Single Chemical Carcinogens

Many different cancers are caused by exposures to single chemicals and many different compounds are known to induce cancers. Some compounds are known to cause more than one cancer type. It is not the purpose of this book to exhaustively explore the hundreds of known single chemical carcinogens and the cancers they induce. Following, however, are some illustrative examples of these. 

Introduction 434 Cancer Incidence Rates 434 Chemical Carcinogens 435 Mechanisms of Carcinogenesis 439 Single Chemical Carcinogens 444... 

Some chemicals are believed to have no threshold above which toxic effects are observed. In other words, a single molecule has the potential to induce an adverse effect. The most common group of hazards in this respect are genotoxic carcinogens. Chemical carcinogens are not normally approved as food additives because an acceptable daily intake cannot be established. 

The in vitro and in vivo test methods available to study combined actions and toxicological and biochemical interactions of chemicals in mixtures are essentially the same as those used for the study of single chemicals in order to examine their potential general toxicity and special effects such as mutagenicity, carcinogenicity, and reproductive toxicity. 

With chemical carcinogens, time is also an important factor, both for the appearance of the effect, which may be measured in years, and for the length of exposure. It appears that some carcinogens do not induce tumors after single exposures or after low doses but others do. In some cases, there seems to be a relationship between exposure and dose, that is, low doses require longer exposure times to induce tumors than high doses, which is as would be expected for irreversible reactions with nucleic acids. For a further discussion of this topic, the reader is referred to the articles by Aldridge (7, chap. 6) and Zbinden (8). 

Cellular oncogenes (VI) can be activated by amplification, by single point mutations, or by transposition. Slowly transforming retroviruses do not contain viral oncogenes but stimulate expression of cellular oncogenes by insertional mutagenicity. Radiation and chemical carcinogens also activate cellular onco-... 

Current data support a clonal origin of most cancers because most tumors arise from a single cell as a result of a spontaneous mutation or after exposure to an oncogenic virus or chemical carcinogen these tumors then expand to a detectable level while continually shedding viable cells. The ability to cure the cancer then depends on numerous variables, the most important being the presence of viable metastasis. The application of chemotherapy arose out of the appreciation that cancer is not commonly a localized process and is thus not amenable to control... 

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