Bioactivation of the nasal toxicant 2,6-dichlorobenzonitrile: An assessment of metabolic activity in human nasal mucosa and identification of indicators of exposure and potential toxicity

The herbicide 2,6-dichlorobenzonitrile (DCBN) is a potent nasal toxicant in rodents; however, it is not known whether DCBN causes similar nasal toxicity in humans. The tissue-selective toxicity of DCBN in mouse nasal mucosa is largely dependent on target tissue bioactivation by CYP2A5. The human orthologues of CYP2A5, CYP2A6 and CYP2A13, are both expressed in nasal mucosa and are capable of activating DCBN. In this study, we directly determined the ability of human nasal mucosa to bioactivate DCBN. We also tested the suitability of a glutathione conjugate of DCBN (GS-DCBN) or its derivatives as biomarkers of DCBN exposure and nasal toxicity in mouse models. We found that human fetal nasal mucosa microsomes catalyze the formation of GS-DCBN, with a K_m value comparable to that of adult mouse nasal mucosa microsomes. The activity of the human nasal mucosa microsomes was inhibited by 8-methoxypsoralen, a known CYP2A inhibitor. GS-DCBN and its metabolites were detected in the nasal mucosa and nasal-wash fluid obtained from DCBN-treated mice, in amounts that increased with escalations in DCBN dose, and they were all still detectable at 24 h after a DCBN treatment (at 10 mg/kg). Further studies in Cyp2a5-null mice indicated that GS-DCBN and its metabolites in nasal-wash fluid were generated in the nasal mucosa, rather than in other organs. Thus, our data indicate for the first time that the human nasal mucosa is capable of bioactivating DCBN and that GS-DCBN and its metabolites in nasal-wash fluid may collectively serve as indicators of DCBN exposure and potential nasal toxicity in humans.