Abstract P2-09-14: Focal adhesion kinase (FAK) inhibitors VS-6063 and VS-4718 target breast cancer stem cells
Authors: Pachter JA , Kolev VN , Wright QG , Vidal CM , Pavdal MV , Ring JE , Shapiro IM , Weaver DT , Keegan M , Horobin JC , Xu Q 
Published: August 2014

Focal adhesion kinase (FAK) is a non-receptor tyrosine kinase that orchestrates cellular signaling through integrins and growth factor receptors and functions in multiple steps of tumorigenesis. Both VS-6063 and VS-4718 are potent, selective, and orally active FAK inhibitors. In a Phase 1 clinical trial, VS-6063 was well tolerated and demonstrated preliminary clinical activity. VS-6063 is currently being tested in multiple clinical studies both as a single agent and in combination with paclitaxel. VS-4718 was shown to inhibit tumor growth and metastasis in preclinical tumor models including triple negative breast cancer and is currently under evaluation in a Phase 1 clinical trial. We report here that the FAK inhibitors VS-6063 and VS-4718 effectively abrogate cancer stem cells (CSCs) in vitro and in vivo in triple negative breast cancer models.

VS-6063 and VS-4718 were evaluated in multiple cancer stem cell assays in vitro. Pre-treatment of SUM159 and MDA-MB-231 breast cancer cells with FAK inhibitors in matrigel reduced the percentage of ALDH-positive cells. The proportion of side population (SP) cancer stem cells was also reduced by FAK inhibition. The expression of ALDH1 in human breast tumor samples has previously been shown to correlate with poor prognosis (Ginestier et al 2007 Cell Stem Cell 1, 555). Significantly, ex vivo treatment of primary human breast tumor tissue with these FAK inhibitors also effectively reduced the proportion of ALDH-positive CSCs. Similar inhibitory effects on CSCs were observed in ovarian cancer cell lines OVCAR-5 and OVCAR-8. In direct contrast, cytotoxic agents such as paclitaxel and carboplatin increased the percentage of cancer stem cells in vitro, suggesting that these agents do not effectively target CSCs. Importantly, combination of either VS-6063 or VS-4718 with cytotoxic agents attenuated the enrichment of cancer stem cells induced by these cytotoxic agents.

The in vivo effect of a FAK inhibitor on cancer stem cells was evaluated in an MDA-MB-231 human breast cancer orthotopic model. Oral administration of VS-4718 significantly reduced the presence of ALDH1-positive cells in tumors as assessed by immunofluorescence. Following dissociation of cells from tumors, CSCs were found to be reduced in VS-4718-treated tumors as measured by multiple assays. Importantly, cells isolated from VS-4718-treated tumors also showed reduced tumor-initiating capability upon re-implantation into immunodeficient mice in limiting dilutions.

Our results provide clear demonstration that while FAK inhibitors significantly inhibit the growth of human breast tumor xenografts, they also preferentially target breast cancer stem cells in direct contrast to standard-of-care agents. These data provide a strong rationale for the clinical development of Verastem's FAK inhibitors in the treatment of breast cancer.

Citation Information: Cancer Res 2013;73(24 Suppl): Abstract nr P2-09-14.