ATPS-75TARGETING THE Mdm2-Akt SIGNALING NETWORK IN COMBINATION WITH TEMOZOLOMIDE IMPROVES EFFICACY IN ECTOPIC AND ORTHOTOPIC PATIENT-DERIVED GLIOBLASTOMA XENOGRAFT MODELS

An elusive goal in the treatment of patients with glioblastoma multiforme (GBM) is the development of therapeutic strategies that prevent emergence of therapy-resistant tumor cells. Our laboratory has identified a novel combination therapy that targets the Akt-Mdm2 signaling network in the context of genotoxic temozolomide (TMZ) therapy. Inhibitors were specifically selected based on structural properties related to blood-brain barrier permeability, and pharmacokinetic profiles of plasma, normal brain, flank/ectopic brain tumors, and intracranial tumors. Isobolograms and combination indices indicated that Akt (GDC0068) and Mdm2 (RG7112) inhibitors either together or in combination with TMZ, resulted in synergistic inhibition of cell growth in both mtp53 and wt53 GBM primary patient cell lines (GBM10, GBM43, and MHBT32). When RG7112 and GDC0068 (at a ratio of 0.5:1) were combined with TMZ, strong synergism was evident at multiple dose-ratios. As downstream measures of blocking Akt and Mdm2, activation of the Forkhead box O-class 1/3a (FoxO) transcription factors and increased p53/p73 proteins were evident following TMZ + RG7112 + GDC0068. For in vivo studies, an intermittent dosing regimen of TMZ + RG7112 + GDC0068 was developed to avoid normal tissue toxicity. Mice with TMZ-resistant GBM43 ectopic tumors were treated 3 × weekly for 3 weeks. Combination of TMZ + RG7112 + GDC0068 significantly delayed tumor progression compared to other treatments (n = 8-9 per group, p < 0.05). TMZ + GDC0068 and TMZ + RG7112 + GDC0068 significantly reduced tumor volume relative to TMZ, and on the last day with surviving TMZ + GDC0068-treated mice (2500 mm3 endpoint), tumor volumes were significantly reduced in TMZ + RG7112 + GDC0068 compared to TMZ + GDC0068 (p < 0.05). In mice with GBM43 intracranial tumors, survival was significantly increased by GDC0068 + RG7112 (13 days), and further increased by TMZ + RG7112 + GDC0068 (24 days) compared to TMZ alone. These data suggest that blocking the Mdm2-Akt signaling network in the context of TMZ-mediated DNA damage holds therapeutic promise. Studies to identify mechanism and appropriate biomarkers of response are in progress.