Abstract A37: MerTK receptor tyrosine kinase is a therapeutic target in pediatric acute leukemia
Authors: Graham Douglas K. , DeRyckere Deborah , Lee-Sherick Alisa , Hill Amanda A. , Minson Katherine , Xiaodong Wnag , Zhang Weihe , Liu Jing , Frye Stephen , Earp H. Shelton 
Published: May 2015

Abstract
Acute myeloid leukemia (AML) and relapsed or refractory acute lymphoblastic leukemia (ALL) continue to be extremely difficult to treat and unacceptably low overall survival rates mandate the exploration of new potential therapies to ameliorate poor clinical response to conventional therapy. Abnormal tyrosine kinase activation has been associated with poor prognosis in patients with AML and ALL, providing strategic targets for novel therapy development. The MerTK receptor tyrosine kinase is overexpressed in a majority of leukemia cell lines and pediatric patient samples at the time of diagnosis and expression is often upregulated at relapse. In contrast, normal bone marrow myeloid and lymphoid precursors express little or no MerTK. Stimulation of leukemia cell lines with Gas6, a MerTK ligand, activates prosurvival and proliferative signaling pathways known to contribute to leukemogenesis, including ERK1/2, p38, MSK1, CREB, ATF1, AKT and STAT6. Inhibition of MerTK using shRNA significantly increased myeloblast apoptosis in response to serum starvation in the Nomo-1 and Kasumi cell lines and decreased colony forming potential. Importantly, these phenotypes were reversed by exogenous expression of wild-type MerTK protein but not by expression of a mutant MerTK that lacks kinase activity, indicating that oncogenic roles for MerTK are mediated its kinase activity. Additionally, NOD-scid-gamma mice transplanted with Nomo-1 myeloblasts or 697 lymphoblasts with reduced levels of MerTK have a significantly prolonged median survival time compared to mice transplanted with control cell lines. These data validate Mer as a potential therapeutic target in acute leukemias. We have recently developed first-in-class MerTK-selective small molecule tyrosine kinase inhibitors (MerTKIs). Treatment with MerTKI phenocopied the effects of shRNA, including inhibition of prosurvival signaling through ERK1/2, Akt, and STAT6. Flow cytometric analysis of myeloblast cell lines following treatment with MerTKI and staining with YoPro-1-iodide and propidium iodide dyes revealed induction of apoptosis in Mer-positive leukemia cell lines (mean of 66±10% apoptotic and dead cells verses 20±10% after treatment with vehicle, p&lt;0.01). In an orthotopic B-ALL murine xenograft model of minimal residual disease, treatment with MerTKI significantly inhibited leukemogenesis and increased median survival from 25 days after inoculation with tumor cells to 65 days (p &lt; 0.0001). In a similar model in which leukemia was established for 11 days prior to initiation of treatment, median survival increased from 25 to 47 days in response to treatment with MerTKI (p &lt; 0.0001). In both models, tumor burden measured by bioluminescent imaging was significantly decreased in mice treated with MerTKI relative to mice treated with vehicle, even after the development of advanced disease in the control animals (0.27 +/- 0.04 x107 photons/second in the minimal residual disease model and 0.98 +/- 0.23 x107 photons/second in the established disease model verses 26.75 +/- 4.54 x107 photons/sec in vehicle-treated mice, p &lt; 0.0001). These data indicate roles for MerTK in pediatric acute lymphoid and myeloid leukemias and validate targeted inhibition of MerTK as a potential therapeutic strategy.
Citation Format: Douglas K. Graham, Deborah DeRyckere, Alisa Lee-Sherick, Amanda A. Hill, Katherine Minson, Wnag Xiaodong, Weihe Zhang, Jing Liu, Stephen Frye, H. Shelton Earp. MerTK receptor tyrosine kinase is a therapeutic target in pediatric acute leukemia. [abstract]. In: Proceedings of the AACR Special Conference on Pediatric Cancer at the Crossroads: Translating Discovery into Improved Outcomes; Nov 3-6, 2013; San Diego, CA. Philadelphia (PA): AACR; Cancer Res 2013;74(20 Suppl):Abstract nr A37.