Dose and timing of the first light fraction in two-fold illumination schemes for topical ALA-mediated photodynamic therapy of hairless mouse skin

A fractionated illumination scheme in which a cumulative fluence of 100 J cm(-2) is delivered in two equal light fractions separated by a dark interval of 2 h has been shown to considerably increase the efficacy of 5-aminolevulinic acid (ALA)-photodynamic therapy (PDT). The efficacy of such a scheme is further increased if the fluence of the first light fraction is reduced to 5 J cm(-2). We have investigated the relationship between the PDT response and the kinetics of protoporphyrin IX (PpIX) fluorescence in the SKH1 HR hairless mouse for first fraction fluences below 5 J cm-2 delivered 4 h after the application of ALA and 10 J cm-2 delivered 2 h after the application of ALA. Illumination is 2 performed using 514 nm at a fluence rate of 50 mW cm(-2). Reducing the fluence of the first fraction to 2.5 J cm(-2) does not result in significantly different visual skin damage. The PDT response, however, is significantly reduced if the fluence is lowered to I J cm(-2), but this illumination scheme (1 + 99 J cm(-2)) remains significantly more effective than a single illumination of 100 J cm(-2). A first light fraction of 10 J cm(-2) can be delivered 2 h earlier, 2 h after the application of ALA, without significant reduction in the PDT response compared with 5 + 95 J cm(-2) delivered 4 and 6 h after the application of ALA. The kinetics of PpIX fluorescence are consistent with those reported previously by us and do not explain the significant increase in PDT response with a twofold illumination scheme. Histological sections of the illuminated volume showed a trend toward increasing extent and depth of necrosis for the two-fold illumination scheme in which the first light fraction is 5 J cm(-2), compared with a single illumination scheme.