Kinetics of photobleaching of protoporphyrin IX in the skin of nude mice exposed to different fluence rates of red light.

The purpose of the present study was to determine the kinetics and the fluence rate dependency of the photobleaching of protoporphyrin IX (PpIX) in normal skin of Balb/c nude mice after systemic and topical application of 5-aminolevulinic acid (ALA). ALA was administered systemically (200 mg/kg body weight, i.p.) and topically (20% w/w ALA cream) to the mice. Fluences of up to 40 J/cm2 were delivered by a dye laser (636 nm) at fluence rates of 37.5, 75, 150, 300 and 500 mW/cm2. The photo-bleaching rate was constant within this range of fluence rates. This result suggests that there is no oxygen effect for PpIX photobleaching in this region for the skin of Balb/c nude mice. During light exposure the fluorescence decay followed neither first- nor second-order kinetics. The decay rate was slightly faster after systemic application than after topical application of ALA, but did not depend on the time (1-8 h) between application and analysis.