The mechanism of photodynamic inactivation of human cells in vitro in the presence of haematoporphyrin.

The photosensitizing effect of haematoporphyrin (HP) on human cells of the established line NHIK 3025 has been studied. Fluorescence measurements show that HP is bound to these cells. Serum proteins also bind HP, and the presence of 10% human serum during incubation with HP (3 X 10(-4)M) reduces the cellular uptake of HP by 75% or more. The photosensitized inactivation is enhanced when the cells are suspended in D2O-buffer during irradiation. This indicates that singlet oxygen is involved in the inactivation. Two findings indicate that the photoinduced damage is repairable: firstly, the fraction of cells surviving a given light dose decreases with decreasing irradiation temperature, and secondly, the survival curves have a shoulder at low exposures of light.