IMMU-07. TACEDINALINE (CI994), A CLASS I HDAC INHIBITOR, TARGETS INTRINSIC TUMOR GROWTH AND LEPTOMENINGEAL DISSEMINATION IN MYC-DRIVEN MEDULLOBLASTOMA WHILE MAKING THEM SUSCEPTIBLE TO ANTI-CD47 INDUCED MACROPHAGE PHAGOCYTOSIS VIA NF-KB-TGM2 DRIVEN TUMOR INFLAMMATION

BACKGROUND: While major advances have been made in improving the quality of life and survival of children with most forms of medulloblastoma (MB), those with MYC-driven tumors (Grp3-MB) still suffer significant morbidity and mortality. Here, we report a cross-entity, epigenetic drug screen to evaluate therapeutic vulnerabilities in MYC amplified medulloblastoma, which sensitizes them to macrophage-mediated phagocytosis upon targeting the CD47-SIRPa innate checkpoint pathway. METHODS: We performed a primary screen including 78 epigenetic inhibitors and a secondary screen including 20 histone deacetylase inhibitors (HDACi) to compare response profiles in atypical teratoid/rhabdoid tumor (AT/RT, n=11), medulloblastoma (n=14), and glioblastoma (n=14). This unbiased approach revealed the preferential activity of HDACi in MYC-driven medulloblastoma. Importantly, the class I selective HDACi, CI-994, showed significant cell viability reduction mediated by induction of apoptosis in MYC-driven medulloblastoma, with little to no activity in non-MYC-driven medulloblastoma, AT/RT, and glioblastoma in vitro. We tested the combinatorial effect of targeting Class I HDACs and the CD47-SIRPa phagocytosis checkpoint pathway using in-vitro phagocytosis assays and in-vivo orthotopic xenograft models. RESULTS: CI-994 displayed anti-tumoral effects at the primary site and the metastatic compartment in two orthotopic mouse models of MYC-driven medulloblastoma. Furthermore, RNA sequencing revealed NFκB pathway induction as a response to CI-994 treatment, followed by TGM2 expression, which enhanced inflammatory cytokine secretion. We further show interferon-gamma (IFN-g) release and cell surface expression of engulfment (“eat-me”) signals (such as calreticulin). Finally, combining CI-994 treatment with an anti-CD47 mAb targeting the CD47-SIRPa phagocytosis checkpoint enhanced in vitro phagocytosis and survival in tumor-bearing mice. CONCLUSION: Together, these findings suggest a dynamic relationship between MYC amplification and innate immune suppression in MYC-amplified medulloblastoma and support further investigation of phagocytosis modulation as a strategy to enhance cancer immunotherapy responses.