OFFICE OF CHEMICAL SAFETY

AND POLLUTION PREVENTION

UNITED STATES ENVIRONMENTAL PROTECTION AGENCY

WASHINGTON, D.C. 20460

RE: Meeting between Dow AgroSciences (DAS) and EPA to Discuss DAS’ 

Enhancements to their Physiologically Based Pharmacokinetic Model (PBPK)

List of Documents

1. Meeting Summary Page 1

2. Meeting Sign in Sheet Page 2

3. DAS PowerPoint Presentations Page 3

Meeting Summary

On Tuesday, March 19, 2013, EPA’s Office of Pesticide Programs met with DAS to discuss their 

PBPK model. DAS wanted to discuss their recent PBPK model enhancements related to dermal 

and oral exposure. In addition, they presented, in the context of the application of the modeling 

tools, their work focusing on dose response. DAS anticipates discussing the progress of future 

enhancements with EPA later in the summer. 
Name 

EPA and Registrant Meeting 

Chlorpyrifos and PBPK 

March 19, 2013 

Affiliation Phone Number E-mail 
USING PBPK MODELS 

TO IMPROVE THE 

CHLORPYRIFOS RISK 

ASSESSMENT 

Michael Bartels

Paul Price

March 19, 2013
| 2

• PBPK Model Development– Michael Bartels

• PBPK Applications – Michael Bartels, Paul 

Price

• Modeling Effects of low dose exposures to 

Chlorpyrifos in infants – Paul Price

• Further Discussion - All

Speakers
| 3

Development of Chlorpyrifos PBPK/PD model

Year Author

Model 

Type Compound

Exposure 

Route Species LifeStages

Metabolite 

Biomarkers

Cholinesterase 

Biomarkers Comments

1984 Nolan P K CPF Oral, 

Dermal

Human Adult Plasma/Urine TCPy NA PK Model

2002 Timchalk PBPK/PD CPF Oral Rat, 

Human

Adult Plasma/Urine TCPy

Blood/Tissue CPF

Blood/Tissue Oxon

Plasma, RBC 

and Tissue ChE

PBPK/PD Model

2006 Timchalk PBPK/PD CPF Oral Rat Neonatal Rat Plasma/Urine TCPy

Blood/Tissue CPF

Blood/Tissue Oxon

Plasma, RBC 

and Tissue ChE

Neonatal Rat Model

2009 Lowe PBPK/PD CPF Oral Rat, 

Human

Late-stage 

gestation: 

Maternal / fetus

Plasma/Urine TCPy

Blood/Tissue CPF

Blood/Tissue Oxon

Plasma, RBC 

and Tissue ChE

Gestation Model

2011-2013 Smith PBPK/PD CPF Oral Human Newborn - Adult Plasma/Urine TCPy

Blood/Tissue CPF

Blood/Tissue Oxon

Plasma, RBC 

and Tissue ChE

Life-Stage Model

2011-2013 Poet PBPK/PD CPF Oral Human Newborn - Adult Plasma/Urine TCPy

Blood/Tissue CPF

Blood/Tissue Oxon

Plasma, RBC 

and Tissue ChE

Variation Model

2013 Poet PBPK/PD Oxon Oral Rat, 

Human

Rat: Adult

Human: Newborn - 

Adult

Plasma/Urine TCPy

Blood/Tissue CPF

Blood/Tissue Oxon

Plasma, RBC 

and Tissue ChE

Oxon Oral Model

2013 Poet PBPK/PD CPF Dermal Human Newborn - Adult Plasma/Urine TCPy

Blood/Tissue CPF

Blood/Tissue Oxon

Plasma, RBC 

and Tissue ChE

Updated CPF Dermal 

Model

Planned Poet PBPK/PD CPF Inhalation Rat, 

Human

Rat: Adult

Human: Adult

Plasma/Urine TCPy

Blood/Tissue CPF

Blood/Tissue Oxon

Plasma, RBC 

and Tissue ChE

CPF Inhalation Model
| 4

Development of Chlorpyrifos PBPK/PD model

• 1984: Nolan one-compartment PK model (TAP: 73, 8-15) 

> oral and dermal routes in adult humans

> models blood / urine TCP 

• 2002: Timchalk PBPK/PD model (Tox Sci: 66, 34-53)

> oral and dermal routes in adult humans, oral route in adult 

rats

> models blood / urine TCP (fits well to 1-compartment 

model)

> models blood & tissue CPF and Oxon levels

> models plasma / RBC / tissue ChE activities 
| 5

Development of Chlorpyrifos PBPK/PD model

• 2006: Timchalk PBPK/PD model (Toxicology: 220, 13-25)

> oral route in neonatal rats

> models: TCP / CPF / Oxon levels and blood / tissue ChE 

activities

• 2009: Lowe Gestation PBPK/PD model (Tox Sci: 108, 

258-272)

> oral route in 3rd trimester mothers and fetus (at birth)

> pregnancy-based changes in tissue partitioning 

incorporated

> models: TCP / CPF / Oxon levels and blood / tissue ChE 

activities

> fit to Mattsson 2000 rat data and Columbia blood levels
| 6

Development of Chlorpyrifos PBPK/PD model

• 2009: Lowe Gestation PBPK/PD model: 

Non-pregnant and pregnant Rat

Maternal and cord blood- human
| 7

Development of Chlorpyrifos PBPK/PD model

• 2011: Smith Life-Stage PBPK/PD model (2013- submitted 

manuscript)

> oral route in humans, ages 6 months – adult

> models blood / urine TCP (fits well to 1-compartment 

model)

> models blood & tissue CPF and Oxon levels

> models plasma / RBC / tissue ChE activities

• 2011: Variation PBPK/PD model (2013- manuscript in 

preparation)

> Utilizes Life-Stage model

> Output based on variations in dietary intake of CPF and 

variations in physiology and age-based changes in 

metabolism of CPF

> Presented to Feb. 2011 SAP
| 8

Development of Chlorpyrifos PBPK/PD model

• 2013: Extensions to Life-Stage PBPK/PD model:

> Oxon oral exposures (Jan 2013 report, MRID 49047701)

• adult rat and human life-stages

• validated with 2010 CCA rat data

0 4 8 12 16 20 24

0

25

50

75

100

125

150

175

Brain

RBC

Plasma

Time

Percent of Control ChE Activity

Single dose (0.5 mg/kg) Repeated dose (0.5 mg/kg)

0 48 96 144 192 240

0

25

50

75

100

125

150

175

RBC

BRAIN

PLASMA

Time
| 9

Development of Chlorpyrifos PBPK/PD model

• 2013: Extensions to Life-Stage PBPK/PD model:

> Oxon oral exposures

Oxon single oral dose (1 μg/kg)- Rat Oxon single oral dose (1 μg/kg)- Human

0 4 8 12

10- 1 2

10- 1 0

10- 8

10- 6

10- 4

10- 2 Intestine

Liver

Plasma

Brain

Time (hr)

μmol/kg

0 4 8 12

10- 1 2

10- 1 0

10- 8

10- 6

10- 4

10- 2

Plasma Limit of Quantitation

Initial Water Concentration Intestine

Liver

Plasma

Brain Time (hr)

• A high degree of presystemic hydrolysis 

of Oxon predicted in both species

• Non-detectable Oxon concentrations 

predicted in all tissues
| 10

Development of Chlorpyrifos PBPK/PD model

• 2013: Extensions to Life-Stage PBPK/PD model:

> Oxon oral exposures

Predictions of Tissue ChE activity in adult human, following repeated oral exposure to Oxon

RBC ChE

OXON

μg/kg/day 0.003 0.03 0.3 3

Peak Inhibition (% of control) 99.99952 99.99521 99.94925 99.11643

CPF

Peak Inhibition (% of control) 99.99994 99.99936 99.99351 99.92987

Brain ChE

OXON

μg/kg/day 0.003 0.03 0.3 3

Peak Inhibition (% of control) 100 99.99997 99.99963 99.9935

CPF

Peak Inhibition (% of control) 100 99.99998 99.99983 99.99828

3 μg/kg/day = 105 μg/L in drinking water

No detectable ChE inhibition in RBC or Brain tissue, from either compound
| 11

Development of Chlorpyrifos PBPK/PD model

• 2013: Extensions to Life-Stage PBPK/PD model:

> CPF dermal exposures (report submitted March 2013)

• human life-stages

• improved dermal uptake compartment- better fits to timecourse

• validated with 1984 Nolan human volunteer data
| 12

Development of Chlorpyrifos PBPK/PD model

0 24 48 72 96 120 144 168

10- 7

10- 6

10- 5

10- 4

10- 3

10- 2

10- 1

100

101 TCPy

CPF-oxon CPF

Time (hr) Plasma TCPy (μmol/L)

0 24 48 72 96 120 144 168

10- 7

10- 6

10- 5

10- 4

10- 3

10- 2

10- 1

100

101

TCPy

CPF

CPF-oxon

Time (hr) Plasma TCPy (μmol/L)

0 48 96 144 192 240 288 336

0

25

50

75

100

Time (hr) Plasma ChE (Percent of Control)

0 48 96 144 192 240 288 336

0

25

50

75

100

Time (hr) Plasma ChE (Percent of Control)

Oral dose (0.5 mg/kg) Dermal dose (5 mg/kg)
| 13

Development of Chlorpyrifos PBPK/PD model

• 2013: Extensions to Life-Stage PBPK/PD model:

> CPF inhalation exposures (report planned Sept 2013)

• adult rat and human life-stages

• will include new measurements of lung and brain metabolism

• will be validated with rat study data: 
| 14

Development of Chlorpyrifos PBPK/PD model

Year Author

Model 

Type Compound

Exposure 

Route Species LifeStages

Metabolite 

Biomarkers

Cholinesterase 

Biomarkers Comments

1984 Nolan P K CPF Oral, 

Dermal

Human Adult Plasma/Urine TCPy NA PK Model

2002 Timchalk PBPK/PD CPF Oral Rat, 

Human

Adult Plasma/Urine TCPy

Blood/Tissue CPF

Blood/Tissue Oxon

Plasma, RBC 

and Tissue ChE

PBPK/PD Model

2006 Timchalk PBPK/PD CPF Oral Rat Neonatal Rat Plasma/Urine TCPy

Blood/Tissue CPF

Blood/Tissue Oxon

Plasma, RBC 

and Tissue ChE

Neonatal Rat Model

2009 Lowe PBPK/PD CPF Oral Rat, 

Human

Late-stage 

gestation: 

Maternal / fetus

Plasma/Urine TCPy

Blood/Tissue CPF

Blood/Tissue Oxon

Plasma, RBC 

and Tissue ChE

Gestation Model

2011-2013 Smith PBPK/PD CPF Oral Human Newborn - Adult Plasma/Urine TCPy

Blood/Tissue CPF

Blood/Tissue Oxon

Plasma, RBC 

and Tissue ChE

Life-Stage Model

2011-2013 Poet PBPK/PD CPF Oral Human Newborn - Adult Plasma/Urine TCPy

Blood/Tissue CPF

Blood/Tissue Oxon

Plasma, RBC 

and Tissue ChE

Variation Model

2013 Poet PBPK/PD Oxon Oral Rat, 

Human

Rat: Adult

Human: Newborn - 

Adult

Plasma/Urine TCPy

Blood/Tissue CPF

Blood/Tissue Oxon

Plasma, RBC 

and Tissue ChE

Oxon Oral Model

2013 Poet PBPK/PD CPF Dermal Human Newborn - Adult Plasma/Urine TCPy

Blood/Tissue CPF

Blood/Tissue Oxon

Plasma, RBC 

and Tissue ChE

Updated CPF Dermal 

Model

Planned Poet PBPK/PD CPF Inhalation Rat, 

Human

Rat: Adult

Human: Adult

Plasma/Urine TCPy

Blood/Tissue CPF

Blood/Tissue Oxon

Plasma, RBC 

and Tissue ChE

CPF Inhalation Model
| 15

Application of PBPK modeling tools

• Design of optimum sampling times for ChE measurements

> 2010 CCA study

• Design of optimum biomonitoring sampling times

> 2012 Dow worker exposure study

• Comparison of in vitro substrate conc. to in vivo exposure levels

> 2012 April SAP

• Derivation of relevant Toxicity Adjustment factor for Oxon oral 

exposures

> 2012 Sept. White paper report and 2013 Jan. model report

• Determination of doses that cause <10% RBC inhibition in 

sensitive humans and the development of chlorpyrifos-specific 

safety factors 

> 2011 February SAP and 2013 manuscript (draft by April 2013)
| 16

Application of PBPK modeling tools

Estimated time-of-peak ChE 

inhibition in PND 11 rat brain

(Expanded)
| 17

Application of PBPK modeling tools

• Design of optimum sampling times for ChE measurements

> 2010 CCA study

• Design of optimum biomonitoring sampling times

> 2012 Dow worker exposure study

• Comparison of in vitro substrate conc. to in vivo exposure levels

> 2012 April SAP

• Derivation of relevant Toxicity Adjustment factor for Oxon oral 

exposures

> 2012 Sept. White paper report and 2013 Jan. model report

• Determination of doses that cause <10% RBC inhibition in 

sensitive humans and the development of chlorpyrifos-specific 

safety factors 

> 2011 February SAP and 2013 manuscript (draft by April 2013)
| 18

Application of PBPK modeling tools

0 24 48 72 96

0.0001

0.001

0.01

0.1

1

10

Time (hr)

Plasma TCPy (μmol/L)

Sampling time
| 19

Application of PBPK modeling tools

• Design of optimum sampling times for ChE measurements

> 2010 CCA study

• Design of optimum biomonitoring sampling times

> 2012 Dow worker exposure study

• Comparison of in vitro substrate conc. to in vivo exposure levels

> 2012 April SAP

• Derivation of relevant Toxicity Adjustment factor for Oxon oral 

exposures

> 2012 Sept. White paper report and 2013 Jan. model report

• Determination of doses that cause <10% RBC inhibition in 

sensitive humans and the development of chlorpyrifos-specific 

safety factors 

> 2011 February SAP and 2013 manuscript (draft by April 2013)
| 20

CPF

CPO

• Variation model used for in vitro – in vivo comparisons

(3.5 ng/g)

(3.5 pg/g)

In vitro substrate concentrations correlated to 

population blood levels, and then dose

ChE inhibition range
| 21

Application of PBPK modeling tools

• Design of optimum sampling times for ChE measurements

> 2010 CCA study

• Design of optimum biomonitoring sampling times

> 2012 Dow worker exposure study

• Comparison of in vitro substrate conc. to in vivo exposure levels

> 2012 April SAP

• Derivation of relevant Toxicity Adjustment factor for Oxon oral 

exposures

> 2012 Sept. White paper report and 2013 Jan. model report

• Determination of doses that cause <10% RBC inhibition in 

sensitive humans and the development of chlorpyrifos-specific 

safety factors 

> 2011 February SAP and 2013 manuscript (draft by April 2013)
| 22

CPF Oral- Gavage

Dose (mg/kg)

Cholinesterase Activity (% Control)

0.01 0.1 1 10 100

0

50

100

RBC

Brain

Oxon Oral- Gavage

Dose (mg/kg)

Cholinesterase Activity (% Control)

0.01 0.1 1 10 100

0

50

100

RBC

Brain

2010 CCA study rat ChE data

Time (hr)

Oxon Plasma Concentraion (uM)

0 10 20 30 40

110- 1 2

110- 1 0

110- 0 8

110- 0 6

110- 0 4

110- 0 2

1100 0

1 ug/kg Oxon

10 ug/kg Oxon

100 ug/kg Oxon

1000 ug/kg Oxon

1 ug/kg CPF

10 ug/kg CPF

100 ug/kg CPF

1000 ug/kg CPF

Predicted Oxon blood levels from CPF or Oxon Oral Dose

Substantially different inhibition profiles seen for CPF & Oxon:

• CPF systemically bioavailable, while no Oxon seen in blood

• Oxon is significantly less toxic to central tissues than CPF

• This is due to extremely rapid, and complete presystemic

hydrolysis of Oxon by ChE and other esterases
| 23

Application of PBPK modeling tools

• Design of optimum sampling times for ChE measurements

> 2010 CCA study

• Design of optimum biomonitoring sampling times

> 2012 Dow worker exposure study

• Comparison of in vitro substrate conc. to in vivo exposure levels

> 2012 April SAP

• Derivation of relevant Toxicity Adjustment factor for Oxon oral 

exposures

> 2012 Sept. White paper report and 2013 Jan. model report

• Determination of doses that cause <10% RBC inhibition in 

sensitive humans and the development of chlorpyrifos-specific 

safety factors 

> 2011 February SAP and 2013 manuscript (draft by April 2013)
| 24

Developing variation model for human life-stages

1. Identify sensitive model inputs 

2. Determine if the inputs vary across individuals

3. Develop distributions for parameters that vary and drive 

model outputs

4. Determine need for correlation of inputs

5. Evaluate predictions of variation

> Determine actual drivers of variation and model uncertainty in 

population variation on drivers

> Comparison to published data on human variation 
| 25

Draft manuscript (April, 2013)

• Revised sensitivity analysis

> Track RBC AChEI, Brain AChEI, and blood oxon concentration

> Multiple (5) days

• Added variation in 

> Intestinal metabolism

• Addressed uncertainty in enzyme values

• Preliminary findings – no significant changes from findings 

presented at SAP
| 26

Extension to inhalation and dermal exposures

• Basic model does not change

> PBPK in key compartments (blood, liver, and brain)

> PBPD equations 

> Modeling time varying exposures

> Model outputs

• Additional components 

> Route-specific PBPK models for dermal and inhalation

> Sensitivity analysis to identify parameters that vary

> New interface to allow data on time varying exposures for one 

individual from multiple routes
| 27

Using model in standard setting

• Determination of POD 

> RBC AChEI will not exceed 10% for periods of greater than 1 hr in 

sensitive individuals of the most sensitive age group

• Estimation of uncertainty factors

> Address inter species and inter individual

> Incorporate SAP recommendations

• Initially for oral route 
| 28

Interspecies

• Compare animal data to model predictions of equivalent 

human data

• Use simulation model to make predictions of central 

tendency and benchmark doses of animal studies

• Develop conceptual model of the similarity of test animal 

and humans in pharmacokinetics and pharmacodynamics of 

chlorpyrifos 

> Toxicity pathway is short for RBC AChEI and similar in humans and 

rat

> Leads to small interspecies differences
| 29

Interindividual

• Develop a qualitative model to describe how individuals 

could differ

> Toxicity pathway to RBC AChEI is short for humans

> Limited number of ways we differ from one another

• Discuss

> Differences in physiology

> Differences in metabolism

> Background stress reflected in variation in initial AChE levels 

> Effect of physical activity

• Age-related variation and interindividual variation at specific 

ages
| 30

Planned publication: April 2013

• Focus on oral route of exposure

• Build on EPA POD in draft HHA

• Use Variation model to assess infants and older age groups

• Characterize response from gestational exposures 

> Use maternal blood levels and empirical finding that fetal blood levels are

similar to maternal blood for a given oral exposure to demonstrate fetal

exposures are the same or lower than levels in infants

> Animal evidence that fetal tissues in neonates (equivalent to late trimester 

humans) are no more sensitive than adult tissues. 

> Conclusion risks to fetus are similar to those of infants and adults 
| 31

Modeling Effects of low dose exposures to Chlorpyrifos 

in infants

• Models address age-related changes in physiology 

> Data indicate that physiology varies less across infants than across adults

> Over 0- 12 months body weight increases but organ-sizes (on a weight 

basis do not change significantly

• Models address age-related changes in metabolism

> Metabolism data does not appear to change for infants >1 month to 12 

months.

• As a result the is little predicted change between birth to one year

• The following examples are for typical infants of various ages between 

birth and 12 months under different dietary scenarios and levels of 

activity 
| 32

Questions

• What does the model predict for:

> Infants <0 to 6 Months assume constant feeding (8 bottles a day)

> Infants 6- 12 months assume constant feeding during12 hours the child is 

awake (four bottles a day)

> Default assumption of a single meal

• How does the model deal with levels of physical activity?

> Resting (sleep)

> Light activity (awake)
| 33

0

0.01

0.02

0.03

0.04

0.05

0 1 2 3 4 5

Percent Inhibition of RBC AChE 

Days 

RBC AChEI over five days 

Birth

Meal once 3 hours apart 

give a smooth response 

curve
| 34

0

0.005

0.01

0.015

0.02

0.025

0.03

0.035

0.04

0.045

0.05

0 1 2 3 4 5

Percent Inhibition of RBC AChE 

Days 

RBC AChEI over five days

Birth

Slight increase in Month 5

response based on agerelated changes in 

physiology
| 35

0

0.005

0.01

0.015

0.02

0.025

0.03

0.035

0.04

0.045

0.05

0 1 2 3 4 5

Percent Inhibition of RBC AChE 

Days 

RBC AChEI over five days

Birth

Month 5

Reduction of the number of Month 6

meals from 8 to 4 results in 

an increase in the peak 

inhibition
| 36

0

0.005

0.01

0.015

0.02

0.025

0.03

0.035

0.04

0.045

0.05

0 1 2 3 4 5

Percent Inhibition of RBC AChE 

Days 

RBC AChEI over five days

Birth

Month 5

Month 6

Little change in Month 12

response predicted 

between 6 months and 

12 months
| 37

0

0.005

0.01

0.015

0.02

0.025

0.03

0.035

0.04

0.045

0.05

0 1 2 3 4 5

Percent Inhibition of RBC AChE 

Days 

RBC AChEI over five days

Resting - Birth

Resting - Six 

Months

Birth (8 bottles/day) and at 6 

month (4 bottles per day) 

constant resting
| 38

0

0.005

0.01

0.015

0.02

0.025

0.03

0.035

0.04

0.045

0.05

0 1 2 3 4 5

Percent Inhibition of RBC AChE 

Days 

RBC AChEI over five days

Active - Birth

Resting - Birth

Resting - Six Months

Active - Six Months

Birth (8 bottles/day) and at 

6 month (4 bottles per day) 

light activity

Resting - Birth

Active - Birth
| 39

0

0.005

0.01

0.015

0.02

0.025

0.03

0.035

0.04

0.045

0.05

0 1 2 3 4 5

Percent Inhibition of RBC AChE 

Days 

RBC AChEI over five days

Birth eight meals

12 Months

Birth (8 bottles/day) and at 12 

month (4 bottles per day) 

constant resting
| 40

0

0.005

0.01

0.015

0.02

0.025

0.03

0.035

0.04

0.045

0.05

0 1 2 3 4 5

Percent Inhibition of RBC AChE 

Days 

RBC AChEI over five days

Modeling default (2011 SAP) – one meal 

a day for birth ( )and 12 months ( )

versus

8 bottles/day at birth ( ) and 4 bottles 

per day at 12 months ( )
| 41

Conclusions

• Age related changes in body weight and physiology have minimum 

impacts for ages < 1 year 

• Dividing a daily dose across multiple meals does have an effect

• The approach of assuming a single meal is conservative –predicts the 

highest peak inhibition 

• The model can consider a range of activity levels

> The impact of physical activity on infants is to slightly lower the estimates of 

peak inhibition

> A conservative assumption would be to assume infants are resting

• Total impacts of meals and activity levels are modest.