Pharmacokinetic Modeling of Environmental Chemicals Part 2: Applications

1. Pharmacokinetic Modeling of Environmental ChemicalsPart 2: Applications Harvey J. Clewell, Ph.D. Director, Center for Human Health Assessment The Hamner Institutes for Health Sciences Research Triangle Park, North Carolina

2. TODAY’S TOPICS • Application of PBPK Models in Risk Assessments Based on Animal Studies • - vinyl chloride • - trichloroethylene • Application of PBPK Models to Understand the Health Implications of Human Biomonitoring Data • - methylmercury • - perfluorooctanoic acid

3. Part 1: RISK ASSESSMENT “The characterization of the potential adverse effects of human exposures to environmental hazards.” - National Academy of Sciences, 1983

4. Risk Assessment Questions • Qualitative: Is the chemical potentially harmful under ANY conditions? • Quantitative: At what human exposure concentration does the RISK become SIGNIFICANT?

5. The Dose is Important “All substances are poisons; there is none which is not a poison. The right dose differentiates a poison and a remedy.” –- Paracelsus, 1493-1541 “Dancing with proper limitations is a salutary exercise, but when violent and long continued in a crowded room it is extremely pernicious, and has hurried many young people to the grave.” --A. Murray, M.D., 1826

6. ?? Risk Dose Dose Response Assessment ?? Agent Dose Exposure Assessment Four Components of Risk Assessment(National Academy of Sciences, 1983) ?? Agent Effect Hazard Identification Risk Characterization

7. Key Definitions In Contemporary Human Health Risk Assessment Default – A generic, conservative (safe-sided) approach, for use when chemical-specific information is lacking Mode of Action - in a broad sense, the critical sequence of events involved in the production of a toxic effect by a chemical Dosimetry – Estimation of the tissue exposure to the form of the chemical (e.g., a reactive metabolite) that is most directly related to the toxic effect

8. Steps in a Toxic Mode of Action Exposure absorption, distribution, metabolism, excretion Tissue Dose local metabolism, binding Molecular Interactions reactivity, DNA adducts, receptor activation Early Cellular Effects cytotoxicity, DNA mutation, increased cell division Toxic Responses toxicity, cancer

9. Mode of Action Considerations • Parent Chemical(ethylene oxide) • vs. Stable Metabolite (trichloroacetic acid from trichloroethylene) • or Reactive Metabolite (methylene chloride) • Physical effect(acute neurotoxicity of solvents) • vs. Reactivity (formaldehyde) • or Receptor Binding (dioxin) • Direct Genotoxicity(mutations from vinyl chloride adducts) • vs. Indirect (oxidative stress) • or Nongenotoxic (arsenic inhibition of DNA repair)

10. Role of PBPK Modeling in Risk Assessments for Chemicals • Define the relationship between external concentration or dose and an internal measure of (biologically effective) exposure: • in experimental animals • in subjects from human studies • in the population of concern

11. Application of Pharmacokinetics in Risk Assessment • Underlying Assumption: Tissue Dose Equivalence • Effects occur as a result of tissue exposure to the toxic form of the chemical. • Equivalent effects will be observed at equal tissue exposure/dose in experimental animals and humans. • Appropriate measure of tissue dose depends critically on the mode of action for the effect of the chemical.

12. Steps for Incorporating PBPK Modeling in Human Health Risk Assessment • Identify toxic effects in animals or human populations • Evaluate available data on mode(s) of action, metabolism, for compound and related chemicals • Describe potential mode(s) of action • Propose relationship between response and tissue dose • Develop/adapt an appropriate PBPK model • Estimate tissue dose during toxic exposures with model • Estimate risk in humans based on assumption of similar tissue response for equivalent target tissue dose

13. Applications of PBPK Modeling in Human Risk Assessment by Regulatory Agencies • Methylene Chloride (EPA, OSHA, ATSDR, Health Canada) • 2-Butoxy Ethanol (EPA, Health Canada) • Vinyl Chloride (EPA) • Chloroform (Health Canada) • Dioxin (EPA) • Trichloroethylene (EPA) • Perchloroethylene (EPA) • Isopropanol (EPA)

14. Considering Pharmacokinetic and Mechanistic Information in Cancer Risk Assessment Examples: Easy: Vinyl Chloride Hard: Trichloroethylene

15. Considering Pharmacokinetic and Mechanistic Information in Cancer Risk Assessment Example 1: Vinyl Chloride • Used to produce plastics; formed in groundwater from • bacterial degradation of other contaminants • Cross-species correspondence of a rare tumor type: liver angiosarcoma in mouse, rat, and human (workers). • Carcinogenic at doses with no evidence of toxicity • DNA-reactive, mutagenic • Likely to be carcinogenic even at low doses

16. Metabolism of Vinyl Chloride Dose metric: concentration of chloroethylene epoxide

17. PBPK Model for Vinyl Chloride (Clewell et al. 2001) Dose metric: production rate of reactive metabolite per gram liver

18. Rats -- Pharmacokinetics

19. Rats -- Metabolism

20. Human -- Subject A

21. Human -- Subject B

23. Comparison of Cancer Risk Estimates for Vinyl Chloride Basis Old EPA -- Animal PBPK -- Animal PBPK -- Human (Epidemiology) Inhalation(1 ug/m3) 84.0 x 10-6 1.1 x 10-6 0.2 - 1.7 x 10-6 Drinking Water(1 ug/L) 54.0 x 10-6 0.7 x 10-6

24. Considering Pharmacokinetic and Mechanistic Information in Cancer Risk Assessment Example 2: Trichloroethylene • Popular solvent for degreasing ; • replaced by perchloroethylene for dry cleaning • Lung and liver tumors in mice but not rats; • kidney tumors in rats but not mice • Equivocal human evidence (contradictory studies) • Tumors generally associated with toxicity • Little evidence of direct interaction with DNA • Unlikely to be carcinogenic at low doses

25. PBPK Model for TCE(Clewell and Andersen, 2004) CI QP CX CV CA Alveolar Air Alveolar Blood QC QC QTB CVTB Tracheo-Bronchial Tissue VMTB, KMTB Lung Toxicity CVF QF Fat Tissue CVR QR Rapidly Perfused Tissue CVS QS Slowly Perfused Tissue KTSD KTD Gut Lumen Stomach Lumen PDose KAD KAS QG Gut Tissue CVG QL CVL Liver Tissue Kidney Toxicity Liver Effects KF VM, KM

26. Comparison of Linear Cancer Risk Estimates (per million) for Vinyl Chloride and TCE Basis Vinyl Chloride: Old EPA PBPK -- Animal PBPK -- Human TCE: Old EPA PBPK -- Animal Inhalation(1 ug/m3) 84.0 1.1 0.2 - 1.7 1.3 3.5 Drinking Water(1 ug/L) 54 0.7 0.32 1.2 So… low-dose risk estimates using PBPK modeling would seem to suggest that TCE is a more potent carcinogen than vinyl chloride! (What’s wrong with this picture?)

27. PBPK modeling can only go so far… Also need an understanding of the toxic mechanism to interpret low-dose risks

28. Part 2: Use of PBPK Modeling to Interpret Human Biomonitoring Data • Issue: • Detection of chemicals in human blood (“chemical trespass”) • Uncertain relationship to doses in animal toxicity studies • Goal: • Reconstruct exposures • Compare to regulatory guidelines (MCL, RfD, etc) • Tools: • Pharmacokinetic (PBPK) models • Monte Carlo analysis of exposure variability and sampling uncertainty • Products: • Margins of safety • Objective interpretation of biomonitoring data

29. Relationship of Human Biomonitoring Data to Animal Toxicity Data Margin of safety Chemical concentrations in human blood from biomonitoring studies Chemical concentrations in animal blood in toxicity studies Reverse dosimetry Forward dosimetry Pharmacokinetic Modeling Pharmacokinetic modeling Human exposures (Chemical concentrations in environment) Animal exposures (Administered doses in toxicity studies) Traditional risk assessment

30. Reconstructing Exposure with a PBPK Model: An Example with Methylmercury • Accidental poisoning episode • Iraq – 1972 • Seed grain, treated with methylmercury fungicide, inadvertently used to prepare bread • Exposures continued over 1- to 3-month period • Symptoms (late walking, late talking, neurological performance) observed in children of asymptomatic mothers exposed during pregnancy

31. PBPK Model for Gestational Exposure to Methylmercury Clewell et al. 1999, Shipp et al. 2000

32. Effect of Changes in Fetal and Maternal Physiology on Dosimetry Non-human primates exposed to a constant daily dose of methylmercury during gestation

33. Exposure Reconstruction With a PBPK Model Iraqi woman exposed during pregnancy to grain contaminated with methylmercury Estimated exposure: 42 ug/kg/day EPA Reference Dose: 0.1 ug/kg/day

34. Exposure Reconstruction for perfluoro-octanoic acid • Perfluoro-octanoic acid (PFOA) is used in the production of “non-stick” surface coatings; it is also a by-product of the production of water- and grease-repellent finshes • PFOA is highly persistent compound that has been found in human blood and in the environment, raising public concerns regarding the possible effects of exposure • In this study, a pharmacokinetic model of PFOA was used to estimate exposures in a population exposed to high concentrations of PFOA in drinking water and in a group of workers exposed to PFOA in the workplace

35. Schematic for a physiologically-motivated renal resorption pharmacokinetic model for PFOA

36. Predicted time course of PFOA in plasma at different exposure levels ng/kg/day: 150 90* 46 Occupational exposure Serum PFOA Concentration (ng/mL) Environmental exposure Blood levels in general population: 5 ng/mL) *Estimated safe exposure based on effects in animal studies

37. Different fractional volume of fat between male and female effects dioxin concentration Transplacental exposure to dioxin in maternal blood Dilution of infant dioxin concentration by rapid growth Application of PBPK Modeling to Predict the Effect Of Age-Dependent PK on Dioxin Blood Levels (Clewell et al., 2004) Predicted blood levels assuming a constant daily exposure throughout life

38. Summary: Use of PBPK Modeling in Risk Assessments for Environmental Chemicals • Pharmacokinetics can be used to improve the accuracy of extrapolations across species, and to estimate exposures associated with human biomonitoring results • BUT: • Mechanistic data is essential for the selection of the appropriate dose metric to use in pharmacokinetic modeling as well as for the selection of the appropriate approach for characterizing the dose-response below the range of experimental observation of toxic effects

39. Physiological Pharmacokinetic Modeling Applications References Andersen, M.E., Clewell, H.J. III, Gargas, M.I., Smith, F.A., and Reitz, R.H. (1987). Physiologically-based pharmacokinetics and the risk assessment process for methylene chloride. Toxicol. Appl. Pharmacol. 87, 185 Clewell, H.J., III and Andersen, M.E. 2004. Applying mode-of-action and pharmacokinetic considerations in contemporary cancer risk assessments: An example with trichloroethylene. Crit Rev Toxicol 34(5):385-445. Clewell, H.J., Gearhart, J.M., Gentry, P.R., Covington, T.R., VanLandingham, C.B., Crump, K.S., and Shipp, A.M. 1999. Evaluation of the uncertainty in an oral Reference Dose for methylmercury due to interindividual variability in pharmacokinetics. Risk Anal 19:547-558. Clewell, H.J., Gentry, P.R., Covington, T.R., Sarangapani, R., and Teeguarden, J.G. 2004. Evaluation of the potential impact of age- and gender-specific pharmacokinetic differences on tissue dosimetry. Toxicol. Sci. 79:381-393. Clewell, H.J., Gentry, P.R., Gearhart, J.M., Allen, B.C., Andersen, M.E., 2001. Comparison of cancer risk estimates for vinyl chloride using animal and human data with a PBPK model. Sci. Total Environ. 274 (1-3), 37–66. Shipp, A.M., Gentry, P.R., Lawrence, G., VanLandingham, C., Covington, C., Clewell, H.J., Gribben, K., and Crump, K. 2000. Determination of a site-specific reference dose for methylmercury for fish-eating populations. Toxicol Indust Health 16(9-10):335-438. Tan, Y.-M., Liao, Kai H., Conolly, R.B., Blount, B.C., Mason, A.M., and Clewell, H.J. 2006. Use of a physiologically based pharmacokinetic model to identify exposures consistent with human biomonitoring data for chloroform. J. Toxicol. Environ. Health, Part A, 69:1727-1756.