Photosensitization of MCF-7 cells with diaziquone using visible light: correlation with DNA strand breaks and free radical production

Visible light damage to Escherichia coli in seawater: oxidative stress hypothesis

The effect of visible light on Escherichia coli H10407 in seawater microcosms was investigated. Light damage was estimated by loss of colony-forming ability. Illumination of E. coli suspended in oligotrophic seawater with visible light at an intensity of about 40 klux caused a drastic decrease of culturable bacteria which turned to a viable but non-culturable state. In seawater E. coli exhibited weak metabolic activity as estimated by 3H methyl-thymidine incorporation in the cell. Visible light did not significantly alter this metabolic activity and did not involve detectable oxidation of lipid membranes as evaluated by gas chromatography analysis of fatty acids. The involvement of oxygen and reactive oxygen species in phototoxicity was studied. A decrease of the toxic effect was observed when E. coli was exposed to visible light under anaerobic conditions. Scavengers of reactive oxygen species exhibited variable protective effects. beta-Carotene, a singlet oxygen scavenger, and superoxide dismutase were equally ineffective. On the other hand, catalase, which eliminates hydrogen peroxide and thiourea, a hydroxyl radical scavenger, showed a net protection. In addition desferrioxamine B, an iron chelator, was also effective in reducing phototoxicity, probably by preventing hydroxyl radical generation by decomposition of hydrogen peroxide in the presence of iron (Fenton reaction). Therefore, hydrogen peroxide and hydroxyl radical seem to be reactive intermediates of oxygen-dependent (type II) photosensitized reactions.

Photodynamic enhancement of doxorubicin cytotoxicity

Argon ion laser irradiation at 514.1 nm and 488 nm dramatically increased doxorubicin cytotoxicity in an L929 cell clonogenic survival assay. The cytotoxicity was dependent on both the drug concentration and the total light energy delivered such that at 5 micrograms doxorubicin/ml and 800 J/cm2, cytotoxicity was enhanced by a factor of > 10(4) relative to that achieved with drug alone. Irradiation times in excess of 2 min and power densities in excess of 100 J/cm2 were required to produce the effect. Beyond this 2-min limit, cytotoxicity was not related to the duration of exposure if the total energy delivered was held constant. The ability of catalase and superoxide dismutase to abolish completely the increase in cytotoxicity produced by laser irradiation suggests that the cytotoxic mechanism may depend on the generation of active oxygen species by the photodynamically excited drug.