Introduction Toxicological testing and hazard analysis are of great importance in the protection of human health and the environment. However, animal welfare is also a major concern. Therefore, validated alterna- tives to animal experiments are constantly being sought. For regulatory purposes, reconstructed human epidermis (RHE) has been introduced for studying adverse skin effects such as corrosion (1–5) and irritation (6–9). Recently, genotoxicity (10–12) and skin sensitisation (13–15) have also become major considerations in in vitro test devel- opment. When procedures have been validated, test guide- lines (TGs) are adopted by the Organisation for Economic Cooperation and Development (OECD). For hazard analysis by percutaneous absorption, the OECD Member Countries accepted TG 428 (16), which is accompanied by Guidance Document (GD) 28 (17), for testing cutaneous uptake in vitro by using human and animal skin. This is of major importance, since, after the gut and the lung, the skin is the third biggest absorption organ, and is particularly relevant for pesticide intoxication (18, 19). According to GD 28, RHE can also be used, given that comparable results are obtained with RHE and the established skin preparations. To overcome the shortage of viable human skin for tox- icity testing, a validation study was performed to establish a test procedure for percutaneous absorp- tion with RHE. These experiments were exclusively based on aqueous solutions (20–22). To further test the experimental protocol, the influence on substance uptake of semi-solid vehicles and penetration enhancers was investigated. For both hazard prediction in toxicology, and for the estimation of the availability of the active agent in drug development, vehicle effects need to be reflected correctly by the test membrane used. However, this issue has been studied only rarely, with conventional application methods (23–25) and with particulate carrier systems (26). Drug release studies performed on a routine basis with artificial Reconstructed Epidermis and Full-Thickness Skin for Absorption Testing: Influence of the Vehicles used on Steroid Permeation Monika Schäfer-Korting, 1 Ashraf Mahmoud, 1 Simone Lombardi Borgia, 1 Barbara Brüggener, 1 Burkhard Kleuser, 1 Sylvia Schreiber 1 and Wolfgang Mehnert 2 1 Freie Universität Berlin, Institut für Pharmazie (Pharmakologie & Toxikologie), Berlin, Germany; 2 Freie Universität Berlin, Institut für Pharmazie (Pharmazeutische Technologie), Berlin, Germany Summary — A protocol for percutaneous absorption studies has been validated, based on the use of reconstructed human epidermis (RHE) and aqueous solutions of test substances. However, it is often the case that it is more-complex formulations of drugs or chemicals which will make contact with the skin sur- face. To investigate whether RHE and the reconstructed full-thickness skin model (FT-model) can be used to predict uptake from formulations, we compared the permeation of hydrocortisone and testosterone when applied in emulsion form and as a solution containing the penetration enhancer, ethanol. Human and pig skin and a non-cornified alveolar model served as references. The results were compared with steroid release from the formulations. The permeation rates of the steroids were ranked as: alveolar model >> RHE > FT-model, pig skin > human skin. In accordance with the rapid hydrocortisone release from the formulations, the permeation rates of this steroid exceeded those of testosterone. Only minor differences were observed when comparing the testosterone formulations, in terms of release and permeation. However, the ranking of the permeation of the hydrocortisone formulations was: solution > w/o emulsion > o/w emulsion, which permitted the elucidation of penetration enhancing effects, which is not possible with drug release studies. Differences in penetration were most obvious with native skin and reconstructed tissues, which exhibited a well-developed penetration barrier. In conclusion, RHE and skin preparations may be useful in the development of topical dermatics, and in the framework of hazard analysis of toxic com- pounds and their various formulations. Key words: alveolar tissue, formulation effects, human skin models, reconstructed full-thickness skin, reconstructed human epidermis, skin absorption. Address for correspondence: M. Schäfer-Korting, Freie Universität Berlin, Institut für Pharmazie (Pharmakologie & Toxikologie), Königin-Luise-Straße 2–4, 14195 Berlin, Germany. E-mail: msk@zedat.fu-berlin.de ATLA 36, 441–452, 2008 441