Introduction: Diffuse midline glioma (DMG) is the most aggressive pediatric brain tumor. Approximately 80% harbor the H3 K27M mutation, increasing reliance on SMARCA4 for tumor proliferation and invasion. BRM014, a SMARCA4 inhibitor, has limited effectiveness due to poor blood-brain barrier (BBB) penetration and systemic administration of the drug has failed to show a survival benefit. This study investigated whether low-intensity focused ultrasound (FUS) to transiently open the BBB can enhance BRM014's efficacy.
Methods: An immune-competent DMG mouse model was used, mirroring human DMG through PDGFB overexpression, p53 deletion, and H3.3 K27M mutation. Mice were divided into: control, FUS alone, and combined FUS and drug treatment. BRM014 was administered systemically. FUS was applied with intravenously-administered microbubbles to transiently open the BBB (1.5 MHz fc, 450 kPa pressure). Survival rates were monitored and post-mortem brain tissues were analyzed using immunofluorescence staining for SOX2 (glioma marker), Ki67 (proliferation marker), and Iba1 (microglia/macrophage marker). Significance was assessed using Welch’s t-tests.
Results: Combination of FUS and BRM014 increased median survival by 10.7% compared to control (p=0.03), while FUS alone did not significantly affect survival. Immunofluorescence showed increased microgliosis in the FUS plus drug group (13% Iba1-positive area) and FUS alone group (12%) versus control (10%) (p=0.02 and p=0.06), suggesting FUS stimulates a myeloid response. FUS alone did not significantly change the Ki67 proliferation index in SOX2-positive tumor cells, but combination treatment reduced it to 0.12 from 0.20 in control and FUS-only groups (p=0.003). SOX2 labeling indices remained consistent, indicating the treatment did not affect stem-like properties of the tumor cells.
Conclusion : Low-intensity FUS to enhance BRM014 delivery in a DMG mouse model led to decreased tumor proliferation and extended survival. The increased infiltration of Iba1-positive myeloid cells implies that FUS-induced BBB opening activates this immune cell population. Further studies are needed to understand the mechanisms of FUS-induced Iba-1 positivity, its impact on tumor progression and survival, and to compare the efficacy of FUS-mediated drug delivery with direct intracranial administration.
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