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Argon laser phototherapy of human malignancies using rhodamine‐123 as a new laser dye: The intracellular role of oxygen

Recent studies demonstrated that the cationic, mito‐chondrial‐specific dye Rhodamine‐123 (Rh‐123), is an efficient tumor photosensitizer for Argon laser treatment of human cancer cells both in vitro1–4 and in tumors grown as xenografts in athymic mice.5–7 To demonstrate the photo‐dynamic mechanism of action of this reaction, the intracellular role of oxygen and temperature changes in treated cells have to be defined. In the current study, a large panel of human tumor cell lines of diverse histologic origin were tested for in vitro sensitivity to Rh‐123 and the Argon laser (514.5 nm) in oxygen, deuterium oxide (D2O), and nitrogen (N2) environment. Tumor cells in suspension were first sensitized to Rh‐123 (1 or 20 μg/ml for 1 hour), cooled on ice to 4°C, and then exposed to the Argon laser (ΔT = 14 ± 1°C). Cell proliferation measured by [3H]‐thymidine uptake 24 hours after sensitization with Rh‐123 and laser treatment was significantly decreased in tumor cells kept in oxygen and D2O atmospheres. No decrease in DNA synthesis was seen in Rh‐123 and laser treated cells kept in an N2 environment. Control tumor cells treated with Rh‐123 or the Argon laser separately did not show any decreased [3H]‐thymidine uptake in oxygen, D2O or N2 environment. These results provide evidence of a photodynamic process since Rh‐123 sensitization and Argon laser activation occur at nonther‐mal levels of energy and are oxygen dependent. The high effectiveness of this technique of photodynamic therapy with the Argon laser, and low toxicity of Rh‐123 could make its clinical use very attractive for the treatment of superficial malignancies.