Department of Animal Physiology, Ruhr-University of Bochum, 44780, Bochum, Germany.
Naunyn Schmiedebergs Arch Pharmacol. 2012 Oct;385(10):969-79. doi: 10.1007/s00210-012-0777-4. Epub 2012 Jul 17.
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, β-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.
光动力疗法广泛应用于治疗皮肤浅层癌症。5-氨基酮戊酸（ALA）及其甲基化形式，即甲氧基乙酰基-L-ALA（MAL），常被用作光敏物质的前体。然而，角朊细胞摄取 ALA 和 MAL 及其穿透皮肤的机制仍存在争议。由于这两种化合物的脂溶性都不足够，无法穿透脂质膜，因此它们必须利用特定的摄取系统，而不同细胞类型之间的摄取系统可能会有所不同。在这里，我们研究了来源于健康细胞（CCD 1106 KERTr 细胞）或角朊细胞瘤（A431 细胞）的角朊细胞系对 ALA 和 MAL 的摄取。ALA 的摄取导致原卟啉 IX（PpIX）的产生速度比 MAL 快。摄取系统的药理学特征表明，GABA 转运体（GAT）底物最有效地减少了 PpIX 的形成。GABA、β-丙氨酸和（S）-SNAP-5114 减少了细胞系中 ALA 和 MAL 的摄取。这些化合物的药理学表明，ALA 和 MAL 被正常和病理性角朊细胞通过 GAT-3 摄取。此外，氨基酸精氨酸、半胱氨酸和组氨酸也抑制了 ALA 的摄取，而对 MAL 的抑制作用更为明显，提示氨基酸转运体的额外参与。为了表明体内的 PpIX 形成仅限于应用部位，这在另一项关于 ALA 的研究中受到质疑，我们将临床使用的 ALA 和 MAL 制剂应用于裸鼠皮肤。与这些先前作者的结果相反，两种制剂的 PpIX 荧光随时间增加，并仅限于应用部位。
