Mutagenicity of merocyanine 540-mediated photosensitization

Merocyanine 540 (MC 540) is a photosensitizing dye with antineoplastic and antiviral properties. Because photoexcited MC 540 generates singlet molecular oxygen and possibly other reactive oxygen species there is concern that MC 540-mediated photosensitization may be mutagenic. In this paper, we report on the induction of ouabain- and 6-thioguanine-resistant mutants in Chinese hamster ovary (CHO) cells by MC 540 and light. Incubation with merocyanine 540 in the dark was not mutagenic. Simultaneous exposure to dye and high-intensity (70-75 W/m2) white or green light caused a small but significant increase in the frequency of both ouabain- and 6-thioguanine-resistant mutants. Similar increases in mutation frequencies were observed when the cells were exposed to white or green light in the absence of dye. When the fluence rate of the white light source was reduced by 50% (i.e., to 35 W/m2) but the duration of exposure doubled to keep the fluence constant, dye-mediated photosensitization and exposure to light alone generated few if any mutants. These results indicated that MC 540-mediated photosensitization is not mutagenic (as defined by the employed assays) as long as a nonmutagenic light source is used to excite the photosensitizer. They caution against the use of very powerful light sources in clinical applications of MC 540-mediated photosensitization.