In chemical risk assessment, default uncertainty factors are used to account for interspecies and interindividual differences, and differences in toxicokinetics and toxicodynamics herein. However, these default factors come with little scientific support. Therefore, our aim was to develop an in vitro method, using acetylcholinesterase (AChE) inhibition as a proof of principle, to assess both interspecies and interindividual differences in toxicodynamics. Electric eel enzyme and human blood of 20 different donors (12 men/8 women) were exposed to eight different compounds (chlorpyrifos, chlorpyrifos-oxon, phosmet, phosmet-oxon, diazinon, diazinon-oxon, pirimicarb, rivastigmine) and inhibition of AChE was measured using the Ellman method. The organophosphate parent compounds, chlorpyrifos, phosmet and diazinon, did not show inhibition of AChE. All other compounds showed concentration-dependent inhibition of AChE, with IC50s in human blood ranging from 0.2–29 µM and IC20s ranging from 0.1–18 µM, indicating that AChE is inhibited at concentrations relevant to the in vivo human situation. The oxon analogues were more potent inhibitors of electric eel AChE compared to human AChE. The opposite was true for carbamates, pointing towards interspecies differences for AChE inhibition. Human interindividual variability was low and ranged from 5–25%, depending on the concentration. This study provides a reliable in vitro method for assessing human variability in AChE toxicodynamics. The data suggest that the default uncertainty factor of ~ 3.16 may overestimate human variability for this toxicity endpoint, implying that specific toxicodynamic-related adjustment factors can support quantitative in vitro to in vivo extrapolations that link kinetic and dynamic data to improve chemical risk assessment.

This external scientific report summarises the results from the article 36 grant GA/EFSA/SCER/2015/01 "Modelling human variability in toxicokinetic and toxicodynamic processes using Bayesian meta-analysis, physiologically-based (PB) modelling and in vitro systems". Extensive literature searches, data collection and modelling of human variability in toxicokinetics (TK) (phase I, Phase II enzymes and transporters) and toxicodynamics (TD) are summarised and further elaborated in supplementary material and EFSA knowledge junction, open source databases (MS Excel) and peer reviewed publications (objective 1 and 2). In addition, in vitro TK and TD information from laboratory studies and literature searches are summarised for a range of case studies relevant to EFSA including pesticides (i.e. triflumuron, chlorpyrifos, phosmet), natural toxins (e.g. microcystin variants, mycotoxins), food additives and polyphenols (i.e. resveratrol, tyrosol), food additives as well as drugs (i.e. amiodarone). These include isoform-specific metabolism and kinetic parameters for single chemicals and inhibition constants for multiple chemicals (TK) and identification of molecular targets (TD). Finally, generic quantitative in vitro in vivo extrapolation (QIVIVE) models, PB-kinetic (PBK) and PBK-dynamic (PBKD) models were developed in the R freeware, calibrated and validated using case studies for single and multiple chemicals. Supplementary material and model codes are available on EFSA knowledge junction. The results are in line with EFSA priorities for the implementation of the use of new approach methodologies (NAMs) for human risk assessment (RA) of chemicals and the need to develop open source TK TD databases data and PB-K and PB-KD models have been identified as key steps of this process. Future perspectives are discussed including the integration of pathway-related variability and generic human QIVIVE and PB-K models and PB-K-D models into TKplate, a Toxicokinetic Modelling Platform under development in EFSA.

UDP-glucuronosyltransferases (UGTs) are involved in phase II conjugation reactions of xenobiotics and differences in their isoform activities result in interindividual kinetic differences of UGT probe substrates. Here, extensive literature searches were performed to identify probe substrates (14) for various UGT isoforms (UGT1A1, UGT1A3, UGT1A4, UGT1A6, UGT1A9, UGT2B7 and UGT2B15) and frequencies of human polymorphisms. Chemical-specific pharmacokinetic data were collected in a database to quantify interindividual differences in markers of acute (Cmax) and chronic (area under the curve, clearance) exposure. Using this database, UGT-related uncertainty factors were derived and compared to the default factor (i.e. 3.16) allowing for interindividual differences in kinetics. Overall, results show that pharmacokinetic data are predominantly available for Caucasian populations and scarce for other populations of different geographical ancestry. Furthermore, the relationships between UGT polymorphisms and pharmacokinetic parameters are rarely addressed in the included studies. The data show that UGT-related uncertainty factors were mostly below the default toxicokinetic uncertainty factor of 3.16, with the exception of five probe substrates (1-OH-midazolam, ezetimibe, raltegravir, SN38 and trifluoperazine), with three of these substrates being metabolised by the polymorphic isoform 1A1. Data gaps and future work to integrate UGT-related variability distributions with in vitro data to develop quantitative in vitro–in vivo extrapolations in chemical risk assessment are discussed.