Background
As a heterogeneous disease, breast cancer is clinically classified by taking into account the expression of estrogen receptor alpha (ERα), progesterone receptor (PR), and the presence/amplification status of the oncogenic human epidermal growth factor receptor 2 (HER2) [
1]. Triple-negative breast cancer (TNBC), i.e., those not expressing ER, PR, or HER2, represent ~ 10–20% of all cases, have poorer prognosis than HER2
+ or hormone receptor-positive tumors, and are generally characterized by an aggressive clinical course [
2]. Furthermore, due to the lack of druggable targets including HER2 and ER, the main therapeutic options remain surgery and systemic chemotherapies, either individually or combined [reviewed in [
3]]. Importantly, epidermal growth factor receptor (EGFR) is more often overexpressed in TNBC than in other breast cancer subtypes, making this a possible target for therapeutic intervention.
The present study investigated the effects of Pan-HER, a novel mixture of six monoclonal antibodies (mAbs) directed against members of the human epidermal growth factor receptor (HER) family EGFR/HER1, HER2, and HER3 [
4], in a preclinical trial of TNBC patient-derived xenografts (PDXs). The HER family, notably EGFR, is well recognized for its pro-oncogenic activity that upon activation by corresponding ligands lead to receptor dimerization [
5]. These effects are mediated through downstream signals including PI3K/AKT, RAS/RAF/MEK/ERK1/2, JAK/STAT3, and PLCγ-pathways ([
5]. Several cancer types have been shown to be associated with either mutation or increased expression of the HER family members, including breast, lung, stomach, colorectal, head and neck, pancreatic carcinomas, and glioblastoma {Roskoski, 2014 #2300, [
6]). Furthermore, accumulating evidence shows that the HER family is important in providing cancer cells with compensatory signals that greatly contribute to the development of resistance and survival in response to therapy [
7‐
9]. Pan-HER was designed based on the hypothesis that simultaneous inhibition and/or downmodulation of multiple members of the HER family may result in effective disruption of tumor growth, preventing the HER-driven cell proliferation, survival, angiogenesis, and invasion [
10]. Earlier studies, including clinical trials [
11,
12], explore the concept of targeting simultaneously either HER family members or combining antibody-based therapy with kinase inhibitors [
13,
14] and have set the stage for further investigation of this mechanistic-related concept. In this context, the potential therapeutic advantage of Pan-HER may reside in its ability to simultaneously target all three receptor tyrosine kinases (RTKs), which may in turn block, or at least significantly delay, the appearance of survival and escape mechanisms [
4,
15].
The effect of Pan-HER has been investigated in a number of cell lines and xenografts representing a diverse number of cancer types including head and neck, lung [
16,
17], HER2
+ breast [
18], and other malignancies shown to have a dependency on one or more of the targets, i.e., EGFR, HER2, or HER3 [
4]. Pan-HER demonstrated stronger activity than single mAbs directed against single HER family members and was capable of overcoming acquired resistance due to increased ligand expression [
4]. Here, we present the results of a preclinical trial of Pan-HER performed in 15 TNBC PDX models. Our results show significant antitumor activity by Pan-HER in all PDX tumor models evaluated, with a noticeable therapeutic response in those TNBC tumors whose dependency on the HER family appeared to be most pronounced.
Discussion
The characteristics of TNBC, i.e., absence of ER/PR/HER2 expression, entail that endocrine and HER2-directed therapies are not appropriate for these types of tumors [
44‐
46]. Furthermore, TNBC will in most cases develop resistance to anthracyclines and taxanes, both considered to be among the most active compounds against breast cancer, thus limiting the therapeutic options even further [
47,
48]. With the discovery several years ago of EGFR being overexpressed in TNBCs, a number of new, targeted therapies were developed, including small-molecule tyrosine kinase inhibitors (TKIs) and mAbs. Several of these are now approved and available, including lapatinib, an oral potent HER2 and EGFR inhibitor [
49]; afatinib, also a dual HER2/EGFR inhibitor [
50,
51]; neratinib, a pan-HER inhibitor that selectively targets HER2, HER4, and EGFR [
52,
53], and the recently introduced erlotinib, an EGFR inhibitor [
54,
55]. Anti-EGFR mAbs include cetuximab and panitumumab, both of which block ligand-induced phosphorylation of EGFR [
56,
57]. However, despite efforts to develop these therapeutic approaches further, these options have not yielded sustained responses and mechanisms of resistance have been encountered after a few months.
In an effort to overcome tumor recurrence and resistance, multiple studies have been addressing the potential use of drug combinations as a way to improve the efficacy of treatment. For example, cetuximab has been clinically tested in combination with cisplatin (metastatic TNBC) [
58] and carboplatin (stage IV TNBC) [
59] and preclinically tested with ixabepilone and other chemotherapeutic agents [
60,
61]. Additional studies have investigated the concept of targeting simultaneously multiple arms of the same HER family, for example by combining a mAb (cetuximab or panitumumab) with small-molecule TKIs [
62‐
64], and established chemotherapeutic agents [
60,
61] or other non-competitive mAbs [
13].
In the present study, Pan-HER, a mixture of six antibodies simultaneously targeting EGFR, HER2, and HER3 [
4,
15], displayed a very promising activity against TNBC xenografts. TNBCs are characterized by the expression of high levels of EGFR [
65‐
67], and the observation that Pan-HER induced a dramatic downregulation of EGFR and HER3, together with their corresponding signaling pathways, is very relevant. In fact, clinical trials with cetuximab combined with either cisplatin [
58] or carboplatin [
59] showed only moderate improvements vs. chemotherapy alone in terms of overall response rate and progression-free survival, suggesting that targeting EGFR only, even in combination with other therapies, may not be sufficient. Other studies have expanded in the past on the concept of using the blockade of HER family members [
13,
68‐
70]. For example, previous studies in TNBC showed the potential therapeutic advantage of blocking both EGFR and HER3, as a way to improve the efficacy of PI3K-Akt inhibitors [
14]. Similarly, the combination of non-competitive anti-EGFR mAbs was found to result in a robust degradation of EGFR also in TNBC cell models, demonstrating a reduction on the tumorigenic growth of cells and derived xenografts [
13]. In this context, the present results on the efficacy of Pan-HER on TNBC PDXs showed that the mixture of HER family antibodies was very effective in all the PDX models tested, and more pronouncedly in those tumors that displayed highly active HER family expression and signaling, where no recurrence was observed even after several weeks the treatment ended. On the other hand, the least responsive models corresponded to those showing negative or very low expression of either EGFR or HER3, most notably the former. These observations provide not only a plausible explanation for the response of TNBC tumors to Pan-HER but also an indication of the potential use of EGFR and associated signaling expression as biomarkers for selecting patients who may benefit from targeting these proteins, including cases where Pan-HER may be tested in combination with chemotherapies. In this sense, results from a recent trial (I-SPY 2) evaluating the predictive value of the HER/PI3K/AKT signaling activation/phosphorylation in response to the HER1/2/4 inhibitor neratinib [
71], demonstrated that activation of HER family phosphoproteins may be indicative of a response to neratinib.
It is important to note that although not among the most common features observed, relapsed TNBCs have shown some changes in the triple-negative status of the primary tumor type [
72]. For example, in a clinical study designed to investigate whether hormonal receptors and HER2 status may be modified throughout tumor progression and therapeutical intervention, it was concluded that these patients experienced changes in the status of hormone receptors and HER2, which could be attributed to adjuvant therapies and may have major impact in survival [
72]. Thus, it is plausible to speculate that targeted activation of HER family pathways may help avoid the appearance of resistance. The fact that Pan-HER as a single therapy (inherent combination) was sufficient to achieve complete tumor regression in some tumor models without signs of recurrence makes these findings very promising.
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