Clinical Experience with Abemaciclib in Patients Previously Treated with Another CDK 4/6 Inhibitor in a Tertiary Hospital: A Case Series Study

The three approved cyclin-dependent kinase 4/6 (CDK4/6) inhibitors, including abemaciclib, have shown differences in their preclinical, pharmacological, and clinical data. Abemaciclib stands out for its broader target range and more rapid and intense activity. It has demonstrated efficacy as a monotherapy or in combination with tamoxifen in endocrine-refractory metastatic breast cancer (MBC) patients with prior chemotherapy. However, the clinical data on abemaciclib after exposure to previous CDK4/6 inhibitors are limited. In this single-center retrospective case series, we identified all patients who received abemaciclib until February 2022 after experiencing documented progression on palbociclib or ribociclib. The safety profile and clinical outcomes of abemaciclib treatment in this specific patient cohort were evaluated. Eleven patients were included in this retrospective case series, nine receiving abemaciclib with tamoxifen. Eight patients had visceral involvement, and the median age was 69 (ranging from 42 to 84). The median time from the end of prior CDK4/6 inhibitor treatment to abemaciclib initiation was 17.5 months (ranging from 3 to 41 months). Patients had undergone a median of three prior therapies (ranging from 1 to 7), including chemotherapy in 54.5% of cases. The median follow-up time was six months (ranging from 1 to 22 months). The median progression-free survival (PFS) was 8 months (95% CI 3.9-12). Five patients continued abemaciclib treatment, and one patient with liver metastases achieved a complete hepatic response. The most common adverse events were diarrhea (72.7%, no grade ≥ 3) and asthenia (27.3%, no grade ≥ 3). Our preliminary findings suggest that abemaciclib could be an effective and safe treatment option for MBC patients who have previously received palbociclib or ribociclib.

Comparison of Endocrine Therapies in Hormone Receptor-Positive and Human Epidermal Growth Factor Receptor 2-Negative Locally Advanced or Metastatic Breast Cancer: A Network Meta-Analysis

We aimed to explore what kind of endocrine treatments are optimal for hormone receptor-positive and human epidermal growth factor receptor 2-negative locally advanced or metastatic breast cancer in some specific clinical situations. We searched randomized controlled trials in Embase, Medline, the Cochrane library, and PubMed from inception to April 1, 2020 and performed a network meta-analysis based on a Bayesian fixed-effects model. Progression-free survival (PFS) with hazard ratios and corresponding 95% confidence interval was defined as the primary endpoint, while overall survival (OS), objective response rate (ORR), clinical benefit rate and serious adverse events were used as secondary endpoints. A total of 35 studies involving 12,285 patients and 24 treatment options were included. In general, most co-treatment options prolonged PFS compared to single-agent therapy, of which aromatase inhibitor (AI) plus everolimus and fulvestrant plus palbociclib were probably the most effective agents, and the latter had the best safety record. However, despite the superior efficacy of fulvestrant plus capecitabine for PFS and OS, palpable toxic effects have been demonstrated for this treatment, so its application must be scrupulously considered. The results of subgroup analysis indicated that fulvestrant combined with palbociclib improved prognosis for phosphatidylinositol 3-kinase (PI3K)-mutated patients, PI3K-unmutated patients, patients with endocrine therapy resistance, and visceral metastatic patients, while no obvious improvement was detected in OS. Moreover, the efficacy of fulvestrant plus cyclin-dependent kinase 4/6 (CDK4/6) inhibitors was slightly better than that of AI plus CDK4/6 inhibitors, while AI plus everolimus was more efficacious than fulvestrant combined with everolimus in terms of PFS, OS, and ORR. In conclusion, our results provide moderate evidence that fulvestrant plus palbociclib and AI plus everolimus were the most effective treatments, while the efficacy and safety of fulvestrant plus palbociclib was obviously superior in some specific clinical situations.

Effectiveness and Tolerability of Nab-paclitaxel in Younger versus Elderly Patients With Metastatic HR-positive/HER2-negative Breast Cancer: Results From the Noninterventional, Prospective Study NABUCCO

BACKGROUND

There are only scarce data on treatment of elderly patients with nab-paclitaxel for metastatic breast cancer, especially from the real-world setting. Here we present data from the noninterventional study NABUCCO with special focus on taxane-induced peripheral neuropathy (TIPN) in younger and elderly patients.

PATIENTS AND METHODS

A total of 407 patients with HR-positive/HER2-negative metastatic breast cancer were enrolled between April 2012 and April 2015 into the prospective, multicenter, noninterventional study NABUCCO. Details on effectiveness, tolerability, and safety of nab-paclitaxel were evaluated for younger (<70 years) and elderly (≥70 years) patients.

RESULTS

Neither median time to progression (TTP, younger 6.0 months, 95% confidence interval [CI], 5.5-7.1; elderly 6.9 months, 95% CI, 5.5-8.6) nor median overall survival (younger 16.4 months, 95% CI, 14.2-18.1; elderly 14.5 months, 95% CI, 11.9-17.4) differed by age group, also not in view of prior treatments. A multivariate regression model revealed that age did not significantly influence the TTP. TIPN was reported by 49.0% younger (44.3% common terminology criteria for adverse events [CTCAE] grade 1/2, 4.7% grade 3/4) and 45.8% elderly patients (41.1% CTCAE grade 1/2, 4.7% grade 3/4). The cumulative nab-paclitaxel dose did not correlate with the severity/grading of TIPN.

CONCLUSION

Treatment with nab-paclitaxel in first- or further-line of metastatic HR-positive/HER2-negative breast cancer resulted in similar effectiveness and safety, irrespective of age. Therefore, nab-paclitaxel is a valid treatment option for elderly and partially heavily pretreated patients. However, incidence of TIPN is high, influencing the patients' quality of life. A close monitoring and awareness for early TIPN symptoms is warranted.

Systemic treatment of hormone receptor positive, human epidermal growth factor 2 negative metastatic breast cancer: retrospective analysis from Leeds Cancer Centre

Background

Study aimed to characterise treatment and outcomes for patients with hormone receptor positive (HR+), human epidermal growth factor 2 negative (HER2-) metastatic breast cancer (MBC) within a large regional cancer centre, as a benchmark for evaluating real-world impact of novel therapies.

Methods

Retrospective longitudinal cohort, using electronic patient records of adult females with a first diagnosis of HR+/HER2- MBC January 2012–March 2018.

Results

One hundred ninety-six women were identified with HR+/HER2- MBC. Median age was 67 years, 85.2% were post-menopausal and median time between primary diagnosis and metastasis was 5.4 years. Most (75.1%) patients received endocrine therapy as first line systemic treatment (1st LoT); use of 1st LoT chemotherapy halved between 2012 and 2017. Patients receiving 1st LoT chemotherapy were younger and more likely to have visceral metastasis (p < 0.01). Median OS was 29.5 months and significantly greater for patients with exclusively non-visceral metastasis (p < 0.01). The adjusted hazard ratio for death of patients with visceral (or CNS) metastasis was 1.91 relative to those with exclusively non-visceral metastasis.

Conclusions

Diverse endocrine therapies predominate as 1st LoT for patients with HR+/HER2- MBC, chemotherapy being associated with more aggressive disease in younger patients, emphasising the importance of using effective and tolerable therapies early.

The PI3K/AKT/mTOR and CDK4/6 Pathways in Endocrine Resistant HR+/HER2- Metastatic Breast Cancer: Biological Mechanisms and New Treatments

Endocrine-based treatments are the normal standard-of-care in women with hormone receptor-positive/Human Epidermal growth factor Receptor 2-negative metastatic breast cancer. Despite the well-known efficacy of these drugs as first-line therapies, about 50% of women develop endocrine resistance and disease progression. The treatment of these patients has represented one of the most important research fields in the last few years, with several multicenter phase II/III trials published or still ongoing. Novel therapies, such as cyclin-dependent kinase (CDK)4/6 and phosphatidylinositol 3-kinase (PI3K)/protein kinase B (AKT)/mammalian target of rapamycin (mTOR) inhibitors, have significantly changed the prognosis of patients progressing to a previous endocrine treatment, allowing a great benefit in terms of progression-free survival and, in some cases, of overall survival. However, identifying response predictors is essential for the rational use of these drugs to avoid unnecessary toxicity and costs, and to ensure the optimal therapeutic sequence is used. In this review, we analyze the PI3K/AKT/mTOR and CDK4/6 pathways and their roles in endocrine resistant metastatic breast cancer. We then focus on the new treatments developed and the roles of these drugs in overcoming endocrine resistance, describing the latest clinical trials that led to the approval of the drugs in clinical practice.

Fulvestrant-Based Combination Therapy for Second-Line Treatment of Hormone Receptor-Positive Advanced Breast Cancer

Fulvestrant is recommended for patients with hormone receptor-positive (HR+) advanced breast cancer (ABC) who progress after aromatase inhibitor therapy. As most patients in this setting have already developed mechanisms of resistance to endocrine therapy, targeting biological pathways associated with endocrine resistance in combination with fulvestrant may improve outcomes. Therefore, evidence supporting a combinatorial treatment approach in the second-line setting was investigated based on a search of PubMed and ClinicalTrials.gov . Twenty-eight studies of targeted therapies plus fulvestrant as second-line treatment for HR+ ABC were identified, including three and six key randomized trials exploring cyclin-dependent kinase 4/6 (CDK4/6) inhibitors and phosphatidylinositol 3-kinase (PI3K)/mammalian target of rapamycin (mTOR) inhibitors plus fulvestrant respectively. Additional combinations with fulvestrant included inhibitors of epidermal growth factor receptors, androgen receptor, and the bromodomain and extra-terminal family of proteins. Across the studies reviewed with available data, the addition of targeted therapies to fulvestrant resulted in clinically meaningful improvements in progression-free survival compared with fulvestrant alone. While some challenging toxicities were observed, most adverse events could be effectively managed. Selection of second-line targeted therapy for use with fulvestrant should consider prior treatment as well as the mutation status of the tumor. In conclusion, available data indicate that fulvestrant combined with agents targeting mechanisms of endocrine resistance is a promising approach. The ongoing trials identified in this review will help further inform the selection of combination treatments with fulvestrant for HR+ ABC.

Health-related quality of life of postmenopausal women with hormone receptor-positive, human epidermal growth factor receptor 2-negative advanced breast cancer treated with ribociclib + letrozole: results from MONALEESA-2

PURPOSE

Evaluate patient-reported outcomes (PROs) for postmenopausal women with hormone receptor-positive (HR+), human epidermal growth factor receptor 2-negative (HER2-) advanced breast cancer treated with first-line ribociclib plus letrozole.

METHODS

In the phase III MONALEESA-2 study (NCT01958021), 668 patients were randomized 1:1 to ribociclib (600 mg/day; 3-weeks-on/1-week-off) plus letrozole (2.5 mg/day) or placebo plus letrozole. PROs were assessed using the European Organisation for Research and Treatment of Cancer core quality-of-life (EORTC QLQ-C30) and breast cancer-specific (EORTC QLQ-BR23) questionnaires. Changes from baseline and time to deterioration in health-related quality of life (HRQoL) were analyzed using linear mixed-effect and stratified Cox regression models, respectively. Exploratory analysis of area-under-the-curve for change from baseline in pain score (AUC-pain) was performed.

RESULTS

On-treatment HRQoL scores were consistently maintained from baseline and were similar between arms. A clinically meaningful (> 5 points) reduction in pain score was observed as early as Week 8 and was maintained up to Cycle 15 in the ribociclib arm. A statistically significant increase in mean AUC-pain was also observed in the ribociclib arm. Scores for all other EORTC QLQ-C30 and EORTC QLQ-BR23 domains were maintained from baseline and were similar between arms.

CONCLUSIONS

HRQoL was consistently maintained from baseline in postmenopausal women with HR+, HER2- advanced breast cancer receiving ribociclib plus letrozole and was similar to that observed in the placebo plus letrozole arm. Together with the improved clinical efficacy and manageable safety profile, these PRO results provide additional support for the benefit of ribociclib plus letrozole in this patient population.

Cdk4/6 inhibitors and overall survival: power of first-line trials in metastatic breast cancer

Palbociclib, ribociclib, and abemaciclib have been investigated in combination with aromatase inhibitors as first-line therapy for metastatic hormone receptor-positive breast cancer (PALOMA-2, MONALEESA-2 and MONALEESA-7, MONARCH-3 trials, respectively); pivotal trials led to absolute median progression-free survival (PFS) gain of about 15 months. We aimed to estimate, for each trial, the statistical power to demonstrate a significant gain in overall survival (OS). Power was calculated with Freedman's formula. Given the allocation ratio and the number of events, power was computed as a function of hazard ratio. We focused on four specific hazard ratio values (0.94, 0.89, 0.81, and 0.77), which are estimated to correspond to absolute 3, 6, 12, and 15 months gain in OS, respectively. For these calculations, the type I error rate was stated at 5% with a two-sided test, and we assumed that the risk of death was constant over time. PALOMA-2 and MONALEESA trials have an almost similar power despite different allocation ratios, while MONARCH-3 has a more limited power. Overall, the power of the four trials to demonstrate a statistically significant improvement in OS is less than 70% if the prolongation in median OS is ≤12 months, whatever the OS data maturity. This analysis shows that OS results are jeopardized by limited powers, and a meta-analysis might be required to demonstrate OS benefit. Conversely, if a significant OS improvement is observed in some but not at all trials, this discrepancy might be more attributable to chance than to a truly different drug efficacy.

CDK4/6 inhibitors in breast cancer therapy: Current practice and future opportunities

Dysregulation of the cyclin dependent kinase pathway in luminal breast cancer creates a new therapeutic opportunity for estrogen receptor positive breast cancer. Initial pan-CDK inhibitors were associated with extensive toxicities but in recent years, the development of potent specific CDK inhibitors with favorable tolerability has driven renewed interests in this class of targeted therapies. Palbociclib, ribociclib and abemaciclib are specific CDK4/6 inhibitors that have been approved by the U.S. Food and Drug Administration for use in combination with endocrine therapy for women with advanced hormone receptor positive breast cancer. These three anticancer therapeutics were approved based on progression free survival benefit seen on phase III trials with the most common grade 3 treatment-related side effects being neutropenia, fatigue, nausea and diarrhea. Except for estrogen receptor positivity, no biomarkers predictive of response to CDK4/6 inhibitors have been identified to date. Based on mechanistic insights here described, CDK4/6 inhibitors are currently being explored in combination with other agents, including targeted therapies, immunotherapy and chemotherapy.

The role of abemaciclib in treatment of advanced breast cancer

Until recently, the mainstay of treatment in the majority of hormone receptor (HR)-positive, human epidermal growth factor 2 receptor (HER2)-negative advanced breast cancer (ABC) has consisted of single-agent endocrine therapy (ET). However, as understanding of endocrine resistance has grown, newer targeted agents have come to the fore. Inhibition of cyclin-dependent kinase complexes 4 and 6 (CDK4/6) combined with ET has shown significant activity in HR+ HER2- ABC, with impressive results in terms of progression-free survival (PFS) when compared with ET alone. This review summarizes the seminal findings pertaining to CDK4/6 inhibition in this population, specifically focusing on abemaciclib, contrasted with palbociclib and ribociclib. Potential directions for future studies are discussed, as a way of addressing outstanding issues such as establishing optimal treatment sequencing and agent combinations, appropriate patient selection to derive maximal benefits, predictive biomarkers and the employment of CDK4/6 inhibition beyond the ABC setting.

Ribociclib for the treatment of hormone receptor-positive, human epidermal growth factor receptor 2-negative advanced breast cancer

INTRODUCTION

The emergence of cyclin-dependent kinases 4 and 6 (CDK4/6) inhibitors marked a significant advancement in the treatment of advanced breast cancer. Ribociclib is an orally bioavailable, highly selective inhibitor of CDK4/6. In combination with various endocrine therapies, ribociclib has demonstrated clinical activity as a first-line therapy for patients with HR+, HER2- advanced breast cancer, without compromising the favorable toxicity profile associated with endocrine therapy. Thus, ribociclib is now considered a new standard of care for HR+, HER2- advanced breast cancer. Areas covered: This review provides a concise overview of the preclinical and clinical development of ribociclib, including evidence of its clinical activity and safety profile when combined with endocrine therapy in HR+, HER2- advanced breast cancer. Expert commentary: CDK4/6 inhibition represents a promising treatment option for patients with HR+ metastatic breast cancer. Ribociclib significantly improved progression-free survival in patients receiving first-line endocrine therapy for HR+, HER2- advanced breast cancer. Planned and ongoing trials investigating ribociclib in combination with other endocrine therapies and in various clinical settings will help to determine the optimal treatment sequence for different patient populations.

Clinical efficacy of ribociclib as a first-line therapy for HR-positive, advanced breast cancer

INTRODUCTION

Breast cancer (BC) remains the most frequently diagnosed cancer and the most common cause of cancer death among women of all races worldwide. Over 80% of BC cases are hormone receptor (HR)-positive, comprised of luminal A and luminal B per molecular subtypes, imposing an urgent need to fully understand the mechanisms behind progression. Ribociclib is a selective cycline-dependent kinase 4 and 6 inhibitor. A phase 1 and a phase 3 trial have established a definitive role of ribociclib as frontline in the treatment of endocrine-sensitive advanced BC. Areas covered: Herein, the authors provide an overview of the data on ribociclib covering all aspects of the drug from its pharmacokinetics to efficacy and safety. The authors also provide their perspectives for the future. Expert opinion: Ribociclib is offering an opportunity to explore a new compound at the crossroads of different molecular activity and cell targets, which focus on endocrine-resistance reversal in multiple settings including early BC. Moreover, its activity against different subtypes of BC is being studied as is its immune-modulating effect. One cautionary note is that, in a market of concomitant similar competitors, a financial discussion will be mandatory.

Estrogen Receptor-Positive Breast Cancer: Exploiting Signaling Pathways Implicated in Endocrine Resistance

Advancements in molecular profiling and endocrine therapy (ET) have led to more focused clinical attention on precision medicine. These advances have expanded our understanding of breast cancer (BC) pathogenesis and hold promising implications for the future of therapy. The estrogen receptor-α is a predominant endocrine regulatory protein in the breast and in estrogen-induced BC. Successful targeting of proteins and genes within estrogen receptor (ER) nuclear and nonnuclear pathways remains a clinical goal. Several classes of antiestrogenic agents are available for patients with early, advanced, or metastatic BC, including selective ER modulators, aromatase inhibitors, and a selective ER degrader. Clinical development is focused upon characterizing the efficacy and tolerability of inhibitors that target the phosphatidylinositol 3 kinase (PI3K)/akt murine thymoma viral oncogene (AKT)/mammalian target of rapamycin inhibitor (mTOR) signaling pathway or the cyclin-dependent kinase 4/6 (CDK4/6) cell cycle pathway in women with hormone receptor-positive, human epidermal growth receptor 2-negative BC who have demonstrated disease recurrence or progression. De novo and acquired resistance remain a major challenge for women with BC receiving antiestrogenic therapy. Therefore, sequential combination of targeted ET is preferred in these patients, and the ever-increasing understanding of resistance mechanisms may better inform the selection of future therapy. This review describes the intricate roles of the PI3K/AKT/mTOR and CDK4/6 pathways in intracellular signaling and the use of endocrine and endocrine-based combination therapy in BC.

IMPLICATIONS FOR PRACTICE

The foundational strategy for treating hormone receptor-positive, human epidermal growth receptor 2-negative, advanced breast cancer includes the use of endocrine therapy either alone or in combination with targeted agents. The use of combination therapy aims to downregulate cell-signaling pathways with the intent of minimizing cellular "crosstalk," which can otherwise result in continued tumorigenesis or progression through redundant pathways. This review provides the clinician with the molecular rationale and clinical evidence for these treatments and refers to evidence-based guidelines to inform the decision-making process.

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.

ASCO 2017: highlights in breast cancer

At the 2017 ASCO Annual Meeting, several pertinent studies in the field of breast cancer were presented and some are deemed as being potentially practice changing. BrighTNess was the first phase III study to investigate the addition of carboplatin to standard neoadjuvant chemotherapy in triple-negative breast cancer; while toxicity was increased in the experimental group, a significantly higher pathologic complete remission (pCR) rate was observed as well suggesting that adding carboplatin to neoadjuvant anthracycline, cyclophosphamide and taxane-containing regimens is efficacious in otherwise healthy patients. In metastatic breast cancer patients harbouring BRCA germ-line mutations, the PARP(poly [ADP-ribose] polymerase)-inhibitor olaparib was superior to conventional chemotherapy defining a potential novel treatment standard in this high-risk population. In the adjuvant setting, the APHINITY trial compared dual HER2-directed antibody therapy with trastuzumab plus pertuzumab to trastuzumab alone. A small benefit in favour of the combination was observed which was more pronounced in node-positive subjects. In hormone-receptor positive metastatic disease, several studies evaluating the role of CDK4/6 (cyclin-dependendent kinases 4 and 6) inhibitors were presented with data again indicating that adding CDK4/6 inhibitors to endocrine therapy results in a clinically relevant prolongation of progression-free survival.

Efficacy and safety in older patient subsets in studies of endocrine monotherapy versus combination therapy in patients with HR+/HER2- advanced breast cancer: a review

PURPOSE

Prospective information regarding the tolerability and efficacy of endocrine therapy (ET) alone and in combination with targeted agents in older patients in the metastatic setting is limited. This review summarizes available trial data in this population.

METHODS

We searched PubMed for Phase 2 or 3 trials with age-stratified patient cohorts (≥ 65 vs. < 65 years in most studies) with hormone receptor-positive (HR+), human epidermal growth factor receptor 2-negative (HER2-) advanced breast cancer treated with ET ± targeted agents.

RESULTS

We identified 19 studies reporting 10 clinical trials. Efficacy was similar in age-stratified subsets. There was a reduced disease progression risk for ET + everolimus, palbociclib, or ribociclib versus ET alone. In the first-line setting, median progression-free survival (mPFS) in older patients was 8.5, 26.2 months, and not reached with letrozole + temsirolimus, palbociclib, and ribociclib, respectively, and in younger patients was 9.0, 18.8 months, and not reached, respectively. In the second-line setting, older patients had mPFS of 6.8 and 9.9 months with everolimus + exemestane and palbociclib + fulvestrant, respectively, and younger patients had mPFS of 8.1 and 9.5 months, respectively. Tolerability was worse for combination therapy versus monotherapy. No age-related differences in discontinuations were observed for CDK4/6 inhibitors, although a higher rate of treatment discontinuation was observed for patients ≥ 70 years receiving everolimus + exemestane. Adverse event rates were similar in age-stratified subsets.

CONCLUSIONS

ET + CDK4/6 or mTOR inhibitors are likely safe and effective in older patients with HR+, HER2- advanced breast cancer.

Strategies and Progress of Endocrine Therapy for Patients with Metastatic Breast Cancer

Breast cancer is one of the most prevalent cancers and the leading causes of cancer mortality in women worldwide and in China. For hormone receptor-positive (HR+) breast cancer, accounting for approximately 60-80% of breast cancer, endocrine therapy (ET) is the primary treatment strategy. For patients with HR+ metastatic breast cancer (MBC), there are many endocrine-based treatment options that can improve long-term outcomes and optimize quality of life. With the emergence and availability of new and effective agents, the options for ET have expanded in the last two decades. Although hormone therapy has been a standard of care for many decades, treatment must be individualized based on tumor biology and extent of disease. For example, the patients with impending organ failure may be treated with induction chemotherapy to improve organ function, followed by ET. For the patients who develop metastatic disease while on adjuvant ET, particularly when associated with organ failure, or for those with low expression of hormone receptors or expression of HER2, chemotherapy again may be a preferred initial treatment. ET blocks estrogen-driven tumor growth through different mechanisms; however, HR+ MBC can be intrinsically resistant or may acquire resistance to the treatment. Several targeted agents have been approved to use in combination with ET to improve response and delay development of resistance.