Miyoung Yoon

Quantitative in vitro to in vivo extrapolation of cell-based toxicity assay results.

The field of toxicology is currently undergoing a global paradigm shift to use of in vitro approaches for assessing the risks of chemicals and drugs in a more mechanistic and high throughput manner than current approaches relying primarily on in vivo testing. However, reliance on in vitro data entails a number of new challenges associated with translating the in vitro results to corresponding in vivo exposures. Physiologically based pharmacokinetic (PBPK) modeling provides an effective framework for conducting quantitative in vitro to in vivo extrapolation (QIVIVE). Their physiological structure facilitates the incorporation of in silico- and in vitro-derived chemical-specific parameters in order to predict in vivo absorption, distribution, metabolism and excretion. In particular, the combination of in silico- and in vitro parameter estimation with PBPK modeling can be used to predict the in vivo exposure conditions that would produce chemical concentrations in the target tissue equivalent to the concentrations at which effects were observed with in vitro assays of tissue/organ toxicity. This review describes the various elements of QIVIVE and highlights key aspects of the process, with an emphasis on extrapolation of in vitro metabolism data to predict in vivo clearance as the key element. Other important elements include characterization of free concentration in the toxicity assay and potential complications associated with intestinal absorption and renal clearance. Examples of successful QIVIVE approaches are described ranging from a simple steady-state approach that is suitable for a high throughput environment to more complicated approaches requiring full PBPK models.

Associations of Perfluoroalkyl Substances (PFAS) with Lower Birth Weight: An Evaluation of Potential Confounding by Glomerular Filtration Rate Using a Physiologically Based Pharmacokinetic Model (PBPK)

Background Prenatal exposure to perfluoroalkyl substances (PFAS) has been associated with lower birth weight in epidemiologic studies. This association could be attributable to glomerular filtration rate (GFR), which is related to PFAS concentration and birth weight. Objectives We used a physiologically based pharmacokinetic (PBPK) model of pregnancy to assess how much of the PFAS–birth weight association observed in epidemiologic studies might be attributable to GFR. Methods We modified a PBPK model to reflect the association of GFR with birth weight (estimated from three studies of GFR and birth weight) and used it to simulate PFAS concentrations in maternal and cord plasma. The model was run 250,000 times, with variation in parameters, to simulate a population. Simulated data were analyzed to evaluate the association between PFAS levels and birth weight due to GFR. We compared simulated estimates with those from a meta-analysis of epidemiologic data. Results The reduction in birth weight for each 1-ng/mL increase in simulated cord plasma for perfluorooctane sulfonate (PFOS) was 2.72 g (95% CI: –3.40, –2.04), and for perfluorooctanoic acid (PFOA) was 7.13 g (95% CI: –8.46, –5.80); results based on maternal plasma at term were similar. Results were sensitive to variations in PFAS level distributions and the strength of the GFR–birth weight association. In comparison, our meta-analysis of epidemiologic studies suggested that each 1-ng/mL increase in prenatal PFOS and PFOA levels was associated with 5.00 g (95% CI: –21.66, –7.78) and 14.72 g (95% CI: –8.92, –1.09) reductions in birth weight, respectively. Conclusion Results of our simulations suggest that a substantial proportion of the association between prenatal PFAS and birth weight may be attributable to confounding by GFR and that confounding by GFR may be more important in studies with sample collection later in pregnancy. Citation Verner MA, Loccisano AE, Morken NH, Yoon M, Wu H, McDougall R, Maisonet M, Marcus M, Kishi R, Miyashita C, Chen MH, Hsieh WS, Andersen ME, Clewell HJ III, Longnecker MP. 2015. Associations of perfluoroalkyl substances (PFAS) with lower birth weight: an evaluation of potential confounding by glomerular filtration rate using a physiologically based pharmacokinetic model (PBPK). Environ Health Perspect 123:1317–1324; http://dx.doi.org/10.1289/ehp.1408837

Physiologically Based Pharmacokinetic Modeling of Fetal and Neonatal Manganese Exposure in Humans: Describing Manganese Homeostasis during Development

Concerns for potential vulnerability to manganese (Mn) neurotoxicity during fetal and neonatal development have been raised due to increased needs for Mn for normal growth, different sources of exposure to Mn, and pharmacokinetic differences between the young and adults. A physiologically based pharmacokinetic (PBPK) model for Mn during human gestation and lactation was developed to predict Mn in fetal and neonatal brain using a parallelogram approach based upon extrapolation across life stages in rats and cross-species extrapolation to humans. Based on the rodent modeling, key physiological processes controlling Mn kinetics during gestation and lactation were incorporated, including alterations in Mn uptake, excretion, tissue-specific distributions, and placental and lactational transfer of Mn. Parameters for Mn kinetics were estimated based on human Mn data for milk, placenta, and fetal/neonatal tissues, along with allometric scaling from the human adult model. The model was evaluated by comparison with published Mn levels in cord blood, milk, and infant blood. Maternal Mn homeostasis during pregnancy and lactation, placenta and milk Mn, and fetal/neonatal tissue Mn were simulated for normal dietary intake and with inhalation exposure to environmental Mn. Model predictions indicate similar or lower internal exposures to Mn in the brains of fetus/neonate compared with the adult at or above typical environmental air Mn concentrations. This PBPK approach can assess expected Mn tissue concentration during early life and compares contributions of different Mn sources, such as breast or cow milk, formula, food, drinking water, and inhalation, with tissue concentration.

Evaluating placental transfer and tissue concentrations of manganese in the pregnant rat and fetuses after inhalation exposures with a PBPK model.

A Physiologically Based Pharmaco Kinetic (PBPK) model, based on a published description of manganese (Mn) kinetics in adult rats, has been developed to describe Mn uptake and tissue distribution in the pregnant dam and fetus during dietary and inhalation exposures. This extension incorporated key physiological processes controlling Mn pharmacokinetics during pregnancy and fetal development. After calibration against tissue Mn concentrations observed during late gestation, the model accurately simulated Mn tissue distribution in the dam and fetus following both diet and inhalation exposures to the pregnant rat. Maternal to fetal transfer of Mn through placenta was described using two pathways: a saturable active transport with high affinity and a simple diffusion. The active transport dominates at basal and lower Mn exposure, whereas at higher Mn exposure, the relative contribution of the diffusion pathway increases. To simulate fetal tissue Mn, tissue-binding parameters and preferential influx/efflux rates in fetal brain were adjusted from the adult model based on differential developmental processes and varying tissue demands for Mn in early life. Model simulations were consistent with observed tissue Mn concentrations in fetal tissues, including brain for diet alone and for combined diet and inhalation. Simulations of Mn in placenta and other maternal tissues in late gestation correlated well with measured tissue concentrations. This model, together with our published models for Mn kinetics during lactation and postnatal development, will help to address concerns about Mn neurotoxicity in potentially sensitive human subpopulation, such as infants and children by providing an estimate of Mn exposure in the population of interest.

Analysis of Manganese Tracer Kinetics and Target Tissue Dosimetry in Monkeys and Humans with Multi-Route Physiologically Based Pharmacokinetic Models

Manganese (Mn) is an essential nutrient with the capacity for toxicity from excessive exposure. Accumulation of Mn in the striatum, globus pallidus, and other midbrain regions is associated with neurotoxicity following high-dose Mn inhalation. Physiologically based pharmacokinetic (PBPK) models for ingested and inhaled Mn in rats and nonhuman primates were previously developed. The models contained saturable Mn tissue-binding capacities, preferential fluxes of Mn in specific tissues, and homeostatic control processes such as inducible biliary excretion of Mn. In this study, a nonhuman primate model was scaled to humans and was further extended to include iv, ip, and sc exposure routes so that past studies regarding radiolabeled carrier-free (54)MnCl(2) tracer kinetics could be evaluated. Simulation results accurately recapitulated the biphasic elimination behavior for all exposure routes. The PBPK models also provided consistent cross-species descriptions of Mn tracer kinetics across multiple exposure routes. These results indicate that PBPK models can accurately simulate the overall kinetic behavior of Mn and predict conditions where exposures will increase free Mn in various tissues throughout the body. Simulations with the human model indicate that globus pallidus Mn concentrations are unaffected by air concentrations < 10 μg/m(3) Mn. The use of this human Mn PBPK model can become a key component of future human health risk assessment of Mn, allowing the consideration of various exposure routes, natural tissue background levels, and homeostatic controls to explore exposure conditions that lead to increased target tissue levels resulting from Mn overexposure.

Lactational transfer of manganese in rats: predicting manganese tissue concentration in the dam and pups from inhalation exposure with a pharmacokinetic model.

Manganese (Mn) is an essential element. However, excess Mn causes neurotoxicity. Fetuses and neonates have been discussed as potentially sensitive subpopulations for Mn. In the present study, a previously published physiologically based pharmacokinetic model for Mn in adult rats was extended to examine exposure conditions that could lead to increased central nervous system Mn in developing rats. The basic structure had saturable tissue binding, homeostatic control of uptake and excretion, and tissue-specific increases in Mn from inhalation. Modifications made for lactating dam and pups included differential tissue-binding capacities in developing pups, increased absorption of dietary Mn in lactating dam, and more efficient gastrointestinal absorption and lower basal biliary excretion in pups. Enhancement of biliary excretion in pups was also required to accurately simulate tissue Mn during early postnatal inhalation. Overall, these changes were concordant with the biology of Mn and other essential metals during development. The resulting model simulations match a variety of published studies on maternal Mn homeostasis during lactation, milk Mn levels, and changing patterns of neonatal tissue Mn for normal dietary intake and with Mn inhalation. Our successful description of Mn kinetics across these life stages suggests that the present model can help describe the relationship between dose of exposure and target tissue Mn concentrations across different developmental stages and its potential risks and assess whether infants and children should be regarded as susceptible populations for Mn inhalation.

Evaluation of simple in vitro to in vivo extrapolation approaches for environmental compounds.

As part of an effort to support in silico/in vitro based risk assessment, we evaluated the accuracy associated with conducting simple in vitro to in vivo extrapolation (IVIVE) for environmental compounds using available in vitro human metabolism data. The IVIVE approach was applied to a number of compounds with a wide range of properties spanning the diversity of characteristics of environmental compounds, and where possible the resulting estimates of the in vivo steady-state blood concentration were compared with estimates derived on the basis of human in vivo kinetic data. There appears to be a systematic bias in the estimation of intrinsic clearance (Clint) from in vitro versus in vivo data, with in vitro based estimates underestimating in vivo clearance for small values of Clint but with the opposite relationship at large values of Clint. Nevertheless, the resulting estimates of Css were in good agreement. The chief drawback of the simple approach used in this study, which performs the IVIVE prediction for the parent compound only, is that it is not applicable for toxicity associated with a metabolite.

Application of a multi-route physiologically based pharmacokinetic model for manganese to evaluate dose-dependent neurological effects in monkeys.

Manganese (Mn) is an essential element that is neurotoxic under certain exposure conditions. Monkeys and humans exposed to Mn develop similar neurological effects; thus, an improved understanding of the dose-response relationship seen in nonhuman primates could inform the human health risk assessment for this essential metal. A previous analysis of this dose-response relationship in experimental animals (Gwiazda, R., Lucchini, R., and Smith, D., 2007, Adequacy and consistency of animal studies to evaluate the neurotoxicity of chronic low-level manganese exposure in humans, J. Toxicol. Environ. Health Part A 70, 594-605.) relied on estimates of cumulative intake of Mn as the sole measure for comparison across studies with different doses, durations, and exposure routes. In this study, a physiologically based pharmacokinetic model that accurately accounts for the dose dependencies of Mn distribution was used to estimate increases in brain Mn concentrations in monkeys following Mn exposure. Experimental studies evaluated in the analysis included exposures by inhalation, oral, iv, ip, and sc dose routes, and spanned durations ranging from several weeks to over 2 years. This analysis confirms that the dose-response relationship for the neurotoxic effects of Mn in monkeys is independent of exposure route and supports the use of target tissue Mn concentration or cumulative target tissue Mn as the appropriate dose metric for these comparisons. These results also provide strong evidence of a dose-dependent transition in the mode of action for the neurological effects of Mn that needs to be considered in risk assessments for this essential metal.

Update on a Pharmacokinetic-Centric Alternative Tier II Program for MMT—Part II: Physiologically Based Pharmacokinetic Modeling and Manganese Risk Assessment

Recently, a variety of physiologically based pharmacokinetic (PBPK) models have been developed for the essential element manganese. This paper reviews the development of PBPK models (e.g., adult, pregnant, lactating, and neonatal rats, nonhuman primates, and adult, pregnant, lactating, and neonatal humans) and relevant risk assessment applications. Each PBPK model incorporates critical features including dose-dependent saturable tissue capacities and asymmetrical diffusional flux of manganese into brain and other tissues. Varied influx and efflux diffusion rate and binding constants for different brain regions account for the differential increases in regional brain manganese concentrations observed experimentally. We also present novel PBPK simulations to predict manganese tissue concentrations in fetal, neonatal, pregnant, or aged individuals, as well as individuals with liver disease or chronic manganese inhalation. The results of these simulations could help guide risk assessors in the application of uncertainty factors as they establish exposure guidelines for the general public or workers.

Use of in vitro data in developing a physiologically based pharmacokinetic model: Carbaryl as a case study

In vitro-derived information has been increasingly used to support and improve human health risk assessment for exposure to chemicals. Physiologically based pharmacokinetic (PBPK) modeling is a key component in the movement toward in vitro-based risk assessment, providing a tool to integrate diverse experimental data and mechanistic information to relate in vitro effective concentrations to equivalent human exposures. One of the challenges, however, in the use of PBPK models for this purpose has been the need for extensive chemical-specific parameters. With the remarkable advances in in vitro methodologies in recent years, in vitro-derived parameters can now be easily incorporated into PBPK models. In this study we demonstrate an in vitro data based parameterization approach to develop a physiologically based pharmacokinetic and pharmacodynamic (PBPK/PD) model, using carbaryl as a case study. In vitro experiments were performed to provide the chemical-specific pharmacokinetic (PK) and pharmacodynamic (PD) parameters for carbaryl in the PBPK model for this compound. Metabolic clearance and cholinesterase (ChE) interaction parameters for carbaryl were measured in rat and human tissues. These in vitro PK and PD data were extrapolated to parameters in the whole body PBPK model using biologically appropriate scaling. The PBPK model was then used to predict the kinetics and ChE inhibition dynamics of carbaryl in vivo. This case study with carbaryl provides a reasonably successful example of utilizing the in vitro to in vivo extrapolation (IVIVE) approach for PBPK model development. This approach can be applied to other carbamates with an anticholinesterase mode of action as well as to environmental chemicals in general with further refinement of the current shortcomings in the approach. It will contribute to minimizing the need for in vivo human data for PBPK model parameterization and evaluation in human risk assessments.