J. Sec. Cosmet. Chem. 23 481-521 (1972) ¸ 1972 Society of Cosmetic Chemists of Great Britain Methods of measuring, and factors affecting, percutaneous absorption P. GRASSO and A. B. G. LANSDOWN* Synopsis--Tests for PERCUTANEOUS ABSORPTION are needed principally for substances that are toxic or biologically active or which accumulate in body tissues. The most sensitive SPECIES for percutaneous absorption tests are the rabbit and the guinea-pig. The technique most widely applicable is the measurement by ANALYTICAL or ISOTOPE techniques of the rate of DISAPPEARANCE of a test substance from the site of its topical application. Urine examination or measurement of blood levels may be useful in certain circumstances. IN VITRO measurements are often useful as a guide to the planning of IN VIVO tests. The principal barrier against percutaneous absorption is the STRATUM CORNEUM. Water and water-soluble substances traverse this layer with difficulty while LIPID-SOLUBLE substances do so with less difficulty. Substances that are soluble in both water and lipids (amphipathic) penetrate readily. Vehicles may retard or enhance percutaneous absorption in relation to their water-lipid solubility. They may also enhance absorption by increasing the permeability of the stratum corneum. In the case of OINTMENTS, partition of the test substances between vehicle and stratum corneum is often an important factor influencing absorption. Temperature and pH of the test preparations are additional factors that influence absorption. INTRODUCTION The penetration through the skin of compounds applied to its surface and the consequences of their systemic absorption has been of interest in the treatment of disease for a number of years (1). As far back as 1899 Mussey (2) demonstrated in the urine a number of drugs that had been applied to the unbroken skin. Two years later Reilly (3) reported that * The British Industrial Biological Research Association, Woodmansterne Road, Carshalton, Surrey. 481

482 JOURNAL OF THE SOCIETY OF COSMETIC CHEMISTS therapeutic effects could be obtained after cutaneous application of such substances as belladonna, mercury, pilocarpine and cod liver oil. This author also noted the presence of salicylic acid and phenol in the urine after their topical application to the skin. Toxic effects, often serious and sometimes fatal, have been reported from substances applied percutaneously. Macht (4) reported deaths in experimental animals following the percutaneous absorption of a number of essential oils and Sannicandaro (5), and Gottlieb and Storey (6), reported deaths in humans from the absorption of salicylic acid and phenol through the intact skin. In recent years several examples of poisoning with organo- phosphorus compounds from percutaneous absorption have been reported both in humans following accidental contamination of the skin (7, 8), and in animals used in experimental studies (9). A number of other compounds have been reported to have caused toxicity in humans through percutaneous absorption. These include the chlorinated hydrocarbon insecticides, carbon tetrachloride, aromatic amines, dinitrophenol and hydrogen cyanide gas (10). These observations have created an awareness that compounds present in many consumer products including toilet preparations, cosmetics and household materials, could produce untoward systemic effects by virtue of percutaneous absorption. Examples of this sort are the biologically active substances, intended for use in cosmetics, that are reported from time to time in the literature. One may recall here the mercaptans used in 'cold- hair wave processes' (11), steroid hormones intended to beautify the skin (12) and antibacterial agents (13). The recent reports indicating that hexachloro- phene, added to a variety of toiletry preparations as an antibacterial agent, is absorbed percutaneously in amounts considered to be of toxicological hazard for babies (14) underlies the importance of keeping under review the risks that may arise from the inclusion of biologicall3, active substances in cosmetic and toiletry preparations. Awareness of the possible toxic effects that may arise from these sources has led to the discovery that some cyclo- siloxanes, originally developed for inclusion into cosmetic creams, are potent gonadotoxic and embryotoxic compounds (15). This was done during preliminary toxicological investigations on animals involving per- cutaneous absorption tests. In order to assess potential toxicological hazard from this route it is important to have an understanding of the principles underlying the methods of testing and of the factors which influence percutaneous absorption. Both these fields have been extensively reviewed over the past two decades, but most reviewers tend to emphasize certain specialized aspects of the problem