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Photosensitizing Effects of the Tricyclic Heteroaromatic Cationic Dyes Pyronin Y and Toluidine Blue O (Tolonium Chloride)¹

Sloan-Kettering Institute for Cancer Research, Walker Laboratory, Rye, New York 10580

Pyronin Y [3,6-bis(dimethylamino)xanthylium chloride; PY] and toluidine blue O [tolonium chloride; 3-amino-7-(dimethylamino)-2-methyl phenothiazin-5-ium chloride; TB] are cationic dyes commonly used in cytochemistry that have affinity to nucleic acids, predominantly to RNA. In live cells these dyes accumulate in mitochondria and sensitize the cells to light. The photosensitizing effects of PY and TB were compared with those of another mitochondrial cationic dye, rhodamine 123, and a noncationic dye, merocyanine 540, which binds to the cell membrane. Ninety % reduction of clonogenicity of human epidermoid carcinoma (A-253) cells pretreated with 3.3 µM PY, 0.67 µM TB, 13 µM rhodamine 123, or 18 µM merocyanine 540 was achieved by cell exposure to 0.7, 1.0, 1.2, or 1.5 J/cm² doses of white light, respectively. The above concentrations of PY, TB, or merocyanine 540 represent the maximal ones at which the effect of each of these dyes alone, in the dark, in reducing cell clonogenicity was less than 12%. Exposure of A-253 cells to light at doses reducing clonogenicity by 50% caused a transient (24 h) arrest of the surviving cell population in the G₁ phase of the cell cycle. In contrast to A-253 cells, Chinese hamster ovary cells were highly resistant to the photosensitizing effects of each of the four dyes. Also, the normal human lung fibroblasts (WI-38) were highly resistant to photosensitization by PY, whereas the simian virus 40-transformed WI-38 cells and another carcinoma line (OV-3) were sensitive. The data suggest that PY and TB, like other mitochondrial dyes, may have a selective antitumor photosensitizing activity.

¹ Supported by Grants R37 CA 23296 and RO1 CA 28704 from the National Cancer Institute.

² To whom requests for reprints should be addressed, at Sloan-Kettering Institute for Cancer Research, Walker Laboratory, 145 Boston Post Road, Rye, NY 10580.

Received 8/11/87. Revised 11/11/87. Accepted 12/ 2/87.