PDT-Induced Variations of Radachlorin Fluorescence Lifetime in Living Cells In Vitro

Variations in the fluorescence lifetimes of Radachlorin photosensitizers in HeLa and A549 cells, caused by photodynamic treatment, were studied using fluorescence lifetime imaging microscopy (FLIM). An analysis of FLIM images of the cells demonstrated a substantial decrease in the mean Radachlorin fluorescence lifetime and intensity as a result of UV irradiation of the photosensitized cells at different doses, with higher doses causing a more pronounced decrease in the mean fluorescence lifetime in cells. The post-treatment decrease in Radachlorin fluorescence intensity was accompanied by the appearance of an additional rapidly decaying fluorescence component and a nonlinear decrease in the weighted fluorescence lifetime obtained from double-exponential fits of time-resolved fluorescence signals. Experiments performed in the aqueous solutions of the photosensitizer revealed similar irreversible changes in the Radachlorin fluorescence lifetime and intensity. Therefore, the observed phenomena occurred most likely due to the photodegradation of the photosensitizer molecules and can be applied for dosimetry and monitoring of irradiation doses in different areas of malignant tissues in the course of photodynamic treatment.