MBRS-19. SYNERGISM OF HDAC AND PARP INHIBITORS IN MYC-DRIVEN GROUP 3 MEDULLOBLASTOMA CELLS

Patients with MYC-driven Group 3 medulloblastoma (MB) show particularly poor outcome. It was previously shown that MYC-driven MBs are highly sensitive to class I histone deacetylase inhibition (HDACi). We studied the molecular effects of the class I HDACi entinostat in MYC-driven MB cells to identify potentially synergistic drug combinations, prioritizing drug clinical availability to enable clinical translation. Gene expression profiles of the MYC-amplified group 3 MB cell line HD-MB03 treated with entinostat were analyzed using bioinformatic approaches, identifying 29 altered biomechanisms. Overlay with a translational drug library of n=76 compounds resulted in 44 compounds targeting 9 biomechanisms. Filtering for publications supporting each drug′s role in MYC-driven entities, or functional interaction with HDACs, without publication of this combination in MBs, resulted in 5 compounds (olaparib, idasanutlin, ribociclib, selinexor, vinblastine). Synergism testing identified olaparib as the drug with the strongest synergism. Validation of the combination olaparib and entinostat by p.H2AX and PI staining as well as trypan blue exclusion showed increased double strand breaks (DSBs), increased cell death, loss of viability and cell numbers. Selectivity of MYC-amplified MB cells was shown by comparison to MYC-non amplified cell lines, which showed higher IC50s, and reacted with cell cycle arrest as opposed to cell death to the combination treatment. The role of HDACis in DNA damage repair was confirmed by increased DSBs when entinostat was added to the combination of olaparib with doxorubicin. Our study identified olaparib as a potential combination partner with entinostat for the treatment of MYC-driven Group 3 MB.