Exposure assessment in in vitro bioassays
Cell-based in vitro bioassays have a potential for application in human health risk assessment provided one can quantitatively predict the in vivo effects. This can be accomplished by quantitative in vitro-to-in vivo extrapolation (QIVIVE). The major challenge is that the results from high-throughput screening (HTS) assays are typically reported as nominal effect concentrations, but the cellular dose or as proxy the freely dissolved concentration (Cfree) in the assay medium should be used as dose-metric for QIVIVE. Different sorption and loss processes like volatilization, sorption to medium proteins and lipids, uptake to the cells, cellular metabolism and diffusion into well plate plastic can influence Cfree. Additionally, Cfree is not easily measurable in the small volume of well plates.We are using a combination of in vitro mass balance models and measurements of Cfree to better understand the exposure to chemicals in our in vitro bioassays. Solid-phase microextraction (SPME) a well-established sample preparation technique is used to measure sorption of neutral and ionizable organic chemicals to medium constituents and cells. We also use SPME for the time-resolved and concentration-dependent measurement of Cfree in the cell-based in vitro bioassays. Our goal is to derive freely dissolved effect concentrations of the test chemicals that can be used as input parameters for QIVIVE models.