Clarification of DoD comment #73 regarding the toxi cokinetic factor for UF A and UF H , if

forestomach hyperplasia is chosen as the critical n oncancer oral effect for benzo[a]pyrene

During the benzo[a]pyrene (BaP) IASD on Aug 4, EPA asked for clarification on DoD’s comment regarding the toxicokinetic factor for UF A and UF H , if the forestomach hyperplasia is chosen as the critical noncancer ora l effect for BaP. Below is more detailed explanation of DoD’s comment and supporting referen ces. DoD Suggested Action, Revision and References #73:

“EPA does not provide sufficient justification for discounting forestomach hyperplasia as the critical effect, and needs to answer the question: how do the reprod uctive and fertility effects better characterize noncancer low dose effects? Please revise the criti cal effect choice and discussion to be consistent w ith EPA guidelines. Without appropriate justification to exclude forestomach hyperplasia, Beland and Culp (1998) would be the principal study and forestomach hyperplasia from this study would be the critical effect for developing the oral RfD for benzo[a]pyre ne. EPA should appropriately qualify its discussio n of this endpoint.

If forestomach hyperplasia were chosen as the criti cal effect, then the toxicokinetic factor for exper imental animal to human and within human variability might not be needed as per EPA’s Acute Exposure Guideline Level methods. EPA authors need to explo re this alternative or more clearly justify the selection of Xu et al. (2010) as the principal stud y and decreases in ovary weight, estrogen, and primordial follicles, and altered estrus cycling as the critical effects.”

Background and reason for comment:

Despite several studies [e.g., De Jong et al. (1999 ), Kroese et al. (2001), Beland and Culp (1998) and Culp et al. (1998)] reporting dose-dependent an d statistically significant increases in the incidence of forestomach hyperplasia, and modeling results identifying this as the most sensitive effect at the lowest dose level, EPA did not select the forestomach effects as the most sensitive measure (p. 208, parag. 1). Instead, EPA selected t he Xu et al. (2010) study as the principal study (see also page 245, line 7, and elsewhere). This la tter study observed biologically and statistically significant decreases in ovary weight , estrogen, and primordial follicles, and altered estrus cycling in treated animals (see p. 208, para g. 1). The rationale for this selection is the statement found on EPA’s draft BaP assessment, pg 2 09, line 5:“Forestomach hyperplasia was not selected as the cr itical effect, even though it was

observed at lower doses compared with other effects , based on the consideration that the reproductive and fertility effects, observed in ani mals and supported by human data, appear to better characterize noncancer low dose ef fects of BaP.”

Further explanation of DoD Comment #73:

DoD believes that the selection of principal study and critical requires additional justification. Given that EPA has selected forestomach hyperplasia as a critical effect for other chemicals, and in this case, this endpoint is the most sensitive n oncancer BaP effect, if EPA feels these that the quantitative analysis of effects observed in the fo restomach is not reasonable, this needs to be appropriately and transparently discussed. We ackn owledge that selection of rat forestomach hyperplasia as the critical endpoint for evaluating human non-cancer effects of BaP is problematic; it may be argued that rodent forestoma ch hyperplasia following gavage

administration of BaP is not relevant to humans and /or that foresomach hyperplasia is not appropriate as a noncancer endpoint (due to potenti ally being a cancer precursor effect). Dose and route extrapolation, tissue concordance, tissue specificity, and mode-of-action (MOA) should all be considered when assessing the relevan ce of rodent forestomach effects on a caseby-case evaluation. This in-depth, BaP-specific, a nalysis may warrant modifications to the uncertainty factors applicable to the point of depa rture (POD).

The emphasis on MOA in the evaluation of dose-respo nse assessment, as typified by EPA’s methods for Reference Dose (RfD) and Reference Conc entration (RfC) (EPA, 2002), further supports the case-by-case consideration of forestom ach hyperplasia as a critical effect. For example, NAS (2001) describes methods for the devel opment of Acute Exposure Guideline Levels (AEGLs), which depend on MOA information to determine the appropriate uncertainty factor for experimental animal to human extrapolati on. Specifically, from Section 2.5.3.2.2:

“If evidence is available indicating that the mecha nism or mode of action, such as directacting irritation or alkylation, is not expected to differ significantly among species, an interspecies UF of 3 is generally used. The rationa le for the selection of a UF should include the following. 1. A description of the mec hanism of action. 2. A discussion ofwhy the mechanism of action is unlikely or likely t o differ. 3. Is bioavailability,

metabolism, detoxification, elimination likely to b e an issue?” (NAS, 2001)

In addition, EPA’s Office of Pesticide Programs is making judgments on uncertainty factors on the basis of MOA understanding, including the use o f irritation data as the critical effect. For example, EPA’s decision on the safe concentration o f chloropicrin after short-term human exposure included the evaluation that the critical effect, trigeminal nerve stimulation resulting in eye irritation, was without toxicokinetic variabili ty. Thus, the uncertainty factor for withinhuman toxicokinetic extrapolation was reduced to 1- fold; no toxicokinetic variation was expected for this irritant effect (EPA, 2008).

DoD has identified two potentially helpful referenc es for evaluation of the MOA and the human relevancy of forestomach hyperplasia; Poet et al. 2 003 and Proctor et al. 2007. Undoubtedly there are others. Please feel free to request addi tional clarification if needed. We appreciate the opportunity to provide comment and to clarify our o fficial comments.

References

National Academy of Sciences (NAS). (2001) Standing operating procedures for developing acute exposure guideline levels for hazardous chemi cals. Subcommittee on Acute Exposure Guideline Levels Committee on Toxicology. Board on Environmental Studies and Toxicology. National Research Council. National Academy Press, Washington, DC. Available at: http://www.epa.gov/opptintr/aegl/pubs/sop.pdf

Poet T.S., Soelberg, J.J., Weitz, K.K., Mast, T.J., Miller, R.A., Thrall, B.D., Corley, R.A. (2003) Mode of action and pharmacokinetic studies of 2-but oxyethanol in the mouse with an emphasis on forestomachdosimetry. Toxicological Sciences 71: 176-189.

Proctor, D.M., Gatto, N.M., Hong, S.J., Allamneni, K.P. (2007) Mode-of-action framework for evaluating the relevance of rodent forestomach tumo rs in cancer assessment. Toxicological Science 98(2):313-326.

U.S. EPA Office of Pesticide Programs Registration Eligibility Decision (RED) for Chloropicrin July 9, 2008. Available at: http://www.epa.gov/opp srrd1/reregistration/REDs/chloropicrinred.pdf