TARGETING REPLICATION STRESS IN PEDIATRIC BRAIN TUMORS

Abstract Previously, we have found that Embryonal Tumors with Multilayered Rosettes (ETMR) tumor cells harboring high levels of R-loops, a potential marker for replication stress and genomic instability, are vulnerable to a combination of topoisomerase and PARP inhibitors. To follow up on this, we investigated whether other pediatric brain tumor types with high levels of R-loops, such as MYC-amplified Group 3 medulloblastoma (MB) and ZFTA-fusion positive ependymoma, are also sensitive to these inhibitors. First, we performed in vitro drug screens using HD-MB03, a Group 3 MB cell line, and the ETMR cell line BT183, and in both screens PARP inhibitors were identified as the most synergistic combination partners for the topoisomerase inhibitor Irinotecan, respectively the active metabolite SN-38. Normal Astrocytes were not sensitive to these combinations. Secondly, we performed in vivo studies using patient-derived xenograft (PDX) models injected subcutaneously or intracranially into NSG mice, and treated with the PARP inhibitor Pamiparib, Irinotecan or a combination of both. For a MYC-amplified Group 3 MB and a ZFTA-fusion positive Ependymoma model, both injected intracranially, treatment with Irinotecan or the combination led to a significant survival benefit and inhibition of tumor growth including transient tumor shrinkage, but addition of Pamiparib did not add any further benefit in vivo, even though intratumoral PARP was inhibited by at least 80%. In contrast, in the subcutaneously injected ETMR model, the combination treatment with Irinotecan and Pamiparib led to a synergistic effect and complete regression of the tumors. Further refinements of the treatment strategy as dose adaptations and the use of a pegylated version of SN-38 (PLX038A) did also not induce a synergistic effect of the drugs for the intracranial tumors. Additional in vivo studies to evaluate the differences in efficacy and whether these are tumor specific or due to incomplete brain penetrance of the drugs are ongoing.