Clinical predictors of benefit from fulvestrant in advanced breast cancer: A Meta-analysis of randomized controlled trials

Updates on the treatment of invasive breast cancer: Quo Vadimus?

Breast cancer is a common malignancy among women. Despite an increase in incidence, breast cancer mortality has drastically dropped over the last 20 years. This change has been attributed to advances in screening, diagnosis, and treatment. This review summarises recent updates in the clinical approach to breast cancer. Advances in genetics have facilitated the stratification of the risk of recurrence in early-stage breast cancer. Advances in biology have led to the development of novel therapies (poly-ADP-ribose polymerase inhibitors, cyclin-dependent kinase 4/6 inhibitors, HER2 targeted agents). Their combination with endocrine (tamoxifen, aromatase inhibitors, GnRH-analogues, fulvestrant) and systematic therapy (anthracyclines, taxanes) in early and advanced disease have improved clinical outcomes. In the near future, neoadjuvant strategies in specific breast cancer subgroups (triple-negative breast cancers) and novel strategies (immune-modulatory agents) could further improve histopathological responses and survival. Radical mastectomies have been widely replaced by breast-conserving operations, while the traditional axillary dissection is being replaced with sentinel node techniques. Breast cancer therapeutics represents a great challenge due to patients' heterogeneous molecular and clinical characteristics, while the identification of reliable and easily reproducible predictive factors could further improve individualized treatment.

Efficacy and Safety of Fulvestrant 500mg in Hormone-receptor Positive Human Epidermal Receptor 2 Negative Advanced Breast Cancer: A Real-world Study in China

Background: Fulvestrant 500mg has proved its clinical effectiveness in previous trials as primary or second line treatment of hormone receptor positive, human epidermal receptor 2 negative (HR+/HER2-) post-menopausal advanced breast cancer. This real-world study aimed to investigate the efficacy and safety of Fulvestrant in HR+/HER2- Chinese advanced breast cancer patients. Method: HR+/HER2- advanced breast cancer patients who received Fulvestrant 500mg from January 2015 to December 2018 in Beijing Cancer Hospital were enrolled in this retrospective study. Progression free survival (PFS), objective response rate (ORR), clinical benefit rate (CBR), overall survival (OS) and adverse events (AEs) of Fulvestrant were investigated. Result: In total 303 enrolled patients [median age was 51 years (range: 21-82)], 255 (84.2%) patients were at postmenopausal status at the start of Fulvestrant treatment and 264 patients (87.1%) had advanced breast cancer. The median PFS (95% confidence interval) was 14.1 months (10.1-18.0) for the first-line, 11.2 months (2.2-20.3) for the second-line and 6.7 months (4.8-8.5) for ≥third-line of Fulvestrant. The ORR and CBR were 3.8% and 86.8% for the first-line, 5.5% and 75.4% for the second-line, 1.1% and 61.1% for ≥third-line of Fulvestrant. The multivariate subgroup analyses showed, PFS was significantly longer for the patients with light tumor burden, less palliative chemotherapy before Fulvestrant and long disease-free interval. For patients receiving Fulvestrant after palliative chemotherapy, the median PFS was numerically greater in maintenance treatment group than those who progressed after chemotherapy. Only 5.0% of patients (15/303) experienced adverse events and majority were grade 1-2. The most common adverse event was headache and palpitation, with merely one patient had severe adverse event (pulmonary embolism). Conclusion: Fulvestrant is an effective, safe and well-tolerated treatment regimen in endocrine therapy for HR+/HER2- metastatic breast cancer. Light tumor burden, less palliative chemotherapy before Fulvestrant and long disease-free survival (DFS) might be the ideal condition of Fulvestrant treatment. Fulvestrant can be effective for premenopausal patients with drug-induced menopause. Patients of different luminal subtypes can benefit from Fulvestrant. For patients with visceral metastases, presence of liver metastases rather than lung metastases was poor prognostic factor. Fulvestrant may also be considered as a maintenance treatment after first-line palliative chemotherapy.

Half-dose fulvestrant plus anastrozole as a first-line treatment for hormone receptor-positive metastatic breast cancer: a cost-effectiveness analysis

OBJECTIVE

The S0226 trial demonstrated that the combination of half-dose fulvestrant (FUL) and anastrozole (ANA) (F&A) caused a significant improvement in overall survival (OS) versus ANA monotherapy for first-line treatment of postmenopausal women with hormone receptor-positive metastatic breast cancer (PMW-MBC (HR+)). The objective of this study was to evaluate the cost-effectiveness of F&A in the first-line treatment for PMW-MBC (HR+) in China.

DESIGN

We constructed a Markov model over a life-time horizon. The clinical outcomes and utility data were obtained from published literature. Cost data were obtained from official Chinese websites. Sensitivity analyses were performed to test result uncertainty.

SETTING

Chinese healthcare system perspective.

POPULATION

A hypothetical cohort of adult patients presenting with PMW-MBC (HR+).

INTERVENTIONS

F&A compared with full-dose FUL and ANAmonotherapy.

MAIN OUTCOME MEASURES

The main outcome of this study was the incremental cost-effectiveness ratio (ICER) and quality-adjusted life-years (QALY).

RESULTS

ANA was estimated to have the lowest cost and minimum life-years. The ICER of F&A versus ANA was US$15 665.891/QALY with incremental cost and QALY of US$12 401.120 and 0.792, respectively, which was less than the willingness-to-pay of US$29 383/QALY. Compared with F&A, FUL yielded a higher cost and a shorter lifetime; hence, it was identified as a dominated strategy. The univariate sensitivity analysis indicated the price of FUL was the most influential factor in our study. The probability that F&A was cost-effective at a threshold of US$29 383/QALY in China was 86.5%.

CONCLUSION

F&A is a cost-effective alternative to FUL and ANA monotherapy for the first-line treatment of PMW-MBC (HR+) in China. F&A is a promising first-line treatment for PMW-MBC (HR+), and more research is needed to evaluate the economy of using F&A in other countries.

Although c‑MYC contributes to tamoxifen resistance, it improves cisplatin sensitivity in ER‑positive breast cancer

Tamoxifen (TAM) resistance is a major challenge in the treatment of estrogen receptor‑positive (ER+) breast cancer. To date, to the best of our knowledge, there are only a few studies available examining the response of patients with TAM‑resistant breast cancer to chemotherapy, and the guidelines do not specify recommended drugs for these patients. In the present study, TAM‑resistant cells were shown to exhibit increased proliferation and invasion compared with the parent cells, and the increased expression of c‑MYC was demonstrated to play an important role in TAM resistance. Furthermore, the TAM‑resistant cells were significantly more sensitive to cisplatin compared with the parent cells, and the silencing of c‑MYC expression desensitized the cells to cisplatin through the inhibition of the cell cycle. An increased c‑MYC expression was observed in 28 pairs of primary and metastatic tumors from patients treated with TAM, and the clinical remission rate of cisplatin‑based chemotherapy was significantly higher compared with other chemotherapy‑based regimens in 122 patients with TAM resistant breast cancer. Taken together, the data of the present study demonstrated that although c‑MYC was involved in TAM resistance, it increased the sensitivity of ER+ breast cancer to cisplatin. Thus, cisplatin may be a preferred chemotherapeutic agent for the treatment of patients with TAM‑resistant breast cancer, particularly in patients where the rapid control of disease progression is required.

Targeting Phosphorylation of Y-Box-Binding Protein YBX1 by TAS0612 and Everolimus in Overcoming Antiestrogen Resistance

Nuclear expression of Y-box-binding protein (YBX1) is closely correlated with clinical poor outcomes and drug resistance in breast cancer. Nuclear translocation of YBX1 is facilitated by YBX1 phosphorylation at serine 102 by AKT, p70S6K, and p90RSK, and the phosphorylated YBX1 (pYBX1) promotes expression of genes related to drug resistance and cell growth. A forthcoming problem to be addressed is whether targeting the phosphorylation of YBX1 overcomes antiestrogen resistance by progressive breast cancer. Here, we found that increased expression of pYBX1 was accompanied by acquired resistance to antiestrogens, fulvestrant and tamoxifen. Forced expression of YBX1/S102E, a constitutive phosphorylated form, resulted in acquired resistance to fulvestrant. Inversely, YBX1 silencing specifically overcame antiestrogen resistance. Furthermore, treatment with everolimus, an mTORC1 inhibitor, or TAS0612, a novel multikinase inhibitor of AKT, p70S6K, and p90RSK, suppressed YBX1 phosphorylation and overcame antiestrogen resistance in vitro and in vivo IHC analysis revealed that expression of pYBX1 and YBX1 was augmented in patients who experienced recurrence during treatment with adjuvant endocrine therapies. Furthermore, pYBX1 was highly expressed in patients with triple-negative breast cancer compared with other subtypes. TAS0612 also demonstrated antitumor effect against triple-negative breast cancer in vivo Taken together, our findings suggest that pYBX1 represents a potential therapeutic target for treatment of antiestrogen-resistant and progressive breast cancer.

Metastatic breast cancer patients with lung or liver metastases should be distinguished before being treated with fulvestrant

BACKGROUND

Endocrine therapy is the preferred treatment for patients with hormone receptor -positive metastatic breast cancer (MBC). While visceral metastasis is a negative prognostic factor, few studies have distinguished between the prognoses of patients with metastases at different visceral sites.

PATIENTS AND METHODS

In total, 398 patients receiving fulvestrant 500 mg at a single center over a 6-year period were analyzed. Logistic regression models were used to identify the prognostic factors associated with progression-free survival (PFS). Kaplan-Meier analysis was used to compare the PFS of patients with lung and liver metastases.

RESULTS

Baseline visceral metastases were present in 233 patients, including 138 with lung^{w/o liver} metastases (lung metastases without liver involvement), 51 with liver^{w/o lung} metastases (liver metastases without lung involvement) and 41 with lung and liver metastases. The median PFS was 6.8 months (5.6 and 9.2 months for visceral and nonvisceral metastases, respectively, P = .028). PFS was longer in patients with lung^{w/o liver} metastases than in those with liver^{w/o lung} metastases or lung and liver metastases (9.6, 3.7 and 3.2 months, respectively, P < .001; liver^{w/o lung} vs. lung^{w/o liver} hazard ratio (HR) 1.70; lung and liver vs. lung^{w/o liver} HR 2.85). Patients with liver metastases experienced significantly worse PFS than those without liver involvement (3.7 vs. 9.2 months, P < .001). PFS benefits were observed in patients with longer disease-free intervals, no liver metastases, and no previous chemotherapy.

CONCLUSION

Fulvestrant treatment benefited patients with lung^{w/o liver} or nonvisceral metastases. When treating hormone receptor-positive/HER2-negative MBC patients with endocrine therapy, it is important to differentiate patients with lung metastases from those with liver metastases.

Patterns of treatment and outcome with 500-mg fulvestrant in postmenopausal women with hormone receptor-positive/HER2-negative metastatic breast cancer: a real-life multicenter Italian experience

Background:

Fulvestrant 500 mg (F500) is the most active endocrine single agent in hormone receptor-positive (HR+)/HER2-negative metastatic breast cancer (MBC). Few data are available regarding the effectiveness of the drug in a real-world setting.

Patients and methods:

This prospective, multicenter cohort study aimed to describe the patterns of treatment and performance of F500 in a large population of unselected women with MBC, focusing on potential prognostic or predictive factors for disease outcome and response. The primary endpoints were progression-free survival (PFS) and clinical benefit rate.

Results:

From January 2011 to December 2015, 490 consecutive patients treated with F500 were enrolled. Overall, three different cohorts were identified and analyzed: the first received F500 after progression from previous chemotherapy (CT) or endocrine therapy; the second received the drug for de novo metastatic disease; and the third was treated as maintenance following disease stabilization or a response from a previous CT line. Median overall survival (OS) in the whole population was 26.8 months, ranging from 32.4 in first line to 22.0 and 13.7 months in second line and subsequent lines, respectively. Both the presence of liver metastasis and the treatment line were significantly associated with a worse PFS, while only the presence of liver metastasis maintained its predictive role for OS in multivariate analysis.

Conclusions:

The effectiveness of F500 was detected in patients treated both upon disease progression and as maintenance. The relevant endocrine sensitivity of 80% of patients included in the study could probably explain the good results observed in terms of outcome.

Oncogenic Y-box binding protein-1 as an effective therapeutic target in drug-resistant cancer

Y-box binding protein-1 (YBX1), a multifunctional oncoprotein containing an evolutionarily conserved cold shock domain, dysregulates a wide range of genes involved in cell proliferation and survival, drug resistance, and chromatin destabilization by cancer. Expression of a multidrug resistance-associated ATP binding cassette transporter gene, ABCB1, as well as growth factor receptor genes, EGFR and HER2/ErbB2, was initially discovered to be transcriptionally activated by YBX1 in cancer cells. Expression of other drug resistance-related genes, MVP/LRP, TOP2A, CD44, CD49f, BCL2, MYC, and androgen receptor (AR), is also transcriptionally activated by YBX1, consistently indicating that YBX1 is involved in tumor drug resistance. Furthermore, there is strong evidence to support that nuclear localization and/or overexpression of YBX1 can predict poor outcomes in patients with more than 20 different tumor types. YBX1 is phosphorylated by kinases, including AKT, p70S6K, and p90RSK, and translocated into the nucleus to promote the transcription of resistance- and malignancy-related genes. Phosphorylated YBX1, therefore, plays a crucial role as a potent transcription factor in cancer. Herein, a novel anticancer therapeutic strategy is presented by targeting activated YBX1 to overcome drug resistance and malignant progression.

Clinical and genetic risk factors for Fulvestrant treatment in post-menopause ER-positive advanced breast cancer patients

Background

Among breast cancer (BC) patients, near 40% are post-menopause, and 70%–80% are hormone receptor (HR)-positive. About 30%–40% BC patients who are diagnosed as invasive carcinoma HR-positive BC would eventually develop metastatic breast cancers. In 2016, FALCON trial proves Fulvestrant as an effective first-line endocrine therapy for post-menopause HR-positive advanced BC (ABC) patients. But even after FALCON published, Fulvestrant is rarely used as first-line in real world ABC patients in China.

Method

In this study, 136 Fulvestrant users were enrolled from 2015. To investigate the clinical and genetic risk factors for Fulvestrant treatment response in real world data, biostatistic and bioinformatic analysis tools were adopted.

Result

KM curves showed that Fulvestrant first-line users had a median progression-free survival (mPFS) of 15.67 months, which was longer than the second-line users and third (or higher)-line users (mPFS = 7.47 and 5.43 months, respectively). 16 s (or higher)-line users were voluntarily received circulating tumor DNA (ctDNA) testing after progression. ctDNA testing results showed that compared to patients with PFS longer than 6 months, Fulvestrant users with PFS less than 6 months had a significantly higher mutation rate of ESR1 or ERBB2 gene (0/6 vs 6/10, Fisher’s Exact p-value = 0.03). Multivariate COX regression analysis showed that clinical features, including lymph node metastasis and HER-2 positive, were significant risk factors for poor PFS [hazard ratio (HR) = 2.396 and 2.863, respectively]; high portion of estrogen receptor-positive cells was significant protective factor (HR = 0.663). Propensity-score matching (PMS) analysis suggested that visceral metastasis, prior palliative chemotherapy, and old age at Fulvestrant usage were not significant influential factor for PFS.

Conclusion

First-line Fulvestrant usage could guarantee a better prognosis than higher-line usage. ESR1 or ERBB2 mutation was found to be related to poor PFS in higher-line Fulvestrant users.

Electronic supplementary material

The online version of this article (10.1186/s12967-018-1734-x) contains supplementary material, which is available to authorized users.

Y-box binding protein YBX1 and its correlated genes as biomarkers for poor outcomes in patients with breast cancer

The enhanced expression of the Y-box binding protein YBX1 is consistently correlated with poor outcomes or reduced survival of breast cancer patients. However, the mechanism underlying the association between increased YBX1 expression and poor outcomes has yet to be revealed. We searched a database for the top 500 genes that are positively or negatively correlated with YBX1 and with ESR1 in breast cancer patients. We further examined the association between YBX1-correlated genes and breast cancer outcomes in patients at Kyushu University Hospital. More than 60% of genes that are positively correlated with YBX1 are also negatively correlated with ESR1. The enhanced expression levels of the top 20 positively correlated genes mostly predict negative outcomes, while the enhanced expression levels of the top 20 negatively correlated genes mostly predict positive outcomes. Furthermore, in breast cancer patients at Kyushu University Hospital, the expression levels of YBX1 and YBX1-positively correlated genes were significantly higher and the expression levels of genes negatively correlated with YBX1 were significantly lower in patients who relapsed after their primary surgery than in those who did not relapse. The expression of YBX1 together with the expression of its positively or negatively correlated genes may help to predict outcomes as well as resistance to endocrine therapies in breast cancer patients. Determining the expression of YBX1 and its closely correlated genes will contribute to the development of precision therapeutics for breast cancer.

Why upfront use of CDK inhibitors for the treatment of advanced breast cancer may be wasteful, and how we can increase their value

Three Cyclin Dependent Kinase 4/6 (CDK) inhibitors have been approved by the United Stated Food and Drug Administration for front line treatment of advanced hormone receptor positive breast cancer based on improvements in progression free survival against endocrine monotherapy. Two clinical trials have so far reported results on overall survival but both are negative. CDK inhibitors are usually tolerated well but they do add to inconvenience and cost - for example, grade III-IV neutropenia occur at a frequency of over 60% requiring frequent blood work at least during the initial months of treatment. These drugs cost over $ 13,500 for a 4-week cycle in the United States, and are responsible for billions of dollars annually in drug cost alone. Importantly, many women with metastatic breast cancer do well for a long time with endocrine therapy alone and CDK inhibitors do not have a predictive marker. Selective use of these agents in later lines may improve substantially the convenience and cost without compromise in overall outcome. However, with results demonstrating impressive improvements in PFS published in major medical journals coupled with patients' natural desire for "best available" options, the trend among oncologists is to prescribe these drugs as the default front-line treatment. In this commentary I caution readers against over interpretation of results from the CDK inhibitor trials, describe adverse consequences of routine front-line use, and explain why selective use in later line may yield a higher value.

The regulatory roles of lncRNAs in the process of breast cancer invasion and metastasis

Breast cancer (BC) is the most common cancer and principal cause of death among females worldwide. Invasion and metastasis are major causes which influence the survival and prognosis of BC. Therefore, to understand the molecule mechanism underlying invasion and metastasis is paramount for developing strategies to improve survival and prognosis in BC patients. Recent studies have reported that long non-coding RNAs (lncRNAs) play critical roles in the regulation of BC invasion and metastasis through a variety of molecule mechanisms that endow cells with an aggressive phenotype. In this article, we focused on the function of lncRNAs on BC invasion and metastasis through participating in epithelial-to-mesenchymal transition, strengthening cancer stem cells generation, serving as competing endogenous lncRNAs, influencing multiple signaling pathways as well as regulating expressions of invasion-metastasis related factors, including cells adhesion molecules, extracellular matrix, and matrix metallo-proteinases. The published work described has provided a better understanding of the mechanisms underpinning the contribution of lncRNAs to BC invasion and metastasis, which may lay the foundation for the development of new strategies to prevent BC invasion and metastasis.

Factors associated with prolonged time to treatment failure with fulvestrant 500 mg in patients with post-menopausal estrogen receptor-positive advanced breast cancer: a sub-group analysis of the JBCRG-C06 Safari study

OBJECTIVE

The JBCRG-C06 Safari study showed that earlier fulvestrant 500 mg (F500) use, a longer time from diagnosis to F500 use, and no prior palliative chemotherapy were associated with significantly longer time to treatment failure (TTF) among Japanese patients with estrogen receptor-positive (ER+) advanced breast cancer (ABC). The objective of this sub-group analysis was to further examine data from the Safari study, focusing on ER + and human epidermal growth factor receptor-negative (HER2-) cases.

METHODS

The Safari study (UMIN000015168) was a retrospective, multi-center cohort study, conducted in 1,072 patients in Japan taking F500 for ER + ABC. The sub-analysis included only patients administered F500 as second-line or later therapy (n = 960). Of these, 828 patients were HER2-. Results Multivariate analysis showed that advanced age (≥65 years; p = .035), longer time (≥3 years) from ABC diagnosis to F500 use (p < .001), no prior chemotherapy (p < .001), and F500 treatment line (p < .001) were correlated with prolonged TTF (median = 5.39 months).

CONCLUSIONS

In ER+/HER2- patients receiving F500 as a second-line or later therapy, treatment line, advanced age, no prior palliative chemotherapy use, and a longer period from ABC diagnosis to F500 use were associated with longer TTF.

Anthropometric, clinical and molecular determinants of treatment outcomes in postmenopausal, hormone receptor positive metastatic breast cancer patients treated with fulvestrant: Results from a real word setting

To characterize determinants of treatment outcome in a real world population of 161 post-menopausal hormone receptor-positive metastatic breast cancer patients treated with fulvestrant. Descriptive statistics for demographics, anthropometrics, clinical and molecular characteristic were compared across subgroups of sensitivity/resistance to prior endocrine therapy and tested in uni/multivariate models. Clinical benefit was more common in sensitive patients with higher estrogen receptor expression and when fulvestrant was given in first line (p=0.02 and 0.046). In resistant patients, PFS was longer with lower BMI (p=0.01). Among endocrine sensitive women, longer PFS was associated with fulvestrant in first-line, single metastasis and no visceral involvement (p=0.01, 0.003 and 0.01). OS was shorter in resistant patients with HER2-positive disease and if fulvestrant was given in second and subsequent line (p=0.03). In sensitive patients, we observed worse OS with multiple metastases (p=0.008). Multivariate analyses confirmed longer PFS in resistant patients with lower BMI and older age (p=0.002 and 0.007). OS in resistant patients was negatively influenced by HER2 positivity and fulvestrant in second and subsequent line (p=0.04). In sensitive women, multiple metastases were associated with poorer survival (p=0.002). This evidence encourages considering patient and disease characteristics in decision making and outcome interpretation for patients candidate to fulvestrant.

Steroid hormone receptors and prostate cancer: role of structural dynamics in therapeutic targeting

Steroid hormone receptors (SHRs) act in cell type- and gene-specific manner through interactions with coregulatory proteins to regulate numerous physiological and pathological processes at the level of gene regulation. Binding of steroid receptor modulator (SRM) ligand leads to allosteric changes in SHR to exert positive or negative effects on the expression of target genes. Due, in part, to the fact that current SRMs generally target ligand binding domain (LBD)/AF2 and neglect intrinsically disordered (ID) N-terminal domain (NTD)/AF1, clinically relevant SRMs lack selectivity and are also prone to the development of resistance over time. Therefore, to maximize the efficacy of SHR-based therapeutics, the possibility of developing unique modulators that act to control AF1 activity must be considered. Recent studies targeting androgen receptor's (AR's) ID AF1 domain for the castration-resistant prostate cancer has provided the possibility of therapeutically targeting ID NTD/AF1 surfaces by allosteric modulations to achieve desired effects. In this review article, we discuss how inter- and intra- molecular allosteric regulations controlled by AR's structural flexibility and dynamics particularly the ID NTD/AF1 is an emerging area of investigation, which could be exploited for drug development and therapeutic targeting of prostate cancer.

Breast Cancer Profile among Patients with a History of Chemoprevention

Purpose. This study identifies women with breast cancer who utilized chemoprevention agents prior to diagnosis and describes their patterns of disease. Methods. Our database was queried retrospectively for patients with breast cancer who reported prior use of chemoprevention. Patients were divided into primary (no history of breast cancer) and secondary (previous history of breast cancer) groups and compared to patients who never took chemoprevention. Results. 135 (6%) of 2430 women used chemoprevention. In the primary chemoprevention group (n = 18, 1%), 39% had completed >5 years of treatment, and fully 50% were on treatment at time of diagnosis. These patients were overwhelmingly diagnosed with ER/PR positive cancers (88%/65%) and were diagnosed with equal percentages (44%) of IDC and DCIS. 117 (87%) used secondary chemoprevention. Patients in this group were diagnosed with earlier stage disease and had lower rates of ER/PR-positivity (73%/65%) than the nonchemoprevention group (84%/72%). In the secondary group, 24% were on chemoprevention at time of diagnosis; 73% had completed >5 years of treatment. Conclusions. The majority of patients who used primary chemoprevention had not completed treatment prior to diagnosis, suggesting that the timing of initiation and compliance to prevention strategies are important in defining the pattern of disease in these patients.

Cotargeting of CYP-19 (aromatase) and emerging, pivotal signalling pathways in metastatic breast cancer

Aromatase inhibition is one of the cornerstones of modern endocrine therapy of oestrogen receptor-positive (ER+) metastatic breast cancer (MBC). The nonsteroidal aromatase inhibitors anastrozole and letrozole, as well as the steroidal aromatase inactivator exemestane, are the preferred drugs and established worldwide in all clinical phases of the disease. However, although many patients suffering from MBC experience an initial stabilisation of their metastatic burden, drug resistance and disease progression occur frequently, following in general only a few months on treatment. Extensive translational research during the past two decades has elucidated the major pathways contributing to endocrine resistance and paved the way for clinical studies investigating the efficacy of novel drug combinations involving aromatase inhibitors and emerging drugable targets like mTOR, PI3K and CDK4/6. The present review summarises the basic research that provided the rationale for new drug combinations involving aromatase inhibitors and the main findings of pivotal clinical trials that have already started to change our way to treat hormone-sensitive MBC. The challenging situation of oestrogen receptor-positive and human epidermal growth factor receptor 2-positive (HER2+) MBC is also shortly reviewed to underline the complexity of the clinical scenario in the heterogeneous subgroups of hormone receptor-positive breast cancer patients and the increasing need for personalised medicine. Finally, we summarise some of the promising findings made with the combination of aromatase inhibitors with other potent endocrine treatment options like fulvestrant, a selective oestrogen receptor downregulator.

Breast Cancer Resistance to Antiestrogens Is Enhanced by Increased ER Degradation and ERBB2 Expression

Endocrine therapies effectively improve the outcomes of patients with estrogen receptor (ER)-positive breast cancer. However, the emergence of drug-resistant tumors creates a core clinical challenge. In breast cancer cells rendered resistant to the antiestrogen fulvestrant, we defined causative mechanistic roles for the transcription factor YBX1 and the levels of ER and the ERBB2 receptor. Enforced expression of YBX1 in parental cells conferred resistance against tamoxifen and fulvestrant in vitro and in vivo Furthermore, YBX1 overexpression was associated with decreased and increased levels of ER and ERBB2 expression, respectively. In antiestrogen-resistant cells, increased YBX1 phosphorylation was associated with a 4-fold higher degradation rate of ER. Notably, YBX1 bound the ER, leading to its accelerated proteasomal degradation, and induced the transcriptional activation of ERBB2. In parallel fashion, tamoxifen treatment also augmented YBX1 binding to the ERBB2 promoter to induce increased ERBB2 expression. Together, these findings define a mechanism of drug resistance through which YBX1 contributes to antiestrogen bypass in breast cancer cells. Cancer Res; 77(2); 545-56. ©2016 AACR.