Species Differences In Toxicity Research Articles

Species-differences in the in vitro biotransformation of trifluoroethene (HFO-1123)

1,1,2-Trifluoroethene (HFO-1123) is anticipated for use as a refrigerant with low global warming potential. Inhalation studies on HFO-1123 in rats indicated a low potential for toxicity (NOAELs ≥ 20,000 ppm). In contrast, single inhalation exposure of Goettingen® minipigs (≥ 500 ppm) and New Zealand white rabbits (≥ 1250 ppm) resulted in severe toxicity. It has been suggested that these pronounced species-differences in toxicity may be attributable to species-differences in biotransformation of HFO-1123 via the mercapturic acid pathway. Therefore, the overall objective of this study was to evaluate species-differences in glutathione (GSH) dependent in vitro metabolism of HFO-1123 in susceptible versus less susceptible species and humans as a basis for human risk assessment. Biotransformation of HFO-1123 to S-(1,1,2-trifluoroethyl)-L-glutathione (1123-GSH) and subsequent cysteine S-conjugate β-lyase-mediated cleavage of the corresponding cysteine conjugate (1123-CYS) was monitored in hepatic and renal subcellular fractions of mice, rats, minipigs, rabbits, and humans. While 1123-GSH formation occurred at higher rates in rat and rabbit liver S9 compared to minipig and human S9, increased β-lyase cleavage of 1123-CYS was observed in minipig kidney cytosol as compared to cytosolic fractions of other species. Increased β-lyase activity in minipig cytosol was accompanied by time-dependent formation of monofluoroacetic acid (MFA), a highly toxic compound that interferes with cellular energy production via inhibition of aconitase. Consistent with the significantly lower β-lyase activity in human cytosols, the intensity of the MFA signal in human cytosols was only a fraction of the signal obtained in minipig subcellular fractions. Even though the inconsistencies between GSH and β-lyase-dependent metabolism do not allow to draw a firm conclusion on the overall contribution of the mercapturic acid pathway to HFO-1123 biotransformation and toxicity in vivo, the β-lyase data suggest that humans may be less susceptible to HFO-1123 toxicity compared to minipigs.

Species differences and human relevance of the toxicity of 4-hydroxyphenylpyruvate dioxygenase (HPPD) inhibitors and a new approach method in vitro for investigation

The mode of action (MoA) of the 4-hydroxyphenylpyruvate dioxygenase (HPPD) inhibitor herbicides in mammals is well described and is generally accepted to be due to a build-up of excess systemic tyrosine which is associated with the range of adverse effects reported in laboratory animals. What is less well accepted is the basis for the marked difference in the effects of HPPD inhibitors that has been observed across experimental species and humans, where some species show significant toxicities whereas in other species exposure causes few effects. The activity of the catabolic enzyme tyrosine aminotransferase (TAT) varies across species including humans and it is hypothesized that this primarily accounts for the different levels of tyrosinemia observed between species and leads to the subsequent differences in toxicity. The previously reported activities of TAT in different species showed large variation, were inconsistent, have methodological uncertainties and could lead to a reasonable challenge to the scientific basis for the species difference in response. To provide clarity, a new method was developed for the simultaneous and systematic measurement of TAT in vitro using robust methodologies in a range of mammalian species including human. The results obtained showed general correlation between high TAT activity and low in vivo toxicity when using a model based on hepatic cytosol and a very convincing correlation when using a primary hepatocyte model. These data fully support the role of TAT in explaining the species differences in toxicity. Moreover, this information should give greater confidence in selecting the most appropriate animal model (the mouse) for human health risk assessment and for key classification and labeling decision-making.

Researches on the evaluation of pesticide safety in humans using a pharmacokinetic approach.

Similar to the pharmaceutical compounds, pesticides require human safety assessment for their registration and distribution; however, it is absolutely impossible to assess human safety by dosing humans with pesticides. Thus, how to appropriately evaluate the safety of pesticides in humans remains a great subject of debate. In this article, we present some examples of pesticide toxicity studies that identify species differences in toxicity and evaluate human safety by applying combinations of novel in vivo, in vitro, and in silico techniques to separately assess the key toxicodynamic (i.e., sensitivity) and/or toxicokinetic (i.e., exposure) factors. Because it is scientifically sound, the safety assessment strategy illustrated for three compounds in this article is expected to play an important role in the human safety assessment of agricultural compounds.

Common questions in veterinary toxicology.

Toxicology is a vast subject. Animals are exposed to numerous drugs, household products, plants, chemicals, pesticides and venomous animals. In addition to the individual toxicity of the various potential poisons, there is also the question of individual response and, more importantly, of species differences in toxicity. This review serves to address some of the common questions asked when dealing with animals with possible poisoning, providing evidence where available. The role of emetics, activated charcoal and lipid infusion in the management of poisoning in animals, the toxic dose of chocolate, grapes and dried fruit in dogs, the use of antidotes in paracetamol poisoning, timing of antidotal therapy in ethylene glycol toxicosis and whether lilies are toxic to dogs are discussed.

Effects of propranolol on heart rate and development in Japanese medaka (Oryzias latipes) and zebrafish (Danio rerio)

Propranolol is a β-adrenergic receptor antagonist (β-blocker) that is frequently used to treat hypertension and other cardiovascular conditions in humans. Detected in surface waters due to discharge of domestic wastewater, propranolol has demonstrated significant species differences in toxicity between fish. The aim of this study was to investigate the effects of propranolol on heart rate and development in embryos of two species of fish; Japanese medaka (JM) Oryzias latipes and zebrafish (ZF) Danio rerio. Parents and fertilized embryos of each species were exposed to nominal (measured) concentrations of 0.1 (0.09), 1 (1.1) and 10 (8.3) μg/L of propranolol. Heart rate was monitored during subsequent exposure in embryos at incremental developmental periods (44, 54, 64h post-fertilization (hpf) for ZF and 68, 116, 164 hpf for JM). Heart development and morphology was examined using whole mount immunostaining with distance measurements between the sinus venosus (SV) and bulbus arteriosis (BV). Morphological measurements were made at 44 hpf for ZF and 164 hpf for JM. In ZF, a significant reduction in heart rate was observed at 0.08μg/L propranolol, along with an increase in the SV–BA distance at 44 hpf. Significant reductions in heart rate were also observed in ZF at 54 and 64 hpf at all concentrations of propranolol. For JM, heart rates generally decreased at all developmental timepoints (68, 116 and 164 hpf) after propranolol treatment, with concentration dependent decreases observed at 164 hpf and a lowest observed effect concentration (LOEC) of 0.09μg/L propranolol at each timepoint. However, significant alterations in cardiac morphology were not observed in JM at 164 hpf. In contrast, heart rates and morphology in ZF were affected with a non-monotonic concentration response in morphology and a LOEC of 0.09μg/L propranolol for morphological alterations at 44 hpf and for heart rate at each timepoint. These data indicated unique developmental stages of susceptibility between species and that combined parental and embryo exposures may lead to greater impairment of cardiac development and function in offspring than separate exposures of adults and embryos.

Abstract 3769: Metabolic activation of batracylin and N-acetylbatracylin to reactive metabolites that bind protein and DNA: Possible role in toxicity

Abstract Batracylin (NSC 320846, BAT) is a heterocyclic arylamine and topoisomerase II inhibitor shown to be active in murine tumor models, but with demonstrated species differences in toxicity associated with metabolism to N-acetylbatracylin (NBAT) by N-acetyltransferase (NAT). BAT is a preferential substrate of human NAT2. Accordingly, production of NBAT should be subject to population variability as a function of NAT2 polymorphisms. A phase I clinical trial of BAT in patients with advanced cancer and slow acetylator NAT2 phenotypes to reduce the risk of NBAT-related toxicity found a dose-limiting toxicity (DLT) of cystitis/hemorrhagic cystitis. In subsequent in vitro and in vivo preclinical studies to elucidate metabolic pathways that might contribute to BAT toxicity, we found oxidative metabolites and thiol conjugates of BAT and NBAT (MM Ames et al, Cancer Res 2011;71(8 Suppl):Abstract nr 1303). Since these data are consistent with metabolic activation of BAT to potentially toxic metabolites, we assessed formation of reactive intermediates as measured by covalent binding (CB) to protein and DNA or by formation of glutathione conjugates. Radiolabeled BAT and NBAT were incubated with 3-MC-induced rat (r), human (h) and dog (d) liver microsomes (LM), as well as with recombinant human cytochromes P450 (CYPs) with or without added DNA. NADPH-dependent BAT protein and DNA CB were detected in all microsomal preparations (see table below). NADPH-dependent NBAT protein CB was detected in all microsomal preparations except hCYP1B1. NADPH-dependent NBAT DNA CB was detected in all microsomal preparations except 3MC-rLM. We also detected a glutathione conjugate of mono-hydroxy BAT by lc/ms/ms analysis when BAT was incubated with hLM in the presence of glutathione and NADPH. Thus, CYP-catalyzed oxidation of BAT yields reactive metabolites that that bind protein and DNA and may contribute to drug-related DLT. Supported in part by NCI Contract N01-CM-52206. Citation Format: {Authors}. {Abstract title} [abstract]. In: Proceedings of the 103rd Annual Meeting of the American Association for Cancer Research; 2012 Mar 31-Apr 4; Chicago, IL. Philadelphia (PA): AACR; Cancer Res 2012;72(8 Suppl):Abstract nr 3769. doi:1538-7445.AM2012-3769

Pharmacological and Toxicological Advances in PPAR-Related Medicines

Pharmacological and Toxicological Advances in PPAR-Related Medicines

Different Metabolites of Human Hepatotoxic Pyrazolopyrimidine Derivative 5‐n‐Butyl‐Pyrazolo[1,5‐a]Pyrimidine Produced by Human, Rat and Monkey Cytochrome P450 1A2 and Liver Microsomes

Different Metabolites of Human Hepatotoxic Pyrazolopyrimidine Derivative 5‐<i>n</i>‐Butyl‐Pyrazolo[1,5‐<i>a</i>]Pyrimidine Produced by Human, Rat and Monkey Cytochrome P450 1A2 and Liver Microsomes

Trends in the study of genetic susceptibility

Genotoxicology investigates the molecular basis of cellular responses to DNA damage. During more than three decades of genetic toxicology testing, a large number of tests with varying sensitivity and specificity have been developed. These genotoxicity tests can predict: the likelihood of a substance to be a (rodent) carcinogen, the mechanism of carcinogenic activity of different substances, if the results of genotoxicity studies only predict carcinogenicity or they are a distinct hazard endpoint (Nohynek, 2005). The current genetic toxicity testing batteries represent: Ames test which is a component of all genetic testing batteries. A mammalian cell mutagenicity assay which should confirm or complete the Ames test. Chromosomal aberrations tests which are based on a different endpoint than gene mutations. Positive in vitro results need to be confirmed by in vivo tests; results from test batteries have higher predictive value than results of a single test (Nohynek, 2005). The human molecular epidemiology represents a new trend in the study of genetic susceptibility and interactions between genetic and environmental factors of risk. Doses and biochemical effects biomonitoring have nowadays a tremendous utility providing an efficient means of measuring human exposure to chemical substances. Human biomonitoring considers all routes of uptake and all sources which are relevant making it an ideal instrument for risk assessment and risk management; blood is by far the most approved matrice. Human biomonitoring can be done for most chemical substances which are in the focus of the worldwide discussion of environmental medicine (Angerer et al., 2007). Molecular epidemiology studies of human populations exposed to potential mutagens have the aim to assess the risk of genetic disease or cancer by analysing the relationship between internal exposure and biological effects in target cells under consideration of confounding factors (Faust et al., 2004). Human biomonitoring can identify new chemical exposures, trends and changes in exposure, establish distribution of exposure among the general population, identify vulnerable groups and populations with higher exposures and identify environmental risks at specific contaminated sites. The sensitivity of methods moreover enables the elucidation of human metabolism and toxic mechanisms of the pollutants (Angerer et al., 2007). Biomarkers of effect that indicate exposure to a causative agent and that reflect the individual risk of disease are numerous. The most commonly used biomarkers in cancer epidemiology include: measurements of DNA damage, such as DNA breaks, altered bases, bulky adducts; chromosomal aberrations (CA), micronuclei (MN) and sister chromatid exchange (SCE), which are also the results of DNA damage (Kassie et al., 2000; Collins, 1998). Chromosomal aberrations and micronuclei are biomarkers of damage due to genetic instability or exposure to environmental mutagens or carcinogens. A recent approach is to associate the biomarkers of genetic susceptibility which take into account cancer susceptibility and interindividual differences in the response to a genotoxic exposure, and the analysis of CA and/or MN, which serves as a biomarker of interactions between the environment and the genetic material of the cell. Information is being gathered on how DNA damage and more particularly the frequency of chromosomal aberrations and/or micronuclei depend on the polymorphisms in genes implicated in xenobiotic metabolism (activation and/or detoxification) and DNA lesion repair. For biomonitoring purposes, numerous confounding factors (age, sex, tobacco consumption, etc.) influence the CA and MN biomarker, and thus associating genetic polymorphisms to CA and MN would be useful to better define the prevention and prediction of risk (Iarmarcovai et al., 2007a; Iarmarcovai et al., 2007b). The in vitro genetic toxicology tests used for regulatory purposes measure formation of gene mutations and chromosomal changes following DNA damage induced by the compounds under test, and are used to predict the carcinogenic potential of pharmaceuticals, industrial chemicals, food additives and cosmetic ingredients (Kirkland et al., 2007). In vitro test systems will aid in the identification of the most sensitive species and strains. Also, in vitro systems are good models for studying qualitative and quantitative species differences in toxicity. Further, in vitro systems are excellent models for characterisation of the mode of action/mechanism for critical effects, but findings need to be validated in vivo. In vitro systems will aid in the extrapolation from high to low dose and from experimental animals to humans (Holme et al., 2002).

The Metabolism and Toxicity of Furosemide in the Wistar Rat and CD-1 Mouse: a Chemical and Biochemical Definition of the Toxicophore

Furosemide, a loop diuretic, causes hepatic necrosis in mice. Previous evidence suggested hepatotoxicity arises from metabolic bioactivation to a chemically reactive metabolite that binds to hepatic proteins. To define the nature of the toxic metabolite, we examined the relationship between furosemide metabolism in CD-1 mice and Wistar rats. Furosemide (1.21 mmol/kg) was shown to cause toxicity in mice, but not rats, at 24 h, without resulting in glutathione depletion. In vivo covalent binding to hepatic protein was 6-fold higher in the mouse (1.57 +/- 0.98 nmol equivalent bound/mg protein) than rat (0.26 +/- 0.13 nmol equivalent bound/mg protein). In vivo covalent binding to mouse hepatic protein was reduced 14-fold by a predose of the cytochrome P450 (P450) inhibitor, 1-aminobenzotriazole (ABT; 0.11 +/- 0.04 nmol equivalent bound/mg protein), which also reduced hepatotoxicity. Administration of [(14)C]furosemide to bile duct-cannulated rats demonstrated turnover to glutathione conjugate (8.8 +/- 2.8%), gamma-ketocarboxylic acid metabolite (22.1 +/- 3.3%), N-dealkylated metabolite (21.1 +/- 2.9%), and furosemide glucuronide (12.8 +/- 1.8%). Furosemide-glutathione conjugate was not observed in bile from mice dosed with [(14)C]furosemide. The novel gamma-ketocarboxylic acid, identified by nuclear magnetic resonance spectroscopy, indicates bioactivation of the furan ring. Formation of gamma-ketocarboxylic acid was P450-dependent. In mouse liver microsomes, a gamma-ketoenal furosemide metabolite was trapped, forming an N-acetylcysteine/N-acetyl lysine furosemide adduct. Furosemide (1 mM, 6 h) became irreversibly bound to primary mouse and rat hepatocytes, 0.73 +/- 0.1 and 2.44 +/- 0.3 nmol equivalent bound/mg protein, respectively, which was significantly reduced in the presence of ABT, 0.11 +/- 0.03 and 0.21 +/- 0.1 nmol equivalent bound/mg protein, respectively. Furan rings are part of new chemical entities, and mechanisms underlying species differences in toxicity are important to understand to decrease the drug attrition rate.

Comparison of Toxicity and Toxicokinetics/Pharmacokinetics of an α1L-Adrenoceptor Agonist in Rats and Rhesus Monkeys

We have investigated the toxicity of an α1L-adrenoceptor agonist, ESR 1150 CL, and compared the toxicokinetics and pharmacokinetics in rats and monkeys. In rats, this compound induced death with remarkable sacculated aneurysms of the aorta in groups given more than 3mg/kg per day in a 4-week repeated oral administration study. On the other hand, these findings were not observed in monkeys during a 2-week repeated oral administration study at doses up to 30 mg/kg per day. Orally administered ESR 1150 CL raised blood pressure transiently and dose-dependently during the 4-week repeated study in rats, whereas no increase of blood pressure was observed during the 2-week oral toxicity study in monkeys. Contrary to our expectation, the exposure level of ESR 1150 CL in rats was not higher than that in monkeys in the toxicokinetic evaluation. Pharmacokinetic evaluation indicated good absorption of the compound, but the bioavailability was very low in both rats and monkeys. These findings suggest that the potent species difference in toxicity of ESR 1150 CL between rats and monkeys does not depend on differences of toxicokinetics/pharmacokinetics. Rather, we suggest that the reason is likely to be species difference in the biological susceptibility of the α1L-adrenoceptor subtypes between rats and monkeys, which would be closely related with the effect on blood pressure by α1L-adrenoceptor agonist.

An evaluation of sodium loss and gill metal binding properties in rainbow trout and yellow perch to explain species differences in copper tolerance.

The main objective of the study was to use a species comparison approach in order to understand sensitivity and tolerance differences to copper. We hypothesized that species differences in toxicity would be reflected by differences in copper binding to high-affinity sites on the gill. Specifically, the strength of copper binding (affinity, logK) and maximum number of binding sites (saturation, Bmax) for copper at the gill surface would vary among different species of fish. Two species that are different in their copper sensitivity are the rainbow trout (Oncorhynchus mykiss) and yellow perch (Perca flavescens). We explicitly compared acute toxicity (median lethal concentrations via 96-h LC50s) and whole-body Na+ loss in both organisms in two distinct water chemistries (i.e., hard and soft water). For both species, the copper binding sites at the gill surface were characterized for their affinity and saturability. The binding properties of the gill were quite similar between the two species in each water chemistry. Based on estimations of the free cupric ion concentration, the affinity, or logK, was 8.4 for both species in soft water, whereas in hard water, the affinity was higher (approximately 9.7). The Bmax value in soft water was 1.88 nmol/g for rainbow trout and yellow perch, while in hard water, saturation occurred at 3.63 nmol/g for rainbow trout and 9.01 nmol/g for yellow perch. More importantly, the amount of copper bound to the gills at 50% mortality (i.e., lethal accumulation; the LA50) was different between the two species (yellow perch LA50s were nine times higher than those of rainbow trout in soft water and hard water), indicating that the copper binding to the yellow perch gill must not have been 'biologically reactive.' According to 96-h LC50s, yellow perch were less sensitive to copper than were rainbow trout; however, the difference between the two species was similar in hard water (1.05 vs 4.16 microM) and soft water (approximately 0.10 vs 0.44 microM). Perch were more tolerant because they lost less sodium upon exposure to copper; yet this mechanism of tolerance was not reflected by the amount of copper at the gill surface. The influence of water chemistry on the binding properties of the gill demonstrates the dynamic nature of the gill in maintaining ionoregulatory homeostasis, a key issue in the future development of the chronic biotic ligand model.

Identification of the cytochromes P450 that catalyze coumarin 3,4-epoxidation and 3-hydroxylation.

Coumarin, a widely used fragrance ingredient, is a rat liver and mouse lung toxicant. Species differences in toxicity are metabolism-dependent, with injury resulting from the cytochrome P450-mediated formation of coumarin 3,4-epoxide (CE). In this study, the enzymes responsible for coumarin activation in liver and lung were determined. Recombinant human and rat CYP1A forms and recombinant human CYP2E1 readily catalyzed CE production. Coinhibition with CYP1A1/2 and CYP2E1 antibodies blocked CE formation by 38, 84, and 67 to 92% (n = 3 individual samples) in mouse, rat, and human hepatic microsomes, respectively. Although CYP1A and 2E forms seem to be the most active catalysts of CE formation in liver, studies conducted with the mechanism-based inhibitor 5-phenyl-pentyne demonstrated that CYP2F2 is responsible for up to 67% of CE formation in whole mouse lung microsomes. In contrast to the CE pathway, coumarin 3-hydroxylation is a minor product of coumarin in liver microsomes from mice, rats, and humans and is catalyzed predominately by CYP3A and CYP1A forms, confirming that CE and 3-hydroxycoumarin are formed via distinct metabolic pathways.

Development of a preliminary physiologically based toxicokinetic (PBTK) model for 1,3-butadiene risk assessment

Potential health effects of human exposure to 1,3-butadiene (BD) are of concern due to the use of BD in industry and its low-level presence throughout the environment. Physiologically based toxicokinetic (PBTK) models of BD in rodents have been developed by multiple research groups in an effort to explain species differences in toxicity (especially carcinogenic potency) through toxicokinetics. PBTK modeling of dose metrics related to a non-cancer endpoint, ovotoxicity in experimental animals, was conducted. The cumulative area under the blood concentration vs. time curve (AUC) for the metabolite diepoxybutane (butadiene diepoxide, DEB) was found to be consistent with ovotoxicity in mice and rats exposed to BD by inhalation or epoxybutene (butadiene monoepoxide, EB) or DEB by intraperitoneal injection. This suggests that cumulative DEB AUC may also be an appropriate metric for possible human risk. A preliminary human PBTK model was assembled for the eventual assessment of reproductive risk to humans and for prioritizing the determination of model parameters. The preliminary model accurately predicted published data on exhaled breath BD concentrations in a human volunteer exposed to BD by inhalation. The fit was relatively insensitive to the rate constant for BD epoxidation. Sensitivity analyses were conducted on this human PBTK model. Using a range of published rate constants, human blood DEB was found to be sensitive to rates of epoxidation of EB to DEB and hydrolysis of EB and DEB, but not BD epoxidation. Because of the large ranges of rates measured in vitro for these reactions, different combinations of in-vitro rates produce varying predictions of blood DEB concentration. Thus, validation of a human PBTK model with human biomonitoring data will be essential to produce a PBTK model that can be applied to risk assessment.

Butadiene – progress under the European Union Existing Substances Regulation

A draft risk assessment report (RAR) for 1,3-butadiene has been produced by the UK under the European Union's Existing Substances Regulation (ESR). In line with the requirements of ESR, the RAR presents an evaluation of the hazards and risks of butadiene to human health (for workers, consumers and the general public) and to the environment. If as a result of the risk assessment, concerns for human health and/or the environment are identified, proposals for measures to reduce those risks may be developed. In relation to the human health elements of the ESR RAR for butadiene, the risk assessment involves a comparison of health effects against exposures in the three population sectors. The position arrived at on risk is described in qualitative rather than quantitative terms. For butadiene, the conclusion reached for the human health hazard assessment has been influenced significantly by both the animal and human data available. Species differences in toxicity, in particular with regard to mutagenicity and carcinogenicity, are considered along with the mechanistic arguments for these differences and their relevance for human health. The information presented in the RAR has been extensively discussed within the EU and a generally agreed position has been reached, with respect to human health hazard and risk. It is concluded that whilst the risks to human health at current exposure levels in the workplace, for consumers and following exposure via the environment are uncertain, there are concerns for human health effects for all three populations. Thus, there is a need to limit the risks. The next stage in the ESR process will be to consider what, if any, additional risk reduction measures may be required, taking into account existing control measures.

Stereochemistry of styrene biotransformation*

Metabolism of styrene, an important industrial monomer, is reviewed. Attention is focused on the stereoselectivity of its oxidation to 7,8-styrene oxide as well as on further stereoselective biotransformation by hydrolytic and mercapturic acid pathway. Toxic effects such as mutagenicity, genotoxicity, hepatotoxicity, and pneumotoxicity may be related to the ratio of styrene oxide enantiomers at the target site. In rats formation of the less mutagenic (S)-styrene oxide and a faster detoxication of the (R)-enantiomer is favored. In mice metabolic activation of styrene favors the formation of (R)-styrene oxide but this more toxic enantiomer is detoxified faster, so that a nearly racemic styrene oxide results. Stereochemistry of biotransformation can contribute to the species differences in toxicity but can hardly be interpreted as a crucial factor. Due to lack of relevant data the stereochemistry of human metabolism cannot be interpreted in relation to the toxic effects.

Metabolism of 2-(4-methylsulphonyl-2-nitrobenzoyl)-1,3-cyclohexanedione (mesotrione) in rat and mouse

1. The metabolic fate of [14 C]-2-(4-methylsulphonyl-2-nitrobenzoyl)-1,3-cyclohexanedione (mesotrione) has been determined in the male and female rat and mouse following a single oral dose of either 1 or 100 mg kg -1, in rat given 14 consecutive oral doses of 1 mg kg -1, and in the surgically prepared, bile duct-cannulated rat following a single oral dose of 50 mg kg -1. The excretion of a single i.v. dose of 1 mg kg -1 in the male and female rat was also investigated. 2. Mesotrione was extensively absorbed and rapidly excreted via urine in both rat and mouse. The absorbed dose was not well metabolized in either species. Unabsorbed material was subject to metabolic action by the gut microflora. 3. The major metabolic pathway was hydroxylation of the aromatic ring. There was evidence for cleavage of the dione and aromatic rings followed by reduction of the nitro group in the gastrointestinal tract. 4. There were no species differences in the metabolism and excretion of mesotrione, which could explain the species differences in toxicity reported for this class of compounds.

Selective toxicity of chlorpyrifos to several species of fish during an environmental exposure: Biochemical mechanisms

AbstractThe organophosphorus insecticide chlorpyrifos is frequently used for termite control. A small pond near a construction site was contaminated with chlorpyrifos via runoff resulting in the death of several species of fish including largemouth bass, bluegill sunfish, and golden shiners, but not mosquitofish. Inhibition of acetylcholinesterase (AChE) and aliesterases (AliE) was determined in the tissues of exposed fish and the presence of chlorpyrifos was confirmed in fish livers by analytical chemistry. Brain AChE was inhibited 87 to 93% in bass, bluegill, and shiners but only 73% in mosquitofish. Skeletal muscle AChE was inhibited 91 to 97% in bass, shiners, and mosquitofish but only 77% in bluegill. Liver AChE and AliE were inhibited greater than 90% in bass, shiners, and mosquitofish but in bluegill, the activities were inhibited only 86% and 77%, respectively. After 60 d, brain AChE activity in exposed mosquitofish was fully recovered, and liver AliE activity was recovered to about 50% of control values, but there was little recovery of skeletal muscle and liver AChE activities. The sensitivity of these enzymes to inhibition by chlorpyrifos‐oxon was assessed by determination of 50% inhibitory concentrations (IC50s). Mosquitofish brain and liver AChE and liver AliE were less sensitive to inhibition than the enzymes from the other species. In contrast, mosquitofish skeletal muscle AChE was more sensitive to inhibition compared to the other species. The lower inhibition of mosquitofish brain AChE and the lower in vitro sensitivity of mosquitofish brain AChE to inhibition by chlorpyrifos‐oxon compared to the brain AChE of the other species suggest that the species difference in toxicity exhibited in this exposure resulted primarily from species differences in the sensitivity of brain AChE to inhibition by chlorpyrifos‐oxon.

Interspecies comparison of kinetic data of chlorinated chemicals of potential relevance to risk assessment

Most noncancer risk assessments focus on the NOAEL (the no observed adverse effect level), an external dose. However, the internal dose considers the ongoing kinetics between chemical intake and target site and will provide a better dose‐response relationship. The purpose of this study was to conduct a literature search for kinetic parameters in animals and humans administered chlorinated chemicals by the oral or inhalation routes. Frequendy used industrial solvents as well as pesticides were evaluated. Animal to human kinetic ratios were calculated and compared to the proposed kinetic default subfactors of Renwick for oral chemicals and of the USEPA for inhaled compounds. Kinetic default subfactors were then replaced by data‐derived values and the interspecies factors were recalculated. Seventy percent of data‐derived kinetic ratios were lower than proposed default values. Thus, incorporation of internal dose can reduce uncertainty in animal‐human extrapolation. Furthermore, metabolites should be examined because for some compounds, species differences in toxicity may be due to metabolites.

Risk assessment of butadiene for the Existing Substances Regulation (ESR) in the European Union

The European Commission's Existing Substances Regulation (ESR) requires an evaluation of the risks to humans (workers, consumers and the general public) and the environment, posed by selected existing chemical substances. The Regulation focuses initially on high tonnage chemicals, and in this first year of the ESR, the UK undertook to produce the risk assessment document for 1,3-butadiene. This was in recognition of a need to reconsider the regulatory position in the light of new and ongoing research. In particular, the risk assessment was an opportunity to address the current debate over the animal toxicity data, where marked species differences exist, and the relevance of these data for human health. The human health part of the risk assessment of butadiene involves a comparison of health effects against exposures in the three population sectors. In this paper, the approach to risk assessment is exemplified for the occupational exposure setting. The conclusion reached for the human health hazard assessment has been influenced significantly by both the available animal and human data. Species differences in toxicity, in particular with regard to mutagenicity and carcinogenicity, have been considered, along with the mechanistic arguments for these differences and their relevance for human health. In addition, the evidence for health effects in humans is considered. The overall conclusion of the health hazard assessment is that there is concern for genotoxicity and for carcinogenicity, for which a threshold of effect cannot be identified. The occupational exposure assessment indicates that exposure in the workplace is generally well-controlled, of the order of 3–5 ppm or below. The overall conclusion of the risk assessment for the occupational setting is that there is concern for the possibility of genotoxicity and carcinogenicity at the exposure levels which occur in some European workplaces.