In vivo skin penetration of salicylic compounds in hairless rats

The in vivo skin penetration of four salicylic compounds was investigated using a hairless rat model, which allowed for non-occluded, finite dose application, and free mobility of the rats throughout the test period. The model compounds were applied in equimolal concentrations of 0.4 mmol/g dimethyl isosorbide. At certain times (0.5–24 h) the rats were killed, and the amount of test compound on the skin surface, in the stratum corneum, and in the deeper viable skin layers was determined. Significant different skin concentrations were found with the following ranking: [ 14C]diethylamine salicylate>[ 14C]salicylic acid>[ 14C]salicylamide>[ 14C]butyl salicylate. In addition, the in vivo percutaneous rate of absorption was in the following order: [ 14C]butyl salicylate>[ 14C]salicylic acid≥[ 14C]salicylamide>[ 14C]diethylamine salicylate. [ 14C]Butyl salicylate was rapidly absorbed and completely depleted from the surface 3 h post application. In comparison with [ 14C]salicylic acid, the ionic [ 14C]diethylamine salicylate had larger surface depots and penetrated the skin at a lower rate. The relatively hydrophilic [ 14C]salicylamide also had larger surface depots but much lower skin levels. For comparison, the in vitro permeation of the formulations was studied through freshly excised hairless rat skin using Franz diffusions cells, and an agreement between the techniques was found.