Isothiocyanates are important as haptens in contact allergy to chloroprene rubber

Contact allergy to chloroprene rubber products is well known. Thiourea compounds are considered the cause of allergy. Diethylthiourea commonly occurs in this type of product and can decompose to the sensitizer ethyl isothiocyanate.

To investigate the clinical importance of degradation products and metabolites from organic thioureas in contact allergy to chloroprene rubber with a focus on isothiocyanates and isocyanates.

Patients with contact allergy to diphenylthiourea were patch tested with phenyl isothiocyanate and phenyl isocyanate. Patients with known contact allergy to diethylthiourea were retested with diethylthiourea, while chemical analyses of their chloroprene rubber products were performed. The stability of diethylthiourea, diphenylthiourea and dibutylthiourea in patch-test preparations was investigated. Liquid chromatography/mass spectrometry and solid-phase microextraction/gas chromatography were used for determination of organic thioureas and isothiocyanates.

All patients allergic to diphenylthiourea reacted to phenyl isothiocyanate, two of eight reacted to phenyl isocyanate and six of eight reacted to diphenylthiourea. Four patients allergic to diethylthiourea reacted at retest; diethylthiourea was detected in all chloroprene rubber samples, with levels of 2-1200 nmol cm^-2 . At 35 °C, ethyl isothiocyanate was emitted from all samples. Patch-test preparations of diethylthiourea, diphenylthiourea and dibutylthiourea all emitted the corresponding isothiocyanate, with diethylthiourea showing the highest rate of isothiocyanate emission.

Thiourea compounds are degraded to isothiocyanates, which are generally strong or extreme sensitizers, thus acting as prehaptens. This process occurs in both chloroprene rubber products and patch-test preparations. Positive reactions to phenyl isocyanate indicate cutaneous metabolism, as the only known source of exposure to phenyl isocyanate is through bioactivation of diphenylthiourea.