There is increasing evidence that in a variety of neoplasia

There is increasing evidence that in a variety of neoplasia, including breast cancer, only a subset of cancer nucleoside analogs are capable of reconstituting the tumor after transplantation. These cells called cancer stem cells (CSCs) or tumor-initiating cells (TICs), have the ability to self-renew and regenerate tumor heterogeneity (Al-Hajj et al., 2003) and show intrinsic resistance to conventional chemotherapies, leading to recurrence or metastasis. In fact, breast tumors from patients who received neoadjuvant chemotherapy are substantially enriched for CSCs compared with tumors of untreated patients (Yu et al., 2007), suggesting that anti-cancer agents kill the bulk of tumor cells, but spare the CSCs (Dean et al., 2005). In breast cancer, a variety of markers (CD44, CD24, EpCAM, CD49f, CD133/2, CD10, and ALDH activity) have been shown to identify CSCs (Al-Hajj et al., 2003; Bachelard-Cascales et al., 2008; Li et al., 2008; Lim et al., 2009; Stingl et al., 2006). However, it is still unclear whether all these markers are appropriate for the different breast cancer subtypes, and further studies are necessary to identify the population of TICs and their functionality in each type of tumors.
Results
Discussion Patient-derived xenograft (PDX) models have emerged as an important intermediate tool between basic research and clinical trials to expedite the translation of basic research findings into effective therapies for patients. We have generated a panel of PDX models that recapitulates the heterogeneity of human breast tumors. Initial collections of breast PDX were reported to remain phenotypically identical to human tumors during serial passages (DeRose et al., 2011; Zhang et al., 2013). However, in agreement with our findings, there is increasing evidence that tumors in PDX are not “static” and can evolve, as observed in patients (Eirew et al., 2015). Our PDX models constitute a unique tool to investigate resistance in cancer as they mimic clinical responses: TNBC tumors are more sensitive to chemotherapy than the luminal tumors, confirming previous clinical results (Berry et al., 2006; Colleoni et al., 2004; Guarneri et al., 2006; Martin et al., 2011), and even initially sensitive tumors develop resistance upon continuous exposure to taxanes. Both basal-like tumors (IDB-01 and IDB-02) derived from metastatic samples that were heavily exposed to multiple treatments including taxanes showed minimal clinical response. Strikingly, sensitivity to docetaxel was restored upon xenografting and was retained for months. Moreover, we observed that in PDX tumors with acquired resistance, sensitivity is partially restored when maintained in the absence of the drug. This regain of sensitivity, the so-called “drug holiday,” has been described for targeted therapies in melanoma (Das Thakur et al., 2013; Sun et al., 2014). We now demonstrate that the same is true for cytotoxics such as docetaxel, with important implications for clinical decisions and drug scheduling, as resistant metastatic disease may benefit from intermittent docetaxel treatment. Our data demonstrate that a pre-existing and chemoresistant CD49f+ subpopulation is present in most sensitive TNBC, expands during long-term therapy, and has the ability to generate novel tumors contributing to recurrence and acquisition of chemoresistance (as shown in the graphical abstract), and importantly that this population shrinks again in the absence of taxanes, restoring drug sensitivity. Previous reports have also shown the increased tumor-initiating ability of CD49f+ cells in breast and other solid tumors (Haraguchi et al., 2013; Lo et al., 2012; Meyer et al., 2010; Vassilopoulos et al., 2014). These findings do not imply that the CD49f+ cells are the CSC in TNBC, but demonstrate that the CD49f+ population is associated with taxane resistance. These findings can be clinically validated in the neoadjuvant setting, evaluating whether an enrichment of the CD49f population is observed in residual disease following taxane-based chemotherapy. However, as the rates of pCR in TNBC are high (30%–40%), a dynamic study of early changes in the CD49f population after the first cycles of taxane treatment and occurrence of pCR could be a better approach. The clinical utility of the biomarker could be tested in a prospective clinical trial in the neoadjuvant setting where patients are randomized based on the biomarker modulation to change treatment or continue with taxane-based therapy. Improvement of clinical outcomes (pCR rates or survival) should be the final objective. The effect of novel drugs can be evaluated in the subgroup of chemoresistant CD49f-enriched TNBC. In clinical series the presence of CD49f+ in breast cancer is associated with a poor clinical outcome (Friedrichs et al., 1995; Ye et al., 2015). Moreover, within several CSC markers (CD44, CD24, ALDH1A3, and CD49f) analyzed by IHC in breast cancer samples, only CD49f retained prognostic value in a multivariate analyses in ER– disease (Ali et al., 2011). Our results provide a functional rationale for the poor outcome associated with CD49f expression in hormone receptor-negative breast cancer. Further studies will reveal whether this population can be manipulated in order to unveil the ever-elusive status of tumor drug resistance and recurrence.