Isocyanates are known to react with thiols, rapidly and reversibly, under physiological conditions. The biological significance of the possible formation of these thiol acid esters following exposure to diisocyanates used in polyurethane production, such as MDI, is not known. Reaction of MDI to cysteine or glutathione produced an insoluble product. A water soluble product was produced when MDI was reacted to cysteine methyl ester (CME). The resultant thiol acid ester (MDI-CME) was characterized by full NMR assignment and mass spectroscopy. Approximately one million fibroblast in the exponential phase of growth were seeded into a 100 mm culture dish and cultured over night. The medium was changed to phosphate buffer saline (PBS) and the cells were exposed for 2 hrs to 1.25,2.5, 5 or 10 ug/ml MDI-CME/PBS. Cells were washed and incubated for 20 hrs in culture medium with the cytokinesis blocker, Cytochalasin B. Cells were then washed, fixed, stained and scored for micronuclei (MN) in binucleated cells. Vehicle (water), CME, and positive vincristine sulfate controls were run, concurrently. MDI-CME caused a dose-dependent increase in MN. Significant increases in MN were noted at >/= 2.5 ug/ml. A decrease in the nuclear division index denoting loss of viability was noted at the highest dose (10 ug/ml). MDI-CME was more potent than the insoluble MDI thiol acid esters, as well as, the MDI hydrolysis product,4,4'-methylenedianaline at inducing MN. Preliminary data also suggest that MN in alveolar macrophages may be increased in mice exposed to MDI-CME by intratracheal installation. These results underscore the need for investigation of the possible in vivo formation of MDI thiol acid esters following exposure to MDI.
The Toxicologist. Society of Toxicology 40th Annual Meeting, March 25-29, 2001, San Francisco, California