Jerry L. Campbell

Competitive Inhibition of Thyroidal Uptake of Dietary Iodide by Perchlorate Does Not Describe Perturbations in Rat Serum Total T4 and TSH

Background Perchlorate (ClO4−) is an environmental contaminant known to disrupt the thyroid axis of many terrestrial and aquatic species. ClO4− competitively inhibits iodide uptake into the thyroid at the sodium/iodide symporter and disrupts hypothalamic–pituitary–thyroid (HPT) axis homeostasis in rodents. Objective We evaluated the proposed mode of action for ClO4−-induced rat HPT axis perturbations using a biologically based dose–response (BBDR) model of the HPT axis coupled with a physiologically based pharmacokinetic model of ClO4−. Methods We configured a BBDR-HPT/ClO4− model to describe competitive inhibition of thyroidal uptake of dietary iodide by ClO4− and used it to simulate published adult rat drinking water studies. We compared model-predicted serum thyroid-stimulating hormone (TSH) and total thyroxine (TT4) concentrations with experimental observations reported in these ClO4− drinking water studies. Results The BBDR-HPT/ClO4− model failed to predict the ClO4−-induced onset of disturbances in the HPT axis. Using ClO4− inhibition of dietary iodide uptake into the thyroid, the model underpredicted both the rapid decrease in serum TT4 concentrations and the rise in serum TSH concentrations. Conclusions Assuming only competitive inhibition of thyroidal uptake of dietary iodide, BBDR-HPT/ClO4− model calculations were inconsistent with the rapid decrease in serum TT4 and the corresponding increase in serum TSH. Availability of bound iodide in the thyroid gland governed the rate of hormone secretion from the thyroid. ClO4− is translocated into the thyroid gland, where it may act directly or indirectly on thyroid hormone synthesis/secretion in the rat. The rate of decline in serum TT4 in these studies after 1 day of treatment with ClO4− appeared consistent with a reduction in thyroid hormone production/secretion. This research demonstrates the utility of a biologically based model to evaluate a proposed mode of action for ClO4− in a complex biological process.

Evaluating pharmacokinetic and pharmacodynamic interactions with computational models in supporting cumulative risk assessment.

Simultaneous or sequential exposure to multiple chemicals may cause interactions in the pharmacokinetics (PK) and/or pharmacodynamics (PD) of the individual chemicals. Such interactions can cause modification of the internal or target dose/response of one chemical in the mixture by other chemical(s), resulting in a change in the toxicity from that predicted from the summation of the effects of the single chemicals using dose additivity. In such cases, conducting quantitative cumulative risk assessment for chemicals present as a mixture is difficult. The uncertainties that arise from PK interactions can be addressed by developing physiologically based pharmacokinetic (PBPK) models to describe the disposition of chemical mixtures. Further, PK models can be developed to describe mechanisms of action and tissue responses. In this article, PBPK/PD modeling efforts conducted to investigate chemical interactions at the PK and PD levels are reviewed to demonstrate the use of this predictive modeling framework in assessing health risks associated with exposures to complex chemical mixtures.

Rat Tissue and Blood Partition Coefficients for n-Alkanes (C8 to C12)

Rat tissue:air and blood:air partition coefficients (PCs) for octane, nonane, decane, undecane, and dodecane (n-C8 to n-C12 n-alkanes) were determined by vial equilibration. The blood:air PC values for n-C8 to n-C12 were 3.1, 5.8, 8.1, 20.4, and 24.6, respectively. The lipid solubility of n-alkanes increases with carbon length, suggesting that lipid solubility is an important determinant in describing n-alkane blood:air PC values. The muscle:blood, liver: blood, brain:blood, and fat:blood PC values were octane (1.0, 1.9, 1.4, and 247), nonane (0.8, 1.9, 3.8, and 274), decane (0.9, 2.0, 4.8, and 328), undecane (0.7, 1.5, 1.7, and 529), and dodecane (1.2, 1.9, 19.8, and 671), respectively. The tissue:blood PC values were greatest in fat and the least in muscle. The brain:air PC value for undecane was inconsistent with other n-alkane values. Using the measured partition coefficient values of these n-alkanes, linear regression was used to predict tissue (except brain) and blood:air partition coefficient values for larger n-alkanes, tridecane, tetradecane, pentadecane, hexadecane, and heptadecane (n-C13 to n-C17).Good agreement between measured and predicted tissue:air and blood:air partition coefficient values for n-C8 to n-C12 offer confidence in the partition coefficient predictions for longer chain n-alkanes.

Comparison of Solvents for Removing Pesticides from Skin Using an In Vitro Porcine Model

This study compared four solvents (1-propanol, polyethylene glycol [avg. MW 400], 10% Ivory Liquid and water, and D-TAM) for their ability to remove selected pesticides from an in vitro porcine skin model using a solvent-moistened wipe. Wipes were performed 90 min after pesticide was applied to the skin. The four pesticides selected (glyphosate, alachlor, methyl parathion, and trifluralin) were chosen because of their differences in water solubility. This study also determined whether pretreatment of skin with a solvent prior to pesticide application would either increase or decrease recovery of the pesticide. Recovery efficiencies for all solvents and pesticides were affected by the amount of contaminant on the skin. Although pesticide recoveries from all four solvents were similar (range: 45-57%), on average 1-propanol had significantly higher recoveries, followed by soap and water. There was no significant difference between polyethylene glycol, and D-TAM. When skin was pretreated with any of the four solvents before pesticide application, the recoveries of the more water soluble compounds, glyphosate and alachlor, decreased. When pretreatment with solvent preceded application of trifluralin, the pesticide with the lowest water solubility, recoveries increased. 1-Propanol or soap and water were more effective in removing pesticides from skin than polyethylene glycol or D-TAM, but the amount of pesticide recovered from skin was affected by the chemical characteristics of the pesticide (such as water solubility) and the amount of pesticide originally on the skin. This study provides information useful to the interpretation of skin wipe sample results collected in field studies.

Physiologically Based Pharmacokinetic/Toxicokinetic Modeling

Physiologically based pharmacokinetic (PBPK) models differ from conventional compartmental pharmacokinetic models in that they are based to a large extent on the actual physiology of the organism. The application of pharmacokinetics to toxicology or risk assessment requires that the toxic effects in a particular tissue are related in some way to the concentration time course of an active form of the substance in that tissue. The motivation for applying pharmacokinetics is the expectation that the observed effects of a chemical will be more simply and directly related to a measure of target tissue exposure than to a measure of administered dose. The goal of this work is to provide the reader with an understanding of PBPK modeling and its utility as well as the procedures used in the development and implementation of a model to chemical safety assessment using the styrene PBPK model as an example.

Challenges in the application of quantitative approaches in risk assessment: a case study with di-(2-ethylhexyl)phthalate.

The constantly evolving science of risk assessment is currently faced with many challenges, not only from the interpretation of the volume of data being generated with new innovative technologies, but also in attempting to quantitatively incorporate this information into understanding potential risk of adverse events in human populations. The objective of the case study described was to use the more recent data for di-(2-ethylhexyl)phthalate (DEHP) to investigate the impact of innovative quantitative approaches on the risk assessment of a compound, specifically as it can be used to move towards the new vision of risk assessment involving the integration of the available toxicological data to understand underlying biological processes. What emerged were several outcomes that demonstrated clearly the importance of the integration of the toxicological data, specifically to understand the biological processes being impacted, because standard statistical modeling approaches may not be adequate to describe the dose–response relationships observed. Alternative approaches demonstrate that a definitive mode of action is not needed to justify the shape of the low-dose region or a threshold, when the integration of the available data assist risk assessors in understanding the shape of the dose–response curve for both noncancer and cancer endpoints. Many of the challenges described as part of this case study would likely be encountered with compounds other than DEHP, especially other receptor-mediated compounds or compounds that “perturb” biological pathways, such as endocrine disruptors. This case study also highlights the importance of communication between risk assessors and the research community to focus on the generation of data most relevant for assessing the potential for chemicals to impact biological systems in the human.

A hybrid CFD-PBPK model for naphthalene in rat and human with IVIVE for nasal tissue metabolism and cross-species dosimetry

Abstract A PBPK model for naphthalene in the rat and human that incorporates a hybrid CFD-PBPK description of the upper respiratory tract was developed to support cross-species dosimetry comparisons of naphthalene concentrations and tissue normalized rate of metabolism in the nasal respiratory and olfactory epithelium, lung and liver. In vitro measurements of metabolic rates from microsomal incubations published for rat and monkey (surrogate for human) were scaled to the specific tissue based on the tissue microsomal content and volume of tissue. The model reproduces time courses for naphthalene blood concentrations from intravenous and inhalation exposures in rats and upper respiratory tract extraction data in both naïve rats and rats pre-treated to inhibit nasal metabolism. This naphthalene model was applied to estimate human equivalent inhalation concentrations (HECs) corresponding to several NOAELs or LOAELs for the non-cancer effects of naphthalene in rats. Two approaches for cross-species extrapolation were compared: (1) equivalence based on tissue naphthalene concentration and (2) equivalence based on amount metabolized per minute (normalized to tissue volume). At the NOAEL of 0.1 ppm, the regional gas dosimetry ratio (RGDR) based on naphthalene concentration was 0.18 for the dorsal olfactory region; however, the RGDR rises to 5.4 when based on the normalized amount metabolized due to the lower of expression of CYP isozymes in the nasal epithelium of primates and humans. The resulting HEC is 0.12 ppm (0.63 mg/m3) continuous exposure at the rat NOAEL of 0.1 ppm (6 h/day, 5 days/week).

Quantitative evaluation of dichloroacetic acid kinetics in human—a physiologically based pharmacokinetic modeling investigation

Dichloroacetic acid is a common disinfection by-product in surface waters and is a probable minor metabolite of trichloroethylene. Dichloroacetic acid (DCA) liver carcinogenicity has been demonstrated in rodents but epidemiological evidence in humans is not available. High doses of DCA ( approximately 50mg/kg) are used clinically to treat metabolic acidosis. Biotransformation of DCA by glutathione transferase zeta (GSTzeta) in the liver is the major elimination pathway in humans. GSTzeta is also inactivated by DCA, leading to slower systemic clearance and nonlinear pharmacokinetics after multiple doses. A physiologically based pharmacokinetic (PBPK) model was developed to quantitatively describe DCA biotransformation and kinetics in humans administered DCA by intravenous infusion and oral ingestion. GSTzeta metabolism was described using a Michaelis-Menten equation coupled with rate constants to account for normal GSTzeta synthesis, degradation and irreversible covalent binding and inhibition by the glutathione-bound-DCA intermediate. With some departures between observation and model prediction, the human DCA PBPK model adequately predicted the DCA plasma kinetics over a 20,000-fold range in administered doses. Apparent inhibition of GSTzeta mediated metabolism of DCA was minimal for low doses of DCA (microg/kg day), but was significant for therapeutic doses of DCA. Plasma protein binding of DCA was assumed to be an important factor influencing the kinetics of low doses of DCA (microg/kg day). Polymorphisms of GSTzeta may help explain inter-individual variability in DCA plasma kinetics and warrants evaluation. In conclusion, using a previously published rodent DCA PBPK model (Keys, D.A., Schultz, I.R., Mahle, D.A., Fisher, J.W., 2004. A quantitative description of suicide inhibition of dichloroacetic acid in rats and mice. Toxicol. Sci. 82, 381-393) and this human DCA PBPK model, human equivalent doses (HEDs) were calculated for a 10% increase in mice hepatic liver cancer (2.1mg/kg day). The HEDs for the dosimetrics, area-under-the-concentration-curve (AUC) for total and free DCA in plasma, AUC of DCA in liver and amount of DCA metabolized per day were 0.02, 0.1, 0.1 and 1.0mg/kg day, respectively. Research on the mechanism of action of DCA and the relevance of mouse liver cancer is needed to better understand which dosimetric may be appropriate for extrapolation from animal studies to human.

In Vitro Rat Hepatic Metabolism of n-Alkanes: Nonane, Decane, and Tetradecane:

Jet propellant 8 (JP-8) jet fuel is a complex mixture of aromatic and aliphatic hydrocarbons. The aim of this study was to determine in vitro metabolic rate constants for semivolatile n-alkanes, nonane (C9), decane (C10), and tetradecane (C14), by rat liver microsomal oxidation. The metabolism was assessed by measuring the disappearance of parent compound by gas chromatography. Various concentrations of n-alkanes were incubated with liver microsomes from adult male F-344 rats. Nonlinear kinetic constants for nonane and decane were V max (nmol/mg protein/min) = 7.26 ± 0.20 and 2.80 ± 0.35, respectively, and K M (μM) = 294.83 ± 68.67 and 398.70 ± 42.70, respectively. Metabolic capacity as assessed by intrinsic clearance (V max/K M) was ~four-fold higher for nonane (0.03 ± 0.005) than for decane (0.007 ± 0.001). There was no appreciable metabolism of tetradecane even with higher microsomal protein concentration and longer incubation time. These results show a negative correlation between metabolic clearance and chain length of n-alkanes. These metabolic rate constants will be used to update existing physiologically based pharmacokinetic (PBPK) models for nonane and decane as part of developing a PBPK model for JP-8.

Wipe testing for surface contamination by tritiated compounds

This study investigated the performance of the wipe test in determining contamination from tritiated triolein or thymidine on various surfaces. Filter papers were saturated with water, methanol, petroleum ether, ethyl acetate or maintained dry, and wipes were taken from lead, stainless steel, polyurethane, wood, painted lead, treated floor tile, Formica, or bench paper that were spotted with either 3H-thymidine or 3H-triolein. The recovery of contamination using the dry wipe test averaged 3% for all surfaces. Recoveries using wet wipes were directly related to the solubility of the tritiated compounds in the wipe solution and the physical nature of the wipe surface.