To what extent should an understanding of a Mode of Action (or Adverse Outcome Pathway or Toxicity Pathway) be developed to support reliance on in vitro findings related to toxic effects? Do you agree that information regarding a shared mode of action or converging adverse outcome pathway is the “gold standard” for informing grouping purposes?

Answer Explanations

• Yes, shared mode of action/adverse outcome pathway should be considered the gold standard for grouping

  Expert 1

  In a world where information on the molecular initiating event is known for all chemicals of interest, using AOPs is arguably the best way to map out the potential interactions and group chemicals based on shared molecular initiating event, key event or outcome. That being said, other than some information on PPARa activation for a handful of PFAS, it does not seem possible to use this approach for the current PFAS mixture issue.

• Expert 7

  Do not believe that is feasible.
  I think there is no basis for such approach and would cause more confusion and discussion than resolve a problem.

• No, shared mode of action/adverse outcome pathway should not be considered the gold standard for grouping

  Expert 4

  While shared mode of action is important, and could be useful for grouping, exposure potential is at least as important, and remains so regardless of whether there are shared modes of action or not. In the short term, exposure information will be easier to generate than mode of action information.

• Yes, shared mode of action/adverse outcome pathway should be considered the gold standard for grouping

  Expert 2

  If we are to develop a PFAS mixture based risk assessment, then the mode of action is crucial to support that approach. Assumption of additivity in describing toxicity of chemical mixture needs to be based on a common mode of action.

• Yes, shared mode of action/adverse outcome pathway should be considered the gold standard for grouping

  Expert 11

  Two caveats. First, it is converging AOP networks not AOPs. Second, assuming groupings are intended to be used as Cumulative Assessment Groups (CAG’s) the following points are relevant. CAGs should ideally be based on having a common molecular initiating event (MIE). Thus, being on the same AOP network may not be sufficient to show that they belong to the same CAG. AOPs are, however, are still works in progress. As a result, often it will be unclear if chemicals causing a common effect operate by a common MEI. In these cases, the assessor may want to conservatively assume that affecting a common AOP network is a basis for a subgroup. This can be equivalent to a finding that the chemicals have the same apical effect.

• Yes, shared mode of action/adverse outcome pathway should be considered the gold standard for grouping

  Expert 10

  Yes, it is often claimed to be the "gold standard" in mixture risk assessment (Goodrum et al., 2021), but it can be justified to group PFAS in the absence of evidence of a common mode of action in many jurisdictions (US EPA, EU, etc.). In the EU, this can be justified based on the precautionary principle while waiting for further research to be conducted.

  Goodrum, P.E.; Anderson, J.K.; Luz, A.L.; Ansell, G.K. (2021) Application of a Framework for Grouping and Mixtures Toxicity Assessment of PFAS: A Closer Examination of Dose-Additivity Approaches. Tox. Sci., 179, 2, 262–278.

• Yes, shared mode of action/adverse outcome pathway should be considered the gold standard for grouping

  Expert 8

  Yes, this is the gold standard. However, the MOA must include a quantitative understanding between the key events and the adverse outcome. Simply having in vitro findings without knowledge of the quantitative link of that specific in vitro result with other key events is insufficient. In addition to the MOA, an understanding of the potential exposure would also be useful.

• Yes, shared mode of action/adverse outcome pathway should be considered the gold standard for grouping

  Expert 3

  Shared MOA / AOP is certainly the preferred approach for grouping in most published literature. If the assessors will be using PFAS data available as of October 2021, they will be will be developing rationales and justification for silver, bronze, and steel standards as well. On a philosophical note, the recent lessons from examination of the two-year rodent bioassay for cancer is that gold standards change. See, for example, Cohen, S.M., Boobis, A.R., Dellarco, V.L., Doe, J.E., Fenner-Crisp, P.A., Moretto, A., Pastoor, T.P., Schoeny, R.S., Seed, J.G., Wolf, D.C., Chemical carcinogenicity
  revisited 3: Risk assessment of carcinogenic potential based on the current state of knowledge of carcinogenesis in humans, Regulatory Toxicology and Pharmacology 103: 100-105, (2019) doi: https://doi.org/10.1016/j.yrtph.2019.01.017

• Yes, shared mode of action/adverse outcome pathway should be considered the gold standard for grouping

  Expert 5

  Understanding a MOA, an AOP or Toxicity Pathway would certainly add confidence to decisions based on interpretations of in vitro, in silico or NAMs data. However, such should ne be a requirement, should not be used to exclude such data from consideration. There should be a defined (known, shown, described) relationship between the measured in vitro/silico event and the health outcome of interest. Whether in vivo the corresponding (same) event characterized in vitro/silico represents a biomarker of effect (e.g., enzyme inhibition) or a biomarker of exposure (e.g., hemoglobin adduct) should be discussed and carefully considered during the interpretation phase.

• Yes, shared mode of action/adverse outcome pathway should be considered the gold standard for grouping

  Expert 6

  The first option is consistent with the points I have made throughout this debate. MOA/AOP information is crucial to understanding how to group PFAS for risk assessment purposes. Whether or not we will ever get sufficient MOA/AOP information from NAMs or conventional toxicological analysis across the spectrum of PFAS is a practical matter, but it does not preclude ranking this as the "gold standard".

• No, shared mode of action/adverse outcome pathway should not be considered the gold standard for grouping

  Expert 9

  It is likely that individual PFAS have different modes of action/adverse outcome pathways across tissues so the challenge is then, which mode/pathway and/or tissue is most relevant? Is it the one that occurs at the lowest dose? The one most closely aligned with a well-described human mode/pathway? The one most likely to affect developing organisms? What if an individual PFAS falls into multiple groups? This means that another approach for grouping would be needed for mode/pathways.