Defining the conformation of the estrogen receptor complex that controls estrogen-induced apoptosis in breast cancer

Mechanistic Effects of Estrogens on Breast Cancer

PURPOSE

Current concepts regarding estrogen and its mechanistic effects on breast cancer in women are evolving. This article reviews studies that address estrogen-mediated breast cancer development, the prevalence of occult tumors at autopsy, and the natural history of breast cancer as predicted by a newly developed tumor kinetic model.

METHODS

This article reviews previously published studies from the authors and articles pertinent to the data presented.

RESULTS

We discuss the concepts of adaptive hypersensitivity that develops in response to long-term deprivation of estrogen and results in both increased cell proliferation and apoptosis. The effects of menopausal hormonal therapy on breast cancer in postmenopausal women are interpreted based on the tumor kinetic model. Studies of the administration of a tissue selective estrogen complex in vitro, in vivo, and in patients are described. We review the various clinical studies of breast cancer prevention with selective estrogen receptor modulators and aromatase inhibitors. Finally, the effects of the underlying risk of breast cancer on the effects of menopausal hormone therapy are outlined.

DISCUSSION

The overall intent of this review is to present data supporting recent concepts, discuss pertinent literature, and critically examine areas of controversy.

Estrogen Receptor Complex to Trigger or Delay Estrogen-Induced Apoptosis in Long-Term Estrogen Deprived Breast Cancer

Antiestrogen therapy of breast cancer has been a "gold standard" of treatment of estrogen receptor (ER)-positive breast cancer for decades. Resistance to antiestrogen therapy may develop, however, a vulnerability in long-term estrogen deprived (LTED) breast cancer cells was discovered. LTED breast cancer cells may undergo estrogen-induced apoptosis within a week of treatment with estrogen in vitro. This phenomenon has been also validated in vivo and in the clinic. The molecular ER-mediated mechanism of action of estrogen-induced apoptosis was deciphered, however, the relationship between the structure of estrogenic ligands and the activity of the ER in LTED breast cancer cells remained a mystery until recently. In this review we provide an overview of the structure-activity relationship of various estrogens with different chemical structures and the modulation of estrogen-induced apoptosis in LTED breast cancer cells resistant to antihormone therapy. We provide analysis of evidence gathered over more than a decade of structure-activity relationship studies by our group on the role of the change in the conformation of the estrogen receptor and the biological activities of different classes of estrogens and the receptor as well in LTED breast cancer.

Turning scientific serendipity into discoveries in breast cancer research and treatment: a tale of PhD students and a 50-year roaming tamoxifen team

PURPOSE

This retrospective, about a single "mobile" laboratory in six locations on two continents, is intended as a case study in discovery for trainees and junior faculty in the medical sciences. Your knowledge of your topic is necessary to expect the unexpected.

HISTORICAL METHOD

In 1972, there was no tamoxifen, only ICI 46, 474, a non-steroidal anti-estrogen with little chance of clinical development. No one would ever be foolish enough to predict that the medicine, 20 years later, would achieve legendary status as the first targeted treatment for breast cancer, and millions of women would benefit from long-term adjuvant tamoxifen therapy. The secret of tamoxifen's success was a translational research strategy proposed in the mid 1970's. This strategy was to treat only patients with estrogen receptor (ER)-positive breast cancer and deploy 5 or more years of adjuvant tamoxifen therapy to prevent recurrence. Additionally, tamoxifen prevented mammary cancer in animals. Could the medicine prevent breast cancer in women?

RESULTS

Tamoxifen and the failed breast cancer drug raloxifene became the first selective estrogen receptor modulators (SERMs): a new drug group, discovered at the University of Wisconsin, Comprehensive Cancer Center. Serendipity can play a fundamental role in discovery, but there must be a rigorous preparation for the investigator to appreciate the possibility of a pending discovery. This article follows the unanticipated discoveries when PhD students "get the wrong answer." The secret of success of my six Tamoxifen Teams was their technical excellence to create models, to decipher mechanisms, that drove the development of new medicines. Discoveries are listed that either changed women's health or allowed an understanding of originally opaque mechanisms of action of potential therapies. These advances in women's health were supported entirely by government-sponsored peer-reviewed funding and major philanthropy from the Lynn Sage Breast Cancer Foundation, the Avon Foundation, and the Susan G. Komen Breast Cancer Foundation. The resulting lives saved or extended, families aided in a time of crisis and the injection of billions of dollars into national economies by drug development, is proof of the value of Federal or philanthropic investment into unencumbered research aimed at saving millions of lives.

Rapid Induction of the Unfolded Protein Response and Apoptosis by Estrogen Mimic TTC-352 for the Treatment of Endocrine-Resistant Breast Cancer

Patients with long-term estrogen-deprived breast cancer, after resistance to tamoxifen or aromatase inhibitors develops, can experience tumor regression when treated with estrogens. Estrogen's antitumor effect is attributed to apoptosis via the estrogen receptor (ER). Estrogen treatment can have unpleasant gynecologic and nongynecologic adverse events; thus, the development of safer estrogenic agents remains a clinical priority. Here, we study synthetic selective estrogen mimics (SEM) BMI-135 and TTC-352, and the naturally occurring estrogen estetrol (E₄), which are proposed as safer estrogenic agents compared with 17β-estradiol (E₂), for the treatment of endocrine-resistant breast cancer. TTC-352 and E₄ are being evaluated in breast cancer clinical trials. Cell viability assays, real-time PCR, immunoblotting, ERE DNA pulldowns, mass spectrometry, X-ray crystallography, docking and molecular dynamic simulations, live cell imaging, and Annexin V staining were conducted in 11 biologically different breast cancer models. Results were compared with the potent full agonist E₂, less potent full agonist E₄, the benchmark partial agonist triphenylethylene bisphenol (BPTPE), and antagonists 4-hydroxytamoxifen and endoxifen. We report ERα's regulation and coregulators' binding profiles with SEMs and E₄ We describe TTC-352's pharmacology as a weak full agonist and antitumor molecular mechanisms. This study highlights TTC-352's benzothiophene scaffold that yields an H-bond with Glu353, which allows Asp351-to-helix 12 (H12) interaction, sealing ERα's ligand-binding domain, recruiting E₂-enriched coactivators, and triggering rapid ERα-induced unfolded protein response (UPR) and apoptosis, as the basis of its anticancer properties. BPTPE's phenolic OH yields an H-Bond with Thr347, which disrupts Asp351-to-H12 interaction, delaying UPR and apoptosis and increasing clonal evolution risk.

Pharmacology and Molecular Mechanisms of Clinically Relevant Estrogen Estetrol and Estrogen Mimic BMI-135 for the Treatment of Endocrine-Resistant Breast Cancer

Long-term estrogen deprivation (LTED) with tamoxifen (TAM) or aromatase inhibitors leads to endocrine-resistance, whereby physiologic levels of estrogen kill breast cancer (BC). Estrogen therapy is effective in treating patients with advanced BC after resistance to TAM and aromatase inhibitors develops. This therapeutic effect is attributed to estrogen-induced apoptosis via the estrogen receptor (ER). Estrogen therapy can have unpleasant gynecologic and nongynecologic adverse events. Here, we study estetrol (E4) and a model Selective Human ER Partial Agonist (ShERPA) BMI-135. Estetrol and ShERPA TTC-352 are being evaluated in clinical trials. These agents are proposed as safer estrogenic candidates compared with 17β-estradiol (E2) for the treatment of endocrine-resistant BC. Cell viability assays, real-time polymerase chain reaction, luciferase reporter assays, chromatin immunoprecipitation, docking and molecular dynamics simulations, human unfolded protein response (UPR) RT2 PCR profiler arrays, live cell microscopic imaging and analysis, and annexin V staining assays were conducted. Our work was done in eight biologically different human BC cell lines and one human endometrial cancer cell line, and results were compared with full agonists estrone, E2, and estriol, a benchmark partial agonist triphenylethylene bisphenol (BPTPE), and antagonists 4-hydroxytamoxifen and endoxifen. Our study shows the pharmacology of E4 and BMI-135 as less-potent full-estrogen agonists as well as their molecular mechanisms of tumor regression in LTED BC through triggering a rapid UPR and apoptosis. Our work concludes that the use of a full agonist to treat BC is potentially superior to a partial agonist given BPTPE's delayed induction of UPR and apoptosis, with a higher probability of tumor clonal evolution and resistance. SIGNIFICANCE STATEMENT: Given the unpleasant gynecologic and nongynecologic adverse effects of estrogen treatment, the development of safer estrogens for endocrine-resistant breast cancer (BC) treatment and hormone replacement therapy remains a priority. The naturally occurring estrogen estetrol and Selective Human Estrogen-Receptor Partial Agonists are being evaluated in endocrine-resistant BC clinical trials. This work provides a comprehensive evaluation of their pharmacology in numerous endocrine-resistant BC models and an endometrial cancer model and their molecular mechanisms of tumor regression through the unfolded protein response and apoptosis.

The Structure-Function Relationship of Angular Estrogens and Estrogen Receptor Alpha to Initiate Estrogen-Induced Apoptosis in Breast Cancer Cells

High-dose synthetic estrogen therapy was the standard treatment of advanced breast cancer for three decades until the discovery of tamoxifen. A range of substituted triphenylethylene synthetic estrogens and diethylstilbestrol were used. It is now known that low doses of estrogens can cause apoptosis in long-term estrogen deprived (LTED) breast cancer cells resistant to antiestrogens. This action of estrogen can explain the reduced breast cancer incidence in postmenopausal women over 60 who are taking conjugated equine estrogens and the beneficial effect of low-dose estrogen treatment of patients with acquired aromatase inhibitor resistance in clinical trials. To decipher the molecular mechanism of estrogens at the estrogen receptor (ER) complex by different types of estrogens-planar [17β-estradiol (E2)] and angular triphenylethylene (TPE) derivatives-we have synthesized a small series of compounds with either no substitutions on the TPE phenyl ring containing the antiestrogenic side chain of endoxifen or a free hydroxyl. In the first week of treatment with E2 the LTED cells undergo apoptosis completely. By contrast, the test TPE derivatives act as antiestrogens with a free para-hydroxyl on the phenyl ring that contains an antiestrogenic side chain in endoxifen. This inhibits early E2-induced apoptosis if a free hydroxyl is present. No substitution at the site occupied by the antiestrogenic side chain of endoxifen results in early apoptosis similar to planar E2 The TPE compounds recruit coregulators to the ER differentially and predictably, leading to delayed apoptosis in these cells. SIGNIFICANCE STATEMENT: In this paper we investigate the role of the structure-function relationship of a panel of synthetic triphenylethylene (TPE) derivatives and a novel mechanism of estrogen-induced cell death in breast cancer, which is now clinically relevant. Our study indicates that these TPE derivatives, depending on the positioning of the hydroxyl groups, induce various conformations of the estrogen receptor's ligand-binding domain, which in turn produces differential recruitment of coregulators and subsequently different apoptotic effects on the antiestrogen-resistant breast cancer cells.

Endoxifen, 4-Hydroxytamoxifen and an Estrogenic Derivative Modulate Estrogen Receptor Complex Mediated Apoptosis in Breast Cancer

Estrogen therapy was used to treat advanced breast cancer in postmenopausal women for decades until the introduction of tamoxifen. Resistance to long-term estrogen deprivation (LTED) with tamoxifen and aromatase inhibitors used as a treatment of breast cancer inevitably occurs, but unexpectedly low-dose estrogen can cause regression of breast cancer and increase disease-free survival in some patients. This therapeutic effect is attributed to estrogen-induced apoptosis in LTED breast cancer. Here, we describe modulation of the estrogen receptor (ER) liganded with antiestrogens (endoxifen and 4-hydroxytamoxifen) and an estrogenic triphenylethylene (TPE), ethoxytriphenylethylene (EtOXTPE), on estrogen-induced apoptosis in LTED breast cancer cells. Our results show that the angular TPE estrogen (EtOXTPE) is able to induce the ER-mediated apoptosis only at a later time compared with planar estradiol in these cells. Using real-time polymerase chain reaction, chromatin immunoprecipitation, western blotting, molecular modeling, and X-ray crystallography techniques, we report novel conformations of the ER complex with an angular estrogen EtOXTPE and endoxifen. We propose that alteration of the conformation of the ER complexes, with changes in coactivator binding, governs estrogen-induced apoptosis through the protein kinase regulated by RNA-like endoplasmic reticulum kinase sensor system to trigger an unfolded protein response.

The molecular, cellular and clinical consequences of targeting the estrogen receptor following estrogen deprivation therapy

During the past 20 years our understanding of the control of breast tumor development, growth and survival has changed dramatically. The once long forgotten application of high dose synthetic estrogen therapy as the first chemical therapy to treat any cancer has been resurrected, refined and reinvented as the new biology of estrogen-induced apoptosis. High dose estrogen therapy was cast aside once tamoxifen, from its origins as a failed "morning after pill", was reinvented as the first targeted therapy to treat any cancer. The current understanding of the mechanism of estrogen-induced apoptosis is described as a consequence of acquired resistance to long term antihormone therapy in estrogen receptor (ER) positive breast cancer. The ER signal transduction pathway remains a target for therapy in breast cancer despite "antiestrogen" resistance, but becomes a regulator of resistance. Multiple mechanisms of resistance come into play: Selective ER modulator (SERM) stimulated growth, growth factor/ER crosstalk, estrogen-induced apoptosis and mutations of ER. But it is with the science of estrogen-induced apoptosis that the next innovation in women's health will be developed. Recent evidence suggests that the glucocorticoid properties of medroxyprogesterone acetate blunt estrogen-induced apoptosis in estrogen deprived breast cancer cell populations. As a result breast cancer develops during long-term hormone replacement therapy (HRT). A new synthetic progestin with estrogen-like properties, such as the 19 nortestosterone derivatives used in oral contraceptives, will continue to protect the uterus from unopposed estrogen stimulation but at the same time, reinforce apoptosis in vulnerable populations of nascent breast cancer cells.

Simulation with cells in vitro of tamoxifen treatment in premenopausal breast cancer patients with different CYP2D6 genotypes

BACKGROUND AND PURPOSE

Tamoxifen is a prodrug that is metabolically activated by 4-hydroxylation to the potent primary metabolite 4-hydroxytamoxifen (4OHT) or via another primary metabolite N-desmethyltamoxifen (NDMTAM) to a biologically active secondary metabolite endoxifen through a cytochrome P450 2D6 variant system (CYP2D6). To elucidate the mechanism of action of tamoxifen and the importance of endoxifen for its effect, we determined the anti-oestrogenic efficacy of tamoxifen and its metabolites, including endoxifen, at concentrations corresponding to serum levels measured in breast cancer patients with various CYP2D6 genotypes (simulating tamoxifen treatment).

EXPERIMENTAL APPROACH

The biological effects of tamoxifen and its metabolites on cell growth and oestrogen-responsive gene modulation were evaluated in a panel of oestrogen receptor-positive breast cancer cell lines. Actual clinical levels of tamoxifen metabolites in breast cancer patients were used in vitro along with actual levels of oestrogens observed in premenopausal patients taking tamoxifen.

KEY RESULTS

Tamoxifen and its primary metabolites (4OHT and NDMTAM) only partially inhibited the stimulant effects of oestrogen on cells. The addition of endoxifen at concentrations corresponding to different CYP2D6 genotypes was found to enhance the anti-oestrogenic effect of tamoxifen and its metabolites with an efficacy that correlated with the concentration of endoxifen; at concentrations corresponding to the extensive metabolizer genotype it further inhibited the actions of oestrogen. In contrast, lower concentrations of endoxifen (intermediate and poor metabolizers) had little or no anti-oestrogenic effects.

CONCLUSIONS AND IMPLICATIONS

Endoxifen may be a clinically relevant metabolite in premenopausal patients as it provides additional anti-oestrogenic actions during tamoxifen treatment.

Differences in the rate of oestrogen-induced apoptosis in breast cancer by oestradiol and the triphenylethylene bisphenol

BACKGROUND AND PURPOSE

Triphenylethylene (TPE)-like compounds were the first agents to be used in the treatment of metastatic breast cancer in postmenopausal women. Although structurally related to the anti-oestrogen, 4-hydroxytamoxifen, TPEs possess oestrogenic properties in fully oestrogenized breast cancer cells but do not induce apoptosis with short-term treatment in long-term oestrogen-deprived breast cancer cells. This study determined the differential effects of bisphenol, a TPE, on growth and apoptosis based on the modulation of the shape of the ligand-oestrogen receptor complex.

EXPERIMENTAL APPROACH

Apoptotic flow cytometric studies were used to evaluate apoptosis over time. Proliferation of the breast cancer cells was assessed using DNA quantification and cell cycle analysis. Real-time PCR was performed to quantify mRNA levels of apoptotic genes. Regulation of cell cycle and apoptotic genes was determined using PCR-based arrays.

KEY RESULTS

Bisphenol induced an up-regulation of cell cycle genes similar to those induced by 17β oestradiol (E2 ). Unlike the changes induced by E2 that occur after 24 h, the apoptosis evoked by bisphenol occurred after 4 days, with quantifiable apoptotic changes noted at 6 days. A prolonged up-regulation of endoplasmic reticulum stress and inflammatory stress response genes was observed with subsequent activation of apoptosis-related genes in the second week of treatment with bisphenol.

CONCLUSIONS AND IMPLICATIONS

The bisphenol: ERα complex induces delayed biological effects on the growth and apoptosis of breast cancer cells. Both the shape of the complex and the duration of treatment control the initiation of apoptosis.

Estrogen receptor mutations found in breast cancer metastases integrated with the molecular pharmacology of selective ER modulators

The consistent reports of mutations at Asp538 and Tyr537 in helix 12 of the ligand-binding domain (LBD) of estrogen receptors (ERs) from antihormone-resistant breast cancer metastases constitute an important advance. The mutant amino acids interact with an anchor amino acid, Asp351, to close the LBD, thereby creating a ligand-free constitutively activated ER. Amino acids Asp 538, Tyr 537, and Asp 351 are known to play a role in either the turnover of ER, the antiestrogenic activity of the ER complex, or the estrogen-like actions of selective ER modulators. A unifying mechanism of action for these amino acids to enhance ER gene activation and growth response is presented. There is a range of mutations described in metastases vs low to zero in primary disease, so the new knowledge is of clinical relevance, thereby confirming an additional mechanism of acquired resistance to antihormone therapy through cell population selection pressure and enrichment during treatment. Circulating tumor cells containing ER mutations can be cultured ex vivo, and tumor tissues can be grown as patient-derived xenografts to add a new dimension for testing drug susceptibility for future drug discovery.

The new biology of estrogen-induced apoptosis applied to treat and prevent breast cancer

The successful use of high-dose synthetic estrogens to treat postmenopausal metastatic breast cancer is the first effective 'chemical therapy' proven in clinical trial to treat any cancer. This review documents the clinical use of estrogen for breast cancer treatment or estrogen replacement therapy (ERT) in postmenopausal hysterectomized women, which can either result in breast cancer cell growth or breast cancer regression. This has remained a paradox since the 1950s until the discovery of the new biology of estrogen-induced apoptosis at the end of the 20th century. The key to triggering apoptosis with estrogen is the selection of breast cancer cell populations that are resistant to long-term estrogen deprivation. However, estrogen-independent growth occurs through trial and error. At the cellular level, estrogen-induced apoptosis is dependent upon the presence of the estrogen receptor (ER), which can be blocked by nonsteroidal or steroidal antiestrogens. The shape of an estrogenic ligand programs the conformation of the ER complex, which, in turn, can modulate estrogen-induced apoptosis: class I planar estrogens (e.g., estradiol) trigger apoptosis after 24 h, whereas class II angular estrogens (e.g., bisphenol triphenylethylene) delay the process until after 72 h. This contrasts with paclitaxel, which causes G2 blockade with immediate apoptosis. The process is complete within 24 h. Estrogen-induced apoptosis is modulated by glucocorticoids and cSrc inhibitors, but the target mechanism for estrogen action is genomic and not through a nongenomic pathway. The process is stepwise through the creation of endoplasmic reticulum stress and inflammatory responses, which then initiate an unfolded protein response. This, in turn, initiates apoptosis through the intrinsic pathway (mitochondrial) with the subsequent recruitment of the extrinsic pathway (death receptor) to complete the process. The symmetry of the clinical and laboratory studies now permits the creation of rules for the future clinical application of ERT or phytoestrogen supplements: a 5-year gap is necessary after menopause to permit the selection of estrogen-deprived breast cancer cell populations to cause them to become vulnerable to apoptotic cell death. Earlier treatment with estrogen around menopause encourages growth of ER-positive tumor cells, as the cells are still dependent on estrogen to maintain replication within the expanding population. An awareness of the evidence that the molecular events associated with estrogen-induced apoptosis can be orchestrated in the laboratory in estrogen-deprived breast cancers now supports the clinical findings regarding the treatment of metastatic breast cancer following estrogen deprivation, decreases in mortality following long-term antihormonal adjuvant therapy, and the results of treatment with ERT and ERT plus progestin in the Women's Health Initiative for women over the age of 60. Principles have emerged for understanding and applying physiological estrogen therapy appropriately by targeting the correct patient populations.

Linking estrogen-induced apoptosis with decreases in mortality following long-term adjuvant tamoxifen therapy

The impressive first results of the Adjuvant Tamoxifen: Longer Against Shorter (ATLAS) and the adjuvant Tamoxifen To offer more (aTTom) trials both demonstrate that 10 years of tamoxifen is superior to five years of treatment. Tamoxifen is a nonsteroidal antiestrogen that blocks estrogen-stimulated tumor growth. Paradoxically, mortality decreases dramatically only in the decade after long-term tamoxifen is stopped. It is proposed that the evolution and clonal selection of micrometastases that acquire tamoxifen resistance now become increasingly vulnerable to endogenous estrogen-induced apoptosis. Laboratory and clinical studies confirm the concept, and supporting clinical evidence from the estrogen-alone trial in the Women's Health Initiative (WHI), demonstrate that long-term estrogen-deprived women given exogenous physiologic estrogen have a decreased incidence of breast cancer and decreased mortality. It is proposed that a natural process of apoptosis is recruited to execute the long-term survival benefit of stopping ten years of adjuvant tamoxifen, but only after clonal selection of vulnerable breast cancer cells in an estrogen-deprived environment.

Acquired resistance to selective estrogen receptor modulators (SERMs) in clinical practice (tamoxifen & raloxifene) by selection pressure in breast cancer cell populations

Tamoxifen, a pioneering selective estrogen receptor modulator (SERM), has long been a therapeutic choice for all stages of estrogen receptor (ER)-positive breast cancer. The clinical application of long-term adjuvant antihormone therapy for the breast cancer has significantly improved breast cancer survival. However, acquired resistance to SERM remains a significant challenge in breast cancer treatment. The evolution of acquired resistance to SERMs treatment was primarily discovered using MCF-7 tumors transplanted in athymic mice to mimic years of adjuvant treatment in patients. Acquired resistance to tamoxifen is unique because the growth of resistant tumors is dependent on SERMs. It appears that acquired resistance to SERM is initially able to utilize either E2 or a SERM as the growth stimulus in the SERM-resistant breast tumors. Mechanistic studies reveal that SERMs continuously suppress nuclear ER-target genes even during resistance, whereas they function as agonists to activate multiple membrane-associated molecules to promote cell growth. Laboratory observations in vivo further show that three phases of acquired SERM-resistance exists, depending on the length of SERMs exposure. Tumors with Phase I resistance are stimulated by both SERMs and estrogen. Tumors with Phase II resistance are stimulated by SERMs, but are inhibited by estrogen due to apoptosis. The laboratory models suggest a new treatment strategy, in which limited-duration, low-dose estrogen can be used to purge Phase II-resistant breast cancer cells. This discovery provides an invaluable insight into the evolution of drug resistance to SERMs, and this knowledge is now being used to justify clinical trials of estrogen therapy following long-term antihormone therapy. All of these results suggest that cell populations that have acquired resistance are in constant evolution depending upon selection pressure. The limited availability of growth stimuli in any new environment enhances population plasticity in the trial and error search for survival.

Selective estrogen-induced apoptosis in breast cancer

Antihormone therapy remains the gold standard of care in the treatment of estrogen receptor (ER) positive breast cancer. However, development of acquired long term antihormone resistance exposes a vulnerability to estrogen that induces apoptosis. Laboratory and clinical studies indicate that successful therapy with estrogens is dependent on the duration of estrogen withdrawal and menopausal status of a woman. Interrogation of estradiol (E2) induced apoptosis using molecular studies indicate treatment of long term estrogen deprived MCF-7 breast cancer cells with estrogen causes an endoplasmic reticulum stress response that induces an unfolded protein response signal to inhibit protein translation. E2 binds to the ER and mediates apoptosis through the classical genomic pathway. Furthermore, the induction of apoptosis by estrogens is dependent on the conformation of the estrogen-ER complex. In this review, we explore the mechanism and the processes involved in the paradox of estrogen induced apoptosis and the new selectivity of estrogen action on different cell populations that is correctly been deciphered for clinical practice.

Influence of the length and positioning of the antiestrogenic side chain of endoxifen and 4-hydroxytamoxifen on gene activation and growth of estrogen receptor positive cancer cells

Tamoxifen has biologically active metabolites: 4-hydroxytamoxifen (4OHT) and endoxifen. The E-isomers are not stable in solution as Z-isomerization occurs. We have synthesized fixed ring (FR) analogues of 4OHT and endoxifen as well as FR E and Z isomers with methoxy and ethoxy side chains. Pharmacologic properties were documented in the MCF-7 cell line, and prolactin synthesis was assessed in GH3 rat pituitary tumor cells. The FR Z-isomers of 4OHT and endoxifen were equivalent to 4OHT and endoxifen. Other test compounds used possessed partial estrogenic activity. The E-isomers of FR 4OHT and endoxifen had no estrogenic activity at therapeutic serum concentrations. None of the newly synthesized compounds were able to down-regulate ER levels. Molecular modeling demonstrated that some compounds would each create a best fit with a novel agonist conformation of the ER. The results demonstrate modulation by the ER complex of cell replication or gene transcription in cancer.