Glioblastoma multiforme (GBM) is the most common and highly aggressive form of brain cancer with less than 5% of patients surviving beyond 5 years from diagnosis (1). The conventional standard of care treatment for GBM is surgical resection followed by radiotherapy combined with the chemotherapeutic alkylating agent Temozolomide (TMZ), that induces DNA damage and cell proliferation arrest as well as apoptotic cell death. However, many GBM tumours are intrinsically resistant to TMZ, making this malignant disease challenging to treat (2). The aim of this study was to evaluate the therapeutic potential of targeting specific pro-survival BCL-2 family proteins with BH3-mimetic drugs in human GBM cells in vitro as single agents, or dual therapy when combined with TMZ or the Bromodomain inhibitor JQ1. We demonstrate that the combination treatment of cells with S63845, inhibiting MCL-1, or ABT-1331852, inhibiting BCL-XL, increased apoptotic cell death compared to TMZ alone or single agent treatment of the BH3 mimetic drugs.
U251 cells lacking both effectors of apoptosis, BAX and BAK, were profoundly resistant to apoptotic death triggered by TMZ and dual BH3 mimetic drug treatment as determined by DAPI and Annexin V staining followed by flow cytometry. Western blot analysis revealed abundant cleavage (activation) of caspase-3 and poly ADP ribose polymerase (PARP) protein in WT U251 cells in response to dual treatment with S63845 and ABT-1331852, TMZ treatment alone or the combination of TMZ with BH3 mimetics. The bromodomain inhibitor JQ1 efficiently killed U251 cells more potently when combined with ABT-1331852 compared to JQ1 treatment as single agent. This correlated with abundant cleavage (activation) of caspase-3 and PARP protein detected by Western blotting. We conclude that the combination of BH3 mimetic drugs with standard of care agent TMZ or the bromodomain inhibitor JQ1 substantially increases apoptotic cell death in human glioblastoma cells and this warrants further investigation of evaluating the therapeutic potential of BH3 mimetic drugs in patient derived xenograft mouse models of GBM.