142 Arginine deprivation improves survival and activates the immune system in mouse models of diffuse midline glioma

Abstract Introduction Diffuse midline glioma (DMG) is a rare but fatal childhood brain tumour with no successful treatments. An inert immune microenvironment is a key factor in the failure to find suitable treatments. Arginine deprivation using ADI-PEG20 has been effective in treating a number of cancers, particularly those which cannot generate their own arginine. Our previous studies demonstrated that ADI-PEG20 combined with radiation eradicated GBM tumours and extended survival of animals indefinitely. ADI-PEG20 reprogrammed the tumour microenvironment in GBM and therefore may be an effective strategy in DMG. Methods In vitro: Three mouse DMG cell lines (H3.3K27M, H3.1K27M, H3WT) were grown as neurospheres, treated with ADI-PEG20 and 5 days later given radiation. Viability assays using CCK8 were performed 8 days after initial treatment. In vivo The same cell lines were intracranially injected into C57BL/6J mice. Once tumours were established, mice were treated weekly with ADI-PEG20, radiation, combination, or saline. Mice were culled when symptoms appeared and brains were harvested and analysed using spatial transcriptomics, IHC and IF. Results Treatment with ADI-PEG20 significantly reduced viability measured by CCK8 in H3.3K27M and H3WT cell lines. In vivo, ADI-PEG20 significantly increased survival compared to saline and radiation treatment in mice with H3WT tumours. Spatial transcriptomics revealed upregulation of a number of immune pathways following treatment with ADI-PEG20, and histology analysis indicated increased activation of microglial cells. Conclusions and future work ADI-PEG20 significantly affects the ability of DMG cells to grow in vitro, and increases survival in mice with H3WT tumours. It activates a number of immune pathways and microglial cells. It therefore warrants testing clinically in DMG patients, and may be effective in combination with immunotherapy.