SCIENTIFIC ISSUES CONCERNING 

HEALTH EFFECTS OF 

CHLORPYRIFOS 

INTRODUCTION

ANNA LOWIT, PH. D.

SENIOR SCIENTIST

USEPA, OFFICE OF PESTICIDE 

PROGRAMS

FIFRA SAP April 10, 2012
Introduction

 NRC Reports

 Toxicity Testing in the 21st Century: A Vision and 

a Strategy (2007): 

 Adverse outcome pathways

2

 Integration across multiple lines of evidence

 Disruption of homeostasis leading to adverse 

outcomes

 Science & Decisions (2009):

 Improved technical analysis (including mode of action)

 Evaluation of alternative interpretations/approaches

 Better align risk assessment & risk management

 Use of improved problem formulation
Regulatory Context: 

Chlorpyrifos

 Organophosphate pesticide first registered in 

1965 

 In June 2000:

 The technical registrants entered into an 

agreement with the Agency to eliminate and 

3

phase out nearly all uses that result in 

residential exposures 

 Some mitigation on food (apples & tomatoes)

 Some mitigation for workers (PPE & 

engineering controls for some uses)

 Registration Eligibility Decision (RED) in 2006

 September 2007 petition from NRDC & PANNA to 

revoke all tolerances & cancel registrations
Recent SAP Reviews

4

 2008: New science on infants, children, & pregnant 

women from experimental laboratory toxicology & 

epidemiology studies

 Evaluated both AChE & non-cholinergic modes of 

action

 2009: Approach for risk assessment of semi-volatile 

pesticides 

 http://www.epa.gov/scipoly/sap/meetings/2009/12010

9meeting.html

 2010: Development of the draft “Framework for 

Incorporating Human Epidemiologic & Incident Data in 

Health Risk Assessment” 

 http://www.epa.gov/scipoly/sap/meetings/2010/02021

0meeting.html
Regulatory Context: 

Chlorpyrifos 5

 Registration review opened in March 2009 and is 

expected to conclude by the end of 2015

 http://www.regulations.gov/#!docketDetail;D=EPA-HQOPP-2008-0850

 July 2011 Preliminary Risk Assessment

 http://www.regulations.gov/#!documentDetail;D=EPA-HQOPP-2008-0850-0025

 Focus on AChE-inhibiting potential

 Most sensitive dose-response data

 Weight of the evidence evaluation integrating data from 

epidemiology, biomonitoring, & experimental toxicology 

studies to be conducted
2008 FIFRA SAP Review

 Preliminary review of scientific 

literature with focus on infants, 

children & pregnant women 

 Epidemiology

6

 Age-dependant metabolism

 Intra-species variability of paraoxonase (PON1)

 AChE/ChE inhibition data

 Various lifestages

 Multiple routes, durations

 In vivo rodent studies evaluating long-term effects

 Biomonitoring studies
2008 FIFRA SAP Review

7

 Key recommendations from SAP

 AChE/ChE data remains the most appropriate data for 

deriving points of departure

 Route specific & lifestage-specific AChE/ChE data 

appropriate

 In vivo In vivo rodent studies on long rodent studies on long-term ef term effects from early fects from early 

life exposure 

 Limited dose- response data 

 Only doses of 1 mg/kg or greater demonstrate significant 

effects on behavior

 Epidemiology studies:

 “Chlorpyrifos likely played a role”

 Only measuring biomarkers (3rd trimester maternal, cord 

blood, meconium) at one point in time 

 Cannot exclude possibility that the effects seen were due to 

chlorpyrifos in combination with other pesticides 
8

Draft Framework for Incorporating Epi

 Concepts based on peer-reviewed, robust 

principles & tools 

 Standard practice in epidemiology, toxicology 

& risk assessment 

 Consistent with recommendations from 

 NRC 2007: Toxicity Testing in the 21st Century 

 NRC 2009: Science & Decisions: Advancing Risk 

Assessment

 Flexibility to incorporate information from 

different sources based on AOP concept

 Transparent tool for organizing, reviewing & 

interpreting complex information
Draft Framework for Incorporating 

Epi

9

 Mode of Action Framework (Modified Bradford Hill 

Criteria)

 Postulated mode of action 

 Identify sequence of key events on the path to health 

outcome

 Experimental support

 Concordance of dose-response for key events 

 Temporal relationships for key events

 Biological plausibility & coherence

 Strength, consistency & specificity

 Other modes of action

 Identify uncertainties

 Conclusion
Adverse Outcome Pathway

10

Chemicals Molecular 

Target

Cellular 

Response

Tissue/ 

Organ Individual Population Pharmacokinetics

In vitro studies

Biomonitoring & 

Exposure data

Structure Activity Relationships

Toxicity Pathways

In vivo studies

Exposure data

Epidemiology

Toxicity Pathways

Human Incidents

Molecular

initiating event

Key events or predictive

relationships spanning

levels of biological 

organization 

Adverse outcome

relevant to 

risk assessment

Greater Toxicological Understanding Greater Risk Relevance
Biologic

inputs

“Normal” 

Biological

Function

Adaptive

Responses

Early cellular

changes

Exposure

Uptake-Delivery to Target Tissues

Perturbation

Cellular response pathway

II. Adverse Outcome Pathways –

definition and example

Adverse

Outcomes

(e.g., mortality, 

Reproductive

Impairment)

Cell inury, 

Inability to 

regulate

Responses

Molecular

initiating event

Perturbed cellular 

response pathway

Adverse outcome

relevant to

risk assessment

Toxicity Pathway

Adverse Outcome Pathway

Modify From NRC 2007
July 2011 Preliminary Risk 

Assessment

12

 AChE/ChE inhibiting potential of chlorpyrifos

 Most sensitive dose-response data available

 Route specific studies (oral, dermal, inhalation)

 Lifestage sensitivity

 Process for deriving points of departure

 In 2008, the Agency performed a comprehensive 

review of the literature for AChE inhibition. 

 2012 review has been supplemented with the 

newest data

 Benchmark dose approach
July 2011 Preliminary Risk 

Assessment

13

 AChE/ChE inhibiting potential of chlorpyrifos

 Acute, oral: BMDL10 0.36 mg/kg for RBC AChE 

inhibition

 PND11 rat pups provide the most sensitive cholinesterase 

inhibition data 

 Consistent with the concept that younger juvenile animals 

are more sensitive due to immature metabolic systems 

 Chronic, oral: BMDL10 0.03 mg/kg/day for RBC AChE 

inhibition

 As the metabolic systems mature at or near adult levels, 

sensitivity disappears 

 In repeated dosing studies, pups exhibit similar sensitivity 

to adult rats

 Pregnant dams provide the most sensitive data
April 2012 

14

 Key scientific questions 

1. Lines of evidence that suggest 

chlorpyrifos exposure results in long-term 

neurodevelopmental effects from early life 

exposure 

2. Lines of evidence that adverse effects 

may be attributed to doses lower than 

those which elicit 10% inhibition of AChE, 

i.e., the dose levels previously used for 

regulatory decision making
April 2012

15

 Compared to 2008, 2012 analysis incorporates 

new data available and/or provides more in-depth 

analysis

 AChE/ChE inhibition 

 Other mechanistic mechanistic data----Evaluation Evaluation of plausible of plausible 

hypotheses on modes of action/adverse outcome 

pathways (MOA/AOP) leading to neurodevelopmental 

outcomes

 Adverse outcomes in rodents, including 

neurodevelopmental studies on behavior and 

cognition effects. 

 Epidemiologic studies from three major children’s 

health cohorts in the U.S. 

 Biomonitoring and physiologically-based 

pharmacokinetic/pharmacodnamic (PBPK/PD) 
2007 NRC Toxicity Testing in the 21st

Century

Compounds Affected

Pathway

Dose Response

Analysis for 

Perturbations

Calibrating 

in vitro and human

Population Based

Studies

Exposure

Guideline

Mode of Action

Chemical

Characterization

Dose Response 

Assessment

Assess

Biological 

Perturbation

Measures of

dose in vitro

Perturbations

of Toxicity 

Pathways

in vitro and human

Dosimetry

Human Exposure

Data 

Exposure

Guideline

Hazard Characterization

Risk Characterization

Exposure Assessment

NRC, 2007. Fig 3-7 Risk Assessment Compo

Metabolite(s)
MOA/AOP

Critical 

Duration

Pathway of 

Exposure Lifestage

Infants & Toddlers 

Direct exposure in 

food, water, air from 

volatization/spray drift, 

AChE

inhibition

Critical duration: 

from 1 day to time 

of metabolic 

maturation

Cholinergic 

toxicity

MOA: Unknown; 

Multiple 

Hypotheses

Critical duration: 

Unknown. 

Neurodevelopment

al outcomes in 

children (learning 

& motor deficits)

Mother

Direct exposure in 

food, water, air from 

volatization/spray 

drift, worker 

(applicator, post 

AChE inhibition 

Critical duration: 

from 1 day up to 

steady state (approx 

2-3 weeks) with a 

max exposure of 9 

mos

Cholinergic toxicity

Pregnant Female

(applicator, post 

app)

max exposure of 9 

mos

Fetus Cross-placental 

exposure

AChE inhibition 

Critical duration: 

from 1 day up to 

9 months 

Equal or less 

sensitive to mother; 

protect for mother’s 

AChE inhibition—

protect for fetus

MOA: Unknown; 

Multiple 

Hypotheses

Critical duration: 

Unknown. 

Neurodevelopme

ntal outcomes in 

children (learning 

& motor deficits)
Outline of Presentations

18

 Introduction (Anna Lowit)

 Adverse Outcome Pathways: Data for 

Chlorpyrifos at Varying Levels of Biological 

Organization (William Mundy)

 Chlorpyrifos Effects on the Developing Brain: 

Animal Studies (Virginia Moser)

 Review of Children’s Health Epidemiology 

Cohort Studies (Carol Christensen)

 Review of Chlorpyrifos Biomonitoring 

Research and Interpretive Approaches 

(Aaron Niman)

 Summary & Next Steps (Anna Lowit)
19 Questions?