Co-targeting estrogen receptor and HER2 pathways in breast cancer.

The Breast xxx (2013) 1e8

Contents lists available at ScienceDirect

The Breast journal homepage: www.elsevier.com/brst

Review

Co-targeting estrogen receptor and HER2 pathways in breast cancer Arjun Mehta a, b, Debu Tripathy a, b, * a b

University of Southern California, Keck School of Medicine, USA USC, Norris Comprehensive Cancer and Department of Medicine, USA

a r t i c l e i n f o

a b s t r a c t

Article history: Received 7 May 2013 Received in revised form 14 September 2013 Accepted 21 September 2013

The estrogen steroid hormone receptor (ER) and human epithelial growth factor receptor 2 membrane tyrosine kinase growth factor receptor (HER2) are the mediators of two key pathways involved in breast carcinogenesis, invasive behavior and cell growth. Co-expression of these receptors results in specific biological features that are not fully understood, but include relative resistance to hormonal therapy and chemotherapy as well as better long-term outcome imparted by ER and worse outcome by HER2 expression. The ER and HER2 signaling pathways interact with each other as do many biological networks, and this creates opportunities for therapeutic co-targeting with agents that modulate these respective pathways. However, relatively few studies have been conducted to test concurrent manipulation of ER and HER2. The avoidance of chemotherapy side effects is an attractive feature that has further spurred explorations in this strategy. Still, the only dually targeted strategy approved by some regulatory agencies is the combination of hormonal therapy using aromatase inhibition and the HER2 kinase inhibitor lapatinib. Other dual combinations have also demonstrated a benefit, although most of the testing has compared hormonal therapy with or without HER2-directed agents and not the other way around, limiting the applicability of this concept in routine clinical practice, especially when chemotherapy is also used. Newer generation signal transduction inhibitors can augment the efficacy of hormonal therapy, with one such example of mTOR blockade using everolimus now in the clinic. The logical extension of ER and HER2 co-targeting is the discovery and clinical testing of “synthetic lethal” combinations attacking diverse pathways that produce quantum improvements over either therapy alone. Molecular annotation of human cancers can further inform personalized combinatorial regimens based on the unique circuitry of an individual patient’s tumor, with the potential to yield much more than incremental gains in survival. Ó 2013 Elsevier Ltd. All rights reserved.

Keywords: HER2 Hormone receptor Estrogen receptor Signal transduction Co-targeting Breast cancer treatment

Introduction Over the last few decades, breast cancer therapy has become more individualized on the basis of biological features. The HER2 (HER-2/neu) receptor and oncoprotein together with estrogen and progesterone hormone receptors (ER and PR) are well established molecular markers playing a role in the pathogenesis of breast cancer that can also be targeted therapeutically. The seminal work conducted by Slamon and colleagues demonstrated that HER2 is amplified and/or over-expressed in 15e20% of breast cancers in women and its amplification drives aggressive behavior [1]. Trastuzumab (Herceptin), a humanized anti-HER2 antibody has been shown to be active as a single agent and improves response,

* Corresponding author. University of Southern California, 1441 Eastlake Avenue, NTT-3429, Los Angeles, CA 90033, USA. Tel.: þ1 (323) 865 3962; fax: þ1 (323) 865 0061. E-mail address: [email protected] (D. Tripathy).

progression-free and overall survival (PFS and OS) when added to chemotherapy for advanced HER2-positive breast cancers [2e4]. In early stage HER2-positive breast cancer, several pivotal trials have shown improvements in disease free and overall survival (DFS, OS), and this has now become standard therapy for most such cases [5e 10]. However, the known biological interplay between hormonal and HER2 receptors has not been extensively explored despite the obvious therapeutic ramifications. Approximately 70% of breast cancers exhibit positivity of either ER or PR, thus referred to as hormone receptor-positive, with a lower frequency in younger or black women and higher grade tumors [11,12]. These two DNA-binding transcription factors are primarily implicated in driving the oncogenesis and growth of hormonally sensitive breast cancer [13]. Hormone receptor-positive cancers respond to therapies that lower estrogen or interfere with its binding to ER by delaying progression in advanced disease or lowering the risk of recurrence and death in early stage disease [14e16]. Tamoxifen, a selective ER modulator, has been in the

0960-9776/$ e see front matter Ó 2013 Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.breast.2013.09.006

Please cite this article in press as: Mehta A, Tripathy D, Co-targeting estrogen receptor and HER2 pathways in breast cancer, The Breast (2013), http://dx.doi.org/10.1016/j.breast.2013.09.006

2

A. Mehta, D. Tripathy / The Breast xxx (2013) 1e8

Fig. 1. Crosstalk between ER and HER pathways. A schematic representation of the interplay between ER and GFR signaling pathways. The growth stimuli including ligands binding to receptors like EGFR an HER2 activate a cascade of intracellular signals mainly via the PI3K/Akt/mTOR and the MAPK/ERK pathways. The mTOR phosphorylates downstream kinases including the S6 kinase which phosphorylates and activates ER, which in terms regulates the transcription of several genes including GFR ligands, cyclin, VEGF and PARP. ER activated by both estradiol and growth factor pathways also mediate effects via a “non-genomic” response outside the nucleus, affecting cellular and mitochondrial membrane proteins. Hence bi-directional interaction between the ER and GFR pathways is reported to exist. ER: Estrogen receptor; EGFR: epidermal growth factor receptor; HER2/ neu: Human epidermal growth factor receptor 2; EGF: epidermal growth factor; TK: tyrosine Kinase; PI3k: phosphatidylinositide 3-kinases; mTOR: mammalian target of rapamycin; IRS: insulin receptor substrate; PTEN: phosphatase and tensin homologue; E: estrogen; MAPK: mitogen activated protein kinases; ERK: Extracellular signal-regulated kinases.

breast cancer treatment armamentarium for many decades. The use of tamoxifen has shown to reduce breast cancer recurrence by up to 50% and mortality by 30% in hormone receptor-positive early stage breast cancer. A meta-analysis of randomized clinical trials by the Early Breast Cancer Trialists’ Collaborative Group (EBCTCG) has demonstrated that 5 years use of adjuvant tamoxifen shows 15year risk reduction of breast cancer recurrence and death [14]. Tamoxifen is the accepted standard treatment for hormone receptor-positive cancers especially in women who are pre or perimenopausal at the time of diagnosis, whereas for post-menopausal women, aromatase inhibitors have become the first option. The ASCO guidelines have underscored the importance of including aromatase inhibitors either as primary therapy for postmenopausal women or switching to an aromatase inhibitor 2e3 years after tamoxifen therapy to reduce recurrence risk [16]. Nevertheless, it is clear that hormone receptor-positive breast cancers do not always respond to hormonal therapies and HER2þ breast cancers can also be primarily resistant or acquire resistance to HER2-targeted therapies over time. This review will explore the relationships between these two critical pathways and insights gained from both

preclinical and clinical studies, particularly those that co-target these pathways, and the relevance of this information in the clinical management of patients with hormone receptor and HER2þ breast cancer. Cross-talk between estrogen and HER2 receptors Approximately 10% of hormone receptor-positive breast cancers are HER2þ, whereas 50% of HER2þ cases are HRþ [17,18]. The fact that cancers can recur after or be resistant to hormonally or HER2targeted therapies, implies that mechanisms of resistance are likely to be multifactorial. Several trials have demonstrated that hormone receptor-positive cancers that are also HER2þ are relatively resistant to hormonal therapy for both advanced and early stage disease compared to HER2-negative cases [19e24]. The ATAC trial comparing adjuvant tamoxifen to the aromatase inhibitor anastrozole showed a greater recurrence rate associated with lower PR and in higher HER2-expressing tumors for both drugs (1.9-fold for tamoxifen and 2.6-fold for anastrozole by multivariate analysis), but no difference in relative benefit of anastrozole over tamoxifen



in any of these subsets [17]. A large NCCN (National comprehensive cancer network) series reported among HER2þ and hormone receptor-negative early stage breast cancer cases showed significantly increased adjusted hazards of early but not late mortality compared to hormone receptor-positive cases (HR at 0e2 years ¼ 1.92; 95% CI ¼ 1.28e2.86; p ¼ 0.002 and HR at 2e5 years ¼ 1.55; 95% CI ¼ 1.19e2.00; p ¼ 0.001 and HR at >5 years ¼ 0.81; 95% CI ¼ 0.55e1.19; p ¼ 0.285) [25]. In this series, there appeared to be additional biological differences with hormone receptor-negative recurrences less likely to be in bone and more likely in brain. These clinical observations suggest that molecular cross-talk exists between ER and the HER-2 pathways and may affect responsiveness to ER and HER2-targeted therapies and other cellular behaviors [26]. This has been confirmed using in vivo experimental models of HER2 transfection of ER þ breast cancer cells [27,28]. Other studies have suggested that activation of HER2 and other receptor tyrosine kinases such as EGFR and insulin and insulin-like growth factor-1 receptors that signal through the PI3K/ Akt/mTOR pathway can induce resistance to hormonal therapy, in part by directly phosphorylating and activating ER [29e32] (Fig. 1). Conversely, the activation of ER upon binding to estrogen leads to transcriptional modulation of several estrogen-responsive genes that include growth factor receptor ligands such as insulin-like growth factor-1 and transforming growth factor-alpha [33]. ER may also work through nontranscriptional mechanisms outside the nucleus exerting actions on the membrane and mitochondria that could affect cell motility, metabolism and proliferation [34]. Therefore, HER2 and related growth factor receptors as well as ER initiate bidirectional cross-talk, providing ample preclinical and clinical justification for combinatorial therapy targeting both estrogen and HER2 pathways concurrently. This review focuses on the clinical data from co-targeting these receptor systems and the implications in the clinical management of breast cancer with coexpression of ER and HER2. Hormone receptor status in pivotal and observational studies of trastuzumab Early studies of trastuzumab given in the absence of hormonal therapy for advanced HER2þ breast cancer showed similar activity irrespective of ER status, while randomized trials showed similar benefits from the addition of trastuzumab to chemotherapy [4,35]. A retrospective series of 227 consecutive patients with HER2þ advanced breast cancer treated with trastuzumab showed no difference in ORR (objective response rate) or PFS based on ER positivity (1% staining), but with staining as a continuous variable among ER þ cases, a lower response and higher PFS was seen with increasing ER expression [36]. In patients with ER þ tumors (N ¼ 36), maintenance endocrine therapy added to trastuzumab was associated with a significant PFS benefit (HR, 0.521; 95% CI ¼ 0.325e0.836; p ¼ 0.007). In the RegistHER study, a large prospective cohort study of 1023 patients with HER2þ advanced breast, longer PFS from time of metastatic diagnosis was seen in the 530 patients with hormone receptor-positive disease compared to the 434 patients with hormone receptor-negative disease (11.7 months; 95% CI ¼ 10.2e12.6 vs. 8.8 months; 95% CI ¼ 7.8e10.3), and a longer OS was also noted (41.5 months; 95% CI ¼ 37.7e44.6 vs. 28.6 months; 95% CI ¼ 26.3e32.0) [18]. Much larger adjuvant trials have similarly shown equivalent impacts on outcome with trastuzumab, independent of hormone receptor status, although in all the adjuvant trials, patients were given hormonal therapy for hormone receptor-positive disease. In the HERA trial comparing 1-year of adjuvant trastuzumab to observation following chemotherapy, a worse DFS was observed in the hormone receptor-negative compared to hormone receptor-

3

positive subset (HR 0.61; 95% CI 0.51e0.74), but the relative benefit of trastuzumab was equivalent in ERþ/PRþ compared to ER/PR cases (HR 0.63; 95% CI 0.43e0.93 vs. HR 0.63; 95% CI ¼ 0.50e0.78) [7]. Similar findings were seen in the two other large randomized adjuvant trials, with lower DFS and OS in hormone receptor-negative compared to positive cases, but similar relative improvements in disease-free and overall survival when compared to hormone receptor-positive group [9,10]. More recent pivotal trials of newer HER2-targeting agents in advanced disease have similarly shown equivalent benefits based on hormone receptor status, including the EGF100151 Study of capecitabine lapatinib (dual HER1/2 tyrosine kinase inhibitor), the CLEOPATRA Study of trastuzumab plus docetaxel pertuzumab (HER2 dimerization-inhibiting antibody), and the EMILIA Study of T-DM1 (trastuzumab-emtansine immunoconjugate) vs. lapatinib þ capecitabine [37e40]. Outcomes in neoadjuvant (pre-operative) trials using trastuzumab are particularly instructive as differences in pathological response (pCR) rate can be ascertained and compared amongst multiple subgroups including hormone receptor status. Moreover, the correlation of pCR with longer term outcome such as DFS has made the neoadjuvant model an attractive platform for research with the caveat that the correlation of pCR to DFS appears to be stronger in hormone receptor-negative compared to positive disease and likely varies amongst biologically defined subsets. Nevertheless, such trials have consistently shown that neoadjuvant therapies containing chemotherapy with trastuzumab, as well as lapatinib and pertuzumab alone or in combination with trastuzumab, lead to higher pCR rates in hormone receptor-negative compared to positive cases for each of the agents as well as combinations of trastuzumab with either lapatinib or pertuzumab [41e 44]. In the NeoSphere Study that compared neoadjuvant docetaxel with either trastuzumab, pertuzumab or both, along with a 4th arm that only contained trastuzumab and pertuzumab, pCR rates were higher for HR negative patients even in the chemotherapy-free arm, as compared to rates in HRþ cases in all treatment groups, further highlighting cross-talk between these pathways [44]. In another such trial, the pCR rate varied to a much greater extent based on HER2 expression in hormone receptor-positive compared to negative cases [45]. Taken together, these studies also suggest an interaction between ER and HER2, but the lack of experience with concurrent hormonal and trastuzumab-based therapy in this setting limits conclusions about nature of cross-talk between these receptor systems. Trials of ER and HER2-targeted therapy Based on emerging data on ER/HER2 cross-talk and relative resistance to hormonal therapy imparted by HER2 overexpression, several trials have examined the addition HER2-targeted therapy to hormonal therapy. A Phase II trial of trastuzumab and letrozole in post-menopausal women with hormone receptor-positive and HER2þ advanced breast cancer enrolled 31 evaluable subjects, 82% of which had previously received tamoxifen [46]. Objective responses were seen in 8 patients (26%, 1 CR [complete response] and 7 PRs [partial responses]) and the clinical benefit rate (CBR) was 52%. The median time to progression (TTP) was 5.8 months and the median duration of response was 20.6þ months, suggesting durable response can be seen with co-targeted combination therapy. One case of congestive heart failure was seen. Subsequent randomized trials have assessed the addition of HER2-directed therapy to hormonal therapy. These studies have confirmed the association of HER2 positivity with resistance to hormonal therapy, with very short times to progression seen with hormonal therapy alone. The TAnDEM Trial randomized 207


4


patients with HER2 and ER co-expression to either the aromatase inhibitor anastrozole alone or in combination with trastuzumab as first-line therapy (prior tamoxifen for advanced disease was allowed), and found an improvement in ORR and PFS, but no difference in OS (see Table 1) [47]. The 2-year PFS rate was about 15% with anastrozole plus trastuzumab compared to 5% with anastrozole alone, and no difference in survival was seen. Clinical benefit rate (CBR) defined as ORR þ stable disease 6 months was significantly higher with trastuzumab plus anastrozole (42.7%; 95% confidence interval [CI], 33.0e52.9%) compared to anastrozole alone (27.9%; 95% CI, 19.5e37.5%; p ¼ 0.026). Combination treatment was well tolerated with mostly mild to moderate side effects including fatigue (21%), diarrhea (20%), vomiting (21%), and pyrexia (17.5%). The incidence of Grade 3/4 adverse events was higher with combination therapy (23% vs. 5%) while the incidence of grade 3 or 4 cardiac events was 2% in each of the arms, but for any grade cardiac events was higher in the combination arm (14 vs. 2%). The smaller eLEcTRA Trial, also for first line therapy, compared the aromatase inhibitor letrozole alone or with trastuzumab, showed a nonsignificant trend toward longer PFS and higher ORR (see Table 1) and CBR (ORR þ stable disease 24 weeks) rate of 65% vs. 39% with combination therapy [48]. In this trial, a separate nonrandomized cohort of 35 patients with HER2-negative disease yielded a PFS of 15.2 months, supporting the relative resistance to hormonal therapy in HER2þ disease. A higher rate of adverse events was observed in the combination arm with the more commonly reported symptoms including fatigue (27%), diarrhea (19%), vomiting (19%) and peripheral edema (19%); most were mild to moderate. While no episodes of clinical congestive heart failure were seen, asymptomatic decrements in cardiac ejection fraction were seen in 3% cases with letrozole and 7% with letrozole plus trastuzumab. The dual HER1/2 (EGFR/HER2) kinase inhibitor lapatinib has also been investigated in combination with hormonal therapy. Lapatinib may offer distinct activity on HER2 pathway inhibition that is distinct from that of trastuzumab, with activity demonstrated in trastuzumab-refractory disease [49e51]. Animal xenograft models using tamoxifen-resistant ER þ human breast cancer cells have shown that lapatinib can restore tamoxifen sensitivity [52]. The EGF30008 trial randomized 1286 patients with ER þ advanced breast cancer to letrozole in combination with lapatinib or placebo as first line therapy. Of these subjects, 219 had HER2þ tumors, shown on Table 1 [53,54]. In the HER2þ population, there was a

large and statistically significant improvement in PFS, with over a 2-fold increase. The CBR was also significantly improved from 29 to 48% with the combination (odds ratio [OR ¼ 0.4; 95% CI, 0.2 to 0.8; p ¼ 0.003]). Similarly the clinical benefit was higher in the combination arm. Among HER-2 negative cases (N ¼ 952), there was no improvement in PFS. In terms of adverse effects in the overall study group, with1278 evaluable patients, there was a higher incidence of diarrhea (grade 1 of 32%, grade 2 of 22% and grade 3 of 9%), skin rash (grade 1 of 28%, grade 2 of 15% and grade 3 of 1%), nausea (grade 1 of 22% and grade 2 of 8%), in the combination arm, leading to treatment discontinuation in 15% of patients. Overall, serious adverse events were seen in 8% and 4% of patients on lapatinib compared to placebo arms, respectively. Symptomatic decline in cardiac ejection fraction was observed in 5 patients (0.8%) on the combination and 2 patients (0.3%) on the placebo arm. This trial also examined the effect of prior endocrine therapy with tamoxifen, on hormonal resistance and whether the addition of lapatinib improved the resistance. HER-2 negative and hormone receptor-positive cases (N ¼ 952) were included for this study. The first group of patients included those who had discontinued hormonal therapy at least 6 months prior or had never received it (considered “tamoxifen sensitive”, N ¼ 752). The second group (considered “tamoxifen resistant”, N ¼ 200) was comprised of patients who had taken hormonal therapy for a median of 2.8 years and discontinued less than 6 months earlier. It was noted in the tamoxifen-sensitive group, that the CBR was similar when they were administered letrozole þ placebo or letrozole þ lapatinib (64% vs. 62%). There was, however, a considerable improvement in the CBR in the tamoxifen-resistant group with values of 44% v 32% in the lapatinib þ letrozole arm vs. letrozole þ placebo arm. This trial along with other preclinical studies indicate that the growth factor receptor pathways may be upregulated in HER2 negative breast cancers and contribute to the development of endocrine resistance in cancer cells [27,55]. Thus, blockade of HER2 in combination with hormonal therapy may be useful in HER2þ as well as HER2negative and hormone resistant disease, although the latter of these requires confirmatory trials. The use of dual agent HER2-targeted therapy has been shown to yield benefits compared with single agent therapy. For example, in patients with advanced HER2þ advanced breast cancer previously exposed to trastuzumab, the combination of lapatinib and trastuzumab led to a superior survival compared to lapatinib alone [56]. In human HER2þ xenograft mouse models, dual HER2 blockade is

Table 1 Randomized trials of co-targeting ER and growth factor receptors. Trial

Treatment arm

N

ORR

CBR

PFS (mo)

HR/p-value

OS (mo)

HR/p-value

TAnDEM

Anastrozole Anastrozole þ trastuzumab Letrozole Letrozole þ Trastuzumab Letrozole þ Placebo Letrozole þ Lapatinib Anastrozole þ Placebo Anastrozole þ Gefitinib Tamoxifen þ Placebo Tamoxifen þ Gefitinib Tamoxifen þ Placebo Tamoxifen þ Gefitinib Exemestane or Fulvestrant þ Placebo Exemestane or Fulvestrant þ Ganitumab

104 103 31 26 108 111 50 43 101 105 36 48 50 106

7%a 20%b 13% 27% 15% 28% 2% 12% 15% 12% 0% 0% 13% 8%

28% 43% 39% 65% 29% 48% 34% 49% 46% 51% 31% 29% 32% 35%

2.4 4.8 3.3 14.1 3.0 8.2 8.4 14.7 10.9 8.8 7.0 5.7 5.7 3.9

0.63 0.0016 0.67 0.23 0.71 0.019 0.55 NRc 0.84 0.314 1.16 0.577 1.14 0.44

23.9 28.5 NR NR 32.3 33.3 NRd NRd NR NR NR NR 23.2 NE

NS

eLEcTRA EGF30008 NCT00077025 NCT00229697 Stratum 1 NCT00229697 Stratum 2 NCT00626106

NS NS NR NR NR 1.78 0.025

ER ¼ estrogen receptor; N ¼ number of patients randomized; ORR ¼ objective response rate (complete þ partial responses); PFS ¼ median progression-free survival; mo ¼ months; HR ¼ hazard ratio; OS ¼ median overall survival; NR ¼ not reported; NS ¼ not significant; NE ¼ not estimable. a from 73 evaluable patients. b from 74 evaluable patients. c p-value not provided, but HR 95% CI ¼ 0.32e0.94. d 7 deaths on anastrozole plus placebo and 6 deaths on anastrozole plus gefitinib.



further enhanced with the addition of hormonal blockade [57]. Adding hormonal therapy to dual HER2-targeted treatment has been tested in a Phase II neoadjuvant trial combining trastuzumab, lapatinib and letrozole for patients with HER2 and ER-positive breast cancer, yielding a complete pathologic response rate, defined as the absence of invasive disease in the breast at surgery following 12 weeks of therapy, of 21% (8 of 39 patients) [58]. When response rate was defined as 1 cm or less of residual invasive tumor, the rate was 54% (21 of 39 patients). In the cohort of patients with HER2þ, ER-negative disease treated with trastuzumab and lapatinib, the complete pathological response rate was 36% (9 of 25 patients) and rate of residual tumor of 1 cm or less was 40% (10 of 25 patients), suggesting that dual HER2 plus hormonal blockade without chemotherapy could yield activity approaching that seen with chemotherapy combinations. The question as to whether concurrent hormonal therapy added to HER2-targeted therapy provides an additional benefit over HER2-based therapy alone has never formally been addressed in a randomized trial. In the retrospective series of 227 HER2þ patients receiving trastuzumab described earlier, among patients with ER þ tumors (1% staining), maintenance endocrine therapy added to trastuzumab upon the completion of chemotherapy was associated with a significantly longer PFS (hazard ratio, 0.521; 95% CI, 0.3325e0.836; p ¼ 0.007), although this could have been due to selection bias since non-responding or progressing patients might never receive maintenance hormonal therapy [36]. In the previously described RegistHER prospective cohort study, the outcomes of the 530 patients with hormone receptor-positive disease were analyzed and considered separately for those treated with trastuzumab and chemotherapy (N ¼ 365) or those who received hormonal therapy without chemotherapy (N ¼ 106) [18]. Compared to patients who received chemotherapy and trastuzumab without hormonal therapy (N ¼ 209), those who received the same with hormonal therapy (N ¼ 156) exhibited a longer median PFS of 20.4 vs. 9.5 months (adjusted HR 0.53; 95% CI ¼ 0.42e0.68; p < 0.001) and longer median OS which was not reached vs. 36.7 months (adjusted HR 0.50; 95% CI ¼ 0.36e0.70; p < 0.001). To assess the bias of sequential hormonal therapy being used preferentially in patients responding to chemotherapy plus trastuzumab, the outcomes of sequential (N ¼ 107) vs. concurrent (N ¼ 49) hormonal therapy were examined and showed a non-significantly longer PFS with sequential use of 21.3 vs. 19.1 months (adjusted HR 0.81; 95% CI ¼ 0.54e1.21; p ¼ 0.303) and significantly longer OS of not reached vs. 43.8 months (adjusted HR ¼ 0.48; 95% CI ¼ 0.26e0.89; p ¼ 0.019). While these figures do support a bias toward sequential hormonal therapy being used in those with more favorable disease characteristics including more responsiveness to chemotherapy plus trastuzumab, the absolute median PFS and OS values are better with the addition of hormonal therapy to chemotherapy plus trastuzumab either sequentially or concurrently. Among patients not receiving chemotherapy, the addition of trastuzumab to hormonal therapy given concurrently (N ¼ 52) compared to hormonal therapy alone (N ¼ 54) yielded a longer median PFS of 13.8 vs. 4.8 months (adjusted HR 0.37; 95% CI ¼ 0.22e0.60; p < 0.001) and longer median OS although non-significant of not reached vs. 35.1 months (adjusted HR 0.55; 95% CI ¼ 0.27e1.14; p ¼ 0.109) [18]. There were insufficient patients receiving trastuzumab alone in this cohort (N ¼ 23) to compare outcomes of trastuzumab alone vs. trastuzumab plus hormonal therapy. Trials of ER and epidermal growth factor (EGFR, or HER1)-targeted therapy In-vitro and animal testing has shown that blocking growth factor receptors other that HER2, including EGFR, improves the

5

activity of tamoxifen in HER2-positive or negative breast cancer [19,59,60]. On this premise, a randomized Phase II trial enrolled 93 postmenopausal patients with hormone receptor-positive advanced breast cancer and no prior hormonal or chemotherapy for advanced disease, to anastrozole plus placebo or anastrozole plus the EGFR kinase inhibitor gefitinib [61]. HER2 positivity was seen in 19% of patients. As shown on Table 1, an improvement in the primary endpoint of PFS was observed with the combination at 14.7 months vs. 8.4 months (HR 0.55; 95% CI ¼ 0.32e0.94). There was evidence of a greater effect of gefitinib in the subgroup of patients who had not previously received hormonal therapy (N ¼ 42; HR 0.39; 95% CI ¼ 0.16e0.97), whereas in patients who received prior adjuvant hormonal therapy, there was no difference in PFS. The clinical benefit rate was 49% vs. 34%, and the objective response rate was 2% vs. 12% with anastrozole plus gefitinib and anastrozole plus placebo, respectively. No evidence of interaction was seen between baseline HER family receptor or downstream biomarker status and response. Grade 3 or 4 diarrhea and skin rash was seen at an incidence of 7% or less for grade 3 or 4 was seen with gefitinib, and no unexpected adverse events were observed. Another randomized Phase II placebo-controlled double-blind trial compared tamoxifen plus placebo to tamoxifen plus gefitinib in patients with metastatic hormone receptor-positive breast cancer [62]. Two parallel cohorts were enrolled, Stratum 1 with no prior therapy for metastatic disease and >1-year since adjuvant tamoxifen, and Stratum 2 with recurrent disease during or after adjuvant aromatase inhibitor therapy or progressing after first-line aromatase inhibitor for metastatic disease. There was a numerical advantage in PFS (10.9 vs. 8.8 months) for tamoxifen plus gefitinib compared with tamoxifen plus placebo in Stratum 1. A small cohort of HER-2þ cases (N ¼ 37) showed a higher numerical PFS advantage for gefitinib compared to placebo (HR 0.54; 95% CI, 0.25e1.15; p ¼ 0.111). In Stratum 2, no responses were seen, PFS was significantly shorter and no impact of gefitinib on PFS, ORR, or CBR was seen. No relationship between PFS or response was seen with any of the biomarkers measured including EGFR, p-27, p-Akt, and p-ERK. There was a trend toward larger benefit from gefitinib with lower ER expression, suggesting that lower ER is a readout or a driver of growth factor-mediated hormonal resistance. Toxicities were consistent with the known side effect profile of EGFR inhibitors. IGF-1 receptor and hormonal therapy Signaling through other membrane receptor tyrosine kinases like insulin-like growth factor-1 receptor (IGF-1R) can also activate resistance to hormonal therapy and could represent a driver of intrinsic or acquired hormonal resistance clinically [63e65]. .This might occur due to common downstream end results via the PI3K/ Akt/mTOR axis or through distinct IGF-1R pathways. Blockade of IGF-1R using both antibodies and small molecule tyrosine kinase inhibitors is now in Phase IeIII testing in several malignancies. A randomized controlled double-blind Phase II trial was carried out using open label choice of hormonal therapy with either the aromatase inhibitor exemestane or the estrogen receptor downregulator fulvestrant plus placebo or the fully human IgG1 antibody against IGF-1R, ganitumab (AMG479), which blocks binding of both IGF-1R ligands, IGF-1 and IGF-2 [66]. A total of 156 patients were enrolled, but no difference was seen in PFS between the ganitumab and placebo groups of 3.9 vs. 5.7 months (HR 1.17; 80% CI ¼ 0.91e 1.50; p ¼ 0.44) nor in the ORR at 8 vs. 13%. The trial was terminated early because overall survival was significantly worse in the ganitumab compared to placebo arm (HR 1.78; 80% CI ¼ 1.27e2$50; p ¼ 0.025). Side effects included asymptomatic neutropenia and hyperglycemia in 11% of patients, but only 6% at grade 3 or 4.


6


Downstream signal transduction modulation and hormonal therapy The PI3K/Akt/mTOR signaling pathway that is downstream of receptor tyrosine kinases has been investigated in the laboratory as well as the clinical setting as single agents or in combination with other growth factor receptor-targeting drugs such as trastuzumab and with hormonal therapy. The mTOR inhibitors temsirolimus and everolimus have been tested the most extensively, and everolimus has been approved by some regulatory agencies in combination with exemestane in postmenopausal advanced hormone receptorpositive breast cancer following progression on an aromatase inhibitor. The pivotal study BOLERO-II randomized this patient population to exemestane with or without everolimus at 10 mg daily [67]. A significant improvement in PFS was seen in the everolimus arm when compared to placebo of 6.9 months vs. 2.8 months (HR 0.43; 95% CI ¼ 0.35 to 0.54; P < 0.001). ORR was 9.5% compared to 0.4% in the everolimus compared to placebo arm and the 18-month update of this trial shows fewer deaths, although formal survival analysis is pending [68]. Grade 3 or 4 toxicities included stomatitis (8%), anemia (6%), dyspnea (4%), hyperglycemia (4%), fatigue (4%) and pneumonitis (3%). A similar placebo-controlled smaller Phase II study “TAMRAD”, analyzed the efficacy and safety of everolimus in combination with tamoxifen in patients who had received prior aromatase inhibitor therapy in the adjuvant or advanced settings and could have received prior chemotherapy in any setting [69]. This trial enrolled 111 patients who were stratified according to primary hormonal resistance defined as those relapsing during or within 6 months of stopping adjuvant aromatase inhibitor treatment or progressing within 6 months of starting treatment in the metastatic setting. Secondary resistance was defined as relapse >6 months after stopping adjuvant aromatase inhibitor therapy or responding the therapy for 6 months in the metastatic setting. The primary endpoint of 6-month CBR was 61% in the everolimus as opposed to 42% with placebo. Median time to progression was 8.6 months with tamoxifen plus everolimus compared to 4.5 months with tamoxifen alone (HR 0.54; 95% CI ¼ 0.36e0.81; p ¼ 0.0021). The time to progression benefit appears to be most pronounced in the secondary hormonal resistance group with median time to progression of 14.8 months with combination therapy compared to 5.5 months with tamoxifen alone (HR 0.46; 95% CI ¼ 0.26e0.83; p ¼ 0.0087) and was not significantly different in the primary hormonal resistance group with time to progression of 5.4 and 3.8 months with combination and tamoxifen only respectively (HR 0.70; 95% CI ¼ 0.40e1.21). Median survival was also significantly improved; it had not been reached in the everolimus plus tamoxifen arm and was 32.9 months with tamoxifen alone (HR 0.45; 95% CI ¼ 0.24e0.81). Grade 3 and 4 adverse events included stomatitis (11%), pain (9%), fatigue (6%), nausea (4%) and rash (4%) [69]. The HORIZON Trial was designed to study the combination of mTOR inhibitor temsirolimus with letrozole vs. letrozole alone in 1112 aromatase inhibitor-naïve patients with HR-positive advanced breast cancer. The trial however failed to meet statistical significance in its primary endpoint of PFS, which in the combination arm was a median of 8.9 months (95%CI ¼ 7.4e9.6) vs. 9.0 months (95% CI ¼ 7.2e9.4) in the letrozole þ placebo arm [70]. Results from this trial showed a stark contrast from the BOLERO-II study which may be attributable in part to the lack of prior aromatase inhibitor exposure on patients on HORIZON trial as opposed to BOLERO-II, where all patients had progressed on prior therapy with aromatase inhibitor. Other explanations that have been cited include the suboptimal dosing and schedule used in this trial. Taken together, mTOR inhibition with everolimus clearly improves outcome in the second line hormonal setting in postmenopausal patients when combined with an aromatase inhibitor

and possibly with tamoxifen as well. However, the side effect profile is clearly not as favorable as with traditional hormonal agents. Trials assessing the effects of Akt, PI3K and dual mTOR/PI3K inhibitors in combination with several hormonal agents are currently being evaluated in Phase III trials. Conclusions Targeting of the estrogen receptor with selective estrogen receptor modulators, estrogen removal with aromatase inhibitors and through downregulation with fulvestrant remains one of the most effective therapies although in the advanced setting, resistance almost always develops after highly variable times amongst patients. Similarly, drugs that interfere with the HER2 receptor, with trastuzumab as the prototype, improve survival in both early stage and advanced disease, although in the latter case, this drug as well as newer and more effective next generation agents are still marred by the development of clinical resistance. Co-targeting of ER and HER appears to provide benefit without a significant increase in toxicity although formal trials assessing combination vs. sequence have not been carried out, nor have trials that compare HER2-based therapy with or without hormonal therapy. Moreover, many HER2 agents are typically used in combination with chemotherapy, which by tradition has not been combined with hormonal treatment. Hence, it has been difficult to determine the most effective way to treat hormone receptor/HER2-positive metastatic disease. It makes sense to at least use hormonal therapy if a break in chemotherapy is used after optimal response, or in situations where HER2 targeted therapy (trastuzumab or lapatinib) is being used without chemotherapy. However, for newer agents e pertuzumab and T-DM1, there are no data upon which to base the use of concurrent hormonal therapy. Numerous lines of evidence support the augmentation of effectiveness and possible reversal resistance when ER and growth factor receptor pathways are targeted simultaneously. However, with EGFR receptors targeting, the benefits are modest, and have not been seen at all with IGF1-R blockage. In contrast, downstream blockade with everolimus (but not temsirolimus) clearly increases the efficacy of hormonal therapy, and is approved with aromatase inhibitor therapy in the second line setting. However, its more prominent toxicity profile must be factored in when making treatment decisions. Other downstream targets such as Akt and PI3K are being tested, and early activity has been seen with CDK 4/6 blockade. Targets not related to growth factor pathways such as histone deacetylase inhibition are also showing promise as synergistic agents with hormonal blockade [71]. Newer targets and strategies are likely to emerge and enter clinical testing as highly annotated genomic data from large population studies become available. Conflict of interest statement No conflict of interests are reported by any of the authors. The work was funded by the USC/Norris Comprehensive Cancer Center Women’s Cancers Program. References [1] Slamon D, Clark GM, Wong SG, Levin WJ, Ullrich A, McGuire WL. Human breast cancer: correlation of relapse and survival with amplification of the HER-2/neu oncogene. Science 1987;23:177e82. [2] Slamon DJ, Leyland-Jones B, Skak S, Fuchs H, Paton V, Bajamonde A, et al. Use of chemotherapy plus a monoclonal antibody against HER2 for metastatic breast cancer that overexpresses HER2. New Engl J 2001;344:783e92. [3] Cobleigh MA, Vogel CL, Tripathy D, Robert NJ, Scholl S, Fehrenbacher L, et al. Multinational study of the efficacy and safety of humanized anti-HER2 monoclonal antibody in women who have HER2-overexpressing metastatic


A. Mehta, D. Tripathy / The Breast xxx (2013) 1e8 breast cancer that has progressed after chemotherapy for metastatic disease. J Clin Oncol 1999;17:2639e48. [4] Vogel CL, Cobleigh MA, Tripathy D, Gutheil JC, Harris LN, Fehrenbacher L, et al. Efficacy and safety of trastuzumab as a single agent in first-line treatment of HER2-overexpressing metastatic breast cancer. J Clin Oncol 2002;20:719e26. [5] Piccart-Gebhart MJ, Procter M, Leyland-Jones B, Goldhirsch A, Untch M, Smith I, et al. Trastuzumab after adjuvant chemotherapy in HER2-positive breast cancer. New Engl J Med 2005;353:1659e72. [6] Smith I, Procter M, Gelber RD, Guillaume S, Feyereislova A, Dowsett M, et al. 2year follow-up of trastuzumab after adjuvant chemotherapy in HER2-positive breast cancer: a randomized controlled trial. Lancet 2007;369:29e36. [7] Gianni L, Dafni U, Gelber R, Azambuja E, Muehlbauer S, Goldhirsch A, et al. Treatment with trastuzumab for 1 year after adjuvant chemotherapy in patients with HER2-positive early breast cancer: a 4-year follow-up of a randomised controlled trial. Lancet Oncol 2011;12:236e44. [8] Romond EH, Perez EA, Bryant J, Suman VJ, Geyer Jr CE, Davidson NE, et al. Trastuzumab plus adjuvant chemotherapy for operable HER2-positive breast cancer. New Engl J Med 2005;353:1673e84. [9] Perez EA, Romond EH, Suman VJ, Jeong JH, Davidson NE, Geyer Jr CE, et al. Four-year follow-up of trastuzumab plus adjuvant chemotherapy for operable human epidermal growth factor receptor 2epositive breast cancer: joint analysis of data from NCCTG N9831 and NSABP B-31. J Clin Oncol 2011;29: 3366e73. [10] Slamon D, Eiermann W, Robert N, Pienkowsky T, Martin M, Press M, et al. Adjuvant trastuzumab in HER2-positive breast cancer. N Engl J Med 2011;365:1273e83. [11] Clark GM, Osborne CK, McGuire WL. Correlations between estrogen receptor, progesterone receptor, and patient characteristics in human breast cancer. J Clin Oncol 1984;2:1102e9. [12] Wenger CR, Beardslee S, Owens MA, Pounds G, Oldaker T, Vendely P, et al. DNA ploidy, S-phase, and steroid receptors in more than 127,000 breast cancer patients. Breast Cancer Res Treat 1993;199(28):9e20. [13] Tyson JJ, Baumann WT, Chen C, Verdugo A, Tavassoly I, Wang Y, et al. Dynamic modelling of oestrogen signalling and cell fate in breast cancer cells. Nat Rev Cancer 2011;11:523e32. [14] Early Breast Cancer Trialists Collaborative Group (EBCTCG). Relevance of breast cancer hormone receptors and other factors to the efficacy of adjuvant tamoxifen: patient-level meta-analysis of randomised trials. Lancet 2011;378: 771e84. [15] Dowsett M, Cuzick J, Ingle J, Coates A, Forbes J, Bliss J, et al. Meta-analysis of breast cancer outcomes in adjuvant trials of aromatase inhibitors versus tamoxifen. J Clin Oncol 2010;28:509e18. 16] Burstein HJ, Prestrud AA, Seidenfeld J, Anderson H, Buchholz TA, Davidson NE, et al. American society of clinical oncology clinical practice guideline: update on adjuvant endocrine therapy for women with hormone receptor-positive breast cancer. J Clin Oncol 2010;28:3784e96. [17] Dowsett M, Allred C, Know J, Quinn E, Salter J, Wale C, et al. Relationship between quantitative estrogen and progesterone receptor expression and human epidermal growth factor receptor 2 (HER-2) status with recurrence in the Arimidex, Tamoxifen, alone or in combination trial. J Clin Oncol 2008;26: 1059e65. [18] Tripathy D, Kaufman PA, Brufsky AM, Mayer M, Yood MU, Yoo B, et al. Firstline treatment patterns and clinical outcomes in patients with HER2-positive and hormone receptor-positive metastatic breast cancer from registHER. Oncologist 2013;18(5):501e10. [19] Musgrove EA, Sutherland RL. Biological determinants of endocrine resistance in breast cancer. Nat Rev Cancer 2009;9:631e43. [20] Wright C, Nicholson S, Angus B, Sainsbury JR, Farndon J, Cairns J. Relationship between c-erbB-2 protein product expression and response to endocrine therapy in advanced breast cancer. Br J Cancer 1992 Jan;65(1):118e21. 21] Borg A, Baldetorp B, Ferno M, Killander D, Olsson H, Ryden S, et al. ERBB2 amplification is associated with tamoxifen resistance in steroid receptor positive breast cancer. Cancer Lett 1994;81:137e44. [22] Houston SJ, Plunkett TA, Barnes DM, Smith P, Rubens RD, Miles DW. Overexpression of c-erbB2 is an independent marker of resistance to endocrine therapy in advanced breast cancer. Br J Cancer 1999 Mar;79(7-8):1220e6. [23] Ellis MJ, Coop A, Singh B, Mauriac L, Llombert-Cussac A, Jänicke F, et al. Letrozole is more effective neoadjuvant endocrine therapy than tamoxifen for ErbB-1- and/or ErbB-2-positive, estrogen receptor-positive primary breast cancer: evidence from a phase III randomized trial. J Clin Oncol 2001 Sep 15;19(18):3808e16. [24] Ellis MJ, Tao Y, Young O, White S, Proia AD, Murray J, et al. Estrogen-independent proliferation is present in estrogen-receptor HER2-positive primary breast cancer after neoadjuvant letrozole. J Clin Oncol 2006;24:3019e25. [25] Vaz-Luis I, Ottesen RA, Hughes ME, Marcom PK, Moy B, Rugo HS, et al. Impact of hormone receptor status on patterns of recurrence and clinical outcomes among patients with human epidermal growth factor-2-positive breast cancer in the National Comprehensive Cancer Network: a prospective cohort study. Breast Cancer Res 2012;14:R129. [26] Osborne CK, Schiff R. Mechanisms of endocrine resistance in breast cancer. Annu Rev Med 2011;62:233e47. [27] Benz CC, Scott GK, Sarup JC, Johnson RM, Tripathy D, Coronado E, et al. Estrogen-dependent, tamoxifen-resistant tumorigenic growth of MCF-7 cells transfected with HER2/neu. Breast Cancer Res Treat 1992;24(2): 85e95.

7

[28] Shou J, Massarweh S, Osborne CK, Wakeling AE, Ali S, Weiss H, et al. Mechanisms of tamoxifen resistance: increased estrogen receptor-HER2/neu crosstalk in ER/HER2-positive breast cancer. J Natl Cancer Inst 2004;96:926e35. [29] Arpino G, Wiechmann L, Osborne CK, Schiff R. Crosstalk between the estrogen receptor and the HER tyrosine kinase receptor family: molecular mechanism and clinical implications for endocrine therapy resistance. Endocr Rev 2008;29:217e33. 2008. [30] Miller TW, Hennessy BT, González-Angulo AM, Fox EM, Mills GB, Chen H, et al. Hyperactivation of phosphatidylinositol-3 kinase promotes escape from hormone dependence in estrogen receptor-positive human breast cancer. J Clin Invest 2010;120:2406e13. [31] Knowlden JM, Hutcheson IR, Barrow D, Gee JM, Nicholson RI. Insulin-like growth factor-I receptor signaling in tamoxifen resistant breast cancer: a supporting role to the epidermal growth factor receptor. Endocrinology 2005;146:4609e18. [32] Wu RC, Smith CL, O’Malley BW. Transcriptional regulation by steroid receptor co-activator phosphorylation. Endocr Rev 2005;26:393e9. [33] Renoir JM. Estradiol receptors in breast cancer cells: associated co-factors as targets for new therapeutic approaches. Steroids 2012;77:1249e61. [34] Levin ER, Pietras RJ. Estrogen receptors outside the nucleus in breast cancer. Breast Cancer Res Treat 2008;108:351e61. [35] Marty M, Cognetti F, Maraninchi D, Snyder R, Mauriac L, Tubiana-Hulin M, et al. Randomized phase II trial of the efficacy and safety of trastuzumab combined with docetaxel in patients with human epidermal growth factor receptor 2-positive metastatic breast cancer administered as first-line treatment: the M77001 study group. J Clin Oncol 2005;23:4265e74. [36] Montemurro F, Rossi V, Rocca MC, Martinello R, Verri E, Redana S, et al. Hormone-receptor expression and activity of trastuzumab with chemotherapy in HER2-positive advanced breast cancer patients. Cancer 2012;118: 17e26. [37] Cameron D, Casey M, Press M, Lindquist D, Pienkowski T, Romieu CG, et al. A phase III randomized comparison of lapatinib plus capecitabine versus capecitabine alone in women with advanced breast cancer that has progressed on trastuzumab: updated efficacy and biomarker analyses. Breast Cancer Res Treat 2008;112:533e43. [38] Cameron D, Casey M, Oliva C, Newstat B, Imwalle B, Geyer CE. Lapatinib plus capecitabine in women with HER-2-positive advanced breast cancer: final survival analysis of a phase III randomized trial lapatinib plus capecitabine in women with HER-2-positive advanced breast. Oncologist 2010;15:924e34. [39] Baselga J, Cortés J, Kim SB, Im SA, Hegg R, Im YH, et al. Pertuzumab plus trastuzumab plus docetaxel for metastatic breast cancer. N Engl J Med 2012;366:109e19. [40] Verma S, Miles D, Gianni L, Krop IE, Welslau M, Baselga J, et al. Trastuzumab emtansine for HER2-positive advanced breast cancer. N Engl J Med 2012;367: 1783e91. [41] Untch M, Rezai M, Loibl S, Fasching PA, Huober J, Tesch H, et al. Neoadjuvant treatment with trastuzumab in HER2-positive breast cancer: results from the GeparQuattro study. J Clin Oncol 2010;28:2024e31. [42] Untch M, Fasching PA, Konecny GE, Hasmuller S, Lebeau A, Kreienberg R, et al. Pathologic complete response after neoadjuvant chemotherapy plus trastuzumab predicts favorable survival in human epidermal growth factor receptor 2-overexpressing breast cancer: results from the TECHNO trial of the AGO and GBG study groups. J Clin Oncol 2011;29:3351e7. [43] Baselga J, Bradbury I, Eidtmann H, Di Cosimo S, de Azambuja E, Aura C, et al. Lapatinib with trastuzumab for HER2-positive early breast cancer (NeoALTTO) :a randomized open-label, multi-centre phase 3 trial. Lancet 2012;379:633e40. [44] Gianni L, Pienkowski T, Im YH, Roman L, Tseng LM, Liu MC. Efficacy and safety of neoadjuvant pertuzumab and trastuzumab in women with locally advanced, inflammatory, or early HER2-positive breast cancer (NeoSphere): a randomised multicentre, open-label, phase 2 trial. Lancet Oncol 2012;13:25e32. [45] Denkert C, Huober J, Loibl S, Prinzler J, Kronenwett R, Darb-Esfahani S, et al. HER2 and ESR1 mRNA expression levels and response to neoadjuvant trastuzumab plus chemotherapy in patients with primary breast cancer. Breast Cancer Res 2013;15(1):R11. [46] Marcom PK, Isaacs C, Harris L, Wong ZW, Kommarreddy A, Novielli N, et al. The combination of letrozole and trastuzumab as first or second-line biological therapy produces durable responses in a subset of HER2 positive and ER positive advanced breast cancers. Breast Cancer Res Treat 2007;102:43e9. [47] Kaufman B, Mackey JR, Clemmons MR, Bapsy PP, Vaid A, Wardley A, et al. Trastuzumab plus anastrozole versus anastrozole alone for the treatment of postmenopausal women with human epidermal growth factor receptor 2Positive, hormone Receptor- Positive metastatic breast cancer: results from the randomized phase III TAnDEM study. J Clin Oncol 2009;27:5529e37. [48] Huober J, Fasching PA, Barsoum M, Petruzelka L, Wallwiener D, Thomssen C, et al. Higher efficacy of letrozole in combination with trastuzumab compared to letrozole monotherapy as first-line treatment in patients with HER2-positive, hormone-receptor-positive metastatic breast cancer e results of the eLEcTRA trial. Breast 2012;21:27e33. [49] Xia W, Mullin RJ, Keith BR, Liu LH, Ma H, Rusnak DW, et al. Anti-tumor activity of GW572016: a dual tyrosine kinase inhibitor blocks EGF activation of EGFR/erbB2 and downstream Erk1/2 and AKT pathways. Oncogene 2002;21:6255e63. [50] Geyer CE, Forster J, Lindquist D, Chan S, Romieu CG, Pienkowski T, et al. Lapatinib plus capecitabine for HER2-positive advanced breast cancer. N Engl J Med 2006;355:2733e43.


8


[51] Opdam FL, Guchelaar H-J, Beijnen JH, Schellens JHM. Lapatinib for advanced or metastatic breast cancer. Oncologist 2012;17:536e42. [52] Chu I, Blackwell K, Chen S, Slingerland J. The dual ErbB1/ErbB2 inhibitor, lapatinib (GW572016), cooperates with tamoxifen to inhibit both cell proliferation-and estrogen-dependent gene expression in antiestrogen-resistant breast cancer. Cancer Res 2005;65:18e25. [53] Johnston S, Pippen Jr J, Pivot X, Lichinitser M, Sadeghi S, Dieras V, et al. Lapatinib combined with letrozole versus letrozole and placebo as first-line therapy for postmenopausal hormone receptor-positive metastatic breast cancer. J Clin Oncol 2009;27:5538e46. [54] Schwartzberg LS, Franco SX, Florance A, O’Rourke L, Maltzman J, Johnston S. Lapatinib plus letrozole as first-line therapy for HER-2+ hormone receptorpositive metastatic breast cancer. Oncologist 2010;15:122e9. [55] Massarweh S, Osborne CK, Creighton CJ, Qin L, Tsimelzon A, Huang S, et al. Tamoxifen resistance in breast tumors is driven by growth factor receptor signaling with repression of classic estrogen receptor genomic function. Cancer Res 2008;68:826e33. [56] Blackwell KL, Burstein HJ, Storniolo AM, Rugo HS, Sledge G, Aktan G, et al. Overall survival benefit with lapatinib in combination with trastuzumab for patients with human epidermal growth factor receptor 2-positive metastatic breast cancer: final results from the EGF104900 study. J Clin Oncol 2012;30: 2585e92. [57] Rimawi MF, Wiechmann LS, Wang YC, Huang C, Migliaccio I, Wu MF, et al. Reduced dose and intermittent treatment with lapatinib and trastuzumab for potent blockade of the HER pathway in HER2/neu-overexpre ssing breast tumor xenografts. Clin Cancer Res 2011;17:1351e61. [58] Rimawi MF, Mayer IA, Forero A, Nanda R, Goetz MP, Rodriguez AA, et al. Multicenter phase II study of neoadjuvant lapatinib and trastuzumab with hormonal therapy and without chemotherapy in patients with human epidermal growth factor receptor 2-overexpressing breast cancer: TBCRC 006. J Clin Oncol 2013 May 10;31(14):1726e31. [59] Arpino G, Gutierrez C, Weiss H, Rimawi M, Massarweh S, Bharwani L, et al. Treatment of human epidermal growth factor receptor 2eoverexpressing breast cancer xenografts with multiagent HER-targeted therapy. J Natl Cancer Inst 2007;99:694e705. [60] Pancholi S, Lykkesfeldt AE, Hilmi C, Banerjee S, Leary A, Drury S, et al. ERBB2 influences the subcellular localization of the estrogen receptor in tamoxifenresistant MCF-7 cells leading to the activation of AKT and RPS6KA2. Endocrine-related Cancer 2008;15:985e1002. [61] Cristofanilli M, Valero V, Mangalik A, Royce M, Rabinowitz I, Arena FP, et al. Phase II, randomized trial to compare anastrozole combined with gefitinib or placebo in postmenopausal women with hormone receptor-

[62]

[63]

[64]

[65]

[66]

[67]

[68]

[69]

[70]

[71]

positive metastatic breast cancer. Clin Cancer Res. 2010 Mar 15;16(6): 1904e14. Osborne CK, Nevin P, Dirix LY, Mackey JR, Robert J, Underhill C, et al. Gefitinib or placebo in combination with tamoxifen in patients with hormone receptorepositive metastatic breast cancer: a randomized phase II study. Clin Cancer Res 2011;17:1147e59. Jones HE, Gee JM, Hutcheson IR, Nicholson RI. Insulin-like growth factor-I receptor signaling and resistance in breast cancer. Expert Rev Endocrin Metab 2006;1:33e46. Fagan DH, Uselman RR, Sachdev D, Yee D. Acquired resistance to tamoxifen is associated with loss of the type I insulin-like growth factor receptor: implications for breast cancer treatment. Cancer Res. 2012 Jul 1;72(13):3372e80. Fox EM, Miller TW, Balko JM, Kuba MG, Sánchez V, Smith RA, et al. A kinomewide screen identifies the insulin/IGF-I receptor pathway as a mechanism of escape from hormone dependence in breast cancer. Cancer Res 2011;71: 6773e84. Robertson JF, Ferrero JM, Bourgeois H, Kennecke H, de Boer RH, Jacot W, et al. Ganitumab with either exemestane or fulvestrant for postmenopausal women with advanced, hormone-receptor-positive breast cancer: a randomised, controlled, double-blind, phase 2 trial. Lancet Oncol 2013 Mar;14(3):228e35. Baselga J, Campone M, Piccart M, Burris 3rd HA, Rugo HS, Sahmoud T, et al. Everolimus in postmenopausal hormone-receptor-positive advanced breast cancer. N Engl J Med 2012;366:520e9. Piccart M, Baselga J, Noguchi S, Burris H, Gnant M, Hortobagyi G, et al. Final progression-free survival analysis of BOLERO-2: a phase III trial of everolimus for postmenopausal women with advanced breast cancer. Cancer Res December 15, 2012;72(24) [Supplement 3; San Antonio Breast Cancer Symposium, Abstract P6-04-02]. Bachelot T, Bourgier C, Cropet C, Ray-Coquard I, Ferrero JM, Freyer G, et al. Randomized phase II trial of everolimus in combination with tamoxifen in patients with hormone receptorepositive, human epidermal growth factor receptor 2enegative metastatic breast cancer with prior exposure to aromatase inhibitors: a GINECO study. J Clin Oncol 2012;30:2718e24. Wolff AC, Lazar AA, Bondarenko I, Garin AM, Brincat S, Chow L, et al. Randomized phase III placebo controlled trial of letrozole plus oral temsirolimus as first-line endocrine therapy in postmenopausal women with locally advanced or metastatic breast cancer. J Clin Oncol 2013;31:195e202. Klein P, Orentlich P, McColloch W, Cruickshank S, Rees M, Yardley DA. Characterization of the overall survival benefit in ENCORE 301, a randomized placebo-controlled phase II study of exemestane with and without entinostat in ERþ postmenopausal women with metastatic breast cancer. J Clin Oncol 2012;30 [Suppl; Abstract # 567].


Change in estrogen receptor, HER2, and Ki-67 status between primary breast cancer and ipsilateral breast cancer tumor recurrence.

Aberrant Splicing of Estrogen Receptor, HER2, and CD44 Genes in Breast Cancer.

Obesity Suppresses Estrogen Receptor Beta Expression in Breast Cancer Cells via a HER2-Mediated Pathway.

Molecular essence and endocrine responsiveness of estrogen receptor-negative, progesterone receptor-positive, and HER2-negative breast cancer.

Abnormal estrogen receptor in clinical breast cancer.

Estrogen receptor variants in human breast cancer.

Estrogen receptor variants in clinical breast cancer.

Her2 Negative Primary Breast Cancer With Triple Negative Breast Cancer.

Collagen gel droplet-embedded culture-drug sensitivity test and Ki67 expression in estrogen receptor-positive and HER2-negative breast cancer.

Estrogen Receptor and HER2 Status on Disseminated Tumor Cells and Primary Tumor in Patients with Early Breast Cancer.

Expression of Androgen Receptor in Estrogen Receptor-positive Breast Cancer.

Quantification of HER2 and estrogen receptor heterogeneity in breast cancer by single-molecule RNA fluorescence in situ hybridization.

Locoregional and Overall Recurrence After Neaodjuvant Endocrine Therapy Versus Chemotherapy in Postmenopausal Women With Estrogen Receptor+ HER2- Breast Cancer.

Hedgehog Acyltransferase as a target in estrogen receptor positive, HER2 amplified, and tamoxifen resistant breast cancer cells.

Estrogen and progesterone receptor proteins in breast cancer.

Estrogen receptor mutations and their role in breast cancer progression.

Managing breast cancers with low estrogen receptor and HER2 by drugging both.

Prognostic values of negative estrogen or progesterone receptor expression in patients with luminal B HER2-negative breast cancer.

The mammalian target of rapamycin complex 1 (mTORC1) in breast cancer: the impact of oestrogen receptor and HER2 pathways.

Distribution patterns of 21-gene recurrence score in 980 Chinese estrogen receptor-positive, HER2-negative early breast cancer patients.

Estrogen receptor and natural course of breast cancer.

Estrogen receptor mutations and functional consequences for breast cancer.

The Intestinal Microbiome and Estrogen Receptor-Positive Female Breast Cancer.

Atmospheric particulate matter and breast cancer survival: estrogen receptor triggered?