Topical 5-aminolevulinic acid-photodynamic therapy of hairless mouse skin using two-fold illumination schemes: PpIX fluorescence kinetics, photobleaching and biological effect

Light fractionation with dark periods of the order of hours has been shown to considerably increase the efficacy of 5-aminolevulinic acid-photodynamic therapy (ALA-PDT). Recent investigations have suggested that this increase may be due to the resynthesis of protoporphyrin IX (PpIX) during the dark period following the first illumination that is then utilized in the second light fraction. We have investigated the kinetics of PpIX fluorescence and PDT-induced damage during PDT in the normal skin of the SKH1 HR hairless m ouse. A single illumination (514 nm), with light fluences of 5, 10 and 50 J cm(-2) was performed 4 h after the application of 20% ALA, to determine th e effect of PDT on the synthesis of PpIX, Results show that the kinetics of PpIX fluorescence after illumination are dependent on the fluence delivere d; the resynthesis of PpIX is progressively inhibited following fluences ab ove 10 J cm(-2), In order to determine the influence of the PpIX fluorescen ce intensity at the time of the second illumination on the visual skin dama ge, 5 + 95 and 50 + 50 J cm(-2) (when significantly less PpIX fluorescence is present before the second illumination), were delivered with a dark inte rval of 2 h between light fractions. Each scheme was compared to illuminati on with 100 J cm(-2) in a single fraction delivered 4 or 6 h after the appl ication of ALA, As we have shown previously greater skin damage results whe n an equal light fluence is delivered in two fractions. However, significan tly more damage results when 5 J cm(-2) is delivered in the first light fra ction. Also, delivering 5 J cm(-2) at 5 mW cm(-2) + 95 J cm(-2) at 50 mW cm (-2) results in a reduction in visual skin damage from that obtained with 5 + 95 J cm(-2) at 50 mW cm-2. A similar reduction in damage is observed if 5 + 45 J cm(-2) are delivered at 50 mW cm(-2), PpM photoproducts are formed during illumination and subsequently photobleached, PQIX photoproducts do not dissipate in the 2 h dark interval between illuminations.