Metabolism of carbosulfan II. Human interindividual variability in its in vitro hepatic biotransformation and the identification of the cytochrome P450 isoforms involved

Development of template systems for ligand interactions of CYP3A5 and CYP3A7 and their distinctions from CYP3A4 template

Starting from established CYP3A4 Template (DMPK. 2019, and 2020), CYP3A5 and CYP3A7 Templates have been constructed to be reliable tools for verification of their distinct catalytic properties. A distinct occupancy was observed on CYP3A4-selective ligands, but not on the non-selective ligands, in simulation experiments. These ligands often invade into Bay-1 region during the migration from Entrance to Site of oxidation in simulation experiments. These results offered an idea of the distinct localization of Bay-1 residue on CYP3A5 Template, in which the Bay-1 residue stayed closely to Template border. The idea also accounted for the higher oxidation rates of CYP3A5, than of CYP3A4, of noscapine and schisantherin E through their enhanced sitting-stabilization. Typical CYP3A7 substrates such as zonisamide and retinoic acids took their placements without occupying a left side region of Template for their metabolisms. In turn, the occupancies of the left-side region were inevitably observed among poor ligands of CYP3A7. Altered extent of IJK-Interaction or localization of a specific residue at the left-side would thus explain distinct catalytic properties of CYP3A7 on Template. These data suggest the alteration of each one of Template region, from CYP3A4 Template, led to the distinct catalytic properties of CYP3A5 and CYP3A7 forms.

Identification of the cytochrome P450 enzymes involved in the oxidative metabolism of trantinterol using ultra high-performance liquid chromatography coupled with tandem mass spectrometry

Trantinterol is a novel β₂-adrenoceptor agonist used for the treatment of asthma. This study aimed to identify the cytochrome P450 enzymes responsible for the metabolism of trantinterol to form 4-hydroxylamine trantinterol (M1) and tert-butyl hydroxylated trantinterol (M2), which was achieved using the chemical inhibition study, followed by the metabolism study of trantinterol in a panel of recombinant CYPs, as well as the kinetic study with the appropriate cDNA-expressed P450 enzymes. A highly selective and sensitive ultra high-performance liquid chromatography tandem mass spectrometry method was developed and validated for the simultaneous determination of M1 and M2. The inhibition study suggested that CYP2C19 and CYP3A4/5 were involved in the formation of M1 and M2, and CYP2D6 only contributed to the formation of M1. Assays with cDNA-expressed CYP enzymes further showed that the relative contributions of P450 isoforms were 2C19 > 3A4 > 2D6 > 2E1 for the formation of M1, and 3A4 > 2C19 > 2D6 for the formation of M2. The enzyme kinetic analysis was then performed in CYP2C19, CYP2D6 and CYP3A4. The kinetic parameters were determined and normalized with respect to the human hepatic microsomal P450 isoform concentrations. All the results support the conclusion that CYP3A4 and CYP2C19 are the major enzymes responsible for formation of M1 and M2, while CYP2D6 and CYP2E1 also engaged to a lesser degree. The results imply that potential drug–drug interactions may be noticed when trantinterol is used with CYP2C19 and CYP3A4 inducers or inhibitors, and we should pay attention to this phenomenon in clinical study.

The first report on the characterization of the main CYP450 enzymes and the kinetic study involved in trantinterol metabolism.

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.

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.

High lethality and minimal variation after acute self-poisoning with carbamate insecticides in Sri Lanka - implications for global suicide prevention

Background: Highly hazardous organophosphorus (OP) insecticides are responsible for most pesticide poisoning deaths. As they are removed from agricultural practice, they are often replaced by carbamate insecticides of perceived lower toxicity. However, relatively little is known about poisoning with these insecticides.

Methods: We prospectively studied 1288 patients self-poisoned with carbamate insecticides admitted to six Sri Lankan hospitals. Clinical outcomes were recorded for each patient and plasma carbamate concentration measured in a sample to confirm the carbamate ingested.

Findings: Patients had ingested 3% carbofuran powder (719), carbosulfan EC25 liquid (25% w/v, 389), or fenobucarb EC50 liquid (50% w/v, 127) formulations, carbamate insecticides of WHO Toxicity Classes Ib, II, and II, respectively. Intubation and ventilation was required for 183 (14.2%) patients while 71 (5.5%) died. Compared with carbofuran, poisoning with carbosulfan or fenobucarb was associated with significantly higher risk of death [carbofuran 2.2%; carbosulfan 11.1%, OR 5.5 (95% CI 3.0–9.8); fenobucarb 6.3%, OR 3.0 (1.2–7.1)] and intubation [carbofuran 6.1%; carbosulfan 27.0%, OR 5.7 (3.9–8.3); fenobucarb 18.9%, OR 3.6 (2.1–6.1)]. The clinical presentation and cause of death did not differ markedly between carbamates. Median time to death was similar: carbofuran 42.3 h (IQR 5.5–67.3), carbosulfan 21.3 h (11.5–71.3), and fenobucarb 25.3 h (17.3–72.1) (p = 0.99); no patients showed delayed onset of toxicity akin to the intermediate syndrome seen after OP insecticide poisoning. For survivors, median duration of intubation was 67.8 h (IQR 27.5–118.8) with no difference in duration between carbamates. Reduced GCS at presentation was associated with worse outcome although some patients with carbosulfan died after presentation with normal GCS.

Conclusions: We did not find carbamate insecticide self-poisoning to vary markedly according to the carbamate ingested although the case fatality varied according to the concentration and formulation of the insecticide. Carbamate poisoning did not appear to be much less toxic than poisoning with some liquid OP insecticide formulations, e.g., chlorpyrifos EC40, that we have previously noted in these same hospitals (Lancet 2005, 366:1452–1459; QJM 2006, 99:513–522). Replacement of WHO Class II Toxicity OP insecticides in agriculture with high-strength liquid carbamate formulations may not substantially reduce case fatality after pesticide poisoning and, therefore, global suicide rates.

A Workflow to Investigate Exposure and Pharmacokinetic Influences on High-Throughput in Vitro Chemical Screening Based on Adverse Outcome Pathways

BACKGROUND

Adverse outcome pathways (AOPs) link adverse effects in individuals or populations to a molecular initiating event (MIE) that can be quantified using in vitro methods. Practical application of AOPs in chemical-specific risk assessment requires incorporation of knowledge on exposure, along with absorption, distribution, metabolism, and excretion (ADME) properties of chemicals.

OBJECTIVES

We developed a conceptual workflow to examine exposure and ADME properties in relation to an MIE. The utility of this workflow was evaluated using a previously established AOP, acetylcholinesterase (AChE) inhibition.

METHODS

Thirty chemicals found to inhibit human AChE in the ToxCast™ assay were examined with respect to their exposure, absorption potential, and ability to cross the blood-brain barrier (BBB). Structures of active chemicals were compared against structures of 1,029 inactive chemicals to detect possible parent compounds that might have active metabolites.

RESULTS

Application of the workflow screened 10 "low-priority" chemicals of 30 active chemicals. Fifty-two of the 1,029 inactive chemicals exhibited a similarity threshold of ≥ 75% with their nearest active neighbors. Of these 52 compounds, 30 were excluded due to poor absorption or distribution. The remaining 22 compounds may inhibit AChE in vivo either directly or as a result of metabolic activation.

CONCLUSIONS

The incorporation of exposure and ADME properties into the conceptual workflow eliminated 10 "low-priority" chemicals that may otherwise have undergone additional, resource-consuming analyses. Our workflow also increased confidence in interpretation of in vitro results by identifying possible "false negatives."

CITATION

Phillips MB, Leonard JA, Grulke CM, Chang DT, Edwards SW, Brooks R, Goldsmith MR, El-Masri H, Tan YM. 2016. A workflow to investigate exposure and pharmacokinetic influences on high-throughput in vitro chemical screening based on adverse outcome pathways. Environ Health Perspect 124:53-60; http://dx.doi.org/10.1289/ehp.1409450.

Metabolic interactions of environmental toxicants in humans

A description of the interactions between environmental toxicants following simultaneous exposure or exposure in close temporal sequence is presented. At the metabolic level, such interactions may be based on induction, inhibition, or activation of xenobiotic-metabolizing enzymes. Cytotoxicity may also play a role, particularly in affecting induction of xenobiotic-metabolizing enzymes. The effects of interactions manifested at the level of the expression of toxic endpoints may result from interactions at the metabolic level or may have other causes. New approaches to genome-wide effects (e.g., microarray studies) are also discussed.

Environmental exposure to organophosphate pesticides: assessment of endocrine disruption and hepatotoxicity in pregnant women

In utero exposure is the first point of contact with environmental xenobiotics that may affect the maternal-placental-fetal balance. Considering that maternal pathophysiological changes affect intrauterine development, this pilot study was conducted to address how environmental exposure to organophosphate pesticides (OPs) during pregnancy may contribute to maternal endocrine disruption and disturbed hepatic function. A prospective study was carried out with pregnant women (n=97) living in a rural area of the Rio Negro province where OPs are intensively applied throughout 6 months of the year. Blood samples were obtained and biomarkers of OPs exposure (cholinesterases and β-glucuronidase), cortisol (CT) and progesterone (PG) levels, as well as glycemia, were determined. Parameters of liver injury were assayed by measuring aspartate aminotransferase (AST) and alanine aminotransferase (ALT); liver function was assayed by measuring albumin. Biomonitoring carried out during the pre-spraying period (PreS) and spraying period (SP) showed that the population studied was exposed to OPs, proven by the fact that plasma (PCh) and erythrocyte cholinesterase (AChE) decreased very significantly (p<0.01) during SP. CT values increased very significantly (p<0.01) in the first trimester of pregnancy during SP with respect to PreS. Individual values above the upper limit of the CT and PG reference range were found both in PreS and SP. This finding could be associated with changes in hormone metabolism pathways produced by OPs exposure. During the second trimester of pregnancy there were increases in ALT values and the AST/ALT ratio in SP, suggesting subclinical hepatotoxicity. In SP, glycemia was unchanged while albuminemia increased. Although anthropometric newborn parameters and pregnancy alterations were within normal values for the general population, the increase in CT in the maternal compartment may lead to impaired newborn health later in life.

The inhibition of major human hepatic cytochrome P450 enzymes by 18 pesticides: comparison of the N-in-one and single substrate approaches

In the present study on human hepatic microsomes, the N-in-one assay with ten probe substrates for nine cytochrome-P450 enzymes (CYPs) was compared with the single substrate assays to investigate pesticides-CYP interactions. CYP inhibition was measured by liquid chromatography-tandem mass spectrometry (LC/MS-MS). As illustrated by the initial screening at 100 μM concentration of 18 pesticides, CYPs are more sensitive to organophosphates (OPs) than to other pesticide groups. Chlorpyrifos and fenitrothion were most effective in inhibiting CYP1A1/2, and CYP2B6. Profenofos was also inhibitory towards multiple CYPs. Pyrethroids, e.g. deltamethrin, fenvalerate and lambda-cyhalothrin, potently inhibited CYP2D6. CYP3A4 activity was moderately inhibited by fenvalerate and potently by alpha-cypermethrin. The correlations between IC50 values obtained from the N-in-one and single substrate approaches were highly significant for CYP2Cs (r(2)=0.94), CYP3A4, omeprazole-sulfoxidation, (r(2)=0.89), followed by CYP1A2 and CYP2B6 (r(2)=0.82), and CYP2D6 (r(2)=0.80). In contrast no correlation was observed with CYP2E1 and CYP3A4 (midazolam-1'-hydroxylation). The N-in-one screening assay seems useful and reliable for most CYP activities when a comprehensive and quick evaluation of potential interactions with CYPs is needed. However, at the present moment, it does not enable discrimination on the basis of mechanism of inhibition. A strict comparison between single and N-in-one assays is a prerequisite for more extensive routine use.

Regulation of cytochrome P4501A by protein kinase C: the role of heat shock protein70

Carbofuran is a pesticide, which is used throughout the world as a nematicide and an acaricide. This pesticide integrates into living organisms through aquatic ecosystem. In earlier report, we had demonstrated that cytochrome P4501A was induced in cultured catfish hepatocytes in response to carbofuran, which might be responsible for the detoxification of this pesticide. As the underlying signaling mechanism associated with induction and regulation of cytochrome P4501A has not yet been well defined, we therefore in the present study have investigated to identify the regulatory network of cytochrome P4501A in catfish liver or cultured hepatocytes by targeting several key signaling molecules such as phosphatidyl inositol (PI) or protein kinase C (PKC), which are critical molecules for many important pathways. PKC and heat shock protein70 (HSP70) have been shown to be induced in response to carbofuran in catfish hepatocytes. Results also indicate that induction of CYP1A is modulated by HSP70 and PKC in fish hepatocytes. Thus our data shed light on the regulation of EROD activity, which has been used as a bio-monitoring tool for measuring aquatic pollution.