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SUMMARY (In accordance with 40 CFR part 152, this summary is available for public release after registration)

STUDY TITLE Benchmark Dose Modeling for Cholinesterase In hibition from Exposure to Chlorpyrifos and Chlorpyrifos-Oxon

DATA REQUIREMENTS

None

AUTHOR(S)
R. Reiss., B. Neal, J. Lamb

STUDY COMPLETED ON
June 30, 2010

PERFORMING LABORATORY Exponent Inc. 1800 Diagonal Road, Suite 500 Alexandria, VA 22310

SPONSOR Dow AgroSciences LLC

9330 Zionsville Road Indianapolis, Indiana 46268-1054

LABORATORY STUDY ID

0900956.000 B0T0 06010Dow AgroSciences LLC Page 2

Benchmark Dose Modeling for Cholinesterase In hibition from Exposure to Chlorpyrifos and Chlorpyrifos-Oxon

SUMMARY

This report presents an analysis of cholines terase inhibition data for chlorpyrifos and chlorpyrifos-oxon. Specifically, benchmark dos e (BMD) modeling methodologies are employed to estimate possible points-of-depa rture (PODs) for risk assessment. The focus of the analysis is on the Marty and Andrus (2010) study. This study includes acute dose tests (i.e., single dose) for chlorpyrifos by gavage (pups and adults), milk (pups), and diet (adults). Also, acute test s were made for chlorpyrifos-o xon, the principal metabolite of chlorpyrifos and the proximal toxic agent, by gava ge for pups and adults. Further, repeated dose tests were conducted for pups from PND 11-21 and adults for 11 days for both chlorpyrifos and chlorpyrifos-oxon via gavage. Additionally, literature data were revi ewed for additional information on the cholinesterase inhibition of chlorpyrifos. Da ta were analyzed from the following studies: Betancourt and Carr (2004); Hoberman et al. (1998a,b); Mattsson et al. (1998, 2000); Maurissen et al. (2000); Moser et al. (2006); Timchalk et al. (2006); and Zheng et al. (2000). The cholinesterase inhibition data were f ound to fit well to the EPA recommended dose response models, leading to mostly reliable BMD estimates. This was particularly true for the Marty and Andrus (2010) study where there were as many as five treated groups for many of the tests. Data for both brain and RBC acetylc holinesterase (AChE) were evaluated. The biological basis for cholinesterase inhi bition was also reviewed. It was concluded that brain cholinesterase inhi bition provides the most appropri ate compartment for establishing PODs for risk assessment. This is particularly true for chlorpyrifos because of data that demonstrate that the NOAEL for peripheral nervous system tissue AC hE inhibition is higher than the NOAEL for RBC AChE inhibition, and consiste nt with or higher than th at for brain AChE inhibition (Marable et al., 2007).Dow AgroSciences LLC Page 3

A meta-analysis was performed between the Mart y and Andrus (2010) data and the available literature data, accounting for any reliability issues across the datasets. From this analysis, the recommended PODs based on the BMDL10 (lower limit of the BMD) for brain cholinesterase inhibition are:

• Acute dose to young or adults: 1.7 mg/kg

• Repeat dose to young: 0.46 mg/kg/day

• Repeat dose to adults: 0.55 mg/kg/day

Comparatively, EPA’s current POD for acute exposures is 0.5 mg/kg. However, the 2008 Scientific Advisory Panel (SAP) agreed with a revised EPA analysis that the acute POD should be 0.1 mg/kg (EPA, 2008). The revised analysis for the SAP came with the caveat that the available data were insufficient to adequately characterize the dose-response near the POD. The Marty and Andrus (2010) study provides sufficient da ta at low doses to accurately estimate the BMDs. The POD recommendation in this report is the result of more accurate BMD estimations with the Marty and Andrus (2010) data and the use of brain cholinesterase inhibition instead of red blood cell (RBC) cholinesterase inhibition to esta blish the POD.

References Betancourt, AM; Carr, RL (2004) The effect of chlorpyrifos and chlorpyrifos-oxon on brain cholinesterase, muscarinic recep tor binding, and neurotrophin leve ls in rats following early postnatal exposure. Toxicological Sciences, 77, 63-71. EPA (2008) Science issue paper: chlorpyrifos hazard and dose response characterization. Report to the FIFRA Scientific Advisory Panel, August, 2008. Hoberman, AM (1998a,b) Developmental neurotoxi city study of chlorpyrifos administered orally via gavage to Crl:CD( R)BR VAF/Plus(R) presumed pre gnant rats. Argus Research Laboratories, Inc., Horsham, Pennsylva nia, Laboratory Study No. 304-001, May, MRID 44556901, 44661001. Marable, B.R; Maurissen, J.P.J.; Mattsson, J.L. ; Billington, R. (2007) Differential sensitivity of blood, peripheral, and central cholinesterases in beagle dogs following dietary exposure to chlorpyrifos. Regulatory Toxi cology and Pharmacology, 47, 240-248.Dow AgroSciences LLC Page 4

Marty, MS; Andrus, AK (2010) Comparison of c holinesterase (ChE) inhibition in young adult and preweanling CD rats afte r acute and repeated chlor pyrifos or chlorpyrifos-oxon exposures. The Dow Chemical Company, MRID 48139301. Mattsson, JL; Maurissen, JP; Spencer, PJ; Brzak, KA; Zablotny, CL (1998) Effects of chlorpyrifos administered via gavage to CD rats during gestation and lactation on plasma, erythrocyte, heart and brain chol inesterase and analytical dete rmination of chlorpyrifos and metabolites. Health and Environmental Resear ch Laboratories, The Dow Chemical Co. for Dow AgroSciences, August. MRID 44648101. Mattsson, JL; Maurissen, JP; Nolan, RJ; Brzak, KA (2000) Lack of differential sensitivity to cholinesterase inhibition in fetuses and neonate s compared to dams treated perinatally with chlorpyrifos. Toxicological Sciences, 53, 438-446. Maurissen, JP; Hoberman, AM; Garman, RH; Hanle y, T (2000) Lack of selective developmental neurotoxicity in rat pups with dams treated by gavage with chlor pyrifos. Toxicological Sciences, 57, 250-263. Moser, VC; Simmons, JE; Gennings, C (2006) Neurotoxicological inte ractions of a five pesticide mixture in preweanling rats. T oxicological Sciences, 92, 235-245. Timchalk, C; Poet, TS; Kousba, AA ( 2006) Age-dependent pharmacokinetic and pharmacodynamic response in preweanling rats following oral exposure to the organophosphorus insecticide chlo rpyrifos. Toxicology, 220, 13-25. Zheng, Q.; Olivier, K.; Won, YK ; Pope, CN (2000) Comparative cholinergic neurotoxicity or oral chlorpyrifos exposures in preweanling and adult rats. Toxicolo gical Sciences. 55, 124-
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