EXTH-17 - Combination of HDAC inhibitor and PI3K/mTOR inhibitor synergistically induces apoptosis in DIPG

Diffuse Intrinsic Pontine Glioma (DIPG) is a pediatric brain tumor characterized by epigenetic dysregulation with median survival of 9-11 months. Currently, radiation is the only therapy option, and therefore calls for a new, effective treatment method. RG2833 is a brain penetrant, selective HDAC 1/3 inhibitor. Western blot confirmed that RG2833 treatment alone downregulates the NFkB pathway in multiple DIPG cell lines. We showed that RG2833 treatment decreases expression of NFkB regulated anti-apoptotic genes XIAP, BCL-2, and BCL-XL in vitro. In vivo results in DIPG flank tumors confirmed that BCL-2 and BCL-XL expression are decreased with RG2833 treatment. We have also confirmed through Western blot that RG2833 treatment induces p16, p21, and p27 expression, which negatively regulate cell cycle progression, in multiple DIPG cell lines. RNAseq data showed that PI3K/mTOR pathway is upregulated in DIPG after RG2833 treatment. Paxalisib is a brain penetrant PI3K/mTOR inhibitor currently in clinical trials for DIPG. We hypothesized that RG2833 and paxalisib combination treatment would yield synergistic effects. RG2833 synergizes with paxalisib to suppress growth as measured by CellTiterBlue. ZIP synergy scores calculated by SynergyFinder were 10.5 for SUDIPG06 and 9.2 for HSJD007. Western blot demonstrated that combination treatment induced c-PARP and FOXO1 in the SUDIPG06 cell line, and decreased p-RB expression in HSJD007 cell line. Induction of c-PARP indicates increased apoptosis, and decreased p-RB expression indicates decreased cell proliferation. In vivo experiments evaluating the efficacy of combination treatment are in progress. Our data suggest the combination of RG2833 and paxalisib may be a promising treatment option for DIPG.