L lines of four different subtypes (Additional file 1). Results of the present study have demonstrated that dasatinib effectively inhibited phosphorylation of c-Src in all cell lines tested (Figures 1, 2 and 3), and preferentially inhibited the growth of breast cancer cells of the basal B subtype (Additional file 2). The IC 50 s of dasatinib in the basal B breast cancer cell lines were approximately 4-6 times lower than the trough concentration (0.6 M) of dasatinib in humans [6]. These findings coincide with those reported by two independent groups [5,6]. To test a hypothesis that dasatinib may enhance antitumor activity of chemotherapeutic agents, we first examined the growth inhibitory effects of seven anticancer agents commonly used for the treatment of breast cancer. According to the IC 50 for each agent, breast cancer cells of the basal B subtype seemed to be more sensitive to DNA-damaging agents such as Eto, PubMed ID:https://www.ncbi.nlm.nih.gov/pubmed/28667899 Dox, Cis, Carb and SN38 than those of other subtypes, in particular, the luminal A and luminal B subtypes (Additional file 2). These findings may be partly explained by the facts that a loss of BRCA1 expression is frequently observed in breast cancer cells of the basal B subtype, and BRCA1 dysfunction enhances antitumor activity of DNA-damaging agents in breast cancer cells. Actually, BRCA1 expression levels measured by the immunocytochemical assay were significantly lower in three basal-like breast cancer cell lines (Additional file 1). One of the DNA-damaging agents, Eto has long been used in the treatment of various malignancies such as lung cancer [26]. Recent clinical studies have also demonstrated that oral Eto in combination with intravenous Cis has a significant antitumor effect on metastatic breast cancer [27]. Since both dasatinib and Eto can be administered orally, breast cancer patients could be treated with these agents without intravenous injections. These findings prompted us to investigate a combined antitumor activity of dasatinib with Eto in breast cancer cells of the basal B subtype. Dasatinib (0.1 M) additively enhanced antitumor activity (Figures 4, 5, 6 and 7). To elucidate the mechanism of action of combined treatment with dasatinib and Eto, the effects of single or combined treatments on cell cycle progression andKurebayashi et al. BMC Cancer 2010, 10:568 http://www.biomedcentral.com/1471-2407/10/Page 8 ofFigure 14 Effects of dasatinib and/or Eto on the proportion of ALDH1-positive cells in breast cancer cells. The Aldefluor assay demonstrated that dasatinib alone and dasatinib with Eto decreased the proportion of ALDH1-positive cells in MDA-MB-231 cells.induction of apoptosis were examined. Unexpectedly, Eto did not show any Lurbinectedin site additive effect to the treatment with dasatinib alone on G1-S cell cycle retardation and increased apoptosis (Figures 8 and 9). In addition, the immunocytochemical and Aldefluor assay showed that Eto did not enhance the decreased proportion of ALDH1-positive cells by dasatinib alone (Figures 10, 11, 12, 13, 14 and 15). These findings suggest that the additive antitumor effect with dasatinib and Eto is unlikelyto be due to enhanced cell cycle retardation, increased apoptosis or decreased proportion of ALDH1-positive, putative cancer stem cells. Further studies are needed to elucidate the mechanism of action responsible for possible additive antitumor activity with dasatinib and a chemotherapeutic agent. Otherwise, similar additive antitumor interactions were observed in the combinat.