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, MelQ, MelQx) and experiment 2 (Trp‐P‐2, Glu‐P‐1, MeAαC, AαC, PhIP) were administered together or individually in the diet at levels of 1/1, 1/5, or 1/25 careinogenic 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.