Exploiting ribosomal biogenesis dependence through RNA polymerase I inhibition in medulloblastoma.

e14106 Background: Medulloblastoma (MB) is the most common malignant pediatric brain tumor. Despite multimodal therapy, high-risk subgroups such as group 3 MB have 5-year overall survival rates of less than 50%, with frequent relapse and significant long-term treatment-related toxicities. Rapid tumor proliferation requires increased ribosome biogenesis, making RNA polymerase I a potential therapeutic target. CX-5461 is a selective inhibitor of RNA polymerase I that has demonstrated antitumor activity in several adult malignancies, but its therapeutic potential in medulloblastoma has not been fully defined. Methods: Group 3 and SHH medulloblastoma cell lines were treated with increasing concentrations of CX-5461. Cell viability, apoptosis, and cell cycle distribution were assessed using standard biochemical and flow cytometric assays. Nucleolar stress and DNA damage response signaling were evaluated by immunoblotting and imaging-based analyses. Therapeutic efficacy was tested using subcutaneous and intracranial xenograft models. Patient-derived xenograft (PDX) studies are currently ongoing. Results: CX-5461 reduced viability of group 3 and SHH medulloblastoma cells with IC50 values ranging from 75 to 150 nM. Treatment induced significant cell cycle arrest and increased apoptosis, accompanied by nucleolar disruption and activation of DNA damage response pathways. In subcutaneous xenograft models, CX-5461 significantly inhibited tumor growth without evidence of major systemic toxicity. In an orthotopic intracranial model, CX-5461 treatment increased survival by up to three weeks compared with control. PDX studies are currently underway to further evaluate therapeutic efficacy across patient tumors. Conclusions: These studies demonstrate that RNA polymerase I inhibition using CX-5461 has potent antitumor activity in high-risk medulloblastoma models, including survival benefit in an intracranial setting. These findings identify ribosome biogenesis as a therapeutic vulnerability in medulloblastoma and support continued translational development of CX-5461 toward future clinical evaluation in children with high-risk medulloblastoma.