Abstract C29: FAK inhibition targets cancer stem cells
Authors: Kolev Vihren , Wang Yan , Sprott Kam , Shapiro Irina , Ring Jennifer , Pachter Jonathan , Weaver David 
Published: May 2016

Abstract
Targeting cancer stem cells (CSCs) holds promise to address key challenges of cancer treatment: chemo- and radiation therapy resistance, metastasis, and recurrence. Increased CSC abundance after neoadjuvant chemotherapy has been associated with a significantly worse outcome. Combining CSC-targeted agents with chemotherapy may lead to more durable response with increased overall survival.
Focal adhesion kinase (FAK), a non-receptor tyrosine kinase, mediates signal transduction by integrins and growth factor receptors to regulate cellular adhesion, proliferation, migration, and survival. Several studies have demonstrated that FAK expression and kinase activity are necessary for survival and maintenance of CSCs. FAK is also upregulated in many epithelial tumors and associated with poor patient prognosis and therefore has been pursued as a promising therapeutic target for cancer. VS-6063 and VS-4718 are orally bioavailable small molecules that impede CSCs through the inhibition of FAK. Both VS-6063 and VS-4718 preferentially kill CSCs in multiple cancer models, including models of breast, ovarian, SCLC, and mesothelioma, and these agents are in clinical development. We demonstrate that treatment of cancer cell lines or mice bearing xenograft tumors with VS-6063 or VS-4718 decreases CSCs as assessed by decreased tumor-initiating capability in limiting dilution assays. Both FAK inhibitors decrease significantly CSCs to a varying degrees across multiple tumor models. Furthermore, treatment of mice with either FAK inhibitor following cessation of chemotherapy delayed tumor regrowth in xenograft and PDX models of TNBC, SCLC, and mesothelioma. A mechanistic investigation in breast cancer cell lines revealed an important crosstalk between FAK and the Wnt/β-catenin pathway, where FAK inhibition blocked tyrosine-654 phosphorylation and β-catenin activation. Importantly, a constitutively active mutant form of β-catenin “rescued” CSCs, suggesting that preferential targeting of CSCs by FAK inhibitors is mediated, at least in part, through attenuation of downstream β-catenin activation.
In breast cancer, CSCs are identified by the expression of aldehyde dehydrogenase 1 (ALDH1) or CD44-high/CD24-low markers. To probe the CSC populations in breast cancer patients, we developed a multiplex immunofluorescence assay with three CSC markers (ALDH1, CD44, and CD24) and an epithelial marker (pan-cytokeratin) for FFPE tissue. Automated image analysis with pathology review was used to evaluate single CSCs within the tumor. This assay may provide a means of monitoring a neoadjuvant CSC-targeted treatment in breast cancer FFPE samples in clinical trials.
In summary, our results show that FAK inhibitors preferentially target cancer stem cells and delay the regrowth of tumor cells post standard-of-care treatment in mouse models providing rationale for the clinical development of FAK inhibitors to achieve more durable responses for cancer patients.
Citation Format: Vihren Kolev, Yan Wang, Kam Sprott, Irina Shapiro, Jennifer Ring, Jonathan Pachter, David Weaver. FAK inhibition targets cancer stem cells. [abstract]. In: Proceedings of the AACR-NCI-EORTC International Conference: Molecular Targets and Cancer Therapeutics; 2015 Nov 5-9; Boston, MA. Philadelphia (PA): AACR; Mol Cancer Ther 2015;14(12 Suppl 2):Abstract nr C29.