Ethylene oxide derived glycol ethers: A review of the alkyl glycol ethers potential to cause endocrine disruption.

The 'ethylene glycol ethers' (EGE) are a broad family of solvents and hydraulic fluids produced through the reaction of ethylene oxide and a monoalcohol. Certain EGE derived from methanol and ethanol are well known to cause toxicity to the testes and fetotoxicity and that this is caused by the common metabolites methoxy and ethoxyacetic acid, respectively. There have been numerous published claims that EGE fall into the category of 'endocrine disruptors' often without substantiated evidence. This review systematically evaluates all of the available and relevant in vitro and in vivo data across this family of substances using an approach based around the EFSA/ECHA 2018 guidance for the identification of endocrine disruptors. The conclusion reached is that there is no significant evidence to show that EGE target any endocrine organs or perturb endocrine pathways and that any toxicity that is seen occurs by non-endocrine modes of action.

Physiologically based pharmacokinetic model for ethyl tertiary-butyl ether and tertiary-butyl alcohol in rats: Contribution of binding to α2u-globulin in male rats and high-exposure nonlinear kinetics to toxicity and cancer outcomes.

In cancer bioassays, inhalation, but not drinking water exposure to ethyl tertiary-butyl ether (ETBE), caused liver tumors in male rats, while tertiary-butyl alcohol (TBA), an ETBE metabolite, caused kidney tumors in male rats following exposure via drinking water. To understand the contribution of ETBE and TBA kinetics under varying exposure scenarios to these tumor responses, a physiologically based pharmacokinetic model was developed based on a previously published model for methyl tertiary-butyl ether, a structurally similar chemical, and verified against the literature and study report data. The model included ETBE and TBA binding to the male rat-specific protein α2u-globulin, which plays a role in the ETBE and TBA kidney response observed in male rats. Metabolism of ETBE and TBA was described as a single, saturable pathway in the liver. The model predicted similar kidney AUC0-∞ for TBA for various exposure scenarios from ETBE and TBA cancer bioassays, supporting a male-rat-specific mode of action for TBA-induced kidney tumors. The model also predicted nonlinear kinetics at ETBE inhalation exposure concentrations above ~2000 ppm, based on blood AUC0-∞ for ETBE and TBA. The shift from linear to nonlinear kinetics at exposure concentrations below the concentration associated with liver tumors in rats (5000 ppm) suggests the mode of action for liver tumors operates under nonlinear kinetics following chronic exposure and is not relevant for assessing human risk. Copyright © 2016 The Authors Journal of Applied Toxicology Published by John Wiley & Sons Ltd.

Modeling bioaccumulation and biomagnification of nonylphenol and its ethoxylates in estuarine-marine food chains.

There are several studies on bioaccumulation and biomagnification of nonylphenol (NP) and its ethoxylates (NPEOs), but their toxico-kinetic mechanisms remain unclear. In the present investigation, we explored the accumulation of NP and NPEOs in estuarine-marine food chains with a bioaccumulation model comprising five trophic levels. Using this model, we estimated uptake and elimination rate constants for NPEOs based on the organisms' weight and lipid content and the chemicals' Kow. Further, we calculated accumulation factors for NP and NPEOs, including biota-sediment accumulation factors (BSAF) and biomagnification factors (BMF), and compared these to independent field measurements collected in the Western Scheldt estuary in The Netherlands and field data reported in the literature. The estimated BSAF values for NP and total NPEOs were below 1 for all trophic levels. The estimated BMF values were around 1 for all trophic levels except for the highest level (carnivorous mammals and birds). For this trophic level, the estimated BMF value varied between 0.1 and 2.4, depending on the biotransformation capacity. For all trophic levels, except primary producers, the accumulation estimates that accounted for biotransformation of NPEOs into NP were closer to the field data than model estimates that did not include biotransformation, indicating that NP formation by biotransformation of NPEOs might occur in organisms.

Subchronic exposure to ethyl tertiary butyl ether resulting in genetic damage in Aldh2 knockout mice.

Ethyl tertiary butyl ether (ETBE) is biofuel additive recently used in Japan and some other countries. Limited evidence shows that ETBE has low toxicity. Acetaldehyde (AA), however, as one primary metabolite of ETBE, is clearly genotoxic and has been considered to be a potential carcinogen. The aim of this study was to evaluate the effects of ALDH2 gene on ETBE-induced genotoxicity and metabolism of its metabolites after inhalation exposure to ETBE. A group of wild-type (WT) and Aldh2 knockout (KO) C57BL/6 mice were exposed to 500ppm ETBE for 1-6h, and the blood concentrations of ETBE metabolites, including AA, tert-butyl alcohol and 2-methyl-1,2-propanediol, were measured. Another group of mice of WT and KO were exposed to 0, 500, 1750, or 5000ppm ETBE for 6h/day with 5 days per weeks for 13 weeks. Genotoxic effects of ETBE in these mice were measured by the alkaline comet assay, 8-hydroxyguanine DNA-glycosylase modified comet assay and micronucleus test. With short-term exposure to ETBE, the blood concentrations of all the three metabolites in KO mice were significantly higher than the corresponding concentrations of those in WT mice of both sexes. After subchronic exposure to ETBE, there was significant increase in DNA damage in a dose-dependent manner in KO male mice, while only 5000ppm exposure significantly increased DNA damage in male WT mice. Overall, there was a significant sex difference in genetic damage in both genetic types of mice. These results showed that ALDH2 is involved in the detoxification of ETBE and lack of enzyme activity may greatly increase the sensitivity to the genotoxic effects of ETBE, and male mice were more sensitive than females.

Toxicokinetics of bis(2-chloroethoxy)methane following intravenous administration and dermal application in male and female F344/N rats and B6C3F1 mice.

In the National Toxicology Program's toxicity studies, rats were more sensitive than mice to Bis(2-chloroethoxy)methane (CEM) - induced cardiac toxicity following dermal application to male and female F344/N rats and B6C3F1 mice. Thiodiglycolic acid (TDGA) is a major metabolite of CEM in rats. It has been implicated that chemicals metabolized to TDGA cause cardiac toxicity in humans. Therefore, the toxicokinetics of CEM and TDGA were investigated in male and female F344/N rats and B6C3F1 mice following a single intravenous administration or dermal application of CEM to aid in the interpretation of the toxicity data. Absorption of CEM following dermal application was rapid in both species and genders. Bioavailability following dermal application was low but was higher in rats than in mice with females of both species showing higher bioavailability than males. CEM was rapidly distributed to the heart, thymus, and liver following both routes of administration. Plasma CEM C(max) and AUC(∞) increased proportionally with dose, although at the dermal dose of 400mg/kg in rats and 600mg/kg in mice non-linear kinetics were apparent. Following dermal application, dose-normalized plasma CEM C(max) and AUC(∞) was significantly higher in rats than in mice (p-value<0.0001 for all comparisons except for C(max) in the highest dose groups where p-value=0.053). In rats, dose-normalized plasma CEM C(max) and AUC(∞) was higher in females than in males: however, the difference was significant only at the lowest dose (p-value=0.009 for C(max) and 0.056 for AUC(∞)). Similar to rats, female mice also showed higher C(max) and AUC(∞) in females than in male: the difference was significant only for C(max) at the lowest dose (p-value=0.002). Dose-normalized heart CEM C(max) was higher in rats than in mice and in females than their male counterparts. The liver CEM C(max) was lower compared to that of heart and thymus in both rats and mice following intravenous administration and in rats following dermal application. This is likely due to the rapid metabolism of CEM in the liver as evidenced by the high concentration of TDGA measured in the liver. Dose-normalized plasma and heart TDGA C(max) values were higher in rats compared to mice. In rats, females had higher plasma and heart TDGA C(max) than males; however, there was no gender difference in plasma or heart TDGA C(max) in mice. These findings support the increased sensitivity of rats compared to mice to CEM-induced cardiac toxicity. Data also suggest that, either CEM C(max) or AUC can be used to predict the CEM-induced cardiac toxicity. Although, both plasma and heart TDGA C(max) was consistent with the observed species difference and the gender difference in rats, the gender difference in mice to cardiac toxicity could not be explained based on the TDGA data. This animal study suggests that toxicologically significant concentrations of CEM and TDGA could possibly be achieved in the systemic circulation and/or target tissues in humans as a result of dermal exposure to CEM.

A phase 1 dose-escalation study: safety, tolerability, and pharmacokinetics of FBS0701, a novel oral iron chelator for the treatment of transfusional iron overload.

BACKGROUND: There is still a clinical need for a well-tolerated and safe iron chelator for the treatment of transfusional iron overload. We describe the pharmacokinetic properties and safety data after 7 days of dosing of FBS0701, a novel oral, once-daily iron chelator. DESIGN AND METHODS: This phase 1b dose-escalation study to assess the safety, tolerability, pharmacokinetics and pharmacodynamics of FBS0701, a novel oral iron chelator for the treatment of transfusional iron overload, was conducted in 16 adult patients with iron overloaded consequent to transfusions. FBS0701 was given daily for 7 days at doses up to 32 mg/kg and was well tolerated at all dose levels. RESULTS: Pharmacokinetics showed dose-proportionality. The maxium plasma concentration (C(max)) was reached within 60-90 minutes of dosing and the drug was rapidly distributed at the predicted therapeutic doses. The plasma elimination half-life (t(1/2)) was approximately 19 hours. There were no serious adverse events associated with the drug. Conclusions On the basis of these safety and pharmacokinetic data, FBS0701 warrants further clinical evaluation in patients with transfusional iron overload. (Clinicaltrials.gov identifier: NCT01186419).

[14C]bis(2-chloroethoxy)methane: comparative absorption, distribution, metabolism and excretion in rats and mice.

bis(2-Chloroethoxy)methane (BCM) is used primarily as a precursor in the synthesis of polysulfide elastomers. After administration of [(14)C]BCM, radioactivity is readily absorbed from the gastrointestinal tract and moderately absorbed through skin. Following absorption, BCM-derived radioactivity is rapidly distributed to all tissues, rapidly metabolized and excreted primarily in urine. Minimal effects of sex, species or dose in the range studied (0.1-10 mg kg(-1)) were observed on the fate of BCM in rats and mice after all routes of administration. The major metabolite (about 40% of the dose) of BCM in rat was isolated and identified as thiodiglycolic acid (TDGA) indicating that the ether linkage of BCM is cleaved to form 2-chloroethyl fragments that may be further metabolized to 2-chloracetaldehyde, conjugated with glutathione and the latter subsequently metabolized to TDGA. 2-chloroacetaldehyde has also been shown to be cardiotoxic, possibly accounting for BCM cardiotoxicity observed in repeated dose studies.

Profiles of short chain oligomers in roach (Rutilus rutilus) exposed to waterborne polyethoxylated nonylphenols.

Nonylphenol polyethoxylates (NPEOs) are common contaminants in the aquatic environment through their use as industrial surfactants and their discharge in wastewaters and effluents. In order to assess the bioavailability of NPEOs to fish, we studied the fate of two different oligomeric mixtures of NPEOs in adult roach Rutilus rutilus. Fish were exposed to [14C] radiolabelled NPEOs, averaging either 3 ethoxy units (NP(3av)EO) or 7 ethoxy units (NP(7av)EO), over a 4-day period in a flow-through aquarium. A method was developed to extract radioactive residues from soft tissues and analyse their composition by normal phase radio-HPLC and GC-MS. Radioactive residues were detected in all soft tissues of the roach dosed with either of the test mixtures, but the concentration of NPEO residues in each tissue were highest in fish exposed to the NP(3av)EO compared with the NP(7av)EO test mixture. Radioactive concentrations in roach dosed with 14C NP(3av)EO were highest in the bile at a mean concentration+/-S.E. of 62+/-3 microg/g wet weight, whereas concentrations in the liver, kidney and brain tissues ranged between 1 and 2 microg/g and concentrations in other soft tissues were between 0.1 and 0.7 microg/g. Analysis of the radioactive residues in bile of roach exposed to the NPEO test mixtures indicated that they were mainly metabolites including glucuronide conjugates, whereas in muscle, ovary and gill tissues they were a mixture of NP and short chain NPEO oligomers comprising 1-4 ethoxy units. This study suggests that short chain NPEO oligomers are taken up via the gills and accumulate in tissues such as gonads which are sensitive to endocrine disruption, whereas waterborne NPEOs containing more than 4 ethoxymers are not bioavailable to roach.

Urinary methoxyacetic acid as an indicator of occupational exposure to ethylene glycol dimethyl ether.

OBJECTIVE: To investigate whether methoxyacetic acid (MAA) is the metabolite of ethylene glycol dimethyl ether (EGdiME) in humans and whether its metabolite in urine can be used as a biomarker for exposure to EGdiME. METHODS: Workers occupationally exposed to EGdiME, as well as nonexposed controls, were studied. Urine samples were collected from 20 control subjects and, on Friday postshift, from 14 workers. The identification and quantification of the metabolite were performed by gas chromatography/mass spectrometry (GC/MS) and GC/FID, respectively. Air samples were collected on activated charcoal tubes by area sampling with battery-operated pumps. The glycol ether was analyzed by GC/FID. RESULTS: GC/MS clearly showed the metabolite of EGdiME to be MAA. Urinary MAA levels in the control subjects (background levels) were 0.0-0.3 mg/g crea. The levels of urinary MAA in the solvent-exposed workers were significantly (P<0.0001) higher than those in the control subjects. In the eight workers exposed to an average of 0.3 ppm of EGdiME and the six workers exposed to an average of 2.9 ppm, the mean urinary MAA level was 1.08 (range 0.6-1.5) mg/g crea and 9.33 (range 5.7-18.1) mg/g crea, respectively. These results can be explained by differences in the exposure intensity. CONCLUSIONS: Our results suggest that MAA is the metabolite of EGdiME, and that MAA in urine may be used for biological monitoring of EGdiME exposures.

Bis(2-chloroethoxy)methane-induced mitochondrial and myofibrillar damage: short-term time-course study.

Cardiotoxicity induced by 2-, 3-, 5-, and 12-day dermal administration of 400 and 600 mg/kg/day of bis(2-chloroethoxy)methane to F344/N male and female rats was characterized. The severity and incidence of lesions were similar among males and females and in all three regions of the heart examined (atrium, ventricle, interventricular septum). Damage induced by bis(2-chloroethoxy)methane consisted of time-related development of myofiber vacuolation, necrosis, mononuclear-cell infiltration, fibrosis, and atrial thrombosis. Changes were pronounced at day 2, increased in severity at day 3, appeared to decrease at day 5, and resolved by study-day 16 that corresponded to 12 dosings. Ultrastructural analysis of 2- and 5-day 600 mg/kg/day-treated females elucidated the primary site of damage, the mitochondrion, and two types of vacuolation, one that formed as damaged mitochondria became devoid of cristae and their bounding double membranes became reduced to singleness, and the other manifested as distention of the sarcoplasmic reticulum. After the initial damage induced by bis(2-chloroethoxy)methane, or its metabolite, thiodiglycolic acid, protective mechanisms within the heart were apparently initiated, enabling it to cope with the continued exposure to the toxicant while eliminating some damaged myofibers.

Growth enhancement of ETBE-degrading bacterial consortium with various carbon sources.

In this study, we evaluated Ethyl tert-Butyl Ether (ETBE)-degrading consortia growths in the presence of diverse carbon sources (alcohols, alkanes, ether compounds and carbohydrates). In a second step, we studied the consortium ability to maintain its ETBE degradation activity after growing on these carbon sources in presence or in absence of ETBE. The results indicate that the bacterial growth of ETBE-degrading consortia is enhanced three times more with addition of ethanol than with ETBE alone, while maintaining its ability to degrade ETBE. The bacterial yield growth rate was 0.504 d(-1) when growing on ETBE alone, 1.728 d(-1) on both ETBE and ethanol and 2.856 d(-1) on ethanol alone. Both ETBE and ethanol are completely degraded at 8.33 mg L(-1) h(-1) and 18.55 mg L(-1) h(-1) respectively for an initial OD of 0.4. The frequency of ethanol addition, as growth co-substrate, was studied to preserve the ETBE-degrading capacity of the consortium, and to observe the stability of the genetic character of the ether degradation.

Prediction of pharmacokinetics prior to in vivo studies. II. Generic physiologically based pharmacokinetic models of drug disposition.

Many in vitro data on physicochemical properties and specific absorption, distribution, metabolism, and elimination (ADME) processes are already available at early stages of drug discovery. These data about new drug candidates could be integrated/connected in physiologically based pharmacokinetic (PBPK) models to estimate a priori the overall plasma and tissue kinetic behaviors under in vivo conditions. The objective of the present study was to illustrate that generic PBPK models integrating such data can be developed in drug discovery prior to any in vivo studies. This approach was illustrated with three example compounds, including two lipophilic bases (diazepam, propranolol) and one neutral more hydrophilic drug (ethoxybenzamide). Distribution and liver metabolism were the processes integrated in the generic rat PBPK models of disposition. Tissue:plasma partition coefficients (P(t:p)s) used for description of distribution were estimated from established tissue composition-based equations, which need only in vitro data on drug lipophilicity and plasma protein binding as sole input parameters. Furthermore, data on intrinsic clearance (CL(int)) determined in vitro with hepatocytes were scaled to the in vivo situation to estimate hepatic metabolic clearance. These prediction approaches were both incorporated in the PBPK models to enable automated estimation of distribution and liver metabolism for each drug studied. The generic PBPK models suggested can simulate a priori concentration-time profiles of plasma and several tissues after intravenous administrations to rat. The results indicate that most of the simulated concentration-time profiles of plasma and 10 tissues are in reasonable agreement with the corresponding experimental data determined in vivo (less than a factor of two). However, some more relevant deviations were observed for specific tissues (brain and gut for diazepam; liver and gut for ethoxybenzamide; lung for propranolol) because of important ADME processes were probably neglected in the PBPK models of these drugs. In this context, generic PBPK models were also used for mechanistic evaluations of pharmacokinetics for generating research hypotheses to understand these deviations. Overall, the present generic and integrative PBPK approach of drug disposition suggested as a tool for a priori simulations and mechanistic evaluations of pharmacokinetics has the potential to improve the selection and optimization of new drug candidates.

Toxicokinetics of ethers used as fuel oxygenates.

The toxicokinetics and biotransformation of methyl-tert.butyl ether (MTBE), ethyl-tert.butyl ether (ETBE) and tert.amyl-methyl ether (TAME) in rats and humans are summarized. These ethers are used as gasoline additives in large amounts, and thus, a considerable potential for human exposure exists. After inhalation exposure MTBE, ETBE and TAME are rapidly taken up by both rats and humans; after termination of exposure, clearance by exhalation and biotransformation to urinary metabolites is rapid in rats. In humans, clearance by exhalation is slower in comparison to rats. Biotransformation of MTBE and ETBE is both qualitatively and quantitatively similar in humans and rats after inhalation exposure under identical conditions. The extent of biotransformation of TAME is also quantitatively similar in rats and humans; the metabolic pathways, however, are different. The results suggest that reactive and potentially toxic metabolites are not formed during biotransformation of these ethers and that toxic effects of these compounds initiated by covalent binding to cellular macromolecules are unlikely.

Human cytochrome P450 2A6 is the major enzyme involved in the metabolism of three alkoxyethers used as oxyfuels.

Methyl t-butyl ether (MTBE), ethyl t-butyl ether (ETBE), and t-amyl methyl ether (TAME) are three alkoxyethers added to gasoline to improve combustion and thereby to reduce the level of carbon monoxide and aromatic hydrocarbons in automobile exhaust. Oxidative demethylation of MTBE and TAME and deethylation of ETBE by CYP enzymes results in the formation of tertiary alcohols and aldehydes, both potentially toxic. The metabolism of these three alkoxyethers was studied in a panel of 12 human liver microsomes. The relatively low apparent Km(1) was 0.25+/-0.17 (mean+/-SD), 0.11+/-0.08 and 0.10+/-0.07 mM and the high apparent Km(2) was 2.9+/-1.8, 5.0+/-2.7 and 1.7+/-1.0 mM for MTBE, ETBE and TAME, respectively. Kinetic data, correlation studies, chemical inhibition and metabolism by heterologously expressed human CYPs support the assertion that the major enzyme involved in MTBE, ETBE and TAME metabolisms is CYP2A6, with a minor contribution of CYP3A4 at low substrate concentration.

Biotransformation of MTBE, ETBE, and TAME after inhalation or ingestion in rats and humans.

The biotransformation of methyl tert-butyl ether (MTBE), ethyl tert-butyl ether (ETBE), and tert-amyl methyl ether (TAME) was studied in humans and in rats after inhalation of 4 and 40 ppm of MTBE, ETBE, and TAME, respectively, for 4 hours, and the biotransformation of MTBE and TAME was studied after ingestion exposure in humans to 5 and 15 mg in water. tert-Butyl alcohol (TBA), a TBA conjugate, 2-methyl-1,2-propanediol, and 2-hydroxyisobutyrate were found to be metabolites of MTBE and ETBE. tert-Amyl alcohol (TAA), free and glucuronidated 2-methyl-2,3-butanediol (a glucuronide of TAA), 2-hydroxy-2-methyl butyrate, and 3-hydroxy-3-methyl butyrate were found to be metabolites of TAME. After inhalation, MTBE, ETBE, and TAME were rapidly taken up by both rats and humans; after termination of exposure, clearance from blood of the ethers by exhalation and biotransformation to urinary metabolites occurred with half-times of less than 7 hours in rats and humans. Biotransformation of MTBE and ETBE was similar in humans and rats after inhalation exposure. 2-Hydroxyisobutyrate was recovered as a major product in urine. All metabolites of MTBE and ETBE excreted with urine were eliminated with half-times of less than 20 hours. Biotransformation of TAME was qualitatively similar in rats and humans, but the metabolic pathways were different. In humans, 2-methyl-2,3-butanediol, 2-hydroxy-2-methyl butyrate, and 3-hydroxy-3methyl butyrate were recovered as major urinary products. In rats, however, 2-methyl-2,3-butanediol and its glucuronide were major TAME metabolites recovered in urine. After ingestion of MTBE and TAME, both compounds were rapidly absorbed from the gastrointestinal tract. Hepatic first-pass metabolism of these ethers was not observed, and a significant part of the administered dose was transferred into blood and cleared by exhalation. Metabolic pathways for MTBE and TAME and kinetics of excretion were identical after ingestion and inhalation exposures. Results of studies presented here suggest (1) that excretion of MTBE, ETBE, and TAME in rats and humans is rapid, (2) that biotransformation and excretion of MTBE and ETBE are identical in rats, and (3) that biotransformation and excretion of TAME is quantitatively different in rats and humans.

Biotransformation and kinetics of excretion of ethyl tert-butyl ether in rats and humans.

Ethyl tert-butyl ether (ETBE) may be used in the future as an additive to gasoline to increase oxygen content and reduce tailpipe emissions of pollutants. Therefore, widespread human exposure may occur. To contribute to the characterization of potential adverse effects of ETBE, its biotransformation was compared in humans and rats after inhalation exposure. Human volunteers (3 males and 3 females) and rats (5 males and 5 females) were exposed to 4 (4.5+/-0.6) and 40 (40.6+/-3.0) ppm ETBE for 4 h in a dynamic exposure system. Urine samples from rats and humans were collected for 72 h at 6-h intervals, and blood samples were taken in regular intervals for 48 h. In urine, ETBE and the ETBE-metabolites tert-butanol (t-butanol), 2-methyl-1,2-propane diol, and 2-hydroxyisobutyrate were quantified; ETBE and t-butanol were determined in blood samples. After the end of the exposure period to inhalation of 40-ppm ETBE, blood concentrations of ETBE were found at 5.3+/-1.2 microM in rats and 12.1+/-4.0 microM in humans. The ETBE blood concentrations, after inhalation of 4-ppm ETBE, were 1.0+/-0.7 microM in rats and 1.3+/-0.7 microM in humans. ETBE was rapidly cleared from blood. After the end of the 40-ppm ETBE exposure period, the blood concentrations of t-butanol were 13.9+/-2.2 microM in humans and 21.7+/-4.9 microM in rats. After 4-ppm ETBE exposure, blood concentrations of t-butanol were 1.8+/-0.2 microM in humans and 5.7+/-0.8 microM in rats. t-Butanol was cleared from human blood with a half-life of 9.8+/-1.4 h in humans after 40-ppm ETBE exposure. In urine samples from controls and in samples collected from the volunteers and rats before the exposure, low concentrations of t-butanol, 2-methyl-1,2-propane diol, and 2-hydroxyisobutyrate were present. In the urine of both humans and rats exposed to ETBE, the concentrations of these compounds were significantly increased. 2-Hydroxy-isobutyrate was recovered in urine as the major excretory product formed from ETBE; t-butanol and 2-methyl-1,2-propane diol were minor metabolites. All metabolites of ETBE excreted with urine were rapidly eliminated in both species after the end of the ETBE exposure. Excretion half-lives for the different urinary metabolites of ETBE were between 10.2 and 28.3 h in humans and 2.6 and 4.7 h in rats. The obtained data indicate that ETBE biotransformation and excretion are similar for rats and humans, and that ETBE and its metabolites are rapidly excreted by both species. Between 41 and 53% of the ETBE retained after the end of the exposure was recovered as metabolites in the urine of both humans and rats.

Physiologically based toxicokinetic modeling of inhaled ethyl tertiary-butyl ether in humans.

A physiologically based toxicokinetic (PBTK) model was developed for evaluation of inhalation exposure in humans to the gasoline additive, ethyl tertiary-butyl ether (ETBE). PBTK models are useful tools to relate external exposure to internal doses and biological markers of exposure in humans. To describe the kinetics of ETBE, the following compartments were used: lungs (including arterial blood), liver, fat, rapidly perfused tissues, resting muscles, and working muscles. The same set of compartments and, in addition, a urinary excretion compartment were used for the metabolite tertiary-butyl alcohol (TBA). First order metabolism was assumed in the model, since linear kinetics has been shown experimentally in humans after inhalation exposure up to 50 ppm ETBE. Organ volumes and blood flows were calculated from individual body composition based on published equations, and tissue/blood partition coefficients were calculated from liquid/air partition coefficients and tissue composition. Estimates of individual metabolite parameters of 8 subjects were obtained by fitting the PBTK model to experimental data from humans (5, 25, 50 ppm ETBE, 2-h exposure; Nihlén et al., Toxicol. Sci., 1998; 46, 1-10). The PBTK model was then used to predict levels of the biomarkers ETBE and TBA in blood, urine, and exhaled air after various scenarios, such as prolonged exposure, fluctuating exposure, and exposure during physical activity. In addition, the interindividual variability in biomarker levels was predicted, in the eight experimentally exposed subjects after a working week. According to the model, raising the work load from rest to heavy exercise increases all biomarker levels by approximately 2-fold at the end of the work shift, and by 3-fold the next morning. A small accumulation of all biomarkers was seen during one week of simulated exposure. Further predictions suggested that the interindividual variability in biomarker levels would be higher the next morning than at the end of the work shift, and higher for TBA than for ETBE. Monte Carlo simulations were used to describe fluctuating exposure scenarios. These simulations suggest that ETBE levels in blood and exhaled air at the end of the working day are highly sensitive to exposure fluctuations, whereas ETBE levels the next morning and TBA in urine and blood are less sensitive. Considering these simulations, data from the previous toxicokinetic study and practical issues, we suggest that TBA in urine is a suitable biomarker for exposure to ETBE and gasoline vapor.

Measuring the biodegradability of nonylphenol ether carboxylates, octylphenol ether carboxylates, and nonylphenol.

We examined the biodegradability of several metabolites of C8- and C9-alkylphenol ethoxylates, including nonylphenoxyacetic acid (NPEC1), nonylphenoxyethoxyacetic acid (NPEC2), octylphenoxyacetic acid (OPEC1), octylphenoxyethoxyacetic acid (OPEC2), and nonylphenol (NP). Using OECD method 301B (modified Sturm method), OPEC1 and OPEC2 are readily biodegradable: both compounds exceeded 60% of theoretical CO2 formation (ThCO2) by day 28, and required less than 10 days to go from 10% to 60% ThCO2. Also using method 301B, NPEC1 and NPEC2 exceeded 60% ThCO2 at day 28, but did not meet the 10 day window. Using OECD method 301F, the manometric respirometry method that measures oxygen consumption, approximately 62% of NP was biodegraded in 28 days, but required more than 10 days to go from 10% to 60% biodegradation. While the validity of the "10-day window" is currently being debated within OECD, the data show that the common metabolites of C8- and C9-APEs are rapidly degraded in the test systems used, which strongly suggests that they would not accumulate or persist in the environment.

Biotransformation of 12C- and 2-13C-labeled methyl tert-butyl ether, ethyl tert-butyl ether, and tert-butyl alcohol in rats: identification of metabolites in urine by 13C nuclear magnetic resonance and gas chromatography/mass spectrometry.

The biotransformation of the fuel oxygenates methyl tert-butyl ether (MTBE) and ethyl tert-butyl ether (ETBE) was studied in rats after inhalation exposure; the biotransformation of the initial metabolite of these ethers, tert-butyl alcohol, was studied after oral gavage. To study ether metabolism, rats were exposed for 6 h to initial concentrations of 2000 ppm of MTBE or ETBE, respectively [2-13C]MTBE and [2-13C]ETBE. Urine was collected for 48 h after the end of the exposure, and urinary metabolites were identified by 13C NMR (13C-labeled ethers) and gas chromatography/mass spectrometry (GC/MS) (12C- and 13C-labeled ethers). To study tert-butyl alcohol metabolism, rats were dosed either with tert-butyl alcohol at natural carbon isotope ratio or with 13C-enriched tert-butyl alcohol (250 mg/kg of body weight), urine was collected, and metabolites were identified by NMR and GC/MS. tert-Butyl alcohol was identified as a minor product of the biotransformation of MTBE and ETBE. In addition, small amounts of a tert-butyl alcohol conjugate, likely a glucuronide, were present in the urine of the treated animals. Moreover, the mass spectra obtained indicate the presence of small amounts of [13C]acetone in the urine of [13C]MTBE and [13C]ETBE-treated rats. 2-Methyl-1,2-propanediol, 2-hydroxyisobutyrate, and another unidentified conjugate of tert-butyl alcohol, most probably a sulfate, were major urinary metabolites of MTBE and ETBE as judged by the intensities of the NMR signals. In [13C]-tert-butyl alcohol-dosed rats, [13C]acetone, tert-butyl alcohol, and its glucuronide represented minor metabolites; as with the ethers, 2-methyl-1,2-propanediol, 2-hydroxyisobutyrate, and the presumed tert-butyl alcohol sulfate were the major metabolites present. In one human individual given 5 mg/kg [13C]-tert-butyl alcohol orally, 2-methyl-1,2-propanediol and 2-hydroxyisobutyrate were major metabolites in urine detected by 13C NMR analysis. Unconjugated tert-butyl alcohol and tert-butyl alcohol glucuronide were present as minor metabolites, and traces of the presumed tert-butyl alcohol sulfate were also present. Our results suggest that tert-butyl alcohol formed from MTBE and ETBE is intensively metabolized by further oxidation reactions. Studies to elucidate mechanisms of toxicity for these ethers to the kidney need to consider potential toxicities induced by these metabolites.