Sensitivity to the aromatase inhibitor letrozole is prolonged after a "break" in treatment

CYP19A1 mediates severe SARS-CoV-2 disease outcome in males

Male sex represents one of the major risk factors for severe COVID-19 outcome. However, underlying mechanisms that mediate sex-dependent disease outcome are as yet unknown. Here, we identify the CYP19A1 gene encoding for the testosterone-to-estradiol metabolizing enzyme CYP19A1 (also known as aromatase) as a host factor that contributes to worsened disease outcome in SARS-CoV-2-infected males. We analyzed exome sequencing data obtained from a human COVID-19 cohort (n = 2,866) using a machine-learning approach and identify a CYP19A1-activity-increasing mutation to be associated with the development of severe disease in men but not women. We further analyzed human autopsy-derived lungs (n = 86) and detect increased pulmonary CYP19A1 expression at the time point of death in men compared with women. In the golden hamster model, we show that SARS-CoV-2 infection causes increased CYP19A1 expression in the lung that is associated with dysregulated plasma sex hormone levels and reduced long-term pulmonary function in males but not females. Treatment of SARS-CoV-2-infected hamsters with a clinically approved CYP19A1 inhibitor (letrozole) improves impaired lung function and supports recovery of imbalanced sex hormones specifically in males. Our study identifies CYP19A1 as a contributor to sex-specific SARS-CoV-2 disease outcome in males. Furthermore, inhibition of CYP19A1 by the clinically approved drug letrozole may furnish a new therapeutic strategy for individualized patient management and treatment.

Co-targeting CDK2 and CDK4/6 overcomes resistance to aromatase and CDK4/6 inhibitors in ER+ breast cancer

Resistance to aromatase inhibitor (AI) treatment and combined CDK4/6 inhibitor (CDK4/6i) and endocrine therapy (ET) are crucial clinical challenges in treating estrogen receptor-positive (ER+) breast cancer. Understanding the resistance mechanisms and identifying reliable predictive biomarkers and novel treatment combinations to overcome resistance are urgently needed. Herein, we show that upregulation of CDK6, p-CDK2, and/or cyclin E1 is associated with adaptation and resistance to AI-monotherapy and combined CDK4/6i and ET in ER+ advanced breast cancer. Importantly, co-targeting CDK2 and CDK4/6 with ET synergistically impairs cellular growth, induces cell cycle arrest and apoptosis, and delays progression in AI-resistant and combined CDK4/6i and fulvestrant-resistant cell models and in an AI-resistant autocrine breast tumor in a postmenopausal xenograft model. Analysis of CDK6, p-CDK2, and/or cyclin E1 expression as a combined biomarker in metastatic lesions of ER+ advanced breast cancer patients treated with AI-monotherapy or combined CDK4/6i and ET revealed a correlation between high biomarker expression and shorter progression-free survival (PFS), and the biomarker combination was an independent prognostic factor in both patients cohorts. Our study supports the clinical development of therapeutic strategies co-targeting ER, CDK4/6 and CDK2 following progression on AI-monotherapy or combined CDK4/6i and ET to improve survival of patients exhibiting high tumor levels of CDK6, p-CDK2, and/or cyclin E1.

Efficacy of a novel orally active SERD AZD9496 against hormone dependent post-menopausal breast cancer depends on inhibition of cellular aromatase activity

Treatment of hormone sensitive breast cancer tumors with endocrine therapy such as antiestrogens or aromatase inhibitors has improved the outcome significantly. Studies including our own have shown that downregulation of ERα with pure antiestrogen fulvestrant in combination with aromatase inhibitors may prolong responsiveness of the tumors to endocrine therapy. Fulvestrant has been studied as second line or first line treatment for post-menopausal hormone receptor positive breast cancers as a single agent or in combination with AIs. Studies have also suggested that further escalation of dose may improve benefit. However, dose escalation of fulvestrant, which is administered via intramuscular injection, is difficult due to its poor solubility. To overcome this shortcoming of an injectable drug, a novel orally active antiestrogen, AZD9496 was developed. In addition to being orally active, AZD9496 is designed as a selective ERα downregulator (SERD). In the current study, we compared the effect of AZD9496 and fulvestrant on the growth of MCF-7Ca (human estrogen receptor positive MCF-7 cells stably transfected with human placental aromatase gene) xenografts grown in ovariectomized athymic nude mice. AZD9496 was also compared to fulvestrant in vitro as a single agent or in combination with anastrozole. Our current study shows that AZD9496 is equally effective as fulvestrant at controlling the growth of hormone sensitive human breast cancer tumors. Similar to fulvestrant, AZD9496 inhibits cellular aromatase activity through ERα mediated signaling. However, unlike fulvestrant, combination of AZD9496 with anastrozole did not produce increased tumor inhibition. Our results show that AZD9496 was significantly better at inhibiting cellular aromatase which contributed to its anticancer activity. Next, we measured the effect of AZD9496 on the mouse uterus. Uterine weight of mice treated with AZD9496 was significantly lower than that for mice treated with androstenedione. This reduction in uterine weight was due to AZD9496 mediated inhibition of aromatase activity and not a direct effect on uterine ERα expression. We also observed that anti-cancer efficacy of AZD9496 depended on its ability to inhibit cellular aromatase. These results suggest that AZD9496 may be a better alternative to fulvestrant due to its selectivity for mammary ER and ability to inhibit aromatase in addition of downregulating ERα that can be obtained upon oral administration. As such, AZD9496 may prove to be a better option than fulvestrant for the treatment of hormone sensitive human breast cancer.

Acquired resistance to aromatase inhibitors: where we stand!

Aromatase inhibitors (AIs) are one of the principal therapeutic approaches for estrogen receptor-positive (ER⁺) breast cancer in postmenopausal women. They block estrogen biosynthesis through aromatase inhibition, thus preventing tumour progression. Besides the therapeutic success of the third-generation AIs, acquired resistance may develop, leading to tumour relapse. This resistance is thought to be the result of a change in the behaviour of ER in these breast cancer cells, presumably by PI3K/AKT pathway enhancement along with alterations in other signalling pathways. Nevertheless, biological mechanisms, such as apoptosis, autophagy, cell cycle modulation and activation of androgen receptor (AR), are also implicated in acquired resistance. Moreover, clinical evidence demonstrated that there is a lack of cross-resistance among AIs, although the reason is not fully understood. Thus, there is a demand to understand the mechanisms involved in endocrine resistance to each AI, since the search for new strategies to surpass breast cancer acquired resistance is of major concern.

Treatment with aromatase inhibitors stimulates the expression of epidermal growth factor receptor-1 and neuregulin 1 in ER positive/HER-2/neu non-amplified primary breast cancers

While estrogens have been shown to modulate EGFR/HER-1 and HER-2/neu expression in experimental systems, the effects of estrogen deprivation on expression levels of the HER-receptors and the neuregulin (NRG)1 ligand in breast cancers remain unknown. Here, we measured EGFR/HER-1-4 and NRG1 mRNA in ER positive tumors from 85 postmenopausal breast cancer patients before and after two weeks (n=64) and three months (n=85) of primary treatment with an aromatase inhibitor (AI). In tumors lacking HER-2/neu amplification, quantitative real-time PCR analyses revealed EGFR/HER-1 and NRG1 to vary significantly between the three time points (before therapy, after 2 weeks and after 3 months on treatment; P≤0.001 for both). Pair-wise comparison revealed a significant increase in EGFR/HER-1 already during the first two weeks of treatment (P=0.049) with a further increase for both EGFR/HER-1 and NRG1 after 3 months on treatment (P≤0.001 and P=0.001 for both comparing values at 3 months to values at baseline and 2 weeks respectively). No difference between tumors responding versus non-responders was recorded. Further, no significant change in any parameter was observed among HER-2/neu amplified tumors. Analyzing components of the HER-2/neu PI3K/Akt downstream pathway, the PIK3CA H1047R mutation was associated with treatment response (P=0.035); however no association between either AKT phosphorylation status or PIK3CA gene mutations and EGFR/HER-1 or NRG1 expression levels were observed. Our results indicate primary AI treatment to modulate expression of HER-family members and the growth factor NRG1 in HER-2/neu non-amplified breast cancers in vivo. Potential implications to long term sensitivity warrants further investigations.

Glyceollin I Reverses Epithelial to Mesenchymal Transition in Letrozole Resistant Breast Cancer through ZEB1

Although aromatase inhibitors are standard endocrine therapy for postmenopausal women with early-stage metastatic estrogen-dependent breast cancer, they are limited by the development of drug resistance. A better understanding of this process is critical towards designing novel strategies for disease management. Previously, we demonstrated a global proteomic signature of letrozole-resistance associated with hormone-independence, enhanced cell motility and implications of epithelial mesenchymal transition (EMT). Letrozole-resistant breast cancer cells (LTLT-Ca) were treated with a novel phytoalexin, glyceollin I, and exhibited morphological characteristics synonymous with an epithelial phenotype and decreased proliferation. Letrozole-resistance increased Zinc Finger E-Box Binding Homeobox 1 (ZEB1) expression (4.51-fold), while glyceollin I treatment caused a -3.39-fold reduction. Immunofluorescence analyses resulted of glyceollin I-induced increase and decrease in E-cadherin and ZEB1, respectively. In vivo studies performed in ovariectomized, female nude mice indicated that glyceollin treated tumors stained weakly for ZEB1 and N-cadherin and strongly for E-cadherin. Compared to letrozole-sensitive cells, LTLT-Ca cells displayed enhanced motility, however in the presence of glyceollin I, exhibited a 68% and 83% decrease in invasion and migration, respectively. These effects of glyceollin I were mediated in part by inhibition of ZEB1, thus indicating therapeutic potential of glyceollin I in targeting EMT in letrozole resistant breast cancer.

A phase II trial of low-dose estradiol in postmenopausal women with advanced breast cancer and acquired resistance to aromatase inhibition

BACKGROUND

High-dose oestrogen (HDE) is effective but toxic in postmenopausal women with advanced breast cancer (ABC). Prolonged oestrogen deprivation sensitises BC cell lines to estrogen and we hypothesised that third-generation aromatase inhibitors (AIs) would sensitise BCs to low-dose estradiol (LDE).

METHODS

A single-arm phase II study of LDE (2 mg estradiol valerate daily) in postmenopausal women with estrogen receptor-positive (ER+) ABC. The primary end-point was clinical benefit (CB) rate. If LDE was ineffective, HDE was offered. If LDE was effective, retreatment with the pre-LDE AI was offered on progression.

RESULTS

Twenty-one patients were recruited before the trial was closed early due to slow accrual; 19 were assessable for efficacy and toxicity. CB was seen in 5 in 19 patients (26%; 95% confidence interval 9.1-51.2%), all with prolonged SD (median duration 16.8 months; range 11.0-29.6). Treatment was discontinued for toxicity in 4 in 19 patients (21%) and 8 in 11 women without hysterectomy experienced vaginal bleeding (VB). After primary LDE failure, three patients received HDE and one achieved a partial response (PR). Following CB on LDE, four patients restarted pre-LDE AI and three achieved CB including one PR. Those with CB to LDE had a significantly longer duration of first-line endocrine therapy for ABC than those without (54.9 versus 16.8 months; p < 0.01) CONCLUSION: LDE is an effective endocrine option in women with evidence of prolonged sensitivity to AI therapy. LDE is reasonably well tolerated although VB is an issue. Re-challenge with the pre-LDE AI following progression confirms re-sensitisation as a true phenomenon.

New approaches for improving outcomes in breast cancer in Europe

Considerable progress has been made in breast cancer treatment in Europe over the past three decades, yet survival rates for metastatic disease remain poor, underlining the need for further advances. While the use of predictive biomarkers for response to systemic therapy could improve drug development efficiency, progress in identifying such markers has been slow. The currently inadequate classification of breast cancer subtypes is a further challenge. Improved understanding of the molecular pathology of the disease has led to the identification of new targets for drug treatment, and evolving classifications should reflect these developments. Further ongoing challenges include difficulties in finding optimal combinations and sequences of systemic therapies, circumventing multidrug resistance and intra-tumor heterogeneity, problems associated with fragmentation in clinical trials and translational research efforts. Adoption of some of the strategies identified in this article may lead to further improvements in outcomes for patients with the disease.

A nude mouse model of obesity to study the mechanisms of resistance to aromatase inhibitors

Obesity is a risk factor for breast cancer progression. Breast cancer patients who are overweight or obese or have excess abdominal fat have an increased risk of local or distant recurrence and cancer-related death. Hormone depletion therapies can also cause weight gain, exacerbating the risk for these patients. To understand the effect of obesity on hormone-dependent human breast cancer tumors, we fed ovariectomized athymic nude mice a diet containing 45% kcal fat and 17% kcal sucrose (high fat sucrose diet (HFSD)), 10% kcal fat (low fat diet (LFD)), or a standard chow diet (chow). The mice fed the HFSD developed metabolic abnormalities consistent with the development of obesity such as weight gain, high fasting blood glucose, and impaired glucose tolerance. These mice also developed hyperinsulinemia and insulin resistance. The obese mice also had a higher tumor growth rate compared to the lean mice. Furthermore, the obese mice showed a significantly reduced responsiveness to letrozole. To understand the role of obesity in this reduced responsiveness, we examined the effect of insulin on the growth of MCF-7Ca cells in response to estrogen or letrozole. The presence of insulin rendered MCF-7Ca cells less responsive to estrogen and letrozole. Exogenous insulin treatment of MCF-7Ca cells also resulted in increased p-Akt as well as ligand-independent phosphorylation of ERα. These findings suggest that diet-induced obesity may result in reduced responsiveness of tumors to letrozole due to the development of hyperinsulinemia. We conclude that obesity influences the response and resistance of breast cancer tumors to aromatase inhibitor treatment.

Extending aromatase inhibitor sensitivity in hormone resistant breast cancer

Aromatase inhibitors (AIs) are first-line treatment for ER+ breast cancer. However, despite responses initially, some patients can eventually acquire resistance. Moreover, 25% of all breast cancer patients do not express the estrogen receptor (ERα) and are innately resistance. In tumors of mouse models with acquired AI letrozole resistance, expression of ERα was reduced whereas HER2/growth factor signaling was enhanced. Treatment of mice with trastuzumab (HER2 antibody) reduced HER2/p-MAPK but restored ERα expression. The addition of trastuzumab to letrozole treatment when tumors progressed resulted in significantly longer tumor suppression than these drugs alone. Thus, inhibition of both HER2 and ERα signaling pathways was necessary to overcome resistance. In ERα-negative tumors, the receptor has been shown to be silenced by epigenetic modifications. Treatment of MDA-MB-231 ER-negative tumors with a histone deacetylase inhibitor, entinostat (ENT) increased expression of ERα and also aromatase. When ENT was combined with letrozole, tumor growth rate was markedly reduced compared with control tumors. ENT plus letrozole treatment also prevented the colonization and growth of MDA-MB-231 cells in the lung with significant reduction in visible and microscopic foci. These novel strategies could improve treatment for patients with acquired and innate resistance to AIs.

Endocrine Resistance in Breast Cancer

Around 70% of all breast cancers are estrogen receptor alpha positive and hence their development is highly dependent on estradiol. While the invention of endocrine therapies has revolusioned the treatment of the disease, resistance to therapy eventually occurs in a large number of patients. This paper seeks to illustrate and discuss the complexity and heterogeneity of the mechanisms which underlie resistance and the approaches proposed to combat them. It will also focus on the use and development of methods for predicting which patients are likely to develop resistance.

Mathematical models of the transitions between endocrine therapy responsive and resistant states in breast cancer

Endocrine therapy, targeting the oestrogen receptor pathway, is the most common treatment for oestrogen receptor-positive breast cancers. Unfortunately, these tumours frequently develop resistance to endocrine therapies. Among the strategies to treat resistant tumours are sequential treatment (in which second-line drugs are used to gain additional responses) and intermittent treatment (in which a 'drug holiday' is imposed between treatments). To gain a more rigorous understanding of the mechanisms underlying these strategies, we present a mathematical model that captures the transitions among three different, experimentally observed, oestrogen-sensitivity phenotypes in breast cancer (sensitive, hypersensitive and independent). To provide a global view of the transitions between these phenotypes, we compute the potential landscape associated with the model. We show how this oestrogen response landscape can be reshaped by population selection, which is a crucial force in promoting acquired resistance. Techniques from statistical physics are used to create a population-level state-transition model from the cellular-level model. We then illustrate how this population-level model can be used to analyse and optimize sequential and intermittent oestrogen-deprivation protocols for breast cancer. The approach used in this study is general and can also be applied to investigate treatment strategies for other types of cancer.

Advances in mechanisms of resistance to aromatase inhibitors

Clinically, there are two distinct types of aromatase inhibitor (AI) resistance, namely acquired and innate resistance. Because the underlying mechanisms of these two types of resistance may not be mutually exclusive, strategies to tackle these resistances may not be effective when used interchangeably. Activation of growth factor receptor pathways is the hallmark of acquired AI resistance. These pathways can be targeted either at the cell surface receptor level or their downstream signaling cascades. Currently, everolimus in combination with exemestane represents a new standard of care for patients progressing on non-steroidal AIs. HDAC inhibitors have also shown promising results For innate resistance, the combination of fulvestrant and AI in the front line setting represents a new treatment option, particularly for patients who present with de novo metastatic disease. A Phase III trial is currently ongoing to evaluate the benefit of CDK 4/6 inhibitor, palbociclib, in the first line setting in combination with AI.

Nonhypoxic regulation and role of hypoxia-inducible factor 1 in aromatase inhibitor resistant breast cancer

Introduction

Although aromatase inhibitors (AIs; for example, letrozole) are highly effective in treating estrogen receptor positive (ER+) breast cancer, a significant percentage of patients either do not respond to AIs or become resistant to them. Previous studies suggest that acquired resistance to AIs involves a switch from dependence on ER signaling to dependence on growth factor-mediated pathways, such as human epidermal growth factor receptor-2 (HER2). However, the role of HER2, and the identity of other relevant factors that may be used as biomarkers or therapeutic targets remain unknown. This study investigated the potential role of transcription factor hypoxia inducible factor 1 (HIF-1) in acquired AI resistance, and its regulation by HER2.

Methods

In vitro studies using AI (letrozole or exemestane)-resistant and AI-sensitive cells were conducted to investigate the regulation and role of HIF-1 in AI resistance. Western blot and RT-PCR analyses were conducted to compare protein and mRNA expression, respectively, of ERα, HER2, and HIF-1α (inducible HIF-1 subunit) in AI-resistant versus AI-sensitive cells. Similar expression analyses were also done, along with chromatin immunoprecipitation (ChIP), to identify previously known HIF-1 target genes, such as breast cancer resistance protein (BCRP), that may also play a role in AI resistance. Letrozole-resistant cells were treated with inhibitors to HER2, kinase pathways, and ERα to elucidate the regulation of HIF-1 and BCRP. Lastly, cells were treated with inhibitors or inducers of HIF-1α to determine its importance.

Results

Basal HIF-1α protein and BCRP mRNA and protein are higher in AI-resistant and HER2-transfected cells than in AI-sensitive, HER2- parental cells under nonhypoxic conditions. HIF-1α expression in AI-resistant cells is likely regulated by HER2 activated-phosphatidylinositide-3-kinase/Akt-protein kinase B/mammalian target of rapamycin (PI3K/Akt/mTOR) pathway, as its expression was inhibited by HER2 inhibitors and kinase pathway inhibitors. Inhibition or upregulation of HIF-1α affects breast cancer cell expression of BCRP; AI responsiveness; and expression of cancer stem cell characteristics, partially through BCRP.

Conclusions

One of the mechanisms of AI resistance may be through regulation of nonhypoxic HIF-1 target genes, such as BCRP, implicated in chemoresistance. Thus, HIF-1 should be explored further for its potential as a biomarker of and therapeutic target.

Tumor metastasis: moving new biological insights into the clinic

As the culprit behind most cancer-related deaths, metastasis is the ultimate challenge in our effort to fight cancer as a life-threatening disease. The explosive growth of metastasis research in the past decade has yielded an unprecedented wealth of information about the tumor-intrinsic and tumor-extrinsic mechanisms that dictate metastatic behaviors, the molecular and cellular basis underlying the distinct courses of metastatic progression in different cancers and what renders metastatic cancer refractory to available therapies. However, integration of such new knowledge into an improved, metastasis-oriented oncological drug development strategy is needed to thwart the development of metastatic disease at every stage of progression.

HDAC inhibitor entinostat restores responsiveness of letrozole-resistant MCF-7Ca xenografts to aromatase inhibitors through modulation of Her-2

We previously showed that in innately resistant tumors, silencing of the estrogen receptor (ER) could be reversed by treatment with a histone deacetylase (HDAC) inhibitor, entinostat. Tumors were then responsive to aromatase inhibitor (AI) letrozole. Here, we investigated whether ER in the acquired letrozole-resistant tumors could be restored with entinostat. Ovariectomized athymic mice were inoculated with MCF-7Ca cells, supplemented with androstenedione (Δ(4)A), the aromatizable substrate. When the tumors reached about 300 mm(3), the mice were treated with letrozole. After initial response to letrozole, the tumors eventually became resistant (doubled their initial volume). The mice then were grouped to receive letrozole, exemestane (250 μg/d), entinostat (50 μg/d), or the combination of entinostat with letrozole or exemestane for 26 weeks. The growth rates of tumors of mice treated with the combination of entinostat with letrozole or exemestane were significantly slower than with the single agent (P < 0.05). Analysis of the letrozole-resistant tumors showed entinostat increased ERα expression and aromatase activity but downregulated Her-2, p-Her-2, p-MAPK, and p-Akt. However, the mechanism of action of entinostat in reversing acquired resistance did not involve epigenetic silencing but rather included posttranslational as well as transcriptional modulation of Her-2. Entinostat treatment reduced the association of the Her-2 protein with HSP-90, possibly by reducing the stability of Her-2 protein. In addition, entinostat also reduced Her-2 mRNA levels and its stability. Our results suggest that the HDAC inhibitor may reverse letrozole resistance in cells and tumors by modulating Her-2 expression and activity.

Effect of selumetinib on the growth of anastrozole-resistant tumors

Despite significant improvement in the treatment outcome of hormone responsive postmenopausal breast cancer, some patients eventually acquire resistance to aromatase inhibitors (AIs). Using our MCF-7Ca xenograft model, we observed that although AIs such as anastrozole initially inhibit tumor growth effectively, tumors eventually began to grow. Our previous data show that anastrozole-resistant tumors upregulate growth factor receptor pathways as they adapt to grow in the low estrogen environment. Therefore, in the current study, we investigated the effect of inhibiting the growth factor receptor pathways with a MEK-1/2 inhibitor selumetinib (AZD6244, ARRY-142866). We treated the mice with anastrozole-resistant tumors with selumetinib alone or in combination with anastrozole. MCF-7Ca cells were inoculated sc into ovariectomized athymic nude mice supplemented throughout the experiment with androstenedione (100 μg/day), the substrate for aromatase conversion to estrogen. Once the tumors reached a measurable size (~300 mm(3)), the mice were treated with anastrozole (200 μg/day), supplemented with androstenedione (Δ(4)A). The tumors in the anastrozole group doubled in volume after 6 weeks, at which time the animals were regrouped to receive the following treatments: (i) anastrozole, (ii) anastrozole withdrawal (Δ(4)A alone), (iii) selumetinib (25 mg/kg/d, bid, po), and (iv) selumetinib + anastrozole, (n = 10 mice/group). The treatments were given for 6 weeks (till week 12) and then the mice were euthanized, the tumors were collected and analyzed. The tumors of mice treated with selumetinib + anastrozole had significantly lower growth rates than those treated with single agents (p = 0.008). Western blot analysis of the tumors showed that treatment with anastrozole resulted in upregulation of proteins in the growth factor receptor cascade such as p-mTOR, pAkt, pMEK, and pMAPK. This was accompanied by downregulation of ERα protein, consistent with previous findings. The treatment of mice with selumetinib resulted in downregulation of activated MAPK, along with p-mTOR, which likely resulted in upregulation of ERα. Our results suggest that inhibition of the growth factor receptor pathway with selumetinib can reverse anastrozole resistance.

Aromatase inhibitors and xenograft studies

Aromatase inhibitors (AIs) have become the front-line choice for treatment of ER+ breast cancer. Nevertheless, although patients are responsive initially, they may acquire resistance and become unresponsive to further treatment. In addition, approximately 25% of breast cancers do not express the estrogen receptor (ERα) and consequently, are innately resistant to endocrine therapy. We have investigated the mechanisms associated with this lack of treatment response using xenograft models. We found that in cells and tumors that acquired resistance to the AI letrozole therapy, expression of the ER was reduced whereas growth factor signally was enhanced, including a marked increase in HER2 expression. Treatment with trastuzumab (HER2 antibody) resulted in a significant down-regulation of HER2 and p-MAPK as well as restoration of ERα expression. Thus, when trastuzumab was added to letrozole treatment at the time of tumor progression, there was significantly prolonged tumor suppression compared to trastuzumab or letrozole alone. This suggests that inhibition of both HER2 and ERα signaling pathways are required for overcoming resistance and restoring treatment sensitivity. ER negative tumors are innately resistant to endocrine therapy. Repression of the ERα has been found to be due to epigenetic modifications such as increased methylation and histone deacetylation. We found that entinostat (ENT), a histone deacetylase inhibitor (HDACi), activated not only expression of ERα but also aromatase in MDA-MB-231 ER-negative breast cancer cells, resulting in their ability to respond to estrogen and letrozole. Treatment with ENT in combination with letrozole significantly reduced tumor growth rate in xenografts compared to control tumors (p<0.001). ENT plus letrozole treatment also prevented the colonization and growth of MDA-MB-231 cells in the lung with a significant reduction (p<0.03) in both visible and microscopic foci. These results provide a strong indication for possible use of AIs in combination with HDAC inhibitors for the treatment of ER-negative breast cancer.

Aromatase, aromatase inhibitors, and breast cancer

Estrogens are known to be important in the growth of breast cancers in both pre and postmenopausal women. As the number of breast cancer patients increases with age, the majority of breast cancer patients are postmenopausal women. Although estrogens are no longer made in the ovaries after menopause, peripheral tissues produce sufficient concentrations to stimulate tumor growth. As aromatase catalyzes the final and rate-limiting step in the biosynthesis of estrogen, inhibitors of this enzyme are effective targeted therapy for breast cancer. Three aromatase inhibitors (AIs) are now FDA approved and have been shown to be more effective than the antiestrogen tamoxifen and are well tolerated. AIs are now a standard treatment for postmenopausal patients. AIs are effective in adjuvant and first-line metastatic setting. This review describes the development of AIs and their current use in breast cancer. Recent research focuses on elucidating mechanisms of acquired resistance that may develop in some patients with long term AI treatment and also in innate resistance. Preclinical data in resistance models demonstrated that the crosstalk between ER and other signaling pathways particularly MAPK and PI3K/Akt is an important resistant mechanism. Blockade of these other signaling pathways is an attractive strategy to circumvent the resistance to AI therapy in breast cancer. Several clinical trials are ongoing to evaluate the role of these novel targeted therapies to reverse resistance to AIs. Article from the special issue on 'Targeted Inhibitors'.

Adaptive changes result in activation of alternate signaling pathways and acquisition of resistance to aromatase inhibitors

Hormone therapy is an effective approach for the treatment of breast cancer. Although the antiestrogen tamoxifen has had a major impact on the treatment of the disease, aromatase inhibitors (AIs), which reduce estrogen synthesis, have recently proved to be more effective. These agents are now used as first-line therapy for postmenopausal breast cancer. Nevertheless, despite the efficacy of these agents, resistance to treatment eventually may occur in some patients. In an effort to overcome this resistance and extend the benefits of AIs, investigators have studied the mechanisms involved in resistance to AIs. Adaptive changes that result in activation of alternate signaling pathways in AI-resistant tumors have been identified in xenograft and cell line models. Expression of estrogen receptor α and aromatase was shown to be decreased in tumors after long-term treatment with AIs. In contrast, increased expression was observed in tyrosine kinase receptors such as Her-2 and insulin-like growth factor receptor, as well as in downstream signaling proteins such as mitogen-activated protein kinase. Functional activation of the mitogen-activated protein kinase pathway and dependency on growth factor receptor signaling have been observed in AI-resistant cells and tumors.

Functional activation of the estrogen receptor-α and aromatase by the HDAC inhibitor entinostat sensitizes ER-negative tumors to letrozole

Approximately 25% of breast cancers do not express the estrogen receptor-α (ERα) and consequently do not respond to endocrine therapy. In these tumors, ERα repression is often due to epigenetic modifications such as methylation and histone deacetylation. For this reason, we investigated the ability of the histone deacetylase inhibitor entinostat (ENT) to trigger reexpression of ERα and aromatase in breast cancer cells, with the notion that this treatment would restore sensitivity to the aromatase inhibitor (AI) letrozole. ENT treatment of tumor cells increased expression of ERα and aromatase, along with the enzymatic activity of aromatase, in a dose-dependent manner both in vitro and in vivo. Notably, ERα and aromatase upregulation resulted in sensitization of breast cancer cells to estrogen and letrozole. Tumor growth rate was significantly lower in tumor xenografts following treatment with ENT alone and in combination with letrozole than in control tumors (P > 0.001). ENT plus letrozole also prevented lung colonization and growth of tumor cells, with a significant reduction (P > 0.03) in both visible and microscopic foci. Our results show that ENT treatment can be used to restore the letrozole responsiveness of ER-negative tumors. More generally, they provide a strong rationale for immediate clinical evaluation of combinations of histone deacetylase and aromatase inhibitors to treat ER-negative and endocrine-resistant breast cancers.

Does tumour dormancy offer a therapeutic target?

The increasing number of cancer survivors is cause for celebration, but this expanding population has highlighted the problem of tumour dormancy, which can lead to relapse. As we start to understand more about the biology of dormant cancer cells, we can begin to address how best to treat this form of disease. Preclinical models and initial clinical trials, as exemplified in patients with breast cancer, are paving the way to address how best to treat long-term cancer survivors to minimize the risk of cancer recurrence.