In a two-stage initiation-promotion experiment the hypothesis was investigated that 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD) equivalents (TE), calculated from data of CYP1A induction in hepatocytes in primary culture, or international TCDD equivalents (ITE) are useful for evaluating the tumor-promoting potency of 1,2,3,4,6,7,8-heptachlorodibenzo-p-dioxin (HpCDD) and of a defined mixture (M2) of 49 polychlorinated dibenzo-p-dioxins (PCDDs) in comparison with TCDD. Therefore, female Wistar rats were treated with an initiating dose of N-nitrosomorpholine, and subsequently received daily doses of 2, 20 and 200 ng TCDD/kg or equivalent doses of HpCDD or M2, based on TE values. After a promotion phase of 13 weeks, hepatic PCDD levels, CYP1A activity and the relative hepatic volume of adenosinetriphosphatase-negative or glutathione S-transferase P-positive preneoplastic foci were determined. After logarithmic transformation, linear PCDD level-response relationships were obtained for induction of CYP1A activity with TCDD, HpCDD or M2. Based on TE values, inducing potencies of both HpCDD and M2 were over-estimated at higher doses, whereas induction was approximately equivalent at the lowest dose. The best fit of PCDD level-response relationships of relative hepatic volumes of preneoplastic lesions was achieved using a four-parameter logistic model. Significantly different functions were calculated for promotion with TCDD or HpCDD. It is concluded that (i) different PCDD level-response relationships exist for the induction of hepatic CYP1A activity and the promotion of preneoplastic liver foci, and (ii) that TE or ITE factors provide only a rough estimate of the tumor-promoting potency of a PCDD mixture but may overestimate the risk from exposure to higher-chlorinated 2,3,7,8-substituted congeners such as HpCDD.
