Abstract LB-B06: Functionally defined therapeutic targets in diffuse intrinsic pontine glioma

Abstract

Abstract Diffuse intrinsic pontine glioma (DIPG) is a fatal childhood cancer that has a limited response to treatment (median survival after diagnosis is only 9 months). Historically, DIPG research has been limited by a dearth of tumor tissue available for study and a lack of experimental model systems. Recently, both cell cultures derived from patients with DIPG and orthotopic xenograft models have been established. We performed a chemical screen in 16 patient-derived DIPG cultures using 83 drugs, selected by pediatric neurooncologists as either promising small-molecule compounds or traditional chemotherapeutic agents. Of these 83 drugs, 14 showed activities against three or more DIPG cultures. Notable ‘misses’ that highlight the resistance of this tumor to traditional chemotherapies included temozolomide, carboplatin, and vincristine, while DIPG cell cultures were reported to have substantial sensitivity to histone deacetylase inhibitors, consistent with the highly recurrent H3K27M driving mutations in ∼80% of DIPG. Panobinostat was one of the most effective drugs screened, with 12/16 DIPG cultures showing sensitivity to this drug. Compared to vorinostat, another HDAC inhibitor, panobinostat exhibited substantially greater potency against DIPG cells. Treatment of DIPG cultures with panobinostat yielded a time and dose-dependent decrease in viability (RNA-mediated knockdown of HDAC1 and HDAC2 confirmed the mechanism of action). Treatment of DIPG cultures with panobinostat also increased H3 acetylation and H3K27 trimethylation, and led to a partial rescue of the H3K27M induced global hypotrimethylation phenotype. Increased K27 trimethylation was an unexpected effect of the drug, but it is consistent with recent findings that acetylated H3K27 can ‘detoxify’ K27M-induced inhibition of PCR2. RNA-seq performed on panobinostat- or vehicle-treated DIPG cells revealed sweeping changes in gene expression, including normalization of the K27M gene expression signature and decrease of the oncogenic MYC target gene-expression signature. In addition, the multi-histone deacetylase inhibitor panobinostat demonstrated efficacy in DIPG orthotopic xenograft models administered using convection enhanced delivery. Combination testing of panobinostat and the histone demethylase inhibitor GSK-J4, recently shown to decrease viability of mutant DIPG cells, revealed that the two had synergistic effects. Together, these data suggest a promising FDA-approved therapeutic strategy for DIPG that could be rapidly translated for use in the clinic, further indicating that epigenetic modifying therapies are emerging as the most promising class of agents for the treatment of DIPG. Citation Format: Catherine S. Grasso. Functionally defined therapeutic targets in diffuse intrinsic pontine glioma. [abstract]. In: Proceedings of the AACR-NCI-EORTC International Conference: Molecular Targets and Cancer Therapeutics; 2015 Nov 5-9; Boston, MA. Philadelphia (PA): AACR; Mol Cancer Ther 2015;14(12 Suppl 2):Abstract nr LB-B06.