Selective Estrogen Receptor Downregulator Fulvestrant Research Articles

Abstract OT2-18-01: Harnessing olaparib, palbociclib, and endocrine therapy (HOPE): Phase I/II trial of olaparib, palbociclib and fulvestrant in patients with BRCA1/2-associated, hormone receptor-positive, HER2-negative metastatic breast cancer

Abstract Background: Hormone receptor-positive (HR+) metastatic breast cancer (MBC) that occurs in the context of a germline or somatic BRCA1 or BRCA2 (BRCA1/2) mutation has dual drivers: hormone receptor signaling and homologous recombination deficiency. The PALOMA-3 trial studied the selective estrogen receptor downregulator fulvestrant and the CDK4 and CDK6 inhibitor (CDK4/6i) palbociclib in patients with HR+ MBC. Compared with fulvestrant and placebo, combination fulvestrant and palbociclib demonstrated significant and clinically meaningful improvements in progression-free survival (PFS) and overall survival. The OlympiAD trial examined patients with HER2-negative MBC and a germline BRCA1/2 mutation who were treated with the poly(ADP-ribose) polymerase inhibitor (PARPi) olaparib or non-platinum chemotherapy. Olaparib resulted in improved PFS, double the response rate, and a lower risk of disease progression or death compared to chemotherapy. Despite these remarkable therapeutic advances, resistance to treatment inevitably develops and contributes to mortality in patients with MBC. Targeting dual therapeutic drivers concurrently may delay or circumvent resistance; however, olaparib and palbociclib have overlapping hematologic toxicity and the safety of combined olaparib, palbociclib and fulvestrant has not been determined. Methods: HOPE (NCT03685331) is a phase I/II clinical trial to evaluate the safety and efficacy of olaparib, palbociclib and fulvestrant in patients with HR+ MBC and a pathogenic or likely pathogenic germline or somatic variant in BRCA1/2. Eligible patients are biological males, postmenopausal females, or premenopausal females on ovarian suppression who have an ECOG performance status 0-1, measureable/evaluable breast cancer, any/no prior endocrine therapy, and 0-2 prior lines of chemotherapy for MBC. Prior platinum chemotherapy is allowed for curative intent treatment if completed at least 12 months prior to diagnosis of metastatic disease, or for MBC if there was no progression during therapy. Prior PARPi and prior CDK4/6i are permitted without restriction during phase I, and are permitted during phase II provided there was no progression on these therapies. Treatment (28-day cycles) consists of: olaparib 300mg by mouth twice daily continuously; fulvestrant 500mg intramuscularly on day 1 of each cycle and day 15 of the first cycle; and oral palbociclib once daily on days 1-21. For phase I, palbociclib dose is based on dosing cohort. Dose Levels (DL) are: DL 0 (starting level), 75mg; DL 1, 100mg; DL 2, 125mg. Phase I treatment begins with a 28-day safety run-in of fulvestrant and olaparib alone. For phase II, palbociclib will be dosed at MTD. Subjects will have archival tissue collection or fresh biopsy at baseline as well as research blood samples for cfDNA analysis at baseline, at progression, and at all scan timepoints (every 3 cycles). The phase I primary endpoint is determination of MTD. A 3+3 dose escalation design will be utilized with a 30% rate of dose limiting toxicity (DLT) deemed acceptable, and 6 patients treated at a dose for it to be declared MTD. This schema yields a minimum of 2 and a maximum of 18 patients on the phase I. For the phase II trial, the primary endpoint is PFS estimated using Kaplan-Meier methods and secondary efficacy endpoints are objective response rate and 24-week clinical benefit rate. The phase II trial will evaluate 54 subjects to provide 80% power to detect an increase in PFS from 7 months with olaparib monotherapy to 10 months. Exploratory objectives include examination of baseline tissue for PARPi predictive biomarkers and measures of tumor immunogenicity as well as serial serum evaluation for reversion mutations. Enrollment has begun. Citation Format: Alexandra Torres, Carey Kokkonen, Mary Oladeji, Kurt D'Andrea, Rosemarie Mick, Vivek Narayan, Michael Mallamaci, Gayle Ewing, Hayley Knollman, Nadine M. Tung, Mark Robson, Katherine L. Nathanson, Susan Domchek, Payal D. Shah. Harnessing olaparib, palbociclib, and endocrine therapy (HOPE): Phase I/II trial of olaparib, palbociclib and fulvestrant in patients with BRCA1/2-associated, hormone receptor-positive, HER2-negative metastatic breast cancer [abstract]. In: Proceedings of the 2021 San Antonio Breast Cancer Symposium; 2021 Dec 7-10; San Antonio, TX. Philadelphia (PA): AACR; Cancer Res 2022;82(4 Suppl):Abstract nr OT2-18-01.

Abstract TMIM-079: INHIBITION OF TUMOR MICROENVIRONMENT CYTOKINE SIGNALING SENSITIZES OVARIAN CANCER CELLS TO ANTIESTROGEN THERAPY

Abstract OBJECTIVE: Ovarian cancer tumor cell estrogen receptor (ER) expression alone is a poor predictor of response to antiestrogen therapy and some patients with ER-negative cancers still demonstrate clinical response to therapy, suggesting modifiers of response that can potentially be targeted to improve outcomes. We have demonstrated that IL6 and LIF cytokine signaling from carcinoma-associated mesenchymal stem cells (CA-MSC) in the tumor microenvironment promotes tumorigenesis and hypothesize that this cytokine signaling also engages in crosstalk with estrogen signaling pathways. Furthermore, we propose that blocking both cytokine and estrogen signaling will improve anti-cancer effects. METHODS: High grade serous ovarian cancer (HGSOC) cell lines were treated with CA-MSC conditioned media or recombinant cytokines IL6 and/or LIF. Ovarian cancer cells were then assessed for (i) activation of estrogen response element (ERE)-driven luciferase reporter constructs (SABiosciences), (ii) changes in levels of reported ER target genes by qRT-PCR, and (iii) ER-alpha expression levels by immunoblotting. ER-alpha expression in CA-MSC was assessed by immunoblotting. We then assessed the effect of treating cells with ruxolitinib, the FDA-approved inhibitor of the Janus-associated kinase (JAK) protein that is downstream of IL6/LIF signaling. Tumor cells in the absence or presence of CA-MSC conditioned media were treated with ruxolitinib without or with anti-estrogen therapy. Anti-estrogens studied include the selective estrogen receptor modulator tamoxifen, the selective estrogen receptor downregulator fulvestrant, and the aromatase inhibitor letrozole. Following drug treatment, cell viability was assessed with the MTT assay and colony forming assays, and signaling cascade protein expression determined by immunoblotting. Synergy was calculated by the Chou-Talalay method using CompuSyn software. RESULTS: IL6 and LIF induce ERE reporter construct activation in HGSOC cell lines to a similar extent as control estradiol treatment and increase the expression of known ER target genes. Additionally, IL6 and LIF increase ER-alpha levels in HGSOC cell lines as detected by immunoblotting. The treatment of HGSOC cells with both ruxolitinib and antiestrogen therapy results in a synergistic decrease in cell viability. Variable effects are noted depending on the specific antiestrogen used and the cell line studied, indicating subtle mechanistic differences between cell lines. CA-MSC express ER-alpha, suggesting antiestrogens may exert their effects at least in part through changes in stromal signaling. Further studies are underway, including characterization of effects in both primary tumor cells and animal models. CONCLUSIONS: IL6 and LIF signaling from the tumor microenvironment promotes ovarian cancer cell estrogen signaling. The combination of inhibiting IL6/LIF signaling with ruxolitinib and antiestrogen therapy results in a synergistic decrease in ovarian cancer tumor cell viability. Given the clinical tolerability of antiestrogen therapy with a low side effect profile, strategies such as ruxolitinib treatment to sensitize tumor cells to antiestrogens are an exciting area that warrant further study. Citation Format: Lijun Tan, Victoria Prince, Jake Erba, Karen McLean. INHIBITION OF TUMOR MICROENVIRONMENT CYTOKINE SIGNALING SENSITIZES OVARIAN CANCER CELLS TO ANTIESTROGEN THERAPY [abstract]. In: Proceedings of the 12th Biennial Ovarian Cancer Research Symposium; Sep 13-15, 2018; Seattle, WA. Philadelphia (PA): AACR; Clin Cancer Res 2019;25(22 Suppl):Abstract nr TMIM-079.

Overall Survival with Fulvestrant plus Anastrozole in Metastatic Breast Cancer

BackgroundWe previously reported prolonged progression-free survival and marginally prolonged overall survival among postmenopausal patients with hormone receptor–positive metastatic breast cancer who had been randomly assigned to receive the aromatase inhibitor anastrozole plus the selective estrogen-receptor down-regulator fulvestrant, as compared with anastrozole alone, as first-line therapy. We now report final survival outcomes.MethodsWe randomly assigned patients to receive either anastrozole or fulvestrant plus anastrozole. Randomization was stratified according to adjuvant tamoxifen use. Analysis of survival was performed by means of two-sided stratified log-rank tests and Cox regression. Efficacy and safety were compared between the two groups, both overall and in subgroups.ResultsOf 707 patients who had undergone randomization, 694 had data available for analysis. The combination-therapy group had 247 deaths among 349 women (71%) and a median overall survival of 49.8 months, as compared with 261 deaths among 345 women (76%) and a median overall survival of 42.0 months in the anastrozole-alone group, a significant difference (hazard ratio for death, 0.82; 95% confidence interval [CI], 0.69 to 0.98; P=0.03 by the log-rank test). In a subgroup analysis of the two strata, overall survival among women who had not received tamoxifen previously was longer with the combination therapy than with anastrozole alone (median, 52.2 months and 40.3 months, respectively; hazard ratio, 0.73; 95% CI, 0.58 to 0.92); among women who had received tamoxifen previously, overall survival was similar in the two groups (median, 48.2 months and 43.5 months, respectively; hazard ratio, 0.97; 95% CI, 0.74 to 1.27) (P=0.09 for interaction). The incidence of long-term toxic effects of grade 3 to 5 was similar in the two groups. Approximately 45% of the patients in the anastrozole-alone group crossed over to receive fulvestrant.ConclusionsThe addition of fulvestrant to anastrozole was associated with increased long-term survival as compared with anastrozole alone, despite substantial crossover to fulvestrant after progression during therapy with anastrozole alone. The results suggest that the benefit was particularly notable in patients without previous exposure to adjuvant endocrine therapy. (Funded by the National Cancer Institute and AstraZeneca; ClinicalTrials.gov number, NCT00075764.)

Intrinsic apoptotic pathway activation increases response to anti-estrogens in luminal breast cancers

Estrogen receptor-α positive (ERα+) breast cancer accounts for approximately 70–80% of the nearly 25,0000 new cases of breast cancer diagnosed in the US each year. Endocrine-targeted therapies (those that block ERα activity) serve as the first line of treatment in most cases. Despite the proven benefit of endocrine therapies, however, ERα+ breast tumors can develop resistance to endocrine therapy, causing disease progression or relapse, particularly in the metastatic setting. Anti-apoptotic Bcl-2 family proteins enhance breast tumor cell survival, often promoting resistance to targeted therapies, including endocrine therapies. Herein, we investigated whether blockade of anti-apoptotic Bcl-2 family proteins could sensitize luminal breast cancers to anti-estrogen treatment. We used long-term estrogen deprivation (LTED) of human ERα+ breast cancer cell lines, an established model of sustained treatment with and acquired resistance to aromatase inhibitors (AIs), in combination with Bcl-2/Bcl-xL inhibition (ABT-263), finding that ABT-263 induced only limited tumor cell killing in LTED-selected cells in culture and in vivo. Interestingly, expression and activity of the Bcl-2-related factor Mcl-1 was increased in LTED cells. Genetic Mcl-1 ablation induced apoptosis in LTED-selected cells, and potently increased their sensitivity to ABT-263. Increased expression and activity of Mcl-1 was similarly seen in clinical breast tumor specimens treated with AI + the selective estrogen receptor downregulator fulvestrant. Delivery of Mcl-1 siRNA loaded into polymeric nanoparticles (MCL1 si-NPs) decreased Mcl-1 expression in LTED-selected and fulvestrant-treated cells, increasing tumor cell death and blocking tumor cell growth. These findings suggest that Mcl-1 upregulation in response to anti-estrogen treatment enhances tumor cell survival, decreasing response to therapeutic treatments. Therefore, strategies blocking Mcl-1 expression or activity used in combination with endocrine therapies would enhance tumor cell death.

Abstract PL1: Management of metastatic ER+ breast cancer

Abstract Estrogen receptor positive (ER+) metastatic breast cancer (MBC) is the most common form of the disease, and its prognosis and outcome depends on a variety of clinical factors relating to disease free interval (DFI), response to prior adjuvant therapy, distribution of metastatic disease, and the intrinsic biological subtype of the ER+ tumor. The goals of treatment are to provide effective anti-cancer therapy that will control cancer growth, whilst at the same time maintaining quality of life. International guidelines recommend that for postmenopausal women with ER+ MBC endocrine therapy should be the 1st-line approach for the majority of patients, unless patients have a symptomatic visceral crisis that requires cytotoxic chemotherapy. Aromatase inhibitors (AIs) have been the standard of care with a median progression-free survival (PFS) of 10-13 months reported in previous studies. Recent 1st-line trials aimed at improving upon AIs have investigated either the selective estrogen receptor downregulator (SERD) fulvestrant, or targeted combination of AIs with cyclin dependent kinase (CDK 4/6) inhibitors. While some trials have seen benefit for fulvestrant over AIs in subgroups of endocrine therapy naïve patients (SWOG-O226) or those with non-visceral metastases (FALCON), significant improvement across all subgroups have been reported in two separate phase III trials for both palbociclib (PALOMA-2) and ribociclib (MONALEESA-2) when combined with the AI letrozole. Although it would appear that both endocrine-sensitive and endocrine-resistant disease benefit from combined AI + CDK 4/6 inhibitor therapy, one of the key issues in routine clinical practice is whether all 1st line patients require this approach. In the 2nd-line setting following progression on non-steroidal AIs, further endocrine monotherapy with either the steroidal aromatase inactivator exemestane or the SERD fulvestrant will yield a median PFS of only 3-4 months (EFECT, SOFEA). Much laboratory and translational research has explored mechanisms of resistance to AIs, and trials have since shown that median PFS can be improved to 9-11 months using targeted combinations of either exemestane with the mTOR antagonist everolimus (BOLERO-2), or fulvestrant together with the CDK 4/6 inhibitor palbociclib (PALOMA-3). Studies in endocrine resistance have looked at the impact of acquired key mutations detected in circulating tumour DNA (ctDNA) (ie. ESR1 or PIK3CA) on endocrine responsiveness, and how these evolve during therapy. Likewise, other signalling pathways have been implicated in causing endocrine resistance, and further combination trials are ongoing. While rapid and real progress has been made in ER+ MBC, integrating these novel, well tolerated, targeted combinations into the treatment algorithm of ER+ MBC is a current challenge. In particular, it will be crucial to understand the optimal sequence of endocrine based treatments that will keep patients free of disease progression for as long as possible with a good quality of life. Citation Format: Johnston RD. Management of metastatic ER+ breast cancer [abstract]. In: Proceedings of the 2016 San Antonio Breast Cancer Symposium; 2016 Dec 6-10; San Antonio, TX. Philadelphia (PA): AACR; Cancer Res 2017;77(4 Suppl):Abstract nr PL1.

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.

Effect of tamoxifen and fulvestrant long-term treatments on ROS production and (pro/anti)-oxidant enzymes mRNA levels in a MCF-7-derived breast cancer cell line.

Reactive oxygen species (ROS) are key players in the apoptotic effects induced by short-term tamoxifen treatment of breast cancer cells, but also in acquired resistance following long-term treatment. Whereas the use of the selective estrogen receptor down-regulator fulvestrant is promising, especially in patients who develop tamoxifen resistance, only few studies addressed its implication in the modulation of cellular redox status. The regulation of (pro/anti)-oxidant players were first investigated at the mRNA level in a MCF-7-derived cell line after short-term (24h) estradiol treatment. Long-term anti-estrogen treated MCF-7 derived cell lines were also developed: 3months of 4-hydroxytamoxifen alone (MCF7L-OHTLT) or followed by 3months of fulvestrant (MCF7L-ICILT). Growth properties, hormone sensitivity, receptor content, ROS production and relative mRNA expression of pro or antioxidant enzymes were evaluated in these long-term treated cell lines. Short-term estradiol treatment showed a hormone sensitivity of Nox2, GPx1, GPx2 and SOD1 mRNA levels. The long-term fulvestrant treatment (3months) of MCF7L-OHTLT led to a reduced level of ROS production accompanied with a drastic drop of the accessory protein p22(phox) mRNA. This ROS reduction, although not clearly related to antioxidant enzymes level, seems to be involved in fulvestrant sensitivity of long-term anti-estrogen treated cells, as suggested by the effects of antiradical tempol treatment. When compared to long-term 4-hydroxytamoxifen-treated breast cancer cells, addition of fulvestrant treatment was able to diminish ROS production and p22(phox) mRNA level, and made cells more sensitive to growth inhibition induced by tempol. These effects may be a valuable asset of the fulvestrant treatment.

Polyendocrine Treatment in Estrogen Receptor–Positive Breast Cancer: A “FACT” Yet to Be Proven

The estrogen receptor (ER) was one of the first molecular targets identified for cancer treatment. Today, endocrine agents that target ER are well-established standards of care for patients with breast cancer. These drugs have different mechanisms of action and can either directly alter ER function or deprive ER of its ligand. The selective ER downregulator fulvestrant is a pure antiestrogen that, on binding ER, induces a conformational change that disrupts ER transcriptional activity and accelerates ER degradation. These features are unique to fulvestrant. Other antiestrogens, such as tamoxifen or raloxifene, retain some estrogenic activity and do not reduce ER levels. 1 The aromatase inhibitors anastrozole, letrozole, and exemestane reduce levels of circulating estrogen in postmenopausal women by inhibiting the peripheral conversion of androgens to estradiol. In the premenopausal setting, a reduction of circulating estrogen levels is achieved via luteinizing hormone-releasing hormone (LHRH) agonists or other means of ovarian ablation. Given the diverse mechanisms of action of endocrine agents, rational drug combinations might improve the effectiveness of endocrine therapy. Some preclinical data support polyendocrine treatment strategies. Before reviewing the results of the preclinical studies that tested this approach, we should consider the preclinical models that were used and some of their potential limitations. Early in vivo studies with fulvestrant were undertaken in a xenograft model of MCF7, the prototypic ER-positive breast cancer cell line. In this model, tumor cells are injected subcutaneously into the mouse, and tumors are initially allowed to develop in an estrogenic environment that is provided by a subcutaneous estrogen pellet. After tumor formation, the estrogen pellet is removed and endocrine treatment is initiated. In this low-estrogen environment, fulvestrant has a more potent antitumor effect compared with estrogen deprivation (that mimics aromatase inhibitor treatment in this model) or tamoxifen. However, fulvestrant-resistant tumors eventually develop that are generally insensitive to subsequent treatment with tamoxifen or combinations of tamoxifen and fulvestrant. 2 In the same model, fulvestrant does not have a significant antitumor effect when mice continue to receive estrogen 3 ; this finding is consistent with data showing that estradiol can displace and therefore reactivate fulvestrant-bound ER. 4 These data suggest that fulvestrant might be more active in lowestrogen conditions. Subsequent studies that investigated the combination of fulvestrant with either tamoxifen or aromatase inhibitors were performed using MCF7 cells that were genetically modified to express high levels of aromatase (MCF7 Ca). MCF7 Ca cells are grown in ovariectomized mice that are supplemented with a source of androgens to allow for autocrine estrogen synthesis. Using this model, the combination of anastrozole plus fulvestrant showed equivalence in terms of tumor growth inhibition compared with anastrozole or fulvestrant alone. Similarly, no differences were observed between the combinations of fulvestrant plus tamoxifen or letrozole versus tamoxifen or letrozole alone. 5 More recently, experiments using the same model system but longer periods of observation of tumor growth and higher drug doses (doses of fulvestrant were increased approximately 100-fold) have shown different results, with the combination of fulvestrant plus an aromatase inhibitor (either letrozole or anastrozole) demonstrating

Twist contributes to hormone resistance in breast cancer by downregulating estrogen receptor-α

The role of estrogen receptor alpha (ER) in breast cancer development and as a primary clinical marker for breast cancer prognosis is well documented. In this study, we identified the oncogenic protein TWIST1 (Twist), which is over-expressed in high-grade breast cancers, as a potential negative regulator of ER expression. Functional characterization of ER regulation by Twist was carried out using Twist low (MCF-7, T-47D) and Twist high (Hs 578T, MDA-MB-231, MCF-7/Twist) expressing cell lines. All Twist high cell lines exhibited low ER transcript and protein levels. By chromatin immunoprecipitation and promoter assays, we demonstrated that Twist could directly bind to E-boxes in the ER promoter and significantly down-regulate ER promoter activity in vitro. Functionally, Twist over-expression caused estrogen independent proliferation of breast cells and promoted hormone resistance to the selective estrogen receptor modulator (SERM) tamoxifen and selective estrogen receptor down-regulator (SERD) fulvestrant. Importantly, this effect was reversible on down-regulating Twist. Additionally, orthotopic tumors generated in mice using MCF-7/Twist cells were resistant to tamoxifen. These tumors had high vascular volume and permeability surface area as determined by magnetic resonance imaging (MRI). Mechanistically, Twist recruited DNA methyltransferase 3B (DNMT3B) to the ER promoter leading to a significantly higher degree of ER promoter methylation compared to parental cells. Furthermore, we demonstrated by co-immunoprecipitation that Twist interacted with histone deacetylase 1 (HDAC1) at the ER promoter, causing histone deacetylation and chromatin condensation, further reducing ER transcript levels. Functional re-expression of ER was achieved using demethylating agent 5-azacytidine and histone deacetylase inhibitor valproic acid. Finally, an inverse relationship was observed between Twist and ER expression in human breast tumors. In summary, the regulation of ER by Twist could be an underlying mechanism for loss of ER activity observed in breast tumors and may contribute to the generation of hormone resistant ER negative breast cancer.

Clinical strategies for rationale combinations of aromatase inhibitors with novel therapies for breast cancer

Improving endocrine responsiveness and preventing the development of resistance is the goal of many current strategies that are looking to combine aromatase inhibitors with novel drugs that target various pathways in estrogen receptor (ER) positive breast cancer. Pre-clinical models of acquired resistance to aromatase inhibitors have suggested an increase in several signaling pathways including peptide growth factor signaling (EGFR, HER2) and activation of the mTOR signaling pathway. These may result in associated ‘cross-talk’ activation of ER-dependent gene transcription, such that dual blockade of ER together with other signaling pathways has become a logical approach to improve endocrine responsivness. Clinical strategies with aromatase inhibitors are looking to prevent activation of these pathways either through combination with the selective ER downregulator fulvestrant, or with various signal transduction inhibitors (STIs) including monoclonal antibodies (trastuzumab), small molecule tyrosine kinase inhibitors (TKIs) against EGFR or HER2 (lapatinib, gefitinib) and mTOR antagonists (temsirolimus). Early clinical data have emerged this year for some of these approaches with mixed results. This article reviews the rationale for these strategies, and discusses the lessons that need to be learnt if we are to successfully integrate these new drugs with aromatase inhibitors in the clinic.

Aromatase inhibitors: Combinations with fulvestrant or signal transduction inhibitors as a strategy to overcome endocrine resistance

There is an increasing rationale for effective combinations of endocrine therapy with novel drugs that target aberrant signal transduction pathways in estrogen receptor (ER) positive breast cancer. Prolonged endocrine therapy can be associated with an acquired increase in peptide growth factor signaling (EGFR, HER2), together with cross-talk activation of ER-dependent gene transcription and cell growth that leads to endocrine resistance. Current approaches to target these pathways include both the selective ER downregulator fulvestrant, and various signal transduction inhibitors (STIs). Fulvestrant can overcome resistance to tamoxifen (TAM-R) and long-term estrogen deprivation (LTED-R) in experimental models by reducing ER expression, and represents a current option for post-menopausal women with endocrine resistant ER+ve breast cancer. Emerging data suggest that fulvestrant's effect may be greater when combined with estrogen deprivation, and several phase III trials are assessing fulvestrant combined with aromatase inhibitors (AIs). Small molecule STIs such as tyrosine kinase inhibitors (TKIs), farnesyltransferase inhibitors (FTIs) and mTOR antagonists are also active in breast cancer. Pre-clinical data suggest that combined endocrine/STI therapy may result in greater growth inhibition than either therapy alone, and thus delay emergence of resistance. Several clinical trials are now examining STIs combined with AIs both in the tamoxifen-resistant and first-line advanced breast cancer setting, while pre-surgical studies are investigating the efficacy of combined endocrine/STI therapy utilising biological primary endpoints. This article reviews the pre-clinical rationale for this strategy and the clinical trials in this area.