Although the effective therapy of cancer is an ultimate goal of medical science, the prevention of cancer is, at our present state of knowledge, the most effective and, relatively, the most inexpensive mode of controlling this disease. The prevention of cancer has been discussed by a number of authors (Schottenfeld, 1981; Hirayama, 1992; Doll, 1996). Optimistically, our knowledge of the incidence of neoplasia in the human suggests that age-specific incidence rates might be reduced by as much as 80%, half of this reduction coming through the application of existing knowledge (Doll, 1996). In fact, such knowledge has already been applied to specific populations with significant results (Hirayama, 1992). As has been noted (Pitot, 1993), cancer prevention may occur passively or actively. Passive prevention of cancer involves a cessation or restriction of exposure to potentially carcinogenic influences, such as the cessation of smoking, dietary modification, and avoidance of excessive sunlight. Active cancer prevention involves a positive activity on the part of the individual by such things as vaccination against oncogenic viruses, dietary modification and supplements, or the administration of agents externally and/or internally that are known to prevent neoplastic development. This last subject is most pertinent to our discussion in this chapter.
Chemoprevention in relation to cancer is "the inhibition or reversal of carcinogenesis (before malignancy) by intervention with chemical agents" (Kelloff et al., 1996). From this definition it is obvious that the active prevention of neoplastic development by the administration of exogenous chemical modifiers of carcinogenesis is effective at the stages of initiation and/or promotion. However, because of the ubiquitous nature and occurrence of the stage of initiation in the mammalian population (Chapter 7), it may not be realistic at the present time to attempt to utilize chemicals to prevent the spontaneous initiation of cells. However, it is clear that a reduction in the number of initiated cells is feasible by a number of the agents noted earlier in this chapter. Thus, for chemoprevention to be effective in most cases, it must exert an effective inhibitory action on
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