Abstract 2535: KX-01, a novel Src kinase inhibitor directed towards the peptide substrate site, induces robust apoptosis and synergizes with tamoxifen and chemotherapy in breast cancer

Proceedings: AACR 101st Annual Meeting 2010‐‐ Apr 17‐21, 2010; Washington, DC New therapeutic regimens are needed to increase efficacy and reduce resistance of endocrine therapy and chemotherapy in breast cancer. c-Src is an oncogenic non-receptor tyrosine kinase that is up-regulated in approximately half of all breast cancer. However the efficacy of existing multi-kinase src inhibitors in breast cancer has been limited. KX-01 (Kinex Pharmaceuticals) is a novel class of clinical non-ATP src inhibitor that targets the peptide binding site. In a panel of breast cancer cell lines, KX-01 resulted in dose dependent inhibition of growth and induction of apoptosis that was independent of p53 status, and was preceded by rapid inhibition of src activity. KX-01 induced apoptosis in MDA-MB-468 cells and BT-549 cell lines reported to be resistant to multi-kinase src inhibitors. KX-01 (50 nM, 6 hours) resulted in significant accumulation of MDA-MB-231 cells (ERα−/PR−/HER2/neu−) and MCF-7 cells (ERα+) in G2/M phase. Immunofluorescent staining for mitotic phase marker phospho-histone 3 indicated that cells had arrested in mitotic phase and many of the mitotic arrested cells were undergoing apoptosis (TUNEL), a novel cell death for a small molecule tyrosine kinase inhibitor. KX-01 induced nuclear accumulation of cyclin B1 and activation of CDK1, MPM2 and Cdc25C that is required for progression past the G2/M checkpoint. KX-01 resulted in cytochrome C release and activation of caspases 6, 7, 8 and 9. Combinations of KX-01 (5-75 nM) with tamoxifen (TAM), paclitaxel (PAC) or doxorubicin (DOX) resulted in synergistic growth inhibition of MCF-7 cells (KX-01 + TAM) and MDA-MB-231 cells (KX-01 + DOX or PAC). In addition, synergistic induction of apoptosis was achieved with combination of KX-01 with DOX, PAC or TAM. c-Src induces phosphorylation of ERα at serines 118 and 167, sites required for full ERα activity. KX-01 combined with TAM resulted in decreased serine 167 phosphorylation and reduced ERα transcriptional activity. In tumor xenografts, KX-01 resulted in a dose dependent inhibition of MDA-MB-231 and MCF-7 tumor growth (1, 2.5 or 5 mg/kg body weight, twice daily by oral gavage). KX-01 exhibited synergistic growth inhibition of MCF-7 tumor xenografts when combined with tamoxifen and caused regression of MDA-MB-231 tumor xenografts when combined with paclitaxel or doxorubicin. Immunohistochemical (IHC) analysis in MCF-7 xenografts revealed that KX-01 + TAM induced robust apoptosis and inhibited neovascularization. KX-01 reduced metastasis to bone (femur) and lung as measured by PCR for detection of human chromosome 17. Interestingly, KX-01 resulted in reexpression of ERα and E-cadherin in MDA-MB-231 xenografts. These data define KX-01 as a potent and unique src kinase inhibitor that synergizes with endocrine therapy and chemotherapy to induce robust cell death, tumor growth inhibition/regression and inhibit metastasis. Citation Format: {Authors}. {Abstract title} [abstract]. In: Proceedings of the 101st Annual Meeting of the American Association for Cancer Research; 2010 Apr 17-21; Washington, DC. Philadelphia (PA): AACR; Cancer Res 2010;70(8 Suppl):Abstract nr 2535.