Using available in vitro metabolite identification and time course kinetics for ß-chloroprene and its metabolite, (1-chloroethenyl) oxirane, to include reactive oxidative metabolites and glutathione depletion in a PBPK model for ß-chloroprene.

Introduction: ß-chloroprene (2-chloro-1,3-butadiene; CP) causes lung tumors after inhalation exposures in rats and mice. Mice develop these tumors at lower exposures than rats. In rats CP exposures cause depletion of lung glutathione (GSH). Methods: PBPK models developed to relate the appearance of mouse lung tumors with rates of CP metabolism to reactive metabolites or total amounts metabolized during exposures have been expanded to include production of reactive metabolites from CP. The extended PBPK model describes both the unstable oxirane metabolite, 2-CEO, and metabolism of the more stable oxirane, 1-CEO, to reactive metabolites via microsomal oxidation to a diepoxide, and linked production of these metabolites to a PK model predicting GSH depletion with increasing CP exposure. Key information required to develop the model were available from literature studies identifying: 1) microsomal metabolites of CP, and 2) in vitro rates of clearance of CP and 1-CEO from active microsomal preparations from mice, rats, hamsters and humans. Results: Model simulation of concentration dependence of disproportionate increases in reactive metabolite concentrations as exposures increases and decreases in tissue GSH are consistent with the dose-dependence of tumor formation. At the middle bioassay concentrations with a lung tumor incidence, the predicted tissue GSH is less than 50% background. These simulations of reduction in GSH are also consistent with the gene expression results showing the most sensitive pathways are Nrf2-regulation of oxidative stress and GSH metabolism. Discussion: The PBPK model is used to correlate predicted tissue exposure to reactive metabolites with toxicity and carcinogenicity of CP.

Derivation of whole blood biomonitoring equivalents for titanium for the interpretation of biomonitoring data.

Biomonitoring equivalents (BEs) have been increasingly applied for biomonitoring purposes by regulatory bodies worldwide. The present report describes the development of a BE for titanium based on a 4-step process: (i) identification of a critical study/point of departure (PoD) supporting an established oral exposure guidance value (OEGV);, (ii) review the available oral PK data and application of a pharmacokinetic model for titanium; (iii) selection of the most appropriate biomarker of exposure in a specific tissue and calculation of steady-state tissue levels corresponding to the PoD in the critical study; and (iv) derivation of BE value adjusting for the uncertainties considered in the original OEGV assessment. Using the above 4-step approach, a blood BE value of 32.5 µg titanium/L was derived. Key components of the analysis included a pharmacokinetic model developed by investigators at the Netherlands National Institute of Public Health (RIVM) and a two-year rodent bioassay of titanium conducted by the US National Cancer Institute. The most sensitive pharmacokinetic parameter involved in the current BE derivation is the oral absorption factor of 0.02%. The provisional BE proposed in this article may be updated as new information on the pharmacokinetics of titanium becomes available.

Toxicity testing in the 21st century: progress in the past decade and future perspectives.

Advances in the biological sciences have led to an ongoing paradigm shift in toxicity testing based on expanded application of high-throughput in vitro screening and in silico methods to assess potential health risks of environmental agents. This review examines progress on the vision for toxicity testing elaborated by the US National Research Council (NRC) during the decade that has passed since the 2007 NRC report on Toxicity Testing in the 21st Century (TT21C). Concomitant advances in exposure assessment, including computational approaches and high-throughput exposomics, are also documented. A vision for the next generation of risk science, incorporating risk assessment methodologies suitable for the analysis of new toxicological and exposure data, resulting in human exposure guidelines is described. Case study prototypes indicating how these new approaches to toxicity testing, exposure measurement, and risk assessment are beginning to be applied in practice are presented. Overall, progress on the 20-year transition plan laid out by the US NRC in 2007 has been substantial. Importantly, government agencies within the United States and internationally are beginning to incorporate the new approach methodologies envisaged in the original TT21C vision into regulatory practice. Future perspectives on the continued evolution of toxicity testing to strengthen regulatory risk assessment are provided.

Assessing children's exposure to manganese in drinking water using a PBPK model.

A previously published human PBPK model for manganese (Mn) in infants and children has been updated with Mn in drinking water as an additional exposure source. Built upon the ability to capture differences in Mn source-specific regulation of intestinal uptake in nursing infants who are breast-fed and formula-fed, the updated model now describes the bioavailability of Mn from drinking water in children of ages 0-18. The age-related features, including the recommended age-specific Mn dietary intake, age-specific water consumption rates, and age-specific homeostasis of Mn, are based on the available human data and knowledge of the biology of essential-metal homeostasis. Model simulations suggest that the impact of adding drinking-water exposure to daily Mn exposure via dietary intake and ambient air inhalation in children is not greater than the impacts in adults, even at a drinking-water concentration that is 2 times higher than the USEPA's lifetime health advisory value. This conclusion was also valid for formula-fed infants who are considered at the highest potential exposure to Mn from drinking water compared to all other age groups. Our multi-route, multi-source Mn PBPK model for infants and children provides insights about the potential for Mn-related health effects on growing children and will thereby improve the level of confidence in properly interpreting Mn exposure-health effects relationships in children in human epidemiological studies.

An interpolatory ansatz captures the physics of one-dimensional confined Fermi systems.

Interacting one-dimensional quantum systems play a pivotal role in physics. Exact solutions can be obtained for the homogeneous case using the Bethe ansatz and bosonisation techniques. However, these approaches are not applicable when external confinement is present. Recent theoretical advances beyond the Bethe ansatz and bosonisation allow us to predict the behaviour of one-dimensional confined systems with strong short-range interactions, and new experiments with cold atomic Fermi gases have already confirmed these theories. Here we demonstrate that a simple linear combination of the strongly interacting solution with the well-known solution in the limit of vanishing interactions provides a simple and accurate description of the system for all values of the interaction strength. This indicates that one can indeed capture the physics of confined one-dimensional systems by knowledge of the limits using wave functions that are much easier to handle than the output of typical numerical approaches. We demonstrate our scheme for experimentally relevant systems with up to six particles. Moreover, we show that our method works also in the case of mixed systems of particles with different masses. This is an important feature because these systems are known to be non-integrable and thus not solvable by the Bethe ansatz technique.

Development of a physiologically based pharmacokinetic model of isopropanol and its metabolite acetone.

A physiologically based pharmacokinetic (PBPK) model for isopropanol (IPA) and its major metabolite, acetone, is described. The structure of the parent chemical model, which can be used for either IPA or acetone by choosing the appropriate chemical-specific parameters, is similar to previously published models of volatile organic chemicals such as styrene. However, in order to properly simulate data on the exhalation of IPA and acetone during inhalation exposures, it was necessary to expand the description of the lung compartment to include a subcompartment for the upper respiratory tract mucus layer. This elaboration is consistent with published PBPK models of other water-soluble vapors in which the mucus layer serves to absorb the chemical during inhalation and then release it during exhalation. In the case of IPA exposure, a similar PBPK structure is used to describe the kinetics of the acetone produced from the metabolism of IPA. The resulting model is able to provide a coherent description of IPA and acetone kinetics in the rat and human for exposures to IPA by several routes: intravenous, intraperitoneal, oral, inhalation, and dermal. It is also able to consistently reproduce kinetic data for exposures of rats or humans to acetone. Thus, the model provides a validated framework for performing chemical-specific route-to-route extrapolation and cross-species dosimetry, which can be used in place of generic default calculations in support of risk assessments for IPA and acetone.

Mode of action and tissue dosimetry in current and future risk assessments.

Two fundamental concepts have emerged to organize contemporary approaches to chemical risk assessment - mode of action and tissue dosimetry. Mode of action specifies the nature of the interactions between the chemical and the body that lead to toxic responses and should, under optimal circumstances, also specify the form of the tissue dose that leads to these effects. This paper highlights recent development of biologically based dose response (BBDR) models for specific toxic endpoints that use knowledge on mode of action to specify measures of dose. These dose measures then are used to support low dose and interspecies extrapolations. We first focus on a series of dose response models developed for several compounds that produce nasal toxicity. These examples demonstrate a range of model structures from simple dosimetry models (methylmethacrylate) to linkage of dosimetry with specific biological processes involved in carcinogenesis (formaldehyde). Two BBDR models with dioxin illustrate the organization of biological and dosimetry information into specific testable hypotheses that could distinguish these different models and lead to a more uniform approach to risk assessment for this compound. A final section discusses the impact of molecular biology and the genomic revolution in relation to development of BBDR models for specific toxic endpoints.

Comparison of cancer risk estimates for vinyl chloride using animal and human data with a PBPK model.

Vinyl chloride (VC) is a trans-species carcinogen, producing tumors in a variety of tissues, from both inhalation and oral exposures, across a number of species. In particular, exposure to VC has been associated with a rare tumor, liver angiosarcoma, in a large number of studies in mice, rats, and humans. The mode of action for the carcinogenicity of VC appears to be a relatively straightforward example of DNA adduct formation by a reactive metabolite, leading to mutation, mistranscription, and neoplasia. The objective of the present analysis was to investigate the comparative potency of a classic genotoxic carcinogen across species, by performing a quantitative comparison of the carcinogenic potency of VC using data from inhalation and oral rodent bioassays as well as from human epidemiological studies. A physiologically-based pharmacokinetic (PBPK) model for VC was developed to support the target tissue dosimetry for the cancer risk assessment. Unlike previous models, the initial metabolism of VC was described as occurring via two saturable pathways, one representing low capacity-high affinity oxidation by CYP2E1 and the other (in the rodent) representing higher capacity-lower affinity oxidation by other isozymes of P450, producing in both cases chloroethylene oxide (CEO) and chloroacetaldehyde (CAA) as intermediate reactive products. Depletion of glutathione by reaction with CEO and CAA was also described. Animal-based risk estimates for human inhalation exposure to VC using total metabolism estimates from the PBPK model were consistent with risk estimates based on human epidemiological data, and were lower than those currently used in environmental decision-making by a factor of 80.

Assessing interaction thresholds for trichloroethylene in combination with tetrachloroethylene and 1,1,1-trichloroethane using gas uptake studies and PBPK modeling.

The volatile organic solvents trichloroethylene (TCE), tetrachloroethylene (perchloroethylene, PERC), and 1,1,1-trichloroethane (methylchloroform, MC) are widely distributed environmental pollutants and common contaminants of many chemical waste sites. To investigate the mode of pharmacokinetic interactions among TCE, PERC, and MC and to calculate defined "interaction thresholds", gas-uptake experiments were performed using a closed-chamber exposure system. In each experiment, two rats (Fischer 344, male, 8-9 weeks old) were exposed to different initial concentrations of TCE, PERC, and MC, applied singly or as a mixture, and their concentration in the gas phase of the chamber was monitored over a period of 6 h. A physiologically based pharmacokinetic (PBPK) model was developed to test multiple mechanisms of inhibitory interactions, i.e., competitive, non-competitive, or uncompetitive. All mixture exposure data were accurately described by a system of equations in which a PBPK model was provided for each chemical and each was regarded as an inhibitor of the others' metabolism. Sensitivity-analysis techniques were used to investigate the impact of key parameters on model output and optimize experimental design. Model simulations indicated that, among these three chemicals, the inhibition was competitive. The PBPK model was extended to assess occupationally relevant exposures at or below the current threshold-limit values (TLVs). Based on 10% elevation in TCE blood levels as a criterion for significant interaction and assuming TCE exposure is set at TLV of 50 ppm, the calculated interaction thresholds for PERC and MC were 25 and 135 ppm, respectively. TLV exposures to binary TCE/PERC mixture were below the 10% significance level. The interaction threshold for TCE and MC co-exposure would be reached at 50 and 175 ppm, respectively. Such interactive PBPK models should be of value in risk assessment of occupational and environmental exposure to solvent mixtures.

Using human data to protect the public's health.

The value of using human data in the assessment and management of risk is evaluated. Although the use of such data has a long and successful history with environmental contaminants and the development of drugs and commercial chemicals, recent deliberations within the Environmental Protection Agency (EPA) have questioned this practice in part. Specifically, we evaluate the degree to which reference doses (RfDs) and reference concentrations (RfCs) derived from human data on EPA's Integrated Risk Information System (IRIS) differ with RfDs and RfCs that we estimate from experimental animal data. We also use several minimal risk levels of the Agency for Toxic Substances and Disease Registry (ATSDR) and tolerable intakes of Health Canada in this comparison. Human-based RfDs are more than threefold lower than the corresponding animal-based RfDs for 23% of the comparisons. Human- based RfDs or RfCs are lower than corresponding animal-based RfDs or RfCs for 36% of the comparisons. Furthermore, for 10 of 43 possible comparisons, insufficient experimental animal data are readily available or data are inappropriate to estimate either RfDs or RfCs. We also discuss human pharmacokinetic data from volunteer studies and mechanistic studies with human tissues in vitro and demonstrate through a series of case discussions that utilization of such data is important when making decisions to protect exposed individuals. Moreover, physiologically based pharmacokinetic (PBPK) modeling evaluates critical information in assessing interindividual variability and identifying at-risk populations. Within the limits of our analysis, we conclude that the direct use and interpretation of human data, in conjunction with data gathered from experimental animals, are public health protective policies that should be encouraged.

Mode-of-action-based dosimeters for interspecies extrapolation of vinyl acetate inhalation risk.

Vinyl acetate is used in the manufacture of many polymers. The Clean Air Act Amendments of 1990 require that an inhalation risk assessment be conducted to assess risks to human health from ambient exposures. Vinyl acetate is a nasal carcinogen in rats and induces olfactory degeneration in rats and mice. Because of the many unique aspects of the rodent nasal cavity compared to that of humans, conventional means for extrapolating dosimetry between species are not appropriate. Physiologically based pharmacokinetic (PBPK) and pharmacodynamic (PD) modeling can address many of these unique aspects. A PBPK/PD model has been developed for vinyl acetate, but the choice of appropriate dosimeter(s) to use for interspecies extrapolation depends on a hypothesis regarding mode of action. This article summarizes the key studies that formulate a mode of action hypothesis for vinyl acetate. Dose-response relationships for vinyl acetate-induced nonneoplastic and neoplastic responses are highly nonlinear, suggesting complex kinetic processes. Carboxylesterase-dependent metabolism of vinyl acetate forms acetic acid, a potent cytotoxicant, and acetaldehyde, a weak clastogen. Cell death, proposed to be the result of intracellular acidification, results in restorative cell proliferation. In conjunction with sufficient genetic damage, induced by spontaneous mutation and acetaldehyde-induced DNA-protein cross-links (DPX), olfactory degeneration progresses to a state of elevated proliferation and eventually, at high vinyl acetate concentrations, to neoplastic transformation. Thus, reduction in intracellular pH (pHi) is proposed as the dosimeter most closely linked to the earliest stages of vinyl acetate toxicity. Consequently, risk assessments that are based on protection of nasal epithelium from intracellular acidification will be protective of all subsequent pathological responses related to vinyl acetate exposure. Proposing a reasonable mode of action is an important step in any risk assessment and is critical to the choice of dosimeter(s) to be used for interspecies dosimetry extrapolation.

Physiologically based clearance/extraction models for compounds metabolized in the nose: an example with methyl methacrylate.

Airstream clearance (with units of volume/time) is the volumetric flow from which chemical would have to be completely removed to account for the net loss in the nose. Extraction is the proportion of airflow from which the chemical is completely removed. Over the past several years we have developed physiologically based clearance-extraction (PBCE) models for the nose to assess the physiological, biochemical, and anatomical factors that control airstream clearance. A generic clearance equation was derived for single airway/tissue compartments that had a separate air region and either one, two, or three underlying tissue regions. For all of these structures, airstream clearance (Cl(sys)) has a common form-Equation (1)-related to tissue clearance (Cltot), gas-phase diffusional clearance (PAgas), airflow (Q), and the mucus air partition coefficient (Hmuc:a). Clsys = CltotHm:aPAgasQ/CltotHm:a(Q + PAgas) + PAgasQ. A physiologically based clearance-extraction (PBCE) model for the whole nose combined three separate nasal tissue regions, each with a four-compartment tissue stack (air, mucus, epithelial tissue, and submucosal region). A steady-state solution of the PBCE model successfully described literature results on the steady-state extraction of methyl methacrylate (MMA) and several other metabolized vapors. Model-derived tissue dosimetry estimates, that is, the amount of MMA metabolized in the target epithelial compartment of the olfactory region, for rats and humans provide dosimetric adjustment factors (DAFs) required in calculating a human reference concentration (RfC) from rodent studies. Depending on the assignment of esterase activities to sustentacular and submucosal regions, the DAFs from the PBCE model varied between 1.6 and 8.0, compared to the default value of 0.145. From the experience with MMA, a minimal data set could be defined for building the PBCE model. It consists of mucus:air and blood:air partition coefficients, metabolic constants for enzymatic hydrolysis in nasal tissues from rat and human tissues, immunohistochemistry of the distribution of these activities in rats and human olfactory tissues, and extraction studies in anesthetized rats to assess the total nasal metabolism of the test compound.

Nasal tissue dosimetry-issues and approaches for "Category 1" gases: a report on a meeting held in Research Triangle Park, NC, February 11-12, 1998.

Three organizations, the Basic Acrylic Monomer Manufacturers (BAMM), Methacrylate Producers Association (MPA), and Vinyl Acetate Toxicology Group (VATG), have sponsored development of physiologically based pharmacokinetic (PBPK) models for nasal tissue dosimetry with, respectively, acrylic acid (AA), methyl methacrylate (MMA), and vinyl acetate (VA). These compounds cause lesions in nasal epithelial tissues and are classified as "Category 1" gases within the U.S. EPA (1994) classification scheme. The National Center for Environmental Assessment in the U.S. EPA Office of Research and Development also has continuing interests in refining its methods for dosimetry adjustments when data on mode of action are available for Category 1 gases. A round-table discussion was held in Research Triangle Park, NC, on 11-12 February 1998, to develop a broader appreciation of the key processes and parameters required in developing nasal tissue dosimetry models. The discussions at the round table drew on these three case studies and several background presentations to assess the manner in which chemical-specific and mode-of-action data can be incorporated into nasal dosimetry models. The round table had representation from the U.S. EPA, academia, and industry. This article outlines the presentations and topical areas discussed at the round table and notes recommendations made by participants to extend models for nasal dosimetry and to develop improved data for modeling. The contributions of several disciplines-toxicology, engineering, and physiologically based pharmacokinetic (PBPK) modeling-were evident in the discussions. The integration of these disciplines in creating opportunities for dosimetry model applications in risk assessments has several advantages in the breadth of skills upon which to draw in model development. A disadvantage is in the need to provide venues and develop cross-discipline dialogue necessary to ensure the understanding of cultural attitudes, terminology, and methods. The round-table discussions were fruitful in achieving such enhanced understanding and communication. Subsequent elaboration of these models will benefit from the interactions of these groups at the round table. The round-table discussions have already led to model improvements-as noted in several recently published articles. Participants emphasized several generic data needs in relation to nasal vapor uptake studies in human subjects, to broader discussion of tissue diffusion models, and to extensions to other classes of gases. The round-table articles that are published separately in this issue and the discussions, captured in this overview, provide a glimpse of the state of the science in nasal dosimetry modeling and a clear indication of the growth of and continuing opportunities in this important research area.

A clonal growth model: time-course simulations of liver foci growth following penta- or hexachlorobenzene treatment in a medium-term bioassay.

A combination of experimental and simulation approaches were used to analyze the clonal growth of preneoplastic, enzyme-altered foci during liver carcinogenesis in an initiation-promotion regimen. Male Fisher 344 rats, 8 weeks of age, were initiated with a single dose (200 mg/kg, i.p.) of diethylnitrosamine (DEN). Beginning 2 weeks later, animals were exposed to daily gavage consisting of 0.1 mmol/kg pentachlorobenzene (PECB) or hexachlorobenzene (HCB) in corn oil vehicle for 6 weeks. Partial hepatectomy was performed 3 weeks after initiation. Experimental data including liver weight, hepatocyte density (number of hepatocytes/unit volume), 5-bromo-2'-deoxyuridine-labeling index for analysis of cell division rate, and number and volume of glutathione-S-transferase pi-positive foci were collected 23, 26, 28, 47, or 56 days after initiation. Model parameters describing liver growth were obtained directly from the experimental data. The probability of mutation/division of normal cells and the growth rate of initiated cells were inferred by a comparison of model outcomes with the observed time courses of foci development. To describe the time-dependent increases in foci volume and the concomitant reduction of foci number observed in all treatment groups, the calibrated model for the DEN controls incorporated the hypothesis of two initiated cell populations (referred to as A and B cells) within the framework of the two-stage model. The B cells are initiated cells that have a selective growth advantage under conditions that inhibit the growth of A cells and normal hepatocytes. The parameter values defined in the DEN controls were used to evaluate experiments involving the administration of PECB or HCB. Both PECB and HCB caused a significant increase in foci volume compared with the DEN controls. HCB treatments resulted in increased proliferation of normal hepatocytes, which was not observed for PECB under the same treatment regimen. The best description of the data resulted from the model incorporating the hypothesis that PECB and HCB promoted the growth of foci via increased net growth rates of B cells. We present here a biologically based clonal growth simulation platform to describe the growth of preneoplastic foci under experimental manipulations of initiation-promotion studies. This simulation work is an example of quantitative approaches that could be useful for the analysis of other initiation-promotion studies.

Physiological modeling reveals novel pharmacokinetic behavior for inhaled octamethylcyclotetrasiloxane in rats.

Octamethylcyclotetrasiloxane (D4) is an ingredient in selected consumer and precision cleaning products. Workplace inhalation exposures may occur in some D4 production operations. In this study, we analyzed tissue, plasma, and excreta time-course data following D4 inhalation in Fischer 344 rats (K. Plotzke et al., 2000, Drug Metab. Dispos. 28, 192-204) to assess the degree to which the disposition of D4 is similar to or different from that of volatile hydrocarbons that lack silicone substitution. We first applied a basic physiologically based pharmacokinetic (PBPK) model (J. C. Ramsey and M. E. Andersen, 1984, Toxicol. Appl. Pharmacol. 73, 159-175) to characterize the biological determinants of D4 kinetics. Parameter estimation techniques indicated an unusual set of characteristics, i.e., a low blood:air (P(b:a) congruent with 0.9) and a high fat:blood partition coefficient (P(f:b) congruent with 550). These parameters were then determined experimentally by equilibrating tissue or liquid samples with saturated atmospheres of D4. Consistent with the estimates from the time-course data, blood:air partition coefficients were small, ranging from 1.9 to 6.9 in six samples. Perirenal fat:air partition coefficients were large, from 1400 to 2500. The average P(f:b) was determined to be 485. This combination of partitioning characteristics leads to rapid exhalation of free D4 at the cessation of the inhalation exposure followed by a much slower redistribution of D4 from fat and tissue storage compartments. The basic PK model failed to describe D4 tissue kinetics in the postexposure period and had to be expanded by adding deep-tissue compartments in liver and lung, a mobile chylomicron-like lipid transport pool in blood, and a second fat compartment. Model parameters for the refined model were optimized using single-exposure data in male and female rats exposed at three concentrations: 7, 70, and 700 ppm. With inclusion of induction of D4 metabolism at 700 ppm (3-fold in males, 1-fold in females), the parameter set from the single exposures successfully predicted PK results from 14-day multiple exposures at 7 and 700 ppm. A common parameter set worked for both genders. Despite its very high lipophilicity, D4 does not show prolonged retention because of high hepatic and exhalation clearance. The high lipid solubility, low blood:air partition coefficient, and plasma lipid storage with D4 led to novel distributional characteristics not previously noted for inhaled organic hydrocarbons. These novel characteristics were only made apparent by analysis of the time-course data with PBPK modeling techniques.

The effect of pregnancy on renal clearance of boron in rats given boric acid orally.

Boric acid (H(3)BO(3)) has been shown to cause developmental abnormalities in the offspring of pregnant rats. Comparative data on the renal clearance of boron (B) in rats and humans, both pregnant and nonpregnant, exposed to boric acid (BA) would reduce uncertainty in interspecies extrapolation from rats to humans. The purpose of this study was to evaluate the effect of pregnancy on the plasma half-life and renal clearance of boron in Sprague-Dawley rats given a single oral dose of boric acid. For the half-life study, nonpregnant and pregnant (gestation day 16) rats were given a single dose of 30 mg/kg of boric acid by gavage, and plasma samples were collected at 2-3 h intervals. The plasma half-life of boron was determined to be 2.9 +/- 0.2 and 3.2 +/- 0.3 h in nonpregnant and pregnant rats, respectively. In the clearance study, nonpregnant and pregnant (GD 16) rats were given a single gavage dose of 0.3, 3, or 30 mg/kg of boric acid. Boron clearance was slightly higher in pregnant rats (3.3 +/- 0.6, 3.2 +/- 0.5, and 3.4 +/- 0.5 ml/min/kg, respectively) compared to nonpregnant rats (3.1 +/- 0.8, 3.0 +/- 0.6, and 3.2 +/- 0.5 ml/min/kg, respectively), but the difference was not statistically significant and not dose-related. Boron clearance was less than creatinine clearance, suggesting tubular reabsorption in both groups. In conclusion, pregnancy did not appear to significantly alter the renal clearance or the plasma half-life of boron in Sprague-Dawley rats under the conditions of this study.

Evaluation of biologically based dose-response modeling for developmental toxicity: a workshop report.

Biologically based dose-response (BBDR) modeling represents a novel approach for quantitative assessment of health risk by incorporating pharmacokinetic and pharmacodynamic characteristics of a chemical and by relating the immediate cellular responses to a cascade of aberrant biological actions that leads to detectable adverse outcomes. The quantitative relationship of each of the intervening events can be described in mathematical forms that are amenable for adjustment and extrapolation over a range of doses and across species. A team of investigators at the Reproductive Toxicology Division of the U.S. Environmental Protection Agency has explored the feasibility of BBDR modeling by examining the developmental toxicity of a known teratogen, 5-fluorouracil. A panel of researchers from academic and industrial laboratories, biomathematical modelers, and risk assessment scientists was convened in a workshop to evaluate the approaches undertaken by the EPA team and to discuss the future prospects of BBDR modeling. This report summarizes the lessons learned from one approach to BBDR modeling and comments from the panelists: while it is possible to incorporate mechanistic information into quantitative dose-response models for the assessment of health risks, the process is enormously data-intensive and costly; in addition, the confidence of the model is directly proportional to our current understanding of basic biology and can be enhanced only through the ongoing novel discoveries. More importantly, the extent of "uncertainty" (inherent with the default assumptions associated with the NOAEL or benchmark approach) reducible by BBDR modeling requires further scrutiny and comparison.

Sensitivity analysis of a physiological model for 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD): assessing the impact of specific model parameters on sequestration in liver and fat in the rat.

2,3,7,8-Tetrachlorodibenzo-p-dioxin (TCDD) has dose-dependent tissue distribution because of induction of CYP1A2, a TCDD-binding protein, in the liver. Induction requires transcriptional activation of the CYP1A2 gene product by TCDD and the Ah receptor. An empirical model for dose-dependent distribution (Carrier et al., 1995, Toxicol. Appl. Pharmacol. 131, 253-266) included two simple descriptors: one for the maximum liver sequestration (Fmax) and the other for body burden leading to half maximum sequestration (Kd). Physiologically based pharmacokinetic (PBPK) models include specific parameters for protein receptors, protein binding, tissue solubility, and protein induction. We have applied a PBPK model to define two macroscopic constants related to these dose-response curves, i.e., the inflection point, and the maximum values of these curves. The dose-response curves generated from the PBPK model were for the proportion sequestered in liver and the liver to fat concentration ratio. Our analysis assessed the specific biological factors in the PBPK model that governed the values of these two macroscopic constants. For the fraction in liver, the Hill coefficient (a shape exponent describing the relationship between the Ah receptor-TCDD complex with the DNA receptor) resulted in the largest shift in inflection when using PBPK model parameters specific for TCDD. For the liver to fat ratio, the inflection point was most affected by the number of available Ah receptors. Conventional normalized sensitivity coefficients for the liver-to-fat ratio at the maximum were highest for the fat-to-blood partition coefficient, CYP1A2 binding affinity, and maximum extent of induction of CYP1A2. A similar pattern was observed for the liver fraction, except that the sensitivity coefficients were much smaller. The behavior of different TCDD congeners was evaluated by altering the value of key parameters. Our results demonstrate that the inflection point is more related to characteristics of DNA binding/induction steps of the Ah receptor-DNA complex than by the CYP1A2 affinity of TCDD or concentrations of CYP1A2. Surprisingly, the maximum is more sensitive to changes in CYP1A2 concentrations and affinity for TCDD. In addition, the analysis showed that the liver-to-fat ratio is a more useful experimental measure than is proportion in liver because the ratio responds with similar sensitivity over a much wider range of input parameters.