Metabolism of carbosulfan. I. Species differences in the in vitro biotransformation by mammalian hepatic microsomes including human

Metabolites in the regulatory risk assessment of pesticides in the EU

A large majority of chemicals is converted into metabolites through xenobiotic-metabolising enzymes. Metabolites may present a spectrum of characteristics varying from similar to vastly different compared with the parent compound in terms of both toxicokinetics and toxicodynamics. In the pesticide arena, the role of metabolism and metabolites is increasingly recognised as a significant factor particularly for the design and interpretation of mammalian toxicological studies and in the toxicity assessment of pesticide/metabolite-associated issues for hazard characterization and risk assessment purposes, including the role of metabolites as parts in various residues in ecotoxicological adversities. This is of particular relevance to pesticide metabolites that are unique to humans in comparison with metabolites found in in vitro or in vivo animal studies, but also to disproportionate metabolites (quantitative differences) between humans and mammalian species. Presence of unique or disproportionate metabolites may underlie potential toxicological concerns. This review aims to present the current state-of-the-art of comparative metabolism and metabolites in pesticide research for hazard and risk assessment, including One Health perspectives, and future research needs based on the experiences gained at the European Food Safety Authority.

Characterization of furathiocarb metabolism in in vitro human liver microsomes and recombinant cytochrome P450 enzymes

Furathiocarb is a carbamate insecticide detected in ecosystems. Its main metabolite carbofuran has been alluded to affect birth outcomes and disturb hormone levels in humans. The metabolism of furathiocarb in humans has not been characterized. The metabolism studies were performed using hepatic microsomes from ten donors and fifteen human cDNA-expressed CYPs. The initial screening and identification of the metabolites were performed by LC-TOF. Quantifications and fragmentations were performed by LC/MS-MS. Furathiocarb was metabolized to eight phase I metabolites via two general pathways, carbofuran metabolic pathway and furathiocarb oxidation pathway. Six metabolites in the carbofuran metabolic pathway (carbofuran, 3-hydroxycarbofuran, 3-ketocarbofuran, 3-keto-7-phenolcarbofuran, 3-hydroxy-7-phenolcarbofuran, and 7-phenolcarbofuran) were identified with the help of authentic standards. The two unidentified metabolites in the furathiocarb oxidation pathway are probably hydroxylated and sulfoxidated derivatives of furathiocarb. The carbofuran metabolic pathway was more predominant than the furathiocarb oxidation pathway, ratios ranged from 24- to 115-fold in a 10-donor panel of hepatic microsomes. On the basis of recombinant CYP studies, the carbofuran pathway was dominated by CYP3A4 (95.9%); contributions by CYP1A2 (1.3%) and CYP2B6 (2.0%) were minor. The minor furathiocarb oxidation pathway was catalyzed by CYP2C19 and CYP2D6 (hydroxylated/sulfoxidated metabolite A) and by CYP3A5, CYP3A4 and CYP2A6 (metabolite B). High and significant correlation between carbofuran metabolic pathway and CYP3A4 marker activities (midazolam-1'-hydroxylation and omeprazole-sulfoxidation) were observed. Ketoconazole, a CYP3A4-inhibitor, inhibited the carbofuran pathway by 32–86% and hydroxylated/sulfoxidated metabolite-B formations by 41–62%. The data suggest that in humans, the carbofuran metabolic pathway is dominant, and CYP3A4 is the major enzyme involved. These results provide useful scientific information for furathiocarb risk assessment in humans.

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Eight Phase I metabolites were detected by LC-TOF-MS/MS.

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The carbofuran pathway was more rapid than the furathiocarb oxidation pathway

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The carbofuran pathway was dominated by CYP3A4 (96%).

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Ketoconazole inhibited the carbofuran pathway by 32–86%.

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The findings provide useful scientific information for furathiocarb risk assessment in humans.

Mechanism insights into the transformation of carbosulfan during apple drying processes

The transformation of carbosulfan (CSN) in apples was investigated during oven-drying, microwave drying, and sun-drying. CSN transformed primarily into carbofuran (COA) during these drying processes. The conversion kinetics of CSN and COA was fitted by curve regression and mainly conformed to quadratic models (R² = 0.70-0.97). Oven-drying promoted the transformation of CSN into COA. Microwave drying resulted in the highest scavenging capacity against CSN and COA (41%-100%). Moreover, a transformation mechanism was proposed on the basis of density functional theory (DFT) calculation. The COA originated from a series of chemical reactions involving hydroxyl substitution, cleavage, and oxidation; this result was further confirmed on the basis of molecular electrostatic potential (MEP) and molecular orbital theory. Furthermore, the toxicity and stability of CSN and COA were evaluated with the T.E.S.T. program. COA was less toxic than CSN to aquatic organisms but more toxic than CSN to rats. Therefore, COA production should be avoided during drying. Microwave drying was found to be the optimum choice for drying apples.

Bioaccumulation and Metabolism of Carbosulfan in Zebrafish (Danio rerio) and the Toxic Effects of Its Metabolites

Carbosulfan is a carbamate insecticide that has been widely used in agriculture. However, studies showed that carbosulfan could be highly toxic to aquatic organisms. The metabolism of carbosulfan in adult zebrafish is still largely unexplored, and the metabolites in individual or in combination may pose a potential threat to zebrafish. In the present study, the bioaccumulation and metabolism of carbosulfan in zebrafish (Danio rerio) were assessed, and the main metabolites, including carbofuran and 3-hydroxycarbofuran, were determined. The toxicity of carbosulfan and its metabolites individually or in combination to zebrafish was also investigated. The bioaccumulation and metabolism experiment indicated that carbosulfan was not highly accumulated in zebrafish, with a bioaccumulation factor of 18 after being exposed to carbosulfan for 15 days, and the metabolism was fast, with a half-life of 1.63 d. The two main metabolites were relatively persistent, with half-lives of 3.33 and 5.68 d for carbofuran and 3-hydroxycarbofuran, respectively. The acute toxicity assay showed that carbofuran and 3-hydroxycarbofuran had 96-h LC₅₀ values of 0.15 and 0.36 mg/L, showing them to be more toxic than carbosulfan (96-h LC₅₀ = 0.53 mg/L). Combinations of binary or ternary mixtures of carbosulfan and its metabolites displayed coincident synergistic effects on acute toxicity, with additive index (AI) values of 1.9-14.3. In the livers and gills of zebrafish exposed to carbosulfan, carbofuran, and 3-hydroxycarbofuran, activities of catalase, superoxide dismutase, and glutathione-S-transferase were significantly changed in most cases, and the content of malondialdehyde was greatly increased, indicating that carbosulfan and its metabolites induced varying degrees of oxidative stress. The metabolites were more persistent and toxic to zebrafish and exhibit coincident synergistic effects in combination. These results can provide evidence for the potential risk of pesticides and highlight the importance of a systematic assessment for the combination of the precursor and its metabolites.

Carbofuran degraded by iron-doped anatase: Weakening the cholinesterase inhibitory activity in the photoproducts mixture

Carbofuran is a toxic carbamate pesticide, and its use has increased in recent years. While marketing information indicates stability in different chemical media, carbofuran exhibits relative photolability. The aim of this research was to decompose carbofuran and to identify the photoproducts achieved when two different doped titania photocatalysts were employed under UV irradiation. The iron-doped TiO₂ materials were obtained (a) via a hydrothermal method and (b) by an ultrasound-assisted sol-gel method. The precursors were TiOSO₄⋅xH₂O and Fe₃(NO₃)·9H₂O. X-ray studies confirmed that the anatase phase of the iron-doped TiO₂ resulted from the two preparation methods. The photocatalytic performance of the prepared materials was monitored by LC/ESI-QTOF-MS, enabling the identification of photoproducts: oxo-carbamates, hydroxylated benzofuranes, a carboxamide, and one amine. By using the iron-doped TiO₂ materials, 2,2-dimethyl-2,3-dihydrobenzofuran-3,7-diol was the most abundant photoproduct, and N,2,2-trimethyl-2,3-dihydrobenzofuran-7-amine was the only compound that had not been previously reported in the photolysis and photocatalysis of carbofuran. The product 3-hydroxy carbofuran, a cholinesterase inhibitor, was quantified and was found to be transformed into compounds that lack this inhibitive property.

Metabolism of methiocarb and carbaryl by rat and human livers and plasma, and effect on their PXR, CAR and PPARα activities

The oxidative, reductive, and hydrolytic metabolism of methiocarb and the hydrolytic metabolism of carbaryl by liver microsomes and plasma of rats or humans were examined. The effects of the metabolism of methiocarb and carbaryl on their nuclear receptor activities were also examined. When methiocarb was incubated with rat liver microsomes in the presence of NADPH, methiocarb sulfoxide, and a novel metabolite, methiocarb sulfone were detected. Methiocarb sulfoxide was oxidized to the sulfone by liver microsomes and reduced back to methiocarb by liver cytosol. Thus, the interconversion between methiocarb and the sulfoxide was found to be a new metabolic pathway for methiocarb by liver microsomes. The product of methiocarb hydrolysis, which is methylthio-3,5-xylenol (MX), was also oxidized to sulfoxide form by rat liver microsomes. The oxidations were catalyzed by human flavin-containing monooxygenase isoform (FMO1). CYP2C19, which is a human cytochrome P450 (CYP) isoform, catalyzed the sulfoxidations of methiocarb and MX, while CYP1A2 also exhibited oxidase activity toward MX. Methiocarb and carbaryl were not enzymatically hydrolyzed by the liver microsomes, but they were mainly hydrolyzed by plasma and albumin to MX and 1-naphthol, respectively. Both methiocarb and carbaryl exhibited PXR and PPARα agonistic activities; however, methiocarb sulfoxide and sulfone showed markedly reduced activities. In fact, when methiocarb was incubated with liver microsomes, the receptor activities were decreased. In contrast, MX and 1-naphthol showed nuclear receptor activities equivalent to those of their parent carbamates. Thus, the hydrolysis of methiocarb and carbaryl and the oxidation of methiocarb markedly modified their nuclear receptor activities.

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.

Evaluating the Impact of Uncertainties in Clearance and Exposure When Prioritizing Chemicals Screened in High-Throughput Assays

The toxicity-testing paradigm has evolved to include high-throughput (HT) methods for addressing the increasing need to screen hundreds to thousands of chemicals rapidly. Approaches that involve in vitro screening assays, in silico predictions of exposure concentrations, and pharmacokinetic (PK) characteristics provide the foundation for HT risk prioritization. Underlying uncertainties in predicted exposure concentrations or PK behaviors can significantly influence the prioritization of chemicals, though the impact of such influences is unclear. In the current study, a framework was developed to incorporate absorbed doses, PK properties, and in vitro dose-response data into a PK/pharmacodynamic (PD) model to allow for placement of chemicals into discrete priority bins. Literature-reported or predicted values for clearance rates and absorbed doses were used in the PK/PD model to evaluate the impact of their uncertainties on chemical prioritization. Scenarios using predicted absorbed doses resulted in a larger number of bin misassignments than those scenarios using predicted clearance rates, when comparing to bin placement using literature-reported values. Sensitivity of parameters on the model output of toxicological activity was examined across possible ranges for those parameters to provide insight into how uncertainty in their predicted values might impact uncertainty in activity.

Interspecies differences in metabolism of deoxypodophyllotoxin in hepatic microsomes from human, monkey, rat, mouse and dog

Deoxypodophyllotoxin (DPT) is a natural lignan product which has drawn much attention due to its pharmacological properties including antitumor effect. The purpose of this study was to investigate interspecies differences in metabolism of DPT in hepatic microsomes from human (HLM), cynomolgus monkey (CyLM), rat (RLM), mouse (MLM) and dog (DLM). Incubation of DPT with hepatic microsomes from five species in the presence of NADPH resulted in formation of seven metabolites, five of which were compared with the synthetic standards. M2 was the most abundant metabolite in microsomes from all species. Rank order of intrinsic clearance for M2 formation was RLM > CyLM > MLM > HLM > DLM. In HLM, sulfaphenazole showed the strongest inhibition effect on M2 formation, but neither ticlopidine nor ketoconazole inhibited M2 formation in HLM. Results from cDNA-expressed human CYP450s experiments showed that clearance of M2 formation was much higher in CYP2C9 and CYP2C19 than that in CYP3A4. Contributions of the three CYP450 isoforms to M2 formation in HLM were estimated using relative activity factor (RAF) method or correction by amount of CYP450 isoforms in HLM. M2 formation in HLM was mainly attributed to CYP2C9, followed by CYP2C19. Involvement of CYP3A4 was minor.

A comparative study on toxicity induced by carbosulfan and malathion in Wistar rat liver and spleen

Organophosphorus (OP) and carbamate (CM) pesticides are widely used in agriculture. These pesticides are highly toxic to humans and their residues in food pose potential threat to human health. In this comparative study, we investigated the effect of subchronic exposure of OPs (malathion, MAL) and CM (Carbosulfan, CB) on rat liver and spleen. Biochemical analysis showed that levels of hepatic enzymes (ALT, ALP, LDH and PAL) changed after exposure to the pesticides. In the liver extracts, lipid peroxidation index increased after the treatment by pesticides. Our results indicated that exposure to MAL and CB leads to alteration of liver redox status. Both pesticides induced focal inflammation and fibrosis in the liver. After subchronic administration of MAL (200 mg/kg) and CB (25 mg/kg), systemic inflammation, as depicted by the increase in IFN-δ activity in liver, was observed in both malathion and carbosulfan treated animals. In addition, the results showed that MAL significantly increased TCD4+ and TCD8+ lymphocyte number. It also decreased INF-δ and IL-4 production. However, CB induced a reduction of TCD8+ number and cytokine production in spleen cells. In conclusion, malathion and carbosulfan had significant immunomodulatory properties in the spleen with inflammation and oxidative stress induction in the liver.

Toxicological evaluation of acute and sub-chronic ingestion of hydroalcoholic extract of Solanum cernuum Vell. in mice

ETHNOPHARMACOLOGICAL RELEVANCE

Solanum cernuum Vellozo is a Brazilian shrub or small tree, restricted to Southwest states of the country. It has been widely used for the treatment of many ailments. The pharmacological activity of the extract on gastric ulcer has been the major therapeutic target proposed by the population investigated.

MATERIALS AND METHODS

In the acute toxicity test was used increasing doses of the extract (2, 4, 8, 12, 16, 20 and 25 g of extract per kilogram of body weight). The animal behavior was observed from 5h after a single administration of the extract and subsequently monitored daily until the fourteenth day, beyond the calculation of the estimated LD50 of the extract. In the test sub-chronic toxicity was used two doses of the extract (0.1 and 1.4 g/kg) and the parameters analyzed over 31 days were: body weight, food intake, behavior, respiratory rate, movement and mortality of animals. After anesthesia, blood samples were collected for hematological and biochemical analysis. The animals were euthanized followed by macroscopic analysis of the stomach and intestine. Liver, lungs and kidneys were removed, weighed and analyzed histopathologically.

RESULTS

In the acute toxicity test was observed a dose-dependent mortality and the value of estimated LD50 was 14.50 g/kg. In the hematological and biochemical analyses there were significant increase in the activities of AST and ALT indicating liver toxicity, but the extract was not able to alter food intake, body weight and organ weights after 31 days of treatment and it did not produce significant histopathological changes.

CONCLUSION

Therefore we can consider the hydroalcoholic extract of Solanum cernuum Vell as practically non-toxic in acute administration and safe in the sub-chronic administration, as hepatotoxicity was observed only with the highest dose used, not with the dose routinely used by the native population.