J. Soc. Cosmet. Chem., 40, 367-373 (November/December 1989) Skin stripping as a potential method to determine in vivo cutaneous metabolism of topically applied drugs JOHANN W. WIECHERS, RENELLA E. HERDER, BEN F. H. DRENTH, and ROKUS A. de ZEEUW, Groningen Centre for Drug Research, Bioanalysis and Toxicology Group, University of Groningen, A. Deusinglaan 2, 9713 A W Groningen, The Netherlands. Received August 25, 1989. Synopsis By chromatographing an extract of the tapes obtained in a skin stripping procedure, cutaneous metabolism of compounds after topical administration may be observable, provided that outward transdermal migration occurs. This method may be helpful, especially in situations where no differentiation between cutaneous and systemic metabolism can be made due to the experimental design or the very low systemic concentra- tions. Through use of this methodology, it can be assessed that the penetration enhancer for percutaneous absorption, Azone ©, is only present as the parent compound in the stratum corneum, whereas the anti-acne agent Cyoctol undergoes cutaneous biotransformation during skin passage. INTRODUCTION In recent years there has been a renewed and growing interest in dermal and trans- dermal drug delivery. This route opens new possibilities for systemic therapy, especially for drugs with short biological half-lives due to extensive first-pass metabolism in the liver. Compounds, however, may also be metabolized in the skin before reaching the systemic circulation (1,2), thereby reducing their bioavailability. For this reason, the cutaneous metabolism of these compounds should be studied and compared to already available systemic biotransformation data. If cutaneous metabolism occurs, additional investigations may be required to determine the pharmacological profile of the dermally formed metabolites. A simple method to establish in vivo cutaneous metabolism of topically applied agents was developed and will be discussed on the basis of two compounds currently under investigation in our laboratories, Azone © and Cyoctol. Both compounds are to exert their action in human skin, Azone as a penetration enhancer for percutaneous absorp- Johann W. Wiechers' present address is Unilever Research, Colworth Laboratory, Sharnbrook, Bedford MK44 1LQ, United Kingdom. 367

368 JOURNAL OF THE SOCIETY OF COSMETIC CHEMISTS tion (3) and Cyoctol, an anti-androgen (4), as an anti-acne drug. In order to be able to follow the metabolic processes, tracer amounts of •4C-labeled compounds were used. The structure of the compounds and the position of the labels is given in Figure 1. MATERIALS AND METHODS MATERIALS •4C-labeled Azone, 1-dodecylazacycloheptan-2-one ([1-•4C]-dodecyl), and Cyoctol, 6- (5-methoxyphept- 1-yl)bicyclo[3.3.0]octan-3-one ([•4C]-carbonyl), were kindly sup- plied by Nelson Research, Irvine, California, and Chantal Pharmaceutical Corporation, Los Angeles, California, respectively. The radiochemical purity was determined by iso- cratic high-performance liquid chromatography (HPLC) to be at least 95.3 and 97.0%, respectively, using the system described below. All other materials were HPLC grade and obtained commercially. METHODS Study performance. In separate studies Azone and Cyoctol were applied to a 24-cm 2 area on the volar aspect of the forearm of healthy human volunteers and left in place under occlusion for 12 and 8 hours, respectively. Azone was dosed in a therapeutic formula- tion (100 mg) to three volunteers at a concentration of 1.6%, containing tritium-la- OCH 3 0 Figure 1. Structures of 1-dodecylazacycloheptan-2-one (Azone ©, top) and 6-(5-methoxyhept-1- yl)bicyclo[3.3.0]octan-3-one (Cyoctol, bottom). The asterisks denote the position of the label.