PHOTOBLEACHING DURING AND RE-APPEARANCE AFTER PHOTODYNAMIC THERAPY OFTOPICAL ALA-INDUCED FLUORESCENCE IN UVB-TREATED MOUSE SKIN

Photodynamic therapy (PDT) using protoporphyrin IX (PpIX) induced by t opically applied 5 aminolevulinic acid (ALA) seems a promising alterna tive for the treatment of superficial non-melanoma skin cancer and act inic keratosis. In this study, the kinetics of new PpIX fluorescence a rising after a PDT treatment that had photobleached the original fluor escence were determined, Our purpose was to examine the feasibility of multiple irradiations, following a single topical ALA application, to increase PDT efficacy, In addition, photobleaching during PDT and the fluorescence spectra during and after PDT were studied, As a model we used hairless mice with and without UVB-induced skin lesions, ALA was applied to the skin for 4 kr, An illumination war; delivered either i mmediately after application or 6 hr after the end of the application (at interval of maximum fluorescence), During PDT, the fluorescence of normal skin decreased at a faster rate than the fluorescence of the s kin lesions, In the fluorescence study after PDT, the areas treated im mediately post-application showed a fluorescence increase over time si milar to that in non-treated areas on the same mice. A remarkable resu lt was that the fluorescence of areas treated at maximum fluorescence increased, whereas the fluorescence of non-treated areas did not incre ase over time, With both treatment intervals the new fluorescence show ed a characteristic PpIX spectrum, Our results demonstrate that a seco nd illumination, when new PpIX fluorescence has been formed, may incre ase PDT efficacy after topical ALA application, This finding has been demonstrated previously Tar systemic ALA administration. (C) 1997 Wile y-Liss, Inc.