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

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.