Hemizygous Tg.AC transgenic mouse as a potential alternative to the two-year mouse carcinogenicity bioassay: evaluation of husbandry and housing factors.

The dermal Tg.AC transgenic mouse model has been proposed as a potential alternative to the conventional (e.g. oral, dermal, parenteral, inhalation, etc.) 2-year rodent bioassay for detecting chemical carcinogenicity. The present study was designed to address a number of technical aspects of this model as well as to augment the database being developed with the Tg.AC system at the NIEHS. Hemizygous Tg.AC mice were implanted s.c. with microchips for identification and housed individually in polycarbonate (i.e. 'plastic') or suspended stainless-steel wire-bottom (i.e. 'metal') cages. Treatment consisted of dermal application of the test or control material in treatment volumes of 200 microl of acetone. Groups of 10 males and 10 females were treated as follows: G1--shaved, no treatment; G2--acetone control seven times a week; G3--100 microl of benzene three times a week; G4--150 microl of benzene three times a week; G5--1.25 microg of phorbol ester (PMA) twice a week. The G1-G5 mice were housed in plastic caging with Alpha-dri bedding. Three additional groups were housed in stainless-steel wire-bottom caging: G6--shaved, no treatment; G7--acetone control seven times a week; G8--1.25 microg of PMA twice a week. The PMA-treated mice (G5 and G8) served as the positive controls. Mice were treated for 20 weeks followed by a 6-week recovery period prior to necropsy. The incidence of dermal papillomas in the shaved area was recorded weekly. There were no spontaneous papillomas in the target area of any of the untreated (G1) or vehicle control (G2) animals in the polycarbonate cages. One papilloma was observed in the untreated mice (G6) and one in the vehicle control group (G7) in the steel cages. This suggests that the type of caging, the shaving process, microchip implantation and daily acetone treatment for 20 weeks are all consistent with a very low background incidence of papillomas in this model. Papillomas were observed in the positive control groups as early as 4 weeks of treatment and increased both in number per mouse and number of mice affected up to a maximum average of 3.5 papillomas per mouse and 55% (11/20) mice with papillomas in G5 and 2.7 and 80% (16/20) in G8. A plateau was reached at about week 13 and the numbers of papillomas remained stable through the rest of the treatment and recovery phases. The low dose of benzene (100 microl) showed no significant effect, whereas the higher dose (150 microl) produced a moderate number of papillomas beginning at about week 11. The results of this study are comparable with earlier studies at the NIEHS and indicate reproducibility between laboratories and that the Tg.AC transgenic mouse model is suitable for use in an industrial pre-clinical safety evaluation context.