McKim Conference on Predictive Toxicology

1
Toxicity Pathways as an Organizing Concept
Gilman Veith
McKim Conference on Predictive Toxicology The Inn
of Lake Superior Duluth, Minnesota September
16-18, 2008
2
Summary of Past Discussions

• Need for a Paradigm Shift Kees, Bradbury
• Need to Integrate Toxicology Veith
• Soft Reactive Toxicity Schultz, Comber
• Need for Organizing Chemistry Mekenyan
• ReproTox Pathways Schmieder, Ankley
• NeuroTox Pathways Fowle, Bushnell,

3
Purpose of McKim 2008

• Status on the new mindset in Europe
• Progress on skin sensitisation models
• Progress on the QSAR Application Toolbox
• Explore organizing methods for targets
• Expand the delineation of toxicity pathways

4
QSAR

• QSAR is the study of how to organize experiments
  of chemical behavior to find order
• Order is found when similarity is well-grounded
  in chemistrysmall changes are often huge.
• The hypothesis-driven paradigm will require
  integration of QSAR with more diagnostic assays

5
Why Build QSAR Methods? Example estimated ER
Binding Affinity for 6,518 High Production
Volume Chemicals
RBA
6
Our Conceptual Framework

Molecular Initiating Events
Chemical Speciation and Metabolism
Measurable Biological Effects
Adverse Outcomes
Parent Chemical
7
Our Conceptual Framework
Molecular Initiating Events
Speciation and Metabolism
Measurable Biological Effects
Adverse Outcomes
Parent Chemical
Response Pathways
QSAR
Chemistry/ Biochemistry
1. Identify Plausible Molecular Initiating
Events 2. Design Database for Abiotic
Binding Affinity/Rates 3. Develop QSARs to
Predict Initiating Event from Structure 4.
Quantify Response Pathways to Downstream Effects
8
Conceptual Framework

Molecular Initiating Events
Chemical Speciation and Metabolism
Measurable Biological Effects
Adverse Outcomes
Parent Chemical
Mortality -systemic toxicity -disease -cancer Impa
ired Development -terata -prenatal
deficits Reproductive Fitness -fertility -viable
offspring

• Interaction
• Mechanisms
• -Nonspecific
• Targets
• Atom Centers
• Targets
• -Receptor
• Targets

Chemical Inventories and Categories (200,
000)
9
Major Pathway for Reactive Toxicants To Fish
Vulnerable Organ Pathology
Molecular Initiating Events
Interaction Mechanisms
In vivo Endpoints
Pathogenesis
Michael Addition Schiff base Formation SN2 Acyl
ation
Atom Centered Irreversible (Covalent) Protein Bi
nding

Death from Suffocation
Any Exposed Surface Changes
Necrosis of the Gill Epithelium
Complexes Membranes, etc
10
At the Molecular Initiating Event

Molecular Initiating Events
Chemical Speciation and Metabolism
Measurable Biological Effects
Adverse Outcomes
Parent Chemical
The QSAR Question is How many other
chemicals can interact at this target?
While the Toxicology Question is What are the
known biological effects from this altered
target?
11
From the Library of Initiating Events

Library Of Molecular Initiating Events
Chemical Speciation and Metabolism
Measurable Biological Effects
Adverse Outcomes
Parent Chemical
OECD Toolbox Chemical Profiler Handles the
Chemistry for QSAR Models
Conformations
Targets Interactions
Metabolic Simulators
Inventories
Structural Requirements
12
Which Metabolite should we use in modeling
interactions?
Simulated 2-Acetylaminofluorene Metabolism
13
Which Conformation should we use to model
interactions?
14
From the Library of Initiating Events

Library Of Molecular Initiating Events
Chemical Speciation and Metabolism
Measurable Biological Effects
Adverse Outcomes
Parent Chemical
OECD Toolbox Libraries Will Link Targets to
Adverse Effects
Altered Gonad Development
Gene Activation
ER Binding
Impaired Reproduction
Protein Production
15
Pathways for Reactive Toxicity from Soft
Electrophiles
Mechanisms
Molecular Initiating Events
In vivo Endpoints

Michael Addition Schiff base Formation SN2 Acyl
ation
Atom Centered Irreversible (Covalent) Protein
Binding
Exposed Surface Irritation
Necrosis Skin Lung/Gills GI Tract
No
Immunogenic
Systemic Immune Responses
Systemic Responses Skin Liver Lung
Yes
16
Major Pathways for Reactive Toxicity from
Moderate Electrophiles
Interaction Mechanisms
Molecular Initiating Events
In vivo Endpoints
Exposed Surface Irritation

Michael Addition Schiff base Formation SN2 Acyl
ation
Atom Centered Irreversible (Covalent) Binding

Necrosis Which Tissues?
Pr-S Adducts GSH Oxidation GSH Depletion NH2
Adducts RN Adducts DNA Adducts
Oxidative Stress
Systemic Responses Skin Liver Lung
Systemic Immune Responses
Dose-Dependent Effects
17
Nonpolar Narcosis
Observable Response Level
NOAELLong Term
Nonspecific Reactive
Receptor Off-Target
Therapeutic Effects
1.0
0.1
0.01
Chemical Activity (or comparable metric)
18
Pathways for Reactive Toxicity
Molecular Initiating Events
In vitro Endpoints
Interaction Mechanisms
In vivo Endpoints
Membrane Alteration _ _ _ Oxidative
Stress _ _ _ Genotoxicity
Death Impaired Growth Impaired
Development Impaired Reproduction
Michael Addition Schiff base Formation SN2 Acyl
ation
Atom Centered Irreversible (Covalent) Binding

Pr-S Adducts GSH Oxidation GSH Depletion NH2
Adducts RN Adducts DNA Adducts
Dose-Dependent Pathways Species/Sex/Life-Stage
19
Two Questions for Building Pathways
Direct Reaction
Effect 1
Pr-S Adducts GSH Oxidation GSH Depletion NH2
Adducts RN Adducts DNA Adducts
Altered Synthesis
Effect 2
Effect 3
Oxidation
How Many Ways to Deplete GSH?
How Many Downstream Effects?
20
Delineation of Toxicity Pathways
Linkages Across Levels of Biological Organization
In vivo Methods
In Silico Methods
In vitro Methods
Molecular/ Subcellular
Electronic
Cell
Organ
Individual
Tissue
Exposure/ Metabolism Penetration Routes Detoxi
fication Pathways Activation Pathways
Chemical Reactivity Profiles Reversible
Nonspecific Binding Reversible Specific
Binding Covalent Binding
Response Pathways
Regulatory Endpoints
Chemical Inventories
Molecular Initiating Events Membranes Energy Ch
arge Nuclear Receptors Protein Synthesis DNA In
tegrity
Lethality Growth Development Reproduction
More Relevant Endpoints
Intrinsic Chemical Attributes
Better Defined Endpoints