Prof. Frank Wania
Department of Physical and Environmental Sciences
University of Toronto
Scarborough, 1265 Military Trail, Toronto, Ontario, Canada M1C 1A4
The project plans to develop an approach for assessing bioaccumulation of neutral hydrophobic organic chemicals in air-breathing species based on physical chemical properties (i.e. KOW and KOA) and in-vitro biotransformation bioassay data using mammalian liver S9 in vitro bioassays. This approach follows up on recent work from our research team (Lee et al. 2017, 2012) on in vitro to in vivo extrapolation of biotransformation rates for assessing bioaccumulation of hydrophobic organic chemicals in mammals and may be further supported and augmented through project ECO44, which aims to develop a toxicokinetic framework for bioaccumulation (B) assessment in mammals.
The proposed approach includes a two-tiered approach. In the first tier, KOW and KOA are used to identify substances, which can be eliminated through processes other than biotransformation (i.e. respiration, fecal excretion, urine excretion) at sufficiently high rates to negate biomagnification in mammals. Such chemicals may not need further testing for bioaccumulation in mammals. In a second tier, the biotransformation rate of chemicals in airbreathing organisms is assessed in in vitro systems. Biotransformation (often reasonably well represented by hepatic biotransformation) can increase depuration rates of the chemical to levels that negate biomagnification. Lee et al. (2017) have suggested that an in vitro depletion rate constant in a rat liver S9 homogenate greater than 0.3 d-1 is sufficient to negate biomagnification in rats. If the substance is not biotransformed at a sufficiently high rate in in vitro bioassays, a third tier of testing may be required. This study will only consider the first two tiers and does not intend to use in vivo animal testing. This study will use the rat as a model representing air-breathing species. This follows the approach for bioaccumulation in fish where the rainbow trout is often the preferred test species. The rat is an appropriate test species because of its wide use in drug and chemical testing. Methods for the
standardization of animal strain, the isolation of S9 and hepatocytes from rat livers, as well as proper storage of rat liver S9 are well developed. Furthermore, adequate rat in vivo data are available for testing an in vitro screening approach. In vivo data are less prevalent for other air-breathing species.
The project’s objectives are:
(i) Review the accuracy of establishing KOA for diverse chemistries by theoretical calculation, including consideration of applicability domain,
(ii) Propose and evaluate a standard, practical screening test to establish KOA,
(iii) Perform literature search reviewing and compiling existing data on bioaccumulation and toxicokinetics in air-breathing species. This will advise on the relevance of KOA and in vitro metabolism data for a tiered testing
approach for B assessment. It may also inform as to which substances to test in (iv),
(iv) Develop and test an in vitro screening approach for assessing bioaccumulation of neutral hydrophobic organic chemicals from mammalian liver S9 bioassays which considers exposure and metabolism, intrinsic clearance rates and elimination pathways in S9 assays of air-breathing species,
(v) Identify and quantify the main factors influencing and affecting the proposed experimental set-up for a tiered in vitro screening, and provide guidance for the standardization and reproducibility of the in vitro tests.