules have a common, single global minimum energy conformer for their central core containing the dichlorotriarylcyclopropyl system, which is similar to that found in their crystal structures. Conformational search of MER25 showed that the molecule can assume a number of low-energy conformers of which two, one anti (A1) and one gauche (G1A), have about the same energy. The anti conformation is similar to the one observed in its crystal structure and resembles the estrogenic E-isomer of Tamoxifen, while the lowest energy gauche conformer of MER25 resembles more closely the antiestrogenic Z-isomer of Tamoxifen. NMR spectroscopic analysis of MER25 showed that the molecule exists predominantly in the anti conformation in solution. A comparative review of the structural features and bioactivities of Tamoxifen, DTACs, and MER25 provides a possible explanation for their low estrogen receptor binding affinity which is common to these compounds together with their antiestrogenic activity.     

The protein truncation test (PTT) as a method of detection for choroideremia mutations

The development of endocrine resistance in previously sensitive, estrogen receptor-positive breast cancers is a major limitation in the treatment of breast cancer. Because antiestrogens have a cell cycle-specific action on breast cancer cells and influence the expression and activity of several cell cycle-regulatory molecules, the development of aberrant cell cycle control mechanisms is a potential mechanism by which cells might develop resistance to antiestrogens. We postulated that overexpression of cyclin D1, which is a common feature of breast cancer, may confer antiestrogen resistance. We addressed this question in vitro by testing the ability of ectopic cyclin D1 overexpression to overcome the growth-inhibitory effects of tamoxifen and the pure steroidal antiestrogens, ICI 164384 and ICI 182780, in T-47D and MCF-7 human breast cancer cells. In cells stably transfected with a human cyclin D1 cDNA under the control of a metal-inducible metallothionein promoter, cyclin D1 expression was increased 2-4-fold following treatment with zinc. Despite the continued presence of antiestrogen, cyclin D1 induction resulted in the formation of active cyclin D1/Cdk4 complexes, concurrent hyperphosphorylation of the retinoblastoma protein, and entry into S phase of cells previously arrested in G1. Elevated cyclin D1 protein levels were first detected 3 h after treatment with zinc, and the proportion of cells in S phase began to increase 6 h later. The S-phase fraction increased 2-3-fold from 13 to 17% in cells treated with antiestrogen alone, to a peak of 33-38% 15 h after zinc treatment. Both the cyclin D1 protein level and the proportion of cells in S phase increased with increasing concentrations of zinc. We conclude that the ectopic overexpression of cyclin D1 reverses the growth-inhibitory effect of antiestrogens in estrogen receptor-positive breast cancer cells, providing a potential mechanism for clinical antiestrogen resistance.     

The Arabidopsis thaliana ACT4/ACT12 actin gene subclass is strongly expressed throughout pollen development

We analyzed the displacement activity of sarpogrelate and its active metabolite (M-1) in the radiolabeled ligand binding to various 5-hydroxytryptamine (5-HT) receptor subtypes using rat brain cortical membranes. Sarpogrelate was shown to have the same affinity as ritanserin for 5-HT2A receptors, with a Ki value of 8.39 nM. The active metabolite of sarpogrelate, M-1, was more active than sarpogrelate itself and of ritanserin, with a Ki value of 1.70 nM. Both sarpogrelate and M-1 had no affinity for 5-HT1A receptors, but these substances, at a concentration of 10 microM, displaced the specific binding to the 5-HT1B receptors of [125I]iodocyanopindolol, resulting in Ki values of 0.881 and 0.859 microM, respectively. The Ki values of sarpogrelate and M-1 are almost the same as that of ritanserin, a specific 5-HT2 receptor antagonist. Sarpogrelate and M-1, as well as ritanserin, are shown to have very low affinity for 5-HT1B receptors. Both sarpogrelate and M-1 had no affinity for 5-HT3 receptor subtypes. In the 5-HT4 receptor binding experiments, sarpogrelate exhibited almost no affinity, while M-1, at the concentration of 10 microM, displaced the binding activity, resulting in a Ki value of 0.838 microM. Both drugs had a weak antagonistic effect on a 5-HT4 receptor-mediated function, i.e., the 5-HT-induced relaxation of rat isolated esophageal tunica muscularis mucosae. In conclusion, sarpogrelate and M-1 have high affinity for 5-HT2A receptors with a relatively high selectivity.     

Incisive canal cysts related to periodontal osseous defects: case reports

Tamoxifen (TAM) is an antiestrogen useful in the treatment and control of breast cancer. Exposure of solutions of TAM to UV irradiation produces mixtures of fluorescent derivatives that are useful in the analytical detection and quantitative determination of TAM. The two major products of such irradiations were isolated and assigned unambiguous structures based on analysis of UV and 1H NMR spectral data. Results were in accordance with earlier studies that indicated these products to be substituted phenanthrenes produced by dehydrogenation of cyclized intermediates subsequent to partial isomerization of TAM. Each of the phenanthrenes suppressed MCF-7 human breast cancer cell growth in a dose-dependent manner, but neither compound was as potent as TAM in this regard. Also, unlike treatment with TAM, cell growth could not be restored in the presence of either of the phenanthrenes by simultaneous exposure to estradiol.     

Estrogen receptor expression in bovine and rat retinas

PURPOSE: To investigate the expression and distribution of estrogen receptor protein and mRNA in bovine and rat retinas. METHODS: Western blot analysis with an antiestrogen receptor monoclonal antibody (mAb) was used to detect estrogen receptor protein in the bovine retina. Immunohistochemistry with an antiestrogen receptor mAb and in situ hybridization with an oligodeoxynucleotide sequence coding for estrogen receptor were applied to study the cellular distribution of estrogen receptor protein and its mRNA in male bovine retina and rat retina of both sexes. RESULTS: Estrogen receptor protein was detected in bovine retina by Western blot analysis. Immunohistochemical staining with the antiestrogen receptor mAb was widespread throughout the neural retina. Specific staining showed extensive distribution localizing in the nerve fiber layer, the ganglion cell layer, the inner nuclear layer, and the outer plexiform layer. Retinal pigment epithelium and choroid were also stained with the antiestrogen receptor mAb. By in situ hybridization, the expression of estrogen receptor mRNA was predominantly observed in ganglion cell layer, the inner nuclear layer, and outer portion of the outer nuclear layer. No significant difference was found between male and female rats in the immunostaining of retinas with the antiestrogen receptor mAb. CONCLUSIONS: This study provides the first evidence for the presence of estrogen receptor in bovine and rat retinas. Expression of estrogen receptor throughout the retina suggests that estrogen may have important functions in the retina.     

Induction of apoptosis by tamoxifen and ICI 182780 in primary breast cancer

Hormonal breast cancer therapies have traditionally been considered cytostatic, but recent pre-clinical data suggest that anti-oestrogens can induce apoptosis. The aim of this study was to assess whether tamoxifen (TAM) and ICI 182780 (ICI) could induce apoptosis in human breast cancer, and whether this was related to oestrogen receptor status. We measured apoptosis in primary breast cancer patients before and after pre-surgical treatment with 20 mg/day TAM (study 1) or 6 or 18 mg/day ICI (study 2). In each study there was a randomised non-treatment (NT) control group. TAM significantly increased apoptotic index (AI) in ER+ but not in ER- tumours. There was a significant increase in AI following treatment with ICI. Insufficient pairs of samples were available to determine whether this change was confined to ER+ tumours, but in a cross-sectional analysis AI was significantly higher in excision biopsies for ICI-treated than NT patients for ER+ but not ER- tumours. Our results provide clinical evidence that apoptosis may be induced in ER+ primary breast cancer by both non-steroidal and steroidal anti-oestrogens.     

Hormonal treatment of advanced breast cancer

Endocrine therapy for breast cancer has been used for almost a century, but because of the enormous success of tamoxifen there has been a resurgence of interest by the pharmaceutical industry to develop new and innovative endocrine therapies. Overall, the strategy is quite simple. Estrogen stimulates growth; therefore, the goal is to deny the breast tumor estrogens. Tamoxifen accomplishes this by blocking the estrogen receptor. The new antiestrogens, toremifene and droloxifene, however, appear to have no greater activity than tamoxifen in the treatment of advanced disease and therefore may ultimately offer no advantages over current therapy. In contrast, the pure antiestrogens hold additional promise as they may produce a more profound inhibitory effect on the tumor, and the response may be maintained longer. An orally active, pure antiestrogen, however, would be an important advance. The strategy of using GnRH agonists for premenopausal patients clearly has merit to produce a chemical oophorectomy. The strategy could be integrated into the general treatment plan for the young premenopausal patient taking tamoxifen who may not have had her menstrual cycles stopped by combination chemotherapy. The GnRH agonists would block the reflex rise in estradiol caused by tamoxifen therapy and ultimately produce a more efficient antihormonal therapy. Indeed, the different specific aromatase inhibitors can also be integrated into the treatment plan to produce a complete estrogen blockade. Whether the use will be found to be superior to pure antiestrogens, however, must await the completion of comparative clinical studies. If all the results of endocrine therapy are therapeutically similar, the final strategy may depend on the acceptability by the patient of an individual delivery method for each pharmaceutical approach.