Biomedical subjects
Publications and source records attributed to J Ashby.
Explore the source record for details and available documents.
Explore the source record for details and available documents.
Explore the source record for details and available documents.
Explore the source record for details and available documents.
The distinction between genotoxic and non-genotoxic mechanisms of chemical carcinogenesis is now generally accepted. This acceptance implies a revised approach to the definition and measurement of genotoxicity. It is proposed that genotoxicity should be assessed as part of the overall evaluation of the toxicity of chemicals, not as a separate and isolated activity. The level and duration of dosing plays a critical role in the observation of genotoxic phenomena, and such parameters should assume greater importance in the interpretation of genotoxicity data. Toxic events associated empirically with the non-genotoxic aspects of chemical carcinogenicity are discussed.
The methylating agent streptozotocin is active in the mouse bone marrow micronucleus assay following a single intraperitoneal injection of 150-180 mg/kg. This correlates with its previously reported toxicity to mouse germ cells when administered by the same route of exposure. The potent mutagenicity of streptozotocin to strain G46 of Salmonella typhimurium is compared with its much weaker activity in strain TA1535. The genotoxicity of streptozotocin in vivo is reviewed.
Perera recently discussed the very real problems that accompany any attempt to classify rodent carcinogens into two groups--genotoxic or nongenotoxic. Not the least of these problems is that no agreed definition of these two terms exist. Nonetheless, the current carcinogen databases, for example, that of the U.S. National Toxicology Program (NTP), clearly comprise two broad groups of carcinogens--DNA reactive, mutagenic and multiply carcinogenic chemicals, and others. The others appear to be nonreactive to DNA, are inactive in the primary mutagenicity assays, and usually elicit highly selective carcinogenic responses in animals. These two classes of carcinogen are illustrated by examples taken from the NTP database and are discussed within the possible context of the latter group not being active in humans or, if they are, only when a threshold dose has been exceeded, chronically.
This article brings together the several current approaches to the use of short-term tests for identifying carcinogens and the several meanings of the term 'genotoxic'. Examples of genotoxic and nongenotoxic carcinogens and of genotoxic and nongenotoxic mechanisms of carcinogenesis are discussed. Emphasis is placed on the determination of genotoxicity as it affects evaluation of the mechanism of action of a new rodent carcinogen.
Explore the source record for details and available documents.
The nitrosourea mustard MeCCNU is the most recent organic chemical to be classified as a human carcinogen by IARC. MeCCNU gave a strong positive response when tested in the mouse bone marrow micronucleus assay. Activity was evident using either ip injection or oral gavage of the test chemical. These results further support the correlation between human carcinogens and their genotoxicity.
Explore the source record for details and available documents.
Before a non-genotoxic mechanism of action is proposed for a rodent carcinogen, or predicted for an untested chemical, adequate knowledge of the genotoxic status of the chemical must exist. The current absence of absolute agreement on the set of assays to use when evaluating genotoxicity suggests that a practical approach should be adopted, as outlined, for example, by Arni et al. (1988). In that approach, the chemical is evaluated for gene mutagenicity to Salmonella, for its ability to induce chromosomal aberrations in cultured mammalian cells, and if appropriate, gene mutations in cultured mammalian cells. Consideration of the chemical structure and the expected metabolism of the test chemical also contribute to the classification of an agent as a putative non-genotoxin. Judgement is required when interpreting isolated positive test responses generated in peripheral short-term tests. When attempting to study the mode of action of an established rodent carcinogen that is thought to operate by a non-genotoxic mechanism of action, genotoxicity data generated in vivo, if possible in the target tissue, are of great value. Compounds defined as non-genotoxic by these methods then become candidates for the study of those non-genotoxic toxicities that may predict or explain non-genotoxic rodent carcinogenesis.
This paper is an extension of compilations published previously in this journal. (Ashby and Tennant, 1988; Ashby et al., 1989). A summary of the rodent carcinogenicity bioassay data on a further 39 chemicals tested by the U.S. National Toxicology Program (NTP) is presented. An evaluation of each chemical for structural alerts to DNA-reactivity is also provided, together with a summary of its mutagenicity to Salmonella. Chemicals with an aliphatic nitro group (-C-NO2) have been added to the composite structure of DNA-reactive sub-groups. The 39 chemicals were numbered and evaluated as an extension of the earlier analysis of 264 NTP chemicals. The activity patterns and conclusions derived from the earlier studies are followed by these 39 chemicals, albeit a detailed analysis of the total database of 301 chemicals is reserved for the succeeding paper.
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 chemical 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 less than 5% were mutagenic, whether they were carcinogens or non-carcinogens (sensitivity 0.04).
Sulphur mustard is clearly mutagenic to the repair-proficient (uvrB+) strain G46 of S. typhimurium. It is also a micronucleus-inducing agent to the mouse bone marrow. These data strengthen the database indicating that most human carcinogens are genotoxic.
Explore the source record for details and available documents.
Groups of male Alderley Park rats were dosed concomitantly with 2-acetylaminofluorene (2AAF) by gavage at doses between 0.01 mg/kg and 40 mg/kg, and livers sampled 2-72 h later. The liver of one group of animals was perfused to yield hepatocytes which were assayed in vitro for unscheduled DNA synthesis (UDS) via incorporation of tritiated thymidine and autoradiography. DNA was extracted from the livers of the other group and DNA adduct levels determined using the 32P-postlabelling technique. The major C-8 2-aminofluorene/guanosine adduct and 3 minor adducts were quantitated, enabling the relative sensitivity of the 2 techniques to be compared. A dose- and time-related UDS response was observed, which, at the most sensitive time-point (12 h) enabled DNA repair to be discerned at a dose level of 0.1-1 mg/kg of 2AAF, a response classified as formally positive at 5 mg/kg 2AAF. Only the C-8 adduct, as determined by 32P-postlabelling, was discernible at 0.01 mg/kg of 2AAF, although other adducts were visible on autoradiograms at higher dose levels. It is concluded that as part of a well-defined dose response, UDS can be discerned with confidence for doses of 2AAF between approximately 0.1 and 5 mg/kg, and DNA adducts for doses of 2AAF between approximately 0.01 and 1 mg/kg. Discernible UDS for 2AAF in the rat liver is apparent at approximately 13 DNA (total) adducts/10(8) nucleotides, or approximately 8 DNA (C-8) adducts/10(8) nucleotides. The presumed C-8 2-acetylaminofluorene/guanosine adduct, prepared by reaction of 2-acetoxy-2-acetylaminofluorene (2AAAF) with DNA, was a significant but unreliable marker of 2AAF/DNA adducts in the rat liver in vivo. DNA repair did not appear to remove DNA adducts selectively, and adducts remained in DNA when discernible DNA repair had ceased.
In contrast to earlier studies conducted at lower dose levels, 2AAF is shown to induce a positive UDS response in the liver of mice dosed orally at dose levels between 500 and 1000 mg/kg. Similarly exposed mice had low levels of 2AAF-related hepatic DNA adducts at dose levels in the range 10-1000 mg/kg 2AAF, as determined by 32P-postlabelling analysis. It is concluded that the attenuated UDS response observed in the mouse liver, as compared to the rat liver, is due primarily to metabolic differences between these two species, coupled to a reduced capacity for UDS in the mouse liver for a given level of total 2AAF-related adducts per unit of DNA. These observations are compared and contrasted with identical studies conducted in the rat and reported in the preceding paper (Gallagher et al., 1991).