Toxicological Risk Assessment Research Articles

Restriction on human exposure to xenobiotics: theory and practice

Exposure to environmental xenobiotics, which influence the everyday life of all humans in our times, has become a subject of intensive analysis by scientists, authorities of environmental hazards, state authorities and authors of legislation. Toxicological risk assessment of chemicals in contaminated food and water as well as polluted air is expressed in basic ‘toxic units’. A broad-scale human biomonitoring (HBM) for environmental toxicants is the strategy of a cause-effect analysis of chemical exposure to environmental xenobiotics. HBM demonstrates the relationships between exposure to xenobiotics and the following health disorders: obesity, impaired reproduction, type 1 diabetes (T1D), autism, cancers and other diseases in the society. Developing effective toxicological tools and legislative standards is expected to help in eliminating endocrine disruptor chemicals (EDCs), which cause infertility. The exceptional category of xenobiotics, which highly influences human health, and is treated as a priority problem to be controlled in the European Union, form genotoxic carcinogens. According to a current assessment, hundreds of chemical xenobiotics and their metabolites, in the minimum detectable quantity, mostly all of anthropogenic origin, can be found in the organisms of inhabitants in western countries. Despite the permanent presence of xenobiotics in human environment, it is worth taking into consideration practical methods to limit and avoid contacts with environmental chemicals. It concerns the provision of water and air filters, the thermal processing of food, the selection of food products, and other aspects of everyday life.

In Silico Prediction of Chemicals Binding to Aromatase with Machine Learning Methods.

Environmental chemicals may affect endocrine systems through multiple mechanisms, one of which is via effects on aromatase (also known as CYP19A1), an enzyme critical for maintaining the normal balance of estrogens and androgens in the body. Therefore, rapid and efficient identification of aromatase-related endocrine disrupting chemicals (EDCs) is important for toxicology and environment risk assessment. In this study, on the basis of the Tox21 10K compound library, in silico classification models for predicting aromatase binders/nonbinders were constructed by machine learning methods. To improve the prediction ability of the models, a combined classifier (CC) strategy that combines different independent machine learning methods was adopted. Performances of the models were measured by test and external validation sets containing 1336 and 216 chemicals, respectively. The best model was obtained with the MACCS (Molecular Access System) fingerprint and CC method, which exhibited an accuracy of 0.84 for the test set and 0.91 for the external validation set. Additionally, several representative substructures for characterizing aromatase binders, such as ketone, lactone, and nitrogen-containing derivatives, were identified using information gain and substructure frequency analysis. Our study provided a systematic assessment of chemicals binding to aromatase. The built models can be helpful to rapidly identify potential EDCs targeting aromatase.

Caenorhabditis elegans as a powerful alternative model organism to promote research in genetic toxicology and biomedicine.

In view of increased life expectancy the risk for disturbed integrity of genetic information increases. This inevitably holds the implication for higher incidence of age-related diseases leading to considerable cost increase in health care systems. To develop preventive strategies it is crucial to evaluate external and internal noxae as possible threats to our DNA. Especially the interplay of DNA damage response (DDR) and DNA repair (DR) mechanisms needs further deciphering. Moreover, there is a distinct need for alternative in vivo test systems for basic research and also risk assessment in toxicology. Especially the evaluation of combinational toxicity of environmentally present genotoxins and adverse effects of clinically used DNA damaging anticancer drugs is a major challenge for modern toxicology. This review focuses on the applicability of Caenorhabditis elegans as a model organism to unravel and tackle scientific questions related to the biological consequences of genotoxin exposure and highlights methods for studying DDR and DR. In this regard large-scale in vivo screens of mixtures of chemicals and extensive parallel sequencing are highlighted as unique advantages of C. elegans. In addition, concise information regarding evolutionary conserved molecular mechanisms of the DDR and DR as well as currently available data obtained from the use of prototypical genotoxins and preferential read-outs of genotoxin testing are discussed. The use of established protocols, which are already available in the community, is encouraged to facilitate and further improve the implementation of C. elegans as a powerful genetic model system in genetic toxicology and biomedicine.

Development of an Adverse Outcome Pathway for the Onset of Hypertension by Oxidative Stress-Mediated Perturbation of Endothelial Nitric Oxide Bioavailability

Abstract Hypertension is a cardiovascular risk factor that has a profound influence on cardiovascular morbidity and mortality. While good progress has been made in terms of identifying and managing this risk factor for patient care, methods to assess the potential of chemical compounds to induce hypertension or to assess the efficacy of consumer products (e.g., e-cigarettes) targeted at reducing disease burden remain largely limited to epidemiological associations and in vivo studies. The field of toxicology has undergone a paradigm shift toward the replacement of in vivo testing in toxicological risk assessment. The adverse outcome pathway (AOP) framework could facilitate improved knowledge-based risk/benefit assessment of chemicals or consumer products, respectively, without the necessity of animal testing. Furthermore, to facilitate a more timely, cost effective, and ethical solution for risk/efficacy assessment purposes, integrated testing strategies are required, which do not heavily rely on in vivo ...

Chemical and physiological metal bioaccessibility assessment in surface bottom sediments from the Deba River urban catchment: Harmonization of PBET, TCLP and BCR sequential extraction methods

In the present study, the physiologically based extraction test PBET (gastric and intestinal phases) and two chemical based extraction methods, the toxicity characteristic leaching procedure (TCLP) and the sequential extraction procedure BCR 701 (Community Bureau of Reference of the European Commission) have been used to estimate and evaluate the bioaccessibility of metals (Fe, Mn, Zn, Cu, Ni, Cr and Pb) in sediments from the Deba River urban catchment. The statistical analysis of data and comparison among physiological and chemical methods have highlighted the relevance of simulate the gastrointestinal tract environment since metal bioaccessibility seems to depend on water and sediment properties such as pH, redox potential and organic matter content, and, primordially, on the form in which metals are present in the sediment. Indeed, metals distributed among all fractions (Mn, Ni, Zn) were the most bioaccessible, followed by those predominantly bound to oxidizable fraction (Cu, Cr and Pb), especially near major urban areas. Finally, a toxicological risk assessment was also performed by determining the hazard quotient (HQ), which demonstrated that, although sediments from mid- and downstream sampling points presented the highest metal bioaccessibilities, were not enough to have adverse effects on human health, Cr being the most potentially toxic element.

Accumulation and toxicological risk assessment of Cd, As, Pb, Hg, and Cu from topsoils of school playgrounds at Obio-Akpor LGA Rivers State Nigeria

The adverse health effects associated with heavy metal pollution have become a subject of topical discussion. Using standard methods, this study evaluated the accumulation and toxicological risks of heavy metal deposition on top soils of school playgrounds in Obio-Akpor LGA. Results obtained from the top soils, sampled from 10 different schools showed that the concentration of heavy metals assessed, followed the trend; Hg<As<Cd<Pb<Cu. Marginal differences between the test and control samples were observed for the geoaccumulation index of the heavy metals except for copper. For the contamination factor, FCUA had the least results for cadmium (0.31) but highest in arsenic (0.016) while OPS, UDPS and RBPS recorded the highest contamination factor for lead, mercury, and copper respectively. The results for the pollution load index and degree of contamination of the test sites showed that the sites were unpolluted and had low contamination status, while the enrichment factors showed the deposition of only cadmium to be of a natural origin, and the rest of the heavy metals resulted from anthropogenic sources. The trend from the hazard quotient was Pb>Hg>Cu>Cd>As while the highest (0.0213) and least (0.0162) total hazard index value occurred at RBPS and MOM respectively. These values obtained for this study indicate that the school playground's soils are still within the recommended heavy metal content safe levels.

Extractables Screening of Polypropylene Resins Used in Pharmaceutical Packaging for Safety Hazards.

Pharmaceutical products are packaged in containers so that they can be manufactured, distributed, and used. Because extractables from such containers are precursors of leachable impurities in the product, extractables represent potential hazards to user safety. Polypropylene resins are frequently used as materials of construction for packaging of liquid parenteral drug products. Thus, extractables profiling of polypropylene resins may be an effective means of hazard identification associated with the resin's safe use. Twenty-one polypropylene resins were extracted using aqueous and organic extraction solvents, and the resulting extracts were screened for extractables using appropriate general chemistry, chromatographic, and spectroscopic methodologies. The resulting extractables profiles were toxicologically reviewed by a defined process to identify potential hazards given a specified therapeutic application involving long-term use of a large-volume aqueous parenteral drug product. The organic extractables profiles of individual polypropylene resins were variable in terms of the individual extractable identified and their extracted levels, consistent with high variability in polypropylene resin formulations and pharmaceutical product manufacturing. However, the profiles were similar in terms of the groups of extractables measured. Thus, for example, all the resins had extractables associated with antioxidants, as all the resins contained antioxidants but the specific extractables for a given resin depended on the specific antioxidants present in that resin. Few of the targeted extractable elements were present in the extracts at measurable levels, although most resins had measurable levels of extracted aluminum, silicon, and alkali and alkaline earths. A worst-case extractables profile (all the extractables measured in individual resins at their highest levels) was toxicologically reviewed considering an aqueous large-volume parenteral drug product. This review established certain extractables as potential hazards whose actual toxicological safety risk assessment would require more rigorous data and a more rigorous process than those used for hazard identification.LAY ABSTRACT: Packages used to contain aqueous parenteral drug products may be made of polypropylene. During storage, extractables from the polypropylene may leach out of the container and accumulate in the drug product, where they could affect product quality and/or safety. In this study, 21 polypropylene resins were characterized with respect to their organic and elemental extractables profiles. A worst-case extractables profile (highest level of all the extractables identified) was toxicologically assessed to establish potentially hazardous extractables. Certain extractables were established as potential hazards whose actual toxicological safety risk assessment would require more rigorous data and a more rigorous process than those used for hazard identification.

Elements of kitchen toxicology to exploit the value of traditional (African) recipes: The case of Egusi Okra meal in the diet of HIV+/AIDS subjects.

The Egusi Okra soup is a traditional African meal that is considered of high nutritional value and protective against weight loss. We introduce the concept of "kitchen toxicology" to analyse the recipe of the Egusi Okra soup and highlight possible mitigation measures for toxic and/or antinutritional effects in the wide spectrum of health and nutritional needs of HIV+/AIDS subjects. In particular, we focus on toxicants (environmental contaminants, process contaminants, substances leaching from food contact materials) dysregulating the immune status, as well as on interactions between nutrients, contaminants, and/or antinutrients which may lead to secondary/conditioned nutritional deficiencies or imbalances; in their turn, these can modulate the ability to cope with toxicants, and increase nutritional requirements. Recommendations are given for practices preserving the Egusi Okra soup from such risk factors, identifying points of particular attention during meal preparation, from purchase of raw ingredients through to food handling, cooking, storage, and consumption. The Egusi Okra soup is discussed in the context of a diet that is asked to mitigate complications (weight loss, opportunistic infections) and support antiretroviral therapy in African countries with high HIV/AIDS prevalence. The paper discusses how nutritional interventions benefit of the integration of kitchen toxicology practices in everyday life. Toxicological risk assessment is crucial to understand the history and status of the person exposed to or affected by infectious diseases.

An assessment of environmental and toxicological risk to pesticide exposure based on a case-based approach to computing

Pesticide environmental fate and toxicity depends on its physical and chemical features, the soil composition, soil adsorption, as well as residues that may be found in different soil slots. Indeed, pesticide degradation in soil may be influenced by either biotic or abiotic factors. In addition, the toxicity of pesticides for living organisms depends on their adsorption, distribution, biotransformation, dissemination of metabolites together with interaction with cellular macromolecules and excretion. Biotransformation may result in the formation of less toxic and/or more toxic metabolites, while other processes determine the balance between toxic and a nontoxic upcoming. Aggregate exposure and risk assessment involve multiple pathways and routes, including the potential for pesticide residues in food and drinking water, in addition to residues from pesticide use in residential and non-occupational environments. Therefore, this work will focus on the development of a decision support system to assess the environmental and toxicological risk to pesticide exposure, built on top of a Logic Programming approach to Knowledge Representation and Reasoning, complemented with a Case Based attitude to computing. The proposed solution is unique in itself, once it caters for the explicit treatment of incomplete, unknown, or even self-contradictory information, either in terms of a qualitative or quantitative setting.

English

Following the introduction of genetically-engineered glyphosate-resistant (GEGR) crops, commercially known as Roundup Ready (RR), no pesticide’s active principle has been used as much as glyphosate; yet its safety measures have been sternly disputed.  After its classification by the International Agency for Research on Cancer (IARC) as probably carcinogenic to humans in 2015, scientists, activists, regulators and the general public revisited voluminous studies that outweighed the risk of this herbicide and raised ferocious concerns that warranted serious attention.  Recently published studies on glyphosate established at least four toxicological principles. First, glyphosate exhibited severe mammalian toxicity at concentrations orders of magnitude lower than its regulatory-promulgated ‘No Observed Adverse Effect Level’ (NOAEL) or even its ‘Chronic Reference Dose’ (cRfD) and ‘Acceptable Daily Intake’ (ADI).  Second, even though not transparently scrutinized or officially required for toxicological testing and risk assessment, glyphosate co-formulants and glyphosate-based herbicides (GBHs) are orders of magnitude more toxic than the principle active ingredient alone. Third, glyphosate and GBHs are cytotoxic and endocrine disruptors, and the latter explains why ultra low concentrations - yet environmentally relevant-cause severe chronic toxicity. Fourth, the endocrine disruption likely leads to epigenotoxicity that may be extended to offspring and unexposed descending generations.  Taken all together, it can be fairly said that confidence in the regulatory-certified ADI values is highly eroded. To resolve the paradoxical discrepancy between regulatory safety measures and elicited toxicities at concentrations far below these measures, ADI was refined using two safety or adjustment factors.  Together, these two factors scale down ADI by four orders of magnitude and bring it to an Adjusted ADI (AADI) value of 2.5 ng/kg bw/day. Contrary to regulatory ADI, the new AADI successfully explains many research findings which demonsted severe mammalian toxicity at concentrations in the neighborhood of nanograms a.i./kg bw/day. This distills confidence in the new AADI value, as well as the magnitude of the proposed safety factors.  Glyphosate uses as per human capita, in two countries representing the extremes of adopting RR crops (the USA) or not-adopting these crops (Egypt), were compared. The comparison confirms the association between growing RR crops and the escalated use of glyphosate, and shows that the American public is likely exposed to glyphosate residue at forty times higher levels than the Egyptian public.   Key words: Acceptable daily intake, adjuvant, chronic reference dose, co-formulants, food quality protection act, hazard, glyphosate, glyphosate-based herbicides, no observed adverse effect level, risk, roundup, roundup ready crops.

It takes more than a coating to get nanoparticles through the intestinal barrier in vitro

Size and shape are crucial parameters which have impact on the potential of nanoparticles to penetrate cell membranes and epithelial barriers. Current research in nanotoxicology additionally focuses on particle coating. To distinguish between core- and coating-related effects in nanoparticle uptake and translocation, two nanoparticles equal in size, coating and charge but different in core material were investigated.Silver and iron oxide nanoparticles coated with poly (acrylic acid) were chosen and extensively characterized by small-angle x-ray scattering, nanoparticle tracing analysis and transmission electron microscopy (TEM). Uptake and transport were studied in the intestinal Caco-2 model in a Transwell system with subsequent elemental analysis. TEM and ion beam microscopy were conducted for particle visualization.Although equal in size, charge and coating, the behavior of the two particles in Caco-2 cells was different: while the internalized amount was comparable, only iron oxide nanoparticles additionally passed the epithelium. Our findings suggest that the coating material influenced only the uptake of the nanoparticles whereas the translocation was determined by the core material.Knowledge about the different roles of the particle coating and core materials in crossing biological barriers will facilitate toxicological risk assessment of nanoparticles and contribute to the optimization of pharmacokinetic properties of nano-scaled pharmaceuticals.

Use of physiologically based kinetic modeling-facilitated reverse dosimetry of in vitro toxicity data for prediction of in vivo developmental toxicity of tebuconazole in rats

Toxicological hazard and risk assessment largely rely on animal testing. For economic and ethical reasons, the development and validation of reliable alternative methods for these animal studies, such as in vitro assays, are urgently needed. In vitro concentration-response curves, however, need to be translated into in vivo dose-response curves for risk assessment purposes. In the present study, we translated in vitro concentration–response data of the antifungal compound tebuconazole, obtained in the ES-D3 cell differentiation assay, into predicted in vivo dose–response data for developmental toxicity using physiologically based kinetic (PBK) modeling-facilitated reverse dosimetry. Using the predicted in vivo dose–response data BMD(L)10 values for developmental toxicity in rat were calculated and compared with NOAEL values for developmental toxicity data in rats as reported in the literature. The results show that the BMDL10 value from predicted dose–response data are a reasonable approximation of the NOAEL values (ca. 3-fold difference). It is concluded that PBK modeling-facilitated reverse dosimetry of in vitro toxicity data is a promising tool to predict in vivo dose-response curves and may have the potential to define a point of departure for deriving safe exposure limits in risk assessment.

Use of Physiologically Based Kinetic Modeling-Based Reverse Dosimetry to Predict in Vivo Toxicity from in Vitro Data.

The development of reliable nonanimal based testing strategies, such as in vitro bioassays, is the holy grail in current human safety testing of chemicals. However, the use of in vitro toxicity data in risk assessment is not straightforward. One of the main issues is that concentration-response curves from in vitro models need to be converted to in vivo dose-response curves. These dose-response curves are needed in toxicological risk assessment to obtain a point of departure to determine safe exposure levels for humans. Recent scientific developments enable this translation of in vitro concentration-response curves to in vivo dose-response curves using physiologically based kinetic (PBK) modeling-based reverse dosimetry. The present review provides an overview of the examples available in the literature on the prediction of in vivo toxicity using PBK modeling-based reverse dosimetry of in vitro toxicity data, showing that proofs-of-principle are available for toxicity end points ranging from developmental toxicity, nephrotoxicity, hepatotoxicity, and neurotoxicity to DNA adduct formation. This review also discusses the promises and pitfalls, and the future perspectives of the approach. Since proofs-of-principle available so far have been provided for the prediction of toxicity in experimental animals, future research should focus on the use of in vitro toxicity data obtained in human models to predict the human situation using human PBK models. This would facilitate human- instead of experimental animal-based approaches in risk assessment.

Risk assessment in combustion toxicology: Should carbon dioxide be recognized as a modifier of toxicity or separate toxicological entity?

To characterize the accumulated hazards associated with the inhalation of gases typical of combustion products, a time-integrated value known as the fractional effective dose (FED) is used. This FED is maintained by the International Organization for Standardization (ISO) and made publicly available as the Standard ISO 13571. The current FED calculation related to asphyxiant gases is based on non-human primate data to estimate the 50% probability of humans to be incapacitated or not being able to execute any escape paradigm from fires. The objective of this paper was to compare two to calculate FEDs of the most common mixture of asphyxiant fire gases CO, HCN, and CO2. The first was based on the current ISO 13571 (draft) standard, the alternative second method applied the conceptual principles established for the derivation of Acute Emergency Response Planning Guideline values. The alternative approach applied one third of the non-lethal threshold concentration (LC01) as the most suitable and robust Point of Departure (POD) to estimate the threshold characterizing ‘impairment of escape’ in the absence of post-exposure mortality. The hyperventilation correction factor for CO2 of ISO 13571 was replaced by a separate term that accounts for the inherent acute toxicity of CO2. This analysis supports the conclusion that the current ISO 13571 standard misjudges the impact of the acute toxicity elicited by concentrations of CO2 exceeding ≈6%. While underestimating the hazards attributable to CO2, the hyperventilation adjustment factor suggested by this standard is biased to markedly overestimate the hazards assigned to CO and HCN in fire effluents.

Current methods in risk assessment of genotoxic chemicals

Chemical contaminants and residues are undesired chemicals occurring in consumer products such as food and drugs, at the workplace and in the environment, i.e. in air, soil and water. These compounds can be detected even at very low concentrations and lead frequently to considerable concerns among consumers and in the media. Thus it is a major challenge for modern toxicology to provide transparent and versatile tools for the risk assessment of such compounds in particular with respect to human health. Well-known examples of toxic contaminants are dioxins or mercury (in the environment), mycotoxins (from infections by molds) or acrylamide (from thermal treatment of food). The process of toxicological risk assessment of such chemicals is based on i) the knowledge of their contents in food, air, water etc., ii) the routes and extent of exposure of humans, iii) the toxicological properties of the compound, and, iv) its mode(s) of action. In this process quantitative dose-response relationships, usually in experimental animals, are of outstanding importance. For a successful risk assessment, in particular of genotoxic chemicals, several conditions and models such as the Margin of Exposure (MoE) approach or the Threshold of Toxicological Concern (TTC) concept exist, which will be discussed.

Relative potency factors for a preliminary toxicological risk assessment of pyrrolizidine alkaloids in food and herbal medicines

Relative potency factors for a preliminary toxicological risk assessment of pyrrolizidine alkaloids in food and herbal medicines

Toxicological risk assessment of micro-organisms used as active substances in plant protection products

The aim of this study was to develop a bacteriocin-producing QPS (Qualified Presumption of Safety) status Bacillus strain as a starter culture for fermented soybean foods as well as food preservation with improved safety and organoleptic qualities. Strain SN7 isolated from Meju, a fermented soybean product of Korea, was selected based on antimicrobial activity against pathogens such as B. cereus and production of bacteriocin. The selected strain SN7 was identified as B. subtilis. Safety assessments of B. subtilis SN7 (hemolytic activity, virulence traits, and antibiotic resistance) indicated that B. subtilis SN7 does not present a risk to human health, and this new isolate can be confidently recommended as a safe starter culture for the food industry. Cheonggukjang, a traditional fermented soybean food of Korea, fermented using B. subtilis SN7 showed high amino-type nitrogen content with good sensory characteristics. As a controlling agent of B. cereus in culture medium and in a food model of Cheonggukjang, B. subtilis SN7 efficiently inhibited growth of B. cereus vegetative cells and inactivated germination of B. cereus spores. Furthermore, B. cereus was not detected in Cheonggukjang stored at 4 °C for 6 months. B. cereus was easily detected in fermented food during storage at low temperature, as endo-spores of B. cereus are resistant to various treatments such as heat treatment and can grow even at 4–5 °C. These results suggest that bacteriocin-producing B. subtilis SN7 has great potential as a controlling agent of B. cereus and as a starter culture in Cheonggukjang.

Development of Toxicological Risk Assessment Models for Acute and Chronic Exposure to Pollutants.

Alert level frameworks advise agencies on a sequence of monitoring and management actions, and are implemented so as to reduce the risk of the public coming into contact with hazardous substances. Their effectiveness relies on the detection of the hazard, but with many systems not receiving any regular monitoring, pollution events often go undetected. We developed toxicological risk assessment models for acute and chronic exposure to pollutants that incorporate the probabilities that the public will come into contact with undetected pollution events, to identify the level of risk a system poses in regards to the pollutant. As a proof of concept, we successfully demonstrated that the models could be applied to determine probabilities of acute and chronic illness types related to recreational activities in waterbodies containing cyanotoxins. Using the acute model, we identified lakes that present a ‘high’ risk to develop Day Away From Work illness, and lakes that present a ‘low’ or ‘medium’ risk to develop First Aid Cases when used for swimming. The developed risk models succeeded in categorising lakes according to their risk level to the public in an objective way. Modelling by how much the probability of public exposure has to decrease to lower the risks to acceptable levels will enable authorities to identify suitable control measures and monitoring strategies. We suggest broadening the application of these models to other contaminants.

Microbial biotransformation of furosemide for environmental risk assessment: identification of metabolites and toxicological evaluation.

Some widely prescribed drugs are sparsely metabolized and end up in the environment. They can thus be a focal point of ecotoxicity, either themselves or their environmental transformation products. In this context, we present a study concerning furosemide, a diuretic, which is mainly excreted unchanged. We investigated its biotransformation by two environmental fungi, Aspergillus candidus and Cunninghamella echinulata. The assessment of its ecotoxicity and that of its metabolites was performed using the Microtox test (ISO 11348-3) with Vibrio fischeri marine bacteria. Three metabolites were identified by means of HPLC-MS and 1H/13C NMR analysis: saluamine, a known pyridinium derivative and a hydroxy-ketone product, the latter having not been previously described. This hydroxy-ketone metabolite was obtained with C. echinulata and was further slowly transformed into saluamine. The pyridinium derivative was obtained in low amount with both strains. Metabolites, excepting saluamine, exhibited higher toxicity than furosemide, being the pyridinium structure the one with the most elevated toxic levels (EC50=34.40±6.84mgL-1). These results demonstrate that biotic environmental transformation products may present a higher environmental risk than the starting drug, hence highlighting the importance of boosting toxicological risk assessment related to the impact of pharmaceutical waste.

Enantioselective metabolism and toxic effects of metalaxyl on primary hepatocytes from rat.

Enantiomers of chiral compounds often exhibit enantioselective adverse effects and biochemical processes in non-target organisms. In this study, enantioselective metabolism and toxic effects of metalaxyl enantiomers on primary rat hepatocytes were investigated. Stereoselectivity was observed on both degradation of metalaxyl and formation of metabolites. (-)-R-metalaxyl eliminated faster than (+)-S-metalaxyl, while the hydroxylmetalaxyl, demethylmetalaxyl, and didemethylmetalaxyl metabolites derived from 50-μM (+)-S-metalaxyl after 24h of incubation were approximately 1.57, 1.43, and 1.86 times more than that of (-)-R-metalaxyl, respectively. According to the methyl tetrazolium (MTT) assay, the EC50 values (24h) for rac-, (+)-S-, and (-)-R-metalaxyl were 1788.22, 2066.73, and 2263.71μM, respectively. An accordant enantioselective effect on oxidative stress suggested that the enantioselective cytotoxicity induced by metalaxyl enantiomers may partly contribute to enantioselective oxidative damage and mitochondrial dysfunction. Such results could be of great importance for credible environmental and toxicological risk assessment of metalaxyl.