An evaluation of the hemizygous transgenic Tg.AC mouse for carcinogenicity testing of pharmaceuticals. I. Evidence for a confounding nonresponder phenotype.

We have completed 2 26-wk studies to evaluate the hemizygous transgenic Tg.AC mouse, which has been proposed as an alternative short term model for testing carcinogenicity. We attempted to evaluate the response to the known rodent carcinogens cyclophosphamide, phenolphthalein, and tamoxifen and to the noncarcinogen chlorpheniramine following topical application. In the first study, a weak response (2/17 animals) was observed to the positive control 12-O-tetradecanoylphorbol 13-acetate (TPA in ethanol, 1.25 micrograms), and no response was observed to cyclophosphamide, phenolphthalein, or chlorpheniramine, despite evidence for skin penetration. The second study compared 1.25 micrograms and 6.25 micrograms of TPA in ethanol and acetone solutions. Tamoxifen was also evaluated in both solvents and orally. No significant response was observed to tamoxifen by skin paint or oral routes. Over 60% of the high dose TPA-treated animals showed no (0 or 1) papilloma response, and 30% of the animals each developed more than 32 papillomas. The heterogenous response to high dose TPA may be related to variability in the responsiveness of hemizygous animals. In light of these findings, further Tg.AC studies should employ homozygous animals, and the underlying cause for heterogeneity in the tumorigenic response of Tg.AC mice should be identified and eliminated.

Aflatoxin Residues in Milk of Dairy Cows after Ingestion of Naturally Contaminated Grain.

Thirty-two lactating Holstein cows, blocked according to level of milk production, were fed cottonseed meal contaminated with aflatoxin B1, (AFB1) (0, 94, 241 and 500 µg/kg) as 20% of their ration (equivalent to 0, 20, 48 and 104 µg/kg in complete feed). Within 12 h, aflatoxin M1 (AFM1) appeared in the milk of all cows receiving contaminated feed. The mean AFM1 concentrations in the milk approached steady-state conditions (0.35, 0.63 and 1.61 µg/L for treatments of 20, 48 and 104 µg AFB1/kg, respectively) at 24 h and returned to the Food and Drug Administration action level of 0.5 µg/L or lower within 24 h after removal of the contaminated feed. The ratio of AFB1 in the feed to AFM1 in the milk averaged 66:1. The mean percent of daily AFB1 intake that was transferred to AFM1 was 1.74. This value was unaffected by the concentration of AFB1 in the feed (1.89, 1.55 and 1.81% transferred for treatments of 20, 48 and 104 µg AFB1/kg, respectively). Although increased milk production had no effect on the concentration of AFM1 in the milk, it had a positive effect (P ≤ 0.01) on the percent of AFB1 intake transferred to AFM1 (2.14 vs 1.35%). In a second trial, 16 additional cows were fed either naturally contaminated cottonseed meal or corn (44 and 49 µg/kg, respectively, on a complete feed basis). The percent of AFB1 intake secreted as AFM1 was affected (P ≤ 0.02) by the source of contamination (1.73 vs. 1.32% for the cottonseed meal and corn treatments, respectively). The AFM1 concentrations in the milk were not significantly different (P>0.05).