Physiologically based pharmacokinetic modeling of 1,3-butadiene, 1,2-epoxy-3-butene, and 1,2:3,4-diepoxybutane toxicokinetics in mice and rats

Treosulfan Pharmacokinetics and its Variability in Pediatric and Adult Patients Undergoing Conditioning Prior to Hematopoietic Stem Cell Transplantation: Current State of the Art, In-Depth Analysis, and Perspectives

Treosulfan is a prodrug that undergoes a highly pH- and temperature-dependent nonenzymatic conversion to the monoepoxide {(2S,3S)-1,2-epoxy-3,4-butanediol 4-methanesulfonate [S,S-EBDM]} and diepoxide {(2S,3S)-1,2:3,4-diepoxybutane [S,S-DEB]}. Currently, treosulfan is tested in clinical trials as an alternative to busulfan in conditioning prior to hematopoietic stem cell transplantation (HSCT). Of note, the optimal dosing of the prodrug is still unresolved, especially in infants. In this paper, the pharmacokinetics of treosulfan, together with its biologically active epoxides, is comprehensively reviewed for the first time, with the focus on conditioning prior to HSCT. Most of the insightful data presented in this review comes from studies that have been conducted in the last 3 years. The article widely discusses the volume of distribution and total clearance of treosulfan. In particular, the interindividual variability of these key parameters in infants, children above 1 year of age, and adults is analyzed, including possible covariates. A clinically important aspect of the formation rate-limited elimination of S,S-EBDM and S,S-DEB is described, including the correlation between the exposure of the prodrug and S,S-EBDM in children. The significance of the elimination half-life of treosulfan and its epoxides for successful conditioning prior to HSCT is also raised. Furthermore, the organ disposition of treosulfan and S,S-EBDM in rats is discussed in the context of the clinical toxicity and myeloablative activity of treosulfan versus busulfan. Moreover, perspectives for future therapeutic drug monitoring of treosulfan are presented. The review is intended to be helpful to pharmacists and doctors in the comprehension of the clinical pharmacokinetics of treosulfan.

Parallelogram based approach for in vivo dose estimation of genotoxic metabolites in humans with relevance to reduction of animal experiments

When employing metabolism studies of genotoxic compounds/metabolites and cancer tests for risk estimation, low exposure doses in humans are roughly extrapolated from high exposure doses in animals. An improvement is to measure the in vivo dose, i.e. area under concentration-time curve (AUC), of the causative genotoxic agent. In the present work, we propose and evaluate a parallelogram based approach for estimation of the AUC of genotoxic metabolites that incorporates in vitro metabolic data and existing knowledge from published in vivo data on hemoglobin (Hb) adduct levels, using glycidamide (GA) as a case study compound that is the genotoxic metabolite of acrylamide (AA). The estimated value of AUC of GA per AUC of AA from the parallelogram approach vs. that from Hb adduct levels measured in vivo were in good agreement; 0.087 vs. 0.23 in human and 1.4 vs. 0.53 in rat, respectively. The described parallelogram approach is simple, and can be useful to provide an approximate estimation of the AUC of metabolites in humans at low exposure levels for which sensitive methods for analyzing the metabolites are not available, as well as aid in reduction of animal experiments for metabolism studies that are to be used for cancer risk assessment.

A preliminary regional PBPK model of lung metabolism for improving species dependent descriptions of 1,3-butadiene and its metabolites

1,3-Butadiene (BD), a volatile organic chemical (VOC), is used in synthetic rubber production and other industrial processes. It is detectable at low levels in ambient air as well as in tobacco smoke and gasoline vapors. Inhalation exposures to high concentrations of BD have been associated with lung cancer in both humans and experimental animals, although differences in species sensitivity have been observed. Metabolically active lung cells such as Pulmonary Type I and Type II epithelial cells and club cells (Clara cells)(1) are potential targets of BD metabolite-induced toxicity. Metabolic capacities of these cells, their regional densities, and distributions vary throughout the respiratory tract as well as between species and cell types. Here we present a physiologically based pharmacokinetic (PBPK) model for BD that includes a regional model of lung metabolism, based on a previous model for styrene, to provide species-dependent descriptions of BD metabolism in the mouse, rat, and human. Since there are no in vivo data on BD pharmacokinetics in the human, the rat and mouse models were parameterized to the extent possible on the basis of in vitro metabolic data. Where it was necessary to use in vivo data, extrapolation from rat to mouse was performed to evaluate the level of uncertainty in the human model. A kidney compartment and description of downstream metabolism were also included in the model to allow for eventual use of available urinary and blood biomarker data in animals and humans to calibrate the model for estimation of BD exposures and internal metabolite levels. Results from simulated inhalation exposures to BD indicate that incorporation of differential lung region metabolism is important in describing species differences in pulmonary response and that these differences may have implications for risk assessments of human exposures to BD.

Determination of partition coefficients n-octanol/water for treosulfan and its epoxy-transformers: an example of a negative correlation between lipophilicity of unionized compounds and their retention in reversed-phase chromatography

For the last decade an alkylating agent treosulfan (TREO) has been successfully applied in clinical trials in conditioning prior to hematopoietic stem cell transplantation. Pharmacological activity of the pro-drug depends on its epoxy-transformers, monoepoxide (S,S-EBDM) and diepoxide (S,S-DEB), which are formed in a non-enzymatic consecutive reaction accompanied by a release of methanesulfonic acid. In the present study partition coefficient n-octanol/water (POW) of TREO as well as its biologically active epoxy-transformers was determined empirically (applying a classical shake-flask method) and in silico for the first time. In vitro the partition was investigated at 37°C in the system composed of the pre-saturated n-octanol and 0.05 M acetate buffer pH 4.4 adjusted with sodium and potassium chloride to ionic strength of 0.16 M. Concentration of the analytes was quantified by reversed-phase high performance liquid chromatography (RP-HPLC) method in which retention time increased from S,S-DEB to TREO. It was shown that neither association nor dissociation of the tested compounds in the applied phases occurred. Calculated logPOW (TREO: -1.58±0.04, S,S-EBDM: -1.18±0.02, S,S-DEB: -0.40±0.03) indicate the hydrophilic character of the all three entities, corresponding to its pharmacokinetic parameters described in the literature. Experimentally determined logPOW of the compounds were best comparable to the values predicted by algorithm ALOGPs. Interestingly, the POW values determined in vitro as well as in silico were inversely correlated with the retention times observed in the endcapped RP-HPLC column. It might be explained by the fact that a cleavage of methansulfonic acid from a small molecule of TREO generates significant changes in the molecular structure. Consequently, despite the common chemical origin, TREO, S,S-EBDM and S,S-DEB do not constitute a 'congeneric' series of compounds. We concluded that this might occur in other low-weight species, therefore measurement of their POW by RP-HPLC had to be applied with a special care.

1,3-Butadiene: I. Review of metabolism and the implications to human health risk assessment

1,3-Butadiene (BD) is a multisite carcinogen in laboratory rodents following lifetime exposure, with mice demonstrating greater sensitivity than rats. In epidemiology studies of men in the styrene-butadiene rubber industry, leukemia mortality is associated with butadiene exposure, and this association is most pronounced for high-intensity BD exposures. Metabolism is an important determinant of BD carcinogenicity. BD is metabolized to several electrophilic intermediates, including epoxybutene (EB), diepoxybutane (DEB), and epoxybutane diol (EBD), which differ considerably in their genotoxic potency (DEB >> EB > EBD). Important species differences exist with respect to the formation of reactive metabolites and their subsequent detoxification, which underlie observed species differences in sensitivity to the carcinogenic effects of BD. The modes of action for human leukemia and for the observed solid tumors in rodents are both likely related to the genotoxic potencies for one or more of these metabolites. A number of factors related to metabolism can also contribute to nonlinearity in the dose-response relationship, including enzyme induction and inhibition, depletion of tissue glutathione, and saturation of oxidative metabolism. A quantitative risk assessment of BD needs to reflect these species differences and sources of nonlinearity if it is to reflect the current understanding of the disposition of BD.

Approaches to acrylamide physiologically based toxicokinetic modeling for exploring child-adult dosimetry differences

Dietary exposure to acrylamide is common as a result of its formation during the cooking of carbohydrate foods. This leads to widespread human exposure in adults and children alike. Acrylamide is neurotoxic and is metabolized by cytochrome P-450 (CYP) 2E1 to a mutagenic epoxide, glycidamide. This article describes a modeling framework for assessing acrylamide and glycidamide dosimetry in rats and human adults and children. The challenges in building a physiologically based toxicokinetic (PBTK) model that is compatible with existing rat and human data are described, with an emphasis on calibration against the hemoglobin adduct database. This exploratory PBTK model was adapted to children by incorporating life-stage-specific parameters consistent with children's changing physiology and metabolic capacity for processes involved in acrylamide disposition in terms of CYP2E1, glutathione conjugation, and epoxide hydrolase. Monte Carlo analysis was used to simulate the distribution of internal doses to gain an initial understanding of the range of child/adult differences possible. This analysis suggests modest dosimetry differences between children and adults, with area-under-the-curve (AUC) doses for the 99th percentile child up to fivefold greater than the median adult for both acrylamide and glycidamide. Early life immaturities tended to exert a greater effect on acrylamide than glycidamide dosimetry because immaturities in CYP2E1 and glutathione counteract one another for glycidamide AUC, but both lead to greater acrylamide dose. The analysis points toward glutathione conjugation parameters as being particularly influential and uncertain in early life, making this a key area for future research.

Development of a physiologically based toxicokinetic model for butadiene and four major metabolites in humans: Global sensitivity analysis for experimental design issues

1,3-Butadiene (BD) is metabolized in humans and rodents to mutagenic and carcinogenic species. Our previous work has focused on developing a physiologically based toxicokinetic (PBTK) model for BD to estimate its metabolic rate to 1,2-epoxy-3-butene (EB), using exhaled breath BD concentrations in human volunteers exposed by inhalation. In this paper, we extend our BD model to describe the kinetics of its four major metabolites EB, 1,2:3,4-diepoxybutane (DEB), 3-butene-1,2-diol (BDD), and 3,4-epoxy-1,2-butanediol (EBD), and to test whether the extended model and experimental data (to be collected for BD and metabolites in humans) are together adequate to estimate the metabolic rate constants of each of the above chemicals. Global sensitivity analyses (GSA) were conducted to evaluate the relative importance of the model parameters on model outputs during the 20min of exposure and the 40min after exposure ended. All model parameters were studied together with various potentially measurable model outputs: concentrations of BD and EB in exhaled air, concentrations of BD and all metabolites in venous blood, and cumulated amounts of urinary metabolites excreted within 24h. Our results show that pulmonary absorption of BD and subsequent distribution and metabolism in the well-perfused tissues compartment are the critical processes in the toxicokinetics of BD and metabolites. In particular, three parameters influence numerous outputs: the blood:air partition coefficient for BD, the metabolic rate of BD to EB, and the volume of the well-perfused tissues. Other influential parameters include other metabolic rates, some partition coefficients, and parameters driving the gas exchanges (in particular, for BD outputs). GSA shows that the impact of the metabolic rate of BD to EB on the BD concentrations in exhaled air is greatly increased if a few of the model's important parameters (such as the blood:air partition coefficient for BD) are measured experimentally. GSA also shows that all the transformation pathways described in the PBTK model may not be estimable if only data on the studied outputs are collected, and that data on a specific output for a chemical may not inform all the transformations involving that chemical.

Toxicology of 1,3-butadiene, chloroprene, and isoprene

The diene monomers, 1,3-butadiene, chloroprene, and isoprene, respectively, differ only in substitution of a hydrogen, a chlorine, or a methyl group at the second of the four unsaturated carbon atoms in these linear molecules. Literature reviewed in the preceding sections indicates that these chemicals have important uses in synthesis of polymers, which offer significant benefits within modern society. Additionally, studies document that these monomers can increase the tumor formation rate in various organs of rats and mice during chronic cancer bioassays. The extent of tumor formation versus animal exposure to these monomers varies significantly across species, as well among strains within species. These studies approach, but do not resolve, important questions of human risk from inhalation exposure. Each of these diene monomers can be activated to electrophilic epoxide metabolites through microsomal oxidation reactions in mammals. These epoxide metabolites are genotoxic through reactions with nucleic acids. Some of these reactions cause mutations and subsequent cancers, as noted in animal experiments. Significant differences exist among the compounds, particularly in the extent of formation of highly mutagenic diepoxide metabolites, when animals are exposed. These metabolites are detoxified through hydrolysis by epoxide hydrolase enzymes and through conjugation with glutathione with the aid of glutathione S-transferase. Different strains and species perform these reactions with varying efficacy. Mice produce these electrophilic epoxides more rapidly and appear to have less adequate detoxification mechanisms than rats or humans. The weight of evidence from many studies suggests that the balance of activation versus detoxification offers explanation of differing sensitivities of animals to these carcinogenic actions. Other aspects, including molecular biology of the many processes that lead through specific mutations to cancer, are yet to be understood. Melnick and Sills (2001) compared the carcinogenic potentials of these three dienes, along with that of ethylene oxide, which also acts through an epoxide intermediate. From the number of tissue sites where experimental animal tumors were detected, butadiene offers greatest potential for carcinogenicity of these dienes. Chloroprene and then isoprene appear to follow in this order. Comparisons among these chemicals based on responses to external exposures are complicated by differences among studies and of species and tissue susceptibilities. Physiologically based pharmacokinetic models offer promise to overcome these impediments to interpretation. Mechanistic studies at the molecular level offer promise for understanding the relationships among electrophilic metabolites and vital genetic components. Significant improvements in minimization of industrial worker exposures to carcinogenic chemicals have been accomplished after realization that vinyl chloride caused hepatic angiosarcoma in polymer production workers (Creech and Johnson 1974; Falk et al. 1974). Efforts continue to minimize disease, particularly cancer, from exposures to chemicals such as these dienes. Industry has responded to significant challenges that affect the health of workers through efforts that minimize plant exposures and by sponsorship of research, including animal and epidemiological studies. Governmental agencies provide oversight and have developed facilities that accomplish studies of continuing scientific excellence. These entities grapple with differences in perspective, objectives, and interpretation as synthesis of knowledge develops through mutual work. A major challenge remains, however, in assessment of significance of environmental human exposures to these dienes. Such exposure levels are orders of magnitude less than exposures studied in experimental or epidemiological settings, but exposures may persist much longer and may involve unknown but potentially significant sensitivities in the general population. New paradigms likely will be needed for toxicological evaluation of these human exposures, which are ongoing but as yet are not interpreted.

A physiological toxicokinetic model for inhaled propylene oxide in rat and human with special emphasis on the nose

Chronic exposure to high concentrations of PO induced inflammation in the respiratory nasal mucosa (RNM) of rodents and, for concentrations >or= 300 ppm, caused nasal tumors. Considering the nose to be the most relevant target organ for PO-induced tumorigenicity, we developed a physiological toxicokinetic model for PO in rats and humans. It includes compartments for arterial, venous, and pulmonary blood, liver, muscle, fat, richly perfused tissues, lung, and nose. It simulates inhalation of PO, its distribution into tissues by blood flow, and its elimination by exhalation and metabolism. In nose, lung, and liver of rats, PO conjugation with glutathione (GSH), PO-induced GSH depletion, and formation of PO adducts to DNA are described. Also modeled are PO adducts to hemoglobin of rats and humans. Required partition coefficients and metabolic parameters were derived experimentally or from publications. In rats, simulated PO concentrations in blood and GSH levels in tissues agreed with measured data. If compared with reported values, levels of adducts with hemoglobin were underpredicted up to a factor of about 2. Adducts with DNA differed up to a factor of 3. Hemoglobin adducts predicted for PO-exposed workers were 1.5-1.9 times higher than the reported ones. Considering identical conditions of PO exposure, similar PO concentrations in RNM were modeled for rats and humans. Also, PO concentrations in blood, about 1/30th of those in RNM, were similar in both species. Since the model was evaluated on all available data in rats and humans, we consider it to be useful for estimating the risk from inhalation exposure to PO.

Genotoxicity sensor response correlated with DNA nucleobase damage rates measured by LC-MS

Responses from "reagentless" DNA-based electrochemical toxicity sensors to DNA alkylating agents styrene oxide (SO), diepoxybutane (DEB), and methyl methanesulfonate (MMS) were compared to formation rates of total alkylated nucleobases in DNA measured by LC-UV-MS. Sensors utilized a catalytic metallopolymer in DNA films previously exposed to the damage agents. To achieve adequate sensitivity, LC-UV-MS analyses were done on DNA in solution reacted with the damage agents, and subsequently hydrolyzed to nucleosides with enzymes. Sensor response correlated well with nucleobase-adduct formation rates obtained by the molecule-specific analyses. Results confirm that the metallopolymer-DNA film sensors can be used to estimate relative DNA damage rates from nucleobase adduct-forming chemicals. Results from both methods correlated well with animal genotoxicity as estimated by TDL(o) values, the lowest dose producing carcinogenicity, in mice and rats. These sensors should be useful for rapid, inexpensive screening of moderately and severely genotoxic new chemicals.

Framework for evaluation of physiologically-based pharmacokinetic models for use in safety or risk assessment

Proposed applications of increasingly sophisticated biologically-based computational models, such as physiologically-based pharmacokinetic models, raise the issue of how to evaluate whether the models are adequate for proposed uses, including safety or risk assessment. A six-step process for model evaluation is described. It relies on multidisciplinary expertise to address the biological, toxicological, mathematical, statistical, and risk assessment aspects of the modeling and its application. The first step is to have a clear definition of the purpose(s) of the model in the particular assessment; this provides critical perspectives on all subsequent steps. The second step is to evaluate the biological characterization described by the model structure based on the intended uses of the model and available information on the compound being modeled or related compounds. The next two steps review the mathematical equations used to describe the biology and their implementation in an appropriate computer program. At this point, the values selected for the model parameters (i.e., model calibration) must be evaluated. Thus, the fifth step is a combination of evaluating the model parameterization and calibration against data and evaluating the uncertainty in the model outputs. The final step is to evaluate specialized analyses that were done using the model, such as modeling of population distributions of parameters leading to population estimates for model outcomes or inclusion of early pharmacodynamic events. The process also helps to define the kinds of documentation that would be needed for a model to facilitate its evaluation and implementation.

Uterine-ovarian biochemical and developmental interactions to the postimplantation treatment with a butadiene metabolite, diepoxybutane, in pregnant rats

An industrial chemical used in synthetic rubber production, 1,3-butadiene, is toxic to reproduction in rats and mice. Bioactivation of butadiene to reactive intermediates, i.e. diepoxybutane and other metabolites, is responsible for this toxicity. The present study examines the biochemical and developmental mechanisms of diepoxybutane at the feto-maternal placental axis during gestation. Female Sprague-Dawley rats were administered four daily intraperitoneal doses of diepoxybutane in groups (0.25, 0.30, 0.35, or 0.40 mmol in sesame oil per kg body weight, n = 6/group) during postimplantation (gestation days 5-8) and euthanized on gestation day 9 or 12 for retrieval of uterine and ovarian tissues, and serum for assays. The results demonstrate that this timely diepoxybutane treatment significantly decreased placental levels of pituitary adenylate cyclase-activating polypeptide mRNA expression that was measured by reverse transcription-polymerase chain reaction and of matrix metalloproteinase-9 activity that was determined by gelatin zymography, and serum progesterone levels on gestation days 9 and 12. From a developmental standpoint, fetal growth and viability were reduced in correlation with treatment-related effects of diepoxybutane on implantation losses and fetal resorptions on gestation day 9. Additionally, fetal mortality was maximally increased due to significantly pronounced, dose-independent effects on these parameters on gestation day 12. This trend towards more severe embryolethal treatment effects from gestation day 9 to 12 suggests that fetal metabolism in the gravid uteri of rats may be more sensitive to diepoxybutane exposure as pregnancy progresses. The inhibitory actions of diepoxybutane on placental pituitary adenylate cyclase-activating polypeptide expression and matrix metalloproteinase activity may contribute towards altering placental molecular support for fetal development and viability. Moreover, the reproductive toxicity of diepoxybutane in rats appears to be linked to progesterone action.

1,3-Butadiene: exposure estimation, hazard characterization, and exposure-response analysis

1,3-Butadiene has been assessed as a Priority Substance under the Canadian Environmental Protection Act. The general population in Canada is exposed to 1,3-butadiene primarily through ambient air. Inhaled 1,3-butadiene is carcinogenic in both mice and rats, inducing tumors at multiple sites at all concentrations tested in all identified studies. In addition, 1,3-butadiene is genotoxic in both somatic and germ cells of rodents. It also induces adverse effects in the reproductive organs of female mice at relatively low concentrations. The greater sensitivity in mice than in rats to induction of these effects by 1,3-butadiene is likely related to species differences in metabolism to active epoxide metabolites. Exposure to 1,3-butadiene in the occupational environment has been associated with the induction of leukemia; there is also some limited evidence that 1,3-butadiene is genotoxic in exposed workers. Therefore, in view of the weight of evidence of available epidemiological and toxicological data, 1,3-butadiene is considered highly likely to be carcinogenic, and likely to be genotoxic, in humans. Estimates of the potency of butadiene to induce cancer have been derived on the basis of both epidemiological investigation and bioassays in mice and rats. Potencies to induce ovarian effects have been estimated on the basis of studies in mice. Uncertainties have been delineated, and, while there are clear species differences in metabolism, estimates of potency to induce effects are considered justifiably conservative in view of the likely variability in metabolism across the population related to genetic polymorphism for enzymes for the critical metabolic pathway.

Prediction of isoprene diepoxide levels in vivo in mouse, rat and man using enzyme kinetic data in vitro and physiologically-based pharmacokinetic modelling

The present study was designed to explain the differences in isoprene toxicity between mouse and rat based on the liver concentrations of the assumed toxic metabolite isoprene diepoxide. In addition, extrapolation to the human situation was attempted. For this purpose, enzyme kinetic parameters K(m) and V(max) were determined in vitro in mouse, rat and human liver microsomes/cytosol for the cytochrome P450-mediated formation of isoprene mono- and diepoxides, epoxide hydrolase mediated hydrolysis of isoprene mono- and diepoxides, and the glutathione S-transferases mediated conjugation of isoprene monoepoxides. Subsequently, the kinetic parameters were incorporated into a physiologically-based pharmacokinetic model, and species differences regarding isoprene diepoxide levels were forecasted. Almost similar isoprene diepoxide liver and lung concentrations were predicted in mouse and rat, while predicted levels in humans were about 20-fold lower. However, when interindividual variation in enzyme activity was introduced in the human model, the levels of isoprene diepoxide changed considerably. It was forecasted that in individuals having both an extensive oxidation by cytochrome P450 and a low detoxification by epoxide hydrolase, isoprene diepoxide concentrations in the liver increased to similar concentrations as predicted for the mouse. However, the interpretation of the latter finding for human risk assessment is ambiguous since species differences between mouse and rat regarding isoprene toxicity could not be explained by the predicted isoprene diepoxide concentrations. We assume that other metabolites than isoprene diepoxide or different carcinogenic response might play a key role in determining the extent of isoprene toxicity. In order to confirm this, in vivo experiments are required in which isoprene epoxide concentrations will be established in rats and mice.

Development of a preliminary physiologically based toxicokinetic (PBTK) model for 1,3-butadiene risk assessment

Potential health effects of human exposure to 1,3-butadiene (BD) are of concern due to the use of BD in industry and its low-level presence throughout the environment. Physiologically based toxicokinetic (PBTK) models of BD in rodents have been developed by multiple research groups in an effort to explain species differences in toxicity (especially carcinogenic potency) through toxicokinetics. PBTK modeling of dose metrics related to a non-cancer endpoint, ovotoxicity in experimental animals, was conducted. The cumulative area under the blood concentration vs. time curve (AUC) for the metabolite diepoxybutane (butadiene diepoxide, DEB) was found to be consistent with ovotoxicity in mice and rats exposed to BD by inhalation or epoxybutene (butadiene monoepoxide, EB) or DEB by intraperitoneal injection. This suggests that cumulative DEB AUC may also be an appropriate metric for possible human risk. A preliminary human PBTK model was assembled for the eventual assessment of reproductive risk to humans and for prioritizing the determination of model parameters. The preliminary model accurately predicted published data on exhaled breath BD concentrations in a human volunteer exposed to BD by inhalation. The fit was relatively insensitive to the rate constant for BD epoxidation. Sensitivity analyses were conducted on this human PBTK model. Using a range of published rate constants, human blood DEB was found to be sensitive to rates of epoxidation of EB to DEB and hydrolysis of EB and DEB, but not BD epoxidation. Because of the large ranges of rates measured in vitro for these reactions, different combinations of in-vitro rates produce varying predictions of blood DEB concentration. Thus, validation of a human PBTK model with human biomonitoring data will be essential to produce a PBTK model that can be applied to risk assessment.

Health risk assessment of 1,3-butadiene as a Priority Substance in Canada

1,3-Butadiene was included in the second list of Priority Substances to be assessed under the Canadian Environmental Protection Act. Potential hazards to human health were characterized on the basis of critical examination of available data on health effects in experimental animals and occupationally exposed human populations, as well as information on mode of action. Based on consideration of all relevant data identified as of April 1998, butadiene was considered highly likely to be carcinogenic to humans, and likely to be a somatic and germ cell genotoxicant in humans. In addition, butadiene may also be a reproductive toxicant in humans. Estimates of the potency of butadiene to induce these effects have been derived on the basis of quantitation of observed exposure-response relationships for the purposes of characterization of risk to the general population in Canada exposed to butadiene in the ambient environment.

First-pass metabolism of 1,3-butadiene in once-through perfused livers of rats and mice

First-pass metabolism of 1,3-butadiene (BD) leading to 1,2-epoxy-3-butene (EB), 1,2:3,4-diepoxybutane (DEB), 3-butene-1,2-diol (B-diol), 3,4-epoxy-1,2-butanediol (EBD) and crotonaldehyde (CA) was studied quantitatively in the once-through BD perfused liver of mouse and rat by means of an all-glass gas-tight perfusion system. Metabolites were analyzed using gas chromatography equipped with mass selective detection. The perfusate consisted of Krebs-Henseleit buffer (pH 7.4) containing bovine erythrocytes (40%v/v) and BD. The perfusion flow rates through the livers were 3-4 ml/min (mouse) and 17-20 ml/min (rat). The BD concentrations in the liver perfusates were 330 nmol/ml (mouse) and 240 nmol/ml (rat) being high enough to reach almost saturation of BD metabolism. The mean rates of BD transformation were about 0.014 and 0.055 mmol/h per liver of a mouse and a rat, respectively, being similar to the values expected from in-vivo measurements. There were marked species differences in the formation of BD metabolites. In the effluent of mouse livers, all three epoxides (EB: 9.4 nmol/ml; DEB: 0.06 nmol/ml; EBD: 0.07 nmol/ml) and B-diol (8.2 nmol/ml) were detected. In the perfusate leaving naïve rat livers, only EB and B-diol were found. In that of rat liver, EB concentration was 8.5 times smaller than in that of mouse liver, whereas B-diol concentrations were similar in the effluent liver perfusate of both species. CA was below the limit of its detection (60 nmol/l) in the liver perfusate of mice and of naïve rats. Of BD metabolized, the sum of the metabolites investigated in the effluent amounted to only 30% (mouse) and 20% (rat). In first experiments with rat liver, glutathione (GSH) was depleted by pretreating the animals with diethylmaleate. With the exception of EBD (not quantifiable due to an interfering peak), all other metabolites including CA were found in the effluent perfusate summing up to about 70 and 100% of BD metabolized, which indicates the quantitative importance of the GSH dependent metabolism. In summary, the results demonstrate the relevance of an intrahepatic first-pass metabolism for metabolic intermediates of BD, which undergo further transformation immediately after their production in the liver before leaving this organ. Hitherto, the occurrence of this first-pass metabolism was only hypothesized. The findings will help to explain the drastic species difference between mice and rats in the carcinogenic potency of BD.

Physiological modeling of butadiene disposition in mice and rats

The earliest physiological models of 1,3-butadiene disposition reproduced uptake of the gas from closed chambers but over-predicted steady-state circulating concentrations of the mutagenic intermediates 1,2-epoxybut-3-ene and 1,2:3,4-diepoxybutane. A preliminary model based on the observation of a transient complex between cytochrome P450 and microsomal epoxide hydrolase on the endoplasmic reticulum membrane reproduced the blood epoxide concentrations as well as the chamber uptake data. This model was enhanced by the addition of equations for the production and detoxication of 3,4-epoxybutane-1,2-diol in the liver, lungs, and kidneys. The model includes flow-restricted delivery of butadiene and its metabolites to compartments for lungs, liver, fat, kidneys, gastrointestinal tract, other rapidly perfused tissues, and other slowly perfused tissues. Blood was distributed among compartments for arterial, venous, and tissue capillary spaces. Channeling of the three bound epoxides to epoxide hydrolase and their release from the endoplasmic reticulum are competing processes in this model. Parameters were estimated to fit data for chamber uptake of butadiene and epoxybutene, steady-state blood concentrations of epoxybutene and diepoxybutane, and the fractions of the inhaled dose of butadiene that appears as various excreted metabolites. The optimal values of the apparent K(m)s of membrane-bound epoxides for epoxide hydrolase were only 5% of the values for the cytosolic substrate, consistent with the observation of a transient complex between epoxide hydrolase and the cytochrome P450 that produces the epoxide. This proximity effect corresponds to the notion that epoxides produced in situ have privileged access to epoxide hydrolase. The model also predicts considerable accumulation of epoxybutanediol, in agreement with the observation that most of the DNA adducts in animals exposed to butadiene arise from this metabolite.

A mechanistic assessment of 1,3-butadiene diepoxide-induced inhibition of uterine deciduoma proliferation in pseudopregnant rats

Butadiene diepoxide (BDE), a reactive metabolite of 1,3-butadiene that is an important industrial chemical used in synthetic rubber production causes a dose-dependent inhibition of deciduoma development in pseudopregnant Sprague-Dawley rats. This study used 4 daily i.p. BDE doses of 0.20, 0.25, 0.30, 0.35, or 0.40 to characterize mechanisms that may be responsible for the antideciduoma effect. Pseudopregnant rats were treated either before (pseudopregnancy [PPG] days 1-4) or after (PPG days 5-9) deciduoma induction by endometrial trauma with a blunt needle. Animals were killed on PPG day 9 and evaluated for serum progesterone and endometrial protein and DNA. RT-PCR was used to measure message for estrogen receptor (ER) alpha and pituitary adenylate cyclase-activating polypeptide (PACAP). Substrate zymography and Western blotting were used respectively to measure matrix metalloproteinase (MMP)-9 and inducible nitric oxide synthase. The antideciduoma effects of BDE were associated with decreases in endometrial weight, protein, and DNA, with decreases in serum progesterone, and with decreases in PACAP message and MMP-9. A reduction in NOS was identified at the highest dose of BDE. Message for estrogen receptor (ER) alpha was not affected at any dose. We conclude that the reduction in decidual proliferation was direct and appeared to be associated with either 1) a decrease in the effectiveness of the deciduogenic stimulation and/or a weakened endometrial sensitivity to the stimulus; or 2) an effect on deciduoma development. Molecular mechanisms that apparently contributed to BDE inhibition of decidual metabolism included the synthesis of protein and DNA involved in decidual growth, the synthesis and activation of a matrix metalloproteinase for degradation of the extracellular matrix that is essential for tissue remodeling during deciduoma development, and the nitric oxide/nitric oxide synthase and pituitary adenylate cyclase-activating peptide systems that are involved in promoting vasodilation and increased vascular permeability to enhance the availability of substrates for maximal deciduoma growth. The ovotoxicity of BDE, which has previously been established, may indirectly affect decidual proliferation by reducing progesterone, the preeminent endocrine regulator of deciduoma development. The findings also suggest that BDE may possess no estrogenic action since it was associated with endometrial weight loss and unaltered levels of the estrogen receptor alpha mRNA expression.

A review of the genetic and related effects of 1,3-butadiene in rodents and humans

In this paper, the metabolism and genetic toxicity of 1,3-butadiene (BD) and its oxidative metabolites in humans and rodents is reviewed with attention to newer data that have been published since the latest evaluation of BD by the International Agency for Research on Cancer (IARC). The oxidative metabolism of BD in mice, rats and humans is compared with emphasis on the major pathways leading to the reactive intermediates 1,2-epoxy-3-butene (EB), 1,2:3, 4-diepoxybutane (DEB), and 3,4-epoxy-1,2-butanediol (EBdiol). Results from recent studies of DNA and hemoglobin adducts indicate that EBdiol may play a more significant role in the toxicity of BD than previously thought. All three metabolites are capable of reacting with macromolecules, such as DNA and hemoglobin, and have been shown to induce a variety of genotoxic effects in mice and rats as well as in human cells in vitro. DEB is clearly the most potent of these genotoxins followed by EB, which in turn is more potent than EBdiol. Studies of mutations in lacI and lacZ mice and of the Hprt mutational spectrum in rodents and humans show that mutations at G:C base pairs are critical events in the mutagenicity of BD. In-depth analyses of the mutational spectra induced by BD and/or its oxidative metabolites should help to clarify which metabolite(s) are associated with specific mutations in each animal species and which mutational events contribute to BD-induced carcinogenicity. While the quantitative relationship between exposure to BD, its genotoxicity, and the induction of cancer in occupationally exposed humans remains to be fully established, there is sufficient data currently available to demonstrate that 1,3-butadiene is a probable human carcinogen.

A physiological toxicokinetic model for exogenous and endogenous ethylene and ethylene oxide in rat, mouse, and human: formation of 2-hydroxyethyl adducts with hemoglobin and DNA

Ethylene (ET) is a gaseous olefin of considerable industrial importance. It is also ubiquitous in the environment and is produced in plants, mammals, and humans. Uptake of exogenous ET occurs via inhalation. ET is biotransformed to ethylene oxide (EO), which is also an important volatile industrial chemical. This epoxide forms hydroxyethyl adducts with macromolecules such as hemoglobin and DNA and is mutagenic in vivo and in vitro and carcinogenic in experimental animals. It is metabolically eliminated by epoxide hydrolase and glutathione S-transferase and a small fraction is exhaled unchanged. To estimate the body burden of EO in rodents and human resulting from exposures to EO and ET, we developed a physiological toxicokinetic model. It describes uptake of ET and EO following inhalation and intraperitoneal administration, endogenous production of ET, enzyme-mediated oxidation of ET to EO, bioavailability of EO, EO metabolism, and formation of 2-hydroxyethyl adducts of hemoglobin and DNA. The model includes compartments representing arterial, venous, and pulmonary blood, liver, muscle, fat, and richly perfused tissues. Partition coefficients and metabolic parameters were derived from experimental data or published values. Model simulations were compared with a series of data collected in rodents or humans. The model describes well the uptake, elimination, and endogenous production of ET in all three species. Simulations of EO concentrations in blood and exhaled air of rodents and humans exposed to EO or ET were in good agreement with measured data. Using published rate constants for the formation of 2-hydroxyethyl adducts with hemoglobin and DNA, adduct levels were predicted and compared with values reported. In humans, predicted hemoglobin adducts resulting from exposure to EO or ET are in agreement with measured values. In rodents, simulated and measured DNA adduct levels agreed generally well, but hemoglobin adducts were underpredicted by a factor of 2 to 3. Obviously, there are inconsistencies between measured DNA and hemoglobin adduct levels.

Comparative metabolism of low concentrations of butadiene and its monoepoxide in human and monkey hepatic microsomes

The chronic (2-yr) inhalation toxicity of 1,3-butadiene (BD), a chemical used in large quantities to make rubber and plastics, differs greatly between mice and rats. Mice develop lung tumors after exposures to concentrations as low as 6.25 ppm, whereas rats develop mammary tumors only after exposures to 1000-8000 ppm BD. Extensive research has been carried out to determine where humans fit into this susceptibility range. Species differences in rates of metabolism of BD have been noted, but inconsistencies in metabolism data from different laboratories and some problems in the fit of physiologically based pharmacokinetic (PBPK) models with experimental data have left uncertainties. The experiments reported here are intended to clarify the issue of human metabolism of BD and to determine if metabolism of BD in cynomolgus monkeys is similar enough to metabolism in humans to use in vivo data from monkeys for PBPK modeling. The results indicate that for the reactions studied (oxidation of BD to the mono- and diepoxide), BD is metabolized substantially the same in monkey and human hepatic microsomes. The human metabolism data agreed with that reported earlier when the in vitro metabolism of BD was studied at low BD concentrations. Finally, BD at high concentrations was found to inhibit the further oxidation of its metabolite, the monoepoxide. Incorporation of this information on the competition between BD and its first oxidation product for CYP2E1 should improve the fit of PBPK models.

Glutathione S-transferase M1 genotype influences sister chromatid exchange induction but not adaptive response in human lymphocytes treated with 1,2-epoxy-3-butene

Induction of sister chromatid exchanges (SCEs) by 1,2-epoxy-3-butene (monoepoxybutene, MEB), an epoxide metabolite of 1,3-butadiene, in human whole-blood lymphocyte cultures has previously been observed to depend on the glutathione S-transferase M1 (GSTM1) and T1 (GSTT1) genotype of the blood donor. Pretreatment of lymphocyte cultures with a low dose of MEB has been shown to reduce the SCE response obtained by later treatment with a higher concentration of MEB. To investigate whether this adaptive response depends on the GSTM1 genotype of the donor, SCE induction by MEB (25 and 250 microM at 48 h for 24 h) was studied from whole-blood lymphocyte cultures of young non-smoking male and female subjects representing GSTM1 positive (n=7) and null (n=7) genotypes, with or without a MEB pretreatment (12.5 microM at 24 h). A higher mean number of induced SCEs per cell at 250 microM MEB was observed in lymphocytes of the GSTM1 null than positive donors, a statistically significant difference being obtained in the presence of the adaptive treatment (9.44 vs. 6.56; results from ethanol-treated controls subtracted). The pretreatment resulted in a statistically significant reduction in the response of the GSTM1 null group at both concentrations of MEB and in the GSTM1 positive group at 250 microM. However, there were no statistically significant differences in the adaptive response of the two genotypes. In conclusion, the present study further supported earlier findings on an increased sensitivity of GSTM1 null donors to SCE induction by MEB, suggesting that GSTM1 is involved in the detoxification of MEB in human lymphocyte cultures. As an adaptive response was observed in both GSTM1 positive and null donors, the phenomenon cannot be explained by GSTM1 induction. It may represent induction of other enzymes operating in MEB detoxification, or activation of DNA repair.

Comparison of the disposition of butadiene epoxides in Sprague-Dawley rats and B6C3F1 mice following a single and repeated exposures to 1,3-butadiene via inhalation

1,3-Butadiene (BD), a compound used extensively in the rubber industry, is a potent carcinogen in mice and a weak carcinogen in rats in chronic carcinogenicity bioassays. While many chemicals are known to alter their own metabolism after repeated exposures, the effect of exposure prior to BD on its in vivo metabolism has not been reported. The purpose of the present research was to examine the effect of repeated exposure to BD on tissue concentrations of two mutagenic BD metabolites, butadiene monoepoxide (BDO) and butadiene diepoxide (BDO2). Concentrations of BD epoxides were compared in several tissues of rats and mice following a single exposure or ten repeated exposures to a target concentration of 62.5 ppm BD. Female Sprague-Dawley rats and female B6C3F1 mice were exposed to BD for 6 h or 6 h x 10 days. BDO and BDO2 were quantified in blood and several other tissues following preparation by cryogenic vacuum distillation and analysis by multidimensional gas chromatography-mass spectrometry. Blood and lung BDO concentrations did not differ significantly (P < or = 0.05) between the two exposure regimens in either species. Following multiple exposures to BD, BDO levels were 5- and 1.6-fold higher (P < or = 0.05) in mammary tissue and 2- and 1.4-fold higher in fat tissue of rats and mice, respectively, as compared with single exposures. BDO2 levels also increased in rat fat tissue following multiple exposures to BD. However, in mice, levels of this metabolite decreased by 15% in fat, by 28% in mammary tissue and by 34% in lung tissue following repeated exposures to BD. The finding that the mutagenic epoxide BDO, which is the precursor to the highly mutagenic BDO2, accumulates in rodent fat may be important in assessing the potential risk to humans from inhalation of BD.