Mutational response at the splenic T-lymphocyte hprt locus in mice treated as neonates: contrasting effects of the carcinogens N-ethyl-N-nitrosourea, dimethylnitrosamine, and 2-amino-1-methyl-6-phenylimidazo[4,5-b]pyridine.

The newborn mouse tumorigenicity assay, which involves the treatment of animals during the first two weeks after birth and monitoring tumor induction after a year, has been suggested as a cost- and time-effective alternative to the conventional two-year rodent bioassay. In order to evaluate whether or not lymphocyte hprt mutant induction is an accurate predictor of carcinogenicity in the assay, we determined the frequencies of 6-thioguanine-resistant (TGr) lymphocytes in the spleens of mice neonatally treated with the carcinogenic mutagens N-ethyl-N-nitrosourea (ENU), dimethylnitrosamine (DMN), and 2-amino-1-methyl-6-phenylimidazo[4,5-b]pyridine (PhIP). Male C57BL/6 pups were injected on postnatal days 8 and 15, and the frequency of TGr T-lymphocytes was measured in groups of three animals, sacrificed periodically up to 31 weeks post-treatment. Compared to background frequencies of 1.1-2.9 x 10(-6), mutant frequencies (MFS) reached 155.1 x 10(-6) following a cumulative dose of 49 mg ENU/kg body weight and 172.3 x 10(-6) following a cumulative dose of 142 mg ENU/kg. These results show that TGr lymphocyte mutations can be induced and measured in mice treated as neonates and that the induced MFs found for mice treated neonatally with ENU are comparable with frequencies reported for the treatment of adult animals with the same chemical. In contrast, treatment with the promutagenic and procarcinogenic compounds DMN (at a maximum concentration of 10.5 mg/kg) and PhIP (26.2 mg/kg) did not result in an increase in lymphocyte MF, suggesting that reactive metabolites of these compounds may not be reaching cells that are sensitive for mutation fixation. The results indicate that the lymphocyte hprt assay may fail to predict the carcinogenicity of some test chemicals in the neonatal mouse bioassay.