Modeling blood/plasma concentrations in dosed feed and dosed drinking water toxicology studies

PBPK modeling to evaluate maximum tolerated doses: A case study with 3-chloroallyl alcohol

Introduction: A physiologically based pharmacokinetic (PBPK) model for 3-chloroallyl alcohol (3-CAA) was developed and used to evaluate the design of assays for the in vivo genotoxicity of 3-CAA. Methods: Model development was supported by read across from a published PBPK model for ethanol. Read across was motivated by the expectation that 3-CAA, which like ethanol is a primary alcohol, is metabolized largely by hepatic alcohol dehydrogenases. The PBPK model was used to evaluate how two metrics of tissue dosimetry, maximum blood concentration (Cmax; mg/L) and area under the curve (AUC; mg-hr/L) vary with dose of 3-CAA and with dose route (oral gavage, drinking water). Results: The model predicted that oral gavage results in a 6-fold higher Cmax than the same dose administered in drinking water, but in similar AUCs. Predicted Cmax provided the best correlation with severe toxicity (e.g., lethality) from 3-CAA, consistent with the production of a reactive metabolite. Therefore, drinking water administration can achieve higher sustained concentration without severe toxicity in vivo. Discussion: This evaluation is significant because cytotoxicity is a potential confounder of mutagenicity testing. The PBPK model can be used to ensure that studies meet OECD and USEPA test guidelines and that the highest dose used is not associated with severe toxicity. In addition, PBPK modeling provides assurance of target tissue (e.g., bone marrow) exposure even in the absence of laboratory data, by defining the relationship between applied dose and target tissue dose based on accepted principles of pharmacokinetics, relevant physiology and biochemistry of the dosed animals, and chemical-specific information.

Integration of paraquat pharmacokinetic data across species using PBPK modelling

Paraquat dichloride (PQ) is a non-selective herbicide which has been the subject of numerous toxicology studies over more than 50 years. This paper describes the development of a physiologically-based pharmacokinetic (PBPK) model of PQ kinetics for the rat, mouse and dog, firstly to aid the interpretation of studies in which no kinetic measurements were made, and secondly to enable the future extension of the model to humans. Existing pharmacokinetic data were used to develop a model for the rat and mouse. Simulations with this preliminary model were then used to identify key data gaps and to design a new blood binding study to reduce uncertainty in critical aspects of the model. The new data provided evidence to support the model structure, and its predictive performance was then assessed against dog and rat datasets not used in model development. The PQ-specific model parameters are the same for all three species, with only the physiological parameters varying between species. This consistency across species provides a strong basis for extrapolation to other species, as demonstrated here for the dog. The model enables a wide range of PQ data to be linked together to provide a broad understanding of PQ pharmacokinetics in rodents and the dog, showing that the key aspects of PQ kinetics in these species are understood and adequately encapsulated within the model.

Modeling energy intake by adding homeostatic feedback and drug intervention

Energy intake (EI) is a pivotal biomarker used in quantification approaches to metabolic disease processes such as obesity, diabetes, and growth disorders. Eating behavior is however under both short-term and long-term control. This control system manifests itself as tolerance and rebound phenomena in EI, when challenged by drug treatment or diet restriction. The paper describes a model with the capability to capture physiological counter-regulatory feedback actions triggered by energy imbalances. This feedback is general as it handles tolerance to both increases and decreases in EI, and works in both acute and chronic settings. A drug mechanism function inhibits (or stimulates) EI. The deviation of EI relative to a reference level (set-point) serves as input to a non-linear appetite control signal which in turn impacts EI in parallel to the drug intervention. Three examples demonstrate the potential usefulness of the model in both acute and chronic dosing situations. The model shifts the predicted concentration-response relationship rightwardly at lower concentrations, in contrast to models that do not handle functional adaptation. A fourth example further shows that the model may qualitatively explain differences in rate and extent of adaptation in observed EI and its concomitants in both rodents and humans.

Dietary administration of paraquat for 13 weeks does not result in a loss of dopaminergic neurons in the substantia nigra of C57BL/6J mice

Several investigations have reported that mice administered paraquat dichloride (PQ·Cl2) by intraperitoneal injection exhibit a loss of dopaminergic neurons in the substantia nigra pars compacta (SNpc). In this study, male and female C57BL/6J mice were administered PQ·Cl2 in the diet at concentrations of 0 (control), 10, and 50ppm for a duration of 13weeks. A separate group of mice were administered 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine (MPTP) during week 12 as positive controls to produce a loss of dopaminergic neurons in the SNpc. The comparative effects of PQ and MPTP on the SNpc and/or striatum were assessed using neurochemical, neuropathological, and stereological endpoints. Morphological and stereological assessments were performed by investigators 'blinded' to the origin of the tissue. Neither dose of PQ·Cl2 (10 or 50 ppm in the diet) caused a loss of striatal dopamine or dopamine metabolite concentrations in the brains of mice. Pathological assessments of the SNpc and striatum showed no evidence of neuronal degeneration or astrocytic/microglial activation. Furthermore, the number of tyrosine hydroxylase-positive (TH(+)) neurons in the SNpc was not reduced in PQ-treated mice. In contrast, MPTP caused a decrease in striatal dopamine concentration, a reduction in TH(+) neurons in the SNpc, and significant pathological changes including astrocytic and microglial activation in the striatum and SNpc. The MPTP-induced effects were greater in males than in females. It is concluded that 13weeks of continuous dietary exposure of C57BL/6J mice to 50ppm PQ·Cl2 (equivalent to 10.2 and 15.6mg PQ ion/kg body weight/day for males and females, respectively) does not result in the loss of, or damage to, dopaminergic neurons in the SNpc.

Differential muscle gene expression as a function of disease progression in Goto-Kakizaki diabetic rats

The Goto-Kakizaki (GK) rat, a polygenic non-obese model of type 2 diabetes, is a useful surrogate for study of diabetes-related changes independent of obesity. GK rats and appropriate controls were killed at 4, 8, 12, 16 and 20 weeks post-weaning and differential muscle gene expression along with body and muscle weights, plasma hormones and lipids, and blood cell measurements were carried out. Gene expression analysis identified 204 genes showing 2-fold or greater differences between GK and controls in at least 3 ages. Array results suggested increased oxidative capacity in GK muscles, as well as differential gene expression related to insulin resistance, which was also indicated by HOMA-IR measurements. In addition, potential new biomarkers in muscle gene expression were identified that could be either a cause or consequence of T2DM. Furthermore, we demonstrate here the presence of chronic inflammation evident both systemically and in the musculature, despite the absence of obesity.

Mechanistic modeling of the effects of glucocorticoids and circadian rhythms on adipokine expression

A mechanism-based model was developed to describe the effects of methylprednisolone (MPL), circadian rhythms, and the glucose/free fatty acid (FFA)/insulin system on leptin and adiponectin expression in white adipose tissue in rats. Fifty-four normal Wistar rats received 50 mg/kg MPL intramuscularly and were sacrificed at various times. An additional set of 54 normal Wistar rats were sacrificed at 18 time points across the 24-h light/dark cycle and served as controls. Measurements included plasma MPL, glucocorticoid receptor (GR) mRNA, leptin mRNA, adiponectin mRNA, plasma leptin, adiponectin, glucose, FFA, and insulin. MPL pharmacokinetics was described by a two-compartment model with two absorption components. All measured plasma markers and mRNA expression exhibited circadian patterns except for adiponectin and were described by Fourier harmonic functions. MPL caused significant down-regulation in GR mRNA with the nadir occurring at 5 h. MPL disrupted the circadian patterns in plasma glucose and FFA by stimulating their production. Plasma glucose and FFA subsequently caused an increase in plasma insulin. Furthermore, MPL disrupted the circadian patterns in leptin mRNA expression by stimulating its production. This rise was closely followed by an increase in plasma leptin. Both leptin mRNA and plasma leptin peaked at 12 h after MPL and eventually returned back to their circadian baselines. MPL and insulin had opposing effects on adiponectin mRNA expression and plasma adiponectin, which resulted in biphasic pharmacodynamic profiles. This small systems model quantitatively describes, integrates, and provides additional insights into various factors controlling adipokine gene expression.

Adipose tissue deficiency and chronic inflammation in diabetic Goto-Kakizaki rats

Type 2 diabetes (T2DM) is a heterogeneous group of diseases that is progressive and involves multiple tissues. Goto-Kakizaki (GK) rats are a polygenic model with elevated blood glucose, peripheral insulin resistance, a non-obese phenotype, and exhibit many degenerative changes observed in human T2DM. As part of a systems analysis of disease progression in this animal model, this study characterized the contribution of adipose tissue to pathophysiology of the disease. We sacrificed subgroups of GK rats and appropriate controls at 4, 8, 12, 16 and 20 weeks of age and carried out a gene array analysis of white adipose tissue. We expanded our physiological analysis of the animals that accompanied our initial gene array study on the livers from these animals. The expanded analysis included adipose tissue weights, HbA1c, additional hormonal profiles, lipid profiles, differential blood cell counts, and food consumption. HbA1c progressively increased in the GK animals. Altered corticosterone, leptin, and adiponectin profiles were also documented in GK animals. Gene array analysis identified 412 genes that were differentially expressed in adipose tissue of GKs relative to controls. The GK animals exhibited an age-specific failure to accumulate body fat despite their relatively higher calorie consumption which was well supported by the altered expression of genes involved in adipogenesis and lipogenesis in the white adipose tissue of these animals, including Fasn, Acly, Kklf9, and Stat3. Systemic inflammation was reflected by chronically elevated white blood cell counts. Furthermore, chronic inflammation in adipose tissue was evident from the differential expression of genes involved in inflammatory responses and activation of natural immunity, including two interferon regulated genes, Ifit and Iipg, as well as MHC class II genes. This study demonstrates an age specific failure to accumulate adipose tissue in the GK rat and the presence of chronic inflammation in adipose tissue from these animals.

Circadian variations in gene expression in rat abdominal adipose tissue and relationship to physiology

Circadian rhythms occur in all levels of organization from expression of genes to complex physiological processes. Although much is known about the mechanism of the central clock in the suprachiasmatic nucleus, the regulation of clocks present in peripheral tissues as well as the genes regulated by those clocks is still unclear. In this study, the circadian regulation of gene expression was examined in rat adipose tissue. A rich time series involving 54 animals euthanized at 18 time points within the 24-h cycle (12:12 h light-dark) was performed. mRNA expression was examined with Affymetrix gene array chips and quantitative real-time PCR, along with selected physiological measurements. Transcription factors involved in the regulation of central rhythms were examined, and 13 showed circadian oscillations. Mining of microarray data identified 190 probe sets that showed robust circadian oscillations. Circadian regulated probe sets were further parsed into seven distinct temporal clusters, with >70% of the genes showing maximum expression during the active/dark period. These genes were grouped into eight functional categories, which were examined within the context of their temporal expression. Circadian oscillations were also observed in plasma leptin, corticosterone, insulin, glucose, triglycerides, free fatty acids, and LDL cholesterol. Circadian oscillation in these physiological measurements along with the functional categorization of these genes suggests an important role for circadian rhythms in controlling various functions in white adipose tissue including adipogenesis, energy metabolism, and immune regulation.

Physiologically based pharmacokinetic modeling of dibromoacetic acid in F344 rats

A novel physiologically based pharmacokinetic (PBPK) model structure, which includes submodels for the common metabolites (glyoxylate (GXA) and oxalate (OXA)) that may be involved in the toxicity or carcinogenicity of dibromoacetic acid (DBA), has been developed. Particular attention is paid to the representation of hepatic metabolism, which is the primary elimination mechanism. DBA-induced suicide inhibition is modeled by irreversible covalent binding of the intermediate metabolite alpha-halocarboxymethylglutathione (alphaH1) to the glutathione-S-transferase zeta (GSTzeta) enzyme. We also present data illustrating the presence of a secondary non-GSTzeta metabolic pathway for DBA, but not dichloroacetic acid (DCA), that produces GXA. The model is calibrated with plasma and urine concentration data from DBA exposures in female F344 rats through intravenous (IV), oral gavage, and drinking water routes. Sensitivity analysis is performed to confirm identifiability of estimated parameters. Finally, model validation is performed with data sets not used during calibration. Given the structural similarity of dihaloacetates (DHAs), we hypothesize that the PBPK model presented here has the capacity to describe the kinetics of any member or mixture of members of this class in any species with the alteration of chemical-and species-specific parameters.

Quantitative pharmacology or pharmacokinetic pharmacodynamic integration should be a vital component in integrative pharmacology

Pharmacodynamics (PD) examines the relationship between drug concentration and onset, intensity, and duration of the pharmacological effect. Pharmacokinetics (PK) is the science of the time course of drugs in the organism. The quantitative pharmacological approach focuses on concentration-response and response-time relationships, with special emphasis on the proposed impact of the drug on the disease. The review aims to raise awareness among pharmacologists with regard to why pharmacokinetic-pharmacodynamic (PKPD) integration is essential in basic pharmacology research to improve interpretation of data. Quantitative pharmacology is vital in drug discovery for target validation, optimizing the development of lead compounds, and scaling compounds to humans and has become mandatory for regulatory bodies. However, its use is still comparatively rare in experimental pharmacology, and its absence diminishes the interpretative value of published experimental data and can allow the presentation of misleading information. A primary requirement for PKPD integration is establishing the inter-relationships between in vitro and in vivo PK and PD properties and extrapolation to the known or possible future clinical use of a compound. This review examines the use of PKPD in experimental pharmacology by reviewing drug exposure measurements, plasma protein binding, exposure-effect relationships, and the measurement of active metabolites. It examines the significance of dosing schedules, the importance of target engagement, and problems in examining time-response relationships. It shows how quantitative pharmacology adds significant value to study design and examines why ignoring pharmacokinetics can lead to misleading results and conclusions. Finally, a guide list of points to be considered when performing studies is provided.

Comparing single and repeated dosimetry data for perfluorooctane sulfonate in rats

Perfluorooctane sulfonate (PFOS) is a member of a class of perfluorinated chemicals used in a variety of consumer and industrial applications because of their oleophobic and hydrophobic properties. It has been shown to cause toxicity in adult and developing laboratory animals. Because PFOS has also been shown to be widely distributed throughout the environment, there have been concerns about its potential health risk to humans. Limited pharmacokinetic data for PFOS are available in rodents and humans, while epidemiological studies of workers and extensive toxicity studies in rodents have been performed. The existing pharmacokinetic and toxicity database in rodents can be useful in the cross-species extrapolations needed to evaluate and interpret internal dosimetry in humans. A mathematical model that describes the disposition of PFOS in adult rats following intravenous, oral, and chronic dietary exposures was developed to gain a better understanding of the pharmacokinetics of PFOS and to determine whether single-dose kinetics are predictive of repeated-dose kinetics. In order to characterize existing time-course data, time-dependent and concentration-dependent changes in the pharmacokinetic parameters for urinary and biliary clearance and liver distribution were needed. Whether these time-dependent changes represent inconsistencies across experiments, effects of aging in the rats, or chemically induced changes in pharmacokinetics remains to be determined.

Predicting maternal rat and pup exposures: how different are they?

Risk and safety assessments for early life exposures to environmental chemicals or pharmaceuticals based on cross-species extrapolation would greatly benefit from information on chemical dosimetry in the young. Although relevant toxicity studies involve exposures during multiple life stages, the mother's exposure dose is frequently used for extrapolation of rodent toxicity findings to humans and represents a substantial source of uncertainty. A compartmental pharmacokinetic model augmented with biological information on factors changing during lactation and early postweaning was developed. The model uses adult pharmacokinetics, milk distribution, and relevant postnatal biology to predict dosimetry in the young for chemicals. The model addressed three dosing strategies employed in toxicity studies (gavage, constant ppm diet, and adjusted ppm diet) and the impact of different pharmacokinetic properties such as rates of clearance, milk distribution, and volume of distribution on the pup exposure doses and internal dosimetry. Developmental delays in clearance and recirculation of chemical in excreta from the pup to mother were evaluated. Following comparison with data for two chemicals, predictions were made for theoretical chemicals with a range of characteristics. Pup exposure was generally lower than the mother's with a shorter half-life, lower milk transfer, larger volume of distribution, and gavage dosing, while higher with longer half-life, higher milk transfer, smaller volume of distribution, and dietary exposures. The present model demonstrated pup exposures do not always parallel the mother's. The model predictions can be used to help design early life toxicity and pharmacokinetic studies and better interpret study findings.

Strategies to assess systemic exposure of chemicals in subchronic/chronic diet and drinking water studies

Strategies were developed for the estimation of systemically available daily doses of chemicals, diurnal variations in blood levels, and rough elimination rates in subchronic feeding/drinking water studies, utilizing a minimal number of blood samples. Systemic bioavailability of chemicals was determined by calculating area under the plasma concentration curve over 24 h (AUC-24 h) using complete sets of data (> or =5 data points) and also three, two, and one selected time points. The best predictions of AUC-24 h were made when three time points were used, corresponding to Cmax, a mid-morning sample, and C(min). These values were found to be 103 +/- 10% of the original AUC-24 h, with 13 out of 17 values ranging between 96 and 105% of the original. Calculation of AUC-24 h from two samples (Cmax and Cmin) or one mid-morning sample afforded slightly larger variations in the calculated AUC-24 h (69-136% of the actual). Following drinking water exposure, prediction of AUC-24 h using 3 time points (Cmax, mid-morning, and Cmin) was very close to actual values (80-100%) among mice, while values for rats were only 63% of the original due to less frequent drinking behavior of rats during the light cycle. Collection and analysis of 1-3 blood samples per dose may provide insight into dose-proportional or non-dose-proportional differences in systemic bioavailability, pointing towards saturation of absorption or elimination or some other phenomenon warranting further investigation. In addition, collection of the terminal blood samples from rats, which is usually conducted after 18 h of fasting, will be helpful in rough estimation of blood/plasma half-life of the compound. The amount of chemical(s) and/or metabolite(s) in excreta and their possible use as biomarkers in predicting the daily systemic exposure levels are also discussed. Determining these parameters in the early stages of testing will provide critical information to improve the appropriate design of other longer-term toxicity studies.

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.

Chloroform: exposure estimation, hazard characterization, and exposure-response analysis

Chloroform has been assessed as a Priority Substance under the Canadian Environmental Protection Act. The general population in Canada is exposed to chloroform principally through inhalation of indoor air, particularly during showering, and through ingestion of tap water. Data on concentrations of chloroform in various media were sufficient to serve as the basis for development of deterministic and probabilistic estimates of exposure for the general population in Canada. On the basis of data acquired principally in studies in experimental animals, chloroform causes hepatic and renal tumors in mice and renal tumors in rats. The weight of evidence indicates that chloroform is likely carcinogenic only at concentrations that induce the obligatory precursor lesions of cytotoxicity and proliferative regenerative response. Since this cytotoxicity is primarily related to rates of formation of reactive, oxidative metabolites, dose response has been characterized in the context of rates of formation of reactive metabolites in the target tissue. Results presented here are from a "hybrid" physiologically based pharmacokinetic (PBPK) animal model that was revised to permit its extension to humans. The relevant measure of exposure response, namely, the mean rate of metabolism in humans associated with a 5% increase in tumor risk (TC05), was estimated on the basis of this PBPK model and compared with tissue dose measures resulting from 24-h multimedia exposure scenarios for Canadians based on midpoint and 95th percentiles for concentrations in outdoor air, indoor air, air in the shower compartment, air in the bathroom after showering, tap water, and food. Nonneoplastic effects observed most consistently at lowest concentrations or doses following repeated exposures of rats and mice to chloroform are cytotoxicity and regenerative proliferation. As for cancer, target organs are the liver and kidney. In addition, chloroform has induced nasal lesions in rats and mice exposed by both inhalation and ingestion at lowest concentrations or doses. The mean rate of metabolism associated with a 5% increase in fatty cysts estimated on the basis of the PBPK model was compared with tissue dose measures resulting from the scenarios already described, and lowest concentrations reported to induce cellular proliferation in the nasal cavities of rats and mice were compared directly with midpoint and 95th percentile estimates of concentrations of chloroform in indoor air in Canada. The degree of confidence in the underlying database and uncertainties in estimates of exposure and in characterization of hazard and dose response are delineated.

A physiologically based pharmacokinetic model of p,p'-dichlorodiphenylsulfone

A physiologically based pharmacokinetic model of the absorption, distribution, metabolism, and elimination of p,p'-dichlorodiphenylsulfone (DDS) in male and female rats and mice is presented. Data used in constructing the model come from single-dose intravenous administration of DDS to male Fischer 344 rats (10 mg/kg, with data taken up to 504 h after administration), from single-dose gavage administration to male rats (10, 100, or 1000 mg/kg, with data up to 72 h after administration), and from chronic feed studies in male and female rats and male and female B6C3F(1) mice (studies of duration from 2 weeks up to 18 months, with feed concentrations of DDS up to 300 ppm). The model uses diffusion-limited kinetic for the distribution of the parent compound. Because fewer data are available for the metabolites of DDS (at least five of which are known to exist in the data), the model groups the metabolites into one metabolic pathway and uses simpler flow-limited kinetics for the metabolites. The data show that the kinetics of DDS are nonlinear. Possible sources of nonlinearity considered in the model were nonlinear (Michaelis-Menten) metabolism, nonlinear absorption of DDS from the gut, and induction by DDS of its own metabolism. A model using Michaelis-Menten metabolism was not found to give a significantly better fit than one using first-order linear metabolism, but omitting either of the other nonlinear effects was found to give a significantly poorer fit to the data. Because the data from mice are limited compared to those from rats, there is more confidence in the model's description of DDS kinetics in rats than in its description of kinetics in mice.

Dichloroacetate (DCA) dosimetry: interpreting DCA-induced liver cancer dose response and the potential for DCA to contribute to trichloroethylene-induced liver cancer

Pharmacokinetic studies with dichloroacetate (DCA) provide insights into the likelihood that trichloroethylene-induced liver cancers arise from formation of DCA as a metabolite and the mode of action by which DCA induces liver cancer. A simple physiologically based pharmacokinetic model was developed to analyze DCA blood concentration data from mice unexposed to or pre-treated with DCA. The large first pass metabolism of DCA in the liver is significantly reduced by DCA pretreatment. Because DCA inhibits its own metabolism, large increases in area under the blood concentration curve occur at lower doses than would be predicted from single-dose pharmacokinetic studies with naive mice. The dose metrics associated with the incidence of liver tumors in contrast to the multiplicity of tumors per animal may be different, suggesting potentially different roles in the cancer process for DCA versus its metabolites. By linking a model for trichloroethylene (TCE) pharmacokinetics with the DCA model, maximum levels of DCA potentially produced from TCE were estimated to be at or below the analytical chemistry detection limits. In addition, the predicted levels of DCA would be too small to produce the observed liver cancers following corn oil gavage exposure of mice to TCE.

Effects of drinking pattern on the peak/trough blood concentrations in drinking water studies

The effects of changes in drinking patterns on the expected peak/trough blood concentrations of test compounds were examined during rodent dosed drinking water studies. They were based on the assumption that the kinetics of the test compound is linear and time-invariant. Results indicate that drinking patterns have minor effects on the expected peak/trough concentrations and the time to reach these concentrations. If a 12-hr light/dark cycle starting at 7.00 is used for all the drinking patterns studied, the peak and trough concentrations will occur in the early morning and late afternoon, respectively. A comparison of the predicted versus experimentally determined pentachlorophenol (PCP) plasma concentrations in a 1-wk rat drinking water study revealed that using a circadian rhythm drinking pattern in the model generated the most satisfactory prediction. Predictions based on a square wave drinking pattern with 90% drinking activities in the night phase were also excellent. Triangular or sinusoidal drinking patterns were least accurate in predictions.

Effects of gavage versus dosed feed administration on the toxicokinetics of benzyl acetate in rats and mice

Effects of gavage versus dosed feed administration on the toxicokinetics of benzyl acetate were studied in male F344 rats and B6C3F1 mice. Benzyl acetate was rapidly hydrolysed to benzyl alcohol and then oxidized to benzoic acid. After gavage administration of benzyl acetate in corn oil at 500 mg/kg (rats) and 1000 mg/kg (mice), high benzoic acid plasma concentrations were observed. In contrast, much lower benzoic acid plasma concentrations were found after dosed feed administration at about 615 mg/kg/day for rats and about 850 mg/kg/day for mice. Results show that although the daily doses of benzyl acetate are comparable, bolus gavage administration effectively saturated the benzoic acid elimination pathway whereas dosed feed administration did not. In contrast, hippuric acid plasma concentrations were similar after both gavage and dosed feed administration due to the depletion of the glycine supply pool. Study results could explain the different toxicity and carcinogenicity responses of benzyl acetate observed in 2-yr chronic gavage and dosed feed studies.

Toxicokinetics of oxazepam in rats and mice

The comparative toxicokinetics of oxazepam were studied in F344 rats, B6C3F1 mice, and Swiss-Webster mice of both sexes after an i.v. dose of 20 mg/kg and oral gavage doses of 50, 200, and 400 mg/kg. In addition, the toxicokinetics of oxazepam in a 3-week dosed-feed study of male B6C3F1 mice at 125 and 2500 ppm were also investigated. Results indicated that the elimination of oxazepam from plasma after i.v. injection in both rats and mice were first-order and could be best described by a two-compartment model with a terminal elimination half-life of 4-5 h for rats and 5-7 h for mice. After oral gavage dosing the peak oxazepam plasma concentrations in most rodents were reached within 2-3.5 h. At all doses studied, female rodents had significantly higher plasma concentrations than males. Absorption of oxazepam was significantly extended at higher oral doses of 200 and 400 mg/kg. At 50 mg/kg, the bioavailability of oxazepam in rats (< 50%) was lower than in Swiss-Webster mice (> 80%). The bioavailability of oxazepam in both B6C3F1 and Swiss-Webster mice decreased with increasing dose. A dose proportionality of Cmax was not observed in rats and mice after gavage doses of 50, 200, and 400 mg/kg. Plasma concentrations of oxazepam in the dosed-feed study increased with the concentration of oxazepam in the feed, a quasi-steady-state of plasma concentrations of oxazepam was reached after approximately 4 days ad libitum exposure. In B6C3F1 mice, the estimated relative bioavailability of oxazepam from dosed feed (relative to gavage study at 50 mg/kg) was about 43%.

Toxicokinetics of pentachlorophenol in the F344 rat. Gavage and dosed feed studies

1. The toxicokinetics of pentachlorophenol (PCP) were studied in the Fischer 344 rat using i.v. and oral (gavage, dosed feed) routes of exposure. 2. Only minor sex differences were observed in the elimination kinetics of PCP after i.v. administration at 5 mg/kg. 3. Absorption of PCP from the gastrointestinal tract after gavage doses of 9.5 and 38 mg/kg in aqueous methylcellulose vehicles was first order with an absorption half-life of about 1.3 h. 4. The absorption rate constant of PCP from doses feed was comparable with that obtained from aqueous methylcellulose gavage formulations. 5. Bioavailability of PCP administered in dosed feed was significantly lower than the bioavailability of PCP administered by gavage. 6. Dose proportionality was established to a dosage of at least 38 mg/kg. 7. Daily fluctuation of PCP plasma concentrations was observed during the dosed feed study with peak and trough concentrations occurring in early morning and late afternoon, respectively. 8. The time course of PCP plasma concentrations during the dosed feed study were simulated using a computer model based on linear theory. The simulations were comparable with the experimentally determined concentrations.