Citation:

Nichols, J., P. Fitzsimmons, A. Hoffman, AND K. Wong. In vitro-in vivo extrapolation of hepatic biotransformation data for fish. III. An in-depth case study with pyrene. ENVIRONMENTAL TOXICOLOGY AND CHEMISTRY. Society of Environmental Toxicology and Chemistry, Pensacola, FL, 42(7):1501-1515, (2023). https://doi.org/10.1002/etc.5626

Impact/Purpose:

The purpose of this work was to evaluate the accuracy of in vitro-in vivo metabolism extrapolation procedures.  In vitro rates of biotransformation for a model polycyclic aromatic hydrocarbon, pyrene, were measured using rainbow trout liver S9 fractions.  These measured rates were then extrapolated to the intact animal and used as inputs to an established one-compartment model to predict a whole-body steady-state bioconcentration factor (BCF).  Pyrene accumulation was then measured (as a BCF) using a standardized in vivo exposure protocol to assess the accuracy of modeled predictions.  Bioaccumulation prediction models are widely used by OPPT and the regional offices to perform bioaccumulation assessments for industrial chemicals regulated under TSCA.  Of special concern are methodologically challenging compounds that possess an extreme degree of hydrophobic behavior, as even modest rates of biotransformation may substantially reduce the extent to which such compounds accumulate in fish.  This research supports the goal of using in vitro and in silico methods to perform screening-level chemical assessments in order to increase chemical throughput and reduce current reliance on whole-animal testing methods.

Description:

Computational models that predict chemical bioaccumulation in fish generally account for the effect of biotransformation using an apparent first order whole-body rate constant (kB; d-1). The use of such models requires, therefore, that methods exist for estimating kB, ideally without the need to expose live animals. One promising approach for estimating kB involves the extrapolation of measured in vitro intrinsic clearance (CLIN VITRO,INT) to the whole animal (in vitro-in vivo extrapolation; IVIVE). To date, however, the accuracy of such predictions has been difficult to assess due to uncertainties associated with one or more extrapolation factors and the mismatch that often exists between fish used to generate in vitro data and those used to conduct in vivo exposures. In the present study we employed a combined in vitro and in vivo experimental approach to provide an in-depth evaluation of the IVIVE procedure using pyrene (PYR) as a model chemical. To the extent possible, measured rates of CLIN VITRO,INT were extrapolated to estimates of kB using extrapolation factors based on measured values. In vitro material (liver S9 fraction) was obtained from fish exposed to PYR in a controlled bioconcentration study protocol. Fish from the same study were then used to estimate in vivo kB values from an analysis of chemical depuration data. Averaged across 4 study groups, kB values estimated by IVIVE underestimated those determined from in vivo data by 2.6-fold. This difference corresponds to a 4.1-fold underestimation of true in vivo intrinsic clearance, assuming the liver is the only site of biotransformation. These findings are consistent with previous work performed using mammals and have important implications for the use of IVIVE procedures as part of a weight-of-evidence approach for bioaccumulation assessments with fish.  

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