Human risk assessment of dermal and inhalation exposures to chemicals assessed by route-to-route extrapolation: The necessity of kinetic data

Human urinary excretion kinetics of the antimycotic climbazole: Biomonitoring of two new metabolites after oral and dermal dosage

Climbazole is an antimycotic compound used in cosmetic products as a preservative or as an active ingredient in anti-dandruff (AD) formulations. In this study we provide human toxicokinetic data on climbazole. Using our previously published analytical method, we investigated the urinary excretion of two climbazole metabolites, (OH)2-climbazole and cx-OH-climbazole, for 48 h after oral ingestion (n = 5, 49-77 µg/kg bw) and for 72 h after dermal application of either a climbazole-containing rinse-off AD shampoo or a leave-on hair tonic (n = 2×3). In total, 23.9 % (18.0-33.4 %) of the oral dose were excreted as the two abovementioned metabolites over 48 h. In one volunteer, who used an over-the-counter phytopharmaceutical, metabolite excretion was about three times lower and we found influences on diastereoselectivity of (OH)2-climbazole formation using a modified analytical method. After dermal application, urinary concentration maxima occurred considerably later than after oral intake. The two different dermal exposure scenarios also revealed a relevance of exposure duration and product formulation on the systemic availability of climbazole. Back-calculated oral-dose-equivalent intakes from the dermal exposures showed a maximum climbazole intake of 18.5 µg/kg bw/d after hair tonic use, or 6.6 µg/kg bw/d after AD shampoo application.

Accessible methods and tools to estimate chemical exposure in humans to support risk assessment: A systematic scoping review

Exposure assessment is a crucial component of environmental health research, providing essential information on the potential risks associated with various chemicals. A systematic scoping review was conducted to acquire an overview of accessible human exposure assessment methods and computational tools to support and ultimately improve risk assessment. The systematic scoping review was performed in Sysrev, a web platform that introduces machine learning techniques into the review process aiming for increased accuracy and efficiency. Included publications were restricted to a publication date after the year 2000, where exposure methods were properly described. Exposure assessments methods were found to be used for a broad range of environmental chemicals including pesticides, metals, persistent chemicals, volatile organic compounds, and other chemical classes. Our results show that after the year 2000, for all the types of exposure routes, probabilistic analysis, and computational methods to calculate human exposure have increased. Sixty-three mathematical models and toolboxes were identified that have been developed in Europe, North America, and globally. However, only twelve occur frequently and their usefulness were associated with exposure route, chemical classes and input parameters used to estimate exposure. The outcome of the combined associations can function as a basis and/or guide for decision making for the selection of most appropriate method and tool to be used for environmental chemical human exposure assessments in Ontology-driven and artificial intelligence-based repeated dose toxicity testing of chemicals for next generation risk assessment (ONTOX) project and elsewhere. Finally, the choice of input parameters used in each mathematical model and toolbox shown by our analysis can contribute to the harmonization process of the exposure models and tools increasing the prospect for comparison between studies and consistency in the regulatory process in the future.

Considerations for Applying Route-to-Route Extrapolation to Assess the Safety of Oral Exposure to Substances

The safety evaluation of oral exposure to substances, such as food ingredients, additives, and their constituents, relies primarily on a careful evaluation and analysis of data from oral toxicity studies. When relevant oral toxicity studies are unavailable or may have significant data gaps that make them inadequate for use in safety evaluations, data from non-oral toxicity studies in animals, such as studies on inhalation, dermal exposure, etc., might be used in support of or in place of oral toxicity studies through route-to-route (R-t-R) extrapolation. R-t-R extrapolation is applied on a case-by-case basis as it requires attention to and comparison of substance-specific toxicokinetic (TK) and toxicodynamic (TD) data for oral and non-oral exposure routes. This article provides a commentary on the utility of R-t-R extrapolation to assess the safety of oral exposure to substances, with an emphasis on the relevance of TK and systemic toxicity data.

Comparative mammalian hazards of neonicotinoid insecticides among exposure durations

Neonicotinoid insecticides have become one of the most widely used insecticides over the past two decades. Recent studies have shown considerable risk of neonicotinoids to beneficial insects, however, their health risks to mammals are still under debate. Limited empirical mammalian toxicity information for neonicotinoids inherently presents challenges to environmental health practitioners performing health hazard and risk assessment. Therefore, we first compiled and examined publicly available hazard data for neonicotinoids, and knowledge gaps on mammals were identified. Probabilistic hazard assessment using chemical toxicity distributions (CTDs) was subsequently conducted, and initial thresholds of toxicological concern were derived for rat, dog, mouse, and rabbit under comparative experimental scenarios. Using the rat model, for example, oral 5% threshold concentrations (TC5s) of 0.11 (0.02, 0.36) and 0.23 (0.001, 3.2) mg/kg bw/day were estimated using chronic developmental and reproductive no observed adverse effect levels (NOAELs), respectively, while acute TC5 of 0.71 (0.25, 1.6) mg/kg bw/day was identified using neurological NOAELs. Comparatively, dermal and inhalational TC5s were estimated as 1583 (1172, 1777) and 451 (294, 615) mg/kg bw/day (equivalent to 486 (322, 622) mg/m³), respectively, using acute median lethal doses. Uncertainty factors (UFs) were also estimated using both CTD comparisons and individual UF probability distribution approaches to test whether rodent oral toxicity information or default 10-fold UF approach can provide sufficient protection for mammals. These initially identified UFs were generally smaller than default values (e.g., 10) employed by regulatory stakeholders, yet larger UFs were occasionally noted. Our findings appear particularly useful for environmental health practitioners when conducting screening-level risk assessment for neonicotinoids, and provide an example for health hazard assessment of pesticides with limited toxicity information.

Kinetic time courses of lambda-cyhalothrin metabolites after dermal application of Matador EC 120 in volunteers

A controlled kinetic study was conducted in volunteers dermally exposed to the widely used lambda-cyhalothrin pyrethroid pesticide to document the time courses of relevant biomarkers of exposure, in order to better assess biomonitoring data in workers. Matador^® EC120 formulation (120 g/l) was applied on 40 cm² of the forearm at a 0.25 mg/kg dose of lambda-cyhalothrin and left without occlusion or washing for 6 h. The application site was then washed thoroughly with soap and water. The kinetic time courses of cis-3-(2-chloro-3,3,3-trifluoroprop-1-en-1-yl)-2,2-dimethylcyclopropane carboxylic acid (CFMP) and 3-phenoxybenzoic acid (3-PBA) metabolites were determined in plasma and urine up to 84 h post-application. Results show that the fraction of lambda-cyhalothrin absorbed in the body was rapidly cleared following dermal contact. According to CFMP and 3-PBA plasma profiles, calculated mean apparent absorption half-lives (t_1/2) were 3 and 7.3 h, respectively, and corresponding mean apparent elimination t_1/2 were 11.2 and 7.6 h. These differences suggest some metabolism at the site-of-entry and storage of metabolites by the dermal route. Toxicokinetic parameters calculated from urinary profiles confirm the values of absorption and elimination rates. Metabolites were almost completely excreted over the 84-h period post-application and, on average, 0.12 and 0.08% of the applied lambda-cyhalothrin dose was recovered in the urine as CFMP and 3-PBA, respectively, indicating a low dermal absorption fraction of this pyrethroid. This study showed the potential use of CFMP and 3-PBA biomarkers for the assessment of dermal exposure to lambda-cyhalothrin pyrethroid.

Fate and exposure modeling in regulatory chemical evaluation: new directions from retrospection

The development and application of fate and exposure modeling has undergone fundamental changes over the last 20 years. This has, in part, been driven by different needs within the regulatory community to address chemicals of concern using different approaches. Here we present a retrospective look at fate and exposure model application over the last two decades keeping an international regulatory perspective and using the Government of Canada's Chemicals Management Plan to illustrate concepts. We discuss the important role fate and exposure modeling has played to help address key data gaps when evaluating the risk of chemicals for both human health and ecological reasons. Yet limitations for more widespread model application within a regulatory context remain. Consequently, we identify specific data gaps and regulatory needs with an eye towards new directions for 21^st century chemical evaluation. We suggest that one factor limiting greater model application is the need for increased awareness and agreement of what chemical exposure assessment encompasses within the risk assessment paradigm. This is of particular importance today because of the increased availability of computational and high-throughput data and methods for chemical assessment allowing evaluators to potentially examine exposure from site of release to site of toxic action, thus linking exposure with toxicology. We further suggest there is a need for discussion at a global level to promote the awareness of new tools and approaches available for fate and exposure modeling and suggest that this could be organized using the aggregate exposure pathways concept.

Applicability of surface sampling and calculation of surface limits for pharmaceutical drug substances for occupational health purposes

Within the context of Occupational Hygiene (OH), surface sampling has been employed as a method to assess surface levels of Active Pharmaceutical Ingredients (APIs). There are potentially a number of reasons surface samples are collected including assessing potential health risks, housekeeping and cleaning effectiveness. There are no internationally accepted standards relating to collecting or interpreting surface samples for OH purposes. In the past, surface sampling results have been applied not only for estimating risks due to dermal contact, but also for other routes of exposure (e.g. inhalation, ingestion, etc). In this publication, we provide a decision tree to support the decision and value of performing surface sampling. For scenarios without conceivable skin exposure due to applied risk mitigation measures or for substances that do not penetrate the skin, surface sampling may not be needed. If the workers' health is determined to be at risk for systemic effects via skin, we propose to use the skin Permitted Daily Exposure (PDE_skin), a safe skin dose independent of the exposure scenario that takes into consideration skin absorption properties of substances. For the purpose of OH monitoring, the likelihood of dermal exposure has to be understood before taking any samples, using both the PDE_skin to calculate the surface limit and appropriate validated monitoring method for the surface.

A test strategy for the assessment of additive attributed toxicity of tobacco products

The new EU Tobacco Product Directive (TPD) prohibits tobacco products containing additives that are toxic in unburnt form or that increase overall toxicity of the product. This paper proposes a strategy to assess additive attributed toxicity in the context of the TPD. Literature was searched on toxicity testing strategies for regulatory purposes from tobacco industry and governmental institutes. Although mainly traditional in vivo testing strategies have been applied to assess toxicity of unburnt additives and increases in overall toxicity of tobacco products due to additives, in vitro tests combined with toxicogenomics and validated using biomarkers of exposure and disease are most promising in this respect. As such, tests are needed that are sensitive enough to assess additive attributed toxicity above the overall toxicity of tobacco products, which can associate assay outcomes to human risk and exposure. In conclusion, new, sensitive in vitro assays are needed to conclude whether comparable testing allows for assessment of small changes in overall toxicity attributed to additives. A more pragmatic approach for implementation on a short-term is mandated lowering of toxic emission components. Combined with risk assessment, this approach allows assessment of effectiveness of harm reduction strategies, including banning or reducing of additives.

Barbecue Fumes: An Overlooked Source of Health Hazards in Outdoor Settings?

Barbecuing or charcoal-grilling has become part of popular outdoor recreational activities nowadays; however, potential human health hazards through outdoor exposure to barbecue fumes have yet to be adequately quantified. To fill this knowledge gap, atmospheric size-fractioned particle and gaseous samples were collected near an outdoor barbecuing vendor stall (along with charcoal-grilled food items) in Xinjiang of Northwest China with a 10-stage micro-orifice uniform deposit impactor and a polyurethane foam (PUF) sampler and were analyzed for particulate matter and polycyclic aromatic hydrocarbons (PAHs). Exposure to PAHs through inhalation and dermal contact by adult consumers who spent 1 h per day near a charcoal-grilling vendor for a normal meal (lunch or dinner) amounted to a BaP equivalent (BaPeq) dosage of 3.0-77 ng day(-1) (inhalation: 2.8-27 ng day(-1) of BaPeq; dermal contact: 0.2-50 ng day(-1) of BaPeq), comparable to those (22-220 ng day(-1) of BaPeq) from consumer exposure through the consumption of charcoal-grilled meat, assumed to be at the upper limit of 50-150 g. In addition, the potential health risk was in the range of 3.1 × 10(-10) to 1.4 × 10(-4) for people of different age groups with inhalation and dermal contact exposure to PAHs once a day, with a 95% confidence interval (7.2 × 10(-9) to 1.2 × 10(-5)) comparable to the lower limit of the potential cancer risk range (1 × 10(-6) to 1 × 10(-4)). Sensitivity analyses indicated that the area of dermal contact with gaseous contaminants is a critical parameter for risk assessment. These results indicated that outdoor exposure to barbecue fumes (particularly dermal contact) may have become a significant but largely neglected source of health hazards to the general population and should be well-recognized.

Implementation of toxicokinetics in toxicity studies--Toxicokinetics of 4-methylanisole and its metabolites in juvenile and adult rats

The current risk assessment of compounds is generally based on external exposure and effect relationships. External doses are often not representative for internal exposure concentrations. The aim of this study was to show how the implementation of toxicokinetics in a scheduled toxicity study contributes to improved data interpretation without additional use of animals and to the three goals of the 3R principles for animal testing. Toxicokinetic analyses were implemented in a rat developmental immunotoxicity study with 4-methylanisole without interfering with the outcome of the study and without the use of additional animals. 4-Methylanisole and its metabolites were analysed in plasma of adult rats and in pups at postnatal day 10. 4-Methylanisole has a short half-life in adult animals and the plasma concentrations increased more than proportional with increasing dose. The metabolic profile appeared to be different at low dose as compared to high dose. This information on the dose-proportionality of the internal exposure is crucial for the interpretation of the toxicity data and helps to identify the toxic agent and the appropriate dose metric. The metabolism was similar in adult and juvenile animals. Large inter-individual variability in adult animals, as observed for 4-methylanisole, may hamper dose-response analyses of the results. In addition, 4-metylanisole was excreted via milk, but concentrations in the juvenile animals appeared to be 20- to 100-fold lower than via direct gavage exposure. The toxicokinetic parameters support the data interpretation, among others by providing better insight into internal exposures. Subsequently, it will help to prevent testing of irrelevant exposure scenarios and exposure concentrations. Overall, implementation of kinetics with limited effort provides useful information to support the interpretation of toxicological data and can contribute to reduction and refinement of animal testing.

The application of global sensitivity analysis in the development of a physiologically based pharmacokinetic model for m-xylene and ethanol co-exposure in humans

Global sensitivity analysis (SA) was used during the development phase of a binary chemical physiologically based pharmacokinetic (PBPK) model used for the analysis of m-xylene and ethanol co-exposure in humans. SA was used to identify those parameters which had the most significant impact on variability of venous blood and exhaled m-xylene and urinary excretion of the major metabolite of m-xylene metabolism, 3-methyl hippuric acid. This analysis informed the selection of parameters for estimation/calibration by fitting to measured biological monitoring (BM) data in a Bayesian framework using Markov chain Monte Carlo (MCMC) simulation. Data generated in controlled human studies were shown to be useful for investigating the structure and quantitative outputs of PBPK models as well as the biological plausibility and variability of parameters for which measured values were not available. This approach ensured that a priori knowledge in the form of prior distributions was ascribed only to those parameters that were identified as having the greatest impact on variability. This is an efficient approach which helps reduce computational cost.