Rapid in vivo assay for topical oral cancer chemopreventive agents

The cancer chemopreventive effect of topically applied phenethyl isothiocyanate (PEIT) was examined in a hamster buccal pouch model, in which squamous cell carcinomas (SCC) are induced at high frequency, by topical application of N-methyl-N-benzylnitrosamine (MBN). The buccal pouches of eleven hamsters were pretreated thrice-weekly for two weeks with corn oil (CO) containing 50 mM PEIT, followed by 22 weeks of twice-weekly application of CO containing MBN and PEIT, both at 50 mM. Under similar conditions, twelve hamsters were pretreated with CO for 2 weeks, followed by MBN in CO. Tumor analysis was performed 19 days after the last application of PEIT and MBN. The incidence of tumors (approximately 90% SCC) in the unprotected and protected groups was 100% and 73%, respectively. Although total tumor incidence was marginally decreased, PEIT inhibited significantly the tumor frequency and tumor burden by 79% and 74%, respectively. In both groups, 43% of the carcinomas exhibited p53 immunohistochemical activity. A short-term experiment was performed to determine whether PEIT inhibits MBN-induced cellular foci expressing gamma-glutamyltranspeptidase histochemical activity (gamma-GT foci). Buccal pouches of four protected hamsters received 50 mM PEIT pretreatment on days 1 and 4, followed on days 6 and 9 by application of CO containing MBN and PEIT, both at 50 mM. Four unprotected hamsters were similarly pretreated with CO, followed by MBN in CO. gamma-GT foci were enumerated in buccal pouch epithelial whole mounts prepared from pairs of protected and unprotected hamsters on days 10 through 13. Whereas the number of gamma-GT foci in unprotected hamsters ranged from 98 to 356 per 10 cm2, the protected hamsters exhibited no more than one focus per 10 cm2. This model may be useful for rapid identification of chemopreventive agents, and combinations of agents, which inhibit initiation and promotion stages of oral carcinogenesis.