Percutaneous penetration of squaric acid and its esters in hairless mouse and human skin in vitro.

Squaric acid diethylester and squaric acid dibutylester have been used in contact sensitization therapy of alopecia areata. This study investigated the application of these esters or squaric acid alone to hairless mouse and human skin in vitro to determine squaric acid flux from the various preparations. Measurable amounts of squaric acid were delivered through skin by squaric acid itself, but flux was lower than for that delivered by the two esters. These results support the proposal by Noster that the esters combine with a protein to form an antigen while squaric acid can not and that this explains why the esters are active in contact sensitization and the acid is not. We suggest that the results of previous studies showing that the diethyl ester of squaric acid was a less effective sensitizer than the dibutyl ester may have been due to decomposition of the ethyl ester to squaric acid.