Protoporphyrin IX Fluorescence Photobleaching during ALA-Mediated Photodynamic Therapy of UVB-Induced Tumors in Hairless Mouse Skin

— Fluorescence photobleaching of protoporphyrin IX (PpIX) during superficial photodynamic therapy (PDT), using 514 nm excitation, was studied in UVB‐induced tumor tissue in the SKH‐HR1 hairless mouse. The effects of different irradiance and light fractionation regimes upon the kinetics of photobleaching and the PDT‐induced damage were examined. Results show that the rate of PpIX photobleaching (i.e. fluorescence intensity vs fluence) and the PDT damage both increase with decreasing irradiance. We have also detected the formation of fluorescent PpIX photoproducts in the tumor during PDT, although the quantity recorded is not significantly greater than generated in normal mouse skin, using the same light regime. The subsequent photobleaching of the photoproducts also occurs at a rate (vs fluence) that increases with decreasing irradiance. In the case of light fractionation, the rate of photobleaching increases upon renewed exposure after the dark period, and there is a corresponding increase in PDT damage although this increase is smaller than that observed with decreasing irradiance. The effect of fractionation is greater in UVB‐induced tumor tissue than in normal tissue and the damage is enhanced when fractionation occurs at earlier time points. We observed a variation in the distribution of PDT damage over the irradiated area of the tumor: at high irradiance a ring of damage was observed around the periphery. The distribution of PDT damage became more homogeneous with both lower irradiance and the use of light fractionation. The therapeutic dose delivered during PDT, calculated from an analysis of the fluorescence photobleaching rate, shows a strong correlation with the damage induced in normal skin, with and without fractionation. The same correlation could be made with the data obtained from UVB‐induced tumor tissue using a single light exposure. However, there was no such correlation when fractionation schemes were employed upon the tumor tissue.