Comparison of the binding potential of various diisocyanates on DNA in vitro

Chronic inhalation exposures of Fischer 344 rats to 1,6-hexamethylene diisocyanate did not reveal a carcinogenic potential

The polyisocyanates of 1,6-hexamethylene diisocyanate (HDI) find widespread commercial use as components of paints and in the formulation of light-stable polyurethane coating materials. This 2-year study assessed the oncogenicity of the diisocyanate monomer HDI in male and female Fischer-344 rats exposed 6 h/day, 5 days/week to mean analytical air concentrations of 0, 0.005, 0.025, and 0.164 ppm HDI. During the in-life phase, transient eye irritation was observed in 0.164 ppm males, and a slight body weight decrease (5%) in the 0.164 ppm females during the second year of exposure. There were no exposure-related effects on mortality. Compound-related, non-neoplastic histopathologic changes were limited to the respiratory tract and changes were characterized by epithelial tissue reaction to the acute irritant properties of HDI vapor. For tissues of the nasal cavity, the major histopathologic findings were degeneration of the olfactory epithelium characterized by destruction of the epithelial architecture often with narrowing or atrophy and occasional focal erosion or ulceration. In addition, there was variable degeneration of the respiratory epithelium with hyperkeratosis of the epithelium, epithelial and mucus secretory cell hyperplasia, squamous metaplasia, chronic-active inflammation, and errosive or ulcerative changes. These tissue effects along with a statistically significant decrease in body weight of female rats demonstrated attainment of a maximum tolerated dose. There was no evidence of progression of these changes in the nasal epithelium to neoplasia nor evidence of any compound-related neoplastic lesions for any of the other tissues examined. Therefore, it is concluded that HDI did not show a carcinogenic potential in this study.

Carcinogenic risk of toluene diisocyanate and 4,4'-methylenediphenyl diisocyanate: epidemiological and experimental evidence

Diisocyanates are highly reactive compounds widely used, for example, in the production of polyurethane foams, elastomers, paints, and adhesives. The high chemical reactivity of these compounds is also reflected in their toxicity: diisocyanates are one of the most important causes of occupational asthma but also other adverse effects, such as irritation and toxic reactions, have been described in exposed subjects. One of the open questions is whether occupational isocyanate exposure is a carcinogenic hazard. The few epidemiological studies available have been based on young cohorts and short follow-up and are not conclusive. Toluene diisocyanate (TDI) has been classified as carcinogenic in animals on the basis of gavage administration studies, but no conclusions are available on inhalation exposure. For 4,4'-methylene diphenyldiisocyanate (MDI) there is suggestive evidence for carcinogenicity in rats. The possible carcinogenic mechanism of TDI and MDI is not clear. Both chemicals have been positive in a number of short-term tests inducing gene mutations and chromosomal damage. The reactive form could be either the diisocyanate itself or may derive from the metabolic activation of the aromatic diamine derivatives formed by hydrolysis. TDI and MDI react with DNA in vivo and in vitro. However, the structure of the adducts has not been identified. Especially from the in vivo experiment it is not known if the adducts are a product from the reaction with the isocyanate or the corresponding amine. In conclusion, both TDI and MDI are highly reactive chemicals that bind to DNA and are probably genotoxic. The alleged animal carcinogenicity of TDI and MDI would suggest that occupational exposure to these compounds is a carcinogenic risk. The few epidemiological studies available have not, however, been able to clarify if TDI and MDI are occupational carcinogens.