The goal of targeted therapy is to match a selective drug with a genetic lesion that predicts for drug sensitivity. ALDH as a marker of cancer stem cells (CSCs) in MPM a cell population thought to mediate tumor relapse after chemotherapy. Whereas pemetrexed and cisplatin standard-of-care agents for MPM enrich for CSCs FAK inhibitor treatment preferentially eliminates these cells. These preclinical results provide the TCN 201 rationale for a clinical trial in MPM patients using a FAK inhibitor as a single agent after first-line chemotherapy. With this design the FAK inhibitor could potentially induce a more durable clinical response due to reduction of CSCs along with a strong antitumor effect. Furthermore our data suggest that patients with Merlin-negative tumors may especially benefit from FAK inhibitor treatment. Introduction Focal adhesion kinase (FAK) is an important cancer target because gene amplification and protein overexpression have been demonstrated in a wide range of malignancies (1). FAK is a non-receptor protein tyrosine kinase that integrates signals from integrins and growth factor receptors to regulate cell proliferation survival migration invasion and cancer stem cell (CSC) renewal (1-3). FAK inhibitors have been shown to decrease tumor growth and metastasis in preclinical models and have shown initial clinical activity in cancer patients (4-6). Although TCN 201 elevated FAK expression is often observed in human tumors no specific mutations or translocations have been identified to predict TCN 201 which patient population is most likely to respond to a FAK inhibitor. Successful targeted therapies that pair small molecule inhibitors with specific activated oncogenes include agents targeting and translocations gene amplification and activating mutations in EGFR and B-RAF (7). Alternatively identification of a synthetic lethal relationship between a drug target and loss of a tumor suppressor is exemplified by the efficacy of PARP inhibitors in breast cancer bearing or mutations (7). An analogous therapeutic strategy could greatly facilitate the clinical development of a FAK inhibitor. The neurofibromatosis type 2 (contribute to development of type 2 neurofibromatosis which is characterized by growth of meningiomas ependymomas and schwannomas (12). In addition is frequently inactivated in human malignant pleural mesothelioma (MPM) where biallelic inactivation IL13 of occurs in 40-50% of tumors (12 13 MPM is an aggressive tumor of the pleural lining of the lung and is often associated with prior exposure to asbestos (13). It has been estimated that as many as 43 0 people worldwide die from MPM each year (14). Median overall survival following frontline chemotherapy with pemetrexed and cisplatin is approximately 12 months (15). New therapies are urgently needed to improve the prognosis of TCN 201 patients with MPM. Cancer stem cells (CSCs) comprise a subpopulation of tumor cells that possess self-renewal capacity exhibit elevated resistance to chemotherapeutic agents and are often responsible for tumor recurrence (16). CSCs have been identified in many cancer types including colorectal breast ovarian pancreatic prostate and head and neck cancers (17). Several studies found cells with CSC properties in MPM (18 19 Moreover an elevated CSC population has been demonstrated in a mouse model of aggressive NF2-deficient asbestos-induced mesothelioma (20). FAK plays a role in self-renewal tumorigenicity and maintenance of mammary CSCs (2). Therefore therapeutic targeting of FAK may diminish CSCs in a variety of malignancies including MPM. In the present study we aimed to identify cancers most sensitive to FAK inhibition and discover biomarkers to identify patients most likely to benefit from a FAK inhibitor treatment. VS-4718 previously known as PND-1186 (21) is a potent and selective FAK inhibitor (Fig. S1). We found that VS-4718 is especially effective against Merlin-negative cell lines of certain cancer types including MPM and and have uncovered a mechanism governing sensitivity to the FAK inhibitor. The preferential inhibitory effect of VS-4718 on CSCs in addition to.