TMIC-72. MECHANISMS TO ENHANCE IMMUNOTHERAPY EFFICACY IN A RAT MODEL OF MEDULLOBLASTOMA

Abstract Medulloblastoma is the most common malignant brain tumor in children. Current treatments are limited by irreversible CNS damage, particularly in high-risk subgroups. Innate immune cells constitute a significant portion of the brain tumor microenvironment, nurturing tumor growth and suppressing key defense mechanisms. Leveraging the innate immune system for improved clinical outcomes remains unrealized. We aimed to determine if Baicalein (Bac), a known tumoricidal M1 macrophage potentiator, improves efficacy in medulloblastoma models. Nude rats (N=10) were implanted with MB002 medulloblastoma intracerebellar patient-derived non-WNT/non-SHH, group 3 xenografts. Baseline T2 and gadolinium-enhanced MRI (Gd-MRI) T1 estimated pre-therapy tumor volumes. Single fraction 2Gy irradiation to the tumor-bearing hemisphere with 20mg/kg temozolomide (CRT) combined with Bac 100mg/kg PO QD daily x8 (N= 3) was administered. CRT alone (N= 3) and no treatment (N= 4) served as controls. Seven days post-therapy, Gd-MRI and histology established tumor volume and CD68 and CD163 expression (T-test; p≤ 0.05). MRI demonstrated the therapeutic effect of Bac+CRT compared to CRT only (P< 0.01) and untreated controls (P= 0.01). This was confirmed via histology volumes; untreated group (P< 0.01) and a trend toward significance vs. CRT only (P= 0.22). The normal brain demonstrated a near significant increase in CD68 expression in the Bac+CRT compared to CRT and control (P< 0.06). A near statistically significant decrease in CD163 expression was observed at the tumor margin for Bac+CRT compared to CRT only (P< 0.06). Likewise when comparing CRT to untreated control (P< 0.05). Preliminary data suggests Bac+CRT uniquely induces an innate immune response within the brain that expresses features of M1 polarization, resulting in effective tumoricidal efficacy in preclinical medulloblastoma models. Current studies are evaluating the mechanisms of efficacious innate immune targeted therapies in larger sample sizes with the eventual goal of clinical translation.