DUAL INHIBITION OF HISTONE DEACETYLATION AND PI3K PATHWAY RADIOSENSITIZES PEDIATRIC HIGH-GRADE GLIOMAS BY BLOCKING RADIATION INDUCED DNA DAMAGE RESPONSES

INTRODUCTION: We sought rational targeted agents that exhibit single agent activity and sensitize pediatric high-grade gliomas (pHGG) to radiation (RT). We investigated a novel dual inhibitor of class I and II histone deacetylases and class I PI3Ks in combination with RT. METHODS: pHGG cells including DIPGs were treated with CUDC-907 alone or in combination with radiation and effects on cell viability, cell cycle, apoptosis, DNA damage, DNA damage repair pathways were determined. Homologous recombination (HR) and non-homologous end joining (NHEJ) were assessed by functional repair assays and loss of formation/resolution of RAD51 (HR) and phospho-DNA-PKCs (NHEJ) foci, respectively. In vivo evaluation of CUDC-907 efficacy was performed using SF8628 (DIPG xenograft) cells implanted in mice brainstems and randomized to: (1) control, (2) RT (0.5 Gy x 6, M-W-F), (3) CUDC-907 (100 mg/kg, oral gavage, daily x 2 weeks), (4) CUDC907+concurrent RT. RESULTS: G1 cell cycle arrest, induction of apoptosis, and accumulation of double strand DNA breaks (DSB) as measured by g-H2AX contribute to CUDC-907-induced radiosensitization. CUDC-907 attenuates radiation-induced DNA damage responses and repressed DSB repair. Specifically, CUDC-907 impairs the key DSB repair pathways of HR and NHEJ through reduced expression of RAD51 and Ku proteins, respectively. CUDC-907-mediated down-regulation of DNA damage responses is partly transcriptional and is driven by RelB-p50-based NFkB complex and FOXM1, activators critical for expression of key DSB repair genes. Following irradiation, RelB-p50-FOXM1 recruitment on the promoters of RAD51 and KU70/80 is enhanced to induce DSB repair, and CUDC-907 reduces RelB-p50-FOXM1 binding at the promoters thus interfering with DSB repair. Radiosensitization by CUDC-907 is also observed in vivo in mice-bearing DIPG xenografts where CUDC907+RT combination treatment significantly prolongs survival compared to monotherapy (p<0.05 for combination vs CUDC-907, RT, or control). CONCLUSION: CUDC-907 impairs key DNA damage repair pathways, augments radiation-induced DNA damage, enhances apoptosis, and radiosensitizes pHGG.