Extending the Clinical Benefit of Endocrine Therapy for Women With Hormone Receptor–Positive Metastatic Breast Cancer: Differentiating Mechanisms of Action

Novel oral selective estrogen receptor degraders (SERDs) to target hormone receptor positive breast cancer: elacestrant as the poster-child

INTRODUCTION

Estrogen receptor positive (ER+) breast cancer is the most common breast cancer subtype, and therapeutic management relies primarily on inhibiting ER signaling. In the metastatic setting, ER signaling is typically targeted by selective estrogen receptor degraders (SERDs) or aromatase inhibitors (AIs), the latter of which prevent estrogen production. Activating ESR1 mutations are among the most common emergent breast cancer mutations and confer resistance to AIs.

AREAS COVERED

Until 2023, fulvestrant was the only approved SERD; fulvestrant is administered intramuscularly, and in some cases may also have limited efficacy in the setting of certain ESR1 mutations. In 2023, the first oral SERD, elacestrant, was approved for use in ESR1-mutated, ER+/HER2- advanced breast cancer and represents a new class of therapeutic options. While the initial approval was as monotherapy, ongoing studies are evaluating elacestrant (as well as other oral SERDs) in combination with other therapies including CDK4/6 inhibitors and PI3K inhibitors, which parallels the current combination uses of fulvestrant.

EXPERT OPINION

Elacestrant's recent approval sheds light on the use of biomarkers such as ESR1 to gauge a tumor's endocrine sensitivity. Ongoing therapeutic and correlative biomarker studies will offer new insight and expanding treatment options for patients with advanced breast cancer.

The crucial role of epigenetic regulation in breast cancer anti-estrogen resistance: Current findings and future perspectives

Breast cancer (BC) cell de-sensitization to Tamoxifen (TAM) or other selective estrogen receptor (ER) modulators (SERM) is a complex process associated with BC heterogeneity and the transformation of ER signalling. The most influential resistance-related mechanisms include modifications in ER expression and gene regulation patterns. During TAM/SERM treatment, epigenetic mechanisms can effectively silence ER expression and facilitate the development of endocrine resistance. ER status is efficiently regulated by specific epigenetic tools including hypermethylation of CpG islands within ER promoters, increased histone deacetylase activity in the ER promoter, and/or translational repression by miRNAs. Over-methylation of the ER α gene (ESR1) promoter by DNA methyltransferases was associated with poor prognosis and indicated the development of resistance. Moreover, BC progression and spreading were marked by transformed chromatin remodelling, post-translational histone modifications, and expression of specific miRNAs and/or long non-coding RNAs. Therefore, targeted inhibition of histone acetyltransferases (e.g. MYST3), deacetylases (e.g. HDAC1), and/or demethylases (e.g. lysine-specific demethylase LSD1) was shown to recover and increase BC sensitivity to anti-estrogens. Indicated as a powerful molecular instrument, the administration of epigenetic drugs can regain ER expression along with the activation of tumour suppressor genes, which can in turn prevent selection of resistant cells and cancer stem cell survival. This review examines recent advances in the epigenetic regulation of endocrine drug resistance and evaluates novel anti-resistance strategies. Underlying molecular mechanisms of epigenetic regulation will be discussed, emphasising the utilization of epigenetic enzymes and their inhibitors to re-program irresponsive BCs.

ESR1 genetic alterations and their association with clinicopathologic characteristics in advanced breast cancer: a single academic institution experience

Estrogen receptor (ER) alpha, a ligand-dependent nuclear transcription factor encoded by the ESR1 gene, is expressed in 70% of breast carcinomas (BCs) and is used as a target for endocrine-based therapies. However, some patients develop resistance to endocrine-based therapies due to ESR1 mutation, which leads to constitutive activation in the absence of ligand. We retrospectively analyzed 223 clinically advanced BCs using the FoundationOne CDX assay and found 13.9% (31/223) of cases had ESR1 genetic alterations (26 mutations and 5 amplifications). All ESR1 mutations occurred within the ligand binding domain, with the most prevalent being Y537S (42.3%) and D538G (38.5%), and all ESR1-mutated cases had a history of aromatase inhibitor use. No significant difference in clinicopathologic features was identified between ESR1-mutated and ESR1-amplified cases except higher frequency of HER2 positivity and TP53 mutations in ESR1-amplified cases. The prevalence of ESR1 mutations in ER-positive BCs was 19.1% (26/136). In comparison to ESR1-nonmutated ER-positive cases, ESR1-mutated cases demonstrated significantly higher percentage of tumor cells with ER and progesterone receptor expression, an increased tendency for overall distant metastasis and liver metastasis, higher frequency of FGF3/4/19 mutations, lower frequency of TP53 mutation, but no difference in overall survival and metastatic/recurrent intervals. In conclusion, our findings suggest that development of ESR1 mutations are selected for under the influence of estrogen deprivation, and a positive correlation between ESR1 mutations and ER protein expression may exist.

Review of concepts in therapeutic decision-making in HER2-negative luminal metastatic breast cancer

Purpose

Hormone receptor (HR)-positive, Human epidermal growth factor receptor 2 (HER2)-negative metastatic breast cancer (MBC) requires a therapeutic approach that takes into account multiple factors, with treatment being based on anti-estrogen hormone therapy (HT). As consensus documents are valuable tools that assist in the decision-making process for establishing clinical strategies and optimize the delivery of health services, this consensus document has been created with the aim of developing recommendations on cretiera for hormone sensitivity and resistance in HER2-negative luminal MBC and facilitating clinical decision-making.

Methods

This consensus document was generated using a modification of the RAND/UCLA methodology, which included the definition of the project and identification of issues of interest, a non-exhaustive systematic review of the literature, an analysis and synthesis of the scientific evidence, preparation of recommendations, and external evaluation with a panel of 64 medical oncologists specializing in breast cancer.

Results

A Spanish panel of experts reached consensus on 32 of the 32 recommendations/conclusions presented in the first round and were accepted with an approval rate of 100% about definition of metastatic disease not susceptible to local curative treatment, definition of hormone sensitivity and hormone resistance in metastatic luminal disease and therapeutic decision-making.

Conclusion

We have developed a consensus document with recommendations on the treatment of patients with HER2-negative luminal MBC that will help to improve therapeutic benefits.

An integrated bioinformatics analysis of potential therapeutic targets among matrix metalloproteinases in breast cancer

Breast cancer (BC) is one of the most aggressive malignancies worldwide among females. Matrix metalloproteinases (MMPs), as the most abundant class of non-serine proteases present in invasive and metastatic tumors, can regulate a variety of alterations in the microenvironment during tumor progression. However, the differential expression of MMPs and its prognostic values in BC is yet to be elucidated. In this research, using the ONCOMINE dataset, The Cancer Genome Atlas, Breast Cancer Gene-Expression Miner v4.1 (Bc-GenExMiner), Kaplan-Meier Plotter and cBioPortal, the transcriptional MMPs and survival outcome data of patients with BC was compared. It was indicated that mRNA levels of MMP1/3/9/10/11/12/13 were increased compared with non-tumor tissues, whereas mRNA expression of MMP2/16/19/23B/28 was lower in BC tissues. Kaplan-Meier plots showed that high mRNA levels of MMP2/10/16/19/20/23B/27 in patients with BC were associated with better recurrence-free survival. In contrast, high MMP1/8/9/11/12 conferred worse RFS rate. Meanwhile, high transcription levels of MMP1/3/11/12/13 predicted shorter distant metastasis-free survival, while high levels of MMP1/12 demonstrated worse overall survival in patients with BC. From Bc-GenExMiner, it was indicated that high expression of MMP16/20 was correlated with better prognosis, while MMP1/9/11/12/13/14/15 exerted a negative effect on patient prognosis. The integrative bioinformatics analysis performed in the present study suggests that MMP1/9/12/16, compared with other MMPs, are potentially appropriate targets for targeted therapy in patients with BC.

The Impact of ESR1 Mutations on the Treatment of Metastatic Breast Cancer

After nearly 20 years of research, it is now established that mutations within the estrogen receptor (ER) gene, ESR1, frequently occur in metastatic breast cancer and influence response to hormone therapy. Though early studies presented differing results, sensitive sequencing techniques now show that ESR1 mutations occur at a frequency between 20 and 40% depending on the assay method. Recent studies have focused on several "hot spot mutations," a cluster of mutations found in the hormone-binding domain of the ESR1 gene. Throughout the course of treatment, tumor evolution can occur, and ESR1 mutations emerge and become enriched in the metastatic setting. Sensitive techniques to continually monitor mutant burden in vivo are needed to effectively treat patients with mutant ESR1. The full impact of these mutations on tumor response to different therapies remains to be determined. However, recent studies indicate that mutant-bearing tumors may be less responsive to specific hormonal therapies, and suggest that aromatase inhibitor (AI) therapy may select for the emergence of ESR1 mutations. Additionally, different mutations may respond discretely to targeted therapies. The need for more preclinical mechanistic studies on ESR1 mutations and the development of better agents to target these mutations are urgently needed. In the future, sequential monitoring of ESR1 mutational status will likely direct personalized therapeutic regimens appropriate to each tumor's unique mutational landscape.

Ongoing unmet needs in treating estrogen receptor-positive/HER2-negative metastatic breast cancer

Estrogen receptor-positive (ER+)/human epidermal growth factor receptor 2 negative (HER2-) advanced or metastatic breast cancer (MBC) is the most common MBC subtype and currently remains incurable, with a median overall survival of 24.8 months (95% confidence interval, 21.3-30.3). Common sites of metastases are bone, viscera, and brain, causing significant symptoms that negatively affect patient functioning, quality of life (QoL), and work productivity. Guidelines state that endocrine therapy (ET) is preferable to chemotherapy as first-line treatment for patients with ER+ MBC, regardless of limited visceral metastases, unless rapid tumor response is required or ET resistance is suspected. Although response rates up to 40% have been reported for first-line MBC treatment, the majority of initial responders eventually develop ET resistance. Notwithstanding the steep decline in efficacy between first and later lines of ET, some patients may receive chemotherapy earlier than necessary. Although new treatments have been approved for patients with ER+/HER2- advanced or MBC in the past decade, neither survival nor QoL appear to have improved significantly. Thus, there remain significant unmet needs for this patient population, including improved survival, maintaining or improving patient QoL, and emphasizing the importance of treatment selection to assist healthcare practitioners managing patient care. In this review, we identify current challenges and unmet needs in this patient population, review cutting-edge treatments, and provide clinically relevant suggestions for treatment selection that can optimize outcomes and patients' health-related QoL.

Long-term management of patients with hormone receptor-positive metastatic breast cancer: Concepts for sequential and combination endocrine-based therapies

Treatment options for hormone receptor-positive (HR-positive) metastatic breast cancer (MBC) continue to increase in parallel with expanding knowledge about the complex biology of breast cancer subtypes and resistance mechanisms to endocrine therapy. For patients with HR-positive MBC, there are now an unprecedented number of endocrine-based treatment options that can improve long-term outcomes, while preserving or optimizing quality of life, and that can be used before selecting more cytotoxic chemotherapeutic regimens. In addition to antiestrogens, steroidal and nonsteroidal aromatase inhibitors, the selective estrogen-receptor degrader, fulvestrant, and new endocrine-based combinations provide significant and clinically meaningful improvements in outcomes in the first line setting and beyond. Also, new clinical scenarios and indications for monotherapy endocrine and targeted therapies continue to be explored. Patients have several therapeutic options when their disease progresses or becomes resistant, although the optimal sequencing of these therapies remains unclear. Ongoing research in the resistant/refractory setting is anticipated to continue improving the outlook for these patients. This review will discuss current and investigational approaches to sequential single-agent endocrine and endocrine-based combination therapy for the long-term management of patients with HR-positive, human epidermal growth factor receptor 2-negative MBC.

Endocrine Therapy in the Current Management of Postmenopausal Estrogen Receptor-Positive Metastatic Breast Cancer

Metastatic breast cancer (MBC) results in substantial morbidity and mortality for women afflicted with this disease. A majority of MBCs are hormone-responsive and estrogen receptor-positive, making endocrine therapy (ET) an integral component of systemic therapy. With a primary goal of minimizing the effects of estrogen on hormone-responsive MBC, ETs are among the first targeted treatments that aim to inhibit the influence of estrogen receptor activation on tumor proliferation. Several biochemical mechanisms have been the focus of drug development for treatment, including selective estrogen-receptor modulation, aromatase inhibition, and selective estrogen-receptor degradation. Treatments that exploit these mechanisms have improved survival and quality of life for women with MBC. However, in many cases, resistance to ET limits their effectiveness. Elucidation of the complex cellular signal cascades involved in the development of acquired resistance to ET and the interrelationship of growth factor signaling and estrogen responsiveness have characterized components of these pathways as attractive targets for drug development. Based on these insights and with the aim of overcoming hormone resistance, targeted therapies are emerging as useful treatments for MBC. This article reviews current endocrine treatments of MBC as well as recent and ongoing study of combination treatments and targeted therapies that interfere with cellular proliferation pathways as means of overcoming resistance. The Oncologist 2017;22:507-517 IMPLICATIONS FOR PRACTICE: This review provides medical oncologists and other oncology health care providers with a current understanding of the rationale for endocrine therapy in estrogen receptor-positive metastatic breast cancer and the efficacy and safety profile of available treatment options. Additionally, current concepts regarding the development of treatment resistance and the treatment strategies for overcoming resistance are discussed. Enhancing the current information and the understanding of these topics will assist clinicians in evaluating optimal treatment options for their patients.

Chemosensitization role of fulvestrant in combination with chemotherapy in postmenopausal hormone receptor positive and human epidermal growth factor negative metastatic breast cancer

In metastatic breast cancer (MBC), hormone receptor positive (HR+), human epidermal growth factor negative (HER2-) subtype accounts for the majority. With various new modalities available to prolong life span in this group of patients, the effect is distant from optimum. Prevalent strategy of treating postmenopausal HR+ HER2- MBC is application of chemotherapy (CT) after progression of disease on endocrine therapy (ET) of several lines. Generally, ET targets HR+ ingredients and CT works better with HR- tumor cells. HR+ MBC, though hormone-sensitive, has HR- portion which reacts poorly to ET. Thus, sequential use of ET and CT neglects its insensitive part and gives rise to drug resistance, while alleviation of tumor burden is the top priority in metastatic setting. Chemohormonal therapy (i.e. concomitant use of ET and chemotherapy) complements for the shortcoming of current therapy strategy targeting both HR+ and HR- ingredients theoretically. Fulvestrant, a pure estrogen receptor antagonist and down-regulator, could be a promising agent using concurrently with CT based on chemosensitizing character shown in preclinical and pilot clinical studies. It is hypothesized in this article that chemohormonal therapy with concurrent fulvestrant and CT would be a promising strategy in postmenopausal HR+ HER2- MBC patients. Proof of this hypothesis would help control evolvement of tumor burden and acquirement of drug resistance over a short period of time.

Treatment challenges for community oncologists treating postmenopausal women with endocrine-resistant, hormone receptor-positive, human epidermal growth factor receptor 2-negative advanced breast cancer

Community-based oncologists are faced with challenges and opportunities when delivering quality patient care, including high patient volumes and diminished resources; however, there may be the potential to deliver increased patient education and subsequently improve outcomes. This review discusses the treatment of postmenopausal women with endocrine-resistant, hormone receptor-positive, human epidermal growth factor receptor 2- negative advanced breast cancer in order to illustrate considerations in the provision of pertinent quality education in the treatment of these patients and the management of therapy-related adverse events. An overview of endocrine-resistant breast cancer and subsequent treatment challenges is also provided. Approved treatment options for endocrine-resistant breast cancer include hormonal therapies and mammalian target of rapamycin inhibitors. Compounds under clinical investigation are also discussed.

Delaying Chemotherapy in the Treatment of Hormone Receptor-Positive, Human Epidermal Growth Factor Receptor 2-Negative Advanced Breast Cancer

Global guidelines for the management of locally advanced or metastatic hormone receptor-positive (HR-positive), human epidermal growth factor 2-negative (HER2-negative) breast cancer recommend endocrine therapy as first-line treatment for all patients, regardless of age or postmenopausal status. However, current practice patterns in the United States and Europe suggest that these modes of therapy are not being used as recommended, and many patients with advanced HR-positive, HER2-negative disease are being treated first-line with chemotherapy or switched to chemotherapy after a single endocrine therapy. Given that chemotherapy is associated with increased toxicity and reduced quality of life (QOL) compared with endocrine therapy, prolonging the duration of response obtained with endocrine therapy may help delay chemotherapy and its attendant toxicities. Several strategies to delay or overcome endocrine resistance and thereby postpone chemotherapy have been explored, including the use of second-line endocrine agents with different mechanisms of action, adding targeted agents that inhibit specific resistance pathways, and adding agents that act in complementary or synergistic ways to inhibit tumor cell proliferation. This review analyzes the different therapy options available to HR-positive, HER2-negative patients with advanced breast cancer that can be used to delay chemotherapy and enhance QOL.

ESR1 mutations—a mechanism for acquired endocrine resistance in breast cancer

Approximately 70% of breast cancers are oestrogen receptor α (ER) positive, and are, therefore, treated with endocrine therapies. However, about 25% of patients with primary disease and almost all patients with metastases will present with or eventually develop endocrine resistance. Despite the magnitude of this clinical challenge, the mechanisms underlying the development of resistance remain largely unknown. In the past 2 years, several studies unveiled gain-of-function mutations in ESR1, the gene encoding the ER, in approximately 20% of patients with metastatic ER-positive disease who received endocrine therapies, such as tamoxifen and aromatase inhibitors. These mutations are clustered in a 'hotspot' within the ligand-binding domain (LBD) of the ER and lead to ligand-independent ER activity that promotes tumour growth, partial resistance to endocrine therapy, and potentially enhanced metastatic capacity; thus, ER LBD mutations might account for a mechanism of acquired endocrine resistance in a substantial fraction of patients with metastatic disease. In general, the absence of detectable ESR1 mutations in patients with treatment-naive disease, and the correlation between the frequency of patients with tumours harbouring these mutations and the number of endocrine treatments received suggest that, under selective treatment pressure, clonal expansion of rare mutant clones occurs, leading to resistance. Preclinical and clinical development of rationale-based novel therapeutic strategies that inhibit these ER mutants has the potential to substantially improve treatment outcomes. We discuss the contribution of ESR1 mutations to the development of acquired resistance to endocrine therapy, and evaluate how mutated ER can be detected and targeted to overcome resistance and improve patient outcomes.