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FIFRA SCIENTIFIC ADVISORY PANEL

Consultation

May 24-26, 2011

Integrated Approaches to Testing and Assessment Strategy: 

Use of New Computational and Molecular Tools

US Environmental Protection Agency

Office of Pesticide Programs

Washington DC

April 25, 2011
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Table of Contents

1. Integrated Approaches to Testing and Assessment Strategy: Use of New 

Computational and Molecular Tools..........................................................................................5

1.1 Purpose of SAP Consultation..........................................................................................................5

1.2 Problem Statement............................................................................................................................6

1.3 Vision and Strategy for a New Direction ....................................................................................8

1.3.1 Integrated Approach to Testing and Assessment..............................................................................................11

1.3.2 Exposure..............................................................................................................................................................................15

1.3.3 Benefits.................................................................................................................................................................................16

1.4 Building the Components of IATA: Key Research Activities ........................................... 17

1.4.1 Developing Databases and Knowledge Bases to Capture Existing Data ................................................17

1.4.2 Defining Adverse Outcome Pathways and Building Predictive Systems..............................................19

1.5 Building the Components of IATA: Key Partnership Activities....................................... 24

1.5.1 Capturing Existing Data and Building Libraries of Adverse Outcome Pathways..............................24

1.5.2 Developing Predictive Tools .......................................................................................................................................25

1.6 Building the Components of IATA: Key Activities Toward Regulatory Adoption ... 26

1.6.1 Components of IATA in Current Use .......................................................................................................................27

1.6.2 Anticipated Near-Term Activities.............................................................................................................................28

1.6.3 Screening using “Threshold of Toxicological Concern” Values ...................................................................30

1.6.4 Maximizing Efficiency of Animal Testing..............................................................................................................31

1.7 Stakeholder Engagement.............................................................................................................. 31

1.8 Metrics................................................................................................................................................. 33

2. Use of ”Omic” Technology to Inform the Risk Assessment A Case Study: 

Propiconazole ................................................................................................................................. 35

2.1 Background ....................................................................................................................................... 35

2.1.1 General Trends with Toxicogenomic Information............................................................................................36

2.1.2 EPA Cancer Guidelines and Mode of Action (MOA) Framework ................................................................37

2.1.3 Toxicogenomic Data in Risk Assessments............................................................................................................38

2.2 Approach ............................................................................................................................................ 40

2.2.1 Data elements supporting the MOA.........................................................................................................................42

2.2.2 Utility of “Omics” to Define the Propiconazole MOA........................................................................................49

2.2.2.1. Identify a series of potential molecular initiating and other key events that are plausibly 

linked to tumorigenesis (MOA/AOP). .......................................................................................49

2.2.2.2. Link a continuum of precursor events that lead to apical responses.................................................50

2.2.2.3. Identify cellular pathways and networks associated with these events for both linkage and 

coherence by enhancing information contained in apical key events. ...............................................................51

2.2.2.4. Infer other key events not measured through apical approaches........................................................51

2.2.2.5. Extend apical dose-response relationships to lower doses......................................................................53

2.2.2.6. Consideration of alternate MOAs. ........................................................................................................................53

2.2.3 Relationship of MOA to AOP/ Incorporation of the generated data into knowledge-based data 

models for prediction of toxicity for new or untested chemicals. ............................................................54

2.2.4 Minimize animal usage..................................................................................................................................................55

2.2.5 In vitro testing ...................................................................................................................................................................56

2.2.6 Conclusions.........................................................................................................................................................................56
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3. Use of an Adverse Outcome Pathway (AOP) to Inform Risk Assessment - Triclosan 

as a Case Study................................................................................................................................ 59

3.1 Background ....................................................................................................................................... 59

3.1.1 Approach .............................................................................................................................................................................60

3.2 Endocrine Effects of Triclosan.................................................................................................... 61

3.2.1 Effects on Thyroid Hormone -- in vivo Studies ...................................................................................................61

3.2.1.1 In vivo studies in weanling/young adult animals..........................................................................................63

3.2.1.2 In vivo studies in pubertal, maternal, and neonatal animals...................................................................67

3.2.1.3 In vivo studies in maternal and perinatal animals.......................................................................................69

3.2.1.4 In vitro/in silico studies.............................................................................................................................................71

3.3 Benchmark Dose Analysis for Effect on T4 ............................................................................ 72

3.4 Adverse Outcome Pathway Development............................................................................... 73

References

Section 1. Integrated Approaches to Testing and Assessment Strategy: Use of New 

Computational and Molecular Tools - References ..................................................... 75

Section 2. Use of ”Omic” Technology to Inform the Risk Assessment A Case Study: 

Propiconazole - References................................................................................................ 77

Submitted MRIDs...............................................................................................................................................................................79

Section 3. Use of an Adverse Outcome Pathway (AOP) to Inform Risk Assessment -

Triclosan as a Case Study - References........................................................................... 80
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LIST OF FIGURES

FIGURE 1. CRITICAL PATH TO AN ENHANCED INTEGRATED APPROACH TO TESTING AND ASSESSMENT..............................................10

FIGURE 2. ADVERSE OUTCOME PATHWAY AND THE DATA STREAMS THAT INFORM THE PATHWAY....................................................12

FIGURE 3. AN ER-MEDIATED AOP IN FISH ..................................................................................................................................................21

FIGURE 4. (Q)SAR-BASED APPROACH TO PREDICT THYROID DISRUPTION BASED ON THE ADVERSE OUTCOME PATHWAY FOR 

THYROID PEROXIDASE (TPO)INHIBITION. ........................................................................................................................................23

FIGURE 5. CURRENT AND FUTURE USE OF IATA IN THE PESTICIDE PROGRAM. ......................................................................................27

FIGURE 6. PROPOSED MODE OF ACTION FOR PROPICONAZOLE ................................................................................................................35

FIGURE 7 PROPOSED MODE OF ACTION (MOA) OF PROPICONAZOLE USING TOXICOGENOMIC DATA..............................................42

FIGURE 8. PRINCIPAL COMPONENT ANALYSIS (PCA) OF DEGS FROM PHENOBARBITAL (PB) AND PROPICONAZOLE (PRO)

TREATMENTS AT 30D. ...........................................................................................................................................................................54

FIGURE 9. PROPOSED ADVERSE OUTCOME PATHWAY (AOP) FOR TRICLOSAN EFFECTS ON THE THYROID HORMONE SYSTEM......60

FIGURE 10. STRUCTURE OF TRICLOSAN AND THYROXINE.........................................................................................................................62

FIGURE 11. SHORT-TERM ORAL EXPOSURE TO TRICLOSAN DECREASED SERUM TOTAL T4 CONCENTRATION. .................................62

FIGURE 12. THE EFFECT OF A 31-DAY EXPOSURE TO TRICLOSAN (MG/KG/DAY) ON MEAN (A) TOTAL SERUM TESTOSTERONE 

AND (B) SERUM ANDROSTENEDIONE CONCENTRATIONS IN THE MALE WISTAR RAT. ................................................................64

FIGURE 13. THE EFFECT OF A 31-DAY EXPOSURE TO TRICLOSAN (MG/KG/DAY) ON (A) MEAN TOTAL SERUM T4 AND (B) MEAN 

TOTAL T4 SHOWN AS PERCENT OF CONTROL. ....................................................................................................................................64

FIGURE 14. TRICLOSAN TREATMENT RESULTS IN DOSE-DEPENDENT DECREASES IN TOTAL SERUM. ................................................65

FIGURE 15. DOSE-EFFECT CURVES FOR THE EFFECTS OF A 4-DAY EXPOSURE TO TRICLOSAN ON LIVER MICROSOMAL EROD

ACTIVITY (A) AND PROD ACTIVITY (B).............................................................................................................................................66

FIGURE 16. DOSE-RESPONSE CURVES FOR THE EFFECTS OF A 4-DAY EXPOSURE TO TRICLOSAN ON LIVER MRNA EXPRESSION OF 

CYP ISOFORMS. .......................................................................................................................................................................................66

FIGURE 17 DOSE-RESPONSE CURVES FOR THE EFFECTS OF A 4-DAY EXPOSURE TO TRICLOSAN ON LIVER MRNA EXPRESSION OF 

CYP ISOFORMS. .......................................................................................................................................................................................67

FIGURE 18. MEAN SERUM (A) ESTRADIOL (B) TOTAL T4 (C) FREE T4 AND (D) TSH ON PND 42 FOLLOWING A 21-DAY 

EXPOSURE TO TRICLOSAN IN THE FEMALE PUBERTAL ASSAY...........................................................................................................68

FIGURE 19. MEAN SERUM TOTAL (A) AND FREE (B) T4 CONCENTRATIONS FOLLOWING A 3-DAY ORAL EXPOSURE IN THE 

UTEROPHIC ASSAY. ..................................................................................................................................................................................69

FIGURE 20. PERCENT OF CONTROL SERUM TOTAL THYROXINE (T4) FOR DAMS (A) AND PUPS (B), BY AGE AND ORAL MATERNAL 

DOSE (MEAN ± SE). ................................................................................................................................................................................70

List of Tables

TABLE 1. POSSIBLE KEY EVENTS SUGGESTING A MITOGENIC MODE OF ACTION (MOA) FROM THE TOXICOLOGY STUDIES 

FINDINGS WITH PROPICONAZOLE--APICAL ENDPOINTS..................................................................................................................40

TABLE 2. TEMPORAL ASSOCIATION- TOXICOLOGY, GENOMIC, PROTEOMIC AND METABOLOMICS STUDIES WITH PROPICONAZOLE

...................................................................................................................................................................................................................44

TABLE 3. SUMMARIZED KEY EVENTS ASSOCIATED WITH TUMOR INDUCTION IN MALE MICE FED DIETS CONTAINING 

PROPICONAZOLE (SUBMITTED MECHANISTIC STUDIES FROM THE OPEN LITERATURE)

A,B

............................................45

TABLE 4. SUMMARIZED KEY EVENTS ASSOCIATED WITH TUMOR INDUCTION IN MALE MICE FED DIETS CONTAINING 

PROPICONAZOLE .....................................................................................................................................................................................47

TABLE 5. SUMMARIZED KEY EVENTS ASSOCIATED WITH TUMOR INDUCTION IN MALE MICE FED DIETS CONTAINING 

PROPICONAZOLE (SUBMITTED STUDIES)

A B

.......................................................................................................................................48

TABLE 6. RESULTS OF BENCHMARK DOSE ANALYSIS USING DATA FROM ZORILLA ET AL.(2009).....................................................72
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IATA is a progressive, tieredevaluation approach that 

logically starts with hazardbased hypotheses about the 

plausible toxicological 

potential of a pesticide or 

group of pesticides based on 

their physical-chemical 

properties. Existing exposure 

and toxicity information is 

then combined with 

computer modeling and ‘new’ 

diagnostic in vitro (nonanimal) assays to target 

information needs specific for 

a chemical or group of 

chemicals.

1. Integrated Approaches to Testing and Assessment Strategy: Use of 

New Computational and Molecular Tools

1.1 Purpose of SAP Consultation

EPA’s Office of Pesticide Programs (OPP) is committed to improving and transforming 

its approaches to pesticide human health and ecological risk assessment and management.

The goals are to increase the efficiency, effectiveness, and accuracy of the testing and 

assessment process by enhancing our abilities to target the effects of concern and to base 

decisions on the information most relevant to the 

assessment. The strategy to attain this goal is to enhance 

and advance our ability to use an integrated approach to 

testing and assessment (IATA) in a consistent manner across 

all pesticide chemicals (OECD 2008). OPP views this as a 

critical time to take advantage of rapidly advancing 

molecular science and emerging in silico and in vitro

technologies. 

The purpose of this consultation is to seek guidance 

from the FIFRA Scientific Advisory Panel (SAP) about OPP’s 

vision, strategy, plans, and initial efforts to incorporate

advances in molecular science and emerging in silico and in 

vitro technologies (e.g., structure-activity relationships 

(SARs), quantitative SARs [(Q)SARs], mechanistically-based 

in vitro assays, high throughput screening (HTS) assays) as 

components of an enhanced IATA. OPP requests your input on whether we have sufficiently 

articulated a sequence of events including research activities and plans to utilize new 

advances and tools that demonstrate a logical progression that will ultimately achieve

increased efficiencies and effectiveness in the risk assessment of pesticides. To some 

extent, OPP already uses in vitro/in silico approaches, (e.g., SARs, read-across, HTS assays) 

in certain aspects of our risk assessments (e.g., for evaluating the toxicity of inert 

ingredients1, metabolites and/or environmental degradates). Thus, OPP plans to build and 

expand on an established foundation of using a variety of tools in a tiered testing and 

assessment framework by systematically adding new tools and methodologies that, in 

particular, advance the biological understanding of the linkages between various levels of 

biological organization along adverse outcome pathways (AOPs) (NRC 2007, Bradbury et 

al. 2004, Dellarco et al. 2010, Ankley et al. 2010). These linkages will better enable the 

Agency to understand the intermediate effects from a molecular initiating event to apical 

adverse effects at the whole organism and/or population level. Once these linkages are 

established, a range of integrative tools can be used to estimate effects on higher levels of 

 

1 An inert ingredient means any substance (or group of similar substances) other than an active 

ingredient that is intentionally included in a pesticide product. Called “inerts” by the law, the name 

does not mean non-toxic.
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biological organization. A series of SAP reviews is anticipated over the next decade as OPP 

prepares to adopt newly available tools, methods, and information. 

Two case studies illustrate the use of mechanistic data to support chemical 

assessments and the importance of AOP knowledge to IATA. The first case study illustrates 

the use of genomic information to assess an AOP using the fungicide propiconazole and its

induction of hepatocarcinogenesis (Nesnow 2009). The second case study illustrates the 

use of AOP information both qualitatively and quantitatively to inform the risk assessment

and to identify susceptible populations using the antimicrobial triclosan as an example. In 

both case studies, the focus is on the AOP as a conceptual construct that portrays existing 

knowledge concerning the linkage between key events and an adverse outcome at a 

biological level of organization relevant to risk assessment. It is important to note that we 

are not considering the risk assessment of these example compounds; rather, we are 

focusing on the application of new approaches to guide data generation to illustrate the 

utility of different data streams within a biological framework as a scientifically rigorous 

approach to risk assessment. In addition, although the case-studies focus on the use of 

IATA approaches as they relate specifically to use in human health risk assessment, the 

approaches have utility for informing ecological risk assessments across multiple taxa as 

well. OPP seeks the SAP’s guidance regarding acceptance of the approaches illustrated in 

the case studies to a general application to evaluating hazard and risk. 

1.2 Problem Statement

Before new pesticides may be used in the United States, OPP is responsible for 

registering these chemicals (active and inert ingredients) under several statutes including 

the Federal Insecticide, Fungicide, and Rodenticide Act (FIFRA), the Federal Food, Drug, 

and Cosmetic Act (FFDCA), the Food Quality Protection Act (which in 1996 amended both 

FIFRA and FFDCA), and the Endangered Species Act (ESA). The Agency also reevaluates the 

safety of existing registered pesticides on a 15-year cycle to ensure they meet current 

standards. Under this Registration Review program, EPA initiates the reevaluation of about 

70 existing pesticides per year 

(http://www.epa.gov/oppsrrd1/registration_review/highlights.htm). OPP is a high volume 

program; there are approximately 1,100 active ingredients (including conventional, 

antimicrobial and biochemical) and 2,500 inert ingredients, comprising approximately 

19,000 registered products with a wide variety of use patterns (including agricultural 

chemicals and consumer products). Each year, OPP makes over 5,000 regulatory decisions

including the registration of dozens of products with new active ingredients. Additionally, 

OPP registers hundreds of new uses of currently registered active ingredients. 

Approximately 10-20 new active ingredients and 75 inert ingredients are evaluated each 

year

(http://www.epa.gov/pesticides/fees/2010annual_report/pria_annual_report_2010.pdf). 
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Each year the Agency makes 

over 5,000 regulatory 

decisions, registers dozens of 

products with new active 

ingredients and hundreds of 

new uses of currently 

registered active ingredients. 

Approximately 10-20 new 

active ingredients and 75

inert ingredients are 

evaluated each year. 

Extensive testing for a wide 

range of endpoints is 

required to support a 

registration action. 

OPP is a unique regulatory program in the EPA in that it has a strong statutory

authority to collect data. Regulations in 40 CFR part 158 specify the types and minimum 

amounts of data necessary for chemicals under consideration for registration and/or 

reevaluation. These data are intended to address potential risks associated with a 

pesticide’s use and include for example, data on a pesticide’s

identity, composition, potential adverse effects, exposure 

and environmental fate. In the risk assessment, OPP 

evaluates potential routes and durations of exposure and 

determines the potential adverse human and environmental

health effects posed by the chemical to different non-target 

species and populations. The current testing paradigm 

requires extensive toxicity testing followed by selection of

endpoints relevant to the risk assessment. For example, in 

vivo animal data are generated for multiple outcomes on 

food use pesticide active ingredients to determine which 

adverse effects are relevant. As such, EPA requires pesticide 

registrants to generate data for a wide range of endpoints 

(e.g., two-generation rat reproductive studies; cancer 

bioassays in mice and rats; avian reproduction tests; fish full 

life cycle studies, etc.). For a conventional agricultural pesticide, the battery of tests, on 

average, costs an applicant approximately $6 million and it takes the government 

approximately 15 to 36 months to review the results, at a cost to the government of 

approximately $1.25 million. Inert ingredients typically have a more limited toxicological 

and environmental database when compared to pesticide active ingredients. Inert 

ingredient risk assessments are based on publicly available data on the chemical of 

interest, data on similar surrogate chemicals or SARs.

Despite the amount of data that may be available on pesticides, complex science 

issues constantly arise in the course of pesticide evaluation. The potential risk associated 

with technological advances, such as in the development of nanomaterials; the concern for

certain toxicities such as the disruption of endocrine function; the identification of 

susceptible/vulnerable populations; the risk resulting from cumulative exposure to 

multiple chemicals; and extrapolation uncertainties across species and to environmental 

levels of exposure exemplify the scientific challenges the Agency regularly confronts. In 

short, the Agency must manage and review large amounts of data and resolve difficult 

scientific problems while adhering to strict schedules to meet statutory deadlines. The time 

it takes to make decisions affects public health, environmental protection and access to the 

overall benefits provided by new pest control products.

The current testing and assessment paradigm has provided a strong basis for risk 

management decisions for many years. However, the current approach is time consuming

and resource intensive in terms of dollars and animal usage. In addition, the current 

approach is not easily adaptable when new issues arise (e.g., novel toxicities associated 

with a particular subpopulation, endocrine disruption, cumulative risk). Thus, there is a 

need for a new agile approach that provides broad coverage of outcomes, life stages, and 

taxa and is readily augmented and changed as our knowledge advances. The end result is 
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OPP’s vision:

Evaluate pesticides more 

efficiently and with greater 

accuracy by enhancing our 

abilities to target the effects 

of concern and focus on the 

information most relevant 

to the assessment. 

an ability to evaluate safety with increased efficiency and effectiveness and focused on the 

information most relevant to an assessment of human and environmental health. The longterm solution to meeting emerging issues will not be the generation of more data faster, 

but rather, the determination of what specific effects data, for which chemicals, which 

exposures, and which populations, are essential to assess risks. In this context, the Agency

would require sufficient, targeted, credible information from which to make a decision, 

rather than an overwhelming amount of information from which only a subset may

ultimately be utilized in the decision-making process. Consistent with this view is the 

consideration of time and cost efficiencies associated with the generation and 

interpretation of toxicity and exposure data and the sound and responsible use of animals 

in testing. With this perspective, OPP's vision to advance an evolution in the risk 

assessment paradigm requires a strong foundation of the best available science. 

1.3 Vision and Strategy for a New Direction

In the future, OPP will evaluate pesticides more efficiently and with greater accuracy

by enhancing our abilities to target the effects of concern and to focus decision-making 

more quickly on the information most relevant to the assessment. This will reduce the 

amount of required in vivo testing, which will reduce the costs associated with bringing 

new pest management tools to the market both in terms of 

dollars and animals and in turn, will reduce the costs 

associated with evaluating that data. Concomitant to reducing 

costs, our increased abilities to quickly adapt the testing 

program to respond to new issues by, for example, examining 

a broader range of effects, will provide greater insight on 

population vulnerabilities, low dose effects and effects from 

mixtures. To that end, OPP envisions a better way to utilize 

information associated with the real-world use of pesticides

such as monitoring data and incident reports in order to inform faster risk management 

adjustments and to provide a feedback mechanism that will improve the methods and tools

that better enable more effective and comprehensive risk assessments. By achieving these 

outcomes, OPP will ensure that society’s resources are focused on those chemicals and 

potential adverse effects of greatest likelihood and concern. 

Technological advances in predicting chemical toxicity and exposure are absolutely 

essential for the Agency to be able to more efficiently and thoroughly review and assess 

pesticides toward anticipating and managing potential risk. EPA and research 

organizations around the world are developing and evaluating evolving technologies in 

molecular, cellular and computational sciences to supplement or replace in vivo testing. 

Research is underway to refine exposure models and develop improved methods for 

integrating monitoring and surveillance data into exposure assessments. EPA is committed 

to incorporating the use of these new techniques into regulatory decision-making as the

science evolves (US EPA 2009, 

http://www.epa.gov/spc/toxicitytesting/docs/toxtest_strategy_032309.pdf and 

http://www.epa.gov/pesticides/science/testing-assessment.html). 
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OPP’s strategy:

Adopt 21st century tools 

and apply them through 

IATA. 

This strategy is consistent with the ambitious vision founded on 21st Century tools 

and methodologies that the National Academy of Sciences (NAS) laid out in its 2007 report 

“Toxicity Testing in the 21st Century: A Vision and Strategy” (NRC 2007). The report

envisions that risk assessment would shift toward the avoidance of significant 

perturbations of normal cellular pathways in exposed organisms by using subcellular- and 

cell-based assays to measure these perturbations. This includes dose-response modeling 

organized around computational systems biology models of the “circuitry” underlying each 

toxicity pathway, and in vitro to in vivo extrapolations based on pharmacokinetic models to 

predict tissue concentrations under specific exposure conditions. As such, information 

from multiple levels of biological organization represented through in silico, in vitro and in 

vivo assays would be integrated to characterize AOPs.

Hierarchical consideration of exposure and effect information in a more tailored 

manner in the context of the information needed for risk management is also consistent 

with the 2009 National Research Council (NRC) report “Advancing Risk Assessment: 

Science and Decisions” (NRC 2009). This report recommended that the utility of a risk 

assessment would be increased by dialogue between risk assessors and risk managers 

early in the planning process, i.e., problem formulation. Furthermore, the 2009 report 

recommended inclusion of other stakeholders in that dialogue to ensure that the technical 

analyses within the risk assessment would align more closely with the risk management 

options and provide a more transparent analysis.

OPP’s critical path focuses on fully utilizing these 21st century tools in an integrated 

approach to testing and assessment (IATA). The long-term goal 

is to move from a paradigm requiring in vivo testing for “every 

possible adverse outcome” toward a hypothesis-driven 

paradigm where in vivo testing is targeted to the most likely 

hazards and risks of concern across taxa. This is a progressive, 

tiered-testing approach that starts with hazard-based hypotheses about the plausible 

toxicological and fate potential of a pesticide or group of pesticides based on their physicalchemical properties (e.g., using read-across, bioavailability, bioaccumulation, and SARs to 

examine exposure and toxicological potential). Existing exposure and toxicity information 

is then combined with refined exposure models, computational toxicological models (e.g., 

(Q)SARs), and diagnostic in vitro assays to narrow requirements for in vivo testing (US EPA 

2009a, Bradbury et al., 2004; van Leeuwen et al. 2009, Schulz 2010). This necessitates an 

improved understanding of chemically-induced AOPs , i.e., the linkages between a 

molecular initiating event and an adverse outcome at the individual or population level 

(Ankley et al. 2010). Figure 1 illustrates the current activities that will enable future risk 

assessments based on targeted in vivo testing and in vitro/in silico based predictions. A 

significant foundation of information and new tools is necessary to enhance OPP’s IATA. 

The critical components of this foundation (highlighted in yellow in Figure 1) – knowledge 

bases and databases of existing information, predictive tools (e.g., (Q)SAR, in-vitro methods, 

HTS assays), and AOP libraries – are currently under development. As OPP considers 

adopting new tools they will be evaluated using existing criteria and methods and through 

an iterative process of case studies and expert review. Ultimately, predictions and targeted 

testing will provide the basis for future risk assessments. 
Page 10 of 81

Figure 1. Critical path to an enhanced integrated approach to testing and assessment 

The goal is to require only those in vivo tests that provide the specific data needed to 

address remaining uncertainties for human health and ecological risk assessments given a 

chemical’s toxicological potential at a relevant expected environmental exposure. Moving 

away from a paradigm that requires extensive hazard testing will be both an evolution and 

a revolution. This process will take time, but as it occurs, it will result in a fundamental shift 

in the information we use to make decisions about pesticides.

Development and implementation of this more integrated approach to the testing 

and assessment of pesticides will be a challenge and will take a long time. This approach 

cannot be adopted and implemented overnight. Scientific tools and knowledge must be 

advanced and public understanding and confidence must be developed. As science 

advances, our regulatory structure also needs to evolve. Any changes we make in our risk 

assessment approaches and regulatory practice must reliably and credibly meet our safety 

standards. Initially, new tools will be used on a case-by-case and pilot program basis

depending on the quality of the underlying science. Ultimately, OPP will explore and make 

changes to policies, regulations and test guidelines as appropriate. Our overarching goal is 

to improve the quality of environmental and public health protection through timely and 

effective analyses.
Page 11 of 81

1.3.1 Integrated Approach to Testing and Assessment

The challenge is to develop the means to move in a scientifically credible and 

transparent manner from a paradigm that requires extensive in vivo hazard testing in 

which some data prove to be of limited utility (given what may already be known about the 

chemical or class of chemicals or that certain information is ultimately not central to the 

assessment), to a paradigm that provides the means to use a risk-based, hypothesis-driven 

approach to identify the specific in vivo information most relevant to the assessment. OPP’s 

goal is to have pesticides tested in animals only for those most relevant endpoints. This will 

require a systematic process to evaluate the potential of each chemical to initiate molecular 

interactions that are the basis for causing adverse effects. To achieve this goal, a more 

comprehensive understanding of pathways leading to toxicity from molecular initiating 

events to adverse outcomes at the whole organism and/or population is required.

An AOP represents existing knowledge concerning the linkages between a molecular 

initiating event and an adverse outcome at the individual or population level (Ankley et al. 

2010). As such, AOPs, by definition, span multiple levels of biological organization. Figure 

2 illustrates the relationship between chemical structure and the initial molecular 

interaction (e.g., receptor binding) and the sequent cellular responses (e.g., gene activation 

and protein expression), organ level responses, and ultimately an adverse outcome (e.g., 

impaired reproduction). The different data streams that inform this pathway include, for 

example, (Q)SARs reflective of the types of chemicals that can initiate a pathway, in vitro

assays that measure the chemical-biological interactions, in vitro assays that confirm the 

sequent cellular responses (e.g., gene expression), and ultimately in vivo tests that measure 

endpoints that are directly relevant to the adverse outcomes (e.g., impaired growth, 

reproduction and/or survival) that drive regulatory decision making. By understanding 

the likelihood of effects (i.e., initiation of a toxicity pathway) at lower levels of biological 

organization (e.g., from SARs and in vitro assays) one could efficiently determine if more 

expensive and time-consuming testing at higher levels of biological organization (i.e., in 

vivo assays) are needed. For a particular pesticide, in vivo tests could be eliminated for 

those outcomes that are not likely to occur because the chemical or its metabolites do not 

interact with the biomolecules identified in the AOP’s molecular initiating event at 

environmentally relevant concentrations.
Page 12 of 81

Figure 2. Adverse outcome pathway and the data streams that inform the pathway

AOP and mode of action (MoA) are similar in concept. MoA is defined as the 

sequence of key events and cellular and biochemical events (measurable parameters), 

starting with the interaction of an agent with the target cell, through functional and 

anatomical changes, resulting in cancer or other adverse health effects (USEPA 2005; 

Boobis et al. 2008). When available, OPP incorporates MoA information into risk 

assessment so as to more accurately characterize what effects may be biologically plausible 

in humans and the environment to inform which susceptible lifestages or subpopulations 

may be more impacted by chemical exposure. For example, OPP has used information on 

metabolism and key events to identify reproductive and neurodevelopmental adverse 

outcome pathways to better characterize potentially susceptible populations (e.g., USEPA, 

2008, USEPA, 2010a, 2010b).

To bring structure, rigor and transparency to the evaluation of MoA data, a 

framework approach is used that was developed and adopted by both EPA (USEPA, 2005) 

and by the International Programme for Chemical Safety (IPCS) (Sonich-Mullin et al., 2001; 

Boobis et al., 2006 also see http://www.who.int/ipcs/methods/harmonization/en/). This 

MoA framework provides a weight-of-evidence approach that is based on considerations 

for causality as originally articulated by Bradford Hill in epidemiologic studies, and 

includes considerations of dose response and temporal concordance, consistency, 

specificity, biological plausibility and coherence (Hill et al., 1965). The MoA framework 

provides an important tool to promote and formalize the use of mode of action data in risk 

assessment regardless of whether the information comes from traditional approaches or 

evolving molecular and in vitro technologies (Seed et al., 2005). 

The development of AOPs builds in part on one of the key concepts of the MoA

framework developed by EPA and IPCS. Once a MoA has been established, the key events 

can be used for read across to other chemicals, both qualitatively and quantitatively. If a 

new compound triggers the key events, it will potentially trigger the adverse effect 

identified in the MoA. The likelihood of an adverse outcome will then simply depend on a 

significant perturbation of key events (i.e., dose response). This allows the use of existing 
Page 13 of 81

knowledge and concepts while grounding the results of new methods in toxicologically 

relevant outcomes. 

Development of the toxicological understanding across a range of AOPs, from their 

causal molecular initiating event and subsequent key events to the adverse outcome will

establish the necessary knowledge to streamline the pesticide testing and assessment to 

more thoroughly integrate, in a tiered fashion, existing data, computer modeling, and in 

vitro models diagnostic of AOPs to target in vivo testing. Thus, establishing AOPs will

reduce animal testing, since, at each level of biological organization (starting with the 

molecular characteristics of the compound), as the potential for a chemical (or a group of 

chemicals) to elicit toxicological effects is refined in the context of likely human or 

environmental species exposure, fewer tests at higher levels of biological organization are 

needed. Therefore, instead of every chemical being tested for every possible endpoint in an 

in vivo testing battery, the only in vivo tests required are those that are essential for 

understanding remaining uncertainties regarding a chemical’s toxicological potential at a 

relevant expected environmental exposure. Furthermore, once an AOP is established the 

information can be used to develop new quantitative predictive tools and qualitative readacross methods. Thus, AOPs enable a shift from a chemical by chemical approach to a 

category based approach. Given that there could be hundreds of AOPs and that varying 

levels of AOP information are needed depending on the decision being made (i.e., the tier of 

the risk assessment), the Agency is seeking the Panel’s guidance on principles for scientific 

acceptance of AOPs and the use of AOPs to move away from chemical by chemical 

approaches. 

IATA approaches have utility for informing ecological risk assessments across 

multiple taxa. For example, an understanding of the biology behind an AOP in mammals 

could be useful for understanding AOPs across a variety of other taxa, and vice-versa. If the 

key molecular event(s) within an AOP are phylogenetically conserved across mammals and 

other taxa, then a mechanistic-based understanding of an AOP in mammals may be 

extrapolated to those other taxa. At a minimum, an understanding of an AOP in one species 

could be a useful starting point for understanding and possibly identifying AOPs in other 

taxa (e.g., birds, fish, invertebrates, and plants) particularly where there is sufficient 

information regarding specific deviations in highly conserved pathways. Successfully 

integrating emerging technologies such as in vitro high through-put assays, in silico

predictive tools such as (Q)SARs, and read-across, along with existing in vivo data across 

multiple taxa will improve our mechanistic-based understanding of the underlying biology. 

Conversely, an understanding of the biology of these various taxa will also enable the use of 

these emerging technologies to establish linkages between various levels of biological 

organization that will allow increased use of data from lower levels of biological 

organization to estimate risk at the whole organism and population level with reduced 

reliance on in vivo testing. The Agency is seeking comment from the Panel on the use of 

highly conserved processes to extrapolate AOP linkages across taxa and to identify 

methodologies to develop AOPs.

Most known AOPs derive from a long history of focused, hypothesis-driven 

experimentation (as described in the conazole case study). Such reductionist 
Page 14 of 81

experimentation, i.e., understanding the nature of adverse outcomes by reducing them to 

the interactions of their parts (key events along an AOP), is critical, particularly for defining 

the cause-effect relationships that underlie the connections between key events at different 

levels of biological organization. However, given that there are potentially hundreds of 

AOPs relevant to regulatory risk assessment decisions; there is a critical need to accelerate 

the pace of AOP discovery and development. Once AOPs have been defined, there is a need 

to evaluate and build confidence in their predictive utility and determine their domains of 

applicability in both chemical and biological (i.e., taxonomic) space. 

EPA envisions that 21st century approaches including omics 2, and HTS can help 

meet these needs. Specifically, omics can be applied to more efficiently survey the breadth 

of molecular/cellular effects elicited (in vivo or in vitro) by specific chemicals. Functional 

interpretation of omic data is generally dependent on annotations derived from existing 

knowledge, thereby placing some limits on the scope of novel discovery. However, as omic 

datasets grow sufficiently large and diverse, network inference and “reverse engineering” 

approaches may facilitate discovery of some novel interactions of toxicological significance. 

Overall, application of omics can lead to novel hypotheses regarding the linkage between 

molecular perturbations and specific chemical exposures or adverse outcomes, which can 

subsequently be tested through focused experimentation, thereby accelerating the 

discovery of novel AOPs. 

High-throughput screening can be similarly effective for increasing the efficiency 

with which a wide breadth of molecular/cellular responses can be surveyed, at least in 

vitro. However, unlike omics, HTS can dramatically enhance the efficiency with which 

chemical space can be surveyed. Thus, HTS is well positioned to identify new/novel 

chemicals that would be predicted to initiate specific AOPs via interaction with specific 

molecular targets (molecular initiating events) or perturbation of cellular response 

pathways. Identification of chemicals associated with molecular initiating events can 

facilitate the development of QSARs. It may also accelerate the pace of AOP evaluation by 

providing rapid identification of chemicals that would be hypothesized to cause specific 

outcomes, based on their AOPs. These hypotheses can then be tested to assess the accuracy 

of AOP-based predictions and the supporting extrapolation tools that would be required to 

make the predictions quantitative.

The current AOP framework employs a simplified depiction of a linear progression 

of events across levels of biological organization that translate a molecular perturbation to 

an adverse outcome (assuming the perturbation is sufficiently severe). However, it is well 

recognized that AOPs actually operate as networks within a systems biology context and

that various AOPs can share key events and interact with one another in a variety of ways. 

Reductionist experimentation has generally been characterized by a more linear mode of 

thinking. In contrast, because omics and HTS (viewed as a large suite or battery of assays 

implemented in a high-throughput manner) can efficiently cover greater breadth of 

molecular initiating events and/or key events and associate them with a given chemical, 

 

2 Omics is used to represent the broad group of toxicogenomics, proteomics, metabolomics, 

transciptomics, etc.
Page 15 of 81

they are, perhaps, better equipped for promoting a network-view. Ultimately, 

understanding of responses to stressors and effectively predicting toxicity, in a systems 

biology context that operates across multiple levels of biological organization, will require 

the kind of network-thinking that HTS and omics promote. Consequently, effective 

integration of data from hypothesis-driven, focused, testing, omics, and HTS represents an 

important foundation for an enhanced IATA. The Agency is seeking input from the Panel on 

how the Agency can take advantage of this foundation to speed the discovery, development, 

and acceptance of AOPs both qualitatively and quantitatively in risk assessments.

1.3.2 Exposure

Although this consultation is focused on toxicity, exposure is a critical consideration 

in fully advancing an IATA for pesticides. For the purposes of context, this section provides 

a brief summary of OPP activities and future plans with respect to 21st century science 

tools for exposure assessment.

Limitations in exposure data and models and issues associated with the use of 

available biomonitoring and population surveillance data confound OPP’s ability to refine 

testing needs. Human biomonitoring data can provide information about the contribution 

of certain chemicals to body burden which informs potential associations with human

disease. Thus, further advances in exposure science are critical to an integrated approach 

to testing and assessment. Exposure data are central to selecting doses for in vitro and 

targeted in vivo testing, interpreting and extrapolating from in vitro screening assay results, 

and identifying adverse outcome pathways. Exposure data provide an understanding of the 

linkage between source pathways and the resulting biological doses and adverse effects. In 

its 2007 report, NAS envisioned that understanding the toxicity pathways for a select group 

of health/environmental effects or molecular mechanisms would guide research to identify 

biomarkers associated with the toxicity-pathway perturbations that could be used for 

biomonitoring surveys and population surveillance. The information gathered through 

biomonitoring surveys and surveillance programs, in turn, would be critical to interpreting 

toxicity data and for evaluating the effectiveness of the new predictive tests and models. 

For example, population surveillance may indicate a human health risk that was not 

detected in toxicity tests. In addition, comparison of human exposure data from 

biomonitoring surveys with concentrations that perturb pathways in screening assays can 

be used to identify potentially important exposures. 

Probabilistic modeling is an example of current work in the context of human 

exposures. The Food Quality Protection Act of 1996 (FQPA) mandated that EPA consider 

aggregate (single-chemical, multi-route/pathway) and cumulative (multi-chemical, multiroute/pathway) human exposure, particularly for infants and children, when making 

pesticide regulatory decisions. Implementation of FQPA necessitated developing new 

methodologies to assess human exposures resulting from food and residential use of

pesticides. Probabilistic models have been recommended by the National Research Council 

because they allow better quantification of exposure at different percentiles of a population 

of interest, as well as the improved characterization of the uncertainty associated with the 

exposure estimates at those percentiles. OPP scientists have worked closely with ORD 
Page 16 of 81

scientists to develop SHEDS-Multimedia (Stochastic Human Exposure and Dose Simulation, 

see http://www.epa.gov/heasd/products/sheds_multimedia/sheds_mm.html). SHEDSMultimedia is a physically-based, probabilistic model that simulates individual exposures 

to chemicals in food and drinking water and through residential exposures over different 

time periods (e.g., daily, weekly, yearly). In addition, ORD scientists are working to link 

daily SHEDS exposure outputs to a sophisticated Bayesian-based PBPK/PD 

(Physiologically-Based Pharmacokinetic/Pharmacodynamic) model which will use the 

output from SHEDS to predict toxicant concentrations as the target organ on a temporal 

basis. 

In the context of ecological risks, OPP typically uses monitoring studies (e.g., 

groundwater and surface water) and incident (i.e., adverse effect) data to ground truth 

estimated exposures and effects. OPP continues to seek out exposure data and to develop 

modeling approaches that allow us to describe ecological risks in both spatially and 

temporally explicit contexts, to support not only pesticide registrations but also 

endangered species assessments. Concurrently, OPP envisions extending the current 'fieldscale' (i.e., small-scale land model) exposure assessment methods to a 'watershed scale' in 

both agricultural and residential/urban settings. With these improved tools, risk 

management decisions can effectively target specific regions of the country and times and 

methods of application that would yield the greatest reductions in risk while maintaining 

needed pest management tools. 

In order to advance 21st century science tools for exposure assessment, EPA has 

commissioned a second NAS committee to provide the Agency with specific feedback and 

advice on critical research and the science and public policy issues associated with 

evaluating pesticide exposure. The NAS will prepare a report with its recommendations in 

2012 (http://dels.nas.edu/Study-In-Progress/Human-Environmental-ExposureScience/DELS-BEST-09-02). OPP’s vision for exposure science is to develop better 

environmental measures of exposure and enhance our ability to use tiered processes that 

integrate new predictive models across various classes and categories of chemicals so as to 

permit refined and characterized ecological and human health risk projections at varying 

biological, spatial, and temporal scales for risk management decisions. 

1.3.3 Benefits

A paradigm shift to a more integrated approach to testing and assessment will 

significantly improve EPA’s ability to carry out its mission of protecting public health and 

the environment. It will focus on the most likely hazards of concern and determine what 

specific data for each chemical and exposure situation are essential to assess and manage 

risks appropriately. As a result, the approach will lower the costs for the government and 

tax payers because the Agency could avoid reviewing unnecessary tests. For the pesticideproducing industry, this approach will reduce and may eventually eliminate complex and 

expensive in vivo testing. It will also refine and reduce the use of animals in testing. This 

approach also has the potential to increase the feasibility of assessing the risks posed by 

mixtures. In the long run, the approach will provide opportunities for improved diagnostic 

biomonitoring and surveillance methods to detect chemical exposures and identify causes 
Page 17 of 81

Knowledge bases are more

than databases (storage 

systems built for simple 

queries) because they are 

designed to help analyze 

information in a relational 

integrated fashion.

of toxic effects. Finally, it will improve health and environmental protection and increase 

efficiency by focusing the regulated community, government, and interested parties on 

chemicals and endpoints of greatest concern. OPP will be able to evaluate more chemicals 

across a broader range of potential effects in a shorter time frame, thereby enhancing the 

quality and efficiency of risk assessment and risk management decisions. In short, by 

focusing resources where they are most needed, this paradigm shift will better meet the 

needs of EPA risk managers, those in the broader community of stakeholders engaged in 

risk assessment and risk management, and – most importantly – the public.

1.4 Building the Components of IATA: Key Research Activities

OPP is closely collaborating with the EPA Office of Research and Development (ORD)

and other Agency regulatory programs to plan the development and evaluation of 

components of IATA. ORD's Chemical Safety and Sustainability (CSS) research program 

(http://yosemite.epa.gov/sab/sabproduct.nsf/BC3D06D94442465C8525784700782CE3/

$File/Kavlock_CSS_SAB_public.pdf) provides the research direction for the Agency to 

support transformative approaches to further improve both the quality and nature of 

information used in managing chemical risks. Regulatory program offices need to be 

engaged early and often in the development of the science to ensure that the new methods 

will meet the needs of chemical risk management. As such, OPP has partnered with ORD on 

a number of activities including the continued development of new tools and 

methodologies including new in vitro, molecular, and computational science approaches to 

evaluate the inherent chemical and biological properties of pesticides as well as providing 

data to support the development of more reliable predictive tools such as (Q)SARs. As 

highlighted in Figure 1, the following critical components are necessary to enhance OPP’s 

IATA:

Databases and knowledge bases to capture existing data (e.g., toxicity, metabolism);

AOP and MoA libraries; and 

Predictive tools (e.g., (Q)SAR, in vitro methods, HTS assays). 

Some significant examples of ongoing Agency research activities on each of these 

components are highlighted in the subsections below. OPP is seeking feedback on whether 

we have sufficiently articulated a sequence of events including research activities and plans 

to implement new advances and tools that demonstrate a logical progression that will 

ultimately achieve increased efficiencies and effectiveness of the program.

1.4.1 Developing Databases and Knowledge Bases to Capture Existing

Data

An important component of IATA is capturing, in a 

searchable format, what we already know, i.e., legacy data. 

The development, maintenance, and use of data warehouses 

and “knowledge” bases are critical to achieve this goal. These 

databases in turn serve as a means to develop predictive 

tools for estimating similar information for data-limited 

chemicals. Thus, their utility will rely on establishing 
Page 18 of 81

databases with training sets that are sufficiently populated with pesticide data by chemical 

class and mode of action. 

OPP makes extensive use of ECOTOX, a comprehensive web-based database 

developed and maintained by ORD‘s National Health and Environmental Effects Research 

Laboratory (NHEERL). It contains data on effects of single chemicals to aquatic and 

terrestrial organism. The data primarily come from studies published in the open 

literature. More details can be found at the following websites: 

http://www.epa.gov/med/Prods_Pubs/ecotox.htm and 

http://cfpub.epa.gov/ecotox/help.cfm?sub=about. 

Several efforts are underway at EPA to build knowledge bases and expert systems; 

these include decision-support tools developed by ORD’s National Center for 

Computational Toxicology (NCCT) that guide users through chemical structural alerts, fate, 

hazard and risk information. Many of these tools are publicly available. For example,

ACToR (Aggregated Computational Toxicology Resource) is a knowledge base available 

online (http://www.epa.gov/actor/). ACToR is made up of 500 public data sources on over 

500,000 chemicals and it provides information on chemical exposure, hazard and potential 

risks to human health and the environment (Judson et al. 2008). It can be used to query a 

specific chemical and find all available public hazard, exposure and risk assessment data as 

well as previously unpublished studies related to cancer, reproductive and developmental 

toxicity.

ACToR is linked to the Toxicity Reference Database (ToxRefDB 

http://actor.epa.gov/toxrefdb), which allows Agency scientists and the public to search 

and download thousands of mammalian toxicity test results on hundreds of chemicals. The 

database captures 30 years of animal in vivo testing data that were previously found only in 

paper documents. The animal toxicity data contained in ToxRefDB, when combined with 

other sources of information such as exposure and metabolism data, provide the basic 

elements for estimating risk.

ACToR is also linked to the Distributed Structure-Searchable Toxicity (DSSTox) 

database network (http://www.epa.gov/ncct/dsstox/index.html). DSSTox provides a 

public forum for publishing downloadable, structure-searchable, standardized chemical 

structure files associated with toxicity data developed from available structure-viewing 

freeware and open-source programming tools. It delivers a simple, easy-to-use structuresearching capability through the chemical inventory of published DSSTox Data Files

(Richard et al. 2002, Richard et al. 2008). 

The metabolism pathways system (METAPATH) is another important knowledge 

base that is being developed by ORD’s NHEERL and the National Exposure Research 

Laboratory (NERL). METAPATH is a software system for collection and analysis of 

metabolism pathway data. Using the METAPATH platform, a database of mammalian liver 

metabolism of pesticide chemicals was created. The database currently contains > 400 

metabolic pathways (maps) for > 200 pesticides. The metabolic pathways were observed in 

various experimental systems, primarily for studies using rats. A wide range of additional 
Page 19 of 81

supporting information to aid interpretation was also coded – test species details, 

experimental treatment conditions, biological compartments sampled and processing 

methods, dosing levels and duration, etc. The METAPATH platform provides very powerful 

and flexible data analysis and search capabilities. Search queries can be defined and 

subsequently combined in complex logical AND, OR, NOT clauses. The search queries can 

be based on experimental treatment group information (e.g., dose level, gender, biological 

sample matrix, etc), chemical structure, or substructure. The system can calculate the 

similarity between maps with regard to a selected parameter of interest, for instance, how 

similar are the metabolites found for two different species exposed to the same chemical, 

or males vs females. Efforts are underway to further populate this knowledge base with 

livestock and plant metabolites as well as environmental degradate data.

As a means of accurately and quickly populating METAPATH, electronic Data 

Evaluation Record (DER) Composers have been developed based on North American Free 

Trade Agreement (NAFTA) harmonized study review templates for rat metabolism studies 

and studies characterizing the nature of the residue in livestock. The DER Composers serve

as external stand-alone applications allowing direct input of summary information from 

studies that can be easily imported into METAPATH or exported as text files for risk 

assessor use in reviewing submitted study information. Populating METAPATH directly 

using the DER Composers allows for more efficient and accurate transfer of data. OPP has 

worked closely with ORD to ensure data accuracy and the development of data verification 

procedures. Making greater use of DER composers to populate other databases will further 

ensure the appropriate level of quality assurance for databases.

METAPATH also provides critical data for the development of metabolic simulators, 

which are being developed to simulate the metabolism of xenobiotic chemicals in biological 

systems(http://cfpub.epa.gov/si/si_public_record_report.cfm?dirEntryId=216644&fed_org

_id=770&sitype=pr&timstype=presentation&showcriteria=0&address=nerl&view=citation

&sortby=pubdate&count=100&datebeginpublishedpresented=10/01/2009). Reliable 

forecasting of chemical metabolism is a critical factor in estimating a chemical’s toxic 

potential. A major challenge for scientists and regulators is accounting for the metabolic 

activation of chemicals, which may lead to increased toxicity. Research is underway to 

develop computer modeling approaches to predict chemical metabolism as a component of 

efforts to screen and categorize chemical risks. The quality of these metabolic predictions 

depends on the amount and quality of data used to train programs to simulate observed 

metabolic transformations. The METAPATH databases provide a source of metabolism 

data consistently collected under OPP guidelines.

1.4.2 Defining Adverse Outcome Pathways and Building Predictive 

Systems

One conceptual approach the Agency is taking to build the scientific foundation for 

expert predictive systems, starts with the description of the chemically-initiated 

perturbation (molecular initiating event) which leads to an adverse effect in the form of an 

AOP. The Estrogen Receptor (ER)-mediated reproductive impairment adverse outcome 

pathway (Schmieder et al., 2004), used as a basis for development of an estrogen receptor 
Page 20 of 81

(ER) binding Expert System for chemical prioritization to target required testing, which 

was reviewed in 2009 both by the FIFRA SAP 

(http://www.epa.gov/scipoly/sap/meetings/2009/082509meeting.html) and by the 

Organization for Economic Cooperation and Development (OECD; OECD, 2009), is an 

illustration of this use of the AOP approach. As shown in Figure 3, this particular AOP 

describes the linkage between the event that initiates the pathway (i.e., chemical binding to 

the ER) and the series of events that occur at successively higher and more complex levels 

of biological organization. The key events in this pathway include: 

Initiation of events by a chemical binding the ER as a result of sufficient chemical 

uptake into the organism and partitioning to a target tissue with ER-containing cells; 

Cell and tissue level gene transcription and translation, e.g., activation/transcription

of ER responsive genes indicated by vitellogenin mRNA induction (Vtg; an egg-yolk 

pre-cursor glycolipoprotein)

Protein production in fish liver; 

Organ effects (e.g., appearance of ova in male fish testicular tissue); and

Adverse reproductive and developmental outcome(s) observed in the individual 

(e.g., change in secondary sex characteristics (feminization of males); 

Cessation of spawning in females; complete sex reversal (i.e., genetic males with 

fully developed and functioning ovaries). 

The boxes in Figure 3 describe responses that may be observed at the various levels of 

biological organization. Binding and activation of the ER pathway was measured in an in 

vitro binding assay. A rainbow trout (Oncorhynchus mykiss) liver slice assay was then used 

to confirm ER-mediated gene activation (production of vitellogenin) within a 

metabolically-competent tissue. By using in vivo data, this pathway could be extended to 

potential consequences to a fish population as represented in the “Population Responses” 

box in Figure 3 where, for instance, sex reversal in individuals could result in skewed sex 

ratios in the population, or where cessation or decrease in spawning in individuals may 

potentially result in reduction in a year class in the population. 
Page 21 of 81

Figure 3. An ER-mediated AOP in Fish (modified from Schmieder et al., 2004)

It should be emphasized that the use of the ER-mediated reproductive impairment 

AOP in this case study was not to predict in vivo potency of ER binding chemicals which is a 

more complex and different problem than chemical prioritization. The ER-mediated AOP in 

fish is used as the scientific basis for focusing on chemical binding to the ER as the 

parameter to prioritize chemicals for in vivo testing, providing plausibility of the binding 

interaction resulting in an adverse outcome. The results of subsequent in vivo tests 

conducted on the priority list of chemicals will then be used to answer the higher tier risk 

assessment question of whether or not reproductive effects are likely to occur upon 

environmental exposure to these chemicals. 

OPP and other offices in EPA are faced with large numbers (hundreds to thousands) 

of chemicals that need to be assessed for their potential to cause endocrine disruption. 

Time and resources dictate that all chemicals cannot be evaluated at once. A challenge is to 

determine which chemicals should be tested first. The chemicals that most need to be 

prioritized and screened are typically those with the least amount of data. The Endocrine 

Disruptor Screening and Testing Advisory Committee 

(http://www.epa.gov/scipoly/oscpendo/pubs/edspoverview/finalrpt.htm), the combined 

Science Advisory Board, and the FIFRA SAP recommended the development of a 

prioritization scheme that includes both an effect and an exposure component 

(http://www.epa.gov/endo/pubs/sab_sap_report.pdf ). Ideally, EPA would be able to put 

into use a hypothesis-based approach that focuses on the potential for adverse effects, and 

prioritizes for testing the chemicals most likely to cause an effect. A predictive system built 

on the ER-mediated reproductive impairment AOP provides a hypothesis-based strategy to 

chemical prioritization. One example for how this could be put into use is the ER binding 

affinity Expert System which was developed to predict which of the regulatory food use 

Xenobiotic

chg. gene, 

mRNA,

protein

expression

chg. 

hormones; 

induc.VTG;

ova-testis

MOLECULAR

Interactions

CELLULAR

Responses

TISSUE/ORGAN

Responses INDIVIDUAL

Responses

POPULATION

Responses

chg. 2’ sex; 

decr. spawn; 

sex reversal

skewed 

sex ratios;

decr. yr. 

class

Incr. Toxicological 

Understanding

Incr. Risk Assessment 

Relevance Levels of Biological Organization

ER-mediated Reproductive Impairment AOP

ER 

binding
Page 22 of 81

inert pesticide ingredients and antimicrobial active ingredients, i.e., the chemical domain 

and structures of interest, could initiate the ER binding AOP. It is known that interference 

with hormone receptors (e.g., the estrogen receptor) can adversely affect reproduction, 

and, further, that diverse chemical structures can bind to the ER. Thus, this model will be 

used to help establish which inert ingredients and antimicrobial active ingredients 

(grouped in chemical categories) should be given higher priority for testing in EPA’s 

Endocrine Screening Program

(http://www.epa.gov/endo/pubs/edspoverview/primer.htm). 

The ORD NHEERL lab that developed the ER binding Expert System is taking a 

similar approach to build an (Q)SAR system to predict which chemicals are potential 

thyroid hormone disruptors. As shown in Figure 4, this AOP would include: 

The initial molecular event, which is inhibition of thyroid peroxidase (TPO) enzyme,

Followed by a decrease in thyroid hormone synthesis,

A consequent reduction in serum thyroid hormone and increase in thyroid 

stimulating hormone (TSH), 

Which leads to arrested metamorphosis in amphibians, and, more generally, to 

alterations in thyroid-driven normal development. 

TPO inhibition is measured in vitro and confirmation of activation of this pathway is 

with a metabolically-competent thyroid gland culture assay. However, though the AOP has 

been elucidated (linking molecular, cellular, tissue effects to in vivo developmental delay), 

this work is not as far along as the ER-Binding expert model. So far, in vitro assays have 

been optimized for a range of industrial–type chemicals (e.g., pesticide inerts and 

antimicrobials). An initial set of chemicals have been tested sufficiently to hypothesize 

possible chemical structural features associated with activity.
Page 23 of 81

Figure 4. (Q)SAR-based approach to predict thyroid disruption based on the adverse outcome pathway for 

thyroid peroxidase (TPO) inhibition (M. Hornung, personal communication).

As described, knowledge of AOPs allows the development of refined (Q)SAR/expert 

systems. In addition to these highly specific (Q)SAR tools, which are being developed to 

predict individual pathways of effects, the Agency is conducting biological profiling using 

high-throughput in vitro assay systems. HTS assays are being investigated as a means to 

efficiently evaluate the ability of chemicals to initiate molecular interactions that are the 

basis for causing adverse effects. Through its ToxCastTM program

(http://www.epa.gov/ncct/toxcast), the Agency uses a battery of relatively inexpensive in 

vitro, HTS assays on a relatively large and diverse chemical space to develop methods to 

predict potential human toxicity of environmental chemicals at a fraction of the cost of fullscale animal testing (Dix et al. 2007; Judson et. al., 2010). ToxCastTM uses advanced science 

tools to help efficiently understand biological processes impacted by chemicals that may 

lead to adverse health effects. ToxCastTM currently includes 500 fast, automated chemical 

screening tests. The major goals of ToxCastTM are to: (1) identify in vitro assays that can 

reliably indicate alterations in biological processes of relevance to in vivo mammalian 

toxicity; (2) develop signatures or prediction models based on multiple assays, along with 

computed or available chemical properties, that can achieve higher predictive power than 

single assays or chemical structure alone; and (3) use these combined computed and in 

vitro assay-based signatures to screen large numbers of previously untested environmental 

chemicals. Results from ToxCastTM are available via ACToR. 
Page 24 of 81

1.5 Building the Components of IATA: Key Partnership Activities 

Achieving our goal of increasing the efficiency and accuracy of chemical risk 

assessment and risk management by implementing a strategy to use enhanced integrative 

approaches to testing and assessment that incorporate emerging technologies will require 

coordinated efforts and acceptance across the Agency’s programs and across regulatory 

authorities globally. 

The Agency is involved in a number of international fora to advance 21st Century 

science approaches in IATA for regulatory purposes (e.g.OECD, NAFTA, World Health 

Organizations IPCS). For example, in 2007, the Agency hosted an OECD “Workshop on 

Integrated Approaches to Testing and Assessment” which provided an international forum 

to exchange information and views on applying the various components of IATA including 

in vivo and in vitro testing, (Q)SAR models, chemical category and read-across assessment 

methodologies, toxicogenomics, and exposure considerations to different kinds of 

chemicals and in different regulatory frameworks 

( http://www.oecd.org/dataoecd/45/52/40705314.pdf ).

The Agency is also partnering with the Pest Management Regulatory Agency 

(PMRA) of Health Canada on the “21st Century Toxicology: Integrated Approaches to 

Testing and Assessment” project approved under the auspices of NAFTA. The objective of 

this project is to “supplement, replace and reduce traditional animal toxicity testing 

methods in risk assessment by using a variety of tools and approaches in combination” and 

through making more efficient use of existing information. 

The intent of this section is not to provide an exhaustive list of all of the 

international activities but to highlight a few pertinent activities related to the 

advancement of IATA. These activities are complimentary to the Agency’s research 

program in developing essential IATA components:

Databases and knowledge bases to capture existing data (e.g., toxicity, metabolism);

AOP and MoA libraries; and 

Predictive tools (e.g., (Q)SAR, in vitro methods, HTS assays). 

Current partnership efforts to put these components of IATA into place are summarized in 

the following subsections.

1.5.1 Capturing Existing Data and Building Libraries of Adverse 

Outcome Pathways

In 2010, EPA proposed a project that was accepted by the OECD Working Group on 

Pesticides to initiate a multi-national collaborative venture to develop and populate a 

comprehensive and searchable metabolism database that stores metabolism data on 

laboratory animal, livestock, plant and environmental transformation products. This 

database, METAPATH (discussed above), facilitates the development of new predictive 
Page 25 of 81

structure-based methods, and allow for a more efficient comparison of metabolic profiles 

across chemicals, chemical classes, and species. 

In 2010, the Agency hosted an OECD Workshop entitled "Using Mechanistic 

Information in Forming Chemical Categories to Fill Data Gaps for Regulatory Purposes”. 

The objective of this workshop was to solicit scientific input to further the development of 

AOPs. The workshop focused on how AOP can be of use in forming toxicologically 

meaningful chemical categories for filling data gaps using existing knowledge (e.g., readacross and SARs). Participants also provided feedback on how scientific information can be 

gathered and generated to aid in developing AOPs and establishing linkages between data 

collected at multiple levels of biological organization, and the best platform for information 

exchange in AOP development. 

1.5.2 Developing Predictive Tools 

The Agency is committed to ensuring a transparent process for the objective 

determination of the reliability of (Q)SAR models in order to further enhance the accuracy 

and regulatory acceptance of (Q)SAR models. To that end, the Agency is engaged in a 

number of partnership activities related to the development and evaluation of predictive 

tools.

OPP has partnered with the U. S. Food and Drug Administration (FDA) Center for 

Food Safety and Applied Nutrition (CFSAN) in sharing publically available toxicity data and 

experience on the application of (Q)SAR in the regulatory risk assessment decision-making 

process. This partnership was formalized in a Memorandum of Agreement signed in 2009 

and since then has resulted in a number of (Q)SAR training seminars for OPP science staff 

members and information sharing meetings between the two agencies.

The Agency is partnering with PMRA of Health Canada to test the predictive 

performance of selected (Q)SAR models for pesticides and develop guidance for the 

application of (Q)SAR models to pesticide risk assessments. 

EPA has also contributed to several OECD reports and guidance documents on 

practical applications of (Q)SARs in specific regulatory contexts by governments and 

industry and has participated in OECD Expert Consultation to evaluate the application of 

the OECD (Q)SAR principles for the use of (Q)SAR expert systems in regulatory assessment 

of chemical safety (http://www.oecd.org/dataoecd/33/37/37849783.pdf and 

http://www.oecd.org/officialdocuments/displaydocumentpdf/?cote=env/jm/mono(2004)

24&doclanguage=en). The Agency continues to aid in the development of the OECD (Q)SAR 

Application Toolbox through contribution of new tools as well as participation in the QSAR 

management group 

(http://www.oecd.org/document/23/0,3343,en_2649_34365_33957015_1_1_1_1,00.html)

Over 20 years ago, OECD member countries endorsed the Mutual Acceptance of 

Data (MAD) agreement (OECD, 1981)

(http://www.oecd.org/document/41/0,3746,en_2649_34365_1890473_1_1_1_1,00.html), 
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which commits member countries to accepting test data that have been produced with 

tests conducted in accordance with the OECD Test Guidelines (TGs) and Principles of Good 

Laboratory Practice. However, it is not realistic or efficient to develop TGs for every 

possible HTS or in vitro system. The Agency is involved in efforts through OECD to work 

toward “performance-based test guidelines” for in vitro systems under the Validation 

Management Group for Non-Animal Testing (VMG-NA) of the OECD Task Force for 

Endocrine Disrupting Testing and Assessment (EDTA). 

To improve chemical risk assessment, there needs to be harmonized principles and 

approaches and agreement that the newer methods deliver protection of human and 

environmental health within a framework of sustainability3. Achieving a fully harmonized 

approach is critical to the implementation of IATA as it will ensure consistency in how 

information is used in regulatory decision making while providing incentives for directing 

the appropriate level of resources to ensure these tools are properly developed, vetted, and 

integrated into the regulatory decision-making process. International cooperation and 

coordination is also critical to making the best use of all available resources, avoiding 

duplicative efforts, and ensuring global public health protection based on the best available 

information.

1.6 Building the Components of IATA: Key Activities Toward Regulatory 

Adoption

Working in partnership with ORD and others, OPP has already begun to harness 

components of IATA. Over the next several years, depending on the readiness of the 

science, OPP plans to operationalize new in vitro and in silico predictive methods for 

priority setting and to enhance its integrated approach to testing and assessment to better 

target what toxicity data are needed to further refine risk assessments for chemicals that 

do not have extensive toxicity information (e.g., inert ingredients and metabolites and 

degradates of pesticide active ingredients). Over the long-term, as we gain experience with 

new in vitro and in silico approaches and as knowledge of AOPs increase, OPP’s view is that

reliance on animal testing will be reduced and highly targeted. Thus, we are actively 

engaged in developing hypothesis-based AOP approaches to phase in these new 

technologies and to phase out the less efficient animal intensive approaches over time. 

IATA provides OPP with the means to systematically link the various tools we currently 

have at our disposal with those that will be developed in the future in an iterative fashion 

to efficiently and effectively evaluate hazard and exposure and to assess risk. OPP has a 

long and proven track record of implementing new test requirements and risk assessment

approaches to ensure the safety of pesticides when used according to the label. Thus, OPP 

already has a number of “tools” in its “toolbox” to accomplish this. OPP will move from the 

application of specific tools to an integrated approach to testing and assessment by 

expanding and integrating that toolbox over time as our knowledge evolves.

 

3

Sustainability is being used in the context of developing approaches or policies that integrate 

environmental, economic, and social values in decision making.
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1.6.1 Components of IATA in Current Use

OPP is already gaining improvement and efficiencies in pesticide risk 

assessment by using searchable databases and predictive systems. Furthermore, in 

the near term, the Pesticide Program plans to adopt new components of IATA in a 

stepwise fashion (Sec 1.6.2). 

Figure 5 illustrates the use of IATA in the pesticide program now versus the future.

Figure 5. Current and future use of IATA in the pesticide program.

An example of a knowledge base currently used by OPP is METAPATH (Sec 1.4.1). 

The system METAPATH, in conjunction with other information, assists OPP risk assessors 

to determine 1) major metabolites of a pesticide that have toxic potential and 2) which 

metabolites to include in the risk assessment and tolerance expression of the pesticide 

active ingredient. The information stored in METAPATH on pesticide metabolism allows

comparisons across maps to identify common metabolites with potential toxicity concerns 

and include these in risk assessment. This information, in conjunction with QSAR 

predictions, is used to judge testing needs for metabolites and degradates of pesticide 

active ingredients.

In certain contexts, OPP has used existing data from similar compounds and 

computational models to evaluate pesticides for a number of years. For example, OPP’s 

ecological assessment process includes extensive use of read-across and QSAR tools as well 

as a tiered testing regime. Tools used by OPP include:

Ecological Structure Activity Relationships (ECOSAR): a component on the 

Estimation Program Interface Suite (EPISuite), which estimates the aquatic toxicity 

of industrial organic chemicals and pesticides.

(http://www.epa.gov/oppt/newchems/tools/21ecosar.htm)

ECOTOX (Sec 1.4.1): chemical toxicity data for aquatic and terrestial plants and 

animals (http://cfpub.epa.gov/ecotox/help.cfm?sub=about)
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Assessment Tools for the Evaluation of Risk (ASTER): used for hazard ranking and 

the development of comprehensive environmental risk assessments for aquatic 

organisms and wildlife. Provides high quality data for discrete chemicals in the 

associated databases (i.e., ECOTOX and EcoChem) and QSAR-based estimates when 

data are lacking. (http://www.epa.gov/med/Prods_Pubs/aster.htm).

As with the Agency’s assessment of industrial chemicals, pesticide inert ingredients 

are frequently evaluated with QSARs and read-across methods to determine whether in 

vivo testing is necessary to make a safety determination. Tools include,

EPI SuiteTM: estimates physical / chemical properties and environmental fate

(http://www.epa.gov/opptintr/exposure/pubs/episuite.htm)

Screening-level tools for exposure 

(http://www.epa.gov/opptintr/exposure/pubs/screen.htm).

Read-across (http://www.epa.gov/hpv/pubs/workshop/wkshebi.htm)

OPP’s assessment program is based on the concept of tiered-testing and targeted 

data needs informed by existing knowledge of the chemical or class of chemicals, including 

an understanding of the mechanism of biological activity, and the proposed use of the 

chemical. When available, OPP incorporates MoA information into its risk assessments so 

as to more accurately characterize what effects may be biologically plausible in humans 

and what susceptible populations may be more likely impacted by chemical exposure. 

1.6.2 Anticipated Near-Term Activities

Over the next five years, the Pesticide Program will adopt new components of IATA 

in a stepwise fashion. In collaboration with ORD and other partners, OPP will evaluate new 

tools as they are developed using available criteria and methods such as the OECD 

principles for (Q)SAR expert systems 

(http://www.oecd.org/dataoecd/33/37/37849783.pdf and 

(http://www.oecd.org/officialdocuments/displaydocumentpdf/?cote=env/jm/mono(2007

)2&doclanguage=en) and the EPA-IPCS mode of action framework (Sonich-Mullin et 

al., 2001; Boobis et al., 2006). As OPP considers adopting new tools they will be evaluated 

through an iterative process of case studies and expert peer review (see Figure 1). Case 

studies will examine questions such as: for a set of chemicals, does the new tool provide 

predictions comparable to available in vivo studies? Each situation will require criteria and 

a determination of acceptable level of uncertainty. Once a tool has been sufficiently 

evaluated by the scientific community, OPP may set up a pilot program to refine the 

assessment and regulatory process associated with the use of the new tool. For example, in 

2009 OPP established a voluntary pilot program for using non-animal methods for testing 

the likelihood and degree of certain antimicrobial pesticides to cause eye irritation

(http://www.epa.gov/oppad001/eye-irritation.pdf). Application will be context specific, 

for example, a new tool may first be used to support non-food uses. OPP anticipates a series 

of SAP reviews as new tools become available. For example, the recent SAP on structure 

activity relationships of estrogen binding affinity to support prioritization of pesticide inert 

ingredients and antimicrobial pesticides for screening and testing
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(http://www.epa.gov/scipoly/sap/meetings/2009/082509meeting.html). The Agency is 

seeking input from the Panel on criteria and methods the Agency can use to reliably 

integrate predictive tools into the process without delay.

Near-term developments could include, for example, an analysis of HTS signaling 

profile alerts as part of the evaluation of pesticide inert ingredients. Also, the Agency is 

expanding the library of QSAR tools with knowledge databases sufficiently relevant to 

pesticide structures to enable their use in an improved evaluation of metabolites and 

degradates of pesticide active ingredients. In the context of EPA’s Endocrine Disruptor 

Screening Program it is envisioned that in vitro and in silico methodologies will also be 

helpful in screening pesticide actives for potential impacts on endocrine-mediated 

processes. EPA plans to use the schedule for re-evaluating registered pesticides in the 

Registration Review program to determine testing schedules for pesticide actives in the 

EDSP. For pesticide inert ingredients, the Agency also plans to employ predictive HTS in 

vitro and in silico tools in the EDSP prioritization process (http://www.epa.gov/endo/). It 

is possible that computational and HTS tools could eventually even serve as a substitute, or 

partial substitute, for screening, which currently consists of primarily (6 of 11) in vivo

studies. 

The upcoming 158W antimicrobial rule is another milestone towards OPP’s longterm vision (http://www.federalregister.gov/articles/2008/10/08/E8-23127/datarequirements-for-antimicrobial-pesticides). Antimicrobial pesticides provide an 

appropriate opportunity for advancing the paradigm shift. Many antimicrobials have both 

pesticidal and non-pesticidal uses and many antimicrobial products are regulated under 

multiple jurisdictions. Thus, many antimicrobial chemicals have been assessed by other

regulatory programs and agencies. The ready availability of published literature and 

publicly-available assessments offers a unique opportunity for the pesticide applicant to 

use the available information as a starting point for fulfilling data requirements, and, when 

appropriate, using computer modeling and in-vitro data to supplement or fulfill data 

requirements. As described in the preamble to the proposed rule, the Agency encourages 

submission of alternative studies e.g. (Q)SAR data.

Consistent with the long-term goals of the NAS vision, OPP is committed to 

advancing the use of biomonitoring data in exposure assessment. Currently, these data are 

used in trend analyses of exposure. For example, cancellation of a pesticide should 

correspond with decreases in urinary concentration of the pesticide in the general 

population. OPP is also beginning to use biomonitoring data such as the Center of Disease 

Control’s National Health and Nutrition Examination Survey (CDC NHANES) data to 

provide more accurate estimates of exposure than modeled estimates. OPP plans to 

continue to incorporate biomonitoring data into assessments when sufficient appropriate 

data are available. 
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1.6.3 Screening using “Threshold of Toxicological Concern” Values 

Combined with simple or more refined exposure scenarios, “Threshold of 

Toxicological Concern” (TTC) values can form the basis of an integrated approach in a 

tiered risk-assessment scheme. The TTC approach is widely acknowledged as a useful 

(Q)SAR based concept for addressing information gaps and has a long history of being used 

in different regulatory contexts for risk assessments of chemicals found at low exposure 

levels including chemical residues in food packaging, flavoring agents, cosmetic 

ingredients, and genotoxic contaminants in pharmaceuticals. (Blackburn, et al., 2005; 

Felter, et. al., 2009; Kroes, et al. 2004, Kroes et al. 2005, Kroes et al. 2007; Munro, 1996; 

Munro, et al., 1996; Munro, et al., 2008). TTC values represent an exposure threshold for 

chemicals below which no significant risk of adverse toxicological effects would be 

expected. The approach is based on extrapolation of toxicity data from an available 

database to a specific chemical for which the structure is known, but for which limited 

toxicity data are available. Thus, the utility of TTC in safety evaluation is dependent on the 

quality, quantity and relevance of the underlying toxicity database (e.g., the chemical 

domain of applicability) and the availability of a reliable and relevant estimation of the

exposure for the durations and routes of interest. 

The Agency is currently participating in an International Life Sciences Institute 

(ILSI) Research Foundation project to develop a TTC-based approach for the evaluation of 

antimicrobial pesticide active ingredients

(http://www.ilsi.org/Europe/Pages/TF_ThresholdToxicological.aspx). The TTC concept is 

being proposed as a means for waiving testing requirements based on knowledge of

exposure limits and known toxicities of chemical classes in order to focus finite resources 

and animal testing on the evaluation of chemicals with a greater potential to pose risks. 

This concept is very similar to that of setting reference doses (RfD) and margins of 

exposure (MOE), however, it expands on the concept by extending it from an individual 

chemical to a class or grouping of chemicals. The ILSI project is being coordinated with the 

European Food Safety Authority’s (EFSA) initiative to investigate broadening the TTC 

concept across a range of programs including its application to metabolites, degradation 

products, and reaction products of active substances of plant protection products. In the 

ILSI project, the TTC approach is being investigated in relation to structural information 

and the toxicological data of antimicrobial chemicals. The development of structural 

categories and corresponding TTC values would be considered for an array of human 

endpoints relevant to the evaluation of these active ingredients including systemic, 

reproductive, and developmental toxicities, and carcinogenicity. The chemical domain for 

antimicrobials has been defined and the existing toxicity data has been housed in a 

searchable database (ToxRefDB) to evaluate whether the chemical structures for 

antimicrobials are adequately represented to derive the exposure thresholds to support 

use of TTC toward chemical testing prioritization and targeted testing strategies, and 

eventually in the risk assessment of the antimicrobial chemicals. Additionally, because TTC 

values are largely based on oral toxicity data, an approach is being developed to adjust the 

oral TTC values to account for dermal exposure. In sum, the TTC approach is a potential 

near-term methodology that could considerably advance OPP’s goals of increasing 

efficiency and reducing animal testing. 
Page 31 of 81

1.6.4 Maximizing Efficiency of Animal Testing 

Another short-term activity is to make the existing animal testing paradigm more 

efficient, reliable, and responsive to risk assessment and management needs. OPP supports 

the development of increasingly effective laboratory animal tests that are designed to 

maximize the information generated about the nature of the effects being studied. For 

example, the ILSI Health and Environmental Sciences Institute’s tiered-testing proposal on 

Agricultural Chemical Safety Assessment (ACSA)

(http://www.24d.org/scientific/Carmichael2006.pdf), published in 2006 in Critical 

Reviews in Toxicology (Carmichael et al. 2006), is consistent with EPA’s vision of a more 

efficient and reliable science-based paradigm. Toward this end, EPA has partnered with 

other countries through the OECD to develop the extended one-generation rat toxicity test 

as a replacement for the two-generation reproductive test. Without compromising the 

quality of information obtained from these studies, adoption of the alternative protocol will 

substantially reduce the number of animals used. Furthermore, modules can be included to 

assess developmental immunotoxicity and developmental neurotoxicity. The alternative 

methodology was adopted in November 2010 at the OECD's Joint Meeting of the Working 

Party on Chemicals, Pesticides and Biotechnology and the Chemicals Committee approved a 

draft version of a test guideline for the Extended One-Generation Reproductive Toxicity 

Study (EOGRTS). Consistent with OPP’s commitment to advancing its vision without delay, 

OPP has already begun to accept the use of EOGRTS when appropriate, in place of the 

combination of the two-generation reproductive assay and the developmental 

neurotoxicology (DNT) assay which are resource intensive studies in terms of their 

conduct, analysis and use of animals. 

In conclusion, OPP has made considerable progress in developing the components 

essential to IATA. OPP seeks guidance on whether we have sufficiently articulated a 

sequence of events including research activities and the early use of new tools and 

methodologies that demonstrates a logical progression toward an enhanced IATA. 

1.7 Stakeholder Engagement

Achieving OPP’s vision requires active, collaborative partnerships with

stakeholders. In order to construct a new paradigm in which stakeholders have confidence, 

it is important that OPP understands the issues that are critical to address from the 

perspectives of a broad range of stakeholders. It is also incumbent upon the Agency to 

ensure that stakeholders understand new tools, methodologies, and processes as they are 

adopted. Communicating and engaging with stakeholders now in a clear, open, and 

transparent manner will enable OPP to take advantage of advances as soon as they are 

available.

To inform and engage stakeholders early in the Agency’s efforts to implement the 

NRC recommendations, the Pesticide Program Dialogue Committee (PPDC) work group on 

“Integrated Testing Strategies/21st Century Toxicology” was established in 2008 

(http://www.epa.gov/pesticides/ppdc/testing/index.html). The workgroup consists of 
Page 32 of 81

members from multiple stakeholder groups, including pesticide manufacturer and user 

organizations, animal rights advocates, local, state, Federal, and international agencies, and 

public health, worker, and environmental advocates. The key objective of this workgroup is 

to advise OPP on communication and transition issues as we move forward to adopt 

components of IATA. The workgroup is concerned with both short- and long-term 

implications of the transition and has focused on identifying critical issues, developing 

potential metrics of success, and examining the biomarkers component of the NRC strategy. 

In December 2010, OPP held a stakeholder workshop on “21st Century Science and 

Integrated Testing and Assessment Strategies: Transitioning Research to Regulatory 

Practice” to broaden the involvement of stakeholders beyond the workgroup. The 

objectives of the workshop were as follows: 

To communicate the strategic vision for integrated approaches to testing and 

assessment through the application of case studies.

To increase the common understanding of emerging 21st Century science tools and 

how they might be applied;

To further OPP’s understanding of stakeholder perspectives, priorities, and 

expectations.

To build an effective, transparent communication strategy. 

In general, stakeholders have responded favorably to the overall direction that OPP

is taking. There is broad support for expected outcomes such as regulatory efficiencies, an 

increased ability to examine a range of adverse effect endpoints, and reduced animal 

testing. However, stakeholders have expressed concerns and raised issues regarding 

potential implications of the transition. Furthermore, stakeholders have provided 

recommendations to the Agency on ways to address some of these concerns and issues. 

Some of the more common issues and concerns expressed include:

How will EPA ensure that the current level of protection of human health and the 

environment is maintained or enhanced? 

Will moving away from the use of a whole animal and towards the use of a limited 

set of in silico and in vitro assays and predictive tools miss important biological 

endpoints?

What are the expected impacts on the reliability of and the uncertainties associated 

with human health and environmental risk assessments and risk management 

decisions?

Will new testing approaches be able to account for differences between individuals 

in human and ecological populations?

Will the use of molecular tools increase our ability to characterize mixtures?

How will the new toxicity-testing paradigm streamline the pesticide regulation 

process? 

What barriers stand in the way of using newer tools and methods and what can be

done to remove these barriers?
Page 33 of 81

Will new tools and methodologies be incorporated into the regulatory process in a 

timely manner? 

How might these changes affect EPA’s efforts to harmonize pesticide testing and 

regulation guidelines with other international regulatory agencies?

Recommendations to the Agency on ways to address certain issues and concerns include:

Use existing data to develop a detailed case study of how future testing, assessment, 

and decision processes would work.

Assess new testing approaches by comparing results to animal testing results.

Conduct prospective and retrospective checks.

Build feedback mechanisms and safeguards into the system. For example, incident 

reporting and surveillance programs would provide a check to ensure effects are 

not being missed. In turn, results from surveillance programs will provide feedback 

to develop improved tools.

Establish a clear process for risk assessment and management.

As we move forward, it will be important for OPP to remain mindful of the issues 

and concerns raised by stakeholders because they will guide us in understanding essential 

process considerations and in developing strategies for evaluating emerging tools and 

methodologies. In light of these stakeholder issues and recommendations, OPP seeks 

advice on potential gaps in our strategy or factors we should consider as we begin to 

develop metrics (Sec 1.8).

1.8 Metrics

To ensure progress toward and successful implementation of a transition to an 

integrative approach for testing and assessment, OPP must develop and employ a set of 

metrics, each specific to a critical factor of the vision. Metrics are useful internally as well 

as being a valuable communications tool. However, OPP is only at the beginning stages of 

determining how best to do this. OPP seeks guidance on the concepts and attributes we 

should consider as we start to develop metrics.

Two types of metrics have been recommended to the Agency by the PPDC 

workgroup: metrics of progress and of success. Metrics of progress track milestones along 

the way to the goal; examples are new test guidelines, new testing or toxicity evaluation 

tools for use by industry and regulators, and identification and acceptance of adverse 

outcome pathways. Metrics of success involve comparing the use of new tools for testing 

and assessment to the existing paradigm, or current activities. Examples are: enhancing the 

quality of assessments by efficiently considering a broader range of endpoints, quantifying 

the development of AOPs and their qualitative and quantitative application, reducing 

testing costs, and reducing the number of animals used in testing,.
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OPP is at the beginning stages of contemplating metrics to demonstrate success in 

achieving our vision. OPP is looking for metrics that are easy to define, can be practically 

measured, are understandable, and easily communicated. In addition, as mentioned above, 

OPP would like to consider issues identified by stakeholders to define concepts for 

potential measures. OPP is interested in the Panel’s initial thinking regarding the concepts, 

considerations, and factors we should take into account as we start to build metrics. 
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2. Use of ”Omic” Technology to Inform the Risk Assessment A Case 

Study: Propiconazole

2.1 Background

This propiconazole case study document and its two appendices were developed to 

supplement the FIFRA Scientific Advisory Panel (SAP) background paper entitled 

“Integrated Approach to Testing and Assessment Strategy: Use of New Computational and 

Molecular Tools”. This case study describes how EPA’s Office of Pesticide Programs (OPP) 

and the Office of Research and Development (ORD) have expanded their research by 

linking key events along a specific adverse outcome pathway (AOP), i.e., the progression of 

cancer in rodents, to define a mode of action (MOA) as illustrated below for propiconazole 

and discussed in detail in the subsequent pages of this document.

Figure 6. Proposed Mode of Action for Propiconazole

Additionally, the case study shows how this approach may increase efficiency and 

reliability of assessment relative to traditional approaches that are heavily reliant on 

resource intensive in vivo studies. EPA hopes to apply this approach more broadly over 

time and, as described in the background papers, the Agency is seeking the SAP’s advice 

and recommendations about this proposal. It is important to note that the purpose of this 

background paper is to illustrate an approach and is not intended to assess the potential 

risks of propiconazole. Key terms used throughout this document are listed and defined as 

follows:

Definitions of Key Terms as Used in This Document

Adverse outcome pathways (AOP) represent existing knowledge concerning the linkage between the 

molecular initiating event, intermediate key events, and an adverse outcome at the individual organism or 

population level.

Mode of action is defined by the USEPA as the sequence of key events and cellular and biochemical 

events (measurable parameters), starting with the interaction of an agent with the target cell, through 

functional and anatomical changes, resulting in cancer or other adverse health effects (USEPA 2005).

Key events are empirically observable, necessary steps in a disease (e.g., cancer) process. 
Page 36 of 81

Apical endpoints are tissue- or organism-based alterations observed in vivo and are often the first 

evidence that is observed after treatment-related chemical exposure; these can include but are not 

limited to tumors, loss of body weight, altered urinary proteins, and blood markers.

Toxicogenomics is a field of science that examines apical endpoints from classical toxicology studies 

and combines them with the collection, interpretation, and storage of information about gene, 

protein, and endogenous metabolite levels within particular cells or tissues of an organism in 

response to a toxic substance. These data provide the molecular underpinnings for many of the 

adverse outcomes and offer novel insights into the adverse events associated with chemical 

exposure.

“Omics” is a general term that is applied to the data generated from genomics (global analysis of 

genes expression), proteomics (global analysis of protein expression) and metabolomics (global 

analysis of endogenous metabolites ).

Toxicity pathways are subcellular and cellular response pathways that can lead to adverse 

outcomes (e.g., health effects) when sufficiently perturbed. 

Molecular initiating event is the initial point of chemical-biological interaction within the organism 

that starts, i.e., initiates, the AOP. Additional key events further along the pathway that lead to, and 

are experimentally or toxicologically associated with the adverse outcome, are referred to as “key 

events”. 

Cytochrome P450: Mixed function oxidases (MFO) also known as “CYPs”, are a host of enzymes, 

referred to as Phase I enzymes that metabolize chemicals, often as part of the body’s defenses to aid 

in the excretion of potentially harmful substances (e.g., pesticides and other toxins).

AROD: Alkoxyresorufin O-dealkylase is one of the MFO enzymes used to assay for up- or downregulation of the enzymatic activities of individual and /or groups of CYPs. 

CAR/PXR: Constitutive androstane receptor/pregnane X receptor, is both part of a nuclear receptor 

subfamily that function as a sensor of xenobiotic substances and in response up-regulates the 

expression of target genes (CAR, e.g., Cyp2b10; PXR e.g., Cyp3a11) to produce the gene product (e.g.,

Cyp2b, Cyp3a), which are responsible for the metabolism of toxic chemicals. Activation of CAR is a 

key necessary step in the hepatocarcinogenic MOA for certain chemicals.

Pathways are fixed and confined groups of genes (or proteins) associated with a specific biological 

function (e.g., retinol metabolism or CAR/PXR activation). Two types of software have been used in 

this analysis: Ingenuity Pathway Analysis (IPA) and GeneGo MetaCore software.

Networks are groups of genes (or proteins) that are linked together in both simple and/or complex 

arrangements. The linking of one gene (or protein) and another gene (or protein) could be one of a 

series of functions such as binding or activating (e.g., phosphorylation). Ingenuity creates networks 

from the list of genes submitted after they are mapped to the knowledge database of relationships. 

Multiple networks are possible from the same groups of genes because of the multiple functions of 

each gene.

IPA Tox List: Functional gene groupings based on critical biological processes and key toxicological 

responses. 

IPA Canonical Pathway: Well-characterized metabolic and cell signaling pathways that have been 

curated by expert scientists. 

High-throughput screens: In vitro biochemical or cellular assays that can be run efficiently on a 

large number of compounds to determine their activity on different biological targets such as ion 

channels, receptors, enzymes, proteins, and signaling pathways Typically involves batch testing of 

chemicals for toxicological endpoints using automated liquid handling, detectors, and data 

acquisition methods.

2.1.1 General Trends with Toxicogenomic Information

The traditional risk assessment paradigm is based on exposure – dose – response 

without necessarily considering the underlying mechanism of action. In this model, the 

individual organism or particular tissue is exposed to chemicals or other stressors at some 

dose/concentration and a response in the organism or tissue is elicited. Thus, conventional 
Page 37 of 81

rodent toxicity studies, which are used to detect such responses, characterize adverse 

effects of a chemical primarily on apical endpoints such as clinical signs or pathological 

states. Data on precursor events such as enzyme induction or target cell proliferation are 

often available and could be used as indicators of the response in place of apical endpoints 

if a sufficient association of these events to the apical outcomes is provided. For many 

decades, the findings and interpretation of frank effects (i.e., tumors) from rodent cancer 

bioassays have formed the basis for human health risk assessments and regulatory 

decisions about carcinogens and noncarcinogens. With the advent of toxicogenomics, 

however, significant advances have been made in the assessment of human/environmental 

health risks. In this case study, we explain how toxicogenomics can provide a more 

efficient and effective means of evaluating the potential toxicity of a chemical by taking 

advantage of knowledge about the chemical’s mode of action (MOA) (NAS, 2007). Given 

the increasing number of chemicals requiring evaluation [more than 80,000 chemicals on 

the market, approximately 700 new chemicals added annually, (GAO, 2005)] and the need 

to provide more timely evaluations to insure appropriate regulation of chemicals deemed 

to pose high risks to human and/or environmental health, there is a need to streamline the 

testing scheme toward a more hypothesis-based, efficient and less resource intensive 

process based on adverse outcome pathways (AOPs).

A case in point is carcinogenesis, a complex process involving multiple levels of 

biological organization including genetic alterations and altered cellular events, 

progressing toward tumor formation, an apical endpoint and adverse outcome. The 

quickly evolving field of toxicogenomics has enabled a more comprehensive understanding 

of this progression starting with a molecular initiating event to intermediate key events at 

various levels of biological organization and culminating in tumor formation. The power of 

toxicogenomics and other molecular profiling technologies (collective referred to as 

“omics”) such as transcriptomics, proteomics, and metabolomics to understand the steps 

leading to cancer is amplified when combined with conservative analytic techniques such 

as biochemical analysis and histopathology (Leighton, 2005; Reynolds, 2005). 

Transcriptomics is the global analysis of mRNA expression; proteomics is the global 

analysis of protein expression; and metabolomics is the global analysis of endogenous 

metabolites. The combination of new and standard practices in toxicology enhances the 

ability to inform mode of action analyses of chemical-induced toxicities such as cancer 

because toxicogenomic investigations have shown the ability to discriminate between 

mutagenic and nonmutagenic MOAs (Fielden et al., 2007). Hence, pathway-associated 

gene, protein, and metabolite networks derived from the use of ”omics” data may prove to 

be a robust approach in predicting mechanisms of toxicity and classifying chemical-specific 

MOAs. Our goal was to use “omics” data in the MOA framework to provide an improved 

scientific basis for determining potential cancer risk. Accordingly, the following 

documents were prepared: 1) Case study, 2) Data supporting the case study (Appendix A), 

and 3) Tables and Figures representing the data (Appendix B).

2.1.2 EPA Cancer Guidelines and Mode of Action (MOA) Framework

An organized approach to risk assessment and research in support of risk 

assessment was presented by the U.S. National Research Council (NRC) that describes how 
Page 38 of 81

the biologically effective dose is related to the precursor biological response that is 

ultimately related to the adverse health consequence from exposure to the stressor of 

concern (NRC 1994). More recent cancer risk assessments have been developed through 

the process of describing an MOA (USEPA 2005), which is the identification of the key (but 

not all) events associated with the development of an adverse effect. In contrast to MOA, 

mechanism of action implies a more detailed understanding and description of events, 

often at the biochemical and molecular level, than is meant by MOA (US EPA 2005).

Since the prediction of human health risks relies heavily on data generated from 

laboratory animals (generally rodents), it is unlikely that complete knowledge at the 

molecular level, (i.e., the mechanism of action, of how an agent causes an adverse effect 

such as cancer) could be determined. As described earlier, this level of understanding is 

not necessary for establishing health protective risk assessments as knowledge of the MOA 

may be sufficient for this purpose. With respect to the potential risk of cancer from a 

chemical, once the animal MOA is established, qualitative and quantitative comparisons of 

each key event between the experimental animal and humans enable a conclusion of the 

likely relevance of the MOA to human risk. Data from the chronic toxicity/carcinogenicity 

studies conducted in laboratory rodents should not be interpreted in isolation. There are 

significant differences between rodents and humans in tumor prevalence, anatomy, or 

tumor histology. This awareness has influenced the U. S. EPA Cancer Guidelines emphasis 

on the use of MOA for carcinogenicity to interpret and quantify the potential cancer risk to 

humans. This awareness has influenced the U. S. EPA Cancer Guidelines emphasis on the 

use of MOA for carcinogenicity to interpret and quantify the potential cancer risk to 

humans. MOA analysis is central to any cancer risk assessment, and toxicogenomics can 

allow new research to be brought to bear particularly in comprehending the carcinogenic 

processes (USEPA, 2005; Cohen et al., 2003; Boobis et al., 2006). Consequently, the 

creation of an MOA to determine how the underlying biology translates across species, life 

stages, and guides the shape of the low dose region of the extrapolation curve has been 

adopted by EPA. The extension of this concept goes far beyond the scope of a cancer risk 

assessment and has been recognized by international bodies such as the International Life 

Science Institute (ILSI) and the International Program on Chemical Safety (IPCS), which 

have fashioned guidance for the use of the MOA framework to elucidate MOAs for both 

cancer and non-cancer endpoints (Cohen et al., 2003; Boobis et al., 2006; 2008).

2.1.3 Toxicogenomic Data in Risk Assessments

Since the EPA 2005 guidelines were promulgated, MOA data have been used, when 

available, for chemicals whose cancer risk assessment is being evaluated. Typically, an 

MOA analysis incorporates data from required toxicology studies and supplemental 

mechanistic data. Recently, toxicogenomic data have been considered as part of the 

weight-of-the-evidence (WOE) to test the application of these data to MOA analysis. At this 

time, the U.S. EPA has no guidance for incorporating genomic data into risk assessments, 

but the U.S. EPA Science Policy Council (SPC) has developed a draft interim guidance on 

various aspects of genomic data (USEPA, 2007). This report is presented to illustrate how 

the new technology can be used to enhance the efficiency and accuracy of our risk 

assessments. In keeping with these objectives, EPA has launched an approach to bring 
Page 39 of 81

about a shift in the way it assesses adverse effects and exposure; this was articulated in the 

strategic plan released by EPA (USEPA, 2009). This new strategy involves the 

incorporation of a wider suite of tools, (e.g., “omics”, high throughput screenings, and 

predictive models) into the effects assessment process and the application of AOP-based 

approaches. The goals of this ambitious undertaking are:

1. To determine the relevance of EPA’s approach for developing a new testing 

strategy that is in concert with the vision laid down by the NAS (2007) for toxicity 

testing in the 21st century through the examination of existing classic toxicology 

results blended with newly generated toxicogenomic data;

2. To demonstrate the value of toxicogenomic data in achieving this goal through 

characterization of a cancer MOA/AOP; 

3. To detect the perturbations (key events) within an MOA/AOP as a result of 

chemical exposure and to understand how they lead to apical effects; 

4. To show how these generated data can be incorporated into knowledge-based 

profiling data models for predicting the hazards of new or untested pesticides;

5. To address the risk uncertainties associated with extrapolation of effect data 

from lower levels of biological organization to whole organisms and/or 

populations in a reliable, time- and cost-effective manner;

6. To show how animal usage can be minimized;

7. To provide a more mechanistically-based process for quantitative risk 

assessments; 

8. To inform and encourage risk assessors on the utility of ”omic” data in chemical–

specific risk assessments. 

To address these goals, the U. S. EPA research laboratory, NHEERL/ORD, developed 

a toxicogenomic dataset for several registered conazoles that lack MOA data for various 

toxicities, including tumorigenicity. Pesticides in this class are fungicides and are 

commonly used in the protection of fruit, vegetable and cereal crops. In addition to their 

widespread agricultural use, many conazoles are also available as pharmaceuticals for 

treatment of local and systemic fungal infections in humans. Antifungal activity is exerted 

through inhibition of a specific cytochrome P450, i.e., CYP51 (lanosterol 14α- demethylase), 

a critical step in the biosynthesis of ergosterol, a steroid required for formation of the 

fungal cell wall. In mammals, many of these conazoles induce a range of responses and 

toxicities that include induction or inhibition of hepatic CYP isoforms, hepatic hypertrophy, 

hepatotoxicity, and liver tumors. The present effort concentrates on propiconazole, which 

induces both liver adenomas and carcinomas in male CD-1 mice. It should be stressed that 

the regulatory status of propiconazole is not the focus of this undertaking; propiconazole

was selected only as a model compound to test and illustrate an approach to incorporate 

toxicogenomic data into the risk assessment process. The data supporting the detailed case 

study for propiconazole are presented in Appendices A and B.
Page 40 of 81

This Chapter is organized into two sections (Background and Approach). The 

approach section presents the strategy developed to examine a chemical with large 

datasets (i.e., propiconazole) using both traditional and toxicogenomic data to investigate 

the MOA of mouse liver tumors. Additionally, ways in which these data can be 

incorporated into the risk assessment process (e.g., definition of the MOA, defining AOPs, 

building of knowledge-based models for predicting the toxicity of new or untested 

pesticides) are discussed. 

2.2 Approach 

Propiconazole was selected based on discussions and on the wealth of toxicology 

studies submitted in response to FIFRA data requirements. The NHEERL studies of Sun et 

al. (2005), Allen et al. (2006), Ward et al. (2006), Goetz et al. (2006), Chen et al. (2008; 

2009), Bruno et al. (2009), Nesnow et al. (2009; 2011), and Ortiz et al. (2010), Ross et al.

(2009, 2010) represent the determination of traditional apical endpoints, non-traditional 

apical endpoints and the assessment of “omic” responses to support concerns regarding 

carcinogenicity.

A proposed phenobarbital type MOA, i.e., microsomal enzyme induction, which is 

shown in Table 1, features the following apical endpoints: increase in body/liver weight (a 

result of hypertrophy); the livers contained increased levels of CYP proteins (i.e., Cyp2b) 

and CYP enzymatic activity (PROD) which led to cell proliferation and tumors. 

Phenobarbital is known to act through a mitogenic MOA (Whysner et al., 1996) where 

mitogenesis is the process that stimulates cell proliferation in the target organ without 

causing obvious cytotoxicity or cell death, and creates a favorable environment for tumor 

development partly because mitogens can act like tumor promoters.

Table 1. Possible Key Events Suggesting a Mitogenic Mode of Action (MOA) from the Toxicology Studies Findings 

with Propiconazole--Apical Endpoints

Key Events:

Increased Liver /Body Weight

Increased Liver Hypertrophy

Increased CYP 2b Protein and Increased PROD Activity

Increased Cell Proliferation

Increased Liver Tumors

PROD = Pentoxyresorufin O-dealkylase; most frequently associated with the cytochrome

P450 protein, Cyp2b

In the NHEERL studies, dose and time course evaluations were conducted using 

experimental conditions equivalent to those used in the 2-year chronic bioassay (Allen et 

al., 2006). Briefly, groups of male CD-1 mice (17-21/dose) were administered dietary 

concentrations of 0, 100, 500 or 2500 ppm propiconazole for 4, 30 or 90 days. 
Page 41 of 81

Toxicological effects included: liver hypertrophy, increased AROD activities, increased 

hepatic cell proliferation, increased serum triglycerides, decreased serum high density 

lipids, and decreased serum cholesterol levels. Although propiconazole was determined to 

be non-genotoxic (non-DNA reactive as parent or metabolite) using short-term tests for 

genotoxicity, it was found to induce in vivo mutagenicity in the livers of Big Blue®

transgenic mice after 4d of treatment at 2500 ppm (Ross et al., 2009; Ross and Leavitt, 

2010), a response possibly linked to oxidative stress-mediated endogenous DNA damage. 

Data that support this hypothesis have recently been presented (Nelson et al., 2011).

Transcriptional analysis of liver tissue revealed: CAR and PXR nuclear receptor 

activation, CYP induction, oxidative stress, dysregulation (abnormal control) of cholesterol 

biosynthesis/metabolism, effects on retinoic acid metabolism, and alterations in cell 

signaling, cell growth, cell proliferation and apoptosis pathways (Ward et al., 2006; Nesnow 

et al. 2009). Proteomic analyses of cytosolic liver fractions confirmed over half of the 

propiconazole-altered transcriptional pathways and identified additional pathways (Ortiz

et al., 2010). Metabolomic analysis of liver homogenates enhanced the knowledge-base on 

the process of metabolism and excretion of propiconazole, identified potential cancer 

biomarkers and extended the data on several key events within the MOA of propiconazole 

(Nesnow et al., 2011).

Propiconazole-induced oxidative stress was confirmed by the identification of 

carbonylated proteins and by biochemical measurements (Bruno et al., 2009). Studies 

confirmed that hepatic retinoic acid levels were reduced by propiconazole treatment and 

that this effect was related to specific CYP induction (Chen et al., 2009). Dysregulation of 

cholesterol biosynthesis and metabolism was confirmed by metabolomic analysis (Nesnow

et al., 2011). Based on these findings, many of the toxicogenomic observations cited above 

were selected as key events and were synthesized into the proposed MOA that elucidated 

the toxicological effects of propiconazole (Figure 7).

The MOA analysis for propiconazole describes a series of key events that lead to 

mouse liver tumors. Many of the effects of these key events could generally be ascribed to 

either CYP induction or CYP inhibition. Key events related to CYP induction include: 

activation of nuclear receptors; hypertrophy, induction of CYP proteins, induction of 

oxidative stress, reduction in the hepatic levels of all-trans retinoic acid and induction of 

mutations (See Figure 7 [Key Events in blue boxes]). Events related to CYP inhibition are: 

dysregulation of cholesterol biosynthesis and metabolism and an increase in the levels of 

mevalonic acid (Figure 7 [Key Events in green boxes]). Propiconazole also decreases 

apoptosis; increases cell proliferation; and leads to liver tumors (Figure 7 [Key Events in 

orange boxes]). In this proposed MOA, propiconazole activates the CAR/PXR nuclear 

receptors, which in turn induce hypertrophy and increase levels and activities of Cyp2b and 

Cyp3a proteins, causing oxidative stress and mutations. Concomitant with this process is 

increases in Cyp2b and Cyp3a enzymatic activities, which trigger increased metabolism of 

all-trans retinoic acid (atRA), reduce atRA hepatic levels and increase cell proliferation. 

Propiconazole also inhibits CYP51, causing dysregulation of cholesterol biosynthesis and 

metabolism leading to increased cholesterol biosynthetic intermediates (e.g., mevalonic 

acid), which results in increased cell proliferation. Propiconazole suppresses apoptosis, 
Page 42 of 81

which in concert with increased mutations and increased cell proliferation, can lead to 

tumor formation.

Figure 7 Proposed Mode of Action (MOA) of Propiconazole Using Toxicogenomic Data

Key Events enclosed in blue boxes are related to CYP induction; Key Events enclosed in green boxes are related to enzyme 

inhibition; Key Events enclosed in orange boxes are result of previous key events or are a direct result of propiconazole 

treatment.

2.2.1 Data elements supporting the MOA

As shown in Tables 2-5, propiconazole is a rich source of apical endpoints and 

toxicogenomic data. The classical toxicology studies serve as the point of reference to 

compare toxicogenomic responses to apical data and demonstrate the proof of concept of 

OPP’s overall integrated approach to develop a MOA and an adverse outcome pathway 

(AOP) using these emerging toxicity testing strategies.

The apical endpoints along with the toxicogenomic data for propiconazole show a 

high degree of concordance with respect to adverse events, time and dose (Table 2). For 

example, CAR/PXR activation was identified by toxicogenomic analysis and was confirmed 

by the AROD data with marked increases in PROD activity, which occurred as early as 4 

days at 500 and 2500 ppm (Allen et al., 2006) (Table 3) and at 850 and 2500 ppm by day 

14 in the sponsor’s submitted mechanistic study (MRID 45215803, Table 4). Cyp2b10

mRNA levels were increased 2.4X by 150 mg/kg/day (≈1000 ppm propiconazole) (Sun et 

al., 2005) and Cyp2b protein levels were increased as well (Chen et al., 2009). PXR/RXR 

activation was identified by toxicogenomic data. Levels of Cyp3a11 mRNA were increased 

Activate CAR, PXR 

Proposed MOA for Propiconazole

Induce high activities of 

Cyp2b, Cyp3a

Increase atRA 

metabolism/ 

decrease atRA 

levels

Increase cell 

proliferation/alter 

cell signaling 

Liver Tumors

Oxidative 

stress 

Dysregulate

cholesterol 

biosynthesis/

metabolism 

Mutation 

in vivo

Propiconazole parent

Hypertrophy

Decrease 

apoptosis

Increase 

mevalonic 

acid levels 
Page 43 of 81

at the same and higher doses after 4 or after 14 days of treatment (Sun et al 2005, Chen et 

al., 2009). Cyp3a protein levels were increased (Chen et al., 2009) and testosterone 16βhydroxylation (a measure of Cyp3a protein activity) was also increased (MRID 45215803). 

A 45% decrease in levels of hepatic all trans-retinoic acid (atRA) was evident in the study 

of Chen et al. (2009) after 4 days of treatment with 2500 ppm propiconazole and this was 

related to the induction of specific CYPs.

The apical data from the subchronic and chronic bioassays showed that reductions 

in the serum cholesterol level (-29 to –64% of control) were noted 4 days after mice 

received dietary preparations containing 850, 1450 or 2500 ppm propiconazole, 

respectively, for 4 days (Table 5). Allen et al. (2006) showed comparable activity at 2500 

ppm after 30 and 90 days of treatment (Table 3).

Cell proliferation was markedly increased after 1 day’s treatment with 850 or 2500 

ppm; peaked at day 2 (37X at 850 ppm and 49X at 2500 ppm, respectively); started to 

decline at 3 days; and generally returned to background by day 14 (Table 4, MRID 

45215802). The 4-day data by Allen et al. (2006) are also in good agreement with the 

findings from the sponsor’s mechanistic study. The overall analysis indicated that the 

toxicogenomic studies conducted by NHEERL were instrumental in further delineating the 

initial MOA laid out for propiconazole, which only considered apical endpoints related to 

hepatocarcinogenesis. While “omic” studies are currently not a part of the typical 

toxicology studies reviewed by risk assessors, they provide important information to 

further characterize the MOA analysis as well as augment other areas of risk assessment. 

The application of “omic” data to refine the MOA and AOPs and other areas of the risk 

assessment is presented below:
Page 44 of 81

Table 2. Temporal Association- Toxicology, Genomic, Proteomic and Metabolomics Studies with Propiconazole

Time, d (feed levels, ppm)

Key event 1 2 3 4 7 14 28/30 56 90 119 2yr

CAR/PXR Activation G, IA (2500) G/IA 

(2500)

G/IA 

(2500)

IA

Increased Cyp2b 

Cyp3a

A, G 500, 

2500)

A A, G 

(500, 

2500)

A, G 

(500, 

2500)

Increased hypertrophy A (2500) A A (2500 IA 

(2500)

A A

Increased atRA 

metabolismdecreased hepatic 

atRA levels

A, G (2500) IG 

(2500)

IG 

(2500)

IA

Increased oxidative 

stress

A, G, P 

(2500)

IA, IG 

(2500)

IA, IG 

(2500)

In vivo mutation A, G (2500) IG 

Inhibition of CYP51-

dysregulation of 

cholesterol 

biosynthesis 

/metabolism and 

reduction in serum 

cholesterol levels

Increase 

hydroxylated 

cholesterols 

and bile 

acids. M 

(1250-2500)

Decrease 

serum 

cholesterol 

levels. A 

(2500)

A (2500) A A 

(2500)

A

Increase hepatic 

mevalonic acid 

levels 

IG (2500)

Increased cell 

proliferation

A A A A (2500) IG IG

Suppression of 

apoptosis

IG (2500) IG IG

Tumors A 

(2500)

Bold = causal key event; Italicized = associative event 

Italicized = associative events

A= apical endpoint measured

G= genomic endpoint measured

P= proteomic endpoint measured

M=metabolomic endpoint

IA= inferred from apical endpoint

IG= inferred from genomic endpoint
Page 45 of 81

Table 3. Summarized Key Events Associated with Tumor Induction in Male Mice Fed Diets Containing 

Propiconazole (Submitted Mechanistic Studies From the Open Literature) a,b

Key Event Dose 

(ppm))

Dose

(mg/kg/da)

Days Reference

4 14-15 30 90

Activation of Nuclear Receptor: CAR/PXRd 

EROD 500 50 1.7Xc1.8X Allen et al., 

2006

75 1.2X Sun et al., 2005

150 1.3X Sun et al., 2005

2500 350 2.1X 3.1X 4X Allen et al., 

2006

PROD 500 50 11.6X 14.0X 10.9X Allen et al., 

2006

75 3.5X Sun et al., 2005

150 5.4X Sun et al., 2005

2500 350 13.3X 37.5X 35.9X Allen et al., 

2006

Cyp2b10 500 50

75

150 2.4X

2.6X

Sun et al., 

2005; Goetz et 

al., 2006

2500 350 -- -- -- --

Cyp2c55 150 27.6X Goetz et al.,

2006

2500 350 -- -- -- --

4 14-15 30 90

150 5.2X Sun et al., 2005

Cyp3a11 2500 350 -- -- -- --

↑ Liver Weight

500 50

75

150 1.5X

1.4X

Sun et al., 2005 

Goetz et al., 

2006

2500 350 1.4X 

1.3X

1.9X 1.95X Allen et al., 

2006 

Bruno et al., 

2009

↑ Liver Hypertrophy

100 10 80% 60% 60% Allen et al., 

2006

500 50 60% 80% 100% Allen et al., 

2006

75

150

2500 350 100% 100% 100% Allen et al., 

2006

↓ Serum Cholesterol (Dsyregulation of cholesterol biosynthesis and metabolism)

2500 350 -- -- -56 % -55% Allen et al., 

2006

↓ Hepatic Retinoic Acid Levels

2500 350 45%↓

Chen et al., 

2009
Page 46 of 81

Key Event Dose 

(ppm))

Dose

(mg/kg/da)

Days Reference

↑ Cell Proliferation

500 50

2500 350 9X 6X 1.8X Allen et al., 

2006

a Data were derived from Allen et al. (2006); Sun et al. (2005); Goetz et al.(2006); Chen et al. (2009) and Bruno et al. (2009),

b N = 4-12 mice/group (Allen et al., 2006); 4 mice/group (Sun et al., 2005) 6 mice/group (Goetz et al., 2006); 3 mice/group 

(Chen et al., 2009); and

5 mice/group (Bruno et al., 2009).

c Values expressed as X= fold increase over control: values expressed as % = %of control. 

d Abbreviations: CAR = Constitutive androstane receptor PXR = Pregnane X receptor EROD = Ethoxyresorufin O-dealkylase

PROD = Pentoxyresorufin O-dealkylase
Page 47 of 81

Table 4. Summarized Key Events Associated with Tumor Induction in Male Mice Fed Diets Containing Propiconazole

(Submitted Mechanistic Studies) a b

Key Event Dose 

(ppm) Days

Reference

1 2 3 4 7 14 28 60

CARc Activation

Total P450 850 3X d45215803

2500 3.9X

EROD 850 2.2X

2500 3.9X

PROD 850 30X

2500 55X

Coumarin 7-

hydroxylase

850 5X

2500 24X

UDPGT 850 1.6X

2500 1.4X

Cytosolic GST 850 1.6X

2500 1.8X

Liver/Body 

Weight

850 1.3X

2500 2X

850 1.1X 1.3X 1.3X 1.3X 1.3X 1.3X 1.4X 1.3X 45215802

2500 1.2X 1.4X 1.5X 1.7X 1.9X 2.1X 2.3X 2X

Hypertrophy 850 Mi Mi Mi Mi Mi Mi Mo-Ma Mo-Ma

2500 Mi Mi Mi Mi Mi Mi Mo-Ma Mo-Ma

Cell Proliferation 850 19X 37X 6X 5X <1X <1X <1X <1X

2500 11X 50X 19X 10X 8X 1X 2X 1X

a Data were derived from MRID Nos. 45215802 and 45215803.

b n generally = 5 mice/group.

c Abbreviations:

CAR = Constitutive androsane receptor UDPGT = Uridine diphosphate glucuronosyl-transferase

EROD = Ethoxyresorufin O-dealkylase GT = Glutathione S-transferase 

PROD = Pentoxyresorufin O-dealkylase 

Mi = Minimal Mo-Ma = Moderate-Marked

d Fold increase
Page 48 of 81

Table 5. Summarized Key Events Associated with Tumor Induction in Male Mice Fed Diets Containing Propiconazole (Submitted Studies) a b

Key Event Dose (ppm) Weeks

4 8 9 13 14 17 52-53 78 100

MRID -502a

-502a-401a-502a-501a-401a-501a-401 a570a-401a-570a

Liver/Body 

Weight 500 b

- - 10% 18% - - - 13% 12% 10% 6%

850 - - 33% 34% - - - 29% - 20% -

1450 - - - 56% - - - - - - -

2500 - - - 94% - - - - 41% - 50%

Hypertrophy

500 b100% 80% c60% c80% - - 20% 60% 18% d56% 33% d

850 90% 100% 100% 100% - - 70% 80% - 58%

1450 100% 100% - 100% - - 100% - - -

2500 90% 100% - 100% - - 100% - 78% d- 86% d

Cholesterol

500 b NC NC -11% NC NC -14% -12% -15% -15% -24% -6%

850 -29% -9% -25%- -30% -10% -24% -24% -34% - -29% -

1450 -37% -49% - -39% -42% - -45% - - - -

2500 -64% -50% - -45% -41% - -44% - -28%- - +44%

Single cell 

necrosis 500 b

40% 80% 10% 25% - - 0 0 0 0 0

850 70% 40% 30% 65% - - 0 0 0

1450 90% 90% - 90% - - 10%

2500 90% 90% - 100% - - 60% 0 0

Necrosis

500 b10% 10% 30% 25% - - 10% 0 0 0 0

850 30% 0 -50% 15% - - 20% 0 0

1450 30% 50% - 30% - - 40%

2500 70% 60% - 30% - - 60% 0 0

Tumorsf 

500 b10% 37%

850 24%

1450

2500 86%

a Data were derived from MRID Nos. 42050502, 42050401, 42050501, 45215801, and 000129570.

b Results for lower doses (≤ 100 ppm) were generally comparable to the vehicle control group. /10 = number of animals

c MRID 45215801: reread of histological slides from MRID 42050502 (8-week) and 44381401 (9-week)

d Listed as “hepatocyte enlargement, presumed to be hepatocyte hypertrophy.

f Combined adenomas and carcinomas
Page 49 of 81

2.2.2 Utility of “Omics” to Define the Propiconazole MOA

One of the obvious advantages of using “omics” data is to gain more insights into 

mechanisms of toxicity. The data generated in the propiconazole case study are illustrative 

of this point and clearly show how the “omics” data more completely clarify the operative 

MOA for propiconazole carcinogenesis (Figure 7) and enhance the knowledge base for 

propiconazole-induced hepatocarcinogenicity in the following ways:

2.2.2.1 Identify a series of potential molecular initiating 

and other key events that are plausibly linked to 

tumorigenesis (MOA/AOP).

The molecular initiating events that were identified from apical measurements were 

elevated expression of Cyp2b and Cyp3a (i.e., CAR/PXR nuclear receptor activation) and 

inhibition of Cyp51 (i.e., dysregulation of cholesterol biosynthesis and metabolism). Other 

key events include: liver cell hypertrophy, mutations in vivo (in transgenic mice), increased 

cell proliferation (hyperplasia), and liver tumors. The ”omic” analyses were an equally rich 

source of potential key events when these results were associated with what is known 

about the carcinogenesis mechanisms. Intermediate key events identified by “omic” 

analyses include: oxidative stress, increase in atRA metabolism, dysregulation of 

cholesterol biosynthesis and metabolism, increased mevalonic acid formation, increased 

cell proliferation, and decreases in apoptosis (Ward et al., 2006; Nesnow et al., 2009)

(Figure 7). The results of the microarray genomic analysis revealed the following:

CAR and PXR canonical pathways (See page 4, Key Terms) altered by propiconazole 

were identified.

The oxidative stress related canonical pathways altered by propiconazole exposure 

were: Nrf2-mediated oxidative stress response, and mitochondrial dysfunction;

The Retinol Metabolism canonical pathway was identified as being altered;

A series of genes in the cholesterol biosynthesis and metabolism pathways were 

also up regulated; 

Decreases in specific genes associated with apoptosis and the alterations of 

pathways associated with apoptosis: Apoptosis and Apoptosis Signaling; DNAdamaged-induced apoptosis; and Regulation of BCL2-associated agonist of cell 

death (BAD) phosphorylation were identified.

In addition to identifying new key events within the MOA, “omic” data provided 

greater depth to the understanding of certain apical key events (Ward et al., 2006; Nesnow

et al., 2009). For example, pathways associated with DNA damage were altered supporting 

the in vivo mutation studies. These included Cdc25 and Chk1 Regulatory Pathways leading 

to DNA damage; Retinoblastoma Tumor Suppressor/Checkpoint Signal leading to DNA 

damage; and ATM/ATR regulation of G2/M checkpoint pathways, associated with cell 
Page 50 of 81

proliferation. Altered cell growth/signaling pathways were: PI3K/AKT Signaling; p38 Map 

Kinase Signaling; PTEN Signaling; TGF-β signaling; mTOR Signaling; and WNT Signaling. 

Other cell proliferation/cell cycle pathways altered by propiconazole were: Cell Cycle: G1/S 

Check Point; Influence of Ras and Rho on G1 to S Transition. Disruption of these pathways 

can eventually lead to tumors. The “omic” data also helped in identifying the distinct 

chemical-biological interactions of the molecular initiating events that are linked with 

tumor formation and are an integral part of the AOP analysis.

2.2.2.2 Link a continuum of precursor events that lead to 

apical responses. 

One method of establishing a continuum of key events is to find differentially 

expressed genes (DEGs) common to each event. Examples are the key events of nuclear 

receptor activation and CYP induction in the livers of propiconazole-treated mice. CAR/PXR 

activation was significantly increased when mapped to the Ingenuity Pathway Analysis 

(IPA) Toxicity List [www.ingenuity.com] (Nesnow et al., 2009). This list contained a 

number of cytochrome P450’s (CYP) Cyp1a2, Cyp2b, and Cyp2c genes. The over expression 

of several of these genes was confirmed by Q-RT-PCR (Chen et al., 2009). This shows that 

genes are common to both apical endpoints (CAR activation and Cyp induction), thus 

linking these events. 

A second example describing a continuum of events is Cyp induction and retinoic 

acid metabolism. Mapping differentially expressed genes (DEGs) to the IPA Canonical 

Pathway, revealed that retinol and retinic acid metabolism were altered in livers from 

propiconazole-treated mice as evidenced by transcriptomic analyses. In addition, several 

Cyps known to be involved in the metabolism of retinoic acid were over expressed: Cyp26a, 

Cyp3a, and Cyp2c.

Retinoids are well-known anticancer agents that induce differentiation and growth 

suppressive signals in normal, premalignant, and malignant cells (Freemantle et al., 2003). 

One of the major roles of retinoids is to control cell proliferation in epithelial cells. Followup experimental studies showed that Cyp3a protein levels and retinoic acid metabolism 

were also increased in the liver by propiconazole (Chen et al., 2009). As retinoic acid 

metabolism was increased, it was hypothesized that retinoic acid levels in the liver would 

decrease. This hypothesis was confirmed by direct measurement of reduced retinol acid 

levels in liver tissues from propiconazole-treated mice (Chen et al., 2009). These 

propiconazole data suggest that the decrease in atRA levels would provide a tumor 

promoting environment (Wang, 2003). Retinoic acid controls cell proliferation and lower 

levels of retinoic acid would be consistent with increases in cell proliferation, which were 

also observed in treated animals.
Page 51 of 81

2.2.2.3 Identify cellular pathways and networks 

associated with these events for both linkage and 

coherence by enhancing information contained in 

apical key events.

An example of the linkage and coherence of the cellular pathways to key events is 

oxidative stress. Propiconazole induced a series of genes associated with oxidative stress 

through the Nrf2 pathway. This was observed in both pathway analysis and by the upregulation of specific genes. Several aldo-keto reductases were also up regulated 

suggesting reactive oxygen species (ROS) formation, lipid peroxidation and reactive 

aldehyde formation. Many apical endpoints related to oxidative stress were increased by 

propiconazole treatment (i.e., protein carbonyl formation, GSSG/GSH ratio; GSH levels and 

glutathione S transferase [GST] activities) (Bruno et al., 2009; Nesnow et al., 2011).

There is also linkage and coherence between the increased mutation frequency with 

a number of up regulated genes associated with DNA damage and repair as well as with the 

formation of ROS and its sequelae, lipid peroxidation and reactive aldehyde formation. 

Propiconazole treatment increased the mutation frequency in the livers of transgenic mice. 

Transcriptomic analysis identified both altered pathways associated with DNA damage and 

repair: Cdc25 and Chk1 Regulatory Pathway to DNA damage; Retinoblastoma Tumor 

Suppressor/Checkpoint Signal to DNA damage; and ATM/ATR regulation of G2/M 

checkpoint. A number of specific DNA repair genes were also up regulated: Parp2; Atr; 

Apex1; and H2afx. Gadd45b was significantly up regulated and is involved in the control of 

cell cycle, DNA repair and apoptosis (Thyss et al., 2005).

2.2.2.4 Infer other key events not measured through 

apical approaches.

One example of this application is the decrease in apoptosis. A series of apoptosisrelated genes with altered expression levels were detected by microarray analyses of livers 

from mice fed propiconazole in their diets. Anti-apoptotic genes (Li et al., 2000; Pandey et 

al., 2000) including Gadd45b, Ikbkb, Ikbke, Ikbkg, Hsp70 Hsp 90 and Tmbim4 were up 

regulated (Ward et al., 2006). A number of the pro-apoptotic genes, Fas, Casp6 and Bmf

were suppressed (Ward et al., 2006). The data strongly suggest that propiconazole 

suppressed apoptosis in mouse livers. The high levels of Gadd45b mRNA induction at each 

time point indicates that high levels of protection against apoptosis occurred in the livers of 

propiconazole-treated mice. These results suggest a mechanism for reduced apoptosis in 

propiconazole-treated mouse liver, which focuses on Gadd45b as the indicator of an antiapoptotic protein. Based on analyses of gene expression studies, the overall results suggest 

a decrease in apoptosis.

Another example of a key event identified by genomic analyses without a direct 

apical endpoint is the dysregulation of cholesterol biosynthesis and metabolism with 

increased mevalonic acid formation as a result of the overexpression of cholesterol 

biosynthetic genes. This is likely a compensatory effect related to cholesterol biosynthesis 
Page 52 of 81

inhibition by propiconazole, an effect attributed to the known ability of propionconazole to 

inhibit CYP51 (lanosterol 14α-demethylase), a key step in the cholesterol, ergosterol 

biosynthesis pathway (Trosken et al., 2006). This direct interaction is a distinct molecular 

initiating event for a second APO, in addition to the APO initiated by CAR/PXR activation.

Microarray analyses of the mRNA from the livers of mice treated with a number of 

conazoles show that the genes involved in the biosynthesis of cholesterol were consistently 

up regulated but to differing extents, presumably through a negative feedback 

compensatory mechanism producing more cholesterol (Ward et al., 2006; Nesnow et al., 

2009). Mice treated with propiconazole (2500 ppm) for 4 days over expressed several key 

genes in the cholesterol anabolic pathway including squalene epoxidase (Sqle, 2.6-fold 

increase), lanosterol α-demethylase (Cyp51, 1.7-fold increase) and sterol-C4-methyl 

oxidase (Sc4mol, 1.6-fold increase), and 7-dehydrocholesterol reductase (Dhcr7, 1.5-fold 

increase) (See Table 12, Support Document for Case Study, Appendix B). Cholesterol is 

further metabolized to hydroxylated cholesterols and converted to bile acids. Several 

genes in these pathways were also up regulated at 4 days of treatment: Cyp7a (cholesterol 

7α-hydroxylase) 1.8-fold increase, aldo-keto reductase family 1, member D1 (Akr1d1, 2.1-

fold increase), and aldehyde dehydrogenase family 1, subfamily A7 (Aldh1a7, 1.8-fold 

increase) (Ward et al., 2006; Nesnow et al., 2009). From proteomic analyses of livers of 

mice treated with propionconazole for 4 days, Aldo-keto reductase family 1, member D1 

(Akr1d1) was increased 1.7-fold and aldehyde dehydrogenase family 1, subfamily A1 

(Aldh1a1) was increased 1.3-fold, thus confirming the transcriptomic results (Ortiz et al., 

2010).

Concomitant with the microarray data, however, results from the submitted studies 

indicated that propiconazole treatment (2500 ppm) consistently reduced the serum 

cholesterol levels in the mice. The effect was first observed at 4 days and continued for at 

least 11⁄2 years (Table 2). An explanation for these observations is the inhibition of 

cholesterol transport. Acyl-coenzyme A (CoA) cholesterol acyltransferases (ACATs) are 

membrane-bound proteins that utilize long-chain fatty acyl-CoA and cholesterol as 

substrates to form cholesteryl esters. ACATs play important roles in cellular cholesterol 

homeostasis in various tissues (Chang et al., 2009), and the inhibition of ACAT activity has 

been associated with decreased plasma cholesterol levels (Leon et al., 2005). 

Transcriptomic data from the livers of mice treated with propiconazole indicated 

significant decreases (-1.7 to -1.9-fold) in Acat1 levels over a 90-day period (Nesnow et al., 

2009) suggesting a continued suppression in cholesteryl ester formation and transport of 

cholesterol out of the hepatocyte. Metabolomic data demonstrated that propiconazole 

treatment increased the levels of hydroxylated cholesterols and a number of bile acids 

(Nesnow et al., 2011) suggesting an increased flux in cholesterol biosynthesis and 

metabolism. The increased flux of cholesterol biosynthesis increased the levels of 

mevalonic acid, an intermediate in the cholesterol biosynthesis pathway. Transcriptomic 

data after 4 days of treatment revealed significant increases in a series of genes within the 

cholesterol biosynthesis and metabolism pathways suggesting a dysregulation of 

cholesterol biosynthesis and metabolism. Cholesterol biosynthesis essentially begins with 

the action of HMG-CoA reductase on 3-hydroxy-3-methylglutaryl-CoA (HMG-CoA) to form 

mevalonic acid. Mevalonate production is a step in the cholesterol biosynthetic pathway 
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and leads to the formation of isopentenyl pyrophosphate, farnesyl pyrophosphate and 

lanosterol. Farnesylation of ras protein by farnesyl pyrophosphate is a posttranslational 

modification that allows attachment of ras to the plasma membrane. Eventually this 

triggers the ras/MAPK signaling pathway that drives malignant transformation through 

mutant ras GTP complexes (End, 1999). Ras regulates cyclin D1 expression and affects the 

G1 to S transition as well as the progression of cells through the G1 checkpoint into S phase, 

which leads to proliferation. Inhibition of mevalonate synthesis would block this process 

since it has been shown that inhibition of mevalonate prevents farnesylation of p21ras 

proteins and arrests cell growth of CHO cells (Goldstein and Brown, 1990). It is, therefore, 

hypothesized that propiconazole-induced cholesterol reduction brings about a negative 

feedback loop increasing hepatic mevalonic acid levels that can result in increased cell 

proliferation through increased ras farnesylation and activation of the MAPK signaling 

pathway. This hypothesis has been tested and verified using immortalized mouse 

hepatocytes (Murphy et al., 2011).

2.2.2.5 Extend apical dose-response relationships to 

lower doses.

There are many examples where the apical responses can be extended to lower 

doses using transcriptomic data. A case in point is the Gstm2 and Gstt3 genes, which are 

associated with oxidative stress and exhibited a dose response of gene expression after 4 

days of treatment with 500 or 2500 ppm propiconazole, glutathione S-transferase (GST) 

activity, which is encoded by these genes, was only evaluated at 2500 ppm propiconazole 

and found to be significantly increased. It is likely that GST activity would also be increased 

at 500 ppm since gene expression was also up regulated at this lower dose.

2.2.2.6 Consideration of alternate MOAs.

An example of the ability of toxicogenomic data to rule out potential alternative 

MOAs is illustrated in the case study for propiconazole. Propiconazole was compared to 

the prototypic reference compound phenobarbital to assess similarities and differences 

using transcriptomic profiling (Nesnow et al., 2009). Targeted transcriptomic analyses 

were conducted on a global level using Principal Components Analysis (PCA) and on the

gene level examining DEGs, and subsets of DEGs (cell cycle genes, genes associated with 

human hepatocellular tumors, and transcription factors). Analyses were also conducted on 

function, pathway and network levels by examining IPA Tox Lists and IPA Canonical

Pathways maps, and GeneGo MetaCore dynamic networks and their central hubs 

www.genego.com. Genes expressed by propiconazole were also comparedwith genes 

associated with human hepatocellular cancer. The data indicated that although 

phenobarbital and propiconazole induced mouse liver tumors and exhibited similar apical

responses, their transcriptional profiles were significantly different. This is shown as a 

Principal Components Analysis (PCA) plot (Figure 8). In this plot, each dot represents the 

total microarray data from one mouse after treatment with propiconazole or 

phenobarbital. Note, the propiconazole samples cluster in one area of space while the 
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phenobarbital samples cluster together in a separate area of space consistent with 

treatment group differences.

Figure 8. Principal Component Analysis (PCA) of DEGs from Phenobarbital (PB) and Propiconazole (Pro) 

Treatments at 30d. Data extracted from Nesnow et al. 2009.

2.2.3 Relationship of MOA to AOP/ Incorporation of the generated data 

into knowledge-based data models for prediction of toxicity for 

new or untested chemicals.

MOA and AOP are similar in concept. MOA is defined as the sequence of key events 

and cellular and biochemical events (measurable parameters), starting with the interaction 

of an agent with the target cell, through functional and anatomical changes, resulting in 

cancer or other adverse health effects (USEPA 2005; Boobis et al. 2008). An AOP 

represents existing knowledge concerning the linkage between a molecular initiating event 

and an adverse outcome at the individual or population levels (Ankley et al., 2009). AOPs 

-4

-2

0

2

4

6

-16.5

-16.0

-15.5

-15.0

-14.5

-14.0

-13.5

-13.0

4 2 0 -2 -4 -6

PCA #3

PCA #1

PCA #2

Phenobarbital

Propiconazole

Triadimefon

PB

Pro
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are built in part on one of the key concepts of the MOA framework developed by EPA and 

IPCS. Once an MOA has been established, the key events can be used for read across to 

other chemicals, both qualitatively and quantitatively. If a new compound triggers the key 

events, it will potentially trigger the adverse effect identified in the MOA. The likelihood of 

an adverse outcome will then depend on a significant perturbation of key events (i.e., dose 

response). This allows the use of existing knowledge and concepts while grounding the 

results of new methods in toxicologically relevant outcomes. This emphasizes the 

advantage of using ”omics”, i.e., in structure-activity relationship (SAR) determinations 

both for chemicals with similar structural features and for chemicals with similar toxicity 

profiles. For example, propiconazole is structurally related to other triazole fungicides, as 

the rudimentary use of SAR presented in Table 16 of the Case Study indicates (See Support 

Document for Case Study, Appendix B). This SAR analysis relied solely on structure, tumor 

response in the mouse liver, and mutagenicity. Many parent conazoles have been shown to 

be tumorigenic in rodents, specifically liver tumors in the mouse, while a number of others 

are inactive. As a group, conazoles are known to inhibit ergosterol biosynthesis through 

inhibition of CYP51 (Vanden Bossche et al., 1989). In mammals, many of these conazoles 

induce a wide range of effects that include induction or inhibition of CYP isoforms, 

hepatotoxicity, and liver tumors. With an increasing array of computational tools available, 

it seems reasonable that a more complex approach to calculate the physical and chemical 

properties of propiconazole would be enhanced with the incorporation of toxicogenomic 

data to inform SAR modeling. Similarly, with the expanded understanding of key toxicity 

pathways afforded by the “omic” findings, and the identification of the likely molecular 

initiating events, it is equally reasonable to assume that propiconazole would be an ideal 

candidate for inclusion as a training set in the computational, SAR and quantitative (Q)SAR 

databases to enhance their ability to predict potential interactions of similar structures 

with cellular molecules and linkage to the biological activities for AOPs . Using such an 

approach, we envisioned that compounds, such as phenobarbital, that exhibit toxicity 

behaviors similar to the conazoles but appear structurally different could be tested in vitro 

for their ability to interact with defined molecular initiating events. These data would then 

be available to develop SARs, group chemicals according to APOs and determine chemical 

properties that are not obvious from two-dimensional chemical structures. Using such a 

plan, chemicals could be prioritized for more apical and comprehensive testing. The 

ultimate goal being to shift away from the emphasis on apical endpoint testing towards a 

more in depth chemical characterization, afforded by the inclusion of computational tools 

early in the screening phase of pesticide registration. This would set the stage for the 

selection of more efficient “omic” assays designed to detect targeted biological 

perturbations.

2.2.4 Minimize animal usage

In addition to providing less of an in depth understanding than “omic” studies, 

traditional toxicology studies exact an enormous price in animal usage. For example, over 

800 animals were used in the FIRFA-approved study designs for traditional toxicology 

testing (Table 5) and 240 mice were tested in the submitted mechanistic studies (Table 4). 

The overall number of sacrificed animals was upwards of 1000 rodents. By contrast, far 

fewer mice were involved in the microarray analysis of Ward et al. (2006). Our experience 
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in this project affirms the belief of many at EPA and the NAS that a new paradigm designed 

to be more efficient and instructive will also help minimize animal testing, and we foresee 

that the model of microarray analysis, which we have presented, has application as a proof 

of concept for 21st century toxicology testing.

2.2.5 In vitro testing

We further foresee that while targeted in vivo testing will continue to play a 

significant role in toxicology testing for the immediate future, movement away from whole 

animal testing is feasible and desirable over time. High throughput (HTP) in vitro screens 

at lower levels of biological organization could be used to estimate apical endpoints and/or 

target appropriate tier testing, thereby gradually reducing the need for more resource 

intensive tests in the future. Both rodent and human cell lines are amenable to generate 

important data. This is exemplified by the in vitro genomic data with primary mouse 

hepatocytes, immortalized mouse hepatocytes, and tumorigenic human and mouse cell 

lines, which have been used in propiconazole MOA studies (Chen et al., 2008; Murphy et al., 

2011). Similarly, as discussed in the MOA analysis section of the case study, metabolism 

does not seem to be a necessary step to demonstrate the genomic activity of propiconazole 

(See Support Document for the Case Study, Appendix A). Thus, the need for an exogenous 

source of mammalian metabolic activation is not necessary. Based on these considerations, 

propiconazole is proposed as an ideal candidate for inclusion as a positive control in test 

systems constructed around mechanistic endpoints of its target organ responses. Such a 

HTP screening model could be designed to assess transcriptomic, proteomic and/or 

metabolomic alterations associated with one or more of the key events in conazolemediated tumorigenesis. With such a prototype in mind, the need for animal testing could 

be circumvented. Thus, the overall goal of generating reliable data in a simple but efficient 

manner that is cost- and time-effective, while minimizing resources, could be realized.

2.2.6 Conclusions

We believe that the approach is reliable, relevant, and offers an innovative means of 

informing and improving risk assessments. It has been presented at many international 

meetings and received enthusiastically. In summary, “omic” data have the capability to 

inform risk assessment in the following ways:

1. Identify potential molecular initiating and other key events that could be linked 

to adverse outcomes. 

2. Link a continuum of precursor events that lead to apical responses. 

3. Identify cellular pathways and networks associated with these events for both 

linkage and coherence by enhancing information contained in apical key events.

4. Infer events not measured through apical approaches.

5. Extend apical dose-response relationships to lower doses.
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6. Link molecular initiating and other key events to adverse outcomes observed in 

animal models to humans thus enhancing human relevance.

7. Consider alternative MOAs.

8. Develop (Q)SAR databases to enhance their ability to predict potential 

interactions of similar structures with cellular molecules and linkage to the 

biological activities for AOPs .

This new technology can improve traditional approaches, which often fail to reveal 

mechanistic insights. Moreover, “omic” data can inform the relevance of a particular 

toxicity finding in animals with regard to what is biologically plausible in humans 

(Leighton, 2005; Reynolds, 2005). Toxicogenomic approaches have been used to address 

specific mechanistic questions in combination with standard toxicology assays and can 

provide detailed assessments of profiles that can distinguish molecular alterations in a 

target organ when compared to normal tissue. These technologies include the 

identification of predictive signatures for toxicity and carcinogenicity, support for low-dose 

extrapolation and characterization of susceptible human populations. There have been 

numerous reports detailing with the application of toxicogenomics methods in the 

assessment and prediction of chemical toxicity (Steiner et al., 2004; Ruepp et al., 2005; 

Ellinger-Ziegelbauer et al., 2008; Ellinger-Ziegelbauer et al., 2009) that serve to 

substantiate this approach.

While measurement of apical endpoints is considered important, they have inherent 

limitations particularly in terms of sensitivity. By contrast,”omics” can identify biological 

perturbations at the molecular level, the pathway level, and the toxicity pathway level (e.g., 

groups of genes that interact giving a toxicity endpoint). These pathways can detect 

alterations in cell-signaling motifs, regulatory pathways and metabolic pathways. 

Additionally,”omics” can link seemingly disparate pieces of information into cellularresponse dynamic networks that uncover new insights into a MOA. Finally,”omics” can 

detect early subtle changes farther upstream of a measurable physiological change in apical 

endpoints, and thus can identify early events associated with exposure. ”Omics” is a 

developing field of science and is expected to impact the risk assessment of chemicals. It 

has the capability to provide both global high-level views of chemically induced alterations 

as well as more detailed views of altered pathways, networks genes, proteins and 

endogenous metabolites. It is envisioned that ”omics” will impact risk assessment 

procedures in the hazard identification and dose- response phases within the MOA 

framework as well as within the framework for human relevance (Sonich-Mullin et al., 

2001; Boobis et al., 2006). The advances in this technology that increase throughput 

screening at reduced costs and minimal usage of animals coupled with advances in 

bioinformatics will provide a significant benefit to the future of risk assessment.
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OPP’s strategic plan is to build on efforts such as those discussed in this document to 

augment the understanding of the MOA for toxicity pathways, to develop the knowledge of 

APOs, and to use this awareness as the basis for risk assessments. We further foresee a 

shift in the way toxicology testing is conducted, with emphasis on replacing the current 

resource intensive protocols with more efficient, informative and focused in vitro

approaches.
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3. Use of an Adverse Outcome Pathway (AOP) to Inform Risk 

Assessment - Triclosan as a Case Study

3.1 Background

This triclosan case study has been developed for the FIFRA Scientific Advisory Panel 

meeting on “Integrated Approach to Testing and Assessment Strategy: Use of New 

Computational and Molecular Tools”. This case study describes how EPA’s Office of 

Pesticide Programs (OPP) and the Office of Research and Development (ORD) have utilized 

in vivo, in vitro, and in silico research on triclosan to link key events and define an adverse 

outcome pathway (AOP) for triclosan’s effect on the thyroid hormone system. This 

approach may be more advantageous in comparison to use of conventional (i.e. apical) 

toxicity endpoints for defining potential adverse outcomes in humans. We intend to apply 

the AOP approach increasingly over time and, as described in this case study, we seek the 

SAP’s advice and recommendations with respect to the proposed approach. It is important 

to note that the purpose of this case study is to illustrate the approach we are taking to 

advance the risk assessment process by making it more efficient and effective and not to 

assess the risk of triclosan. Key terms used throughout this document are listed and 

defined as follows:

Key Terms Presented in This Document

Mode of action is defined by the USEPA as the sequence of key events and cellular and biochemical 

events (measurable parameters), starting with the interaction of an agent with the target cell, through 

functional and anatomical changes, resulting in cancer or other adverse health effects (USEPA 2005).

Adverse outcome pathways (AOP) represents existing knowledge concerning the linkage between the 

molecular initiating event and an adverse outcome at the individual or population level. 

Key events are empirically observable, necessary steps in the disease (cancer) process. 

Apical endpoints are tissue-based alterations observed in vivo and are often the first evidence that is 

observed after treatment-related chemical exposure and include tumors, loss of body weight, altered urinary 

proteins, and blood markers.

Toxicity pathways are cellular response pathways that can lead to adverse health effects when sufficiently 

perturbed. 

Molecular initiating event is the initial point of chemical-biological intersection within the organism that 

starts the AOP. Additional key events further along the pathway that lead to, and are experimentally or 

toxicologically associated with the adverse outcome are referred to as “key events”. 

Cytochrome P450: Mixed function oxidases (MFO) also known as CYPs, are a host of enzymes, referred 

to as Phase I enzymes that metabolize chemicals, often as part of the body’s defenses to dispose of 

potentially harmful substances (pesticides and other toxins.

EROD/PROD: Ethoxyresorufin/Pentoxyresorufin O-deethylase: These assays measure the enzymatic 

activities of individual and /or groups of CYPs. 

CAR/PXR: Constitutive androstane receptor/pregnane X receptor, are part of a nuclear receptor 

subfamily, functions as a sensor of xenobiotic substances and in response up-regulates the expression of 

target genes (CAR e.g, Cyp2b10; PXR e.g, Cyp3a11) to produce the gene product (e.g., Cyp2b, Cyp3a), 

which are responsible for the metabolism of harmful substances. 

Pathways are fixed and confined groups of genes (or proteins) associated with a specific biological 

function (e.g., retinal metabolism or CAR/PXR activation). 
Page 60 of 81

High-throughput screening: in vitro biochemical or cellular assays that can be run efficiently on a large number of 

compounds to determine their activity on different biological targets such as ion channels, receptors, enzymes, proteins, 

and signaling pathways. Typically involves batch testing of chemicals for toxicological endpoints using 

automated liquid handling, detectors, and data acquisition methods.

The case-study presented here focuses on the use of IATA approaches as they relate 

specifically to use in human health risk assessment; however, their application is not 

limited to evaluating potential adverse effects in humans. The approaches have utility for 

informing ecological risk assessments across multiple taxa as well. For example, an 

understanding of the biology behind an AOP in mammals could be useful for understanding 

AOPs across a variety of other taxa. If the key molecular event(s) within an AOP are 

phylogenetically conserved across mammals and other taxa, then a mechanistic-based 

understanding of an AOP in mammals may be extrapolated to those other taxa. At a 

minimum, an understanding of an AOP in mammals could be a useful starting point for 

understanding and possibly identifying AOPs in other taxa (e.g., birds, fish, invertebrates, 

and plants) particularly where there is sufficient information regarding specific deviations 

in highly conserved pathways. Successfully integrating emerging technologies such as in 

vitro/in silico high through-put assays based on omics, predictive tool such as QSARs and 

read-across, along with existing in vivo data across multiple taxa, will improve our 

mechanistic-based understanding of the underlying biology. Conversely, an understanding 

of the biology of these various taxa will also enable the use of these emerging technologies 

to establish linkages between various levels of biological organization that will allow 

increased use of data from lower levels of biological organization to estimate risk at the 

whole organism and population level with reduced reliance on in vivo testing.

3.1.1 Approach

Since 2008, the Agency, in collaboration with the Office of Research and 

Development’s National Health and Environmental Effects Research 

Laboratory (NHEERL), conducted research investigations into the potential 

endocrine effects of triclosan, with an interest in identifying adverse outcome 

pathways that would facilitate characterization of precursor events for 

human health risk assessment. These research efforts have involved 

investigation of triclosan’s effect on the estrogen, androgen, and thyroid 

hormone systems using, in vitro and in vivo testing methodologies consistent 

with the Agency’s goal under the Endocrine Disruptor Screening Program 

(EDSP) to screen pesticides, commercial chemicals, and environmental 

contaminants for their potential to disrupt the endocrine system. Research 

results on triclosan to date have identified precursor events involving the 

effect of triclosan on the thyroid hormone system that form the basis for the 

adverse outcome pathway for this case study. This proposed pathway is 

outlined below (Figure 9).

Figure 9. Proposed adverse outcome pathway (AOP) for triclosan effects on the thyroid hormone system.
Page 61 of 81

From these data, it is proposed that the molecular initiating event in the adverse 

outcome pathway for triclosan’s effect on the circulating thyroxine (T4) is activation of the 

pregnane-X-receptor (PXR) and/or the constitutive androstane receptor (CAR) in the liver. 

From activation of these receptors, up-regulation of hepatic phase I and phase II enzymes 

and hepatic transporters occurs. The up-regulation of these enzymes and transporters 

results in increased catabolism of thyroxine (T4) with a resulting decrease in circulating 

T4, tissue T4, and T3. The significance of this AOP for humans is that adverse 

neurodevelopmental outcomes in children is a well known adverse outcome from thyroid 

hornmone insufficieny during brain development (Haddow, 1999; van Wassenaer, 2002). 

Thus, identification of this AOP for triclosan represents a significant step to refinement of 

the existing human health assessment, where the AOP is relevant to potential adverse 

effects in humans in comparison to apical endpoints. The use of the experimental evidence 

to define an AOP and to use it in assessment of human health is consistent with the 

Agency’s goal of moving toward 21st century toxicity principles that emphasize a 

mechanistic understanding of how chemicals perturb normal biological pathways and 

subsequent reliance on assays that are diagnostic of key events within an adverse outcome 

pathway. This approach begins with formulation of evidence-based hypotheses about the 

plausible toxicological potential of a chemical or group of chemicals based on their 

physical-chemical and biological properties, and using knowledge of key events on a causal 

path to target animal testing focuses on the most likely hazards of concern. The Agency is 

seeking the Science Advisory Panel’s advice on the scientific soundness of using AOP and 

knowledge of the qualitative and quantitative relationship of the causal key events 

identified in the triclosan case study.

3.2 Endocrine Effects of Triclosan 

The endocrine effects of triclosan have been studied in a variety of systems, 

including in vitro systems (Gee et al. 2007; Ahn et al. 2008; Chen et al. 2007; Christen et al. 

2010; James et al. 2008) and in sub-mammalian species (Foran et al, 2000; Ishibashi et al. 

2004; Dodson 2005; Veldhoen et al. 2006; Fort et al 2010). The focus of this case study is 

in identification of an adverse outcome pathway involving disruption of thyroid hormone 

levels and downstream consequences, most significantly adverse neurodevelopmental 

outcomes in humans. 

3.2.1 Effects on Thyroid Hormone -- in vivo Studies

Triclosan’s structural similarity with thyroxine, the evidence for interaction of 

triclosan with hormone systems in non-mammalian species, and published evidence of 

widespread environmental occurrence all led to the need for further investigation of 

triclosan’s potential effect on mammalian hormone systems. Alterations in thyroid 

homeostasis is of particular concern as disruption of circulating concentrations of thyroid 

hormones have been associated with adverse outcomes in humans, including altered 

neurodevelopment (Zoeller and Crofton, 2000; Mitchell and Klein, 2004; Morreale de 

Escobar et al., 2004; Koibuchi and Iwasaki, 2006).
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Figure 10. Structure of Triclosan and Thyroxine

Crofton et al., (2007) examined the effects of short-term (4-day) oral exposure to 

triclosan on thyroxine (T4) levels in adult rats at doses of 0, 10, 30, 100, 300, and 1000 

mg/kg. At doses of 100 mg/kg and higher, a dose-related decrease in serum total thyroxine 

concentrations was observed. Decreases of 28, 34, and 58% were observed at doses of 100, 

300, and 1000 mg/kg/day (Figure 11).

Figure 11. Short-term oral exposure to triclosan decreased serum total T4 concentration. 

Data are expressed as group mean values (±S.E.), V, corn oil vehicle-only control. Absolute T4 group mean value for the control 

group (0 mg/kg/day triclosan) is 41.8 ± 2.2 ng total T4/ml serum. The Symbol (*) shows significant difference from the 

respective control (p<0.05); n=16 for all groups except 10 and 1000 mg/kg/day where n=8/group.

At that time, there were several possible mechanisms suggested by which this effect 

could be produced, including increases in sulfation and glucuronidation activity through 

activation of PXR-linked genes. Activation of hepatic xenobiotic receptors (eg., PXR, CAR) 

results in induction of Phase I enzymes (e.g., cytochrome P-450s), and Phase II enzymes 

(e.g., UGTs, SULTs), as well as Phase III enzyme induction (e.g., cellular transporters) 

(Omiensinsky, 2011). Upregulation of these enzymes and transporters can result in 

decreased circulating hormone levels via increased turnover rates (Capen, 1997; McClain, 

1995; Hood and Klassen, 2000).
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3.2.1.1 In vivo studies in weanling/young adult 

animals

Zorilla et al., (2008) extended the findings of Crofton (2007) by investigating the 

effects of oral triclosan exposure in weanling male rats using the Tier I Endocrine 

Disruption Screening Program (EDSP) male pubertal protocol 

(http://www.epa.gov/endo/pubs/pubertalmale_fs.pdf). Rats were administered triclosan 

orally from post-natal day (PND) 23 to PND 42. Two experiment blocks were used: Block 

one received doses of 0, 3, 30, and 300 mg/kg/day, block two received doses of 0, 100, and 

200 mg/kg/day. Assessments included preputial separation beginning on post-natal day 

33, measurement of serum testosterone, androstenedione, leutenizing hormone (LH), 

thyroxine (T4), triiodothyroxine (T3), thyroid stimulating hormone (TSH), and prolactin 

following treatment, histology of the testes, epididymides, and thyroid gland, and hepatic 

EROD, PROD, and UGT activities. No visible signs of toxicity or effects on body weight were 

observed in this study. Liver weight was significantly increased at 100 and 300 mg/kg as 

well as anterior pituitary weight at 3 and 300 mg/kg. Preputial separation was not affected 

at any dose in this study. Serum testosterone was significantly decreased in a non-dosedependent manner at 200 mg/kg, but not at 300 mg/kg, as compared with the controls 

(Figure 12), and serum LH and prolactin were not affected at any dose level. No significant 

histological lesions were observed in testes or epididymides following triclosan exposure. 

Mean serum T4 concentrations were observed to be decreased in a dose-dependent 

manner from 30-300 mg/kg (Figure 13), while serum T3 was affected only at 200 mg/kg. 

No effect on serum TSH was observed in this study. 
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Figure 12. The effect of a 31-day exposure to triclosan (mg/kg/day) on mean (A) total serum testosterone and 

(B) serum androstenedione concentrations in the male Wistar rat.

*p < 0.05 as compared with total mean.

Figure 13. The effect of a 31-day exposure to triclosan (mg/kg/day) on (A) mean total serum T4 and (B) mean 

total T4 shown as percent of control.

In a more detailed mechanistic study, Paul et al., (2010) characterized the mode-of-action 

of the hypothyroxinemia induced by triclosan. This work hypothesized that triclosan activated 

hepatic CAR or PXR receptors, leading to upregulation of catabolic enzymes (i.e., UGTs or 

SULTs), that caused increased biliary excretion of conjugated thyroxine, with subsequent 

declines in serum concentrations. In this study, female Long-Evans rats 27-29 days of age were 
Page 65 of 81

exposed to oral doses of triclosan for four days at dose levels of 0, 10, 30, 100, 300, and 1000 

mg/kg/day. Measurements post-treatment included serum total T4 and total T3, hepatic 

microsomal EROD and PROD activity, UGT activity for T4, and mRNA gene expression of 

specific genes involved in hepatic catabolism and transport. Results of this study showed 

increased liver weight (12%) at 1000 mg/kg, and decreased total serum T4 and T3 levels in a 

dose-responsive manner at 100, 300, and 1000 mg/kg, but no change in TSH levels relative to 

controls (Figure 14). 

Figure 14. Triclosan treatment results in dose-dependent decreases in total serum T4 (A) and T3 (B), with no

change in TSH (C), represented as group mean values (± SE) percent of vehicle control for each study cohort V 1⁄4 

vehicle control, corn oil; for T4: n 1⁄4 24 for V, 30, 100, and 300 mg/kg/day dose groups, n 1⁄4 16 for 1000 dose 

group, and n 1⁄4 8 for 10 dose group; for T3 and TSH: n 1⁄4 8 for V, 30, 100, 300, and 1000 mg/kg/day dose groups; 

*significantly different from vehicle controls, p > 0.05.
Page 66 of 81

As noted in Paul (2010), “expression levels of hepatic microsomal CYP mRNA were 

differentially altered by triclosan exposure.” Cyp1a1 mRNA expression was not altered by 

triclosan treatment, but Cyp 2b1/2 and Cyp 3a1/23 were increased in a dose-dependent manner. 

Triclosan increased hepatic pentoxyresorufin-o-deethylase (PROD) activity, but not ethoxy-odeethylase (EROD) activity (Figure 15). T4-glucuronidation activity increased in a dosedependent manner (Figure 16), and isoform-specific increases in UGT and SULT mRNA 

expression were also observed in triclosan-treated rats (Figure 17). These findings are consistent 

with the hypothesized MOA, and the pattern of effects on hepatic Phase 1 and 2 mRNA and 

enzyme activities and supports the conclusion that the molecular initiating event in the mode of 

action for triclosan-induced hypothyroxinemia is activation of hepatic CAR and/or PXR 

receptors. 

Figure 15. Dose-effect curves for the effects of a 4-day exposure to triclosan on liver microsomal EROD activity 

(A) and PROD activity (B). Data are presented as group mean (± SE) percent of vehicle control for each respective 

study block. Activity 1⁄4 picomoles resorufin/mg protein/min; V 1⁄4 vehicle control, corn oil; PC 1⁄4 positive control, 

single 10 lg/kg 2,3,7,8-tetrachlorodibenzo-p-dioxin dose and single 300 mg/kg Aroclor dose; n 1⁄4 24 for V, 30, 

100, and 300 mg/kg/day dose groups, n 1⁄4 16 for 1000 dose group, and n 1⁄4 8 for 10 dose group; *significantly 

different from vehicle control, p > 0.05.

Figure 16. Dose-response curves for the effects of a 4-day exposure to triclosan on liver mRNA expression of CYP 

isoforms. Data are plotted as the mean (± SE) fold change from control for the genes assayed: Cyp1a1 (A), Cyp2b2 

(B), and Cyp3a1/23 (C). V 1⁄4 vehicle control, corn oil; n 1⁄4 8 per dose group; *significantly different from vehicle 

control, p > 0.05.
Page 67 of 81

Figure 17 Dose-response curves for the effects of a 4-day exposure to triclosan on liver mRNA expression of CYP 

isoforms. Data are plotted as the mean (± SE) fold change from control for the genes assayed: Cyp1a1 (A), Cyp2b2 

(B), and Cyp3a1/23 (C). V 1⁄4 vehicle control, corn oil; n 1⁄4 8 per dose group; *significantly different from vehicle 

control, p > 0.05.

3.2.1.2 In vivo studies in pubertal, maternal, and neonatal 

animals

Additional investigations were conducted in reports published by Paul et al., (2010) 

and Stoker et al., (2010) that extended the research on the effects of triclosan on thyroid 

hormone levels by examining the effect in pubertal and maternal animals, as well as 

offspring. It is known that regulation of thyroid hormones during development is critical to 

development and maturation of the nervous system in mammalian species, including 

humans, and that there are adverse impacts of subclinical thyroxine concentrations during 
Page 68 of 81

human development (cited in Paul et al., 2010). Therefore, investigation into the effect of 

triclosan on the developing mammalian organism is of significance. 

Stoker et al. (2010) conducted measurement of thyroid hormones as part of a Tier I 

EDSP female pubertal study with triclosan. In this study, weanling female rats received 

triclosan by gavage at doses of 0, 9.375, 37.5, 75, or 150 mg/kg from PND 22 through PND 

42. Total and free serum T4 and serum TSH as well as serum estradiol were measured at 

the end of the 21-day exposure period. 

In the female pubertal assay, mean serum estradiol concentrations were 

significantly decreased in females from 37.5-150 mg/kg triclosan following the 21-day 

exposure. Mean serum T4 concentrations were also decreased in a dose-dependent 

manner from 37.5 to 150 mg/kg following the 21-day exposure ,while free serum T4 was 

decreased at 75 and 150 mg/kg. There was no significant effect on serum TSH 

concentrations on PND 42 (Figure18). In the uterotrophic assay, there was a dosedependent decrease in both mean serum total T4 and free T4 at doses of 18.75 mg/kg and 

higher (Figure 19).

Figure 18. Mean serum (A) estradiol (B) total T4 (C) free T4 and (D) TSH on PND 42 following a 21-day exposure 

to triclosan in the female pubertal assay.

Mean ± SEM. *p < 0.05 for significance compared with control mean.
Page 69 of 81

Figure 19. Mean serum total (A) and free (B) T4 concentrations following a 3-day oral exposure in the uterophic 

assay. 

Mean ± SEM. *p < 0.05 for significance compared with control mean.

In both the uterotrophic study and the pubertal study, there was a dose response 

suppression of total and free T4. In the uterotrophic study, a dose of 18.75 mg/kg triclosan 

decreased total and free thyroxine by 20% six hours following the third daily exposure in 

the weanling female rat. This effect was dose-dependent, with the dose of 75 mg/kg 

suppressing T4 by more than 50%. A no observed adverse effect level (NOAEL) value of 

9.38 mg/kg was identified in this study for the thyroid effect of triclosan. 

3.2.1.3 In vivo studies in maternal and perinatal 

animals

Paul et al., (2010) investigated the hypothesis that perinatal triclosan exposure 

would alter circulating thyroid hormone levels in pups during early postnatal development 

and in dams at the conclusion of lactation. In this study, dams received oral doses of 0, 30, 

100, and 300 mg/kg/day from gestation day 6 through post-natal day 21. Pups were 

sacrificed on PND 4, 14, and 21 for examination of serum T4. Dams were sacrificed on PND 
Page 70 of 81

22. EROD, PROD and uridine diphosphate glucuronyltransferase (UDPGT) activities were 

also measured in liver microsomes in both maternal animals and offspring.

Perinatal exposure to triclosan in this study had no effect on gestation length, litter 

size, viability, or sex ratio. Body weight decrease of 7-10% were observed in dams at the 

300 mg/kg dose level. There were no effects on pup body weights at any dose level. At 300 

mg/kg, serum thyroxine was decreased in maternal animals on GD20 and PND 22 by 

approximately 30%. Decreases in thyroxine in maternal animals were not observed at 

lower doses. Serum T4 in pups was decreased 27% on PND4 at 300 mg/kg triclosan, but 

not on PND14 or PND21 at any dose level. Maternal exposure to triclosan resulted in 

decreased serum T4 in neonates, but only on PND 4 at the 300 mg/kg dose, where a 27% 

decrease in serum T4 was observed relative to controls. No effects of triclosan were 

observed in pups on PND 14 or PND 21 at any dose level tested in this study (Figure 20). At 

the 300 mg/kg dose, triclosan concentrations in fetal and neonatal serum as well as liver 

were observed to decrease with animal age from PND4 to PND21, suggesting that the lack 

of effect on T4 at PND14 and PND21 are due to lower exposures at these ages. 

Figure 20. Percent of control serum total thyroxine (T4) for dams (A) and pups (B), by age and oral maternal 

dose (mean ± SE). 

For dams and postnatal day (PND) 21, n = 10, 10, 9, 8 for the 0, 30, 100, and 300 mg/kg/d, respectively; for PND14, n = 10, 10, 

8, 8 for the 0, 30, 100 and 300 mg/kg/day, respectively; for PND4, n= 9, 9, 8, 8 for the 0, 30, 100 and 300 mg/kg/day, 

respectively; * = significantly different from vehicle control by ANOVA, p > 0.05. Raw T4 values in ng/ml (mean ± SE) for the 

0 mg/kg/d and 300 mg/kg/d groups, respectively, were 49.6 ± 2.2 and 34.1 ± 3.7 for PND22 dams, 9.63 ± 0.49 and 7.08 ±0.64 for

PND4 pups, and 48.4 ± 2.2 and 40.6 ± 2.3 for PND14 pups, and 38.9 ± 2.0 and 36.0 ± 1.8 for PND21 pups. V= vehicle control.

PROD activity was observed to be increased in pups on PND4 to 220% of control 

and in dams on PND22 (309% of control) at 300 mg/kg triclosan. There were no effects on 

PROD observed in neonates on PND14 or PND21. Hepatic UDPGT activity was observed to 

be increased by 1.5-fold in dams on PND22 at 300 mg/kg triclosan. Up-regulated 

expression of Cyp2b and Cyp3a isoforms in dams was observed in dams only. Measurement 

of serum triclosan levels in both maternal animals and neonates showed decreased 

concentrations in offspring on PND14 and PND21 compared to concentrations found in 

offspring on PND4 and fetuses on GD20. The results of this study showed a significant 

effect of triclosan on serum T4 levels in neonates on PND 4, but not at later post-natal 

sampling periods. This could be the result of toxicodynamic and/or toxicokinetic factors 
Page 71 of 81

that require additional examination, such as the toxicokinetic distribution of triclosan into 

maternal milk that may limit its exposure with increasing post-natal age, the degree of 

placental transfer to the fetus, and effects of triclosan on biotransformation enzymes that 

have been shown to be upregulated in adult animals. 

3.2.1.4 In vitro/in silico studies

Triclosan has also been observed to affect a number of biochemical processes 

involved in thyroid hormone homeostasis in mammalian systems. As summarized in 

Crofton et al. (2007), triclosan has been shown to induce hepatic EROD and PROD activity, 

and cytochromes P450 2B1/2 (Hanioka et al., 1996; Jinno et al., 1997). Inhibition of 

iodothyronine sulfotransferase activity has also been demonstrated in both rat and human 

hepatic microsomes (Schuur et al., 1998; Wang et al., 2004). Activation of human 

pregnane-X receptor (PXR) by triclosan in a transient transfection study with human PXR 

in a human hepatoma cell line has also been shown (Jacobs et al., 2005). PXR activation 

leads to increases in Phase I and II hepatic biotransformation enzymes, including 

glucuronidases that catabolize thyroid hormones (THs) (You, 2004). These enzymes are 

important in maintaining euthyroid hormone concentrations (Capen, 1994; Hill et al., 1998; 

DeVito et al., 1999).

Recent work from Paul (2011) investigated the molecular basis of the up-regulation 

of the biomarkers Cyp2b2, Cyp3a1, Ugt1a1, Sult1c1, and T4 glucuronidation. As the 

expression of these Phase I and Phase II enzymes is regulated by nuclear receptors 

including CAR and PXR, cell-based nuclear reporter assays were used to measure the ability 

of triclosan to activate human and rat PXR, the biologically active variants of human CAR, 

while the human CAR2 variant was strongly activated by triclosan and human CAR3 was 

moderately activated. At the human CAR1 receptor, triclosan acted as an inverse agonist. 

There was no activation of rat PXR by triclosan, but triclosan instead acted as a modest 

inverse agonist. These data suggest that triclosan may be capable of interacting with rat 

CAR and can activate human CAR and PXR to up-regulate hepatic catabolism of triclosan in 

both species, consistent with in vivo observations in the rat.

Additional evidence for this action of triclosan comes from data collected through 

EPA’s ToxCastTM program (Judson et al., 2010; http://www.epa.gov/ncct/toxcast/), where 

triclosan was shown to be active in a Nova ScreenTM Biosciences PXR binding assay. In 

addition, there was increased transcription of target genes for PXR and CAR nuclear 

receptors in the quantitative nuclease protection assay (qNPA) run by CellzDirect on 

primary human hepatocytes. Although cytotoxicity of triclosan was found to be a 

confounding factor at some concentrations tested, the results suggest that triclosan may 

have PXR and possibly CAR agonist activity, which are generally consistent with the results 

observed from in vivo and in vitro testing of triclosan.
Page 72 of 81

3.3 Benchmark Dose Analysis for Effect on T4

Knowledge of the AOP can provide useful information in the dose–response 

assessment for a substance. If the quantitative relationships are understood for precursor 

events or key events within a causal path leading to a disease or adverse outcome, the key 

event(s) can serve as the basis of for the dose response analysis. In this case, the reduction 

of serum thyroid hormones was subjected to benchmark dose (BMD) modeling. This 

approach is preferred over use of the NOAEL/ loest adverse effect level (LOAEL) approach 

in this particular case, as it is suspected that a certain level of perturbation is necessary for 

disruption of homeostatic mechanisms of thyroid regulation leading to an adverse 

outcome.

Benchmark doses were identified that corresponded to low- and no-effect levels in 

the rat for potential use in dose response analysis from the data of Zorrilla et al. (2009) and 

Stoker et al. (2010) that investigated the effect of triclosan on thyroid hormone 

concentrations in male and female weanling and pubertal rats . The data of from these two 

studies were used for BMD analysis because the studies were considered representative of 

the most sensitive lifestage with respect to perturbation of thyroid hormone levels and 

provided data at the lower region of the dose response curve. These studies consisted of 

two blocks of male rats and one block of female rats. The first block included 10 males per 

treatment at doses of 0, 100 or 200 mg/kg triclosan, and the second block included 15 

males per treatment at doses of 0, 3, 30 and 300 mg/kg. One block of females was 

administered 0, 9.375, 37.5, 75 or 150 mg/kg triclosan (n=10) for 20 days.

Serum T4 data from all three blocks of triclosan exposure were evaluated using 

EPA's Bench Mark Dose Software (BMDS) to determine the BMD and lower limit estimate 

(BMDL). Because the doses were run in three blocks, the T4 was rescaled for control 

values in each block. The unconstrained Hill coefficient is a useful model to make an 

approximation to be used in the observed range. Setting the bench mark response (BMR) 

at 10% using an unconstrained Hill estimate to fit the data, the BMD was 10.37 mg/kg and 

the BMDL was 3.47 and at 20% BMR using the same model, the BMD was 19.05 mg/kg and 

the BMDL was 8.07 mg/kg (Table 6).

Table 6. Results of benchmark dose analysis using data from Zorilla et al. (2009)

and Stoker et al. (2010).

Both the BMR10 and BMR20 are represented. 

The 20% BMR was included in the BMD modeling results based on the clinical 

literature indicating perceived toxicological significance of a change in the measured 

endpoint (decreased T4 concentration) and general clinical knowledge about adverse 

outcomes associated with decreases in circulating thyroid hormones. The 10% BMR is 

selected as a default approach and to provide a lower end of biological changes. US EPA 
Page 73 of 81

guidance on setting the BMR suggests using “a minimal level of change in the endpoint that 

is generally considered to be biologically significant (for example, a change in average adult 

body weight of 10%, or the doubling of average level for some liver enzyme”. 

(http://www.epa.gov/ncea/bmds/bmds_training/toc.htm). There are some data that 

associate the degree of disruption of thyroid function to adverse neurodevelopmental 

outcomes where the perturbations are mild to moderate. Haddow et al (1999) reported a 

25% decrease in maternal free T4 during the second trimester in women and associated 

this with neurodevelopmental and cognitive deficits in children. Henrichs et al. (2010) 

associated maternal hypothyroxinemia with higher risk of verbal and nonverbal cognitive 

delay in early childhood. It should be noted that a 10% change is within normal 

experiment-to-experiment variation in control values, whereas the BMR of 20% provides a 

more reliable BMR. The range of 10-20% is interpreted as a balance between the biological 

and statistical significance of the reported changes in T4 and associated adverse outcomes. 

This range is consistent with previous work in both animals and humans for changes in 

thyroid hormones and precursor events such as iodine uptake inhibition (Strawson et al., 

2004; Zorrilla et al., 2008; Zhou et al., 2001; Paul et al., 2010). For this reason, the BMD at 

both 10% and 20% were calculated to provide a lower and upper range of BMRs. 

3.4 Adverse Outcome Pathway Development 

The effect of triclosan on the thyroid hormone system has been studied in in vivo

experiments in both adult and weanling experimental animals as well as fetal/neonatal 

animals. Additional evidence for the key events and molecular initiating event of this effect 

comes from in vitro and in silico studies examining the effect of triclosan on up-regulation 

of specific hepatic receptors (PXR and CAR) leading to downstream induction of enzymes 

involved in biotransformation of thyroid hormones and reduction in circulating levels in 

vivo . It is known that the developing nervous system is dependent upon adequate amounts 

of thyroid hormones and that significant neurological damage can occur when the 

deficiency is present during brain development (Obregon, 2007). It has also been 

demonstrated that deficiencies of thyroid hormone at birth can result in deficiencies in 

mental and neurologic outcome at 2 and 5 years of age (van Wassenaer, 2002). Indeed, 

25% decreases in maternal T4 levels during the second trimester are associated with a 7 

point decreases in IQ at 7 to 9 years of age (Haddow et al., 1998). Thus, the thyroid effects 

observed from studies on triclosan in experimental animal systems are relevant for 

interpreting potential adverse neurodevelopmental effects in humans. Data presented in 

this case study have shown that triclosan causes hypothyroxinemia, while in vitro studies 

have provided supporting data on the molecular initiating event and downstream 

molecular events resulting in decreased circulating T4 levels, events that are plausible in 

humans. This case study that presents the AOP proposed for triclosan and supported by 

the experimental evidence allows for refinement of the current human health assessment 

and is significant for its relevance to human health. 

Additional published data prompt the discussion of relative species sensitivity to 

perturbations of thyroid hormone levels. In humans and other primates, thyroxine-binding 

globulin (TBG) is the principal protein that binds T4 (Dohler et al., 1979). It has a very high 

affinity for T4 (only about 0.03% of the T4 in serum is in the free unbound form (Hill et al. 
Page 74 of 81

1989)). By comparison, most T4 in adult rat serum is bound to albumin and transthyretin 

(TTR). There is a developmental switch in binding proteins in rats where TBG is the major 

binding protein until approximately postnatal day 35 when the TTR becomes the dominant 

binding protein. The binding affinity of T4 for TBG is more than 100-fold greater than that 

of albumin or transthyretin (Hill et al., 1989) and this difference contributes to the higher 

rate of T4 clearance in adult rats. The increased clearance also contributes to the need for a 

higher rate of production of T4 per unit of body weight in rats to maintain normal 

concentrations of T4 (Dohler et al., 1979). These differences are hypothesized to be 

responsible to the increased susceptibility of rats to thyroid follicular tumors compared to 

humans (Capen, 1997; McClain, 1995). In addition, it has also led to speculation that that 

rats may be a more sensitive species to perturbations of circulating thyroid hormone 

compared to humans.

There is also evidence for species differences in the metabolism and toxicokintecs of 

triclosan. Data in both rats and humans using single and repeated doses of triclosan 

(Scientific Committee on Consumer Products, Opinion on Triclosan, 2009; van Dijk, 1994, 

1995, 1996; cited in SCCP, 2009; Lucker et al., 1990; cited in SCCP, 2009) that have 

measured kinetics of triclosan show that rats attain higher values for area under the curve 

(AUC) [63.9 μg*hr/mL] compared to humans [0.2-7.2 μg*hr/mL] and higher plasma levels 

(4,450 ng/mL in rats vs 23-574 ng/mL in humans) at comparable oral doses. These data 

suggest a higher overall exposure to triclosan in rats compared to humans. The 

disposition of triclosan in rats also differs from humans, with the majority of an oral dose of 

triclosan (81-84%) eliminated in feces, whereas urinary elimination is the major route in 

humans (57-87%). Humans also demonstrate a pronounced first-pass effect from triclosan 

exposure, resulting in less systemic exposure to free triclosan than in rats (SCCP, 2009). 

Enteroehepatic circulation of triclosan has also been observed from experimental studies 

in rats (consistent with feces as the major excretory route in rats vs. urine in humans), 

which could result in longer residence time for triclosan in blood and higher tissue 

exposures to triclosan in rats compared to humans (SCCP, 2009).

In the only data available on the impact of triclosan exposure on thyroid hormones 

in humans , Allmyr et al. (2009) reported no significant effect on serum T4 after repeated 

(14 day) use of triclosan-containing toothpaste in adults. These data, while from a shortterm dosing study, suggest that normal use of toothpaste containing triclosan is not likely 

to cause adverse effects on thyroid function.

In addition to understanding of the molecular and key events for the proposed AOP 

for thyroid hormone perturbation between experimental animals and humans, the 

understanding of the physiological differences in thyroid hormone regulation and 

pharmacokinetic differences in rats versus humans is important to informing the factors 

applied for interspecies and intraspecies extrapolation.
Page 75 of 81

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