Comparative tumor-promoting activities of phenobarbital, amobarbital, barbital sodium, and barbituric acid on livers and other organs of male F344/NCr rats following initiation with N-nitrosodiethylamine.

Tumor-promoting abilities of four barbiturates, phenobarbital [(PB) CAS: 50-06-6], amobarbital [(AB) CAS: 57-43-2], barbital sodium [(BB) CAS: 144-02-5], and barbituric acid [(BA) CAS: 67-52-7], on the development of neoplasms in livers and other organs of rats following initiation with N-nitrosodiethylamine [(DENA) CAS: 55-18-5] were compared. Four-week-old F344/NCr male rats were given a single ip injection of 75 mg DENA/kg body weight. Beginning 2 weeks later, they were given either tap water (group 1) or drinking water containing 500 ppm of PB (group 2), the sodium salt of BB (group 3), AB (group 4), or BA (group 5) for the remaining experimental period. Control groups (groups 6-10) received an ip injection of saline alone and 2 weeks later were given either tap water or drinking water containing barbiturates as listed above. Animals were sacrificed at either 52 weeks or 78 weeks. None of the barbiturates altered the growth and survival of animals. PB and BB increased liver weights and significantly enhanced the development of hepatocellular foci and hepatocellular adenomas at 52 weeks and hepatocellular foci, hepatocellular adenomas, and trabecular carcinomas at 78 weeks in DENA-treated rats. No such enhancing effects were observed with AB or BA. PB or BB did not significantly enhance the incidence of nonhepatic tumors at 52 weeks. However, at 78 weeks BB significantly enhanced the development of renal tubular adenomas and carcinomas, while PB enhanced the development of thyroid follicular cell neoplasms in DENA-treated rats. These results clearly showed that barbiturates exhibited structure-promoting activity relationships and that their promoting abilities were not restricted to liver alone. Substitution of both hydrogen atoms at the C-5 position of the pyrimidine ring by alkyl or aryl groups appears to be essential but not sufficient for tumor-promoting activity of barbiturates.