Electron transfer reactions in rhodamine: Potential use in photodynamic therapy

Abstract Rhodamine (R) compounds are photosensitizers with low production of 1O2, so these compounds have not been considered very often for photodynamic therapy (PDT). Rhodamine-6G (R6G) undergoes electron transfer reactions with folic acid (FA). This reaction converts this form of R into a potential photosensitizer (PS) for PDT. The occurrence of this reaction with other R or oxygen has not yet been proven. The low penetrability of visible light into the skin has led to the proposal of a PDT light source alternative to Cerenkov radiation (CR). This work evaluates and compares the occurrence of type-I transfer reactions of R6G, R123, and RB with FA and oxygen when irradiated at 532 nm. The potential use of these compounds as type-I photosensitizers for PDT was also evaluated in vitro by excitation with CR. R electron transfer reactions with FA and oxygen were monitored by emission fluorescence and ultraviolet/visible (UV/Vis) absorption of 2,6-dichlorophenol-indophenol (DCPIP) as a redox indicator, respectively. 177Lu and 18F were used as CR sources. The results showed that the three R compounds underwent type-I transfer reactions with FA in the order R123 > R6G > RB when irradiated at 532 nm. Excitation of R6G using CR was demonstrated. Only R6G showed a type-I reaction with O2. By irradiating T47D tumor cells with a 532 nm laser light, cell death occurred in the order R6G>R123>RB. The same trend was found when cells were irradiated with CR. The possibility of using the type-I reaction between R compounds and biomolecules provides a new perspective in the use of these dyes in PDT.