Structure-related differences in plasma protein binding of per- and polyfluoroalkyl substances in mice and humans

Understanding interspecies and structure-based differences in the toxicokinetics of perfluoroalkyl substances (PFAS) is important to explain their persistence and improve health risk assessment. Since renal clearance is the main elimination pathway, this study measured the free (protein-unbound) fraction (f_u) of PFAS in human and mouse plasma using the Dianorm^R system, selected for its robustness and rapid equilibrium. Sixteen PFAS were tested, including perfluoroalkyl carboxylic acids (PFCA), sulfonic acids (PFSA), and ether derivatives (PFECA). Mean f_u values ranged from 0.21 % (PFOS) to 50 % (PFPeA) in mice and from 0.02 % (PFHpS) to 8.5 % (PFPeA) in humans. Interspecies differences were most pronounced for short-chain PFAS and PFECA, but not observed for longer chains. A U-shaped relationship between f_u and molecular weight (MW) was found, with the lowest values near 400-500 g/mol. These results highlight plasma protein binding as a key determinant of PFAS persistence and provide predictive models linking f_u to MW.