Medium-term bioassays for carcinogenicity of chemical mixtures

Carcinogenic effects of chemical mixtures were examined with a medium-term liver bioassay for carcinogens or a multiorgan medium-term bioassay using m ale F344 rats. in the medium-term liver bioassay, rats were initially treat ed with diethylnitrosamine (DEN) at 200 mg/kg body weight, ip, after 2 week s they received chemical mixtures such as 10 different heterocyclic amines at one-tenth or one-hundredth the dose levels used in carcinogenicity studi es and the mixtures of 20 different pesticides, each at acceptable daily in take (ADI) levels or a mixture of 100 rimes ADI levels. All animals were su bjected to two-thirds partial hepatectomy at week 3 and were sacrificed at week 8. The numbers and areas of glutathione S-transferase placental form ( GST-P) positive foci (preneoplastic lesions in the liver) were compared bet ween respective groups. When 10 heterocyclic amines were mixed in the diet at one-tenth dose level, clear synergism was observed, but no combined effe cts were evident with the one-hundredth dose levels. In the pesticide exper iment, treatment of rats with the 20-pesticide mixture at the ADI dose leve l did not enhance GST-P-positive foci. In contrast, a mixture of 100 times the ADI significantly increased those values. In a multiorgan bioassay of 2 8 weeks, mixtures of 40 high-volume compounds and 20 pesticides (suspected carcinogens) added together at their respective ADI levels did not enhance carcinogenesis in any organs initiated by five different carcinogens (DEN, N-methylnitrosourea, dimethylhydrazine, N-butyl-N-(4-hydroxybutyl)nitrosami ne, and dihydroxy-di-n-propyinitrosamine) in combination. The combination e ffect of low dietary levels of five antioxidants, butylated hydroxyanisole, caffeic acid, sesamol, 4-methoxyphenol, and catechol, were also examined u sing the multiorgan bioassay. The incidence of forestomach papillomas was s ignificantly increased only in the combination group and the results indica te that combination of the five antioxidants can exert additive/synergistic effects on tumorigenesis in the multiorgan bioassay. These results indicat e that chemical mixtures at very low doses did not enhance preneoplastic le sions synergistically but the mixtures at certain doses show synergism in t he target organ. The medium-term bioassays are particularly useful tools fo r this purpose.