Abstract 5111: Rational synergistic radiosensitization strategies for group 3 medulloblastoma

Abstract Medulloblastoma (MB) is the most common malignant pediatric brain tumor. MB is comprised of 4 groups, termed Wnt, Shh, Group 3 and Group 4, each characterized by distinct molecular and clinical profiles. MYC-amplified Group 3 MB is associated imminent fatal metastatic relapse following standard treatment approaches, which currently include surgery, craniospinal irradiation and chemotherapy, highlighting an urgent need for identification of therapeutic vulnerabilities for these tumours. As such, using 4 novel patient-derived models of Group 3 MB that faithfully recapitulate the molecular and histological tumor characteristics, we characterised response to radiation of Group 3 MB in vitro to show that these tumors do not possess intrinsic radiation resistance. Furthermore, cumulative radiation experiments do not increase bulk radiation resistance in MB cells, highlighting the contribution of dynamic cell states to radiation response. In contrast, exposure to radiation leads to changes in cell clonogenic capacity as measured by limited dilution assays. Furthermore, RNA-seq data uncovered altered transcriptomic programs in response to cumulative radiation treatment. In particular, we observe differential regulation of genes encoding potassium ion channels and their regulators as well as genes encoding ECM and cell adhesion proteins. As part of our goal to evaluate the current multimodal treatment strategy of MB, we also show that a set of standard chemotherapeutic agents (etoposide, thiotepa and lomustine) currently used in the clinic do not produce a potent response at optimal doses in vitro, potentially accounting for abysmal survival outcomes seen in chemotherapy treated MB patients. As such, we conducted a screen of 144 FDA approved cancer drugs and identified therapeutics that selectively target Group 3 MB cells in contrast to normal tissue controls (foetal neural stem cells) in the low nanomolar dose range. In particular, topoisomerase inhibitor topotecan produces a potent response specific to MYC-amplified Group 3 MB, and sensitizes 2 patient-derived MB models to gamma irradiation. Altogether, our data characterizes Group 3 MB molecular and cellular responses to current treatment modalities and provides rationale for new clinical strategies to sensitize patients to radiation that will potentially allow to reduce the required radiation doses to treat these aggressive tumors, reduce the incidence of radiation treatment-related morbidity as well as improve cure rates. Citation Format: Julija Povilaikaite, Kaitlin Stitz, Nikhita Austin, Vijay Ramaswamy. Rational synergistic radiosensitization strategies for group 3 medulloblastoma abstract. In: Proceedings of the American Association for Cancer Research Annual Meeting 2024; Part 1 (Regular Abstracts) ; 2024 Apr 5-10; San Diego, CA. Philadelphia (PA): AACR; Cancer Res 2024;84 (6Suppl): Abstract nr 5111.