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

Early phase clinical trials in oncology: Realising the potential of seamless designs

BACKGROUND

The pharmaceutical industry's productivity has been declining over the last two decades and high attrition rates and reduced regulatory approvals are being seen. The development of oncology drugs is particularly challenging with low rates of approval for novel treatments when compared with other therapeutic areas. Reliably establishing the potential of novel treatment and the corresponding optimal dosage is a key component to ensure efficient overall development. A growing interest lies in terminating developments of poor treatments quickly while enabling accelerated development for highly promising interventions.

METHODS

One approach to reliably establish the optimal dosage and the potential of a novel treatment and thereby improve efficiency in the drug development pathway is the use of novel statistical designs that make efficient use of the data collected.

RESULTS

In this paper, we discuss different (seamless) strategies for early oncology development and illustrate their strengths and weaknesses through real trial examples. We provide some directions for good practices in early oncology development, discuss frequently seen missed opportunities for improved efficiency and some future opportunities that have yet to fully develop their potential in early oncology treatment development.

DISCUSSION

Modern methods for dose-finding have the potential to shorten and improve dose-finding and only small changes to current approaches are required to realise this potential.

From Pure Antagonists to Pure Degraders of the Estrogen Receptor: Evolving Strategies for the Same Target

Pure antiestrogens, or selective estrogen receptor degraders (SERDs), have proven to be effective in treating breast cancer that has progressed on tamoxifen and/or aromatase inhibitors. However, the only FDA-approved pure antiestrogen, fulvestrant, is limited in efficacy by its low bioavailability. The search for orally bioavailable SERDs has continued for nearly as long as the clinical history of the injection-only fulvestrant. Oral SERDs that have been developed and tested in patients ranged from nonsteroidal ER binders containing an acrylic acid or amino side chain to bifunctional proteolysis-targeting chimera (PROTAC) pure ER degraders. Structural evolution in the development of oral SERD molecules has been closely associated with quantifiable ER-degrading potency, as seen in the structural comparison analysis of acrylic acid and basic amino side-chain-bearing SERDs. Failure to improve on fulvestrant in the clinical trials by numerous acidic SERDs and early basic SERDs is blamed on tolerability and/or insufficient efficacy, which will likely be overcome by the new-generation basic SERD molecules and PROTAC ER degraders with improved oral bioavailability, low toxicity, and superior efficacy of receptor degradation.

GLL398, an oral selective estrogen receptor degrader (SERD), blocks tumor growth in xenograft breast cancer models

PURPOSE

Selective estrogen receptor degrader (SERD) has proven clinically effective in treating advanced or metastatic breast cancer since the approval of fulvestrant by FDA in 2002. Recent expansion of indications as a first line monotherapy and as combination therapy with CDK4/6 inhibitors further extends its clinical utility as an efficacious breast cancer endocrine regimen. However, the poor pharmacokinetic properties of fulvestrant and its injection-only administration route has driven continued efforts to develop orally bioavailability SERD that could potentially improve clinical response to SERD treatment. GLL398, a boron-modified GW5638 analog, showed superior oral bioavailability, while retaining both antiestrogenic activity and ER degrading efficacy at a potency level comparable to the more active metabolite of GW5638, GW7604.

METHODS

Here we used molecular modeling, ER (Y537S) binding assay, MCF-7 Xenograft tumor, and patient-derived xenograft (PDX) tumor model to conduct further studies on the pharmacology and metabolism of GLL398.

RESULTS

Consistent with GLL398's robust activities in breast cancer cells that either are tamoxifen resistant or express constitutively active, mutant ESR1 (Y537S), it was found to bind the mutant ER^Y537S with high affinity. Molecular modeling of the binding mode of GLL398 to ER also found its molecular interactions consistent with the experimentally determined high binding affinity towards WT ER and ER^Y537S. To test the in vivo efficacy of GLL398, mice bearing MCF-7-derived xenograft breast tumors and patient-derived xenograft tumors harboring ER^Y537S were treated with GLL398 which potently inhibited tumor growth in mice.

CONCLUSIONS

This study demonstrates GLL398 is an oral SERD that has therapeutic efficacy in clinically relevant breast tumor models.

Therapeutic advances in hormone-dependent cancers: focus on prostate, breast and ovarian cancers

Hormonal cancers affect over 400,000 men and women and contribute collectively to over 100,000 deaths in the United States alone. Thanks to advances in the understanding of these cancers at the molecular level and to the discovery of several disease-modifying therapeutics, the last decade has seen a plateauing or even a decreasing trend in the number of deaths from these cancers. These advanced therapeutics not only effectively slow the growth of hormonal cancers, but also provide an insight on how these cancers become refractory and evolve as an altogether distinct subset. This review summarizes the current therapeutic trends in hormonal cancers, with focus on prostate, breast and ovarian cancers. The review discusses the clinical drugs being used now, promising molecules that are going through various stages of development and makes some predictions on how the therapeutic landscape will shift in the next decade.

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.

Fulvestrant in management of hormone receptor-positive metastatic breast cancer

Fulvestrant is a steroidal selective estrogen receptor degrader that was approved by the US FDA in 2002 for treatment of ER-positive metastatic breast cancer (ER + MBC) post-progression on aromatase inhibitors. In 2017, the label was updated to include endocrine therapy naive ER + MBC. While initially fulvestrant was thought to be equivalent to aromatase inhibitors with monthly dose of 250 mg intramuscular injection, several postmarketing trials challenged this understanding. Subsequently, the recommended dose changed to 500 mg monthly plus loading dose, and this was proven to be superior to anastrozole in efficacy. Most recently the results of FALCON trial have further challenged the way fulvestrant is viewed and used. This report provides a historical perspective on this compound, its evolving role in management of ER + MBC and what the future may hold for this drug.

Fulvestrant for the treatment of advanced breast cancer

INTRODUCTION

The current issues with endocrine therapy for treatment of advanced breast cancer include balance of efficacy of therapy versus tolerability as well as hormone resistance. The efficacy of fulvestrant, a selective oestrogen receptor degrader (SERD), has been demonstrated in hormone receptor positive patients previously untreated or treated with hormonal therapy. Areas covered: This article discusses the journey of fulvestrant licensing, its efficacy in combination with other endocrine therapies and the future role it may have within breast cancer treatment. Expert commentary: Within phase III trials, fulvestrant has demonstrated equivalent or improved clinical efficacy when compared with established endocrine agents. In the recent decade, fulvestrant has achieved licensing as a second line agent in non-operative advanced breast cancer at initially 250mg, increasing to 500mg. Presently, fulvestrant is licensed globally as first line endocrine management for advanced breast cancer in post-menopausal women. Early combination trials of fulvestrant and cyclin dependent kinase 4/6 inhibitors have demonstrated good clinical efficacy with improved progression free survival when compared to fulvestrant alone.

Endogenous Estrogen Regulates Somatostatin-Induced Rebound GH Secretion in Postmenopausal Women

BACKGROUND

Systemic concentrations of T, estradiol (E₂), GH, IGF-1, and IGF binding protein-3 decline in healthy aging individuals. Conversely, T and E₂ stimulate GH and IGF-1 production in hypogonadal patients.

HYPOTHESIS

Because E₂ stimulates GH secretion, putatively via the nuclear estrogen receptor-α and E₂ and GH fall with menopause, we postulated that diminished endogenous E₂ contributes to low GH output in older women.

LOCATION

The study was conducted at the Mayo Center for Clinical and Translational Science.

STUDY DESIGN

This was a randomized, double-blind, controlled study in 60 healthy postmenopausal women treated with the following: 1) double placebo; 2) anastrozole, a potent inhibitor of aromatase-enzyme activity, which mediates E₂ synthesis from T; and/or 3) fulvestrant, a selective estrogen receptor-α antagonist.

METHODS

GH pulse generation was quantified by frequent GH sampling before and after short-term iv somatostatin infusion, thought to induce hypothalamic GHRH-mediated rebound-like GH secretion.

RESULTS

On anastrozole, E₂ fell from 3.1 ± 0.35 pg/mL to 0.36 ± 0.04 pg/mL, and estrone from 13 ± 1.4 pg/mL to 1.9 ± 0.01 pg/mL (P < .001) by mass spectrometry. Estrogen values were unchanged by fulvestrant. T concentrations did not change. One-hour peak GH rebound after somatostatin infusion declined markedly during both estrogen-deprivation schedules (P < .001). Mean (150 min) maximal GH rebound decreased comparably (P < .001). Measures of GH rebound correlated negatively with computed tomography-estimated abdominal visceral fat (all P < .05).

CONCLUSION

These data suggest a previously unrecognized dependence of hypothalamo-pituitary GH regulation on low levels of endogenous estrogen after menopause.

The role of fulvestrant in endometrial cancer

INTRODUCTION

Endometrial cancer is the most common malignancy of the female genital tract in industrialized countries. The traditional treatment of endometrial cancer is based on a surgical approach. In recent years, systemic endocrine therapy has demonstrated good efficacy in recurrent or metastatic setting, delaying progression, ameliorating quality of life and palliating symptoms. Areas covered: Phase I and II studies on selective estrogen receptor down-regulators used for the treatment of endometrial cancer treatment have been reviewed. The pharmacokinetic and pharmacodynamic features of selective receptor down-regulators have been also investigated. Expert opinion: Selective estrogen receptor down-regulators may exhibit clinical efficacy in the treatment of gynecological malignancies due to their pure estrogen receptor antagonist properties. However, up to now data are still limited and some unsolved questions remain. Fulvestrant has poor oral bioavailability and low pharmacodynamic characteristics. Further trials are required to examine new selective estrogen receptor down-regulator agents with better pharmacodynamic and pharmacokinetic profiles.

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.

Stability Indicating HPLC Method for the Determination of Fulvestrant in Pharmaceutical Formulation in Comparison with Linear Sweep Voltammetric Method

This paper describes two rapid, sensitive and specific methods for the determination of fulvestrant in pharmaceutical preparations by high performance liquid chromatography (HPLC) and linear sweep voltammetry (LSV). HPLC method was used to study the degradation behaviour. Fulvestrant was subjected to degradation under the conditions of hydrolysis (acid and alkali), oxidation (30% H₂O₂). The linearity was established over the concentration range of 5-50 m g mL^-1 for LSV and 0.5-20 m g mL^-1 for HPLC method. The intra- and inter-day relative standard deviation (RSD) was less than 3.96 and 3.07% for LSV and HPLC, respectively. Limits of quantification were determined as 5.0 and 0.50 m g mL^-1 for LSV and HPLC, respectively. No interference was found from tablet excipients at the selected assay conditions. The methods were applied for the quality control of commercial fulvestrant dosage form to quantify the drug and to check the formulation content uniformity.

Estrogen-like potentiation of ghrelin-stimulated GH secretion by fulvestrant, a putatively selective ER antagonist, in postmenopausal women

CONTEXT

Hyposomatotropism in healthy aging women reflects in part physiological estrogen (estradiol [E2]) depletion associated with menopause.

OBJECTIVE AND DESIGN

The purpose of this study was to test the hypothesis that low concentrations of endogenous E2 after menopause continue to drive GH secretion.

SETTING

The study was performed at the Mayo Center for Clinical and Translational Science.

PARTICIPANTS

The participants were 24 postmenopausal women (aged 50-77 years with body mass index of 19-32 kg/m(2)).

INTERVENTIONS

This was a randomized, double-blind, placebo-controlled, parallel-cohort treatment study with placebo (PL) (n = 14) or the antiestrogen fulvestrant (FUL) (n = 10) for 3 weeks, followed by infusion of l-arginine with saline, GHRH, ghrelin, or both peptide secretagogues.

OUTCOMES

GH concentrations were measured over 6 hours with 10-minute sampling and mass spectrometry measures of testosterone, E2, and estrone.

RESULTS

Concentrations of testosterone, E2, estrone, SHBG, IGF-I, LH, and FSH were not influenced by antiestrogen treatment. In contrast, GH rose from 0.096 ± 0.018 (PL) to 0.23 ± 0.063 μg/L (FUL, P = .033), and IGF-I binding protein type 3 (IGFBP-3) from 3.6 ± 0.18 to 4.0 ± 2.0 mg/L (P = .041). Conversely, prolactin fell from 7.1 ± 0.69 (PL) to 5.5 ± 0.57 μg/L (FUL) (P = .05), and IGF-I binding protein type 1 (IGFBP-1) fell from 44 ± 9.4 to 27 ± 4.3 μg/L (P = .048). Moreover, FUL vs PL potentiated mean GH responses to l-arginine/saline (P = .007), l-arginine/ghrelin (P = .008), and l-arginine/GHRH + ghrelin (P = .031), but not l-arginine/GHRH.

CONCLUSION

The potent antiestrogen, FUL, amplifies fasting and secretagogue-driven GH secretion and IGFBP-3 concentrations in postmenopausal women without altering SHBG or sex steroid levels. FUL also suppresses prolactin and IGFBP-1, without altering IGF-I. Thus, a major antiestrogen mediates 3 actions of estrogen: agonism (GH), neutral effects (sex steroids), and estrogen antagonism (prolactin and IGFBP-1).

Estrogen and bisphenol A affect male rat enamel formation and promote ameloblast proliferation

Bisphenol A (BPA) is a widespread endocrine disrupting chemical (EDC) strongly suspected to have adverse health effects. Numerous tissues and cells are affected by BPA, and we showed recently that BPA targets include ameloblasts and enamel. We therefore investigated the effects of BPA on ameloblasts and the possible involvement of the estrogen signaling pathway. Rats were exposed daily to low-dose BPA, and developed enamel hypomineralization similar to human molar incisor hypomineralization (MIH). BPA increased ameloblast proliferation in vivo and in vitro. The proliferation of the rat dental epithelial cell line HAT-7 was also increased by estrogen (E2). Ameloblasts express ERα but not ERβ both in vivo and in vitro. The ER antagonist ICI 182,780 was used to inactivate ERα and abolished the effects of E2 on cell proliferation and transcription, but only partially reduced the effects of BPA. In conclusion, we show, for the first time, that: 1) BPA has ER-dependent and ER-independent effects on ameloblast proliferation and gene transcription; 2) the estrogen signaling pathway is involved in tooth development and the enamel mineralization process; and 3) BPA impacts preferentially amelogenesis in male rats. These results are consistent with the steroid hormones having effect on ameloblasts, raising the issues of the hormonal influence on amelogenesis and possible differences in enamel quality between sexes.

An overview of current and emerging SERMs

Selective estrogen receptor modulators (SERMs) are compounds that exhibit tissue-specific estrogen receptor (ER) agonist or antagonist activity, and are used for various indications, including treatment of breast cancer, osteoporosis, and menopausal symptoms. Endometrial safety has been a key differentiator between SERMs in clinical practice. For example, tamoxifen exhibits ER agonist activity in the uterus, resulting in an increased risk of endometrial hyperplasia and malignancy, whereas raloxifene and bazedoxifene have neutral effects on the uterus. Based on their efficacy and long-term safety, SERMs are increasingly being prescribed for women who cannot tolerate other treatment options and for younger women at an increased risk of fracture who may remain on therapy for long periods of time. Continuing advances in the understanding of SERM mechanisms of action and structural interactions with the ER may lead to the development of new agents and combinations of agents to provide optimal treatments to meet the varying needs of postmenopausal women. One such example is the tissue selective estrogen complex, which partners a SERM with 1 or more estrogens, with the aim of blending the desired estrogen-receptor agonist activities of estrogens on vasomotor symptoms, vulvar-vaginal atrophy, and loss of bone mass with the tissue selectivity of a SERM.

Fulvestrant in the treatment of postmenopausal women with advanced breast cancer

Fulvestrant is a new type of estrogen receptor antagonist with no agonist effects, and represents a valuable addition to the range of endocrine treatments available to treat postmenopausal women with advanced breast cancer. Fulvestrant binds, blocks and degrades the estrogen receptor, thereby retarding growth and progression of hormone-sensitive tumors. Two Phase III trials have demonstrated that fulvestrant is at least as effective as the third-generation aromatase inhibitor anastrozole in the treatment of advanced breast cancer following progression on antiestrogen therapy. In addition, a Phase III trial comparing fulvestrant with tamoxifen in postmenopausal women with advanced breast cancer showed similar efficacy and tolerability. Clinical trials are ongoing to evaluate different endocrine sequence options for postmenopausal women with breast cancer, and to evaluate the efficacy of fulvestrant in combination with anticancer agents that target other signaling pathways.

PET imaging of oestrogen receptors in patients with breast cancer

Oestrogen receptors are overexpressed in around 70% of all breast cancers, and are a target for endocrine therapy. These receptors can be visualised on PET with use of 16α-[(18)F]-fluoro-17β-oestradiol ((18)F-FES) as a tracer. Compared with biopsy, which enables assessment of individual sites, whole-body (18)F-FES-PET enables quantification of oestrogen-receptor expression in all metastases. In several studies, measurement of tumour protein expression in oestrogen receptors by (18)F-FES-PET, concurrent with biopsy, detected oestrogen-receptor-positive tumour lesions with a sensitivity of 84% and specificity of 98%. Roughly 45% of patients with metastatic breast cancer have discordant oestrogen-receptor expression across lesions (ie, (18)F-FES-positive and (18)F-FES-negative metastases). Low tumour (18)F-FES uptake in metastases can predict failure of hormonal therapy in patients with oestrogen-receptor-positive primary tumours. Finally, (18)F-FES-PET has shown that oestrogen-receptor binding capacity changes after intervention with hormonal drugs, but findings need to be confirmed. Factors other than oestrogen-receptor expression, including menopausal status and concomitant therapies, that can affect tumour (18)F-FES uptake must be taken into account.

The potential for estrogens in preventing Alzheimer's disease and vascular dementia

Estrogens are the best-studied class of drugs for potential use in the prevention of Alzheimer's disease (AD). These steroids have been shown to be potent neuroprotectants both in vitro and in vivo, and to exert effects that are consistent with their potential use in prevention of AD. These include the prevention of the processing of amyloid precursor protein (APP) into beta-amyloid (Aß), the reduction in tau hyperphosphorylation, and the elimination of catastrophic attempts at neuronal mitosis. Further, epidemiological data support the efficacy of early postmenopausal use of estrogens for the delay or prevention of AD. Collectively, this evidence supports the further development of estrogen-like compounds for prevention of AD. Several approaches to enhance brain specificity of estrogen action are now underway in an attempt to reduce the side effects of chronic estrogen therapy in AD.

Fulvestrant in the management of postmenopausal women with advanced, endocrine-responsive breast cancer

Fulvestrant is a pure estrogen antagonist that binds, blocks and downgrades the estrogen receptor (ER). Its unique mechanism of action is its antitumor activity after progression on prior endocrine therapy. Fulvestrant has shown activity in ER-dependent cells that are ligand independent. Fulvestrant has been approved at 250 mg/month for postmenopausal women with hormone-sensitive advanced breast cancer after progression or recurrence on antiestrogen therapy. The fulvestrant 500 mg regimen has just received approval by the EMA and the US FDA, supported by dose-dependent ER downregulation and the recent results of the clinical trial CONFIRM. Fulvestrant in combination with systemic lowering of estrogen has shown no improvement over fulvestrant alone. Combination therapy with inhibitors of growth factor signaling may have greater efficacy and is under exploration. To enhance the benefit of fulvestrant and improve outcomes for individuals with ER-positive breast cancer, greater understanding of resistance mechanisms is required. A key issue is identification of patients with ER-positive disease who retain sensitivity to antiestrogen therapy after progression on tamoxifen and/or aromatase inhibitors.

Fulvestrant: a review of its use in the management of hormone receptor-positive metastatic breast cancer in postmenopausal women

Fulvestrant (Faslodex®) is an intramuscularly administered steroidal estrogen receptor antagonist that is devoid of any known estrogen agonist effects. It is indicated as second-line therapy for the treatment of postmenopausal women with hormone receptor-positive advanced breast cancer who have progressed following prior endocrine therapy. In well designed, randomized clinical trials, regimens of fulvestrant 250 and 500 mg provided effective second-line therapy for postmenopausal women with advanced breast cancer who had progressed following prior endocrine therapy. Moreover, fulvestrant 250 mg monthly (with or without a loading dose) was as effective as aromatase inhibitor therapy. However, fulvestrant is absorbed slowly, and greater steady-state concentrations are achieved more rapidly when using a higher dosage with a loading dose regimen. Consequently, a regimen of fulvestrant 500 mg monthly with a loading dose was significantly more effective than a regimen of 250 mg monthly in postmenopausal women with disease progression. Limited data also indicate a potential role for the fulvestrant 500 mg regimen as first-line therapy. Fulvestrant is generally well tolerated with no additional adverse events noted with the high-dose regimen compared with the 250 mg regimens. Furthermore, the incidence of joint disorders was shown to be significantly lower with fulvestrant 250 mg monthly than with anastrozole. Treatment with fulvestrant is not associated with any clinically significant effects on endometrial thickening, bone-specific turnover markers or sex hormone levels. In conclusion, a monthly regimen of intramuscular fulvestrant 500 mg with a loading dose provides effective and well tolerated second-line therapy for postmenopausal women with advanced breast cancer who have progressed following prior endocrine therapy and is now the approved optimal dose.

Estrogen stimulates proliferation and differentiation of neural stem/progenitor cells through different signal transduction pathways

Our previous study indicated that both 17β-estradiol (E2), known to be an endogenous estrogen, and bisphenol A (BPA), known to be a xenoestrogen, could positively influence the proliferation or differentiation of neural stem/progenitor cells (NS/PCs). The aim of the present study was to identify the signal transduction pathways for estrogenic activities promoting proliferation and differentiation of NS/PCs via well known nuclear estrogen receptors (ERs) or putative membrane-associated ERs. NS/PCs were cultured from the telencephalon of 15-day-old rat embryos. In order to confirm the involvement of nuclear ERs for estrogenic activities, their specific antagonist, ICI-182,780, was used. The presence of putative membrane-associated ER was functionally examined as to whether E2 can activate rapid intracellular signaling mechanism. In order to confirm the involvement of membrane-associated ERs for estrogenic activities, a cell-impermeable E2, bovine serum albumin-conjugated E2 (E2-BSA) was used. We showed that E2 could rapidly activate extracellular signal-regulated kinases 1/2 (ERK 1/2), which was not inhibited by ICI-182,780. ICI-182,780 abrogated the stimulatory effect of these estrogens (E2 and BPA) on the proliferation of NS/PCs, but not their effect on the differentiation of the NS/PCs into oligodendroglia. Furthermore, E2-BSA mimicked the activity of differentiation from NS/PCs into oligodendroglia, but not the activity of proliferation. Our study suggests that (1) the estrogen induced proliferation of NS/PCs is mediated via nuclear ERs; (2) the oligodendroglial generation from NS/PCs is likely to be stimulated via putative membrane-associated ERs.

Benefits of early and prolonged fulvestrant treatment in 848 postmenopausal advanced breast cancer patients

Response to fulvestrant and survival in postmenopausal hormone-sensitive advanced breast cancer was investigated within a non-randomized, In-Practice Evaluation Program, with the aim of optimizing treatment decisions. 848 patients (median age 64 years; 52% co-morbidity; 78% prior palliative therapy; median 4 prior regimens) received monthly fulvestrant injections (250 mg/month) and were followed-up three-monthly for 9 months. Clinical benefit (PFS ≥ 24 weeks) occurred in 532/848 (62.7%); stable disease (SD) in 627/848 patients (74%), including 62 complete and 177 partial responses. Best response was delayed in 115 patients. Estimated 9-month overall survival (OS) was 89%; 9-month event-free survival (EFS) was 71%. Indicators of disease aggressiveness affected response and survival, but number of fulvestrant cycles was the key OS and EFS determinant. The patients with SD at 3 months benefitted from continued fulvestrant. Excluding deaths, 7 serious adverse events occurred (none attributable to fulvestrant). No new or unexpected safety issues arose; 90% of the patients and physicians rated fulvestrant tolerability as "very good" or "good". In the largest prospective, fulvestrant-treated cohort to date, advanced breast cancer patients achieving SD or better after 3 months of treatment gained survival benefit by prolonging fulvestrant therapy-independent of disease and treatment history.

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.

Estrogen utilization of IGF-1-R and EGF-R to signal in breast cancer cells

As breast cancer cells develop secondary resistance to estrogen deprivation therapy, they increase their utilization of non-genomic signaling pathways. Our prior work demonstrated that estradiol causes an association of ERalpha with Shc, Src and the IGF-1-R. In cells developing resistance to estrogen deprivation (surrogate for aromatase inhibition) and to the anti-estrogens tamoxifen, 4-OH-tamoxifen, and fulvestrant, an increased association of ERalpha with c-Src and the EGF-R occurs. At the same time, there is a translocation of ERalpha out of the nucleus and into the cytoplasm and cell membrane. Blockade of c-Src with the Src kinase inhibitor, PP-2 causes relocation of ERalpha into the nucleus. While these changes are not identical in response to each anti-estrogen, ERalpha binding to the EGF-R is increased in response to 4-OH-tamoxifen when compared with tamoxifen. The changes in EGF-R interactions with ERalpha impart an enhanced sensitivity of tamoxifen-resistant cells to the inhibitory properties of the specific EGF-R tyrosine kinase inhibitor, AG 1478. However, with long term exposure of tamoxifen-resistant cells to AG 1478, the cells begin to re-grow but can now be inhibited by the IGF-R tyrosine kinase inhibitor, AG 1024. These data suggest that the IGF-R system becomes the predominant signaling mechanism as an adaptive response to the EGF-R inhibitor. Taken together, this information suggests that both the EGF-R and IGF-R pathways can mediate ERalpha signaling. To further examine the effects of fulvestrant on ERalpha function, we examined the acute effects of fulvestrant, on non-genomic functionality. Fulvestrant enhanced ERalpha association with the membrane IGF-1-receptor (IGF-1-R). Using siRNA or expression vectors to knock-down or knock-in selective proteins, we further demonstrated that the ERalpha/IGF-1-R association is Src-dependent. Fulvestrant rapidly induced IGF-1-R and MAPK phosphorylation. The Src inhibitor PP2 and IGF-1-R inhibitor AG1024 greatly blocked fulvestrant-induced ERalpha/IGF-1-R interaction leading to a further depletion of total cellular ERalpha induced by fulvestrant and further enhanced fulvestrant-induced cell growth arrest. More dramatic was the translocation of ERalpha to the plasma membrane in combination with the IGF-1-R as shown by confocal microscopy. Taken in aggregate, these studies suggest that secondary resistance to hormonal therapy results in usage of both IGF-R and EGF-R for non-genomic signaling.

Preclinical and Clinical Experience with Fulvestrant (Faslodex) in Postmenopausal Women with Hormone Receptor-Positive Advanced Breast Cancer

Fulvestrant (Faslodex) is a new type of estrogen receptor (ER) antagonist that binds, blocks, and degrades the ER, leading to reduced expression of the progesterone receptor (PgR). Unlike the selective ER modulator tamoxifen, fulvestrant is devoid of any known agonist activity. Fulvestrant has a steroidal structure that competitively binds to the ER with an affinity much greater than that of tamoxifen. After binding to fulvestrant, degradation of the ER is accelerated, ultimately resulting in a reduction in cellular ER. Immunohistochemical studies have demonstrated that a single intramuscular injection of fulvestrant results in a dose-dependent decrease in ER and PgR indices and in Ki-67 expression. At the approved 250 mg dose, the decrease in receptor positivity with fulvestrant treatment was greater than that with tamoxifen. Phase III clinical trials have demonstrated the clinical benefit of fulvestrant in the endocrine treatment of breast cancer. Among patients who progressed during adjuvant or first-line endocrine therapy, fulvestrant was at least as effective as anastrozole as second-line treatment for the primary endpoints of objective response and time to progression, and was well tolerated. After a median follow-up of 24.5 months, a combined survival analysis from two Phase III studies has shown that fulvestrant may be considered similar to anastrozole for time to death (27.4 months versus 27.7 months, respectively). For the first-line therapy of advanced breast cancer in postmenopausal women, fulvestrant was shown to be active and well tolerated in a trial that compared fulvestrant 250 mg once monthly and tamoxifen 20 mg once daily. Treatment with fulvestrant has also demonstrated clinical efficacy among patients who progressed following treatment with tamoxifen followed by nonsteroidal aromatase inhibitors. The efficacy of fulvestrant in sequential endocrine therapy and in combination with other agents appears promising and active investigations are ongoing to explore the clinical potential of this novel antiestrogen.

Rational management of endocrine resistance in breast cancer: a comprehensive review of estrogen receptor biology, treatment options, and future directions

Endocrine therapy for breast cancer was introduced more than 100 years ago. In the last 30 years, it has been demonstrated that tamoxifen significantly improves outcomes for patients with hormone-responsive breast tumors. Aromatase inhibitors, which suppress the production of estrogen, are recognized today as an effective alternative for estrogen-receptor-positive breast cancer in postmenopausal women. However, despite an initial response to treatment, many tumors eventually recur or progress. When selecting subsequent endocrine therapy, it is helpful to understand the mechanisms of hormone resistance, consider the goals of treatment, and evaluate the clinical potential of each available drug. The objective of this article was to review the underlying mechanisms of action and resistance for each type of hormone therapy, evaluate the most recent data regarding the use of endocrine agents after disease progression or recurrence, and explore potential combinations of hormone therapies with novel molecules that target key growth factor signaling pathways.

Interrogating the genome to understand oestrogen-receptor-mediated transcription

Defining transcription mediated by the oestrogen (estrogen) receptor (ER) in breast cancer cell models has been an area of interest for many years. Initial studies focused on promoter regions of putative target genes and revealed significant insight into the basis of ER binding to DNA. More recently, the complexities of ER transcription are starting to become apparent. It is now clear that ER can regulate gene targets from significant distances and that cooperating transcription factors play an integral role in ER activity. It is also clear that the sequence information defining an in vivo ER-binding site is more complicated than initially thought. However, contemporary genomic tools based on chromatin immunoprecipitation (ChIP) – such as ChIP-on-chip and ChIP–sequencing – and gene expression profiling have allowed us to redefine the underlying properties of ER biology on a genomic scale. The advances in technology that have permitted a better understanding of how and where ER can bind to DNA are discussed in this review. The possible clinical implications of these findings for understanding the role of oestrogen in breast cancer are also briefly considered.

Growth factor signalling in endocrine and anti-growth factor resistant breast cancer

Growth factors provide powerful mitogenic and survival signals to breast cancer cells and it is therefore not surprising that they are able to subvert inhibitory responses to anti-hormonal drugs. In this review we discuss several mechanisms by which this may be achieved and expand our observations to encompass recently emerging anti-growth factor treatments. The information presented is underpinned by inhibitor studies that show the targeting of such mechanisms in advance of anti-hormone or anti-growth factor resistance development is able to substantially delay this event, thus pointing the way forward to intelligent combination therapies relevant to the future management of breast cancer.

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.

Breast cancer cells with acquired antiestrogen resistance are sensitized to cisplatin-induced cell death

Antiestrogens are currently used for treating breast cancer patients who have estrogen receptor-positive tumors. However, patients with advanced disease will eventually develop resistance to the drugs. Therefore, compounds effective on antiestrogen-resistant tumors will be of great importance for future breast cancer treatment. In this study, we have investigated the effect of the chemotherapeutic compound cisplatin using a panel of antiestrogen-resistant breast cancer cell lines established from the human breast cancer cell line MCF-7. We show that the antiestrogen-resistant cells are significantly more sensitive to cisplatin-induced cell death than antiestrogen-sensitive MCF-7 cells and we show that cisplatin induces cell death by activating both the caspase and lysosomal death pathways. The antiestrogen-resistant cell lines express lower levels of antiapoptotic Bcl-2 protein compared with parental MCF-7 cells. Our data show that Bcl-2 can protect antiestrogen-resistant breast cancer cells from cisplatin-induced cell death, indicating that the reduced expression of Bcl-2 in the antiestrogen-resistant cells plays a role in sensitizing the cells to cisplatin treatment.

Fulvestrant in heavily pre-treated patients with advanced breast cancer: results from a single compassionate use programme centre

PURPOSE

Fulvestrant ('Faslodex') is an oestrogen receptor (ER) antagonist with no agonist effects. The drug was administered to heavily pre-treated patients with advanced breast cancer (ABC). Patients received Fulvestrant after disease progression (PD) on a previous endocrine treatment or as maintenance treatment after chemotherapy.

MATERIAL AND METHODS

Fifty-seven postmenopausal women with ER and/or progesterone receptor-positive ABC resistant to previous endocrine treatments prospectively received fulvestrant 250 mg via intramuscular injection q 28.

RESULTS

Twenty-seven patients received fulvestrant after PD and 30 received it as maintenance therapy after chemotherapy. All patients received fulvestrant as second-up to eight-line endocrine treatment for ABC. One patient (2%) had a partial response (PR) and 24 patients (42%) had stable disease > or =12 weeks (SD), including 11 patients who had SD > or =24 weeks. Thirty-two patients (56%) had de novo PD. Clinical benefit (CB; PR + SD > or =24 weeks) occurred in 12 patients (21%). Patients treated as maintenance and treated upon PD had 0 and 4% PR, 43 and 41% SD (including 20 and 19% SD > or =24 weeks), 57 and 55% PD, respectively. Overall, median time to progression (TTP) was 3 months. No differences in CB rate (20% vs. 23%), TTP (3 months vs. 3 months) and time to treatment failure (3 months vs. 3 months) were observed between patients receiving fulvestrant as maintenance therapy and those treated at PD on prior endocrine treatment. No grade 2-4 NCI-CTC toxicity was recorded.

CONCLUSIONS

Fulvestrant treatment was associated with prolonged CB and was well tolerated in this group of heavily pre-treated patients with ABC. The outcomes appeared to be similar for patients treated upon PD and those receiving fulvestrant as maintenance therapy.

Postmenopausal advanced breast cancer: Options for therapy after tamoxifen and aromatase inhibitors

All patients receiving endocrine treatment for advanced breast cancer (ABC) eventually progress, resulting in a need for new therapies that lack cross-resistance with existing agents. Oestrogen receptor (ER) modulators such as toremifene and raloxifene have poor efficacy following tamoxifen failure, whereas the non-steroidal aromatase inhibitors (AIs), anastrozole and letrozole and the steroidal AI exemestane are effective. Fulvestrant is a new ER antagonist with no agonist effects that is as effective as anastrozole in treating patients who have progressed on tamoxifen. AIs are replacing tamoxifen as first-line treatments for ABC and in the adjuvant setting, necessitating a re-evaluation of optimal sequencing. Preliminary data suggest that tamoxifen, exemestane and fulvestrant have activity in patients who have progressed on non-steroidal AIs and hence could be considered for use in this setting. Due to the apparent lack of cross-resistance between non-steroidal and steroidal AIs, non-steroidal AIs could also be effective following steroidal AI failure. Clinical trials are underway to assess the most appropriate treatment sequence following non-steroidal AI failure, with comparisons of fulvestrant and exemestane of major interest.

Estrogenic agonist activity of ICI 182,780 (Faslodex) in hippocampal neurons: implications for basic science understanding of estrogen signaling and development of estrogen modulators with a dual therapeutic profile

The present study sought to determine the characteristics of ICI 182,780 (Faslodex) action in rat primary hippocampal neurons. We first investigated the neuroprotective efficacy of ICI 182,780 against neurodegenerative insults associated with Alzheimer's disease and related disorders. Dose-response analyses revealed that ICI 182,780, in a concentration-dependent manner, significantly promoted neuron survival following exposure to either excitotoxic glutamate (200 muM)- or beta-amyloid(1-42) (1.5 muM)-induced neurodegeneration of hippocampal neurons. At a clinically relevant concentration of 50 ng/ml, ICI 182,780 exerted nearly maximal neuroprotection against both insults with efficacy comparable with that induced by the endogenous estrogen 17beta-estradiol. Thereafter, we investigated the impact of 50 ng/ml ICI 182,780 on mechanisms of 17beta-estradiol-inducible neuronal plasticity and neuroprotection. Results of these analyses demonstrated that ICI 182,780 directly induced a series of rapid intracellular Ca(2+) concentration ([Ca(2+)](i)) oscillations in a pattern comparable with that of 17beta-estradiol. In addition, ICI 182,780 exerted dual regulation of the glutamate-induced rise in [Ca(2+)](i) identical to that induced by 17beta-estradiol. Further analyses demonstrated that ICI 182,780 induced significant activation of extracellular signal-regulated kinase 1/2 and Akt (protein kinase B) and significantly increased expression of spinophilin and Bcl-2, with efficacy comparable with neurons treated with 17beta-estradiol. Taken together, results of these in vitro analyses of ICI 182,780 provide direct evidence of an estrogenic agonist profile of ICI 182,780 action in rat hippocampal neurons. Therapeutic development of neuroselective estrogen receptor modulators that mimic ICI 182,780 is discussed with respect to the potential of safe and efficacious alternatives to estrogen therapy for the prevention of postmenopausal cognitive decline and late-onset Alzheimer's disease.

Fulvestrant: pharmacologic profile versus existing endocrine agents for the treatment of breast cancer

OBJECTIVE

To compare the pharmacologic profile of fulvestrant with that of tamoxifen and the aromatase inhibitors with respect to the choice of treatment for advanced breast cancer (ABC).

DATA SOURCES

Principal literature and review articles were obtained from MEDLINE (1991-March 2006). Key search terms included fulvestrant, tamoxifen, aromatase inhibitors, pharmacology, and breast cancer. Further data sources were identified from the bibliographies of selected articles.

STUDY SELECTION AND DATA EXTRACTION

English-language preclinical and clinical research and review articles reporting pharmacologic and safety data for fulvestrant, tamoxifen, and the aromatase inhibitors were evaluated to identify relevant information. Randomized clinical trial data were preferred over preclinical or Phase I and II trial data.

DATA SYNTHESIS

A total of 52 clinical papers (including 10 reviews) and 17 clinical abstracts were evaluated reporting results from controlled Phase I-III studies and pilot studies. Eleven preclinical papers (including 2 reviews) and 6 preclinical abstracts were also included. Fulvestrant has little effect on sex hormone endocrinology, bone metabolism, and lipid biochemistry and appears unlikely to be the subject or cause of CYP3A4-mediated drug interactions. Tamoxifen has a protective effect on bone (due to its partial estrogen agonist activity) and reduces plasma low-density lipoprotein cholesterol but increases triglyceride levels. The aromatase inhibitors have variable effects on lipid profiles and sex hormone endocrinology but have detrimental effects on bone due to inhibition of estrogen synthesis. Drug interactions have been noted between tamoxifen and anticoagulants and tamoxifen and aromatase inhibitors, which may be due to CYP-mediated mechanisms.

CONCLUSIONS

Fulvestrant appears to have little effect on sex hormone endocrinology, bone metabolism, and lipid biochemistry and is unlikely to be subject to or the cause of CYP3A4-mediated drug-drug interactions. As such, fulvestrant represents a valuable new endocrine therapy for the treatment of ABC and broadens the options available to clinicians in the treatment of this disease.

Second-line treatment of postmenopausal women with advanced breast carcinoma

Breast cancer is the most prevalent cancer in women and, currently, there is no standard of care for the treatment of metastatic disease. Treatment options are based on a number of tumor- and patient-related factors. This review explores some of these options, including the use of hormonal manipulation in the treatment of hormone-positive disease, current chemotherapy options and the use of targeted therapies, such as trastuzumab.

Estrogen induces death of tamoxifen-resistant MCF-7 cells: contrasting effect of the estrogen receptor downregulator fulvestrant

A common problem in breast cancer therapy is resistance to the antiestrogen tamoxifen. However, tamoxifen-resistant breast tumors can still respond to other hormonal therapies. In animal models of tamoxifen-resistant breast cancer cells, physiological levels of estrogen can induce tumor regression. Recently, the estrogen receptor downregulator fulvestrant was shown to promote tumor growth of tamoxifen-resistant cells when added in combination with physiological levels of estrogen. Here, we show, using a cell culture model, that continuous exposure of tamoxifen-resistant cells to physiological levels of estrogen leads to cell death. Addition of the estrogen receptor downregulator fulvestrant prevents estrogen-induced death in a dose-dependent manner. Our data indicate that endogenous levels of estrogen affect the response of tamoxifen-resistant cells to fulvestrant. These results suggest that failure of fulvestrant to inhibit tumor growth in some tamoxifen-resistant patients may be due to endogenous estrogen levels. Moreover, these studies support short-term treatment with estrogen as a second-line hormonal therapy for tamoxifen-resistant breast cancer.

Bidirectional cross talk between ERalpha and EGFR signalling pathways regulates tamoxifen-resistant growth

We have previously demonstrated that oestrogen receptor alpha (ERalpha) modulates epidermal growth factor receptor (EGFR)/mitogen-activated protein kinase (MAPK) signalling efficiency in a tamoxifen-resistant MCF-7 breast cancer cell line (Tam-R). In the present study we have investigated whether this cross-talk between EGFR/MAPK and ERalpha signalling pathways is bidirectional by examining the effects of EGFR/MAPK activity on ER functionality in the same cell line. Elevated expression levels of phosphorylated serine 118 (S118) ERalpha were observed in the Tam-R compared to the parental wild type MCF-7 cell line (WT-MCF-7) under basal growth conditions. Phosphorylation of ERalpha at S118 was regulated by the EGFR/MAPK pathway in Tam-R cells being increased in response to amphiregulin (AR) and inhibited by the selective EGFR tyrosine kinase inhibitor, gefitinib and the MEK1/2 inhibitor, PD184352. Recruitment of the co-activators p68 RNA helicase and SRC1 to ERalpha, oestrogen response element (ERE) activity and Tam-R cell growth were similarly EGFR/MAPK-regulated. Chromatin immunoprecipitation (ChIP) studies revealed that in Tam-R cells the ERalpha assembled on the AR gene promoter and this was associated with elevated basal expression of AR mRNA. Furthermore, AR mRNA expression was under the regulation of the EGFR/MAPK and ERalpha signalling pathways. Neutralising antibodies to AR inhibited EGFR/ERK1/2 activity, reduced S118 ERalpha phosphorylation and reduced AR mRNA expression in TAM-R cells. These findings suggest that ERalpha function in Tam-R cells is maintained as a consequence of EGFR/MAPK-mediated phosphorylation at serine residue 118 resulting in the generation of a self-propogating autocrine growth-regulatory loop through the ERalpha-mediated production of AR.

Clinical development of fulvestrant ("Faslodex")

This paper outlines the development of fulvestrant, the first in a new class of antioestrogen agents with no agonist effects, to be used for the treatment of hormone receptor-positive advanced breast cancer in postmenopausal women. The role of the oestrogen receptor in breast cancer growth and development and the evolution of pharmacological strategies to manipulate it are also discussed. Preclinical and clinical evidence for the efficacy of fulvestrant are also reviewed, along with the tolerability profile of this agent in relation to other endocrine therapies. Further research will define the role of this exciting new agent in the endocrine treatment of breast cancer.

Recent advances in endocrine therapy for postmenopausal women with early breast cancer: implications for treatment and prevention

BACKGROUND

In the treatment of advanced breast cancer, third-generation aromatase inhibitors (AIs) have shown superior efficacy and tolerability compared with tamoxifen and megestrol acetate, the previous standard endocrine therapies in the first- and second-line settings, respectively. AIs are now being assessed in the adjuvant and prevention settings.

DESIGN

Literature review (PubMed search).

RESULTS

Tamoxifen is currently the only endocrine option available for adjuvant therapy and chemoprevention in postmenopausal women. However, results from the ATAC ('Arimidex', Tamoxifen, Alone or in Combination) trial have shown anastrozole to be more effective than tamoxifen as adjuvant therapy for postmenopausal women with hormone-responsive early breast cancer. Other third-generation AIs, including letrozole and exemestane, are also being investigated as adjuvant therapies. In the chemoprevention setting, tamoxifen is the only available endocrine option for women at high risk of breast cancer but, given that these are healthy subjects, is associated with an unacceptable rate of adverse events. Raloxifene is being further assessed in the STAR (Study of Tamoxifen and Raloxifene) trial, while anastrozole is being evaluated in the second IBIS-II (International Breast Intervention Study II).

CONCLUSIONS

AIs, in particular anastrozole, are set to change the way that early breast cancer is treated. Effective and better-tolerated endocrine alternatives for breast cancer prevention may become available in the future.

Endocrinology and hormone therapy in breast cancer: selective oestrogen receptor modulators and downregulators for breast cancer - have they lost their way?

Although tamoxifen has been an effective treatment for breast cancer, several novel anti-oestrogen compounds have been developed with a reduced agonist profile on breast and gynaecological tissues. These include selective oestrogen receptor modulators (SERMs; both 'tamoxifen-like' and 'fixed-ring' SERMs) and selective oestrogen receptor downregulators (SERDs), although none has been proved superior in efficacy to tamoxifen in various advanced breast cancer trials. Thus, many have questioned whether a need for SERMs in breast cancer still exists, although chemoprevention remains a possible niche setting. In contrast, SERDs may have useful efficacy following aromatase inhibitors because of their unique mechanism of action, and clinical trials to determine their optimal use or sequence are ongoing.

Role of Fulvestrant in Sequential Hormonal Therapy for Advanced, Hormone Receptor—Positive Breast Cancer in Postmenopausal Women

The introduction of antiendocrine agents with differing mechanisms of action now mandates the design of rational sequential hormonal regimens for breast cancer. The aromatase inhibitors (AIs), including the nonsteroidal compounds anastrozole and letrozole and the steroidal compound exemestane, are important alternatives or adjuncts to the antiestrogen agent tamoxifen in postmenopausal women with hormone receptor-positive breast cancer in the first-line management of advanced disease and in the adjuvant treatment of early-stage disease. These and other endocrine agents, including the newer estrogen receptor antagonist fulvestrant and also tamoxifen itself, have not been extensively evaluated within the context of hormonal sequencing. Based on a retrospective analysis of data from 3 phase III trials, patients treated with fulvestrant in the first- or second-line hormonal management of advanced breast cancer may derive further clinical benefit from subsequent treatment with an endocrine agent from another class. The need for prospective investigation of post-AI hormonal therapy is intensifying as a result of the increasing clinical use of the AIs. Sophisticated sequencing regimens designed to exploit different mechanisms of action have the potential to confer greater clinical benefit than the historical approach of selecting the agent with the next highest single-agent clinical activity upon disease progression.

Steroid hormone receptors as targets for the therapy of breast and prostate cancer--recent advances, mechanisms of resistance, and new approaches

Surgical ovariectomy and orchiectomy, first proposed over a century ago, are effective in breast and prostate cancer therapy, respectively. Later, the discovery of steroid hormones and their nuclear receptors led to the concept that inhibition of steroid receptor function by an antagonist prevents tumour growth. While the first anti-hormones, cyproteroneacetate (CPA) and tamoxifen were found accidentally, deeper understanding of nuclear receptors as transcription factors enabled more rational, structure-activity based drug discovery. Results from a drug-finding program on pure anti-estrogens will be reported. These new steroidal anti-estrogens are highly active, pure ER-antagonists that lead to an efficient degradation of the estrogen receptor alpha (ERalpha) protein without any agonistic activity. Data obtained in preclinical tumour models in mice and rats showed a high potency in growth inhibition of ERalpha-positive breast cancer. In parallel, by comparing three independently generated anti-estrogen-resistant breast cancer cell lines, it was our intention to gain insight into the mechanisms of endocrine resistance which will allow to define new approaches for the treatment of endocrine-resistant breast cancer. Candidate proteins potentially involved in mechanisms of anti-estrogen-resistant growth of breast cancer cell lines were analyzed. ERalpha and progesterone receptor (PR) expressions were lost on the protein level in all three anti-estrogen-resistant cell lines, whereas binding of epidermal growth factor (EGF) and protein expression of epidermal growth factor receptor (EGFR) were increased. Loss of ERalpha expression may be linked to the acquisition of anti-estrogen resistance and enhanced expression of the EGFR and of members of the S100 family of Ca2+-binding proteins may contribute to the outgrowth of resistant cells. Furthermore, we describe the pharmacological development of a novel, highly potent progesterone receptor antagonist. In rat mammary tumour models, treatment with the PR antagonist completely suppressed the growth of established tumours and prevented the development of breast tumours. Advanced prostate cancer is effectively treated by androgen ablation. However, this therapy becomes inefficient although the androgen receptor (AR) is still functionally expressed. One novel strategy for the treatment of advanced prostate cancer could be the selective inhibition of AR protein expression by anti-sense oligonucleotides or small interfering RNA (siRNA) molecules. Down-regulation of the human AR caused significant inhibition of LNCaP prostate cancer growth in vivo. Taken together, many promising alternatives for endocrine therapy of breast and prostate cancer are arising.

Selective oestrogen receptor modulators/new antioestrogens: a clinical perspective

Following tamoxifen, the first selective oestrogen receptor modulator (SERM), a number of other antioestrogens have been developed. The first-generation SERMs exhibit cross-resistance with tamoxifen and have agonist effects on the uterus. Toremifene has equal efficacy to tamoxifen and may be useful as a tamoxifen alternative. Efficacy results for droloxifene and idoxifene were disappointing and their clinical development ceased. Response rates for second-generation SERMs such as raloxifene and arzoxifene are also not high, although raloxifene shows promise in the chemoprevention of breast cancer. Paradoxically, high-dose oestrogens are proving to be effective breast cancer treatment with similar responses to tamoxifen in postmenopausal women with advanced disease, although these drugs are not well tolerated. Fulvestrant is a new type of oestrogen receptor (ER) antagonist with no agonist effects, which binds, blocks and degrades the ER. Fulvestrant produces high response rates compared with the SERMs, is not cross-resistant with SERMs or aromatase inhibitors (AIs) and is equally as effective as the AI anastrozole in the treatment of postmenopausal women with advanced breast cancer who have progressed after prior antioestrogen therapy. Pure antioestrogens such as the ER antagonist fulvestrant provide opportunities for therapeutic sequencing with tamoxifen and AIs and offer exciting possibilities for the future treatment of breast cancer.

ICI182,780 induces p21Waf1 gene transcription through releasing histone deacetylase 1 and estrogen receptor alpha from Sp1 sites to induce cell cycle arrest in MCF-7 breast cancer cell line

We used the estrogen-responsive MCF-7 breast cancer cell line as a relevant model to study the anti-proliferative effects of ICI182,780 and identified the negative cell cycle regulator p21Waf1 as a specific target of ICI182,780. Furthermore, silencing of the p21Waf1 expression by small interfering RNA overcame the G0/G1 cell cycle arrest induced by ICI182,780, suggesting that the induction of p21Waf1 expression has a direct role in mediating the ICI182,780-induced G0/G1 arrest. We further demonstrated that the induction of p21Waf1 by ICI182,780 is mediated at transcriptional and gene promoter levels through the proximal Sp1 sites located near the transcription start site. Co-immunoprecipitation, DNA "pull-down," and chromatin immunoprecipitation experiments together showed that in cycling cells, estrogen receptor alpha and histone deacetylase 1 (HDAC1) are recruited to the proximal Sp1 sites of the promoter to repress p21Waf1 expression. In the presence of ICI182,780, estrogen receptor alpha and HDACs are dissociated from Sp1, resulting in increased histone acetylation and de-repression of the p21Waf1 promoter and induction of p21Waf1 expression. The fact that p21Waf1 expression is normally repressed by HDAC activity in cycling cells is further demonstrated by the finding that p21Waf1 transcription can be induced by the silencing of HDACs with small interfering RNA or treatment with HDAC inhibitors.