Carcinogenesis

The likelihood of a substance to cause cancer is called carcinogenicity; this is determined in several ways. The first is to see if cells grown in vitro (in a test tube) with the potential carcinogen develop cell-structure abnormalities in the new-grown cells. The second is to see if the substance will result in cancers in animals. The third is to look at the clinical course of a disease in a human patient. Finally, the outcomes in a group of people exposed to the substance will be compared to outcomes in those not exposed—either by following the natural course of history in a population through time in cohort studies or in case-control.

In spite of these methods, however, identifying carcinogens is complex and difficult. One reason is the long time delay between use of a carcinogenic chemical and the appearance of cancer symptoms; intervals of 20-30 years are not unusual. As a result, carcinogenic effects are rarely detected in early studies of new medications. Secondly, carci-nogenesis appears to be a multistep process involving environmental, nutritional, and genetic factors as well as use of a carcinogenic drug.

Carcinogens are classified as genotoxic or epigenetic. Genotoxic carcinogens act directly on the DNA in human cells, in effect producing abnormal cells. Almost all identified drug carcinogens, however, fall into the second category, the epigenetic carcinogens. These substances encourage the proliferation of latent tumor cells in a tissue or organ.

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