Oxygen dependence of hypericin-induced phototoxicity to EMT6 mouse mammary carcinoma cells.

The photodynamic effect of hypericin on EMT6 mouse mammary carcinoma cells was investigated in vitro under aerobic and hypoxic conditions. Under aerobic conditions, hypericin-induced photocytotoxicity was dose dependent within a 1-50 microM range. Under hypoxic conditions, cells were resistant to hypericin-induced phototoxicity. In the dark, no cytotoxicity was observed at any hypericin concentration tested either aerobically or hypoxically. Cellular accumulation of hypericin, examined by chemical extraction and spectroscopy, occurred under both hypoxic and aerobic conditions. Fluorescence photomicrographs of cells exposed to hypericin corroborate drug uptake in the plasma membrane and subcellular regions. Our results demonstrate that hypericin cytotoxicity to EMT6 mouse mammary carcinoma cells in vitro is both light and oxygen-dependent. These results suggest that EMT6 cell kill caused by photoactivated hypericin is mediated by an oxygen-dependent mechanism, rather than by a type I oxygen-independent mechanism.