Harnessing Repurposed Drugs to Enhance Temozolomide Efficacy in Glioblastoma

Glioblastoma (GB) is the most aggressive primary malignant brain tumor in adults and remains associated with poor survival despite surgical resection followed by radiotherapy and temozolomide (TMZ) chemotherapy. Intrinsic and acquired resistance to TMZ, including MGMT-dependent DNA repair and activation of pro-survival pathways, could decrease treatment efficacy. Drug repurposing offers an attractive strategy to identify agents that may enhance TMZ activity because these drugs already have known pharmacokinetic and safety profiles. This narrative review summarizes the available evidence on repurposed drugs investigated as potential modulators of TMZ response in GB.

A range of repurposed agents, including chloroquine, valproic acid, levetiracetam, metformin, aspirin, amlodipine, atorvastatin, chlorpromazine, melatonin, disulfiram, bortezomib, and verteporfin, have been evaluated in GB models and selected clinical studies. Reported mechanisms include modulation of MGMT expression, autophagy, oxidative stress, apoptosis, DNA-damage responses, cancer stem-cell properties, and signaling pathways such as PI3K/AKT/mTOR, AMPK, EGFR, STAT3, ERK1/2, and NF-κB. Several agents have enhanced TMZ-associated cytotoxicity in cell culture and animal models. However, clinical evidence remains limited, and the results are inconsistent for some drugs. Blood-brain barrier penetration, achievable intratumoral drug exposure, toxicity, treatment scheduling, and molecular heterogeneity of GB remain major translational challenges.

Repurposed drugs may provide useful candidates for improving TMZ-based therapy in GB. However, most evidence remains preclinical, and further studies are needed to clarify blood-brain barrier penetration, optimal dosing, toxicity, predictive biomarkers, and clinical efficacy. Well-designed prospective clinical trials are required before these combinations can be incorporated into routine GB treatment.