ORIGINAL ARTICLE Aminolaevulinic acid diffusion characteristics in ‘in vitro’ normal human skin and actinic keratosis: implications for topical photodynamic therapy Nabla McLoone 1 , Ryan F. Donnelly 1 , Maureen Walsh 2 , Olivia M. Dolan 3 , Sean McLoone 4 , Kevin McKenna 5 & Paul A. McCarron 1 1 School of Pharmacy, Medical Biology Centre, Queen’s University Belfast, Belfast, UK, 2 Department of Pathology, The Royal Hospitals, Belfast, UK, 3 Department of Dermatology, The Royal Hospitals, Belfast, UK, 4 Department of Electronic Engineering, National University of Ireland Maynooth, Maynooth, Co. Kildare, Ireland, and 5 Department of Dermatology, Belfast City Hospital, Belfast, UK Key words: actinic keratosis; aminolaevulinic acid; normal skin Correspondence: Paul A. McCarron, School of Pharmacy, Medical Biology Centre, Queen’s University Belfast, 97 Lisburn Road, Belfast BT9 7BL, UK. Tel: 144 2890 335800 Fax: 144 2890 247794 e-mail: p.mccarron@qub.ac.uk Accepted for publication: 17 January 2008 Conflicts of interest: None declared. Summary Background: The response rate of aminolaevulinic acid (ALA)-based photodynamic therapy (PDT) in certain subtypes of actinic keratosis (AK), such as hypertrophic and hyperkeratotic lesions, is variable, an effect attributable to a supposed lack of ALA penetration. A detailed and depth-related profile of spatial ALA permeation in AK following drug administration would lead to a greater understanding of concentrations achievable before protoporphyrin IX biosynthesis and subsequent PDT. Methods: ALA penetration through excised normal human skin (NS) and AK lesions was evaluated using a cryostatic sectioning technique and radio-isotope counting following drug delivery using a novel, bioadhesive patch, loaded with 19, 38 or 50 mg/cm 2 ALA. Results: Distinct differences in ALA concentration with respect to depth between AK and NS samples were shown, particularly within the superficial layers of the tissue structure, down to a depth of 1.0 mm. Patch application times were shown to influence ALA concentrations in tissue, but there was no clear correlation between ALA penetration in AK lesions taken from different body locations and from patients of different age. Similarly, the thickness of stratum corneum was not related to the ALA distribution profiles. Conclusions: Sizable variation in ALA concentration was a prominent feature of profiles through AK lesions, which may explain the variation of observed protoporphyrin IX production seen in the clinical implementation of AK PDT. That said, the results of this study show sufficient ALA penetration to a depth of 1.0 mm, which should be satisfactory for successful treatment of the majority of non-hyperkeratotic, hypertrophic AK using patch- based delivery methods. A minolaevulinic acid-based photodynamic therapy (ALA- PDT) is a well-established treatment modality for pre- malignant and malignant skin lesions, with high success rates reported for actinic keratoses (AK), Bowen’s disease and superficial basal cell carcinomas. The photodynamic effect results in the formation of 1 O 2 , a highly reactive species that is the major cause of PDT-induced cytotoxicity (1). Several different mechanisms are involved in this cytotoxicity and include direct cell killing by apoptosis or necrosis, vascular ischaemia, inflammation and induction of a host immune response. Effective PDT involves selective targeting of the diseased tissue or tumor and sparing of normal tissues from PDT cytotoxicity. The adequate penetration of ALA into the full depth of skin lesions and the production of protoporphyrin IX (PpIX) throughout its entirety is therefore an important factor in the success of topical ALA-PDT. The excellent response rate of AK to ALA-PDT is attributed to the effective accumulation of PpIX noted within the lesions following topical ALA application (2–4). It is believed that increased penetration of ALA through a defective lesional stratum corneum (SC) barrier contributes to the high PpIX accumulation. AK are typically induced by chronic ultraviolet (UV) exposure and barrier dysfunction has been shown to be present in chronic UVB-exposed mice skin (5). This would support the hypothesis of a defective SC barrier in AK leading to an increased tissue concentration of ALA within lesions. However, data confirming this is lacking. Hypertrophic, hyperkeratotic AK do not respond well to topical ALA-PDT (6, 7). This poor response has been accredited to limited ALA penetration through the thickened keratin barrier and lesion. These assumptions are based on poor fluorescence observed within lesions and are attributed to subtherapeutic PpIX concentrations. 183 r 2008 The Authors Journal compilation r 2008 Blackwell Munksgaard  Photodermatology, Photoimmunology & Photomedicine 24, 183–190