Potential for chemoprevention.

Chemopreventive agents are natural or synthetic agents administered to prevent, inhibit or reverse one or more of the stages of carcinogenesis. Natural agents include vitamins, coumarin, indoles, flavones, plant sterols and selenium salts. Synthetic agents include retinoids, phenolic antioxidants, protease inhibitors and prostaglandin synthesis inhibitors. All have demonstrated inhibition of tumor models in vitro or in vivo. These agents may work in different ways; by stimulating the immune response, by inducing gene suppression, by detoxifying carcinogens, by blocking oxidative damage to DNA or by other mechanisms. Human trials are being carefully initiated. Retinoids have demonstrated effectiveness in controlling some proliferative skin tumors. In the gastrointestinal tract, ascorbic acid has demonstrated a modest suppressive effect on adenomas of the rectum in patients with polyposis. There is initial evidence that 13-cis-retinoic acid is effective in modifying leukoplakia of the oral cavity.