PERSPECTIVES IN CANCER CHEMOPREVENTION

Cancer chemoprevention can be defined as prevention of cancer by the a dministration of one or more chemical entities, either as individual d rugs or as naturally occurring constituents of the diet. Based largely on the time period that chemopreventive agents exhibit activity in an imal models of carcinogenesis, they can be classified as inhibitors of carcinogen formation, blocking agents, and suppressing agents. The ma jority of compounds that inhibit the formation of carcinogens prevent the formation of nitrosamines from secondary amines and nitrite in an acidic environment. Blocking agents are inhibitors of tumor initiation , while suppressing agents are inhibitors of tumor promotion/progressi on. Many well-characterized chemopreventive agents act at one or more steps in both tumor initiation and promotion/progression. The objectiv e of this paper is to provide a general discussion of the mechanisms t hrough which chemopreventive agents inhibit carcinogenesis. Examples o f agents that act through these mechanisms are given; however. a compl ete listing of effective chemopreventive agents is not possible within the context of this paper. At the conclusion is a brief discussion of future prospects in cancer chemoprevention and obstacles to overcome.