A QSAR study of the acute toxicity of some industrial organic chemicals to goldfish. Narcosis, electrophile and proelectrophile mechanisms

1. A baseline toxicity QSAR model was derived for the 24-h LC50 to the goldfish, Carassius auratus. 2. The QSAR-predicted LC50 values for six epoxide derivatives were 2.8-985 times greater than measured. The excess toxicity of these epoxides and other compounds was ascribed to an electrophile molecular mechanism involving SN2 reaction with sulphydryl and other neucleophile groups present in enzymes and other biological macromolecules. 3. The excess toxicities of allyl alcohol and pentaerythritol triallyl ether were interpreted in terms of proelectrophile mechanisms. For the latter compound, this involves a monooxygenase-mediated free radical proton abstraction to a stable allyl radical. The allyl-free radical can undergo enzymic free radical hydroxylation to afford the corresponding acetal, which on decomposition yields the Michael acceptor electrophile acrolein.     

A QSAR study of the acute toxicity of some industrial organic chemicals to goldfish. Narcosis,electrophile and proelectrophile mechanisms

1. A baseline toxicity QSAR model was derived for the 24-h LC₅₀ to the goldfish, Carassius auratus.

2. The QSAR-predicted LC₅₀ values for six epoxide derivatives were 2.8–985 times greater than measured. The excess toxicity of these epoxides and other compounds was ascribed to an electrophile molecular mechanism involving S_N² reaction with sulphydryl and other neucleophile groups present in enzymes and other biological macromolecules.

3. The ex?ess toxicities of allyl alcohol and pentaerythritol triallyl ether were interpreted in terms of proelectrophile mechanisms. For the latter compound, this involves a monooxygenase-mediated free radical proton abstraction to a stable allyl radical. The allylfree radical can undergo enzymic free radical hydroxylation to afford the corresponding acetal, which on decomposition yields the Michael cceptor electrophile acrolein.     

A comparison of the acute toxicity of chemicals to fish, rats and mice

The acute toxicity of chemicals to rainbow trout, as shown by intraperitoneal injections (IP LD50), oral dosing (oral LD50) and aqueous exposure (LC50) was compared with published values for IP LD50S and oral LD50S of mice and rats. The method of comparison was by simple linear regression analyses of log-transformed data, modified to recognize that X (fish toxicity) was neither fixed nor measured without error. Within-species comparisons demonstrated very strong linear correlations (r = 0.866-0.998) between IP and oral LD50S. Variability was least for the fish data since it was all generated in one laboratory. Comparisons between species of IP and oral LD50S gave correlation coefficients ranging from 0.59 to 0.95 with the majority over 0.80. Correlations were best (r = 0.83-0.94) between fish LD50S and rat and mice IP LD50S. Correlations were poorest between fish and mammalian oral LD50S (r = 0.59-0.66) because the sample sizes and the ranges of values were very small. In all cases, the slopes were close to, or equalled, 1.0. Comparisons of fish LC50S to fish or mammalian LD50S were not as successful. Correlation coefficients ranged from 0.19 to 0.83. Presumably the cause was the aqueous exposure. Interactions of the chemicals with water (e.g. dissociation) and with lipid membranes (partitioning) should cause considerable variations in uptake efficiency. However, adjustments of LC50S for dissociation constants and partition coefficients did not improve these correlations, probably because there were few chemicals for which all data were available. These comparisons demonstrate a potential for a wider use of surrogate species in toxicity testing and for adapting existing data from mammalian toxicology to aquatic hazard assessments.     

The joint acute toxicity to Daphnia magna of industrial organic chemicals at low concentrations

The joint acute toxicity of a mixture of 50 nonreactive organic chemicals towards Daphnia magna was investigated. It was found that the observed toxicity is accurately predicted by the model of concentration addition, even at very low concentrations of the individual compounds. The implications these findings may have on the prediction of the toxicity of mixtures of chemicals occurring in nature are briefly discussed.     

Chemical structure-biodegradation inhibition and fish acute toxicity relationships for narcotic industrial chemicals

Biodegradation of each primary alcohol from methanol (C₁. OH) to 1-dodecanol (1–C₁₂. OH) was manometrically measured as a function of alcohol concentration. The study employed resting cells prepared from the growth of a mixed microbial culture on the respective alcohol substrate. Biodegradation rates at each alcohol concentration were calculated in terms of μmol oxygen consumed/μg cell protein/h. Maximum biodegradation rates of all alcohols were correlated with the concentrations of respective alcohols. Gaussian curves were obtained for all alcohols, except 1-undecanol and 1-dodecanol. From the rate inhibition data, concentrations that reduced maximum observed biodegradation rates by 50% (experimental EC₅₀) were statistically derived for 10 alcohols (C₁. OH to 1-C₁₀. OH). The logarithms of 1-octanol/water partition coefficients (Log P) and EC₅₀ values of these alcohols were correlated. This relationship was parabolic and defined by the polynomial regression equation: Log EC₅₀ (mol/L) = –0.88 (Log P) + 0.07 (Log P)²–0.23. This equation was used for predicting EC₅₀ values of 19 narcotic industrial chemicals based on their Log P values. Experimental EC₅₀ values of these test chemicals were derived in the same manner as described for alcohols. The EC₅₀ values were compared for each test chemical by taking a ratio of predicted to experimental EC₅₀ (mol/L). The mean value of these ratios was 1.16 ± 0.35. The logarithms of microbial respirometric EC₅₀ and fathead minnow acute toxicity (96-h LC₅₀) values of 7 primary alcohols were correlated. This relationship was linear and defined by the exponential equation: LC₅₀ (mg/L) = 0.0001 (EC₅₀ mg/L)^1.76. This equation was used for predicting LC₅₀ values of 12 narcotic chemicals. Predicted and experimental (from literature) LC₅₀ values of each chemical were compared by appropriately taking a ratio. The average of these ratios was about 1.2.     

The ChemScreen project to design a pragmatic alternative approach to predict reproductive toxicity of chemicals

There is a great need for rapid testing strategies for reproductive toxicity testing, avoiding animal use. The EU Framework program 7 project ChemScreen aimed to fill this gap in a pragmatic manner preferably using validated existing tools and place them in an innovative alternative testing strategy. In our approach we combined knowledge on critical processes affected by reproductive toxicants with knowledge on the mechanistic basis of such effects. We used in silico methods for prescreening chemicals for relevant toxic effects aiming at reduced testing needs. For those chemicals that need testing we have set up an in vitro screening panel that includes mechanistic high throughput methods and lower throughput assays that measure more integrative endpoints. In silico pharmacokinetic modules were developed for rapid exposure predictions via diverse exposure routes. These modules to match in vitro and in vivo exposure levels greatly improved predictivity of the in vitro tests. As a further step, we have generated examples how to predict reproductive toxicity of chemicals using available data. We have executed formal validations of panel constituents and also used more innovative manners to validate the test panel using mechanistic approaches. We are actively engaged in promoting regulatory acceptance of the tools developed as an essential step towards practical application, including case studies for read-across purposes. With this approach, a significant saving in animal use and associated costs seems very feasible.     

A testing strategy to evaluate the mutagenic activity of industrial chemicals in cultured mammalian cells

Based on the published guidelines of regulatory agencies and the latest scientific findings, an experimental strategy for the routine testing of the mutagenic activity of industrial chemicals in mammalian cells in vitro is designed. The strategy includes three individual experiments: (1) a range-finding experiment for cytotoxicity to determine the appropriate doses to be used in subsequent mutagenicity testing; (2) mutagenicity testing at limited doses with different concentrations of Aroclor 1254-induced rat liver homogenate (S9); and (3) a more extensive dose-response experiment using an S9 concentration determined to be optimum in mutagenicity. Besides satisfying the guidelines of regulatory agencies, this strategy should yield data that allow sound scientific judgment on the mutagenicity of the chemicals tested.     

A hierarchical approach to the evaluation of chemicals for estrogenic and other endocrine-disrupting properties

The emergence of estrogenicity/endocrine-disruption as an important endpoint in the toxicological assessment of chemicals presents a series of problems to overcome before regulatory control of such agents can be enacted. A framework is presented by which progress in this endeavour can be expedited. A hierarchial approach to testing is proposed, together with consideration of the types of information required to transform test data into human risk estimations. The approach is based broadly on current methods for defining potential human carcinogens and mutagens, and if found acceptable, would dramatically accelerate regulatory progress on this subject. However, several questions must be answered, using focused data, before the approach can be endorsed or transferred into a regulatory context. The importance of early consideration of all aspects of this complex new toxicity, including the unexpected observation of synergism between synthetic estrogens, is emphasized.     

Methods for the assessment of the toxicity of environmental chemicals to earthworms

In view of the impending publication of standards for earthworm toxicity testing by the Commission of the European Communities, a review has been made of the recent literature on earthworm toxicology. Relevant studies are reviewed from the standpoints of methods used, reproducibility of results, and ability to extrapolate laboratory results to field situations. Eisenia foetida, a commonly used test species, is much less sensitive to agricultural chemicals than other, native earthworms and is of doubtful utility for extrapolating laboratory data to field conditions, but when native soil organisms are used, such extrapolations show good general agreement. Standardization of test conditions and broadening of the data base are encouraged.     

A new approach to practical acute toxicity testing

A method for the investigation of the acute toxicity of an unknown chemical substance, with an estimation on the LD50, is described. Using this, it is possible to obtain with 13 experimental animals adequate information on the acute toxicity and on the LD50. This method has no limitations and applies to drugs, agricultural and industrial chemicals. It can be used for every route of administration.     

Comparative analysis of pharmaceuticals versus industrial chemicals acute aquatic toxicity classification according to the United Nations classification system for chemicals. Assessment of the (Q)SAR predictability of pharmaceuticals acute aquatic toxicity and their predominant acute toxic mode-of-action

Pharmaceuticals have been reported to be ubiquitously present in surface waters prompting concerns of effects of these bioactive substances. Meanwhile, there is a general scarcity of publicly available ecotoxicological data concerning pharmaceuticals. The aim of this paper was to compile a comprehensive database based on OECD's standardized measured ecotoxicological data and to evaluate if there is generally cause of greater concern with regards to pharmaceutical aquatic toxicological profiles relative to industrial chemicals. Comparisons were based upon aquatic ecotoxicity classification under the United Nations Global Harmonized System for classification and labeling of chemicals (GHS). Moreover, we statistically explored whether the predominant mode-of-action (MOA) for pharmaceuticals is narcosis. We found 275 pharmaceuticals with 569 acute aquatic effect data; 23 pharmaceuticals had chronic data. Pharmaceuticals were found to be more frequent than industrial chemicals in GHS category III. Acute toxicity was predictable (>92%) using a generic (Q)SAR ((Quantitative) Structure Activity Relationship) suggesting a narcotic MOA. Analysis of model prediction error suggests that 68% of the pharmaceuticals have a non-specific MOA. Additionally, the acute-to-chronic ratio (ACR) for 70% of the analyzed pharmaceuticals was below 25 further suggesting a non-specific MOA. Sub-lethal receptor-mediated effects may however have a more specific MOA.     

The acute oral toxicity of 369 pesticidal, pharmaceutical and other chemicals to wild birds

The acute po toxicity of 369 chemicals was determined for 1 or more species of wild birds, always including either red-winged blackbirds or starlings. Of these, 180 chemicals were toxic to 1 or more species at 100 mg/kg or less. Statistical comparison of redwing, starling and rat data indicated that redwings were more sensitive to chemicals than starlings, and both were more sensitive than rats.     

Application of liquid chromatography-mass spectrometry to biomonitoring of exposure to industrial chemicals

Recent advances on biomarker research are discussed, primarily relying on experience gained with technologies based on liquid chromatography-tandem mass spectrometry (LC-MS-MS). Determination of urinary metabolites of industrial chemicals (n-hexane, benzene, toluene, and styrene) in samples from occupationally exposed workers and controls was performed by LC-MS-MS with either electrospray (ESI) or atmospheric pressure chemical ionization (APCI), as appropriate. Both phase I and II metabolites (glucuronides, sulfates, and mercapturic acids) can be detected with little or no sample manipulation, thus allowing the identification of a number of artifacts and "new" metabolites. However, experimental evidence indicates the need for properly addressing the matrix effect, which is always associated with the analysis of biological samples. Both efficient sample preparation and the use of isotopically labeled internal standards seem to be necessary to develop validated quantitative methods.     

A combined approach to drug metabolism and toxicity assessment.

The challenge of predicting the metabolism or toxicity of a drug in humans has been approached using in vivo animal models, in vitro systems, high throughput genomics and proteomics methods, and, more recently, computational approaches. Understanding the complexity of biological systems requires a broader perspective rather than focusing on just one method in isolation for prediction. Multiple methods may therefore be necessary and combined for a more accurate prediction. In the field of drug metabolism and toxicology, we have seen the growth, in recent years, of computational quantitative structure-activity relationships (QSARs), as well as empirical data from microarrays. In the current study we have further developed a novel computational approach, MetaDrug, that 1) predicts metabolites for molecules based on their chemical structure, 2) predicts the activity of the original compound and its metabolites with various absorption, distribution, metabolism, excretion, and toxicity models, 3) incorporates the predictions with human cell signaling and metabolic pathways and networks, and 4) integrates networks and metabolites, with relevant toxicogenomic or other high throughput data. We have demonstrated the utility of such an approach using recently published data from in vitro metabolism and microarray studies for aprepitant, 2(S)-((3,5-bis(trifluoromethyl)benzyl)-oxy)-3(S)phenyl-4-((3-oxo-1,2,4-triazol-5-yl)methyl)morpholine (L-742694), trovofloxacin, 4-hydroxytamoxifen, and artemisinin and other artemisinin analogs to show the predicted interactions with cytochromes P450, pregnane X receptor, and P-glycoprotein, and the metabolites and the networks of genes that are affected. As a comparison, we used a second computational approach, MetaCore, to generate statistically significant gene networks with the available expression data. These case studies demonstrate the combination of QSARs and systems biology methods.     

Fish toxicity tests with mixtures of more than two chemicals: A proposal for a quantitative approach and experimental results

The problem of the toxic action of mixtures of many chemicals has been studied. Starting with the classification of Plackett and Hewlett, the types of mixtures for which toxicity can be predicted is discussed. A Mixture Toxicity Index (MTI) has been proposed as a quantification of the result of mixture toxicity experiments. Toxicity experiments with guppies have been conducted, using 6 mixtures of 3–50 chemicals. The toxicity of mixtures of simple similarly acting chemicals could be predicted by Concentration Addition. The concentrations of the separate chemicals in a mixture of 50 compounds giving 50% mortality, were 0.02 × LC₅₀.     

Toxicological characteristics of endocrine-disrupting chemicals: developmental toxicity, carcinogenicity, and mutagenicity

It is generally accepted that endocrine-disrupting chemicals (EDCs) play a role in a variety of adverse health effects in an intact organism or its progeny as a consequence of changes in the endocrine system. Primary toxic effects of EDCs were reported to be related to infertility, reduction in sperm count, and teratogenicity, but other important toxic effects of EDCs such as carcinogenicity and mutagenicity have also been demonstrated. The aim of the present study was to systematically analyze the toxicological characteristics of EDCs in pesticides, industrial chemicals, and metals. A comprehensive literature survey on the 48 EDCs classified by the Centers for Disease Control and Prevention (CDC) was conducted using a number of databases which included Medline, Toxline, and Toxnet. The survey results revealed that toxicological characteristics of EDCs were shown to produce developmental toxicity (81%), carcinogenicity (79%, when positive in at least one animal species; 48%, when classified based on IARC evaluation), mutagenicity (79%), immunotoxicity (52%), and neurotoxicity (50%). Regarding the hormone-modulating effects of the 48 EDCs, estrogenic effects were the most predominant in pesticides, while effects on thyroid hormone were found for heavy metals. EDCs showing estrogen-modulating effects were closely related to carcinogenicity or mutagenicity with a high degree of sensitivity. Systematic information on the toxicological characteristics of the EDCs will be useful for future research directions on EDCs, the development of new screening methods, legal regulation, and for investigations of their mechanism of action.     

A decision tree approach to the regulation of food chemicals associated with irreversible toxicities

A decision tree approach to the regulation of foodborne carcinogen-associated chemicals is presented. In the first stage, this approach places emphasis on the question of whether a chemical is capable of generating chemically reactive metabolitesin vivo. Such metabolites are considered to havepotential to initiate not only carcinogenesis but also other serious irreversible toxicities. In the second stage, the scheme suggests that the ability of a compound to enhance the development of the carcinogenic process (positive modifiers) should be evaluated. Those compounds which enhance that process should then be evaluated for efficacy. And finally it is argued that the various decision options should depend both on the degree of risk aversion desired by the consumer and the feasibility of marketplace control.