Final Report Dated May 26, 2011

April 13-14, 2011 EPA Human Studies Review Board Meeting Report

Appendix 1 

Reconsideration of Two Concerns Previously Raised by the HSRB in its J une 2009 Review 

of a Pre-Rule Intentional Human Dosing Study Involving Chlorpyrifos (Kisicki et al. 1999)

In addition to the other topics considered at the April 2011 meeting, the Board considered 

additional information regarding the Kisicki et al. (1999) study on chlorpyrifos, originally 

reviewed by the HSRB in June 2009 (EPA HSRB 2009). Additional information was supplied 

by Dow AgroSciences about two concerns raised by the HSRB at the June 2009 meeting, but this 

information was not supplied to the HSRB until after the report of that meeting had been 

finalized.

In its June 2009 report, the Board raised four concerns about the Kisicki et al. (1999) 

study. Two of these concerns were addressed in the additional information provided by Dow 

AgroSciences. 

The first of these concerns was about the substantially lower level of oral absorption of 

chlorpyrifos calculated in the Kisicki et al. (1999) study as compared with the level of absorption 

calculated in an earlier study conducted Nolan et al. (1982). The Kisicki et al. study reported 

35% absorption as compared with the 70% absorption reported by Nolan et al. for the same 

dosing level (0.5 mg/kg). The Board was skeptical as to whether the reason presented for the 

differences between the level of oral absorption between the two studies, namely that the gelatin 

capsule used in the Kisicki et al. study would have taken longer to dissolve and would therefore 

have resulted in lower absorption, were valid. 

The second of these concerns was the lack of documentation that urine samples were 

subjected to acid hydrolysis. Acid hydrolysis would be necessary to liberate the chlorpyrifos 

metabolite trichloropyridinol (TCP) from any conjugates in the sample. Had acid hydrolysis not 

been performed, the subsequent quantitation of TCP in the urine samples would have 

underestimated the levels of TCP. In its June 2009 report (EPA HSRB 2009) the Board 

concluded that, had the hydrolysis step not been performed, the lower oral absorption reported in 

Kisicki et al. (1999) might have been the result of inaccurate measurement of urinary TCP 

levels.

The information provided to the Agency by Dow AgroSciences addressed both of these 

concerns. This supplementary information indicated that the urine was indeed subjected to acid 

hydrolysis using methods that had been previously shown to free TCP from any conjugates. The 

information provided by Dow AgroSciences also indicated that the chlorpyrifos used in the 

Kisicki et al. (1999) study was placed in a gelatin capsule in a crystalline form, with the 

remainder of the capsule filled with lactose. By contrast, in the Nolan et al. (1982) study the 

chlorpyrifos was dissolved in methylene chloride and applied as a solution to the lactose tablets. 

The particle size of the chlorpyrifos used in the Kisicki et al. study thus was larger than that used 

in the Nolan et al. study. It is possible that these larger particles could have been absorbed more 

slowly than the smaller particles. 
Final Report Dated May 26, 2011

After considering this addition information, the Board concluded that its original 

recommendations about the Kisicki et al. (1999) study should be amended as follows:

1. It is logical that larger particles of a material such as chlorpyrifos would be absorbed 

more slowly than smaller particles. The differences in absorption between the Nolan 

et al. (1982) and Kisicki et al. (1999) studies may have resulted, at least in part, from 

the different sizes of chlorpyrifos particles in the two formulations.

2. The quantitation of urinary TCP was accurate because the urine was subjected to acid 

hydrolysis and heat to liberate conjugated TCP.

The Board also concluded, however, that two other issues discussed by the HSRB at the June 

2009 meeting remain unaddressed. These issues still raise concern about the reliability and utility 

of the blood and urine measurements of chlorpyrifos and/or TCP from Kisicki et al. (1999) for 

risk assessment purposes. 

REFERENCES

EPA Human Studies Review Board. 2009. June 24-25, 2009 Human Studies Review Board 

Meeting Report.

Kisicki, J, C. Seip, and M. Combs. A Rising Dose Toxicological Study to Determine the NoObservable-Effect-Levels (NOEL) For Erythrocyte Acetylcholinesterase (AChE) Inhibition and 

Cholinergic Signs and Symptoms of Chlorpyrifos at Three Dose Levels. Dated April 15-19, 

1999. Unpublished study prepared by MDS Harris Laboratories under Project No. 21438 and 

Dow AgroSciences Study No. DR K-0044793-284. MRID 44811002.

Nolan, R.J., D.L. Rick, R.L. Freshour, et al. 1982. Chlorpyrifos: Pharmacokinetics in Human 

Volunteers Following Single Oral and Dermal Doses. Dated August 1982. Unpublished study 

prepared by the Dow Chemical Company under Protocol HEB-DR-0043-4946-4. MRID 124144.