Dosimetry of O6-methylguanine in rat DNA after low-dose, chronic exposure to N-nitrosodimethylamine (NDMA). Implications for the mechanism of NDMA hepatocarcinogenesis.

Groups of female Wistar Furth/NCr rats, aged 6 weeks or 7 months at the start of the experiment, were administered drinking water containing N-nitrosodimethylamine (NDMA) for up to 28 days at concentrations in the range 0.2-2.64 p.p.m., resulting in daily intakes in the range 28-372 micrograms/kg/day at age 10 weeks. The levels of the premutagenic DNA adduct O6-methylguanine (O6-meG) in liver and blood leukocyte DNA were measured at different times during this exposure as well as on the days immediately following cessation of exposure. The adduct was found to accumulate rapidly in both tissues, reaching within 2-7 days steady states in the range 0.08-0.45 mumol/molG, similar in young and adult animals. Accumulation of O6-meG in blood leukocytes was approximately 30% lower than in the liver. Following cessation of NDMA treatment, adducts were lost rapidly from the DNA of both tissues, with an apparent t1/2 of approximately 19-23 h for the liver and 30-35 h for blood leukocytes. No change in liver O6-alkylguanine-DNA alkyltransferase (AGT) took place throughout this treatment. The steady-state adduct levels were approximately linearly related to NDMA dose-rate, except that a clear break (corresponding to a 2.6-fold lower slope) was observed at dose rates > 0.4 p.p.m. (approximately 56 micrograms/kg/day). This dose-response relationship is in contrast to the sharp increase in the liver tumour induction in rats chronically treated with similar concentrations of NDMA reported by Peto et al. (Cancer Res., 51, 6415-6451) and suggests that accumulation of O6-meG cannot by itself account for the hepatocarcinogenic efficacy of NDMA in the rat. Following i.g. administration of single doses of NDMA in a range approximately corresponding to the daily intake during the above mentioned chronic exposure study (25, 50 or 100 micrograms/kg), repair of O6-meG followed sharply biphasic kinetics in both liver and blood leukocytes. In the liver, an initial rapid phase (t1/2 approximately 1.5-1.7 h) was followed by much slower repair (t1/2 approximately 20.7-24.9 h) despite the absence of any change in AGT. Extrapolation of the two segments of the repair kinetics plots back to 0 time suggests that approximately 52-61% of the adducts originally formed in the liver and 33-35% of those formed in blood leukocytes belonged to the rapidly repaired category.(ABSTRACT TRUNCATED AT 400 WORDS)