In order to study the reliability of in vivo sister chromatid exchange (SCE) assays for predicting carcinogenicity, several known animal carcinogens were tested in a multicellular in vivo SCE assay and an in vivo/in vitro murine lymphocyte assay. The methylating agents 1,2-dimethylhydrazine.2 HCl (DMH), dimethylnitrosamine (DMN), methylnitrosourea (MNU), methyl methane-sulphonate (MMS), and methylazoxymethanol acetate (MAM) were tested for SCE induction in several murine tissues in vivo, including bone marrow, alveolar macrophages, regenerating and intact liver, and kidney from B6D2F1 mice. In all cell types, clear dose-responses were observed following exposure of mice to subcytotoxic fractions of the LD50 dose of DMH, MNU, or MMS. DMN (0.03-0.27 mmol/kg) produced small, although not dose-related, increases in SCE in all cell types. At the doses tested (0.06 and 0.08 mmol/kg), MAM did not induce elevated SCE in the various cell types. Following a series of multiple i.p. injections of low, non-toxic doses of DMH (0.15 mmol/kg, once a week, for 10 weeks), significant differences were observed in intact vs. regenerating liver and in single vs. multiple injections in regenerating liver. Following exposure of B6D2F1 mice to a single i.p. injection of 0.25 mmol/kg DMN, DMH, or MMS or 0.19 mmol/kg MNU, SCE responses were evaluated in Concanavalin A (Con A)- and LPS-stimulated blood and spleen lymphocytes. Considerable cytotoxicity was observed in blood lymphocytes. In Con A- and LPS-stimulated spleen lymphocytes, DMH-, and DMN- and MMS-induced SCE frequencies were approximately 1.5-2 x baseline levels and MNU-induced SCE were approximately three- to fourfold higher than baseline values in cultures initiated at 1 and 24 h postexposure. At 48 and 72 h after an i.p. injection of 0.131 mmol/kg MNU, SCE responses in lymphocytes were approximately 2 x baseline levels. At 24 h following one, two, or four injections (one/week) of 0.075 mmol/kg MNU dose-related increases in SCE were observed in spleen lymphocytes. These studies illustrate that carefully designed in vivo SCE assays may have the capacity to predict the tumorigenic potential of chemical agents.
Animals Biotransformation