Persistence of DNA single-strand breaks and other tests as indicators of the liver carcinogenicity of 1-nitroso-5, 6-dihydrouracil and the noncarcinogenicity of 1-nitroso-5, 6-dihydrothymine

The cyclic nitrosourea 1-nitroso-5, 6-dihydrothymine [(NDHT) 1-nitrosodihydrothymine] was not significantly carcinogenic when it was administered for 1 year in drinking water (206 mg/liter) to MRC-Wistar rats. In acute toxicity tests, ip injection of saline solutions of 1-nitroso-5, 6-dihydrouracil [(NDHU) CAS: 16813-36-8; 1-nitrosohydrouracil], a strong liver carcinogen in rats, produced only mild liver toxicity but marked focal degeneration of myocardial fibers. NDHU injected ip in water solution produced subcapsular liver damage. NDHU, but not NDHT, induced unscheduled DNA synthesis in hepatocyte primary cultures. NDHU, NDHT, and methylnitrosourea [(MNU) CAS: 684-93-5; N-methyl-N-nitrosourea], a liver carcinogen only under special conditions, were tested for their ability, when injected ip into rats, to produce liver DNA damage measured as strand breaks by alkaline sucrose gradient centrifugation. The three nitrosoureas produced similar maximum DNA damage of 2.2–3.2 strand breaks/10⁸ daltons. Eighty percent of the damage due to NDHU persisted for 7 days, and the damage at that time was significantly greater than that produced by NDHT and MNU. The varying persistence of liver DNA damage may explain why NDHU, but not NDHT, is a liver carcinogen.