Plasma Protein Binding Evaluations of Per- and Polyfluoroalkyl Substances for Category-Based Toxicokinetic Assessment

New approach methodologies (NAMs) that make use of in
vitro screening
and in silico approaches to inform chemical evaluations rely on in
vitro toxicokinetic (TK) data to translate in vitro bioactive concentrations
to exposure metrics reflective of administered dose. With 1364 per-
and polyfluoroalkyl substances (PFAS) identified as of interest under
Section 8 of the U.S. Toxic Substances Control Act (TSCA) and concern
over the lack of knowledge regarding environmental persistence, human
health, and ecological effects, the utility of NAMs to understand
potential toxicities and toxicokinetics across these data-poor compounds
is being evaluated. To address the TK data deficiency, 71 PFAS selected
to span a wide range of functional groups and physico-chemical properties
were evaluated for in vitro human plasma protein binding (PPB) by
ultracentrifugation with liquid chromatography-mass spectrometry analysis.
For the 67 PFAS successfully evaluated by ultracentrifugation, fraction
unbound in plasma (fup) ranged from less
than 0.0001 (pentadecafluorooctanoyl chloride) to 0.7302 (tetrafluorosuccinic
acid), with over half of the PFAS showing PPB exceeding 99.5% (fup < 0.005). Category-based evaluations revealed
that perfluoroalkanoyl chlorides and perfluorinated carboxylates (PFCAs)
with 6–10 carbons were the highest bound, with similar median
values for alkyl, ether, and polyether PFCAs. Interestingly, binding
was lower for the PFCAs with a carbon chain length of ≥11.
Lower binding also was noted for fluorotelomer carboxylic acids when
compared to their carbon-equivalent perfluoroalkyl acids. Comparisons
of the fup value derived using two PPB
methods, ultracentrifugation or rapid equilibrium dialysis (RED),
revealed RED failure for a subset of PFAS of high mass and/or predicted
octanol–water partition coefficients exceeding 4 due to failure
to achieve equilibrium. Bayesian modeling was used to provide uncertainty
bounds around fup point estimates for
incorporation into TK modeling. This PFAS PPB evaluation and grouping
exercise across 67 structures greatly expand our current knowledge
and will aid in PFAS NAM development.