Squaric-acid-diethylester - a strong sensitizer

A patient developed a circumscribed contact dermatitis after a single contact with squaric-acid-diethylester (SADE). Patch tests with low concentrations of SADE were positive. Based on these observations, experimental studies on the sensitizing capacity of SADE were performed on guinea-pigs. The results disclosed that this compound is a strong sensitizer. Derivatives of SADE, such as the nearly insoluble squaric acid, squaric acid-1,2-diamide and dithio-N,N'-dicyclohexyle-1,2-diamide were not able to produce allergic skin reactions in the sensitized patient nor in guinea pigs. The chemistry and the range of uses of squaric acid and its derivatives are briefly mentioned. A hypothesis on the molecular mechanism of conjugation of SADE to human proteins and thereby on sensitization is discussed. A warning is given for taking precautionary measures before handling this substance.