EFFECTIVE PHOTODYNAMIC-ACTION BY RHODAMINE-123 LEADING TO PHOTOSENSITIZED KILLING OF CHINESE-HAMSTER OVARY CELLS IN TISSUE-CULTURE AND A PROPOSED MECHANISM

The effectiveness of rhodamine 123 (R123) as a photosensitizer of cell killing is relatively low and correlates with its inefficient production of singlet oxygen. The known selective retention of R123 in the mitochondria of epithelially derived carcinoma cells, however, is a selective feature that could lead to a more useful therapeutic ratio if photosensitizing effectiveness could be increased. Chinese hamster ovary (CHO) cells,in tissue culture were therefore exposed to R123 shortly before and during illumination under conditions controlled for oxygen concentration and temperature. Effective photosensitization of cell killing, as judged by colony formation, was produced by 95% but not by 19% O2 during illumination of cells at 5-degrees-C or 37-degrees-C, and this was additionally enhanced at the sublethal temperature of 42-degrees-C. Two CHO cell lines were examined; one line, CHO-AA8, was proficient in the repair of DNA damage and the parent to the second line, CHO-EM9, that was deficient in the repair of DNA strand breaks. Cells of both lines incorporated R123 to a similar degree and were similarly photosensitized by the presence of high oxygen concentration. Furthermore, plasma membrane damage as judged by the exclusion of trypan blue was not observed immediately after illumination in the presence of R123, but was seen in the presence of meso-tetra-(4-sulfonatophenyl)-porphine (TPPS4). The extent of damage to the plasma membrane by TPPS, was greater in the presence of 95% compared to 19% 02 during illumination. Photodynamic action at the level of the plasma membrane appears to contribute to photosensitization by TPPS, but not by R123 soon after exposure of cells to these sensitizers. It is hypothesized that photodynamic action by R123 is the primary mechanism causing the observed photosensitization of cell killing, and that mitochondria are the site of photosensitized damage responsible for this killing.