Abstract 1740: Development of a novel small molecule MER tyrosine kinase inhibitor with therapeutic activity in cell culture and mouse models of acute lymphoblastic leukemia

Proceedings: AACR Annual Meeting 2014; April 5-9, 2014; San Diego, CA In children, acute lymphoblastic leukemia (ALL) is treated with an intense regimen of chemotherapy yielding cure rates near 85%, yet significant hurdles remain, including decreasing toxicity and improving patient outcome in relapsed or refractory disease. Alternative strategies using available drugs are unlikely to provide significant improvements while more targeted therapies may reduce the risk of severe toxicities including infertility, organ damage, and secondary malignancy. We previously demonstrated ectopic expression of MER, a member of the TAM-family of receptor tyrosine kinases, in pediatric B- and T-cell ALL. Using shRNA-mediated MER knockdown, we also demonstrated anti-leukemia effects of MER inhibition in B- and T-ALL models, implicating MER as a novel therapeutic target. Here we report preclinical testing of a novel, first-in-class MER-selective small molecule tyrosine kinase inhibitor (UNC TKI) as a potential therapy for MER-expressing ALL. UNC TKI mediates potent inhibition of MER in enzymatic assays (IC50 = 0.74 nM), has ≥10-fold selectivity for MER over other TAM-family members, and has limited off-target activity against other tyrosine kinases, with the exception of FLT3. In 697 B-ALL cells, UNC TKI inhibited phosphorylation/activation of MER with an IC50 of 2.6 nM and decreased downstream signaling through the ERK and AKT pathways, leading to induction of apoptosis and reduced colony-formation in methylcellulose in MER-expressing ALL cell lines. In mouse models, UNC TKI is orally bioavailable and inhibits MER phosphorylation/activation in leukemic blasts in the bone marrow. In an orthotopic B-ALL xenograft model of minimal residual disease, treatment with UNC TKI resulted in a dose-dependent reduction in tumor burden and increased median survival from 27 days after inoculation with tumor cells to 70 days (p < 0.0001). In a similar model of existent disease in which leukemia was established for 14 days prior to initiation of treatment, median survival increased from 27.5 to 45 days in response to treatment with UNC TKI (p < 0.0001). In both models, tumor burden measured by bioluminescent imaging was significantly decreased in mice treated with UNC TKI relative to mice treated with vehicle, even after the development of advanced disease in the control animals. In addition, treatment with UNC TKI in combination with methotrexate, a chemotherapy that is currently in clinical use for treatment of pediatric ALL, resulted in reduced tumor burden and increased tumor-free survival relative to mice treated with either agent alone. The very high potency, relative selectivity, oral bioavailability, and demonstrated target inhibition and therapeutic efficacy in murine ALL models, both alone and in combination with chemotherapy, identify UNC TKI as an excellent candidate for clinical development in patients with MER-expressing ALL. Citation Format: Deborah A. DeRyckere, Amanda A. Hill, Xiaodong Wang, Weihe Zhang, Michael A. Stashko, Susan Sather, Christopher Cummings, Dmitri Kireev, William P. Janzen, Stephen V. Frye, H. Shelton Earp, Douglas K. Graham. Development of a novel small molecule MER tyrosine kinase inhibitor with therapeutic activity in cell culture and mouse models of acute lymphoblastic leukemia. [abstract]. In: Proceedings of the 105th Annual Meeting of the American Association for Cancer Research; 2014 Apr 5-9; San Diego, CA. Philadelphia (PA): AACR; Cancer Res 2014;74(19 Suppl):Abstract nr 1740. doi:10.1158/1538-7445.AM2014-1740