The globally accepted model for bioaccumulation determination currently is fish (c.f. OECD 305). The basis of bioaccumulation potential (B) screening and assessment, and subsequent testing requests, is driven by the Octanol-Water Partition Coefficient (KOW).
In the recent revamping of REACh Guidance Chapter R11: PBT / vPvB Assessment (latest version at time of writing RfP is Draft to CARACAL on 28/03/2017), ECHA has included B / vB screening threshold criteria for air-breathing organisms linked to logKOW and logKOA (KOW >2 in conjunction with KOA >5). (see the PDF below attached)
Chapter R.11 further states: An efficiently absorbed, non-biotransformed neutral organic substance with a log KOA ≥ 5 in combination with a log KOW ≥ 2 has the potential to biomagnify in terrestrial food chains and air-breathing marine wildlife as well as in humans, while the substances with log KOW < 2 are being quickly eliminated by the urinary excretion, and therefore do not biomagnify even though their KOA is high.
At the same time, it is acknowledged that considerations of absorption efficiency and biotransformation rates1, 2 are also necessary for bioaccumulation assessment.
This additional evaluation approach to include KOA as an evaluation screen, for potential bioaccumulation in air-breathing (terrestrial) organisms, may lead to unnecessary regulation and testing of substances without sufficient scientific justification. At present, there is no tiered testing approach to facilitate the testing and regulation of chemicals with regards to potential bioaccumulation in air-breathing organisms.
In addition, although standardized test guidelines exist to determine the Octanol-Water Partition Coefficient (KOW – OECD 107, 117, 123), no such disposition exists for the experimental determination of the KOA, the establishment of which is, currently, a function of the Octanol-Water Partition Coefficient and Henry’s Law Constant. There is also a need to critically examine the applicability of the KOA/KOW screening criteria set out in the draft update of R.11.
To start to address the lack of a suitable KOA test method and clear subsequent testing strategy for air-breathing organisms, there is a need to consider the following:
- Review the accuracy of the theoretical calculation approach for KOA, and develop a standard and practical Phys-Chem screening test to establish KOA.
- Develop a Tier 1.5* in vitro screening approach to determine metabolic turnover kinetics and adsorption efficiency for air-breathing organisms.
To date, the development and validation of in vitro testing and implementation of recognized test guidelines has been restricted to the field of mammalian toxicity. Whilst mammalian metabolism rate and toxicokinetic data is relevant in the bioaccumulation assessment of air-breathing organisms, it is necessary to also develop in vitro screening approaches adapted to the environmental / ecotoxicological-relevant domain.
Fish liver hepatocytes and S9 cells have been successfully employed to demonstrate metabolic turnover of chemicals under in vitro test conditions and have been used to construct IVIVE (In vitro to In vivo Extrapolation) models 3, 4, 5, 6, 7. Draft OECD test guidelines are currently under development for this approach, which, in time, will serve as a Tier 1.5 evaluation step, in respect of the 3Rs (Reduction, Refinement, Replacement) criteria, before an eventual need to move to a full in vivo study, and remaining within the realms of additional 3Rs (reproducibility / reliability, ecological relevance and regulatory acceptance).
The Octanol-Water partition coefficient (KOW) is not a robust indicator for the prediction of a chemical’s potential to bioaccumulate, where metabolism and elimination are the main driving factors. In addition, current in silico models inadequately simulate metabolism, and, thus, rely heavily on KOW to generate their output.
It is envisaged that a similar approach to the fish liver hepatocyte / S9 cells could also be considered to develop knowledge on the metabolic turnover and adsorption efficiency of ingested chemicals in air-breathing species.
The objectives of the project are:
- Review the accuracy of establishing KOA for diverse chemistries via the theoretical calculation approach 8, including consideration of applicability domain (this will be partly via literature review, but experimental data from (ii) will also be relevant);
- Propose and evaluate a standard and practical Phys-Chem screening test to establish KOA – via a) review of existing state-of-the-science with respect to KOA; b) compare the validity and representativeness of the generator-column method 9, 10, 11, 12, the GC multi-column retention time approach13,14, headspace sampling techniques15, and, any other practical approaches for experimentally measuring KOA which might be applicable and could be taken forward for proposal as an OECD Series 100 Draft test guideline;
- Perform literature search to review and compile existing data on bioaccumulation and toxicokinetics in air-breathing species. This will advise on the relevance of the 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);
- Develop and validate a Tier 1.5* in vitro screening approach, inspired from fish in vitro liver hepatocyte and S9 metabolism / bioaccumulation, and corresponding IVIVE models 3, 4, 5, 6, 7, relevant to air-breathing species. These will consider exposure and metabolism, intrinsic clearance rates and elimination pathways in liver hepatocytes and/or S9 cells of air-breathing species 16.
- Identify and quantify the main factors influencing and affecting the proposed experimental set-up for the Tier 1.5* in vitro screening, and standardize procedures to permit high level of reproducibility (standardization of hepatocyte, S9 isolation, storage, activity; optimization of exposure concentration; effect of addition of co-factors; analytics; etc.).
This CEFIC LRi project intends to address the lack of a tiered testing approach to facilitate the testing and regulation of chemicals with regards to potential bioaccumulation in air-breathing organisms. This will be done by proposing a standard and practical Phys-Chem screening test to establish KOA, and the development and validation of a Tier 1.5* in vitro screening approach.
Cefic LRI project monitors and the research team for this project will discuss the range of chemistries to be covered at the project start, and appropriate substances will be defined based on appropriate property threshold criteria.
Download here the full version of the RfP LRI-ECO41.