Abstract P6-15-17: Synergy of Ixabepilone and Dasatinib in Triple Negative Breast Cancer Cell Lines

Abstract

Abstract Background: Treatment options are very limited for breast cancers that do not express HER2 or estrogen and progesterone receptors (Triple Negative Breast Cancer; TNBC). Dasatinib is an orally active small molecule inhibitor of the src kinase. It has been proposed that dasatinib inhibits growth of breast cancer cells by modulating epidermal growth factor receptor (EGFR) signaling. Approximately 70% of breast cancers over express EGFR family members as well as c-Src. Src is co-expressed with at least one of the EGFRs. Both src and EGFR1 expression have been associated with TNBC. Dasatinib has previously shown activity as a single agent in TNBC cell lines. However, dasatinib has limited single agent activity in a cohort of chemotherapy refractory TNBC patients according to a phase I/II study. We believe that dasatinib may sensitize breast cancer cells to anti-mitotic agents. Ixabepilone is a potent anti-mitotic agent that has shown significant response in TNBC patients, both in the neoadjuvant setting and in the metastatic setting, including those cancers that are taxane refractory. Therefore, combining these two drugs may have utility in this setting and may lead to inhibition of tumor cell proliferation, invasion, metastasis, angiogenesis, and tumor cell survival. Objectives: To study the activity of dasatinib combined with Ixabepilone in breast cancer cell lines. Methods: A panel of 8 breast cancer cell lines was used in these experiments. These included MCF-7(ER+), T-47D (ER +), SKBR3 (HER2+), and five triple negative cell lines: BT-549, HS578T, MDA-MB-157, MDA-MB-231, MDA-MB-436. First, these cell lines were evaluated for src expression by Western blot as well as sensitivity to dasatinib alone, ixabepilone alone and then the combination of these two drugs using a high-throughput, 384- well CellTiter-Blue cell viability assay. Percent viability was calculated from four replicate cell wells after 72-hour drug incubation and IC50s were determined. Experiments were repeated a minimum of three times. Synergistic, additive, or antagonistic effects were determined using the combination index (CI) method developed by Chou and Talalay. Results: TNBC cell lines expressed higher levels of src protein than cell lines with positive expression of HER2, ER and/or PR by Western blot analysis. Viability assays suggested a correlation between src expression and dasatinib IC50. Combinations of dasatinib and ixabepilone were determined to have combinational indexes <1 in all cell lines tested, suggestive of synergistic activity. Conclusion: This study demonstrates that in a range of breast cell lines with varying levels of src expression, dasatinib combined with ixabepilone is a synergistic combination and may prove to be beneficial to patients with very limited treatment options. Figure available in online version. Citation Information: Cancer Res 2010;70(24 Suppl):Abstract nr P6-15-17.