Abstract 952: Preclinical efficacy of maternal embryonic leucine-zipper kinase (MELK) inhibition in acute myeloid leukemia
Authors: Alachkar Houda , Mutonga Martin , Chung Suyoun , Matsuo Yo , Stock Wendy , Nakamura Yusuke 
Published: May 2015

Abstract
Maternal embryonic leucine-zipper kinase (MELK), a member of the serine-threonine kinases snf1/AMP-activated protein family is involved in mammalian embryonic development. MELK is aberrantly upregulated in several types of solid cancer including glioblastoma and breast cancer, and implicated in formation and maintenance of cancer stem cells. Little is known about the relevance of this kinase in hematological malignancies. Our study aimed to explore the role of MELK in acute myeloid leukemia (AML) and identify whether targeting this kinase in leukemia stem cells may have therapeutic relevance. In order to characterize the expression of MELK in AML, we examined the protein levels in AML cell lines and primary blasts by western blot. MELK protein was expressed at variable levels in 10 out of 11 AML cell lines and primary blasts obtained from AML patients (n=5), representing diverse molecular and cytogenetic abnormalities of the disease. Importantly, we also found that MELK protein levels were higher in CD34+ cells compared with the more differentiated CD34- primary blasts from AML patient. To determine the role of MELK in AML, we utilized a loss of function approach in three AML cell lines (KG1, MV4-11 and U937 cells) and assessed cell viability by MTS. Cells transfected with MELK-siRNA showed significant decrease in cell viability (40-60%, P&lt;0.05) compared with cells transfected with control-siRNA. In addition, knockdown of MELK expression resulted in increased apoptosis in MV4-11 and U937 cells assessed by AnnexinV staining.
OTS167, a small molecule MELK kinase inhibitor is currently undergoing phase I clinical testing in patients with advanced solid malignancies. To examine the anti-leukemia activity of OTS167 in AML, we treated nine AML cell lines with this compound and assessed cells viability by MTS assay. Decrease in cell viability following treatment with increasing doses of OTS167 was time and dose dependent. Variable sensitivities among AML cell lines (IC50∼10-40nM at 48hrs) were observed. In addition, the compound promoted cell differentiation as assessed by CD11b staining in U937 cells. Treatment with (25 and 50nM) of OTS167 induced apoptosis by ∼30 and 40% in MV4-11 and MOLM13 cells at 48 hours post treatment (P=0.01 and 0.05, respectively). Similarly, OTS167 cytotoxicity (IC50 ∼ 10-40nM; n=3) and induced apoptosis was observed in primary AML blasts.
In conclusion, MELK is expressed in AML cell lines and AML blasts with higher levels observed in less differentiated cells. MELK knockdown results in growth inhibition, apoptosis, and cell death suggesting that this kinase may act as a potential therapeutic target. MELK kinase inhibitor (OTS167) causes cytotoxicity in AML cell lines and primary blasts. These results warrant further preclinical evaluation of this compound as a novel targeted therapy for patients with AML.
Citation Format: Houda Alachkar, Martin Mutonga, Suyoun Chung, Yo Matsuo, Wendy Stock, Yusuke Nakamura. Preclinical efficacy of maternal embryonic leucine-zipper kinase (MELK) inhibition in acute myeloid 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 952. doi:10.1158/1538-7445.AM2014-952