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J Ashby

Publications and source records attributed to J Ashby.

At least 289 records · Page 16Linked to original sources
PubMed1986

Inactivity of glycerol formal in a mouse micronucleus assay: relationship to its teratogenicity.

Glycerol formal (GF) has been reported to be teratogenic to the rat. Its mechanism of action in this respect has not been defined, but it has been discussed within the context of its being a possible slow formaldehyde-releasing agent. In order to evaluate this possibility, the hydrolysis of GF has been studied, and its activity as a mouse bone marrow clastogen assessed using the micronucleus assay. Neither was hydrolysis observed, nor was the test agent active as a clastogen in vivo. It is therefore concluded that the teratogenic activity of GF is unlikely to be associated with the release of formaldehyde in utero. However, GF bears a close structural similarity to the rodent teratogen, ethyleneglycol monomethylether (EGME), and this may provide a basis for understanding its teratogenic activity and for predicting similar activity for other cyclic ether alcohols.

Animals↗
PubMed1986

Clastogenicity in vitro of the Na, K, Ca and Mg salts of saccharin; and of magnesium chloride; consideration of significance.

The sodium, potassium, calcium and magnesium salts of saccharin, and magnesium chloride, have been shown to be clastogenic to Chinese hamster lung (CHL) fibroblasts in vitro, but only at elevated dose levels (8-16 mg/ml). Saccharin acid was inactive to the limits of its solubility (4 mg/ml). When the data are expressed in terms of ionic concentration, each salt showed a similar clastogenic potency. This suggests that ionic effects induced by these salts in the assay medium may be the critical determinant of the clastogenic effects seen, rather than that the saccharin moiety presents a genotoxic insult to the chromosomes of the cells. The metal-chelating agents EDTA and EGTA were non-clastogenic, but the disodium salt of EDTA showed weak activity prior to toxicity at 0.5 mg/ml. The absence of a clastogenic response for the salts of saccharin at dose levels lower than 4 mg/ml is discussed within the context of the threshold-dependent tumour-promoting activity of high dose levels of sodium saccharin to the bladder of male rats. The doubtful value of conducting in vitro clastogenicity studies at dose levels greater than 10(-2) M is discussed.

Animals↗
PubMed1986

Slide preparation and sampling as a major source of variability in the mouse micronucleus assay.

This paper describes the results of a study in which mouse bone marrow micronucleus assay slides were assessed for homogeneity of micronucleated polychromatic erythrocytes (MPE) among polychromatic erythrocytes (PE). The slides were prepared by 3 distinct methods and several methods of slide reading were assessed. Observations made using our slides were confirmed by re-analysis of slides from 3 independent laboratories. It is concluded that the method of slide preparation and assessment can significantly influence the variability of data obtained from a study. The extent of this variability casts doubt upon the validity of certain assumptions concerning this assay--such as sex differences in MPE incidence, responder variability, etc. Results are discussed within the context of the very recent literature for this assay. Some laboratories appear to have adequate methods of slide preparation and data accumulation, while others do not. Methods to improve the sensitivity of this assay are suggested within the context of the recommendations made by the Gene-Tox review group. In particular, it is suggested that individual investigators present evidence of the adequacy of their data accumulation techniques in order to enhance the value of future studies.

Animals↗
PubMed1986

Potent mitogenic activity of 4-acetylaminofluorene to the rat liver.

Administration of 4-acetylaminofluorene (4AAF) to rats by oral gavage (1000 mg/kg) produces a wave of S-phase activity in the liver 36 h later, followed by a wave of mitoses at 48 h. These events were monitored by autoradiography of isolated hepatocytes and by histopathology, respectively. DNA-labelling was shown to occur following both in vivo and in vitro radiolabelling. The level of S-phases observed approached that reported following partial hepatectomy. These effects were not accompanied by unscheduled DNA synthesis (UDS) nor was any frank histopathological damage to the liver evident. 2-Acetylaminofluorene (2AAF) elicited a very weak S-phase response at a dose level of 50 mg/kg, but gave marked UDS between 12 and 48 h.

2-Acetylaminofluorene↗
PubMed1986

The disposition and in vivo covalent binding to liver DNA of the monoazodyes 6-(p-dimethylaminophenylazo)benzothiazole (6BT) and 5-(p-dimethylaminophenylazo)indazole (5I) after administration to the rat.

6-(p-Dimethylaminophenylazo)benzothiazole (6BT) and 5-(p-dimethylaminophenylazo)indazole (5I) comprise respectively a carcinogen/non-carcinogen pair of monoazo dyes related to the hepatocarcinogen, butter yellow (DAB). While both members of the pair are potent bacterial mutagens in vitro, only 6BT induces unscheduled DNA synthesis in rat liver in vivo. To investigate factors responsible for these divergent activities we have determined in rats: relative rates of uptake from the gut after direct injection of 14C-labelled compound into intestine in situ, and after administration p.o.; distribution in selected tissues and elimination in urine, faeces and bile; binding of both compounds in vivo to liver DNA. The results revealed that, although 5I was taken up from the gut to a lesser extent than 6BT, comparable labelling associated with both compounds was detected in the presumed target organ (the liver). 5I binds in vivo to DNA much less effectively than 6BT. Therefore it would seem that other factors, such as differential metabolism in vivo, are more important than differences in uptake and distribution in accounting for the divergent activities of 6BT and 5I.

Animals↗
PubMed1986

The prospects for a simplified and internationally harmonized approach to the detection of possible human carcinogens and mutagens.

It is proposed that the many sets of Regulatory Guidelines for the assessment of chemical carcinogenicity and mutagenicity should be simplified and harmonized in light of current experimental data. Data are discussed which illustrate that an absolute distinction would be drawn between assays conducted in vitro from those in vivo, and that the genotoxicity of a chemical can be adequately defined using a combination of the Salmonella mutation assay and one for the assessment of chromosome aberrations in vitro. It is specifically recommended that once a chemical has shown a clear positive response in vitro, further short-term assays should be conducted in vivo; this avoids considering the 'weight of evidence' of in vitro data, the dangers of which are illustrated. It has now been unequivocally established that not all in vitro genotoxins prove carcinogenic to mammals. It is therefore recommended that all new in vitro genotoxins should be assessed in vivo using the mouse bone marrow micronucleus assay, and if a negative response is observed, a liver genotoxicity test. At present an assay for the induction of unscheduled DNA synthesis (UDS) in the liver is the most well developed for this purpose. Current data indicate that an in vitro genotoxin found to be inactive in these two in vivo assays will be neither carcinogenic nor mutagenic to the germ cells of mammals. Equally, genotoxicity produced in mammals indicates a carcinogenic and mutagenic potential which can usually only be countered by appropriate chronic bioassays. The use of short-term in vivo assays in this critical role requires attention to the selection of appropriate dose-levels and routes of exposure - these issues are discussed. The above testing strategy will not detect certain animal carcinogens, some of which are specifically discussed. These carcinogens have been variously referred to in the literature as epigenetic/non-genotoxic/hormonal/toxic/ambiguous or ambivalent carcinogens. It is suggested that they present a minor potential hazard to man when compared with that of genotoxic carcinogens and that their short-term detection can only be achieved by the development of new whole mammal assays employing non-genetic endpoints. This is in contrast to the present tendency to employ additional genotoxicity assays for their detection in the unjustified belief that they possess an exquisite specificity of genotoxic action. This article represents a personal view, but the testing strategy proposed is based to a large extent on the original three-tier approach of Bridges.(ABSTRACT TRUNCATED AT 400 WORDS)

Animals↗
PubMed1986

Mutagenicity to Salmonella of four derivatives of the azo mutagen 5I: some implications for structure--activity databases and the evaluation of combinations of mutagens.

A structure--activity study is described in which four new derivatives of the potent bacterial mutagen 5-dimethylaminophenylazoindazole (5I) have been evaluated for mutagenicity to Salmonella. As expected, monodemethylation of the -NMe2 group of 5I increased its mutagenic potency while replacement of the -NMe2 with a cyclic amine reduced it. However, replacement of the aromatic indazole -NH group (of 5I) by an -NMe group (yielding NMe5I) dramatically attenuated mutagenic potency, a reduction which was both unexpected and not reversed in the monomethyl analogue (NMeMA5I). In competition experiments NMe5I had an inhibitory effect on the mutagenic potency of 5I itself and on that of the nonazo mutagen 2-acetylaminofluorene 2AAF. The results illustrate some of the problems associated with evaluating mixtures for mutagenicity and of assuming simple structure--activity relationships in the absence of relevant experimental data.

Azo Compounds↗
PubMed1985

Significance of the genotoxic activities observed in vitro for 35 of 70 NTP noncarcinogens.

A speculative analysis is presented of the in vitro genotoxicity data reported by Shelby and Stasiewicz for 70 chemicals defined as noncarcinogenic to rodents by the National Toxicology Program. It is concluded that the genotoxic activities observed are probably subject to logical explanation. It is suggested that short-term genotoxicity assays conducted in vivo on newly defined in vitro genotoxins may have a useful role to play in discriminating animal carcinogens from noncarcinogens. It is clear from the results reported that genotoxic activities observed in vitro for a new test chemical only provide evidence of its possible animal carcinogenicity; they are not definitive of carcinogenicity--the difference may be negligible in general but might prove unacceptable in the particular.

Animals↗
PubMed1985

Investigations into the reported ability of cimetidine to initiate UDS in rat hepatocyte primary cultures.

Cimetidine is widely prescribed as a palliative or cure for gastric disorders in man. It is known to be noncarcinogenic to rodents and has been shown to be inactive in a wide range of in vitro and in vivo genotoxicity assays. It was therefore of concern when an injectable formulation of this drug (Cimetex) was reported to initiate UDS in cultured primary rat hepatocytes [Martelli et al, 1983]. We have confirmed the ability of Cimetex to elicit UDS in primary hepatocytes and established that cimetidine itself is inactive. These data indicate that it is not cimetidine per se, but rather its protonated formulation which is responsible for the activity. These observations pose fundamental questions regarding the validity of the UDS end-point when testing salts in vitro, rather than presenting a challenge to the nongenotoxic status of cimetidine.

Animals↗
PubMed1985

Non-genotoxicity of 2,4,6-trinitrotoluene (TNT) to the mouse bone marrow and the rat liver: implications for its carcinogenicity.

2,4,6-Trinitrotoluene (TNT) is structurally related to the rat liver carcinogen 2,4-dinitrotoluene (technical grade), and both compounds are known to be mutagenic to bacteria in vitro. TNT is therefore established as a potential rodent carcinogen; the present paper describes experiments designed to assess if this potential is likely to be expressed in appropriately exposed animals. TNT gave a negative response in the mouse bone marrow micronucleus assay and in an in vivo/in vitro rat liver assay for unscheduled DNA synthesis (UDS). In the latter assay animals are exposed to the test chemical in vivo and their hepatocytes subsequently evaluated for UDS in vitro. The negative response observed for TNT in the liver assay at dose-levels up to 1000 mg/kg was accompanied by a positive response for the hepatocarcinogen 2,4-dinitrotoluene at the lower dose-level of 200 mg/kg. In contrast, the dinitro compound gave a negative response in the micronucleus assay, as was also observed for TNT. It is concluded that the negative response observed for TNT in the liver assay indicates that it is unlikely to be a rat hepatocarcinogen. Nonetheless, high levels of methaemoglobin were observed in the TNT-treated rats and their urine was coloured red. These facts, together with the known toxicities of this agent suggest a possible carcinogenic hazard to the haemopoetic and urinary tissues of animals exposed chronically to it at toxic dose-levels.

Animals↗
PubMed1985

Tabulation and assessment of 113 human surveillance cytogenetic studies conducted between 1965 and 1984.

There is an increasing tendency to monitor human exposure to genotoxic chemicals by the assessment of chromosomal aberrations or sister-chromatid exchanges (SCEs) in peripheral blood lymphocytes. In order to assess the sensitivity of these techniques, and to discern minimal criteria for their conduct, a survey of 113 human lymphocyte cytogenetic surveillance studies conducted between 1965 and 1984 has been undertaken. The present survey indicates the urgent need for standardization of study protocols. It is suggested that a common method of reporting chromosomal aberrations should be adopted, and that this should be based on the system described by Scott et al. It is also suggested that a minimum acceptable size of control and exposed populations should be agreed, and that potentially important factors such as the gender, the period and extent of exposure and individual smoking habits be defined in advance of the commission of future surveillance studies. As general awareness of the possible hazard presented by exposure of man to genotoxic chemicals increases, so appropriate preventative industrial hygiene measures will be instituted. This implies that future human cytogenetic surveillance studies may yield either weakly positive or negative data. This emphasizes the current need for agreement on appropriate study protocols. The formation of a central repository for control databases, and its subsequent updating and use by those involved in human cytogenetic surveillance studies, is recommended. Minimal experimental criteria for the design of future studies are also outlined.

Carcinogens, Environmental↗