HGG-30. BRAIN PENETRANT HDAC INHIBITOR RG2833 SYNERGIZES WITH LOMUSTINE AND RADIATION TO INDUCE DIPG CELL DEATH

Diffuse intrinsic pontine glioma (DIPG) is driven by epigenetic dysregulation. The pan-HDAC inhibitor panobinostat showed pre-clinical efficacy against DIPG, but was limited by toxicity in clinical trials. RG2833 (RGFP109) is a selective HDAC1/3 inhibitor with established brain penetration. RG2833 reverses temozolomide-resistance in glioblastoma through downregulation of the NFĸB pathway. As this pathway is upregulated in DIPG and may contribute to tumorigenesis, we hypothesized that RG2833 would kill DIPG cells. We found that RG2833 treatment inhibits cell growth in the 4 to 9μM range in both autopsy and biopsy-derived DIPG cell lines (MTS assay for HSJD007 p=0.0004, JHH-DIPG1 p=0.001, SF-7761 p=0.04, SU-DIPG13 p=0.01 by t-test). Western blot confirmed on target activity by increased histone 3 acetylation at IC50 doses. RG2833 induces apoptosis (cPARP Western blot, cleaved caspase 3 immunofluorescence HSJD007 p<0.003, JHH-DIPG1 p=0.0026 by t-test) and slows cell proliferation (phospho-Rb Western blot, BrdU immunofluorescence HSJD007 p=0.008, JHH-DIPG1 p=0.0002 by t-test) in multiple DIPG cell lines. RG2833 disrupts the NFĸB pathway through acetylation of p65, resulting in decreased expression of NFĸB regulated pro-survival genes (Western blot for BCL2, BCL-xL, and XIAP). In a DIPG flank tumor mouse model, treatment with RG2833 alone for 1 week suppresses flank tumor growth (p< 0.005 by t-test) and induces apoptosis (Western blot for cPARP). We next assessed whether RG2833 combines synergistically with conventional therapies. We found that RG2833 has strong synergism with both lomustine and radiation to slow DIPG cell growth in vitro (ZIP synergy scores by SynergyFinder for RG2833+lomustine in JHH-DIPG1 17.8 and HSJD007 17.7, RG2833+radiation in JHH-DIPG1 9.7 and JHH-DIPG16A 10.9). Evaluation of combination treatment for apoptotic effects in vitro and in vivo are ongoing. This data identifies selective HDAC inhibitor RG2833 as a promising therapy for DIPG that combines synergistically with conventional therapies.