DIPG-05. PRECLINICAL EFFICACY OF MELK INHIBITION IN DIFFUSE INTRINSIC PONTINE GLIOMA

Abstract Maternal embryonic leucine zipper kinase (MELK) is a highly overexpressed kinase in diffuse intrinsic pontine glioma (DIPG). Inhibition of MELK by the small molecule OTSSP167 effectively reduces proliferation and induces cell death in primary DIPG cell lines at low nanomolar concentrations. RNA sequencing of DIPG cells treated with OTSSP167 reveals upregulation of genes associated with the PPAR transcription factors as well as upregulation of PPARγ target genes. Western blot and immunofluorescence assays demonstrate reduced inhibitory phosphorylation and increased nuclear localization of PPARγ upon MELK inhibition. Consequently, combined treatment of DIPG cells with OTSSP167 and PPARγ agonists synergistically reduces cell survival. Pharmacokinetic analyses reveal OTSSP167 to be a strong substrate of both MDR1 and BCRP, multidrug transporters on the blood-brain barrier, limiting the brain bioavailability of the compound. To demonstrate the potential of MELK inhibition for the treatment of DIPG, Mdr1a/b -/-; Bcrp1-/- athymic nude mice were xenografted with primary DIPG tumors and treated with a low dose of OTSSP167 for six weeks. In these models, treatment of the mice with OTSSP167 resulted in a significant delay in tumor growth, induction of remissions in almost half of the mice, and significantly prolonged survival of the xenograft-bearing mice. Efficacy was comparable among pre-treatment biopsy-derived and post-treatment autopsy-derived xenograft models. These strong preclinical therapeutic effects of OTSSP167 demonstrate the potential of MELK inhibition for the treatment of DIPG and encourage the development of BBB-penetrable MELK inhibitors.