Response after withdrawal of tamoxifen and progestogens in advanced breast cancer

Homopterocarpanes as bridged triarylethylene analogues: synthesis and antagonistic effects in human MCF-7 breast cancer cells

A series of new compounds structurally derived from 6a,12a-dihydro-6H,7H-[1]-benzopyran-[4,3-b]-benzopyran (homopterocarpane) was efficiently synthesized by reduction of the corresponding pyrilium salts obtained by treatment of selected flavanones and aldehydes with anhydrous HClO4. Cytotoxic effects on the human breast cancer cell line MCF-7 and antiestrogenic activity (only for compounds which resulted more active than tamoxifen (TAM)) on MCF-7 cells stimulated by 17beta-estradiol were evaluated. In vivo antiestrogenic activity and the relative binding affinity were also assessed. Some of the new compounds (4c, 4h, 4i and 4l) showed a biological activity in the micromolar range, and were more potent than TAM taken as the reference.

Raloxifene rebound regression

The first case of clinical remission of metastatic breast after the withdrawal of raloxifene is reported. A postmenopausal woman was treated for stage II breast cancer with a modified radical mastectomy and adjuvant cyclophosphamide, doxorubicin, and 5-flurouracil followed by tamoxifen for 5 years. One year following the cessation of tamoxifen, osteopenia was noted and raloxifene was begun. Two years following the start of raloxifene, supraclavicular adenopathy and lung metastases developed, which regressed after discontinuation of raloxifene. Although raloxifene is not frequently given to women with previous breast cancer, increasing use in osteopenic women may lead to cancer developing in these raloxifene-treated women; this despite the major reduction in the incidence of breast cancer in raloxifene-treated versus placebo-treated women. A clinically significant response to the withdrawal of a hormone, used either for the treatment of breast cancer or for some other reason, is not a rare event, and the therapeutic usefulness of observation only for a rebound regression is a frequently overlooked strategy in the treatment of breast cancer.

The consequences of exhaustive antiestrogen therapy in breast cancer: estrogen-induced tumor cell death

Forty years ago, the endocrine treatment for breast cancer was a last resort at palliation before the disease overwhelmed the patient (1). Ovarian ablation was the treatment of choice for the premenopausal patient, whereas either adrenalectomy or, paradoxically, high-dose synthetic estrogen therapy were used for treatment in postmenopausal patients. A reduction or an excess of estrogen provoked objective responses in one out of three women. Unfortunately, there was no way of predicting who would respond to endocrine ablation, and because so few patients responded there was no enthusiasm for developing new endocrine agents. All hopes for a cure for breast cancer turned to appropriate combinations of cytotoxic chemotherapy. Today tamoxifen, a nonsteroidal antiestrogen (2), has proven to be effective in all stages of premenopausal and postmenopausal breast cancer, and several new endocrine strategies, including aromatase inhibitors, luteinizing-hormone releasing hormone (LHRH) superagonists, and a pure antiestrogen (fulvestrant), are now available for breast cancer treatment. Additionally, tamoxifen and raloxifene, a related compound, are used to reduce the risk of breast cancer and osteoporosis, respectively, in high-risk groups (3). Hormonal modulation and strategies to prevent the actions of estrogen in the breast are ubiquitous. However, with successful changes in treatment strategies comes the consequence of change. This minireview will describe the current strategies for the treatment and prevention of breast cancer and present emerging new concepts about the consequences of exhaustive antiestrogen treatment on therapeutic resistance.

Endocrinology and hormone therapy in breast cancer: aromatase inhibitors versus antioestrogens

Endocrine therapies act by either blocking or downregulating the oestrogen receptor or by reducing oestrogen concentrations around and within the cancer cell. In postmenopausal women, oestrogen suppression is achieved by inhibition of the enzyme aromatase by aromatase inhibitors (AIs). Modern AIs (anastrozole, letrozole and exemestane) are more potent than earlier ones and suppress oestradiol levels in plasma to virtually undetectable concentrations. Recent comparisons of AIs with the most widely used oestrogen receptor blocking drug tamoxifen indicate that, in general, AIs result in increased response rates and greater durations of response. Here, we summarize data supporting the difference between the two types of treatment and attempt to account for the underlying mechanisms that favour AIs.

Activity and Safety of the Antiestrogen EM-800, the Orally Active Precursor of Acolbifene, in Tamoxifen-Resistant Breast Cancer

Purpose

To determine the efficacy and safety of EM-800 (SCH-57050), the precursor of acolbifene, a new, highly potent, orally active, pure antiestrogen in the mammary gland and endometrium, for the treatment of tamoxifen-resistant breast cancer.

Patients and Methods

Forty-three post menopausal/ovariectomized women with breast cancer who had received tamoxifen, either for metastatic disease or as adjuvant to surgery for ≥ 1 year, and had relapsed were treated in a prospective, multicenter, phase II study with EM-800 (20 mg/d [n = 21] or 40 mg/d [n = 22] orally).

Results

Thirty-seven patients had estrogen receptor (ER)-positive tumors (>10 fmol/mg; mean, 146 fmol/mg cytosolic protein), three patients had ER-negative/progesterone receptor-positive tumors, and three patients had undetermined ER status. The objective response rate to EM-800 was 12%, with one complete response and four partial responses. Ten patients (23%) had stable disease for ≥ 3 months, and 7 patients (16%) had stable disease for ≥ 6 months. With a median follow-up of 29 months, median duration of response was 8 months (range, 7 to 71+ months). Treatment with EM-800 was well tolerated. No significant adverse events related to the study drug were observed clinically or biochemically.

Conclusion

EM-800 produced responses in a significant proportion of patients with tamoxifen-resistant breast cancer, thus showing that this highly potent, selective estrogen receptor modulator, which lacks estrogenic activity in the mammary gland and endometrium, has incomplete cross-resistance with tamoxifen, thus suggesting additional benefits in the treatment of breast cancer.

Short-term biologic response to withdrawal of hormone replacement therapy in patients with invasive breast carcinoma

BACKGROUND

The biologic effect of continuing hormone replacement therapy (HRT) after a diagnosis of breast carcinoma is unclear. The goal of rhe current study was to determine the short-term effect of HRT withdrawal on invasive breast carcinoma using biologic surrogate markers of tumor response.

METHODS

The study was performed between 1996 and 2000 and comprised 140 women who had been using HRT at the time of breast carcinoma diagnosis by core needle biopsy. The breast tumors were removed a median of 17 days later (range, 2-31 days). Of these women, 125 women stopped HRT at the time of core needle biopsy and 15 continued to receive HRT until surgery. In addition, 55 women with breast carcinoma from the same time period, who were not receiving HRT at diagnosis, were studied. Changes in expression of Ki-67 (a measure of epithelial cell proliferation), progesterone receptor (PR), p27KIP-1 (a cyclin-dependent kinase inhibitor), and cyclin D1 (a cell cycle-related protein) were determined by immunohistochemistry on paired sections of the core needle biopsy and surgical specimens from each patient.

RESULTS

In women who stopped HRT, a significant decrease in Ki-67 expression was observed between core needle biopsy and surgery in estrogen receptor (ER)-positive (n = 106; P < 0.001), but not in ER-negative tumors (n = 19; P = 0.58), with an associated reduction in PR (P < 0.001) and cyclin D1 expression (P < 0.001) and an increase in p27KIP-1 (P = 0.03). These changes in Ki-67 and PR expression occurred irrespective of c-erb-B2 status. No change was observed in any parameter in the other groups of patients.

CONCLUSIONS

ER-positive invasive breast carcinomas demonstrated a favorable biologic response to withdrawal of HRT. Therefore, HRT should be stopped at the time of diagnosis and was subsequently contraindicated.

Changing role of the oestrogen receptor in the life and death of breast cancer cells

The oestrogen receptor (ER) has proven to be an extraordinarily successful target for breast cancer treatment and prevention. The clinical use of tamoxifen, a nonsteroidal antioestrogen, demonstrated (1) that the strategic use of adjuvant tamoxifen in ER-positive patients could save lives and (2) that a selective ER modulator (SERM) could reduce the incidence of breast cancer in high-risk women. The ER is now the target for new and safer therapies such as the aromatase inhibitors and the pure antioestrogens that either block oestrogen synthesis or destroy the ER. However, the use of raloxifene, a SERM to prevent osteoporosis with the potential to prevent breast cancer has introduced a new dimension into preventive oncology. The widespread use of endocrine modulators (SERMs, aromatase inhibitors, and pure antioestrogens) raised the question of drug resistance. It is now clear that endocrine resistance can evolve through stages. Once a breast tumour becomes resistant to SERMs, the growth is stimulated by either the SERM or oestrogen. This is why an aromatase inhibitor is effective following SERM resistance and withdrawal. However, the extended use of repeated endocrine therapies now supersensitized the cells to oestrogen that causes apoptosis through the ER. We suggest that future clinical treatment strategies incorporate an 'oestrogen purge' to both enhance the actions of chemotherapy or completely reverse endocrine resistance and restore endocrine sensitivity. These new data build on the idea that breast cancer can be controlled as a chronic disease and will permit patients to live long and productive lives during targeted maintenance treatment.

Endocrine-resistant breast cancer: underlying mechanisms and strategies for overcoming resistance

Estrogen plays important roles in the development and progression of breast cancer. However, one-third of breast cancers fail to respond to endocrine therapy and most endocrine-responsive breast cancers subsequently become resistant to endocrine therapy. A tremendous effort has been made to elucidate the mechanisms responsible for the development of endocrine-resistance in breast cancer. Since the main target molecule of estrogen in breast cancer is estrogen receptor (ER)-alpha, most studies have focused on investigating quantitative and qualitative changes in ER-alpha in endocrine-resistant breast cancer. Breast cancers expressing no ER-alpha fail to respond to endocrine therapy. Some breast cancers expressing ER-alpha also fail to respond to endocrine therapy and most breast cancers with acquired endocrine resistance retain ER-alpha expression, which suggests that the disappearance of ER-alpha in breast cancer cells is not a common cause of resistance to endocrine therapy. Recent molecular biological studies have shown evidence that qualitative and functional changes, such as gene mutations and phosphorylation of ER-alpha, cause endocrine resistance in breast cancer. In addition, it has been suggested that endocrine resistance could be induced by epigenetic changes, such as hypoxia, in breast cancer tissues. Understanding the precise mechanisms that underlie endocrine resistance may enable clinicians to develop new strategies for retarding or overcoming endocrine resistance in breast cancer.

Effect of treatment sequence with radiotherapy and the antiestrogen EM 800 on the growth of ZR 75 1 human mammary carcinoma in nude mice

We demonstrated previously that continuous administration of EM-800, a SERM having pure antiestrogenic activity in the mammary gland and endometrium in combination with monthly radiotherapy caused a greater inhibition of human ZR 75 1 tumor growth in nude mice than either therapy used alone. To further optimize therapy, we have now examined the effect of various treatment sequences to determine the optimal treatment regimen in the same model. EM 800 was given at the maximally effective oral dose of 300 microg daily. External beam radiation therapy (RTX) was carried out (2 Gy/tumor/day, 5 days per week for 3 weeks) for a total of 30 Gy/tumor delivered directly to the tumor while shielding the rest of the animal body. There was no evidence of RTX-related morbidity. Continuous treatment with EM 800 was initiated either 3 weeks before or at the same time as RTX, immediately after RTX, or 3 weeks before and immediately after RTX. After 156 days of treatment, EM 800 alone caused a 75% decrease in average tumor area, an effect equivalent to that achieved by ovariectomy. RTX alone, on the other hand, caused a transient 30% decrease in tumor area regardless of treatment sequence, whereas combined treatment with EM 800 and RTX was superior to either treatment alone. Combined treatment with EM 800 and RTX both started on Day 1 caused the greatest (88%), most rapid (50% in 2 weeks) and sustained decrease in tumor size. The present data indicate that optimal reduction in breast tumor size is achieved by continuous administration of EM 800 and RTX started simultaneously on Day 1.

A mechanism of drug resistance to tamoxifen in breast cancer

Drug resistance to tamoxifen (Tam) is a significant clinical problem but the mechanism through which this occurs remains elusive. We have developed a number of xenograft models of Tam-stimulated growth that model breast cancer progression using estrogen receptor positive MCF-7 or T47D breast cancer cells. When estrogen-stimulated T47D:E2 tumors are treated long term with Tam, Tam-stimulated tumors develop (T47D:Tam) that are stimulated by both estrogen and Tam. When HER-2/neu status is determined, it is clear that the T47D:Tam tumors express significantly higher levels of HER-2/neu protein by immunohistochemistry and mRNA as measured by real-time RT-PCR. The T47D:Tam tumors also express higher levels of estrogen receptor and progesterone receptor protein than their estrogen-stimulated T47D:E2 counterparts. We compared out results to the MCF-7 model of Tam-stimulated growth. The MCF-7:Tam ST (estrogen- and Tam-stimulated) and MCF-7:Tam LT (estrogen-inhibited, Tam-stimulated) were bilaterally transplanted to account for any mouse to mouse variation and characteristic growth patterns were observed. TUNEL staining was performed on MCF-7:Tam LT treated with either estrogen or Tam and it was concluded that estrogen-inhibited tumor growth was a result of increased apoptosis. Three phases of tumor progression are described that involve increases in HER-2/neu expression, de-regulation of estrogen receptor expression and increases in apoptosis which in concert determine the phenotype of drug resistance to Tam.

Fulvestrant (Faslodex): current status in the therapy of breast cancer

Fulvestrant (Faslodex, formerly ICI 182,780) is a potent steroidal antiestrogen that mediates its effects by estrogen receptor downregulation. It appears to act as a pure antiestrogen and exhibits none of the negative side effects associated with the partial agonist activity of tamoxifen. It has been shown to be as effective as the oral aromatase inhibitor anastrozole in postmenopausal women with advanced breast cancer who have progressed on prior endocrine therapy, principally tamoxifen. It therefore provides the clinician with an alternative therapeutic strategy following the development of tamoxifen resistance. Fulvestrant might also have potential as a follow-on therapy after tamoxifen in an adjuvant setting and help alleviate some of the concerns surrounding long-term (up to 5 years) tamoxifen therapy.

Where do selective estrogen receptor modulators (SERMs) and aromatase inhibitors (AIs) now fit into breast cancer treatment algorithms?

The agents used for endocrine therapy in patients with breast cancer have changed markedly over the past decade. Tamoxifen remains the anti-oestrogen of choice, but could be replaced by the oestrogen receptor down-regulator ICI 182780 or by the fixed ring triphenylethylene arzoxifene (previously SERM III) soon. Whilst aminoglutethimide and 4-OH androstenedione were the aromatase inhibitors of choice, they have been replaced by non-steroidal (anastrozole and letrozole) and steroidal (exemestane) inhibitors of high potency and low side effect profile. Previously, often used treatments such as progestogens (megestrol acetate and medroxyprogesterone acetate) and androgens are now rarely used or confined to fourth or fifth line treatments. The LHRH agonist, goserelin, remains the treatment of choice for pre-menopausal patients with advanced breast cancer although recent randomised trials indicate a response, time to progression and survival advantage for the combination of goserelin and tamoxifen compared with goserelin alone. The newer treatments have led to questions concerning the optimum sequence of agents to use in advanced breast cancer and as neo-adjuvant and adjuvant therapy in relation to surgery. Two trials of anastrozole compared with tamoxifen and one trial of letrozole compared with tamoxifen indicate that the new triazole aromatase inhibitors have a significant advantage over the anti-oestrogen with respect to time to progression and survival. Similarly, triazole aromatase inhibitors give faster and more complete responses compared with tamoxifen when used in post-menopausal women before surgery. Major research questions remain with respect to the aromatase inhibitors used as adjuvant therapy. Anastrozole is being tested alone or in combination with tamoxifen compared with tamoxifen in the 'so-called' ATAC trial. Over 9000 patients have been randomised to this important study: the results will be available late-2001. A similar study comparing letrozole and tamoxifen started recently under the auspices of the Breast International Group. Importantly, this trial is also comparing the sequence of tamoxifen followed by letrozole (or vice versa). A similar trial of exemestane given after 2-3 years of tamoxifen compared with 5 years of tamoxifen is recruiting well as is a study comparing letrozole (or placebo) for 5 years after 5 years of adjuvant tamoxifen. These studies may show that aromatase inhibitors are superior to tamoxifen or that a sequence is preferable.ICI 182780 causes complete oestrogen receptor down-regulation leading to a the lack of agonist activity of the drug. Two trials of ICI 182780 compared with anastrozole for advanced disease will report later this year and a comparison with tamoxifen next year. Arzoxifene (SERM III) is being tested against tamoxifen. These studies are likely to result in new anti-oestrogens being introduced into the clinic. Most of our endocrine treatments deprived the tumour cell of oestradiol. In vitro experiments with MCF-7 cells indicate that tumour cells can adapt and then grow in response to low oestrogen concentrations in the tissue--culture medium. Importantly, the cells were shown to apoptose in response to high oestrogen concentrations. A recent clinical trial has demonstrated a high response rate to stilboestrol given after a median of four previous oestrogen depriving endocrine therapies. These data and the newer treatments available indicate a need to re-think our general approach to endocrine therapy and endocrine prevention.

Faslodex (ICI 182, 780), a novel estrogen receptor downregulator--future possibilities in breast cancer

Tamoxifen is an effective treatment for breast cancer; however, as well as exerting antagonistic effects on the estrogen receptor (ER), tamoxifen acts as a partial agonists in estrogen-sensitive tissues, resulting in stimulation of the endometrium and tumor growth in some patients who become resistant to treatment.ICI 182, 780 (Faslodex), a steroidal estrogen antagonist, is the first in a new class of agent-an estrogen receptor downregulator. Pre-clinical breast cancer models show that ICI 182, 780 leads to a prolonged duration of response, and that it exerts its effects via a different mode of action to tamoxifen. This was confirmed in a small clinical study involving 19 post-menopausal advanced breast cancer patients, where ICI 182, 780 was highly effective after tamoxifen failure. Definitive evidence of the differing modes of action of ICI 182, 780 and tamoxifen, were provided in a study involving post-menopausal women with primary breast cancer, where analyses of tumor samples following short-term exposure to both drugs, showed that ICI 182, 780 reduced tumor ER levels in a dose-dependent manner, and to a significantly greater extent than tamoxifen. Additionally, unlike tamoxifen, ICI 182, 780 did not promote ER-mediated progesterone receptor expression, indicating that it lacks estrogen agonist activity. Ongoing studies in post-menopausal women with advanced breast cancer are comparing ICI 182, 780 to anastrozole and tamoxifen, respectively. Future studies being considered are whether ICI 182, 780 may also be effective in breast cancer in pre-menopausal women, in early breast cancer and in ductal carcinoma in situ in the breast, in combination with other hormonals, cytotoxics and biological modifiers.

Chemoprevention with antiestrogens: the beginning of the end for breast cancer. Daniel G. Miller Lecture

In the 1960s, compounds known as nonsteroidal antiestrogens were identified as potential contraceptives, but the drugs caused the induction of ovulation in subfertile women. Tamoxifen and clomiphene were marketed for this indication. However, tamoxifen was advanced for the treatment of breast cancer in the 1970s through a close cooperation between the laboratory and the clinical trials community. The extensive use of long-term adjuvant tamoxifen has resulted in saving the lives of 400,000 women with breast cancer. Tamoxifen is a selective estrogen receptor modulator (SERM) that produces antiestrogenic actions in the breast but estrogen-like actions in bone and lowers serum cholesterol. These properties not only allowed the application of tamoxifen as the first chemopreventive in high-risk pre- and postmenopausal women but also the development of raloxifene to prevent osteoporosis with the potential to prevent breast cancer in postmenopausal women. The future development of SERMs holds the promise of preventing osteoporosis and coronary heart disease as well as breast and endometrial cancer.

EM-652 (SCH57068), a pure SERM having complete antiestrogenic activity in the mammary gland and endometrium

In order to minimize the risks of endometrial cancer and the development of resistance to antiestrogen therapy, we have synthesized the orally active antiestrogen EM-652 which is the most potent of the known antiestrogens and exerts pure antiestrogenic activity in the mammary gland and endometrium. EM-652 inhibits the AF-1 and AF-2 functions of both ERalpha and beta while the inhibitory action of OH-TAM is limited to AF-2. EM-652, thus, inhibits Ras-induced transcriptional activity and blocks SRC-1-stimulated activity of the two receptors. The absence of blockade of AF-1 by OH-TAM could explain why resistance develops to Tamoxifen treatment. Not only the development, but also the growth of established DMBA-induced mammary carcinoma is inhibited by treatment with EM-800, the prodrug of EM-652. EM-652 is the most potent antiestrogen to inhibit the growth of human breast cancer ZR-75-1, MCF-7 and T-47D cells in vitro. When incubated with human Ishikawa endometrial carcinoma cells, EM-800 has no stimulatory effect on the estrogen-sensitive parameter alkaline phosphatase activity. When administered to ovariectomized animals, EM-800 prevents bone loss, and lowers serum cholesterol and triglyceride levels. EM-800 has shown benefits in women with breast cancer who had failed Tamoxifen. The above-summarized preclinical and clinical data clearly suggest the interest of studying this compounds in the neoadjuvant and adjuvant settings and, most importantly, for the prevention of breast and uterine cancer.

Estrogen receptor alpha mediated induction of the transforming growth factor alpha gene by estradiol and 4-hydroxytamoxifen in MDA-MB-231 breast cancer cells

The selective estrogen receptor modulator, 4-hydroxytamoxifen (4-OHT) is a full agonist at the transforming growth factor (TGF) alpha gene in ER negative breast cancer cells stably transfected with ER alpha cDNA (Levenson et al., Br. J. Cancer 77 (1998) 1812-1819). E(2) and 4-OHT increase TGF alpha mRNA and protein in a concentration dependent manner. The responses to E(2) and 4-OHT are blocked by the pure antiestrogen ICI 182,780, which does not induce TGF alpha. Transfected MDA-MB-231 cells contain functional ER alpha but no ER beta function was detected. Neo transfected cells that did not express ER alpha or cells stably transfected with the DNA binding domain mutant C202R/E203V which prevents gene activation did not induce TGF alpha mRNA after either E(2) or 4-OHT treatment. An examination of the time course for either 10 nM E(2) or 1 microM 4-OHT for MDA-MB-231 cells stably transfected with cDNA for ER alpha showed increases in TGF alpha mRNA within 2 or 3 h respectively. Cells pretreated with cycloheximide (1 microg/ml) showed induced TGF alpha mRNA in response to E(2) or 4-OHT but TGF alpha mRNA induction was blocked by actinomycin D (1 microg/ml). We conclude that both E(2) and 4-OHT induce TGF alpha by direct interaction of ER alpha with DNA and that ER beta is not involved in the estrogen-like response to 4-OHT in the MDA-MB-231 cells.

Molecular mechanism of action at estrogen receptor alpha of a new clinically relevant antiestrogen (GW7604) related to tamoxifen

Tamoxifen is the endocrine treatment of choice for all stages of estrogen receptor (ER)-positive breast cancer, and it is the first drug approved to reduce the incidence of breast cancer in high-risk women. Unfortunately, tamoxifen also possesses some estrogen-like effects in the uterus that cause a modest increase in the risk of endometrial cancer. GW5638 is a tamoxifen derivative with a novel carboxylic acid side chain with no uterotropic activity in the rat (Willson et al., J Med Chem, 1994, 37:1550-1552). We have compared and contrasted the actions of 4-hydroxytamoxifen (4-OHT, the active metabolite of tamoxifen) with GW7604 [the presumed metabolite of GW5638 in breast (MCF-7) and endometrial (ECC-1) cell lines in vitro]. GW7604 did not cause the growth of ECC-1 cells at any concentration (10(-11)-10(-6) M), but 4-OHT was weakly estrogen-like at low concentrations (10(-11)-10(-10) M). Compounds (10(-7) M) blocked the growth promoting action of estradiol (10(-10) M) in both ECC-1 and MCF-7 cells. Western blotting was used to show that GW7604 and raloxifene did not affect ER levels significantly, compared with controls, in MCF-7 cells; whereas the pure antiestrogen ICI182,780 decreased ER levels (P < 0.05). An assay system was used that can classify compounds into tamoxifen-like, raloxifene-like, or pure antiestrogens. The assay depends on the activation of the transforming growth factor alpha (TGFalpha) gene in situ by wild-type or D351Y mutant ER stably transfected into MDA-MB-231 cells (MacGregor-Schafer et al., Cancer Res, 1999, 59:4308-4313). GW7604 inhibited both estradiol (10(-9) M) and 4-OHT (10(-8), 10(-7) M) induction of TGFalpha in a concentration related manner (10(-9)-10(-6) M). GW7604 and raloxifene stimulated TGFalpha with the D351Y ER. In contrast, ICI 182,780 (10(-6) M) did not initiate TGFalpha and blocked the induction of TGFalpha with GW7604, raloxifene, and 4-OHT in D351Y-transfected cells. Using computer-assisted molecular models of ER complexes, we found that the antiestrogenic side chain of 4-OHT weakly interacted with the surface amino acid 351 (aspartate), but the carboxylic acid of GW7604 caused a strong repulsion of aspartate 351. We propose that GW7604 is less estrogen-like than 4-OHT, because it disrupts the surface charge around aa351 required for coactivator docking in the 4-OHT:ER complex. This charge is restored in the D351Y ER, thus converting GW7604 from an antiestrogen to an estrogen-like molecule.

Resistance to endocrine therapy of breast cancer: recent advances and tomorrow's challenges

The role of endocrine therapy in early as well as advanced breast cancer cannot be overrated. Long-term tamoxifen exposure (5 years) in the adjuvant setting has been shown to be effective not only in improving relapse-free and overall survival but also in reducing the incidence of contralateral cancers. Promising results have been achieved in breast cancer prevention with use of antiestrogens. Novel aromatase inhibitors and inactivators have been found superior to conventional treatment in metastatic disease and are currently being evaluated in the adjuvant setting to improve relapse-free and overall survival. If potential health hazards from estrogen deprivation with regard to cardiovascular disease as well as bone metabolism can be addressed, adjuvant endocrine therapy may include such drugs in the future. However, while endocrine therapy of breast cancer has become more and more important in the clinic, the major problems in hormonal therapy are primary and acquired resistance to endocrine manipulations. The causes for endocrine resistance and possible ways to delay or avoid this phenomenon are only allusively understood. Elucidation of the mechanisms underlying endocrine resistance in vivo represents the key to improve our treatment strategies. Due to intense use of in vitro models and animal systems, many potential mechanisms of endocrine resistance have been described; however, our understanding of the problem of drug resistance in vivo remains limited. Hopefully, ongoing programs on translational research in the neoadjuvant, adjuvant, and palliative settings will provide information that will improve our understanding of the biology of endocrine resistance in vivo and, thus, provide us with a better rationale to improve early as well as late endocrine therapy in breast cancer patients. The present publication summarizes the state of the art with respect to endocrine resistance.

Control of the estrogen-like actions of the tamoxifen-estrogen receptor complex by the surface amino acid at position 351

Tamoxifen is a valuable therapeutic agent with applications in the treatment and prevention of breast cancer. However, the development of drug resistance limits the usefulness of tamoxifen therapy. One form of drug resistance in breast cancer is tamoxifen-stimulated growth. We have addressed a mechanism how the tamoxifen-estrogen receptor (ER) complex can convert from being a blocking to stimulatory signal in breast cancer. We have described an effective assay system to study the action of antiestrogen-ER complex through the activation of transforming growth factor alpha gene in situ. The MDA-MB-231 breast cancer cells were stably transfected with cDNAs for wtER (D351), mutant Asp351Tyr ER (D351Y) and mutant Asp351Gly ER (D351G). The D351Y ER can enhance the estrogenic properties of 4OHT and change the pharmacology of raloxifene by converting it from antiestrogen to estrogen. We hypothesized that alterations in the charge of amino acid (aa) 351, and changes in the interaction with the side chain of an antiestrogen, are critical for the subsequent estrogenicity of the complex. Our goal was (1) to modulate the estrogenicity of the antiestrogen-ER complex by different aa substitutions at position 351 and (2) to examine the role of alterations in the side chain of antiestrogens on the estrogenicity of the complex. Substitution of tyrosine for aspartate at aa351 results in increased estrogenicity for a series of tamoxifen derivatives-ER complexes and the conversion of EM 652-ER and GW 7604-ER complexes from antiestrogenic to estrogen-like. Substitution of glycine for aspartate at aa 351 results in the conversion of 4OHT-ER complex from estrogen-like to antiestrogenic. We propose that the side chain of antiestrogens either neutralizes or displaces the charge at aspartate 351 thereby removing a charged site for the opportunistic binding of a novel coactivator. If no charge is present (D351G) then no coactivator can bind and the complex with any antiestrogen is not estrogen-like. However, if the charge is extended beyond the reach of an antiestrogen side chain (D351Y), then the coactivators bind and compounds are estrogen-like. The establishment of a relationship between the structure of the antiestrogen-ER complex and its function will enhance the development of novel compounds with unique biological activities and potentially avoid premature drug resistance.

Molecular and pharmacological aspects of antiestrogen resistance

Endocrine therapy is effective in approximately one-third of all breast cancers and up to 80% of tumors that express both estrogen and progesterone receptors. Despite the low toxicity, good overall response rates, and additional benefits associated with its partial agonist activity, most Tamoxifen-responsive breast cancers acquire resistance. The development of new antiestrogens, both steroidal and non-steroidal, provides the opportunity for the development of non-cross-resistant therapies and the identification of additional mechanisms of action and resistance. Drug-specific pharmacologic mechanisms may confer a resistance phenotype, reflecting the complexities of both tumor biology/pharmacology and the molecular endocrinology of steroid hormone action. However, since all antiestrogens will be effective only in cells that express estrogen receptors (ER), many mechanisms will likely be directly related to ER expression and signaling. For example, loss of ER expression/function is likely to confer a cross-resistance phenotype across all structural classes of antiestrogens. Altered expression of ERalpha and ERbeta, and/or signaling from transcription complexes driven by these receptors, may produce drug-specific resistance phenotypes. We have begun to study the possible changes in gene expression that may occur as cells acquire resistance to steroidal and non-steroidal antiestrogens. Our preliminary studies implicate the altered expression of several estrogen-regulated genes. However, resistance to antiestrogens is likely to be a multigene phenomenon, involving a network of interrelated signaling pathways. The way in which this network is adapted by cells may vary among tumors, consistent with the existence of a highly plastic and adaptable genotype within breast cancer cells.

Antiaromatase agents after adjuvant tamoxifen: rationale and clinical implications

Although tamoxifen is considered standard adjuvant hormonal therapy in receptor-positive, stage I and II breast cancer, information on its optimal duration of administration has only been reported recently and, for many, the subject is still a matter of scientific debate. Data suggest that there is a time period beyond which, if tamoxifen is continued, it may become ineffective or even detrimental to the patient. Tamoxifen is usually discontinued after approximately 5 years of therapy, at which time most patients are thought to be disease free; however, some patients may harbor residual micrometastases. A fraction of these patients will have micrometastatic tumor cells that are still responsive to tamoxifen, and tamoxifen discontinuation could result in cancer cell growth. The preponderance of clinical data, however, indicate that a greater fraction of patients will have micrometastatic tumor cells that have become progressively resistant to tamoxifen. In fact, tumor cell growth could be stimulated by continued therapy with the drug. Although in some patients the micrometastatic tumor cells may have become hormonally unresponsive, in most cases (tamoxifen-responsive or tamoxifen-stimulated micrometastases), the tumor remains hormonally responsive. Therefore, the use of anti-aromatase agents to reduce the level of estrogenic stimulation and, as a result, the risk of recurrence may prove to be a valuable approach at the time of tamoxifen discontinuation. The National Surgical Adjuvant Breast and Bowel Project (NSABP) is in the final stages of developing a clinical trial (NSABP B-33) to evaluate the effect of administering 2 years of therapy with the aromatase inactivator exemestane to postmenopausal, receptor-positive patients who have completed 5 years of tamoxifen therapy and are disease free at the time of tamoxifen discontinuation.

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.

ICI 182,780 (Faslodex): development of a novel, "pure" antiestrogen

BACKGROUND

The nonsteroidal antiestrogen tamoxifen is well established as an effective treatment for patients with breast carcinoma, both for the treatment of metastatic disease and as an adjuvant to surgery for patients with primary breast carcinoma. In addition to exerting antagonistic effects on the estrogen receptor, tamoxifen and its derivatives act as partial agonists on certain tissues. These agonistic effects, for example, endometrial stimulation and stimulation of tumor growth after previous response to tamoxifen, may limit their clinical efficacy. ICI 182,780 (Faslodex) from AstraZeneca (Cheshire, United Kingdom) is a novel, steroidal estrogen antagonist that was designed to be devoid of estrogen agonist activity in preclinical models.

METHODS

ICI 182,780 was tested in a large number of in vitro and in vivo preclinical models, and its value was assessed clinically when administered before surgery for breast carcinoma and hysterectomy for benign conditions and after failure of tamoxifen in patients with advanced breast carcinoma.

RESULTS

All data indicated that ICI 182,780 is devoid of agonist activity in preclinical models and in clinical trials. It inhibits growth of the breast and endometrium. In animal models, it does not cross the blood-brain barrier and appears to be neutral with respect to lipids and bone. ICI 182,780 down-regulates the estrogen receptor and is active in tamoxifen-resistant breast carcinoma. In a small, Phase II study, durable responses were seen: Phase III clinical trials are in progress comparing ICI 182,780 with anastrozole and tamoxifen in the treatment of patients with advanced breast carcinoma.

CONCLUSIONS

ICI 182,780 specifically down-regulates the estrogen receptor and, thus, represents the first of a new class of therapeutic agents. In this report, the authors present the current evidence that distinguishes ICI 182,780 from tamoxifen and related nonsteroidal compounds and establishes ICI 182,780 as the first in a new class of therapeutic agents.

Prognostic significance of oestrogen receptor beta in breast cancer

BACKGROUND

Endocrine therapy for breast cancer is now well established and with the identification of a second oestrogen receptor, ERbeta, 3 years ago it is timely to review the possible significance of this receptor in breast cancer management.

METHODS

An up-to-date review of the current literature concerning the role and possible implications of ERbeta in human breast cancer was undertaken.

RESULTS

Wild-type and variant ERbeta are expressed in human breast tumours. Expression of ERbeta correlates with accepted prognostic indicators including lymph node status and tumour grade. Furthermore, levels of ERbeta messenger RNA alter during carcinogenesis and are upregulated in breast tumours that develop antioestrogen resistance.

CONCLUSION

ERbeta has potential as a novel clinical prognostic marker in breast cancer, particularly in determining tumours that are resistant to tamoxifen.

Altering the estrogenic milieu of breast cancer with a focus on the new aromatase inhibitors

Aromatase is a dual-enzyme complex that catalyzes the synthesis of estrogen from androgenic precursors. Although evidence implicates estrogens in the pathogenesis of breast cancer, recent findings suggest that deregulation of aromatase may be a crucial link between these hormones and this neoplasm. Whereas tamoxifen is the endocrine therapy of choice, selective inhibition of aromatase may be equally effective, and possibly less toxic, in the management of patients with hormone-responsive breast tumors.

Long-term inhibitory effect of the orally active and pure antiestrogen EM-800 on the growth of human breast cancer xenografts in nude mice

The antiproliferative effect of the new antiestrogen EM-800 has been studied during 40 weeks of treatment on human breast carcinoma ZR-75-1 xenografts in ovariectomized nude mice supplemented with estrone (0.5 microg, s.c. daily). At the daily 50 microg (approximately 2.5 mg/kg) oral dose, EM-800 caused a complete inhibition of the 680% stimulatory effect of estrone on the growth of the ZR-75-1 human breast cancer xenografts. Complete response, defined as the complete disappearance of the tumors, was observed in 41% of tumors following treatment with the 50 microg dose of the antiestrogen, while a value of 26% was found in ovariectomized animals. The proportion of tumors showing progression at the end of 40 weeks of treatment decreased from 94% in the estrone-supplemented animals to 62%, 61% and 19% in the animals receiving the 5 microg, 20 microg and 50 microg daily doses of the antiestrogen, respectively. None of the tumors that showed a complete or a partial response progressed at later time intervals. The 50 microg daily dose of EM-800 nearly completely (93%) or completely (28% below the value in ovariectomized animals) reversed the stimulatory effect of estrone on uterine and vaginal weight, respectively. The disappearance of 41% of tumors in the group of animals that received the 50 microg daily dose of EM-800 indicates that the antiestrogen induces cell death or apoptosis in ZR-75-1 human breast cancer cells and that its action is cytotoxic and not only cytostatic.

EM-652 (SCH 57068), a third generation SERM acting as pure antiestrogen in the mammary gland and endometrium

Breast cancer is the most frequent cancer in women while it is the second cause of cancer death. Estrogens are well recognized to play the predominant role in breast cancer development and growth and much efforts have been devoted to the blockade of estrogen formation and action. The most widely used therapy of breast cancer which has shown benefits at all stages of the disease is the use of the antiestrogen Tamoxifen. This compound, however, possesses mixed agonist and antagonist activity and major efforts have been devoted to the development of compounds having pure antiestrogenic activity in the mammary gland and endometrium. Such a compound would avoid the problem of stimulation of the endometrium and the risk of endometrial carcinoma. We have thus synthesized an orally active non-steroidal antiestrogen, EM-652 (SCH 57068) and the prodrug EM-800 (SCH57050) which are the most potent of the known antiestrogens. EM-652 is the compound having the highest affinity for the estrogen receptor, including estradiol. It has higher affinity for the ER than ICI 182780, hydroxytamoxifen, raloxifene, droloxifene and hydroxytoremifene. EM-652 has the most potent inhibitory activity on both ER alpha and ER beta compared to any of the other antiestrogens tested. An important aspect of EM-652 is that it inhibits both the AF1 and AF2 functions of both ER alpha and ER beta while the inhibitory action of hydroxytamoxifen is limited to AF2, the ligand-dependent function of the estrogen receptors. AF1 activity is constitutive, ligand-independent and is responsible for mediation of the activity of growth factors and of the ras oncogene and MAP-kinase pathway. EM-652 inhibits Ras-induced transcriptional activity of ER alpha and ER beta and blocks SRC-1-stimulated activity of the two receptors. EM-652 was also found to block the recruitment of SRC-1 at AF1 of ER beta, this ligand-independent activation of AF1 being closely related to phosphorylation of the steroid receptors by protein kinase. Most importantly, the antiestrogen hydroxytamoxifen has no inhibitory effect on the SRC-1-induced ER beta activity while the pure antiestrogen EM-652 completely abolishes this effect, thus strengthening the need to use pure antiestrogens in breast cancer therapy in order to control all known aspects of ER-regulated gene expression. In fact, the absence of blockade of AF2 by hydroxytamoxifen could explain why the benefits of tamoxifen observed up to 5 years become negative at longer time intervals and why resistance develops to tamoxifen. EM-800, the prodrug of EM-652, has been shown to prevent the development of dimethylbenz(a)anthracene (DMBA)-induced mammary carcinoma in the rat, a well-recognized model of human breast cancer. It is of interest that the addition of dehydroepiandrosterone, a precursor of androgens, to EM-800, led to complete inhibition of tumor development in this model. Not only the development, but also the growth of established DMBA-induced mammary carcinoma was inhibited by treatment with EM-800. An inhibitory effect was also observed when medroxyprogesterone was added to treatment with EM-800. Uterine size was reduced to castration levels in the groups of animals treated with EM-800. An almost complete disappearance of estrogen receptors was observed in the uterus, vaginum and tumors in nude mice treated with EM-800. EM-652 was the most potent antiestrogen to inhibit the growth of human breast cancer ZR-75-1, MCF-7 and T-47D cells in vitro when compared with ICI 182780, ICI 164384, hydroxytamoxifen, and droloxifene. Moreover, EM-652 and EM-800 have no stimulatory effect on the basal levels of cell proliferation in the absence of E2 while hydroxytamoxifen and droloxifene had a stimulatory effect on the basal growth of T-47D and ZR-75-1 cells. EM-652 was also the most potent inhibitor of the percentage of cycling cancer cells. (ABSTRACT TRUNCATED)

Characterization of the effects of the novel non-steroidal antiestrogen EM-800 on basal and estrogen-induced proliferation of T-47D, ZR-75-1 and MCF-7 human breast cancer cells in vitro

Since estrogens play a predominant role in the development and growth of human breast cancer, antiestrogens represent a logical approach to the treatment of this disease. The present study compares the effects of the novel nonsteroidal anti-estrogen EM-800 and related compounds with those of a series of anti-estrogens on basal and 17 beta-estradiol (E2)-induced cell proliferation in human breast cancer cell lines. In the absence of added E2, EM-800 and related compounds failed to change basal cell proliferation, thus showing the absence of intrinsic estrogenic activity in the ER-positive T-47D, ZR-75-1 and MCF-7 cell lines. The stimulation of T-47D cell proliferation induced by 0.1 nM E2 was competitively blocked by a simultaneous incubation with EM-652, EM-800, OH-tamoxifen, OH-toremifene, ICI 182780, ICI 164384, droloxifene, tamoxifen and toremifene at apparent Ki values of 0.015, 0.011-0.017, 0.040-0.054, 0.043, 0.044, 0.243 and 0.735 nM, approx. 10 nM and > 10 nM, respectively. Similar data were obtained in ZR-75-1 and/or MCF-7 cells. Moreover, EM-652 was 6-fold more potent than OH-Tamoxifen in inhibiting the proportion of cycling MCF-7 cells. Our data show that EM-800 and EM-652 are the most potent known antiestrogens in human breast cancer cells in vitro and that they are devoid of the estrogenic activity of OH-tamoxifen and droloxifene suggested by stimulation of cell growth in the absence of estrogens in ZR-75-1 and MCF-7 cells.

The role of estrogen receptor mutations in tamoxifen-stimulated breast cancer

During the past 20 years, the hormonal therapy of choice for the treatment of breast cancer has been the antiestrogen, tamoxifen. The use of tamoxifen has been proved to produce a favorable response and survival advantage in patients whose tumors are classified as estrogen receptor-positive (ER+)/progesterone receptor-positive (PR+). Additionally, tamoxifen is the only drug known to reduce the incidence of contralateral disease. This drug produces relatively few harmful side effects, while exhibiting several beneficial effects such as maintaining bone density and reducing the incidence of myocardial infarction in the postmenopausal woman. However, tumors eventually acquire a tamoxifen-resistant or tamoxifen-stimulated phenotype, resulting in disease recurrence. Several mechanisms have been proposed to account for tamoxifen-resistant breast cancer, in the hope of developing a more effective first-line or perhaps second-line treatment strategy. One popular theory is the occurrence of a mutation in the estrogen receptor, the drug target. A plethora of studies have reported the detection of estrogen receptor mRNA splice variants, and it has been suggested that the accumulation of these variant mRNAs are responsible for the development of tamoxifen-resistant breast cancer. In this review, several questions will be posed to address the suitability of both laboratory and clinical evidence to support this hypothesis. Although there is adequate data generated in the laboratory, there is, as yet, no compelling evidence to suggest that mutation of the estrogen receptor is the molecular mechanism producing tamoxifen-stimulated growth in human breast and endometrial cancer.

Steroid hormone withdrawal syndromes. Pathophysiology and clinical significance

The steroid withdrawal syndrome has brought a new dimension to the treatment of advanced prostate cancer not only in the way we treat androgen-independent disease but also in terms of insights into the development of hormonal resistance. The data now confirm that 30% of cases have a meaningful subjective, biochemical, and objective response to the withdrawal of a steroid hormone as the first maneuver after primary hormonal therapy failure. Larger studies are needed to define further the withdrawal effect related to the other steroid hormone family members and to determine the objective response proportions. Although controversy surrounds the cause of the steroid withdrawal phenomenon, studies suggest that the androgen receptor plays a pivotal role. Molecular studies of the androgen receptors involving larger number of patients are paramount if we are going to develop a better understanding of the evolution of the withdrawal effect and hormone resistance.

Pharmacokinetics, pharmacological and anti-tumour effects of the specific anti-oestrogen ICI 182780 in women with advanced breast cancer

We have assessed the pharmacokinetics, pharmacological and anti-tumour effects of the specific steroidal anti-oestrogen ICI 182780 in 19 patients with advanced breast cancer resistant to tamoxifen. The agent was administered as a monthly depot intramuscular injection. Peak levels of ICI 182780 occurred a median of 8-9 days after dosing and then declined but were above the projected therapeutic threshold at day 28. Cmax during the first month was 10.5 ng/ml-1 and during the sixth month was 12.6 ng ml-1. The AUCs were 140.5 and 206.8 ng day ml-1 on the first and sixth month of dosing respectively, suggesting some drug accumulation. Luteinising hormone (LH) and follicle-stimulating hormone (FSH) levels rose after withdrawal of tamoxifen and then plateaued, suggesting no effect of ICI 182780 on the pituitary-hypothalamic axis. There were no significant changes in serum levels of prolactin, sex hormone-binding globulin (SHBG) or lipids. Side-effects were infrequent. Hot-flushes and sweats were not induced and there was no apparent effect of treatment upon the endometrium or vagina. Thirteen (69%) patients responded (seven had partial responses and six showed "no change' responses) to ICI 182780, after progression on tamoxifen, for a median duration of 25 months. Thus ICI 182780, given by monthly depot injection, and at the drug levels described, is an active second-line anti-oestrogen without apparent negative effects on the liver, brain or genital tract and warrants further evaluation in patients with advanced breast cancer.

New endocrine therapies for breast cancer

How do the new endocrine therapies stand up to the aims of modern endocrine therapy outlined in Table 1? We wish to see increased efficacy, decreased toxicity and improved general health in women taking a new agent. None of the new non-steroidal anti-oestrogens have shown unequivocal evidence of improved efficacy in the clinic to mirror their improved profiles over tamoxifen in preclinical studies. We know that toremifene is equivalent to tamoxifen, but we do not have any phase III data from the other four compounds in development. The specific steroidal antioestrogen, ICI 182,780, looks very promising, but is early in its developmental programme. The new aromatase inhibitors are likely to prove equal to tamoxifen or progestagens, but it is disappointing that improved oestrogen suppression has not led, to date, to improved efficacy. No comment can be made about adjuvant or preventative therapy for any of the new agents, although trials are planned for the new aromatase inhibitors in this clinical situation. Currently, the antiprogestins are disappointing and we will need to wait a considerable time for new agents in preclinical testing to reach the clinic. Many of the new agents are associated with decreased toxicity. It is likely that the NSAEs will be equitoxic with tamoxifen. The steroidal antioestrogen looks particularly non-toxic as do the new aromatase inhibitors, and thus we have an advance in terms of reduced toxicity. The effects of the new agents on the uterus, lipids and bone are in the early stages of testing. Raloxifene, ICI 182,780 and the new aromatase inhibitors are expected to have no proliferative effects on the endometrium, but only the new NSAEs are expected to have beneficial cardiovascular and skeletal effects. If the steroidal anti-oestrogens and new aromatase inhibitors become adjuvant therapies of choice, other agents to prevent osteoporosis and cardiovascular events may also have to be administered.

Endocrine resistance in advanced breast cancer

Although about half of patients with advanced breast cancer show either an objective response or disease stabilization on first line endocrine therapy, virtually all eventually relapse. Few of those patients that fail to respond to first-line treatment, respond to challenge with a second agent. In most cases, the cause of this de novo resistance appears to be the presence of only very low levels of oestrogen receptor (ER) and presumed growth dependence on other pathways. Patients who develop acquired resistance after an initial response, have approximately a 50% chance of responding to a further agent. The most frequently used first-line agent is tamoxifen, and the understanding of acquired endocrine resistance mainly relates to this agent. Selection of ER-ve clones of cells does not appear to occur frequently, and there is little clinical evidence to support the role of ER variants or mutants. There is evidence, however, that in some patients the intratumoural concentration of tamoxifen is substantially reduced at relapse, despite no change in plasma levels.

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.

Increasing the number of tandem estrogen response elements increases the estrogenic activity of a tamoxifen analogue

There have been several reports of women who have tumor relapse while on tamoxifen therapy, followed by tumor regression after tamoxifen withdrawal. In such apparently tamoxifen-stimulated tumors, there is likely a genetic change which increases the estrogenicity of tamoxifen. In this study, we determine if increasing the number of estrogen response elements (EREs) in the promoter region of a reporter gene can alter the agonistic activity of fixed-ring 4-hydroxytamoxifen. We show that increasing the number of EREs in the promotor region increases the transcriptional response of the reporter plasmid to estradiol. We also find that while fixed-ring 4-hydroxytamoxifen is unable to stimulate transcription when one ERE is present, transcriptional activation can occur with multiple EREs. These results demonstrate that ERE amplification could explain the agonistic properties of tamoxifen, and suggests a novel mechanism to explain tamoxifen-stimulated breast cancer growth.

Mechanisms of action of antiestrogens

Investigators from laboratories worldwide have spent nearly 40 years studying the mechanisms by which the diverse class of compounds known as antiestrogens exert their effects. In this review we present an overview of the work to date that has led to a greater understanding of both the classical and the sometimes unexpected actions which an antiestrogenic compound can have on the growth of a cell. In addition, we review work which has begun to explain the means by which some cells can ultimately become resistant to the action of antiestrogens. We conclude with a discussion of the current directions being followed by researchers in this area, as well as with several comments regarding what physiological activities might be desired in an 'ideal' antiestrogenic compound.