The Current Status and Future Applicability of Quantitative Structure-activity Relationships (QSARs) in Predicting Toxicity

Mark T.D. Cronin

The current status of quantitative structure-activity relationships (QSARs) in predicting toxicity is assessed. Widespread use of these methods to predict toxicity from chemical structure is possible, both by industry to develop new compounds, and also by regulatory agencies. The current use of QSARs is restricted by the lack of suitable toxicity data available for modelling, the unsuitability of simplistic modelling approaches for the prediction of certain endpoints, and the poor definition and utilisation of the applicability domain of models. Suggestions to resolve these issues are made.
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SAR Modelling of Complex Phenomena: Probing Methodological Limitations

Herbert S. Rosenkranz

The increased acceptance of the use of structure–activity relationship (SAR) approaches to toxicity modelling has necessitated an evaluation of the limitations of the methodology. In this study, the limit of the capacity of the MULTICASE SAR program to model complex biological and toxicological phenomena was assessed. It was estimated that, provided the data set consists of at least 300 chemicals, divided equally between active and inactive compounds, the program is capable of handling phenomena that are even more “complex” than those modelled up to now (for example, allergic contact dermatitis, Salmonella mutagenicity, biodegradability, inhibition of tubulin polymerisation). However, within the data sets currently used to generate SAR models, there are limits to the complexity that can be handled. This may be the situation with regard to the modelling of systemic toxicity (for example, the LD50).
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A Methylcellulose Microculture Assay for the In Vitro Assessment of Drug Toxicity on Granulocyte/macrophage Progenitors (CFU-GM)

Augusto Pessina, Cristina Croera, Maria Bayo, Ilaria Malerba, Laura Passardi, Loredana Cavicchini, Maria G. Neri and Laura Gribaldo

In a recent prevalidation study, the use of a methylcellulose colony-forming unit-granulocyte/macrophage (CFU-GM) macroassay for two independent in vitro tests (human and murine cell based) was suggested for quantifying the potential haematotoxicity of xenobiotics. In this paper, we describe the transfer of the macroassay to a 96-well plate microassay, in which the linearity of the response was studied (both in terms of CFU-GM and optical density [OD] versus the number of cells cultured), and the inhibitory concentration (IC) values for doxorubicin, 5-fluorouracil and taxol were determined and compared with those obtained by using the original macroassay. Fresh murine bone marrow and human umbilical cord blood mononuclear cells were used as a source of myeloid progenitors. The cells were cultured in methylcellulose containing ranulocyte/macrophage-colony-stimulating factor, and in the presence of increasing drug concentrations. The cloning capacity of the progenitors was measured both as the number of colonies counted manually (CFU-GM), and as OD evaluated with an automated plate reader in an MTT test. Our results show that, in the microassay, up to 20 colonies/well could be easily counted, and that this range (20 to zero) gave a regression line from which IC values were calculated, which were very close to those obtained by using the macroassay (where the range of colony numbers was from 100 to zero). The test did not give good results when the OD (instead of the colony count) was used as the endpoint, because, although a high coefficient of determination was obtained, the OD values ranged from 0.6 to zero and the IC values determined were not comparable to those obtained by manual counts. The use of the microassay dramatically reduces the quantity of methylcellulose needed, and permits hundreds of cultures to be processed in the same experiment, contributing to significant reductions in both the work involved and the cost. A further important benefit is a reduction of the amount of drug needed for testing, which is crucial for screening new molecules, when many different toxicological tests have to be carried out. The microassay is therefore a useful and reproducible tool for screening compounds (chemicals, drugs and xenobiotics) for potential haematotoxicity directly on human myeloid progenitors, and could contribute significantly to reducing the use of animals in toxicity testing.
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The Role of Biomembranes in Chromium (III)-induced Toxicity In Vitro

Emil Rudolf and Miroslav Cervinka

The role of biomembranes in the chronic toxicity of environmentally occurring chromium acetate hydroxide was investigated by using primary human fibroblasts. Transport of chromium acetate hydroxide across the plasma membrane of the cell, and the effects of chromium (III) ions on the plasma membrane as well as other intracellular membranes, were determined during six weeks of continuous exposure by using atomic absorption spectrometry, observation of cell morphology, membrane integrity assays (for lactate dehydrogenase leakage and lysosomal membrane disruption), and mitochondrial assays (for mitochondrial dehydrogenase activity and mitochondrial transmembrane potential analysis). The type of cell death induced by long-term exposure was determined in terms of phosphatidylserine externalisation, caspase-3 activation, and chromatin fragmentation. Chromium acetate hydroxide, at a concentration of 100μmol/l, accumulated in exposed cells, inflicting plasma membrane damage and suppressing mitochondrial function. Antioxidant co-enzyme Q, at a concentration of 10μmol/l, partially prevented plasma membrane damage and mitochondrial dysfunction. Exposure to chromium acetate hydroxide produced apoptosis, necrosis and an intermediate type of cell death in primary human fibroblasts. These results show that the plasma membrane and mitochondrial membrane are important targets for chronic chromium acetate hydroxide toxicity, and that this in vitro system holds promise for studying the toxicity resulting from long-term exposure to metal ions.
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The Relevance of Genetically Altered Mouse Models of Human Disease

Nirmala Bhogal and Robert Combes

The impetus to develop useful models of human disease and toxicity has resulted in a number of large-scale mouse mutagenesis programmes. This, in turn, has stimulated considerable concern regarding the scientific validity and welfare of genetically altered mice, and the large numbers of mice that are required by such programmes. In this paper, the scientific advantages and limitations of genetically altered mice as models of several human diseases are discussed. We conclude that, while the use of some such mouse models has contributed considerably to an understanding of human disease and toxicity, other genetically altered mouse models have limited scientific relevance, and fewer have positively contributed to the development of novel human medicines. Suggestions for improving this unsatisfactory situation are made.
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The UK Food Standards Agency Draft Report on Variability and Uncertainty in Toxicology: A Response by FRAME1

Nirmala Bhogal and Robert Combes

At the request of the Food Standards Agency, the Committee on Toxicity of Chemicals in Food, Consumer Products and the Environment (COT) established a Working Group on Variation and Uncertainty in Toxicology (WG VUT). In April 2006, the WG VUT produced a draft report for public consultation. FRAME made a submission in response to this consultation in July 2006. We commend the WG VUT for its comprehensive account of many of the problems associated with risk assessment, and for making recommendations about the problems that need to be addressed. We were particularly encouraged by the WG VUT’s recognition of the need for guidelines on how toxicological studies should be conducted and data analysed. However, we believe that the report has not achieved all of its objectives. It does not adequately consider how modern technologies, experimental design, statistical analysis and species extrapolation can be used in practice to address variability and uncertainty. There is a disproportionate focus on the sources of variability and uncertainty in human data, with relatively little consideration of how variation and uncertainty due to animal tests can be addressed. Furthermore, it is clear that, until the advantages and limitations of all toxicological methods are fully appraised and testing strategies and guidelines are agreed, the scope for improving the existing approaches to risk assessment will be severely limited. Hence, the use of alternative methods for hazard identification and characterisation merit more consideration than they were given in the draft report.
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The Effects of Heavy Metals on Common Carp White Blood Cells In Vitro

Malgorzata Witeska and Marta Wakulska

The in vitro effects of cadmium, copper, lead and zinc, and various cadmium compounds (chloride, sulphate and nitrate) on common carp (Cyprinus carpio) lymphocyte viability and phagocyte activity, were evaluated. The percentage of dead lymphocytes was determined after Trypan blue staining, and phagocyte activity was measured by using the nitroblue tetrazolium (NBT) reduction test. Lead was the most toxic to lymphocytes — the maximum mortality exceeded 30%, and was significantly higher at 1μM of lead, compared to the control. The maximum mortality caused by cadmium was below 10%, but was significantly elevated with 5μM or more of cadmium. Zinc induced lymphocyte mortality from 10μM, whilst no effect was observed with copper. The incubation of full blood with the three cadmium compounds (at 5mg/l of cadmium) showed that cadmium nitrate and cadmium sulphate were more toxic (over 35% and 25% mortality, respectively) than cadmium chloride (about 15% mortality). This was confirmed by the results of tests on isolated cells —1mg/l of cadmium as nitrate and sulphate increased lymphocyte mortality compared to the control and cadmium chloride. Phagocytic activity was less sensitive to heavy metals than was lymphocyte viability. It was significantly reduced following exposure to 50μM and 100μM cadmium, and 100μM zinc, but no effects were observed with either lead or copper.
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Environmental and Synthetic Sulphydryl Group Inhibitors: Effects on Bioluminescence and Respiration in Vibrio fischeri

Virginija Kalciene. and Anolda C∨etkauskaite

Elemental sulphur (as S0 and S8) is abundant in anaerobic sediments and soil, and is highly toxic in the Vibrio fischeri bioluminescence test. This mode of S0 action remains uncertain. The objective of this research was the analysis of the toxic effects of S0 on bioluminescence and respiration in V. fischeri, in joint action with N-ethylmaleimide (NEM) or 2,4-dithio-DL-threitol (DTT), which are –SH group inhibiting and maintaining synthetic agents, respectively. Non-toxic DTT immediately protected cell bioluminescence against S0 inhibition at low (5.5ppb) and high (55ppb) concentrations of S0, whilst restoration of the inhibitory effect of S0 took up to 30 minutes. NEM (62.5ppb) diminished cell bioluminescence by up to 50% after 5 minutes, but after 60 minutes, the inhibition reached 100%. DTT restored the bioluminescence function inhibited in vivo and in vitro by S0 and NEM. Enhancement of cell respiration by up to 20% and 33% was observed at 2.2ppm of S0 and 36.8ppm of 2,4-dinitrophenol (2,4-DNP; an uncoupler of oxidative phosphorylation), respectively; whilst NEM (3.1ppm) caused a reduction of up to 40%. This comparative analysis confirmed that S0 has multiple modes of action — it acts as both an –SH group inhibitor and an uncoupler of oxidative phosphorylation in V. fischeri cells.
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The Use of Bioassays for the Risk Assessment of Toxic Leachates: An Experimental Study

Natalya Irha and Irina Blinova

Solid wastes from the oil-shale industry produce leachates containing toxic compounds such as heavy metals and persistent polycyclic aromatic hydrocarbons (PAH). The hazard to the environment represented by waste leachates depends not only on their chemical composition, but also on the mobility and bioavailability of toxic contaminants in soils. We evaluated the applicability of bioassays for toxicity assessment of the bioavailable fraction of heavy metals and PAH in soils, in experiments with samples of four different soil types (Rendzina, Brown pseudopodzolic, Typical brown, Sodpodzolic), the pH of which ranged from 6.2 to 7.2. The toxicity of the bioavailable fraction of the soil contaminants was assessed with the dehydrogenase enzyme activity assay, and with a Toxkit microbiotest with the crustacean, Thamnocephalus platyurus, after treatment of the soil samples with an artificial solution containing chromium (III), lead (II),
copper (II), cadmium (II) and pyrene. The test results confirm those of earlier experiments, which characterised the sorption potential of investigated soils for the same compounds. Both tests turned out to be sufficiently sensitive, and hence can be recommended as effective and useful tools for the assessment of the bioavailable fraction of soil contaminants.
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An Evaluation of a Novel Chick Cardiomyocyte Micromass Culture Assay with Two Teratogens/Embryotoxins Associated with Heart Defects

Helena S. Hurst, Richard H. Clothier and Margaret Pratten

This study was aimed at determining whether the chick cardiomyocyte micromass (MM) system could be employed to predict the teratogenicity/embryotoxicity of exogenous chemicals. Two documented teratogens/embryotoxins, sodium valproate (the sodium salt of valproic acid; VPA) and all-trans retinoic acid (tRA), were used in the initial phase of the study. White Leghorn 5-day-old embryo hearts were dissociated to produce a cardiomyocyte suspension in Dulbecco’s Modified Eagle’s Medium. Cultures were incubated at 37°C in 5% CO2 in air, and observations were made every 24 hours over 5 days, for the detection of beating. Culture viability was assessed by using the resazurin reduction assay for determining culture activity and the kenacid blue assay for determining cell number. It was found that tRA significantly reduced cell activity and beating, whilst not affecting total cell number. VPA up to 500μM induced no cytotoxicity in the MM cardiomyocyte cultures, whilst all the VPA concentrations tested reduced beating. The results demonstrate the potential of the chick cardiomyocyte MM culture assay to identify teratogens/embryotoxins that alter functionality, which may result in a teratogenic outcome, whilst not causing cytotoxicity (direct embryotoxicity). This could form part of a screen for developmental toxicity related to cardiac function, whilst limb cultures and brain cultures based on the same system could be relevant to teratogenic effects on those tissues.
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