[Chemoprevention of cancer--present status, problems and trends]

The most desirable way of eliminating the cancer incidence is by prevention. While the removal of causative agents is the primary goal of cancer prevention, for the foreseeable future it is likely to be incomplete. Therefore the development of a second line of cancer prevention based on chemoprevention assumes importance. During the past decade a growing number of naturally occurring and synthetic chemicals have been found which prevent or reduce the occurrence of cancer especially in experimental animals. These compounds belong to more than 20 different classes of chemicals. The mechanisms of action of most of them are up to now poorly understood. Classifying inhibitors of carcinogenesis according to the time during the carcinogenic process at which they are effective it is possible to divide them into 3 main categories: Substances that prevent the formation of carcinogens from precursors, blocking agents and suppressing agents. There are also several classes of chemicals that inhibit the phase of tumor promotion. But many anticarcinogens only act in combination with specific carcinogens or in distinct species, specific organs, tissues or situations, and completely different effects under other circumstances cannot be ruled out. In terms of application of the existing data to prevention of human cancer there has been a focus on 2 types of efforts: To select compounds that have little toxicity (i.e. ascorbic acid, beta-carotene), but which have not been shown to have broad inhibitory capacities, or to select compounds (i.e. retinoids) which have inhibitory efficacy but also can produce toxic side effects. When one evaluate a chemopreventive compound, considerations of its projected use are highly relevant. If the compound seems to be very potent but has significant toxicity, it may be useful for high-risk individuals but not for the general population.