Promoting effects of phenobarbital and barbital on development of thyroid tumors in rats treated with N-bis(2-hydroxypropyl)nitrosamine.

Phenobarbital (PB) and barbital (BB) promoted the development of thyroid tumors in rats treated with a sub-effective dose of N-bis(2-hydroxypropyl)nitrosamine (DHPN) for thyroid tumorigenesis. Rats were given s.c. injections of 70 mg DHPN/100 g body weight once a week for 4 or 6 weeks with or without diet containing 500 p.p.m. PB or BB for the next 12 weeks. The incidences of thyroid tumors at the end of week 20 of the experiment were 66% in rats given DHPN for 4 weeks and then PB, 23% in rats given DHPN for 4 weeks and then BB, 100% in rats given DHPN for 6 weeks and then PB, 45% in rats given DHPN for 6 weeks and then BB, and 23% in rats given DHPN for 6 weeks. Rats given only DHPN for 4 weeks or only PB or BB had no thyroid tumors after 20 weeks.