Induction of Mammary Cancer by 6-Nitrochrysene in Female CD Rats

6-Nitrochrysene (6-NC) is a pollutant generated in diesel exhaust In order to study its propensity to induce mammary cancer, we injected 6-NC into the mammary glands of female CD rats. 4-Nitropyrene (4-NP), the most active mammary carcinogen among the mononitropyrene isomers, was used as a positive control (K. Imaida et al., Cancer Res., 51: 29022907,1991). A total of 2.04 μmol of each compound in dimethyl sulfoxide was injected into each of the six mammary glands on the left side of weanling rats. The corresponding glands on the right side received injections of dimethyl sulfoxide. The thoracic glands were treated on Day 1 and those located in the inguinal area were treated on Day 2. Rats were sacrificed after 43 wk. 6-NC induced fibroadenomas, adenocarcinomas, and spindle cell sarcomas of the mammary glands in a high percentage of the rats. The numbers of animals with mammary tumors and the numbers of malignant tumors were significantly higher in the group treated with 6-NC than in those receiving 4-NP or dimethyl sulfoxide alone. The results of this study, taken together with those of previous bioassays, demonstrate the remarkable activity of 6-NC as a mammary, colon, and lung carcin-ogen in rodents.