Disinfection by-products (DBPs) are formed when disinfectants such as chlorine, chloramine, and ozone react with organic and inorganic matter in water. The observations that some DBPs such as trihalomethanes (THMs), di-/trichloroacetic acids, and 3-chloro-4-(dichloromethyl)-5-hydroxy-2(5H)-furanone (MX) are carcinogenic in animal studies have raised public concern over the possible adverse health effects of DBPs. To date, several hundred DBPs have been identified. To prioritize research efforts, an in-depth, mechanism-based structure-activity relationship analysis, supplemented by extensive literature search for genotoxicity and other data, was conducted for ranking the carcinogenic potential of DBPs that met the following criteria: a) detected in actual drinking water samples, b) have insufficient cancer bioassay data for risk assessment, and c) have structural features/alerts or short-term predictive assays indicative of carcinogenic potential. A semiquantitative concern rating scale of low, marginal, low-moderate, moderate, high-moderate, and high was used along with delineation of scientific rationale. Of the 209 DBPs analyzed, 20 were of priority concern with a moderate or high-moderate rating. Of these, four were structural analogs of MX and five were haloalkanes that presumably will be controlled by existing and future THM regulations. The other eleven DBPs, which included halonitriles (6), haloketones (2), haloaldehyde (1), halonitroalkane (1), and dialdehyde (1), are suitable priority candidates for future carcinogenicity testing and/or mechanistic studies.
Increased understanding of the mechanistic basis of chemical carcinogenesis and the relationship between molecular structure and carcinogenic activity provides opportunities not only for identifying suspect carcinogens but also for designing chemicals with lower carcinogenic potential. One of the chemical classes in which the structural and molecular basis of carcinogenicity is the most clearly understood is the aromatic amines. This paper summarizes the bioactivation mechanisms and structural criteria for aromatic amine carcinogenesis; a strategic approach of risk reduction through mechanism-based molecular design of aromatic amine dyes with lower carcinogenic potential is discussed. With our increasing knowledge of the mechanisms and structure-activity relationships, it should be possible to develop safer products for other chemical classes using similar molecular design approaches.
Mechanisms of Chemical Carcinogenesis and Structure-Activity RelationshipsConsiderable knowledge of the mechanisms of chemical carcinogenesis has accrued since the pioneer work by James A. and Elizabeth C. Miller at the University of Wisconsin beginning in the 1940's, particularly concepts on the metabolic activation of chemicals to reactive electrophilic intermediates that interact with cellular nucleophiles to initiate carcinogenesis (7,2). For many carcinogen classes, the molecular basis of carcinogenic activity is now known in considerable detail and the concept of electrophiles provides the most probable rationale for their carcinogenic action. Some of the electrophilic, reactive intermediates believed to be responsible for the carcinogenicity of genotoxic chemicals include: carbonium, aziridium, episulfonium, oxonium, nitrenium or arylamidonium ions, free radicals, epoxides, lactones, aldehydes, semiquinones/quinoneimines, and acylating moieties. The metabolic activation of various genotoxic chemicals to their ultimate carcinogenic metabolites has been reviewed (3).
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