In vitro predictions of skin absorption of caffeine, testosterone, and benzoic acid: a multi-centre comparison study

Enhanced transdermal permeation of caffeine through a skin model using electric field-induced lipid vesicles: a novel approach for drug transport

Caffeine is a highly beneficial compound for human health, known for its anticancer, anti-inflammatory, and antioxidant properties, particularly in protecting the skin from UVB radiation damage. Although caffeine shows excellent potential for transdermal delivery, its hydrophilic nature often requires a chemical enhancer for effective transport. Traditional methods like iontophoresis and electroporation utilize external electric fields to create micro-pores in the skin, enhancing the delivery of hydrophilic molecules. While electroporation is well understood, the molecular mechanisms of iontophoresis are unclear. This investigation presents an innovative mechanism for caffeine transport from an aqueous solution without chemical enhancers using lipid vesicles generated by external electric fields. To investigate the caffeine transdermal transport process, we combined our iontophoresis methodology with molecular dynamics simulations using Gromacs and the Martini force field alongside a practical custom experiment. Our approach employed a constant electric field of 22-25 mV nm-1, successfully generating lipid vesicles that transport caffeine molecules. Notably, alternating electric fields at 306 K (physiological skin temperature) increased caffeine transport by 20%, and at 323 K, we achieved an impressive 300% increase compared to scenarios without electric fields. Our homemade Franz cell setup showed a permeation rate dependent on the electric field correlated with vesicle formation. Additionally, hyperspectral Raman mapping identified unsaturated carbon and C-N groups as key contributors to vesicle and pore instability. This groundbreaking approach offers significant potential for enhancing transdermal drug delivery systems.

Finite Dose In Vitro Permeation Testing: Significance of Occluding the Donor compartment, a Case study

The evaporation of the solvent induces significant changes in formulation, directly impacting its performance. The performance of topical products is determined by the interplay between their inherent quality attributes and the transformations that occur due to solvent evaporation when applied to the skin in clinically relevant doses. To accurately assess, in vivo performance, it is advisable to apply smaller doses to the skin and keep the donor compartment open to enable evaporation of solvents while carrying out in vitro permeation tests. This manuscript highlights the critical role of solvent evaporation in differentiating the performance of two compositionally distinct products. One gel formulation contained alcohol, while the other did not. Although both exhibited similar quality attributes, their drying profiles varied significantly. Permeation studies conducted with closed donor compartments (Evaporation-disabled (ED)) failed to reveal these differences. However, when the donor compartments were exposed to the atmosphere to allow evaporation (Evaporation-enabled (EN)), the performance differences between the two products became evident.

In vitro human skin absorption of ethyl salicylate, pentyl salicylate, and (Z)-3-hexenyl salicylate from topical formulations: Effects on permeation and distribution

In vitro human skin permeation and distribution of the fragrance materials ethyl salicylate (CAS 118-61-6, ES), (Z)-3-hexenyl salicylate (CAS 65405-77-8, HS) and pentyl salicylate (CAS 2050-08-0, PS) from separate 0.5 % (w/w) cream formulations were determined under unoccluded and occluded conditions for 24 h. For PS only, a 0.5 % (w/v) solution in 70/30 (v/v) ethanol/water was also assessed. Consumer relevant finite formulation doses were applied (5 mg/cm2 or 5 μl/cm2) with salicylate application of ∼25 μg/cm2. Although specifically assessing metabolism was not an aim, the common hydrolysis product salicylic acid (SA) was also quantified and included in overall test compound absorption values. For ES, absorbed doses (mean ± standard error, SE) were 12.0 ± 1.0 and 24.7 ± 1.3 % applied dose under unoccluded and occluded conditions, respectively. For HS these values were 7.28 ± 0.52 and 11.1 ± 0.7 % applied dose. For the PS cream, corresponding values were 4.43 ± 0.48 and 7.52 ± 0.63 % applied dose. Whilst for the PS solution values were 8.26 ± 0.31 and 16.1 ± 0.7 % applied dose. The salicylate structure, application vehicle and level of occlusion impacted on the observed skin absorption. Considering salicylates are commonly used fragrance ingredients and have limited skin absorption data, the current research will be helpful in risk assessment to determine systemic exposure that is realistic and fill data gaps.

Formulation Study of a Poly(amino methacrylate) Film-Forming Solution for Transdermal Administration

BACKGROUND/OBJECTIVES

The objective of this paper is to design a novel film-forming system (FFS) based on Eudragit® E PO (EuE) polymeric solutions, differing in volatile solvents (i.e., isopropanol and ethanol) and plasticizers (i.e., tributylcitrate, glycerine, triacetin and PEG 400).

METHODS

The physicochemical and mechanical properties of the FFS and dried films were evaluated in terms of formation time, stickiness, Tg, tensile strength, break elongation and Young's modulus. The in vitro skin permeation studies were conducted on formulations containing caffeine and testosterone.

RESULTS

The FFS, consisting of EuE and PEG400 in isopropyl alcohol and ethanol (80:20, v/v), exhibited rapid film formation within about 5 min and the dried film allowed a high skin permeability compared to other formulations due to the ability to increase the thermodynamic activity of both drugs. When triiodothyronine (T3) was loaded as a model of a very low soluble drug, tocopherol polyethylene glycol succinate (TPGS) was added as a co-solvent and it allowed for the improvement of T3 retention in the skin.

CONCLUSIONS

Among the formulative variables, the nature and the amount of plasticizer represent the most critical variables to obtain an EuE-based film with satisfying physical and biopharmaceutical properties.

Towards the understanding of the IVPT results variability-Development, verification and validation of the PBPK model of caffeine in vitro human skin permeation

In the context of evaluating the safety and efficacy of dermal products, pharmacokinetic (PK) studies face considerable challenges, particularly concerning topically applied formulations. This underscores the necessity for alternative methods, such as in vitro permeation tests (IVPT) and physiologically based pharmacokinetic (PBPK) modelling, to better understand the dermal pharmacokinetics of a product. The purpose of this study was to modify, verify, and validate the PBPK model of caffeine permeation through human skin previously developed by Patel et al. (2022), and compare simulation results with experimental data from IVPT studies. Moreover, the study aimed to analyse the IVPT data variability and explore the potential of using the PBPK model to understand the influence of biological and drug-related factors on the IVPT results. In total, eight manuscripts describing nine experiments were included. The overall shapes of the permeation curves were considered acceptable based on visual checks for all analysed experiments. Five out of nine experiments met the predefined standard 2-fold difference criterion for comparison of the cumulative amount of caffeine in the receptor solution.. Our investigation highlights challenges in validating PBPK models for IVPT experiments, as the quality and consistency of experimental results pose significant hurdles. Despite access to data on caffeine permeation in scientific literature, reliable model validation is currently infeasible. Inter-laboratory variation suggests that alternative validation methods may be needed. Further studies should focus on issues with other compounds, especially lipophilic ones.

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).

In Vitro and In Vivo Anti-Aging Effect of Coffee Berry Nanoliposomes

Encapsulation of bioactive compounds in the liposome system provides several advantages, such as enhancing the stability and lowering the toxicity of active compounds. Coffee berry extract (CBE) has previously been established to have in vitro anti-aging properties and to retard the aging of human skin. The purposes of this study were to encapsulate CBE in nanoliposomes and to assess its stability and in vitro anti-aging potential in human dermal fibroblasts (HDF), as well as in healthy human skin. In the HDF model, anti-aging potential was determined by nitric oxide (NO) and collagenase inhibition assays and a superoxide dismutase (SOD) activity assay, whereas in healthy human skin (in vivo), the skin elasticity and brightness were examined. First, liposomal CBE (L-CBE) was created with a particle size of 117.33 ± 2.91 nm, a polydispersity index (PDI) of 0.36 ± 0.03, and a zeta potential of -56.13 ± 1.87 mV. The percentages of encapsulation efficacy (%EE) and loading efficacy (%LE) were 71.26 ± 3.12% and 2.18 ± 0.18%, respectively. After undergoing a 12-week stability test, the L-CBE retained more phenolic content than the free CBE when stored at 4 °C, room temperature, and 45 °C. Compared to free CBE, the L-CBE demonstrated a more consistent, elevated, and prolonged release of phenolics from the lipid system. In human dermal fibroblasts, L-CBE showed lower toxicity, and at its maximum nontoxic concentration (10 mg/mL), it exhibited slightly higher anti-aging effects than CBE, including NO inhibition, enhanced SOD activity, and anti-collagenase activities. In clinical trials (30 volunteer subjects), none of the participants' skin was irritated when the L-CBE, the CBE, or base creams were applied. After 2 weeks of application, the L-CBE and CBE creams both demonstrated an improvement in skin elasticity and a reduction in melanin levels, and after 4 weeks, L-CBE cream showed a significantly greater improvement in skin elasticity and lightening. The results demonstrate that the encapsulation of the CBE in liposomal systems could increase its stability and skin penetration, reduce its toxicity, and maintain its anti-aging effect, which is powerful enough to be exploited in anti-aging and whitening agents for application in cosmetics and cosmeceuticals.

Recent Options and Techniques to Assess Improved Bioavailability: In Vitro and Ex Vivo Methods

Bioavailability assessment in the development phase of a drug product is vital to reveal the disadvantageous properties of the substance and the possible technological interventions. However, in vivo pharmacokinetic studies provide strong evidence for drug approval applications. Human and animal studies must be designed on the basis of preliminary biorelevant experiments in vitro and ex vivo. In this article, the authors have reviewed the recent methods and techniques from the last decade that are in use for assessing the bioavailability of drug molecules and the effects of technological modifications and drug delivery systems. Four main administration routes were selected: oral, transdermal, ocular, and nasal or inhalation. Three levels of methodologies were screened for each category: in vitro techniques with artificial membranes; cell culture, including monocultures and co-cultures; and finally, experiments where tissue or organ samples were used. Reproducibility, predictability, and level of acceptance by the regulatory organizations are summarized for the readers.

Confocal Raman Spectroscopy for Assessing Bioequivalence of Topical Formulations

The evaluation of bioequivalence (BE) for topical dermatological drug products is challenging, and there has been significant interest from regulatory authorities in developing new BE methodologies in recent years. Currently, BE is demonstrated by comparative clinical endpoint studies; these are costly and time-consuming and often lack sensitivity and reproducibility. Previously, we reported excellent correlations between in vivo Confocal Raman Spectroscopy in human subjects and in vitro skin permeation testing (IVPT) with the human epidermis for skin delivery of ibuprofen and a number of excipients. The aim of the present proof-of-concept study was to evaluate CRS as a method to assess BE of topical products. Two commercially available formulations, Nurofen Max Strength 10% Gel and Ibuleve Speed Relief Max Strength 10% Gel, were selected for evaluation. Delivery of ibuprofen (IBU) to the skin was determined in vitro and in vivo by IVPT and CRS, respectively. The formulations examined were found to deliver comparable amounts of IBU across the skin over 24 h in vitro (p > 0.05). Additionally, the formulations resulted in similar skin uptake values measured with CRS in vivo, either at 1 h or 2 h after application (p > 0.05). This is the first study to report the capability of CRS for the demonstration of BE of dermal products. Future studies will focus on the standardisation of the CRS methodology for a robust and reproducible pharmacokinetic (PK)-based evaluation of topical BE.

A Comparative Analysis of Biological and Synthetic Skin Models for Drug Transport Studies

Ethical restrictions as well as practical or economic issues related to use of animal and human skin has been the main reason the growth in the number of investigations with alternative models. Reconstructed skin models, for example, have been useful to evaluate the in vitro toxicity of compounds; however, these models usually overestimate the amount of drug permeated due to impaired barrier properties. In this review, the performance of synthetic and biological skin models in transport studies was compared by considering two compounds with different physicochemical properties. The advantages and limitations of each skin model are discussed in detail. Although synthetic and reconstructed skin models have shown to be useful in the formulation optimization step, they present many limitations: (1) impaired barrier properties; (2) lack of follicular transport; (3) no metabolism in synthetic membranes; (4) differences in terms of lipid organization; (5) more affected by formulation constituents. Therefore, animal and human tissues should still be prioritized in drug transport studies until new advances in alternative models are achieved. Investigations of the impact of cell-culture conditions on skin formation, in turn, bring perspectives related to the development of unhealthy/injured skin models (an aspect that still deserves attention).

Nanoparticles for Topical Application in the Treatment of Skin Dysfunctions-An Overview of Dermo-Cosmetic and Dermatological Products

Nanomaterials (NM) arouse interest in various fields of science and industry due to their composition-tunable properties and the ease of modification. They appear currently as components of many consumer products such as sunscreen, dressings, sports clothes, surface-cleaning agents, computer devices, paints, as well as pharmaceutical and cosmetics formulations. The use of NPs in products for topical applications improves the permeation/penetration of the bioactive compounds into deeper layers of the skin, providing a depot effect with sustained drug release and specific cellular and subcellular targeting. Nanocarriers provide advances in dermatology and systemic treatments. Examples are a non-invasive method of vaccination, advanced diagnostic techniques, and transdermal drug delivery. The mechanism of action of NPs, efficiency of skin penetration, and potential threat to human health are still open and not fully explained. This review gives a brief outline of the latest nanotechnology achievements in products used in topical applications to prevent and treat skin diseases. We highlighted aspects such as the penetration of NPs through the skin (influence of physical-chemical properties of NPs, the experimental models for skin penetration, methods applied to improve the penetration of NPs through the skin, and methods applied to investigate the skin penetration by NPs). The review summarizes various therapies using NPs to diagnose and treat skin diseases (melanoma, acne, alopecia, vitiligo, psoriasis) and anti-aging and UV-protectant nano-cosmetics.

Dermal uptake: An important pathway of human exposure to perfluoroalkyl substances?

Per- and polyfluoroalkyl substances (PFAS) have been produced and used in a broad range of products since the 1950s. This class, comprising of thousands of chemicals, have been used in many different products ranging from firefighting foam to personal care products and clothes. Even at relatively low levels of exposure, PFAS have been linked to various health effects in humans such as lower birth weight, increased serum cholesterol levels, and reduced antibody response to vaccination. Human biomonitoring data demonstrates ubiquitous exposure to PFAS across all age groups. This has been attributed to PFAS-contaminated water and dietary intake, as well as inadvertent ingestion of indoor dust for adults and toddlers. In utero exposure and breast milk have been indicated as important exposure pathways for foetuses and nursing infants. More recently, PFAS have been identified in a wide range of products, many of which come in contact with skin (e.g., cosmetics and fabrics). Despite this, few studies have evaluated dermal uptake as a possible route for human exposure and little is known about the dermal absorption potential of different PFAS. This article critically investigates the current state-of-knowledge on human exposure to PFAS, highlighting the lack of dermal exposure data. Additionally, the different approaches for dermal uptake assessment studies are discussed and the available literature on human dermal absorption of PFAS is critically reviewed and compared to other halogenated contaminants, e.g., brominated flame retardants and its implications for dermal exposure to PFAS. Finally, the urgent need for dermal permeation and uptake studies for a wide range of PFAS and their precursors is highlighted and recommendations for future research to advance the current understanding of human dermal exposure to PFAS are discussed.

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.

Comparison Between Franz Diffusion Cell and a novel Micro-physiological System for In Vitro Penetration Assay Using Different Skin Models

In vitro diffusive models are an important tool to screen the penetration ability of active ingredients in various formulations. A reliable assessment of skin penetration enhancing properties, mechanism of action of carrier systems, and an estimation of a bioavailability are essential for transdermal delivery. Given the importance of testing the penetration kinetics of different compounds across the skin barrier, several in vitro models have been developedThe aim of this study was to compare the Franz Diffusion Cell (FDC) with a novel fluid-dynamic platform (MIVO) by evaluating penetration ability of caffeine, a widely used reference substance, and LIP1, a testing molecule having the same molecular weight but a different lipophilicity in the two diffusion chamber systems. A 0.7% caffeine or LIP1 formulation in either water or propylene glycol (PG) containing oleic acid (OA) was topically applied on the Strat-M® membrane or pig ear skin, according to the infinite-dose experimental condition (780 ul/cm²). The profile of the penetration kinetics was determined by quantify the amount of molecule absorbed at different time-points (1, 2, 4, 6, 8 hours), by means of HPLC analysis. Both diffusive systems show a similar trend for caffeine and LIP1 penetration kinetics. The Strat-M® skin model shows a lower barrier function than the pig skin biopsies, whereby the PGOA vehicle exhibits a higher penetration, enhancing the effect for both diffusive chambers and skin surrogates. Most interestingly, MIVO diffusive system better predicts the lipophilic molecules (i.e. LIP1) permeation through highly physiological fluid flows resembled below the skin models.

Dermal uptake of chlorinated organophosphate flame retardants via contact with furniture fabrics; implications for human exposure

The chlorinated organophosphate flame retardants (Cl-PFRs): tris-(2-chloroethyl)-phosphate (TCEP), tris-(1-chloro-2-propyl)-phosphate (TCIPP) and tris-(1,3-dichloropropyl)-phosphate (TDCIPP), have been widely used in upholstered furniture despite their carcinogenic potential. Although Cl-PFRs are mainly added to furniture foam, they are present in the fabrics likely due to migration from the foam. While several studies have assessed human exposure to Cl-PFRs via different pathways, no information exists on dermal uptake of these chemicals through contact with fabrics. In the current study, dermal absorption of TCEP, TCIPP and TDCIPP from 3 UK domestic furniture fabrics was experimentally assessed for the first time using in vitro 3D-human skin equivalents (EpiSkin™) under different real-life exposure scenarios. Results revealed all 3 target Cl-PFRs were dermally bioavailable to varying degrees (3.5%-25.9% of exposure dose) following 24 h contact with the studied fabrics. Estimated permeability coefficients (K_P, cm h^-1) showed TCEP had the highest percutaneous penetration potential followed by TCIPP, then TDCIPP. Further investigation revealed human dermal uptake of Cl-PFRs can be influenced by several factors including: the specific physicochemical properties of the compound, the type of exposure matrix, the exposure dose and the degree of skin hydration at the point of contact. Exposure assessment revealed UK adults and toddlers can be exposed to 20.4 and 14.1 ng TCIPP/kg bw/day via contact with furniture fabrics in summer, which is higher than international average exposures via inhalation and dust ingestion for adults and dietary exposure for toddlers. Therefore, risk assessment studies for Cl-PFRs and future replacements should consider dermal contact with consumer products (e.g. furniture fabrics) as a potential significant human exposure pathway.

An OECD TG 428 study ring trial with 14C-Caffeine demonstrating repeatability and robustness of the dermal absorption in vitro method

The dermal absorption potential of ¹⁴C-Caffeine applied as a 4 mg/mL concentration (10 μL/cm² finite dose) was investigated in six laboratories under Good Laboratory Practice conditions using an OECD TG 428-compliant in vitro assay with flow-through cells and split-thickness human skin. Potential sources of variation were reduced by a standardized protocol, test item and skin source. Particularly, skin samples from same donors were distributed over two repeats and between labs in a non-random, stratified design. Very similar recovery was achieved in the various assay compartments between laboratories, repeats and donors, demonstrating that the assay can be robustly and reliably performed. The absorption in one laboratory was 5-fold higher than in the others. This did not clearly correlate with skin integrity parameters but might be associated with an accidental COVID-19 pandemic-related interruption in sample shipment. It is possible that other factors may affect dermal absorption variation not routinely assessed or considered in the current method. The mean receptor fluid recovery, potential absorption (recovery in receptor fluid and skin except tape strips 1 and 2) and mass balance of caffeine was 6.99%, 7.14% and 99.13%, respectively, across all and 3.87%, 3.96% and 99.00% in the subset of five laboratories.

Comparative study of the percutaneous permeation and bioaccumulation of the cyclic siloxane using frozen-thawed and nonfrozen ex vivo human skin

Literature shows contradictory information regarding the effect of freezing the excise skin ex vivo on the diffusion of substances into the skin. Few studies indicate that storing the human or animal skin in a frozen state decreases the barrier properties after thawing. Therefore, to understand the properties of frozen skin, we evaluated the effect of storage of ex vivo human skin (2 weeks at -20 °C) on the penetration of stratum corneum and permeation into deeper skin layers (epidermis, and dermis) as well as to the receptor fluid by octamethylcyclotetrasiloxane (D4) a representative test compound of cyclic siloxanes. The main research were preceded by checking the integrity of nonfrozen ex vivo human skin in comparison to the frozen-thawed one by using the Electrical Resistance technique (ER) and the fluorescence microscopy. Samples collected in the skin absorption experiment were analyzed by gas chromatography equipped with a flame ionization detector (GC-FID). The results of this study demonstrated that freezing of excised ex vivo human skin at -20 °C for up to 14 days does not alter the permeability of D4 in a statistically significant manner. Thus, our results confirmed the validity of using skin storage conditions for testing the penetration and permeation of xenobiotics recommended by the OECD, EMA, and WHO guidelines.

α-Linolenic acid and linoleic acid modulate the lipidome and the skin barrier of a tissue-engineered skin model

Polyunsaturated fatty acids (PUFAs) play an important role in the establishment and the maintenance of the skin barrier function. However, the impact of their derived lipid mediators remains unclear. Skin substitutes were engineered according to the self-assembly method with a culture medium supplemented with 10 μM of both α-linolenic acid (ALA) and linoleic acid (LA). The supplementation with ALA and LA decreased testosterone absorption through a tissue-engineered reconstructed skin model, thus indicating an improved skin barrier function following supplementation. The exogenously provided fatty acids were incorporated into the phospholipid and triglyceride fractions of the skin substitutes. Indeed, the dual supplementation increased the levels of eicosapentaenoic acid (EPA) (15-fold), docosapentaenoic acid (DPA) (3-fold), and LA (1.5-fold) in the epidermal phospholipids while it increased the levels of ALA (>20-fold), DPA (3-fold) and LA (1.5-fold) in the epidermal triglycerides. The bioactive lipid mediator profile of the skin substitutes, including prostaglandins, hydroxy-fatty acids, N-acylethanolamines and monoacylglycerols, was next analyzed using liquid chromatography-tandem mass spectrometry. The lipid supplementation further modulated bioactive lipid mediator levels of the reconstructed skin substitutes, leading to a lipid mediator profile more representative of the one found in normal human skin. These findings show that an optimized supply of PUFAs via culture media is essential for the establishment of improved barrier function in vitro. STATEMENT OF SIGNIFICANCE: Supplementation of the culture medium with 10 μM of both α-linolenic acid (ALA) and linoleic acid (LA) improved the skin barrier function of a tissue-engineered skin model. The exogenously provided fatty acids were incorporated into the phospholipid and triglyceride fractions of the skin substitutes and further modulated bioactive lipid mediator levels, including prostaglandins, hydroxy-fatty acids, N-acylethanolamines and monoacylglycerols. These findings highlight the important role of ALA and LA in skin homeostasis and show that an optimized supply of polyunsaturated fatty acids via culture media is essential for the establishment of improved barrier function in vitro.

A review of the success and challenges in characterizing human dermal exposure to flame retardants

Since organic flame retardants (FRs) have several industrial applications, they have been largely detected in environmental and biological samples, and humans have been highly exposed to them. Although the effects of oral and inhaled FRs have been well studied, dermal exposure to them has only recently been pointed out as a potential route of human exposure. Consequently, the effects of FRs on the skin and secondary target organs have been poorly investigated. This review article summarizes the main findings regarding dermal exposure to FRs, points the limitation of the published studies, and suggests future perspectives for better understanding of how dermal exposure to FRs impacts the human health. This review lists some gaps that must be filled in future studies, including characterization of the bioavailable fraction and assessment of exposure for new FRs, to establish their physiological significance and to improve the development of 3D dermal tissue for more reliable results to be obtained.

How to define the experimental conditions of skin permeation assays for drugs presenting biopharmaceutical limitations? The experience with testosterone

Cutaneous permeation assays are crucial to attest the performance or bioequivalence of topical or transdermal products. Although the official guidelines (e.g., FDA/EMA) play a key role in harmonizing the experimental design, alternative methods are often proposed by the scientific community, which makes it difficult to compare results from different studies. In this review, permeation assays with testosterone (TST) were selected to show this high variability in drug transport rate. The main sources of variation discussed were tissue thickness, animal model, donor and receptor fluid constitution, type of solubilizing agent used in aqueous fluids, drug concentration, degree of supersaturation, skin lipid content, number of experimental times and the physical-chemical stability of the molecule in test fluids. This variation becomes even more critical for molecules that present biopharmaceutical limitations such as TST. In addition, the skin presents specific receptors for this hormone due to its physiological action in this region of the body, which makes the evaluation of the TST transport rate in this tissue even more challenging. The impact of each experimental parameter mentioned above on the flux or permeation coefficient of TST is discussed in detail in the review. Assays used to evaluate tissue integrity are also presented.

A Physiologically Based Pharmacokinetic Model for Naphthalene With Inhalation and Skin Routes of Exposure

Naphthalene, a volatile organic compound present in moth repellants and petroleum-based fuels, has been shown to induce toxicity in mice and rats during chronic inhalation exposures. Although simpler default methods exist for extrapolating toxicity points of departure from animals to humans, using a physiologically based pharmacokinetic (PBPK) model to perform such extrapolations is generally preferred. Confidence in PBPK models increases when they have been validated using both animal and human in vivo pharmacokinetic (PK) data. A published inhalation PBPK model for naphthalene was previously shown to predict rodent PK data well, so we sought to evaluate this model using human PK data. The most reliable human data available come from a controlled skin exposure study, but the inhalation PBPK model does not include a skin exposure route; therefore, we extended the model by incorporating compartments representing the stratum corneum and the viable epidermis and parameters that determine absorption and rate of transport through the skin. The human data revealed measurable blood concentrations of naphthalene present in the subjects prior to skin exposure, so we also introduced a continuous dose-rate parameter to account for these baseline blood concentration levels. We calibrated the three new parameters in the modified PBPK model using data from the controlled skin exposure study but did not modify values for any other parameters. Model predictions then fell within a factor of 2 of most (96%) of the human PK observations, demonstrating that this model can accurately predict internal doses of naphthalene and is thus a viable tool for use in human health risk assessment.

Effect of Application Amounts on In Vitro Dermal Absorption Test Using Caffeine and Testosterone

Dermal absorption of chemicals is a key factor in risk assessment. This study investigated the effects of different amounts of application on dermal absorption and suggested an appropriate application dose for proper dermal absorption. Caffeine and testosterone were chosen as test compounds. An in vitro dermal absorption test was performed using a Franz diffusion cell. Different amounts (5, 10, 25, and 50 mg (or µL)/cm²) of semisolid (cream) and liquid (solution) formulations containing 1% caffeine and 0.1% testosterone were applied to rat and minipig (Micropig^®) skins. After 24 h, the concentrations of both compounds remaining on the skin surface and in the stratum corneum, dermis and epidermis, and receptor fluid were determined using LC-MS / MS or HPLC. Dermal absorption of both compounds decreased with increasing amounts of application in both skin types (rat and minipig) and formulations (cream and solution). Especially, dermal absorptions (%) of both compounds at 50 mg (or µL)/cm² was significantly lower compared to 5 or 10 mg (or µL)/cm² in both rat and minipig skins. Therefore, a low dose (5 or 10 mg (or µL)/cm²) of the formulation should be applied to obtain conservative dermal absorption.

Development and evaluation of a quality control system based on transdermal electrical resistance for skin barrier function in vitro

BACKGROUND

In vitro skin permeation experiments are highly relevant for pharmaceutical, cosmetic, agricultural developments, and regulatory evaluation. A key requirement is the skin barrier integrity, that is accompanied by an intact stratum corneum (SC) which implements high skin quality. A variety of integrity tests are currently available, for example, measurement of transepidermal water loss, monitoring the permeation of tritiated water and the measurement of transdermal electrical resistance (TER).

MATERIALS AND METHODS

We aimed for a non-destructive examination of barrier integrity as quality control system, based on TER. Therefore, the in-house developed instrument SkinTER measures electrical resistance on excised human skin samples in a non-invasive and easy-to-use pattern. In this proof of concept study, we compared three human in vitro skin models with focus on their TER and permeation properties. The skin integrity was impaired to mimic conditions of skin during age, lifestyle (eg, shaving) or diseases (eg, obesity, psoriasis, and atopic dermatitis). The OECD permeation marker caffeine was correlated to the corresponding TER value.

RESULTS

A correlation between both was obtained by having a Pearson coefficient of -0.830. Hereby, a minimum TER value for intact skin samples of ~1.77 kΩ*cm² was suggested. Intact samples are significantly different (α = ≤0.05) to their impaired counterparts in flux and TER values.

CONCLUSION

The new SkinTER instrument gives a quick and non-invasive feedback on skin quality before a permeation experiment.

In vitro human skin percutaneous penetration: does a second topical application effect flux of first application?

INTRODUCTION

Skin care products are often utilized in conjunction with topical treatment for skin disease. However, these appears insufficient body of experimental data to advise the health care worker or patient. Examine penetration effect (hence potential for altering efficacy and toxicity) - dosing with one topical after another and how quickly such a potential phenomenon might occur.

METHODS

A marketed moisturizer applied on human skin with an in vitro diffusion system. At 1, 15, and 30 min post application, [¹⁴C]-benzoic acid dosed the same skin site for 24 h. Amounts of chemical retained in skin and permeation flux rats were measured to determine penetration effect of the prior moisturizer application.

RESULTS

Exposure of a water-enriched moisturizer before a hydrophilic chemical immediate application favors the chemical penetration, especial in 1 and 15 min moisturizer exposure groups. The enhancement effect was expressed as an earlier lag time and a rapid absorption peak when compared to related non-moisturizer control.

CONCLUSION

This experiment opens up a large door: what would be the result with many complex topical products and different tracers was used here, of varying hydrophilicity and lipophilicity. We do not wish to overgeneralize until such studies are confirmed in vivo.

Caffeine and Its Pharmacological Benefits in the Management of Androgenetic Alopecia: A Review

Caffeine, particularly after ingestion, is well known to exert various pharmacological effects. A growing body of evidence implicates the ingestion of caffeine with beneficial effects on several diseases. The easy penetration of caffeine across the skin barrier and into human skin makes caffeine an ideal compound for topical application. Hair loss is known to negatively affect the quality of life and predispose to depression and anxiety. Androgenetic alopecia (AGA) is the most common type of hair loss in both men and women. To date, only few approved drug-based treatments for AGA exist, and these are inevitably associated with side effects. Therefore, the development of topical treatments based on well-tolerated natural ingredients such as caffeine to alleviate hair loss may provide a much-needed alternative to drug-based approaches.

Investigation of TEMPO partitioning in different skin models as measured by EPR spectroscopy - Insight into the stratum corneum

Electron paramagnetic resonance (EPR) spectroscopy represents an established tool to study properties of microenvironments, e.g. to investigate the structure and dynamics of biological and artificial membranes. In this study, the partitioning of the spin probe 2,2,6,6-tetramethylpiperidine-1-oxyl (TEMPO) in ex vivo human abdominal and breast skin, ex vivo porcine abdominal and ear skin as well as normal and inflammatory in vitro skin equivalents was investigated by EPR spectroscopy. Furthermore, the stratum corneum (SC) lipid composition (as determined by high-performance thin-layer chromatography), SC lipid chain order (probed by infrared spectroscopy) and the SC thickness (investigated by histology) were determined in the skin models. X-band EPR measurements have shown that TEMPO partitions in the lipophilic and hydrophilic microenvironment in varying ratios in different ex vivo and in vitro skin models. Ex vivo human abdominal skin exhibited the highest amount of TEMPO in the lipophilic microenvironment. In contrast, the lowest amount of TEMPO in the lipophilic microenvironment was determined in ex vivo human breast skin and the inflammatory in vitro skin equivalents. Individual EPR spectra of epidermis including SC and dermis indicated that the lipophilic microenvironment of TEMPO mainly corresponds to the most lipophilic part of the epidermis, the SC. The amount of TEMPO in the lipophilic microenvironment was independent of the SC lipid composition and the SC lipid chain order but correlated with the SC thickness. In conclusion, EPR spectroscopy could be a novel technique to determine differences in the SC thickness, thus suitably complementing existing methods.

Measurement of the penetration of 56 cosmetic relevant chemicals into and through human skin using a standardized protocol

OECD test guideline 428 compliant protocol using human skin was used to test the penetration of 56 cosmetic‐relevant chemicals. The penetration of finite doses (10 μL/cm²) of chemicals was measured over 24 hours. The dermal delivery (DD) (amount in the epidermis, dermis and receptor fluid [RF]) ranged between 0.03 ± 0.02 and 72.61 ± 8.89 μg/cm². The DD of seven chemicals was comparable with in vivo values. The DD was mainly accounted for by the amount in the RF, although there were some exceptions, particularly of low DD chemicals. While there was some variability due to cell outliers and donor variation, the overall reproducibility was very good. As six chemicals had to be applied in 100% ethanol due to low aqueous solubility, we compared the penetration of four chemicals with similar physicochemical properties applied in ethanol and phosphate‐buffered saline. Of these, the DD of hydrocortisone was the same in both solvents, while the DD of propylparaben, geraniol and benzophenone was lower in ethanol. Some chemicals displayed an infinite dose kinetic profile; whereas, the cumulative absorption of others into the RF reflected the finite dosing profile, possibly due to chemical volatility, total absorption, chemical precipitation through vehicle evaporation or protein binding (or a combination of these). These investigations provide a substantial and consistent set of skin penetration data that can help improve the understanding of skin penetration, as well as improve the prediction capacity of in silico skin penetration models.

The penetration of 56 chemicals was tested in human skin using a standard protocol. Dermal delivery correlated with the amount in the receptor fluid (RF). The impact of solvent on penetration was evaluated. Despite finite doses being applied, different profiles of cumulative absorption kinetics into the RF were observed. These data may help understand skin penetration and improve the prediction capacity of in silico skin penetration models.

Applications of Tea (Camellia sinensis) and its Active Constituents in Cosmetics

Studies on the cosmetic applications of plant extracts are increasingly appearing in the scientific literature, which is due to the growing popularity of skincare products around the world. In the light of the observed changes, a return to natural treatment and skincare with cosmetics free of harmful substances or toxic preservatives is visible. Currently, tea extracts, due to their rich composition and various biological actions, play an important role among the dietary supplements and cosmetics. This review is intended to collect the reports on the properties of the tea plant, its extracts and preparations in cosmetology: for skin care products and for the treatment of selected dermatological diseases. Particular attention is paid to its antioxidant, anti-hyaluronidase, anti-inflammatory, slimming, hair-strengthening, photoprotective and sealing blood vessels properties.

Cerium Oxide Nanoparticles Absorption through Intact and Damaged Human Skin

Cerium oxide (CeO2) nanoparticles (NPs) are used in polishing products and absorbents, as promoters in wound healing, and as organopesticide decontaminants. While systemic bioaccumulation and organ toxicity has been described after inhalation, data on CeO2 NPs' transdermal permeation are lacking. Our study was an in vitro investigation of the permeation of 17-nm CeO2 NPs dispersed in synthetic sweat (1 g L-1) using excised human skin on Franz cells. Experiments were performed using intact and needle-abraded skin, separately. The average amount of Ce into intact and damaged skin samples was 3.64 ± 0.15 and 7.07 ± 0.78 µg cm-2, respectively (mean ± SD, p = 0.04). Ce concentration in the receiving solution was 2.0 ± 0.4 and 3.3 ± 0.7 ng cm-2 after 24 h (p = 0.008). The Ce content was higher in dermal layers of damaged skin compared to intact skin (2.93 ± 0.71 µg cm-2 and 0.39 ± 0.16 µg cm-2, respectively; p = 0.004). Our data showed a very low dermal absorption and transdermal permeation of cerium, providing a first indication of Ce skin uptake due to contact with CeO2.

In Vitro Skin Permeation Methodology for Over-The-Counter Topical Dermatologic Products

For topically applied over-the-counter (OTC) products, the association of unwanted systemic exposure and adverse events may be difficult to ascertain without a recognition or determination of in vivo absorption. Evaluation of skin permeability using a validated in vitro permeation methodology can provide important information for both initial formulation selection and reformulation during the product life cycle. Additionally, a comparison of permeation rates between formulations using a validated methodology could reduce the number of nonclinical studies needed as part of reformulation. However, many in vitro permeation tests (IVPTs) have produced results with high variability and low reproducibility between study sites. It is unclear if this is due to a lack of a standardized protocol, or lack of control of multiple key experimental factors including skin source, preparation, receptor fluid, and study design. This review presents the authors perspective on the potential regulatory utility of IVPT and proposes steps to improve the accuracy and reproducibility of IVPT. The focus of this review is on topical dermatologic drugs with an initial emphasis on the OTC marketplace where reformulations are more common.

[Follicular penetration of nanocarriers is an important penetration pathway for topically applied drugs]

BACKGROUND

The hair follicle represents a significant penetration route for topically applied substances.

ISSUE

The percutaneous absorption of substances can be significantly increased and accelerated by the involvement of hair follicles. In addition, nanoparticles have the characteristic to penetrate deeply and effectively into the hair follicles.

MATERIALS AND METHODS

An optimization of drug delivery for topically applied substances is possible if the nanoparticles act solely as a carrier to transport active ingredients into the hair follicle. Once the nanocarrier has penetrated into the hair follicle, the active substance must be released there. This can be triggered by various mechanisms.

RESULTS

The released drug can thus pass into the living tissue surrounding the hair follicle independently. With the help of this innovative strategy, the bioavailability of topically applied substances can be significantly improved.

CONCLUSION

The transport of active ingredients into the hair follicles with the help of particles and the release of active substances there is a very effective new method for transporting active substances through the skin barrier.

Modulation of the Lipid Profile of Reconstructed Skin Substitutes after Essential Fatty Acid Supplementation Affects Testosterone Permeability

Skin models with efficient skin barrier function are required for percutaneous absorption studies. The contribution of media supplementation with n-3 and n-6 polyunsaturated fatty acids (PUFAs) to the development of the skin barrier function of in vitro skin models remains incompletely understood. To investigate whether PUFAs, alpha-linolenic acid (ALA, n-3 PUFA) and linoleic acid (LA, n-6 PUFA), could enhance the impermeability of a three-dimensional reconstructed human skin model, skin substitutes were produced according to the self-assembly method using culture media supplemented with either 10 μM ALA or 10 μM LA. The impact of PUFAs on skin permeability was studied by using a Franz cell diffusion system to assess the percutaneous absorption of testosterone and benzoic acid. Our findings showed that ALA supplementation induced a decrease in the absorption of testosterone, while LA supplementation did not significantly influence the penetration of testosterone and benzoic acid under present experimental conditions. Both ALA and LA were incorporated into phospholipids of the skin substitutes, resulting in an increase in n-3 total PUFAs or n-6 total PUFAs. Collectively, these results revealed the under-estimated impact of n-3 PUFA supplementation as well as the importance of the n-6 to n-3 ratio on the formation of the skin barrier of in vitro reconstructed human skin models.

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

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

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.

Formulation and Artificial Sebum Effects on the Percutaneous Absorption of Zinc Pyrithione through Excised Human Skin

BACKGROUND

Zinc pyrithione (ZnPT) is deposited on the skin as a fine particulate and must reach microorganisms localized in the stratum corneum and hair follicles in molecular form to exert its broad-spectrum antimicrobial/antifungal activity. Dissolution of ZnPT particles followed by molecular speciation results in the organic portion, i.e. pyrithione, being more susceptible to skin penetration than the inorganic component, i.e. zinc, or the chelate itself, i.e. ZnPT.

OBJECTIVES

To further test the hypothesis that ZnPT skin penetration is rate-limited by dissolution and molecular speciation, the effect of different formulations and artificial sebum on the in vitro percutaneous absorption of radiolabel associated with Zn[14C]PT was investigated.

METHOD

In vitro penetration of [14C]PT into and through excised human skin was measured following application of Zn[14C]PT prepared as suspensions in distinct vehicles including water-based carboxymethylcellulose (CMC), diluted body wash comprised of surfactants, and castor oil, in the presence and absence of artificial sebum.

RESULTS

The steady-state flux and cumulative absorption of Zn[14C]PT increased 4- to 5-fold when deposited from a body wash or castor oil compared to a water-based CMC suspension. Tritiated water flux measured before and after treatment showed that neither the surfactant vehicle nor castor oil significantly altered barrier function versus water alone. An artificial sebum layer on the skin potentiated Zn[14C]PT and 3H2O absorption when dosed from both aqueous formulations, but not from castor oil.

CONCLUSION

These data are consistent with the hypothesis that ZnPT percutaneous absorption, as measured by [14C]PT kinetics, is controlled by particle dissolution and molecular speciation.

Another Reason for Using Caffeine in Dermocosmetics: Sunscreen Adjuvant

The excessive exposure to ultraviolet (UV) radiation is the main cause of skin cancer, the most commonly diagnosed cancer in the world. In this context, the development of innovative and more effective sunscreens, with bioactive compounds like caffeine, displaying antioxidant and anticancer potential, is required. This research work assessed in vitro and in vivo the efficacy and safety of topical sunscreen formulations containing caffeine as an adjuvant of the UV filters. Sunscreens were prepared with 2.5% w/w caffeine or in the absence of this compound. In order to evaluate the safety of these formulations, stratum corneum hydration, skin barrier and colorimetry were assessed in vivo in healthy subjects before and after skin treatment with the samples. The efficacy of the sunscreens was assessed in vitro, using PMMA plates and a spectrophotometer equipped with an integrating sphere; and in vivo by the determination of the sun protection factor (SPF). None of the formulations caused erythema or impaired the skin barrier function. The in vitro functional characterization showed higher SPF values for the caffeine formulation. The in vivo studies also confirmed the higher SPF value of the formulation combining caffeine with the filters, compared to the caffeine-free sample. This improvement contributed to an increase of, approximately, 25% in the in vivo anti-UVB protection. In conclusion, caffeine was well tolerated by the skin and increased the photoprotective activity, being a new alternative adjuvant in sunscreens formulation.

Skin toxicology and 3Rs-Current challenges for public health protection

Driven by the fast paced development of complex test systems in vitro, mass spectrometry and omics, we finally have the tools to unravel the molecular events that underlie toxicological adversity. Yet, timely regulatory adaptation of these new tools continues to pose major challenges even for organs readily accessible such as skin. The reasons for this encompass a need for conservatism as well as the need of tests to serve an existing regulatory framework rather than to produce scientific knowledge. It is important to be aware of this in order to align regulatory skin toxicity with the 3R principles more readily. While most chemical safety testing is still based on animal data, regulatory frameworks have seen a strong push towards non-animal approaches. The endpoints corrosion, irritation, sensitisation, absorption and phototoxicity, for example, can now be covered in vitro with the corresponding test guidelines (TGs) being made available by the OECD. However, in vitro approaches tend to be more reductionist. Hence, a combination of several tests is usually preferable to achieve satisfying predictivity. Moreover, the test systems and their combined use need to be standardised and are therefore subject not only to validation but also to the ongoing development of so-called integrated approaches to testing and assessment (IATAs). Concomitantly, skin models are being refined to deliver the complexity required for increased applicability and predictivity. Given the importance of regulatory applicability for 3R-derived approaches to have a long-lasting impact, this review examines the state of regulatory implementation and perspectives, respectively.

Predicting Skin Permeability by Means of Computational Approaches: Reliability and Caveats in Pharmaceutical Studies

The skin is the main barrier between the internal body environment and the external one. The characteristics of this barrier and its properties are able to modify and affect drug delivery and chemical toxicity parameters. Therefore, it is not surprising that permeability of many different compounds has been measured through several in vitro and in vivo techniques. Moreover, many different in silico approaches have been used to identify the correlation between the structure of the permeants and their permeability, to reproduce the skin behavior, and to predict the ability of specific chemicals to permeate this barrier. A significant number of issues, like interlaboratory variability, experimental conditions, data set building rationales, and skin site of origin and hydration, still prevent us from obtaining a definitive predictive skin permeability model. This review wants to show the main advances and the principal approaches in computational methods used to predict this property, to enlighten the main issues that have arisen, and to address the challenges to develop in future research.