Fulvestrant: pharmacokinetics and pharmacology

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.

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.

A systematic and prospectively validated approach for identifying synergistic drug combinations against malaria

Background

Nearly half of the world’s population (3.2 billion people) were at risk of malaria in 2015, and resistance to current therapies is a major concern. While the standard of care includes drug combinations, there is a pressing need to identify new combinations that can bypass current resistance mechanisms. In the work presented here, a combined transcriptional drug repositioning/discovery and machine learning approach is proposed.

Methods

The integrated approach utilizes gene expression data from patient-derived samples, in combination with large-scale anti-malarial combination screening data, to predict synergistic compound combinations for three Plasmodium falciparum strains (3D7, DD2 and HB3). Both single compounds and combinations predicted to be active were prospectively tested in experiment.

Results

One of the predicted single agents, apicidin, was active with the AC50 values of 74.9, 84.1 and 74.9 nM in 3D7, DD2 and HB3 P. falciparum strains while its maximal safe plasma concentration in human is 547.6 ± 136.6 nM. Apicidin at the safe dose of 500 nM kills on average 97% of the parasite. The synergy prediction algorithm exhibited overall precision and recall of 83.5 and 65.1% for mild-to-strong, 48.8 and 75.5% for moderate-to-strong and 12.0 and 62.7% for strong synergies. Some of the prospectively predicted combinations, such as tacrolimus-hydroxyzine and raloxifene-thioridazine, exhibited significant synergy across the three P. falciparum strains included in the study.

Conclusions

Systematic approaches can play an important role in accelerating discovering novel combinational therapies for malaria as it enables selecting novel synergistic compound pairs in a more informed and cost-effective manner.

Electronic supplementary material

The online version of this article (10.1186/s12936-018-2294-5) contains supplementary material, which is available to authorized users.

Effective combination therapies in preclinical endocrine resistant breast cancer models harboring ER mutations

Although endocrine therapy is successfully used to treat patients with estrogen receptor (ER) positive breast cancer, a substantial proportion of this population will relapse. Several mechanisms of acquired resistance have been described including activation of the mTOR pathway, increased activity of CDK4 and activating mutations in ER. Using a patient derived xenograft model harboring a common activating ER ligand binding domain mutation (D538G), we evaluated several combinatorial strategies using the selective estrogen receptor degrader (SERD) fulvestrant in combination with chromatin modifying agents, and CDK4/6 and mTOR inhibitors. In this model, fulvestrant binds WT and MT ER, reduces ER protein levels, and downregulated ER target gene expression. Addition of JQ1 or vorinostat to fulvestrant resulted in tumor regression (41% and 22% regression, respectively) though no efficacy was seen when either agent was given alone. Interestingly, although the CDK4/6 inhibitor palbociclib and mTOR inhibitor everolimus were efficacious as monotherapies, long-term delayed tumor growth was only observed when co-administered with fulvestrant. This observation was consistent with a greater inhibition of compensatory signaling when palbociclib and everolimus were co-dosed with fulvestrant. The addition of fulvestrant to JQ1, vorinostat, everolimus and palbociclib also significantly reduced lung metastatic burden as compared to monotherapy. The combination potential of fulvestrant with palbociclib or everolimus were confirmed in an MCF7 CRISPR model harboring the Y537S ER activating mutation. Taken together, these data suggest that fulvestrant may have an important role in the treatment of ER positive breast cancer with acquired ER mutations.

Local delivery of hormonal therapy with silastic tubing for prevention and treatment of breast cancer

Broad use of germline testing has identified an increasing number of women at risk for breast cancer with a need for effective chemoprevention. We report a novel method to selectively deliver various anti-estrogens at high drug levels to the breast tissue by implanting a device comprised of silastic tubing. Optimized tubing properties allow elution of otherwise poorly bioavailable anti-estrogens, such as fulvestrant, into mammary tissue in vitro and in vivo with levels sufficient to inhibit estrogen receptor activation and tumor cell proliferation. Implantable silastic tubing delivers fulvestrant selectively to mouse mammary fat tissue for one year with anti-tumor effects similar to those achieved with systemic fulvestrant exposure. Furthermore, local delivery of fulvestrant significantly decreases cell proliferation, as assessed by Ki67 expression, most effectively in tumor sections adjacent to tubing. This approach may thereby introduce a potential paradigm shift and offer a promising alternative to systemic therapy for prevention and early interception of breast cancer.

Estrogenic effects following larval exposure to the putative anti-estrogen, fulvestrant, in the fathead minnow (Pimephales promelas)

The objective of the present study was to investigate the consequences of early-life exposure to fulvestrant on estrogenic gene expression in fathead minnow larvae. To address this objective, fathead minnow larvae were exposed to fulvestrant (ICI 182,780) during the window of sexual differentiation between 0 to 30 days post-hatch (dph). The four treatment groups in this study included: filtered water controls (never exposed), solvent controls (ethanol 0.01%), and nominally low (0.10μg/L) and high (10.0μg/L) doses of fulvestrant. Following 30 d exposure to their respective treatment, larvae were transferred to filtered water aquaria and assessed for alterations in endocrine-responsive gene expression (i.e., RT-qPCR), body size and survival. The remaining fish depurated in filtered water until reaching sexual maturity (180dph) for assessment of persistent effects on sex characteristics, reproductive performance and sex ratio. Following the 30-d early life exposure, larvae showed upregulations of the endocrine-responsive genes ar, erβ and vtg in response to both low and high doses of fulvestrant, but showed no differences in survival or body mass. Upon reaching sexual maturity under depuration conditions, male minnows previously exposed to fulvestrant as larvae showed reductions in gonad mass along with the feminization of secondary sex characteristics with no observed effects in females. Exposure to fulvestrant had no effects on gonadal histology, reproductive performance or final sex ratio as adults. Results from this study demonstrate that aqueous exposure to fulvestrant is estrogenic in fathead minnow larvae and is capable of feminizing male fish as adults following early life exposure.

Controlling Sulfuryl-Transfer Biology

In humans, the cytosolic sulfotransferases (SULTs) catalyze regiospecific transfer of the sulfuryl moiety (-SO3) from 3'-phosphoadenosine 5'-phosphosulfate to thousands of metabolites, including numerous signaling small molecules, and thus regulates their activities and half-lives. Imbalances in the in vivo set points of these reactions leads to disease. Here, with the goal of controlling sulfonation in vivo, molecular ligand-recognition principles in the SULT and nuclear receptor families are integrated in creating a strategy that can prevent sulfonation of a compound without significantly altering its receptor affinity, or inhibiting SULTS. The strategy is validated by using it to control the sulfonation and estrogen receptor (ER) activating activity of raloxifene (a US Food and Drug Administration-approved selective estrogen receptor modulator) and its derivatives. Preventing sulfonation is shown to enhance ER-activation efficacy 10(4)-fold in studies using Ishikawa cells. The strategy offers the opportunity to control sulfuryl transfer on a compound-by-compound basis, to enhance the efficacy of sulfonated drugs, and to explore the biology of sulfuryl transfer with unprecedented precision.

Lead Optimization of Benzoxepin-Type Selective Estrogen Receptor (ER) Modulators and Downregulators with Subtype-Specific ERα and ERβ Activity

Estrogen receptor α (ERα) is an important target for the design of drugs such as tamoxifen (2a) and fulvestrant (5). Three series of ER-ligands based on the benzoxepin scaffold structure were synthesized: series I containing an acrylic acid, series II with an acrylamide, and series III with a saturated carboxylic acid substituent. These compounds were shown to be high affinity ligands for the ER with nanomolar IC₅₀ binding values. Series I acrylic acid ligands were generally ERα selective. In particular, compound 13e featuring a phenylpenta-2,4-dienoic acid substituent was shown to be antiproliferative and downregulated ERα and ERβ expression in MCF-7 breast cancer cells. Interestingly, from series III, the phenoxybutyric acid derivative compound 22 was not antiproliferative and selectively downregulated ERβ. A docking study of the benzoxepin ligands was undertaken. Compound 13e is a promising lead for development as a clinically relevant SERD, while compound 22 will be a useful experimental probe for helping to elucidate the role of ERβ in cancer cells.

Novel Pieces for the Emerging Picture of Sulfoximines in Drug Discovery: Synthesis and Evaluation of Sulfoximine Analogues of Marketed Drugs and Advanced Clinical Candidates

Sulfoximines have gained considerable recognition as an important structural motif in drug discovery of late. In particular, the clinical kinase inhibitors for the treatment of cancer, roniciclib (pan-CDK inhibitor), BAY 1143572 (P-TEFb inhibitor), and AZD 6738 (ATR inhibitor), have recently drawn considerable attention. Whilst the interest in this underrepresented functional group in drug discovery is clearly on the rise, there remains an incomplete understanding of the medicinal-chemistry-relevant properties of sulfoximines. Herein we report the synthesis and in vitro characterization of a variety of sulfoximine analogues of marketed drugs and advanced clinical candidates to gain a better understanding of this neglected functional group and its potential in drug discovery.

Tamoxifen and ICI 182,780 activate hypothalamic G protein-coupled estrogen receptor 1 to rapidly facilitate lordosis in female rats

In the female rat, sexual receptivity (lordosis) can be facilitated by sequential activation of estrogen receptor (ER) α and G protein-coupled estrogen receptor 1 (GPER) by estradiol. In the estradiol benzoate (EB) primed ovariectomized (OVX) rat, EB initially binds to ERα in the plasma membrane that complexes with and transactivates metabotropic glutamate receptor 1a to activate β-endorphin neurons in the arcuate nucleus of the hypothalamus (ARH) that project to the medial preoptic nucleus (MPN). This activates MPN μ-opioid receptors (MOP), inhibiting lordosis. Infusion of non-esterified 17β-estradiol into the ARH rapidly reduces MPN MOP activation and facilitates lordosis via GPER. Tamoxifen (TAM) and ICI 182,780 (ICI) are selective estrogen receptor modulators that activate GPER. Therefore, we tested the hypothesis that TAM and ICI rapidly facilitate lordosis via activation of GPER in the ARH. Our first experiment demonstrated that injection of TAM intraperitoneal, or ICI into the lateral ventricle, deactivated MPN MOP and facilitated lordosis in EB-primed rats. We then tested whether TAM and ICI were acting rapidly through a GPER dependent pathway in the ARH. In EB-primed rats, ARH infusion of either TAM or ICI facilitated lordosis and reduced MPN MOP activation within 30min compared to controls. These effects were blocked by pretreatment with the GPER antagonist, G15. Our findings demonstrate that TAM and ICI deactivate MPN MOP and facilitate lordosis in a GPER dependent manner. Thus, TAM and ICI may activate GPER in the CNS to produce estrogenic actions in neural circuits that modulate physiology and behavior.

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.

Identification of markers of cancer in urban sewage through the use of a suspect screening approach

The administration of anticancer drugs during chemotherapy treatments has increased considerably in recent years, and based on the growing incidence of cancer worldwide there is a foreseen increase in their use over the coming years. Many anticancer drugs are not removed by conventional wastewater treatment plants (WWTPs) and can therefore reach the aquatic environment and potentially threaten aquatic life. The objective of this work was to apply a suspect screening methodology to detect chemotherapy and radiotherapy drugs and their related compounds such metabolites and/or biomarkers in wastewater. The use of logical pre-determined criteria to refine the suspect list down to a relatively small number of relevant compounds greatly improved the efficiency of the analysis. Mass accuracy, isotopic patterns and predicted retention time were used to tentatively identify the suspects. Successful identification of cancer-related suspects included two antineoplastic hormones, two X-ray contrast agents and a pyrrolizidine alkaloid related to an herbal medicine. This is the first time that a suspect screening paradigm has been successfully applied to the identification of pharmaceuticals and biomarkers related to chemotherapy in wastewater.

Combining Fulvestrant with Low-Dose Capecitabine is Effective and Tolerable in Woman with Metastatic Breast Cancer

Although the use of endocrine therapy in combination with intravenous chemotherapy has not been standardized, the combination of fulvestrant and chemotherapy may be promising. A 62-year-old woman came to our hospital's outpatient clinic with extensive ascites. Approximately 10 years earlier, she had undergone mastectomy and sentinel lymph node biopsy. Pathologically invasive lobular carcinoma, with a maximum diameter of 28 mm, had been diagnosed in the left breast. The cancer had a histological grade of 2, was positive for estrogen receptor (95% or more positive cells), and was negative for both progesterone receptor (less than 1% positive cells) and human epidermal growth factor receptor 2. For 5 years the patient underwent adjuvant endocrine therapy with tamoxifen and then with anastrozole. Four years 2 months after adjuvant endocrine therapy had been completed, she felt abdominal distention, and her symptoms gradually worsened. A series of intensive examinations indicated that the invasive lobular carcinoma had metastasized to the peritoneum, pleura, uterus, and bone. Aromatase inhibitor was administered as a first-line therapy for the metastatic disease and was accompanied by denosumab injected every 28 days. For 2 months after the start of treatment with anastrozole, the ascites did not decrease and tumor markers increased. Because anastrozole had not been effective, fulvestrant (500 mg) and low-dose capecitabine (500 mg) were administered for the first 21 days of a 28-day cycle; this regimen had been shown by a phase 2 trial to be effective and tolerable in patients with metastatic breast cancer. The patient felt an improvement in abdominal distention, and the tumor markers decreased 2 weeks after the start of this combination therapy. By 10 months after the start of the combined therapy the ascites had decreased and pleural effusion had completely disappeared. The uterine wall became thinner, and the endometrial cavity became smaller. Tumor markers continued decreasing. No adverse events were observed. The combination of fulvestrant and low-dose capecitabine is promising because of its efficacy and tolerability for the treatment of patients with estrogen receptor-positive, human epidermal growth factor receptor 2-negative metastatic breast cancer.

AZD9496: An Oral Estrogen Receptor Inhibitor That Blocks the Growth of ER-Positive and ESR1-Mutant Breast Tumors in Preclinical Models

Fulvestrant is an estrogen receptor (ER) antagonist administered to breast cancer patients by monthly intramuscular injection. Given its present limitations of dosing and route of administration, a more flexible orally available compound has been sought to pursue the potential benefits of this drug in patients with advanced metastatic disease. Here we report the identification and characterization of AZD9496, a nonsteroidal small-molecule inhibitor of ERα, which is a potent and selective antagonist and downregulator of ERα in vitro and in vivo in ER-positive models of breast cancer. Significant tumor growth inhibition was observed as low as 0.5 mg/kg dose in the estrogen-dependent MCF-7 xenograft model, where this effect was accompanied by a dose-dependent decrease in PR protein levels, demonstrating potent antagonist activity. Combining AZD9496 with PI3K pathway and CDK4/6 inhibitors led to further growth-inhibitory effects compared with monotherapy alone. Tumor regressions were also seen in a long-term estrogen-deprived breast model, where significant downregulation of ERα protein was observed. AZD9496 bound and downregulated clinically relevant ESR1 mutants in vitro and inhibited tumor growth in an ESR1-mutant patient-derived xenograft model that included a D538G mutation. Collectively, the pharmacologic evidence showed that AZD9496 is an oral, nonsteroidal, selective estrogen receptor antagonist and downregulator in ER(+) breast cells that could provide meaningful benefit to ER(+) breast cancer patients. AZD9496 is currently being evaluated in a phase I clinical trial. Cancer Res; 76(11); 3307-18. ©2016 AACR.

Optimization of a Novel Binding Motif to (E)-3-(3,5-Difluoro-4-((1R,3R)-2-(2-fluoro-2-methylpropyl)-3-methyl-2,3,4,9-tetrahydro-1H-pyrido[3,4-b]indol-1-yl)phenyl)acrylic Acid (AZD9496), a Potent and Orally Bioavailable Selective Estrogen Receptor Downregulator and Antagonist

The discovery of an orally bioavailable selective estrogen receptor downregulator (SERD) with equivalent potency and preclinical pharmacology to the intramuscular SERD fulvestrant is described. A directed screen identified the 1-aryl-2,3,4,9-tetrahydro-1H-pyrido[3,4-b]indole motif as a novel, druglike ER ligand. Aided by crystal structures of novel ligands bound to an ER construct, medicinal chemistry iterations led to (E)-3-(3,5-difluoro-4-((1R,3R)-2-(2-fluoro-2-methylpropyl)-3-methyl-2,3,4,9-tetrahydro-1H-pyrido[3,4-b]indol-1-yl)phenyl)acrylic acid (30b, AZD9496), a clinical candidate with high oral bioavailability across preclinical species that is currently being evaluated in phase I clinical trials for the treatment of advanced estrogen receptor (ER) positive breast cancer.

A Screening Assay Cascade to Identify and Characterize Novel Selective Estrogen Receptor Downregulators (SERDs)

Here, we describe an approach to identify novel selective estrogen receptor downregulator (SERD) compounds with improved properties such as oral bioavailability and the potential of increased efficacy compared to currently marketed drug treatments. Previously, methodologies such as Western blotting and transient cell reporter assays have been used to identify and characterize SERD compounds, but such approaches can be limited due to low throughput and sensitivity, respectively. We have used an endogenous cell-imaging strategy that has both the throughput and sensitivity to support a large-scale hit-to-lead program to identify novel compounds. A screening cascade with a suite of assays has been developed to characterize compounds that modulate estrogen receptor α (ERα)-mediated signaling or downregulate ERα levels in cells. Initially, from a focused high-throughput screening, novel ERα binders were identified that could be modified chemically into ERα downregulators. Following this, cellular assays helped determine the mechanism of action of compounds to distinguish between on-target and off-target compounds and differentiate SERDs, selective estrogen receptor modulator (SERM) compounds, and agonist ERα ligands. Data are shown to exemplify the characterization of ERα-mediated signaling inhibitors using a selection of literature compounds and illustrate how this cascade has been used to drive the chemical design of novel SERD compounds.

3'-Phosphoadenosine 5'-phosphosulfate allosterically regulates sulfotransferase turnover

Human cytosolic sulfotransferases (SULTs) regulate the activities of thousands of small molecules—metabolites, drugs, and other xenobiotics—via the transfer of the sulfuryl moiety (-SO₃) from 3′-phosphoadenosine 5′-phosphosulfate (PAPS) to the hydroxyls and primary amines of acceptors. SULT1A1 is the most abundant SULT in liver and has the broadest substrate spectrum of any SULT. Here we present the discovery of a new form of SULT1A1 allosteric regulation that modulates the catalytic efficiency of the enzyme over a 130-fold dynamic range. The molecular basis of the regulation is explored in detail and is shown to be rooted in an energetic coupling between the active-site caps of adjacent subunits in the SULT1A1 dimer. The first nucleotide to bind causes closure of the cap to which it is bound and at the same time stabilizes the cap in the adjacent subunit in the open position. Binding of the second nucleotide causes both caps to open. Cap closure sterically controls active-site access of the nucleotide and acceptor; consequently, the structural changes in the cap that occur as a function of nucleotide occupancy lead to changes in the substrate affinities and turnover of the enzyme. PAPS levels in tissues from a variety of organs suggest that the catalytic efficiency of the enzyme varies across tissues over the full 130-fold range and that efficiency is greatest in those tissues that experience the greatest xenobiotic “load”.

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.

Paradigms of sulfotransferase catalysis: the mechanism of SULT2A1

Human cytosolic sulfotransferases (SULTs) regulate the activities of thousands of signaling small molecules via transfer of the sulfuryl moiety (-SO3) from 3'-phosphoadenosine 5'-phosphosulfate (PAPS) to the hydroxyls and primary amines of acceptors. Sulfonation controls the affinities of ligands for their targets, and thereby regulates numerous receptors, which, in turn, regulate complex cellular responses. Despite their biological and medical relevance, basic SULT mechanism issues remain unresolved. To settle these issues, and to create an in-depth model of SULT catalysis, the complete kinetic mechanism of a representative member of the human SULT family, SULT2A1, was determined. The mechanism is composed of eight enzyme forms that interconvert via 22 rate constants, each of which was determined independently. The result is a complete quantitative description of the mechanism that accurately predicts complex enzymatic behavior. This is the first description of a SULT mechanism at this resolution, and it reveals numerous principles of SULT catalysis and resolves previously ambiguous issues. The structures and catalytic behaviors SULTs are highly conserved; hence, the mechanism presented here should prove paradigmatic for the family.

Role of G protein-coupled estrogen receptor 1 in modulating transforming growth factor-β stimulated mesangial cell extracellular matrix synthesis and migration

Estrogen has been demonstrated to exert beneficial effects on kidney; however, the role of G protein-coupled estrogen receptor 1 (GPER) is still uncertain. In the present study, we investigated the effect of 17β-estradiol and GPER agonist Fulvestrant on extracellular matrix production under transforming growth factor-β1 (TGF-β1) stimulation in human and rat mesangial cells. As a result, 17β-estradiol and Fulvestrant inhibit TGF-β1-induced type IV collagen and fibronectin expression in a dose-dependent manner, by suppressing acute Smad2/3 phosphorylation and Smad4 complex formation. Furthermore, estrogen and Fulvestrant also down-regulate Smad signaling by promoting ubiquitin/proteasome-dependent Smad2 degradation. These effects could be abrogated by receptor antagonist G-15 or GPER gene knockdown. GPER is also required for estrogen and Fulvestrant to regulate mesangial cell migration in response to TGF-β1. To conclude, GPER is crucial in modulating glomerular mesangial cell function including extracellular matrix production and migration.

Nuclear receptors and their selective pharmacologic modulators

Nuclear receptors are ligand-activated transcription factors and include the receptors for steroid hormones, lipophilic vitamins, sterols, and bile acids. These receptors serve as targets for development of myriad drugs that target a range of disorders. Classically defined ligands that bind to the ligand-binding domain of nuclear receptors, whether they are endogenous or synthetic, either activate receptor activity (agonists) or block activation (antagonists) and due to the ability to alter activity of the receptors are often termed receptor "modulators." The complex pharmacology of nuclear receptors has provided a class of ligands distinct from these simple modulators where ligands display agonist/partial agonist/antagonist function in a tissue or gene selective manner. This class of ligands is defined as selective modulators. Here, we review the development and pharmacology of a range of selective nuclear receptor modulators.

The gate that governs sulfotransferase selectivity

Human cytosolic sulfotransferases (SULTs) transfer the sulfuryl moiety (-SO(3)) from activated sulfate [3'-phosphoadenosine 5'-phosphosulfate (PAPS)] to the hydroxyls and primary amines of numerous metabolites, drugs, and xenobiotics. Receipt of the sulfuryl group often radically alters acceptor-target interactions. How these enzymes select particular substrates from the hundreds of candidates in a complex cytosol remains an important question. Recent work reveals PAPS binding causes SULT2A1 to undergo an isomerization that controls selectivity by constricting the opening through which acceptors must pass to enter the active site. The enzyme maintains an affinity for large substrates by isomerizing between the open and closed states with nucleotide bound. Here, the molecular basis of the nucleotide-induced closure is explored in equilibrium and nonequilibrium molecular dynamics simulations. The simulations predict that the active-site "cap," which covers both the nucleotide and acceptor binding sites, opens and closes in response to nucleotide. The cap subdivides into nucleotide and acceptor halves whose motions, while coupled, exhibit an independence that can explain the isomerization. In silico weakening of electrostatic interactions between the cap and base of the active site causes the acceptor half of the cap to open and close while the nucleotide lid remains shut. Simulations predict that SULT1A1, the most abundant SULT in human liver, will utilize a similar selection mechanism. This prediction is tested using fulvestrant, an anti-estrogen too large to pass through the closed pore, and estradiol, which is not restricted by closure. Equilibrium and pre-steady-state binding studies confirm that SULT1A1 undergoes a nucleotide-induced isomerzation that controls substrate selection.

Sulfation of fulvestrant by human liver cytosols and recombinant SULT1A1 and SULT1E1

Fulvestrant (Faslodex™) is a pure antiestrogen that is approved to treat hormone receptor-positive metastatic breast cancer in postmenopausal women. Previous studies have demonstrated that fulvestrant metabolism in humans involves cytochromes P450 and UDP-glucuronosyltransferases (UGTs). To date, fulvestrant sulfation has not been characterized. This study examined fulvestrant sulfation with nine recombinant sulfotransferases and found that only SULT1A1 and SULT1E1 displayed catalytic activity toward this substrate, with K(m) of 4.2 ± 0.99 and 0.2 ± 0.16 μM, respectively. In vitro assays of 104 human liver cytosols revealed marked individual variability that was highly correlated with β-naphthol sulfation (SULT1A1 diagnostic substrate; r = 0.98, P < 0.0001), but not with 17β-estradiol sulfation (SULT1E1 diagnostic substrate; r = 0.16, P = 0.10). Fulvestrant sulfation was correlated with both SULT1A1*1/2 genotype (P value = 0.023) and copy number (P < 0.0001). These studies suggest that factors influencing SULT1A1/1E1 tissue expression and/or enzymatic activity could influence the efficacy of fulvestrant therapy.

Hormonal modulation in the treatment of breast cancer

This article explores the history of endocrine therapy for the treatment of breast cancer, the clinical evidence behind the current standards of care, and controversies that may change these standards in the future.

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.

The breast cancer continuum in hormone-receptor-positive breast cancer in postmenopausal women: evolving management options focusing on aromatase inhibitors

There are now a number of highly effective options for the treatment of hormone-receptor-positive breast cancer. Although tamoxifen was the standard hormonal treatment for many years, we now have another option for postmenopausal women: the third-generation aromatase inhibitors (AIs) anastrozole, exemestane and letrozole. A number of trials have investigated the use of third-generation AIs compared with tamoxifen throughout the continuum of treatment settings for postmenopausal women with breast cancer. In the neoadjuvant setting, letrozole, given for 4 months, resulted in better overall clinical response and breast-conserving surgery rates than tamoxifen. The Immediate Preoperative Anastrozole Tamoxifen or Combined with Tamoxifen trial gave anastrozole for 3 months with no difference in clinical response but significantly improved breast-conserving surgery rates. Compared with tamoxifen, anastrozole and letrozole significantly improved disease-free survival as early adjuvant treatment for hormone-receptor-positive disease. Switching to anastrozole or exemestane after 2 to 3 years of adjuvant tamoxifen for a total of 5 years of therapy was also more effective than continued tamoxifen. All three agents are approved in the early adjuvant or switching setting in the USA. Letrozole following 5 years of tamoxifen as extended adjuvant treatment improved disease-free survival and, in the node-positive subgroup, overall survival when compared with placebo. Anastrozole and letrozole are both approved for the first-line treatment of hormone-sensitive advanced breast cancer in postmenopausal women; letrozole showed an improved response rate compared with tamoxifen. Anastrozole, letrozole and exemestane are all indicated for the second-line treatment of advanced breast cancer. In summary, third-generation AIs have been shown to have superior efficacy over tamoxifen in the metastatic, neoadjuvant and adjuvant settings and to improve outcome as extended adjuvant therapy following 5 years of tamoxifen. Ongoing studies will further define the role of sequential adjuvant treatment. Appropriate duration of treatment is another important area of investigation. This review will cover hormonal therapy for postmenopausal women with breast cancer and will not address the treatment of premenopausal women.

Hormonal therapy for advanced breast cancer

Hormonal therapy for advanced breast cancer has evolved significantly in the more than 100 years since the first publications documenting the effect of ovarian ablation on advanced breast cancer in premenopausal women. Since that time, not only have we developed the methods to measure estrogen and progesterone receptors in cancer cells, but more recently we have understood that expression of these receptors determines response to hormone therapy. The availability of more selective antiestrogen therapies has changed and significantly improved the treatment options for women who have advanced hormone-responsive breast cancer. Current research is focusing on reversing resistance to hormone therapy with the addition of targeted biologic agents to standard hormonal treatment.

Inactivation of the Pure Antiestrogen Fulvestrant and Other Synthetic Estrogen Molecules by UDP-Glucuronosyltransferase 1A Enzymes Expressed in Breast Tissue

Fulvestrant (Faslodex) is administered by intramuscular injection and is converted into ketone, sulfate, sulfone and glucuronide metabolites. Glucuronidation, catalyzed by 18 members of the UDP-glucuronosyltransferase (UGT) enzyme family, plays a major role in the elimination of natural estrogens. The present study was aimed at identifying and characterizing human UGT enzymes involved in the glucuronidation of this antiestrogen as well as other synthetic estrogen derivatives with aliphatic chains on the E2 molecule. In contrast to E2, which is conjugated by UGT1A1, -1A3, -1A8, -1A10, and -2B7, fulvestrant is glucuronidated by UGT1A1, -1A3, -1A4, and -1A8. The four UGT1A-fulvestrant conjugating enzymes glucuronidate this substrate at position 3, whereas only UGT1A8 also produces fulvestrant-17-glucuronide. For E2, only UGT1A3 and UGT2B7 are capable to conjugate at 17-hydroxyposition. These observations indicate that addition of an aliphatic chain to the E2 molecule modifies the specificity of the UGT enzymes toward the C18 molecules. To further investigate the specificity of these enzymes, a series of E2 derivatives with aliphatic or phenyl chains at position 2, 7alpha, and 11beta was also tested for its conjugation with human UGT enzymes. It was observed that, in addition to UGT1A3, UGT1A1 and UGT1A8 also played important roles for the glucuronidation of these compounds. This suggests that the basic structure of E2 is one of the major determinants for the glucuronidation catalyzed by this group of enzymes. Considering the high level of UGT1A3 and -1A4 expression in the gastrointestinal tract and mammary gland, our results suggest that fulvestrant can be inactivated both in intestine and in its target tissue.

The History and Mechanism of Action of Fulvestrant

Fulvestrant is a pure antiestrogen that emerged from a systematic medicinal chemistry strategy of modification of long-chain alkyl substitutes in the 7a-position of estradiol. Fulvestrant has no uterotrophic effects on the immature or ovariectomized rat and blocks the agonistic effects of estradiol and tamoxifen in a dose-dependent manner. In in vivo and in vitro breast cancer models, fulvestrant has anticancer activity at least as good as tamoxifen and is superior to tamoxifen in some models. Fulvestrant requires intramuscular administration in a proprietary formulation of castor oil and alcohols. When fulvestrant binds to estrogen receptor monomers it inhibits receptor dimerization, activating function 1 (AF1) and AF2 are rendered inactive, translocation of receptor to the nucleus is reduced, and degradation of the estrogen receptor is accelerated. This results in pure antiestrogenic effects. There is substantial preclinical evidence that the nonsteroidal hormone-dependent mechanisms of estrogen receptor activation and regulation via growth factor receptors and their signal transduction pathways are important in the development of breast cancer hormonal resistance. Methods of exploiting the interactions between these nonsteroidal hormone-dependent mechanisms of resistance and hormonal agents such as fulvestrant are an active area for drug development and clinical investigation.

A proteomic microarray approach for exploring ligand-initiated nuclear hormone receptor pharmacology, receptor selectivity, and heterodimer functionality

Nuclear hormone receptors (NHRs) are major regulators of development and homeostasis in multiple organ systems. These proteins are ligand-modulated transcription factors that regulate gene expression in response to changes in circulating levels of their cognate hormones or hormone analogs. When NHRs bind ligands, they adopt distinct conformations that enable or disable the binding of coregulator proteins in a manner that reflects the agonist versus antagonist character of the ligand. Using the estrogen receptor ligand binding domain as a representative member of the NHR family, we show the development of functional protein microarrays and use them to explore coactivator recruitment and NHR homo- and heterodimer functionality. These NHR protein microarrays can be fabricated in either a forward mode (coactivator recruited to printed NHR) or a reversed mode (NHR recruited to printed coactivator). From these microarrays, we can predict the potency and pharmacological character of various NHR ligands through the nature of their coactivator recruitment. Additionally different coactivator proteins can be functionally classified and their affinity for NHRs can be quantified. NHR-selective antagonist ligands and small molecule coactivator mimics disrupt the coactivator-NHR complex. This novel proteomic approach was also used to assess coactivator recruitment to explore heterodimer functionality. Heterodimers of the estrogen receptor were found only to recruit coactivators when both monomers are bound with agonist ligands, an observation that provides an insight into the complex biology of hormones that act on tissues containing both NHR subtypes. We can extend this NHR proteomic approach to the analysis of multidomain full-length NHR constructs and can concurrently monitor the activation state of different classes of NHRs with a mixture of endogenous or synthetic ligands of varying NHR selectivity and pharmacology.