Formation of DNA adducts of the food-derived mutagen 2-amino-9H-pyrido-[2,3-b]indole (A(α)C) and bioassay of mammary gland carcinogenicity in Sprague–Dawley rats

2-amino-9H-pyrido[2,3-b]indole (AαC) is a heterocyclic amine found at relatively high concentrations in barbecued or grilled meats. In the current study, the mammary gland carcinogenicity of AαC was examined in female Sprague–Dawley rats given 10 doses of AαC (75 mg/kg, orally, once per day starting at 43 days of age) and placed on a defined high-fat diet (23.5% corn oil), a strong promotional factor for rat mammary gland carcinogenesis. Within 1 year, one out of 20 rats dosed with AαC developed a tubulopapillary carcinoma, indicating that the bioassay was largely negative. As DNA adduct formation is considered to play a role in carcinogenesis, AαC–DNA adduct levels were measured in the mammary gland and other tissues by the 32P-postlabelling method. Under intensification conditions, one major adduct and up to three minor adducts were detected in isolated mammary gland epithelial cells and other tissues (liver, stomach, small intestine, colon and kidney) of AαC-treated rats; the adduct patterns were similar in all tissues examined. The major adduct, comprising 60–100% of total DNA adduct levels in tissues, was chromatographically identical to the principal adduct found in 3′-dGp-AαC (synthesized by reacting 3′-phospho-2′-deoxyguanosine (3′-dGp) with N-acetoxy-AαC). Of the tissues examined, the highest AαC–DNA adduct levels were found in the liver. In male rats given a single dose of AαC (75 mg/kg, orally, 3 hr prior to necropsy), no AαC–DNA adducts were detected in extrahepatic tissues. In female rats given a single dose or 12 daily doses of AαC, hepatic DNA adduct levels were at least 12–13-fold higher than those in any other tissue. Mean total AαC–DNA adduct levels in mammary gland epithelial cells and liver from female rats given multiple doses of AαC were 3.5 and 50.7 (RAL×107), respectively. Although factors in addition to DNA adduct formation are likely to play a role in mammary gland carcinogenesis, the results suggest that the weak mammary gland carcinogenicity of AαC may in part be associated with low AαC–DNA adduct levels in the mammary gland epithelium.