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. 2022 Nov 17;10(11):700.
doi: 10.3390/toxics10110700.

A Physiologically Based Pharmacokinetic (PBPK) Modeling Framework for Mixtures of Dioxin-like Compounds

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A Physiologically Based Pharmacokinetic (PBPK) Modeling Framework for Mixtures of Dioxin-like Compounds

Rongrui Liu et al. Toxics. .

Abstract

Humans are exposed to persistent organic pollutants, such as dioxin-like compounds (DLCs), as mixtures. Understanding and predicting the toxicokinetics and thus internal burden of major constituents of a DLC mixture is important for assessing their contributions to health risks. PBPK models, including dioxin models, traditionally focus on one or a small number of compounds; developing new or extending existing models for mixtures often requires tedious, error-prone coding work. This lack of efficiency to scale up for multi-compound exposures is a major technical barrier toward large-scale mixture PBPK simulations. Congeners in the DLC family, including 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD), share similar albeit quantitatively different toxicokinetic and toxicodynamic properties. Taking advantage of these similarities, here we reported the development of a human PBPK modeling framework for DLC mixtures that can flexibly accommodate an arbitrary number of congeners. Adapted from existing TCDD models, our mixture model contains the blood and three diffusion-limited compartments-liver, fat, and rest of the body. Depending on the number of congeners in a mixture, varying-length vectors of ordinary differential equations (ODEs) are automatically generated to track the tissue concentrations of the congeners. Shared ODEs are used to account for common variables, including the aryl hydrocarbon receptor (AHR) and CYP1A2, to which the congeners compete for binding. Binary and multi-congener mixture simulations showed that the AHR-mediated cross-induction of CYP1A2 accelerates the sequestration and metabolism of DLC congeners, resulting in consistently lower tissue burdens than in single exposure, except for the liver. Using dietary intake data to simulate lifetime exposures to DLC mixtures, the model demonstrated that the relative contributions of individual congeners to blood or tissue toxic equivalency (TEQ) values are markedly different than those to intake TEQ. In summary, we developed a mixture PBPK modeling framework for DLCs that may be utilized upon further improvement as a quantitative tool to estimate tissue dosimetry and health risks of DLC mixtures.

Keywords: AHR; PBPK; TCDD; TEQ; dioxin-like compounds; mixture.

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Conflict of interest statement

The authors declare no conflict of interest.

Figures

Figure 1
Figure 1
Schematic illustration of the structure and molecular key events of the DLC mixture PBPK model. (A) The overall structure of the DLC mixture PBPK model containing four blood circulation-connected compartments: Blood, Fat, RB, and Liver. The latter three are further divided into blood and tissue subcompartments to account for diffusion-limited distribution processes. (B) Details of the molecular key events in Liver Tissue: Competitive, reversible binding of DLCs for AHR (parameterized with kfAHR and kbAHR) and CYP1A2 (kf1A2 and kb1A2), transcriptional induction of CYP1A2 mRNA by DLCs-AHR complexes governed by a Hill function (n, CYP1A2_1EMAX, CYP1A2_1EC50, ktranscription_1A2), degradation of CYP1A2 mRNA (kdegCYP1A2_mRNA), protein translation of CYP1A2 (ktranslation_1A2), degradation of CYP1A2 (kdegCYP1A2), CYP1A2-mediated elimination of DLCs (kelim), and fast equilibrium between free and non-specific bound fractions of DLCs (as defined by the partition coefficients PL). Φ denotes degradation or elimination. Open arrow head: flux, solid arrow head: regulation.
Figure 2
Figure 2
Performance comparison of the mixture model and Emond model. (A) Predicted time−course tissue concentrations of TCDD for the Blood (CA), Fat (CF), RB (CRB), and Liver (free: CLfree, total: CL) compartments, and the induced liver CYP1A2 levels for lifetime exposure to two doses by the two models as indicated. (B) Scatter plot of Log10-transformed tissue TCDD and CYP1A2 concentrations at select ages (year 1, 10, 20, 30, 40, 50, 60, and 70) predicted by the two models in (A).
Figure 3
Figure 3
Simulation results for binary mixture exposures to DLCs. (A) Predicted time-course tissue concentrations of CA, CF, CRB, and CL for lifetime exposure to single 2,3,4,7,8-PeCDF (dashed blue line), single (dashed green line) 2,3,7,8-TCDD or binary mixture (solid blue and green lines, respectively) at low dose (0.0007 ng/kg bw/day). (B,C) Lifetime-averaged ratios (indicated by the heatmap color) of CA, CF, CRB, and CL in a binary exposure to the corresponding concentrations in single exposures for all pair-wise combinations of 11 DLCs as indicated for low-dose exposure of 0.0007 ng/kg bw/day. The diagonal represents unity. Except on the diagonal, each square contains two vertical bins each corresponding to the concentration ratio for one of the two congeners in a binary mixture. (DF) Same as (AC) except for high-dose exposure at 0.02 ng/kg bw/day.
Figure 4
Figure 4
Simulation results for mixture exposure to 11 DLCs. (A) Predicted time-course of CA, CF, CRB, and CL for lifetime exposure to mixture of 11 DLCs (solid line) or single DLC (dashed line) as indicated at a dose of 0.0007 ng/kg bw/day for each congener. (B) Tissue TEQ values calculated based on the concentrations in (A) using systemic TEF. (C,D) Same as (A,B) but for high-dose exposure at 0.02 ng/kg bw/day for each congener.
Figure 5
Figure 5
Monte Carlo simulation of 1000 human individuals exposed to 11 DLCs. (A) Inter-individual variations in lifetime body weight (BW) changes and three biochemical parameters as indicated. (B) The mean (orange solid line) and 2.5–97.5 percentile (orange dashed line) levels of blood or tissue TEQ as indicated, overlaid with 100 randomly selected individuals (gray lines). (C) Percentage contributions of individual DLC congeners to daily average mass dose (top-left) or lifetime cumulative mass dose (bottom-left) as indicated. Percentage contributions of individual DLC congeners to daily average TEQ (top-right) or lifetime cumulative TEQ (bottom-right) as indicated. (D) Percentage contributions of individual DLC congeners to lifetime cumulative blood or tissue TEQ as indicated. The color scheme of the congeners is indicated at the bottom.

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References

    1. White S.S., Birnbaum L.S. An overview of the effects of dioxins and dioxin-like compounds on vertebrates, as documented in human and ecological epidemiology. J. Environ. Sci. Health C. Environ. Carcinog. Ecotoxicol. Rev. 2009;27:197–211. doi: 10.1080/10590500903310047. - DOI - PMC - PubMed
    1. Van den Berg M., Birnbaum L.S., Denison M., De Vito M., Farland W., Feeley M., Fiedler H., Hakansson H., Hanberg A., Haws L., et al. The 2005 World Health Organization reevaluation of human and Mammalian toxic equivalency factors for dioxins and dioxin-like compounds. Toxicol. Sci. 2006;93:223–241. doi: 10.1093/toxsci/kfl055. - DOI - PMC - PubMed
    1. Dopico M., Gómez A. Review of the current state and main sources of dioxins around the world. J. Air Waste Manag. Assoc. 2015;65:1033–1049. doi: 10.1080/10962247.2015.1058869. - DOI - PubMed
    1. Erickson M.D., Kaley R.G., 2nd Applications of polychlorinated biphenyls. Environ. Sci. Pollut. Res. Int. 2011;18:135–151. doi: 10.1007/s11356-010-0392-1. - DOI - PubMed
    1. ATSDR . Toxicological Profile for Chlorinated Dibenzo-p-Dioxins (CDDs) U.S. Department of Health and Human Services, P.H.S.; Atlanta, GA, USA: 1998. [(accessed on 14 November 2022)]. Available online: https://www.atsdr.cdc.gov/toxprofiles/tp104.pdf.

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