Enhancement of thyroid and hepatocarcinogenesis by 1,4-bis[2-(3,5- dichloropyridyloxy)]benzene in rats at doses that cause maximal induction of CYP2B.

To investigate the promoting effects of 1,4-bis[2-(3,5- dichloropyridyloxy)]benzene (TCPOBOP) on liver and thyroid carcinogenesis of rats at doses that cause maximal induction of hepatic CYP2B, 5-week-old male F344 rats were given either a single i.p. dose of 75 mg N-nitrosodiethylamine (NDEA)/kg body wt in saline or saline alone. After 2 weeks the rats were fed control diet or a diet containing 330 or 1000 p.p.m. TCPOBOP or 500 p.p.m. phenobarbital (PB; a positive control group). A total of four sequential sacrifices (9, 30, 52 and 79 weeks of age) was performed. At 30 weeks the mean volume (mm3) of hepatocellular foci in NDEA-initiated rats exposed to either dose of TCPOBOP or to PB was significantly increased as compared with rats exposed to NDEA followed by control diet (P < 0.05). In addition, the volume percentage of liver occupied by foci was significantly greater in NDEA-initiated/1000 p.p.m. TCPOBOP-promoted rats as compared with rats exposed to NDEA alone (P < 0.05, n = 6). At 52 weeks of age the incidences (and multiplicities, in units of tumors per tumor-bearing rat) of hepatocellular adenomas were 0, 83 (2.6 +/- 1.3), 100 (3.4 +/- 2.1) or 67% (2.5 +/- 1.9) in rats exposed to NDEA alone or NDEA followed by 330 or 1000 p.p.m. TCPOBOP or 500 p.p.m. PB respectively (n = 12). Hepatocellular carcinomas were found only in rats given 1000 p.p.m. TCPOBOP (17% incidence) or PB (8% incidence) following NDEA initiation. The incidences of thyroid follicular cell adenomas were 0, 17, 33 or 8% in rats exposed to NDEA alone or NDEA followed by 330 or 1000 p.p.m. TCPOBOP or 500 p.p.m. PB respectively. Between 53 and 79 weeks of age 38% of rats treated with NDEA alone developed multiple (1.5 +/- 0.8) hepatocellular adenomas. This incidence was enhanced to 100% in rats exposed to NDEA followed by either 330 or 1000 p.p.m. TCPOBOP. Multiplicities of hepatocellular adenomas were also increased significantly (10.5 +/- 3.9, 10.4 +/- 7.0 and 10.1 +/- 6.7 respectively) in rats promoted with 330 or 1000 p.p.m. TCPOBOP or 500 p.p.m. PB. None of the rats exposed to NDEA alone developed hepatocellular carcinomas, while multiple hepatocellular carcinomas occurred in 38% of the rats exposed to 330 p.p.m. and 78% of the rats given 1000 p.p.m. TCPOBOP following NDEA initiation. Thyroid follicular cell tumors occurred at 79 weeks in more than 40 and 50% incidences in rats exposed to NDEA followed by 330 or 1000 p.p.m. TCPOBOP respectively. Also, a significant decrease in serum levels of triiodothyronine and thyroxine were observed in non-initiated 79-week-old rats fed 1000 p.p.m. TCPOBOP, compared with age-matched untreated controls (n = 6). Increases in hepatic CYP2B-mediated benzyloxyresorufin O-dealkylase activity detected in rats exposed to 330 and 1000 p.p.m. TCPOBOP for 2 or 23 weeks were similar in magnitude to those caused by 500 p.p.m. PB. Thus TCPOBOP at maximal CYP2B induction doses exhibits a strong promoting activity for both liver and thyroid of rats.