EXPERIMENTAL STUDIES OF THE TRANSCUTANEOUS ABSORPTION OF VITAMIN A* By Joan A. KILLIAN, Ph.D., and TP, INITA RveR^, PH.D. KillJan Research Laboratories, New York, N.Y. DURING THE past two decades, the topic of absorption of drugs and nutrients into and through intact skins of human subjects and the mechanisms involved in this process have been subjects of vital interest to cosmeticians, pharmacologists, and therapeutists. In his text, "The Principles and Practice of Modern Cosmetics," Harry (1) has presented a comprehensive and criti- cal review of the extensive and, oftentimes, contradictory literature which has been accumulated on the cosmetic, biochemical, and derma- tological phases of skin absorption and penetration. Much of the confusion in both cosmetic and medical literature concerning the nutritional and the therapeutic merits of ingredients of cosmetics and of medicaments ap- plied to cutaneous surfaces is at- tributable to (a) inadequate meth- ods of testing and (b) the use of terms "absorption" and "penetra- tion" as synonyms. In this presentation, the authors * Read before the Society of Cosmetic Chemists, in London, by Dr. W. A. Taylor on Oct. 8, 1948. have made no attempt to discuss limitations of experimental pro- cedures which have been utilized by different investigators for deter- minations of comparative absorb- abilities of drugs and nutrients into and through skin. A task of this magnitude and responsibility would extend far beyond the scope of a brief report of which the primary objective is the discussion of results of a series of experimental studies of absorption of vitamin A through skins of animals. As a secondary objective of this communication, the authors wish to emphasize the need of the adoption of a uniform terminology for differ- entiation between (a) the passage into skin and (b) the passage through skin of drugs and of in- gredients of cosmetics applied to the surface of the epidermis. The firkt of these two phenomena has been designated "intracutaneous absorp- tion" and the latter "transcutaneous absorption." Obviously, intracutaneous ab- sorption is a prerequisite to trans- cutaneous absorption. Also, it is realized that some transcutaneous 169

170 JOURNAL OF THE SOCIETY OF COSMETIC CHEMISTS absorption will be an invariable "sequitur" of intracutaneous ab- sorption. The purpose in the adop- tion of these two terms was not to distinguish between absorption into and absorption through skin as in- dependent and separable phenom- ena. On the contrary, the sole reason for the introduction of the two qualifications of absorption was to enable an investigator to define clearly the objectives and results of his experimental or clinical research- es. If the investigator's interest is limited to effects upon skin within areas of topical application, he is concerned only with intracutaneous absorption despite the fact that the product under test may pass through the skin. On the other hand, if the experimentalist or the clinician directs his attention to systemic evidences of either toxic or beneficial effects of the product applied to skin, then his objective is the demonstration of transcutaneous absorption. The group of experiments de- scribed in this report was one of several series which were carried out, during a period of five years, fo.r the purposes of assaying the com- parative effects of different ingredi- ents in skin creams and lotions upon the intracutaneous and transcuta- neous absorption of vitamin A. In the series of experiments presented at this time, the objective was the determination of the transcuta- neous absorbability of the vitamin, in a petrolatum base, applied to skins of rats, under carefully controlled conditions. EXPERIMENTAL PROCEDURE In an experimental plan directed to determinations of transcutaneous absorption of any substance from sources applied to animals' skins, an obvious prerequisite is the elim- ination of possibilities of oral con- sumption of the product under test. Of the several methods which had been tried out, only one was found to be entirely satisfactory and this ' . procedure has been adopted in all of the experiments on rats which are presented in this report. Sources of vitamin A were applied to the abdominal and chest areas of rats' skins which had been clipped free of fur by means of an electric razor. Care was taken to avoid abrasions of the skin. The rats were held firmly on their backs in racks to which their feet and heads were se- curely fastened. Definite areas (1 sq. in.) for applications were out- lined with surgeons' silk pasted on the skin. The purpose of this bor- der of silk was to prevent seepage of the applied materials beyond the dehaired area of application. Petrolatum, conforming with the specifications of the Pharmacopteia of the U.S. (2), was the base in which varying amounts of concen- trates of vitamin A were incorpo- rated by manual'mixing. Potencies of the vitamin in these preparations were determined at regular intervals by the 'Cart-Price method (3). Results obtained by this colorimet- tic procedure were checked by both spectrophotometric analyses (4) and biologic assays (5). Samples of'the preparations were utilized only so