RETENTION AND PHOTOTOXICITY OF TETRA(4‐SULFONATOPHENYL)PORPHINE IN CULTIVATED HUMAN CELLS. THE EFFECT OF FRACTIONATION OF LIGHT

— Human cervix carcinoma cells of the line NHIK 3025 were incubated for 18 h with tetra(4‐sulfonatophenyl)porphine (TPPS 4 ) and further incubated for 1–29 h in sensitizer free medium before exposure to light. After 1 h in sensitizer free medium only a 20% further loss of TPPS 4 was observed within the next 28 h. During the time in senzitizer free medium, each TPPS 4 molecule became more efficient in sensitizing single cells to photoinactivation. This enhanced photosensitizing efficiency of TPPS 4 correlated well with the enhanced fluorescence yield of TPPS 4 . In some experiments the cells were exposed to a light dose inactivating 10% of the cells after incubation for 1 h in sensitizer free medium and a second graded light dose given 4–28 h later. Exposure of the cells to the first light dose led to loss of 60% of TPPS 4 from the cells. Despite the significant loss of sensitizer from the cells the fluorescence yield of TPPS 4 from each cell was found to increase (e.g. by 100% 4 h after light exposure). The enhanced fluorescence yield of cell bound TPPS 4 was followed by a 1.6–2.5‐fold increase in sensitivity of each cell to a second light dose. Thus, a small light dose increased the photosensitivity of TPPS 4 ‐loaded NHIK 3025 cells for several hours after the first light exposure. The advantageous effect of light fractionation was reduced by a significantly enhanced loss of sensitizer induced by the first light exposure. The optimal time between the two fractions of light seems to be 30–90 min.