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Toxicity Pathways to Assessment Endpoints

Toxicity Pathways to Assessment Endpoints. P. Schmieder, S. Bradbury, G. Veith, J. McKim. Toxicity Pathway . WHAT:.

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Toxicity Pathways to Assessment Endpoints

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  1. Toxicity Pathways to Assessment Endpoints P. Schmieder, S. Bradbury, G. Veith, J. McKim

  2. Toxicity Pathway WHAT: • A concept; a way of depicting a chain of events starting with a molecular initiating event (site of chemical –biological interaction) and ending with an adverse effect manifested in an individual, or higher level – population, community, ecosystem • May include a biochemical/signaling pathway, but goes beyond, to at least hypothesize how something observed at one level of biological organization is linked to response manifested at another level. • Chemical similarity is defined in the context of biological similarity • “Similar” chemicals, by definition, invoke the same toxicity pathway (within a specified biological model) • QSARs are developed for “similar” chemicals from a known or hypothesized “mode/mechanism” of action; hypothesis is tested to refine the models • QSAR requires a well-defined biological system WHY:

  3. Effects of toxicants occur at different levels of biological organization. Toxic effects are best known and understood at the cell and organ level, while the ecosystem and community level are least understood although most relevant. (Haux and Forlin, 1988) Organ Population Individual Ecosystem Community Cell Chronic toxicity Reproduction Growth Productivity Energy Flow Contaminant dynamics in microcosms Acute toxicity Lethal Sublethal Respiration Osmoregulation Structural changes Induction TOXIC CHEMICAL Understanding Relevance

  4. Toxicity Pathway Uses • Assess knowledge gaps - what we know and what we don’t know about a chemical’s toxicity (toxicodynamics) • Assess the plausibility that a series of events are linked, i.e., degree of connectedness; • degree of specificity/certainty needed depends upon intended use • prioritization for further testing – correlation; “good” hypothesis? • quantitative RA - confirm cause and effect? • Pinpoint molecular initiating event for chemical extrapolation • QSAR – can be based on in vivo endpt if system is simple enough, e.g., fish acute/chronic for narcotic chemicals where applied chem conc is directly related to chemical activity in blood and further to the whole organism effect • Measurements closer to molecular initiating event will be more definitive for QSAR but some degree of relevance should be established (Linkage across levels of biological organization) • Basis for species extrapolation • Shifting RA paradigm - predict most likely tox pathways for a chemical to pinpoint most appropriate testing

  5. Well-Defined Biological System(Know what you know and what you don’t know) • Metabolism • Is the system used for collection of empirical data capable of xenobiotic metabolism? • Is what you’re measuring due to parent chemical or a metabolite? • Kinetics • What do you understand about the chemical kinetics within the system? • Is the chemical in solution • Bound and unavailable • Loss to hydrolysis Measure chemical form and concentration in your system

  6. Fathead Minnow Acute Toxicity Database 0 Narcosis I -2 Narcosis III -4 Narcosis II Log Fathead Molar Toxicity (1LC50) Uncoupler -6 -8 -10 -2 0 2 4 6 8 Log P

  7. Sorting Modes of Action (Toxicity Pathways) Fish Acute Toxicity Syndromes - respiratory/cardiovascular responses (RBT) Behavioral observations (FHM) Mixture studies (FHM)

  8. Nonpolar Narcotic Toxicants

  9. Assigning Chem Toxicol. Similarity for QSAR In vivo Assays Delineating Toxicity Pathways Across Levels of Biological Organization: Acute Nonpolar Narcosis Xenobiotic MOLECULAR TARGETS/RESPONSES TISSUE/ORGAN SYSTEM PHYSIOLOGY INDIVIDUAL -Decreased Respiration -Decreased Circulation -Faulty Osmoregulation Membrane Partitioning Ion Gradient Interruption Failed ATP Production Lethality Toxicological Understanding Risk Assessment Relevance

  10. Uncoupling Toxicants Water Solubility LC50-96hr MATC-30 day LC50-96hr MATC-30 day

  11. Assigning Chem Toxicol. Similarity for QSAR In vivo Assays Delineating Toxicity Pathways Across Levels of Biological Organization: Acute Uncoupling of Oxidative Phosphorylation Xenobiotic TISSUE/ORGAN SYSTEM PHYSIOLOGY MOLECULAR TARGETS INDIVIDUAL -Increased Respiration -Increased O2 Consumption -Decreased O2 Utilization Lethality Chemical Partitioning Membrane Proteins/ Ion Channels Toxicological Understanding Risk Assessment Relevance

  12. Reactive Toxicants

  13. Sorting Modes of Action (Toxicity Pathways) Fish Acute Toxicity Syndromes - respiratory/cardiovascular responses (RBT) Behavioral observations (FHM) Mixture studies (FHM) Biochemical responses – in vitro

  14. Effects of toxicants occur at different levels of biological organization. Toxic effects are best known and understood at the cell and organ level, while the ecosystem and community level are least understood although most relevant. (Haux and Forlin, 1988) Organ Population Individual Ecosystem Community Cell Chronic toxicity Reproduction Growth Productivity Energy Flow Contaminant dynamics in microcosms Acute toxicity Lethal Sublethal Respiration Osmoregulation Structural changes Induction TOXIC CHEMICAL Understanding Relevance

  15. Defining Toxicity Pathways Across Levels of Biological Organization: Redox cycling_Arylation Assigning Chem Toxicol. Similarity for QSAR In vivo Assays In vitro Assays Xenobiotic CELLULAR GSH Oxidation PrSH Oxidation ROS Production Decr. Energy Chg Disrupt Cytoskel. (MT;IF); Blebbing Altered Cell Signaling Cell Death TISSUE/ORGAN INDIVIDUAL Liver Toxicity Multiple Organ System Toxicities/Disease MOLECULAR Lethality Impaired Growth Binding to cytoskeletal components -Redox cycling - SH Arylation Toxicological Understanding Risk Assessment Relevance

  16. Chemical Class is not MOA for Industrial Chemical Acute Tox

  17. Knoxville Workshop Framework for Predicting Reactive Toxicity Speciation and Metabolism Molecular Initiating Events Measurable System Effects Adverse Outcomes Parent Chemical Rather than developing statistical models of complex endpoints, molecular initiating events are identified as well-defined QSAR endpoints…..and used to estimate the probabilities for important downstream biological effects based on transparent assumptions

  18. Steps to the Development of QSAR for Reactive Toxicants Speciation and Metabolism Molecular Initiating Events Measurable System Effects Adverse Outcomes Parent Chemical Systems Biology QSAR 1. Establish Plausible Molecular Initiating Events 2. Design Database for Abiotic Binding Affinity/Rates 3. Explore Correlations/Pathways to Downstream Effects 4. Explore QSARs to Predict Initiating Event from Structure

  19. Delineation of Toxicity Pathways Linkages Across Levels of Biological Organization In Silico Methods In vitro Methods In vivo Methods Electronic Molecular Cellular Organ Individual Chemical Reactivity Profiles Receptor binding DNA alteration Proteins adducts Membrane effects Gene Activation Protein Syn/deg Cell Signaling GSH balance Respiration Osmoregulation Liver Function Gonad Devel Lethality Growth Development Reproduction

  20. Understanding “Specific” Toxicities Endocrine Disruptors: -Receptor-Mediated Toxicity Pathways ER, AR, TR? -Enzyme Inhibition (aromatase) -Steroidogenesis (altered steroid metab)

  21. QSAR In vivo Assays In vitro Assays Delineating Toxicity Pathways Across Levels of Biological Organization: Direct Chemical Binding to ER Xenobiotic INDIVIDUAL POPULATION TISSUE/ORGAN Skewed Sex Ratios, Altered Repro. Chg 2ndry Sex Char, Altered Repro. CELLULAR Altered Hormone Levels, Ova-testis MOLECULAR Altered Protein Expression ER Binding Toxicological Understanding Risk Assessment Relevance

  22. Xenopus Metamorphosis Model for Thyroid System Disruption Molecular Tissue Individual Cellular Gene/Protein Expression Thyroid Histology Altered Morphology Circulating TH Status Thyroid Gland Thyroid Hormone Synthesis Peripheral Tissues Deiodination Morphology Pituitary Gland TSH Release Hypothalamus TRH (CRH) Release

  23. Conceptual Overview of Project Increasing Diagnostic (Screening) Utility Increasing Ecological Relevance Levels of Biological Organization • Molecular • Gene expression • Protein levels • Receptor binding • Enzyme activities Cellular Alterations in production of signalling molecules • Organ • Functional changes • Structural changes • (Pathology) Individual Altered reproduction or development Population Decreased numbers of animals Small teleost model, well characterized genome, low ecological / regulatory relevance Phase 2. Zebrafish genomics proteomics Computational modeling HPG Systems modeling Population modeling Small teleost model, poorly characterized genome, high ecological / regulatory relevance Phase 3. Fathead minnow molecular markers metabonomics Phase 1. Fathead minnow 21 d reproduction test →’s Depict the flow of information

  24. Chemical Risk Assessments Linkages Across Levels of Biological Organization Receptor-Mediated Pathways Organ Chemical 2-D Structure/ Properties Individual Molecular Cellular Gonad Development (Ova-Testis) Altered Hormone Levels Impaired Kidney Function Gene Activation Protein Production Receptor/ Ligand Interaction Impaired Reproduction Chemical 3-D Structure/ Properties Metabolism Understanding Relevance

  25. In vivo Toxicological Understanding Risk Assessment Relevance Toxicokinetics Toxicodynamics Molecular/ Sub-Cellular Xenobiotic Chemical Cell Organ/Tissue Individual Changes in Gene/Protein Expression Leading to Altered Cell Function Chemical- Receptor Binding Initiating Altered Gene/Protein Expression Impaired Reproduction Altered Organ Growth and Function

  26. Chemical Kinetics In vivo Molecular/ Sub-Cellular Xenobiotic Chemical Cell Organ/Tissue Individual Gene/Protein Cell Function Receptor Binding Gene/Protein Expression Reproduction Growth and Function Toxicological Understanding Risk Assessment Relevance

  27. Chemical Kinetics In vivo Xenobiotic Chemical Uptake Molecular/ Sub-Cellular Cell Organ/Tissue Individual Trout Toxicological Understanding Risk Assessment Relevance

  28. Chemical Kinetics In vivo Xenobiotic Chemical Uptake Distribution/Metabolism Molecular/ Sub-Cellular Cell Organ/Tissue Individual Trout Toxicological Understanding Risk Assessment Relevance

  29. Chemical Kinetics In vivo Xenobiotic Chemical Uptake Distribution/Metabolism/Excretion Molecular/ Sub-Cellular Cell Organ/Tissue Individual Trout Toxicological Understanding Risk Assessment Relevance

  30. In vivo Metabolism studies across levels of biological organization Linkages must be established Xenobiotic Chemical In vitro Uptake Distribution/Metabolism/Excretion Molecular/ Sub-Cellular Cell Organ/Tissue Individual Isolated Hepatocytes Celllines Microsomes S9 Purified enzymes Trout Isolated Perfused Liver Tissue Slices Toxicological Understanding Risk Assessment Relevance

  31. (E2) (E2-gluc) (E2) (gluc)

  32. Chemical Kinetics In vivo Xenobiotic Chemical Uptake Distribution/Metabolism/Excretion Molecular/ Sub-Cellular Cell Organ/Tissue Individual Gene/Protein Expression Cell Function Receptor Binding Gene/Protein Expression Reproduction Growth and Function Toxicological Understanding Risk Assessment Relevance

  33. Project Goal: Enhance Metabolic Simulator for EPA Regulatory Lists Predicted inactive parent; “activated” metabolites Existing ER Binding Model OPP Chemicals Existing ER Binding Model Expert Judgement Existing Metabolism Simulator improve ER model enhance simulator Prioritized Chemicals Verified ER activation Verified maps Predicted Metabolites Rat liver microsomes,S9 Trout liver slice Analytical methods MED; NERL-Athens; LMC

  34. Toxicity Pathways A useful concept for organizing toxicity data across levels of biological organization -Linking toxicological understanding to risk assessment relevance A conceptual framework for: - chemical extrapolation - molecular initiating events are the key to linking chemical reactivity continuum to biological response continuum - species extrapolation A useful concept in Predictive Toxicology - Predict most likely tox pathway for a chemical to pinpoint most appropriate testing

  35. Mapping Toxicity Pathways to Adverse Outcomes Structure Individual Cellular Molecular Organ Chemical 2-D Structure Altered Reproduction/ Development ER Transctivation VTG mRNA Vitellogenin Induction Sex Steroids ER Binding Initiating Events Impaired Reproduction/Development Chemical 3-D Structure/ Properties Libraries of Toxicological Pathways

  36. Mapping Toxicity Pathways to Adverse Outcomes Adverse Outcomes Initiating Events Libraries of Toxicological Pathways

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