Chemopreventive effects of a flavonoid antioxidant Silymarin on N-butyl-N-(4-hydroxybutyl)nitrosamine-induced urinary bladder carcinogenesis in male ICR mice

The modifying effects of dietary administration of a flavonoid antioxidant, silymarin, a mixture of three flavonoids isolated from milk thistle seeds, on N-butyl-N-(4-hydroxybutyl)nitrosamine (OH-BBN)-induced urinary bladder carcinogenesis were examined in male ICR mice. Animals were divided into 5 groups, and groups 1 to 3 were given OH-BBN (500 ppm) in drinking water for 6 weeks. Mice in group 2 were fed a diet containing 1000 ppm silymarin for 8 weeks during the initiation phase starting I week before OH-BBN exposure, and mice in group 3 were fed the diet for 24 weeks during the postinitiation phase. Animals in group 4 were given only the test compound, and those in group 5 were given the basal diet alone throughout the experiment. Animals were sacrificed at the end of week 32. The frequency of bladder lesions, cell proliferation and cell cycle progression activity estimated in terms of the 5-bromodeoxyuridine (BrdU) labeling index or cyclin D1-positive cell ratio were compared among the groups. Administration of silymarin in the initiation or postinitiation phase significantly decreased the incidences of bladder neoplasms and preneoplastic lesions. Dietary exposure to this agent significantly reduced the labeling index for BrdU and the cyclin D1-positive cell ratio in various bladder lesions. These findings suggest that silymarin is effective in preventing OH-BBN-induced bladder carcinogenesis in mice.