Transdermal delivery of a tetrapeptide: Evaluation of passive diffusion Heather A. E. Benson 1 *, Rima Caccetta 1 , Yan Chen 1 , Philip Kearns 2 & Istvan Toth 2 1 Western Australian Biomedical Research Institute, School of Pharmacy, Curtin University, Perth, Western Australia 6845, Australia; 2 School of Biological Chemistry and Pharmacy, University of Queensland, Brisbane, Queensland, Australia (* Author for correspondence, e-mail: h.benson@curtin.edu.au, Fax: 61 8 9266 2769, Tel: 61 8 9266 2338) Received 9 January 2004; Accepted 10 March 2004 Key words: epidermis, peptide, psoriasis, stratum corneum, transdermal delivery Summary Skin penetration of the tetrapeptide Ac-Ala-Ala-Pro-Val-NH 2 was assessed. This peptide sequence fits the P-P 1 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 epider- mis, which had been removed from the dermis by heat separation. The influence of 75% aqueous etha- nol 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 lg cm )2 h )1 was achieved. The study demonstrates that the stratum corneum is the main barrier to tetrapeptide skin pene- tration and must be overcome if therapeutically relevant amounts of tetrapeptide are to be delivered to the skin. Introduction Chemicals are applied to the skin for a variety of purposes, such as the treatment of skin and underlying tissues (topical) or for systemic drug delivery (transdermal). The rationale for topical application is to target the site of action with minimal systemic uptake and side effects. Exam- ples of target sites for chemicals applied to the skin are: the surface (e.g., sunscreens and insect repellents), epidermis/dermis (e.g., infections, psoriasis, etc.), nerve and muscle (e.g., non-ste- roidal anti-inflammatory drugs NSAIDs). In addition, a number of drugs are applied transder- mally for systemic action, for example, oestradiol and testosterone for hormone replacement ther- apy, nicotine to aid smoking cessation, glyceryl trinitrate for angina and fentanyl for pain. The major limitation of the skin as a route of transder- mal drug delivery is the relative impermeability of the skin. Skin penetration of small and relatively hydrophobic molecules is favoured. This has also limited the skin as a potential site for application of biotechnology products such as peptides and proteins. Considerable research effort has been focused on the development of skin penetration enhance- ment techniques. These include chemicals such as Azone, fatty acids, surfactants and aprotic sol- vents, physical techniques such as iontophoresis, phonophoresis and electroporation, chemical modification such as ion-pairs and prodrugs, and formulation methods such as liposomes and transferomes [1–4]. However, many of these tech- niques have been limited by insufficient penetra- tion enhancement and/or induced irritancy. An example of a potentially useful topical peptide application is Cyclosporin A, a powerful immunomodulating agent with application in the treatment of atopic dermatitis and psoriasis Letters in Peptide Science, 10: 615–620, 2003. KLUWER / ESCOM 615 Ó 2004 Kluwer Academic Publishers. Printed in the Netherlands.