OFFICE OF CHEMICAL SAFETY AND POLLUTION PREVENTION UNITED STATES ENVIRONMENTAL PROTECTION AGENCY WASHINGTON, D.C. 20460

RE : Meeting between Dow AgroScience s (DAS) and EPA to Discuss DAS’ Enhancements to the ir Physio logically Based Pharmacokinetic Model (PBPK) List of Documents 1. Meeting Summary Page 1 2. Meeting Sign in Sheet Page 2 3. DAS PowerPoint Presentation s Page 3 Meet ing 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 o f 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| 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 3 rd 

### 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 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 • 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 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 time course • validated with 1984 Nolan human volunteer data| 12 

### Development of Chlorpyrifos PBPK/PD model

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| 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| 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 2010 CCA study rat ChE data 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 l ife - 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 m anuscript (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 p harmacodynamics 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 

### Planne d publicati on: April 201 3

• Foc us on or al ro ute of exposu re • B uild on E P A P OD in dr aft HH A • Us e Variation model to as se ss infants and older age gr oups • Ch ar ac ter ize re sp ons e from ges tat ional exposu re s > Us e ma tern a l b lo o d le ve ls a n d emp iri ca l fin d in g tha t feta l b lo o d le ve ls a re simi la r t o ma tern al bl oo d for a gi ven ora l ex po sure to de mon strate fetal ex po sure s are the same or lo w er tha n le vel s in in fan ts > An ima l evi de nce tha t fe tal tissu es in ne on ate s (eq ui val en t to la te t rime ster hu man s) are n o more sen sitive tha n a du lt tissue s. > Co ncl usi on risks to f etu s are simil ar to t ho se o f in fan ts an d ad ul ts| 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 Month 5 

### Slight increase in 

### response based on age related 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 Month 6 Reduction 

### of the number of 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 Month 12 

### Little change in 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 

### Con clusions

• Age re lated ch ange s in body we ight and physiolog y have minimum impacts for ages < 1 y ear • Dividing a daily do se ac ro ss multi ple meals does have an eff ec t • The appr oac h of as su ming a sin gle meal is co ns er vat iv e – pr edic ts the highest peak inhibit ion • The model ca n co ns ider a rang e of ac tiv ity lev els > T he imp act of phy sica l activi ty on in fan ts is to slig htly lo w er the estima tes of pe ak in hi bi tion > A con serva tive assu mptio n w ould be t o a ssume infa nts are restin g • Total impac ts of meals and ac tiv ity levels are modes t.