Influence of penetration enhancers on topical delivery of 5-aminolevulinic acid from bioadhesive patches

Abstract

The inclusion of chemical penetration enhancers in a novel patch-based system for the delivery of 5-aminolevulinic acid (ALA) was examined in vitro and in vivo. Poor penetration of ALA has been implicated as the primary factor for low response rates achieved with topical ALA-based photodynamic therapy of thicker neoplastic lesions, such as nodular basal cell carcinomas. Several chemical permeation enhancers (dimethylsulfoxide, Labrafac CC, Labrafac PG and Labrafil M1944CS) were incorporated into bioadhesive patches tailored to deliver 19 mg ALA/cm(2). In-vitro depth penetration studies into excised porcine skin showed that high concentrations of ALA (>9 micromol/cm(3)) could be delivered to a depth of 1.875 mm. However, inclusion of permeation enhancers did not significantly increase ALA delivery, relative to the control (P > 0.05). In-vivo studies were in strong agreement with in-vitro results, with formulations containing chemical enhancers showing no improvement in delivery compared with the control. The patches designed in this work are suited to defineable ALA delivery without the need to immobilise patients for up to 6 h, as is common with the cream-under-occlusion approach. Overall, permeation enhancers were not found to markedly enhance the topical delivery of ALA. However, chemical penetration enhancers may have a greater effect on the delivery of more lipophilic ALA prodrugs, which are thought to primarily permeate the stratum corneum via the intercellular pathway.