An evaluation of mathematical models for predicting skin permeability

Delivery of small hydrophilic molecules across the stratum corneum: Identification of model systems and parameters to study topical delivery of free amino acids

Free amino acids (FAAs) constitute the largest component (∼40 %) of the so-called natural moisturizing factors of the skin. Their level declines in dry skin conditions and one strategy to overcome this problem may involve the topical delivery of FAAs through appropriate strategy. The objective of the present study was therefore to identify alternative skin models and study the corneocyte-water partition coefficients (KCOR/W) and permeation coefficient (KP) of 18 FAAs. The KCOR/W was studied using standard protocols and the permeation studies were conducted using Franz diffusion cell. The results indicate that the FAAs have high partitioning behavior to the corneocytes. The KCOR/W values of the human COR and that of pig ear skin were better correlated with each other than that of keratin isolated from chicken feathers. The presence of lipid in the stratum corneum (SC), initial concentration of the FAAs, and permeation enhancers affect the KCOR/W. The FAAs have low permeation into the SC which suggests the need for permeation enhancers in designing dosage form containing these compounds. Even though the investigated mathematical models show good prediction of the Kp values, better prediction could be obtained by considering factors such as the possible entrapment of the FAAs by the CORs.

Development of a Mass Spectrometry Imaging Method to Evaluate the Penetration of Moisturizing Components Coated on Surgical Gloves into Artificial Membranes

Skin dryness and irritant contact dermatitis induced by the prolonged use of surgical gloves are issues faced by physicians. To address these concerns, manufacturers have introduced surgical gloves that incorporate a moisturizing component on their inner surface, resulting in documented results showing a reduction in hand dermatitis. However, the spatial distribution of moisturizers applied to surgical gloves within the integument remains unclear. Using matrix-assisted laser desorption/ionization (MALDI)-mass spectrometry imaging (MSI), we investigated the spatial distribution of moisturizers in surgical gloves within artificial membranes. Recently, dermal permeation assessments using three-dimensional models, silicone membranes, and Strat-M have gained attention as alternative approaches to animal testing. Therefore, in this study, we established an in vitro dermal permeation assessment of commercially available moisturizers in surgical gloves using artificial membranes. In this study, we offer a methodology to visualize the infiltration of moisturizers applied to surgical gloves into an artificial membrane using MALDI-MSI, while evaluating commercially available moisturizer-coated surgical gloves. Using our penetration evaluation method, we confirmed the infiltration of the moisturizers into the polyethersulfone 2 and polyolefin layers, which correspond to the epidermis and dermis of the skin, after the use of surgical gloves. The MSI-based method presented herein demonstrated the efficacy of evaluating the permeation of samples containing active ingredients.

Model-Informed Drug Development: In Silico Assessment of Drug Bioperformance following Oral and Percutaneous Administration

The pharmaceutical industry has faced significant changes in recent years, primarily influenced by regulatory standards, market competition, and the need to accelerate drug development. Model-informed drug development (MIDD) leverages quantitative computational models to facilitate decision-making processes. This approach sheds light on the complex interplay between the influence of a drug's performance and the resulting clinical outcomes. This comprehensive review aims to explain the mechanisms that control the dissolution and/or release of drugs and their subsequent permeation through biological membranes. Furthermore, the importance of simulating these processes through a variety of in silico models is emphasized. Advanced compartmental absorption models provide an analytical framework to understand the kinetics of transit, dissolution, and absorption associated with orally administered drugs. In contrast, for topical and transdermal drug delivery systems, the prediction of drug permeation is predominantly based on quantitative structure-permeation relationships and molecular dynamics simulations. This review describes a variety of modeling strategies, ranging from mechanistic to empirical equations, and highlights the growing importance of state-of-the-art tools such as artificial intelligence, as well as advanced imaging and spectroscopic techniques.

Understanding Drug Skin Permeation Enhancers Using Molecular Dynamics Simulations

Our skin constitutes an effective permeability barrier that protects the body from exogenous substances but concomitantly severely limits the number of pharmaceutical drugs that can be delivered transdermally. In topical formulation design, chemical permeation enhancers (PEs) are used to increase drug skin permeability. In vitro skin permeability experiments can measure net effects of PEs on transdermal drug transport, but they cannot explain the molecular mechanisms of interactions between drugs, permeation enhancers, and skin structure, which limits the possibility to rationally design better new drug formulations. Here we investigate the effect of the PEs water, lauric acid, geraniol, stearic acid, thymol, ethanol, oleic acid, and eucalyptol on the transdermal transport of metronidazole, caffeine, and naproxen. We use atomistic molecular dynamics (MD) simulations in combination with developed molecular models to calculate the free energy difference between 11 PE-containing formulations and the skin's barrier structure. We then utilize the results to calculate the final concentration of PEs in skin. We obtain an RMSE of 0.58 log units for calculated partition coefficients from water into the barrier structure. We then use the modified PE-containing barrier structure to calculate the PEs' permeability enhancement ratios (ERs) on transdermal metronidazole, caffeine, and naproxen transport and compare with the results obtained from in vitro experiments. We show that MD simulations are able to reproduce rankings based on ERs. However, strict quantitative correlation with experimental data needs further refinement, which is complicated by significant deviations between different measurements. Finally, we propose a model for how to use calculations of the potential of mean force of drugs across the skin's barrier structure in a topical formulation design.

Evaluating Neutral PFAS for Potential Dermal Absorption from the Gas Phase

Exposures to per- and polyfluoroalkyl substances (PFAS) are of increasing concern. Assessments typically focus only on ingestion and inhalation exposure due to a lack of generally accepted approaches for estimating dermal absorption. Prior work indicates limited dermal absorption of ionic PFAS, but absorption of neutral PFAS has not been examined from the liquid vehicle or from vapor. Partitioning of semivolatile organic compounds from the gas phase to the skin surface (i.e., stratum corneum) is well known, but the potential for partitioning of neutral PFAS from the gas phase to the stratum corneum has yet to be estimated. The SPARC-estimated physicochemical properties were used to calculate transdermal permeability coefficients (k_{p_g}) and dermal-to-inhalation (D/I) exposure ratios for two groups of neutral PFAS, including those on a U.S. Environmental Protection Agency PFAS list. 11 neutral PFAS gave calculated D/I ratios >5, indicating that direct transdermal absorption may be an important exposure pathway compared to inhalation. Data on consumer products or indoor air is needed for the 11 neutral PFAS, followed by possible biomonitoring to experimentally verify dermal absorption from air. Additional PFAS should be estimated by the protocol used here as they are identified in commercial products.

Skin permeability prediction with MD simulation sampling spatial and alchemical reaction coordinates

A molecular-level understanding of skin permeation may rationalize and streamline product development, and improve quality and control, of transdermal and topical drug delivery systems. It may also facilitate toxicity and safety assessment of cosmetics and skin care products. Here, we present new molecular dynamics simulation approaches that make it possible to efficiently sample the free energy and local diffusion coefficient across the skin's barrier structure to predict skin permeability and the effects of chemical penetration enhancers. In particular, we introduce a new approach to use two-dimensional reaction coordinates in the accelerated weight histogram method, where we combine sampling along spatial coordinates with an alchemical perturbation virtual coordinate. We present predicted properties for 20 permeants, and demonstrate how our approach improves correlation with ex vivo/in vitro skin permeation data. For the compounds included in this study, the obtained log KPexp-calc mean square difference was 0.9 cm2 h-2.

A Mathematical Approach Using Strat-M® to Predict the Percutaneous Absorption of Chemicals under Finite Dose Conditions

Estimation of the percutaneous absorption is essential for the safety assessment of cosmetic and dermopharmaceutical products. Currently, an artificial membrane, Strat-M^®, has been focused on as the tool which could obtain the permeation parameters close to the skin-derived values. Nevertheless, few practical methodologies using the permeation parameters for assessing percutaneous absorption under in-use conditions are available. In the present study, based on Fick's first law of diffusion, a novel mathematical model incorporating the permeation parameters as well as considering the water evaporation (T_eva) was constructed. Then, to evaluate the applicability domain of our model in the case where Strat-M^®-derived parameters were used, the permeation parameters were compared between the skin from edible porcine and Strat-M^®. Regarding chemicals (-0.2 ≤ Log K_ow ≤ 2.0), their permeation profiles were equivalent between Strat-M^® and porcine skin. Therefore, for these chemicals, the percutaneous absorption was calculated using our model with the permeation parameters obtained using Strat-M^® and the T_eva determined by measuring the solution weight. The calculated values revealed a good correlation to the values obtained using porcine skin in finite dose experiments, suggesting that our mathematical approach with Strat-M^® would be useful for the future safety assessment of cosmetic and dermopharmaceutical products.

IAM Chromatographic Models of Skin Permeation

Chromatographic retention factor log k_IAM obtained from IAM HPLC chromatography with buffered aqueous mobile phases and calculated molecular descriptors (surface area—S_a; molar volume—V_M; polar surface area—PSA; count of freely rotable bonds—FRB; H-bond acceptor count—HA; energy of the highest occupied molecular orbital—E_HOMO; energy of the lowest unoccupied orbital—E_LUMO; and polarizability—α) obtained for a group of 160 structurally unrelated compounds were tested in order to generate useful models of solutes’ skin permeability coefficient log K_p. It was established that log k_IAM obtained in the conditions described in this study is not sufficient as a sole predictor of the skin permeability coefficient. Simple put, potentially useful models based on log k_IAM and readily available calculated descriptors, accounting for 85 to 91% of the total variability, were generated using Multiple Linear Regression (MLR).The models proposed in the study were tested on a group of 20 compounds with known experimental log K_p values.

Effect of Cosmetics Use on the In Vitro Skin Absorption of a Biocide, 1,2-Benzisothiazolin-3-one

1,2-Benzisothiazolin-3-one (BIT) is a commonly used organic biocide containing an isothiazolone ring. However, it may have adverse effects on human health and its risk needs to be properly evaluated. Dermal exposure is the main route of BIT exposure, and co-exposed substances may affect its absorption. The dermal permeation profile of BIT has not been well-studied. This study aimed to investigate the dermal permeation profiles of BIT with or without cosmetic use. Dermal permeation profiles of BIT were investigated after infinite- (100 μg/cm²), or a finite-dose (10 μg/cm²) application with or without cosmetics using a minipig skin and Strat-M^®, an artificial membrane. A cream, lotion, and essence (namely, face serum) were pre-treated as representative cosmetics on minipig skin for 30 min, with BIT treatment afterward. After the treatment, BIT left on the skin surface was collected by cotton swabbing, BIT in the stratum corneum, by sequential tape stripping, and BIT retained in the remaining skin was extracted after cutting the skin into pieces before LC-MS/MS analysis. When an infinite dose was applied, permeation coefficients (K_p, cm/h) for minipig skin and Strat-M^® were 2.63 × 10⁻³ and 19.94 × 10⁻³, respectively, reflecting that skin permeation was seven to eight times higher in Strat-M^® than in the minipig skin. BIT, in the presence of cosmetics, rapidly permeated the skin, while the amount in the stratum corneum and skin deposit was reduced. We performed a risk assessment of dermally applied BIT in the absence or presence of cosmetics by calculating the skin absorption rate at 10 h based on the toxicological data from several references. The risk level was higher in the presence of essence as compared to lotion, which was higher than cream, which was higher than the control (non-treated). However, all of the margins of safety values obtained were greater than 100, suggesting that BIT is safe for use in dermally exposed consumer products. We believe that this research contributes to a greater understanding of the risk assessment of isothiazolinone biocides.

RP-18 TLC and Computational Descriptors of Skin Permeability of Sunscreens

Abstract The relationships between the reversed-phase thin layer chromatographic retention parameters obtained on octadecyl-modified silica (RP-18) sorbent for mobile phases containing water and one of six water-miscible organic modifiers (acetone, methanol, acetonitrile, tetrahydrofurane, N,N-dimethylformamide, 1,4-dioxane) and skin permeability coefficients were studied for a group of 21 cosmetic raw materials, mainly organic sunscreens and preservatives. The correlations between the skin permeability coefficients log Kd calculated in silico using EpiSuite software and the RP-18 thin layer chromatographic retention parameters are mostly linear, especially for compounds of lower-to-medium lipophilicity.. It was established that skin permeability coefficient models based on retention parameters collected for mobile phases containing acetone or dioxane (75% v/v), proposed for structurally unrelated cosmetic raw materials are also applicable to other actives, as shown using a test set of compounds whose in vivo log Kd data are available. Skin permeability models developed in this study have the benefit of being based on easily obtained, chromatographic descriptors and their applicability extends beyond cosmetic chemistry.

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.

Ability of mathematical models to predict human in vivo percutaneous penetration of steroids

Human skin is a common route for topical steroids to enter the body. To aid with risk management of therapeutic steroid usage, the US Environmental Protection Agency estimates percutaneous penetration using mathematical models. However, it is unclear how accurate are mathematical models in estimating percutaneous penetration/absorption of steroids. In this study, accuracy of predicted flux (penetration/absorption) by the main mathematical model used by the EPA, the Potts and Guy model based on in vitro data is compared to actual human in vivo data from our laboratory of percutaneous absorption of topical steroids. We focused on steroids due to the availability of steroid in vivo human data in our laboratory. For most steroids the flux was underestimated by a factor 10-60. However, within the group itself, there was an association between the Potts and Guy model and experimental human in vivo data (Pearson Correlation = 0.8925, p = 0.000041). Additionally, some physiochemical parameters used in the Potts and Guy equation, namely log Kp (Pearson Correlation = 0.7307, p = 0.0046) and molecular weight (Pearson correlation = -0.6807, p = 0.0105) correlated significantly with in vivo flux. Current mathematical models used in estimating percutaneous penetration/absorption did not accurately predict in vivo flux of steroids. Why? Proposed limitations to mathematical models currently used include: not accounting for volatility, lipid solubility, hydrogen bond effects, drug metabolism, as well as protein binding. Further research is needed in order to increase the predictive nature of such models for in vivo flux.

Reassessment of the experimental skin permeability coefficients of polycyclic aromatic hydrocarbons and organophosphorus pesticides

Human exposure to polycyclic aromatic hydrocarbons (PAHs) and organophosphorus pesticides (OPPs) by dermal route is a continuing concern in environmental and occupational toxicology. Diverse authors have measured in vitro the absorption flux and permeability coefficient (k_P) of those compounds delivered on skin surface using volatile solvents. However, there isn't a harmonized method to obtain k_P when the test substance is deposited on the skin as a solid. Consequently, varied experimental k_Ps have been reported for PAHs and OPPs, most in clear disagreement with the values predicted by well-established mathematical models. In this work, we collected the permeation fluxes reported for these toxicants through human skin and calculated the (aqueous) k_Ps using a method based on the maximum flux and water solubility. The reanalyzed fluxes and recalculated k_Ps show improved consistency between the different experimental works and mathematical models. Notably, the recalculated k_P of benzo[a]pyrene, among others, was approximately 100 times higher than it had been previously considered. Suggestions are given to generalize the method in studies with other solvent-deposited toxicants and drugs.

Physiologically-Based Pharmacokinetic Modeling to Support Determination of Bioequivalence for Dermatological Drug Products: Scientific and Regulatory Considerations

Physiologically-based pharmacokinetic (PBPK) modeling and simulation provides mechanism-based predictions of the pharmacokinetics of an active ingredient following its administration in humans. Dermal PBPK models describe the skin permeation and disposition of the active ingredient following the application of a dermatological product on the skin of virtual healthy and diseased human subjects. These models take into account information on product quality attributes, physicochemical properties of the active ingredient and skin (patho)physiology, and their interplay with each other. Regulatory and product development decision makers can leverage these quantitative tools to identify factors impacting local and systemic exposure. In the realm of generic drug products, the number of US Food and Drug Administratioin (FDA) interactions that use dermal PBPK modeling to support alternative bioequivalence (BE) approaches is increasing. In this report, we share scientific considerations on the development, verification and validation (V&V), and application of PBPK models within the context of a virtual BE assessment for dermatological drug products. We discuss the challenges associated with model V&V for these drug products stemming from the fact that target-site active ingredient concentrations are typically not measurable. Additionally, there are no established relationships between local and systemic PK profiles, when the latter are quantifiable. To that end, we detail a multilevel model V&V approach involving validation for the model of the drug product of interest coupled with the overall assessment of the modeling platform in use while leveraging in vitro and in vivo data related to local and systemic bioavailability.

RP-18 TLC Chromatographic and Computational Study of Skin Permeability of Steroids

The skin permeability of steroids, as investigated in this study, is important because some of these compounds are, or could, be used in preparations applied topically. Several models of skin permeability, involving thin layer chromatographic and calculated descriptors, were generated and validated using K_p reference values obtained in silico and then tested on a group of solutes whose experimental K_p values could be found (log K_p^exp). The study established that the most applicable log K_p model is based on RP-18 thin layer chromatographic data (R_M) and the calculated descriptors V_M (molar volume) and PSA (polar surface area). Two less efficient, yet simple, equations based on PSA or V_M combined with HD (H-donor count) can be used with caution for rapid, rough estimations of compounds’ skin permeability prior to their chemical synthesis.

Assessment of Finite and Infinite Dose In Vitro Experiments in Transdermal Drug Delivery

Penetration, usually with finite dosing, provides data about the total active amount in the skin and permeation, being the most used methodology, usually with infinite dosing, leads to data about pharmacokinetic parameters. The main objective of this work is to assess if results from permeation, most of them at finite dose, may be equivalent to those from penetration usually at infinite dose. The transdermal behavior of four drugs with different physicochemical properties (diclofenac sodium, ibuprofen, lidocaine, and caffeine) was studied using penetration/finite and kinetic permeation/infinite dose systems using vertical Franz diffusion cells to determine the relationships between permeation and penetration profiles. Good correlation of these two in vitro assays is difficult to find; the influence of their dosage and the proportion of different ionized/unionized compounds due to the pH of the skin layers was demonstrated. Finite and infinite dose regimens have different applications in transdermal delivery. Each approach presents its own advantages and challenges. Pharmaceutical industries are not always clear about the method and the dose to use to determine transdermal drug delivery. Being aware that this study presents results for four actives with different physicochemical properties, it can be concluded that the permeation/infinite results could not be always extrapolated to those of penetration/finite. Differences in hydrophilicity and ionization of drugs can significantly influence the lack of equivalence between the two methodologies. Further investigations in this field are still needed to study the correlation of the two methodologies and the main properties of the drugs that should be taken into account.

Digital Twins for Tissue Culture Techniques—Concepts, Expectations, and State of the Art

Techniques to provide in vitro tissue culture have undergone significant changes during the last decades, and current applications involve interactions of cells and organoids, three-dimensional cell co-cultures, and organ/body-on-chip tools. Efficient computer-aided and mathematical model-based methods are required for efficient and knowledge-driven characterization, optimization, and routine manufacturing of tissue culture systems. As an alternative to purely experimental-driven research, the usage of comprehensive mathematical models as a virtual in silico representation of the tissue culture, namely a digital twin, can be advantageous. Digital twins include the mechanistic of the biological system in the form of diverse mathematical models, which describe the interaction between tissue culture techniques and cell growth, metabolism, and the quality of the tissue. In this review, current concepts, expectations, and the state of the art of digital twins for tissue culture concepts will be highlighted. In general, DT’s can be applied along the full process chain and along the product life cycle. Due to the complexity, the focus of this review will be especially on the design, characterization, and operation of the tissue culture techniques.

Permeability coefficients and vapour pressure determination for fragrance materials

OBJECTIVE

This study aims to correlate new experimental data relevant to the description of the combined evaporation/permeation process of a perfume applied onto the skin.

METHODS

The vapour pressure data were measured by thermogravimetric analysis (TG-DTA). The Antoine constants and the Clarke and Glew parameters were determined for the same set of fragrance molecules to describe its low vapour pressures at new temperature ranges. The permeability coefficient of a set of 14 fragrance molecules in ethanolic solution was determined by Franz diffusion cell experiments, using porcine skin. The samples were analysed by gas chromatography with a flame ionization detector (GC/FID) and high-performance liquid chromatography with UV visible detector (HPLC/UV). A QSAR model was proposed to correlate the experimental data.

RESULTS

The Antoine constants were determined and presented low standard deviations. The Clarke and Glew physically significant parameters were obtained along with its statistical analysis. The fitting is good since the magnitude order is in accordance with the literature, associated with the low correlation between the estimated parameters and low standard deviations. The presented correlation, based on a mixture using only ethanol as solvent, showed better results than previous QSAR models with a standard relative deviation ( σ r ) of 0.190, a standard error (SE) of 0.397 and a determination coefficient (R² ) of 0.7786.

CONCLUSION

The dataset is still small compared to larger and more general QSAR models; however, it is much more specific as to the type of solvent and class of materials studied. This work represents an advance for the modelling of the perfume diffusion process since it specifies important properties that until then had been treated in a more general way.

Application of RP-18 TLC Retention Data to the Prediction of the Transdermal Absorption of Drugs

Several chromatographic parameters (R_M⁰ and S obtained from RP-18 TLC with methanol—pH 7.4 phosphate buffer mobile phases by extrapolation to zero concentration of methanol; R_f and R_M obtained from RP-18 TLC with acetonitrile—pH 7.4 phosphate buffer 70:30 v/v as a mobile phase) and calculated molecular descriptors (molecular weight—M_W; molar volume—V_M; polar surface area—PSA; total count of nitrogen and oxygen atoms—(N+O); H-bond donor count—HD; H-bond acceptor count—HA; distribution coefficient—log D; total energy—E_T; binding energy—E_b; hydration energy—E_h; energy of the highest occupied molecular orbital—E_HOMO; energy of the lowest unoccupied orbital—E_LUMO; electronic energy—E_e; surface area—S_a; octanol-water partition coefficient—log P; dipole moment—DM; refractivity—R, polarizability—α) and their combinations (R_f/PSA, R_M/M_W, R_M/V_M) were tested in order to generate useful models of solutes’ skin permeability coefficient log K_p. It was established that neither R_M⁰ nor S obtained in the conditions used in this study is a good predictor of the skin permeability coefficient. The chromatographic parameters R_f and R_f/PSA were also unsuitable for this purpose. A simple and potentially useful, purely computational model based on (N+O), log D and HD as independent variables and accounting for ca. 83% of total variability was obtained. The evaluation of parameters derived from R_M (R_M, R_M/M_W, R_M/V_M) as independent variables in log K_p models proved that R_M/V_M is the most suitable descriptor belonging to this group. In a search for a reliable log K_p model based on this descriptor two possibilities were considered: a relatively simple model based on 5 independent variables: (N+O), log D, R_M/V_M, E_T and E_h and a more complex one, involving also E_b, M_W and PSA.

HuskinDB, a database for skin permeation of xenobiotics

Skin permeation is an essential biological property of small organic compounds our body is exposed to, such as drugs in topic formulations, cosmetics, and environmental toxins. Despite the limited availability of experimental data, there is a lack of systematic analysis and structure. We present a novel resource on skin permeation data that collects all measurements available in the literature and systematically structures experimental conditions. Besides the skin permeation value k_p, it includes experimental protocols such as skin source site, skin layer used, preparation technique, storage conditions, as well as test conditions such as temperature, pH as well as the type of donor and acceptor solution. It is important to include these parameters in the assessment of the skin permeation data. In addition, we provide an analysis of physicochemical properties and chemical space coverage, laying the basis for applicability domain determination of insights drawn from the collected data points. The database is freely accessible under https://huskindb.drug-design.de or https://doi.org/10.7303/syn21998881 .

Finite Element Analysis for Predicting Skin Pharmacokinetics of Nano Transdermal Drug Delivery System Based on the Multilayer Geometry Model

Background

Skin pharmacokinetics is an indispensable indication for studying the drug fate after administration of transdermal drug delivery systems (TDDS). However, the heterogeneity and complex skin structured with stratum corneum, viable epidermis, dermis, and subcutaneous tissue inevitably leads the drug diffusion coefficient (Kp) to vary depending on the skin depth, which seriously limits the development of TDDS pharmacokinetics in full thickness skin.

Methods

A multilayer geometry skin model was established and the Kp of drug in SC, viable epidermis, and dermis was obtained using the technologies of molecular dynamics simulation, in vitro permeation experiments, and in vivo microdialysis, respectively. Besides, finite element analysis (FEA) based on drug Kps in different skin layers was applied to simulate the paeonol nanoemulsion (PAE-NEs) percutaneous dynamic penetration process in two and three dimensions. In addition, PAE-NEs skin pharmacokinetics profile obtained by the simulation was verified by in vivo experiment.

Results

Coarse-grained modeling of molecular dynamic simulation was successfully established and the Kp of PAE in SC was 2.00×10⁻⁶ cm²/h. The Kp of PAE-NE in viable epidermis and in dermis detected using penetration test and microdialysis probe technology, was 1.58×10⁻⁵ cm²/h and 3.20×10⁻⁵ cm²/h, respectively. In addition, the results of verification indicated that PAE-NEs skin pharmacokinetics profile obtained by the simulation was consistent with that by in vivo experiment.

Discussion

This study demonstrated that the FEA combined with the established multilayer geometry skin model could accurately predict the skin pharmacokinetics of TDDS.

Analysis of consumer behavior for the estimation of the exposure to chemicals in personal care products

In this study, the main objective was to implement an integrative modelling framework in order to support the prioritization and screening of chemicals present in personal care products (PCPs) regarding their potential to expose users across multiple possible pathways. Here, we implemented an exposure-based framework based on product intake fractions (PiFs) calculated using a two-compartment model reproducing the skin uptake and the competing volatilization of chemicals applied on skin during PCP use. The implemented framework enabled to simultaneously and comprehensively accommodate coupled chemical specific parameters (i.e. physical and chemical properties of the candidate chemicals), exposure information specific for product-chemical combinations, and survey data informing on consumer behavior. A case-study, based on the usage pattern data of 22 PCPs investigated among Swiss individuals (Garcia-Hidalgo et al., 2017a) and 113 candidate chemicals chosen for their suspected presence in the PCP categories of interest was defined to evaluate the applicability of the framework. Nonnegative matrix factorization (NMF) and hierarchical clustering were subsequently applied to identify chemicals with the highest exposure potential and to highlight most relevant mixtures of chemicals on the basis of the specific usage patterns of the considered survey individuals.

Computational Modeling on Aquaporin-3 as Skin Cancer Target: A Virtual Screening Study

Aquaporin-3 (AQP3) is one of the aquaglyceroporins, which is expressed in the basolateral layer of the skin membrane. Studies have reported that human skin squamous cell carcinoma overexpresses AQP3 and inhibition of its function may alleviate skin tumorigenesis. In the present study, we have applied a virtual screening method that encompasses filters for physicochemical properties and molecular docking to select potential hit compounds that bind to the Aquaporin-3 protein. Based on molecular docking results, the top 20 hit compounds were analyzed for stability in the binding pocket using unconstrained molecular dynamics simulations and further evaluated for binding free energy. Furthermore, examined the ligand-unbinding pathway of the inhibitor from its bound form to explore possible routes for inhibitor approach to the ligand-binding site. With a good docking score, stability in the binding pocket, and free energy of binding, these hit compounds can be developed as Aquaporin-3 inhibitors in the near future.

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.

Percutaneous absorption of thirty-eight organic solvents in vitro using pig skin

Percutaneous absorption is highly variable between chemicals but also within chemicals depending on exposure conditions and experimental set up. We tested a larger number of organic solvents with the same experimental set up, using skin from new-born piglets and static diffusion cells. Thirty-six common organic solvents were studied neat (and 31 of them also in water dilution): acetone, acetonitrile, n-butanol 2-butanone 2-butoxyethanol, 1-butoxy-2-propanol, n-butyl acetate, butyl acrylate, cyclohexane, cyclohexanone, 1,2-dichloroethane, dichloromethane, ethanol, 2-ethoxyethanol, ethyl acetate, ethyl acrylate, ethylbenzene, furfuryl alcohol, n-hexane, 2-hexanone, 2-isopropoxyethanol, methanol, 1-methoxy-2-propanol, methyl acrylate, 3-methyl-1-butanol, methyl tertiary butyl ether, 4-metyl-2-pentanol, methyl methacrylate, 2-propanol, 2-propen-1-ol, 2-propoxyethanol, 1-propoxy-2-propanol, styrene, trichloromethane, toluene and m-xylene. In addition, a mixture of 2-methylbutyl acetate and n-pentyl acetate was tested. For most of the solvents, little or no percutaneous absorption data have been published. Lag times, steady-state fluxes and apparent permeability coefficients were obtained from the time courses of solvent appearance in the receptor medium, as measured by gas chromatography. The use of the same methodology and kind of skin resulted in small variability within experiments, underlining the need for consistent methodology for useful results for developing predictive models. Furthermore, a comparison of the neat and diluted data shows that water dilution affects all these variables and that the direction and magnitude of the effects vary between chemicals. This comparison strongly supports that prediction of percutaneous absorption of neat and water diluted chemicals requires different models.

Aggregate consumer exposure to isothiazolinones via household care and personal care products: Probabilistic modelling and benzisothiazolinone risk assessment

Consumers regularly use household care and personal care products (HC&PCPs). Isothiazolinones are included in HC&PCPs as preservatives and are being held responsible for an epidemic rise in allergic contact dermatitis (ACD). The objective of this study was to assess the origin and extent of dermal exposure in order to evaluate the risk of ACD from isothiazolinones in HC&PCP. Individual-based aggregate dermal exposure to four isothiazolinones was estimated using the newly proposed Probabilistic Aggregated Consumer Exposure Model-Kinetic, Dermal (PACEM-KD) by combining the reported individual use patterns for HC&PCP in Switzerland (N = 669 (558 adults), ages 0-91) with isothiazolinone concentrations measured in products used by the individual person. PACEM-KD extends the original PACEM by considering exposure duration, product dilution and skin permeability. PACEM-KD-based higher-tier exposure on palms (99th percentile) was 15.4 ng/cm², 1.3 ng/cm², 0.9 ng/cm², and 0.08 ng/cm² for the isothiazolinones 1,2‑Benzisothiazol‑3‑(2H)‑one (BIT), 2‑Octyl‑3(2H)‑isothiazolinone (OIT), 2‑Methylisothiazolin‑3(2H)‑one (MI), and 5‑Chloro‑2‑methyl‑4‑isothiazolin‑3‑one (CMI), respectively. Major sources of exposure to BIT included all-purpose cleaners, dishwashing detergent, and kitchen cleaner, while exposure to OIT mainly stems from a fungicide. For MI, the main contributors were dishwashing detergent and all-purpose wet wipes, and for CMI all-purpose cleaner. A Quantitative Risk Assessment (QRA) for BIT using Sensitization Assessment Factors (SAFs) indicates that around 1% of the Swiss population is at risk to be sensitized by BIT in cosmetics and household chemicals. For isothiazolinones in general the presented higher-tier modelling approach suggests that household cleaners are currently more important sources of exposure than cosmetics.

Salt water and skin interactions: new lines of evidence

In Health Resort Medicine, both balneotherapy and thalassotherapy, salt waters and their peloids, or mud products are mainly used to treat rheumatic and skin disorders. These therapeutic agents act jointly via numerous mechanical, thermal, and chemical mechanisms. In this review, we examine a new mechanism of action specific to saline waters. When topically administered, this water rich in sodium and chloride penetrates the skin where it is able to modify cellular osmotic pressure and stimulate nerve receptors in the skin via cell membrane ion channels known as "Piezo" proteins. We describe several models of cutaneous adsorption/desorption and penetration of dissolved ions in mineral waters through the skin (osmosis and cell volume mechanisms in keratinocytes) and examine the role of these resources in stimulating cutaneous nerve receptors. The actions of salt mineral waters are mediated by a mechanism conditioned by the concentration and quality of their salts involving cellular osmosis-mediated activation/inhibition of cell apoptotic or necrotic processes. In turn, this osmotic mechanism modulates the recently described mechanosensitive piezoelectric channels.

Predicting drug permeability through skin using molecular dynamics simulation

Understanding and predicting permeability of compounds through skin is of interest for transdermal delivery of drugs and for toxicity predictions of chemicals. We show, using a new atomistic molecular dynamics model of the skin's barrier structure, itself validated against near-native cryo-electron microscopy data from human skin, that skin permeability to the reference compounds benzene, DMSO (dimethyl sulfoxide), ethanol, codeine, naproxen, nicotine, testosterone and water can be predicted. The permeability results were validated against skin permeability data in the literature. We have investigated the relation between skin barrier molecular organization and permeability using atomistic molecular dynamics simulation. Furthermore, it is shown that the calculated mechanism of action differs between the five skin penetration enhancers Azone, DMSO, oleic acid, stearic acid and water. The permeability enhancing effect of a given penetration enhancer depends on the permeating compound and on the concentration of penetration enhancer inside the skin's barrier structure. The presented method may open the door for computer based screening of the permeation of drugs and toxic compounds through skin.

Evaluating Molecular Properties Involved in Transport of Small Molecules in Stratum Corneum: A Quantitative Structure-Activity Relationship for Skin Permeability

The skin permeability (Kp) defines the rate of a chemical penetrating across the stratum corneum. This value is widely used to quantitatively describe the transport of molecules in the outermost layer of epidermal skin and indicate the significance of skin absorption. This study defined a Kp quantitative structure-activity relationship (QSAR) based on 106 chemical substances of Kp measured using human skin and interpreted the molecular interactions underlying transport behavior of small molecules in the stratum corneum. The Kp QSAR developed in this study identified four molecular descriptors that described the molecular cyclicity in the molecule reflecting local geometrical environments, topological distances between pairs of oxygen and chlorine atoms, lipophilicity, and similarity to antineoplastics in molecular properties. This Kp QSAR considered the octanol-water partition coefficient to be a direct influence on transdermal movement of molecules. Moreover, the Kp QSAR identified a sub-domain of molecular properties initially defined to describe the antineoplastic resemblance of a compound as a significant factor in affecting transdermal permeation of solutes. This finding suggests that the influence of molecular size on the chemical’s skin-permeating capability should be interpreted with other relevant physicochemical properties rather than being represented by molecular weight alone.

A Measurement and Modeling Study of Hair Partition of Neutral, Cationic, and Anionic Chemicals

Various neutral, cationic, and anionic chemicals contained in hair care products can be absorbed into hair fiber to modulate physicochemical properties such as color, strength, style, and volume. For environmental safety, there is also an interest in understanding hair absorption to wide chemical pollutants. There have been very limited studies on the absorption properties of chemicals into hair. Here, an experimental and modeling study has been carried out for the hair-water partition of a range of neutral, cationic, and anionic chemicals at different pH. The data showed that hair-water partition not only depends on the hydrophobicity of the chemical but also the pH. The partition of cationic chemicals to hair increased with pH, and this is due to their electrostatic interaction with hair increased from repulsion to attraction. For anionic chemicals, their hair-water partition coefficients decreased with increasing pH due to their electrostatic interaction with hair decreased from attraction to repulsion. Increase in pH did not change the partition of neutral chemicals significantly. Based on the new physicochemical insight of the pH effect on hair-water partition, a new quantitative structure property relationship model has been proposed, taking into account of both the hydrophobic interaction and electrostatic interaction of chemical with hair fiber.

Surging footprints of mathematical modeling for prediction of transdermal permeability

In vivo skin permeation studies are considered gold standard but are difficult to perform and evaluate due to ethical issues and complexity of process involved. In recent past, a useful tool has been developed by combining the computational modeling and experimental data for expounding biological complexity. Modeling of percutaneous permeation studies provides an ethical and viable alternative to laboratory experimentation. Scientists are exploring complex models in magnificent details with advancement in computational power and technology. Mathematical models of skin permeability are highly relevant with respect to transdermal drug delivery, assessment of dermal exposure to industrial and environmental hazards as well as in developing fundamental understanding of biotransport processes. Present review focuses on various mathematical models developed till now for the transdermal drug delivery along with their applications.

Molecular Dynamics as a tool for in silico screening of skin permeability

Prediction of skin permeability can have manifold applications ranging from drug delivery to toxicity prediction. Along with the semi-empirical or mechanistic models proposed in the last decades, Molecular Dynamics simulations have recently become a fruitful tool for investigating membrane permeability, in particular as they allow the involved mechanisms to be modelled at a molecular level. Despite their significant structural complexity, Molecular Dynamics simulations can also be utilized to study permeation through the lipid matrix that characterizes the stratum corneum. In this work, Steered Molecular Dynamics simulations are performed on a suitably developed stratum corneum lipid matrix model. Regardless of their actual tortuous path within the stratum corneum, the permeants, taken from a Fully Validated dataset of 80 compounds of known permeability coefficient, are moved through the bilayer along its normal. This allows the exploration of all the possible conformational and physicochemical constraints the molecule experiences when moving through the bilayer. The so performed Steered Molecular Dynamics simulations are then utilized to extract the corresponding lipophilicity and diffusion parameters as computed by subdividing the entire path in 18 regions of different polarity and composition. Correlative analyses showed that the water-lipids interface is the best performing region and that significant enhancements can be gained by including parameters accounting for the temperature effect. Taken together, the developed models possess an enhanced predictive power compared to the existing equations and statistics are approaching the best possible results, given the uncertainty in the utilized permeability data.

Dermal uptake and percutaneous penetration of ten flame retardants in a human skin ex vivo model

The dermal uptake and percutaneous penetration of ten organic flame retardants was measured using an ex vivo human skin model. The studied compounds were DBDPE, BTBPE, TBP-DBPE, EH-TBB, BEH-TEBP, α, β and γ-HBCDD as well as syn- and anti-DDC-CO. Little or none of the applied flame retardants was recovered in either type of the receptor fluids used (physiological and worst-case). However, significant fractions were recovered in the skin depot, particularly in the upper skin layers. The primary effect of the worst-case receptor fluid was deeper penetration into the skin. The recovered mass was used to calculate lower- and upper-bound permeability coefficients kp. Despite large structural variation between the studied compounds, a clear, significant decreasing trend of kp was observed with increasing log Kow. The results indicate that the dermis may provide a significant barrier for these highly lipophilic compounds. However, based on our results, dermal uptake should be considered in exposure assessments, though it may proceed in a time-lagged manner compared to less hydrophobic compounds.

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.

Multi-pathway exposure modeling of chemicals in cosmetics with application to shampoo

We present a novel multi-pathway, mass balance based, fate and exposure model compatible with life cycle and high-throughput screening assessments of chemicals in cosmetic products. The exposures through product use as well as post-use emissions and environmental media were quantified based on the chemical mass originally applied via a product, multiplied by the product intake fractions (PiF, the fraction of a chemical in a product that is taken in by exposed persons) to yield intake rates. The average PiFs for the evaluated chemicals in shampoo ranged from 3×10(-4) up to 0.3 for rapidly absorbed ingredients. Average intake rates ranged between nano- and micrograms per kilogram bodyweight per day; the order of chemical prioritization was strongly affected by the ingredient concentration in shampoo. Dermal intake and inhalation (for 20% of the evaluated chemicals) during use dominated exposure, while the skin permeation coefficient dominated the estimated uncertainties. The fraction of chemical taken in by a shampoo user often exceeded, by orders of magnitude, the aggregated fraction taken in by the population through post-use environmental emissions. Chemicals with relatively high octanol-water partitioning and/or volatility, and low molecular weight tended to have higher use stage exposure. Chemicals with low intakes during use (<1%) and subsequent high post-use emissions, however, may yield comparable intake for a member of the general population. The presented PiF based framework offers a novel and critical advancement for life cycle assessments and high-throughput exposure screening of chemicals in cosmetic products demonstrating the importance of consistent consideration of near- and far-field multi-pathway exposures.

In vitro models to estimate drug penetration through the compromised stratum corneum barrier

OBJECTIVES

The phospholipid vesicle-based permeation assay (PVPA) is a recently established in vitro stratum corneum model to estimate the permeability of intact and healthy skin. The aim here was to further evolve this model to mimic the stratum corneum in a compromised skin barrier by reducing the barrier functions in a controlled manner.

METHODS

To mimic compromised skin barriers, PVPA barriers were prepared with explicitly defined reduced barrier function and compared with literature data from both human and animal skin with compromised barrier properties. Caffeine, diclofenac sodium, chloramphenicol and the hydrophilic marker calcein were tested to compare the PVPA models with established models.

RESULTS AND DISCUSSIONS

The established PVPA models mimicking the stratum corneum in healthy skin showed good correlation with biological barriers by ranking drugs similar to those ranked by the pig ear skin model and were comparable to literature data on permeation through healthy human skin. The PVPA models provided reproducible and consistent results with a distinction between the barriers mimicking compromised and healthy skin. The trends in increasing drug permeation with an increasing degree of compromised barriers for the model drugs were similar to the literature data from other in vivo and in vitro models.

CONCLUSIONS

The PVPA models have the potential to provide permeation predictions when investigating drugs or cosmeceuticals intended for various compromised skin conditions and can thus possibly reduce the time and cost of testing as well as the use of animal testing in the early development of drug candidates, drugs and cosmeceuticals.

Validation of an aggregate exposure model for substances in consumer products: a case study of diethyl phthalate in personal care products

As personal care products (PCPs) are used in close contact with a person, they are a major source of consumer exposure to chemical substances contained in these products. The estimation of realistic consumer exposure to substances in PCPs is currently hampered by the lack of appropriate data and methods. To estimate aggregate exposure of consumers to substances contained in PCPs, a person-oriented consumer exposure model has been developed (the Probabilistic Aggregate Consumer Exposure Model, PACEM). The model simulates daily exposure in a population based on product use data collected from a survey among the Dutch population. The model is validated by comparing diethyl phthalate (DEP) dose estimates to dose estimates based on biomonitoring data. It was found that the model's estimates compared well with the estimates based on biomonitoring data. This suggests that the person-oriented PACEM model is a practical tool for assessing realistic aggregate exposures to substances in PCPs. In the future, PACEM will be extended with use pattern data on other product groups. This will allow for assessing aggregate exposure to substances in consumer products across different product groups.

OpenVirtualToxLab--a platform for generating and exchanging in silico toxicity data

The VirtualToxLab is an in silico technology for estimating the toxic potential--endocrine and metabolic disruption, some aspects of carcinogenicity and cardiotoxicity--of drugs, chemicals and natural products. The technology is based on an automated protocol that simulates and quantifies the binding of small molecules towards a series of currently 16 proteins, known or suspected to trigger adverse effects: 10 nuclear receptors (androgen, estrogen α, estrogen β, glucocorticoid, liver X, mineralocorticoid, peroxisome proliferator-activated receptor γ, progesterone, thyroid α, thyroid β), four members of the cytochrome P450 enzyme family (1A2, 2C9, 2D6, 3A4), a cytosolic transcription factor (aryl hydrocarbon receptor) and a potassium ion channel (hERG). The toxic potential of a compound--its ability to trigger adverse effects--is derived from its computed binding affinities toward these very proteins: the computationally demanding simulations are executed in client-server model on a Linux cluster of the University of Basel. The graphical-user interface supports all computer platforms, allows building and uploading molecular structures, inspecting and downloading the results and, most important, rationalizing any prediction at the atomic level by interactively analyzing the binding mode of a compound with its target protein(s) in real-time 3D. Access to the VirtualToxLab is available free of charge for universities, governmental agencies, regulatory bodies and non-profit organizations.

Skin permeation of organic gunshot residue: implications for sampling and analysis

Traditional gunshot residue (GSR) analysis is based on detection of particulates formed from metals found in the primer. Recent concerns regarding the interpretation of GSR evidence has led to interest in alternatives such as the organic constituents (organic gunshot residue, OGSR) found in propellants. Previous work has shown OGSR to be detectable on hands for several hours after a firing event, and given the lipophilic nature of these compounds, it was expected that losses due to secondary transfer (an issue with GSR particulates) would be negligible. However, other loss mechanisms have been identified, specifically skin permeation and evaporation. This paper describes experimental and modeling studies used to elucidate characteristics of skin permeation of 5 compounds present in OGSR. Pharmaceutical methods were adapted to characterize skin permeation using a skin surrogate and Franz diffusion cells. The amount of compounds deposited on skin after an authentic firing event (1 and 2 shots) was experimentally determined and applied for the permeation experiments. A fully validated selected ion monitoring GC/MS method was developed for quantitative analysis, and easily accessible online tools were employed for modeling. Results showed that OGSR residues should be detectable on skin for many hours after a firing event of as few as one or two shots, with detection capability being a function of the efficacy of sampling and sample preparation and the instrumental method employed. The permeation rates of the OGSR compounds were sufficiently different to suggest the potential to develop methods to approximate time-since-deposition.

PBTK modelling platforms and parameter estimation tools to enable animal-free risk assessment: recommendations from a joint EPAA--EURL ECVAM ADME workshop

Information on toxicokinetics is critical for animal-free human risk assessment. Human external exposure must be translated into human tissue doses and compared with in vitro actual cell exposure associated to effects (in vitro-in vivo comparison). Data on absorption, distribution, metabolism and excretion in humans (ADME) could be generated using in vitro and QSAR tools. Physiologically-based toxicokinetic (PBTK) computer modelling could serve to integrate disparate in vitro and in silico findings. However, there are only few freely-available PBTK platforms currently available. And although some ADME parameters can be reasonably estimated in vitro or in silico, important gaps exist. Examples include unknown or limited applicability domains and lack of (high-throughput) tools to measure penetration of barriers, partitioning between blood and tissues and metabolic clearance. This paper is based on a joint EPAA--EURL ECVAM expert meeting. It provides a state-of-the-art overview of the availability of PBTK platforms as well as the in vitro and in silico methods to parameterise basic (Tier 1) PBTK models. Five high-priority issues are presented that provide the prerequisites for wider use of non-animal based PBTK modelling for animal-free chemical risk assessment.