You are here:
In vitro-in vivo extrapolation of hepatic and gastrointestinal biotrasnformation rates of hydrophobic chemicals in rainbow trout
Citation:
Saunders, L., P. Fitzsimmons, J. Nichols, AND F. Gobas. In vitro-in vivo extrapolation of hepatic and gastrointestinal biotrasnformation rates of hydrophobic chemicals in rainbow trout. AQUATIC TOXICOLOGY. Elsevier Science Ltd, New York, NY, 228:1-12, (2020). https://doi.org/10.1016/j.aquatox.2020.105629
Impact/Purpose:
Hepatic in vitro biotransformation assays used to support chemical bioaccumulation assessments for fish do not account for extrahepatic metabolism, potentially resulting in underestimation of true, in vivo levels of metabolic activity toward hydrophobic organic substrates. In this study, we evaluated the metabolic activity of S9 fractions derived from liver and the intestinal epithelium of rainbow trout using two well-known polycyclic aromatic hydrocarbons, pyrene (PYR) and benzo[a]pyrene (BAP), and two organic ultraviolet filters (UVFs), 2-ethylhexyl-4-methoxycinnamate (EHMC) and octocrylene (OCT). For PYR, BAP, and EHMC, measured rates of activity in liver S9 fractions were substantially higher than those in intestinal S9 fractions. For OCT, the reverse was true. To evaluate the potential significance of these findings, an existing in vitro-in vivo extrapolation (IVIVE) model for trout was expanded to consider biotransformation in both hepatic and gastrointestinal tissues. Apparent whole-body biotransformation rates (kmet) for each chemical were generated using the ?‘liver only’ and ?‘liver and GIT’ models. Predictions obtained for PYR, BAP, and EHMC using both models were generally similar and exhibited reasonable agreement with empirically determined values. In contrast, kmet predictions obtained for OCT using the ?‘liver only’ model were substantially lower than those obtained using the ?‘liver and GIT’ model. Predictions obtained using the ?‘liver and GIT’ model also exhibited much better agreement with empirical values. The results of this study suggest that current ?‘liver only’ IVIVE models may underestimate the true capacity of fish to metabolize some chemicals. As such, these findings have important implications for future use of IVIVE methods when performing chemical bioaccumulation assessments with fish.
Description:
Hepatic in vitro biotransformation assays, in combination with in vitro-in vivo extrapolation (IVIVE) and bioaccumulation modeling, can support regulatory bioaccumulation assessments. In most applications, however, these methods ignore the possibility of extrahepatic metabolism. Here we evaluated intestinal biotransformation in rainbow trout using S9 fractions prepared from the upper intestinal (GIT) epithelium. Measured levels of activity determined using standard substrates for phase I and phase II biotransformation enzymes were within 2-fold of activities measured in hepatic S9 fractions. In vitro intrinsic clearance rates for 2-ethylhexyl-4-methoxycinnamate (EHMC; an organic sunscreen agent) and two polycyclic aromatic hydrocarbons (pyrene [PYR] and benzo(a)pyrene [BAP]) were significantly higher in liver S9 fractions than in GIT S9 fractions. For octocrylene (OCT; a second sunscreen agent), however, in vitro intrinsic clearance rates were higher in GIT S9 fractions compared to liver S9 fractions. An existing ?‘liver only’ IVIVE model was expanded to consider biotransformation in both the liver and GIT. Relevant IVIVE scaling factors were developed by morphological, histological, and biochemical evaluation of trout intestines. For chemicals biotransformed at higher rates by hepatic S9 fractions (i.e., BAP, PYR, EHMC), the ?‘liver & GIT’ model yielded whole-body biotransformation rate constants (kMET) that were within 1.2 to 1.5-fold of those estimated using the ?‘liver only’ model. In contrast to these findings, the mean kMET for OCT obtained using the ?‘liver & GIT’ model was 3.3 times higher than the mean kMET derived using the ?‘liver only’ model. This higher predicted kMET exhibited good agreement with empirical kMET estimates (<10% difference). The results of this study suggest that current ?‘liver only’ IVIVE approaches may underestimate in vivo biotransformation rates for chemicals that undergo substantial biotransformation in the GIT.
URLs/Downloads:
DOI: In vitro-in vivo extrapolation of hepatic and gastrointestinal biotrasnformation rates of hydrophobic chemicals in rainbow trout