 Glioblastoma GBM is the most common and aggressive brain cancer, and despite significant advances in treatment the majority of patients die within two years of diagnosis. During routine animal studies we serendipitously observed that fenbendazole, a benzimidazole antihelminthic used for pinworms, prevented tumor engraftment. Subsequent in vitro and in vivo experiments with benzimidazoles identified mebendazole as the drug having the best results in preclinical testing 1. In GBM cell lines, mebendazole displayed cytotoxicity with IC50s ranging from 0.1-0.3 μM. Mebendazole disrupted microtubule formation in GBM cells and its in vitro activity was correlated with reduced tubulin polymerization. In two orthotopic mouse glioma models, one syngeneic and one xenograft, mebendazole significantly extended average survival up to 63 compared to untreated controls 1. 