Photodynamic effectiveness and vasoconstriction in hairless mouse skin after topical 5-aminolevulinic acid and single- or two-fold illumination

Several options were investigated to increase the efficacy of photodynamic therapy (PDT) using protoporphyrin IX (PpIX) induced by topically applied 5 -aminolevulinic acid (ALA). Hairless mice with normal skin or UVB-light-ind uced skin changes were used as a model. In the first part of the study anim als were illuminated immediately (t = 4) or 6 h (t = 10, PpIX fluorescence maximum) after the end of a 3 h ALA application. A total incident light flu ence of 100 J/cm(2) (514.5 nm) was delivered at a fluence rate of 100 or 50 mW/cm(2). The PDT-induced damage to normal skin was more severe after trea tment at t = 10 than at t = 4, Illumination at 50 mW/cm(2) caused significa ntly more visible damage than the same light fluence given at 100 mW/cm2. F or UVB-illuminated skin, different intervals or fluence rates made no signi ficant difference in the severity of damage, although some qualitative diff erences occurred. In situ fluence rate measurements during PDT indicated va soconstriction almost immediately after the start of the illumination, A fl uorescein exclusion assay after PDT demonstrated vasoconstriction that was more pronounced in UVB-treated skin than in normal skin. The second part of the study examined the effect of two illuminations. The first illumination bleaches the PpIX fluorescence. At the start of the second illumination, n ew PpIX had been formed. Light of 514.5 nm was delivered at 100 mW/cm(2) to a total incident light fluence of 200 J/cm(2) at t = 4 (single illuminatio n) or 100 J/cm(2) at t = 4 plus 100 J/cm(2) at t = 10. There was no visual difference in skin damage between 100 and 200 J/cm(2) single illumination. Two-fold illumination (100 + 100 J/cm(2)) caused significantly more skin da mage, indicating a potentially successful option for increasing the efficac y of topical ALA-PDT.