1. The strong inhibition in the rat of hepatic carcinogenesis due to the feeding of 0.054 per cent of 3′-methyl-4-dimethylaminoazobenzene (3′-methyl-DAB) by the administration of 0.0033 per cent of 3-methylcholanthrene (MC) has been confirmed. At equimolar levels 3,4-benzpyrene and 1,2,5,6-dibenzanthracene also inhibited hepatic carcinogenesis strongly, 1,2-benzanthracene was a moderately strong inhibitor, 9,10-dimethyl-1,2-benzanthracene and its photooxide were weak inhibitors, and pyrene was without effect. One-half the level of 3′-methyl-DAB (0.027 per cent) induced few tumors in this assay in which the compounds were fed for only 3 months. The inhibitory effect of MC was overcome by increasing the level of 3′-methyl-DAB to 0.108 per cent. Administration of MC with 4′-fluoro-DAB or 2′,4′-difluoro-DAB also inhibited their hepatocarcinogenic actions.

  2. Administration of MC with 2-acetylaminofluorene or 7-fluoro-2-acetylaminofluorene strongly inhibited the induction in rats of tumors at the four sites (liver, mammary gland, ear duct, and small intestine) normally affected by these compounds under our conditions.

  3. Metabolic studies on rats fed 3′-methyl-DAB alone or with one of the hydrocarbons indicated that the hydrocarbons protected against the induction of tumors by causing the liver to maintain high levels of certain enzyme systems which metabolize the dye to less active or inactive derivatives. Thus, administration of 0.054 per cent of 3′-methyl-DAB resulted in a marked loss of the ability of liver homogenates to N-demethylate and to reduce the azo linkage of aminoazo dyes. A similar loss of activity occurred when pyrene was fed with the dye, but not when a hydrocarbon protecting against carcinogenesis was administered. When one of the protective hydrocarbons was fed with 0.054 per cent of 3′-methyl-DAB, the levels of free aminoazo dye in the liver and blood and of protein-bound dye in the liver were similar to those found in rats fed only one-half as much dye. The levels of free and bound dye were altered to a lesser extent by the feeding of 1,2-benzanthracene and were not affected by feeding pyrene with the dye.

  4. The hepatic riboflavin levels of rats fed 3′-methyl-DAB alone or with the various hydrocarbons could not be correlated with the incidences of tumors obtained under these conditions.

*

This investigation was supported in part by Grant C355 of the National Cancer Institute, United States Public Health Service, and by a grant from the Alexander and Margaret Stewart Trust Fund. Parts of these data were presented before meetings of the American Association for Cancer Research (16, 20).

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