Vorinostat enhances the cytotoxic effects of the topoisomerase I inhibitor SN38 in glioblastoma cell lines

Histone deacetylase (HDAC) inhibitors represent a promising class of anti-cancer agents that are actively being evaluated in the context of clinical trials in solid tumors, including glioblastoma. What makes these agents particularly attractive is their capacity to enhance the activity of commonly used cytotoxics in cancer therapy, including both chemotherapy and ionizing radiation. As recent investigations suggest HDAC inhibitors may potentiate the cytotoxicity of topoisomerase inhibitors, which continue to be a commonly used class of agents in the treatment of glioblastoma, we performed preclinical studies to determine if this combination may be a promising strategy in glioblastoma. The effects of the HDAC inhibitor vorinostat and SN38, which is the active metabolite of the topoisomerase I inhibitor CPT-11, was evaluated using the clonogenic assay. Various treatment schedules were tested to determine optimum drug sequencing. Induction of DNA double strand breaks (DSBs) with the combination of vorinostat and SN38 was evaluated using the neutral comet assay, and their subsequent repair was evaluated by γH2AX foci kinetics using immunofluorescent cytochemistry. Vorinostat enhanced the cytotoxicity of SN38 in glioblastoma cell lines. Optimal treatment schedules involved maximal concurrent administration of agents. Pretreatment with either agent did not enhance cytotoxicity. Vorinostat potentiated SN38-induced DNA DSBs and attenuated their subsequent repair. These results indicate vorinostat enhances the cytotoxicity of SN38 in glioblastoma cell lines, suggesting this combination may be a worthwhile strategy to test in the context of a clinical trial.