Protoporphyrin IX fluorescence kinetics and localization after topical application of ALA pentyl ester and ALA on hairless mouse skin with UVB-induced early skin cancer

In order to improve the efficacy of 5-aminolevulinic acid-based (ALA) photo dynamic therapy (PDT), different ALA derivatives are presently being invest igated. ALA esters are more lipophilic and therefore may have better skin p enetration properties than ALA, possibly resulting in enhanced protoporphyr in IX (PpIX) production, In previous studies it was shown that ALA pentyl e ster (ALAPE) does considerably enhance the PpIX production in cells in vitr o compared with ALA, We investigated the in vivo PpIX fluorescence kinetics after application of ALA and ALAPE to hairless mice with and without UVB-i nduced early skin cancer. ALA and ALAPE (20% wt/wt) were applied topically to the mouse skin and after 30 min, the solvent was wiped off and PpIX fluo rescence was followed in time with in vivo fluorescence spectroscopy and im aging. At 6 and 12 h after the 30 min application, skin samples of visible lesions and adjacent altered skin (UVB-exposed mouse skin) and normal mouse skin were collected for fluorescence microscopy. From each sample, frozen sections were made and phase contrast images and fluorescence images were r ecorded. The in vivo fluorescence kinetics showed that ALAPE induced more P pIX in visible lesions and altered skin of the UVB-exposed mouse skin, but not in the normal mouse skin. In the microscopic fluorescence images, highe r ALAPE-induced PpIX levels were measured in the stratum corneum, but not i n the dysplastic layer of the epidermis, In deeper layers of the skin, PpIX levels were the same after ALA and ALAPE application. In conclusion, ALAPE does induce higher PpIX fluorescence levels in vivo in our early skin canc er model, but these higher PpIX levels are not located in the dysplastic la yer of the epidermis.