The effect of formulations and experimental conditions on in vitro human skin permeation—Data from updated EDETOX database

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.

An update of skin permeability data based on a systematic review of recent research

The cutaneous absorption parameters of xenobiotics are crucial for the development of drugs and cosmetics, as well as for assessing environmental and occupational chemical risks. Despite the great variability in the design of experimental conditions due to uncertain international guidelines, datasets like HuskinDB have been created to report skin absorption endpoints. This review updates available skin permeability data by rigorously compiling research published between 2012 and 2021. Inclusion and exclusion criteria have been selected to build the most harmonized and reusable dataset possible. The Generative Topographic Mapping method was applied to the present dataset and compared to HuskinDB to monitor the progress in skin permeability research and locate chemotypes of particular concern. The open-source dataset (SkinPiX) includes steady-state flux, maximum flux, lag time and permeability coefficient results for the substances tested, as well as relevant information on experimental parameters that can impact the data. It can be used to extract subsets of data for comparisons and to build predictive models.

New approaches build upon historical studies in dermal toxicology

These are my personal reflections on the history of approaches to understanding dermal toxicology brought together for the Paton Prize Award. This is not a comprehensive account of all publications from in vivo studies in humans to development of in vitro and in silico approaches but highlghts important progress. I will consider what is needed now to influence approaches to understanding dermal exposure with the current development and use of NAMs (new approach methodologies).

High Throughput Read-Across for Screening a Large Inventory of Related Structures by Balancing Artificial Intelligence/Machine Learning and Human Knowledge

Read-across is an in silico method applied in chemical risk assessment for data-poor chemicals. The read-across outcomes for repeated-dose toxicity end points include the no-observed-adverse-effect level (NOAEL) and estimated uncertainty for a particular category of effects. We have previously developed a new paradigm for estimating NOAELs based on chemoinformatics analysis and experimental study qualities from selected analogues, not relying on quantitative structure-activity relationships (QSARs) or rule-based SAR systems, which are not well-suited to end points for which the underpinning data are weakly grounded in specific chemical-biological interactions. The central hypothesis of this approach is that similar compounds have similar toxicity profiles and, hence, similar NOAEL values. Analogue quality (AQ) quantifies the suitability of an analogue candidate for reading across to the target by considering similarity from structure, physicochemical, ADME (absorption, distribution, metabolism, excretion), and biological perspectives. Biological similarity is based on experimental data; assay vectors derived from aggregations of ToxCast/Tox21 data are used to derive machine learning (ML) hybrid rules that serve as biological fingerprints to capture target-analogue similarity relevant to specific effects of interest, for example, hormone receptors (ER/AR/THR). Once one or more analogues have been qualified for read-across, a decision theory approach is used to estimate confidence bounds for the NOAEL of the target. The confidence interval is dramatically narrowed when analogues are constrained to biologically related profiles. Although this read-across process works well for a single target with several analogues, it can become unmanageable when, for example, screening multiple targets (e.g., virtual screening library) or handling a parent compound having numerous metabolites. To this end, we have established a digitalized framework to enable the assessment of a large number of substances, while still allowing for human decisions for filtering and prioritization. This workflow was developed and validated through a use case of a large set of bisphenols and their metabolites.

Biodegradable core-multishell nanocarrier: Topical tacrolimus delivery for treatment of dermatitis

Two challenges in topical drug delivery to the skin include solubilizing hydrophobic drugs in water-based formulations and increasing drug penetration into the skin. Polymeric core-multishell nanocarrier (CMS), particularly the novel biodegradable CMS (bCMS = hPG-PCL_1.1K-mPEG_2k-CMS) have shown both advantages on excised skin ex vivo. Here, we investigated topical delivery of tacrolimus (TAC; > 500 g/mol) by bCMS in a hydrogel on an oxazolone-induced model of dermatitis in vivo. As expected, bCMS successfully delivered TAC into the skin. However, in vivo they did not increase, but decrease TAC penetration through the stratum corneum compared to ointment. Differences in the resulting mean concentrations were mostly non-significant in the skin (epidermis: 35.7 ± 20.9 ng/cm² for bCMS vs. 92.6 ± 62.7 ng/cm² for ointment; dermis: 76.8 ± 26.8 ng/cm²vs 118.2 ± 50.4 ng/cm²), but highly significant in blood (plasma: 1.1 ± 0.4 ng/ml vs 11.3 ± 9.3 ng/ml; erythrocytes: 0.5 ± 0.2 ng/ml vs 3.4 ± 2.4 ng/ml) and liver (0.01 ± 0.01 ng/mg vs 0.03 ± 0.01 ng/mg). bCMS were detected in the stratum corneum but not in viable skin or beyond. The therapeutic efficacy of TAC delivered by bCMS was equivalent to that of standard TAC ointment. Our results suggest that bCMS may be a promising carrier for the topical delivery of TAC. The quantitative difference to previous results should be interpreted in light of structural differences between murine and human skin, but highlights the need as well as potential methods to develop more a complex ex vivo analysis on human skin to ensure quantitative predictive value.

The importance of metrological tools to implementation of alternative method OECD TG 428

Usually, if percutaneous absorption tests are conducted in accordance with OECD Guideline 428, in vitro determination is accepted by mainly regulatory agencies. In this paper, we focus on the lack of comparability of the results regarding the permeation parameter/flow rate, although it is widely discussed in the literature. This work sought to evaluate the absorption of caffeine using Franz-type diffusion cell with porcine ear skin samples, varying the storage duration and the way to handle them. Metrological tools were used for caffeine quantification such as certified reference material candidate, calibrated instruments, and validated methodology. Our results corroborate with the recommendation that membranes should be freshly prepared or frozen for short periods. Samples frozen for approximately one year should not be used because they present high cutaneous absorption. The results obtained for the absorption rate (J) are comparable to the results obtained by previous studies using similar experimental conditions. The evidence of the barrier characteristic promoted by the stratum corneum and the effect promoted by the storage time is shown through J = 6.25 ± 0.48 μg/cm²/h. We demonstrated the importance of metrological tools to guarantee reproducibility and comparability of the results between different laboratories.

Occupational exposure assessment with solid substances: choosing a vehicle for in vitro percutaneous absorption experiments

Percutaneous occupational exposure to industrial toxicants can be assessed in vitro on excised human or animal skins. Numerous factors can significantly influence skin permeation of chemicals and the flux determination. Among them, the vehicle used to solubilize the solid substances is a tricky key step. A "realistic surrogate" that closely matches the exposure scenario is recommended in first intention. When direct transposition of occupational exposure conditions to in vitro experiments is impossible, it is recommended that the vehicle used does not affect the skin barrier (in particular in terms of structural integrity, composition, or enzymatic activity). Indeed, any such effect could alter the percutaneous absorption of substances in a number of ways, as we will see. Potential effects are described for five monophasic vehicles, including the three most frequently used: water, ethanol, acetone; and two that are more rarely used, but are realistic: artificial sebum and artificial sweat. Finally, we discuss a number of criteria to be verified and the associated tests that should be performed when choosing the most appropriate vehicle, keeping in mind that, in the context of occupational exposure, the scientific quality of the percutaneous absorption data provided, and how they are interpreted, may have long-range consequences. From the narrative review presented, we also identify and discuss important factors to consider in future updates of the OECD guidelines for in vitro skin absorption experiments.

Evaluation of Two Novel Hydantoin Derivatives Using Reconstructed Human Skin Model EpiskinTM: Perspectives for Application as Potential Sunscreen Agents

This study aimed to assess two novel 5-arylideneimidazolidine-2,4-dione (hydantoin) derivatives (JH3 and JH10) demonstrating photoprotective activity using the reconstructed human skin model Episkin^TM. The skin permeability, irritation, and phototoxicity of the compounds was evaluated in vitro. Moreover, the in vitro genotoxicity and human metabolism of both compounds was studied. For skin permeation and irritation experiments, the test compounds were incorporated into a formulation. It was shown that JH3 and JH10 display no skin irritation and no phototoxicity. Both compounds did not markedly enhance the frequency of micronuclei in CHO-K1 cells in the micronucleus assay. Preliminary in vitro studies with liver microsomes demonstrated that hydrolysis appears to constitute their important metabolic pathway. Episkin^TM permeability experiments showed that JH3 permeability was lower than or close to currently used UV filters, whereas JH10 had the potential to permeate the skin. Therefore, a restriction of this compound permeability should be obtained by choosing the right vehicle or by optimizing it, which should be addressed in future studies.

Topical drug delivery: History, percutaneous absorption, and product development

Topical products, widely used to manage skin conditions, have evolved from simple potions to sophisticated delivery systems. Their development has been facilitated by advances in percutaneous absorption and product design based on an increasingly mechanistic understanding of drug-product-skin interactions, associated experiments, and a quality-by-design framework. Topical drug delivery involves drug transport from a product on the skin to a local target site and then clearance by diffusion, metabolism, and the dermal circulation to the rest of the body and deeper tissues. Insights have been provided by Quantitative Structure Permeability Relationships (QSPR), molecular dynamics simulations, and dermal Physiologically Based PharmacoKinetics (PBPK). Currently, generic product equivalents of reference-listed products dominate the topical delivery market. There is an increasing regulatory interest in understanding topical product delivery behavior under 'in use' conditions and predicting in vivo response for population variations in skin barrier function and response using in silico and in vitro findings.

Assessment of Vehicle Volatility and Deposition Layer Thickness in Skin Penetration Models

Systemic disposition of dermally applied chemicals is often formulation-dependent. Rapid evaporation of the vehicle can result in crystallization of active compounds, limiting their degree of skin penetration. In addition, the choice of vehicle can affect the permeant’s degree of penetration into the stratum corneum. The aim of this study is to build a predictive, mechanistic, dermal absorption model that accounts for vehicle-specific effects on the kinetics of permeant transport into skin. An existing skin penetration model is extended to explicitly include the effect of vehicle volatility over time. Using in vitro measurements of skin penetration by chemicals applied in both a saline and an ethanol solvent, the model is optimized to learn two vehicle-specific quantities: the solvent evaporation rate and the extent of permeant deposition into the upper stratum corneum immediately following application. The dermal disposition estimates of the trained model are subsequently compared against those of the original model using further in vitro measurements. The trained model showed a 1.5-fold improvement and a 19-fold improvement in overall goodness of fit among compounds tested in saline and ethanol solvents, respectively. The proposed model structure can thus form a basis for in vitro to in vivo extrapolations of dermal disposition for skin formulations containing volatile components.

CPE-DB: An Open Database of Chemical Penetration Enhancers

The cutaneous delivery route currently accounts for almost 10% of all administered drugs and it is becoming more common. Chemical penetration enhancers (CPEs) increase the transport of drugs across skin layers by different mechanisms that depend on the chemical nature of the penetration enhancers. In our work, we created a chemical penetration enhancer database (CPE-DB) that is, to the best of our knowledge, the first CPE database. We collected information about known enhancers and their derivatives in a single database, and classified and characterized their molecular diversity in terms of scaffold content, key chemical moieties, molecular descriptors, etc. CPE-DB can be used for virtual screening and similarity search to identify new potent and safe enhancers, building quantitative structure-activity relationship (QSAR) and quantitative structure-property relationship (QSPR) models, and other machine-learning (ML) applications for the prediction of biological activity.

Potential application of novel liquid crystal nanoparticles of isostearyl glyceryl ether for transdermal delivery of 4-biphenyl acetic acid

Novel liquid crystal nanoparticles (LCNs) composed of isostearyl glyceryl ether (GE-IS) and ethoxylated hydrogenated castor oil (HCO-60) were developed for the enhanced transdermal delivery of 4-biphenyl acetic acid (BAA). The physical properties and pharmaceutical properties of the LCNs were measured. The interaction between the intercellular lipid model of the stratum corneum and the LCNs was observed to elucidate the skin permeation mechanism. In the formulation, the LCNs form niosomes with mean particles sizes of 180-300 nm. The skin absorption mechanisms of LCNs are different, depending upon the application and buffer concentration. The LCNs composed of GE-IS and HCO-60 are attractive tools for use as transdermal drug delivery systems carriers for medicines and cosmetics, due to their high efficiency and safety.

Evaluation of Formulation Parameters on Permeation of Ibuprofen from Topical Formulations Using Strat-M® Membrane

Topical drug delivery is an attractive alternative to conventional methods because of advantages such as non-invasive delivery, by-pass of first pass metabolism, and improved patient compliance. However, several factors such as skin, physicochemical properties of the drug, and vehicle characteristics influence the permeation. Within a formulation, critical factors such as concentration of drug, physical state of drug in the formulation, and organoleptic properties affect the flux across the skin. The aim of the study was to develop and investigate topical semisolid preparations (creams and gels) with ibuprofen as the model drug and investigate the effect of various formulation parameters on the in-vitro performance across the Strat-M® membrane using flow-through cells. In addition, the physical stability of the developed formulations was investigated by studying viscosity, pH, and appearance. All the formulations developed in the study had appealing appearance with smooth texture and no signs of separation. Viscosity and pH of the formulations were acceptable. Cumulative amount of drug permeated at the end of 24 h was highest for clear gel (3% w/w ibuprofen; F6: 739.6 ± 36.1 µg/cm2) followed by cream with high concentration of ibuprofen in suspended form (5% w/w; F3: 320.8 ± 17.53 µg/cm2), emulgel (3% w/w ibuprofen; F5: 178.5 ± 34.5 µg/cm2), and cream with solubilized ibuprofen (3% w/w; F2A: 163.2 ± 9.36 µg/cm2). Results from this study showed that permeation of ibuprofen was significantly influenced by formulation parameters such as concentration of ibuprofen (3% vs. 5% w/w), physical state of ibuprofen (solubilized vs. suspended), formulation type (cream vs. gel), mucoadhesive agents, and viscosity (high vs. low). Thus, findings from this study indicate that pharmaceutical formulation scientists should explore these critical factors during the early development of any new topical drug product in order to meet pre-determined quality target product profile.

Effect of propylene glycol on the skin penetration of drugs

Propylene glycol (PG) has been used in formulations as a co-solvent and/or to enhance drug permeation through the skin from topical preparations. Two skin in vitro permeation approaches are used to determine the effect of PG on drug penetration. The in vitro Skin-PAMPA was performed using 24 actives applied in aqueous buffer or PG. PG modulates permeability by increasing or diminishing it in the compounds with poor or high permeability, respectively. Percutaneous absorption using pigskin on Franz diffusion cells was performed on seven actives and their commercial formulations. The commercial formulations evaluated tend to have a lower permeability than their corresponding PG solutions but maintain the compound distribution in the different strata: stratum corneum, epidermis and dermis. The results indicate the enhancer properties of PG for all compounds, especially for the hydrophilic ones. Additionally, the Synchrotron-Based Fourier Transform Infrared microspectroscopy technique is applied to study the penetration of PG and the molecular changes that the vehicle may promote in the different skin layers. Results showed an increase of the areas under the curve indicating the higher amount of lipids in the deeper layers and altering the lipidic order of the bilayer structure to a more disordered lipid structure.

Transdermal absorption of active substances from cosmetic vehicles

BACKGROUND

Cosmetic products mean any substance or mixture intended to be placed in contact with the external parts of the human body (eg, epidermis, lips) and should not pass to the lower parts and penetrate to the skin. As a part of evaluation of cosmetic safety, the transdermal absorption of substances should be investigated.

MATERIALS AND METHODS

In vitro absorption was investigated with Franz diffusion cells on untreated porcine skin and specimens of the same treated with 15%wt. SLS. The integrity of the skin was discerned by gauging transdermal electrical conductivity (TEC), the concentration of caffeine absorbed by the samples of skin membrane by liquid chromatography, which took place by applying an emulsion and/or a gel containing active hydration agents (urea, sodium hyaluronate, and sericin).

RESULTS

The greatest extent of caffeine penetration was seen for pretreatment with just SLS; similar results were in skin treated with the base gel with 10%wt. urea. In the skin treated with the base emulsion only, the amount of caffeine absorbed was twofold less; this increased after adding the active hydration substances. The values measured for TEC corresponded with the amount of caffeine absorbed.

CONCLUSION

The gel proved to be the more potent vehicle for the active ingredient, as it demonstrated greater transdermal caffeine penetration than the emulsions, correlating with the degree of damage to the skin as detected by TEC.

In silico prediction of dermal absorption of pesticides - an evaluation of selected models against results from in vitro testing

Current guidance for the estimation of dermal absorption (DA) of pesticides recommends the use of default values, read-across of information between formulations and in vitro testing. While QSARs exist to estimate percutaneous absorption, their use is currently not encouraged. Therefore, the potential of publicly available models for DA estimation was investigated based on data from 564 human in vitro DA experiments on pesticides. The classic Potts Guy model, the correction of Cleek Bunge for highly lipophilic chemicals, the mechanistic model of Mitragotri, and the COSMOS model were used to estimate the permeability coefficient k_p. Different approaches were explored to calculate the percentage of external dose absorbed. IH SkinPerm was examined as stand-alone model. The models generally failed to accurately predict experimental values. For 30-40% of the predictions, there was overestimation by one order of magnitude. Three models underpredicted >10% of the cases, the remaining models <5%. DA of hydrophilic substances was typically underpredicted. Overprediction was more prominent for solid preparations and suspensions. The molecular weight, irritation potential and skin thickness did not correlate with the models' predictivity. Of the models investigated, IH SkinPerm performed best with 38% of the predictions within one order of magnitude and 2% underpredicted cases.

Dermal permeation data and models for the prioritization and screening-level exposure assessment of organic chemicals

High-throughput screening (HTS) models are being developed and applied to prioritize chemicals for more comprehensive exposure and risk assessment. Dermal pathways are possible exposure routes to humans for thousands of chemicals found in personal care products and the indoor environment. HTS exposure models rely on skin permeability coefficient (KP; cm/h) models for exposure predictions. An initial database of approximately 1000 entries for empirically-based KP data was compiled from the literature and a subset of 480 data points for 245 organic chemicals derived from testing with human skin only and using only water as a vehicle was selected. The selected dataset includes chemicals with log octanol-water partition coefficients (KOW) ranging from -6.8 to 7.6 (median=1.8; 95% of the data range from -2.5 to 4.6) and molecular weight (MW) ranging from 18 to 765g/mol (median=180); only 3% >500g/mol. Approximately 53% of the chemicals in the database have functional groups which are ionizable in the pH range of 6 to 7.4, with 31% being appreciably ionized. The compiled log KP values ranged from -5.8 to 0.1cm/h (median=-2.6). The selected subset of the KP data was then used to evaluate eight representative KP models that can be readily applied for HTS assessments, i.e., parameterized with KOW and MW. The analysis indicates that a version of the SKINPERM model performs the best against the selected dataset. Comparisons of representative KP models against model input parameter property ranges (sensitivity analysis) and against chemical datasets requiring human health assessment were conducted to identify regions of chemical properties that should be tested to address uncertainty in KP models and HTS exposure assessments.

In silico models to predict dermal absorption from complex agrochemical formulations

Dermal absorption is a critical part in the risk assessment of complex mixtures such as agrochemical formulations. To reduce the number of in vivo or in vitro absorption experiments, the present study aimed to develop an in silico prediction model that considers mixture-related effects. Therefore, an experimental 'real-world' dataset derived from regulatory in vitro studies with human and rat skin was processed. Overall, 56 test substances applied in more than 150 mixtures were used. Descriptors for the substances as well as the mixtures were generated and used for multiple linear regression analysis. Considering the heterogeneity of the underlying data set, the final model provides a good fit (r² = 0.75) and is able to estimate the influence of a newly composed formulation on dermal absorption of a well-known substance (predictivity Q²Ext = 0.73). Application of this model would reduce animal and non-animal testings when used for the optimization of formulations in early developmental stages, or would simplify the registration process, if accepted for read-across.

Consolidating exposure scenario information for mixtures--experiences and challenges

INTRODUCTION

The requirements of the European Union Regulation on the Registration, Evaluation, Authorisation and Restriction of Chemicals (REACH), including the preparation of exposure scenarios for the communication of safe use, are focused on single substances. Since the chemical products used at workplaces are typically mixtures, it is important to ensure that accurate information is available regarding their safe use. The focus of the present study was on the methods for consolidating (combining) the information presented in the exposure scenarios of the individual components of a mixture.

METHODS

We tested the applicability and comparability of three methods for consolidating the exposure scenario information on six real products, focusing on occupational exposure. Two of the reviewed methods were based on the identification of the critical components that determine the health effects and risk management of the product, and one was based on the selection of the most stringent risk management measures presented in the exposure scenarios of the components.

RESULTS

The differences in the critical components recognised by the two critical component-based methods revealed limitations in both of the methods. The quality and comprehensiveness of the consolidated scenarios produced by the two methods were generally of the same level, depended directly on the content of the exposure scenarios of the critical components. Choosing the most stringent risk management measures improved the comprehensiveness of the scenarios. All of the examined methods run the risk of communicating over-precautionary measures if the concentration of the components determining the level of protection is not taken into consideration.

CONCLUSIONS

Of the three methods considered, we recommend the selection of the most stringent operational conditions and risk management measures as a conservative default approach for consolidating exposure scenario information. If a critical component-based approach is used, checking the adequacy of the consolidated scenario for the other components of the mixture is recommended. With all of the methods, further modification of the consolidated scenario may be necessary to achieve the intended level of protection.