Initiation and post-initiation chemopreventive effects of beta-carotene in toad liver carcinogenesis

Hepatocellular carcinoma were recognized in the toad, Bufo viridis, in 14 cases out of 50 by injection of 1 mg 7,12 dimethylbenz(a)anthracene (DMBA)/toad twice/week for 12 weeks. In contrast, toads treated with DMB at the same dose level and beta-carotene (betaC), 0.05 mg (3 hr prior to the carcinogen)/toad, twice/week for 12 weeks showed no tumor incidence. However, betaC at the same dose level was less effective when administered 3 hr. after the carcinogen (DMBA). In 8 cases out of 50 cases neither tumor growth nor neoplastic changes were observed in toads treated with betaC alone or olive oil. It is concluded that betaC completely blocked hepatocarcinogenesis in toads when given 3 hr. before initiation.