In vitro skin permeation and retention of 5-aminolevulinic acid ester derivatives for photodynamic therapy.

In photodynamic therapy (PDT), 5-aminiolevulinic acid (5-ALA) applied topically is converted, via the heme cycle, into protoporphyrin IX (PpIX), a photosensitizing agent, which upon excitation with light can induce tumor destruction. Due to its hydrophilic and zwitterionic characteristics, 5-ALA has limited penetration into the skin. More lipophilic 5-ALA ester derivatives are expected to cross stratum corneum more easily than 5-ALA. According to the determination of the partition coefficients of 5-ALA methyl, n-butyl, n-hexyl and n-octyl esters, these compounds showed an increased affinity to the SC, with 5-ALA hexyl ester and 5-ALA-octyl ester having the highest partition coefficients. Our in vitro skin permeation studies demonstrated an increased permeated amount for hexyl-ALA after 6 h of incubation, compared to other esters and 5-ALA. After 6 h, more 5-ALA-hexyl ester and -octyl ester were retained at viable epidermis and dermis than 5-ALA. According to these results, and considering that the conversion of 5-ALA into PpIX occurs preferentially in epidermis, it can be supposed that topical use of ester derivatives with longer chains (C(6) or C(8)) is an interesting proposal to optimize topical 5-ALA-PDT