Development of a physiologically-based pharmacokinetic model of 2-phenoxyethanol and its metabolite phenoxyacetic acid in rats and humans to address toxicokinetic uncertainty in risk assessment.

2-Phenoxyethanol (PhE) has been shown to induce hepatotoxicity, renal toxicity, and hemolysis at dosages ≥ 400 mg/kg/day in subchronic and chronic studies in multiple species. To reduce uncertainty associated with interspecies extrapolations and to evaluate the margin of exposure (MOE) for use of PhE in cosmetics and baby products, a physiologically-based pharmacokinetic (PBPK) model of PhE and its metabolite 2-phenoxyacetic acid (PhAA) was developed. The PBPK model incorporated key kinetic processes describing the absorption, distribution, metabolism and excretion of PhE and PhAA following oral and dermal exposures. Simulations of repeat dose rat studies facilitated the selection of systemic AUC as the appropriate dose metric for evaluating internal exposures to PhE and PhAA in rats and humans. Use of the PBPK model resulted in refinement of the total default UF for extrapolation of the animal data to humans from 100 to 25. Based on very conservative assumptions for product composition and aggregate product use, model-predicted exposures to PhE and PhAA resulting from adult and infant exposures to cosmetic products are significantly below the internal dose of PhE observed at the NOAEL dose in rats. Calculated MOEs for all exposure scenarios were above the PBPK-refined UF of 25.

Nonlinear responses for chromosome and gene level effects induced by vinyl acetate monomer and its metabolite, acetaldehyde in TK6 cells.

Vinyl acetate monomer (VAM) produced rat nasal tumors at concentrations in the hundreds of parts per million. However, VAM is weakly genotoxic in vitro and shows no genotoxicity in vivo. A European Union Risk Assessment concluded that VAM's hydrolysis to acetaldehyde (AA), via carboxylesterase, is a critical key event in VAM's carcinogenic potential. In the following study, we observed increases in micronuclei (MN) and thymidine kinase (Tk) mutants that were dependent on the ability of TK6 cell culture conditions to rapidly hydrolyze VAM to AA. Heat-inactivated horse serum demonstrated a high capacity to hydrolyze VAM to AA; this activity was highly correlated with a concomitant increase in MN. In contrast, heat-inactivated fetal bovine serum (FBS) did not hydrolyze VAM and no increase in MN was observed. AA's ability to induce MN was not impacted by either serum since it directly forms Schiff bases with DNA and proteins. Increased mutant frequency at the Tk locus was similarly mitigated when AA formation was not sufficiently rapid, such as incubating VAM in the presence of FBS for 4 hr. Interestingly, neither VAM nor AA induced mutations at the HPRT locus. Finally, cytotoxicity paralleled genotoxicity demonstrating that a small degree of cytotoxicity occurred prior to increases in MN. These results established 0.25 mM as a consistent concentration where genotoxicity first occurred for both VAM and AA provided VAM is hydrolyzed to AA. This information further informs significant key events related to the mode of action of VAM-induced nasal mucosal tumors in rats.

Reconstruction of Exposure to m-Xylene from Human Biomonitoring Data Using PBPK Modelling, Bayesian Inference, and Markov Chain Monte Carlo Simulation.

There are numerous biomonitoring programs, both recent and ongoing, to evaluate environmental exposure of humans to chemicals. Due to the lack of exposure and kinetic data, the correlation of biomarker levels with exposure concentrations leads to difficulty in utilizing biomonitoring data for biological guidance values. Exposure reconstruction or reverse dosimetry is the retrospective interpretation of external exposure consistent with biomonitoring data. We investigated the integration of physiologically based pharmacokinetic modelling, global sensitivity analysis, Bayesian inference, and Markov chain Monte Carlo simulation to obtain a population estimate of inhalation exposure to m-xylene. We used exhaled breath and venous blood m-xylene and urinary 3-methylhippuric acid measurements from a controlled human volunteer study in order to evaluate the ability of our computational framework to predict known inhalation exposures. We also investigated the importance of model structure and dimensionality with respect to its ability to reconstruct exposure.

TK Modeler version 1.0, a Microsoft® Excel®-based modeling software for the prediction of diurnal blood/plasma concentration for toxicokinetic use.

TK Modeler 1.0 is a Microsoft® Excel®-based pharmacokinetic (PK) modeling program created to aid in the design of toxicokinetic (TK) studies. TK Modeler 1.0 predicts the diurnal blood/plasma concentrations of a test material after single, multiple bolus or dietary dosing using known PK information. Fluctuations in blood/plasma concentrations based on test material kinetics are calculated using one- or two-compartment PK model equations and the principle of superposition. This information can be utilized for the determination of appropriate dosing regimens based on reaching a specific desired C(max), maintaining steady-state blood/plasma concentrations, or other exposure target. This program can also aid in the selection of sampling times for accurate calculation of AUC(24h) (diurnal area under the blood concentration time curve) using sparse-sampling methodologies (one, two or three samples). This paper describes the construction, use and validation of TK Modeler. TK Modeler accurately predicted blood/plasma concentrations of test materials and provided optimal sampling times for the calculation of AUC(24h) with improved accuracy using sparse-sampling methods. TK Modeler is therefore a validated, unique and simple modeling program that can aid in the design of toxicokinetic studies.

Statistical methodology to determine kinetically derived maximum tolerated dose in repeat dose toxicity studies.

Several statistical approaches were evaluated to identify an optimum method for determining a point of nonlinearity (PONL) in toxicokinetic data. (1) A second-order least squares regression model was fit iteratively starting with data from all doses. If the second order term was significant (α<0.05), the dataset was reevaluated with successive removal of the highest dose until the second-order term became non-significant. This dose, whose removal made the second order term non-significant, is an estimate of the PONL. (2) A least squares linear model was fit iteratively starting with data from all doses except the highest. The mean response for the omitted dose was compared to the 95% prediction interval. If the omitted dose falls outside the confidence interval it is an estimate of the PONL. (3) Slopes of least squares linear regression lines for sections of contiguous doses were compared. Nonlinearity was suggested when slopes of compared sections differed. A total of 33 dose-response datasets were evaluated. For these toxicokinetic data, the best statistical approach was the least squares regression analysis with a second-order term. Changing the α level for the second-order term and weighting the second-order analysis by the inverse of feed consumption were also considered. This technique has been shown to give reproducible identification of nonlinearities in TK datasets.

Assessment of diurnal systemic dose of agrochemicals in regulatory toxicity testing--an integrated approach without additional animal use.

Integrated toxicokinetics (TK) data provide information on the rate, extent and duration of systemic exposure across doses, species, strains, gender, and life stages within a toxicology program. While routine for pharmaceuticals, TK assessments of non-pharmaceuticals are still relatively rare, and have never before been included in a full range of guideline studies for a new agrochemical. In order to better understand the relationship between diurnal systemic dose (AUC(24h)) and toxicity of agrochemicals, TK analyses in the study animals is now included in all short- (excluding acute), medium- and long-term guideline mammalian toxicity studies including reproduction/developmental tests. This paper describes a detailed procedure for the implementation of TK in short-, medium- and long-term regulatory toxicity studies, without the use of satellite animals, conducted on three agrochemicals (X11422208, 2,4-D and X574175). In these studies, kinetically-derived maximum doses (KMD) from short-term studies instead of, or along with, maximum tolerated doses (MTD) were used for the selection of the high dose in subsequent longer-term studies. In addition to leveraging TK data to guide dose level selection, the integrated program was also used to select the most appropriate method of oral administration (i.e., gavage versus dietary) of test materials for rat and rabbit developmental toxicity studies. The integrated TK data obtained across toxicity studies (without the use of additional/satellite animals) provided data critical to understanding differences in response across doses, species, strains, sexes, and life stages. Such data should also be useful in mode of action studies and to improve human risk assessments.

In vitro metabolism, glutathione conjugation, and CYP isoform specificity of epoxidation of 4-vinylphenol.

4-Vinylphenol (4VP) has been identified as a minor urinary metabolite of styrene in rat and human volunteers. This compound has been shown to be more hepatotoxic and pneumotoxic than both styrene and styrene oxide at lower doses in rats and mice. To explore the possible toxicity mechanism of 4VP, the current study was conducted to investigate the metabolism of 4VP, the glutathione (GSH) conjugation of the metabolites of 4VP and its cytochrome P(450) (CYP) specificity in epoxidation in different microsomes in vitro. Incubations of 4VP with mouse lung microsomes afforded two major metabolites which were identified as 4-(2-oxiranyl)-phenol of 4VP (4VPO) and 4VP catechol. 4VPO was found to react with GSH to form GSH conjugate and 4VP catechol was found to further be metabolized to electrophilic species which react with GSH to form the corresponding 4VP catechol GSH conjugates. Relative formation rates for those GSH conjugates and the regioisomer formation of 4VPO-GSH conjugates with both inhibitors of CYP 2F2 and CYP 2E1 in microsomal incubation condition were also investigated. This present study provides better insight on the lung toxicity seen with 4VP, the toxic metabolite of commercial styrene.

In vitro metabolism and covalent binding of ethylbenzene to microsomal protein as a possible mechanism of ethylbenzene-induced mouse lung tumorigenesis.

This study was conducted to determine species differences in covalent binding of the reactive metabolites of ethylbenzene (EB) formed in the liver and lung microsomes of mouse, rat and human in the presence of NADPH. These data further the understanding of the mechanism by which EB causes mouse specific lung toxicity and a follow-up to our earlier report of the selective elevation, although minor, of the ring-oxidized reactive metabolites in mouse lung microsomes (Saghir et al., 2009). Binding assays were also conducted with or without 5-phenyl-1-pentyne (5P1P), an inhibitor of CYP 2F2, and diethyldithiocarbamate (DDTC), an inhibitor of CYP 2E1 to evaluate their role in the formation of the related reactive metabolites. Liver and lung microsomes were incubated with (14)C-EB (0.22 mM) in the presence of 1mM NADPH under physiological conditions for 60 min. In lung microsomes, binding activity was in the order of mouse (812.4+/-102.2 pmol/mg protein)>>rat (57.0+/-3.2 pmol/mg protein). Human lung microsomes had little binding activity (15.7+/-1.4 pmol/mg protein), which was comparable to the no-NADPH control (9.9-16.7 pmol/mg protein). In liver microsomes, mouse had the highest activity (469.0+/-38.5 pmol/mg protein) followed by rat (148.3+/-14.7 pmol/mg protein) and human (89.8+/-3.0 pmol/mg protein). Presence of 5P1P or DDTC decreased binding across species and tissues. However, much higher inhibition was observed in mouse (86% [DDTC] and 89% [5P1P]) than rat (56% [DDTC] and 59% [5P1P]) lung microsomes. DDTC showed approximately 2-fold higher inhibition of binding in mouse and human liver microsomes than 5P1P (mouse=85% vs. 40%; human=59% vs. 36%). Inhibition in binding by DDTC was much higher (10-fold) than 5P1P (72% vs. 7%) in rat liver microsomes. These results show species, tissue and enzyme differences in the formation of reactive metabolites of EB. In rat and mouse lung microsomes, both CYP2E1 and CYP2F2 appear to contribute in the formation of reactive metabolites of EB. In contrast, CYP2E1 appears to be the primary CYP isozyme responsible for the reactive metabolites of EB in the liver.

The effect of plasma lipids on the pharmacokinetics of chlorpyrifos and the impact on interpretation of blood biomonitoring data.

Lipophilic molecules, like chlorpyrifos (CPF), present a special problem for interpretation of biomonitoring data because both the environmental dose of CPF and the physiological (pregnancy, diet, etc.) or pathological levels of blood lipids will affect the concentrations of CPF measured in blood. The objective of this study was to investigate the distribution of CPF between plasma and tissues when lipid levels are altered in late pregnancy. CPF was sequestered more in the low-density lipid fraction of the blood during the late stages of gestation in the rat and returned to nonpregnant patterns in the dam after birth. Plasma partitioning of CPF increased with increases in plasma lipid levels and the increased partitioning of CPF into plasma lipids resulted in less CPF in other tissue compartments. Gavage dosing with corn oil also increased plasma lipids that led to a moderate increase of CPF partitioning into the plasma. To mechanistically investigate the potential pharmacokinetic effects of blood lipid changes, an existing CPF physiologically based pharmacokinetic/pharmacodynamic model for rats and humans was modified to account for altered lipid-tissue partition coefficients and for major physiological and biochemical changes of pregnancy. The model indicated that plasma CPF levels are expected to be proportional to the well-known changes in plasma lipids during gestation. There is a rapidly growing literature on the relationship of lipid profiles with different disease conditions and on birth outcomes. Increased blood concentrations of lipophilic chemicals like CPF may point to altered lipid status, as well as possibly higher levels of exposure. Thus, proper interpretation of blood biomonitoring data of lipophilic chemicals requires a careful consideration of blood lipids.

Mechanism of ethylbenzene-induced mouse-specific lung tumor: metabolism of ethylbenzene by rat, mouse, and human liver and lung microsomes.

This study was conducted to determine species differences in the metabolism of ethylbenzene (EB) in liver and lung. EB (0.22-7.0mM) was incubated with mouse, rat and human liver and lung microsomes and the formation of 1-phenylethanol (1PE), acetophenone (AcPh), 2-ethylphenol (2EP), 4-ethylphenol (4EP), 2,5-ethylquinone, and 3,4-ethylquinone were measured. Reactive metabolites (2,5-dihydroxyethylbenzene-GSH [2EP-GSH] and 3,4-dihydroxyethylbenzene-GSH [4EP-GSH]) were monitored via glutathione (GSH) trapping technique. None of the metabolites were formed at detectable levels in incubations with human lung microsomes. Percent conversion of EB to 1PE ranged from 1% (rat lung; 7.0mM EB) to 58% (mouse lung; 0.22 mM EB). More 1PE was formed in mouse lung than in mouse liver microsomes, although formation of 1PE by rat liver and lung microsomes was similar. Metabolism of EB to 1PE was in the order of mouse > rat > human. Formation of AcPh was roughly an order of magnitude lower than 1PE. Conversion of EB to ring-hydroxylated metabolites was much lower (0.0001% [4EP-GSH; rat lung] to 0.6% [2EP-GSH; mouse lung]); 2EP-GSH was typically 10-fold higher than 4EP-GSH. Formation of 2EP-GSH was higher by lung (highest by mouse lung) than liver microsomes and the formation of 2EP-GSH by mouse liver microsomes was higher than rat and human liver microsomes. Increasing concentrations of EB did lead to a decrease in amount of some formed metabolites. This may indicate some level of substrate- or metabolite-mediated inhibition. High concentrations of 2EP and 4EP were incubated with microsomes to further investigate their oxidation to ethylcatechol (ECat) and ethylhydroquinone (EHQ). Conversion of 2EP to EHQ ranged from 6% to 9% by liver (mouse > human > rat) and from 0.1% to 18% by lung microsomes (mouse >> rat >> human). Conversion of 4EP to ECat ranged from 2% to 4% by liver (mouse > human approximately rat) and from 0.3% to 7% by lung microsomes (mouse >> rat >> human). Although ring-oxidized metabolites accounted for a relatively small fraction of overall EB metabolism, its selective elevation in mouse lung microsomes is nonetheless consistent with the hypothesized mode of action for observed preferential toxicity of EB to the lung in this species.

Simulation of repeated dose kinetics of methyl isobutyl ketone in humans from experimental single-dose inhalation exposure.

Methyl isobutyl ketone (MIBK) is a solvent used in numerous products and processes and may be present in the air of the workplace as a vapor. The American Conference of Governmental Industrial Hygienists (ACGIH) threshold limit value-time-weighted average (TLV-TWA) and TLV-short term exposure limit (TLV-STEL) for MIBK are 50 and 75 ppm, respectively. These workplace air concentration limits were set to protect workers from irritation, neurasthenic symptoms and possible adverse effects to their livers and kidneys. A recent revision of the ACGIH limit value has been proposed, to reduce the current TLV-TWA to 30 ppm. This article predicts the kinetics and accumulation of MIBK in humans exposed repeatedly in various exposure scenarios (8, 12, and 24h/day for 7 days) to the current ACGIH TLV-TWA of 50 ppm. The kinetic parameters of the model were derived from published human time-course blood MIBK data from a single 2h inhalation exposure to 48.9 ppm MIBK. The model correctly simulated single exposure experimental data with a rapid rise in blood concentration to 1.06 microg/ml within 1h and approached >or=99% steady-state blood level in 4h of exposure. MIBK was predicted to be rapidly eliminated from blood after terminating the exposure, reaching 0.53 microg/ml and 0.13 microg/ml within 0.5 and 2h post-exposure, respectively. Within 4h after the termination of exposure, blood concentration would be expected to <1% of the steady-state concentration. On the basis of these results, it is concluded that accumulation of MIBK in workers due to repeated inhalation exposure is not likely to occur at the current TLV-TWA concentration of 50 ppm.

Determination of the dietary absorption efficiency of hexachlorobenzene with the channel catfish (Ictalurus punctatus).

This study was designed to experimentally measure the assimilation efficiency of hexachlorobenzene (HCB) in a warm-water, benthic-feeding fish species, the channel catfish (Ictalurus punctatus). Catfish were exposed to (14)C-radiolabeled HCB in catfish food over a 28-day exposure period, followed by a 14-day clearance period. Over the experimental period, the total (14)C residues were measured in fish tissue and a simple two-box kinetic model was applied to the data to simulate uptake and clearance dynamics. No detectable metabolism of HCB by catfish was found. A two-box kinetic model effectively modeled the uptake and clearance of (14)C-HCB in catfish, with a calculated assimilation efficiency of the chemical into the whole catfish of 67% (growth corrected). The growth-corrected pseudo first-order elimination half-life of (14)C-HCB from whole catfish was determined to be 29 days (k(2)=0.024 day(-1)).

Investigations of oxidative stress, antioxidant response, and protein binding in chlorpyrifos exposed rat neuronal PC12 cells.

ABSTRACT Chlorpyrifos (CPF) is a widely used organophosphate insecticide. In addition to its known properties of cholinesterase inhibition, the production of reactive oxygen species (ROS) has been suggested as a possible toxic mechanism. To investigate CPF-generated ROS, rat neuronal PC12 cells were exposed to CPF concentrations of 0 to 5000 mug/mL in Krebs buffered media (KRH), KRH + 4% bovine serum albumin (BSA), and KRH + 25 muM of the antioxidant Trolox for 0 to 5 h. Paraquat served as a positive control for ROS. The fluorescent probe 2,7-dichlorodihydro-fluorescein and the MTS assay were used to measure ROS and cytotoxicity, respectively. Examinations into CPF-albumin binding were also conducted. CPF was not strongly cytotoxic to PC12 cells, causing only mild cytotoxicity at 5000 mug/ml. In KRH media, CPF-generated ROS was observed at 4 and 5 h at 500 and 1000 mug/mL, and at 1 to 5 h at 5000 mug/mL CPF. In KRH + 4% BSA, ROS was seen only at 5 h in 5000 mug/mL CPF. Trolox significantly reduced CPF- and paraquat-induced ROS. Calculated CPF-albumin binding at 1, 10, and 100 mug/mL CPF in 4% BSA was 96%, 75%, and 15%. These data show CPF at >/=500 mug/mL induced ROS in PC12 cells, but the addition of the antioxidant Trolox and 4% BSA dramatically reduced ROS levels.