DIPG-15. EFFECTIVE PRECLINICAL TREATMENT OF DIFFUSE INTRINSIC PONTINE GLIOMA BY MELK INHIBITION

AbstractMaternal embryonic leucine zipper kinase (MELK) is the second highest expressed 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 PPAR target genes. Western blot and immunofluorescence demonstrates reduced inhibitory phosphorylation and increased nuclear localization of PPARγ upon MELK inhibition. Combined treatment of DIPG cells with OTSSP167 and PPARγ agonists synergistically reduced cell survival. Given the importance of the blood-brain barrier in DIPG, brain penetration of OTSSP167 was investigated by administration of the compound to wild-type, Abcb1a/b -/-, Abcg2-/- and Abcb1a/b -/-; Abcg2-/- mice. Brain bioavailability was severely reduced by both Abcb1a/b and Abcg2 efflux activity. To demonstrate the potential of MELK inhibition for the treatment of DIPG, Abcb1a/b -/-; Abcg2-/- athymic nude mice were xenografted with primary DIPG tumors and treated with a low dose of OTSSP167 for six weeks. After 15 days of treatment, half the mice carrying DIPG xenografts showed remission of the tumors, with several showing long-term stable disease even after cessation of treatment. To our knowledge, this is the first time treatment of DIPG xenografts resulted in remissions, urging the development of brain penetrable MELK inhibitors for clinical trials.