Characterization of developmental toxicity and Adverse Outcome Pathways for emerging PFAS - individual compounds and mixtures

There is growing global concern about the health effects of per- and polyfluorinated alkyl substances (PFAS) due to widespread exposure from a variety of sources. Human biomonitoring data from multiple countries have indicated that virtually all humans carry residues of multiple PFAS. Despite the evidence of co-exposure there has been relatively little mixture-based PFAS toxicity research. Studies in experimental animals have indicated that legacy and emerging PFAS induce developmental toxicity from in utero exposure, however the mechanism(s) and key events producing these effects have not been clearly defined. We are developing an AOP network for PFAS from the literature and from our own developmental studies with rats. The goal is to link in vitro Molecular Initiating Events (MIEs) and in vivo Key Events (KEs) to adverse fetal and postnatal effects of individual PFAS and mixtures of PFAS. We have investigated peroxisome proliferator activated receptor (PPAR) alpha and gama activation in vitro molecular initiating events (MIEs) and found many PFAS activate both receptor types with varying potency. Further, we have conducted in vivo studies to identify critical key events (KEs) related to PFAS developmental toxicity using emerging PFAS with documented human exposure but little or no published toxicity data, including hexafluoropropylene oxide-dimer acid (HFPO-DA or GenX), Nafion byproduct 2 (NBP2), and perfluoromethoxyacetic acid (PFMOAA). To date, both GenX and NBP2 reduce neonatal viability, alter maternal/fetal carbohydrate and lipid metabolism, and reduce maternal thyroid hormones, but have disparate effects on birthweight, liver weight, and liver gene expression profiles. Recently we exposed rats to fixed-ratio dilutions of an equipotent mixture of HFPO-DA, NBP2, and PFOS where the top dose (100%) contained each chemical at their respective ED50 for neonatal mortality, followed by dilutions of 33, 10, 3, 1 and 0%. Multiple effects were observed in all dose groups and mixture model analyses indicated that dose addition accurately predicted the mixture ED50 for neonatal mortality; whereas, response addition considerably underpredicted toxicity by 3.8-fold. Results of the study support the additive effects of PFAS co-exposure and a cumulative mixtures-based risk assessment approach. Abstract does not necessarily reflect the views or policy of USEPA.

Impact/Purpose

Per- and poly-fluoroalkyl substance (PFAS) research is an area of critical need due to issues associated with environmental persistence, widespread occurrence, biological half-life, toxicity, and nearly ubiquitous human and environmental exposure. Perfluoroalkyl ether acids (PFEAs) are a sub-class of PFAS and include the compounds hexafluoropropylene oxide dimer acid (GenX) and Nafion byproduct 2 (NBP2). PFEAs are currently used in the production of fluoropolymers following the phase-out of perfluorooctanoic acid (PFOA) and perfluorooctane sulfonic acid (PFOS), presumably due to more favorable fate and toxicity profiles. However, monitoring studies have detected GenX, NBP2, and others in a wide range of matrices including surface water, drinking water, and/or human serum. Little published research is available regarding the potential toxicity of these compounds compared to the legacy PFAS or cumulative effects of exposure to PFAS mixtures. We hypothesized that in utero exposure to GenX would impact the maternal liver during gestation and potentially lead to adverse neonatal effects, similar to other PFAS, and that mixture exposures would produce dose additive effects. The data from this project will be useful to state, federal, and other regulatory agencies in the development of hazard assessments for GenX and NBP2, among other PFAS and combined exposure to multiple PFAS. This study characterizes the toxic effects of oral GenX, NBP2, and a PFAS mixture exposure during pregnancy to both the mother and the offspring (fetal and newborn) using a laboratory rat model. We found multiple, severe effects for GenX and NBP2 that were consistent with other PFAS, such as PFOA and PFOS. Importantly, it appears that NBP2 is only slightly less potent than PFOS and GenX is only slightly less potent than PFOA based on the orally administered dose. Further, combined exposure to multiple PFAS produce dose additive effects. The data presented here will be highly valuable for regulators to make scientifically based decisions on the potential adverse effects PFAS and PFAS mixtures.

Citation

Gray, E. AND J. Conley. Characterization of developmental toxicity and Adverse Outcome Pathways for emerging PFAS - individual compounds and mixtures. SOT Risk Assessment and Mixtures Specialty Sections Joint Webinar Series-Virtual, NA, Virtual, January 13, 2021.