Combination experiments with very low doses of three genotoxic N-nitrosamines with similar organotropic carcinogenidty in rats

Abstract

The study was designed to assess the syncarcinogenic activity of very low doses of N-nitrosodiethylamine (NDEA), N-nitrosopyrrolidine (NPYR) and N-nitrosodiethanolamine (NDElA) in the liver of 1800 male Sprague-Dawley rats. The N-nitrosamines were administered throughout the rats' lives individually and in combination at three logarithmically spaced dose levels contained in drinking water. The dose levels in the individual dose-response experiments ranged from the lowest concentrations of previous experiments (NDEA, 0.1 mg/kg; NPYR, 0.4 mg/kg; NDElA, 2.0 mg/kg) to dosages 10 times lower and comprised a high, medium and low dose (escalation factor: 3.16). The high dose of the combination contained the three nitrosamine concentrations used as the medium doses of the individual nitrosamines. The medium combination dose resulted from the combined administration of the three lowest dosages, and the low combination dose consisted of three nitrosamine dosages which amounted to one-third of the low dosages respectively. Administration of these dosages was associated with a dose-dependent incidence of liver cancer: NDEA induced 45, 3.8 and 2.5%; NPYR caused 21.3, 5 and 1.3%; NDElA generated 7.5, 1.3 and 2.5%; and the combinations induced 16, 4.2 and 1.7% respectively. Untreated controls showed 0.6% liver cancer incidence. Besides the liver, the gastrointestinal tract, the neurogenic tissue, the urinary tract and the hematopoietic and lymphatic tissue were affected by tumor incidences increased over that of controls. There was, however, no well-defined dose dependency as with the liver tumors. These results indicate dose dependency of liver tumor formation even at very low exposure levels of the individual agents. The carcinogenic effects of the hepatotropic N-nitrosamines summed up in combination. The observed additivity was linear. Dose levels, which alone would presumably not have been carcinogenic, effected a significant cancer risk in combination.

Carcinogenic Effects of Mixtures of Chemicals

“…However, at low dose levels, this is not always the case; thus significant findings were that a low dose combination of three carcinogenic nitrosamines did not e x e r t s u m m a t i o n o r s y n e r g i s m r e g a r d i n g t u m o r development 10 , and a combination of 20 or 40 pesticides at A D I l e v e l s r e s u l t e d i n n o e n h a n c e m e n t o f hepatocarcinogenesis, but at doses at the NOEL, removing the safety factor of ×100, synergistic enhancement of GST-P positive foci was demonstrated 21 . From the theoretical point of view, the safety factor reflects both the possibility of an increased sensitivity of humans relative to laboratory animals and the variation in susceptibility within the human population, and that for ADI used by the Japanese Ministry of Health, Labour and Welfare and the FAO/WHO is usually 100.…”

“…Berger et al 10 For combination studies, three different dose levels were set, the high dose combination being composed of the middle doses of individual carcinogens, the middle dose combination being constituted by the lowest doses of each given individually and the low dose combination consisting of one-third of each of the lowest doses. They found that all carcinogens individually as well as in combination induced liver cancers in a dose-dependent manner.…”

Genotoxicity of N -nitrosodicyclohexylamine in V79 cells in the sister chromatid exchange test and the single cell gel assay

“…For example, in vitro genotoxicity studies on N-nitrosodiphenylamine [CAS: 86±30±6] were inconclusive or negative (McGregor 1994) with the exception of one study, where microcolony induction in S. typhimurium TA104 was observed in the presence of arochlor-1254 induced S9 at pH 6.5 (Zielenska and Guttenplan 1988). The potent experimental carcinogen N-nitrosopyrrolidine (Berger et al 1987) can only be eectively activated to Ames test positive metabolites by pyrazole-induced rat liver S9 (Burke et al 1994). Pyrazole is known to induce CYP2E1 in rat liver.…”

The Fifth Plot of the Carcinogenic Potency Database: Results of Animal Bioassays Published in the General Literature through 1988 and by the National Toxicology Program through 1989

“…The TD5o, our numerical index of carcinogenic potency, has been fully described (1,5,6) and may be briefly defined as follows: Fora given target site(s), if there are no tumors in control animals, then TDso is the chronic dose rate in mg/kg body weight/day that would induce tumors in half the test animals at the end of a standard lifespan for the species. Because the tumor(s) of interest often does occur in control animals, TD5o is more precisely defined as the chronic dose rate that will halve the probability of remaining tumor-free throughout the standard life span.…”

“…(carcinogenic potency) and its statistial significance, dose response, author's opinion about carcinogenicity, and literature citation. We refer the reader to the 1984 publications (1,5,6) for a guide to the plot of the database, a complete description of the numerical index ofcarcinogenic potency, and a discussion ofthe sources of data, the rationale for the inclusion of particular experiments and particular target sites, and the conventions adopted in summarizing the literature. The five plots of the database are to be used together, as results ofindividual experiments that were published earlier are not repeated.…”