Kinetic parameters of OPT pesticide desulfuration by c-DNA expressed human CYPs

Flubendiamide provokes oxidative stress, inflammation, miRNAs alteration, and cell cycle deregulation in human prostate epithelial cells: The attenuation impact of synthesized nano-selenium using Trichodermaaureoviride

Flubendiamide (FBD) is a novel diamide insecticide extensively used with potential human health hazards. This research aimed to examine the effects of FBD on PrEC prostate epithelial cells, including Oxidative stress, pro-inflammatory responses, modifications in the expression of oncogenic and suppressor miRNAs and their target proteins, disruption of the cell cycle, and apoptosis. Additionally, the research investigated the potential alleviative effect of T-SeNPs, which are selenium nanoparticles biosynthesized by Trichoderma aureoviride, against the toxicity induced by FBD. Selenium nanoparticles were herein synthesized by Trichoderma aureoviride. The major capping metabolites in synthesized T-SeNPs were Isochiapin B and Quercetin 7,3',4'-trimethyl ether. T-SeNPs showed a spherical shape and an average size between 57 and 96.6 nm. FBD exposure (12 μM) for 14 days induced oxidative stress and inflammatory responses via overexpression of NF-κB family members. It also distinctly caused upregulation of miR-221, miR-222, and E2F2, escorted by downregulation of miR-17, miR-20a, and P27kip1. FBD encouraged PrEC cells to halt at the G1/S checkpoint. Apoptotic cells were drastically increased in FBD-treated sets. Treatment of T-SeNPs simultaneously with FBD revealed its antioxidant, anti-inflammatory, and antitumor activities in counteracting FBD-induced toxicity. Our findings shed light on the potential FBD toxicity that may account for the neoplastic transformation of epithelial cells in the prostate and the mitigating activity of eco-friendly synthesized T-SeNPs.

Suitability of human HepaRG cells and liver spheroids as in vitro model to investigate the bioactivation of the organothiophosphate pesticide parathion

Organophosphorus compounds (OP) constitute a large group of chemicals including pesticides and nerve agents. Organothiophosphate pesticides require cytochrome P450-mediated oxidative desulphuration in the liver to form corresponding oxons, which are potent inhibitors of the enzyme acetylcholinesterase (AChE). Human HepaRG cells are a promising tool to study liver-specific functions and have been shown to maintain drug metabolizing enzymes. This research describes for the first time the in vitro metabolic activation of an organothiophosphate to its active oxon by two different HepaRG cell-based models. Monolayer cultures and liver spheroids were exposed to the model OP parathion and the quantification of the corresponding oxon was performed with an AChE inhibition assay. Our results showed a time- and dose-dependent cytochrome P450 catalyzed bioactivation and a superior metabolism capacity of the monolayer HepaRG model in comparison with the liver spheroids. Finally, HepaRG cells can be assessed as a metabolically competent cell model intermediate between cell-free preparations and intact animals and as suitable to study OP metabolism in the human liver.

Prediction of in vivo prenatal chlorpyrifos exposure leading to developmental neurotoxicity in humans based on in vitro toxicity data by quantitative in vitro-in vivo extrapolation

Introduction: Epidemiological studies in children suggested that in utero exposure to chlorpyrifos (CPF), an organophosphate insecticide, may cause developmental neurotoxicity (DNT). We applied quantitative in vitro-in vivo extrapolation (QIVIVE) based on in vitro concentration and non-choline esterase-dependent effects data combined with Benchmark dose (BMD) modelling to predict oral maternal CPF exposure during pregnancy leading to fetal brain effect concentration. By comparing the results with data from epidemiological studies, we evaluated the contribution of the in vitro endpoints to the mode of action (MoA) for CPF-induced DNT. Methods: A maternal-fetal PBK model built in PK-Sim^® was used to perform QIVIVE predicting CPF concentrations in a pregnant women population at 15 weeks of gestation from cell lysate concentrations obtained in human induced pluripotent stem cell-derived neural stem cells undergoing differentiation towards neurons and glia exposed to CPF for 14 days. The in vitro concentration and effect data were used to perform BMD modelling. Results: The upper BMD was converted into maternal doses which ranged from 3.21 to 271 mg/kg bw/day. Maternal CPF blood levels from epidemiological studies reporting DNT findings in their children were used to estimate oral CPF exposure during pregnancy using the PBK model. It ranged from 0.11 to 140 μg/kg bw/day. Discussion: The effective daily intake doses predicted from the in vitro model were several orders of magnitude higher than exposures estimated from epidemiological studies to induce developmental non-cholinergic neurotoxic responses, which were captured by the analyzed in vitro test battery. These were also higher than the in vivo LOEC for cholinergic effects. Therefore, the quantitative predictive value of the investigated non-choline esterase-dependent effects, although possibly relevant for other chemicals, may not adequately represent potential key events in the MoA for CPF-associated DNT.

Experimental and Established Oximes as Pretreatment before Acute Exposure to Azinphos-Methyl

Poisoning with organophosphorus compounds (OPCs) represents an ongoing threat to civilians and rescue personal. We have previously shown that oximes, when administered prophylactically before exposure to the OPC paraoxon, are able to protect from its toxic effects. In the present study, we have assessed to what degree experimental (K-27; K-48; K-53; K-74; K-75) or established oximes (pralidoxime, obidoxime), when given as pretreatment at an equitoxic dosage of 25% of LD01, are able to reduce mortality induced by the OPC azinphos-methyl. Their efficacy was compared with that of pyridostigmine, the only FDA-approved substance for such prophylaxis. Efficacy was quantified in rats by Cox analysis, calculating the relative risk of death (RR), with RR=1 for the reference group given only azinphos-methyl, but no prophylaxis. All tested compounds significantly (p ≤ 0.05) reduced azinphos-methyl-induced mortality. In addition, the efficacy of all tested experimental and established oximes except K-53 was significantly superior to the FDA-approved compound pyridostigmine. Best protection was observed for the oximes K-48 (RR = 0.20), K-27 (RR = 0.23), and obidoxime (RR = 0.21), which were significantly more efficacious than pralidoxime and pyridostigmine. The second-best group of prophylactic compounds consisted of K-74 (RR = 0.26), K-75 (RR = 0.35) and pralidoxime (RR = 0.37), which were significantly more efficacious than pyridostigmine. Pretreatment with K-53 (RR = 0.37) and pyridostigmine (RR = 0.52) was the least efficacious. Our present data, together with previous results on other OPCs, indicate that the experimental oximes K-27 and K-48 are very promising pretreatment compounds. When penetration into the brain is undesirable, obidoxime is the most efficacious prophylactic agent already approved for clinical use.

Human Family 1-4 cytochrome P450 enzymes involved in the metabolic activation of xenobiotic and physiological chemicals: an update

This is an overview of the metabolic activation of drugs, natural products, physiological compounds, and general chemicals by the catalytic activity of cytochrome P450 enzymes belonging to Families 1-4. The data were collected from > 5152 references. The total number of data entries of reactions catalyzed by P450s Families 1-4 was 7696 of which 1121 (~ 15%) were defined as bioactivation reactions of different degrees. The data were divided into groups of General Chemicals, Drugs, Natural Products, and Physiological Compounds, presented in tabular form. The metabolism and bioactivation of selected examples of each group are discussed. In most of the cases, the metabolites are directly toxic chemicals reacting with cell macromolecules, but in some cases the metabolites formed are not direct toxicants but participate as substrates in succeeding metabolic reactions (e.g., conjugation reactions), the products of which are final toxicants. We identified a high level of activation for three groups of compounds (General Chemicals, Drugs, and Natural Products) yielding activated metabolites and the generally low participation of Physiological Compounds in bioactivation reactions. In the group of General Chemicals, P450 enzymes 1A1, 1A2, and 1B1 dominate in the formation of activated metabolites. Drugs are mostly activated by the enzyme P450 3A4, and Natural Products by P450s 1A2, 2E1, and 3A4. Physiological Compounds showed no clearly dominant enzyme, but the highest numbers of activations are attributed to P450 1A, 1B1, and 3A enzymes. The results thus show, perhaps not surprisingly, that Physiological Compounds are infrequent substrates in bioactivation reactions catalyzed by P450 enzyme Families 1-4, with the exception of estrogens and arachidonic acid. The results thus provide information on the enzymes that activate specific groups of chemicals to toxic metabolites.

Integrating biokinetics and in vitro studies to evaluate developmental neurotoxicity induced by chlorpyrifos in human iPSC-derived neural stem cells undergoing differentiation towards neuronal and glial cells

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Human iPSC-derived NSCs undergoing differentiation possess some metabolic competence.

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CPF entered the cells and was biotrasformed into its two main metabolites (CPFO and TCP).

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After repeated exposure, very limited bioaccumulation of CPF was observed.

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Treatment with CPF decreased neurite outgrowth, synapse number and electrical activity.

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Treatment with CPF increased BDNF levels and the percentage of astrocytes.

For some complex toxicological endpoints, chemical safety assessment has conventionally relied on animal testing. Apart from the ethical issues, also scientific considerations have been raised concerning the traditional approach, highlighting the importance for considering real life exposure scenario. Implementation of flexible testing strategies, integrating multiple sources of information, including in vitro reliable test methods and in vitro biokinetics, would enhance the relevance of the obtained results. Such an approach could be pivotal in the evaluation of developmental neurotoxicity (DNT), especially when applied to human cell-based models, mimicking key neurodevelopmental processes, relevant to human brain development.

Here, we integrated the kinetic behaviour with the toxicodynamic alterations of chlorpyrifos (CPF), such as in vitro endpoints specific for DNT evaluation, after repeated exposure during differentiation of human neural stem cells into a mixed culture of neurons and astrocytes. The upregulation of some cytochrome P450 and glutathione S-transferase genes during neuronal differentiation and the formation of the two major CPF metabolites (due to bioactivation and detoxification) supported the metabolic competence of the used in vitro model. The alterations in the number of synapses, neurite outgrowth, brain derived neurotrophic factor, the proportion of neurons and astrocytes, as well as spontaneous electrical activity correlated well with the CPF ability to enter the cells and be bioactivated to CPF-oxon. Overall, our results confirm that combining in vitro biokinetics and assays to evaluate effects on neurodevelopmental endpoints in human cells should be regarded as a key strategy for a quantitative characterization of DNT effects.

Phosmet bioactivation by isoform-specific cytochrome P450s in human hepatic and gut samples and metabolic interaction with chlorpyrifos

Data on the bioactivation of Phosmet (Pho), a phthalimide-derived organophosphate pesticide (OPT), to the neurotoxic metabolite Phosmet-oxon (PhOx) in human are not available. The characterization of the reaction in single human recombinant CYPs evidenced that the ranking of the intrinsic clearances was: 2C18>2C19>2B6>2C9>1A1>1A2>2D6>3A4>2A6. Considering the average human hepatic content, CYP2C19 contributed for the great majority (60%) at relevant exposure concentrations, while CYP2C9 (33%) and CYP3A4 (31%) were relevant at high substrate concentration. The dose-dependent role of the active isoforms was confirmed in human liver microsomes by using selective CYP inhibitors. This prominent role of CYP2C in oxon formation was not shared by other OPTs. The pre-systemic Pho bioactivation measured in human intestinal microsomes was relevant accounting for ¼ of that measured in the liver showing two reaction phases catalysed by CYP2C and CYP3A4. Phosmet efficiently inhibited CPF bioactivation and detoxication, with Ki values (≈30 μM) relevant to pesticide concentrations achievable in the human liver, while the opposite is unlikely (Ki ≈ 160 μM) at the actual exposure levels, depending on the peculiar isoform-specific Pho bioactivation. Kinetic information in humans can support the development of quantitative in vitro/in vivo extrapolation and in silico models for risk assessment refinement for single and multiple pesticides.

Metabolism of triflumuron in the human liver: Contribution of cytochrome P450 isoforms and esterases

Triflumuron (TFM) is a benzoylurea insecticide commonly used in Tunisian agriculture and around the world to control crop pests and flies as a promising alternative to conventional insecticides for its arthropod specificity and low toxicity. From the evidence available in animal models, it can be expected that the metabolism of TFM is catalyzed by cytochrome P450 (CYP) and esterases. However, no data are available on human metabolism of TFM with regards to phase I metabolism and CYP isoform specificity. Hence, this manuscript describes experimental investigations to underpin in vitro phase I TFM metabolism in human samples for the first time. TFM biotransformation by recombinant human CYPs was characterized, then human liver microsomes (HLM) and chemical specific inhibitors have been used to identify the relative contribution of CYPs and esterases. Our results showed that all CYP isoforms were able to metabolize TFM with different affinity and efficiency. The relative contribution based both on the kinetic parameters and the CYP hepatic content was 3A4 > >2C9 > 2C8 > 2A6 > 1A2 > 2B6 > 2D6 > 2C19 > 2C18 > 1A1 at low TFM concentration, whilst at high TFM concentration it was 1A2 > >2C9 = 3A4 = 2A6 > 2C19 > 2B6 = 2C8 > 2D6 > 1A1 > 2C18. Experiments with HLMs confirmed the involvement of the most relevant CYPs in the presence of specific chemical inhibitors with a catalytic efficiency (Cliapp) lower by an order of magnitude compared with recombinant enzymes. Esterases were also relevant to the overall TFM kinetics and metabolism, with catalytic efficiency higher than that of CYPs. It is foreseen that such isoform-specific information in humans will further support in silico models for the refinement of the human risk assessment of single pesticides or mixtures.

The Experimental Oxime K027-A Promising Protector From Organophosphate Pesticide Poisoning. A Review Comparing K027, K048, Pralidoxime, and Obidoxime

Poisoning with organophosphorus compounds (OPCs) is a major problem worldwide. Standard therapy with atropine and established oxime-type enzyme reactivators (pralidoxime, obidoxime) is unsatisfactory. In search of more efficacious broad-spectrum oximes, new bispyridinium (K-) oximes have been synthesized, with K027 being among the most promising. This review summarizes pharmacokinetic characteristics of K027, its toxicity and in vivo efficacy to protect from OPC toxicity and compares this oxime with another experimental bisquaternary asymmetric pyridinium aldoxime (K048) and two established oximes (pralidoxime, obidoxime). After intramuscular (i.m.) injection, K027 reaches maximum plasma concentration within ∼30 min; only ∼2% enter the brain. Its intrinsic cholinesterase inhibitory activity is low, making it relatively non-toxic. In vitro reactivation potency is high for ethyl-paraoxon-, methyl-paraoxon-, dichlorvos-, diisopropylfluorophosphate (DFP)- and tabun-inhibited cholinesterase. When administered in vivo after exposure to the same OPCs, K027 is comparable or more efficacious than pralidoxime and obidoxime. When given as a pretreatment before exposure to ethyl-paraoxon, methyl-paraoxon, DFP, or azinphos-methyl, it is superior to the Food and Drug Administration-approved compound pyridostigmine and comparable to physostigmine, which because of its entry into the brain may cause unwanted behavioral effects. Because of its low toxicity, K027 can be given in high dosages, making it a very efficacious oxime not only for postexposure treatment but also for prophylactic administration, especially when brain penetration is undesirable.

Novel Clinical Toxicology and Pharmacology of Organophosphorus Insecticide Self-Poisoning

Organophosphorus insecticide self-poisoning is a major global health problem, killing over 100,000 people annually. It is a complex multi-organ condition, involving the inhibition of cholinesterases, and perhaps other enzymes, and the effects of large doses of ingested solvents. Variability between organophosphorus insecticides—in lipophilicity, speed of activation, speed and potency of acetylcholinesterase inhibition, and in the chemical groups attached to the phosphorus—results in variable speed of poisoning onset, severity, clinical toxidrome, and case fatality. Current treatment is modestly effective, aiming only to reactivate acetylcholinesterase and counter the effects of excess acetylcholine at muscarinic receptors. Rapid titration of atropine during resuscitation is lifesaving and can be performed in the absence of oxygen. The role of oximes in therapy remains unclear. Novel antidotes have been tested in small trials, but the great variability in poisoning makes interpretation of such trials difficult. More effort is required to test treatments in adequately powered studies.

Reversible cholinesterase inhibitors as pretreatment for exposure to organophosphates. A review

Organophosphorus compounds (OPCs), inhibitors of acetylcholinesterase (AChE), are useful agents as pesticides, but also represent a serious health hazard. Standard therapy with atropine and established oxime-type enzyme reactivators (pralidoxime, obidoxime) is unsatisfactory. Better therapeutic results are obtained, when reversible AChE inhibitors are administered before OPC exposure. This review summarizes the history of such a pretreatment approach and sums up a set of experiments undertaken in search of compounds that are efficacious when given before a broad range of OPCs. The prophylactic efficacy of 10 known AChE inhibitors, either already used clinically for different indications (physostigmine, pyridostigmine, ranitidine, tiapride, tacrine, amiloride, metoclopramide, methylene blue) or developed for possible therapeutic use in the future (7-methoxytacrine, K-27) was compared, when administered before exposure to six chemically diverse OPCs in the same experimental setting: ethyl-paraoxon, methyl-paraoxon, diisopropylfluorophosphate, terbufos sulfone, azinphos-methyl and dicrotophos. The experimental oxime K-27 was the most efficacious compound, affording best protection, when administered before terbufos sulfone, azinphos-methyl and dicrotophos, second best before ethyl- and methyl-paraoxon exposure and third best before diisopropylfluorophosphate administration. This ranking was similar to that of physostigmine, which was superior to the Food and Drug Administration-approved pretreatment for soman with pyridostigmine. Tiapride, amiloride, metoclopramide, methylene blue and 7-methoxytacrine did not achieve protection. No correlation was observed between the IC₅₀ of the reversible AChE inhibitors and their protective efficacy. These studies indicate that K-27 can be considered a very promising broad-spectrum prophylactic agent in case of imminent organophosphate exposure, which may be related to its AChE reactivating activity rather than its AChE inhibition.

A framework and case studies for evaluation of enzyme ontogeny in children's health risk evaluation

Knowledge of the ontogeny of Phase I and Phase II metabolizing enzymes may be used to inform children's vulnerability based upon likely differences in internal dose from xenobiotic exposure. This might provide a qualitative assessment of toxicokinetic (TK) variability and uncertainty pertinent to early lifestages and help scope a more quantitative physiologically based toxicokinetic (PBTK) assessment. Although much is known regarding the ontogeny of metabolizing systems, this is not commonly utilized in scoping and problem formulation stage of human health risk evaluation. A framework is proposed for introducing this information into problem formulation which combines data on enzyme ontogeny and chemical-specific TK to explore potential child/adult differences in internal dose and whether such metabolic differences may be important factors in risk evaluation. The framework is illustrated with five case study chemicals, including some which are data rich and provide proof of concept, while others are data poor. Case studies for toluene and chlorpyrifos indicate potentially important child/adult TK differences while scoping for acetaminophen suggests enzyme ontogeny is unlikely to increase early-life risks. Scoping for trichloroethylene and aromatic amines indicates numerous ways that enzyme ontogeny may affect internal dose which necessitates further evaluation. PBTK modeling is a critical and feasible next step to further evaluate child-adult differences in internal dose for a number of these chemicals.

A Survey of Pesticide Accumulation in a Specialist Feeder, the Koala (Phascolarctos cinereus)

To maintain profitability in Australia's agricultural and urban landscapes pesticides are used throughout the range of koala habitats. The koala is a specialist feeder, reliant on metabolic enzyme capacities to utilise a toxic diet of eucalypt leaves and is potentially prone to adverse effects when xenobiotic interactions between dietary and anthropogenic xenobiotics occur. The aim of this study was to investigate accumulation of frequently used pesticides in wild koalas in 4 areas of New South Wales and Queensland. Liver samples of 57 deceased koalas were collected from care facilities and analysed using a modified QuEChERS extraction method followed by GCMSMS, HRLCMS and LCMSMS. No accumulation of any of the 166 investigated pesticides was found. Data indicate hepatic accumulation of pesticides in this species is uncommon even with close interactions with intensive land use. Despite the lack of hepatic bioaccumulation, this study cannot exclude a direct effect on hepatocellular metabolic pathways.

An investigation into the formation of tebufenozide's toxic aromatic amine metabolites in human in vitro hepatic microsomes

Tebufenozide is a nonsteroid ecdysone agonist that causes premature and incomplete molting in Lepidoptera. Studies conducted so far have shown the low toxicity of tebufenozide in mammals, birds and invertebrates. Tebufenozide potential metabolites such as aromatic amines are known to induce methemoglobinemia disorder in humans, most likely by the formation of N-hydroxy metabolites; therefore, the aim of this research is to investigate the formation of the potential toxic N-hydroxy derivatives in pooled human hepatic microsomal fractions. Analyses of metabolites by high performance liquid chromatography equipped by a time-of-flight detector (HPLC/TOF) indicated the formation of a hydroxylated metabolite (exact mass=369; retention time: 6.65min) and two de-dimethylethyl metabolites (exact masses=313; retention times: 5.76 and 6.22min). Hydroxylated tebufenozide metabolite resulted from hydroxylation at either the 3 or 5 position of the dimethylbenzoic acid moiety to form either 3-hydroxymethyl-5-methylbenzoic acid 1-(1,1-dimethylethyl)-2-(4-ethylbenzoyl) or 3-methyl-5-hydroxymethylbenzoic acid 1-(1,1-dimethylethyl)-2-(4-ethylbenzoyl), respectively. The two de-dimethylethyl-tebufenozide derivatives were 3,5-dimethylbenzoic acid-2-(4-hydroxyethylbenzoyl) and 3-hydroxymethyl-5-methylbenzoic acid-2-(4-ethylbenzoyl) or 3-methyl-5-hydroxymethylbenzoic acid-2-(4-ethylbenzoyl). Generally the metabolite formation rates increased with incubation time. The rate of hydroxylation of the dimethylbenzoic acid moiety was approximately 12 times higher than the hydroxylation of the ethylbenzoyl moiety. Tebufenozide does not appear to produce the toxic aromatic amine metabolites in human in vitro hepatic microsomes. This suggests that the fate of tebufenozide in humans is a process of detoxification rather than activation.

Reversible cholinesterase inhibitors as pre-treatment for exposure to organophosphates: assessment using azinphos-methyl

Pre-treatment with reversible acetylcholinesterase (AChE) inhibitors before organophosphorous compound (OPC) exposure can reduce OPC-induced mortality. However, pyridostigmine, the only substance employed for such prophylaxis, is merely efficacious against a limited number of OPCs. In search of more efficacious and broad-range alternatives, we have compared in vivo the ability of five reversible AChE inhibitors (pyridostigmine, physostigmine, ranitidine, tacrine and K-27) to reduce mortality induced by the OPC azinphos-methyl. Protection was quantified using Cox analysis by determining the relative risk (RR) of death in rats that were administered these AChE inhibitors in equitoxic dosage (25% of LD01) 30 min before azinphos-methyl exposure. Azinphos-methyl-induced mortality was significantly reduced by all five tested compounds as compared with the reference group that was only exposed to azinphos-methyl without prior pre-treatment (RR = 1). The most efficacious prophylactic agents were K-27 (RR = 0.15) and physostigmine (RR = 0.21), being significantly more efficacious than ranitidine (RR = 0.62) and pyridostigmine (RR = 0.37). Pre-treatment with tacrine (RR = 0.29) was significantly more efficacious than pre-treatment with ranitidine, but the difference between tacrine and pyridostigmine was not significant. Our results indicate that prophylactic administration of the oxime K-27 may be a promising alternative in cases of imminent OPC exposure.

Cholinesterase and carboxylesterase inhibition in Planorbarius corneus exposed to binary mixtures of azinphos-methyl and chlorpyrifos

Though pesticide mixtures are commonly encountered in fresh water systems, the knowledge of their effects on non-target aquatic species is scarce. The present investigation was undertaken to explore the in vivo inhibition of the freshwater gastropod snail Planorbarius corneus cholinesterase (ChE) and carboxylesterases (CES) activities by the organophosphorus pesticides azinphos-methyl (AZM) and chlorpyrifos (CPF). To this end, snails were exposed for 48 h to different concentrations of AZM and CPF in single-chemical and binary-mixture studies, and ChE and CES activities were measured in the whole soft body tissues and hemolymph. ChE activity was measured with acetylthiocholine as substrate and CES activity was measured with four substrates: p-nitrophenyl acetate, p-nitrophenyl butyrate, 1- and 2-naphthyl acetate. Single-chemical experiments showed that CPF was a more potent inhibitor of ChE activity than AZM (350 and 27 times for the whole soft tissue and hemolymph, respectively). CES were 15 times more sensitive than ChE when the activities were measured in the whole soft tissue of the animals exposed to AZM. Based on a default assumption of concentration addition, P. corneus snails were exposed to mixtures of AZM+CPF designed to yield predicted soft tissue ChE inhibitions of 31% (mixture 1), 50% (mixture 2) and 61% (mixture 3). Results showed that ChE inhibition produced by mixture 1 followed a model of concentration addition. In contrast, the other mixtures showed synergism, both in whole soft tissue and hemolymph. In addition, results obtained when the snails were exposed sequentially to the pesticides showed that the sequence AZM/CPF produced at 48 h a higher ChE inhibition than the sequence CPF/CPF. A range of metabolic pathways and responses associated with bioactivation or detoxification may play important roles in organophosphorus interactions. In particular, the data presented in the present study indicate that CES enzymes would be important factors in determining the effects of the mixtures of OPs on P. corneus ChE activity.

Bioactivation of chlorpyrifos by CYP2B6 variants

Chlorpyrifos (CPF), an organophosphorus (OP) pesticide, is bioactivated by cytochrome P450s (CYPs) to the active metabolite chlorpyrifos oxon (CPF-O). Given that human CYP2B6 has the highest intrinsic clearance (CL(int)) for CPF bioactivation, CYP2B6 polymorphisms may impact human susceptibility to CPF at real world environmental and occupational CPF exposure levels. CYP2B6.4,.5,.7, and .18 were over-expressed in mammalian COS-1 cells to assess the impact of CYP2B6 variants on the K(m) and V(max) for bioactivation of CPF. Cell lysates were incubated with CPF (0-100 μM) and the production of CPF-O was measured via HPLC analysis. CYP2B6 content was determined by western blot. CYP2B6.18 had neither detectable protein nor activity levels. The V(max) value for each remaining variant was significantly higher than wild-type (CYP2B6.1, V(max) 4.13 × 10(4) pmol/min/nmol CYP2B6), with CYP2B6.4,.5, and .7 having V(max) values of 4.52 × 10(5), 1.82 × 10(5), and 9.60 × 10(4) pmol/min/nmol CYP2B6, respectively. The K(m) values for these variants ranged from 0.39 to 1.09 μM and were not significantly different from wild-type. All active variants examined had significantly higher CL(int) than CYP2B6.1. Variants of CYP2B6 have altered capacity to bioactivate CPF and may affect individual susceptibility by altering the V(max) for CPF-O formation.

Human glutathione transferases catalyzing the conjugation of the hepatoxin microcystin-LR

Many cyanobacterial species are able to produce cyanotoxins as secondary metabolites. Among them, microcystins (MC) are a group of around 80 congeners of toxic cyclic heptapeptides. MC-LR is the most studied MC congener, in view of its high acute hepatotoxicity and tumor promoting activity. Humans may be exposed to cyanotoxins through several routes, the oral one being the most important. The accepted pathway for MC-LR detoxication and excretion in the urine is GSH conjugation. The GSH adduct (GS-MCLR) formation has been shown to occur spontaneously and enzymatically, catalyzed by glutathione transferases (GSTs). The enzymatic reaction has been reported but not characterized both in vitro and in vivo in animal and plant species. No data are available on humans. In the present work, the MC-LR conjugation with GSH catalyzed by five recombinant human GSTs (A1-1, A3-3, M1-1, P1-1, and T1-1) has been characterized for the first time. All GSTs are able to catalyze the reaction; kinetic parameters K(m), k(cat), and their relative specific activities to form GS-MCLR were derived (T1-1 > A1-1 > M1-1 > A3-3 ≫ P1-1). In the range of MC tested concentrations used (0.25-50 μM) GSTT1-1 and A1-1 showed a typical saturation curve with similar affinity for MC-LR (≈80 μM; k(cat) values 0.18 and 0.10 min(-1), respectively), A3-3 and M1-1 were linear, whereas GSTP1-1 showed a temperature-dependent sigmoidal allosteric curve with a k(cat) = 0.11 min(-1). The enzymes mainly expressed in the liver and gastrointestinal tract, GSTA1-1, T1-1, and M1-1, seemed to be mainly involved in the MC-LR detoxification after oral exposure, whereas P1-1 kinetics and location in the skin suggest a role related to dermal exposure. Considering the high frequency of some GST polymorphism, especially M1 and T1 gene deletion, with complete loss in activity, this information could be the first step to identify groups of individual at higher risk associated with MC exposure.

An evaluation of the cytochrome P450 inhibition potential of selected pesticides in human hepatic microsomes

The goal of this work was to study the ability of 18 pesticides to inhibit selective model activities for all major xenobiotic-metabolizing enzymes, namely CYP1A1/2, 2A6, 2B6, 2C8, 2C9, 2C19, 2D6, 2E1 and 3A4. Generally organophosphorus insecticides were the most potent and extensive inhibitors, especially towards CYP1A1/2 (IC(50) values of chlorpyrifos, fenitrothion and profenofos approximately 3 micro M), CYP2B6 (IC(50) values of chlorpyrifos and fenitrothion 2.5 micro M), CYP2C8 (fenitrothion 4.3 micro M), CYP2C9 (fenitrothion and malathion 4.8 and 2.5 micro M, respectively), CYP2D6 (chlorpyrifos and phenthoate approximately 3 micro M) and CYP3A4 (chlorpyrifos, fenitrothion and phenthoate 3-4 micro M). Otherwise there were quite considerable differences in potency and extent of inhibition between different organophosphates. Pyrethroids were in general very weak or inactive. Deltamethrin and fenvalerate were potent inhibitors of CYP2D6 (IC(50) values of approximately 3 micro M) while lambda-cyhalothrin potently inhibited both CYP2D6 and CYP3A4-mediated activities (IC(50)'s about 3-4 micro M). Some pesticides caused relatively potent inhibitions sporadically (carbendazim, CYP2D6, IC(50) = 12 micro M; atrazine, CYP3A4, IC(50) = 2.8 micro M; glyphosate, CYP2C9, IC(50) = 3.7 micro M; hexaflumuron, IC(50) = 6.0 micro M). With the exceptions of alpha-cypermethrin, cypermethrin, isoproturon, carbaryl and abamectin, most pesticides inhibited relatively potently at least one CYP-selective activity, which may have relevance for potential interactions in occupational exposures and for further studies on the CYP-associated metabolism of respective pesticides.

Biotransformation of chlorpyrifos and diazinon by human liver microsomes and recombinant human cytochrome P450s (CYP)

The cytochrome P450 (CYP)-mediated biotransformation of the organophosphorothioate insecticides chlorpyrifos and diazinon was investigated. Rates of desulphuration to the active oxon metabolite (chlorpyrifos-oxon and diazinon-oxon) and dearylation to non-toxic hydrolysis products were determined in human liver microsome preparations from five individual donors and in recombinant CYP enzymes. Chlorpyrifos and diazinon underwent desulphuration in human liver microsome with mean Km = 30 and 45 microM and V(max) = 353 and 766 pmol min(-1) mg(-1), respectively. Dearylation of these compounds by human liver microsome proceeded with Km = 12 and 28 microM and V(max) = 653 and 1186 pmol min(-1) mg(-1), respectively. The apparent intrinsic clearance (V(max)/Km) of dearylation was 4.5- and 2.5-fold greater than desulphuration for chlorpyrifos and diazinon, respectively. Recombinant human CYP2B6 possessed the highest desulphuration activity for chlorpyrifos, whereas CYP2C19 had the highest dearylation activity. In contrast, both desulphuration and dearylation of diazinon were catalysed at similar rates, in the rank order CYP2C19 > CYP1A2 > CYP2B6 > CYP3A4. Both organophosphorothioates were more readily detoxified (dearylation) than bioactivated (desulphuration) in all human liver microsome preparations. However, the role of individual CYP enzymes in these two biotransformation pathways varied according to the structure of the organophosphorothioate, which was reflected in different activation/detoxification ratios for chlorpyrifos and diazinon. Variability in activity of individual CYP enzymes may influence interindividual sensitivity to the toxic effects of chlorpyrifos and diazinon.

The participation of human hepatic P450 isoforms, flavin-containing monooxygenases and aldehyde oxidase in the biotransformation of the insecticide fenthion

Although fenthion (FEN) is widely used as a broad spectrum insecticide on various crops in many countries, very scant data are available on its biotransformation in humans. In this study the in vitro human hepatic FEN biotransformation was characterized, identifying the relative contributions of cytochrome P450 (CYPs) and/or flavin-containing monooxygenase (FMOs) by using single c-DNA expressed human enzymes, human liver microsomes and cytosol and CYP/FMO-specific inhibitors. Two major metabolites, FEN-sulfoxide and FEN-oxon (FOX), are formed by some CYPs although at very different levels, depending on the relative CYP hepatic content. Formation of further oxidation products and the reduction of FEN-sulfoxide back to FEN by the cytosolic aldehyde oxidase enzyme were ruled out. Comparing intrinsic clearance values, FOX formation seemed to be favored and at low FEN concentrations CYP2B6 and 1A2 are mainly involved in its formation. At higher levels, a more widespread CYP involvement was evident, as in the case of FEN-sulfoxide, although a higher efficiency of CYP2C family was suggested. Hepatic FMOs were able to catalyze only sulfoxide formation, but at low FEN concentrations hepatic FEN sulfoxidation is predominantly P450-driven. Indeed, the contribution of the hepatic isoforms FMO(3) and FMO(5) was generally negligible, although at high FEN concentrations FMO's showed activities comparable to the active CYPs, accounting for up to 30% of total sulfoxidation. Recombinant FMO(1) showed the highest efficiency with respect to CYPs and the other FMOs, but it is not expressed in the adult human liver. This suggests that FMO(1)-catalysed sulfoxidation may represent the major extra-hepatic pathway of FEN biotransformation.

Metabolic interactions of agrochemicals in humans

Agrochemicals and other xenobiotics are metabolized by xenobiotic-metabolizing enzymes (XMEs) to products that may be more or less toxic than the parent chemical. In this regard, phase-I XMEs such as cytochrome P450s (CYPs) are of primary importance. Interactions at the level of metabolism may take place via either inhibition or induction of XMEs. Such interactions have often been investigated, in vitro, in experimental animals, using subcellular fractions such as liver microsomes, but seldom in humans or at the level of individual XME isoforms. The authors have been investigating the metabolism of a number of agrochemicals by human liver microsomes and recombinant CYP isoforms and have recently embarked on studies of the induction of XMEs in human hepatocytes. The insecticides chlorpyrifos, carbaryl, carbofuran and fipronil, as well as the repellant DEET, are all extensively metabolized by human liver microsomes and, although a number of CYP isoforms may be involved, CYP2B6 and CYP3A4 are usually the most important. Permethrin is hydrolyzed by esterase(s) present in both human liver microsomes and cytosol. A number of metabolic interactions have been observed. Chlorpyrifos and other phosphorothioates are potent inhibitors of the CYP-dependent metabolism of both endogenous substrates, such as testosterone and estradiol, and exogenous substrates, such as carbaryl, presumably as a result of the interaction of highly reactive sulfur, released during the oxidative desulfuration reaction, with the heme iron of CYP. The hydrolysis of permethrin in human liver can be inhibited by chlorpyrifos oxon and by carbaryl. Fipronil can inhibit testosterone metabolism by CYP3A4 and is an effective inducer of CYP isoforms in human hepatocytes.

Management of acute organophosphorus pesticide poisoning

Organophosphorus pesticide self-poisoning is an important clinical problem in rural regions of the developing world, and kills an estimated 200,000 people every year. Unintentional poisoning kills far fewer people but is a problem in places where highly toxic organophosphorus pesticides are available. Medical management is difficult, with case fatality generally more than 15%. We describe the limited evidence that can guide therapy and the factors that should be considered when designing further clinical studies. 50 years after first use, we still do not know how the core treatments--atropine, oximes, and diazepam--should best be given. Important constraints in the collection of useful data have included the late recognition of great variability in activity and action of the individual pesticides, and the care needed cholinesterase assays for results to be comparable between studies. However, consensus suggests that early resuscitation with atropine, oxygen, respiratory support, and fluids is needed to improve oxygen delivery to tissues. The role of oximes is not completely clear; they might benefit only patients poisoned by specific pesticides or patients with moderate poisoning. Small studies suggest benefit from new treatments such as magnesium sulphate, but much larger trials are needed. Gastric lavage could have a role but should only be undertaken once the patient is stable. Randomised controlled trials are underway in rural Asia to assess the effectiveness of these therapies. However, some organophosphorus pesticides might prove very difficult to treat with current therapies, such that bans on particular pesticides could be the only method to substantially reduce the case fatality after poisoning. Improved medical management of organophosphorus poisoning should result in a reduction in worldwide deaths from suicide.

Evidences for CYP3A4 autoactivation in the desulfuration of dimethoate by the human liver

Dimethoate (DIM) is an organophosphorothionate (OPT) pesticide used worldwide as a systemic insecticide and acaricide. It is characterized by low-to-moderate acute mammalian toxicity; similarly to the other OPT pesticides, its mode of action is mediated by the inhibition of acetylcholinesterase (AChE), exerted by its toxic metabolite dimethoate-oxon or omethoate (OME), which is also used as a direct acting pesticide. Human hepatic DIM bioactivation to the toxic metabolite OME has been characterized by using c-DNA expressed human CYPs and human liver microsomes (HLM) also in the presence of CYP-specific chemical inhibitors, with a method based on AChE inhibition. The obtained kinetic parameters and AChE IC(50) have been compared with those previously obtained with other OPTs, indicating a lower efficiency in DIM desulfuration reaction and a lower potency in inhibiting AChE. Results showed that, similarly to the other OPTs tested so far, at low DIM concentration OME formation is mainly catalysed by CYP1A2, while the role of 3A4 is relevant at high DIM levels. Differently from the other OPTs, DIM desulfuration reaction showed an atypical kinetic profile, likely due to CYP3A4 autoactivation. The sigmoidicity degree of the activity curve increased with the level of CYP3A4 in HLM or disappeared in the presence of a CYP3A4 chemical inhibitor. This atypical kinetic behaviour can be considered one of the possible explanations for the recent findings that among patients hospitalized following OPT intoxication, DIM ingestion gave different symptoms and more severe poisoning (23.1% of fatal cases versus total) than chlorpyrifos (8% of deaths), which has a lower LD(50) value. Since DIM-poisoned patients poorly responded to pralidoxime, the possibility to use CYP3A4 inhibitors could be considered as a complementary treatment.

The importance of cytochrome P450 2B6 in the human metabolism of environmental chemicals

Cytochrome P450 (CYP) 2B6 (CYP2B6) is a human CYP isoform found in variable amounts in the liver and other organs. It is known to be inducible and polymorphic and has a wide range of xenobiotic substrates. Studies of CYP2B6 to date have concentrated heavily on clinical drugs. In the present communication, however, we concentrate on its role in the metabolism of environmental xenobiotics. The term environment is used, in its broadest sense, to include natural ecosystems and agroecosystems as well as the industrial and indoor domestic environments. In essence, this excludes only clinical drugs and drugs of abuse. Many of these chemicals, including agrochemicals and industrial chemicals, can serve as substrates, inhibitors and/or inducers of CYP2B6, these activities being often modified by the existence of polymorphic variants. Metabolism-based interactions between environmental chemicals are discussed, as well as the emerging possibility of metabolic interactions between environmental chemicals and clinical drugs.

Foetal and adult human CYP3A isoforms in the bioactivation of organophosphorothionate insecticides

In humans organophosphorothionate pesticides (OPT) prenatal exposure has been demonstrated. Since OPT-induced neurodevelopmental effects may be due to in situ bioactivation by foetal enzymes, the catalytic activity of the foetal CYP3A7 toward chlorpyrifos (CPF), parathion (PAR), malathion (MAL) and fenthion (FEN) has been assessed by using recombinant enzymes. A comparison with the adult isoforms CYP3A4 and CYP3A5 has been also carried out. CYP3A7 was able to produce significant levels of oxon or sulfoxide from the four OPTs in the range of tested concentrations (0.05-200 microM). When the efficiencies of CYP3A isoforms were compared, the ranking, expressed as CLi values, were: CPF=3A4>3A5>3A7; PAR=3A4>>3A7>>3A5; MAL=3A4>3A7>3A5; FEN (sulfoxide formation)=3A4>3A5>>3A7. The CYP3A5 efficiency appeared to be more dependent on the single insecticide than its related isozyme CYP3A4. Our results indicate that the levels of toxic metabolite formed in situ by CYP3A7 from CPF, MAL and PAR but not from FEN have the chance to inhibit acetylcholinesterase, following prenatal exposure to OPTs. However, due to the smaller weight of foetal liver, the contribution to total OPT biotransformation is relatively low. On the other hand, our results clearly indicate that at low CPF concentrations, the formation of the non-toxic metabolites is highly favoured in the foetus.

Diazinon, chlorpyrifos and parathion are metabolised by multiple cytochromes P450 in human liver

This research describes both the activation and detoxification of diazinon, chlorpyrifos and parathion by recombinant P450 isozymes and by human liver microsomes that had been characterised for P450 marker activities. Wide variations in activity were found for diazinon (50 microM; 500 microM) activation to diazoxon, chlorpyrifos (100 microM) to chlorpyrifos oxon and parathion (5 microM, 20 microM and 200 microM) to paraoxon in NADPH-dependent reactions. In parallel, the dearylated metabolites pyrimidinol (IHMP), trichloro-2-pyridinol (TCP) and p-nitrophenol (PNP) were produced from diazinon, chlorpyrifos and parathion, respectively, with similarly wide variations in activity. There were significant correlations between diazoxon formation from diazinon (50 microM; 500 microM) with the three CYP3A4/5 marker reactions, while IHMP formation correlated significantly with the three CYP3A4/5 reactions, the CYP2C8 marker reaction (p<0.05) and the CYP2C19 marker (p<0.01). Chlorpyrifos oxon formation from chlorpyrifos did not correlate with any of the P450 markers but TCP formation correlated with one of the CYP3A4/5 reactions (p<0.01) and CYP2C8 (p<0.01), CYP2C19 (p<0.01) and CYP1A2 (p<0.01) mediated reactions. There were significant relationships between paraoxon formation from parathion (5 microM, 20 microM and 200 microM) and the CYP3A4/5, CYP2C8 and CYP1A2 mediated reactions, although only the latter two isoforms correlated significantly with the lowest parathion concentration. Recombinant CYPs 2D6, 2C19, 3A5, 3A4 were most efficient in producing diazoxon and IHMP from diazinon; CYPs 2D6, 3A5, 2B6 and 3A4 were best at producing chlorpyrifos-oxon and CYPs 2C19, 2D6, 3A5 and 3A4 at producing TCP from chlorpyrifos (100 microM). These data strongly suggest that CYPs 3A4/5, 2C8, 1A2, 2C19 and 2D6 are primarily involved in the metabolism of all three OPs, although the profile of participating isoforms was different for each of the pesticides suggesting that chemical structure influences which P450s mediate the reaction. The marked inter-individual variation in expression of the various P450 isozymes may result in sub-populations of individuals that produce higher systemic oxon levels with increased susceptibility to OP toxicity.

A case study of acute human chlorpyrifos poisoning: Novel aspects on metabolism and toxicokinetics derived from liquid chromatography–tandem mass spectrometry analysis of urine samples

The metabolic fate of the organophosphorothioate-type insecticide chlorpyrifos (CP) in an acutely intoxicated 59 years old female was investigated by liquid chromatography-electrospray ionisation-tandem mass spectrometry (LC-ESI-MS/MS) analysis of urine samples. Fifteen metabolites of CP and its bioactivated intermediate chlorpyrifos-oxon (CPO), respectively, of which only three have been described in man so far, were identified on the basis of characteristic MS/MS transitions, precursor/product ion and/or neutral loss scans, chlorine isotopomer patterns, and partly by synthesis of reference compounds and subsequent structure confirmation. Three distinct biotransformation routes of CP are proposed: (1) cleavage reactions at the aromatic phosphoester bond, (2) cleavage reactions at the alkyl phosphoester bonds, and (3) glutathione (GSH) dependent nucleophilic substitution of the 6-chlorine at the aromatic moiety. Route (2) has not been reported in humans before and (3) is a hitherto completely unknown scheme of CP metabolism. Urinary markers of the latter were chiefly cysteine S-conjugates of mono-dechlorinated CP, CPO, mono-O-deethyl CP, and mono-O-deethyl CPO as well as the 6-mercapturic acid conjugate of 3,5-dichloro-2-pyridinol. The presence of 3,5-dichloro-6-methylthio-2-pyridinol as well as its O-glucuronide suggests further a cysteine S-conjugate beta-lyase mediated degradation. In addition to the qualitative LC-MS/MS screening the renal elimination profiles of the primary products of scheme (1), i.e. diethyl thiophosphate (DETP), diethyl phosphate (DEP), and 3,5,6-trichloro-2-pyridinol (TCP), were monitored over 14 days (n=21). A biphasic first-order excretion mechanism with half-lives of 21.5h (initial fast excretion phase) and 119.5h (terminal phase) for the sum of free DETP and DEP was found. TCP was hardly eliminated in its free form (O-glucuronide identified as phase II conjugate) and half-lives calculated for the total amount of TCP (acidic hydrolysis of urine samples) were 40.8 and 150.7h. The present study gives a more detailed view on the biotransformation of CP and together with the obtained kinetic data adds novel aspects to the limited knowledge of human metabolism of this xenobiotic, in particular at high dosage.

Pharmacokinetic and pharmacodynamic interaction for a binary mixture of chlorpyrifos and diazinon in the rat

Chlorpyrifos (CPF) and diazinon (DZN) are two commonly used organophosphorus (OP) insecticides and a potential exists for concurrent exposures. The primary neurotoxic effects from OP pesticide exposures result from the inhibition of acetylcholinesterase (AChE). The pharmacokinetic and pharmacodynamic impact of acute binary exposures of rats to CPF and DZN was evaluated in this study. Rats were orally administered CPF, DZN, or a CPF/DZN mixture (0, 15, 30, or 60 mg/kg) and blood (plasma and RBC), and brain were collected at 0, 3, 6, 12, and 24 h postdosing, urine was also collected at 24 h. Chlorpyrifos, DZN, and their respective metabolites, 3,5,6-trichloro-2-pyridinol (TCP) and 2-isopropyl-4-methyl-6-hydroxypyrimidine (IMHP), were quantified in blood and/or urine and cholinesterase (ChE) inhibition was measured in brain, RBC, and plasma. Coexposure to CPF/DZN at the low dose of 15/15 mg/kg did not alter the pharmacokinetics of CPF, DZN, or their metabolites in blood. A high binary dose of 60/60 mg/kg increased the C(max) and AUC and decreased the clearance for both parent compounds, likely due to competition between CPF and DZN for CYP450 metabolism. At lower doses, most likely to be encountered in occupational or environmental exposures, the pharmacokinetics were linear. A dose-dependent inhibition of ChE was noted in tissues for both the single and coexposures, and the extent of inhibition was plasma > RBC > or = brain. The overall relative potency for ChE inhibition was CPF/DZN > CPF > DZN. A comparison of the ChE response at the low binary dose (15/15 mg/kg), where there were no apparent pharmacokinetic interactions, suggested that the overall ChE response was additive. These experiments represent important data concerning the potential pharmacokinetic and pharmacodynamic interactions for pesticide mixtures and will provide needed insight for assessing the potential cumulative risk associated with occupational or environmental exposures to these insecticides.

MALATHION BIOACTIVATION IN THE HUMAN LIVER: THE CONTRIBUTION OF DIFFERENT CYTOCHROME P450 ISOFORMS

Among organophosphorothioate (OPT) pesticides, malathion is considered relatively safe for use in mammals. Its rapid degradation by carboxylesterases competes with the cytochrome P450 (P450)-catalyzed formation of malaoxon, the toxic metabolite. However, impurities in commercial formulations are potent inhibitors of carboxylesterase, allowing a dramatic increase in malaoxon formation. Malathion desulfuration has been characterized in human liver microsomes (HLMs) with a method based on acetylcholinesterase inhibition that is able to detect nanomolar levels of oxon. The active P450 isoforms have been identified by means of a multifaceted strategy, including the use of cDNA-expressed human P450s and correlation, immunoinhibition, and chemical inhibition studies in a panel of phenotyped HLMs. HLMs catalyzed malaoxon formation with a high level of variability (>200-fold). One or two components (K(mapp1) = 53-67 microM; K(mapp2) = 427-1721 microM) were evidenced, depending on the relative specific P450 content. Results from different approaches indicated that, at low malathion concentration, malaoxon formation is catalyzed by CYP1A2 and, to a lesser extent, 2B6, whereas the role of 3A4 is relevant only at high malathion levels. These results are in line with those found with chlorpyrifos, diazinon, azynphos-methyl, and parathion, characterized by the presence of an aromatic ring in the molecule. Since malathion has linear chains as substituents at the thioether sulfur, it can be hypothesized that, independently from the chemical structure, OPTs are bioactivated by the same P450s. These results also suggest that CYP1A2 and 2B6 can be considered as possible metabolic biomarkers of susceptibility to OPT-induced toxic effects at actual human exposure levels.

Incorporating pharmacokinetic differences between children and adults in assessing children's risks to environmental toxicants

Children's risks from environmental toxicant exposure can be affected by pharmacokinetic factors that affect the internal dose of parent chemical or active metabolite. There are numerous physiologic differences between neonates and adults that affect pharmacokinetics including size of lipid, and tissue compartments, organ blood flows, protein binding capacity, and immature function of renal and hepatic systems. These factors combine to decrease the clearance of many therapeutic drugs, which can also be expected to occur with environmental toxicants in neonates. The net effect may be greater or lesser internal dose of active toxicant depending upon how the agent is distributed, metabolized, and eliminated. Child/adult pharmacokinetic differences decrease with increasing postnatal age, but these factors should still be considered in any children's age group, birth through adolescence, for which there is toxicant exposure. Physiologically based pharmacokinetic (PBPK) models can simulate the absorption, distribution, metabolism, and excretion of xenobiotics in both children and adults, allowing for a direct comparison of internal dose and risk across age groups. This review provides special focus on the development of hepatic cytochrome P-450 enzymes (CYPs) in early life and how this information, along with many factors unique to children, can be applied to PBPK models for this receptor population. This review describes a case study involving the development of neonatal PBPK models for the CYP1A2 substrates caffeine and theophylline. These models were calibrated with pharmacokinetic data in neonates and used to help understand key metabolic differences between neonates and adults across these two drugs.

In vitro human phase I metabolism of xenobiotics I: pesticides and related compounds used in agriculture and public health, May 2003

This is the first revision of a database covering human phase I enzymes and their isoforms that metabolize pesticides and related compounds. The original version included enzymes that metabolize chloroacetamide and triazine herbicides, and organophosphorus insecticides. This revision also includes carbamate, nicotinoid, and pyrethroid insecticides and insect repellents.

Developmental exposure to chlorpyrifos alters reactivity to environmental and social cues in adolescent mice

Neonatal mice were treated daily on postnatal days (pnds) 1 through 4 or 11 through 14 with the organophosphate pesticide chlorpyrifos (CPF), at doses (1 or 3 mg/kg) that do not evoke systemic toxicity. Brain acetylcholinesterase (AChE) activity was evaluated within 24 h from termination of treatments. Pups treated on pnds 1-4 underwent ultrasonic vocalization tests (pnds 5, 8, and 11) and a homing test (orientation to home nest material, pnd 10). Pups in both treatment schedules were then assessed for locomotor activity (pnd 25), novelty-seeking response (pnd 35), social interactions with an unfamiliar conspecific (pnd 45), and passive avoidance learning (pnd 60). AChE activity was reduced by 25% after CPF 1-4 but not after CPF 11-14 treatment. CPF selectively affected only the G(4) (tetramer) molecular isoform of AChE. Behavioral analysis showed that early CPF treatment failed to affect neonatal behaviors. Locomotor activity on pnd 25 was increased in 11-14 CPF-treated mice at both doses, and CPF-treated animals in both treatment schedules were more active when exposed to environmental novelty in the novelty-seeking test. All CPF-treated mice displayed more agonistic responses, and such effect was more marked in male mice exposed to the low CPF dose on pnds 11-14. Passive avoidance learning was not affected by CPF. These data indicate that developmental exposure to CPF induces long-term behavioral alterations in the mouse species and support the involvement of neural systems in addition to the cholinergic system in the delayed behavioral toxicity of CPF.

CYP-specific bioactivation of four organophosphorothioate pesticides by human liver microsomes

The bioactivation of azinphos-methyl (AZIN), chlorpyrifos (CPF), diazinon (DIA), and parathion (PAR), four widely used organophosphorothioate (OPT) pesticides has been investigated in human liver microsomes (HLM). In addition, the role of human cytochrome P450 (CYPs) in OPT desulfuration at pesticide levels representative of human exposure have been defined by means of correlation and immunoinhibition studies. CYP-mediated oxon formation from the four OPTs is efficiently catalyzed by HLM, although showing a high variability (>40-fold) among samples. Two distinct phases were involved in the desulfuration of AZIN, DIA, and PAR, characterized by different affinity constants (K(mapp1) = 0.13-9 microM and K(mapp2) = 5- 269 microM). Within the range of CPF concentrations tested, only the high-affinity component was evidenced (K(mapp1) = 0.27-0.94 microM). Oxon formation in phenotyped individual HLM showed a significant correlation with CYP1A2-, 3A4-, and 2B6-related activities, at different levels depending on the OPT concentration. Anti-human CYP1A2, 2B6, and 3A4 antibodies significantly inhibited oxon formation, showing the same OPT concentration dependence. Our data indicated that CYP1A2 is mainly involved in OPT desulfuration at low pesticide concentrations, while the role of CYP3A4 is more significant to the low-affinity component of OPT bioactivation. The contribution of CYP2B6 to total hepatic oxon formation was relevant in a wide range of pesticide concentrations, being a very efficient catalyst of both the high- and low-affinity phase. These results suggest CYP1A2 and 2B6 as possible metabolic biomarkers of susceptibility to OPT toxic effect at the actual human exposure levels.