Walid Maho

First insights in the metabolism of phosphate flame retardants and plasticizers using human liver fractions

Phosphate flame retardants and plasticizers (PFRs) are additives used in a wide range of polymers. Important representatives, such as tris(2-butoxyethyl) phosphate (TBOEP), triphenyl phosphate (TPHP), tris(2-chloroethyl) phosphate (TCEP), tris(1-chloro-2-propyl) phosphate (TCIPP), tris(1,3-dichloro-2-propyl) phosphate (TDCIPP), have been found in the indoor environment at high levels. Biotransformation of these PFRs needs to be investigated because it can be a major determinant of their bioavailability and toxicity in humans. TBOEP, TPHP, TCEP, TCIPP and TDCIPP were incubated with human liver S9 fraction and microsomes. Supernatants were analyzed using a liquid chromatography coupled to a quadrupole-time-of-flight mass spectrometer. Chromatograms were scanned for the presence of Phase-I and Phase-II metabolites and tentatively identified based on mass accuracy of the molecular formula, isotopic pattern, and MS/MS spectra. The two major metabolites of TBOEP were products of O-dealkylation and of hydroxylation, respectively. TPHP was mainly transformed to its diester metabolite by O-dearylation and to a hydroxylated metabolite. TCEP was poorly metabolized into its diester and a product of oxidative dehalogenation. The major metabolite of TCIPP was a product of oxidative dehalogenation. TDCIPP was mainly transformed into its diester and a glutathione S-conjugate. The metabolites identified in the present study are candidate biomarkers for future human biomonitoring studies.

Application of a sewage-based approach to assess the use of ten illicit drugs in four Chinese megacities.

Sewage-based epidemiology was applied for the first time to a number of mainland Chinese megacities. The application monitored influents to 9 sewage treatment plants (STPs) to estimate the use of illicit drugs in Beijing, Guangzhou, Shenzhen, and Shanghai. Altogether, 11.4 million inhabitants were covered during September-October 2012. 24-h composite raw sewage samples were collected for 4 consecutive days at each STP. Each collected sample was analyzed for cocaine, benzoylecgonine, ecgonine methylester, methadone, 2-ethylidene-1,5-dimethyl-3,3-diphenylpyrrolidine, 6-monoacetylmorphine, amphetamine, methamphetamine, ecstasy, mephedrone, methylenedioxypyrovalerone, 11-nor-9-carboxy-delta-9-tetrahydrocannabinol, ketamine, and norketamine. Through the analysis of these chemical residues, the use of amphetamine, cannabis, cocaine, ecstasy, heroin, mephedrone, methadone, methamphetamine, methylenedioxypyrovalerone and ketamine among Chinese urban inhabitants was monitored. The results obtained demonstrated in a quantitative way that the drug use patterns of Chinese are different from their European counterparts. Abuse of methamphetamine and ketamine was particularly noteworthy in China, while consumption of cocaine and ecstasy, the most popular drugs in Europe, was very low among the sampled Chinese inhabitants. Further, the use of most drugs demonstrated a geographical trend, since their use was much higher in the southern cities of Shenzhen and Guangzhou than it was in Beijing and Shanghai. Interestingly, the exclusive, but minor, metabolite of heroin, 6-monoacetylmorphine, was detected only sporadically. This would suggest that the use of heroin among Chinese urban users sampled in the study was low. Further, the patterns of drug use observed during the study are largely consistent with trends reported by the United Nations Office on Drugs and Crime. Overall, our study suggests that sewage-based epidemiology can readily be used to monitor the use of illicit drugs in those countries/regions where traditional means to monitor drug use patterns have only yielded limited or information of questionable reliability.

Sewage-based epidemiology in monitoring the use of new psychoactive substances: Validation and application of an analytical method using LC-MS/MS.

Sewage-based epidemiology (SBE) employs the analysis of sewage to detect and quantify drug use within a community. While SBE has been applied repeatedly for the estimation of classical illicit drugs, only few studies investigated new psychoactive substances (NPS). These compounds mimic effects of illicit drugs by introducing slight modifications to chemical structures of controlled illicit drugs. We describe the optimization, validation, and application of an analytical method using liquid chromatography coupled to positive electrospray tandem mass spectrometry (LC-ESI-MS/MS) for the determination of seven NPS in sewage: methoxetamine (MXE), butylone, ethylone, methylone, methiopropamine (MPA), 4-methoxymethamphetamine (PMMA), and 4-methoxyamphetamine (PMA). Sample preparation was performed using solid-phase extraction (SPE) with Oasis MCX cartridges. The LC separation was done with a HILIC (150 x 3 mm, 5 µm) column which ensured good resolution of the analytes with a total run time of 19 min. The lower limit of quantification (LLOQ) was between 0.5 and 5 ng/L for all compounds. The method was validated by evaluating the following parameters: sensitivity, selectivity, linearity, accuracy, precision, recoveries and matrix effects. The method was applied on sewage samples collected from sewage treatment plants in Belgium and Switzerland in which all investigated compounds were detected, except MPA and PMA. Furthermore, a consistent presence of MXE has been observed in most of the sewage samples at levels higher than LLOQ.

In vitro biotransformation of tris(2-butoxyethyl) phosphate (TBOEP) in human liver and serum.

Tris(2-butoxyethyl) phosphate (TBOEP) is a plasticizer present in indoor dust, reaching levels of several micrograms per gram. Such levels could lead to significant daily exposure of adults and children. Currently, no toxicokinetic data are available to estimate TBOEP clearance in humans after uptake and therefore, one objective of this study was to investigate intrinsic clearance of TBOEP by human liver microsome (HLM) and serum enzymes. Another objective was to generate information to identify and prioritize several metabolites of TBOEP for investigation of human exposure by biomonitoring. 1D and 2D-NMR methodologies were successfully applied on a mixture of the metabolites to confirm the structure of 3-HO-TBOEP (bis(2-butoxyethyl) 3-hydroxyl-2-butoxyethyl phosphate) and to tentatively assign structures to 1-HO-TBOEP and 2-HO-TBOEP. HO-TBOEP isomers and bis(2-butoxyethyl) phosphate (BBOEP), bis(2-butoxyethyl) hydroxyethyl phosphate (BBOEHEP) were further monitored by liquid chromatography-tandem mass spectrometry. Rates of formation of BBOEHEP and HO-TBOEP metabolites by liver enzymes were best described by the Michaelis-Menten model. Apparent Km values for BBOEHEP, 3-HO-TBOEP, and sum of 1- and 2-HO-TBOEP isomer formation were 152, 197 and 148μM, respectively. Apparent Vmax values for the formation of BBOEHEP, 3-HO-TBOEP, and the sum of 1- and 2-HO-TBOEP isomers were 2560, 643, and 254pmol/min/mg protein, respectively. No detectable formation of BBOEP occurred with liver or serum enzymes. Our findings indicate that intrinsic clearance of TBOEP is mainly catalyzed by oxidative enzymes in the liver and that its major in vitro metabolite is BBOEHEP. These findings can be applied in human biomonitoring studies and risk assessment.

A high throughput passive dosing format for the Fish Embryo Acute Toxicity test.

High throughput testing according to the Fish Embryo Acute Toxicity (FET) test (OECD Testing Guideline 236) is usually conducted in well plates. In the case of hydrophobic test substances, sorptive and evaporative losses often result in declining and poorly controlled exposure conditions. Therefore, our objective was to improve exposure conditions in FET tests by evaluating a passive dosing format using silicone O-rings in standard 24-well polystyrene plates. We exposed zebrafish embryos to a series of phenanthrene concentrations until 120h post fertilization (hpf), and obtained a linear dilution series. We report effect values for both mortality and sublethal morphological effects based on (1) measured exposure concentrations, (2) (lipid normalized) body residues and (3) chemical activity. The LC50 for 120hpf was 310μg/L, CBR50 (critical body residue) was 2.72mmol/kg fresh wt and La50 (lethal chemical activity) was 0.047. All values were within ranges expected for baseline toxicity. Impaired swim bladder inflation was the most pronounced morphological effect and swimming activity was reduced in all exposure concentrations. Further analysis showed that the effect on swimming activity was not attributed to impaired swim bladder inflation, but rather to baseline toxicity. We conclude that silicone O-rings (1) produce a linear dilution series of phenanthrene in the 120hpf FET test, (2) generate and maintain aqueous concentrations for reliable determination of effect concentrations, and allow for obtaining mechanistic toxicity information, and (3) cause no toxicity, demonstrating its potential as an extension of the FET test when testing hydrophobic chemicals.

Impaired anterior swim bladder inflation following exposure to the thyroid peroxidase inhibitor 2-mercaptobenzothiazole part I: Fathead minnow

Disruption of the thyroid hormone (TH) system, an important mode of action, can lead to ecologically relevant adverse outcomes, especially during embryonic development. The present study characterizes the effects of disruption of TH synthesis on swim bladder inflation during zebrafish early-life stages using 2-mercaptobenzothiazole (MBT), a thyroid peroxidase (TPO) inhibitor. Zebrafish were exposed to different MBT concentrations until 120/168h post fertilization (hpf) and 32days post fertilization (dpf), in two sets of experiments, to investigate the effects of TPO inhibition on posterior and anterior swim bladder inflation respectively, as well as whole body thyroid hormone concentrations (triiodothyronine (T3) and its prohormone, thyroxine (T4)). At 120hpf, MBT did not directly impair posterior chamber inflation or size, while anterior chamber inflation and size was impaired at 32dpf. As previously shown in amphibians and mammals, we confirmed that MBT inhibits TPO in fish. Whole-body T4 decreased after MBT exposure at both time points, while T3 levels were unaltered. There was a significant relationship between T4 levels and the anterior chamber surface at 32dpf. The absence of effects on posterior chamber inflation can possibly be explained by maternal transfer of T4 into the eggs. These maternally derived THs are depleted at 32dpf and cannot offset TPO inhibition, resulting in impaired anterior chamber inflation. Therefore, we hypothesize that TPO inhibition only inhibits swim bladder inflation during late development, after depletion of maternally derived T4. In a previous study, we showed that iodothyronine deiodinase (ID) knockdown impaired posterior chamber inflation during early development. Our findings, in parallel with similar effects observed in fathead minnow (see part I, this issue) suggest that thyroid disruption impacts swim bladder inflation, and imply an important distinction among specific subtypes of TH disrupting chemicals. However, the existence of another - yet unknown - mode of action of MBT impacting swim bladder inflation cannot be excluded. These results can be helpful for delineating adverse outcome pathways (AOPs) linking TPO inhibition, ID inhibition and other TH related molecular initiating events, to impaired swim bladder inflation in fish during early life stages. Such AOPs can support the use of in vitro enzyme inhibition assays for predicting reduced survival due to impaired posterior and anterior chamber inflation.

Potential impact of sitagliptin on collagen-derived dipeptides in diabetic osteoporosis

It is known that diabetes coincides with an increased risk of osteoporosis. While a disturbed collagen metabolism is proposed as a possible cause, much remains unknown about the enzymes involved and changes in the collagen-derived dipeptides and amino acids. Therefore, we sought to study this intricate pathway and the effect of dipeptidyl peptidase 4 (DPP4) inhibitors. Control and streptozotocin-nicotinamide-induced diabetic rats were treated for 12 weeks with vehicle or sitagliptin, a DPP4 inhibitor (Con/VH, Con/SG, DM/VH and DM/SG). The activities of four key enzymes involved in collagen breakdown were determined in serum (DPP4, matrix metalloproteinase 2 and 9 and prolidase). Dipeptide (Ala-Pro, Gly-Pro, Pro-Pro and Pro-Hyp) and amino acid (Pro and Hyp) concentrations were measured by liquid chromatography coupled to mass spectrometry. We found three-fold higher MMP9 activities in DM/VH than in controls, while in DM/SG this rise was attenuated. MMP2 and prolidase did not differ in the investigated groups. Furthermore, we are the first to report on two-fold higher Ala-Pro and Pro-Pro levels in diabetes compared to controls. In contrast, Pro-Hyp concentrations were lower in diabetes (DM/VH and DM/SG). DPP4 inhibition does not seem to have a direct influence on the collagen metabolism in streptozotocin-nicotinamide-induced diabetic rats. Instead, it probably acts through its effect on osteoprotective substrates. In diabetes, increased MMP9 activities seem to favour the production of Ala-Pro and Pro-Pro containing collagen fragments. The high Pro-Hyp levels in untreated controls might have a bone-stimulating effect. Nevertheless, the biological significance of these dipeptides is not yet clear and should be further investigated.

Phenotypic and biomarker evaluation of zebrafish larvae as an alternative model to predict mammalian hepatotoxicity

Zebrafish phenotypic assays have shown promise to assess human hepatotoxicity, though scoring of liver morphology remains subjective and difficult to standardize. Liver toxicity in zebrafish larvae at 5 days was assessed using gene expression as the biomarker approach, complementary to phenotypic analysis and analytical data on compound uptake. This approach aimed to contribute to improved hepatotoxicity prediction, with the goal of identifying biomarker(s) as a step towards the development of transgenic models for prioritization. Morphological effects of hepatotoxic compounds (acetaminophen, amiodarone, coumarin, methapyrilene and myclobutanil) and saccharin as the negative control were assessed after exposure in zebrafish larvae. The hepatotoxic compounds induced the expected zebrafish liver degeneration or changes in size, whereas saccharin did not have any phenotypic adverse effect. Analytical methods based on liquid chromatography–mass spectrometry were optimized to measure stability of selected compounds in exposure medium and internal concentration in larvae. All compounds were stable, except amiodarone for which precipitation was observed. There was a wide variation between the levels of compound in the zebrafish larvae with a higher uptake of amiodarone, methapyrilene and myclobutanil. Detection of hepatocyte markers (CP, CYP3A65, GC and TF) was accomplished by in situ hybridization of larvae to coumarin and myclobutanil and confirmed by real‐time reverse transcription–quantitative polymerase chain reaction. Experiments showed decreased expression of all markers. Next, other liver‐specific biomarkers (i.e. FABP10a and NR1H4) and apoptosis (i.e. CASP‐3 A and TP53) or cytochrome P450‐related (CYP2K19) and oxidoreductase activity‐related (ZGC163022) genes, were screened. Links between basic mechanisms of liver injury and results of biomarker responses are described. Copyright

Biotransformation of three phosphate flame retardants and plasticizers in primary human hepatocytes: untargeted metabolite screening and quantitative assessment

Tris(2‐butoxyethyl) phosphate (TBOEP), triphenyl phosphate (TPHP) and tris(1‐chloro‐2‐propyl) phosphate (TCIPP) are current high‐volume organophosphate flame retardants/plasticizers (PFRs) and are abundant in the indoor environment. While recent in vitro research has indicated potential toxic effects in the endocrine system, biotransformation of these compounds is still underexplored. In this study, we aimed to characterize the metabolite formation for three PFRs in primary human hepatocytes, an in vitro system that mimics in vivo liver metabolism more closely than hepatic subcellular fractions or cell lines. Cryopreserved human hepatocytes were thawed and suspended in media with 50 μm TBOEP or TCIPP, or 20 μm TPHP up to 2 h. Extracts were analyzed by liquid chromatography–quadrupole‐time‐of‐flight‐mass spectrometry. Quantification of biotransformation products in hepatocytes exposed for 2 h revealed that bis(1‐chloro‐2‐propyl) phosphate and diphenyl phosphate corresponded to less than half of the depletion of TCIPP and TPHP, respectively, while bis(2‐butoxyethyl) 2‐hydroxyethyl phosphate compared to 40–66% of the depletion of TBOEP. Other metabolite structures of these PFRs were produced at 4‐ to 10‐fold lower rates. These findings help interpret biological levels of the major metabolites and relate it to levels of their parent PFR. Percentage of substrate depletion was largest for TBOEP followed by comparable values for TPHP and TCIPP, indicating that hepatic clearance of TPHP and TCIPP would be slower than that of TBOEP. The resulting higher levels and longer presence of TPHP in the circulation after exposure, would allow TPHP a larger time window to exert its suspected adverse effects compared to TBOEP. Copyright