Abstract C262: Malignant mesothelioma lacking merlin shows enhanced sensitivity to the FAK inhibitor defactinib (VS-6063): Elucidation of the merlin-FAK relationship.
Authors: Shapiro Irina M. , Kolev Vihren N. , Kadariya Yuwaraj , Vidal Christian M. , Wright Quentin G. , Ring Jennifer E. , Menges Craig , Testa Joseph R. , Xu Qunli , Pachter Jonathan A. 
Published: August 2014

Abstract
Malignant pleural mesothelioma (MPM) is an aggressive tumor in the lining of the lung often resulting from prior exposure to asbestos. Median overall survival with the standard of care chemotherapy is only 12 months from diagnosis. Thus, new therapeutic modalities are urgently needed to improve the prognosis of MPM patients. ∼50% of MPM patients exhibit biallelic inactivation of the NF2 tumor suppressor gene by mutation and/or deletion resulting in lack of expression of functional merlin protein. Merlin inactivation is a critical step in MPM tumorigenesis and is related, at least in part, to up-regulation of focal adhesion kinase (FAK) activity. FAK has also been demonstrated to play a role in maintenance of cancer stem cells (CSCs), a subpopulation of tumor cells associated with drug resistance and tumor recurrence. Therefore, therapeutic targeting of FAK using defactinib (VS-6063), a potent, orally active FAK kinase inhibitor, may benefit cancer patients through targeting of both CSCs and bulk tumor cells.
Among a panel of MPM cell lines in 3-dimensional Matrigel culture, MPM lines lacking expression of merlin were found to be especially sensitive to defactinib. In contrast, MPM cell lines with wild-type merlin expression were less sensitive with average EC50 values of 5.1 μM. Oral dosing of defactinib induced significant tumor growth inhibition in a merlin-negative MPM model pre-implanted in the lungs of mice. Using Aldefluor activity as a marker for CSCs in MPM, we find that chemotherapeutic agents enriched for Aldefluor-positive CSCs, while the FAK inhibitor defactinib reduced the proportion of CSCs. When used in combination with pemetrexed or cisplatin, defactinib blocked the enrichment of CSCs cells by these chemotherapeutic agents.
Disruption of cell-cell or cell-extracellular matrix (ECM) contacts with blocking antibodies allowed us to propose a molecular model to account for the observed merlin-FAK inhibitor synthetic lethal relationship. We find that weak cell-cell adhesions in merlin-negative MPM cells lead to a greater dependence of tumor cell survival and proliferation on cell-ECM-induced FAK signaling rendering merlin-negative cells especially vulnerable to FAK inhibitor treatment. Taken together, these data support the clinical development of defactinib in an ongoing Phase 2 registration-directed clinical trial evaluating the activity of defactinib in MPM patients (stratified by merlin status) with a confirmed response (PR or SD) following first line platinum/pemetrexed therapy.
Citation Information: Mol Cancer Ther 2013;12(11 Suppl):C262.
Citation Format: Irina M. Shapiro, Vihren N. Kolev, Yuwaraj Kadariya, Christian M. Vidal, Quentin G. Wright, Jennifer E. Ring, Craig Menges, Joseph R. Testa, Qunli Xu, Jonathan A. Pachter. Malignant mesothelioma lacking merlin shows enhanced sensitivity to the FAK inhibitor defactinib (VS-6063): Elucidation of the merlin-FAK relationship. [abstract]. In: Proceedings of the AACR-NCI-EORTC International Conference: Molecular Targets and Cancer Therapeutics; 2013 Oct 19-23; Boston, MA. Philadelphia (PA): AACR; Mol Cancer Ther 2013;12(11 Suppl):Abstract nr C262.