CHEMICAL INTERACTION - ENHANCEMENT AND INHIBITION OF CLASTOGENICITY

Most environmental exposures involve concurrent or sequential exposure to multiple chemicals in air, water, and food. Interactive effects in carcinogenesis have been described for certain combinations of agents . They are described in terms of enhancement or inhibition of carcinog enesis. Enhancement effects have been documented for cigarette smoking in combination with exposure to asbestos, radon, alcohol, or other ex posures. A variety of inhibitors of carcinogenesis have also been desc ribed. They are classified into agents preventing formation of carcino gens; blocking agents; and suppressing agents. Assessment of risk from exposure to multiple agents can be derived either from epidemiologica l studies in relation to actual exposure or from laboratory studies af ter controlled exposure to different agents. Prediction of how toxic c omponents or mixtures will interact should be based on an understandin g of the mechanisms of such interactions. Compounds may interact chemi cally, yielding new toxic components or causing a change in the biolog ical availability of the existing components or metabolites. In human great individual variability in response is to be expected because of genetic heterogeneity or acquired host susceptibility factors. Interac tion is thus a key component in the risk assessment process. In this p aper, the definition of interaction and the theoretical basis for diff erent types of interaction in cancer causation are reviewed. Epidemiol ogical and experimental studies showing interactive effects of two che mical carcinogens are also presented.