High-dose metformin plus temozolomide shows increased anti-tumor effects in glioblastoma in vitro and in vivo compared with monotherapy

Purpose The purpose of the study is to investigate the efficacy of combined treatment with temozolomide (TMZ) and metformin for glioblastoma (GBM) in vitro and in vivo. Materials and Methods We investigated the efficacy of combined treatment with TMZ and metformin using cell viability and apoptosis assays. A GBM orthotopic mice model was established by inoculation of 5×10⁵ U87 cells and treated with metformin, TMZ, and the combination for 4 weeks. Western blotting and immunofluorescence of tumor specimens were analyzed to investigate AMPactivated protein kinase (AMPK) and AKT pathway. Results The combination of TMZ and metformin showed higher cytotoxicity than single agents in U87, U251, and A172 cell lines. A combination of high-dose metformin and TMZ showed the highest apoptotic activity. The combination of TMZ and metformin enhanced AMPK phosphorylation and inhibited mammalian target of rapamycin phosphorylation, AKT phosphorylation, and p53 expression. The median survival of each group was 43.6, 55.2, 53.2, 65.2, and 71.3 days for control, metformin treatment (2 mg/25 g/day or 10 mg/25 g/day), TMZ treatment (15 mg/kg/day), combination treatment with low-dose metformin and TMZ, and combination treatment with high-dose metformin and TMZ, respectively. Expression of fatty acid synthase (FASN) was significantly decreased in tumor specimens treated with metformin and TMZ. Conclusion The combination of metformin and TMZ was superior to monotherapy using metformin or TMZ in terms of cell viability in vitro and survival in vivo. The combination of high-dose metformin and TMZ inhibited FASN expression in an orthotopic model. Inhibition of FASN might be a potential therapeutic target of GBM.