MODL-18. Enhancing anti-CD47 mAb efficacy with radiotherapy for Group 3 paediatric medulloblastoma in preclinical models

Abstract Medulloblastoma is the most common paediatric brain cancer. Standard treatment approaches, including craniospinal irradiation (CSI), can result in severe lifelong side effects and have not changed for decades resulting in a stagnation of survival outcomes for children with aggressive medulloblastoma. Despite successes in other cancers, no immunotherapies have been approved for use in paediatric medulloblastoma. Unlike other solid tumours, medulloblastomas are myeloid dominant, and immunotherapies must be rationally designed with the tumour microenvironment in mind. Anti-CD47 antibody therapy activates macrophages against cancer cells by blocking anti-phagocytic signalling mediated by CD47-SIRPa ligation and has shown preclinical efficacy in brain cancer models. We have developed preclinical CSI protocols that mimic clinical treatment response using a small animal radiotherapy platform. We show that CSI depletes adaptive immune cells in the brain, increasing the proportional abundance of myeloid cells, suggesting an opportunity to combine radiation with myeloid-targeted immunotherapy. We show that anti-CD47 therapy is ineffective as a single agent against a patient-derived xenograft model of Group 3 medulloblastoma (SJ_MB002), and that while the CSI protocol causes temporary tumour regression, the combination of anti-CD47 with CSI results in marked and persistent tumour regression. To enhance our preclinical evaluation of CSI and anti-CD47, we have developed new mouse models that more accurately reflect the developing microenvironment of children and show that immune populations in paediatric brain are distinct from adult mouse brain. Future work will elucidate the mechanisms by which radiotherapy alters the medulloblastoma microenvironment to enhance the anti-tumour activity of myeloid immune cells in the brain. By evaluating this novel combination of immunotherapy with standard medulloblastoma treatments, in age-appropriate models, our research should facilitate the rational selection and rapid translation of optimised treatment combinations for future medulloblastoma clinical trials.