Updated project metadata. The outcome for patients afflicted with glioblastoma (GBM), the most common and malignant of primary brain tumors in adults, has changed little despite decades of clinical, translational, and basic research. Any effective, systemically administered GBM therapy needs to target cellular components that are indispensable for the malignant phenotype with drugs that cross the blood brain barrier (BBB) and have manageable toxicities. However, while a number of signaling pathways have been shown to drive the malignant phenotype in GBM, and while relatively non-toxic, CNS permeant inhibitors of several of these have been identified, their efficacy in GBM has been disappointing. In this study, we examine the mechanism of resistance to the Kif11 inhibitor ispinesib in murine and human GBM. We find that development of resistance in these tumors occurs by a mechanism not previously described for Kif11 inhibitors, is associated with broad scale transcriptomic and phenotypic changes, and can be reversed with drugs that are FDA approved or in clinical investigation. Our findings also point to ways of enhancing the efficacy of Kif11 inhibitors that are directly translatable into a clinical setting.The outcome for patients afflicted with glioblastoma (GBM), the most common and malignant of primary brain tumors in adults, has changed little despite decades of clinical, translational, and basic research. Any effective, systemically administered GBM therapy needs to target cellular components that are indispensable for the malignant phenotype with drugs that cross the blood brain barrier (BBB) and have manageable toxicities. However, while a number of signaling pathways have been shown to drive the malignant phenotype in GBM, and while relatively non-toxic, CNS permeant inhibitors of several of these have been identified, their efficacy in GBM has been disappointing. In this study, we examine the mechanism of resistance to the Kif11 inhibitor ispinesib in murine and human GBM. We find that development of resistance in these tumors occurs by a mechanism not previously described for Kif11 inhibitors, is associated with broad scale transcriptomic and phenotypic changes, and can be reversed with drugs that are FDA approved or in clinical investigation. Our findings also point to ways of enhancing the efficacy of Kif11 inhibitors that are directly translatable into a clinical setting.