In vitro metabolism of 5-ALA esters derivatives in hairless mice skin homogenate and in vivo PpIX accumulation studies

Purpose. In topical photodynamic therapy, 5-ALA and its esters are enzymatically converted in the endogenous photosensitizing compounds such as, for example, protoporphyrin IX (PpIX). In order to elucidate in more detail their enzymatic fate, we have determined in vitro the enzymatic degradation of methyl, butyl, hexyl, and octyl-5-ALA ester derivatives in skin homogenate. Furthermore, in vivo porphyrin accumulation was measured in healthy hairless mice skins. Methods. Hairless mouse skins were homogenized in isotonic phosphate buffer pH 7.4. 5-ALA esters were added, and aliquots were colleted for HPLC-fluorimetric determinations of remaining content of 5-ALA esters. Furthermore, oil-in-water emulsions containing esters were topically applied to mice skin for 6 h, and the amount of accumulated PpIX in the treated areas was determined by quantitative extraction and confocal fluorescence microscopy. Results. The enzymatic degradation of esters follows pseudo first-order kinetics. The octyl ester had the largest rate constant for enzymatic degradation, followed by hexyl-, butyl-, and methyl-ALA. The long-chained 5-ALA esters, butyl-, hexyl-, and octyl ester, induced significantly more porphyrins than 5-ALA and 5-ALA methyl ester as shown by confocal microscopy and quantitative extraction studies. Conclusions. 5-ALA derivatives differ widely with respect to their enzymatic degradation. The presence of alkyl chains in 5-ALA esters significantly influences the in vitro enzymatic metabolism and the in vivo PpIX formation in healthy hairless mice skins.