Aromatase destabilizer: novel action of exemestane, a food and drug administration-approved aromatase inhibitor

Aromatase inhibitors for the treatment of breast cancer: An overview (2019-2023)

Aromatase inhibition is considered a legitimate approach for the treatment of ER-positive (ER+) breast cancer as it accounts for more than 70% of breast cancer cases. Aromatase inhibitor therapy has been demonstrated to be highly effective in decreasing tumour size, increasing survival rates, and lowering the chance of cancer recurrence. The present review deliberates the pathophysiology and the role of aromatase in estrogen biosynthesis. Estrogen biosynthesis, various androgens, and their function in the human body have also been discussed. The salient aspects of the aromatase active site, its mode of action, and AIs, along with their intended interactions with presently FDA-approved inhibitors, have been briefly discussed. It has been detailed how different reported AIs were designed, their SAR investigations, in silico analysis, and biological evaluations. Various AIs from multiple origins, such as synthetic and semi-synthetic, have also been discussed.

In Vitro Effects of Combining Genistein with Aromatase Inhibitors: Concerns Regarding Its Consumption during Breast Cancer Treatment

INTRODUCTION

The third-generation of aromatase inhibitors (AIs)-Exemestane (Exe), Letrozole (Let), and Anastrozole (Ana)-is the main therapeutic approach applied for estrogen receptor-positive (ER+) breast cancer (BC), the most common neoplasm in women worldwide. Despite their success, the development of resistance limits their efficacy. Genistein (G), a phytoestrogen present in soybean, has promising anticancer properties in ER+ BC cells, even when combined with anticancer drugs. Thus, the potential beneficial effects of combining G with AIs were investigated in sensitive (MCF7-aro) and resistant (LTEDaro) BC cells.

METHODS

The effects on cell proliferation and expression of aromatase, ERα/ERβ, and AR receptors were evaluated.

RESULTS

Unlike the combination of G with Ana or Let, which negatively affects the Ais' therapeutic efficacy, G enhanced the anticancer properties of the steroidal AI Exe, increasing the antiproliferative effect and apoptosis relative to Exe. The hormone targets studied were not affected by this combination when compared with Exe.

CONCLUSIONS

This is the first in vitro study that highlights the potential benefit of G as an adjuvant therapy with Exe, emphasizing, however, that soy derivatives widely used in the diet or applied as auxiliary medicines may increase the risk of adverse interactions with nonsteroidal AIs used in therapy.

Cannabidiol as a Promising Adjuvant Therapy for Estrogen Receptor-Positive Breast Tumors: Unveiling Its Benefits with Aromatase Inhibitors

BACKGROUND

Estrogen receptor-positive (ER⁺) breast cancer is the most diagnosed subtype, with aromatase inhibitors (AIs) being one of the therapeutic drug types used in the clinic. However, endocrine resistance may develop after prolonged treatment, and different approaches, such as combining endocrine and targeted therapies, have been applied. Recently, we demonstrated that cannabidiol (CBD) induces anti-tumor actions in ER⁺ breast cancer cells by targeting aromatase and ERs. Considering this, we studied, in vitro, whether CBD when combined with AIs could improve their effectiveness.

METHODS

MCF-7aro cells were used and the effects on cell viability and on the modulation of specific targets were investigated.

RESULTS

CBD when combined with anastrozole (Ana) and letrozole (Let) caused no beneficial effect in comparison to the isolated AIs. In contrast, when combined with the AI exemestane (Exe), CBD potentiated its pro-cell death effects, abolished its estrogen-like effect, impaired ERα activation, and prevented its oncogenic role on the androgen receptor (AR). Moreover, this combination inhibited ERK_1/2 activation, promoting apoptosis. The study of the hormonal microenvironment suggests that this combination should not be applied in early stages of ER⁺ breast tumors.

CONCLUSIONS

Contrary to Ana and Let, this study highlights the potential benefits of combining CBD with Exe to improve breast cancer treatment and opens up the possibility of new therapeutic approaches comprising the use of cannabinoids.

An Exemestane Derivative, Oxymestane-D1, as a New Multi-Target Steroidal Aromatase Inhibitor for Estrogen Receptor-Positive (ER+) Breast Cancer: Effects on Sensitive and Resistant Cell Lines

Around 70-85% of all breast cancer (BC) cases are estrogen receptor-positive (ER⁺). The third generation of aromatase inhibitors (AIs) is the first-line treatment option for these tumors. Despite their therapeutic success, they induce several side effects and resistance, which limits their efficacy. Thus, it is crucial to search for novel, safe and more effective anti-cancer molecules. Currently, multi-target drugs are emerging, as they present higher efficacy and lower toxicity in comparison to standard options. Considering this, this work aimed to investigate the anti-cancer properties and the multi-target potential of the compound 1α,2α-epoxy-6-methylenandrost-4-ene-3,17-dione (Oxy), also designated by Oxymestane-D1, a derivative of Exemestane, which we previously synthesized and demonstrated to be a potent AI. For this purpose, it was studied for its effects on the ER⁺ BC cell line that overexpresses aromatase, MCF-7aro cells, as well as on the AIs-resistant BC cell line, LTEDaro cells. Oxy reduces cell viability, impairs DNA synthesis and induces apoptosis in MCF-7aro cells. Moreover, its growth-inhibitory properties are inhibited in the presence of ERα, ERβ and AR antagonists, suggesting a mechanism of action dependent on these receptors. In fact, Oxy decreased ERα expression and activation and induced AR overexpression with a pro-death effect. Complementary transactivation assays demonstrated that Oxy presents ER antagonist and AR agonist activities. In addition, Oxy also decreased the viability and caused apoptosis of LTEDaro cells. Therefore, this work highlights the discovery of a new and promising multi-target drug that, besides acting as an AI, appears to also act as an ERα antagonist and AR agonist. Thus, the multi-target action of Oxy may be a therapeutic advantage over the three AIs applied in clinic. Furthermore, this new multi-target compound has the ability to sensitize the AI-resistant BC cells, which represents another advantage over the endocrine therapy used in the clinic, since resistance is a major drawback in the clinic.

Differential biological effects of aromatase inhibitors: Apoptosis, autophagy, senescence and modulation of the hormonal status in breast cancer cells

Estrogen receptor-positive (ER⁺) breast carcinomas are the most common subtype, corresponding to 60% of the cases in premenopausal and 75% in postmenopausal women. The third-generation of aromatase inhibitors (AIs), the non-steroidal Anastrozole (Ana) and Letrozole (Let) and the steroidal Exemestane (Exe), are considered a first-line endocrine therapy for postmenopausal women. Despite their clinical success, the development of resistance is the major setback in clinical practice. Nevertheless, the lack of cross-resistance between AIs hints that these drugs may act through distinct mechanisms. Therefore, this work studied the different effects induced by AIs on biological processes, such as cell proliferation, death, autophagy and senescence. Moreover, their effects on the regulation of the hormonal environment were also explored. The non-steroidal AIs induce senescence, through increased YPEL3 expression, on aromatase-overexpressing breast cancer cells (MCF-7aro), whereas Exe promotes a cytoprotective autophagy, thus blocking senescence induction. In addition, in a hormone-enriched environment, the non-steroidal AIs prevent estrogen signaling, despite up-regulating the estrogen receptor alpha (ERα), while Exe down-regulates ERα and maintains its activation. In these conditions, all AIs up-regulate the androgen receptor (AR) which blocks EGR3 transcription in Exe-treated cells. On the other hand, in hormone-depleted conditions, a crosstalk between AR and ERα occurs, enhancing the estrogenic effects of Exe. This indicates that Exe modulates both ERα and AR, while Ana and Let act as pure AIs. Thus, this study highlights the potential clinical benefit of combining AR antagonists with Exe and discourages the sequential use of Exe as second-line therapy in postmenopausal breast cancer.

Insights on the interaction mechanism of exemestane to three digestive enzymes by multi-spectroscopy and molecular docking

Exemestane is an irreversible steroidal aromatase inhibitor, typically used to treat breast cancer. As an anti-tumor drug, exemestane has more obvious side effects on the gastrointestinal tract. The purpose of this work is to investigate the combination of exemestane with three important digestive enzymes including pepsin (Pep), trypsin (Try) and α-Chymotrypsin (α-ChT) so as to analyze the mechanism of the gastrointestinal adverse effects causing by exemestane binding. Enzyme activity experiment showed that the enzyme activity of Pep was decreased in the presence of exemestane. Fluorescence spectra revealed that exemestane formed stable complexes with digestive enzymes, and the quenching mechanism of drug-digestive enzymes interaction were all static quenching. The binding constants of Pep, Try and α-ChT at 298 K were 2.34 × 10⁵, 1.45 × 10⁵, and 2.05 × 10⁵ M^-1, respectively. Synchronous fluorescence and 3D fluorescence spectroscopy showed that the conformation of exemestane was slightly changed after combining with digestive enzymes, and non-radiative energy transfer occurred. Circular dichroism results indicated that exemestane could change the secondary structure of digestive enzymes via increase the α-helix content and decrease in the β-sheet content. Thermodynamic parameters (ΔH⁰, ΔS⁰, and ΔG⁰) revealed that exemestane interacted with α-ChT through electrostatic force, and the binding force with Pep and Try was van der Waals interactions and hydrogen, which was basically consistent with the molecular docking results.

A Fluorometric CYP19A1 (Aromatase) Activity Assay in Live Cells

Inhibition of estrogen synthesis is an integral component of the frontline pharmacologic therapy for the treatment of estrogen receptor positive cancers. However, there is currently no direct, high-throughput-ready assay for aromatase (also known as CYP19A1) that can be performed in live cells. Herein we present a cell-based assay that allows for multiplexed assessment of enzyme activity, protein half-life, cell viability, and identification of inhibitors with slow off-rates.

Unveiling the mechanism of action behind the anti-cancer properties of cannabinoids in ER+ breast cancer cells: Impact on aromatase and steroid receptors

Breast cancer is the leading cause of cancer-related death in women worldwide. In the last years, cannabinoids have gained attention in the clinical setting and clinical trials with cannabinoid-based preparations are underway. However, contradictory anti-tumour properties have also been reported. Thus, the elucidation of the molecular mechanisms behind their anti-tumour efficacy is crucial to better understand its therapeutic potential. Considering this, our work aims to clarify the molecular mechanisms underlying the anti-cancer properties of the endocannabinoid anandamide (AEA) and of the phytocannabinoids, cannabidiol (CBD) and Δ⁹-tetrahydrocannabinol (THC), in estrogen receptor-positive (ER⁺) breast cancer cells that overexpress aromatase (MCF-7aro). Their in vitro effects on cell proliferation, cell death and activity/expression of aromatase, ERα, ERβ and AR were investigated. Our results demonstrated that cannabinoids disrupted MCF-7aro cell cycle progression. Unlike AEA and THC that induced apoptosis, CBD triggered autophagy to promote apoptotic cell death. Interestingly, all cannabinoids reduced aromatase and ERα expression levels in cells. On the other hand, AEA and CBD not only exhibited high anti-aromatase activity but also induced up-regulation of ERβ. Therefore, all cannabinoids, albeit by different actions, target aromatase and ERs, impairing, in that way, the growth of ER⁺ breast cancer cells, which is dependent on estrogen signalling. As aromatase and ERs are key targets for ER⁺ breast cancer treatment, cannabinoids can be considered as potential and attractive therapeutic compounds for this type of cancer, being CBD the most promising one. Thus, from an in vitro perspective, this work may contribute to the growing mass of evidence of cannabinoids and cannabinoids-based medicines as potential anti-cancer drugs.

Estrogen and sex-dependent loss of the vocal learning system in female zebra finches

Sex hormones alter the organization of the brain during early development and coordinate various behaviors throughout life. In zebra finches, song learning is limited to males, with the associated song learning brain pathways only maturing in males and atrophying in females. While this atrophy can be prevented by treating females with exogenous estrogen during early post-hatch development, the requirement of estrogen during normal male song system development is uncertain. For the first time in songbirds, we administered exemestane, a potent third generation estrogen synthesis inhibitor, from the day of hatching until adulthood in order to reassess the role of estrogen in song circuit development. We examined the behavior, brain anatomy, and transcriptomes of individual song nuclei in these pharmacologically manipulated animals. We found that males with long-term exemestane treatment had diminished male-specific plumage and impaired song learning, but minimal effect on song nuclei sizes and their specialized transcriptome. Consistent with prior findings, females with long-term estrogen treatment retained a functional song system with song nuclei that had specialized gene expression similar, but not identical to males. We also observed that different song nuclei responded to estrogen manipulation differently, with Area X in the striatum being the most altered by estrogen modulation. These findings support the hypothesis that song learning is an ancestral trait in both sexes that was subsequently suppressed in females of some species and that estrogen has come to play a critical role in modulating this suppression as well as refinement of song learning.

Discovery of a multi-target compound for estrogen receptor-positive (ER+) breast cancer: Involvement of aromatase and ERs

Despite intense research, breast cancer remains the leading cause of cancer-related death in women worldwide, being estrogen receptor-positive (ER⁺) the most common subtype. Nowadays, aromatase inhibitors (AIs), the selective estrogen receptor modulator (SERM) tamoxifen and the selective estrogen receptor down-regulator (SERD) fulvestrant are used as therapeutic options for ER⁺ breast cancer, since they interfere directly with the production of estrogens and with the activation of estrogen-dependent signaling pathways. Despite the success of these treatments, the occurrence of resistance limits their clinical efficacy, demanding the development of novel therapies. Recently, multi-target compounds emerged as promising therapeutic strategies for ER⁺ breast cancer, as they can potentially modulate several important targets simultaneously. In line with this, in this work, the anti-cancer properties and multi-target action of 1,1-Bis(4-hydroxyphenyl)-2-phenylbut-1-ene, tamoxifen bisphenol (1,1-BHPE), were evaluated in an ER⁺ breast cancer cell model (MCF-7aro cells). Molecular docking analysis predicted that 1,1-BHPE was able to bind to aromatase, ERα and ERβ. In vitro studies showed that, although it did not present anti-aromatase activity, 1,1-BHPE reduced aromatase protein levels and interfered with ERα and ERβ signaling pathways, acting as an ERα antagonist and inducing ERβ up-regulation. Through these mechanisms, 1,1-BHPE was able to impair breast cancer growth and induce apoptosis. This represents an important therapeutic advantage because the main players responsible for estrogen production and signaling are modulated by a single compound. To the best of our knowledge, this is the first study describing the anti-cancer properties of 1,1-BHPE as a multi-target compound specific for ER⁺ breast cancer.

New small-molecule compound Hu-17 inhibits estrogen biosynthesis by aromatase in human ovarian granulosa cancer cells

Estrogen-dependent cancers (breast, endometrial, and ovarian) are among the leading causes of morbidity and mortality in women worldwide. Aromatase is the main enzyme that catalyzes the biosynthesis of estrogen, which drives proliferation, and antiestrogens can inhibit the growth of these estrogen-dependent cancers. Hu-17, an aromatase inhibitor, is a novel small-molecule compound that suppresses viability of and promotes apoptosis in ovarian cancer cells. Therefore, this study aimed to predict targets of Hu-17 and assess its intracellular signaling in ovarian cancer cells. Using the Similarity Ensemble Approach software to predict the potential mechanism of Hu-17 and combining phospho-proteome arrays with western blot analysis, we observed that Hu-17 could inhibit the ERK pathway, resulting in reduced estrogen synthesis in KGN cells, a cell line derived from a patient with invasive ovarian granulosa cell carcinoma. Hu-17 reduced the expression of CYP19A1 mRNA, responsible for producing aromatase, by suppressing the phosphorylation of cAMP response element binding-1. Hu-17 also accelerated aromatase protein degradation but had no effect on aromatase activity. Therefore, Hu-17 could serve as a potential treatment for estrogen-dependent cancers albeit further investigation is warranted.

Changes in serum estrogenic activity during neoadjuvant therapy with letrozole and exemestane

The aromatase inhibitors (AIs), letrozole (Femar®/Femara®) and exemestane (Aromasin®), are widely used to treat estrogen receptor (ER) positive breast cancer in postmenopausal patients. In the setting of metastatic breast cancer, these drugs may be used after another causing new responses in selected patients after progressing on the first choice. The precise explanation for this "lack of cross resistance" is still missing. NEOLETEXE is a neoadjuvant, randomized, open-label, cross-over trial. Postmenopausal patients with ER-positive, HER-2 negative, locally advanced breast cancer were enrolled. All patients were randomized to treatment starting with either letrozole or exemestane for at least 2 months followed by another 2 months on the alternative AI. The total estrogenic activities in blood samples were determined using the AroER tri-screen assay developed in the Chen laboratory. Using this highly sensitive assay, estrogenic activity was detected at three time points for all patients. Importantly, a significantly higher total estrogenic activity was found during therapy with exemestane compared to letrozole in 21 out of 26 patients. When letrozole was included in the AroER tri-screen assay, the estrogenic activities in most samples collected during exemestane treatment were further reduced, suggesting that low levels of androgens remained in specimens obtained after exemestane treatment. Our results suggest the AroER tri-screen to be a very sensitive method to estimate the overall estrogen-mediated activity in human samples even during therapy with highly potent aromatase inhibitors. In the present study, serum estrogen activity was significantly higher during exemestane therapy when compared to letrozole therapy.

The potential clinical benefit of targeting androgen receptor (AR) in estrogen-receptor positive breast cancer cells treated with Exemestane

The development of acquired resistance to the aromatase inhibitors (AIs) used in clinic is being considered the major concern in estrogen-receptor positive (ER⁺) breast cancer therapy. Recently, androgen receptor (AR) has gained attention in the clinical setting, since it has been implicated in AIs-resistance, although, different roles for AR in cell fate have been described. In this work, our group elucidates, for the first time, the oncogenic role of AR in sensitive and resistant ER⁺ breast cancer cells treated with the potent third-generation steroidal AI Exemestane (Exe). We demonstrate that Exe promotes an overexpression/activation of AR, which has an oncogenic and pro-survival role in Exe-sensitive and Exe-resistant cells. Moreover, we also disclose that targeting AR with bicalutamide (CDX) in Exe-treated cells, enhances the efficacy of this AI in sensitive cells and re-sensitizes resistant cells to Exe treatment. Furthermore, by targeting AR in Exe-resistant cells, it is also possible to block the activation of the ERK₁/₂ and PI3K cell survival pathways, hamper ERα activation and increase ERβ expression. Thus, this study, highlights a new mechanism involved in Exe-acquired resistance, implicating AR as a key molecule in this setting and suggesting that Exe-resistant cells may have an AR-dependent but ER-independent mechanism. Hence we propose AR antagonism as a potential and attractive therapeutic strategy to overcome Exe-acquired resistance or to enhance the growth inhibitory properties of Exe on ER⁺ breast cancer cells, improving breast cancer treatment.

Effects of new C6-substituted steroidal aromatase inhibitors in hormone-sensitive breast cancer cells: Cell death mechanisms and modulation of estrogen and androgen receptors

Estrogen receptor-positive (ER⁺) breast cancers require estrogens for their growth. Aromatase inhibitors (AIs) are considered the first-line therapy for this type of tumours. Despite the well-established clinical benefit of this therapy, the search for novel potent AIs that present higher efficacy and fewer side effects is still demanded. Thus, taking into account the known interactions of the natural substrate, androstenedione, within the aromatase active-site, a range of new steroidal compounds have been designed, synthesized and studied by our group. In this work, it was evaluated in MCF-7aro, an ER⁺ breast cancer cell line that overexpress aromatase, the anti-aromatase efficacy and the biological effects of eight new AIs: 6α-methyl-5α-androst-3-en-17-one (1a), 6α-methyl-3α,4α-epoxy-5α-androstan-17-one (3a), 6α-methylandrost-4-ene-3,17-dione (9), 6α-allylandrosta-1,4-diene-3,17-dione (13), 6α-allylandrost-4-ene-3,17-dione (15), 6α-allylandrost-4-en-17-one (17), 6β-hydroxyandrost-4-ene-3,17-dione (19) and 6α-hydroxyandrost-4-ene-3,17-dione (20). Their anti-cancer properties were elucidated, as well as, the dependence of their mechanism of action on aromatase inhibition and/or on steroid receptors modulation, such as estrogen and androgen receptors, which are key targets for this type of cancer. Results demonstrate that the studied AIs present high anti-aromatase activity, disrupt MCF-7aro cell cycle progression and induce apoptosis, through the mitochondrial pathway. Compounds 1a, 3a, 9, 13, 15 and 17 exhibited an aromatase-dependent effect on cells and, interestingly, steroids 9 and 13 displayed the ability to decrease aromatase protein levels without affecting CYP19A1 mRNA levels. Furthermore, the effects of compounds 1a, 3a and 15 were dependent on ER and on AR modulation, whereas compounds 9 and 19 were only dependent on AR modulation. From a clinical point of view, these actions can be considered as a therapeutic advantage for this type of tumours. Thus, new promising AIs that impair ER⁺ breast cancer cell growth, by acting on aromatase, and even, on ER and AR were discovered. Furthermore, new insights on the most favourable structural modifications in the steroidal core structure were provided, helping to a more rational drug design of new and potent AIs.

Preclinical evaluation of exemestane as a novel chemotherapy for gastric cancer

CYP19A1/aromatase (Ar) is a prognostic biomarker of gastric cancer (GCa). Ar is a critical enzyme for converting androstenedione to oestradiol in the steroidogenesis cascade. For decades, Ar has been targeted with Ar inhibitors (ARIs) in gynaecologic malignancies; however, it is unexplored in GCa. A single‐cohort tissue microarray examination was conducted to study the association between Ar expression and disease outcome in Asian patients with GCa. The results revealed that Ar was a prognostic promoter. Bioinformatics analyses conducted on a Caucasian‐based cDNA microarray databank showed Ar to be positively associated with GCa prognosis for multiple clinical modalities, including surgery, 5‐Fluorouracil (5‐FU) for adjuvant chemotherapy, or HER2 positivity. These findings imply that targeting Ar expression exhibits a potential for fulfilling unmet medical needs. Hence, Ar‐targeting compounds were tested, and the results showed that exemestane exhibited superior cancer‐suppressing efficacy to other ARIs. In addition, exemestane down‐regulated Ar expression. Ablating Ar abundance with short hairpin (sh)Ar could also suppress GCa cell growth, and adding 5‐FU could facilitate this effect. Notably, adding oestradiol could not prevent exemestane or shAr effects, implicating a nonenzymatic mechanism of Ar in cancer growth. Regarding translational research, treatment with exemestane alone exhibited tumour suppression efficacy in a dose‐dependent manner. Combining subminimal doses of 5‐FU and exemestane exerted an excellent tumour suppression effect without influencing bodyweight. This study validated the therapeutic potentials of exemestane in GCa. Combination of metronomic 5‐FU and exemestane for GCa therapy is recommended.

Hormone-dependent breast cancer: Targeting autophagy and PI3K overcomes Exemestane-acquired resistance

The leading cause of cancer death in women around the world is breast cancer. The aromatase inhibitors (AIs) are considered - as first-line treatment for estrogen receptor-positive (ER⁺) breast tumors, in postmenopausal women. Exemestane (Exe) is a powerful steroidal AI, however, despite its therapeutic success, Exe-acquired resistance may occur leading to tumor relapse. Our group previously demonstrated that autophagy acts as a pro-survival process in Exe-induced cell death of ER⁺ sensitive breast cancer cells. In this work, the role of autophagy and its relationship with the PI3K/AKT/mTOR pathway in Exe-acquired resistance was explored. In that way, the mechanism behind the effects of the combination of Exe with pan-PI3K, or autophagic inhibitors, was studied in a long-term estrogen deprived ER⁺ breast cancer cell line (LTEDaro cells). Our results indicate that Exe induces autophagy as a cytoprotective mechanism linked to acquired resistance. Moreover, it was demonstrated that by inhibiting autophagy and/or PI3K pathway it is possible to revert Exe-resistance through apoptosis promotion, disruption of cell cycle, and inhibition of cell survival pathways. This work provides new insights into the mechanisms involved in Exe-acquired resistance, pointing autophagy as an attractive therapeutic target to surpass it. Thus, it highlights new targets that together with aromatase inhibition may improve ER⁺ breast cancer therapy, overcoming AIs-acquired resistance.

Identification and Quantification of Novel Major Metabolites of the Steroidal Aromatase Inhibitor, Exemestane

Exemestane (EXE) is an aromatase inhibitor used for the prevention and treatment of estrogen receptor–positive breast cancer. Although the known major metabolic pathway for EXE is reduction to form the active 17β-dihydro-EXE (17β-DHE) and subsequent glucuronidation to 17β-hydroxy-EXE-17-O-β-D-glucuronide (17β-DHE-Gluc), previous studies have suggested that other major metabolites exist for exemestane. In the present study, a liquid chromatography–mass spectrometry (LC-MS) approach was used to acquire accurate mass data in MS^E mode, in which precursor ion and fragment ion data were obtained simultaneously to screen novel phase II EXE metabolites in urine specimens from women taking EXE. Two major metabolites predicted to be cysteine conjugates of EXE and 17β-DHE by elemental composition were identified. The structures of the two metabolites were confirmed to be 6-methylcysteinylandrosta-1,4-diene-3,17-dione (6-EXE-cys) and 6-methylcysteinylandrosta-1,4-diene-17β-hydroxy-3-one (6-17β-DHE-cys) after comparison with their chemically synthesized counterparts. Both underwent biosynthesis in vitro in three stepwise enzymatic reactions, with the first involving glutathione conjugation. The cysteine conjugates of EXE and 17β-DHE were subsequently quantified by liquid chromatography–mass spectrometry in the urine and matched plasma samples of 132 subjects taking EXE. The combined 6-EXE-cys plus 6-17β-DHE-cys made up 77% of total EXE metabolites in urine (vs. 1.7%, 0.14%, and 21% for EXE, 17β-DHE, and 17β-DHE-Gluc, respectively) and 35% in plasma (vs. 17%, 12%, and 36% for EXE, 17β-DHE, and 17β-DHE-Gluc, respectively). Therefore, cysteine conjugates of EXE and 17β-DHE appear to be major metabolites of EXE in both urine and plasma.

Hemisynthesis, computational and molecular docking studies of novel nitrogen containing steroidal aromatase inhibitors: testolactam and testololactam

Combined DFT and molecular docking studies of synthesized steroidal lactams reveal their potential as aromatase inhibitors.

Comprehensive and Automated Linear Interaction Energy Based Binding-Affinity Prediction for Multifarious Cytochrome P450 Aromatase Inhibitors

Cytochrome P450 aromatase (CYP19A1) plays a key role in the development of estrogen dependent breast cancer, and aromatase inhibitors have been at the front line of treatment for the past three decades. The development of potent, selective and safer inhibitors is ongoing with in silico screening methods playing a more prominent role in the search for promising lead compounds in bioactivity-relevant chemical space. Here we present a set of comprehensive binding affinity prediction models for CYP19A1 using our automated Linear Interaction Energy (LIE) based workflow on a set of 132 putative and structurally diverse aromatase inhibitors obtained from a typical industrial screening study. We extended the workflow with machine learning methods to automatically cluster training and test compounds in order to maximize the number of explained compounds in one or more predictive LIE models. The method uses protein-ligand interaction profiles obtained from Molecular Dynamics (MD) trajectories to help model search and define the applicability domain of the resolved models. Our method was successful in accounting for 86% of the data set in 3 robust models that show high correlation between calculated and observed values for ligand-binding free energies (RMSE < 2.5 kJ mol-1), with good cross-validation statistics.

Anti-tumor efficacy of new 7α-substituted androstanes as aromatase inhibitors in hormone-sensitive and resistant breast cancer cells

The majority of breast cancer cases are estrogen receptor positive (ER⁺). Although, third-generation aromatase inhibitors (AIs) are used as first-line treatment in post-menopausal women, they cause endocrine resistance and bone loss, which limits their success. Therefore, there is a demand to discover new potent molecules, with less toxicity that can circumvent these drawbacks. Our group has previously demonstrated that new 7α-substituted steroidal molecules, 7α-(2ξ,3ξ-epoxypropyl)androsta-1,4-diene-3,17-dione (3), 7α-allylandrost-4-ene-3,17-dione (6), 7α-allylandrost-4-en-17-one (9), 7α-allyl-3-oxoandrosta-1,4-dien-17β-ol (10) and 7α-allylandrosta-1,4-diene-3,17-dione (12) are potent AIs in placental microsomes. In this work, it was investigated their anti-aromatase activity and in vitro effects in sensitive and resistant breast cancer cells. All the steroids efficiently inhibit aromatase in breast cancer cells, allowing to establish new structure-activity relationships for this class of compounds. Moreover, the new AIs can inhibit breast cancer cell growth, by causing cell cycle arrest and apoptosis. The effects of AIs 3 and 12 on sensitive cells were dependent on aromatase inhibition and androgen receptor (AR), while for AI 9 and AI 10 were AR- and ER-dependent, respectively. In addition, it was shown that all the AIs can sensitize resistant cancer cells being their behavior similar to the sensitive cells. In summary, this study contributes to the understanding of the structural modifications in steroidal scaffold that are translated into better aromatase inhibition and anti-tumor properties, providing important information for the rational design/synthesis of more effective AIs. In addition, allowed the discovery of new potent 7α-substituted androstane molecules to inhibit tumor growth and prevent endocrine resistance.

Role of the UGT2B17 deletion in exemestane pharmacogenetics

Exemestane (EXE) is an aromatase inhibitor used for the prevention and treatment of breast cancer. The major metabolic pathway for EXE is reduction to form the active 17β-dihydro-EXE (17β-DHE) and subsequent glucuronidation to 17β-hydroxy-EXE-17-O-β-D-glucuronide (17β-DHE-Gluc) by UGT2B17. The aim of the present study was to determine the effects of UGT2B17 copy number variation on the levels of urinary and plasma 17β-DHE-Gluc and 17β-DHE in patients taking EXE. Ninety-six post-menopausal Caucasian breast cancer patients with ER+ breast tumors taking 25 mg EXE daily were recruited into this study. UGT2B17 copy number was determined by a real-time PCR copy number variant assay and the levels of EXE, 17β-DHE and 17β-DHE-Gluc were quantified by UPLC/MS in patients’ urine and plasma. A 39-fold decrease (P<0.0001) in the levels of creatinine-adjusted urinary 17β-DHE-Gluc was observed among UGT2B17 (*2/*2) subjects vs. subjects with the UGT2B17 (*1/*1) genotype. The plasma levels of 17β-DHE-Gluc was decreased 29-fold (P<0.0001) in subjects with the UGT2B17 (*2/*2) genotype vs. subjects with UGT2B17 (*1/*1) genotype. The levels of plasma EXE-adjusted 17β-DHE was 28% higher (P=0.04) in subjects with the UGT2B17 (*2/*2) genotype vs. subjects with the UGT2B17 (*1/*1) genotype. These data indicate that UGT2B17 is the major enzyme responsible for 17β-DHE-Gluc formation in vivo and that the UGT2B17 copy number variant may play a role in inter-individual variability in 17β-DHE levels in vivo.

Unravelling exemestane: From biology to clinical prospects

Aromatase inhibitors (AIs) are anti-tumor agents used in clinic to treat hormone-dependent breast cancer. AIs block estrogens biosynthesis by inhibiting the enzyme aromatase, preventing tumor progression. Exemestane, a third-generation steroidal AI, belongs to this class of drugs and is currently used in clinic to treat postmenopausal women, due to its high efficacy and good tolerability. Here, its pharmacological and biological aspects as well as its clinical applications and comparison to other endocrine therapeutic agents, are reviewed. It is also focused the benefits and risks of exemestane, drawbacks to be overcome and aspects to be explored.

Structural and functional characterization of aromatase, estrogen receptor, and their genes in endocrine-responsive and -resistant breast cancer cells

Aromatase and estrogen receptor α (ER) are two key proteins for the proliferation of endocrine-responsive and -resistant breast cancers. Aromatase is an enzyme involved in the conversion of androgen (such as testosterone) to estrogen (such as 17β-estradiol). It is also a very effective therapeutic target for the treatment of endocrine-responsive breast cancer. Comparing endocrine-responsive and -resistant breast cancer, aromatase protein levels do not change significantly. Aromatase activity; however, can be increased via PI3K/Akt/IGFR signaling pathways in endocrine resistant cells. The activity of aromatase has been reported to be modulated by phosphorylation. The ER is an important steroid nuclear receptor in the proliferation of both endocrine-responsive and -resistant cells. Although the mutation or amplification of ER can cause endocrine resistance, it is not commonly found. Some point mutations and translocation events have been characterized and shown to promote estrogen-independent growth. Phosphorylation by cross-talk with growth factor pathways is one of the main mechanisms for ligand-independent activation of ER. Taken together, both ER and aromatase are important in ER-dependent breast cancer and the development of endocrine resistance.

Cell-Based High-Throughput Screening for Aromatase Inhibitors in the Tox21 10K Library

Multiple mechanisms exist for endocrine disruption; one nonreceptor-mediated mechanism is via effects on aromatase, an enzyme critical for maintaining the normal in vivo balance of androgens and estrogens. We adapted the AroER tri-screen 96-well assay to 1536-well format to identify potential aromatase inhibitors (AIs) in the U.S. Tox21 10K compound library. In this assay, screening with compound alone identifies estrogen receptor alpha (ERα) agonists, screening in the presence of testosterone (T) identifies AIs and/or ERα antagonists, and screening in the presence of 17β-estradiol (E2) identifies ERα antagonists. Screening the Tox-21 library in the presence of T resulted in finding 302 potential AIs. These compounds, along with 31 known AI actives and inactives, were rescreened using all 3 assay formats. Of the 333 compounds tested, 113 (34%; 63 actives, 50 marginal actives) were considered to be potential AIs independent of cytotoxicity and ER antagonism activity. Structure-activity analysis suggested the presence of both conventional (eg, 1, 2, 4, - triazole class) and novel AI structures. Due to their novel structures, 14 of the 63 potential AI actives, including both drugs and fungicides, were selected for confirmation in the biochemical tritiated water-release aromatase assay. Ten compounds were active in the assay; the remaining 4 were only active in high-throughput screen assay, but with low efficacy. To further characterize these 10 novel AIs, we investigated their binding characteristics. The AroER tri-screen, in high-throughput format, accurately and efficiently identified chemicals in a large and diverse chemical library that selectively interact with aromatase.

Anti-tumor effect of Shu-Gan-Liang-Xue decoction in breast cancer is related to the inhibition of aromatase and steroid sulfatase expression

ETHNOPHARMACOLOGICAL RELEVANCE

Shu-Gan-Liang-Xue Decoction (SGLXD), a traditional Chinese herbal formula used to ameliorate the hot flushes in breast cancer patients, was reported to have anti-tumor effect on breast cancer. Estrogen plays a critical role in the genesis and evolution of breast cancer. Aromatase and steroid sulfatase (STS) are key estrogen synthesis enzymes that predominantly contribute to the high local hormone concentrations. The present study was to evaluate the anti-tumor effect of SGLXD on estrogen receptor (ER) positive breast cancer cell line ZR-75-1, and to investigate its underlying mechanisms both in vitro and in vivo.

MATERIALS AND METHODS

The anti-tumor activity of SGLXD in vitro was investigated using the MTT assay. The in vivo anti-tumor effect of SGLXD was evaluated in non-ovariectomized and ovariectomized athymic nude mice. The effect of SGLXD on enzymatic activity of aromatase and STS was examined using the dual-luciferase reporter (DLR) based on bioluminescent measurements. Aromatase and STS protein level were assessed using Western blot assay.

RESULTS

SGLXD showed dose-dependent inhibitory effect on the proliferation of ZR-75-1 cells with IC50 value of 3.40 mg/mL. It also suppressed the stimulating effect on cell proliferation of testosterone and estrogen sulfates (E1S). Oral administration of 6 g/kg of SGLXD for 25 days resulted in a reduction in tumor volume in non-ovariectomized and ovariectomized nude mice. The bioluminescent measurements confirmed that SGLXD has a dual-inhibitory effect on the activity of aromatase and STS. Western blot assay demonstrated that the treatment of SGLXD resulted in a decrease in aromatase and STS protein levels both in vitro and in vivo.

CONCLUSION

Our results suggested that SGLXD showed anti-tumor effect on breast cancer cells both in vitro and in vivo. The anti-tumor activity of SGLXD is related to inhibition of aromatase and STS via decreasing their expression. SGLXD may be considered as a novel treatment for ER positive breast cancer.

Exploring novel strategies for AIDS protozoal pathogens: α-helix mimetics targeting a key allosteric protein-protein interaction in C. hominis TS-DHFR

The bifunctional enzyme thymidylate synthase-dihydrofolate reductase (TS-DHFR) from the protozoal parasite Cryptosporidium hominis is a potential molecular target for the design of antiparasitic therapies for AIDS-related opportunistic infections. The enzyme exists as a homodimer with each monomer containing a unique swap domain known as a "crossover helix" that binds in a cleft on the adjacent DHFR active site. This crossover helix is absent in species containing monofunctional forms of DHFR such as human. An in-depth understanding of protein-protein interactions between the crossover helix and adjacent DHFR active site that might modulate enzyme integrity or function would allow for insights into rational design of species-specific allosteric inhibitors. Mutational analysis coupled with structural studies and biophysical and kinetic characterization of crossover helix mutants identifies this domain as essential for full enzyme stability and catalytic activity, and pinpoints these effects to distinct faces of the crossover helix important in protein-protein interactions. Moreover, targeting this helical protein interaction with α-helix mimetics of the crossover helix leads to selective inhibition and destabilization of the C. hominis TS-DHFR enzyme, thus validating this region as a new avenue to explore for species-specific inhibitor design.

Inhibitory effect of luteolin on estrogen biosynthesis in human ovarian granulosa cells by suppression of aromatase (CYP19)

Inhibition of aromatase, the key enzyme in estrogen biosynthesis, is an important strategy in the treatment of breast cancer. Several dietary flavonoids show aromatase inhibitory activity, but their tissue specificity and mechanism remain unclear. This study found that the dietary flavonoid luteolin potently inhibited estrogen biosynthesis in a dose- and time-dependent manner in KGN cells derived from human ovarian granulosa cells, the major source of estrogens in premenopausal women. Luteolin decreased aromatase mRNA and protein expression in KGN cells. Luteolin also promoted aromatase protein degradation and inhibited estrogen biosynthesis in aromatase-expressing HEK293A cells, but had no effect on recombinant expressed aromatase. Estrogen biosynthesis in KGN cells was inhibited with differing potencies by extracts of onion and bird chili and by four other dietary flavonoids: kaempferol, quercetin, myricetin, and isorhamnetin. The present study suggests that luteolin inhibits estrogen biosynthesis by decreasing aromatase expression and destabilizing aromatase protein, and it warrants further investigation as a potential treatment for estrogen-dependent cancers.

Apoptosis and autophagy in breast cancer cells following exemestane treatment

Aromatase inhibitors (AIs), which block the conversion of androgens to estrogens, are used for hormone-dependent breast cancer treatment. Exemestane, a steroidal that belongs to the third-generation of AIs, is a mechanism-based inhibitor that binds covalently and irreversibly, inactivating and destabilizing aromatase. Since the biological effects of exemestane in breast cancer cells are not totally understood, its effects on cell viability, cell proliferation and mechanisms of cell death were studied in an ER-positive aromatase-overexpressing breast cancer cell line (MCF-7aro). The effects of 3-methyladenine (3-MA), an inhibitor of autophagy and of ZVAD-FMK, an apoptotic inhibitor, in exemestane treated cells were also investigated. Our results indicate that exemestane induces a strong inhibition in MCF-7aro cell proliferation in a dose- and time-dependent manner, promoting a significant cell cycle arrest in G(0)/G1 or in G(2)/M phases after 3 and 6 days of treatment, respectively. This was accompanied by a decrease in cell viability due to activation of cell death by apoptosis, via mitochondrial pathway and the occurrence of autophagy. Inhibition of autophagy by the autophagic inhibitor, 3-MA, resulted in a reduction of cell viability and activation of caspases. All together the results obtained suggest that exemestane induced mitochondrial-mediated apoptosis and autophagy, which act as a pro-survival process regulating breast cancer cell apoptosis.

[Hormone therapy and breast cancer]

The nuclear estrogen receptors (ER) are the major targets for endocrine treatment of hormone-dependent breast cancers. Hormone therapy blocked endogenous estrogen activation of ER, either by competitive inhibition of endogenous estrogens (selective estrogen receptor modulators - SERM or selective estrogen receptor down regulators - SERD) or by inhibition of estrogen synthesis (aromatase inhibitors) from adrenal androgens in post-menopausal women. The efficacy of these treatments has been shown on large series of breast cancer patients. However de novo or acquired resistance to treatment occurs. The better knowledge of the mechanism of action of such treatment may help to better understand them, and also for the determinism of adverse side effects of the different class of molecules.

Molecular Docking of Aromatase Inhibitors

Aromatase is an enzyme that plays a critical role in the development of estrogen receptor positive breast cancer. As aromatase catalyzes the aromatization of androstenedione to estrone, a naturally occurring estrogen, it is a promising drug target for therapeutic management. The undesirable effects found in aromatase inhibitors (AIs) that are in clinical use necessitate the discovery of novel AIs with higher selectivity, less toxicity and improving potency. In this study, we elucidate the binding mode of all three generations of AI drugs to the crystal structure of aromatase by means of molecular docking. It was demonstrated that the docking protocol could reliably reproduce the interaction of aromatase with its substrate with an RMSD of 1.350 Å. The docking study revealed that polar (D309, T310, S478 and M374), aromatic (F134, F221 and W224) and non-polar (A306, A307, V370, L372 and L477) residues were important for interacting with the AIs. The insights gained from the study herein have great potential for the design of novel AIs.

An "omics" approach to determine the mechanisms of acquired aromatase inhibitor resistance

Aromatase inhibitors (AIs) are the major types of drugs to treat hormone-dependent breast cancer. Although these drugs work effectively, cancer still recurs in many patients after treatment as a result of acquired resistance to the AIs. To characterize the resistant mechanisms, a set of MCF-7aro cell lines that acquired resistance to the AIs was generated. Through an "Omics" approach, we found that the resistance mechanisms of the three AIs (anastrozole, letrozole, and exemestane) differ and activation of estrogen receptor alpha (ERα) is critical for acquired AI resistance. Our results reveal that growth factor/signal transduction pathways are upregulated after ERα-dependent pathways are suppressed by AIs, and ERα can then be activated through different crosstalk mechanisms.

Insulin-like growth factor I enhances the expression of aromatase P450 by inhibiting autophagy

Aromatase, a key enzyme of estrogen biosynthesis, is transcriptionally regulated by many growth factors. IGF-I enhances aromatase activity in a variety of cells, but the mechanism of action has not been determined. We herein report our finding of a novel mechanism of action for IGF-I. IGF-I enhanced the dexamethasone (DEX)-induced aromatase activity by 30% in serum-starved THP-1 cells. The increase was associated with a corresponding increase in the level of aromatase protein but not with any change in the mRNA level. Metabolic labeling experiments revealed that IGF-I inhibited the degradation of aromatase. We identified pepstatin A as the most effective inhibitor of aromatase degradation by in vitro assay. Using a nontoxic concentration of pepstatin A, we examined IGF-I's action on aromatase distribution in microsomes and lysosomes. In the presence of pepstatin A, DEX caused an increase in the amount of aromatase in both microsomes and lysosomes, and IGF-I attenuated the DEX-induced accumulation of aromatase in lysosomes and, conversely, enhanced its accumulation in the microsomes. The addition of serum abolished the IGF-I-induced changes. The transport from microsome to lysosome was fluorescently traced in cells using a recombinant aromatase. IGF-I selectively reduced the aromatase signal in the lysosomes. Finally, we observed that IGF-I enhanced the aromatase activity by 50% as early as 1 h after treatment; furthermore, rapamycin, an enhancer of autophagy, completely negated the effect of IGF-I on the enzyme. These results indicate that IGF-I enhances aromatase by the inhibition of autophagy.

The HDAC inhibitor LBH589 (panobinostat) is an inhibitory modulator of aromatase gene expression

Aromatase converts androgens to estrogens. Although third-generation aromatase inhibitors (AIs) are important drugs in hormonal therapy for breast cancer in postmenopausal women, there are concerns about the side effects associated with the estrogen deprivation achieved with AIs. Expression of aromatase in breast cancer tissue is driven by different promoters than those in noncancer tissues; thus, suppression of aromatase expression in cancer tissues through the down-regulation of breast tumor-specific promoters would reduce the side effects associated with whole-body suppression of estrogen biosynthesis by AIs. We report that histone deacetylase inhibitor LBH589 (panobinostat) is a potent inhibitor of aromatase expression (with an IC(50) value < 25 nM). LBH589 selectively suppresses human aromatase gene promoters I.3/II, which are preferentially used in breast cancer tissue. Furthermore, using the H295R cell culture model, we found that achieving the same degree of inhibition of aromatase activity required only one-fifth as much letrozole (an AI) in the presence of 25 nM LBH589 as in the absence of LBH589. We also used an H295R/MCF7 coculture model to demonstrate the synergistic interaction of LBH589 + letrozole in suppressing the proliferation of hormone-responsive breast cancer cells. Finally, our results also indicate that LBH589 down-regulates the activity of promoters I.3/II in an epigenetic fashion. LBH589 reduces the levels of C/EBPdelta, decreases the binding of C/EBPdelta, and increases the levels and binding of acetyl-histones to the promoters I.3/II. These findings provide an important basis for future clinical evaluations of LBH589 in hormone-dependent breast cancer.

Molecular characterization of aromatase inhibitor-resistant, tamoxifen-resistant and LTEDaro cell lines

To determine potential genes involved in mediating resistance to aromatase inhibitors (AIs), a microarray study was performed using MCF-7aro (aromatase overexpressing) cells that are resistant to letrozole (T+LET R), anastrozole (T+ANA R) and exemestane (T+EXE R), as well as LTEDaro and tamoxifen-resistant (T+TAM R) lines for comparison. Based on hierarchical clustering, estrogen-responsive genes were found to be differentially expressed in AI-resistant lines versus LTEDaro and T+TAM R. Additional genome-wide analysis showed that gene expression profiles of the non-steroidal AI-resistant lines were most closely correlated and that T+EXE R lines exhibit differing profiles. Also, LTEDaro and T+TAM R lines are inherently different from expression profiles of AI-resistant lines. Further characterization of these resistant lines revealed that T+LET R, T+ANA R and LTEDaro cells contain a constitutively active estrogen receptor alpha (ERalpha) that does not require the ligand estrogen for activation. Ligand-independent activation of ERalpha does not activate identical estrogen-responsive gene profiles in AI-resistant lines as in LTEDaro lines, thereby establishing differing mechanisms of resistance. This ligand-independent activation of ER was not observed in the parental cell lines MCF-7aro, T+EXE R or T+TAM R cells. Based on the steroidal structure of EXE, our laboratory has shown that this AI has weak estrogen-like properties, and that EXE resistance involves an ER-dependent crosstalk with EGFR growth factor signaling. Recent studies in our laboratory pertaining to pre-clinical models of AI treatment revealed that intermittent use of EXE delays the onset of acquired resistance in comparison to continuous treatment. Specific molecular mechanisms involved in intermittent use of EXE are currently being explored, based on microarray gene expression profiling. Lastly, our laboratory has initiated a study of microRNAs and their potential role in regulating target genes involved in AI-resistance. Overall, we propose a model of acquired resistance that progresses from hormone-dependence (T+TAM R and T+EXE R) to hormone-independence (T+LET R and T+ANA R), eventually resulting in hormone-independence that does not rely on conventional ER signaling (LTEDaro).

Antiproliferative effect of exemestane in lung cancer cells

BACKGROUND

Recent evidence suggests that estrogen signaling may be involved in the pathogenesis of non-small cell lung cancer (NSCLC). Aromatase is an enzyme complex that catalyses the final step in estrogen synthesis and is present in several tissues, including the lung. In the current study we investigated the activity of the aromatase inhibitor exemestane in human NSCLC cell lines H23 and A549.

RESULTS

Aromatase expression was detected in both cell lines. H23 cells showed lower protein and mRNA levels of aromatase, compared to A549 cells. Exemestane decreased cell proliferation and increased apoptosis in both cell lines, 48 h after its application, with A549 exhibiting higher sensitivity than H23 cells. Aromatase protein and mRNA levels were not affected by exemestane in A549 cells, whereas an increase in both protein and mRNA levels was observed in H23 cells, 48 h after exemestane application. Moreover, an increase in cAMP levels was found in both cell lines, 15 min after the administration of exemestane. In addition, we studied the effect of exemestane on epidermal growth factor receptor (EGFR) localization and activation. Exemestane increased EGFR activation 15 min after its application in H23 cells. Furthermore, we demonstrated a translocation of EGFR from cell membrane, 24 h after the addition of exemestane in H23 cells. No changes in EGFR activation or localization were observed in A549 cells.

CONCLUSION

Our findings suggest an antiproliferative effect of exemestane on NSCLC cell lines. Exemestane may be more effective in cells with higher aromatase levels. Further studies are needed to assess the activity of exemestane in NSCLC.

Exemestane: a review of its use in postmenopausal women with breast cancer

Exemestane (Aromasin) is an orally active steroidal irreversible inactivator of the aromatase enzyme indicated as an adjuvant treatment in postmenopausal women with estrogen receptor-positive early-stage breast cancer following 2-3 years of adjuvant treatment with tamoxifen, and for the treatment of advanced breast cancer in postmenopausal women whose disease has progressed following tamoxifen or other antiestrogen therapy. Exemestane is effective for the treatment of postmenopausal women with early-stage or advanced breast cancer. In early-stage disease, switching to exemestane for 2-3 years after 2-3 years of adjuvant tamoxifen treatment was more effective in prolonging disease-free survival than continuing tamoxifen therapy, although it was not associated with an overall survival benefit, except in those with estrogen receptor-positive or unknown receptor status disease when nodal status, hormone replacement therapy (HRT) and chemotherapy use were adjusted for. Moreover, preliminary data suggest that the efficacy of exemestane is generally no different to that of tamoxifen in the primary adjuvant treatment of early-stage breast cancer, although exemestane may be better in prolonging the time to distant recurrence. In advanced disease, exemestane showed equivalent efficacy to megestrol in patients with disease refractory to tamoxifen and an efficacy not significantly different from that of fulvestrant in those refractory to a nonsteroidal aromatase inhibitor. Available data, some of which are limited, suggest exemestane is also effective in the first-line hormonal treatment of advanced breast cancer in postmenopausal women. Exemestane is generally well tolerated, although the potential bone fracture risk of the drug requires further investigation. Results from directly comparative trials indicating the efficacy, tolerability and bone fracture risk of exemestane relative to third-generation aromatase inhibitors and other agents in both early-stage and advanced disease, as well as the optimal sequence of endocrine therapies, are awaited with interest. In the meantime, switching to exemestane should be considered in postmenopausal women who have received 2-3 years of adjuvant tamoxifen treatment for early-stage breast cancer, and is an emerging treatment option for postmenopausal women with advanced breast cancer refractory to one or more antiestrogen therapies.

Lack of complete cross-resistance between different aromatase inhibitors; a real finding in search for an explanation?

While third-generation aromatase inhibitors (anastrozole, letrozole and exemestane) are successfully implemented as adjuvant and first-line therapy for hormone-sensitive breast cancer in postmenopausal women, important questions remain to be addressed. An issue of particular interest is the question about lack of complete cross-resistance between steroidal and non-steroidal compounds. Although the studies reporting this phenomenon in general contain a small number of patients, the findings across the different reports seem consistent. While several potential mechanisms have been suggested, so far we lack scientific proof what mechanisms may be responsible for this finding. Finally, we do not know whether lack of cross-resistance actually signals an improved efficacy for certain compounds or may be due to alternative mechanisms of action. Neither do we know whether some tumours are more sensitive to particular drugs. This paper summarizes clinical findings up to now with respect to lack of cross-resistance and discuss potential mechanisms involved.

Aromatase is phosphorylated in situ at serine-118

Phosphorylation of the cytochrome P450 aromatase has been proposed as a switch to rapidly modulate enzymatic activity and estrogen biosynthesis. Herein, we demonstrate that aromatase serine-118 is a potential phosphorylation site in mammalian cells. The amino acid context surrounding S118 is highly conserved among diverse animal species and suggests that an AGC-like kinase may phosphorylate aromatase. Mutation of S118 to Ala blocked phosphorylation. Mutation of S118 to either Ala or Asp destabilized aromatase, indicating an important structural role for S118. The phosphomimetic S118D mutant showed decreased specific enzymatic activity, decreased Vmax, and increased Km, while the S118A phospho-inhibiting mutant showed opposite effects. Our findings suggest that phosphorylation of S118 may decrease aromatase activity, presenting a mechanism whereby kinase signaling may modulate estrogen production and hormone balance.

Genome-wide analysis of aromatase inhibitor-resistant, tamoxifen-resistant, and long-term estrogen-deprived cells reveals a role for estrogen receptor

Acquired resistance to either tamoxifen or aromatase inhibitors (AI) develops after prolonged treatment in a majority of hormone-responsive breast cancers. In an attempt to further elucidate mechanisms of acquired resistance to AIs, MCF-7aro cells resistant to letrozole (T+LET R), anastrozole (T+ANA R), and exemestane (T+EXE R), as well as long-term estrogen deprived (LTEDaro) and tamoxifen-resistant (T+TAM R) lines were generated. This is the first complete panel of endocrine therapy-resistant cell lines, which were generated as multiple independent biological replicates for unbiased genome-wide analysis using affymetrix microarrays. Although similarities are apparent, microarray results clearly show gene signatures unique to AI-resistance were inherently different from LTEDaro and T+TAM R gene expression profiles. Based on hierarchical clustering, unique estrogen-responsive gene signatures vary depending on cell line, with some genes up-regulated in all lines versus other genes up-regulated only in the AI-resistant lines. Characterization of these resistant lines showed that LTEDaro, T+LET R, and T+ANA R cells contained a constitutively active estrogen receptor (ER)alpha that does not require estrogen for activation. This ligand-independent activation of ER was not observed in the parental cells, as well as T+EXE R and T+TAM R cells. Further characterization of these resistant lines was performed using cell cycle analysis, immunofluorescence experiments to visualize ER subcellular localization, as well as cross-resistance studies to determine second-line inhibitor response. Using this well-defined model system, our studies provide important information regarding differences in resistance mechanisms to AIs, TAM, and LTEDaro, which are critical in overcoming resistance when treating hormone-responsive breast cancers.

Aromatase inhibitors: are there differences between steroidal and nonsteroidal aromatase inhibitors and do they matter?

Aromatase inhibitors (AIs) are approved for use in both early- and advanced-stage breast cancer in postmenopausal women. Although the currently approved "third-generation" AIs all powerfully inhibit estrogen synthesis, they may be subdivided into steroidal and nonsteroidal inhibitors, which interact with the aromatase enzyme differently. Nonsteroidal AIs bind noncovalently and reversibly to the aromatase protein, whereas steroidal AIs may bind covalently and irreversibly to the aromatase enzyme. The steroidal AI exemestane may exert androgenic effects, but the clinical relevance of this has yet to be determined. Switching between steroidal and nonsteroidal AIs produces modest additional clinical benefits, suggesting partial noncrossresistance between the classes of inhibitor. In these circumstances, the response rates to the second AI have generally been low; additional research is needed regarding the optimal sequence of AIs. To date, clinical studies suggest that combining an estrogen-receptor blocker with a nonsteroidal AI does not improve efficacy, while combination with a steroidal AI has not been evaluated. Results from head-to-head trials comparing steroidal and nonsteroidal AIs will determine whether meaningful clinical differences in efficacy or adverse events exist between the classes of AI. This review summarizes the available evidence regarding known differences and evaluates their potential clinical impact.

Characterization of the weak estrogen receptor alpha agonistic activity of exemestane

Third generation aromatase inhibitors (AI) have shown good clinical efficacy in comparison to the anti-estrogen tamoxifen. The steroidal AI, exemestane (EXE) has previously been shown to act as an androgen, but this report demonstrates the estrogen-like activity of EXE. Based on genome-wide microarray analysis, high correlation was seen between EXE-Only (EXE O, hormone-free) and hormone-containing AI-resistant lines. In addition, the top regulated genes in the EXE O lines were mostly estrogen-responsive genes. This estrogen-like activity of EXE was further validated using estrogen receptor (ER) activity assays, where in comparison to 17beta-estradiol (E2), EXE was able to induce ER activity, though at a higher concentration. Also, this EXE-mediated ER activity was blocked by the ER antagonist ICI as well as the ERalpha-specific antagonist methyl-piperidino-pyrazole (MPP). Similarly, EXE was able to induce proliferation of breast cancer cell lines, MCF-7 and MCF-7aro, as well as activate transcription of known estrogen-responsive genes, i.e., PGR, pS2 and AREG. These results suggest that EXE does have weak estrogen-like activity.

MCF-7aro/ERE, a novel cell line for rapid screening of aromatase inhibitors, ERalpha ligands and ERRalpha ligands

We have previously generated a breast cancer cell line, MCF-7aro, which over-expresses aromatase and is also ER positive. Recently, this MCF-7aro cell line was stably transfected with a promoter reporter plasmid, pGL3-Luc, containing three repeats of estrogen responsive element (ERE). Experiments using MCF-7aro/ERE have demonstrated that it is a novel, non-radioactive screening system for aromatase inhibitors (AIs), ERalpha ligands and ERRalpha ligands. The screening is carried out in a 96-well plate format. To evaluate this system, the cells were cultured overnight in charcoal-dextran stripped FBS medium supplemented with 0.1 nM testosterone or 17beta-estradiol, and various concentrations of antiestrogens or AIs. We found that the luciferase activity was induced when the cells were cultured either in the presence of testosterone or 17beta-estradiol. Furthermore, a 50% decrease in luciferase activity could be achieved when the cells were cultured in the presence of testosterone together with letrozole, anastrozole, tamoxifen or fulvestrant (concentrations being 75 nM, 290 nM, 100 nM, and 5 nM, respectively), compared to the testosterone-only cultured cells. Using this assay system, we confirmed that 3(2'-chlorophenyl)-7-methoxy-4-phenylcoumarin is an agonist of ER. Furthermore, genestein has been shown to be a ligand of ERRalpha because its binding could be blocked by an ERRalpha inverse agonist, XCT790. These results indicate that MCF-7aro/ERE is a novel cell line for rapid screening of AIs, ERalpha ligands and ERRalpha ligands.

The role of amphiregulin in exemestane-resistant breast cancer cells: evidence of an autocrine loop

Exemestane-resistant breast cancer cell lines (i.e., ExeR), derived from MCF-7 cells expressing a high level of aromatase (MCF-7aro), were generated in our laboratory. The epidermal growth factor (EGF)-like protein amphiregulin (AREG) was highly expressed in ExeR cells based on cDNA microarray analysis. The high levels of AREG mRNA in ExeR cell lines were confirmed by real-time reverse transcription-PCR. The high levels of AREG protein in ExeR cell lysates and culture media were confirmed by Western blot analysis and ELISA, respectively. Furthermore, our Western blot analysis showed that whereas no AREG was detected in the DMSO control, overnight treatment of parental MCF-7aro cells with 1 micromol/L exemestane strongly induced the expression of AREG. This induction was totally blocked by 100 nmol/L of pure antiestrogen ICI 182,780, implying estrogen receptor (ER) dependence of exemestane-induced AREG expression. MCF-7aro cells were not able to proliferate in hormone-free medium, but were able to proliferate in conditioned medium from ExeR cells, similar to the treatment of recombinant human AREG. Small interference RNA targeting AREG inhibited ExeR proliferation, confirming that AREG is truly functioning as a growth factor of ExeR cells. The specific inhibitors to ER (ICI 182,780), EGF receptor (EGFR; AG1478), and mitogen-activated protein kinase (MAPK; U0126) all showed dose-dependent suppression of the proliferation of ExeR cells, indicating the involvement of the ER, EGFR, and MAPK pathways. Based on these findings, we propose a possible mechanism that underlies exemestane resistance: exemestane induces AREG in an ER-dependent manner. AREG then activates the EGFR pathway and leads to the activation of the MAPK pathway that drives cell proliferation.