Quantitative in vitro-in vivo extrapolation of biotransformation rates for bioaccumulation assessment: Focus on organic sunscreen agents in rainbow trout

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In vitro-in vivo extrapolation
Intestinal metabolism
Organic ultraviolet filters

An improved understanding of chemical biotransformation has been identified as a key requirement for the bioaccumulation assessment of commercial chemicals. In vitro biotransformation assays, in combination with in vitro-in vivo extrapolation (IVIVE) represents one initiative to generate chemical biotransformation rates for use in bioaccumulation modeling efforts. However, rigorous evaluation of the IVIVE approach requires studies with well-matched animals to ensure in vitro tests adequately predict in vivo biotransformation potential. Therefore, the overarching objective of this thesis was to evaluate factors that may influence the extrapolation of hepatic in vitro biotransformation rate constants (kdep) using well-matched studies with rainbow trout. Hydrophobic organic ultraviolet filters (UVFs) 4-methylbenzylidene camphor, 2-ethylhexyl-4-methoxycinnamate (EHMC), and octocrylene (OCT) represented model chemicals in this investigation. The first study showed that measured kdep values for UVFs were highly dependent on the selected assay concentration. Modeled bioconcentration factors (BCF) derived from kdep measured at concentrations well below corresponding Michaelis-Menten constants (KM) were closer to empirical BCFs than those calculated from kdep measured at higher test concentrations. A corresponding in vivo study demonstrated that during standardized dietary exposures that measured UVF concentrations in trout were well below the previously derived KM values This demonstrated that biotransformation pathways in trout operate under first-order conditions and that working at an appropriate concentration range in in vitro assays (i.e., C0 << KM) can be expected to improve estimates of in vivo biotransformation potential. In a final study, an existing IVIVE model was expanded to consider biotransformation in both the intestinal epithelia and liver. For chemicals biotransformed at higher rates by hepatic S9 fractions (e.g., EHMC), the ‘liver only’ IVIVE model was sufficient in estimating whole-body biotransformation rate constants (kMET). For chemicals biotransformed at higher rates in intestinal S9 fractions (i.e., OCT), the inclusion of both hepatic and intestinal activities improved estimates of kMET relative to the in vivo data generated here. The results of this study indicate that current ‘liver only’ IVIVE approaches may underestimate kMET for chemicals that undergo substantial intestinal biotransformation. The presented findings suggest that the future use of quantitative IVIVE methods for bioaccumulation assessment require greater consideration of extrahepatic biotransformation.

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This thesis may be printed or downloaded for non-commercial research and scholarly purposes. Copyright remains with the author.
Frank Gobas
Science: Biological Sciences Department
Thesis type: 
(Thesis) Ph.D.