Synergistic effects of camptothecin with gefitinib or NSC 23766 [ras-related C3 botulinum toxin substrate 1 inhibitor (RAC1)], as well as imatinib with DAPT or NSC 23766 and additive effects of temozolomide with gefitinib or PF-573228 (focal kinase inhibitor) to sensitize glioblastoma cells to cytotoxic chemotherapy causing cancer cell death and growth arrest. The gene discussed is RAC1; the disease is glioblastoma.