Endocrine Disruptor 

Screening Program (EDSP)

Weight-of-Evidence Process

Thomas M. Steeger, Ph.D.

Environmental Fate and Effects Division

Office of Pesticide Programs

Office of Chemical Safety and Pollution Prevention

U.S. Environmental Protection Agency
Topics

Focus of the SAP review (what is the question being 

addressed)

Rationale for Case Study Selection

Overview of the 2011 EDSP Weight-of-Evidence 

Guidance

 Assembling and evaluating the quality and interpreting responses 

of individual studies

 Formulating hypotheses and integrating data along a biological 

continuum

• Adverse outcome pathway (AOP)/mode of action (MOA) concepts

• Evaluating degree and nature of complementarity and redundancy 

in responses

 Characterization of the conclusions

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Focus of the SAP

Seek comment from the SAP on the 

implementation of the basic interpretive 

process as illustrated with five case studies

The purpose of this weight-of-evidence (WoE) 

analysis is not to evaluate the 2011 EDSP 

WoE guidance itself but rather its proposed 

implementation

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Focus of the SAP Review

WoE analysis is focused on data interpretation as it 

relates to these scientific questions:

 Do the data provide relevant, robust, and consistent evidence that 

the chemical has the potential to interact with the normal function 

of the E, A, or T signaling pathways?

 If the data indicate a potential of the chemical to interact with E, A, 

or T signaling pathways, at what level of biological organization 

(e.g., molecular, cellular, tissue/organ response, organism level) is 

that interaction expressed?

 and

 Under what conditions does the chemical perturb the endocrine 

system (e.g., in an organism with an intact HPG axis, at what 

dose and duration, presence of overt toxicity, etc.)?

Addressing these questions is fundamental and provides 

a critical basis for Tier 2 testing decisions

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Case Study Selection Rationale

May 2013 – Subsample of 21 chemicals from

List 1 that reflected a range of pesticide classes 

and physical-chemical properties (e.g., molecular 

weights, water solubilities, log Kow, Koc, volatility)

Four Case Studies selected from the 21-chemical 

subsample; the fifth Case Study did not receive 

test orders

• Reflects different pesticidal classes and modes of 

action

• Illustrative of the range of responses that will be 

observed across the Tier 1 assays and OSRI

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Case Study Selection

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EDSP WoE Guidance

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Fundamental steps

Assembling and evaluating the scientific quality of the 

individual studies

Formulating hypotheses and integrating data at different 

levels of biological organization

• Evaluating extent and nature of the complementarity 

and redundancy in responses

Characterizing main lines of evidence and conclusions
Adverse Outcome Pathways (AOPs)

Structure Activity 

Relationships In vitro Studies In vivo Studies

Population 

Studies/Models

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EDSP WoE Guidance

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Assembling and evaluating the scientific quality of the 

individual studies

 Tier 1 assays primary source of data

• May 2013 SAP — Data reliable

–Performance criteria met or only minor deviations

–In some situations, Tier 1 in vivo responses coincident with 

overt toxicity

 Other Scientifically Relevant Information (OSRI)

• Journal articles, Test Guideline Data submitted in response to 

Part 158

–Did not conduct literature searches

–Relied on OSRI identified in response to List 1 Test Orders

–Chemical X did not receive Test Orders and limited literature 

search was conducted
EDSP WoE Guidance

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Method Selection on Evaluating Endpoints in 

Individual Studies

• May 2013 SAP
1AR: Androgen Receptor Binding Assay (OCSPP Guideline 890.1150)

2ER: Estrogen Receptor (OCSPP Guideline 890.1250)

3ERTA: Estrogen Receptor Transcriptional Activation (OCSPP Guideline 890.1300)

Decision Logic for Interpretation for a 

Chemical’s Potential for Endocrine Interaction

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Decision Logic for Interpretation for a 

Chemical’s Potential for Endocrine Interaction

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Decision Logic for Interpretation for a 

Chemical’s Potential for Endocrine Interaction

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*Histology may be required by some regulatory authorities despite significant 

differences in advanced and asynchronous development. 

Decision Logic for Interpretation of the 

Amphibian Metamorphosis Assay

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Decision Logic for Interpretation of the

Fish Short-Term Reproduction Assay

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EDSP WoE Guidance

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Formulating hypotheses and integrating data at 

different levels of biological organization

Evaluating extent and nature of the complementarity 

and redundancy in responses

• Provides a basis to evaluate “biological plausibility and 

coherence, strength, and consistency of the body of 

evidence.”

–“complementarity” describes the concordance of 

endpoints within an assay measuring multiple endpoints; 

and,

–“redundancy” describes the concordance of endpoints 

across the assays
Structure Activity 

Relationships

In vitro 

Studies

In vivo 

Studies

Formulating Hypotheses and Integrating Data 

at Different Levels of Biological Organization 

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Population 

Studies/Models

Greater 

mechanistic 

understanding

Greater relevance 

for humans and 

wildlife

Adverse Outcome Pathways (AOPs) Concept
Application Hypothesis-Based Approach

Androgen Antagonism

• Interaction of chemical with androgen receptor (measured in in vitro

AR binding assay), which leads to

• Altered functional cellular response such as increased VTG production 

in males (measured in FSTRA), and/or a decrease in serum 

testosterone (measured in the male rat pubertal assay),

• Corroborated by an altered structural response at the tissue or organ 

level, such as increase in gonado-somatic index and decrease in mating 

tubercles in male fish (measured in FSTRA), and/or decrease in 

androgen sensitive tissue weights (as measured in the Hershberger or 

male pubertal assays).

• Ultimately, androgen receptor antagonism may lead to an adverse 

outcome at the whole organism level, such as decreased fecundity (fish) 

or a delay in sexual maturation (rat).

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Androgen Antagonism

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WoE Analysis

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