Response to a specific antioestrogen (ICI 182780) in tamoxifen-resistant breast cancer

Discovery of as a Potent and Orally Efficacious Estrogen Receptor PROTAC Degrader with Strong Antitumor Activity

Estrogen receptor α (ERα) is a prime target for the treatment of ER-positive (ER+) breast cancer. Despite the development of several effective therapies targeting ERα signaling, clinical resistance remains a major challenge. In this study, we report the discovery of a new class of potent and orally bioavailable ERα degraders using the PROTAC technology, with ERD-3111 being the most promising compound. ERD-3111 exhibits potent in vitro degradation activity against ERα and demonstrates high oral bioavailability in mice, rats, and dogs. Oral administration of ERD-3111 effectively reduces the levels of wild-type and mutated ERα proteins in tumor tissues. ERD-3111 achieves tumor regression or complete tumor growth inhibition in the parental MCF-7 xenograft model with wild-type ER and two clinically relevant ESR1 mutated models in mice. ERD-3111 is a promising ERα degrader for further extensive evaluations for the treatment of ER+ breast cancer.

Accelerating drug development in breast cancer: New frontiers for ER inhibition

The estrogen receptor (ER) is an important driver in the proliferation, tumorigenesis, and progression of breast cancers, and targeting ER signaling at different levels is a successful strategy in the control of hormone receptor positive (HR+) breast cancer. Endocrine therapy has been the treatment of choice for HR+ breast cancer in the early and advanced stages with multiple agents, including selective estrogen receptor modulators (SERMS), selective estrogen receptor degraders (SERDs), and aromatase inhibitors (AIs), which vary in their mechanisms of action and pharmacokinetics. Combination strategies also employ cyclin dependent kinase 4 and 6 and phosphatidylinositol 3-kinase to maximize the benefits of endocrine therapy. This paper reviews the clinical development of SERDs and other novel ER inhibitors, as well as combination strategies to overcome mechanisms of ER pathway escape. It also assesses the advantages of newer oral ER inhibitors with increased bioavailability, improved therapeutic index, better administration, and increased efficacy, as well as discussing future directions in the field.

Endocrine resistance and epigenetic reprogramming in estrogen receptor positive breast cancer

Despite the enormous advances during the last three decades, breast cancer continues to be the most frequent type of cancer as well as one of the most frequent cancer-related causes of death in women. Therapeutic management of patients with hormone receptor-positive breast cancer becomes very often a challenge, since de novo or acquired resistance deprives a significant percentage of the patients from the clinical benefit of the well-tolerated hormone therapy. Several molecular mechanisms are implicated in resistance to endocrine therapy, including changes in hormone receptor signaling, activation of parallel signaling pathways, modifications of cell cycle regulators, activation of different transcription factors as well as changes in stem cells activity. In addition, a growing number of studies supports the pivotal role of epigenetic changes not only in the initiation and progression of breast cancer, but also in resistance to endocrine therapy. These changes refer to DNA methylation, histone post-translational modifications as well as to ncRNAs alterations. In this review, we provide an overview of epigenetic mechanisms underlying the endocrine resistance focusing exclusively on breast cancer patients.

The Breast Cancer Stem Cells Traits and Drug Resistance

Drug resistance is a major challenge in breast cancer (BC) treatment at present. Accumulating studies indicate that breast cancer stem cells (BCSCs) are responsible for the BC drugs resistance, causing relapse and metastasis in BC patients. Thus, BCSCs elimination could reverse drug resistance and improve drug efficacy to benefit BC patients. Consequently, mastering the knowledge on the proliferation, resistance mechanisms, and separation of BCSCs in BC therapy is extremely helpful for BCSCs-targeted therapeutic strategies. Herein, we summarize the principal BCSCs surface markers and signaling pathways, and list the BCSCs-related drug resistance mechanisms in chemotherapy (CT), endocrine therapy (ET), and targeted therapy (TT), and display therapeutic strategies for targeting BCSCs to reverse drug resistance in BC. Even more importantly, more attention should be paid to studies on BCSC-targeted strategies to overcome the drug resistant dilemma of clinical therapies in the future.

The oral selective oestrogen receptor degrader (SERD) AZD9496 is comparable to fulvestrant in antagonising ER and circumventing endocrine resistance

Background

The oestrogen receptor (ER) is an important therapeutic target in ER-positive (ER+) breast cancer. The selective ER degrader (SERD), fulvestrant, is effective in patients with metastatic breast cancer, but its intramuscular route of administration and low bioavailability are major clinical limitations.

Methods

Here, we studied the pharmacology of a new oral SERD, AZD9496, in a panel of in vitro and in vivo endocrine-sensitive and -resistant breast cancer models.

Results

In endocrine-sensitive models, AZD9496 inhibited cell growth and blocked ER activity in the presence or absence of oestrogen. In vivo, in the presence of oestrogen, short-term AZD9496 treatment, like fulvestrant, resulted in tumour growth inhibition and reduced expression of ER-dependent genes. AZD9496 inhibited cell growth in oestrogen deprivation-resistant and tamoxifen-resistant cell lines and xenograft models that retain ER expression. AZD9496 effectively reduced ER levels and ER-induced transcription. Expression analysis of short-term treated tumours showed that AZD9496 potently inhibited classic oestrogen-induced gene transcription, while simultaneously increasing expression of genes negatively regulated by ER, including genes potentially involved in escape pathways of endocrine resistance.

Conclusions

These data suggest that AZD9496 is a potent anti-oestrogen that antagonises and degrades ER with anti-tumour activity in both endocrine-sensitive and endocrine-resistant models.

Clinical utility of fulvestrant in the treatment of breast cancer: a report on the emerging clinical evidence

Fulvestrant is the first selective estrogen receptor (ER) downregulator available in clinical practice. It is a pure antiestrogen with no agonistic effects, leading to degradation of ER alpha, with activity in tamoxifen-resistant breast cancer (BC) models. Pharmacokinetic and pharmacodynamic studies and several postmarketing clinical trials led to the definition of the optimal dose at 500 mg intramuscularly on days 1, 15, and 29 and then every 28 days. Targeting ER alpha, fulvestrant is a cornerstone of treatment in luminal BCs, whose growth is largely driven by the ER pathway. In endocrine therapy-naïve patients with hormone receptor-positive, HER2− advanced BC (ABC), fulvestrant yielded significantly longer progression-free survival compared to anastrozole in the Phase III FALCON study. Due to its mechanism of action and pharmacokinetic properties, fulvestrant is an ideal backbone for combination therapies. Preclinical studies have shown synergism with drugs acting on signaling pathways involved in the development of endocrine resistance, among which the cyclin D/cyclin-dependent kinase 4-6/retinoblastoma pathway and the phosphatidylinositol 3-kinase (PI3K)/Akt/mammalian target of rapamycin pathway, contributing to overcoming or delaying endocrine resistance. In the Phase III PALOMA-3 trial, a combination of the cyclin-dependent kinase 4/6 inhibitor palbociclib with fulvestrant significantly improved progression-free survival over fulvestrant alone in women with hormone receptor positive, HER2− ABC progressing during prior endocrine therapy. This led to approval of the combination in this clinical setting. Similar results were obtained with abemaciclib and ribociclib. Combination with pan-PI3K inhibitors, though showing some efficacy, was hampered by the toxicity of these agents, and studies in combinations with more selective inhibitors of the α-catalytic subunit of PI3K are ongoing. Fulvestrant has shown partial activity also in patients with tumors harboring mutations of the ESR1 gene. It is thus a key drug in the treatment of ABC, whose role in combination with new targeted agents is still evolving.

Hypoxia and Hormone-Mediated Pathways Converge at the Histone Demethylase KDM4B in Cancer

Hormones play an important role in pathophysiology. The hormone receptors, such as estrogen receptor alpha and androgen receptor in breast cancer and prostate cancer, are critical to cancer cell proliferation and tumor growth. In this review we focused on the cross-talk between hormone and hypoxia pathways, particularly in breast cancer. We delineated a novel signaling pathway from estrogen receptor to hypoxia-inducible factor 1, and discussed the role of this pathway in endocrine therapy resistance. Further, we discussed the estrogen and hypoxia pathways converging at histone demethylase KDM4B, an important epigenetic modifier in cancer.

Enhancement of the bioavailability of a novel anticancer compound (acetyltanshinone IIA) by encapsulation within mPEG-PLGA nanoparticles: a study of formulation optimization, toxicity, and pharmacokinetics

The Poly (ethylene glycol) methyl ether-block-poly (lactide-co-glycolide) (mPEG-PLGA) nanoparticles carrying acetyltanshinone IIA (ATA), a novel anti-breast cancer agent, were prepared by ultrasonic emulsion method to enhance the bioavailability and reduce the toxicity. Systematic optimization of encapsulation process was achieved using an orthogonal design. Drug efficacy analysis showed that ATA nanoparticles were as effective as free ATA against estrogen receptor positive breast cancer cells, but much less toxic towards human endothelial cells. Furthermore, in zebrafish, ATA nanoparticles displayed much lower toxicity than free ATA. More importantly, the blood concentration of ATA nanoparticles indicated by 24 hour-area under the curve (AUC_0-24h) was 10 times higher than free ATA. These results indicated the potential of ATA-loaded mPEG-PLGA nanoparticles for the delivery of ATA in a clinical formulation, and their potential for use in tumor therapy in the future.

Designer interface peptide grafts target estrogen receptor alpha dimerization

The nuclear transcription factor estrogen receptor alpha (ERα), triggered by its cognate ligand estrogen, regulates a variety of cellular signaling events. ERα is expressed in 70% of breast cancers and is a widely validated target for anti-breast cancer drug discovery. Administration of anti-estrogen to block estrogen receptor activation is still a viable anti-breast cancer treatment option but anti-estrogen resistance has been a significant bottle-neck. Dimerization of estrogen receptor is required for ER activation. Blocking ERα dimerization is therefore a complementary and alternative strategy to combat anti-estrogen resistance. Dimer interface peptide "I-box" derived from ER residues 503-518 specifically blocks ER dimerization. Recently using a comprehensive molecular simulation we studied the interaction dynamics of ERα LBDs in a homo-dimer. Based on this study, we identified three interface recognition peptide motifs LDKITDT (ERα residues 479-485), LQQQHQRLAQ (residues 497-506), and LSHIRHMSNK (residues 511-520) and reported the suitability of using LQQQHQRLAQ (ER 497-506) as a template to design inhibitors of ERα dimerization. Stability and self-aggregation of peptide based therapeutics poses a significant bottle-neck to proceed further. In this study utilizing peptide grafted to preserve their pharmacophoric recognition motif and assessed their stability and potential to block ERα mediated activity in silico and in vitro. The Grafted peptides blocked ERα mediated cell proliferation and viability of breast cancer cells but did not alter their apoptotic fate. We believe the structural clues identified in this study can be used to identify novel peptidometics and small molecules that specifically target ER dimer interface generating a new breed of anti-cancer agents.

Fulvestrant for the treatment of endometrial cancer

INTRODUCTION

About one third of patients with endometrial cancer (EC) relapse and face a limited prognosis, if surgery or radiotherapy are not feasible. The remaining therapeutic options are chemotherapy and endocrine therapy.

AREAS COVERED

This review summarizes the development of the first selective estrogen receptor (ER) down-regulator fulvestrant. This article provides its mechanism of action, pharmacokinetics and the available preclinical and clinical data. Furthermore, this review provides an overview of the market of treatments for recurrent or metastatic EC (RMEC) while also taking into account studies of fulvestrant in metastatic breast cancer.

EXPERT OPINION

Even if fulvestrant showed only marginal activity in two phase II trials, it shouldn't be abandoned but instead further developed in EC. Firstly, the dose of fulvestrant used in these trials was too low from today's point of view. Secondly, the available literature on other endocrine agents is full of limitations and does not provide a gold standard. Furthermore, given the activity of mTOR inhibitors in EC, there may also be synergistic effects, given the cross-regulation of ER and the PI3K/AKT/mTOR pathway. The authors suggest that a prospective, phase II trial in ER positive RMEC would help to further explore the efficacy and tolerability of fulvestrant together with a mTOR inhibitor.

Estrogen receptor-α directly regulates the hypoxia-inducible factor 1 pathway associated with antiestrogen response in breast cancer

A majority of breast cancers are driven by estrogen via estrogen receptor-α (ERα). Our previous studies indicate that hypoxia-inducible factor 1α (HIF-1α) cooperates with ERα in breast cancer cells. However, whether ERα is implicated in the direct regulation of HIF-1α and the role of HIF-1α in endocrine therapy response are unknown. In this study we found that a subpopulation of HIF-1α targets, many of them bearing both hypoxia response elements and estrogen response elements, are regulated by ERα in normoxia and hypoxia. Interestingly, the HIF-1α gene itself also bears an estrogen response element, and its expression is directly regulated by ERα. Clinical data revealed that expression of the HIF-1α gene or a hypoxia metagene signature is associated with a poor outcome to endocrine treatment in ERα(+) breast cancer. HIF-1α was able to confer endocrine therapy resistance to ERα(+) breast cancer cells. Our findings define, for the first time to our knowledge, a direct regulatory pathway between ERα and HIF-1α, which might modulate hormone response in treatment.

Effects of exercise training together with tamoxifen in reducing mammary tumor burden in mice: Possible underlying pathway of miR-21

Exercise training has an anti-tumor effect and can reduce tumor growth; however, the exact underlying mechanisms of its protective effects are still obscure. MicroRNA (miR)-21 is a predictor in cancer survival, and has a potential use as an indicator of therapeutic outcome in breast malignancies. Forty-eight female BALB/c mice were equally divided into six groups to investigate the effects of interval exercise training with tamoxifen on miR-21 expression and its possible assumed mechanisms in an estrogen receptor-positive breast cancer model. ELISA, immunohistochemistry, western blot, qRT-PCR assays were performed at the end of the study. Tumor size was significantly declined in exercise training and tamoxifen groups compared to tumor group (P<0.05). Expression of miR-21 was significantly down-regulated in trained and tamoxifen treated mice in comparison with tumor group (P<0.05). Exercise training was as effective as tamoxifen treatment in decreasing serum estradiol and ER-α expression (P<0.05). Exercise training and tamoxifen reduced tumor IL-6 levels, NF-kB and STAT3 expressions, and up-regulated TPM1 and PDCD4 expressions (P<0.05). Both exercise and tamoxifen had synergistic effects in reducing miR-21 and Bcl-2, and up-regulating PDCD4 expression. Results showed that interval exercise training may reduce mammary tumor burden in mice through possible underlying pathway of miR-21.

Aurora kinase B is important for antiestrogen resistant cell growth and a potential biomarker for tamoxifen resistant breast cancer

Background

Resistance to antiestrogen therapy is a major clinical challenge in the treatment of estrogen receptor α (ER)-positive breast cancer. The aim of the study was to explore the growth promoting pathways of antiestrogen resistant breast cancer cells to identify biomarkers and novel treatment targets.

Methods

Antiestrogen sensitive and resistant T47D breast cancer cell lines were used as model systems. Parental and fulvestrant resistant cell lines were subjected to a kinase inhibitor library. Kinase inhibitors preferentially targeting growth of fulvestrant resistant cells were identified and the growth inhibitory effect verified by dose–response cell growth experiments. Protein expression and phosphorylation were investigated by western blot analysis. Cell cycle phase distribution and cell death were analyzed by flow cytometry. To evaluate Aurora kinase B as a biomarker for endocrine resistance, immunohistochemistry was performed on archival primary tumor tissue from breast cancer patients who have received adjuvant endocrine treatment with tamoxifen.

Results

The selective Aurora kinase B inhibitor barasertib was identified to preferentially inhibit growth of fulvestrant resistant T47D breast cancer cell lines. Compared with parental cells, phosphorylation of Aurora kinase B was higher in the fulvestrant resistant T47D cells. Barasertib induced degradation of Aurora kinase B, caused mitotic errors, and induced apoptotic cell death as measured by accumulation of SubG1 cells and PARP cleavage in the fulvestrant resistant cells. Barasertib also exerted preferential growth inhibition of tamoxifen resistant T47D cell lines. Finally, high percentage of Aurora kinase B positive tumor cells was significantly associated with reduced disease-free and overall survival in 261 ER-positive breast cancer patients, who have received tamoxifen as first-line adjuvant endocrine treatment.

Conclusions

Our results indicate that Aurora kinase B is a driving factor for growth of antiestrogen resistant T47D breast cancer cell lines, and a biomarker for reduced benefit of tamoxifen treatment. Thus, inhibition of Aurora kinase B, e.g. with the highly selective kinase inhibitor barasertib, could be a candidate new treatment for breast cancer patients with acquired resistance to antiestrogens.

ACK1/TNK2 tyrosine kinase: molecular signaling and evolving role in cancers

Deregulated tyrosine kinase signaling alters cellular homeostasis to drive cancer progression. The emergence of a non-receptor tyrosine kinase, ACK1 as an oncogenic kinase, has uncovered novel mechanisms by which tyrosine kinase signaling promotes cancer progression. While early studies focused on ACK1 (also known as activated Cdc42-associated kinase 1 or TNK2) as a cytosolic effecter of activated transmembrane receptor tyrosine kinases (RTKs), wherein it shuttles between the cytosol and the nucleus to rapidly transduce extracellular signals from the RTKs to the intracellular effectors, recent data unfold a new aspect of its functionality as an epigenetic regulator. ACK1 interacts with the Estrogen Receptor (ER)/histone demethylase KDM3A (JHDM2a) complex, modifies KDM3A by tyrosine phosphorylation to regulate transcriptional outcome at HOXA1 locus to promote the growth of tamoxifen-resistant breast cancer. It is also well established that ACK1 regulates the activity of Androgen Receptor (AR) by tyrosine phosphorylation to fuel the growth of hormone-refractory prostate cancers. Further, recent explosion in genomic sequencing has revealed recurrent ACK1 gene amplification and somatic mutations in a variety of human malignancies, providing a molecular basis for its role in neoplastic transformation. In this review, we will discuss the various facets of ACK1 signaling, including its newly uncovered epigenetic regulator function, which enables cells to bypass the blockade to major survival pathways to promote resistance to standard cancer treatments. Not surprisingly, cancer cells appear to acquire an `addiction’ to ACK1 mediated survival, particularly under stress conditions, such as growth factor deprivation or genotoxic insults or hormone deprivation. With the accelerated development of potent and selective ACK1 inhibitors, targeted treatment for cancers harboring aberrant ACK1 activity may soon become a clinical reality.

ACK1 tyrosine kinase interacts with histone demethylase KDM3A to regulate the mammary tumor oncogene HOXA1

Hormone therapy with the selective estrogen-receptor modulator tamoxifen provides a temporary relief for patients with estrogen receptor α (ER)-positive breast cancers. However, a subset of patients exhibiting overexpression of the HER2 receptor tyrosine kinase displays intrinsic resistance to tamoxifen therapy. Therefore, elucidating the mechanisms promoting the estrogen (E2)-independent ER-regulated gene transcription in tamoxifen-resistant breast tumors is essential to identify new therapeutic avenues to overcome drug resistance and ameliorate poor prognosis. The non-receptor tyrosine kinase, ACK1 (also known as TNK2), has emerged as a major integrator of signaling from various receptor tyrosine kinases including HER2. We have uncovered that heregulin-mediated ACK1 activation promoted ER activity in the presence of tamoxifen, which was significantly down-regulated upon ACK1 knockdown or inhibition of ACK1 by small molecule inhibitors, AIM-100 or Dasatinib. We report that ACK1 phosphorylates the ER co-activator, KDM3A, a H3K9 demethylase, at an evolutionary conserved tyrosine 1114 site in a heregulin-dependent manner, even in the presence of tamoxifen. Consistent with this finding, ACK1 activation resulted in a significant decrease in the deposition of dimethyl H3K9 epigenetic marks. Conversely, inhibition of ACK1 by AIM-100 or Dasatinib restored dimethyl H3K9 methylation marks and caused transcriptional suppression of the ER-regulated gene HOXA1. Thus, by its ability to regulate the epigenetic activity of an ER co-activator KDM3A, ACK1 modulates HOXA1 expression in the absence of E2, conferring tamoxifen resistance. These data reveal a novel therapeutic option, suppression of ACK1 signaling by AIM-100 or Dasatinib, to mitigate HOXA1 up-regulation in breast cancer patients displaying tamoxifen resistance.

Advances in the approach to novel drug clinical development for breast cancer

INTRODUCTION

In the post-genomic era clinical development of new agents to treat breast cancer (BC) can be a real challenge. Different from chemotherapy agents, with a broad but not specific spectrum of activity, novel drugs are being developed as 'targeted' agents, potentially benefiting a subgroup of patients. In BC, different clinically identifiable subtypes are now separately addressed in specific clinical trials.

AREAS COVERED

In this review, the authors discuss the clinical development of targeted drugs that have become part of the current treatment of BC. They also highlight the challenges that in other cases determined the failure of promising compounds. Furthermore, the article reports on how combinations of targeted agents have emerged as valid strategies to overcome acquired resistance. It also provides discussion of how 'old' therapies can be retargeted to certain patient populations or 'reinvented' as safer and more effective with the creation of drug conjugates. They also discuss how novel clinical trial designs are emerging to accelerate the successful matching of targeted drugs to the right patient population.

EXPERT OPINION

It is important not to forget that the development of BC therapeutics is a 'moving target', as its biology evolves in time under the pressure of ongoing treatments. There are currently a finite number of resources available for the development of new therapeutics, which means that resources need to be carefully allocated. There is also a need to prioritize clinical trials that can reduce the number of patients who are candidates for expensive treatments.

The p53-estrogen receptor loop in cancer

Tumor suppressor p53 maintains genome stability by regulating diverse cellular functions including cell cycle arrest, apoptosis, senescence and metabolic homeostasis. Mutations in the p53 gene occur in almost all human cancers with a frequency of up to 80%. However, it is only 20% in breast cancers, 18% in endometrial cancers and 1.5% in cervical cancers. Estrogen receptor alpha (ERα) plays a pivotal role in hormone-dependent cancer development and the status of ERα is used for designing treatment strategy and for prognosis. A closer look at the cross-talk between p53 and ERα has revealed that their activities are mutually regulated. This review will summarize the current body of knowledge on p53, ERα and ERβ in cancer. Clinical correlations between estrogen receptors and p53 status have also been reported. Thus, this review will discuss the relationship between p53 and ERs at both the molecular and clinical levels.

A novel anti-cancer agent, acetyltanshinone IIA, inhibits oestrogen receptor positive breast cancer cell growth by down-regulating the oestrogen receptor

In this paper we show that acetyltanshinone IIA (ATA), a novel anti-cancer agent, preferentially inhibits cell growth of oestrogen receptor positive (ER+) breast cancer cells and that it is more potent than the commonly used anti-breast cancer agent, tamoxifen. The metabolic product of ATA, hydroquinone tanshinone IIA (HTA) binds to the ERα and causes its degradation mainly in the nucleus via an ubiquitin-mediated proteasome-dependent pathway. In addition, ATA also reduced the mRNA levels of the ERα encoding gene, ESR1, distinguishing ATA from another anti-breast cancer drug, fulvestrant. Finally, ATA reduced the transcription of an ER-responsive gene, GREB1.

A novel anthracene derivative, MHY412, induces apoptosis in doxorubicin-resistant MCF-7/Adr human breast cancer cells through cell cycle arrest and downregulation of P-glycoprotein expression

New potential chemotherapeutic strategies are required to overcome multidrug resistance (MDR) in cancer. This study investigated the anticancer effect of a novel anthracene derivative MHY412 on doxorubicin-resistant human breast cancer (MCF-7/Adr) cells. We measured cell viability and the expression of apoptosis-related genes; in addition, the antitumor activity of MHY412 was confirmed using an in vivo tumor xenograft model. MHY412 significantly inhibited the proliferation of MCF-7/Adr and MCF-7 cells in a concentration-dependent manner. Notably, the half-maximal inhibitory concentration (IC50) values of MHY412 in MCF-7/Adr (0.15 µM) and MCF-7 (0.26 µM) cells were lower than those of doxorubicin (MCF-7/Adr, 13.6 µM and MCF-7, 1.26 µM) after treatment for 48 h. MHY412 at low concentrations induced S phase arrest, but at high concentrations, the number of MCF-7/Adr cells in the sub-G1 phase significantly increased. MHY412-induced sub-G1 phase arrest was associated with inhibition of cyclin, cyclin-dependent kinase 2 (CDK2) and p21 expression in MCF-7/Adr cells. MHY412 markedly reduced P-glycoprotein (P-gp) expression and increased apoptotic cell death in MCF-7/Adr cells. Cleavage of poly-ADP ribose polymerase, reduced Bcl-2 expression, and increased in cytochrome c release in MCF-7/Adr cells confirmed the above results. In addition, MHY412 markedly inhibited tumor growth in a tumor xenograft model of MCF-7/Adr cells. Our data suggest that MHY412 exerts antitumor effects by selectively modulating the genes related to cell cycle arrest and apoptosis. In particular, MHY412 is a new candidate agent for the treatment of Bcl-2 overexpressed doxorubicin-resistant human breast cancer.

Fulvestrant for advanced breast cancer: a meta-analysis

BACKGROUND

Fulvestrant is an endocrine agent which degrades the estrogen receptor, thereby downregulating its signaling. Trials of fulvestrant are limited by inconsistent study populations and drug dosing. The optimal use of fulvestrant in advanced breast cancer is therefore unclear.

METHODS

A systematic review of electronic databases was conducted to identify randomized trials of fulvestrant versus other endocrine therapy. The hazard ratios (HR) for time to progression (TTP) and the odds ratios (OR) for serious adverse events (SAEs), discontinuation of treatment due to toxicity and commonly reported toxicities (hot flashes, venous thrombosis, gastrointestinal disturbance, arthralgia, and asthenia) were pooled in a meta-analysis. Meta-regression explored heterogeneity in study population and fulvestrant dosing.

RESULTS

Eight studies were included in the analysis. Overall, there was no difference in TTP between fulvestrant and control groups (HR: 0.94, p=0.18). On meta-regression, fulvestrant showed reduced hazards for TTP compared to aromatase inhibitors (AI) if used in first line, in studies where fewer patients received adjuvant endocrine therapy and at higher doses. Rates of SAEs and treatment discontinuation were similar for fulvestrant and control groups, but fulvestrant monotherapy was associated with significantly less arthralgia (OR: 0.73, p=0.02). The addition of fulvestrant to AI was not associated with improved TTP, but led to increased toxicity.

CONCLUSION

In unselected patients, fulvestrant monotherapy is associated with similar efficacy, but reduced arthralgia compared with other endocrine therapy options. Use of high dose fulvestrant monotherapy in first line or in patients with limited prior exposure to adjuvant endocrine therapy may delay progression compared with AI.

Biological mechanisms and clinical implications of endocrine resistance in breast cancer

Endocrine therapy represents the first and most efficacious targeted treatment for women with estrogen receptor-positive (ER+) breast cancer. In the last four decades several hormonal agents have been successfully introduced in clinical practice as both palliative therapy for advanced disease and adjuvant treatment for prevention of tumor relapse. Nevertheless, the intrinsic and acquired resistance occurs in a significant proportion of patients, limiting the efficacy of endocrine treatments. Several molecular mechanisms have been proposed to be responsible for endocrine resistance. Loss of ER expression, altered activity of ER coregulators, deregulation of apoptosis and cell cycle signaling, and hyperactive receptor tyrosine kinase (RTK) and stress/cell kinase pathways can collectively orchestrate the development and sustenance of pharmacologic resistance to endocrine therapy. Preclinical and clinical evidence documents the plasticity in ER expression levels and signaling. As such, ER can either drive gene transcription and tumor progression directly or crosstalk with alternate RTK and cellular kinase signaling pathways, resulting in modulation of its own expression levels and transcriptional program. For this reason a therapeutic approach based on the combination of agents targeting both ER and RTK signaling represents a promising strategy to be tested. Among many RTKs, EGFR, HER2, and PI3K have been found to be viable targets for the combination therapy strategy, at least in the preclinical setting. However, early results from clinical trials testing combination strategies have been discordant, suggesting the need for better approaches to simultaneously inhibit multiple escape pathways and to select the patients who may benefit the most from these strategies.

Role of fulvestrant in the management of postmenopausal breast cancer

Fulvestrant is a form of endocrine therapy used in the treatment of postmenopausal breast cancer. It has a unique mechanism of action in that it causes the degradation of estrogen receptor and therefore has been labeled a selective estrogen receptor downregulator. Unlike the selective estrogen receptor modulator tamoxifen, it has no agonistic properties and is therefore a pure anti-estrogen. Given its low level of bioavailability and presystemic metabolism, it has been formulated as an intramuscular injection. A number of dosing regimens have been utilized - these include a dose of 250 mg monthly ('approved dose'), an initial 500 mg followed by 250 mg on days 14 and 28, and thereafter 250 mg every 28 days ('loading dose'), or 500 mg on days 0, 14 and 28, and thereafter every 28 days ('high dose'). This article will review its unique mode of action and preclinical data, as well as clinical data for different dosing regimens and data for its combination with aromatase inhibitors. Fulvestrant is a well-tolerated drug and its toxicities will also be reviewed. The optimal position of fulvestrant in sequential endocrine therapy has yet to be defined.

Low-dose dietary genistein negates the therapeutic effect of tamoxifen in athymic nude mice

The present study examined the effect of dietary genistein, a soy isoflavone, on breast cancer patients who take tamoxifen, an antiestrogen treatment, using a preclinical model. The interaction of various doses of genistein with tamoxifen on the growth of estrogen receptor-positive breast cancer MCF-7 cells was investigated by subcutaneously injecting MCF-7 cells into the flank of ovariectomized athymic mice. Animals were randomized into eight experimental groups with 10-13 mice per group: control (C), estrogen (E) (0.08 mg E implant), tamoxifen (T) (3 mg T implant), estrogen + tamoxifen (E + T), tamoxifen + 500 p.p.m. genistein (T + G500), estrogen + tamoxifen + 250 p.p.m. genistein (E + T + G250), estrogen + tamoxifen + 500 p.p.m. genistein (E + T + G500) and estrogen + tamoxifen + 1000 p.p.m. genistein (E + T + G1000). Treatment of tamoxifen significantly reduced the estrogen-induced MCF-7 tumor prevalence and tumor size. This inhibitory effect of tamoxifen was significantly negated by the low doses of dietary genistein (250 and 500 p.p.m.), whereas the 1000 p.p.m. genistein did not have the same effect. Cells harvested from tamoxifen-treated tumors retained estrogen responsiveness of their progenitor MCF-7 cells, indicating that the abrogating effect of genistein on tamoxifen-treated tumor growth was not caused by a diminished tamoxifen response but directly by genistein. The low doses of dietary genistein abrogated the inhibitory effect of tamoxifen potentially by acting on the tumor cell proliferation/apoptosis ratio and the messenger RNA (mRNA) expression of cyclin D1 in addition to regulating the mRNA expression of progesterone receptor. Therefore, data from the current study suggest that caution is warranted regarding the consumption of dietary genistein by breast cancer patients while on tamoxifen therapy.

GPER mediates the Egr-1 expression induced by 17β-estradiol and 4-hydroxitamoxifen in breast and endometrial cancer cells

Early growth response-1 (Egr-1) is an immediate early gene involved in relevant biological events including the proliferation of diverse types of cell tumors. In a microarray analysis performed in breast cancer cells, 17β-estradiol (E2) and the estrogen receptor antagonist 4-hydroxitamoxifen (OHT) up-regulated Egr-1 through the G protein-coupled receptor named GPR30/GPER. Hence, in this study, we aimed to provide evidence regarding the ability of E2, OHT and the selective GPER ligand G-1 to regulate Egr-1 expression and function through the GPER/EGFR/ERK transduction pathway in both Ishikawa (endometrial) and SkBr3 (breast) cancer cells. Interestingly, we demonstrate that Egr-1 is involved in the transcription of genes regulating cell proliferation like CTGF and cyclin D1 and required for the proliferative effects induced by E2, OHT, and G-1 in both Ishikawa and SkBr3 cells. In addition, we show that GPER mediates the expression of Egr-1 also in carcinoma-associated fibroblasts (CAFs). Our data suggest that Egr-1 may represent an important mediator of the biological effects induced by E2 and OHT through GPER/EGFR/ERK signaling in breast and endometrial cancer cells. The results obtained in CAFs provide further evidence regarding the potential role exerted by the GPER-dependent Egr-1 up-regulation in tumor development and progression. Therefore, Egr-1 may be included among the bio-markers of estrogen and antiestrogen actions and may be considered as a further therapeutic target in both breast and endometrial tumors.

Management of patients with metastatic breast cancer

Hormone treatment is one of the key strategies in the management of metastatic breast cancer. Hormone treatment is one of the key strategies in the management of metastatic breast cancer. Aromatase inhibitors (AI) have been extensively studied in this setting. This section summarizes the key data regarding the use of AI in advanced breast cancer. In postmenopausal women, AI are the first line of treatment for untreated patients, or those who had prior AI treatment and progress after 12 months of adjuvant therapy. A longer disease-free interval and absence of visceral disease is associated with a better response. If tumors recur in less than 12 months, it is recommended that tamoxifen (TAM) or the estrogen-receptor antagonist fulvestrant (FUL) treatment be initiated. In the second-line setting, the best option after progression is the administration of either FUL or TAM. In the third-line setting, reintroduction of AI is considered an acceptable option. In premenopausal women who have not received prior treatment or who have progressed after 12 months following adjuvant treatment, it is recommended to initiate therapy with a combination of TAM and a luteinizing hormone-releasing hormone (LHRH) analog. If there is treatment failure with the use of this combination, megestrol acetate or an LHRH agonist plus an AI may be reasonable alternatives. Intensive research is ongoing to understand the mechanisms of resistance to hormone therapy. In human epidermal growth factor receptor 2 positive-patients, combinations with HER2 antagonists are associated with significant clinical activity.

Emerging data on the efficacy and safety of fulvestrant, a unique antiestrogen therapy for advanced breast cancer

INTRODUCTION

Fulvestrant is an antiestrogen therapy with a unique mechanism of action. Unlike the selective estrogen receptor modulator tamoxifen, fulvestrant has no known estrogen agonist activity and is considered a pure antiestrogen. Its primary mechanism of action is thought to result from downregulation of the estrogen receptor (ER). Considerable data have demonstrated the efficacy of fulvestrant in postmenopausal women with ER-positive advanced breast cancer, both in the first-line setting and following disease progression on tamoxifen or aromatase inhibitors. Recent studies report improved benefit with alternative dosing strategies. At all administration schedules, fulvestrant has an excellent safety profile with no significant adverse effects.

AREAS COVERED

This article provides a review of the mechanism of action of fulvestrant and the preclinical and clinical data evaluating its use as a form of endocrine therapy. The reader will gain insight into the pharmacologic properties of the drug and its role in the treatment of advanced hormone receptor-positive breast cancer in postmenopausal women.

EXPERT OPINION

Based on data demonstrating the efficacy of fulvestrant, including prolonged clinical benefit in many patients, this well-tolerated antiestrogen is an important therapy for breast cancer. The optimal position of fulvestrant in the sequence of endocrine therapies for postmenopausal women and its role in combination regimens are not yet resolved.

The molecular mechanisms underlying the pharmacological actions of ER modulators: implications for new drug discovery in breast cancer

Our understanding of the molecular mechanisms underlying the pharmacological actions of estrogen receptor (ER) ligands has evolved considerably in recent years. Much of this knowledge has come from a detailed dissection of the mechanism(s) of action of the Selective Estrogen Receptor Modulators (SERMs) tamoxifen and raloxifene, so called for their ability to function as ER agonists or antagonists depending on the tissue in which they operate. These mechanistic insights have had a significant impact on the discovery of second generation SERMs, some of which are in late stage clinical development for the treatment/prevention of breast cancer as well as other estrogenopathies. In addition to the SERMs, however, have emerged the Selective Estrogen Degraders (SERDs), which as their name suggests, interact with and facilitate ER turnover in cells. One drug of this class, fulvestrant, has been approved as a third line treatment for ER-positive metastatic breast cancer. Whereas the first generation SERMs/SERDs were discovered in a serendipitous manner, this review will highlight how our understanding of the molecular pharmacology of ER ligands has been utilized in the development of the next generation of SERMs/SERDs, some of which are likely to have a major impact on the pharmacotherapy of breast cancer.

Growth factor stimulation induces a distinct ER(alpha) cistrome underlying breast cancer endocrine resistance

Estrogen receptor α (ERα) expression in breast cancer is predictive of response to endocrine therapy; however, resistance is common in ERα-positive tumors that overexpress the growth factor receptor ERBB2. Even in the absence of estrogen, ERα can be activated by growth factors, including the epidermal growth factor (EGF). EGF induces a transcriptional program distinct from estrogen; however, the mechanism of the stimulus-specific response is unknown. Here we show that the EGF-induced ERα genomic targets, its cistromes, are distinct from those induced by estrogen in a process dependent on the transcription factor AP-1. The EGF-induced ERα cistrome specifically regulates genes found overexpressed in ERBB2-positive human breast cancers. This provides a potential molecular explanation for the endocrine therapy resistance seen in ERα-positive breast cancers that overexpress ERBB2. These results suggest a central role for ERα in hormone-refractory breast tumors dependent on growth factor pathway activation and favors the development of therapeutic strategies completely antagonizing ERα, as opposed to blocking its estrogen responsiveness alone.

IFNgamma restores breast cancer sensitivity to fulvestrant by regulating STAT1, IFN regulatory factor 1, NF-kappaB, BCL2 family members, and signaling to caspase-dependent apoptosis

Antiestrogens are effective therapies for the management of many estrogen receptor-alpha (ER)-positive breast cancers. Nonetheless, both de novo and acquired resistance occur and remain major problems in the clinical setting. IFNgamma is an inflammatory cytokine that induces the expression and function of IFN regulatory factor 1 (IRF1), a tumor suppressor gene that can increase antiestrogen responsiveness. We show that IFNgamma, but not IFNalpha, IFNbeta, or fulvestrant (ICI; ICI 182,780; Faslodex), induces IRF1 expression in antiestrogen-resistant MCF7/LCC9 and LY2 cells. Moreover, IFNgamma restores the responsiveness of these cells to fulvestrant. Increased IRF1 activation suppresses NF-kappaB p65 (RELA) activity, inhibits the expression of prosurvival (BCL2, BCL-W), and induces the expression of proapoptotic members (BAK, mitochondrial BAX) of the BCL2 family. This molecular signaling is associated with the activation of signal transducer and activator of transcription 1 and leads to increased mitochondrial membrane permeability; activation of caspase-7 (CASP7), CASP8, and CASP9; and induction of apoptosis but not autophagy. Whereas antiestrogen-resistant cells are capable of inducing autophagy through IFN-mediated signaling, their ability to do so through antiestrogen-regulated signaling is lost. The abilities of IFNgamma to activate CASP8, induce apoptosis, and restore antiestrogen sensitivity are prevented by siRNA targeting IRF1, whereas transient overexpression of IRF1 mimics the effects of IFNgamma treatment. These observations support the exploration of clinical trials combining antiestrogens and compounds that can induce IRF1, such as IFNgamma, for the treatment of some ER-positive breast cancers.

Jostling for position: optimizing linker location in the design of estrogen receptor-targeting PROTACs

Estrogen receptor-alpha (ER) antagonists have been widely used for breast cancer therapy. Despite initial responsiveness, hormone-sensitive ER-positive cancer cells eventually develop resistance to ER antagonists. It has been shown that in most of these resistant tumor cells, the ER is expressed and continues to regulate tumor growth. Recent studies indicate that tamoxifen initially acts as an antagonist, but later functions as an ER agonist, promoting tumor growth. This suggests that targeted ER degradation may provide an effective therapeutic approach for breast cancers, even those that are resistant to conventional therapies. With this in mind, we previously demonstrated that proteolysis targeting chimeras (PROTACs) effectively induce degradation of the ER as a proof-of-concept experiment. Herein we further refined the PROTAC approach to target the ER for degradation. The ER-targeting PROTACs are composed of an estradiol on one end and a hypoxia-inducing factor 1alpha (HIF-1alpha)-derived synthetic pentapeptide on the other. The pentapeptide is recognized by an E3 ubiquitin ligase called the von Hippel Lindau tumor suppressor protein (pVHL), thereby recruiting the ER to this E3 ligase for ubiquitination and degradation. Specifically, the pentapeptide is attached at three different locations on estradiol to generate three different PROTAC types. With the pentapeptide linked through the C7alpha position of estradiol, the resulting PROTAC shows the most effective ER degradation and highest affinity for the estrogen receptor. This result provides an opportunity to develop a novel type of ER antagonist that may overcome the resistance of breast tumors to conventional drugs such as tamoxifen and fulvestrant (Faslodex).

Fulvestrant: a unique antiendocrine agent for estrogen-sensitive breast cancer

IMPORTANCE OF THE FIELD

The role of estrogen deprivation for the treatment of breast cancer has been understood since the 1800s. Pharmacologic advances in the field in the past decades, including tamoxifen and the aromatase inhibitors, have contributed significantly to the reduced mortality of estrogen-sensitive breast cancer. However, this subtype of breast cancer still presents with relapses and, once metastatic, progression to hormone-refractory state and loss of disease control remain an expected disease course. Fulvestrant, a pure estrogen receptor downregulator, is a new addition to the antiestrogen therapeutic armamentarium since its FDA approval in 2002. Its unique mechanism of action offers potential advantages over other estrogen targeted therapies.

AREAS COVERED IN THIS REVIEW

Published scientific literature, including presented abstracts, on fulvestrant from 1985 to the present were reviewed with selected publications included.

WHAT THE READER WILL GAIN

This review addresses current issues and therapies for estrogen-sensitive breast cancer, highlights the role of fulvestrant in current treatment guidelines and outlines some of the ongoing investigations of this compound.

TAKE HOME MESSAGE

Fulvestrant is an effective and well-tolerated drug for treatment of metastatic estrogen-sensitive breast cancer. Work is underway to enhance its clinical benefit to patients as a single agent and in combination with other therapies.

BCL2 and CASP8 regulation by NF-kappaB differentially affect mitochondrial function and cell fate in antiestrogen-sensitive and -resistant breast cancer cells

Resistance to endocrine therapies remains a major problem in the management of estrogen receptor-alpha (ER)-positive breast cancer. We show that inhibition of NF-kappaB (p65/RELA), either by overexpression of a mutant IkappaB (IkappaBSR) or a small-molecule inhibitor of NF-kappaB (parthenolide; IC(50)=500 nM in tamoxifen-resistant cells), synergistically restores sensitivity to 4-hydroxytamoxifen (4HT) in resistant MCF7/RR and MCF7/LCC9 cells and further sensitizes MCF-7 and MCF7/LCC1 control cells to 4HT. These effects are independent of changes in either cell cycle distribution or in the level of autophagy measured by inhibition of p62/SQSTM1 expression and cleavage of LC3. NF-kappaB inhibition restores the ability of 4HT to decrease BCL2 expression, increase mitochondrial membrane permeability, and induce a caspase-dependent apoptotic cell death in resistant cells. Each of these effects is reversed by a caspase 8 (CASP8)-specific inhibitor that blocks enzyme-substrate binding. Thus, increased activation of NF-kappaB can alter sensitivity to tamoxifen by modulating CASP8 activity, with consequent effects on BCL2 expression, mitochondrial function, and apoptosis. These data provide significant new insights into how molecular signaling affects antiestrogen responsiveness and strongly suggest that a combination of parthenolide and tamoxifen may offer a novel therapeutic approach to the management of some ER-positive breast cancers.

Perturbation of estrogen receptor alpha localization with synthetic nona-arginine LXXLL-peptide coactivator binding inhibitors

The interaction of estrogen receptor alpha (ERalpha) with the consensus LXXLL motifs of transcriptional coactivators provides an entry for functional ERalpha inhibition. Here, synthetic cell-permeable LXXLL peptide probes are brought forward that allow evaluation of the interaction of specific recognition motifs with ERalpha in the context of the cell. The probes feature a nona-arginine tag that facilitates cellular entry and induces probe localization in nucleoli. The nucleoli localization provides an explicit tool for evaluating the LXXLL motif interaction with ERalpha. The probes compete with coactivators, bind ERalpha, and recruit it into the nucleoli. The physical inhibition of the ERalpha-coactivator interaction by the probes is shown to be correlated with the inhibition of ERalpha-mediated gene transcription. This chemical biology approach allows evaluating the ERalpha-coactivator interaction and inhibitor binding directly in cells.

G protein-coupled receptor 30 expression is up-regulated by EGF and TGF alpha in estrogen receptor alpha-positive cancer cells

In the present study, we evaluated the regulation of G protein-coupled receptor (GPR)30 expression in estrogen receptor (ER)-positive endometrial, ovarian, and estrogen-sensitive, as well as tamoxifen-resistant breast cancer cells. We demonstrate that epidermal growth factor (EGF) and TGF alpha transactivate the GPR30 promoter and accordingly up-regulate GPR30 mRNA and protein levels only in endometrial and tamoxifen-resistant breast cancer cells. These effects exerted by EGF and TGF alpha were dependent on EGF receptor (EGFR) expression and activation and involved phosphorylation of the Tyr(1045) and Tyr(1173) EGFR sites. Using gene-silencing experiments and specific pharmacological inhibitors, we have ascertained that EGF and TGF alpha induce GPR30 expression through the EGFR/ERK transduction pathway, and the recruitment of c-fos to the activator protein-1 site located within GPR30 promoter sequence. Interestingly, we show that functional cross talk of GPR30 with both activated EGFR and ER alpha relies on a physical interaction among these receptors, further extending the potential of estrogen to trigger a complex stimulatory signaling network in hormone-sensitive tumors. Given that EGFR/HER2 overexpression is associated with tamoxifen resistance, our data may suggest that ligand-activated EGFR could contribute to the failure of tamoxifen therapy also by up-regulating GPR30, which in turn could facilitates the action of estrogen. In addition, important for resistance is the ability of tamoxifen to bind to and activate GPR30, the expression of which is up-regulated by EGFR activation. Our results emphasize the need for new endocrine agents able to block widespread actions of estrogen without exerting any stimulatory activity on transduction pathways shared by the steroid and growth factor-signaling networks.

Gene network signaling in hormone responsiveness modifies apoptosis and autophagy in breast cancer cells

Resistance to endocrine therapies, whether de novo or acquired, remains a major limitation in the ability to cure many tumors that express detectable levels of the estrogen receptor alpha protein (ER). While several resistance phenotypes have been described, endocrine unresponsiveness in the context of therapy-induced tumor growth appears to be the most prevalent. The signaling that regulates endocrine resistant phenotypes is poorly understood but it involves a complex signaling network with a topology that includes redundant and degenerative features. To be relevant to clinical outcomes, the most pertinent features of this network are those that ultimately affect the endocrine-regulated components of the cell fate and cell proliferation machineries. We show that autophagy, as supported by the endocrine regulation of monodansylcadaverine staining, increased LC3 cleavage, and reduced expression of p62/SQSTM1, plays an important role in breast cancer cells responding to endocrine therapy. We further show that the cell fate machinery includes both apoptotic and autophagic functions that are potentially regulated through integrated signaling that flows through key members of the BCL2 gene family and beclin-1 (BECN1). This signaling links cellular functions in mitochondria and endoplasmic reticulum, the latter as a consequence of induction of the unfolded protein response. We have taken a seed-gene approach to begin extracting critical nodes and edges that represent central signaling events in the endocrine regulation of apoptosis and autophagy. Three seed nodes were identified from global gene or protein expression analyses and supported by subsequent functional studies that established their abilities to affect cell fate. The seed nodes of nuclear factor kappa B (NFkappaB), interferon regulatory factor-1 (IRF1), and X-box binding protein-1 (XBP1)are linked by directional edges that support signal flow through a preliminary network that is grown to include key regulators of their individual function: NEMO/IKKgamma, nucleophosmin and ER respectively. Signaling proceeds through BCL2 gene family members and BECN1 ultimately to regulate cell fate.

Endocrine therapy of metastatic breast cancer

Breast cancer growth and dissemination is regulated by estrogen and different growth factor receptor signalling pathways. The increasing knowledge of the biology of breast cancer regarding the interaction of these signalling pathways provides a tool to understand endocrine therapies response and resistance mechanisms. In patients with slowly progressive disease, no visceral involvement, and minimal symptoms, endocrine therapy could be the strategy of choice, even if the tumor has low estrogen receptor expression. Ovarian suppression and tamoxifen are recommended for premenopausal patients whether aromatase inhibitors are the option for postmenopausal ones. Chemotherapy still remains as the right alternative for hormone unresponsive or resistant patients. This is a review focused on the different strategies and combinations of endocrine therapies for metastatic breast cancer patients considering the potential strategies clinically tested to overcome resistance and the different treatments of choice available for each scenario of disseminated disease.

Development of new predictive markers for endocrine therapy and resistance in breast cancer

Today, the decision to treat breast cancer patients with endocrine therapy relies solely on tumor expression of two predictive factors, the estrogen receptor and the progesterone receptor. Expression of these hormone receptors are, however, not a guarantee for a response to treatment and patients who experience response at first may become resistant after prolonged treatment. This paper describes the use of preclinical models to identify mechanisms and new markers for endocrine sensitivity and resistance and the translation of these data to clinical utility.

Hormone therapy for breast cancer, with an emphasis on the pure antiestrogen fulvestrant: mode of action, antitumor efficacy and effects on bone health

Breast cancer is a major health problem in women of developed Western countries. Whereas estrogen receptor (ER) may be involved in many cases in breast carcinogenesis, its expression in breast tumors may predict a favorable response to hormone therapy. In this review, we report the role played by ER in breast cancer and compare the effects and mechanisms of action of partial (tamoxifen) and pure (fulvestrant) antiestrogens, as well as of aromatase inhibitors. Moreover, as ER also has a critical role in bone metabolism, we review the beneficial and adverse effects of breast cancer hormone therapy on bone health, with a particular emphasis on fulvestrant, the only pure antiestrogen recently approved by the FDA for Phase III clinical trials. We conclude that, because of its therapeutic efficacy and its seemingly minimal effect on bone integrity, fulvestrant represents a new option for the hormonal treatment of breast cancer that deserves further clinical evaluation.

Activation of ErbB3, EGFR and Erk is essential for growth of human breast cancer cell lines with acquired resistance to fulvestrant

Seven fulvestrant resistant cell lines derived from the estrogen receptor alpha positive MCF-7 human breast cancer cell line were used to investigate the importance of epidermal growth factor receptor (ErbB1-4) signaling. We found an increase in mRNA expression of EGFR and the ErbB3/ErbB4 ligand heregulin2 (hrg2) and a decrease of ErbB4 in all resistant cell lines. Western analyses confirmed the upregulation of EGFR and hrg2 and the downregulation of ErbB4. Elevated activation of EGFR and ErbB3 was seen in all resistant cell lines and the ErbB3 activation occurred by an autocrine mechanism. ErbB4 activation was observed only in the parental MCF-7 cells. The downstream kinases pAkt and pErk were increased in five of seven and in all seven resistant cell lines, respectively. Treatment with the EGFR inhibitor gefitinib preferentially inhibited growth and reduced the S phase fraction in the resistant cell lines concomitant with inhibition of Erk and unaltered Akt activation. In concert, inhibition of Erk with U0126 preferentially reduced growth of resistant cell lines. Treatment with ErbB3 neutralizing antibodies inhibited ErbB3 activation and resulted in a modest but statistically significant growth inhibition of two resistant cell lines. These data indicate that ligand activated ErbB3 and EGFR, and Erk signaling play important roles in fulvestrant resistant cell growth. Furthermore, the decreased level of ErbB4 in resistant cells may facilitate heterodimerization of ErbB3 with EGFR and ErbB2. Our data support that a concerted action against EGFR, ErbB2 and ErbB3 may be required to obtain complete growth suppression of fulvestrant resistant cells.

Cell cycle machinery: links with genesis and treatment of breast cancer

Loss of normal growth control is a hallmark of cancer. Thus, understanding the mechanisms of tissue-specific, normal growth regulation and the changes that occur during tumorigenesis may provide insights of both diagnostic and therapeutic importance. Control of cell proliferation in the normal mammary gland is steroid hormone (estrogen and progestin)-dependent, involves complex interactions with other hormones, growth factors and cytokines and ultimately converges on activation of three proto-oncogenes (c-Myc, cyclin D1 and cyclin E1) that are rate limiting for the G1 to S phase transition during normal cell cycle progression. Mammary epithelial cell-specific overexpression of these genes induces mammary carcinoma in mice, while cyclin D1 null mice have arrested mammary gland development and are resistant to carcinoma induced by the neu/erbB2 and ras oncogenes. Furthermore, c-Myc, cyclins D1, E1 and E2 are commonly overexpressed in primary breast cancer where elevated expression is often associated with a more aggressive disease phenotype and an adverse patient outcome. This may be due in part to overexpression of these genes conferring resistance to endocrine therapies since in vitro studies provide compelling evidence that overexpression of c-Myc and to a lesser extent cyclin D1 and cyclin E1, attenuate the growth inhibitory effects of SERMS, antiestrogens and progestins in breast cancer cells. Thus, abnormal regulation of the expression of cell cycle molecules, involved in the steroidal control of cell proliferation in the mammary gland, are likely to be directly involved in the development, progression and therapeutic responsiveness of breast cancer. Furthermore, a more detailed understanding of these pathways may identify new targets for therapeutic intervention particularly in endocrine-unresponsive and endocrine-resistant disease.

Oestrogen-receptor-mediated transcription and the influence of co-factors and chromatin state

Oestrogen receptor-alpha (ERalpha)-regulated transcription in breast cancer cells involves protein co-factors that contribute to the regulation of chromatin structure. These include co-factors with the potential to regulate histone modifications such as acetylation or methylation, and therefore the transcriptional state of target genes. Although much of the information regarding the interaction of specific co-factors with ER has been generated by studying specific promoter regions, we now have an improved understanding of the nature of these interactions and are better placed to relate these with ER activity and potentially with the activity of breast cancer drugs, including tamoxifen.

Classification of anti-estrogens according to intramolecular FRET effects on phospho-mutants of estrogen receptor alpha

Anti-estrogen resistance is a major clinical problem in the treatment of breast cancer. In this study, fluorescence resonance energy transfer (FRET) analysis, a rapid and direct way to monitor conformational changes of estrogen receptor alpha (ERalpha) upon anti-estrogen binding, was used to characterize resistance to anti-estrogens. Nine different anti-estrogens all induced a rapid FRET response within minutes after the compounds have liganded to ERalpha in live cells, corresponding to an inactive conformation of the ERalpha. Phosphorylation of Ser(305) and/or Ser(236) of ERalpha by protein kinase A (PKA) and of Ser(118) by mitogen-activated protein kinase (MAPK) influenced the FRET response differently for the various anti-estrogens. PKA and MAPK are both associated with resistance to anti-estrogens in breast cancer patients. Their respective actions can result in seven different combinations of phospho-modifications in ERalpha where the FRET effects of particular anti-estrogen(s) are nullified. The FRET response provided information on the activity of ERalpha under the various anti-estrogen conditions as measured in a traditional reporter assay. Tamoxifen and EM-652 were the most sensitive to kinase activities, whereas ICI-182,780 (Fulvestrant) and ICI-164,384 were the most stringent. The different responses of anti-estrogens to the various combinations of phospho-modifications in ERalpha elucidate why certain anti-estrogens are more prone than others to develop resistance. These data provide new insights into the mechanism of action of anti-hormones and are critical for selection of the correct individual patient-based endocrine therapy in breast cancer.

Human X-box binding protein-1 confers both estrogen independence and antiestrogen resistance in breast cancer cell lines

Human X-box binding protein-1 (XBP1) is an alternatively spliced transcription factor that participates in the unfolded protein response (UPR), a stress-signaling pathway that allows cells to survive the accumulation of unfolded proteins in the endoplasmic reticulum lumen. We have previously demonstrated that XBP1 expression is increased in antiestrogen-resistant breast cancer cell lines and is coexpressed with estrogen receptor alpha (ER) in breast tumors. The purpose of this study is to investigate the role of XBP1 and the UPR in estrogen and antiestrogen responsiveness in breast cancer. Overexpression of spliced XBP1 [XBP1(S)] in ER-positive breast cancer cells leads to estrogen-independent growth and reduced sensitivity to growth inhibition induced by the antiestrogens Tamoxifen and Faslodex in a manner independent of functional p53. Data from gene expression microarray analyses imply that XBP1(S) acts through regulation of the expression of ER, the antiapoptotic gene BCL2, and several other genes associated with control of the cell cycle and apoptosis. Testing this hypothesis, we show that overexpression of XBP1(S) prevents cell cycle arrest and antiestrogen-induced cell death through the mitochondrial apoptotic pathway. XBP1 and/or the UPR may be a useful molecular target for the development of novel predictive and therapeutic strategies in breast cancer.