P1.09-007 Targeting MET/TAM Receptors in Mesothelioma: Are Multi-TKIs Superior to Specific TKI?

Malignant pleural mesothelioma (MPM) is an aggressive inflammatory cancer associated with exposure to asbestos, and most patients die within 24 months of diagnosis. There is an urgent need to identify new therapies for treating MPM patients. Targeting “addicted” receptor tyrosine kinase (RTK) signaling networks has become a critical therapy option in cancer therapy. RTK hetero-dimerization may however, be a key element in the development of resistance to such therapy. As such Tyrosine kinase inhibitors (TKIs) with the ability to target multiple receptors may have superior efficacy to those targeting individual receptors. We and others have identified c-MET, MST1R (also known as RON), Axl and Tyro3 as RTKs frequently overexpressed and activated in MPM, making these attractive candidate targets. Several agents have been developed which target these. LCRF0004 specifically targets MST1R, whereas BMS-777607, RXDX-106 or Merestinib (LY2801653) are orally bioavailable small molecule inhibitors which inhibit c-MET, MST1R, Axl and Tyro3 at nM concentrations. These drugs may therefore have clinical utility in the treatment/management of MPM. Expression of RON/MET/TAM and associated ligands were assessed in a cohort of patient samples and MPM cell lines comprising benign, epithelial, biphasic, and sarcomatoid histologies. In vitro and in vivo experiments were undertaken to determine the efficacy of single and multi RTK targeting agents (LCRF0004, RXDX-106, BMS-777607). The effects of LCRF0004 and BMS-777607 were subsequently examined in an in vivo SQ xenograft tumor model. mRNA expression of the RON/MET/TAM family and associated ligands (MSP, GAS6) was detected in a large panel of normal pleural and MPM cell lines. In a cohort of patient samples, mRNA levels of c-MET, Axl, Tyro3 and various isoforms of MST1R (flRON, sfRON, t-ΔRON) and MSP but not Gas6 or MERTK were increased in tumors compared with benign pleural samples (p<0.05). No MET Exon 14 skipping mutations were detected. RTK targeting agents displayed in vitro efficacy in terms of reduced proliferation. In vivo, the multi-target TKI (BMS-777607) demonstrated superior anti-tumor activity compared with LCRF0004 (MST1R specific compound). IHC analysis of the xenograft tumors showed high cytoplasmic expression of Vimentin, Cytokeratin and Calretinin, with significant necrosis in many. Our data suggests that a multi-TKI, targeting the RON/MET/TAM signaling network, is superior to selective RTK inhibition as an interventional strategy in MPM.