Effect of elevating the skin temperature during topical ALA application on in vitro ALA penetration through mouse skin and in vivo PpIX production in human skin

Abstract

An approach to induce increased protoporphyrin IX (PpIX) production in aminolevulinic acid (ALA)-based photodynamic therapy (PDT) of skin lesions is to elevate the skin temperature during topical ALA application. Increased skin temperature may increase the (depth of) penetration of ALA into the skin, which may in turn increase PpIX production (in deeper layers). The effect of skin temperature on in vitro ALA penetration into mouse skin was determined in an in vitro percutaneous penetration model at two different temperatures. The effect of skin temperature on PpIX production in human skin during ALA application was measured with in vivo fluorescence spectroscopy in temperature-controlled areas (5 different temperatures). The data from the experiment with the in vitro percutaneous penetration model clearly show that the penetration of ALA into skin is temperature dependent. The penetration of ALA through the mouse skin was higher when its temperature was maintained at 37 °C than through skin that was kept at 32 °C. The fluorescence data from the in vivo experiment show that the PpIX fluorescence increases with increasing temperature of the skin during the application period. The overall activation energy (E_a) for PpIX production was obtained for each hour of the ALA application period from the fluorescence data using the Arrhenius equation. The E_a value in the first hour of ALA application was not significant, indicating that the PpIX production in that period is dominated by processes that are not temperature dependent, like the passive diffusion of ALA across the stratum corneum. In the second, third and fourth hours of ALA application, the E_a for PpIX production proved to be significant, which indicates that the PpIX production in these time intervals is dominated by temperature-dependent processes. In conclusion, the data from the present study indicate that improving ALA-based PDT of skin lesions might be achieved by elevating the skin temperature during the ALA application.