Triple-negative breast cancer (TNBC), which accounts for 20 % of all breast cancers, is characterized by the absence of estrogen receptor (ER), progesterone receptor (PR), and human epidermal growth factor receptor 2 (HER2) expression. They are histologically high grade, aggressive, and lethal tumor types that lack targeted therapeutic options. Patients with TNBC are associated with relatively poor prognosis and are at a significant risk of relapse and frequent metastases [
1,
2]. Triple-negative and basal-like breast cancers display a similar profile of cell-surface markers of breast cancer stem cells (CSCs) [
3]. CSCs are defined as rare tumor cells that are capable of self-renewal and give rise to multipotent progenitor cells, which ultimately differentiate into all cell types within the tumor [
4‐
6]. CSCs have been identified by cell sorting technologies using various surface markers in acute myeloid leukemia and solid tumors, including breast tumors [
7]. Studying tumorigenic cells separated in vitro, from malignant human breast cancer-derived pleural effusions, Al Hajj and colleagues isolated a cell population characterized by high CD44 expression and low or undetectable levels of CD24 (CD44
+/CD24
−/low). These cells had classic features of bona fide stem cells, including the capacity for self-renewal and generation of heterogeneous progeny [
8]. This subpopulation can form mammospheres in vitro and were shown to be enriched for tumorigenic cells by their ability to form xenograft tumors in immunocompromised mice [
8]. Ginestier et al. demonstrated that aldehyde dehydrogenase 1 is an alternative marker for breast CSCs [
8]. We have recently shown that CD44
+/CD24
−/low and ALDH
+ phenotypes reflect different epithelial-mesenchymal transition states in CSCs [
9]. Identification of breast CSCs from tumor samples or breast cancer cell lines has been based mainly on CD44
+/CD24
−/low or ALDH
+ phenotypes. We have previously reported that breast CSCs are a subpopulation of cells within the primary tumor responsible for tumor initiation and metastases, and are associated with resistance to chemotherapy in human breast cancers following neoadjuvant chemotherapy [
10]. In addition, it has been shown that epidermal growth factor receptor (EGFR) signaling may be required for cancer self-renewal [
11]. EGFR is more commonly overexpressed in TNBC than in other breast cancer subtypes [
12,
13]. Also, TNBC can be classified as basal type cancer defined by EGFR and cytokeratin 5/6 staining.
Ixabepilone is a new-generation microtubule-stabilizing agent and has more efficacious anti-tumor effects than taxanes [
14,
15]. It is an analog of epothilone B, a naturally occurring microtubule stabilizer with very high cytotoxic activities against a wide range of tumor types, including drug-resistant tumors. For example, anthracycline- and taxane-resistant metastatic breast cancers (MBCs) are known to be highly sensitive to Ixabepilone as a single agent or in combination with Capecitabine [
13]. Importantly, significant anticancer activity was seen in ER, PR, HER2 negative TNBC patients with MBC. This is consistent with the preclinical activity of Ixabepilone against human cancer cell lines resistant to taxanes and other agents [
16].
Combination therapy is a mainstay of anticancer treatment, with optimal combinations producing synergistic antitumor responses. This is achieved by combining agents with established safety profiles and non-overlapping mechanisms of action. Thus, this study seeks to evaluate the combination therapy of combining Cetuximab and Ixabepilone, which might be more effective at targeting cancer stem cells than other antitubulins, as a possible way to increase antitumor activity. In the present study, we investigated whether the breast CSC population enriched for tumorigenic CD44+CD24−/low cells could be eradicated when treated with Cetuximab and Ixabepilone, as opposed to chemotherapy alone, in TNBC xenografts. Our findings suggest that Ixabepilone produces therapeutic synergism with Cetuximab only in a small subset of TNBCs and provides additional evidence that clinical trials using Cetuximab in combination with Ixabepilone should be applied with caution.