Breast cancer (BC) is the most common malignancy and the leading cause of cancer death in women worldwide. Approximately 75% of BCs are hormone receptor positive (HR+), HER2-negative (HER2-); systemic therapy with endocrine agents represents the mainstay of treatment both in the early and advanced stages of disease (1,2).

Considering the efficacy and the favorable safety profile of endocrine-directed agents, sequential lines of endocrine therapy (ET) should be the preferred treatment strategy in the advanced setting, except in the case of immediate life-threatening disease or rapid visceral recurrence during adjuvant ET (3,4).

Despite the effectiveness of ET in HR+ advanced BC, disease progression occurs in the majority of patients due to primary or acquired ET resistance.

In the last two decades, loss of estrogen receptor (ER) expression, ER mutations, alterations in co-regulatory proteins, and the upregulation of different signal transduction pathways have been identified as mechanisms leading to ET failure (5).

Among these mechanisms of resistance, cross-talk between the phosphatidylinositol 3-kinase (PI3K)/protein kinase B (AKT)/mTOR axis and ER signaling plays a key role in BC proliferation and progression and confers endocrine insensitivity to ET.

Preclinical and clinical studies have shown that co-targeting downstream elements of this pathway using mammalian target of rapamycin (mTOR) inhibitors may synergistically increase the antitumoral activity of ET and overcome anti-hormone therapy resistance in BCs (6–8).

Everolimus (Eve) is a mTOR inhibitor which induces apoptosis by blocking S6K1 and 4E-BP1 activation and inhibits cell growth, proliferation, and G1-S transition. Eve in combination with Exemestane (Exe), a steroidal aromatase inhibitor (SAI), was approved in 2012 based on the results of the pivotal BOLERO-2 trial. In this study, dual-blockade significantly improved progression-free survival (PFS), as compared with Exe alone, in non-steroidal AI (NSAI)-pretreated post-menopausal patients affected by HR+/HER2-MBC, while maintaining health-related quality of life (HRQoL) (9–11).

Over the past two years, a new class of drugs, namely, CDK4/6 inhibitors, in combination with letrozole or fulvestrant as first or second-line therapy, respectively, has increasingly been used in the treatment of luminal MBC disease. Despite the impressive results from recently published studies reporting a significant PFS benefit with CDK4-6 inhibitors (palbociclib, ribociclib, abemaciclib) in combination with ET compared with ET alone, overall survival (OS) data are still awaited and the optimal sequence of ET, as monotherapy or in combination with targeted agents, is still not well established (12–15). However, combination of CDK4-6 inhibitors and letrozole is reasonably expected to become the new gold standard as first-line therapy in post-menopausal patients with HR+/HER2-endocrine sensitive MBC.

Nevertheless, it is crucial to acquire data about long-term outcomes and toxicity profile of therapeutic options currently used in everyday clinical practice in order to define the best therapeutic strategy in HR+/HER2-advanced breast cancer (ABC). Of note, the ongoing phase III randomized trial GIM-16 FEVEX (EudraCT n. 2014-004035-38), where MBCs have been randomized to receive Exe-Eve, followed, in case of progression disease, by fulvestrant or vice versa, should define the best therapeutic sequence in second and third line.

In our multicenter observational study, we retrospectively analyzed data from 264 HR+/HER2-MBC patients who received Exe-Eve combination, following NSAI failure, in different lines of hormonal treatment to evaluate the efficacy and tolerability of this combination in the ‘real world’ setting.

The main aim of our retrospective study was the analysis of activity, efficacy, and safety of Exe-Eve treatment according to the line of therapy. From January 2012 to January 2017, 264 patients with HR+/HER-MBC who received Exe-Eve as first or further line of ET, were eligible for the final analysis. For all patients, inclusion criteria were histologically confirmed diagnosis of HR+ and HER-locally advanced or MBC, age ≥18 years, Eastern Cooperative Oncology Group (ECOG) performance status (PS) ≤2, basal screening for hepatitis B and C, administration of at least one cycle of Exe-Eve until disease progression, unacceptable toxicity or patient refusal, availability of clinical-pathological, radiologic, and laboratory parameters before Exe-Eve treatment, response evaluation, and survival data.

Furthermore, prior therapy with AI was permitted if recurrence had occurred during or within 12 months of completion of adjuvant therapy, or in case of progression during treatment for advanced stage disease; likewise, previous chemotherapy for metastatic disease and palliative radiotherapy on bone and/or brain were permitted. Eve starting dose was 10 mg once a day orally; a dose reduction (5 mg) was chosen by the caring physician in some instances. Supportive measures were allowed and implemented according to individual daily practice. Toxicity data were collected monthly at each patient visit and were classified according to the National Cancer Institute Common Criteria for Adverse Events (NCI-CTCAE), version 4 (16).

Accordingly, in case of toxicity, dose adjustments or temporary interruptions of therapy as well as timing and modality of evaluation were independently implemented by each investigator (as a general rule, Eve dose was reduced for toxicity of grade 2 or higher, while temporary interruption of treatment was proposed for mild toxicities; tumor assessment was carried out every 3 months). Data were retrieved after an anonymization procedure from a centralized database at the Breast Unit of Cardarelli Hospital. Four patients were lost to follow-up, and the study was completed by January 31, 2017. All patients provided written informed consent about the use of their data for future medical research. The Institutional Review Board at ‘F. Magrassi’ Department of Clinical and Experimental Medicine of ‘Luigi Vanvitelli’ University of Campania, Naples (Italy) approved the study.

The outcome of MBC patients is continuously improving because of the availability of new active therapeutic options. The challenge for clinicians is to balance treatment-related toxicity with the likelihood of benefit and cancer-related symptom relief deriving from therapy. For this reason, current clinical guidelines advocate the use of ET as the preferred treatment for HR+, HER2-MBC, unless visceral crisis or concern/proof of endocrine resistance is observed.

Recently, the introduction of CDK4/6 inhibitors in combination with endocrine agents resulted in a significant PFS benefit, however, overall survival data are still missing and there is no consensus on the optimal sequence of ET, that is, as monotherapy or in combination with targeted agents (12–15).

Currently, the combination of Eve and Exe is a widely used regimen for treatment of endocrine sensitive postmenopausal MBC patients progressing after NSAIs therapy. However, current international guidelines suggest taking into account the relevant class-effect AEs of this regimen with regard to the decision to treat, which nonetheless should be made on a case by case basis.

For this purpose, more experience has to be collected on toxicity profile and efficacy in different subsets of patients (i.e., differing in metastatic sites, performance status, number of prior therapies etc.) in order to help physicians to optimize the therapeutic strategy for each patient.

The main aim of our observational study was to explore the efficacy and tolerability of Eve-Exe combination, according to the line of therapy, in an unselected population from nine Cancer Centers in Campania region (South Italy). The hypothesis of the association between the onset of side effects and efficacy was also addressed in our analysis.

The pivotal phase III BOLERO-2 trial showed that dual-blockade based on the association of Eve plus Exe doubled the median PFS compared to Exe alone (7.8 vs. 3.2 by investigator review and 11 vs. 4.1 months by central review, respectively) in patients with HR+, HER2-MBC progressing after prior NSAIs. The clinical benefit rate (CBR), defined as CR+PR+SD ≥24 months, was also better in the Eve-Exe arm with respect to Exe alone arm (51.3% vs. 26.4%, respectively).

OS was a secondary endpoint but did not meet statistical significance despite it was shown to increase (4.4 months) following addition of Eve to Exe (31.0 months in the experimental arm vs. 26.6 months in patients receiving Exe alone) (9–10,18).

The lack of a statistically significant survival gain could be due to the fact that BOLERO-2 trial was powered to detect only an eight-month OS improvement. Furthermore, the small imbalance in post-study salvage chemotherapy use (63 vs. 53%, control vs. experimental arm, respectively) has probably influenced OS results leading to a reduction in the survival gap between the two groups.

In our retrospective analysis, Exe-Eve combination was shown to be active in all lines of therapy. Particularly, although no complete response was observed, DCR was 73.1%, with no statistically significant difference between the different settings. At a median follow-up of 42 months, median PFS was 11.6, 9.7, and 7.5 months for patients treated with Exe-Eve as first, second, or third line of treatment, respectively. Median OS was 33.0 months; at a median follow-up of 67 months, median OS was 43.1, 31.7, and 27.9 months for patients treated with Exe-Eve as first, second or third line of treatment, respectively. No statistically significant difference in terms of PFS and OS between the different subgroups was detected. Our results confirmed the considerable activity and efficacy of Eve-Exe observed in BOLERO-2, although the safety profile of this combination remains debated. In the BOLERO-2 trial, the most commonly reported AEs, affecting at least one-third of patients in the Eve-Exe arm, were stomatitis, rash, fatigue, diarrhea, nausea, decreased appetite, and pneumonitis. While most AEs were low grade (G1-2), one-half of patients in the Eve-Exe arm experienced grade 3-4 toxicities vs. 27% of patients in the placebo plus Exe arm. The discontinuation rate due to AEs was higher with Eve-Exe (9%) compared with the control arm (3%); the most common toxiticies leading to treatment discontinuation in the experimental group included pneumonitis, stomatitis, dyspnea, and fatigue (19). Consistent with BOLERO-2 results, in our analysis, the most frequently described toxicities (all grades) were stomatitis (54.8%), fatigue (39.3%), rash (33.3%), and diarrhea (33.2%), which occurred in more than one third of patients. Treatment discontinuation was registered only in 5.7% of patients and the most common AEs leading to treatment discontinuation were pneumonitis, stomatitis, and diarrhea. In the Italian expanded-access, multicenter BALLET trial, permanent treatment discontinuation due to side effects was reported to be more frequent (17.1% of patients) with respect to our series and was mainly caused by non infectious pneumonitis (NIP), stomatitis, asthenia, and dyspnea. The majority of these toxicities occurred within the first 12 weeks of start of therapy, consistently across all clinical trials (20). Since 2012 approval, several retrospective trials have been carried out to assess the role of Eve-Exe combination in daily clinical practice, providing clinicians with valuable additional data to guide treatment decisions. To our knowledge, five large observational studies have strengthened the role of this combination in patients with HR+, HER2-MBC. With the exception of the Italian trial by Moscetti et al (21), the remaining studies were only presented as abstracts at scientific international conferences on breast cancer.

The Austrian non-interventional phase 4 STEPAUT study aimed at evaluating efficacy and safety of Eve-Exe according to clinical routine. The second interim analysis on 225 out of 300 enrolled patients was recently presented at San Antonio Breast Cancer Symposium in 2016. Overall, median PFS was 9.5 months, in line with our PFS data (9.1 months). A subgroup analysis was also performed according to Eve dosing and, interestingly, PFS was lower for the 5 mg-group compared to the 10-mg group (6.5 vs. 9.1 months, respectively). However, it should be noted that patients receiving the 5 mg starting dose had unfavorable prognostic factors as well as worse ECOG PS and more prior therapies. The safety profile was consistent with data previously reported in BOLERO-2 trial and, noteworthy, in accordance with our findings, occurrence of stomatitis did not negatively affect PFS. The EVEREXES phase IIIb trial investigated safety and tolerability profiles as primary endpoints and efficacy as a secondary objective. In this trial, 232 post-menopausal women affected by HR+, HER2-MBC were recruited in Eastern countries. The planned interim analysis presented at San Antonio Breast Cancer Symposium by Im et al in December 2015 showed a median PFS of 9.45 months, which is also consistent with our results. The majority of AEs were grade 1-2 and the most common G3-4 toxicities, as expected, included stomatitis, fatigue, hyperglycemia, and NIP. Therefore, this trial confirmed the role of Eve-Exe combination even for the treatment of patients from Asia, Africa and middle Est, a population poorly represented in BOLERO-2 (<10% of all series). Furthermore, a large, multicenter, non interventional study from Germany (BRAWO trial) including 3,000 patients provided data on the routine clinical use of Eve-Exe therapy. The second interim analysis of BRAWO, presented at the European Society of Medical Oncology (ESMO) in September 2014 by Fasching et al was carried out on 500 patients: A median PFS of 8 months was recorded, with a higher median PFS (10.1 months) for patients treated with Exe-Eve in the first-line setting. These data are still in accordance with our results as an increased PFS of 11.6 months was observed in patients receiving Eve-Exe as first line treatment, even though no statistically significant correlation in terms of risk of disease progression between the different lines of treatment was found. Of note, the third interim analysis of BRAWO study, presented at ESMO in 2015, showed a significantly longer duration of treatment in patients with younger age, better ECOG performance status, and no comorbidities. 4EVER is another German phase IIIb study evaluating the efficacy and safety of Eve-Exe in a broader patient population than that of BOLERO-2. The final efficacy analysis, calculated on 281 patients, was presented at the San Antonio Breast Cancer Symposium in 2015. Efficacy data were poorer compared with those from BOLERO-2, being the median PFS equal to 5.6 months. This may be due to the different patient population enrolled in this trial. In fact, unlike the BOLERO-2 trial, there were less limitations in enrollment criteria, including the number of previous CT lines, the prior use of Exe, or the time to progression after NSAI therapy. Finally, Moscetti et al (21) recently published an Italian retrospective trial assessing the safety of Eve-Exe combination and the possible association between toxicities and previous treatments in 181 unselected patients. On multivariate analysis, no association between the number of prior therapies and toxic events was found, which is in line with the real-life data of the Italian BALLETT-related cohort (20). In this study, a switch from 10 mg Eve starting dose to 5 mg, due to AEs, was reported in 27% of patients, which is a figure quite similar to that reported in BOLERO-2 and EAP. Conversely, in our series, Eve dosage was reduced to 5 mg in about 10% of patients only, perhaps due to the wide use of prophylactic measures and therapeutic interventions for stomatitis management.

Another important aspect to focus on is the correlation between adverse events and patient characteristics and response to treatment. In this regard, our multivariate analysis showed diabetes and previous chemotherapy for metastatic disease as the variables related to a worse outcome; conversely, the presence of mucositis correlated with long-term survival. Recently, a meta-analysis of data from seven randomized phase III trials of Eve assessing the clinical impact of stomatitis on efficacy in different solid tumors was published (22). Specifically, in BOLERO-2 study, the median PFS was 8.5 vs. 6.9 months for patients receiving Eve with or without stomatitis within 8 weeks, respectively. These results and those of the above mentioned STEPAUT trial are in accordance with our findings and suggest a careful and close monitoring of patients in the first weeks of treatment in order to better manage and prevent stomatitis. In this scenario, the SWISH trial demonstrated that a prophylactic use of a commercially available, non-expensive oral dexamethasone mouthwash resulted in a marked reduction in the incidence and severity of stomatitis in patients receiving Eve-Exe combination for treatment of HR+, HER2-MBC, when compared with data from BOLERO-2. On the basis of these findings, this measure can be considered as a new standard of oral care for BC patients receiving Eve and Exe therapy (23).

In conclusion, with the limitations due to the observational nature of our findings, addition of Eve to Exe is an effective therapeutic option for HR+, HER2-MBC patients after failure of NSAI therapy in the real word scenario. Patient characteristics, costs, and side effects have to be integrated in the treatment decision making process.