Mechanistic considerations in the evaluation of chemopreventive data.

Possible chemopreventive mechanisms include carcinogen-blocking activities, antioxidant/anti-inflammatory activities and antiproliferation/antiprogression activities. Carcinogen-blocking activities encompass inhibition of carcinogen uptake, inhibition of carcinogen formation or activation, deactivation or detoxification of carcinogens, prevention of carcinogen binding to DNA, and enhancement of the level or fidelity of DNA repair. Antioxidant/anti-inflammatory activities include scavenging of reactive electrophiles and oxygen radicals, and inhibition of arachidonic acid metabolism. Antiproliferation/antiprogression activities comprise modulation of signal transduction, modulation of hormonal and growth factor activity, inhibition of aberrant oncogene activity, inhibition of polyamine metabolism, induction of terminal differentiation, restoration of immune responses, enhancement of intercellular communication, restoration of tumour suppressor function, induction of apoptosis, telomerase inhibition, correction of DNA methylation imbalances, inhibition of angiogenesis, inhibition of basement membrane degradation, and activation of antimetastasis genes. In evaluating the potential efficacy of chemopreventive agents several mechanistic parameters are weighed: (1) the number of chemoprevention-related pharmacological activities, (2) the impact of the agent on likely carcinogenesis pathways to the targeted cancer, (3) pharmacodynamics, and (4) specificity for chemopreventive activity compared with interference with normal cellular function. Mechanistic data are important throughout the development process for chemopreventive drugs, and they are particularly important in the earlier phases of identifying promising candidate agents and characterizing efficacy. In vitro mechanistic assays are a first step in evaluating chemopreventive potential. Mechanistic considerations are also useful in defining animal efficacy models and in interpreting the results of assays in these models. Mechanistic data are also applied in designing short-term Phase II clinical chemoprevention trials that use reductions in intermediate biomarkers of cancer rather than cancer incidence as end points. The basis for identifying and evaluating these biomarkers is in understanding carcinogenesis and chemopreventive mechanisms.