Comparison of the uptake of 5-aminolevulinic acid and its methyl ester in keratinocytes and skin

Photodynamic therapy is widely used in the treatment of superficial skin cancers. 5-Aminolevulinic acid (ALA) and its methylated form, methyl-ALA (MAL), are frequently used as precursors to photosensitizing substances. Nevertheless, the mechanism of the uptake of ALA and MAL in keratinocytes and of their skin penetration is still controversial. Since both compounds are not sufficiently lipophilic to penetrate through lipid membranes, they must employ specific uptake systems which may vary between different cell types. Here, we studied ALA and MAL uptake in keratinocyte cell lines originating from healthy cells (CCD 1106 KERTr cells) or keratinocyte tumors (A431 cells). ALA uptake resulted in faster protoporphyrin IX (PpIX) production than MAL uptake. A pharmacological characterization of the uptake systems revealed that PpIX formation was most efficiently reduced with GABA transporter (GAT) substrates. GABA, beta-alanine, and (S)-SNAP-5114 reduced ALA uptake and, to a lesser extent, MAL uptake in the cell lines. The pharmacology of these compounds indicates that ALA and MAL are taken up by normal and pathological keratinocytes via GAT-3. Furthermore, the amino acids arginine, cysteine, and histidine also inhibited the uptake of ALA, and even more so MAL, suggestive of an additional involvement of amino acid transporters. To show that PpIX formation in vivo is restricted to the application site, which has been questioned for ALA in one other report, we applied clinically used ALA and MAL formulations to the skin of nude mice. Contrary to the results of these previous authors, the resulting PpIX fluorescence increased over time and was restricted to the application site for both preparations.