Carcinogenesis by nitrosobis-(2-oxopropyl)amine labeled with deuterium and by nitroso-2-hydroxypropyl-2-oxopropylamine in rats and hamsters

Studies of initiation and promotion of carcinogenesis by N-nitroso compounds

Complete carcinogens must possess both initiating and promoting properties. Most N-nitroso compounds are mutagens and are considered to be initiators, but some are not mutagenic and yet are complete carcinogens. To investigate the two activities, brief treatments of male F344 rats with each of three mutagens, nitrosodimethylurea, nitrosodiethylurea and nitrosobis-(2-oxopropyl)-amine, were followed by chronic treatment (40 weeks) with one of four non-mutagens, nitrosomethyl-3-carboxypropylamine, nitrosodiethanolamine, nitrosomethyl-2-hydroxypropylamine or phenobarbital, the last being a well-known promotor of liver tumors in rats. Each treatment group consisted of 18 animals and there were control groups of 15 animals without initiation and 15 animals without promotion. All surviving rats were sacrificed at week 78. There were almost no tumors in untreated controls or in groups treated with the promotors, other than bladder tumors in one group. Certain tumors were numerous in the initiated groups, but there were only a few instances of increased incidences after treatment with the promotors. The action of the initiators appeared to be the dominant factor and there was scant indication in this experiment of the induction of tumors by the promotors of promotion of initiated cells in most organs (e.g. the liver). This indicates that it is unlikely that non-genotoxic carcinogens induce tumors by promotion of already-initiated cells, but that some other mechanism prevails.

Local and systemic carcinogenic effects of alkylating carcinogens in rats treated by intravesicular administration

Several nitrosamines and an azoxyalkane have been administered intravesically to groups of 12 female F344 rats, twice a week for 20 or 30 weeks. Many of the nitrosamines were as efficacious in giving rise to the same tumors of internal organs as when similar doses were administered orally, showing that absorption from the bladder was as rapid as from other sites. The tumors produced included lung and kidney tumors by nitrosodimethylamine, colon and Zymbal gland tumors by azoxymethane, liver tumors by methylnitrosoethylamine (but not by nitrosodimethylamine), liver and esophagus tumors by nitrosodiethylamine, liver and lung tumors by methylnitrosamino-3-pyridylbutanone, liver tumors by nitrosomorpholine, and tumors of the esophagus by methylnitroso-n-butylamine, 2,6-dimethyl-nitrosomorpholine and methylnitrosamino-N,N-dimethylethylamine. Bladder tumors were induced by intravesicular administration of only low doses of nitrosobis-(2-oxopropyl)amine and to a lesser extent by methylnitroso-n-hexylamine and nitroso-(2-hydroxypropyl)(2-oxopropyl)amine, which all induced tumors systemically in addition. The bladder mucosa seemed to lack enzymes necessary to activate most nitrosamines to locally acting proximate carcinogens, but was quite transparent to the passage of carcinogenic nitrosamines present in the urine into the body to induce tumors in distant organs.

DNA alkylation by nitrosobis-(2-oxopropyl)amine in rats of different ages

We have examined the methylation of liver DNA (O6- and N7-methylguanine) by nitrosobis-(2-oxopropyl)amine (BOP) in male and female rats at various ages, following treatment with 2.5 mg of BOP; this dose given twice weekly for 30 weeks induces tumors in all animals. Except in young rats there was more methylation in female rat liver than in male rat liver, when adjusted for different sizes of the animals. There were differences in the extent of methylation between young (4 weeks) and older rats, but not between young adult (20 weeks) and old adult (65 weeks) males; the latter developed liver tumors when treated with BOP, and the former did not. There was no obvious relation between increased susceptibility to liver tumor induction by BOP and the extent of alkylation of liver DNA. Methylation of DNA was lower in the kidney than in the liver and, here, there was little difference between the sexes. In the testis there was N7-methylation of guanine in DNA, but no O6-methylguanine was detected.