P11.15 Selective inhibition of BCL-XL increases sensitivity of pediatric tumour models to the anti-mitotic agent MLN8237

BACKGROUND: Small molecule inhibitors of the mitotic kinases, including the Aurora kinase inhibitor MLN8237, are being investigated as novel therapeutic agents for the treatment of solid cancers including malignant brain tumours. However, resistance has been observed and anti-apoptotic proteins of the BCL-2 family have been shown to promote cell survival following treatment. In this study, we investigated the effect of disrupting anti-apoptotic protein function in medulloblastoma and pediatric glioblastoma (pGBM) models using BH3-mimetics in combination with MLN8237. METHODS: The effects of single agent and combination treatment on cell viability were determined by MTT/WST-1 assays and flow cytometry. Effects on cell phenotype, cell cycle progression and cellular ploidy were determined by live cell imaging and DNA content analysis. Induction of apoptosis was monitored in real-time, by time-lapse microscopy, using a fluorescent, polarity-sensitive phosphatidylserine probe, and confirmed by measuring caspase-3/7 activity. Knockdown of BAX/BAK was used to confirm these effects. Results: ABT-263 in combination with MLN8237 reduced mitotic slippage and polyploidy, significantly reduced cell viability and promoted the elimination of mitotically defective cells. This occurred through a BAX/BAK-dependent, caspase-mediated apoptotic pathway. The BCL-xL antagonist, WEHI-539, which targets BCL-2, significantly augmented tumour cell killing when used in combination with MLN8237. It also sensitized resistant brain tumour cells to a novel BAX activator, SMBA1. In addition, siRNA mediated knockdown of BCL-xL sensitized pGBM and medulloblastoma cells to MLN8237, and mimicked the effect of combination drug treatment. CONCLUSION: Combination treatment using a pro-apoptotic agent enhanced the cytotoxicity of MLN8237 in paediatric tumours and may overcome an important resistance mechanism.