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 flue nce rate dependency of the photobleaching of protoporphyrin IX (PpIX) in no rmal skin of Balb/c nude mice after systemic and topical application of 5-a minolevulinic 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 u p to 40 J/cm(2) were delivered by a dye laser (636 nm) at fluence rates of 37.5, 75, 150, 300 and 500 mW/cm(2). The photobleaching rate was constant w ithin this range of fluence rates. This result suggests that there is no ox ygen effect for PpIX photobleaching in this region for the skin of Balb/c n ude mice. During light exposure the fluorescence decay followed neither fir st- nor second-order kinetics. The decay rate was slightly faster after sys temic application than after topical application of ALA, but did not depend on the time (1-8 h) between application and analysis.