Chemical behaviour of seven aromatic diisocyanates (toluenediisocyanates and diphenylmethanediisocyanates) under in vitro conditions in relationship to their results in the Salmonella/microsome test

There are conflicting results on the mutagenicity of toluenediisocyanate (T DI) and diphenylmethanediisocyanate (MDI). It was found that the organic so lvent chosen to dissolve the compounds dictates the outcome of the bacteria l tests. The Salmonella/microsome tests showed uniformly mutagenic effects for all the compounds that were predissolved in DMSO. Due to the instabilit y of aromatic diisocyanates in DMSO this solvent was replaced by ethylenegl ycoldimethylether (EGDE). TDI and MDI endured the dissolving and were there fore still available for the subsequent bacterial tests. Furthermore, no ar omatic diamines (TDA or MDA) could be detected in EGDE prior to the start o f the assays. The Salmonella/microsome tests, however, revealed unexpected differences between TDI and MDI. As previously published the four types of MDI showed negative results, whereas the data presented in this paper demon strated mutagenic effects of all three types of TDI if EGDE is the solvent. To gain deeper insight into the chemical changes that occurred during the Salmonella/microsome test, the possible reactions were modelled in the labo ratory by mixing predissolved diisocyanates with a defined surplus of water and monitoring the progress of the chemical reactions by analytical method s. Additionally, the quality of the model was checked by exposing solutions of 2,6-TDI and 4,4'-MDI to the real biological test environment. In both c ases, the reaction patterns of TDI were different to those of MDI. Within 1 min, which is the maximum time needed to mix the predissolved compounds wi th water before they are poured onto the agar plate, the TDI content was re duced in favour of different ureas and TDA. In addition water was replaced by the complete set of test ingredients. While the TDA content remained mor e or less constant, the amount of residual TDI was reduced considerably. Re actions of MDI were markedly slower than those of TDI. More than 90% of the predissolved MDI remained intact when it was mixed with water. The biologi cal test ingredients accelerated the reduction of the MDI content. Within 4 5 s, more than two thirds of the MDI disappeared. Evidently, the chemical r eactions continue during incubation. It is assumed that the contrasting res ults of TDI and MDI in the Salmonella/microsome test are due to the differe nt reaction patterns-and reaction products-of the predissolved diisocyanate s created under the specific conditions of the test. These findings indicat e that the chemical interactions between reactive test compounds and solven ts or test media need to be considered in the interpretation of the relevan ce of test results.(C) 1999 Elsevier Science B.V. All rights reserved.