Topological structural alerts modulations of mammalian cell mutagenicity for halogenated derivatives

Genotoxicity is a key toxicity endpoint for current regulatory requirements regarding new and existing chemicals. However, genotoxicity testing is time-consuming and costly, and involves the use of laboratory animals. This has motivated the development of computational approaches, designed to predict genotoxicity without the need to conduct laboratory tests. Currently, many existing computational methods, like quantitative structure-activity relationship (QSAR) models, provide limited information about the possible mechanisms involved in mutagenicity or predictions based on structural alerts (SAs) do not take statistical models into account. This paper describes an attempt to address this problem by using the TOPological Substructural MOlecular Design (TOPS-MODE) approach to develop and validate improved QSAR models for predicting the mutagenicity of a range of halogenated derivatives. Our most predictive model has an accuracy of 94.12%, exhibits excellent cross-validation and external set statistics. A reasonable interpretation of the model in term of SAs was achieved by means of bond contributions to activity. The results obtained led to the following conclusions: primary halogenated derivatives are more mutagenic than secondary ones; and substitution of chlorine by bromine increases mutagenicity while polyhalogenation decreases activity. The paper demonstrates the potential of the TOPS-MODE approach in developing QSAR models for identifying structural alerts for mutagenicity, combining high predictivity with relevant mechanistic interpretation.

Quantitative structure-activity relationship modelling of the carcinogenic risk of nitroso compounds using regression analysis and the TOPS-MODE approach

Worldwide, legislative and governmental efforts are focusing on establishing simple screening tools for identifying those chemicals most likely to cause adverse effects without experimentally testing all chemicals of regulatory concern. This is because even the most basic biological testing of compounds of concern, apart from requiring a huge number of test animals, would be neither resource nor time effective. Thus, alternative approaches such as the one proposed here, quantitative structure-activity relationship (QSAR) modelling, are increasingly being used for identifying the potential health hazards and subsequent regulation of new industrial chemicals. This paper follows up on our earlier work that demonstrated the use of the TOPological Substructural MOlecular DEsign (TOPS-MODE) approach to QSAR modelling for predictions of the carcinogenic potency of nitroso compounds. The data set comprises 56 nitroso compounds which have been bio-assayed in female rats and administered by the oral water route. The QSAR model was able to account for about 81% of the variance in the experimental activity and exhibited good cross-validation statistics. A reasonable interpretation of the TOPS-MODE descriptors was achieved by means of bond contributions, which in turn afforded the recognition of structural alerts (SAs) regarding carcinogenicity. A comparison of the SAs obtained from different data sets showed that experimental factors, such as the sex and the oral administration route, exert a major influence on the carcinogenicity of nitroso compounds. The present and previous QSAR models combined together provide a reliable tool for estimating the carcinogenic potency of yet untested nitroso compounds and they should allow the identification of SAs, which can be used as the basis of prediction systems for the rodent carcinogenicity of these compounds.

Early microdose drug studies in human volunteers can minimise animal testing: Proceedings of a workshop organised by Volunteers in Research and Testing

Testing the safety and efficacy of a successful human medicine involves many laboratory animals, which can sometimes be subjected to considerable suffering and distress. Also, it is necessary to extrapolate from the test species to humans. UK and European legislation requires that Replacement, Reduction and Refinement of animal procedures (the Three Rs) are implemented wherever possible. Over the last decade, there has been substantial progress with applying in vitro and in silico methods to both drug efficacy and safety testing. This paper is a report of the discussions and recommendations arising from a workshop on the role that might be played by human volunteer studies in the very early stages of drug development. The workshop was organised in November, 2001 by Volunteers in Research and Testing, a group of individuals in the UK which launched an initiative in 1994 to identify where and how human volunteers can participate safely in biomedical studies to replace laboratory animals. It was considered that conducting pre-Phase I very low dose human studies (sub-toxic and below the dose threshold for measurable pharmacological or clinical activity) could enable drug candidates to be assessed earlier for in vivo human pharmacokinetics and metabolism. Moreover, accelerator mass spectrometry (AMS), nuclear magnetic resonance (NMR) spectroscopy and positron emission tomography (PET) are potentially useful spectrometric and imaging methods that can be used in conjunction with such human studies. Some, limited animal tests would still be required before pre-Phase I microdose studies, to take account of the potential risk posed by completely novel chemicals. The workshop recommended that very early volunteer studies using microdoses should be introduced into the drug development process in a way that does not compromise volunteer safety or the scientific quality of the resulting safety data. This should improve the selection of drug candidates and also reduce the likelihood of later candidate failure, by providing in vivo human ADME data, especially for pharmacokinetics and metabolism, at an earlier stage in drug development than is currently the case.

The welfare problems associated with using transgenic mice to bioassay for bovine spongiform encephalopathy

Prion diseases are fatal neurodegenerative disorders, epitomized by the the recent bovine spongiform encephalopathy (BSE) epidemic in cattle and the emergence of a novel variant of Creutzfeldt-Jacob disease (vCJD) in humans. In prion disease, the agent of infection is believed to be composed of proteinaceous particles, termed prions, which are converted from a normal isoform into a pathogenic isoform during pathogenesis. A bioassay to detect pathogenic prions of BSE in bovine products consumed by humans was unattainable until the development of transgenic mice, due to the significantly lower susceptibility of wild-type mice to BSE. Transgenic mice have now been generated which express the bovine prion protein and are susceptible to BSE. Following an intracerebral injection with brain homogenate of BSE-infected cattle, transgenic mice develop numerous clinical signs of prion disease, including truncal ataxia (inability to coordinate the torso's muscular activity), increased tone of the tail, generalized tremor, and lack of a forelimb extensor response. In this study, the ethical score system devised by Porter (1992) was applied to the BSE bioassay as a tool for identifying welfare issues affecting animals used in the bioassay. We acknowledge that there are limitations to the use of the information arising from the application of the Porter scoring scheme for assessing the justification to proceed with any animal experiment; notwithstanding these problems, however, our application of the Porter model to the BSE bioassay enabled us to identify potential targets for refinement: pain involved, duration of distress and the duration of the experiment. This was despite lenient scoring for the duration of distress and pain experienced by the mice, and optimal scoring for the quality of animal care. The targets identified for refinement are discussed in relation to the method of inoculation, the duration of the bioassay, and the duration of the clinical phase, with the objective of exploring ways of reducing the severity of the bioassay.

Ethical investment--what is it, and what are the implications for industry funding of research into alternatives?

This paper is intended to be a critical appraisal of ethical investment with respect to animal experimentation. It is aimed at a wide readership, ranging from scientists in the field and laypersons interested in laboratory animal welfare, potential investors, to senior management in industries directly or indirectly involved in animal testing.

The use of structure-activity relationships and markers of cell toxicity to detect non-genotoxic carcinogens

In contrast to the situation for genotoxic carcinogens, few in vitro tests exist that can detect early markers of the events thought to be associated with non-genotoxic carcinogenesis. Also, comparatively little is known about the quantitative structure-activity relationships (Q)SARs of these agents. This review discusses published SAR studies conducted on non-genotoxic carcinogens, in relation to the use of several markers of in vitro cell toxicity (inhibition of gap-junctional intercellular communication, inhibition of tubulin polymerization, modulation of apoptosis and induction of cell proliferation), which are used as endpoints for screening this class of carcinogen. Much of the work has involved the identification of new biophores (substructural features of molecules associated with toxicity), as well as other structural features, which are thought to predispose the chemicals to ligand binding with specific target molecules acting as possible receptors (e.g. protein kinase C, the oestrogen, peroxisome-proliferator and tubulin protein receptors), implicated in the mechanism of toxicity involved. It is concluded that (a) there is an urgent need for more information on (Q)SARs for non-genotoxic carcinogens; (b) this information should be acquired by using several different approaches in a variety of laboratories; and (c) such research should proceed together with more studies on the mechanisms of cell toxicity caused by these chemicals, including the identification and characterisation of further specific receptors involved in mediating the various types of cell toxicity associated with this type of carcinogenesis.

The use of L5178Y mouse lymphoma cells to assess the mutagenic, clastogenic and aneugenic properties of chemicals

Guidelines have been proposed to assess the potential of chemicals to affect human health. Written into these guidelines is the requirement that information be submitted on mutagenic activity. Although regulatory agencies accept mutagenicity data from both the hprt and tk loci in mammalian cells, many studies suggest that the L5178Y mouse lymphoma assay at the thymidine kinase locus is likely to detect a greater spectrum of mutagenic lesions. Thus, there is increasing emphasis being placed on this assay in many proposed and published guidelines. The L5178Y mouse lymphoma suspension protocol produces both small and large colonies which are the products of mutants growing at different rates. There is a reduction in the proportion of slowly growing mutants with respect to the total population of cells when expression is carried out in suspension. This potentially leads to quantitatively inaccurate assessments of the mutagenic activity of chemicals. Therefore an in situ procedure was developed that more accurately assesses the mutagenic activity of chemicals by maximizing the detection of small colonies. Many guidelines recommend tests that assess the clastogenic activity of chemicals. Some regulatory agencies accept data from the mouse lymphoma mutation assay to detect clastogens if the protocol is optimized for the detection of small colonies or if colony sizing data are submitted. The conventional suspension assay protocol is not sufficiently validated for this purpose. The in situ protocol has greater potential to meet these requirements.(ABSTRACT TRUNCATED AT 250 WORDS)

Interaction of cimetidine with cytochrome P450 and effect on mixed-function oxidase activities of liver microsomes

The histamine H2-receptor antagonist drug, cimetidine (CM), was investigated to determine its effect on the metabolism of 'model' alkoxyphenoxazone substrates ethoxyresorufin (ER) and pentoxyresorufin (PR). The investigation was carried out under different conditions in rat liver microsomes from rodents pretreated with various classical cytochrome P450 inducers. CM exerted a dual effect on uninduced and PB-induced liver microsomal ethoxyresorufin-O-de-ethylase (EROD) activities; it was initially stimulatory but became inhibitory. However, when 3-MC-induced preparations were used. CM only exerted an inhibitory effect on EROD activity over the whole concentration range (0.01 microM-20 mM). Pre-incubating PB-induced and uninduced liver microsomes with NADPH before the addition of ER and CM decreased the stimulatory effect of CM and increased the inhibitory effect in the concentration range (5-20 mM). With 3-MC-induced preparations, the inhibition of EROD was only marginally potentiated. Overall conclusions for the diphasic effects of CM on biological activities were due to CM binding with differing affinities to different P450s. Subsequent decreases and increases in stimulation and inhibition, respectively, on pre-incubation with NADPH were thought to be due to an increased affinity of CM for reduced cytochrome P450.

Alternatives to Aroclor 1254-induced S9 in in vitro genotoxicity assays

A working party was set up by the UK Environmental Mutagen Society to consider alternatives to Aroclor 1254 (Aroclor)-induced S9 in in vitro genotoxicity assays, with the aims of considering whether a replacement for Aroclor in its role in general screening assays could be readily identified. The working party concluded that there was sufficient support in the literature to justify the use of an appropriate phenobarbital/beta-naphthoflavone regime as an acceptable alternative to Aroclor.

Inhibition of dinitropyrene mutagenicity in vitro and in vivo using Salmonella typhimurium and the intrasanguinous host-mediated assay

Dinitropyrenes (DNP), present in polluted air, are potent direct-acting mutagens in Salmonella typhimurium TA98. This mutagenicity is markedly reduced in the presence of rat-liver S9 or microsomes. This has now been confirmed using mouse hepatic fractions. Since most in vitro test systems do not adequately simulate conditions encountered in the intact animal, we have investigated dinitropyrene mutagenicity to Salmonella in the host-mediated assay. 1,8-Dinitropyrene (1,8-DNP) given p.o. to BALB/c mice induced a weak mutagenic effect in S. typhimurium TA98 recovered from the liver 1 h after i.v. administration (optimum time). Over the entire dose range tested no toxicity to bacterial cells was detected. Mutation induction in vivo was dose-related with maximum response at 1 mg DNP/kg body weight. This optimum dose, however, was non-mutagenic to strains TA98/1,8-DNP6 (O-transacetylase-deficient) or TA98NR/1,8-DNP6 (nitroreductase- and O-transacetylase-deficient). 1,3-Dinitropyrene and 1,6-dinitropyrene were weakly mutagenic to TA98 at doses similar to 1,8-DNP. Studies with [14C]1,8-DNP showed that 1 h after oral dosing (1 mg/kg), over 100 ng of 1,8-DNP equivalents were present in the liver (= 0.73% dose). However, only about 5.5 ng were present in the bacterial pellet, suggesting that hepatic components in vivo, as in vitro, bind to DNP, thus interfering with its interaction with Salmonella.

Interaction with microsomal lipid as a major factor responsible for S9-mediated inhibition of 1,8-dinitropyrene mutagenicity

1,8-Dinitropyrene (1,8-DNP), present in polluted air, is a rodent carcinogen and a potent, direct-acting mutagen in salmonella typhimurium TA98. This mutagenicity is markedly reduced in the presence of mammalian hepatic S9 or microsomes. We demonstrate that at least a substantial part of this effect is attributable to non-enzymatic processes. The microsomal-dependent inhibition was unaffected by omission of an NADPH-generating system or when the cytochrome P-450 inhibitor, SKF-525A, or the cytochrome P-448 inhibitor, ellipticine, was incorporated in the metabolic activation system, suggesting that mixed function oxidases are not involved. Heat inactivation partially decreased the ability of induced S9 to reduce DNP mutagenicity. Substitution of S9 with a similar concentration of bovine serum albumin did not affect DNP activity. Thus non-specific binding to microsomal protein is not involved. However, when lipids, derived from uninduced microsomes, were added to incubations of DNP and S. typhimurium TA98, mutagenicity was decreased. Furthermore, substitution of microsomal lipids with a suspension of phosphatidylcholine (PC), a major lipid constituent of microsomes, affected DNP mutagenicity similarly. An increase in PC concentration resulted in a greater inhibitory effect. The reduction in DNP mutagenicity observed with microsomal lipids or with PC was less than that detected with uninduced S9, whilst the mutagenicity of 2-nitrofluorene was reduced to an approximately equal extent by lipids and S9. This phenomenon may be responsible for the S9-mediated detoxification of other mutagenic nitroaromatic compounds and may have important implications for mutagenicity testing.

Distribution and excretion of 1,8-dinitro[14C]pyrene in the mouse

The disposition of the environmental pollutant and potent mutagen and carcinogen, 1,8-dinitropyrene (DNP) in female BALB/c mice was investigated. In the first 48 h after oral administration of 1,8-dinitro[4,5,9,10-14C]pyrene ([14C]DNP), 42% of the dose was eliminated in the faeces and 12% in the urine. Faeces was the major pathway of excretion with 45% of the dose being eliminated by this route in 9 days. Distribution of DNP in various tissues (blood, liver, spleen, lungs, kidneys, stomach, small and large intestine) was studied over 9 days. There was a linear increase in the concentration of radioactive material in the blood, liver and kidneys up to 6 h after [14C]DNP administration, representing 0.27, 2.9 and 0.21% of the radioactive dose, respectively. The corresponding figures after 24 h decreased to 0.1, 1.6 and 0.12%, respectively. In comparison, radioactivity present in the spleen and lungs was low and did not significantly change with time. In studies with ligated sections of the gastrointestinal tract, DNP absorption was from the small and large intestine and there was none from the stomach. The rate of absorption of DNP from the small intestine was greater than that from the large intestine, although overall uptake of the compound was poor (more than 80% of the original dose was recovered from the ligated small intestine after 120 min). The data from these studies suggest that although absorption of orally administered DNP is slow, the compound or its metabolites persist in the body for long periods and the liver should be considered as the major target organ.

Activation and detoxification of 1,8-dinitropyrene by mammalian hepatic fractions in the Salmonella mutagenicity assay

1,8-Dinitropyrene (DNP) is a potent, direct-acting mutagen to Salmonella typhimurium TA98 due to activation by bacterial metabolism. We have studied the effects of uninduced and Aroclor-induced rodent liver post-mitochondrial supernatant (S9), microsomes or cytosolic preparations on its activity. With the plate incorporation protocol, the reported S9-mediated inhibition of mutagenicity was confirmed and shown to be greater after induction when equivalent levels of protein were plated. Cytosol increased mutagenicity considerably whereas the inhibitory effect of S9 could be attributed to the microsomal fraction. The activating and detoxifying capacities of the various hepatic fractions derived from male Sprague-Dawley rats and female BALB/c mice were similar. In the absence of hepatic preparations, DNP mutagenicity was much lower in TA98/1,8DNP6 (deficient in transacetylase) than in TA98, although mutagenic activity could be still increased in the former strain by addition of uninduced cytosol. When DNP was pre-incubated in a liquid assay with S9 before addition of TA98, inhibition occurred and was time dependent. Thus the overall detoxifying effect of S9 is independent of bacterial metabolism. Furthermore, inhibition occurred under conditions where cytosol and microsomal components of S9 were kept together, indicating that the detoxifying effect of the microsomes predominates. The data therefore suggest that the microsomal-mediated detoxification by S9 may be due to the presence of an inhibitory component, enzymatic or otherwise, whose concentration is increased after Aroclor-induction. Nitro-reductase activities detected in TA98 and TA98/1,8-DNP6 were 10-fold higher than those in the mammalian fractions, although the activities in the latter increased to the bacterial enzyme levels after incubation of induced S9 or microsomes, but not cytosol, with an NADPH-generating system.(ABSTRACT TRUNCATED AT 250 WORDS)

DNA-binding studies with 6BT and 5I: implications for DNA-binding/carcinogenicity and DNA-binding/mutagenicity correlations

The divergent activities of a reported carcinogen/noncarcinogen pair of monoazo dyes related to the hepatocarcinogen Butter Yellow (DAB) are currently under investigation in our laboratories. As part of these studies we have determined (a) target organ distribution after oral dosing to rats and (b) covalent binding of 14C-labelled compound to DNA. In DNA-binding studies, 3 rat liver-metabolising systems were employed: in vivo (whole liver), isolated intact hepatocytes, and liver subcellular fractions. Distribution studies revealed that comparable levels of both compounds were detected in the liver at similar times after dosing, and these in vivo tissue concentrations were used for in vitro DNA-binding studies. At this 'in vivo equivalent dose', the carcinogen was consistently bound to DNA more effectively, and the difference (ratio of DNA binding) between the 2 compounds was far greater in vivo. In subsequent studies, covalent DNA binding to bacterial (Salmonella) DNA was assessed at the in vivo equivalent dose. In contrast to the afore-mentioned findings in mammalian systems, the carcinogen was bound less effectively to DNA, and gave fewer revertant counts/plate when the 2 compounds were bound to an equivalent extent. These data are discussed in view of their implications for DNA-binding/carcinogenicity correlations, and with respect to the relationship between DNA binding and mutagenicity in the Salmonella assay.

Brown FK and the colouring of smoked fish--a risk-benefit analysis

A risk-benefit analysis of using Brown FK to colour smoked fish is presented with reference to the available toxicological information. Both the application of the dye itself and the process of smoking can result in the presence in the food of potentially toxic compounds. However, at present it is impossible to ascertain whether or not the use of the dye causes an overall decrease in the levels of such compounds due to the corresponding reduction in the severity of smoking. Recommendations are made for further investigations the results of which will facilitate an appraisal of the possible hazards of adding Brown FK to fish prior to smoking.

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.

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.

Activation of benzo[a]pyrene and aflatoxin B1 to mutagenic chemical species by microsomal preparations from rat liver and small intestine in relation to microsomal epoxide hydrolase

Rat small intestinal microsomes have been compared with liver preparations for their ability to activate promutagens using the Salmonella mutagenicity assay. Induced levels of arylhydrocarbon hydroxylase and cytochrome P-450 in intestinal microsomes are significantly lower than the corresponding amounts in liver microsomes. Greater activation of benzo[a]pyrene (BP) by liver extracts would thus be expected. Although this was observed at greater than 1 microgram BP/plate, at lower doses comparatively high levels of activation were obtained with intestinal microsomes. This could be due to preferential formation of the mutagenic 4,5-oxide with intestinal microsomes, as opposed to the putative major active metabolite, the 7,8-diol-9,10-epoxide. Microsomal epoxide hydrolase inactivates the K-region epoxide by forming the corresponding dihydro-diol. Differences in the levels of these metabolites may thus be a result of higher activity of the enzyme in liver extracts. This hypothesis has been studied using the epoxide hydrolase inhibitor, 1,2-epoxy-3,3,3-trichloropropylene oxide (TCPO). Enzyme activity has been measured using [3H]-BP-4,5-oxide as substrate. Since aflatoxin B1 (AFB) may also be activated via analogous epoxide intermediates, the effects of TCPO on activation of AFB were also investigated. Intestinal microsomal epoxide hydrolase activities were significantly lower than those in liver preparations obtained from animals pre-treated with enzyme inducers. Enzyme activity and promutagen activation ability of intestinal microsomes, respectively, were less susceptible to and not inhibited by TCPO. However, TCPO strongly inhibited microsomal epoxide hydrolase activity and activation of BP and AFB due to liver microsomes.(ABSTRACT TRUNCATED AT 250 WORDS)

Characterization of a microsomal fraction from rat small intestine for metabolic activation of some promutagens

Rat small intestine possesses cytochrome P-450 mixed function oxidase activity and is thus potentially capable of activating procarcinogens during absorption. The low spontaneous tumour incidence at this site may be due partially to the detoxification activity of the intestinal mucosa. To study the contribution of the intestine to the metabolism of foreign chemicals, microsomes have been obtained from rat small intestine by a procedure facilitating recovery of preparations with consistently high enzyme activities and abilities to activate some selected promutagens in the Salmonella mutagenicity assay. Intestinal microsomes from animals with and without pretreatment with inducers have been used for investigations of biochemical properties and ability to activate several mutagens in the Salmonella plate incorporation assay. The effects of microsomal protein concentration and inhibitors were also studied. The results are compared with data obtained using liver microsomes from the same animals. Despite the induction of lower numbers of revertants, intestinal microsomes were at least as efficient as liver preparations for the activation of all the promutagens used when the data were corrected for cytochrome P-450 contents. Differences in dose-response curves for certain mutagens using liver and intestinal microsomes are discussed in relation to variation in metabolism of promutagens. Activation was linearly dependent on microsomal protein concentration, for both liver and small intestinal microsomes. The intestine was generally less susceptible to the effects of cytochrome P-450 and P-448 inducers, although sensitivity to orally administered phenobarbitone was increased by extending treatment times. SKF525A and beta-naphthoflavone inhibited microsomes from both sources, equal inhibition being observed for each type following incorporation of the inhibitor, although they differed in their ability to activate 2-acetylaminofluorene in the presence of the deacylase inhibitor, NaF. The data are discussed in relation to the possible role of the small intestine in metabolic activation in vivo and the utility of microsomes therefrom for the in vitro detection of putative dietary carcinogens.

Mutagenicity studies of urine and faecal samples from rats treated orally with the food colourings Brown FK and Red 2G

Urine and faecal extracts from rats given Brown FK or Red 2G orally (800 mg/kg body weight) were investigated for mutagenicity. Extracts were subjected to liquid fluctuation and plate incorporation assays with Salmonella typhimurium strains TA98 and TA100 in the presence and absence of liver microsomes and/or a beta-glucuronidase-sulphatase preparation. Urine from Red 2G-treated rats only exhibited direct activity when coloured fractions from polyamide-column concentrates were tested with TA100. All other urines, as well as aqueous and ether faecal extracts from animals receiving either colouring, were no more mutagenic than the respective control extracts obtained from the same animals prior to dosing.

Evaluation of a methodology for the use of preparations from rat small intestine in the Salmonella/microsome assay

A methodology is evaluated for the use in the Ames assay of a microsomal metabolising system derived from villous tip cells of rat small intestine. The procedure involved high frequency vibration of everted gut segments followed by gentle lysis and homogenisation. This technique, which has previously been shown to result routinely in high levels of cytochrome P450 and linked enzymes, has now been investigated for its ability to yield preparations capable of activating several promutagens in the Salmonella/plate incorporation test. The data obtained have been compared with results observed with standard rat liver metabolising fractions. In the presence of intestinal microsomes, 2-aminoanthracene, 2-aminofluorene, 2-acetylaminofluorene, aflatoxin B1, benzo[a]pyrene and cyclophosphamide all caused dose-related increases in revertants, the maximum yields of which were lower than those detected with liver microsomes or S9 mix. These and other differences in dose-responses have been discussed in relation to the levels of microsomal protein and cytochrome P450 plated and with respect to the activities of relevant enzymes in the tissue extracts.

Studies on the mutagenicity of commercial preparations of the food colour Red 2G after purification or oxidation

The UK-permitted, monoazo food colour Red 2G (C.I. 18050) has previously been shown to induce transition mutations in bacteria when tested with metabolic activation. Whilst this result has now been confirmed, later commercial food-grade batches of the dye have also been found to exhibit dose-dependent direct activity in fluctuation tests with E. coli WP2 uvr A. HPLC analyses revealed the presence of contaminants several of which were either eliminated or reduced in amount after laboratory recrystallization. A concomitant loss of mutagenicity occurred which was gradually regained with storage of purified preparations. Since peroxidation of standard samples significantly potentiated activity over a number of doses it is concluded that commercial mixtures contain at least one mutagenic impurity which may arise by spontaneous oxidation. The data are discussed in relation to the effects of oxidation on other aromatic amines and with respect to the use of this colour in foods.

An analysis of the efficacy of bacterial DNA-repair assays for predicting genotoxicity

Several bacterial DNA-repair tests have been developed as rapid assays for genotoxicity. In these systems, a positive result is generally considered as evidence for interaction of a chemical with DNA and an indication of carcinogenicity. Recently much data have been published on the responses to suspect carcinogens of such systems in the presence and absence of added exogenous metabolising fractions. An analysis of this literature is presented and it is concluded that bacterial repair tests provide a useful prediction of potential carcinogenicity especially for certain classes of compounds. However, a significant number of procarcinogens and indirect-acting mutagens does not require mammalian metabolism in order to exhibit activity in repair assays. Possible reasons for this discrepancy as well as recommendations concerning the value of bacterial repair systems in carcinogenicity screening are discussed.

A review of the genotoxicity of food, drug and cosmetic colours and other azo, triphenylmethane and xanthene dyes

The genetic toxicology of the major dyestuffs used in foods, drugs and cosmetics has been reviewed. Published data for azo, triphenylmethane and xanthene dyes from short-term assays for muta-carcinogenicity have been summarized and discussed according to usage, current and previous worldwide legislative status. Certain other synthetic food dyes, commercial mixtures, natural and polymeric colourants as well as a section on aminoazobenzene and its derivatives have been included. Genotoxicity has been discussed with reference to structural chemistry, levels of exposure, absorption and metabolism and to epidemiological information. The extent of agreement between data from different tests and correlations with animal cancer assays have been considered. Synthetic dyes from the 3 major structural classes exhibit genotoxicity, whilst only 2 natural colours have proved active. Activity may be due to the presence of certain functional groups, notably nitro- and amino-substituents which are metabolized to ultimate electrophiles that may be stabilized by electronic interaction with aryl rings. Metabolic processes such as azo-reduction may be activating or detoxifying. the low but significant correlation between animal carcinogenicity and short-term test data may be increased with further screening, especially involving chromosome assays. It is suggested that a human cancer hazard may exist where significant quantities of finished benzidine dye samples are handled. Such risks from exposures to other colours and the possibility of human germ-line mutation induction by dyestuffs cannot be meaningfully assessed.

Studies on the genotoxicity of some fluorescein dyes

The activities of 2,4,5,7-tetraiodofluorescein, disodium salt (erythrosine) and 2 phloxine dyes (2,4,5,7-tetrabromo-12,15-dichlorofluorescein, dipotassium salt and the disodium salt of 2,4,5,7-tetraiodo-12,15-dichlorofluorescein) have been determined using DNA-repair, fluctuation and treat-and-plate assays. Tests were conducted with and without illumination from a daylight fluorescent lamp. Both phloxine dyes were active in a rec assay but only in the absence of a rat-liver microsomal metabolising system. Erythrosine was inactive under all conditions. Although the results agreed with some of the published data for these foods and cosmetic colours, previous reports of photodynamic activation and mutagenicity were not confirmed. In the light of recent concern over the efficacy of bacterial DNA-repair tests, it is considered that the results obtained are not at present conclusive evidence for genotoxic hazard of any of the dyes studied.

Methodology for the testing of food dyes for genotoxic activity: experiments with red 2G (C.I. 18050)

A methodology for investigating genotoxicity of food colours using the fluctuation and DNA-repair assays with bacteria is described. In addition, a liquid repair test, developed to permit incorporation of microsomes and the quantitative estimation of cell viability, has been characterised with a number of positive control agents. Results obtained in these systems suggest that the food colour Red 2G induces repairable DNA damage and base-substitution mutation, but only in the presence of a rat-liver microsomal preparation. The significance of the data in the light of other toxicological information is discussed.

Absence of mutation following ultrasonic treatment of Bacillus subtilis cells and transforming deoxyribonucleic acid

Possible mutagenic effects of ultrasound at medical dosages have been assessed using genetic systems of Bacillus subtilis. The induction of mutations, after treatment of cells and of extracted transforming DNA with ultrasound has been tested. High-frequency (2 MHz diagnostic regime and higher intensities) ultrasound was unable to increase significantly the spontaneous frequency of back-mutation of an auxotrophic strain. Moreover, high-frequency treatments (1.5 MHz diagnostic and therapeutic regimes) were incapable of producing detectable levels of mutagenic lesions after in vitro irradiation of transforming DNA. Slight decreases in transforming activity of the treated DNA were apparent while the degree of linkage between two contiguous markers was unaffected. It is concluded that the ultrasound treatments employed under the conditions pertaining do not result in production of detectable mutagenic effects in cells or in vitro treated DNA. Before extrapolating such results to the human hazard situation, it is suggested that tests using genetic systems of higher organisms should be carried out.