Perspective on the current state-of-knowledge
of mode of action as it relates to the dose
response assessment of cancer and
noncancer toxicity
Jennifer Seed, PhD
Office of Pollution Prevention & Toxics
US EPA
This presentation represents the view of presenter & does not
necessarily represent the decisions or stated policies of the EPA.
Evolution of Risk Assessment
1980s
2000s
1990s
21st
Century
RA/RM Paradigm
Mode of Action
Toxicity Pathways
Guidelines/Methods
Susceptible Populations
Integrated Approaches
Dosimetry/PbPK
Mixtures
CompTox
1983
2007
1994
2
2009
Mode of Action information is core to the risk
assessment paradigm and its evolution into the 21st
century
Understand
Human Relevance
Susceptibility
Dose Extrapolation
Mode of Action
Toxicity Pathways
Identify Key Events
Characterize
Uncertainty
Promote
Harmonized
approach for all
endpoints
Inform
Multiple Chemical
Risks
3
How do you determine the Weight of Evidence
for establishing MoA?
 Needs to be based around specific hypothesis judged
against data
International Framework
 Introduced by USEPA & IPCS (1999, 2001)
 ILSI builds on EPA/IPCS work (2003, 2005)
 IPCS continues work (2006, 2008)
Based on Bradford Hill Criteria for Causality
Distinguishes MoA vs Mechanism
Human Relevance (Concordance) Analysis
4
Mode of Action/Human Relevance (MoA/HR)
Framework
 Why is it important?
– Increases predictive capacity and utility of risk
assessment
– Provides rigor and structure to analysis and
consistency of documentation
– Provides transparency
– Guides and focuses research/testing
5
5
Assemble Health Effects Data
Endpoint Assessment
• Identify adverse effects, focusing on those of concern for exposed
populations
• Identify precursors and other upstream indicators of toxicity
• Identify gaps – for example, endpoints or lifestages under assessed or
not assessed
MOA Assessment
(for each endpoint of concern)
• Research MOAs for
endpoints observed in
animals and humans
• Evaluate the sufficiency of
the MOA evidence
• Evaluate endogenous
processes contributing to MOA
Vulnerable Populations
Assessment
Identify potentially vulnerable
groups and individuals,
considering endpoints, the
potential MOA, background
rate of health effect, and other
risk factors
Background Exposure
Assessment
• Identify possible
background exogenous and
endogenous exposures
• Conduct screening level
exposures and analysis focusing
on high end exposure groups
Conceptual Model Selection
Develop or select conceptual model:
• From linear conceptual models unless data sufficient to reject low dose linearity
• From non-linear conceptual models otherwise
Dose Response Method Selection
Select dose response model and method based on:
• Conceptual model
• Data availability
• Risk management needs for form of risk characterization
Dose-Response Modeling
and Results Reporting
Figure 5.8 New unified process for selecting approach and methods for dose-response assessment for cancer and noncancer end points.
Mode of Action Human Relevance
(MoA/HR) Framework (v.IPCS)
Dose Response &
Species Extrapolations
Exposure Assessment
Compare
“Key Events”
& relevant
biology
between
animals &
humans
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Question 1: Sufficient Weight of Evidence to
establish MoA in animals?
 Postulated MoA (hypothesis)
– Describe sequence of Key Events (measurable)
– Consider other possible MoAs
 Experimental support for key events
– Concordance of dose-response relationships
– Temporal association
– Strength, consistency and specificity of association of
toxicological effect with key events
– Biological plausibility and coherence
 Uncertainties, inconsistencies, missing data
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Cacodylic Acid: Sequence of “Key Events” in MOA
Measurable Key Events in Target Tissue
DMAIII
Metabolite
Urothelial
Toxicity
Urinary bladder from a female
F344 rat treated with 100 ppm
DMAV
Sustained
Regenerative
Proliferation
Other MoAs:
DNA damage
via ROS?
BrdU Labeling
Urinary
Bladder
Tumors
Hyperplasia
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Q1: WoE Sufficient to Establish MoA?

Temporal
Postulated mode of
action
6 hrs
– Sequence of key
events
Experimental support
– Relationship of
key events &
adverse effects
– Dose-response
– Temporal
response

Biological plausibility &
coherence

Strength, consistency
& specificity
Metabolism
DMAVDMAIII
+
0.2
(wk 3-0.03 ± 0.01
uM)
(2 ppm)
Dose Response

Dose
(mg/kg
bw/day)
+
1
(wk 3-0.12 ± 0.02
uM)
(10 ppm)
+
4
(wk 3-0.28 ± 0.09
uM)
(40 ppm)
8 wk
104 wk
Regenerative
Proliferation
Urothelial
Hyperplasia
Transitional
Cell
Carcinoma
-
-
-
-
-
+
(wk 10-6/10, grade
3 or 4)
+
(wk 3-2/7, grade 3)
(wk- 10; 8/10, grade
3 or 4)
slight
(wk 10-1.5X inc)
+
(wk 3-7/7, grade 3)
(wk 10-5/10, grade
3 or 4)
+
(wk 10-4.3X inc)
+
(wk 10- 4/10)
-
+
+
9.4
(wk 3-0.55 ± 0.15
uM)
(100 ppm)

Urothelial
Toxicity
1 wk
Identify uncertainties
10
(6 hrs-6/7, grade 3)
(24 hrs-4/7, grade 3
or 4)
(wk 2 6/10, grade
5)(wk 10-0/10,
grade 4 or 5)
+
(wk 1- 2.2X inc) (wk 23.9X inc) (wk 10-4.2X
inc)
+
+
(wk 8-7/10)
(wk 10-9/10)
(papilloma first
obs at wk 107;
carcinoma first
obs at wk 87)
Q.2 & 3 Evaluation of Human Relevance
Q1. Weight of
evidence sufficient to
establish MoA in animals?
Compare
Q2. Fundamental qualitative
“Key Events”
differences in key events?
& relevant
biology
Q3. Fundamental quantitative
between
differences in key events?
animals &
humans
*Use all existing knowledge: chemical specific & generic
(e.g., genetic/disease models, related cpds)
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Q. 2 & 3 The Concordance Analysis
Qualitative Concordance
Key Event
Animals
Bladder
tumours
Strength
Humans
Yes,
Data
DMAIII present
following
exposure to iAs;
Direct exposure
to DMAV?
Considerable
In animals; but
limited data in
humans
Yes,
Data
Unknown:
Potential if
sufficient DMAIII
and cytotoxicity
is produced
Considerable No data
in animals,
possible in
humans but no
Presence of
DMAIII in urine
Sustained
urothelial cell
damage and
regenerative
proliferation
Humans
Quantitative Concordance
Yes,
Data
PbPK Model--based on use of
DMAIII dosimetry at the target
tissue because it represents the
rate-limiting event leading to
proliferation
data
Possible
Considerable
in animals;
plausible in
Limited evidence indicates
significantly less DMA III
produced
humans
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Carrying MoA information forward
into the dose-response analysis
MoA-Based vs Default
Q1. Weight of
evidence sufficient to
establish MoA in animals?
Q2. Fundamental qualitative
differences in key events?
 Dose extrapolation
based on MoA
understanding
 Key events - shape of
the dose-response curve
& points of departure
Q3. Fundamental quantitative
differences in key events?
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Point of Departure for Cacodylic Acid: Benchmark
dose modeling using key events
Key Events
BMD10
(mg/kg/day)
Urothelial cytotoxicity
(3 wk; 10 wk)
Proliferation (10 wks)
0.7; 0.02
BMDL10
(mg/kg/day)
0.2; 0.0008
PoD that provides a level of
exposure that
0.9will not trigger
regenerative proliferation
0.4
.
Hyperplasia (10 wks)
Tumors (104 wks)
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2.0
1.5
7.7
6.0
Carrying MoA information forward Implications for inter- and intra- species extrapolation
Default = 10X
Default = 10X
Interspecies
Dynamics (2.5)
Human Variability
in Disposition (3)
Interspecies
Kinetics (4)
Human Variability
in Sensitivity (3)
Data Derived Extrapolation Factors
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IPCS Harmonization Guidance Relevant to
MoA-Based Risk Assessment

Conceptual Framework for Evaluating a Mode of
Action for Chemical Carcinogenesis

Human Relevance Framework for Cancer

Non-cancer Human Relevance Framework

Principles of Characterizing and Applying PBPK
Models in Risk Assessment

Chemical Specific Adjustment Factors

Combined Exposures to Multiple Chemicals
information can be found at: http://www.who.int/ipcs/methods/harmonization/en/index.html
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2007 NRC Toxicity Testing in the 21st Century
Dose Response
Assessment
Chemical
Characterization
Mode of Action
Compounds
Affected
Pathway
Assess
Biological
Perturbation
Metabolite(s)
Population Based
Studies
Dose Response
Analysis for
Perturbations
of Toxicity
Pathways
Calibrating
in vitro and human
Dosimetry
Measures of
dose in vitro
Human Exposure
Data
Hazard Characterization
Exposure Assessment
Risk Characterization
Fig 3-7 Risk Assessment Components
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Exposure
Guideline