Where do selective estrogen receptor modulators (SERMs) and aromatase inhibitors (AIs) now fit into breast cancer treatment algorithms?

Role of Exemestane in the Treatment of Estrogen-Receptor-Positive Breast Cancer: A Narrative Review of Recent Evidence

INTRODUCTION

Breast cancer (BC) is the most common type of cancer diagnosed among women worldwide with an estimated 2.3 million new cases every year. Almost two-thirds of all patients with BC have estrogen receptor-positive (ER+) tumors. In this review, the clinical evidence of exemestane in different treatment settings in ER+ BC is presented and summarized.

SEARCH STRATEGY

A search strategy with the keywords "breast cancer [MeSH Terms]" AND "exemestane [Title/Abstract]" was devised and a search was performed in PubMed.

RESULTS

The efficacy of exemestane in different treatment settings has been established by numerous clinical studies. Exemestane is recommended as an adjuvant treatment in postmenopausal women previously treated with tamoxifen in trials comparing 5 years of tamoxifen with 2-3 years of tamoxifen combined with 2-3 years of exemestane, which proved that treatment with exemestane provided better survival outcomes. Similarly, exemestane could be considered as a safe treatment option for neoadjuvant treatment, prevention of chemotherapy, and treatment of advanced BC either alone or in combination with other targeted therapy drugs in both pre- and postmenopausal women.

CONCLUSION

Exemestane could be considered as a reasonable therapeutic option in the treatment of ER+ BC at any stage in pre- and postmenopausal women.

Development and implementation of the SUM breast cancer cell line functional genomics knowledge base

Several years ago, the SUM panel of human breast cancer cell lines was developed, and these cell lines have been distributed to hundreds of labs worldwide. Our lab and others have developed extensive omics data sets from these cells. More recently, we performed genome-scale shRNA essentiality screens on the entire SUM line panel, as well as on MCF10A cells, MCF-7 cells, and MCF-7LTED cells. These gene essentiality data sets allowed us to perform orthogonal analyses that functionalize the otherwise descriptive genomic data obtained from traditional genomics platforms. To make these omics data sets available to users of the SUM lines, and to allow users to mine these data sets, we developed the SUM Breast Cancer Cell Line Knowledge Base. This knowledge base provides information on the derivation of each cell line, provides protocols for the proper maintenance of the cells, and provides a series of data mining tools that allow rapid identification of the oncogene signatures for each line, the enrichment of KEGG pathways with screen hit and gene expression data, an analysis of protein and phospho-protein expression for the cell lines, as well as a gene search tool and a functional-druggable signature tool. Recently, we expanded our database to include genomic data for an additional 27 commonly used breast cancer cell lines. Thus, the SLKBase provides users with deep insights into the biology of human breast cancer cell lines that can be used to develop strategies for the reverse engineering of individual breast cancer cell lines.

Effect of androgens on different breast cancer cells co-cultured with or without breast adipose fibroblasts

About 70% of breast tumors express androgen receptors. In addition, there is clinical evidence suggesting that androgens can inhibit mammary epithelial proliferation. Vice versa, there is also significant evidence indicating that androgens can increase the risk of breast cancer via multiple mechanisms, e.g. direct conversion to estrogens that can bind to the estrogen receptor and thereby stimulate cell proliferation. We examined the effect of testosterone (T) and dihydroxytestosterone (DHT) on cell proliferation, pS2 and Ki-67 expression in three different breast cancer cell lines alone or in co-culture with primary human breast adipose fibroblasts (BAFs) obtained from breast cancer patients. In the co-cultures, T induced cell proliferation, pS2 and Ki-67 expression in the estrogen receptor positive (ER(+)) MCF-7 and T47D cells. This was not observed in the (ER(-)) MDA-MB-231 cells. The differences might be explained by the high expression of aromatase, which converts androgens to estrogens in BAFs followed by ER-mediated cell proliferation. In line with this absence of increased cell proliferation, pS2 and Ki-67 expression was observed in the presence of DHT, which is not a substrate for aromatase. In contrast, DHT caused a significant suppression of cell proliferation (68% and 38%), pS2 and Ki-67 expression in the (ER(+)) MCF-7 and T47D cells. More importantly, DHT decreased cell proliferation in (ER(-)) MDA-MB-231 cells by 38%. The results suggest that androgens that cannot be aromatized, like DHT, may provide a perspective for treatment of breast cancer patients, especially those with triple negative breast cancer.

Effect of Berry Extracts and Bioactive Compounds on Fulvestrant (ICI 182,780) Sensitive and Resistant Cell Lines

Fulvestrant (ICI 182,780; ICI) is approved for the treatment of advanced metastatic breast cancer that is unresponsive to other endocrine therapies. Berries are frequently consumed for their antioxidant, anti-inflammatory, and anticancer potential. In this study, we tested the efficacy of two berry extracts (Jamun-EJAE and red raspberry-RRE) and their bioactive compounds (Delphinidin-Del and Ellagic acid-EA) to inhibit cell proliferation with or without a sublethal dose of ICI in various breast cancer cell lines. ICI-sensitive (LCC1, ZR75-1, and BT474) and -resistant (LCC9, ZR75-1R) cells were subjected to treatment with berry extracts alone (0.1–100 μg/mL) or with a sub-lethal dose of ICI (<IC₅₀ dose; 1 nM for sensitive; 1 μM for resistant cells). Extracts and Del enhanced the effect of ICI in sensitive ZR75-1 and BT474 cells primarily in an additive fashion (measured by relative index (RI)~1). In ZR75-1R cells, both EJAE and RRE synergistically enhanced the effects of ICI (15–50%; P < 0.05; RI > 1). EA, in doses tested, did not have any significant effects on any of the cell lines. Finally, we found that the extracts were more effective at lower, physiologically relevant concentrations than at higher experimental doses.

Women live longer than men: understanding molecular mechanisms offers opportunities to intervene by using estrogenic compounds

Women live longer than men. Moreover, females live longer than males in some, but not all, experimental animals. The differences in longevity between genders are related to free radical production. Indeed, females produce less radicals only in animal species in which they live longer than males. This is because estrogens upregulate antioxidant longevity-related genes. These considerations have led us to postulate an extended concept of antioxidant in biology: an antioxidant is any nutritional, physiological, or pharmacological manipulation that increases the expression and activity of antioxidant genes or proteins. Phytoestrogens or other selective estrogen receptor modulators lower age-related diseases and prolong life span, at least in experimental animals. This provides rational bases to study their action in humans further.

Regulation of aryl hydrocarbon receptor function by selective estrogen receptor modulators

Selective estrogen receptor modulators (SERMs), such as tamoxifen (TAM), have been used extensively for the treatment and prevention of breast cancer and other pathologies associated with aberrant estrogen receptor (ER) signaling. These compounds exhibit cell-selective agonist/antagonist activities as a consequence of their ability to induce different conformational changes in ER, thereby enabling it to recruit functionally distinct transcriptional coregulators. However, the observation that SERMs can also regulate aspects of calcium signaling and apoptosis in an ER-independent manner in some systems suggests that some of the activity of drugs within this class may also arise as a consequence of their ability to interact with targets other than ER. In this study, we demonstrate that 4-hydroxy-TAM (4OHT), an active metabolite of TAM, directly binds to and modulates the transcriptional activity of the aryl hydrocarbon receptor (AHR). Of specific interest was the observation, that in the absence of ER, 4OHT can induce the expression of AHR target genes involved in estradiol metabolism, cellular proliferation, and metastasis in cellular models of breast cancer. The potential role for AHR in SERM pharmacology was further underscored by the ability of 4OHT to suppress osteoclast differentiation in vitro in part through AHR. Cumulatively, these findings provide evidence that it is necessary to reevaluate the relative roles of ER and AHR in manifesting the pharmacological actions and therapeutic efficacy of TAM and other SERMs.

6beta,19-Bridged androstenedione analogs as aromatase inhibitors

Inhibition of aromatase is an efficient approach for the prevention and treatment of breast cancer. New 6beta,19-bridged steroid analogs of androstenedione, 6beta,19-epithio- and 6beta,19-methano compounds 11 and 17, were synthesized starting from 19-hydroxyandrostenedione (6) and 19-formylandrost-5-ene-3beta,17beta-yl diacetate (12), respectively, as aromatase inhibitors. All of the compounds including known steroids 6beta,19-epoxyandrostenedione (4) and 6beta,19-cycloandrostenedione (5) tested were weak to poor competitive inhibitors of aromatase and, among them, 6beta,19-epoxy steroid 4 provided only moderate inhibition (K(i): 2.2 microM). These results show that the 6beta,19-bridged groups of the inhibitors interfere with binding in active site of aromatase.

Chemical aromatization of 19-hydroxyandrosta-1,4-diene-3,17-dione with acid or alkaline: elimination of the 19-hydroxymethyl group as formaldehyde

In order to determine whether or not a 19-hydroxymethyl group of 19-hydroxyandrosta-1,4-diene-3,17-dione (2, 19-hydroxy ADD), an intermediate of aromatase-catalyzed estrone formation from ADD, a suicide substrate of aromatase, is eliminated as formaldehyde, we examine chemical nature of removal of the 19-hydroxymethyl group. 19-acetate 3 and 19-tert-butyldimethylsiloxy compound 4 are known to convert rapidly to estrone with treatment of NaOH or n-Bu4NF. Since compound 2 was unstable and unobtainable under these conditions, compounds 3 and 4 as equivalents to compound 2 were used in this study. The acetate 3 with 5 mol/l HCl in acetone and 10% KOH in MeOH along with the silyl ether 4 with 5 mol/l HCl in acetone and 1 mol/l n-Bu4NF in THF gave formaldehyde and estrone in which a ratio of the aldehyde to estrone was near 1. This result indicates that the 19-hydroxymethyl groups of compound 3 and 4 are eliminated as formaldehyde along with estrone derived from the steroid skeleton under the acid or base treatment. The findings suggest that a single hydroxylation at the 19 carbon of ADD (1) would be, chemically, all that was required for estrone formation.

Inhibition of Cytochromes P450: Existing and New Promising Therapeutic Targets

Mammalian cytochromes P450 have been shown to play highly important roles in the metabolism of drugs and xenobiotics as well as in the biosynthesis of a variety of endogenous compounds, many of them displaying hormonal function. The role of P450s as therapeutic targets is still inadequately recognized although several P450 inhibitors became efficient drugs that even reached blockbuster status. Here, we try to give a comprehensive overview on cytochromes P450s, which are already well-established targets - particularly focussing on the treatment of infectious diseases, metabolic disorders and cancer - and on those, which have a high potential to become successful targets. In addition, the design of inhibitors of cytochromes P450 will be discussed.

Aromatase inhibitors: past, present and future in breast cancer therapy

Estrogen has been implicated in promoting breast cancer in a majority of women. Endocrine therapy controlling estrogen production has been the guiding principle in treating breast cancer for more than a century. A greater understanding of this disease at a molecular level has led to the development of molecules that inhibit estrogen production by inhibiting the aromatase enzyme, that is the primary source of estrogen in postmenopausal women. This review examines the evolution of aromatase inhibitor (AI) based therapies over the past three decades. The third generation aromatase inhibitors (anastrozole, letrozole and exemestane), which have been found to be extremely specific and effective in an adjuvant/neoadjuvant/extended adjuvant setting are discussed from a biochemical and clinical perspective. A comprehensive discussion of the past, present, and future of aromatase inhibitors is conducted in this review.

Cyclin D1 is necessary for tamoxifen-induced cell cycle progression in human breast cancer cells

Despite the success of tamoxifen in treating hormone-responsive breast cancer, its use is limited by the development of resistance to the drug. Understanding the pathways involved in the growth of tamoxifen-resistant cells may lead to new ways to treat tamoxifen-resistant breast cancer. Here, we investigate the role of cyclin D1, a mediator of estrogen-dependent proliferation, in growth of tamoxifen-resistant cells using a cell culture model of acquired resistance to tamoxifen. We show that tamoxifen and 4-hydroxytamoxifen (OHT) promoted cell cycle progression of tamoxifen-resistant cells after growth-arrest mediated by the estrogen receptor down-regulator ICI 182,780. Down-regulation of cyclin D1 with small interfering RNA blocked basal cell growth of tamoxifen-resistant cells and induction of cell proliferation by OHT. In addition, pharmacologic inhibition of phosphatidylinositol 3-kinase/Akt or mitogen-activated protein kinase/extracellular signal-regulated kinase 1/2 pathways decreased basal cyclin D1 expression and impaired OHT-mediated cyclin D1 induction and cell cycle progression. These findings indicate that cyclin D1 expression is necessary for proliferation of tamoxifen-resistant cells and for tamoxifen-induced cell cycle progression. These results suggest that therapeutic strategies to block cyclin D1 expression or function may inhibit development and growth of tamoxifen-resistant tumors.

Identification of a novel, orally bioavailable estrogen receptor downregulator

Tamoxifen has been widely used for the treatment of estrogen receptor (ER)-positive breast cancer, but its partial agonist activity is considered to limit the efficacy, and cause tumor flare and endometrial cancer. Fulvestrant, on the other hand, binds and degrades ER, thereby acting as a pure anti-estrogen without partial agonist activity. However, due to its low oral bioavailability, fulvestrant has to be intramuscularly administered to patients, which limits the convenience of the drug, and causes pain and inflammation at the site of injection. In search of a patient- friendly pure anti-estrogen, we screened and identified an ER antagonist, CH4893237, which bound to ER with an IC50 value of 1.4 muM and, by oral administration, inhibited estrogen-stimulated uterine growth in ovariectomized mice. CH4893237 reduced the amount of ER at the protein level and impaired the nuclear accumulation of ER, indicating an orally active pure anti-estrogen. Furthermore, CH4893237 inhibited the estrogen-stimulated proliferation of MCF-7, ZR-75-1 and BT-474 cells, and caused a marked growth inhibition of the MCF-7 xenograft in vivo. Thus, CH4893237 will provide an additional option for second-line hormone treatment of breast cancer.

Tamoxifen--what next?

Most patients with advanced breast cancer (ABC) ultimately die due to disease progression. Consequently, treatments for ABC are predominantly palliative in nature and, therefore, the tolerability profile of a given treatment is particularly relevant in these patients. While cytotoxic chemotherapy and endocrine therapy exhibit efficacy in hormone-sensitive, advanced disease, it is endocrine therapy that combines efficacy with minimal acute toxicity. Tamoxifen has been the chosen endocrine therapy for postmenopausal, hormone-sensitive, ABC for over 20 years. More recently, new endocrine agents with different mechanisms of action from tamoxifen have been introduced. Evidence indicates that the aromatase inhibitors anastrozole (Arimidex; AstraZeneca; Wilmington, DE), letrozole (Femara; Novartis Pharmaceuticals Corp.; East Hanover, NJ) and exemestane (Aromasin; Pharmacia Corp.; Peapack, NJ) offer superior efficacy and tolerability to tamoxifen in the first-line treatment of postmenopausal, hormone-sensitive ABC. Similarly, after tamoxifen failure, fulvestrant (Faslodex; AstraZeneca), a new estrogen receptor (ER) antagonist that downregulates the ER, is at least as effective as anastrozole, is well tolerated, and is not cross-resistant with tamoxifen. Unlike tamoxifen, fulvestrant has no known agonist effects. The sequential use of such agents may prolong the time during which endocrine therapies can be used, thereby avoiding the more acute toxicities associated with cytotoxic chemotherapy. Indeed, a series of studies has shown that this sequential use is a relevant, active, and well-tolerated option. Establishing the comparative efficacies and optimal sequences that incorporate the newer endocrine agents will be central in determining the future role of hormonal therapy in ABC; the results of this work will determine the relative place of tamoxifen in what is a rapidly changing therapeutic environment.

Cyclin D1 expression is dependent on estrogen receptor function in tamoxifen-resistant breast cancer cells

The development of resistance to tamoxifen, the most common antiestrogen used in the treatment of breast cancer, is a frequent and severe clinical problem. Tamoxifen-resistant tumors are still capable of responding to other hormonal therapies such as those that downregulate estrogen receptor expression. Mechanisms leading to acquisition of tamoxifen-resistant but hormone-sensitive growth are not completely understood. In tamoxifen-sensitive breast cancer cells, tamoxifen inhibits, whereas estrogen induces, expression of cyclin D1, a key cell cycle regulatory protein. Ectopic expression of cyclin D1 can lead to antiestrogen resistance. Thus, to determine whether cyclin D1 is involved in the growth of tamoxifen-resistant cells, we developed several tamoxifen-resistant variants from MCF-7 cells. These variants grow in the absence of estrogen or in the presence of tamoxifen, but their growth is inhibited by estrogen receptor downregulators. We show here that cyclin D1 expression is maintained at comparable levels in all tamoxifen-resistant variants, whereas pS2, another estrogen-regulated protein, is not. The addition of physiological levels of estrogen further stimulates cyclin D1 expression and proliferation. In contrast, treatment with estrogen receptor downregulators decreases cyclin D1 expression and proliferation. Thus, changes in cyclin D1 expression upon second-line hormonal therapy may predict hormonal sensitivity of tamoxifen-resistant tumors. These studies suggest that estrogen receptor mediates cyclin D1 expression and growth of tamoxifen-resistant tumors.

Breast cancer models to study the expression of estrogen receptors with small animal PET imaging

Different animal models of estrogen positive tumors (ER+) were evaluated for their suitability to follow tumor response after various treatment protocols, using small animal positron emission tomography (PET). ER+ human breast cancer cell lines MCF-7 and T-47D, using MDA-MB-231 as ER-; control, and murine mammary ductal carcinomas MC4-L2, MC4-L3, and MC7-L1, were compared for their in vivo growth rate and retention of ER+ status. Tumor metabolic activity was estimated from the relative uptake (% injected dose/g) of [18F]fluorodeoxyglucose (FDG) uptake, whereas ER content was determined from 16alpha-[18F]fluoroestradiol (FES) retention. F-18 activity values were obtained by small animal PET imaging and confirmed by tissue sampling and radioactivity counting. Reliable uptake measurements could be obtained for tumors of 200 microl or over. The human cell lines grew at a slower rate in vivo and failed to accumulate FES; in contrast, the Balb/c MC7-L1 and MC4-L2 grew well and showed good uptake of both FDG and FES. Chemotherapy and hormone therapy delayed the growth of MC7-L1 and MC4-L2 tumors, confirming their suitability as an ER+ model for therapeutic interventions. MC4-L3 tumors also showed promising results but required the presence of progestative pellets to grow. These data demonstrate that murine MC7-L1 and MC4-L2 tumors are suitable models for the monitoring of ER+ breast cancer therapy using small animal PET imaging.

Current limits of knowledge in adjuvant and neoadjuvant endocrine therapy of breast cancer: the need for more clinical research

Adjuvant endocrine therapy following surgical resection of early, endocrine sensitive breast cancer has proven benefits in reducing risk of recurrence and death, as demonstrated in many mature well controlled clinical trials. The introduction of new endocrine therapies as potential alternatives to tamoxifen or ovarian ablation and the incorporation of neoadjuvant endocrine therapy into the overall management strategy continue to provide exciting challenges for clinical research. In this article the focus is on as yet unanswered questions pertinent to adjuvant or neoadjuvant endocrine therapy for breast cancer. In the process, we broadly outline the current limits of knowledge as we understand it. Many relevant and current clinical trials are ongoing and a list of these with contact details or references are provided. Definitive data is urgently needed in many areas and, when available, will provide important evidence on which the management of breast cancer patients in future can be based. Participation in relevant clinical trials is vital for future progress.

Postmenopausal women who progress on fulvestrant ('Faslodex') remain sensitive to further endocrine therapy

PURPOSE

This retrospective evaluation of data from two randomized, multicenter trials examined whether tumor responses to further endocrine therapy were seen in postmenopausal women with advanced breast cancer who had progressed on both initial endocrine therapy, usually tamoxifen, and on the estrogen receptor (ER) antagonist fulvestrant ('Faslodex').

PATIENTS AND METHODS

A combined total of 423 patients received fulvestrant 250 mg as a monthly intramuscular injection. After progression on fulvestrant, some patients received another endocrine therapy. Responses to subsequent endocrine therapy were assessed using a questionnaire sent to the trial investigators. Best responses were classified as a complete or partial response (CR or PR), stable disease (SD) lasting > or = 24 weeks, or disease progression.

RESULTS

Follow-up data were available for 54 patients who derived clinical benefit (CB, defined as CR, PR or SD) from fulvestrant and who received subsequent endocrine therapy, resulting in a PR in 4 patients, SD in 21 patients, and disease progression in 29 patients. Data were available for 51 patients who derived no CB from fulvestrant and who received further endocrine therapy, resulting in a PR in 1 patient, SD in 17 patients, and disease progression in 33 patients. Aromatase inhibitors were used as subsequent endocrine therapy in > 80% of patients.

CONCLUSIONS

After progression on fulvestrant, patients may retain sensitivity to other endocrine agents. Fulvestrant provides an additional option to existing endocrine therapies for the treatment of advanced or metastatic breast cancer in postmenopausal women, and may provide the opportunity to extend the sequence of endocrine regimens before cytotoxic chemotherapy is required.

The effects of estradiol and selective estrogen receptor modulators on gene expression and messenger RNA stability in immortalized sheep endometrial stromal cells and human endometrial adenocarcinoma cells

The purpose of this study was to identify an endometrial cell line that maintained the E2 up-regulation of estrogen receptor (ER) mRNA by enhanced message stability and to assess its dependence on ER protein. Estradiol (E2) effects on gene expression were measured in three cell lines: one immortalized from sheep endometrial stroma (ST) and two from human endometrial adenocarcinomas (Ishikawa and ECC-1). E2 up-regulated ER mRNA levels in ST and Ishikawa cells, but down-regulated ER mRNA levels in ECC-1 cells. E2 up-regulated progesterone receptor (PR), glyceraldehyde 3-phosphate dehydrogenase (GAPDH), and transforming growth factor-alpha (TGF-alpha) in both Ishikawa and ECC-1 cells. The selective estrogen receptor modulator ICI 182,780 antagonized the E2-induced up-regulation of ER and/or PR mRNA levels in all three cells, while another, GW 5638, antagonized the up-regulation of PR mRNA in Ishikawa and ECC-1 cells. In mechanistic studies, E2 had no effect on ER mRNA stability in ST cells and it destabilized ER mRNA in ECC-1 cells. Thus, Ishikawa cells appear to be the most physiologically relevant cell line in which to study the up-regulation of ER mRNA levels by enhanced mRNA stability. Its antagonism by ICI 182,780 reveals that ER protein is involved in this E2 response.

Breast cancer management: quality-of-life and cost considerations

The purpose of this article was to provide a literature-based extensive overview of the quality-of-life and cost issues posed by the management of breast cancer. Incidence and mortality rates vary widely in different countries. Breast cancer accounts approximately for one-fifth of all deaths in women aged 40-50 years. The 1994-1998 incidence rate in the US population was on average 114.3 per 100 000 women. Treatment options include surgery, radiotherapy and drug therapy (cytotoxic and endocrine drugs). All treatment options affect patients' health-related quality of life (HR-QOL) in various ways. The use of cytotoxic agents has a particularly large HR-QOL impact. HR-QOL questionnaires are complex tools, not routinely used in breast cancer trials.Worldwide, around 10 million individuals develop cancer each year; this figure is expected to increase to 15 million in 2020. For all cancers, the total economic burden of this disease worldwide was projected by the authors to be in the range of $US 300-400 billion in 2001 (about $US 100-140 billion as direct costs and the remainder as indirect costs [morbidity and mortality]). According to the National Institute of Health (NIH), the total cost of cancer was estimated at $US 156.7 billion in 2001 in US ($US 56.4 billion as direct costs, $US 15.6 as indirect morbidity costs, and $US 84.7 billion as indirect mortality costs). Based on limited information, in the US, breast cancer can be projected to account for about one-fifth/one-fourth of the total cost of cancer. Breast cancer treatment costs are higher in the US than in other developed countries. Both direct and indirect costs are dependent on disease stage. The per-patient costs for initial care in 1992 were estimated at $US 10 813, for continuing care at $US 1084 and for terminal care at $US 17 886. Stage-specific costs provide information for cost-effectiveness analyses of cancer-control initiatives, such as screening programmes. Economic studies on breast cancer are heterogeneous, and the cost estimates made are not easily generalisable. The cost of treatment for breast cancer in developing countries is < or =5% of that in developed regions.