Naturally occurring and synthetic vitamin A metabolites and analogs (retinoids) inhibit tumor development in a variety of cellular, animal, and patient studies, They suppress transformation of cells in vitro and inhibit carcinogenesis in various organs in animal models. In a mouse skin carcinogenesis model, topical retinoids exhibit suppressive effects on tumor promotion, but have no effect on tumor initiation. In other models, retinoids administered in the diet suppress tumor development even in an adjuvant setting after excision of the first tumor. Retinoids suppress carcinogenesis in individuals with premalignant lesions and a high risk to develop cancer of the aerodigestive tract. Likewise, retinoids prevent the development of second primary cancers in head/neck and lung cancer patients who had been treated for the first primary, The mechanisms underlying the anticarcinogenic activity of retinoids appear to be associated with the ability of retinoids to modulate the growth, differentiation, and apoptosis of normal, premalignant, and malignant cells in vitro and in vivo. Most of these effects are mediated by nuclear retinoid receptors, but other mechanisms may also be involved, These studies indicate that retinoids are potentially useful agent for cancer chemoprevention.
