J. Soc. Cosmet. Chem. 26 47-63 (1975) ¸ 1975 Society of Cosmetic Chemists of Great Britain The percutaneous absorption of some anionic surfactants D. HOWES* Presented on 28th August 1974 in London at the IFSCC VIIIth International Congress on 'Cosmetics--Quality and Safety' organized by the Society of Cosmetic Chemists of Great Britain. Synopsis--The irritant action of a SURFACTANT to SKIN may be related to the ability of that surfactant to penetrate the STRATUM CORNEUM and act upon the underlying viable tissues. The PERCUTANEOUS ABSORPTION of some [• 4C] labelled anionic surfactants has been measured in vivo in rats, after both consumer-type applications and applications of longer dura- tion, and the results have been compared with those from in vitro studies using isolated RAT skin and human epidermis. The methodology for both the in vivo and in vitro studies will be outlined and results will be presented from experiments with a series of sodium soaps of normal FATTY ACIDS, sodium LAURYL SULPHATE, sodium LAUROYL ISETHIONATE and sodium DODECYLBEN- ZENE SULPHONATE. The in vivo techniques can also provide information as to the metabolic fate of topically applied surfactants under user type conditions. The usefulness of the in vitro techniques and their shortcomings will be discussed. INTRODUCTION Saponified fats of animal and plant origin are the traditional sur- factants used in toilet soaps and in recent years these have been reinforced by synthetic surfactants of higher surface activity. The function of these surfactants is to solubilize and remove sebum, deposited soil and skin debris, but the fate of topically applied surfactants is not fully documented * Unilever Research Laboratory, Colworth House, Sharnbrook, Bedford. 47

48 JOURNAL OF THE SOCIETY OF COSMETIC CHEMISTS especially the amounts which remain on the skin surface or penetrate the skin. Techniques have been developed to study the percutaneous absorption of both therapeutic and toxic agents through skin but most of these are in vitro methods (1). Although good comparative data are often obtained using in vitro techniques, the extrapolation of these data to the in vivo situation is difficult and Wahlberg (2) using guinea-pig skin indicated that there was little correlation between the two situations. Maibach and his associates (3-5) have used radiotracer techniques for studies in vivo with a wide variety of compounds on both humans and experimental animals and have shown regional and interspecies variations in percutaneous absorption. Sprott (6) measured the urinary recovery of 35S after topical application of [35S] labelled n-alkyl sulphate to rat skin but no other data on surfactant penetration in vivo has been published. This study reports the results from a series of experiments where the in vivo fate of topically applied [x4C] labelled surfactant solutions was compared with in vitro experiments using rat skin and human epidermis. The turnover of the [x4C] labelled surfactants administered intraperitoneally and subcutaneously to rats is also reported. MATERIALS AND METHODS Surfactants Decanoic acid (C101 0), dodecanoic acid (C19•: 0), tetradecanoic acid (Cla: 0), hexadecanoic acid (C16: 0) and octadecanoic acid (C18: 0) were obtained from B.D.H. (Poole, Dorset) and were specially pure Biochemical grade. These were converted to their sodium salts by neutralization to pH 9.5 with sodium hydroxide. These acids were also obtained [1-•aC] labelled from the Radiochemical Centre (Amersham, Bucks). These [•C] labelled acids were incorporated into a model soap system described below. Sodium [1-•C] dodecyl sulphate 5.11 [tCi mg -• (14C-SDS), sodium [1-1aC] dodecoyl isethionate 1.7 liCi mg -1 (laC-SDI)and sodiump-l-[1-1aC] dodecylbenzenesulphonate 8.5 ltCi mg -1 (•C-DOBS) were synthesized in this laboratory and were shown to be chemically and radiochemically pure by thin layer chromatography and isotope dilution analysis.