Anti-myeloma activity of MELK inhibitor OTS167: effects on drug-resistant myeloma cells and putative myeloma stem cell replenishment of malignant plasma cells
Authors: A T Stefka, J-H Park, Y Matsuo, S Chung, Y Nakamura, A J Jakubowiak, S Rosebeck
Journal: Blood Cancer Journal (2016)

Currently considered incurable, multiple myeloma (MM) is characterized by proliferation of malignant plasma cells (PC) predominantly in the bone marrow, which overproduce monoclonal immunoglobulin proteins, and a perturbed tumor microenvironment, which promotes PC survival, inhibits osteoblast activity, increases osteoclast activity, and leads to hallmark osteolytic bone disease, all of which contribute to the clinical manifestations of the disease.Disrupting these sequelae by incorporation of proteasome inhibitors and immunomodulatory drugs into treatment regimens has improved overall survival of MM patients; however, the majority of patients will become refractory to the most effective currently available therapeutic options and relapse. 1 It is hypothesized that a drug-resistant population of myeloma stem cells promotes disease relapse, but the characteristics and identity of this cell type(s) remain uncertain. 2Expression of maternal embryonic leucine zipper kinase (MELK) is increased in a number of cancers and is associated with poorer prognosis.MELK activity modulates many cellular and biological processes, including proliferation, apoptosis, hematopoiesis and oncogenesis, and is believed to have a critical role in cancer stem cell maintenance. 3e assessed the expression of MELK mRNA in malignant PC derived from MM patients and human myeloma cell lines (HMCL) and effects of the MELK inhibitor OTS167 on myeloma cells, including drug-resistant subclones.The effects of OTS167 were also tested in an in vitro cell culture model that recapitulates the bone marrow microenvironment and a malignant PC outgrowth model using peripheral blood mononuclear cells (PBMC) from patients with frank MM.MELK gene expression analysis was performed on publically available data sets GSE5900, 4 GSE2658 (refs 5,6) andGSE6477 (ref.7) and demonstrated significantly increased MELK mRNA expression in newly diagnosed MM PC (n = 628) compared with either normal PC (nPC; n = 37; P = 0.0189), monoclonal gammopathy of undetermined significance (MGUS) CD138+ PC (n = 65; Po0.0001), or PC from smoldering MM patients (n = 36; P = 0.0003; Figure 1a).There were no significant differences in MELK expression between nPC and either MGUS or sMM PC.Protein and mRNA expression of MELK were investigated in a panel of 26 patients from whom CD138+ MM PC were derived as well as 11 HMCL.Overall, MELK levels were variable with limited concordance between mRNA and protein (Supplementary Figures 1A andB).Next, we tested the anti-myeloma effects of a potent, smallmolecule inhibitor of MELK kinase activity, OTS167. 8Treatment decreased cell viability in a dose-dependent manner across 11 HMCL, which included dexamethasone-resistant MM1R, doxorubicin/bortezomib/carfilzomib cross-resistant 8226 Dox40 cells and carfilzomib-resistant KMS-34 CFZ cells (Figure 1b). 9-12OTS167 treatment had comparable effects in both parental MM1S and KMS-34 and the resistant subclones MM1R and KMS-34 CFZ.Crossresistant 8226 Dox40 cells, which overexpress the multidrug resistance channel ABCB1, were more resistant than parental 8226