C18-Coated Solid-Phase Microextraction Fibers for the Quantification of Partitioning of Organic Acids to Proteins, Lipids, and Cells

The effects measured with in vitro cell-based bioassays are typically reported as nominal effect concentrations (C-nom), but the freely dissolved concentration in the exposure medium (C-w) and the total cellular concentration (C-cell) are considered more quantitative dose metrics that allow extrapolation to the whole-organism level. To predict C-w and cell, the partitioning of the test chemicals to medium proteins and lipids and cells has to be known. In this study, we developed a solid-phase microextraction (SPME) method based on C18-coated fibers to quantify the partitioning of diclofenac, 2,4-dichlorophenoxyacetic acid (2,4-D), ibuprofen, naproxen, torasemide, warfarin, and genistein to bovine serum albumin (BSA), phospholipid liposomes, fetal bovine serum (FBS), and cells. For ibuprofen, 2,4-D, naproxen, and warfarin, the partitioning to the SPME fibers was found to be concentration dependent, which had to be considered for the calculation of distribution ratios to biological materials. The sorption isotherms to FBS were nonlinear for diclofenac, 2,4-D, ibuprofen, naproxen, and warfarin. The FBS isotherms could be described by assuming that the total amount of chemical bound to FBS is the sum of the amount specifically bound to the binding sites of albumin and nonspecifically bound to all medium proteins and lipids. The determined cell-water distribution ratios (D-cell/w) differed considerably between four different cell lines (up to 1.83 log-units) and also between different batches of the same cell line (up to 0.48 log-units). The relative importance of protein and lipid content for D-cell/w was evaluated with a mass balance model and different types of cellular proteins and lipids as input parameters. Existing in vitro mass balance models may underestimate C-w because they do not account for saturable protein binding and overestimate C-cell for organic acids, if BSA is used as surrogate for cellular proteins.