Synthesis and Spectroscopic Characterization of 4-Chlorophenyl Isocyanate (= 1-chloro-4-isocyanatobenzene) Adducts with Amino Acids as Potential Dosimeters for the Biomonitoring of Isocyanate Exposure

Biomarkers for Isocyanate Exposure: Synthesis of Isocyanate DNA Adducts

Isocyanates are important intermediates in industrial manufacturing. DNA adducts and protein adducts are important tools to biomonitor people exposed to xenobiotics. In the present work, the formation of DNA adducts deriving from 4-chlorophenyl isocyanate (4CPI) and 4-methylphenyl isocyanate (4MPI) were explored. The adducts of 4CPI and/or 4MPI with 2'-deoxyadenosine, 2'-deoxyguanosine, and 2'-deoxycytidine were synthesized and characterized by NMR and MS. For low level detection, an LC-MS/MS method was developed. The formation of DNA adducts was confirmed in in vitro reactions with DNA.

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.

N-methylcarbamoylated valine of hemoglobin in humans after exposure to N,N-dimethylformamide: evidence for the formation of methyl isocyanate?

N,N-Dimethylformamide (DMF) is reported to cause testicular germ-cell tumors in exposed workers. The reports, however, are not in line with results obtained in animal and in vitro experiments, where DMF was shown not to be mutagenic and also not to be carcinogenic. Considerable interest raised on the formation of a reactive intermediate, presumably methyl isocyanate (MIC), during metabolism of DMF in humans over the last years. We report the formation of N-methylcarbamoylated valine of hemoglobin (Hb) in blood samples from workers exposed to DMF in the polyacrylic fiber industry. N-Methylcarbamoylated Hb was formed by the reaction of MIC with Hb. For this purpose, Hb adducts were monitored by means of a modified Edman degradation involving the release of the N-terminal valine adduct in form of 3-methyl-5-isopropylhydantoin (MIH). For internal standardization of the method, 3-ethyl-5-isopropylhydantoin (EIH) was used. Separation and analysis of MIH and EIH were carried out by gas chromatography and mass spectrometry with electron impact ionization (GC/EI-MS). Hb adducts in form of MIH were quantified in blood samples from exposed persons in concentrations between 26.1 and 412.0 nmol of MIH/g of globin. The observed adducts were proven to be identical to those derived from the in situ reaction between Hb and MIC. Taken together with the fact that only N-methylcarbamoylated Hb can undergo ring closure to the corresponding hydantoin, the reaction is indirect evidence for the occurrence of MIC in vivo. The formation of MIC directly in the cell and its possible distribution through the human body may lead to critical effects after exposure to DMF. Adducts were determined not to be totally specific for exposure to DMF since an identical adduct was also found in blood samples from the general population. However, concentrations were lower by a factor of about 100. The sources for background adducts are currently unknown.

Isocyanate-specific hemoglobin adduct in rats exposed to 4, 4'-methylenediphenyl diisocyanate

4,4'-Methylenediphenyl diisocyanate (MDI) is the most important of the isocyanates used as intermediates in the chemical industry. Among the main types of damage after exposure to low levels of MDI are lung sensitization and asthma. Protein adducts of MDI might be involved in the etiology of sensitization reactions. It is therefore necessary to have sensitive and specific methods for monitoring the isocyanate exposure of workers. To date, urine metabolites or protein adducts have been used as biomarkers in workers exposed to MDI. However, with these methods it is not possible to determine if the biomarkers result from exposure to MDI or to the parent aromatic amine 4,4'-methylenedianiline (MDA). This work presents a procedure for quantitating isocyanate-specific hemoglobin adducts. Blood proteins are used as markers of exposure and possibly as markers of dose size for the modifications of macromolecules in the target organs where the disease develops. For the quantitation of hemoglobin adducts, N(1)-[4-(4-isocyanatobenzyl)phenyl]acetamide (AcMDI) was reacted with the tripeptide valyl-glycyl-glycine and with valine yielding N-[4-(4-acetylaminobenzyl)phenyl]carbamoyl]valyl-glycyl-glycine and N-[4-[4-(acetylaminobenzyl)phenyl]carbamoyl]valine, respectively. N-[4-[4-(Acetylamino-3,5-dideuteriobenzyl)-2, 6-dideuteriophenyl]carbamoyl]valine was synthesized from valine, as was N(1)-[4-(4-isocyanato-3,5-dideuteriobenzyl)-2, 6-dideuteriophenyl]acetamide, for use as an internal standard. These adducts were cleaved in 2 M HCl to yield the corresponding hydantoins, 3-[4-(4-aminobenzyl)phenyl]-5-isopropyl-1, 3-imidazoline-2,4-dione (MDA-Val-Hyd) and 3-[4-(4-amino-3, 5-dideuteriobenzyl)-2,6-dideuteriophenyl]-5-isopropyl-1, 3-imidazoline-2,4-dione, respectively. In globin of rats exposed to MDI, MDA-Val-Hyd could be found in a dose-dependent manner. The adduct was identified by HPLC/MS/MS and quantified by GC/MS after derivatization with heptafluorobutyric anhydride. The amount of MDA-Val-Hyd found after acid hydrolysis of globin at 100 degrees C is about 12 times larger than the sum of N-acetyl-4, 4'-methylenedianiline (AcMDA) and MDA obtained from mild base hydrolysis of hemoglobin. The MDA-Val-Hyd is an isocyanate-specific adduct. MDA and AcMDA released after mild base hydrolyses result most likely from a sulfinamide adduct which is a typical adduct of arylamines. According to these results, higher amounts of isocyanate adducts than arylamine adducts should be expected in workers exposed to isocyanates.