Repurposing triamterene as chloride intracellular channel 1 inhibitor via ligand-based approach for glioblastoma

Identification of effective therapies for glioblastoma is an unmet need in clinical oncology. The antidiabetic drug metformin has been recommended as a repurposed treatment for glioblastoma, mainly acting as a blocker of chloride intracellular channel 1 (CLIC1), a key regulator of glioblastoma stem cell (GSC) proliferation. However, metformin's complex pharmacokinetics and low antitumor potency have limited its clinical translation. Using a ligand-based approach and computational studies, we identified the diuretic triamterene as a previously unrecognized CLIC1 blocker. Whole-cell electrophysiology experiments showed that triamterene inhibits CLIC1 activity in GSCs, with half-maximal inhibitory concentration about 200-fold lower than that of metformin. This effect correlated with reduced GSC proliferation in 2D and 3D cultures, decreased invasiveness in 3D models, and antitumor activity in vivo in GSC-xenografted zebrafish embryos. Triamterene also selectively depleted GSCs in limiting dilution assays and 3D cultures, while only modest viability reduction was observed in human immortalized astrocytes, oligodendrocytes, and brain endothelial cells. Finally, triamterene increased temozolomide antitumor potency and enhanced the cytotoxicity induced by combined radiation/temozolomide treatment. In conclusion, triamterene emerges as a repurposed anti-glioblastoma agent with higher potency and tumor selectivity than metformin. Together with its well-known favorable pharmacokinetic profile, these properties make triamterene a promising candidate for clinical translation in glioblastoma patients.