LIVER-TUMOR PROMOTING ACTIVITY OF 3,4,5,3',4'-PENTACHLOROBIPHENYL ANDITS INTERACTION WITH 2,3,7,8-TETRACHLORODIBENZO-P-DIOXIN

This study was undertaken to compare the tumour promoting effects indu ced by 3,4,5,3',4'-pentachlorobiphenyl (PCB 126) and 2,3,7,8-tetrachlo rodibenzo-p-dioxin (TCDD). In addition, interactive effects in rats tr eated with combinations of PCB 126 and TCDD were studied. Partially he patectomized female Sprague-Dawley rats were initiated with nitrosodie thylamin. After 5 weeks of recovery the promotion treatment started an d continued for 20 weeks. The results from the present study demonstra te that PCB 126 elicit approximately 10% of TCDD's tumour promoting ac tivity measured as enhancement of the development of gamma-glutamyl-tr anspeptidase-positive altered hepatic foci in the liver. The factor re quired for the PCB to match the response of TCDD was adopted as a toxi c equivalency factor and was in this case 0.1, which is the same as th e factor suggested by Ahlborg et al. (1994). In the groups treated wit h a mixture of PCB 126 and TCDD the tumour promoting effect indicated an additive response. This result suggests that PCB 126 and TCDD act b y the same mechanistical pathway, which in turn, supports that the tox ic equivalency factor-concept can be used for TCDD-like tumour promote rs.