Transdermal Delivery of a Tetrapeptide: Evaluation of Passive Diffusion

Skin penetration of the tetrapeptide Ac-Ala-Ala-Pro-Val-NH₂ was assessed. This peptide sequence fits the P-P₁ subsites of elastase and inhibits human neutrophil elastase competitively. Consequently this peptide may be therapeutically useful in a variety of inflammatory disorders, including psoriasis, in which elevated levels of human neutrophil elastase have been reported. Peptide penetration was assessed across whole human skin, whole skin with the stratum corneum removed by tape stripping and epidermis, which had been removed from the dermis by heat separation. The influence of 75 aqueous ethanol as a potential penetration enhancer of the tetrapeptide across epidermis was also assessed. The tetrapeptide did not penetrate whole human skin or epidermis, even under the influence of 75 aqueous ethanol. However, when the stratum corneum was removed tetrapeptide flux of 73.39 μg cm² h⁻¹ was achieved. The study demonstrates that the stratum corneum is the main barrier to tetrapeptide skin penetration and must be overcome if therapeutically relevant amounts of tetrapeptide are to be delivered to the skin.