Covalent binding of [14C]-2,6-dimethylaniline to DNA of rat liver and ethmoid turbinate

The xylidide 2,6-dimethylaniline (2,6-DMA) has produced carcinomas and papillary adenomas in the nasal cavity of rats at high dietary doses (3000 ppm) in a 2-yr bioassay. The objective of the present study was to measure the covalent binding of 2,6-DMA to DNA of rat ethmoid turbinate tissues and, for comparison, to DNA of rat liver. The potent hepatocarcinogen 2-acetylaminofluorene (AAF) was studied as a positive control for adduct formation and covalent binding index (CBI) calculation. Both 2,6-DMA and AAF were administered as 14C-(ring)-labeled agents to naive rats and to rats pretreated for 9 d with unlabeled 2,6-DMA or AAF. The CBI value for 2,6-DMA adduct formation with ethmoid turbinate DNA was below the assay's sensitivity limit in nonpretreated rats, but increased to 41.9 in rats pretreated with unlabeled 2,6-DMA. It also increased from 0.6 in nonpretreated to 7.9 in liver of pretreated rats. The opposite pattern, however, was observed for AAF. In nonpretreated rats considerable adduct formation was observed in liver (CBI = 271.5) and modest values (CBI = 39.3) were calculated for ethmoid turbinate tissues. Pretreatment with unlabeled AAF caused a significant decrease in CBI values, to 18.3 for liver and less than 0.5 for ethmoid turbinate. The results suggest that there may be value in conducting DNA covalent binding assays in both naive animals and animals pretreated with the test article.