THE COMPARATIVE ABSORPTION OF CERTAIN SALICYLATE ESTERS BY THE HUMAN SKIN

¹ From the Medical Research Division, Edgewood Arsenal, Maryland

1. The comparative degree of cutaneous absorption of the salicyl esters calculated in terms of unity for methyl salicylate was as follows: with technique 1, spirosal 2.17, methyl salicylate 1.00, mesotan 0.56, propyl salicylate trace, butyl salicylate trace; with technique 2, spirosal 1.20, methyl salicylate 1.00, mesotan 0.49, ethyl salicylate 0.34, propyl salicylate 0.20, butyl salicylate 0.26, amyl salicylate 0.15.

2. The quantity absorbed from combined solutions and suspensions of spirosal in hot water was strikingly higher than that from analogous applications of methyl salicylate. The skin was more permeable to spirosal under these conditions than from direct application with massage. These two conclusions are tentative, however, in view of the limited number of tests.

3. The absorption of methyl salicylate from 5 per cent solutions in liquid petrolatum and olive oil was practically nil but was considerable from a 0.2 per cent solution of spirosal in liquid petrolatum and a 4 per cent solution in lanoline. The results from continuous massage with 50 per cent solutions of spirosal in lard and olive oils exceeded analogous results with methyl salicylate to a marked degree.

4. A possible correlation between the degree of cutaneous absorption of the pure esters and physical properties was studied. A trend to increasing absorption coincided with a downward trend of the partition coefficients, that is, with a rising tendency to pass from the oil to the aqueous phase. Vapour pressure was negligible; also hydrolysis except in the case of mesotan. With the homologous esters there was a correlation between increasing absorption and rising surface tension and the reverse relation held for viscosity. The degree of absorption from pure methyl salicylate and solutions of the ester in lanoline, liquid petrolatum and benzoinated lard bore no relation to physical properties.

5. Certain of the physical properties of the members of an homologous series of compounds may influence the trend to cutaneous absorption but this does not hold for diverse substances such as boric acid, iothion and sodium salicylate.