Abstract 1314: MERTK and BCL-2 as potential therapeutic targets in early T-precursor acute lymphoblastic leukemia
Authors: Summers Ryan J. , Minson Katherine A. , Vasileiadi Eleana , Wang Xiaodong , Frye Steven V. , Earp H. Shelton , DeRyckere Deborah , Graham Douglas K. 
Published: August 2019

Abstract
Background: Early T-precursor acute lymphoblastic leukemias (ETP-ALL) account for 15% of pediatric T-cell ALL (T-ALL) cases and are characterized by an immature phenotype, resistance to therapy, and high relapse rates. MERTK receptor tyrosine kinase is ectopically expressed in ~50% of T-ALLs, particularly those with an immature T cell phenotype, suggesting a role in ETP-ALL. The anti-apoptotic protein B-cell lymphoma-2 (BCL-2) is specifically expressed in immature T cell precursors, is preferentially expressed in ETP-ALL compared to other T-ALLs, is essential for ETP-ALL cell survival, and is regulated downstream of MERTK in acute leukemia cells. Thus, combination therapies targeting these two proteins may be particularly effective to treat ETP-ALL.
Methods: MERTK and BCL-2 mRNA expression was assessed in T-ALL patient samples using publicly available data. Loucy and PEER ETP-ALL cell lines were cultured with vehicle or MRX-2843, a dual MERTK/FLT3 inhibitor, alone or in combination with the BCL-2 inhibitor venetoclax. Phosphorylated and total MERTK were assessed by immunoblot. Cells were stained with PoPro-1-iodide and propidium iodide dyes and analyzed by flow cytometry to assess cell death. Relative cell numbers were assessed using Presto Blue reagent. Orthotopic xenografts were established in NSG or NSGS mice using luciferase-expressing Loucy cells or an ETP-ALL patient sample and leukemia burden was monitored by bioluminescence imaging or flow cytometry. MRX-2843 or saline vehicle were administered orally once daily. Median survival was determined by Kaplan-Meier analysis.
Results: MERTK and BCL-2 mRNAs were expressed at significantly higher levels in ETP-ALL patient samples relative to other T-ALLs. MRX-2843 mediated a dose-dependent decrease in phosphorylated MERTK and induced dose-dependent cell death (43.2% vs 16% in vehicle-treated cultures, p&lt;0.01) in PEER cells. Moreover, in a patient-derived ETP-ALL xenograft model, treatment with MRX-2843 reduced peripheral blood disease burden by 83% (p&lt;0.001) and decreased spleen weight by 64% (p&lt;0.001) compared to vehicle-treated mice. Furthermore, treatment with MRX-2843 prolonged median survival by 11 days compared to control mice (n=8/group, p=0.0016). In a cell line-derived xenograft model, MRX-2843 reduced disease burden by 60% (n=10/group, p&lt;0.0001) compared to vehicle. Treatment with a combination of MRX-2843 and venetoclax decreased cell density more effectively than either single agent in cultures of the Loucy and PEER cell lines (p&lt;0.05).
Conclusions: MERTK and BCL-2 are preferentially expressed in ETP-ALL relative to T-ALL and MRX-2843 has robust therapeutic activity in cell culture and xenograft models of ETP-ALL. These data validate MRX-2843 as a novel agent with potential for clinical application in patients with ETP-ALL. Combined targeting of MERTK and BCL-2 may be particularly effective.
Citation Format: Ryan J. Summers, Katherine A. Minson, Eleana Vasileiadi, Xiaodong Wang, Steven V. Frye, H. Shelton Earp, Deborah DeRyckere, Douglas K. Graham. MERTK and BCL-2 as potential therapeutic targets in early T-precursor acute lymphoblastic leukemia [abstract]. In: Proceedings of the American Association for Cancer Research Annual Meeting 2019; 2019 Mar 29-Apr 3; Atlanta, GA. Philadelphia (PA): AACR; Cancer Res 2019;79(13 Suppl):Abstract nr 1314.