Inter‐ and intralaboratory variation of in vitro diffusion cell measurements: An international multicenter study using quasi‐standardized methods and materials

Decontamination of Actinide-contaminated Injured Skin with Ca-DTPA Products Using an Ex Vivo Rat Skin Model

ABSTRACT

Skin contamination by α-emitting actinides such as plutonium and americium is a risk for workers during nuclear fuel production and reactor decommissioning. Decontamination of skin is an important medical countermeasure to limit potential internal contamination, particularly in the case of injured skin. Current recommendations include undressing of the victim followed by skin washing using soap or chelating agents, such as diethylene triamine pentaacetic acid (DTPA). The goal of the present work is to assess the efficacy of a novel Ca-DTPA loaded gel to decontaminate injured skin exposed to plutonium or americium as compared to recommended treatments. For decontaminant testing on injured skin, whole body skin was obtained from euthanized rats and lesions created using a metallic brush. Delimited test areas were contaminated with plutonium or americium solutions of known properties. Various protocols were tested including time before contamination, duration of gel application, washing steps, as well as the concomitant addition or not of dressings. Activity was measured in each decontamination product and in skin. Data indicate that healthy skin was easier to decontaminate than damaged skin. On injured skin, we demonstrated an increased decontamination efficacy of the Ca-DTPA gel formulation as compared to the solution. Importantly, gel application alone was effective, and further gel applications could be used for residual activity.

In vitro permeation testing for the evaluation of drug delivery to the skin

This review considers the role of in vitro permeation testing (IVPT) for the evaluation of drug delivery from topical formulations applied to the skin. The technique was pioneered by Franz in the 1970's and today remains an important tool in the development, testing and optimization of such topical formulations. An overview of IVPT as well as selection of skin for the experiment, integrity testing of the membrane, and required number of replicate skin samples is discussed. In the literature many researchers have focused solely on permeation and have not reported amounts of the active remaining on and in the skin at the end of the IVPT. Therefore, a particular focus of this article is determination of the complete mass balance of the drug. It is noteworthy that for the evaluation of bioequivalence of topical formulations the draft guideline issued by the European Medicines Agency (EMA) requires the IVPT method to report on both the skin deposition and distribution of the active in the skin as well as amount permeated. Other aspects of current guidance from the EMA and United States Food and Drug Agency for IVPT are also compared and contrasted. Ultimately, harmonisation of IVPT protocols across the regulatory agencies will expedite the development process for novel topical formulations as well as the availability of generic products.

Alternatives to Conventional Topical Dosage Forms for Targeted Skin Penetration of Diclofenac Sodium

Skin penetration of an active pharmaceutical ingredient is key to developing topical drugs. This penetration can be adjusted for greater efficacy and/or safety through the selection of dosage form. Two emerging dosage forms, cream-gel and gel-in-oil emulsion, were tested for their ability to deliver diclofenac into the skin, with the target of maximising skin retention while limiting systemic exposure. Prototypes with varying amounts of solvents and emollients were formulated and evaluated by in vitro penetration testing on human skin. Cream-gel formulas showed better skin penetration than the emulgel benchmark drug even without added solvent, while gel-in-oil emulsions resulted in reduced diffusion of the active into the receptor fluid. Adding propylene glycol and diethylene glycol monoethyl ether as penetration enhancers resulted in different diclofenac penetration profiles depending on the dosage form and whether they were added to the disperse or continuous phase. Rheological characterisation of the prototypes revealed similar profiles of cream-gel and emulgel benchmark, whereas gel-in-oil emulsion demonstrated flow characteristics suitable for massaging product into the skin. This study underlined the potential of cream-gel and gel-in-oil emulsions for adjusting active penetration into the skin, broadening the range of choices available to topical formulation scientists.

Lipid Biomimetic Models as Simple Yet Complex Tools to Predict Skin Permeation and Drug-Membrane Biophysical Interactions

The barrier function of the skin is primarily determined by its outermost layer, the Stratum Corneum (SC). The SC consists of corneocytes embedded in a lipid matrix composed mainly of ceramides, cholesterol, and free fatty acids in equimolar proportions and is organised in a complex lamellar structure with different periodicities and lateral packings. This matrix provides a diffusion pathway across the SC for bioactive compounds that are administered to the skin. In this regard, and as the skin administration route has grown in popularity, there has been an increase in the use of lipid mixtures that closely resemble the SC lipid matrix, either for a deeper biophysical understanding or for pharmaceutical and cosmetic purposes. This review focuses on a systematic analysis of the main outcomes of using lipid mixtures as SC lipid matrix models for pharmaceutical and cosmetic purposes. Thus, a methodical evaluation of the main outcomes based on the SC structure is performed, as well as the main recent developments in finding suitable new in vitro tools for permeation testing based on lipid models.

Effect of Receptor Solution in Studies of In Vitro Permeation Test (IVPT)

In Vitro Permeation Test (IVPT) is commonly used to evaluate skin penetration of chemicals and performance of dermatological products. For a permeant with low aqueous solubility, an additive that is expected not to alter the skin barrier can be used in the receptor solution to improve permeant solubility. The objective of this study was to (a) evaluate the effects of these additives in IVPT receptor solution on skin permeability of model permeants and skin electrical resistance and (b) determine the solubility of the permeants in these receptor solutions. Bovine serum albumin (BSA), 2-hydroxypropyl-beta-cyclodextrin (HPCD), ethanol, nonionic surfactant Brij-98, and propylene glycol were the additives, and phosphate buffered saline (PBS) was the control. Steady-state skin permeability coefficients and resistances were determined. The receptor solutions examined in this study did not cause a significant increase in skin permeability or decrease in resistance (less than 40 % changes) except 25 % ethanol. The receptor solution containing 25 % ethanol induced an approximately twofold average increase in skin permeability and reduced skin electrical resistance by approximately threefold. The receptor solution of 2.5 % HPCD provided the highest levels of solubility for the model lipophilic permeants, while 0.2 % Brij-98 and 5 % ethanol showed the lowest solubility enhancement from those in PBS.

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).

Human epidermal in vitro permeation test (IVPT) analyses of alcohols and steroids

This study examined a number of factors that can impact the outcomes of in vitro human epidermal permeation coefficients for aliphatic alcohols and steroids, including receptor phase composition and study conditions. We determined experimentally the solubilities and IVPT permeation of a homologous series of ¹⁴C labeled aliphatic alcohols (ethanol, propanol, pentanol, heptanol, octanol and decanol) in different receptor fluids as recommended by Organisation Economic Co-operation and Development (OECD). We used human epidermal membranes at 25°C and phosphate-buffered saline (PBS), 2% w/v bovine serum albumin (2%w/v BSA), 50% v/v ethanol and 0.1, 2 and 6% w/v Oleth-20 receptor phases. We also explored and confirmed the discrepancies between in vitro human epidermal permeability coefficients (k_p) and diffusion lag times for steroids from Scheuplein's group with our own work and that of others. The main reason for the observed differences is not clear but is likely to be multifactorial, including the effects of diffusion cell design, receptor phase solubility, unstirred receptor phase effects, epidermal membrane hydration, diffusion cell configuration, transport through appendageal pathways and steroid lipophilicity. We conclude with a summary of experimental conditions that should be considered in undertaking IVPT studies.

Permeability of Buccal Mucosa

The buccal mucosa provides an alternative route of drug delivery that can be more beneficial compared to other administration routes. Although numerous studies and reviews have been published on buccal drug delivery, an extensive review of the permeability data is not available. Understanding the buccal mucosa barrier could provide insights into the approaches to effective drug delivery and optimization of dosage forms. This paper provides a review on the permeability of the buccal mucosa. The intrinsic permeability coefficients of porcine buccal mucosa were collected. Large variability was observed among the published permeability data. The permeability coefficients were then analyzed using a model involving parallel lipoidal and polar transport pathways. For the lipoidal pathway, a correlation was observed between the permeability coefficients and permeant octanol/water partition coefficients (Kow) and molecular weight (MW) in a subset of the permeability data under specific conditions. The permeability analysis suggested that the buccal permeation barrier was less lipophilic than octanol. For the polar pathway and macromolecules, a correlation was observed between the permeability coefficients and permeant MW. The hindered transport analysis suggested an effective pore radius of 1.5 to 3 nm for the buccal membrane barrier.

Poly(dimethylsiloxane): A Sustainable Human Skin Alternative for Transdermal Drug Delivery Prediction

Despite the advantages of transdermal drug delivery (TDD), which makes it a fast-growing area of research in pharmaceutics, numerous challenges affect their development, which limits exploring the full potential of this alternate drug delivery route. In trying to address one of these problems, it is strongly believed that the need for a sustainable skin alternative is paramount. Efforts made in an attempt to provide a sustainable alternative to employing skin in pharmaceutical analysis, by better utilising a polymer membrane, namely poly(dimethylsiloxane), also known as PDMS are discussed. Several combined properties of this polymer, which includes its relative stability in comparison with human skin, make it a good candidate for the replacement of skin. Modifications undertaken to this polymer membrane (to create an enhanced skin mimic for permeation analysis) are discussed and reviewed in this paper, including the improved ability to predict permeability for both hydrophobic and hydrophilic drugs. Optimisations related to studying TDD including limitations encountered are also documented and reviewed. It is hoped that such developments in this field will ultimately lead to researchers replacing skin with optimised polymer-based alternatives to predict transdermal drug delivery.

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.

Hybrid in vitro diffusion cell for simultaneous evaluation of hair and skin decontamination: temporal distribution of chemical contaminants

Most casualty or personnel decontamination studies have focused on removing contaminants from the skin. However, scalp hair and underlying skin are the most likely areas of contamination following airborne exposure to chemicals. The aim of this study was to investigate the interactions of contaminants with scalp hair and underlying skin using a hybrid in vitro diffusion cell model. The in vitro hybrid test system comprised “curtains” of human hair mounted onto sections of excised porcine skin within a modified diffusion cell. The results demonstrated that hair substantially reduced underlying scalp skin contamination and that hair may provide a limited decontamination effect by removing contaminants from the skin surface. This hybrid test system may have application in the development of improved chemical incident response processes through the evaluation of various hair and skin decontamination strategies.

Inter-laboratory study of the skin distribution of 4-n-butyl resorcinol in ex vivo pig and human skin

4-n-butyl resorcinol (4-nBR) is a highly effective tyrosinase inhibitor, and can be used in cosmetic product for depigmentation purpose. Its efficacy correlates with 4-nBR that absorbed by skin. In this study, skin distribution of 4-nBR within either human or pig skin ex vivo was studied and compared by three independent laboratories. Good agreement was observed in each compartment considering usual inter-lab variability. This study supports the use of pig skin as an alternative source of skin when the availability of human skin is a limiting factor.

Inter-laboratory skin distribution study of 4-n-butyl resorcinol: The importance of liquid chromatography/mass spectrometry (HPLC-MS/MS) bioanalytical validation

In the present study, three laboratories independently compared percutaneous absorption and distribution of 4-n-butylresorcinol, using human skin from five donors. Each laboratory used the same protocol for percutaneous absorption studies but different LC-MS/MS analytical methods to quantify the test compound. All laboratories respected the mass balance criteria (i.e. 100±15%; average 96.5-102% of applied dose). Regarding usual inter-lab variability, good agreement was observed for all compartments with the greatest difference in the epidermis: 3.3 fold increase. The data obtained demonstrate that robustness of skin absorption data rely on properly validated analytical methods including sample extraction and LC-MS/MS method. It also includes clearly defined cutaneous absorption protocol for dose skin preparation, application, washing and tape stripping.

Multi-chamber microfluidic platform for high-precision skin permeation testing

The established in vitro tool used for testing the absorption and penetration of chemicals through skin in pharmacology, toxicology and cosmetic science is the static Franz diffusion cell. While widespread, Franz cells are relatively costly, low-throughput and results may suffer from poor reproducibility. Microfluidics has the potential to overcome these drawbacks. In this paper, we present a novel microfluidic skin permeation platform and validate it rigorously against the Franz cell by comparing the transport of 3 model chemicals of varying lipophilicity: caffeine, salicylic acid and testosterone. Permeation experiments through silicone membranes show that the chip yields higher sensitivity in permeant cumulative amounts and comparable or lower coefficients of variation. Using a skin organotypic culture, we show that the chip decreases the effect of unstirred water layers that can occur in static Franz cells. The validation reported herein sets the stage for efficient skin permeation and toxicity screening and further development of microfluidic skin-on-chip devices.

Evaluation of absorbent materials for use as ad hoc dry decontaminants during mass casualty incidents as part of the UK's Initial Operational Response (IOR)

The UK’s Initial Operational Response (IOR) is a revised process for the medical management of mass casualties potentially contaminated with hazardous materials. A critical element of the IOR is the introduction of immediate, on-scene disrobing and decontamination of casualties to limit the adverse health effects of exposure. Ad hoc cleansing of the skin with dry absorbent materials has previously been identified as a potential means of facilitating emergency decontamination. The purpose of this study was to evaluate the in vitro oil and water absorbency of a range of materials commonly found in the domestic and clinical environments and to determine the effectiveness of a small, but representative selection of such materials in skin decontamination, using an established ex vivo model. Five contaminants were used in the study: methyl salicylate, parathion, diethyl malonate, phorate and potassium cyanide. In vitro measurements of water and oil absorbency did not correlate with ex vivo measurements of skin decontamination. When measured ex vivo, dry decontamination was consistently more effective than a standard wet decontamination method (“rinse-wipe-rinse”) for removing liquid contaminants. However, dry decontamination was ineffective against particulate contamination. Collectively, these data confirm that absorbent materials such as wound dressings and tissue paper provide an effective, generic capability for emergency removal of liquid contaminants from the skin surface, but that wet decontamination should be used for non-liquid contaminants.

Establishing the importance of oil-membrane interactions on the transmembrane diffusion of physicochemically diverse compounds

The diffusion process through a non-porous barrier membrane depends on the properties of the drug, vehicle and membrane. The aim of the current study was to investigate whether a series of oily vehicles might have the potential to interact to varying degrees with synthetic membranes and to determine whether any such interaction might affect the permeation of co-formulated permeants: methylparaben (MP); butylparaben (BP) or caffeine (CF). The oils (isopropyl myristate (IPM), isohexadecane (IHD), hexadecane (HD), oleic acid (OA) and liquid paraffin (LP)) and membranes (silicone, high density polyethylene and polyurethane) employed in the study were selected such that they displayed a range of different structural, and physicochemical properties. Diffusion studies showed that many of the vehicles were not inert and did interact with the membranes resulting in a modification of the permeants' flux when corrected for membrane thickness (e.g. normalized flux of MP increased from 1.25±0.13μgcm(-1)h(-1) in LP to 17.94±0.25μgcm(-1)h(-1)in IPM). The oils were sorbed differently to membranes (range of weight gain: 2.2±0.2% for polyurethane with LP to 105.6±1.1% for silicone with IHD). Membrane interaction was apparently dependent upon the physicochemical properties including; size, shape, flexibility and the Hansen solubility parameter values of both the membranes and oils. Sorbed oils resulted in modified permeant diffusion through the membranes. No simple correlation was found to exist between the Hansen solubility parameters of the oils or swelling of the membrane and the normalized fluxes of the three compounds investigated. More sophisticated modelling would appear to be required to delineate and quantify the key molecular parameters of membrane, permeant and vehicle compatibility and their interactions of relevance to membrane permeation.

In vitro human skin penetration model for organophosphorus compounds with different physicochemical properties

A flow-through diffusion cell was validated for in vitro human epidermal penetration studies of organophosphorus compounds (OPCs) applied by infinite dosing. By testing OPCs with similar molecular weight but different physicochemical properties, it was shown that hydrophilic and lipophilic properties are major determinants for the penetration rate. Lipophilic OPCs displayed maximum cumulative penetration in the 20-75% agent concentration range whereas the hydrophilic OPCs displayed maximum cumulative penetration at 10 or 20% agent concentration. Low penetration was observed for all agents at 1% agent concentration or when applied as neat agents. The impact of the receptor solution composition was evaluated by comparing the penetration using receptor solutions of different ratios of ethanol and water. For diluted OPCs, a high concentration of ethanol in the receptor solution significantly increased the penetration compared to lower concentrations. When OPCs were applied as neat agents, the composition of the receptor solution only affected the penetration for one of four tested compounds. In conclusion, the flow-through diffusion cell was useful for examining the penetration of OPCs through the epidermal membrane. It was also demonstrated that the penetration rates of OPCs are strongly influenced by dilution in water and the receptor fluid composition.

Preliminary evaluation of military, commercial and novel skin decontamination products against a chemical warfare agent simulant (methyl salicylate)

Rapid decontamination is vital to alleviate adverse health effects following dermal exposure to hazardous materials. There is an abundance of materials and products which can be utilised to remove hazardous materials from the skin. In this study, a total of 15 products were evaluated, 10 of which were commercial or military products and five were novel (molecular imprinted) polymers. The efficacies of these products were evaluated against a 10 µl droplet of (14)C-methyl salicylate applied to the surface of porcine skin mounted on static diffusion cells. The current UK military decontaminant (Fuller's earth) performed well, retaining 83% of the dose over 24 h and served as a benchmark to compare with the other test products. The five most effective test products were Fuller's earth (the current UK military decontaminant), Fast-Act® and three novel polymers [based on itaconic acid, 2-trifluoromethylacrylic acid and N,N-methylenebis(acrylamide)]. Five products (medical moist-free wipes, 5% FloraFree™ solution, normal baby wipes, baby wipes for sensitive skin and Diphotérine™) enhanced the dermal absorption of (14)C-methyl salicylate. Further work is required to establish the performance of the most effective products identified in this study against chemical warfare agents.

A Microfluidic Diffusion Cell for Fast and Easy Percutaneous Absorption Assays

PURPOSE

Percutaneous absorption assays of molecules for pharmaceutical and cosmetology purposes are important to determine the bioavailability of new compounds, once topically applied. The current method of choice is to measure the rate of diffusion through excised human skin using a diffusion cell. This method however entails significant drawbacks such as scarce availability and poor reproducibility of the sample, low sampling rate, and tedious assay setup.

METHODS

The objective of the present work is to propose an alternative method that overcomes these issues by integrating an experimental model of the skin (artificial stratum corneum) and online optical sensors into a microfluidic device.

RESULTS

The measurement of the diffusion profile followed by the calculation of the permeability coefficients and time lag were performed on seven different molecules and obtained data positively fit with those available from literature on human skin penetration. The coating of the lipid mixture to generate the artificial stratum corneum also proved robust and reproducible. The results show that the proposed device is able to give fast, real-time, accurate, and reproducible data in a user-friendly manner, and can be produced at a large scale.

CONCLUSION

These assets should help both the cosmetics and pharmaceutics fields where the skin is the target or a pathway of a formulated compound, by allowing more candidate molecules or formulations to be assessed during the various stages of their development.

The application of machine learning to the modelling of percutaneous absorption: an overview and guide

Machine learning (ML) methods have been applied to the analysis of a range of biological systems. This paper reviews the application of these methods to the problem domain of skin permeability and addresses critically some of the key issues. Specifically, ML methods offer great potential in both predictive ability and their ability to provide mechanistic insight to, in this case, the phenomena of skin permeation. However, they are beset by perceptions of a lack of transparency and, often, once a ML or related method has been published there is little impetus from other researchers to adopt such methods. This is usually due to the lack of transparency in some methods and the lack of availability of specific coding for running advanced ML methods. This paper reviews critically the application of ML methods to percutaneous absorption and addresses the key issue of transparency by describing in detail - and providing the detailed coding for - the process of running a ML method (in this case, a Gaussian process regression method). Although this method is applied here to the field of percutaneous absorption, it may be applied more broadly to any biological system.

Liposomes for (trans)dermal drug delivery: the skin-PVPA as a novel in vitro stratum corneum model in formulation development

Penetration potential of vesicles destined for trans(dermal) administration remains to be of great interests both in respect to drug therapy and cosmetic treatment. This study investigated the applicability of the phospholipid vesicle-based permeation assay (PVPA) as a novel in vitro skin barrier model for screening purposes in preformulation studies. Various classes of liposomes containing hydrophilic model drug were examined, including conventional liposomes (CLs), deformable liposomes (DLs) and propylene glycol liposomes (PGLs). The size, surface charge, membrane deformability and entrapment efficiency were found to be affected by the vesicle lipid concentration, the presence of the surfactant and propylene glycol. All liposomes exhibited prolonged drug release profiles with an initial burst effect followed by a slower release phase. The permeation of the drug from all of the tested liposomes, as assessed with the mimicked stratum corneum--PVPA model, was significantly enhanced as compared to the permeability of the drug in solution form. Although the DLs and the PGLs exhibited almost the same membrane elasticity, the permeability of the drug delivered by PGLs was higher (6.2 × 10⁻⁶ cm/s) than DLs (5.5 × 10⁻⁶ cm/s). Therefore, this study confirmed both the potential of liposomes as vesicles in trans(dermal) delivery and potential of the newly developed skin-PVPA for the screening and optimization of liposomes at the early preformulation stage.

Mathematical models for skin toxicology

INTRODUCTION

Our skin is exposed daily to substances; many of these are neutral and safe but others are potentially harmful. In order to estimate the degree of toxicity and damage to skin tissues when exposed to harmful substances, skin toxicology studies are required. If these studies are coupled with suitably designed mathematical models, they can provide a powerful tool that allows appropriate interpretation of data. This work reviews mathematical models that can be employed in skin toxicology studies.

AREAS COVERED

Two types of mathematical models and their suitability for assessing skin toxicology are covered in this review. The first is focused on predicting penetration rate through the skin from a solute's physicochemical properties, while the second type of models transport processes in skin layers using appropriate equations with the specific aim of predicting the concentration of a given solute in viable skin tissues.

EXPERT OPINION

Mathematical models are an important tool for accurate valuation of skin toxicity experiments, estimation of skin toxicity and for developing new formulations for skin disease therapy. Comprehensive mathematical models of drug transport in skin, especially those based on more physiologically detailed mechanistic considerations of transport processes, are required to further enhance their role in assessing skin toxicology.

Polystyrene microsphere and 5-fluorouracil release from custom-designed wound dressing films

Custom-designed wound dressing films of chitosan and alginate have been prepared by a casting/solvent evaporation method for hydrophobic therapeutic agent encapsulation. In this parametric study, the propylene glycol (PG) and calcium chloride (CaCl2) concentrations were varied for chitosan and alginate films, respectively. Mechanical and chemical inter-related responses under observations included thickness (th), elasticity (E), tensile strength (TS), sorption ability (S%) and kinetics of in-vitro drug release, specifically in terms of membrane time to burst (t B ) and duration of release (t R ). As shown by results of a one tailed t-test significance testing at the 95% confidence interval (α = 0.05), alginate films were significantly more elastic (p = 0.003), thinner (p = 0.004) and more susceptible to osmotic burst (p = 0.011) and characterized by a longer duration of release (p = 0.03). Meanwhile chitosan films exhibited superior moisture permeability (p = 0.006) and sorption characteristics (p = 0.001), indicative of higher hydrophilicity. There were no significant differences in tensile strength (p = 0.324) for alginate and chitosan-based formulations. Preliminary testing was conducted using 0.71 μm in diameter microspheres for modeling film dissolution into Lactated Ringer's solution. Experimental release profiles were modeled for each film from which the average release from alginate films (M AGCa  = 81%) was estimated to be twice the percentage associated with chitosan films (M CD  = 42%). The film comprised of 2.5% (w/v) medium MW chitosan/dextran 70 kDa (5:1) was selected for studying the release of 5-Fluorouracil (5-FU) as a model hydrophobic drug. Diffusion coupled with film disintegration is immediate (t B  = 0) in case of encapsulated 5-FU as compared to the control film encapsulating microspheres characterized by t B  = 70 min ± 7 min. This shift in release profile and the ability to modulate the timing of membrane burst can be attributed to the approximate ratio (1: 505) in molecular size between drug and microsphere. This hypothesis has been validated by the film pore size measured to be 430 nm ± 88 nm using atomic force microscopy.

The Contributions of the Celtic Masters and Their Associates

This article summarizes the work of 4 researchers in the field of percutaneous absorption - Keith Brain, Mark Cronin, Dermot McCafferty and John Pugh. It summarizes their main achievements in this field and reviews their major contributions to the broader subject area.

An evaluation of the potential of linear and nonlinear skin permeation models for the prediction of experimentally measured percutaneous drug absorption

Objectives

The developments in combinatorial chemistry have led to a rapid increase in drug design and discovery and, ultimately, the production of many potential molecules that require evaluation. Hence, there has been much interest in the use of mathematical models to predict dermal absorption. Therefore, the aim of this study was to test the performance of both linear and nonlinear models to predict the skin permeation of a series of 11 compounds.

Methods

The modelling in this study was carried out by the application of both quantitative structure permeability relationships and Gaussian process-based machine learning methods to predict the flux and permeability coefficient of the 11 compounds. The actual permeation of these compounds across human skin was measured using Franz cells and a standard protocol with high performance liquid chromatography analysis. Statistical comparison between the predicted and experimentally-derived values was performed using mean squared error and the Pearson sample correlation coefficient.

Key findings

The findings of this study would suggest that the models failed to accurately predict permeation and in some cases were not within two- or three-orders of magnitude of the experimentally-derived values. However, with this set of compounds the models were able to effectively rank the permeants.

Conclusions

Although not suitable for accurately predicting permeation the models may be suitable for determining a rank order of permeation, which may help to select candidate molecules for in-vitro screening. However, it is important to note that such predictions need to take into account actual relative drug candidate potencies.

ECVAM and new technologies for toxicity testing

The development of alternative empirical (testing) and non-empirical (non-testing) methods to traditional toxicological tests for complex human health effects is a tremendous task. Toxicants may potentially interfere with a vast number of physiological mechanisms thereby causing disturbances on various levels of complexity of human physiology. Only a limited number of mechanisms relevant for toxicity ('pathways' of toxicity) have been identified with certainty so far and, presumably, many more mechanisms by which toxicants cause adverse effects remain to be identified. Recapitulating in empirical model systems (i.e., in vitro test systems) all those relevant physiological mechanisms prone to be disturbed by toxicants and relevant for causing the toxicity effect in question poses an enormous challenge. First, the mechanism(s) of action of toxicants in relation to the most relevant adverse effects of a specific human health endpoint need to be identified. Subsequently, these mechanisms need to be modeled in reductionist test systems that allow assessing whether an unknown substance may operate via a specific (array of) mechanism(s). Ideally, such test systems should be relevant for the species of interest, i.e., based on human cells or modeling mechanisms present in humans. Since much of our understanding about toxicity mechanisms is based on studies using animal model systems (i.e., experimental animals or animal-derived cells), designing test systems that model mechanisms relevant for the human situation may be limited by the lack of relevant information from basic research. New technologies from molecular biology and cell biology, as well as progress in tissue engineering, imaging techniques and automated testing platforms hold the promise to alleviate some of the traditional difficulties associated with improving toxicity testing for complex endpoints. Such new technologies are expected (1) to accelerate the identification of toxicity pathways with human relevance that need to be modeled in test methods for toxicity testing (2) to enable the reconstruction of reductionist test systems modeling at a reduced level of complexity the target system/organ of interest (e.g., through tissue engineering, use of human-derived cell lines and stem cells etc.), (3) to allow the measurement of specific mechanisms relevant for a given health endpoint in such test methods (e.g., through gene and protein expression, changes in metabolites, receptor activation, changes in neural activity etc.), (4) to allow to measure toxicity mechanisms at higher throughput rates through the use of automated testing. In this chapter, we discuss the potential impact of new technologies on the development, optimization and use of empirical testing methods, grouped according to important toxicological endpoints. We highlight, from an ECVAM perspective, the areas of topical toxicity, skin absorption, reproductive and developmental toxicity, carcinogenicity/genotoxicity, sensitization, hematopoeisis and toxicokinetics and discuss strategic developments including ECVAM's database service on alternative methods. Neither the areas of toxicity discussed nor the highlighted new technologies represent comprehensive listings which would be an impossible endeavor in the context of a book chapter. However, we feel that these areas are of utmost importance and we predict that new technologies are likely to contribute significantly to test development in these fields. We summarize which new technologies are expected to contribute to the development of new alternative testing methods over the next few years and point out current and planned ECVAM projects for each of these areas.

Dermal absorption of benzene in occupational settings: estimating flux and applications for risk assessment

There is growing emphasis in the United States and Europe regarding the quantification of dermal exposures to chemical mixtures and other substances. In this paper, we determine the dermal flux of benzene in neat form, in organic solvents, and in aqueous solutions based on a critical review and analysis of the published literature, and discuss appropriate applications for using benzene dermal absorption data in occupational risk assessment. As part of this effort, we synthesize and analyze data for 77 experimental results taken from 16 studies of benzene skin absorption. We also assess the chemical activity of benzene in simple hydrocarbon solvent mixtures using a thermodynamic modeling software tool. Based on the collective human in vivo, human in vitro, and animal in vitro data sets, we find that the steady-state dermal flux for neat benzene (and benzene-saturated aqueous solutions) ranges from 0.2 to 0.4 mg/(cm²·h). Observed outlier values for some of the animal in vivo data sets are possibly due to the use of test species that have more permeable skin than humans or study conditions that resulted in damage to the skin barrier. Because relatively few dermal absorption studies have been conducted on benzene-containing organic solvents, and available test results may be influenced by study design or vehicle effects, it is not possible to use these data to quantify the dermal flux of benzene for other types of solvent mixtures. However, depending on the application, we describe several potential approaches that can be used to derive a rough approximation of the steady-state benzene dermal flux for these mixtures. Important limitations with respect to quantifying and evaluating the significance of dermal exposures to benzene in occupational settings include a lack of data on (1) factors that affect the dermal uptake of benzene, (2) the dermal flux of benzene for different organic solvent mixtures, (3) meaningful metrics for evaluating the dermal uptake of benzene, (4) steady-state versus non-steady-state dermal flux values for benzene, (5) the effect of skin damage on the dermal flux of benzene, (6) standardized test methods for estimating the dermal flux of benzene, and (7) robust estimates of the evaporation rate of benzene from different liquid vehicles.

Validation of a static Franz diffusion cell system for in vitro permeation studies

Over the years, in vitro Franz diffusion experiments have evolved into one of the most important methods for researching transdermal drug administration. Unfortunately, this type of testing often yields permeation data that suffer from poor reproducibility. Moreover, this feature frequently occurs when synthetic membranes are used as barriers, in which case biological tissue-associated variability has been removed as an artefact of total variation. The objective of the current study was to evaluate the influence of a full-validation protocol on the performance of a tailor-made array of Franz diffusion cells (GlaxoSmithKline, Harlow, UK) available in our laboratory. To this end, ibuprofen was used as a model hydrophobic drug while synthetic membranes were used as barriers. The parameters investigated included Franz cell dimensions, stirring conditions, membrane type, membrane treatment, temperature regulation and sampling frequency. It was determined that validation dramatically reduced derived data variability as the coefficient of variation for steady-state ibuprofen permeation from a gel formulation was reduced from 25.7% to 5.3% (n = 6). Thus, validation and refinement of the protocol combined with improved operator training can greatly enhance reproducibility in Franz cell experimentation.

Alcohol enhanced permeation in model membranes. Part I. Thermodynamic and kinetic analyses of membrane permeation

While it is well recognised that formulation components influence drug permeation, few studies have addressed the influence of vehicles on drug transport in artificial or biological membranes Previously we have investigated the effects of temperature on the uptake of model vehicles (i.e. alcohols) into silicone membrane. The present study evaluates the permeation of the model drug methyl paraben in the presence of butanol or heptanol. Drug permeation through silicone membranes was studied at different temperatures for each vehicle. Thermodynamic and kinetic analyses of the permeation data were conducted to elucidate the possible mechanisms of drug transport. Independent examination of the partition and diffusion coefficients estimated for the permeation studies at different temperatures showed a break point occurring near 20 degrees C for butanol, but not heptanol. This transition temperature separated two different mechanisms of solute diffusion and partitioning, which may be associated with a change in the properties of the solvent. This was not observed from an analysis of flux data, owing to compensatory influences on the diffusion and partition behaviour of the drug. The study underlines the importance of appropriate temperature control when studying drug permeation.

Nonlinear quantitative structure-property relationship modeling of skin permeation coefficient

The permeation coefficient characterizes the ability of a chemical to penetrate the dermis, and the current study describes our efforts to develop structure-based models for the permeation coefficient. Specifically, we have integrated nonlinear, quantitative structure-property relationship (QSPR) models, genetic algorithms (GAs), and neural networks to develop a reliable model. Case studies were conducted to investigate the effects of structural attributes on permeation using a carefully characterized database. Upon careful evaluation, a permeation coefficient data set consisting of 333 data points for 258 molecules was identified, and these data were added to our extensive thermophysical database. Of these data, permeation values for 160 molecular structures were deemed suitable for our modeling efforts. We employed established descriptors and constructed new descriptors to aid the development of a reliable QSPR model for the permeation coefficient. Overall, our new nonlinear QSPR model had an absolute-average percentage deviation, root-mean-square error, and correlation coefficient of 8.0%, 0.34, and 0.93, respectively. Cause-and-effect analysis of the structural descriptors obtained in this study indicates that that three size/shape and two polarity descriptors accounted for approximately 70% of the permeation information conveyed by the descriptors.

Skin Penetration And Lag Times Of Neat And Aqueous Diethyl Phthalate, 1,2-Dichloroethane And Naphthalene

Cutaneous exposures to occupational chemicals may cause toxic effects. For any chemical, the potential for systemic toxicity from dermal exposure depends on its ability to penetrate the skin. Most laboratory studies measure chemical penetration from an aqueous solution through isolated human or laboratory animal skin, although most exposures are not from pure aqueous solutions. The US EPA Interagency Testing Committee (ITC) mandated by the Toxic Substances Control Act, has required industry to measure the in vitro penetration of 34 chemicals in their pure or neat form (if liquid). The goal of the present study was to measure skin permeability and lag time for three neat chemicals of industrial importance, representing the general types of chemicals to be studied by the ITC (non-volatile liquids, volatile liquids, and solids), and to examine interlaboratory variation from these studies. Steady state fluxes and lag times of diethyl phthalate (DEP, slightly volatile), 1,2-dichloroethane (DCE, highly volatile), and naphthalene (NAP, solid) were studied in two different laboratories using different analytical methods. One lab also measured fluxes and lag times from saturated aqueous vehicle. Static diffusion cells, dermatomed hairless guinea pig skin, and gas chromatography were used to measure skin penetration. In the two laboratories, the steady state fluxes (mean+/-SD; microg cm(-2)hour(-1)) of DEP applied neat were: 11.8+/-4.1 and 23.9+/-7.0; fluxes of DCE (neat) were 6280+/-1380 and 3842+/-712; fluxes of NAP from powder were 30.4+/-2.0 and 7.5+/-4.7. Compared with neat fluxes measured in the same laboratory, flux from saturated aqueous solution was higher with DEP (1.9 x) but lower with DCE (0.17 x) and NAP (0.45 x). The three chemicals studied including a dry powder, demonstrate the potential for significant dermal penetration.

Evaluation of in-vivo animal and in-vitro models for prediction of dermal absorption in man

Risk assessment of dermal exposure to chemicals requires percutaneous absorption data to link the external exposure to the systemic uptake. The most reliable data on percutaneous absorption are obtained from in-vivo human volunteer studies. In addition to ethical constrains, the conduct of these studies is not feasible for the large number of industrial chemicals in use today. Therefore, there is an increasing need for alternative methods to determine percutaneous absorption such as in-vitro assays and methods performed in vivo in experimental animals. In this article, recent comparative in-vitro and in-vivo studies on percutaneous absorption have been addressed with emphasis on the factors that may affect the predictive value of the in-vitro models. Furthermore, the use of animal models, in particular the rat skin, in prediction of percutaneous absorption in the human skin has been reviewed. In-vitro assays showed to be largely influenced by the experimental circumstances, such as type and thickness of the skin, receptor fluid, and the way in which percutaneous absorption is calculated. Rat skin showed consistently to be more permeable than human skin. However, the difference between human and rat skin does not show a consistent pattern between chemicals hampering prediction of human percutaneous absorption. To increase predictive value of in-vitro and animal models, the influence of experimental factors on the percutaneous absorption should be systematically investigated by comparison with human in-vivo data, resulting in more prescriptive guidelines.

From dermal exposure to internal dose

Exposure scenarios form an essential basis for chemical risk assessment reports under the new EU chemicals regulation REACH (Registration, Evaluation, Authorisation and restriction of Chemicals). In case the dermal route of exposure is predominant, information on both exposure and dermal bioavailability is necessary for a proper risk assessment. Various methodologies exist to measure dermal exposure, providing quantitative or semiquantitative information. Although these studies may provide very specific and relevant information, it should be realized that case by case in-depth exposure assessment would be a very expensive process. Dermal bioavailability data are most often obtained from in vitro studies or animal experiments. For the design of studies, which generate data relevant for chemical risk assessment, detailed information on the exposure conditions is crucial (skin surface exposed, exposure duration, dose and physical state of the chemical). Results from non-testing methods for skin absorption, such as (Q)SARs, have been used only to a very limited extent for regulatory purposes. Suggestions are made in order to extend the use these methods to dermal risk assessment of chemical substances, thereby improving the practicability of REACH.

In vitro percutaneous penetration of organophosphorus compounds using full-thickness and split-thickness pig and human skin

Organophosphorus compounds (OPs), such as pesticides and chemical warfare agents like sarin (GB), soman (GD) and VX, are highly toxic compounds. The OP vapours and their liquid forms are readily absorbed through the skin, therefore, protecting the skin of people who are potentially exposed to these agents is crucial. The development of effective countermeasures relies on a better knowledge of the percutaneous penetration of such molecules. The purpose of this present study is to determine the in vitro percutaneous penetration parameters of two pesticides DSM and DFP, as potential simulants of V and G agents, respectively, using four in vitro systems: full-thickness and split-thickness human abdominal and pig-ear skin membranes mounted on static diffusion cells. Based on the toxicokinetic parameters of the percutaneous penetration of DSM and DFP, we demonstrated that (a) pig-ear skin is a relevant model to predict the in vitro human skin permeability taking into account a 2-fold difference between these two species (b) both full and split-thickness skin membranes could be used indiscriminately, (c) DSM and DFP would be appropriate surrogates for V and G agents to perform skin permeation studies.

Variability in Human Skin Permeability In Vitro: Comparing Penetrants with Different Physicochemical Properties

Appreciating and compensating for the inherent variability associated with percutaneous absorption is essential in optimizing (trans)dermal therapy. In this study, the variability in human skin permeability associated with model penetrants of differing lipophilicity (caffeine (CF), methyl paraben (MP), and butyl paraben (BP)) was examined in a standardized intra-laboratory study (Franz cell experiments) using epidermal tissue from various donors. Experimentally derived permeability coefficients (K(P)) were also compared to that derived from two skin permeation models namely, Potts & Guy and Robinson (revised) models in order to further validate the Franz diffusion method employed and also elucidate the potential permeation pathway(s) employed by the model penetrants. Intra-subject variability associated with skin permeation of the model penetrants was generally found to be lower than inter-subject variability. Experimental K(P) values were found to be the same order of magnitude as predicted by the mathematical models. Calculated residual variance suggested the Potts and Guy's model to be relatively accurate in predicting skin permeability of the two parabens whilst the Robinson (revised) model was more effective for CF. The high variability in CF permeation compared to the parabens may suggest the in vitro skin permeation of solutes becomes more sensitive to intra- and/or inter-subject variation in skin lipid content, appendageal density, and imperfections (pores, cracks) as the hydrophilic nature of the solute increases. Such variability in skin permeability suggests a difference in CF permeation kinetics relative to the parabens. As such when performing in vitro drug permeation studies, it is essential that the variability in the absorption of the model permeants, according to their physicochemical properties, is considered when they are used to normalize or standardize any resulting data.

Evidence that drug flux across synthetic membranes is described by normally distributed permeability coefficients

Over recent decades, the use of in vitro diffusion cell studies to assess skin permeability has evolved into a major research tool, providing key insights into the relationships between skin, drug and formulation. Sometimes, such studies involve synthetic membranes as this approach can yield useful inferences with respect to drug-skin partitioning and diffusion phenomena. Yet despite the popularity of such studies, it is still not at all known whether typical solute transport across synthetic barriers results in a normal distribution of permeability coefficients or alternatively some type of skewed distribution. The present study aims to shed light on this issue. To this end, five compounds (testosterone, oestradiol, corticosterone, aldosterone and adenosine) exhibiting a broad range of octanol-water partition coefficient values were selected as test penetrants. The protocol involved taking multiple replicate measurements of each drug's passive steady state flux through poly(dimethylsiloxane) membrane. Each penetrant's resultant permeability coefficient database was subjected to a Kolmogorov-Smirnov (KS) test for normality. It was found that the permeability coefficients of all five drugs were distributed in a Gaussian-normal fashion. The theoretical significance and practical impact of these findings are discussed.

Permeation of 4-cyanophenol and methyl paraben from powder and saturated aqueous solution through silicone rubber membranes and human skin

The objectives were to compare permeation from neat powder and saturated aqueous solution of two model compounds into homogeneous silicone rubber (polydimethylsiloxane) membranes (SRM) and human skin, which is heterogeneous, and to test the common assumption that solid chemicals do not absorb unless liquid is present. The steady-state flux of 4-cyanophenol (CP) through SRM from the powder (0.0684 +/- 0.0040 mg/cm2 x h) was almost the same as from a saturated solution (0.0789 +/- 0.0064 mg/cm2 x h, indicating that solid chemicals can absorb without the presence of liquids. The steady-state flux of CP through skin of a single subject was much smaller from the powder (0.0118 +/- 0.0064 mg/cm2 x h) than from the saturated solution (0.168 +/- 0.033 mg/cm2 x h). The average flux for powder relative to the saturated aqueous solution was 7.24% in skin compared with 87.2% in SRM for CP and 9.02% in skin compared with 99.9% in SRM for methyl paraben. It is evident that absorption into SRM and skin can occur from powdered chemicals and that surface oils or moisture are unnecessary. However, SRM proved to be a poor surrogate for dermal permeation from powders of CP and MP.

The influence of water mixtures on the dermal absorption of glycol ethers

Glycol ethers are solvents widely used alone and as mixtures in industrial and household products. Some glycol ethers have been shown to have a range of toxic effects in humans following absorption and metabolism to their aldehyde and acid metabolites. This study assessed the influence of water mixtures on the dermal absorption of butoxyethanol and ethoxyethanol in vitro through human skin. Butoxyethanol penetrated human skin up to sixfold more rapidly from aqueous solution (50%, 450 mg/ml) than from the neat solvent. Similarly penetration of ethoxyethanol was increased threefold in the presence of water (50%, 697 mg/ml). There was a corresponding increase in apparent permeability coefficient as the glycol ether concentration in water decreased. The maximum penetration rate of water also increased in the presence of both glycol ethers. Absorption through a synthetic membrane obeyed Fick's Law and absorption through rat skin showed a similar profile to human skin but with a lesser effect. The mechanisms for this phenomenon involves disruption of the stratum corneum lipid bilayer by desiccation by neat glycol ether micelles, hydration with water mixtures and the physicochemical properties of the glycol ether-water mixtures. Full elucidation of the profile of absorption of glycol ethers from mixtures is required for risk assessment of dermal exposure. This work supports the view that risk assessments for dermal contact scenarios should ideally be based on absorption data obtained for the relevant formulation or mixture and exposure scenario and that absorption derived from permeability coefficients may be inappropriate for water-miscible solvents.

A new technique to assess dermal absorption of volatile chemicals in vitro by thermal gravimetric analysis

Potential health hazards of dermal exposure, variability in reported dermal absorption rates and potential losses from the skin by evaporation indicate a need for a simple, inexpensive and standardized procedure to measure dermal absorption and desorption of chemical substances. The aim of this study was to explore the possibility to measure dermal absorption and desorption of volatile chemicals using a new gravimetric technique, namely thermal gravimetric analysis (TGA), and trypsinated stratum corneum from pig. Changes in skin weight were readily detected before, during and after exposure to vapours of water, 2-propanol, methanol and toluene. The shape and height of the weight curves differed between the four chemicals, reflecting differences in diffusivity and partial pressure and skin:air partitioning, respectively. As the skin weight is highly sensitive to the partial pressure of volatile chemicals, including water, this technique requires carefully controlled conditions with respect to air flow, temperature, chemical vapour generation and humidity. This new technique may help in the assessment of dermal uptake of volatile chemicals. Only a small piece of skin is needed and skin integrity is not necessary, facilitating the use of human samples. The high resolution weight-time curves obtained may also help to elucidate the characteristics of absorption, desorption and diffusion of chemicals in skin.