by Mel Andersen
Comparing bioactivity signatures of compounds can augment classical read across approaches.
The Need for Balanced Regulatory Strategies for PFAS Compounds: Perfluorooctanoic and perfluorooctane sulfonic acid (PFOA and PFOS), two well-studied compounds with a wide variety of commercial uses, have well established toxicity in animals and are persistent in the environment, recently receiving labels as “forever chemicals”. With advances in analytical chemistry these compounds are found throughout the environment and in most human biomonitoring studies, raising calls for stricter measures to reduce their use and human exposures. Companies have invested in developing chemistries that use compounds like these PFAS compounds that have lower toxicity and lower persistence, including use of shorter chain PFAS or inclusion of ether linkages, trifluoromethyl residues or non-fluorinated carbon atoms. The goal of evaluating alternative chemistries is preservation of utility in important commercial products while significantly reducing toxicity and risks. At the same time, however, there is increasing public pressure to lump all PFAS-like chemicals into a single category and move either to highly restrictive exposure standards for all PFAS, irrespective of differences in toxic potency or persistence, or to banning the compounds from commerce. US EPA now has a listing of some 176 PFAS chemicals of concern in their toxic release inventory (TRI).
Past experience in regulating classes of similar compounds: Experience with polyhalogenated aromatics, including polychlorinated dioxins, furans and dioxin-like PCBs, provides some guidance for developing ranking profiles for compounds based on factors related to biological potency and relative toxicity. Optimally, ranking for PFAS compounds should include key factors related to modes of action (MOAs), pharmacokinetics, and persistence in the environment. With polychlorinated dioxin-like compounds, these approaches produced toxic equivalency factors (TEFs) based on potency for interacting with a key protein, the Aryl Hydrocarbon Receptor (AhR). The application of these TEFs to mixtures of compounds was based on dose additivity of effects caused through a common mode of action (doi: 10.1093/toxsci/kfl10550). Optimally, the use of TEFs for a class of compounds needs to consider MOAs, biological half-life and stability in the environment. The polyhalogenated aromatics TEFs were primarily restricted to potency for induction through AhR.
Possibilities for TEF-like Approaches with PFAS: With the prototypical PFAS compounds, PFOA and PFOS, the primary biological activity is related to activation of pathways of fatty acid metabolism, and, at higher doses, they cause liver toxicity, rodent liver cancer and other forms of toxicity. Studies using new approach methodologies, including toxicogenomics, a platform discussed in a recent ScitoVation blog
, promise to more rapidly screen and prioritize various PFAS compounds based on potency in short-term test in cells or in multi-cellular, 3-dimensional tissue constructs, such as those described in a ScitoVation Webinar by Drs. Steve Ferguson and Ella Atlas. Coupling dose-response studies and toxicogenomics would provide both measures of potency and insights into MOAs.
The second hurdle in a TEF-like approach is estimating differences in persistence in the body and in the environment. The long half-life of these compounds in humans is associated with resorption from the urinary filtrate back into the kidney cells and then from the renal cells into blood. Half-lives of PFOA and PFOS in humans are considerably longer than in rodent species. In vitro measures of protein binding and renal transporter activity toward PFAS could assist in estimating persistence in humans and allow in vitro-in vivo extrapolation of dose response information. While tools for estimating whole body clearance based on kidney resorption have not been widely used with PFAS, short-term in vivo approaches may be useful for establishing similarities of MOAs and assessing relative half-lives. Short duration in vivo studies with 1 to 2 weeks dosing in rats, could provide information on toxicogenomic responses in sentinel organs, such as liver and kidney, and measure blood levels over time to estimate half-lives. Gwinn and colleagues showed the value of these short-term studies by evaluating responses following 5-day exposures of rats to various chemicals including PFOA. These protocols did not include determination of chemicals in blood or plasma. Comparing 1 or 2-day blood levels with those at end of exposure could provide estimates of relative half-lives and comparisons with PFOS and PFOA.
Going Forward: New higher throughput alternative methods (NAMs) and shorter-term in life studies promise to provide better information for ranking likely risks of PFAS compounds and avoiding lumping a wide variety of compounds into a single, higher risk category. The tools for evaluating MOA and dose response with toxicogenomics are available and, with further development of short-term in life measures of pharmacokinetics and refined assays for kidney transporter activity, they could support a TEF-like ranking for PFAS.