Effect of varying the duration of exposure to phenobarbital on its enhancement of N-bis(2-hydroxypropyl)nitrosamine-induced thyroid tumorigenesis in male Wistar rats.

Male Wistar rats were injected s.c. with a single dose of 280 mg N-bis(2-hydroxypropyl)nitrosamine (DHPN) per 100 g body weight and the effect of subsequent exposure to 500 p.p.m. phenobarbitone (PB) for 6, 12 and 19 weeks on development of thyroid tumors was studied. Exposure to PB for 12 or 19 weeks enhanced the development of thyroid tumors in the rats. The incidences of thyroid tumors in DHPN-treated rats at the end of the experiment for 20 weeks were 87%, 83%, 42% and 37%, respectively, in rats given PB for 19 weeks, 12 weeks, and 6 weeks, and in those given no PB. The incidence of thyroid cancers was 12% in rats given DHPN and then PB for 19 weeks. The total numbers of thyroid tumors were 241, 125, 65 and 23, respectively, in DHPN-treated rats given PB for 19 weeks, 12 weeks, and 6 weeks, and in those given no PB. Thus, the total numbers of thyroid tumors were correlated with the period of PB treatment.