Effects of feminization of male F344 rats on induction of tumors and on nucleic acid alkylation by nitrosobis-(2-oxopropyl)amine

The anomalous biological activity of nitroso-2-oxopropyl compounds

The carcinogenic action of a set of N-nitroso compounds containing the 2-oxopropyl group was considered in relation to their metabolism and their activity as alkylating agents for DNA. In contrast with the great carcinogenic potency of methylnitrosourea and ethylnitrosourea, comparable with the corresponding dialkylnitrosamines, 2-oxopropylnitrosourea is a weak carcinogen with a limited range of target organs in rats and hamsters. 2-Oxopropylnitrosochloroethylurea was somewhat weaker than 2-oxopropylnitrosourea and similarly induced spleen hemangiosarcomas in hamsters, but few tumors of any kind in rats. The relatively much more potent carcinogenicity of nitrosobis-(2-oxopropyl)amine, nitroso-(2-hydroxypropyl) (2-oxopropyl) amine and methylnitroso-2-oxopropylamine suggests that the activity of an oxopropylating agent is not involved in carcinogenesis by nitroso-2-oxopropylamines. The nitrosamines are likely to undergo extensive metabolism to form proximate carcinogenic moieties, probably including the methyldiazonium ion, which are responsible for the induction of a broad range of tumors in rats and hamsters. These include tumors of the liver, pancreas ducts, lung and nasal mucosa in hamsters, and esophagus, liver, lung, thyroid, kidney, trachea, bladder and nasal mucosa in rats.

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.

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

The effects of the labeling with deuterium of the alpha-methylene groups of the carcinogen nitrosobis-(2-oxopropyl)amine (NBOP) on its carcinogenic effectiveness in rats and hamsters have been studied. The greater strength of the C-D bond compared with the C-H bond often leads to slower metabolism and lesser carcinogenic activity. When NBOP and NBOP-d4 were given to male and female rats in drinking water at equimolar doses, the mortality rate from tumors was lower in the rats given the deuterium-labeled compound, although the results were statistically significant (P = 0.012) only in males. The incidences of tumors of several groups was similar for NBOP and NBOP-d4, but there was a marked difference between males and females, females having a high incidence of liver tumors, and males very few. When NBOP and NBOP-d4 were given by gavage to rats or Syrian hamsters at identical doses there was no difference in rate of mortality from tumors, or in the pattern of tumors induced by either compound. In rats, both compounds were given at two dose rates, and in neither was a difference seen. To complement the studies with NBOP, a normal reduction product formed metabolically in vivo, nitroso-(2-hydroxypropyl) (2-oxopropyl)amine (NHPOP) was administered to rats in drinking water at the same dose rate. In male rats, the mortality rate was lower with NHPOP than with NBOP, while with female rats the opposite was the case (P less than 0.01 in both cases) and there was little difference in the pattern of tumors induced in either sex. NHPOP appears to have a quite distinct carcinogenic effect from NBOP, suggesting that the metabolic conversion of one to the other does not play a large role. The weak deuterium-isotope effect of NBOP-d4 given to rats in drinking water, but not detected in rats or hamsters treated by gavage, suggests that alpha-oxidation of NBOP is not likely to be a rate-limiting step in carcinogenesis by NBOP.

Carcinogenesis and nucleic acid alkylation by some oxygenated nitrosamines in rats and hamsters

A comparison has been made of the carcinogenic effects of nitroso-2,6-dimethylmorpholine and several hydroxylated acyclic nitrosodialkylamines derived from it or related to it in rats and Syrian hamsters. In rats nitrosodimethylmorpholine was the most potent, inducing mainly esophageal tumors. Nitrosodiethanolamine was the weakest of the five nitrosamines in both rats and hamsters. Tumors of the pancreas ducts were induced by four of the five compounds, but only in hamsters, and esophageal tumors appeared only in rats. Most of the nitrosamines induced tumors of liver and lung in both rats and hamsters. A study of alkylation of nucleic acids of the liver following treatment of rats and hamsters with the radiolabeled nitrosamines showed that nitrosodiethanolamine alkylated liver nucleic acids in rats to only a very small extent. The other four nitrosamines all gave rise to 7-methylation and O6-methylation of guanine residues in DNA of hamster liver and all but nitrosodimethylmorpholine in rat liver DNA, which corresponded quite well with the induction of liver tumors in the two species. Quantitatively, however, there was not a good correlation between liver DNA alkylation and the potency of the nitrosamine in inducing tumors.