BI-907828, a novel potent MDM2 inhibitor, inhibits glioblastoma brain tumor stem cells in vitro and prolongs survival in orthotopic xenograft mouse models

The tumor suppressor TP53 (p53) is frequently mutated, and its downstream effectors inactivated in many cancers, including glioblastoma (GBM). In tumors with wild-type status, p53 function is frequently attenuated by alternate mechanisms including amplification and overexpression of its key negative regulator, MDM2. We investigated the efficacy of the MDM2 inhibitor, BI-907828, in GBM patient-derived brain tumor stem cells (BTSCs) with different amplification statuses of MDM2, in vitro and in orthotopic xenograft models.In vitro growth inhibition and on-target efficacy of BI-907828 were assessed by cell viability, co-immunoprecipitation assays, and western blotting. In vivo efficacy of BI-907828 treatments was assessed with qPCR, immunohistochemistry, and in intracranial xenograft models.BI-907828 decreases viability and induces cell death at picomolar concentrations in both MDM2 amplified and normal copy number TP53 wild-type BTSC lines. Restoration of p53 activity, including robust p21 expression and apoptosis induction, was observed in TP53 wild-type but not in TP53 mutant BTSCs. shRNA-mediated knock-down of TP53 in wild-type BTSCs abrogated the effect of BI-907828, confirming the specificity of the inhibitor. Pharmacokinetic-pharmacodynamic studies in orthotopic tumor-bearing severe combined immunodeficiency (SCID) mice demonstrated that a single 50 mg/kg p.o. dose of BI-907828 resulted in strong activation of p53 target genes p21 and MIC1. Long-term weekly or bi-weekly treatment with BI-907828 in orthotopic BTSC xenograft models was well-tolerated and improved survival both as a single-agent and in combination with temozolomide, with dose-dependent efficacy observed in the MDM2 amplified model.BI-907828 provides a promising new therapeutic option for patients with TP53 wild-type primary brain tumors.