Analysis of elimination half-lives in MamTKDB 1.0 related to bioaccumulation: Requirement of repeated administration and blood plasma values underrepresent tissues

When building the novel public mammalian toxicokinetic database (MamTKDB) we collected and included 3927 elimination half-lives (elimt1/2) for 1407 xenobiotics in various species (rat, human, mouse, dog, monkey, rabbit, cattle, pig, sheep, guinea pig, hamster, horse and goat) with specification of compartment (whole body, organ/tissue, cell type, medium) studied. Here we describe and analyse the collected data in MamTKDB 1.0. Most elimt1/2 are for humans and rats and their data differ in some ways: whereas the rat data are mainly for pesticides, the human data are mainly for pharmaceuticals and environmental contaminants. There are also differences in types of compartments studied and in metabolites followed: human elimt1/2 are mainly whole body based (i.e. based on blood plasma or excretion), animal data are additionally for various organs/tissues, cells or media. Contrary to human studies, animal studies regularly administrate radiolabeled (e.g. 14C) substances and distribution of both parent and eventual metabolites are followed, measuring the radioactivity. In rats, substances had been given through single, preconditioning or repeated administration. Single administration studies dominated, but repeated studies generally had longer elimt1/2 than single or preconditioning studies for which elimt1/2 were similar. Repeated administration studies should better ascertain steady state conditions throughout the body, a process involving time-dependent tissue loading, and the data show that for most substances, repeated studies are required to address bioaccumulation potential. About 65% of the substances in MamTKDB 1.0 fulfilled the octanol–water and octanol–air partitioning-based screening criteria (log Kow > 2 and log Koa > 5) for further bioaccumulation assessment and/or testing, and most of the substances with long elimt1/2 in both humans and rats fulfill these criteria. Of note, however, there are also many chemicals with log Kow > 2 with intermediate or short elimt1/2. Per- and polyfluoroalkyl substances (PFAS) stand out in that they often have log Koa < 5. Rats are poor toxicokinetic test models for perfluoroalkyl acids (PFAAs) for which pigs (and possibly mice) elimt1/2 data resemble those of humans better. Perfluorinated carboxylic acids (PFCAs) and perfluorinated sulfonic acids (PFSAs) of similar molecular weight had similar elimt1/2 in the species tested. For polychlorinated biphenyls (PCBs), elimt1/2 increases with the degree of chlorination in humans. In relation to other compartments, blood plasma/serum had among the shortest elimt1/2 in rats and often underrepresent elimt1/2 in tissues. Rat data were divided into 38 compartment (tissue or media) types out of which 20 had sufficient data for correlational tests. In general, there was a strong degree of correlation of rat elimt1/2 in-between most compartments, but there were also exceptions. Surprisingly, the correlation between brain and white fat was relatively weak. Interestingly, several substances or their metabolites bound to haemoglobin in red blood cells. MamTKDB 1.0 allows investigation on how certain chemical characteristics influence elimt1/2 and is a promising database for assessment of bioaccumulation potential

Report of the EPAA-ECVAM Workshop on Validation of Integrated Testing Strategies (ITS)

The use of Integrated Testing Strategies (ITS) enables the combination of diverse types of chemical and toxicological data for hazard identification and characterisation. In November 2008, the European Partnership for Alternative Approaches to Animal Testing (EPAA), together with ECVAM, held a workshop on ‘Overcoming Barriers to Validation of Non-animal Partial Replacement Methods / Integrated Testing Strategies’ in Ispra, Italy to discuss to what extent current ECVAM approaches to validation can be used to evaluate partial-replacement in vitro test methods (i.e. as potential ITS components) and ITS themselves. The main conclusion of these discussions was that formal validation was only considered necessary for regulatory purposes (e.g. replacement of a test guideline) and that current ECVAM approaches to validation should be adapted to accommodate such test methods (Kinsner-Ovaskainen et al, 2009a). With these conclusions in mind, a follow-up EPAA-ECVAM workshop was held in October 2009 to discuss to what extent existing validation principles are applicable to validation of ITS test methods and to develop a draft approach for validation of such test methods and/or overall ITS for regulatory purposes. This report summarises the workshop discussions which started with a review of current validation methodologies and the presentation of two case studies (skin sensitisation and acute toxicity) before covering the definition of ITS components and ITS themselves, their validation and their regulatory acceptance. The following conclusions/recommendations were made: 1) the validation of a partial replacement test method (for application as part of a testing strategy) should be differentiated from the validation of an in vitro test method for application as a stand-alone replacement, especially with regard to its predictive capacity. In the former case, the predictive capacity of the whole testing strategy (rather than of the individual test methods) would be more important, in particular if the individual test methods have a high biological relevance; 2) an ITS allowing for flexible and ad hoc approaches cannot be validated, whereas the validation of a clearly defined ITS would be feasible, although practically quite difficult; 3) test method developers should be encouraged to develop and submit to ECVAM not only full replacement test methods, but also partial replacement methods to be placed in a testing strategy. The added value of formal validation of testing strategies and the requirements needed in view of regulatory acceptance of the data requires further informed discussion within the EPAA forum on the basis of case studies provided by industry.JRC.I.4-Nanobioscience