Inter- and intra-individual variability in human skin barrier function: A large scale retrospective study

Discussion of EMA Draft Guideline on Quality and Equivalence of Topical Products Based on Comparison of Approved Mometasone Furoate Drugs

INTRODUCTION

Today, the approval for a generic topical product includes the presentation of therapeutic equivalence to the originator based on clinical trials. To facilitate this procedure, in 2018 the European Medicines Agency (EMA) published a draft guideline on quality and equivalence of topical products, which includes request parameters regarding the quality of the newly developed generic product and test protocols for the implementation of equivalence tests regarding efficacy.

METHODS

To date, no data are available on the quality and evidence of the proposed test conditions. In this study, we performed an in vitro penetration test (IVPT) following the terms of the EMA draft guideline on two authorized topical products for which therapeutic equivalence was already proven during the approval process.

RESULTS

The complex biometric data processing revealed that in vitro equivalence could not be observed for all skin sections for either originator or generic product. Moreover, the necessity of the negative control proposed in the draft guideline is more than questionable. From the results presented, there were indications that a reduced number of skin donors would be sufficient to achieve statistically significant equivalence in the comparison of all applied formulations. Here, n = 7 donors was proposed instead of n ≥ 12 as the EMA draft guideline demands, decreasing the degree of biodiversity simultaneously. Moreover, a higher number of independent replicates (n > 2) was proposed for proper statistics.

CONCLUSION

This bioequivalence study shows insufficient parameters, which should be discussed together with the EMA draft guideline.

Observations on the Tritiated Water and TEWL Skin Integrity Tests: Relevance to In Vitro Permeation Testing (IVPT)

BACKGROUND

The in vitro permeation test (IVPT) using ex vivo human skin is a sensitive and robust model system that has been vital in elucidating the fundamental parameters surrounding the absorption of both therapeutic agents and industrial chemicals through skin. FDA and OECD IVPT Guidances recommend that each skin section selected for study should be screened prior to use to ensure that the stratum corneum integrity is retained. Three methods are currently considered acceptable: 1) transepidermal water loss (TEWL), 2) electrical resistance, and 3) tritiated water (3H2O) absorption.

METHODS

A retrospective analysis of data from the authors' laboratory has been performed with the objective of addressing a number of questions regarding the 3H2O and TEWL integrity tests, and the population attributes of a large database consisting of 17,330 individual skin sections obtained from 459 skin donors. The applicability and usefulness of these tests, when compared to companion permeation data obtained from 25 topical drug products, has also been examined.

RESULTS

Both integrity tests found water permeability to be equal in White and Hispanic races but higher than in Blacks, 3H2O being more discriminating than TEWL. Male skin is more permeable than female and there is a slight decrease in permeability with advancing age in both groups. Correlation between 3H2O absorption and drug absorption revealed a minimal relationship between the two in most cases, the Pearson correlation coefficient ranging from -0.417 to 0.953. Additionally, drug outliers were not always identified with a failing integrity test.

CONCLUSION

The results call for a critical reexamination of the value of the 3H2O integrity test, and by extension, TEWL, for use in IVPT studies.

A comparative study of the in vitro dermal absorption of radiolabeled benzophenone through human skin

Octocrylene is a common sun filter ingredient used to protect the skin from damaging UV rays. Benzophenone is an impurity found in formulations containing octocrylene. [14C]-Benzophenone was spiked (0.1 g/L) into 2 commercial sunscreen formulations; Neutrogena® Beach Defense Sunscreen Spray Broad Spectrum SPF 70 Aerosol, Neutrogena® Ultra Sheer Body Mist Sunscreen Broad Spectrum SPF 30 Aerosol, and an acetone vehicle. The formulations were applied (ca 2 μL/cm2) to dermatomed human skin mounted in static diffusion cells in vitro. Receptor fluid was collected up to 24 h post dose. All samples were analyzed by liquid scintillation counting. The dermal delivery of [14C]-Benzophenone was 10.02, 9.04 and 5.19% for the 3 formulations. However, the [14C]-Benzophenone mass balances were low; 81.5, 85.3 and 8.02%, respectively. A volatility test was performed replacing skin with aluminum foil for the sunscreen formulations only. The [14C]-Benzophenone mass balance at dosing was 99% but fell to 56.9 and 60.6% at 24 h post dose, confirming the losses were due to [14C]-Benzophenone volatility. A conservative dermal absorption value of 12.42% was proposed to cover [14C]-Benzophenone containing formulations.

In vitro evaluation of the percutaneous absorption of progesterone in anhydrous permeation-enhancing base using the Franz skin finite dose model and mass spectrometry

Progesterone is used for hormone replacement therapy through various routes of administration. This study was conducted to (a) evaluate the stability of progesterone in a proprietary anhydrous permeation-enhancing base (APEB) and the efficiency of its skin permeation, and (b) determine the appropriateness of mass spectrometry as a method of analysis for permeated progesterone. Using a proven stability-indicating ultra-performance liquid chromatographic method, the compounded hormone (100 mg progesterone/g APEB gel) was determined to be physically and chemically stable at room temperature for six months. Skin permeation analysis using the Franz skin finite dose model and mass spectrometry imaging showed an optical density of 1699 for the permeated progesterone compounded in APEB and 550 for the permeated progesterone in a water containing VBC, which is a statistically significant different (P = 0.029). The study suggests that APEB can be used as a compounding base for effective skin permeation of progesterone, and mass spectrometry is a reliable method for visualization and quantitative analysis of permeated progesterone.

An open-label feasibility trial of transdermal cannabidiol for hand osteoarthritis

Hand osteoarthritis (OA) is an irreversible degenerative condition causing chronic pain and impaired functionality. Existing treatment options are often inadequate. Cannabidiol (CBD) has demonstrated analgesic and anti-inflammatory effects in preclinical models of arthritis. In this open-label feasibility trial, participants with symptomatically active hand OA applied a novel transdermal CBD gel (4% w/w) three times a day for four weeks to their most painful hand. Changes in daily self-reported pain scores were measured on a 0-10 Numeric Pain Rating Scale (NPRS). Hand functionality was determined via daily grip strength measures using a Bluetooth equipped squeeze ball and self-report questionnaire. Quality of life (QoL) ratings around sleep, anxiety, stiffness and fatigue were also measured. All self-report measures and grip strength data were gathered via smartphone application. Urinalysis was conducted at trial end to determine systemic absorption of CBD. Eighteen participants were consented and 15 completed the trial. Pain ratings were significantly reduced over time from pre-treatment baseline including current pain (- 1.91 ± 0.35, p < 0.0001), average pain (- 1.92 ± 0.35, p < 0.0001) and maximum pain (- 1.97 ± 0.34, p < 0.0001) (data represent mean reduction on a 0-10 NPRS scale ± standard error of the mean (SEM)). A significant increase in grip strength in the treated hand (p < 0.0001) was observed although self-reported functionality did not improve. There were significant (p < 0.005) improvements in three QoL measures: fatigue, stiffness and anxiety. CBD and its metabolites were detected at low concentrations in all urine samples. Measured reductions in pain and increases in grip strength seen during treatment reverted back towards baseline during the washout phase. In summary, pain, grip strength and QoL measures, using smartphone technology, was shown to improve over time following transdermal CBD application suggesting feasibility of this intervention in relieving osteoarthritic hand pain. Proof of efficacy, however, requires further confirmation in a placebo-controlled randomised trial.Trial registration: ANZCTR public trials registry (ACTRN12621001512819, 05/11/2021).

Skin barrier function for regulatory skin absorption tests and effects on testosterone and sucrose absorption

In vitro absorption through human skin is a critical component in the safety assessment of chemicals, crop protection products, consumer healthcare products and cosmetics. A barrier integrity assay is used to identify skin samples which are potentially damaged. A retrospective analysis of 9978 electrical resistance (ER) measurements generated in a single laboratory (DTL) over a 15-year period was performed. Skin absorption experiments were performed using two model penetrants, testosterone and sucrose, utilising no ER acceptance criteria, and the results assessed. Using a barrier integrity test, to remove potentially damaged samples, was offset against one that can be used to remove intact skin samples with a poorer barrier function (i.e. false positives). The previously identified barrier integrity limit (10 kΩ for a 2.54 cm2 diffusion cell; Davies et al., 2004) was demonstrated to identify half of all samples tested, many of which would be false positive samples. This retrospective analysis identified 5.0 kΩ (17.5th percentile) as an acceptance criterion for a 2.54 cm2 diffusion cell, whilst not considerably changing results generated in skin absorption studies. This was confirmed from the cumulative absorption of the model penetrants tested. Using this limit would, therefore, provide suitable skin samples for regulatory skin absorption studies.

Evaluation of the in vitro human skin percutaneous absorption of ketoprofen in topical anhydrous and aqueous gels

BACKGROUND

Ketoprofen is a nonsteroidal anti-inflammatory drug used for the treatment of acute and chronic pain associated with inflammatory conditions. This study aims to evaluate the in vitro percutaneous absorption of ketoprofen 10% formulated in proprietary anhydrous and aqueous gels using the Franz skin finite dose model.

MATERIALS AND METHODS

The anhydrous gel was initially characterized for cytotoxicity using EpiDerm skin tissue model by cell proliferation assay and Western blot analysis. The Ultra Performance Liquid Chromatography method for measuring ketoprofen was validated and the stability of ketoprofen 10% in the anhydrous gel formulation was evaluated at 5°C and 25°C for 181 days. The percutaneous absorption of ketoprofen was determined using donated human skin. The tissue sections were mounted within Franz diffusion cells. A variable finite dose of each ketoprofen formulation in either anhydrous or aqueous gel was applied to the skin sections and receptor solutions were collected at various time points.

RESULTS

Cell proliferation assay showed minimal cell death when EpiDerm skin tissue was exposed to the anhydrous gel for 24 h; the levels of protein markers of cell proliferation were not affected after 17-h exposure. Ketoprofen was stable in the anhydrous gel when stored at 5°C and 25°C. When compounded in the anhydrous and aqueous gels, ketoprofen had mean flux rate of 2.22 and 2.50 μg/cm2 /h, respectively, after 48 h. The drug was distributed to the epidermis and dermis sections of the skin. Both the anhydrous and aqueous gels facilitated the percutaneous absorption of ketoprofen without statistically significant differences.

CONCLUSION

The anhydrous gel can be used as a base to facilitate the transdermal delivery of ketoprofen. Although the anhydrous and aqueous gels can deliver a similar amount of ketoprofen, the anhydrous gel (water activity below 0.6) allows for extended default beyond-use-date of compounding preparations.

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).

Visualisation of drug distribution in skin using correlative optical spectroscopy and mass spectrometry imaging

A correlative methodology for label-free chemical imaging of soft tissue has been developed, combining non-linear optical spectroscopies and mass spectrometry to achieve sub-micron spatial resolution and critically improved drug detection sensitivity. The approach was applied to visualise the kinetics of drug reservoir formation within human skin following in vitro topical treatment with a commercial diclofenac gel. Non-destructive optical spectroscopic techniques, namely stimulated Raman scattering, second harmonic generation and two photon fluorescence microscopies, were used to provide chemical and structural contrast. The same tissue sections were subsequently analysed by secondary ion mass spectrometry, which offered higher sensitivity for diclofenac detection throughout the epidermis and dermis. A method was developed to combine the optical and mass spectrometric datasets using image registration techniques. The label-free, high-resolution visualisation of tissue structure coupled with sensitive chemical detection offers a powerful method for drug biodistribution studies in the skin that impact directly on topical pharmaceutical product development.

The Global Bioequivalence Harmonisation Initiative (GBHI): Report of the fifth international EUFEPS/AAPS conference

The series of conferences of the Global Bioequivalence Harmonisation Initiative (GBHI) was started in 2015 by the European Federation for Pharmaceutical Sciences (EUFEPS). All GBHI meetings so far were co-organised together with the American Association of Pharmaceutical Scientists (AAPS). Beginning with the 3rd workshop US-FDA joined as co-sponsor - to support global harmonisation of regulatory recommendations for bioequivalence (BE) assessment. At the 5th GBHI conference, the following BE topics were intensively discussed, and the following main conclusions were drawn: (1) Statistical considerations for BE assessment in specific situations covering scaling approaches for highly variable drug (HVD) products, two-stage adaptive design and opportunities of modelling and simulation to support BE: even though special BE study concepts like adaptive designs are not often used in practise so far, a majority of the workshop participants were in favour of a more frequent application of such approaches. The regulatory conditions relevant in this context need further concretisation and harmonisation between the regions. Moreover, modelling and simulation were considered as a promising and evolving approach, also for BE development programmes. (2) Fed versus fasting conditions in BE trials: Findings that BE between generic products could be confirmed only after fasted administration but failed under fed conditions seem more an exception than the rule. Obviously, BCS class IV compounds are most problematic in this context. Differences in critical excipients such as surfactants or pH-modifiers may be relevant reasons for different sensitivity for interactions in fasted versus fed conditions. Consequently, such deviations in composition of generic preparations should be avoided. Moreover, confirmation of BE may be generally difficult comparing different dosage forms, such like capsules versus tablets, especially in fed state. (3) BE assessment of locally acting drug products applied topically to the skin: Appropriateness and potential benefit of in-vitro tests as alternatives to clinical efficacy studies have been comprehensively discussed. In addition to the already well-established in-vitro release and permeation tests, other techniques were suggested, e.g., Raman spectroscopy or dermal open flow microperfusion. Validation of those methods is challenging and, despite significant progress already achieved during previous years, more research is needed before they may be fully accepted for regulatory purposes. (4) BE evaluation of narrow therapeutic index (NTI) drugs: The discrepancies amongst regulatory agencies in necessity of tighter BE acceptance ranges, the recommendations for inclusion of peak and total drug exposure into BE assessment with more restrictive criteria and the importance of comparison of the product-related within-subject variability for NTI drugs were debated. Arguments in favour and against the different approaches were presented and discussed but need further consideration before harmonisation can be achieved. The highly interactive meeting and extensive exchange between regulators and scientists from industry and academia resulted in useful progress in open BE issues and supported the goal of science-driven harmonisation.

Examining the Systemic Bioavailability of Cannabidiol and Tetrahydrocannabinol from a Novel Transdermal Delivery System in Healthy Adults: A Single-Arm, Open-Label, Exploratory Study

INTRODUCTION

Transdermal cannabinoids may provide better safety and bioavailability profiles compared with other routes of administration. This single-arm, open-label study investigated a novel topical transdermal delivery system on the pharmacokinetics of cannabidiol (CBD) and tetrahydrocannabinol (THC).

METHODS

Participants were 39.5 ± 7.37 years old and healthy, based on a review by the Medical Director. Blood was collected pre-dose and 10, 20, 30, and 45 min, and 1, 1.5, 2, 2.5, 3, 4, 5, 6, 8, and 12 h after topical application of 100 mg CBD:100 mg THC. Psychoactive effects were assessed prior to each timepoint. Area-under-the-curve (AUC_0-12 h), maximum concentration (C_max), time to maximum concentration (T_max), area-under-the-curve to infinity (AUC_I), terminal elimination rate constant (λ), terminal half-life (t½), and absorption rate constant (k_a) were measured individually for CBD and THC. Safety was assessed by clinical chemistry, hematology, and adverse events.

RESULTS

AUC_0-12 h for CBD and THC was 3329.8 ± 3252.1 and 2093.4 ± 2090.6 pg/mL/h, with C_max of 576.52 ± 1016.18 and 346.57 ± 776.85 pg/mL, respectively. T_max for CBD and THC was 8 h, ranging from 2.5 h to 12 h and 10 min to 12 h, respectively. AUC_I for CBD and THC was 6609.2 ± 7056.4 and 3721.0 ± 3251.7 pg/mL/h, with t_1/2 of 5.68 ± 1.5 and 5.38 ± 1.25 h, respectively. CBD was absorbed at a faster rate compared with THC (123.36 ± 530.97 versus 71.5 ± 1142.19 h^-1) but with similar λ (0.12 ± 0.029 versus 0.13 ± 0.03 h^-1). No psychoactive effects were reported. Transdermal cannabinoid delivery was safe and well tolerated in the population studied.

CONCLUSION

To our knowledge, this is the first pharmacokinetic study in humans that demonstrated CBD and THC entering systemic circulation via transdermal administration . This study represents an important contribution to understanding the pharmacokinetics of transdermal cannabinoids.

CLINICAL TRIAL REGISTRATION

ClinicalTrials.gov Identifier-NCT05121506 (November 16, 2021).

Evaluation of an Anhydrous Permeation-Enhancing Vehicle for Percutaneous Absorption of Hormones

The efficiency and safety of hormone delivery through the skin partly depend on the appropriate choice of vehicle and the type of formulation. The present study reports the skin cytotoxicity, irritancy, and safety of a newly developed anhydrous permeation-enhancing base (APEB) and the percutaneous absorption of progesterone, testosterone, estriol, and estradiol in APEB formulations. Using the human skin EpiDerm model, cell death was not observed after 4 h of exposure to APEB and was 48% after 24 h, indicating its mild to non-irritating property. APEB did not change the expression level of skin cell proliferation markers including PCNA, MCL-1, iNOS, and NFκB proteins, and apoptosis was minimal after 8-h exposure. The in vivo skin irritation and sensitization evaluation of APEB using a Human Repeat Insult Patch Test showed no adverse reaction of any kind during the course of the study. These results indicate the safety of APEB on skin tissues. The hormone percutaneous absorption was performed using human cadaver abdomen skin tissues and the Franz diffusion system, and hormone concentrations were determined by ELISA. Absorption was observed as early as 2 h of application and accumulated after 24 h to 2851 ± 66 ng/cm², 2338 ± 594 ng/cm², 55 ± 25 ng/cm², and 341 ± 122 ng/cm² for progesterone, testosterone, estriol, and estradiol, respectively. A steady flux rate of absorption of the hormones was observed within 24 h of application. These results suggest that APEB can be used as a vehicle to deliver these hormones through the skin and into the bloodstream for hormone replacement therapy.

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.

Variability of Skin Pharmacokinetic Data: Insights from a Topical Bioequivalence Study Using Dermal Open Flow Microperfusion

Purpose

Dermal open flow microperfusion (dOFM) has previously demonstrated its utility to assess the bioequivalence (BE) of topical drug products in a clinical study. We aimed to characterize the sources of variability in the dermal pharmacokinetic data from that study.

Methods

Exploratory statistical analyses were performed with multivariate data from a clinical dOFM-study in 20 healthy adults evaluating the BE, or lack thereof, of Austrian test (T) and U.S. reference (R) acyclovir cream, 5% products.

Results

The overall variability of logAUC values (CV: 39% for R and 45% for T) was dominated by inter-subject variability (R: 82%, T: 91%) which correlated best with the subject’s skin conductance. Intra-subject variability was 18% (R) and 9% (T) of the overall variability; skin treatment sites or methodological factors did not significantly contribute to that variability.

Conclusions

Inter-subject variability was the major component of overall variability for acyclovir, and treatment site location did not significantly influence intra-subject variability. These results support a dOFM BE study design with T and R products assessed simultaneously on the same subject, where T and R treatment sites do not necessarily need to be next to each other. Localized variation in skin microstructure may be primarily responsible for intra-subject variability.

Reflections on the OECD guidelines for in vitro skin absorption studies

At the 8th conference of Occupational and Environmental Exposure of the Skin to Chemicals (OEESC) (16-18 September 2019) in Dublin, Ireland, several researchers performing skin permeation assays convened to discuss in vitro skin permeability experiments. We, along with other colleagues, all of us hands-on skin permeation researchers, present here the results from our discussions on the available OECD guidelines. The discussions were especially focused on three OECD skin absorption documents, including a recent revision of one: i) OECD Guidance Document 28 (GD28) for the conduct of skin absorption studies (OECD, 2004), ii) Test Guideline 428 (TGD428) for measuring skin absorption of chemical in vitro (OECD, 2004), and iii) OECD Guidance Notes 156 (GN156) on dermal absorption issued in 2011 (OECD, 2011). GN156 (OECD, 2019) is currently under review but not finalized. A mutual concern was that these guidance documents do not comprehensively address methodological issues or the performance of the test, which might be partially due to the years needed to finalize and update OECD documents with new skin research evidence. Here, we summarize the numerous factors that can influence skin permeation and its measurement, and where guidance on several of these are omitted and often not discussed in published articles. We propose several improvements of these guidelines, which would contribute in harmonizing future in vitro skin permeation experiments.

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.

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.

In vitro percutaneous absorption and metabolism of Bisphenol A (BPA) through fresh human skin

Bisphenol A (BPA) is a high production volume compound. It is mainly used as a monomer to make polymers for various applications including food-contact materials. The primary route of exposure to BPA in the general population is through oral intake (EFSA 2015) however, other potential sources of exposure have also been identified, such as dermal contact. In the present study, the percutaneous absorption through human skin has been investigated in an in vitro study according to OECD TG 428 (Skin Absorption: In Vitro Method). In order to investigate potential dermal BPA metabolism during absorption, radiolabelled BPA was applied to fresh, metabolically competent, human skin samples (ring labelled ¹⁴C BPA concentrations tested were 2.4, 12, 60 and 300mg/L). Measured as total radioactivity the mean absorbed dose (receptor compartment) ranged from 1.7-3.6% of the applied doses and the dermal delivery (epidermis+dermis+receptor compartment), sometimes also named bioavailable dose was 16-20% of the applied doses, with the majority of the radioactivity associated with epidermis compared to dermis and receptor fluid. No metabolism was observed in any of the epidermis samples; however some metabolism was observed in dermis and receptor fluid samples with formation of BPA-glucuronide and BPA-sulfate, and some polar metabolites.

The Tritiated Water Skin Barrier Integrity Test: Considerations for Acceptance Criteria with and Without 14C-Octanol

PURPOSE

A study was designed to assess barrier integrity simultaneously using separate compounds (probes) for polar and non-polar pathways through the skin, ³H₂O and ¹⁴C-octanol, respectively; and to determine whether the two probe approach could better define barrier integrity.

METHODS

A 5-min dose of water containing ³H₂O and ¹⁴C -octanol was applied to ex vivo human skin mounted in Franz diffusion cells. The receptor solution was sampled at 30 min, analyzed for ³H and ¹⁴C content, and the correlation between water and octanol absorption was determined by statistical tests suitable for non-normally distributed data. This study was conducted on skin from 37 donors with from 3 to 30 replicate skin sections per donor (a total of 426 sections).

RESULTS

The correlation between ³H₂O and ¹⁴C-octanol absorption was low (Pearson correlation coefficient = 0.3485). The ³H₂O absorption cutoff used in this study to select for a normal skin barrier rejected some sections in which ¹⁴C-octanol absorption was within normal limits and accepted others in which ¹⁴C-octanol absorption was abnormally high. The converse was true for ³H₂O absorption when the ¹⁴C-octanol-based cutoff was used.

CONCLUSIONS

The results of the ³H₂O test or of similar tests that primarily assess the permeability of polar pathways through the skin may not necessarily provide information relevant to the absorption of highly lipophilic compounds. Octanol, or another molecule that more closely matches the physicochemical attributes of the test compound, may characterize properties of the skin barrier that are more relevant to compounds of low water solubility.

Chronological age affects the permeation of fentanyl through human skin in vitro

AIM

To study the influence of chronological age on fentanyl permeation through human skin in vitro using static diffusion cells. Elderly individuals are known to be more sensitive to opioids and obtain higher plasma concentrations following dermal application of fentanyl compared to younger individuals. The influence of age - as an isolated pharmacokinetic term - on the absorption of fentanyl has not been previously studied.

METHOD

Human skin from 30 female donors was mounted in static diffusion cells, and samples were collected during 48 h. Donors were divided into three age groups: <30 years of age (n = 6), ≥30 and <60 years of age (n = 18) and ≥60 years of age (n = 6).

RESULTS

The youngest group had a significantly higher mean absorption (3,100 ng/cm(2)) than the two other groups (2,000 and 1,475 ng/cm(2), respectively) and a significant larger AUC (young age group: 9,393 ng; middle and old age groups: 5,922 and 4,050 ng, respectively). Furthermore, the lag time and absorption rate were different between the three groups, with a significantly higher rate in the young participants versus the oldest participants.

CONCLUSION

We demonstrate that fentanyl permeates the skin of young individuals in greater amounts and at a higher absorption rate than in middle-aged and old individuals in vitro.

Finite and infinite dosing: difficulties in measurements, evaluations and predictions

Due to the increased demand for reliable data regarding penetration into and permeation across human skin, assessment of the absorption of xenobiotics has been gaining in importance steadily. In vitro experiments allow for determining these data faster and more easily than in vivo experiments. However, the experiments described in literature and the subsequent evaluation procedures differ considerably. Here we will give an overview on typical finite and infinite dose experiments performed in fundamental research and on the evaluation of the data. We will point out possible difficulties that may arise and give a short overview on attempts at predicting skin absorption in vitro and in vivo.

Dermal penetration of ethylene glycol through human skin in vitro

This study was conducted to determine the in vitro dermal absorption of ethylene glycol (EG) through dermatomed human abdominal skin (containing epidermis and dermis), obtained from cadavers within 24 hours of death and kept frozen until processed. Three formulations of EG (neat, 50%, and 10% aqueous solutions) were applied in triplicate to skin samples from 6 donors, and placed in Teflon Bronaugh flow-through diffusion cells. Barrier integrity of each sample was evaluated with (3)H-H(2)O prior to applying EG and only data from samples passing the test were used. A physiological receptor fluid was pumped beneath the skin samples and collected in a fraction collector at predetermined time points through 24 hours. Possible volatilized EG was trapped in a charcoal basket located above each skin sample. Each skin sample was treated with an infinite dose of 500 microL of EG formulation/cm(2). At the end of 24 hours, volatilized EG trapped in the headspace was collected, the unabsorbed dose was removed from the skin and the skin was rinsed, tape stripped, and solubilized along with a rinse of the flow-through cells, and total radioactivity was determined. Only a small fraction (</=1%) of the applied EG was absorbed in 24 hours, of which <0.7% penetrated through the skin and ~0.4% remained in the skin. Recovery (mass balance) of the applied EG was between 93% and 99%, which further validated the observed low dermal penetration of EG. The net penetration of the applied EG over 24 hours was concentration proportional, comprising 2.97 +/- 0.78, 1.75 +/- 0.62, and 0.23 +/- 0.12 mg/cm(2) of the neat, 50%, and 10% formulations, respectively. The steady-state flux of EG was established between 16 and 24 hours. The mean steady-state flux of EG through dermatomed skin was 217, 129, and 15 microg/cm(2)h for the neat, 50%, and 10% aqueous formulations, respectively, consistent with concentration-proportional penetration of EG. The steady-state permeation coefficient (Kp) for EG was low, between 1.5 x 10(-4) and 2.6 x 10(-4) cm/h. These findings demonstrate that EG dermal penetration is expected to be very low and to be slow, indicating very limited systemic or internal dose of EG due to dermal exposure.

Effects of microneedle length, density, insertion time and multiple applications on human skin barrier function: assessments by transepidermal water loss

Microneedle (MN) arrays have attracted considerable attention in recent years due to their ability to facilitate effective transdermal drug delivery. Despite appreciable research, there is still debate about how different MN dimensions or application modes influence permeabilization. This study aimed to investigate this issue by taking transepidermal water-loss measurements of dermatomed human skin samples following the insertion of solid polymeric MNs. Insertions caused an initial sharp drop in barrier function followed by a slower incomplete recovery - a paradigm consistent with MN-generation of microchannels that subsequently contract due to skin elasticity. While 600 μm-long MNs were more skin-perturbing than 400 μm MNs, insertion of 1000 μm-long MNs caused a smaller initial drop in integrity followed by a degree of long term permeabilization. This is explainable by the longest needles compacting the tissue, which then decompresses over subsequent hours. Multiple insertions had a similar effect as increasing MN length. There was some evidence that increasing MN density suppressed the partial barrier recovery caused by tissue contraction. Leaving MNs embedded in skin seemed to reduce the initial post-insertion drop in barrier function. Our results suggest that this in vitro TEWL approach can be used to rapidly screen MN-effects on skin.