Definitive relationships among chemical structure, carcinogenicity and mutagenicity for 301 chemicals tested by the U.S. NTP

An analysis is presented in which are evaluated correlations among chemical structure, mutagenicity to Salmonella, and carcinogenicity to rats and mice among 301 chemicals tested by the U.S. NTP. Overall, there was a high correlation between structural alerts to DNA reactivity and mutagenicity, but the correlation of either property with carcinogenicity was low. If rodent carcinogenicity is regarded as a singular property of chemicals, then neither structural alerts nor mutagenicity to Salmonella are effective in its prediction. Given this, the database was fragmented and new correlations sought between the derived sub-groups. First, the 301 chemicals were segregated into six broad chemical groupings. Second, the rodent cancer data were partially segregated by target tissue. Using the previously assigned structural alerts to DNA reactivity (electrophilicity), the chemicals were split into 154 alerting chemicals and 147 non-alerting chemicals. The alerting chemicals were split into three chemical groups; aromatic amino/nitro-types, alkylating agents and miscellaneous structurally-alerting groups. The non-alerting chemicals were subjectively split into three broad categories; non-alerting, non-alerting containing a non-reactive halogen group, and non-alerting chemicals with minor concerns about a possible structural alert. The tumor data for all 301 chemicals are re-presented according to these six chemical groupings. The most significant findings to emerge from comparisons among these six groups of chemicals were as follows: (a) Most of the rodent carcinogens, including most of the 2-species and/or multiple site carcinogens, were among the structurally alerting chemicals. (b) Most of the structurally alerting chemicals were mutagenic; 84% of the carcinogens and 66% of the non-carcinogens. 100% of the 33 aromatic amino/nitro-type 2-species carcinogens were mutagenic. Thus, for structurally alerting chemicals, the Salmonella assay showed high sensitivity and low specificity (0.84 and 0.33, respectively). (c) Among the 147 non-alerting chemicals < 5% were mutagenic, whether they were carcinogens or non-carcinogens (sensitivity 0.04). From these facts we conclude that the concepts of genotoxic and non-genotoxic rodent carcinogenicity are worthy of continued attention. Also, that it is meaningless to discuss the sensitivity/specificity of the Salmonella assay without defining the broad chemical classes under discussion. This last conclusion is important to any model for screening environmental chemicals for potential carcinogens. Some rodent tissues, such as the lung and Zymbal's gland, are uniquely associated with genotoxic carcinogenesis, while others are equally susceptible to non-genotoxic carcinogenesis. Four such tissues are currently studied as possible sites of non-genotoxic carcinogenicity, and these were separately considered; male rat kidney-specific carcinogenic effects, rodent leukaemogens, rodent thyroid gland carcinogens and mouse liver carcinogens (the latter being the largest group, 97 of the 301 chemicals having increased tumor incidences in this tissue). Chemicals inducing tumors in these tissues were of disparate chemical classes and were predominantly non-mutagenic. These facts, together with the specificity of teh carcinogenic effects, is indicative of carcinogenicity resulting from a specific interaction between the chemical and the tissue, rather than it being an intrinsic and unique property of the chemical. Even when tumours in these four tissues were eliminated from the database, the Salmonella assay was only positive for 67% of the remaining 113 carcinogens (derived from a total of 162 carcinogens in the database). This indicates that a range of additional sites are subject to tissue-specific carcinogenesis by putative non-genotoxins. A distribution chart is presented which represents the 301 chemicals according to the 6 chemical groupings and the level of carcinogenic effect. From this it becomes apparent that the NTP database is dominated by two major groups of chemicals. First, a group of structurally-alerting and mutagenic carcinogens that are predominantly active in both species and/or at multiple sites, and second a group of non-alerting, non-mutagenic non-carcinogens. In between these two groups is a diffusely spread group of species/sex/tissue specific carcinogens, only some of which are mutagenic and/or structurally alerting. It is among the last group of carcinogens that reseach is required to understand their mechanism of action and their significance to man. The in vivo mammalian cell genotoxicity database of the TNP failed to distinguish these last carcinogens from the non-carcinogens, and this endorses that research into the mode of action of these carcinogens should not be concerned with their genotoxicity. Our overall conclusion is that rodent carcinogenicity can no longer be regarded as a single entity. Structural alerts and mutagenicity to Salmonella are useful but non-definitive indicators of the overt carcinogens in the database, and the activity of the remaining (putative non-genotoxic) carcinogens is no predictable using current techniques. To pool rodent carcinogens and to attempt to find a single method for their prediction is no longer tenable. Rather, it is suggested that genotoxic carcinogens should be predicted by reference to chemical structure and the intelligent use of in vitro and in vivo genotoxicity assays, and that non-genotoxic carcinogens require basic studies to understand the subtle effects that occur in rodents upon protracted dosing with chemicals. It will also be necessary to consider which of these effects may be ancillary and which are critical to increases in tumor incidences. Some such indicators are emerging (e.g. peroxisome proliferation in the rodent liver), but much uncertainty remains in this area. The suspected nature of non-genotoxic carcinogenesis indicates that studies into its mechanisms and prediction will be most effectively progressed in vivo, rather than in vitro.