Analysis of synergism in hepatocarcinogenesis based on preneoplastic foci induction by 10 heterocyclic amines in the rat.

The effects of simultaneous treatment with 5 or 10 heterocyclic amines at low dose levels on hepatocarcinogenesis in rats were investigated using a medium-term liver bioassay protocol based on the two-stage carcinogenesis hypothesis with diethylnitrosamine initiation (200 mg/kg, i.p.). Five carcinogenic heterocyclic amines in experiment 1 (Trp-P-1, Glu-P-2, IQ, MeIQ, MeIQx) and experiment 2 (Trp-P-2, Glu-P-1, MeAalphaC, AalphaC, PhIP) were administered together or individually in the diet at levels of 1/1, 1/5, or 1/25 carcinogenic doses, and all 10 chemicals were given at 1/10 or 1/100 levels in experiment 3. Induction of preneoplastic glutathione S-transferase placental form (GST-P)-positive foci in the liver was generally increased in the combination groups over the sums of the 5 or 10 individual effects. Thus, based on the heteroadditive concept, synergism was observed for each combination, being most obvious in the group given all 10 chemicals at the 1/10 dose levels. However, the values for the combined groups were generally close to the averages of the 5 or 10 data gained for the heterocyclic amines alone at the corresponding higher doses, indicating the possibility of isoadditivity. Based on these findings, we propose here a new statistical method for analysis of combined effects of multiple chemicals, and, using this, we demonstrated (true) synergism with some heterocyclic amine combinations. The importance of dose-response curves for evaluation of combination effects is discussed.