An important limitation of topical 5-aminolevulinic acid (ALA)-based photodetection and photodynamic therapy is that the amount of the fluorescing and photosensitizing product protoporphyrin IX (PpIX) formed is limited. The reason for this is probably the limited diffusion of ALA through the stratum corneum. A solution to this problem might be found in the use of ALA derivatives, as these compounds are more lipophilic and therefore might have better penetration properties than ALA itself. Previous studies have shown that ALA hexyl ester (ALAHE) is more successful than ALA for photodetection of early (pre)malignant lesions in the bladder. However, ALA pentyl ester slightly increased the in vivo PpIX fluorescence in early (pre)malignant lesions in hairless mouse skin compared to ALA. The increased PpIX fluorescence is located in the stratum corneum and not in the dysplastic epidermal layer. In the present study, ALA- and ALAHE-induced PpIX fluorescence kinetics are compared in the normal nude mouse skin, of which the permeability properties differ from the bladder. Application times and ALA(HE) concentrations were varied, the effect of a penetration enhancer and the effect of tape stripping the skin before or after application were investigated. Only during application for 24 h, did ALAHE induce slightly more PpIX fluorescence than ALA. After application times ranging from 1 to 60 min, ALA-induced PpIX fluorescence was higher than ALAHE-induced PpIX fluorescence. ALA also induced higher PpIX production than ALAHE after 10 min of application with concentrations ranging from 0.5 to 40%. The results of experiments with the penetration enhancer and tape stripping indicated that the stratum corneum acts a barrier against ALA and ALAHE. Use of penetration enhancer or tape stripping enhanced the PpIX production more in the case of ALAHE application than in the case of ALA application. This, together with the results from the different application times and concentrations indicates that ALAHE diffuses more slowly across the stratum corneum than ALA.

The differences in skin penetration properties could be (co-)responsible for the finding that ALA esters do not induce substantially higher PpIX levels in in vivo skin.

An important limitation of topical 5‐aminolevulinic acid (ALA)‐based photodetection and photodynamic therapy is that the amount of the fluorescing and photosensitizing product protoporphyrin IX (PpIX) formed is limited. The reason for this is probably the limited diffusion of ALA through the stratum corneum. A solution to this problem might be found in the use of ALA derivatives, as these compounds are more lipophilic and therefore might have better penetration properties than ALA itself. Previous studies have shown that ALA hexyl ester (ALAHE) is more successful than ALA for photodetection of early (pre)malignant lesions in the bladder. However, ALA pentyl ester slightly increased the in vivo PpIX fluorescence in early (pre)malignant lesions in hairless mouse skin compared to ALA. The increased PpIX fluorescence is located in the stratum corneum and not in the dysplastic epidermal layer. In the present study, ALA‐ and ALAHE‐induced PpIX fluorescence kinetics are compared in the normal nude mouse skin, of which the permeability properties differ from the bladder. Application times and ALA(HE) concentrations were varied, the effect of a penetration enhancer and the effect of tape stripping the skin before or after application were investigated. Only during application for 24 h, did ALAHE induce slightly more PpIX fluorescence than ALA. After application times ranging from 1 to 60 min, ALA‐induced PpIX fluorescence was higher than ALAHE‐induced PpIX fluorescence. ALA also induced higher PpIX production than ALAHE after 10 min of application with concentrations ranging from 0.5 to 40%. The results of experiments with the penetration enhancer and tape stripping indicated that the stratum corneum acts a barrier against ALA and ALAHE. Use of penetration enhancer or tape stripping enhanced the PpIX production more in the case of ALAHE application than in the case of ALA application. This, together with the results from the different application times and concentrations indicates that ALAHE diffuses more slowly across the stratum corneum than ALA.

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 [degree]C than through skin that was kept at 32 [degree]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.