98 JOURNAL OF THE SOCIETY OF COSMETIC CHEMISTS METHODS AND MATERIALS During the last two years a large number of measurements of percutane- ous absorption of salicylates were made employing rabbit preparations. Most of our tests were done with methyl salicylate because it is widely used in analgesic ointments and lotions. In this method the rabbit was anesthe- tized by the oral administration of 5 g. urethane per animal and the ventral surface was carefully clipped but not shaved. An incision was made in the neck to permit the insertion of a glass cannula into the common carotid in order to facilitate the drawing of blood samples. The various preparations tested were applied with a spatula without rubbing at the rate of 2 g./Kg. to an area of 75 cm.2/kg. Male and female rabbits averaging 3 kg. in weight were employed. A minimum of five rabbit preparations were used for each test. The statistical significance level on all comparisons in this study was set at a = 0.05. At this probability level, the experimenter may erroneously assert a real difference exists between points about 5 per cent of the times. In some cases, however, a was much smaller than 0.05. In this study some special vehicles were employed. These were obtained as follows: alkyl phosphates [including "LNa" trilauryl (tetraglycol- ether) phosphate] and alkyl sulfates from Farbwerke Hoechst AG. "Trans- dermine," an alkyl phosphate, from van Ameringen Haebler Co. "Lantrol," a purified lanolin derivative, from Maimstrom Chemical Corporation "Polylan" and "Acetulan," lanolin derivatives, from American Cholesterol Products, Inc. and silicone, designated Fluid//200, from Dow Corning Cor- poration. 6.0 5.0 4..0 3.0 2.0 1.0 o I I 5 15 30 (MIN.) I I I 2 TIME (HOURS) Fig. 1.--/•g salicylic acid/mi. plasma subsequent to intravenous re- jection of 10 ml. of 55 rag. methyl sal./100 ml. 0.9% NaC1 solution.

THE PERCUTANEOUS ABSORPTION OF SALICYLATES 99 Blood salicylate determinations were performed with a slight modifica- tion of the method of Brodie, Udenfriend and Coburn (5). In order to im- prove the sensitivity of the method, twice the amounts of plasma and all of the reagents described were employed and therefete it was possible to use 50 min. cuvettes with Bechmann DU spectrophotometer. This method could not be used to detect methyl salicylate because esters of salicylic acid do not react with the ferric nitrate reagent but rather remain in the ethylene chloride layer. Determinations of ester salicylate were made by measuring the optical density of this layer at 307 m • and therefore it was possible to determine free salicylate and ester salicylate simultaneously. Recently, Stolar, Rossi and Barr (10,11) have used rabbit plasma levels to study the percutaneous absorption of salicylic acid and sodium salicyl- ate. Their method differed from the present one in that the animals were unanesthetized and the absorbing area was covered by a specially designed bandage. This system or the similar one described here offers a useful tool for studying the percutaneous absorption of salicylates. RESULTS Following the intravenous injection of methyl salicylate, the plasma level of salicylic acid decreases as anticipated (Fig. 1). THE EFFECT or INCREASING THE CONCENTRATION OF METHYL SALICYL- ATE ON ITS PERCUTANEOUS ABSORPTION Figure 2 shows a family of curves corresponding to the increase in plasma salicylic acid following the topical administration of various concen- n 60 4o • 2o o 100% I ' 20 % I _ I0% 5% - . z.5O/o Ig.•--.-'-- ' I I I 5 I0 30 I 2 3 (MIN.) TIME (HOURS) Fig. 2.--Effect of varying concentration on the absorption of methyl salicylate in mineral oil.