Assessment of in vitro human dermal absorption studies on pesticides to determine default values, opportunities for read-across and influence of dilution on absorption

Data collection initiatives of the crop protection industry - A mission to improve non-dietary risk assessment in Europe

Plant protection products (PPPs) undergo rigorous safety assessments. In Europe, non-dietary risk assessments for operators, workers, bystanders and residents are highly conservative as this area of exposure science has historically been data poor. CropLife Europe (CLE) companies have collaborated to generate new data and pool existing data to refine the approaches prescribed by the European Food Safety Authority (EFSA) guidance on non-dietary exposure (2022). This article summarises key activities, beginning with the development of the Agricultural Operator Exposure Model (AOEM) and covers projects which refine current approaches to bystander, resident and re-entry worker assessment, including the Bystander Resident Orchards Vineyards (BROV) project, improvements to the Bystander and Resident Exposure Assessment Model for spray drift (BREAM), proposals for refined vapour inhalation assessments, and a meta-analysis of Dislodgeable Foliar Residue (DFR) data. A study quantifying the benefits of using closed transfer systems, an appraisal of the inherent compounded conservatism in current risk assessment paradigms and the development of a new seed treatment model by the SeedTROPEX taskforce are also introduced. These industry-led activities underscore the critical role of non-dietary exposure in the registration process for PPPs and reflect an ongoing commitment to provide farmers with effective crop protection solutions while ensuring safety.

In Vitro Human Dermal Absorption Studies on Pesticides in Complex Mixtures: Investigation of Guidance Criteria and Possible Impact Parameters

Pesticides must not pose unacceptable risks to human health, so risk assessments are conducted before products are authorised. Dermal exposure is often the main route of intake, so estimating realistic and trustworthy dermal absorption values is crucial for risk assessment. Although there are agreed test guidelines for in vitro dermal absorption studies, not every product is tested due to cost reasons. The present dataset consists of 945 individual in vitro experiments on the dermal absorption of human skin with 179 active substances of pesticides in 353 different mixtures, including concentrates and dilutions. The dataset was evaluated to identify the possible impacts of experimental conditions and physico-chemical properties on dermal absorption. The dataset was also analysed to assess the appropriateness of the pro rata correction for untested dilutions, and the set concentration cut-off to decide on the dilution status for choosing a default value on dermal absorption. The study found that the implementation of specific guidelines improved the harmonisation of study conduct, with support for approaches such as pro rata correction and default values. Further analysis of the specific co-formulants may identify influencing factors that may be more important than the experimental variables.

A proposed NAM-based tiered phototoxicity testing and human risk assessment framework for agrochemicals

Phototoxicity testing is required by European regulations for agrochemicals with UV/visible molar extinction/absorption coefficient (MEC) higher than 10 L x mol^-1 x cm^-1 in the 290-700 nm wavelength range. Furthermore, regulations identify a need of considering human exposure in case of positive results. While in vitro OECD test guidelines are available for hazard characterisation, there is no guidance on how to utilise positive results in human exposure risk assessments. Our goal was to take a first step towards developing a NAM based tiered testing approach and a framework for non-dietary acute human dermal risk assessment for phototoxicity to agrochemicals. The proposed framework can be divided into a few steps: 1) use the OECD updated MEC values of 1000 L x mol^-1 x cm^-1 as trigger for phototoxicity testing; 2) establish a reference concentration (RfC) from in vitro phototoxicity studies using BMC approach, 3) estimate potential exposure to skin, target organ for phototoxicity, using EFSA exposure models, product specific labels and skin penetration values, and 4) phototoxicity risk assessment; 5) refinement to RfC and/or exposure estimates can be considered. Finally, case studies of a nematicide and an herbicide active substance are provided to illustrate the proposed framework.

A new conceptional model for deriving average dermal absorption estimates from studies with multiple tested concentrations for non-dietary risk assessment of pesticides

Dermal absorption values are used to translate external dermal exposure into potential systemic exposure for non-dietary risk assessment of pesticides. While the Environmental Protection Agency of the United States of America (US EPA) derives a common dermal absorption factor for active substances covering all related products, the European Food Safety Authority (EFSA) requests specific product-based estimates for individual concentrations covering the intended use rates. The latter poses challenges, because it disconnects exposure dose from applied dose in absorption studies, which may not be suitable in scenarios where concentration is not relevant. We analyzed the EFSA dermal absorption database, collected 33 human in vitro studies from CropLife Europe (CLE) companies, where ≥3 in-use dilution concentrations were tested, and 15 dermal absorption triple pack datasets. This shows that absolute dermal absorption correlates with absolute applied dose on a decadic logarithm-scale, which is concordant with the toxicological axiom that risk is driven by exposure dose. This method is radically different from the current European approach focused on concentrations and offers new insights into the relationship of internal and external exposure doses when utilizing data from in vitro studies. A single average dermal absorption value can be simply derived from studies with multiple tested concentrations, by calculating the y-intercept of a linear model on a decadic logarithm scale while assuming a slope of 1. This simplifies risk assessment and frees resources to explore exposure refinements. It also serves as a basis to harmonize dermal absorption estimation globally for use in exposure-driven risk assessments.

Compounded conservatism in European re-entry worker risk assessment of pesticides

We review the risk parameters and drivers in the current European Union (EU) worker risk assessment for pesticides, for example considering crop maintenance, crop inspection or harvesting activities, and show that the current approach is very conservative due to multiple worst-case default assumptions. As a case study, we compare generic exposure model estimates with measured worker re-entry exposure values which shows that external cumulative exposure is overpredicted by about 50-fold on average. For this exercise, data from 16 good laboratory practice (GLP)-compliant worker exposure studies in 6 crops were evaluated with a total number of 184 workers. As generic overprediction does not allow efficient risk management or realistic risk communication, we investigate how external exposure can be better predicted within the generic model, and outline options for possible improvements in the current methodology. We show that simply using averages achieves more meaningful exposure estimates, while still being conservative, with an average exposure overprediction of about 9-fold. Overall, EU risk assessment includes several numerically unaccounted "hidden safety factors", which means that workers are well protected; but simultaneously risk assessments are biased towards failing due to compounded conservatism. This should be considered for further global or regional guidance developments and performing more exposure-relevant risk assessment.

Interspecies differences in cytochrome P450-mediated metabolism of neonicotinoids among cats, dogs, rats, and humans

Neonicotinoid insecticides are used for agricultural and non-agricultural purposes worldwide. Pets are directly exposed to neonicotinoids in veterinary products and through environmental contamination. Cytochrome P450 (CYP) is among the most significant xenobiotic metabolizing enzymes that oxidizes several chemicals, including neonicotinoids. However, CYP activities and metabolite compositions of neonicotinoid metabolites are unknown in most domesticated pet species. Our objectives were to reveal the differences in metabolites of neonicotinoids (imidacloprid, clothianidin, and acetamiprid) and CYP activities among common pet species (cats and dogs), humans, and rats. The results indicated that the CYP-mediated neonicotinoid metabolism was different depending on species and each neonicotinoid. Among these four species, the kinetics of imidacloprid metabolism indicated that rats have the highest rate of oxidation of imidacloprid to 4OH-imidacloprid, while the greatest enzyme kinetics of imidacloprid metabolism to 5OH-imidacloprid were found in rats and humans. Clothianidin was rapidly metabolized to 1-methyl-3-nitroguanidine and dm-clothianidin in rats, but cats and humans showed the lowest formation of dm-clothianidin. CYP activities in metabolism of acetamiprid to dm-acetamiprid and N-acetyl-acetamiprid were determined to be significantly higher in humans compared to other species. However, further studies should be targeted at identifying the differences in hepatic metabolism of neonicotinoids in these species using recombinant CYP enzymes.

Dermal absorption study OECD TG 428 mass balance recommendations based on the EFSA database

The European Food Safety Authority (EFSA) guidance (EFSA, 2017) for dermal absorption (DA) studies recommends stringent mass balance (MB) limits of 95-105%. EFSA suggested that test material can be lost after penetration and requires that for chemicals with <5% absorption the non-recovered material must be added to the absorbed dose if MB is <95%. This has huge consequences for low absorption pesticides. Indeed, one third of the MBs in the EFSA DA database are outside the refined criteria. This is also true for DA data generated by Cosmetics Europe (Gregoire et al., 2019), indicating that this criterion is often not achieved even when using highly standardized protocols. While EFSA hypothesizes that modern analytical and pipetting techniques would enable to achieve this criterion, no scientific basis was provided. We describe how protocol procedures impact MB and evaluate the EFSA DA database to demonstrate that MB is subject to random variation. Generic application of "the addition rule" skews the measured data and increases the DA estimate, which results in unnecessary risk assessment failure. In conclusion, "missing material" is just a random negative deviation to the nominal dose. We propose a data-driven MB criterion of 90-110%, fully in line with OECD recommendations.

Exposure of amateur gardeners to pesticides via the non-gloved skin per day

To predict a risk to gardeners not wearing protective gloves, the dermal absorption of three active insecticides was assessed in vitro using porcine ear-skin simulating 1-h handling of diluted plant protection products. Acetamiprid and Pirimicarb were found in the receptor fluid immediately after 1-h skin exposure, whereas Chlorpyrifos-methyl absorbed in the skin was not released into the receptor fluid even after 23 hours. The Estimated Gardener Exposure Level (EGEL) at 23 hours after 1-h exposure for two worst-case scenarios (i) non-gloved hands; (ii) non-gloved hands/uncovered forearms, was (i) 0.002, 0.042, and 0.057; (ii) 0.006, 0.101, and 0.135 mg/kg bw/day for Acetamiprid, Pirimicarb, and Chlorpyrifos-methyl, respectively, although the systemically available Chlorpyrifos-methyl amount, due to retention in the skin, is probably lower than determined. The Gardener Exposure Risk (GER), as a ratio of Acceptable Operator Exposure Level (databased values) to EGEL, for Acetamiprid was (i) 35 and 12-fold higher than the limit 1, so the risk via the skin is assumed to be low. Based on the GER values of (i) 0.83 and 0.18; (ii) 0.34 and 0.07 (i.e.<1) for Pirimicarb and Chlorpyrifos-methyl, respectively there is a level of concern regarding the health risk to gardeners handling pesticide products without skin protection.

Guidance on dermal absorption

This guidance on the assessment of dermal absorption has been developed to assist notifiers, users of test facilities and Member State authorities on critical aspects related to the setting of dermal absorption values to be used in risk assessments of active substances in Plant Protection Products (PPPs). It is based on the 'scientific opinion on the science behind the revision of the guidance document on dermal absorption' issued in 2011 by the EFSA Panel on Plant Protection Products and their Residues (PPR). The guidance refers to the EFSA PPR opinion in many instances. In addition, the first version of this guidance, issued in 2012 by the EFSA PPR Panel, has been revised in 2017 on the basis of new available data on human in vitro dermal absorption for PPPs and wherever clarifications were needed. Basic details of experimental design, available in the respective test guidelines and accompanying guidance for the conduct of studies, have not been addressed but recommendations specific to performing and interpreting dermal absorption studies with PPPs are given. Issues discussed include a brief description of the skin and its properties affecting dermal absorption. To facilitate use of the guidance, flow charts are included. Guidance is also provided, for example, when there are no data on dermal absorption for the product under evaluation. Elements for a tiered approach are presented including use of default values, data on closely related products, in vitro studies with human skin (regarded to provide the best estimate), data from experimental animals (rats) in vitro and in vivo, and the so called 'triple pack' approach. Various elements of study design and reporting that reduce experimental variation and aid consistent interpretation are presented. A proposal for reporting data for assessment reports is also provided. The issue of nanoparticles in PPPs is not addressed. Data from volunteer studies have not been discussed since their use is not allowed in EU for risk assessment of PPPs.

In vitro determination of transdermal permeation of synthetic musks and estimated dermal uptake through usage of personal care products

Synthetic musks, chemical constituents of personal care products, enter the human body through dermal contact. Elucidation of the mechanisms underlying transdermal permeation of synthetic musks should enhance our understanding of their uptake and distribution in human skin and allow accurate evaluation of associated human exposure. Here, the transdermal permeation dynamics and distribution of galaxolide (HHCB) and tonalide (AHTN) were investigated using an in vitro skin diffusion model. The transdermal permeation amounts of HHCB and AHTN increased rapidly during the first 6 h. The applied HHCB and AHTN amounts did not affect percutaneous absorption rates. HHCB and AHTN remained primarily in the stratum corneum, accounting for 70.0% and 70.3% of the totals during the 24-h period, respectively. The percutaneous absorption rate of both chemicals was ∼11%. HHCB, AHTN, musk ketone, musk xylene, and Musk-T were detected in 29 personal care products. The average total concentrations of the musks were 3990, 54.0, 17.7, and 9.8 μg g^-1 in perfume, shampoo, lotion, and shower gel, respectively. Among the four product categories, HHCB was dominant (57.4%-99.6%), followed by AHTN. The data clearly indicate that polycyclic and nitro musks are most commonly used in personal care products. The total estimated dermal intake (51.6 μg kg^-1_bw day^-1) was markedly higher than total dermal uptake (5.9 μg kg^-1_bw day^-1) when percutaneous absorption rates of the chemicals were added into the calculation. Uptake of HHCB and AHTN via dermal contact of personal care products was significantly higher than that from dust inhalation calculated according to earlier literature data.

Assessment of an extended dataset of in vitro human dermal absorption studies on pesticides to determine default values, opportunities for read-across and influence of dilution on absorption

Dermal absorption is a key parameter in non-dietary human safety assessments for agrochemicals. Conservative default values and other criteria in the EFSA guidance have substantially increased generation of product-specific in vitro data and in some cases, in vivo data. Therefore, data from 190 GLP- and OECD guideline-compliant human in vitro dermal absorption studies were published, suggesting EFSA defaults and criteria should be revised (Aggarwal et al., 2014). This follow-up article presents data from an additional 171 studies and also the combined dataset. Collectively, the data provide consistent and compelling evidence for revision of EFSA's guidance. This assessment covers 152 agrochemicals, 19 formulation types and representative ranges of spray concentrations. The analysis used EFSA's worst-case dermal absorption definition (i.e., an entire skin residue, except for surface layers of stratum corneum, is absorbed). It confirmed previously proposed default values of 6% for liquid and 2% for solid concentrates, irrespective of active substance loading, and 30% for all spray dilutions, irrespective of formulation type. For concentrates, absorption from solvent-based formulations provided reliable read-across for other formulation types, as did water-based products for solid concentrates. The combined dataset confirmed that absorption does not increase linearly beyond a 5-fold increase in dilution. Finally, despite using EFSA's worst-case definition for absorption, a rationale for routinely excluding the entire stratum corneum residue, and ideally the entire epidermal residue in in vitro studies, is presented.

Estimation of in vivo dose of dermally applied chemicals leading to estrogen/androgen receptor-mediated toxicity from in vitro data--Illustration with four reproductive toxicants

We present a quantitative in vitro-in vivo extrapolation framework enabling the estimation of the external dermal exposure dose from in vitro experimental data relevant to a toxicity pathway of interest. The framework adapts elements of the biological pathway altering dose (BPAD) method [Judson et al. Chem Res Toxicol 2011;24:451] to the case of dermal exposure. Dermal doses of four toxicants equivalent to concentrations characterizing their effect on estrogen receptor α or androgen receptor activity in chemical-activated luciferase expression (CALUX) assays are estimated. The analysis shows that dermal BPADs, calculated from one in vitro concentration, can differ by up to a factor of 55, due to the impact applied dose and dermal exposure scenarios can have on skin permeation kinetics. These features should therefore be taken into account in risk assessment of dermally applied chemicals.

Evaluation of in vitro vs. in vivo methods for assessment of dermal absorption of organic flame retardants: a review

There is a growing interest to study human dermal exposure to a large number of chemicals, whether in the indoor or outdoor environment. Such studies are essential to predict the systemic exposure to xenobiotic chemicals for risk assessment purposes and to comply with various regulatory guidelines. However, very little is currently known about human dermal exposure to persistent organic pollutants. While recent pharmacokinetic studies have highlighted the importance of dermal contact as a pathway of human exposure to brominated flame retardants, risk assessment studies had to apply assumed values for percutaneous penetration of various flame retardants (FRs) due to complete absence of specific experimental data on their human dermal bioavailability. Therefore, this article discusses the current state-of-knowledge on the significance of dermal contact as a pathway of human exposure to FRs. The available literature on in vivo and in vitro methods for assessment of dermal absorption of FRs in human and laboratory animals is critically reviewed. Finally, a novel approach for studying human dermal absorption of FRs using in vitro three-dimensional (3D) human skin equivalent models is presented and the challenges facing future dermal absorption studies on FRs are highlighted.