Abstract
Outcomes are poor in patients with advanced or relapsed Ewing sarcoma (EWS) and current treatments have significant short- and long-term side effects. New, less toxic and more effective treatments are urgently needed. MER proto-oncogene tyrosine kinase (MERTK) promotes tumor cell survival, metastasis, and resistance to cytotoxic and targeted therapies in a variety of cancers. MERTK was ubiquitously expressed in five EWS cell lines and five patient samples. Moreover, data from CRISPR-based library screens indicated that EWS cell lines are particularly dependent on MERTK. Treatment with MRX-2843, a first-in-class, MERTK-selective tyrosine kinase inhibitor currently in clinical trials, decreased the phosphorylation of MERTK and downstream signaling in a dose-dependent manner in A673 and TC106 cells and provided potent anti-tumor activity against all five EWS cell lines, with IC50 values ranging from 178 to 297 nM. Inhibition of MERTK correlated with anti-tumor activity, suggesting MERTK inhibition as a therapeutic mechanism of MRX-2843. Combined treatment with MRX-2843 and BCL-2 inhibitors venetoclax or navitoclax provided enhanced therapeutic activity compared to single agents. These data highlight MERTK as a promising therapeutic target in EWS and provide rationale for the development of MRX-2843 for the treatment of EWS, especially in combination with BCL-2 inhibitors.