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

RE : Meeting between Dow AgroScience s (DAS) and EPA to Discuss DAS’ Possible Enhancements to the ir Physio logically Based Pharmacokinetic Model (PBPK) and Their Views on the Exposure and Potency of the Chlorpyrifos Oxon List of Documents 1. Meeting Summary Page 1 2. Meeting Sign in Sheet Page 2 3. DAS PowerPoint Presentations a. Using PBPK Models to Improve the Chlorpyrifos Risk Assessment Page 3 b. Chlorpyrifos -Oxon Potency: Discussion of Toxicity Adjustment Factors Used for Risk Assessment Page 32 c. Chlorpyrifos -Oxon Expo sure in Drinking Water from Surface Water Sources Page 46 Meeting Summary On Wednesday, July 18 , 2012, EPA’s Office of Pesticide Programs met with DAS to discuss their PBPK model and their views on the exposure and potency of the chlorpyrifos oxon . DAS prov ided further detail and timeline for anticipated enhancements to the PBPK model . DAS will discuss the progress of any enhancements with EPA as the work progresses. DAS also discussed their views related to the exposure and potency of the chlorpyrifos oxon in contrast to what EPA presented in the Chlorpyrifos Preliminary Human Health Risk Assessment released for public comment in July 2011. The assessment is available at http://www.regulations.gov/#!documentDetail;D=EPA - HQ - OPP -2008-0850-0025 .Name

Vt c**k~ ~-t**  krco**M~~~~ 'l4t~rt~~**  EPA and Registrant Meeting Chlorpyrifos and PBPK July 18, 2012 Affiliation Phone Number 3/7-337-'J?lL E-mail

# Using PBPK Models to Improve the Chlorpyrifos 

# Risk Assessment Paul Price Michael Bartels 

### July 18, 2012

### • PBPK Applications – Paul Price • PBPK Research – Michael Bartels • Oxon – Hazard – Mike Bartels • Oxon – Exposure – Nick Poletika 

## • Further Discussion - All 

## Speakers

## Proposed Activities • Extend models

– To other routes

– To oxon

– To infants and in utero exposures

– To address lactational pathway • Develop chemical specific uncertainty factors for:

– Oral, inhalation, dermal routes

– Chlorpyrifos and oxon • Use models to explore biomonitoring data in the Columbia study (to be discussed at August meeting)

## Why address Inter-species and Inter-individual 

# uncertainty factors? • Refine approaches for remaining existing and emerging 

## risk assessments

– MOEs for bystanders (CPF and oxon?)

– MOEs for drinking water (oxon)

– Finalize aggregate assessment

## Relationship between proposed modeling and 

# key issues Development of PBPK Models Compound Specific Uncertainty Factors Current Question? Inhalation Route Dermal

Route Extension to earlier ages Oxon model Oral CPF Oral Oxon Dermal CPF Inhalation

CPF Bystander Assessments X X X X X Drinking water Assessment X X Epidemiology Findings X X X

## Developing CPF-specific uncertainty factors

• Address SAP issues on developing CPF-specific uncertainty factors • Develop a 2D oral model (separate variability and uncertainty) • Follow the approach presented in DAS report prepared for the SAP

– Use 2D oral model to construct the human equivalent of the animal POD

– Use 2D oral model to determine the dose that protects a large fraction of the population at a specific level of confidence from an RBC AChEI of 10%

– Ratios of the doses provide the basis for interindividual and interspecies uncertainty factors

## Proposed Timeline for PBPK model 

# Enhancements

Timeline dependent on approval for use of Kisicki human volunteer study data

## Details of PBPK model Enhancements

• Oral Intake of Oxon

– Model basis: CPF Life-Stage PBPK/PD

– Initial Oxon PK parameters will be estimated from CCA study TK data

– Sensitivity analysis will be conducted on these parameters to evaluate need for further Oxon kinetic studies

– Model parameters will be optimized to fit to CCA study Oxon TK data

– Simulations will be conducted for Oxon bioavailability and ChE effects from Drinking Water exposure in rats & humans

## Details of PBPK model Enhancements

• Dermal Uptake of CPF

– Model basis: CPF Life-Stage PBPK/PD

– Dermal uptake will be based on Surface Area x Formulation Concentration x Kp value, as per 2002 Timchalk model

– Dermal penetration flux values from in vitro study of Griffin (2000) and in vivo volunteer studies of Griffin (1999) and Nolan (1984)

– Model parameters will be optimized to fit to Griffin (1999) and Nolan (1984) datasets

– Model will be used to simulate occupational/residential exposure via dermal route

## Details of PBPK model Enhancements

• Inhalation Exposure to CPF

– Model basis: CPF Life-Stage PBPK/PD

– model modifications will include incorporation of CPF and Oxon metabolism in lung tissue • minimum: average metabolism rates from pooled donor tissues • full variation evaluation: individual metabolism rates from ~20 donor tissues • in silico only: de novo evaluation of lung metabolism via computational assessment of CYP isoform contributions

– evaluation of contribution and variation in CYP-based metabolism in brain tissue will also be conducted same minimum/full/in silico options, as defined for lung metabolism

– validate rat version of model with 2010 Hotchkiss Inhalation toxicity study- TK data

## Details of PBPK model Enhancements

• Late-stage Gestation / In Utero Exposure to CPF

– Model basis: CPF Life-Stage PBPK/PD and Lowe 2009 end-stage PBPK/PD model • will include growth of fetus and maternal changes through 3 rd trimester • model will be validated against available mother/newborn datasets

– Model will be used to simulate in utero and maternal exposures from various routes (oral, dermal, possibly inhalation)

## References

• Griffin, P., H. Mason, K. Heywood, and J. Cocker. 1999. Oral and dermal absorption of chlorpyrifos: a human volunteer study. Occupational and Environmental Medicine. 56:10-13. • Griffin, P., M. Payne, H. Mason, E. Freedlander, A.D. Curran, and J. Cocker. 2000. The in vitro percutaneous penetration of chlorpyrifos. Hum Exp Toxicol. 19:104-107. • Nolan, R.J., D.L. Rick, N.L. Freshour, and J.H. Saunders. 1984. Chlorpyrifos: pharmacokinetics in human volunteers. Toxicology and Applied Pharmacology. 73:8-15. • Timchalk, C., R.J. Nolan, A.L. Mendrala, D.A. Dittenber, K.A. Brzak, and J.L. Mattsson. 2002. A Physiologically based pharmacokinetic and pharmacodynamic (PBPK/PD) model for the organophosphate insecticide chlorpyrifos in rats and humans. Toxicological Sciences: An Official Journal of the Society of Toxicology. 66:34-53.

Page 12

## BREAK HERE MOVE TO BARTELS SLIDES • COME BACK AFTER CAROL’s Presentation

Page 13

## Understanding the Columbia Studies
• The findings raise questions:

– They suggest effects at levels 3 orders of magnitude lower than those from animal models and lower than levels shown to have effects in in vitro testing.



## – The biomonitoring data are only a random snapshot of exposures and can not inform prior exposures

### Blood levels associated developmental effects are 1000 fold lower than animal models, 100 fold lower than ChE 

## inhibition and 5-1000 fold lower than in vitro non-ChE effects.

0.1 1 10 100 1000 10000 100000 1000000 10000000 100000000

Maternal levels in the

Columbia study Conc. in rats (1 - 5 mg/kg) 10% RBC AChEI Conc. adults and infants Range of in vitro conc.Concentration in Blood pg/g 

# Columbia is an Outlier

## Key question posed in May meeting • Do data on chlorpyrifos suggest Columbia cohort mothers were exposed to levels that could have caused 

## significant inhibition of RBC or brain ACHE? Considering:

– Data published by the authors

– Data published in earlier studies of exposures from residential uses

– Collected by Dow as part of the registration process

## What do we know?

• Biomonitoring date indicates majority of Columbia participants did not have body burdens that differed from the general public’s levels from dietary exposures • The max blood level was 15-fold higher then the upper bound level predicted for the dietary exposures general population (35 vs. 2 ng/l) • Dow workers receiving similar doses did not have RBC AChEI • Modeling suggest that sensitive adults require blood concentrations that are in the range of 10,000 to 300,000 ng/l to cause RBC AChEI

### Blood Conc. of Chlorpyrifos vs. RBC and Plasma AChEI

1.00E - 02 1.00E - 01 1.00E+00 1.00E+01 1.00E+02

1 10 100 1000 10000 100000 1000000 10000000Maximum ChEI % of control

Maximum Blood Concentration ng/l Blood levels associated with 10 - 100% RBC AChEI Range of blood levels reported in Wyatt, 2004 for years < 2002

## Estimates of exposures from residential uses

• Generally, historical estimates are combination of modeled 

### risk assessment assumptions mingled with measured observations.

– Estimates of aggregate doses for adults in treated homes at the time of treatment are less than 0.5 ug/kg/d. (TAS, 1996)

– Estimates of doses from high (worst case) residential dermal and inhalation exposures was found to 17-37 ug/kg (DAS, 1993) 

### • These doses (if administered orally) would have produced 

### blood levels in the 10-1000 ng/l range • These levels do not affect RBC AChE

## Approaches to Columbia

• Option 1:

– Use the new findings of Huen et al. to test the hypothesis that chlorpyrifos blood levels in the range of 1-35 ng/l in blood cause developmental effects

– Huen et al. found higher chlorpyrifos blood levels than Whyatt

– Explore if Huen found a correlation to developmental effects

## Responding to Columbia
• Option 2 Obtain data from Columbia

– Use a PBPK model to predict blood concentrations at time of arrival to hospital • Use data in regression model (test hypothesis that 

### duration of time between admittance and delivery was a confounder in analysis)



### – Review regression model and determine if alternative 

## analyses are justified and if they produce different results.

## PBPK Capabilities Needed
• PBPK model of maternal and fetal blood levels for early and late stage pregnancy 

## • Oral, dermal, and inhalation routes of exposure • Ability to track individuals for periods of 270 days

## Questions

## EXTRA slides

### Means of the highest 

## and lowest quartiles of 1999-2000 cord blood range from 0.5 to 13 

## pg/g over all mean of 4 pg/g 

## Columbia Data

0.1 1 10 100 1000 10000 100000 1000000 10000000 100000000

Cord levels in the Columbia studyConcentration in Blood pg/g

### Frank developmental and behavioral effects in rats are associated with dose of 1- 5 mg/kg/day Blood levels in dams and neonates (PND 1) given an oral doss of 1-5 mg/kg range from 350,000 to 1,800,000 pg/g (Mattsson et al., 2000) 

# Rat blood levels associated with behavioral and developmental effects

0.1 1 10 100 1000 10000 100000 1000000 10000000 100000000

Cord levels in the Columbia

study Conc. in rats (1 - 5 mg/kg)Concentration in Blood pg/g

### PBPK model (Timchalk 2002; Hinderliter et al., 2011; DAS, 2010) predicted blood levels associated with 10% RBC inhibition. Age-specific metabolism 

## and physiology in 5,000 adults and 5,000 infants (6 months) 

## Blood levels that cause a 10% inhibition of RBC 

# AChE

0.1 1 10 100 1000 10000 100000 1000000 10000000 100000000

Cord levels in the

Columbia study Conc. in rats (1 - 5

mg/kg) 10% RBC AChEI

Conc. adults and infantsConcentration in Blood pg/g

### In vitro concentrations causing initiating effects other than ChE inhibition. 

## Blood levels necessary to cause in vitro effects

0.1 1 10 100 1000 10000 100000 1000000 10000000 100000000

Cord levels in the Columbia

study Conc. in rats (1 - 5 mg/kg) 10% RBC AChEI Conc. adults and infants Range of in vitro conc.Concentration in Blood pg/g

### • Need to address:



### – Comments from the SAP on the models

– Other ages (late stage pregnancy and neonates)



### – The lactation pathway • Extend the model to oxon

– Addresses water issues • Dermal and inhalation

– Worker/bystanders safety factors



### – Epidemiology 

## Development of compound-specific uncertainty 

## factors

## Chlorpyrifos-Oxon Potency: Discussion 

## of Toxicity Adjustment Factors used for

Risk Assessment Michael J. Bartels and Daland R. Juberg July 18, 2012

### Chlorpyrifos-Oxon Potency

• Overview: • Review underst anding of pharmacokinet ic differences bet ween chlorpyrifos (CPF) and Oxon in mammalian species • Scientific rationale for utilizing the target organ of brain for biological effects arising 

### from exposure to Oxon in drinking water • Excellent data for this approach from recent CCA study (Marty, 2012) 

### • Recommendations

### Chlorpyrifos-Oxon Potency

GI Tract CPF

Portal vein

- CPF stable in blood - Majority of absorbed CPF reaches the liver Oxon - > TCP Systemic circulation - 99% TCP * - 1% CPF

- 0.01% Oxon Liver - high capacity for CPF metabolism and Oxon hydrolysis - inhibition of AChE & BuChE occurs from metabolic Oxon

TCP, CPF , Oxon 

### CPF is known t o undergo first pass met abolism via t he oral rout e

CPF - > TCP CPF - > Oxon

* Timchalk, 2002

### Chlorpyrifos-Oxon Potency

GI Tract Oxon

Portal vein

- Blood flow transit time 0.35-1.7 min * - oxon half-life in blood = 10 sec - little or no oxon reaches the liver - however, substantial AChE and BuChE inhibition occurs

Oxon - > TCP Systemic circulation - No detectable Oxon in CCA study ** Liver - extremely high capacity for Oxon hydrolysis TCP TCP 

### In cont rast , presyst emic binding of Oxon 

### result s in complet e det oxificat ion of t his compound

* Munk 2003, ** Marty, 2012

### Chlorpyrifos-Oxon Potency 

## 2010 CCA study: • Direct administration of Chlorpyrifos or Oxon to neonatal 

## and adult rats • Measurements of cholinesterase (ChE) inhibition in plasma, RBC and brain tissue • Additional blood measurements 

## of Chlorpyrifos and Oxon

### Chlorpyrifos-Oxon Potency

Conclusions: • Oxon is significantly less toxic to peripheral tissues than CPF • This is due to extremely rapid, and complete presystemic hydrolysis of Oxon by ChE and other esterases

### Chlorpyrifos-Oxon Potency

• Summary 

### • Oral exposure to Oxon results in substantial inhibition of RBC cholinesterase activity. • However, no inhibition in brain ChE occurs 

### at dose levels up to 10 mg/kg bw , • Also, no Oxon t est mat erial was found in syst emic TK blood samples • These data show complete presystemic 

### hydrolysis of this chemical in the rat. • This efficient hydrolysis may be due, in part, to slow transit time for absorbed Oxon 

### through blood capillaries, coupled with the extremely short half-life for this compound in blood

### Chlorpyrifos-Oxon Potency



## • CPF- Effects via oral administration

### Chlorpyrifos-Oxon Potency



## • Oxon- Effects via oral administration

### Chlorpyrifos-Oxon Potency

• Conclusions 

### • Profile of Oxon effect s subst ant ially different t han t hose arising from CPF • Orally absorbed Oxon causes no effects in 

### peripheral tissues, due to lack of systemic bioavailability of this chemical up to dose levels of 10 mg/kg • Effects on RBC inhibition are due to presystemic 

### exposure only, and are not as appropriate for Risk Assessment as Brain (peripheral tissue) • As a result, no increased Toxicity Adjustment factor should be applied to Oxon 

### • POD > 10 mg/kg for acute- adult rat

### Chlorpyrifos-Oxon Potency

• Proposal for Supplemental Work • To better understand exposure and biological effects from Oxon in drinking water: • PBPK model for CPF would be modified to incorporate oral exposures to Oxon • Additional PK studies might be conducted to refine Oxon kinetic parameters • Simulations would be conducted to evaluate possible systemic exposures and biological effects in RBC and the target organ of brain, from worst-case intake of Oxon in drinking water (0.1-13 μg/kg/day) • Note- these Drinking Water levels are 1000x lower than the relevant animal POD

### Chlorpyrifos-Oxon Potency

• References • Marty, M.S., A.K. Andrus, M.P. Bell, J.K. Passage, A.W. Perala, K.A. Brzak, M.J. Bartels, M.J. Beck, and D.R. Juberg. 2012. Cholinesterase inhibition and toxicokinetics in immature and adult rats after acute or repeated exposures to chlorpyrifos or chlorpyrifos-oxon. Regul Toxicol Pharmacol. 63:209- 224.
• Munk, O. L., Keiding, S. and Bass, L. (2003). Impulse-response function of splanchnic circulation with model-independent constraints: theory and experimental validation. American Journal of Physiology. Gastrointestinal and Liver Physiology 285, G671- 680.
• Timchalk, C., R.J. Nolan, A.L. Mendrala, D.A. Dittenber, K.A. Brzak, and J.L. Mattsson. 2002. A Physiologically based pharmacokinetic and pharmacodynamic (PBPK/PD) model for the organophosphate insecticide chlorpyrifos in rats and humans. Toxicological Sciences: An Official Journal of the Society of Toxicology. 66:34-53 %U http://www.ncbi.nlm.nih.gov/pubmed/11861971.

### Chlorpyrifos-Oxon Potency

### Chlorpyrifos-Oxon Potency

Summary of Chlorpyrifos (CPF) and Chlorpyrifos-Oxon Cholinesterase (ChE) Inhibition in Adult Female CD Rats After Single Gavage Exposure in Corn Oil (Marty, 2012)

# Chlorpyrifos-Oxon Exposure in 

# Drinking Water from Surface Water

Sources Nick Poletika and Cheryl Cleveland

18 Jul 2012

1

# Current Acute Dietary Risk • The 2011 CPF dietary risk for food is highly refined by exposure information from PDP with risk estimates

– 9% aPAD for children 

## • The 2011 CPF dietary risk for water is 

## unrefined using modeled numbers for drinking water estimates

– Infants range from 210 to 2700% of aPAD for oxon
2

# Current Chronic Dietary Risk • The 2011 CPF dietary risk for food is highly refined by exposure information from PDP with risk estimates

– 11% cPAD for children (with FHF) 

## • The 2011 CPF dietary risk for water is 

## unrefined using modeled numbers for drinking water estimates

– Infants range from 69 to 270% of cPAD for oxon
3

# Dilemma • Unrefined modeled DW numbers vs . highly refined monitoring Food numbers • Lines of evidence from water modeling and monitoring

– Currently • Maximum predicted concentration range is 10.6 to 112 μg/L • Maximum reported concentration is 0.5 μg/L from all monitoring datasets reviewed

– How can the DW contribution be refined? • New modeling? • Interpretation of existing monitoring data? • HED policies for actual drinking water relative to modeled surface water? • Another approach?
4

# DAS View on Contribution of Water to Dietary Exposure • DAS continues to recommend use of the maximum reported chlorpyrifos concentration of 0.5 μg/L from all monitoring datasets reviewed as 

## the appropriate highest input level for acute risk assessment. • This value can be attributed to parent or to the oxon, depending on assumptions regarding oxidative conversion through disinfection 

## treatment and fate during distribution.
5

## EPA Responses to DAS Comments on 30 Jun 2011 DW Memo? • Interested in OPP views, particularly in three areas

– Crops and use patterns for modeling



### – Use of oxon fate studies (hydrolysis + others in DCI)

– Weight of monitoring data as a line of evidence 

## • We have specific questions and more information in back-up slides, if helpful for 

## discussion
6

# Questions • What is the mechanism for EPA to respond to positions stated in the DAS Oct 2011 report? • Agreement that soybean apple, alfalfa, citrus and wheat are representative crops? 

### • Agreement with details of current use patterns for model inputs? • Intended use of oxon fate studies? 

### • Maximum concentration from Heidelberg University NCWQR data set corrected? • Utility of 2001 targeted Syngenta OP DW monitoring 

### study?
7

# Questions • Evaluation of survey statistics analysis of NAWQA/NASQAN monitoring data set? • View of complete PDP DW data set as an indicator? • Utility of all lines of evidence to refine DW 

## exposure estimates?

8