Thomas J. Flammang

METABOLIC ACTIVATION CAPACITY OF NEONATAL MICE IN RELATION TO THE NEONATAL MOUSE TUMORIGENICITY BIOASSAY

The neonatal mouse tumorigenicity bioassay is a well-developed animal model that has recently been recommended as an alternative tumorigenicity bioassay by the International Conference on Harmonization (ICH) for Technical Requirements for the Registration of Pharmaceuticals for Human Use. There are sufficient data to conclude that this animal model is highly sensitive to genotoxic chemical carcinogens that exert their tumorigenicity through mechanisms involving the formation of covalently bound exogenous DNA adducts that lead to mutation. On the other hand, it is not sensitive to chemical carcinogens that exert tumorigenicity through a secondary mechanism. The metabolizing enzymes present in the neonatal mouse, particularly the cytochromes P450, are critical factors in determining the tumorigenic potency of a chemical tested in this bioassay. However, compared to the metabolizing enzymes of the adult mouse and rat, the study of the metabolizing enzymes in neonatal mouse tissues has been relatively limited.

NEONATAL MOUSE TUMORIGENICITY BIOASSAY

The International Conference on Harmonisation (ICH) has suggested the use of the newborn mouse bioassay as an alternative tumorigenicity assay. There are sufficient data to conclude that this animal model is highly sensitive to chemical carcinogens that exert their action through mechanisms involving the formation of covalently bound DNA adducts (exogenous adducts) of the administered compound and the processing of these adducts to mutations. Mechanistic studies, including metabolism, DNA adduct formation, and ras-oncogene activation, presented in this paper aid in the interpretation of tumor experiments. By comparison, the data thus far obtained suggest that this bioassay is very likely insensitive to some indirect-acting chemical carcinogens. Ongoing studies are focused on the sensitivity of this bioassay to indirect-acting carcinogens that are believed to exert their tumorigenicity through secondary mechanisms, including: 1. induction of lipid peroxidation and increases in endogenous DNA adducts, 2. endocrine disruption, and 3. induction of peroxisome proliferation. A systematic approach to validate the neonatal mouse tumorigenicity bioassay as a part of a risk identification procedure is proposed. This approach combines the tumorigenicity bioassay with several simple and convenient supportive mechanistic experiments of study so that direct-acting chemical carcinogens, indirect-acting carcinogens, and noncarcinogens can be distinguished.

Development of an Avidin-Biotin Amplified Enzyme-Linked Immunoassay for Detection of DNA Adducts of the Human Bladder Carcinogen 4-Aminobiphenyl

4-Aminobiphenyl (ABP) is a known human urinary bladder carcinogen which is present in tobacco smoke and may be ubiquitous in the environment. As a biological monitor of carcinogen exposure, we have developed an immunological method for measuring the predominant carcinogen-DNA adduct of ABP, N-(deoxyguanosin-8-yl)-ABP (dG-C8-ABP). Rabbits were immunized with keyhole limpet hemocyanin (KLH) conjugate prepared by a periodate oxidation and coupling of N-(guanosin-8-yl)-ABP (rG-C8-ABP) to the protein. The resulting polyclonal antisera was systematically characterized using dual inhibitor methodology augmented by specialized computer and software support; and a competitive avidin-biotin enzyme-linked immunoassay (A-B ELISA) assay employing polyclonal rabbit anti-KLH-(rG-C8-ABP) was developed. Under the assay conditions described, the detection limit for dG-C8-ABP was 18 fmol/well. The relative lack of reactivity toward ABP, N-acetyl-4-aminobiphenyl, N-(deoxyadenosin-8-yl)-ABP, N-(deoxyguanosin-8-yl)-2-aminofluorene and deoxyguanosine as inhibitors indicated that primary specificity involves epitopes found on the purine and biphenyl rings. Results emphasize the need to define polyclonal anti-adduct sera operationally in the context of the antigen/assay system used to evaluate it. Assay sensitivity was achieved by decreasing the amount of antibody and solid-phase antigen in the competitive portion of the assay and the use of avidin-biotin as well as enzymatic amplification. This methodology is a useful alternative to other ultrasensitive techniques and should be directly applicable to the detection of ABP-DNA adducts in exposed human populations.