Accelerating drug development in breast cancer: New frontiers for ER inhibition

The current landscape of aromatase inhibitors for the treatment of estrogen receptor-positive breast carcinoma

Estrogen receptor-positive (ER+) breast carcinoma represents a significant portion of breast cancer cases and is characterized by the presence of estrogen receptors that promote tumor growth upon estrogen binding. ER + breast cancer progression involves hormonal influences, interactions within the tumor microenvironment, and genetic mutations that may lead to treatment resistance. Successful therapeutic options include hormonal therapies, particularly aromatase inhibitors (AIs), which aim to block the effects of estrogen or reduce its synthesis. With higher efficacy than tamoxifen, AIs such as anastrozole, letrozole, and exemestane have become widely employed in adjuvant and first-line treatments for advanced breast cancer. AIs function by inhibiting the enzyme aromatase, which converts androgens into estrogens in the peripheral tissues. Because too much estrogen might promote tumor growth, this decrease in estrogen levels is essential for treating ER+ malignancies. To provide a comprehensive overview of AIs in the treatment of ER+ breast cancer, this study examined the pharmacokinetics, clinical uses, mechanisms of action, and problems with treatment resistance. To maximize therapeutic approaches and enhance patient outcomes in the treatment of ER breast cancer, it is imperative to understand these characteristics.

Oral selective estrogen receptor degraders (SERDs) in hormone receptor-positive HER2-negative metastatic breast cancer after progression with CDK4/6 inhibitors

INTRODUCTION

Hormone receptor-positive (HR+), HER2-negative metastatic breast cancer (mBC) remains a prevalent and challenging disease. Endocrine therapy (ET) combined with CDK4/6 inhibitors is the first-line standard of care, yet resistance mechanisms, including ESR1 mutations, drive disease progression. Novel oral selective estrogen receptor degraders (SERDs) have emerged as promising therapeutic agents after progression with CDK4/6 inhibitors secondary to ESR1 mutations. However, the available studies on SERDs differ in design, study population, and outcomes, necessitating a critical review of available data.

AREAS COVERED

This review explores the mechanisms, clinical efficacy, and safety profiles of oral SERDs in HR-positive, HER2-negative mBC, particularly following progression on CDK4/6 inhibitors. Recent key clinical trials, including EMERALD, SERENA-2, EMBER-3 and AMEERA-3, are analyzed, highlighting their efficacy in overcoming resistance, especially in ESR1-mutant populations.

EXPERT OPINION

Oral SERDs offer enhanced bioavailability and convenience compared to fulvestrant, representing a critical advancement in endocrine therapy. Their integration into treatment strategies, particularly in combination regimens and ctDNA-driven approaches, may improve patient outcomes and address resistance mechanisms. However, other than ESR1 mutations, clinical refinement for patient selection is limited. Further trials are needed to optimize patient selection for oral SERD use and define the most effective combination strategies with oral SERDs.

Post-progression treatment options after CDK4/6 inhibitors in hormone receptor-positive, HER2-negative metastatic breast cancer

The combination of cyclin-dependent kinase 4/6 inhibitors (CDK4/6i) and endocrine therapy (ET) is the standard first-line treatment for hormone receptor-positive (HR + ) and HER2-negative metastatic breast cancer (mBC). Despite their efficacy, resistance inevitably develops, necessitating alternative therapeutic strategies post-progression. This review explores current and emerging treatment options following progression on CDK4/6i, focusing on endocrine therapies, targeted therapies, combination approaches, and the continued use of CDK4/6i. Endocrine therapies, including fulvestrant and novel oral selective estrogen receptor degraders (SERDs) like elacestrant, show promise, especially in patients with ESR1 mutations. Targeted therapies such as PI3K/AKT/mTOR inhibitors, exemplified by alpelisib and capivasertib, offer potential by addressing downstream signaling pathways involved in resistance. Additionally, FGFR inhibitors like erdafitinib are under investigation for their role in overcoming specific resistance mechanisms. Combination strategies involving CDK4/6 inhibitors with immune checkpoint inhibitors or other targeted agents are also being explored, with early trials suggesting possible synergistic effects, although further validation is required. Continuation of CDK4/6 inhibitors beyond progression has shown potential benefits in selected patients, but the data are heterogeneous, and further studies are needed to clarify their role. While chemotherapy remains a standard option for patients who progress on these treatments, the goal is to delay its use through the effective utilization of endocrine and targeted therapies. Understanding resistance mechanisms and tailoring treatment to individual patient profiles is crucial for optimizing outcomes. Ongoing clinical trials are expected to provide deeper insights, guiding the development of more effective post-progression therapeutic strategies. This evolving landscape highlights the need for continuous research and individualized patient care to improve survival and quality of life in HR + mBC patients.

Oral selective estrogen receptor degraders for breast cancer treatment: focus on pharmacological differences

PURPOSE

The management of hormone receptor-positive (HR +) breast cancer (BC) relies on endocrine therapy (ET), with a primary focus on disrupting estrogen receptor (ER) signaling due to its critical role in BC tumorigenesis and progression. While effective for both early-stage and advanced breast cancers, ET frequently encounters resistance mechanisms, including both ligand-dependent and ligand-independent trajectories, ultimately leading to disease progression.

METHODS

We searched PubMed, EMBASE and Scopus databases to review the current evidence on the use of novel oral selective estrogen receptor degraders (SERDs) for the treatment of HR+ BC.

CONCLUSIONS

Somatic activating mutations of the estrogen receptor 1 (ESR1) gene are known to sustain ER activity, boost ER-dependent gene transcription, and foster resistance to ET. The most significant gap remains after treatment failure with ET and cyclin-dependent kinases 4 and 6 (CDK4/6) inhibitors, where fulvestrant monotherapy typically results in a median progression-free survival of 2-3 months. Novel compounds, including oral SERDs, have been explored for their potential to overcome therapeutic resistance, both as monotherapy and in combination with other targeted therapies. Herein, we provide an overview on the latest findings on oral SERDs, examining their mechanism of action, safety data, and pharmacokinetics and pharmacodynamics profiles.

Discovery of Thiochroman Derivatives as Potent, Oral Selective Estrogen Receptor Degraders and Antagonists for the Treatment of Endocrine-Resistant Breast Cancer

Selective estrogen receptor degraders (SERDs) deplete the ER signaling pathway via antagonism and degradation of ERα and represent a promising strategy to tackle endocrine resistance. Here, we report a new class of SERDs by pharmacological evolution of a selective estrogen receptor modulator, lasofoxifene. The structure-activity relationship study and efforts to circumvent the issue of human ether-a-go-go-related gene led to the identification of compounds 51. This bifunctional compound displayed broad activity across a vast array of cell backgrounds and was capable of effectively degrading and antagonizing wild-type ERα and clinically relevant ERα mutants. 51 exhibited favorable pharmacokinetic properties and good brain penetration, with a brain/plasma ratio of 3.05, and significantly suppressed the growth of tumor in a tamoxifen-resistant MCF-7 Tam1 xenograft model. Overall, the study demonstrates 51 as a highly potent, oral, and brain penetrant ER degrader and pure antagonist, showing a good potential in overcoming endocrine resistance.

Discovery of ERD-1233 as a Potent and Orally Efficacious Estrogen Receptor PROTAC Degrader for the Treatment of ER+ Human Breast Cancer

Despite the development of highly effective therapies for the treatment of estrogen receptor α (ERα)-positive human breast cancer, clinical resistance to current therapies requires the development of novel therapeutic strategies. Herein, we report the discovery of ERD-1233 as a potent and orally efficacious ERα degrader designed using the PROTAC technology. ERD-1233 was developed based on Lasofoxifene as the ER binding moiety and a novel cereblon ligand through extensive optimization of the linker. ERD-1233 potently and effectively reduces the ERα protein in vitro and achieves excellent oral bioavailability in mice and rats. Oral administration of ERD-1233 effectively reduces ER protein in ER+ tumors and achieves tumor regression in the ER wild-type MCF-7 xenograft tumor model and strong tumor growth inhibition in the ESR1Y537S mutated model in mice. Our data demonstrate that ERD-1233 is a promising ER PROTAC degrader for extensive evaluation as a new therapy for the treatment of ER+ human breast cancer.

Endocrine therapy for early breast cancer in the era of oral selective estrogen receptor degraders: challenges and future perspectives

PURPOSE OF REVIEW

Growth and survival of hormone receptor positive breast cancer cells are dependent on circulating hormones (e.g., estrogen and progesterone). Endocrine therapy improved outcomes in both early and advanced hormone receptor positive breast cancer. These treatments include drugs with different mechanisms of action, namely selective estrogen receptor modulators (SERM), aromatase inhibitors, and selective estrogen receptor degraders (SERDs). SERDs represent estrogen receptor antagonists, favoring its degradation and thus interfering with proliferation genes transcription and activation. Fulvestrant is the first approved SERD, administered intramuscularly for treating advanced breast cancer.

RECENT FINDINGS

Oral SERDs have been tested to overcome the limitation of the intramuscular administration, and to increase SERD bioavailability. Recently, an oral SERD, Elacestrant, has been approved by the Food and Drug Administration (FDA) for patients carrying an ESR1 mutation. In fact, oral SERDs seem to be effective in tumors harboring ESR1 mutations, a well known mechanism of resistance to endocrine therapy (especially aromatase inhibitors).

SUMMARY

More recently, oral SERDs have been tested in patients with early hormone receptor positive breast cancer, although their impact on survival and in this curative setting compared to standard endocrine therapy still needs to be elucidated. The best timing and duration of SERD administration and specific biomarkers in (neo)adjuvant setting remain largely unknown.

Precision therapeutics and emerging strategies for HR-positive metastatic breast cancer

Anti-oestrogen-based therapies, often combined with a CDK4/6 inhibitor, are the current standard-of-care first-line therapy for patients with advanced-stage hormone receptor-positive (HR+) breast cancer. Resistance to anti-oestrogen agents inevitably occurs, mediated by oestrogen receptor (ER)-dependent or ER-independent mechanisms that drive tumour progression. Emerging endocrine therapies include, but are not limited to, next-generation oral ER degraders and proteolysis targeting chimeras, which might be particularly effective in patients with ESR1-mutant breast cancer. Furthermore, cancers harbouring driver alterations in oncogenic signalling pathways, including AKT and PI3K, might be susceptible to novel combination strategies involving targeted inhibitors. Next-generation CDK2/4 inhibitors are an area of active clinical investigation, and efforts are ongoing to evaluate the role of sequential CDK inhibition. Approved and emerging antibody-drug conjugates exploiting novel target antigens have also demonstrated promising clinical activity. These novel agents, as well as further identification and characterization of predictive biomarkers, will hopefully continue to improve clinical outcomes, reduce the incidence of toxicities, and limit the extent of overtreatment in this population. In this Review, we describe the evolving treatment paradigm for patients with metastatic HR+ breast cancer in light of the growing armamentarium of drugs and biomarkers that will help to shape the future therapeutic landscape. These strategies are expected to involve tumour molecular profiling to enable the delivery of precision medicine.

Adverse event signal mining and serious adverse event influencing factor analysis of fulvestrant based on FAERS database

Fulvestrant, as the first selective estrogen receptor degrader, is widely used in the endocrine treatment of breast cancer. However, in the real world, there is a lack of relevant reports on adverse reaction data mining for fulvestrant. To perform data mining on adverse events (AEs) associated with fulvestrant and explore the risk factors contributing to severe AEs, providing a reference for the rational use of fulvestrant in clinical practice. Retrieved adverse event report information associated with fulvestrant from the U.S. Food and Drug Administration (FDA) Adverse Event Reporting System (FAERS) database, covering the period from market introduction to September 30, 2023. Suspicious AEs were screened using the reporting odds ratio (ROR) and proportional reporting ratio methods based on disproportionality analysis. Univariate and multivariate logistic regression analyses were conducted on severe AEs to explore the risk factors associated with fulvestrant-induced severe AEs. A total of 6947 reports related to AEs associated with fulvestrant were obtained, including 5924 reports of severe AEs and 1023 reports of non-severe AEs. Using the disproportionality analysis method, a total of 210 valid AEs were identified for fulvestrant, with 45 AEs (21.43%) not listed in the product labeling, involving 11 systems and organs. The AEs associated with fulvestrant were sorted by frequency of occurrence, with neutropenia (325 cases) having the highest number of reports. By signal strength, injection site pruritus showed the strongest signal (ROR = 658.43). The results of the logistic regression analysis showed that concurrent use of medications with extremely high protein binding (≥ 98%) is an independent risk factor for severe AEs associated with fulvestrant. Age served as a protective factor for fulvestrant-related AEs. The co-administration of fulvestrant with CYP3A4 enzyme inhibitors did not show statistically significant correlation with the occurrence of severe AEs. Co-administration of drugs with extremely high protein binding (≥ 98%) may increase the risk of severe adverse reactions of fulvestrant. Meanwhile, age (60-74 years) may reduce the risk of severe AEs of fulvestrant. However, further clinical research is still needed to explore and verify whether there is interaction between fulvestrant and drugs with high protein binding through more clinical studies.

The discovery of potent USP2/USP8 dual-target inhibitors for the treatment of breast cancer via structure guided optimization of ML364

USP2 and USP8 are crucial in the development and progression of breast cancer, primarily through the stabilization of protein substrates such as Her2 and ERα. The dual-target inhibitor ML364, targeting both USP2 and USP8, has garnered significant interest in recent research. In this study, we developed a series of ML364 derivatives using ligand-based drug design strategies. The standout compound, LLK203, demonstrated enhanced inhibitory activity, showing a 4-fold increase against USP2 and a 9-fold increase against USP8, compared to the parent molecule. In MCF-7 breast cancer cells, LLK203 effectively degraded key proteins involved in cancer progression and notably inhibited cell proliferation. Moreover, LLK203 exhibited potent in vivo efficacy in the 4T1 homograft model, while maintaining a low toxicity profile. These results underscore the potential of LLK203 as a promising dual-target inhibitor of USP2/USP8 for breast cancer treatment.

Current and emerging treatment approaches for hormone receptor-positive/human epidermal growth factor receptor 2-negative metastatic breast cancer

In the past decade, significant progress was made in treating hormone receptor-positive (HR+), human epidermal growth factor receptor 2-negative (HER2-) metastatic breast cancer (mBC), but many clinical questions remain. Cyclin-dependent kinase 4/6 inhibitors are now widely used in combination with endocrine therapy (ET) as standard of care, having demonstrated significant progression-free survival versus ET, and also significant overall survival benefits in the metastatic setting. Inhibition of the PI3K/AKT/mTOR intracellular signaling pathway coupled with ET typically follows first-line therapies. Novel endocrine options including oral selective estrogen receptor down-regulators (SERDs) are in late phases of development, with elacestrant being the first oral SERD to be approved for ESR1-mutant mBC. However, endocrine-refractory disease is inevitable in most patients and represents an area of unmet need, with current recommended options offering poor efficacy, undesirable toxicity, and reduced quality of life. Breakthrough advances in the metastatic setting came via the development of antibody-drug conjugates, which have the advantage of delivering cytotoxic payloads to tumor cells with higher tumor selectivity. Trastuzumab deruxtecan offers a novel therapeutic option for patients with HR+/HER2-low mBC and sacituzumab govitecan is a novel therapeutic option for patients with HR+/HER2- mBC, including those with unmet treatment need in the later-line endocrine-refractory setting. Data gaps still exist regarding optimal sequencing of these novel agents; additional studies into mechanisms of resistance in the metastatic setting would provide further insights. Herein, we describe the current treatment options for HR+/HER2- mBC, including the latest practice-impacting data, and provide commentary on future directions.

Oral SERD, a Novel Endocrine Therapy for Estrogen Receptor-Positive Breast Cancer

Breast cancer is the most common cancer among women worldwide, and estrogen receptor-positive (ER+) breast cancer accounts for a significant proportion of cases. While various treatments are available, endocrine therapies are often the first-line treatment for this type of breast cancer. However, the development of drug resistance poses a significant challenge in managing this disease. ESR1 mutations have been identified as a common mechanism of endocrine therapy resistance in ER+ breast cancer. The first-generation selective estrogen receptor degrader (SERD) fulvestrant has shown some activity against ESR1 mutant tumors. However, due to its poor bioavailability and need for intramuscular injection, it may not be the optimal therapy for patients. Second-generation SERDs were developed to overcome these limitations. These newer drugs have improved oral bioavailability and pharmacokinetics, making them more convenient and effective for patients. Several oral SERDs are now in phase III trials for early and advanced ER+ breast cancer. This review summarizes the background of oral SERD development, the current status, and future perspectives.

Therapies for the Treatment of Advanced/Metastatic Estrogen Receptor-Positive Breast Cancer: Current Situation and Future Directions

The hormone receptor-positive (HR+) type is the most frequently identified subtype of breast cancer. HR+ breast cancer has a more positive prognosis when compared to other subtypes, such as human epidermal growth factor protein 2-positive disorder and triple-negative disease. The advancement in treatment outcomes for advanced HR+ breast cancer has been considerably elevated due to the discovery of cyclin-dependent kinase 4/6 inhibitors and their combination effects with endocrine therapy. However, despite the considerable effectiveness of tamoxifen, a selective estrogen receptor modulator (SERMs), and aromatase inhibitors (AI), the issue of treatment resistance still presents a significant challenge for HR+ breast cancer. As a result, there is a focus on exploring new therapeutic strategies such as targeted protein degradation and covalent inhibition for targeting ERα. This article discusses the latest progress in treatments like oral selective ER degraders (SERDs), complete estrogen receptor antagonists (CERANs), selective estrogen receptor covalent antagonists (SERCAs), proteolysis targeting chimera (PROTAC) degraders, and combinations of CDK4/6 inhibitors with endocrine therapy. The focus is specifically on those compounds that have transitioned into phases of clinical development.

A new treatment for breast cancer using a combination of two drugs: AZD9496 and palbociclib

In this study, we examined a mathematical model of breast cancer (BC) treatment that combines an oral oestrogen receptor inhibitor, AZD9496 with Palbociclib, a selective inhibitor of cyclin- dependent kinases CDK4 and CDK6. Treatment is described by analytical functions that enable us to control the dosage and time interval of the treatment, thus personalising the treatment for each patient. Initially, we investigated the effect of each treatment separately, and finally, we investigated the combination of both treatments. By applying numerical simulations, we confirmed that the combination of AZD9496 with palbociclib was the optimal treatment for BC. The dosage of AZD9496 increased and decreased throughout the treatment period, while the intervals were constant between treatments. Palbociclib changed almost cyclically, whereas the time intervals remained constant. To investigate the mathematical model, we applied the singularly perturbed homotopy analysis method, which is a numerical algorithm. The significant advantage of this method is that the mathematical model does not have to contain a small parameter (as is standard in perturbation theory). However, it is possible to artificially introduce a small parameter into the system of equations, making it possible to study the model using asymptotic methods.

The Anti-Cancer Activity of the Naturally Occurring Dipeptide Carnosine: Potential for Breast Cancer

Carnosine is an endogenous dipeptide composed of β-alanine and L-histidine, possessing a multimodal pharmacodynamic profile that includes anti-inflammatory and anti-oxidant activities. Carnosine has also shown its ability to modulate cell proliferation, cell cycle arrest, apoptosis, and even glycolytic energy metabolism, all processes playing a key role in the context of cancer. Cancer is one of the most dreaded diseases of the 20th and 21st centuries. Among the different types of cancer, breast cancer represents the most common non-skin cancer among women, accounting for an estimated 15% of all cancer-related deaths in women. The main aim of the present review was to provide an overview of studies on the anti-cancer activity of carnosine, and in particular its activity against breast cancer. We also highlighted the possible advantages and limitations involved in the use of this dipeptide. The first part of the review entailed a brief description of carnosine's biological activities and the pathophysiology of cancer, with a focus on breast cancer. The second part of the review described the anti-tumoral activity of carnosine, for which numerous studies have been carried out, especially at the preclinical level, showing promising results. However, only a few studies have investigated the therapeutic potential of this dipeptide for breast cancer prevention or treatment. In this context, carnosine has shown to be able to decrease the size of cancer cells and their viability. It also reduces the levels of vascular endothelial growth factor (VEGF), cyclin D1, NAD+, and ATP, as well as cytochrome c oxidase activity in vitro. When tested in mice with induced breast cancer, carnosine proved to be non-toxic to healthy cells and exhibited chemopreventive activity by reducing tumor growth. Some evidence has also been reported at the clinical level. A randomized phase III prospective placebo-controlled trial showed the ability of Zn-carnosine to prevent dysphagia in breast cancer patients undergoing adjuvant radiotherapy. Despite this evidence, more preclinical and clinical studies are needed to better understand carnosine's anti-tumoral activity, especially in the context of breast cancer.

Tailoring Potential Natural Compounds for the Treatment of Luminal Breast Cancer

Breast cancer (BC) is the most diagnosed cancer worldwide, mainly affecting the epithelial cells from the mammary glands. When it expresses the estrogen receptor (ER), the tumor is called luminal BC, which is eligible for endocrine therapy with hormone signaling blockade. Hormone therapy is essential for the survival of patients, but therapeutic resistance has been shown to be worrying, significantly compromising the prognosis. In this context, the need to explore new compounds emerges, especially compounds of plant origin, since they are biologically active and particularly promising. Natural products are being continuously screened for treating cancer due to their chemical diversity, reduced toxicity, lower side effects, and low price. This review summarizes natural compounds for the treatment of luminal BC, emphasizing the activities of these compounds in ER-positive cells. Moreover, their potential as an alternative to endocrine resistance is explored, opening new opportunities for the design of optimized therapies.

Breast-conserving surgery is an appropriate procedure for centrally located breast cancer: a population-based retrospective cohort study

BACKGROUND

The evidence of breast-conserving therapy (BCT) applied in centrally located breast cancer (CLBC) is absent. This study aims to investigate the long-term survival of breast-conserving therapy (BCT) in centrally located breast cancer (CLBC) compared with mastectomy in CLBC and BCT in non-CLBC.

METHODS

Two hundred ten thousand four hundred nine women with unilateral T1-2 breast cancer undergoing BCT or mastectomy were identified from the Surveillance, Epidemiology, and End Results database. Kaplan-Meier survival curves were assessed via log-rank test. Propensity score matching (PSM) was used to balance baseline features, and the multivariable Cox model was used to estimate the adjusted hazard ratio [HR] and its 95% confidence interval [CI] for breast cancer-specific survival (BCSS) and overall survival (OS).

RESULTS

With a median follow-up of 91 months, the BCSS and OS rates in patients who received BCT were greater than those patients treated with mastectomy in the entire CLBC set. Multivariable Cox analyses showed that CLBC patients who received BCT had better BCSS (HR = 0.67, 95%CI: 0.55-0.80, p < 0.001) and OS (HR = 0.78, 95%CI: 0.68-0.90, p = 0.001) than patients who received a mastectomy, but there were no significant differences of BCSS (HR = 0.65, 95%CI: 0.47-0.90, p = 0.009) and OS (HR = 0.82, 95%CI: 0.65-1.04, p = 0.110) after PSM. In patients treated with BCT, CLBC patients had a similar BCSS (HR = 0.99, 95%CI: 0.87-1.12, p = 0.850) but a worse OS (HR = 1.09, 95%CI: 1.01-1.18, p = 0.040) compared to that of the non-CLBC patient, but there was no significant difference both BCSS (HR = 1.05, 95%CI: 0.88-1.24, p = 0.614) and OS (HR = 1.08, 95%CI: 0.97-1.20, p = 0.168) after PSM.

CONCLUSION

Our findings revealed that BCT should be an acceptable and preferable alternative to mastectomy for well-selected patients with CLBC.

Therapeutic resistance to anti-oestrogen therapy in breast cancer

The hormone receptor oestrogen receptor-α (ER) orchestrates physiological mammary gland development, breast carcinogenesis and the progression of breast tumours into lethal, treatment-refractory systemic disease. Selective antagonism of ER signalling has been one of the most successful therapeutic approaches in oncology, benefiting patients as both a cancer preventative measure and a cancer treatment strategy. However, resistance to anti-oestrogen therapy is a major clinical challenge. Over the past decade, we have gained an understanding of how breast cancers evolve under the pressure of anti-oestrogen therapy. This is best depicted by the case of oestrogen-independent mutations in the gene encoding ER (ESR1), which are virtually absent in primary breast cancer but highly prevalent (20-40%) in anti-oestrogen-treated metastatic disease. These and other findings highlight the 'evolvability' of ER+ breast cancer and the need to understand molecular processes by which this evolution occurs. Recent development and approval of next-generation ER antagonists to target ESR1-mutant breast cancer underscores the clinical importance of this evolvability and sets a new paradigm for the treatment of ER+ breast cancers.

Recent Developments in the Therapeutic Landscape of Advanced or Metastatic Hormone Receptor-Positive Breast Cancer

Hormone receptor-positive (HR+) disease is the most frequently diagnosed subtype of breast cancer. Among tumor subtypes, natural course of HR+ breast cancer is indolent with favorable prognosis compared to other subtypes such as human epidermal growth factor protein 2-positive disease and triple-negative disease. HR+ tumors are dependent on steroid hormone signaling and endocrine therapy is the main treatment option. Recently, the discovery of cyclin-dependent kinase 4/6 inhibitors and their synergistic effects with endocrine therapy has dramatically improved treatment outcome of advanced HR+ breast cancer. The demonstrated efficacy of additional nonhormonal agents, such as targeted therapy against mammalian target of rapamycin and phosphatidylinositol 3-kinase signaling, poly(ADP-ribose) polymerase inhibitors, antibody-drug conjugates, and immunotherapeutic agents have further expanded the available therapeutic options. This article reviews the latest advancements in the treatment of HR+ breast cancer, and in doing so discusses not only the development of currently available treatment regimens but also emerging therapies that invite future research opportunities in the field.

Estrogen Receptor Signaling in Breast Cancer

Estrogen receptor (ER) signaling is a critical regulator of cell proliferation, differentiation, and survival in breast cancer (BC) and other hormone-sensitive cancers. In this review, we explore the mechanism of ER-dependent downstream signaling in BC and the role of estrogens as growth factors necessary for cancer invasion and dissemination. The significance of the clinical implications of ER signaling in BC, including the potential of endocrine therapies that target estrogens' synthesis and ER-dependent signal transmission, such as aromatase inhibitors or selective estrogen receptor modulators, is discussed. As a consequence, the challenges associated with the resistance to these therapies resulting from acquired ER mutations and potential strategies to overcome them are the critical point for the new treatment strategies' development.

The Treatment Landscape of Elderly Patients with Hormone Receptor-Positive Her2 Negative Advanced Breast Cancer: Current Perspectives and Future Directions

Breast cancer (BC) in elderly women is an increasing health issue due to demographic changes. BC tends to present later and may receive less than standard treatment options. More often, BC in elderly patients is endocrine-positive (HR+). The treatment of elderly patients with metastatic BC (mBC) represents a therapeutic challenge. In recent years, the treatment landscape of patients that are HR+/Her2-negative has changed due to the introduction in clinical practice of new targeted drugs, which have improved patient outcomes. Elderly patients are a small percentage of all patients enrolled in clinical trials and, to date, there are no standardized guidelines that define the best treatment option for this patient population. This can lead to undertreatment or overtreatment, impacting patient morbidity and mortality. Geriatric Assessment tools to tailor the treatment in elderly patients are underused because they are long and difficult to apply in a busy routine clinical practice. For all these reasons, there is an urgent need to produce data about the best treatment for elderly patients with HR+ mBC. Herein, we report data from randomized clinical trials and real-world evidence on the therapeutic options for HR+ Her2-negative mBC elderly patients and explore future treatment directions.

Discovery of as a Potent and Orally Efficacious Estrogen Receptor PROTAC Degrader with Strong Antitumor Activity

Estrogen receptor α (ERα) is a prime target for the treatment of ER-positive (ER+) breast cancer. Despite the development of several effective therapies targeting ERα signaling, clinical resistance remains a major challenge. In this study, we report the discovery of a new class of potent and orally bioavailable ERα degraders using the PROTAC technology, with ERD-3111 being the most promising compound. ERD-3111 exhibits potent in vitro degradation activity against ERα and demonstrates high oral bioavailability in mice, rats, and dogs. Oral administration of ERD-3111 effectively reduces the levels of wild-type and mutated ERα proteins in tumor tissues. ERD-3111 achieves tumor regression or complete tumor growth inhibition in the parental MCF-7 xenograft model with wild-type ER and two clinically relevant ESR1 mutated models in mice. ERD-3111 is a promising ERα degrader for further extensive evaluations for the treatment of ER+ breast cancer.

Emerging systemic therapy options beyond CDK4/6 inhibitors for hormone receptor-positive HER2-negative advanced breast cancer

Endocrine therapy (ET) with cyclin-dependent kinase 4/6 inhibitor (CDK4/6i) is currently the standard first-line treatment for most patients with hormone receptor (HR) positive, human epidermal growth factor receptor (HER2) negative advanced breast cancer. However, resistance to ET and CDK4/6i inevitably ensues. The optimal post-progression treatment regimens and their sequencing continue to evolve in the rapidly changing treatment landscape. In this review, we summarize the mechanisms of resistance to ET and CDK4/6i, which can be broadly classified as alterations affecting cell cycle mediators and activation of alternative signaling pathways. Recent clinical trials have been directed at the targets and pathways implicated, including estrogen and androgen receptors, PI3K/AKT/mTOR and MAPK pathways, tyrosine kinase receptors such as FGFR and HER2, homologous recombination repair pathway, other components of the cell cycle and cell death. We describe the findings from these clinical trials using small molecule inhibitors, antibody-drug conjugates and immunotherapy, providing insights into how these novel strategies may circumvent treatment resistance, and discuss how some have not translated into clinical benefit. The challenges posed by tumor heterogeneity, adaptive rewiring of signaling pathways and dose-limiting toxicities underscore the need to elucidate the latest tumor biology in each patient, and develop treatments with improved therapeutic index in the era of precision medicine.

Research progress on tamoxifen and its analogs associated with nuclear receptors

Tamoxifen, a triphenylethylene-based selective estrogen-receptor modulator, is a landmark drug for the treatment of breast cancer and is also used for treating liver cancer and osteoporosis. Structural studies of tamoxifen have led to the synthesis of more than 20 novel tamoxifen analogs as receptor modulators, including 16 ERα modulators 2-17, an ERRβ inverse agonist 19 and six ERRγ inverse agonists 20-25. This paper summarizes the research progress and structure-activity relationships of tamoxifen analogs modulating these three nuclear receptors reported in the literature, and introduces the relationship between these three nuclear receptor-mediated diseases and tamoxifen analogs to guide the research of novel tamoxifen analogs.

A comprehensive update of hormone-related pharmacokinetic variations associated with breast cancer drugs

INTRODUCTION

Drugs available for the treatment of breast cancer are increasing, yielding improved oncological outcomes. The efficacy and safety of anticancer drugs significantly depend on pharmacokinetic profiles, which could be influenced by several factors, such as sex hormones.

AREAS COVERED

This article discusses the potential hormone-related pharmacokinetic influences on novel breast cancer pharmacotherapies.

EXPERT OPINION

Recently approved drugs for the treatment of breast cancer belong to different classes, each with unique pharmacokinetic profile. The impact of hormones, such as estrogen and progesterone, may occur at different steps of drug metabolism. Key effects of sex hormones ha ve been reported on multidrug-resistant transporters and enzymes involved in the liver metabolism of drugs, such as cytochromes. Nevertheless, no data is currently available to establish hormone-related metabolic interactions that may account for variability in drug scheduling and selection. Whereas we recognize influences may occur, we do not assume hormones alone can yield clinically significant metabolic changes. Rather, we believe that hormonal influences should be considered along with other elements that may affect drugs metabolism, such as concomitant medications, age-related pharmacokinetic changes, and genetic polymorphisms, in order to deliver treatment personalization and ensure better tolerability and safety of anticancer treatments.

Targeting the Estrogen Receptor for the Treatment of Breast Cancer: Recent Advances and Challenges

Estrogen receptor alpha (ERα) is a well-established therapeutic target for the treatment of ER-positive (ER+) breast cancers. Despite the tremendous successes achieved with tamoxifen, a selective ER modulator, and aromatase inhibitors (AIs), resistance to these therapies is a major clinical problem. Therefore, induced protein degradation and covalent inhibition have been pursued as new therapeutic approaches to target ERα. This Perspective summarizes recent progress in the discovery and development of oral selective ER degraders (SERDs), complete estrogen receptor antagonists (CERANs), selective estrogen receptor covalent antagonists (SERCAs), and proteolysis targeting chimera (PROTAC) ER degraders. We focus on those compounds which have been advanced into clinical development.

Comparative in vitro and in silico study on the estrogenic effects of 2,2-bis(4-chlorophenyl)ethanol, 4,4'-dichlorobenzophenone and DDT analogs

DDT and its transformation products (DDTs) are frequently detected in environmental and biological media. Research suggests that DDT and its primary metabolites (DDD and DDE) could induce estrogenic effects by disturbing estrogen receptor (ER) pathways. However, the estrogenic effects of DDT high-order transformation products, and the exact mechanisms underlying the differences of responses in DDT and its metabolites (or transformation products) still remain unknown. Here, besides DDT, DDD and DDE, we selected two DDT high-order transformation products, 2,2-bis(4-chlorophenyl) ethanol (p,p'-DDOH) and 4,4'-dichlorobenzophenone (p,p'-DCBP). We aim to explore and reveal the relation between DDTs activity and their estrogenic effects by receptor binding, transcriptional activity, and ER-mediated pathways. Fluorescence assays showed that the tested 8 DDTs bound to the two isoforms (ERα and ERβ) of ER directly. Among them, p,p'-DDOH exhibited the highest binding affinity, with IC₅₀ values of 0.43 μM and 0.97 μM to ERα and ERβ, respectively. Eight DDTs showed different agonistic activity toward ER pathways, with p,p'-DDOH exhibiting the strongest potency. In silico studies revealed that the eight DDTs bound to either ERα or ERβ in a similar manner to 17β-estradiol, in which specific polar and non-polar interactions and water-mediated hydrogen bonds were involved. Furthermore, we found that 8 DDTs (0.0008-5 μM) showed distinct pro-proliferative effects on MCF-7 cells in an ER-dependent manner. Overall, our results revealed not only for the first time the estrogenic effects of two DDT high-order transformation products by acting on ER-mediated pathways, but also the molecular basis for differential activity of 8 DDTs.

Elacestrant: a new FDA-approved SERD for the treatment of breast cancer

Elacestrant (RAD-1901), a selective estrogen receptor degrader, was approved by USFDA on January 27, 2023, for the treatment of breast cancer. It has been developed by Menarini Group under the brand name Orserdu®. Elacestrant showed anticancer activity both in vitro and in vivo in ER+ HER2-positive breast cancer models. The present review delebrates the development stages of Elacestrant, with its medicinal chemistry, synthesis, mechanism of action, and pharmacokinetic studies. Clinical data and safety profile has also been discussed, including data from randomized trials.

Novel endocrine therapies: What is next in estrogen receptor positive, HER2 negative breast cancer?

Endocrine therapy (ET) is the cornerstone of management in hormone receptor (HR)+ breast cancer (BC). Indeed, targeting the estrogen receptor (ER) signaling at different levels is a successful strategy, since BC largely relies on the ER signaling as a driver of tumorigenesis and progression. In metastatic BC, progression of disease typically occurs due to either ligand-independent ER signaling, which favors tumor proliferation and survival in the absence of hormonal stimuli, or an ER-independent signaling, which exploits alternative transcription pathways. For instance, estrogen receptor 1 (ESR1) mutations induce constitutive ER activity, in turn upregulating ER-dependent gene transcription and causing resistance to estrogen depleting therapies. The largest unmet need lies after progression on ET + cyclin-dependent kinases 4 and 6 (CDK4/6) inhibitors, where fulvestrant alone provides an average 2-3-month PFS. In this context, novel oral selective estrogen receptor degraders (SERDs) and other next-generation ETs are being investigated, both as single agents and in combination with targeted therapies. Elacestrant, the next generation ET in most advanced clinical development and the first to be FDA approved, demonstrated improved outcomes compared to standard ETs in ET pre-treated HR+/HER2- metastatic BC in the phase 3 EMERALD clinical trial. Additionally, other agents are showing promising results in both preclinical and early phase clinical settings. In this review, emerging data related to oral SERDs and other novel ETs in managing HR+/HER2- BC are presented. Major challenges and future perspectives related to the optimal sequence of therapeutic options and the molecular landscape of endocrine resistance are also provided.

Targeting Breast Cancer: An Overlook on Current Strategies

Breast cancer (BC) is one of the most widely diagnosed cancers and a leading cause of cancer death among women worldwide. Globally, BC is the second most frequent cancer and first most frequent gynecological one, affecting women with a relatively low case-mortality rate. Surgery, radiotherapy, and chemotherapy are the main treatments for BC, even though the latter are often not aways successful because of the common side effects and the damage caused to healthy tissues and organs. Aggressive and metastatic BCs are difficult to treat, thus new studies are needed in order to find new therapies and strategies for managing these diseases. In this review, we intend to give an overview of studies in this field, presenting the data from the literature concerning the classification of BCs and the drugs used in therapy for the treatment of BCs, along with drugs in clinical studies.

Mechanisms of Endocrine Resistance in Hormone Receptor-Positive Breast Cancer

Hormone receptor-positive (HR+) breast cancer (BC) accounts for approximately 70% of all breast invasive tumors. Endocrine therapy (ET) represents the standard treatment for HR + BC. Most patients, however, eventually develop resistance to ET, which limits their effectiveness and poses a major challenge for the management of HR + BC. Several mechanisms that contribute to ET resistance have been described. One of the most common mechanisms is the upregulation of alternative signaling pathways that can bypass estrogen dependency, such as activation of the PI3K/Akt/mTOR as well as mitogen-activated protein kinase (MAPK) and the insulin-like growth factor 1 receptor (IGF-1R) pathways. Another common mechanism of endocrine resistance is the acquisition of activating mutations of ESR1, which encodes for the estrogen receptor, that lead to structural changes of the receptor, prevent the binding to anti-estrogen drugs and result in constitutive activation of the receptor, even in the absence of estrogens. Epigenetic changes, such as DNA methylation and histone modifications, can also contribute to ET resistance by altering the expression of genes that are involved in estrogen signaling. Understanding the mechanisms of resistance to ET is crucial for the development of new therapies that can overcome resistance and improve outcomes for patients with HR + BC.

Uncovering the molecular mechanism of Gynostemma pentaphyllum (Thunb.) Makino against breast cancer using network pharmacology and molecular docking

Because of their strong anti-cancer efficacy with fewer side effects, traditional Chinese medicines (TCM) have attracted considerable attention for their potential application in treating breast cancer (BC). However, knowledge about the underlying systematic mechanisms is scarce. Gynostemma pentaphyllum (Thunb.) Makino (GP), a creeping herb, has been regularly used as a TCM to prevent and treat tumors including BC. Again, mechanisms underlying its anti-BC properties have remained elusive. We used network pharmacology and molecular docking to explore the mechanistic details of GP against BC. The TCM systems pharmacology database and analysis platform and PharmMapper Server database were used to retrieve the chemical constituents and potential targets in GP. In addition, targets related to BC were identified using DrugBank and Therapeutic Target Database. Protein-protein interaction network, Gene Ontology, and Kyoto Encyclopedia of Genes and Genomes pathway enrichment analyses of crucial targets were performed using the Search Tool for the Retrieval of Interacting Genes/Proteins and database for annotation, visualization, and integrated discovery databases, whereas the network visualization analysis was performed using Cytoscape 3.8.2. In addition, the molecular docking technique was used to validate network pharmacology-based predictions. A comparison of the predicted targets of GP with those of BC-related drugs revealed 26 potential key targets related to the treatment of BC, among which ALB, EGFR, ESR1, AR, PGR, and HSP90AA1 were considered the major potential targets. Finally, network pharmacology-based prediction results were preliminarily verified by molecular docking experiments. In addition, chemical constituents and potential target proteins were scored, followed by a comparison with the ligands of the protein. We provide a network of pharmacology-based molecular mechanistic insights on the therapeutic action of GP against BC. We believe that our data will serve as a basis to conduct future studies and promote the clinical applications of GP.

Identification of Human Breast Adipose Tissue Progenitors Displaying Distinct Differentiation Potentials and Interactions with Cancer Cells

Breast adipose tissue (AT) participates in the physiological evolution and remodeling of the mammary gland due to its high plasticity. It is also a favorable microenvironment for breast cancer progression. However, information on the properties of human breast adipose progenitor cells (APCs) involved in breast physiology or pathology is scant. We performed differential enzymatic dissociation of human breast AT lobules. We isolated and characterized two populations of APCs. Here we report that these distinct breast APC populations selectively expressed markers suitable for characterization. The population preferentially expressing ALPL (MSCA1) showed higher adipogenic potential. The population expressing higher levels of INHBA and CD142 acquired myofibroblast characteristics upon TGF-β treatment and a myo-cancer-associated fibroblast profile in the presence of breast cancer cells. This population expressed the immune checkpoint CD274 (PD-L1) and facilitated the expansion of breast cancer mammospheres compared with the adipogenic population. Indeed, the breast, as with other fat depots, contains distinct types of APCs with differences in their ability to specialize. This indicates that they were differentially involved in breast remodeling. Their interactions with breast cancer cells revealed differences in the potential for tumor dissemination and estrogen receptor expression, and these differences might be relevant to improve therapies targeting the tumor microenvironment.