Enhancement of radiosensitivity in human malignant glioma cells by hypericin in vitro

Hypericin, an antidepressant and antiviral agent being evaluated in phase I and II trials for patients with HIV infection, is known to be a potent protein kinase C inhibitor. We have investigated its effects on cellular response to radiation via a tetrazolium-formazan cell growth rate assay using 3-(4,5-dimethylthiazol-2-yl)-2, 5-diphenyltetrazolium bromide and clonogenic assay in three human glioblastoma cell lines, U87-MG, A-172, and T98G, and a low-passage malignant glioma culture, 93-492. At a concentration of 5 microM, hypericin inhibited these cells slightly but caused significant radiosensitization (e.g., the cell survival rate after the radiation treatment was 50.2 and 26.0% in cells treated with 6 Gy and 6 Gy plus 5 microM hypericin in U87-MG cells, respectively; P = 0.0285). Hypericin also enhanced the radiosensitivity significantly in the low-passage glioma 93-492 cells. These findings suggest that hypericin represents a potential new agent in combination with radiation therapy of malignant gliomas.