Morphologic basis for a pore-pathway in mammalian stratum corneum

Mechanisms of penetration and diffusion of drugs and cosmetic actives across the human Stratum corneum

Based on the structure of the Stratum corneum (SC) the potential penetration/diffusion pathways of drugs and cosmetic actives through the SC are presented and discussed. The well-known lipophilic pathway across the SC is presented and relevant examples are used to show that highly lipophilic molecules such as glucocorticoids, coenzyme Q10 etc. are accumulated in the SC and penetrate into the inner liquid like layer of the SC lipid bilayer by lateral diffusion. The diffusion into and across the SC of highly hydrophilic drugs and active substances such as urea, amino acids and peptides is still under discussion. Another diffusion pathway for the highly hydrophilic molecules via the corneocytes and the corneodesmosomes is presented and discussed, the corneocytary diffusion pathway.

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.

Quantitative Analysis of Acoustic Pressure for Sonophoresis and Its Effect on Transdermal Penetration

Ultrasound facilitates the penetration of macromolecular compounds through the skin and offers a promising non-invasive technique for transdermal delivery. However, technical difficulties in quantifying ultrasound-related parameters have restricted further analysis of the sonophoresis mechanism. In this study, we devise a bolt-clamped Langevin transducer-based sonophoresis device that enables us to measure with a thin lead zirconate titanate (PZT) sensor. One-dimensional acoustic theory accounting for wave interaction at the skin interface indicates that the acoustic pressure and cavitation onset on the skin during sonophoresis are sensitive to the subcutaneous support, meaning that there is a strong need to perform the pressure measurement in an experimental environment replacing the human body. From a series of the experiments with our new device, the transdermal penetration of polystyrene, silica and gold nanoparticles is found to depend on the size and material of the particles, as well as the hardness of the subcutaneous support material. We speculate from the acoustic pressure measurement that the particles' penetration results from the mechanical action of cavitation.

Impact of physiologically relevant temperatures on dermal absorption of active substances - an ex-vivo study in human skin

Skin temperature plays a certain role in the dermal absorption of substances, but the extent and mechanisms of skin temperatures-induced modulation in ranges caused by physiological thermoregulation or environmental conditions are largely unknown. The influence of dermal temperature on the absorption of the model lipophilic compound (anisole) and the model hydrophilic compounds (1,4-dioxane, ethanol) through human skin was investigated at three dermal temperatures (25, 32 and 39 °C) in an ex-vivo diffusion cell model. The substances were applied to the skin and transdermal penetration was monitored. All substances showed temperature dependent variations in their penetration behavior (3 h: 25-39 °C: 202-275% increase in cumulative, transdermally penetrated amounts). The relative differences in absorption in relation to temperature were greatest within 45 min after exposure (25-39 °C: 347-653% rise in cumulated penetration), although absolute amounts absorbed were small (45 min vs. 3 h: 4.5-14.5%). Regardless of blood circulation, skin temperature significantly influences the amount and kinetics of dermal absorption. Substance-dependent, temperature-related changes of the lipid layer order or the porous pathway may facilitate penetration. The early-stage modulation of transdermal penetration indicates transappendageal absorption, which may be relevant for short-term exposures. For both, toxicological evaluation and perfusion cell studies, it is important to consider the thermal influence on absorption or to perform the latter at a standardized temperature (32±1 °C).

Coexistence of Lipid Phases Stabilizes Interstitial Water in the Outer Layer of Mammalian Skin

The lipid matrix in the outer layer of mammalian skin, the stratum corneum, has been previously investigated by multiple biophysical techniques aimed at identifying hydrophilic and lipophilic pathways of permeation. Although consensus is developing over the microscopic structure of the lipid matrix, no molecular-resolution model describes the permeability of all chemical species simultaneously. Using molecular dynamics simulations of a model mixture of skin lipids, the self-assembly of the lipid matrix lamellae has been studied. At higher humidity, the resulting lamellar phase is maintained by partitioning excess water into isolated droplets of controlled size and spatial distribution. The droplets may fuse together to form intralamellar water channels, thereby providing a pathway for the permeation of hydrophilic species. These results reconcile competing data on the outer skin's structure and broaden the scope of molecular-based methods to improve the safety of topical products and to advance transdermal drug delivery.

Fundamentals of fractional laser-assisted drug delivery: An in-depth guide to experimental methodology and data interpretation

In the decade since their advent, ablative fractional lasers have emerged as powerful tools to enhance drug delivery to and through the skin. Effective and highly customizable, laser-assisted drug delivery (LADD) has led to improved therapeutic outcomes for several medical indications. However, for LADD to reach maturity as a standard treatment technique, a greater appreciation of its underlying science is needed. This work aims to provide an in-depth guide to the technology's fundamental principles, experimental methodology and unique aspects of LADD data interpretation. We show that drug's physicochemical properties including solubility, molecular weight and tissue binding behavior, are crucial determinants of how laser channel morphology influences topical delivery. Furthermore, we identify strengths and limitations of experimental models and drug detection techniques, interrogating the usefulness of in vitro data in predicting LADD in vivo. By compiling insights from over 75 studies, we ultimately devise an approach for intelligent application of LADD, supporting its implementation in the clinical setting.

Assessment of skin permeability to topically applied drugs by skin impedance and admittance

OBJECTIVE

Pharmacokinetic and pharmacodynamic studies of topically applied drugs are commonly performed by sampling of interstitial fluid with dermal open flow microperfusion and subsequent analysis of the samples. However, the reliability of results from the measured concentration-time profile of the penetrating drug suffers from highly variable skin permeability to topically applied drugs that is mainly caused by inter- and intra-subject variations of the stratum corneum. Thus, statistically significant results can only be achieved by performing high numbers of experiments. To reduce the expenditures needed for such high experiment numbers we aimed to assess the correlation between skin permeability and skin impedance/skin admittance.

APPROACH

We performed an ex vivo drug penetration study with human skin, based on the hypothesis that inter-subject variations of the respective concentration-time profiles can be correlated with variations of the passive electrical properties of the skin. Therefore, skin impedance and skin admittance were related to the skin permeability to the model drug Clobetasol-17-proprionate.

MAIN RESULTS

The measured low frequency skin impedance and the skin admittance correlated linearly with the drug concentration-time profiles from dermal sampling.

SIGNIFICANCE

Skin permeability can be assessed by measuring the passive electrical properties of the skin, which enables correction of skin permeability variations. This allows reduction of experiment numbers in future pharmacokinetic and pharmacodynamic studies with human skin ex vivo and in vivo and leads to diminished study costs.

Skin barrier and dry skin in the mature patient

Dry skin is the most common clinical manifestation of dermatologic diseases, and it presents with itching, redness, and desquamation-signs and clinical manifestations that are not only physically uncomfortable but also affect patients psychologically. The water content in the stratum corneum is largely dependent on the composition and amount of the intercellular lipids, which regulate the loss of water from the skin, and on the levels of hygroscopic substances of the natural moisturizing factors, which are responsible for retention of water in the stratum corneum. Prevention of water loss and penetration of potentially toxic substances and microorganisms into the body are the most important functions of the skin, which acts as a natural frontier between the inner organism and the environment. Skin barrier defects occur in several skin diseases, but the influence of aging on the skin barrier function is largely unknown and conflicting results have been reported. In this review, the structure and function of the barrier in relation to the aging process are discussed.

Deformable Nanovesicles Synthesized through an Adaptable Microfluidic Platform for Enhanced Localized Transdermal Drug Delivery

Phospholipid-based deformable nanovesicles (DNVs) that have flexibility in shape offer an adaptable and facile method to encapsulate diverse classes of therapeutics and facilitate localized transdermal delivery while minimizing systemic exposure. Here we report the use of a microfluidic reactor for the synthesis of DNVs and show that alteration of input parameters such as flow speeds as well as molar and flow rate ratios increases entrapment efficiency of drugs and allows fine-tuning of DNV size, elasticity, and surface charge. To determine the ability of DNV-encapsulated drug to be delivered transdermally to a local site, we synthesized, characterized, and tested DNVs carrying the fluorescently labeled hydrophilic bisphosphonate drug AF-647 zoledronate (AF647-Zol). AF647-Zol DNVs were lyophilized, resuspended, and applied topically as a paste to the calvarial skin of mice. High-resolution fluorescent imaging and confocal microscopy revealed significant increase of encapsulated payload delivery to the target tissue-cranial bone-by DNVs as compared to nondeformable nanovesicles (NVs) or aqueous drug solutions. Interestingly, NV delivery was not superior to aqueous drug solution. Our studies show that microfluidic reactor-synthesized DNVs can be produced in good yield, with high encapsulation efficiency, reproducibility, and stability after storage, and represent a useful vehicle for localized transdermal drug delivery.

Infrared spectroscopic imaging tracks lateral distribution in human stratum corneum

PURPOSE

To demonstrate the efficacy of infrared (IR) spectroscopic imaging for evaluation of lateral diffusion in stratum corneum (SC) and for elucidation of intermolecular interactions between exogenous agents and SC constituents.

METHODS

In separate experiments, acyl chain perdeuterated oleic acid (OA-d) and deuterated dimethyl sulfoxide (DMSO-d) were applied to the surface of isolated human SC. The lateral distribution of permeant concentrations was monitored using the time-dependence of IR images. Diffusion coefficients (D) were estimated from Fick's second law. Interactions between the exogenous agents and the SC were tracked from changes in CD2 and Amide I stretching frequencies.

RESULTS

Networked glyphs served as the major pathway for lateral distribution of OA-d. In glyph-poor regions, D values from 0.3-1 × 10(-8) cm(2)/s bracketed the OA-d data and apparently decreased with time. Although diffusion of DMSO-d is relatively fast compared to our experimental measurement time, the results suggest values of ~10(-7) cm(2)/s. OA-d spectral changes suggest penetration into the ordered lipids of the SC; DMSO-d penetration results in perturbation of SC keratin structure.

CONCLUSIONS

IR imaging provides concentration profiles, diffusion coefficients, and unique molecular level information about structural changes in the endogenous SC constituents and exogenous agents upon their mutual interaction. Transport along glyphs is the dominant mode of distribution for OA-d.

Design, synthesis of novel lipids as chemical permeation enhancers and development of nanoparticle system for transdermal drug delivery

In the present study, we designed and developed novel lipids that include (Z)-1-(Octadec-9-en-1-yl)-pyrrolidine (Cy5T), 1, 1-Di-((Z)-octadec-9-en-1-yl)pyrrolidin-1-ium iodide (Cy5), (Z)-1-(Octadec-9-en-1-yl)-piperidine (Cy6T), and 1, 1-Di-((Z)-octadec-9-en-1-yl) piperidin-1-ium iodide (Cy6) to enhance the transdermal permeation of some selected drugs. Firstly, we evaluated the transdermal permeation efficacies of these lipids as chemical permeation enhancers in vehicle formulations for melatonin, ß-estradiol, caffeine, α-MSH, and spantide using franz diffusion cells. Among them Cy5 lipid was determined to be the most efficient by increasing the transdermal permeation of melatonin, ß-estradiol, caffeine, α-MSH, and spantide by 1.5 to 3.26-fold more at the epidermal layer and 1.3 to 2.5-fold more at the dermal layer, in comparison to either NMP or OA. Hence we developed a nanoparticle system (cy5 lipid ethanol drug nanoparticles) to evaluate any further improvement in the drug penetration. Cy5 lipid formed uniformly sized nanoparticles ranging from 150–200 nm depending on the type of drug. Further, Cy5 based nanoparticle system significantly (p<0.05) increased the permeation of all the drugs in comparison to the lipid solution and standard permeation enhancers. There were about 1.54 to 22-fold more of drug retained in the dermis for the Cy5 based nanoparticles compared to OA/NMP standard enhancers and 3.87 to 66.67-fold more than lipid solution. In addition, epifluorescent microscopic analysis in rhodamine-PE permeation studies confirmed the superior permeation enhancement of LEDs (detection of fluorescence up to skin depth of 340 μm) more than lipid solution, which revealed fluorescence up to skin depth of only 260 μm. In summary the present findings demonstrate that i) cationic lipid with 5 membered amine heterocyclic ring has higher permeating efficacy than the 6 membered amine hertocyclic ring. ii) The nanoparticle system prepared with Cy5 showed significant (p<0.05) increase in the permeation of the drugs than the control penetration enhancers, oleic acid and NMP.

Penetration and haptenation of p-phenylenediamine

Although p-phenylenediamine (PPD) has been recognized as an extreme sensitizer for many years, the exact mechanism of sensitization has not been elucidated yet. Penetration and the ability to bind to proteins are the first two hurdles that an allergen has to overcome to be able to sensitize. This review is an overview of studies regarding PPD penetration through skin (analogues) and studies on the amino acids that are targeted by PPD. To complete this review, the auto-oxidation and N-acetylation steps involved in PPD metabolism are described. In summary, under normal hair dyeing exposure conditions, <1% of the applied PPD dose penetrates the skin. The majority (>80%) of PPD that penetrates will be converted into the detoxification products monoacetyl-PPD and diacetyl-PPD by the N-acetyltransferase enzymes. The small amount of PPD that does not become N-acetylated is susceptible to auto-oxidation reactions, yielding protein-reactive PPD derivatives. These derivatives may bind to specific amino acids, and some of the formed adducts might be the complexes responsible for sensitization. However, true in vivo evidence is lacking, and further research to unravel the definite mechanism of sensitization is needed.

Ablative fractional resurfacing in topical drug delivery: an update and outlook

BACKGROUND

The effective delivery of therapeutic molecules to varied targets in the skin and elsewhere has been an area of ongoing research and development.

OBJECTIVE

To review the structure of the skin with an emphasis on topical drug delivery and to present the rationale for the use of ablative and nonablative fractional resurfacing in assisted drug delivery.

METHODS AND MATERIALS

Review of the currently available scientific literature on laser-assisted drug delivery.

RESULTS

A number of strategies can be employed to enhance topical drug delivery. Ablative fractional resurfacing (AFR) has been demonstrated to be effective in enhancing drug delivery. Further studies are needed to assess the use of nonablative fractional resurfacing in assisted drug delivery.

CONCLUSION

AFR-assisted drug delivery is a promising tool for the future of dermatology. We expect to see a number of agents to be paired with AFR for enhanced drug delivery. Further investigation is necessary to evaluate appropriate drug specific channel density and depth parameters. Factors that must be considered include the physicochemical properties of the drug, the target tissue, skin wounding, and cost when evaluating the drugs and conditions that will most benefit from this promising new drug delivery system.

Structure and function of the stratum corneum extracellular matrix

The stratum corneum extracellular matrix (ECM) is enriched in lipids that are organized into lamellar bilayers, whose molecular architecture is now known. Although these bilayers are important for the permeability barrier, the ECM contains not only lipids but also enzymes, structural proteins, and antimicrobial peptides that impact barrier function. Yet, how such diverse components affect barrier function remains largely unknown. Static models of the epidermis may not do justice to the ECM, which is metabolically active, as it changes both structure and function as it transits to the surface.

Solar UV radiation reduces the barrier function of human skin

The ubiquitous presence of solar UV radiation in human life is essential for vitamin D production but also leads to skin photoaging, damage, and malignancies. Photoaging and skin cancer have been extensively studied, but the effects of UV on the critical mechanical barrier function of the outermost layer of the epidermis, the stratum corneum (SC), are not understood. The SC is the first line of defense against environmental exposures like solar UV radiation, and its effects on UV targets within the SC and subsequent alterations in the mechanical properties and related barrier function are unclear. Alteration of the SC's mechanical properties can lead to severe macroscopic skin damage such as chapping and cracking and associated inflammation, infection, scarring, and abnormal desquamation. Here, we show that UV exposure has dramatic effects on cell cohesion and mechanical integrity that are related to its effects on the SC's intercellular components, including intercellular lipids and corneodesmosomes. We found that, although the keratin-controlled stiffness remained surprisingly constant with UV exposure, the intercellular strength, strain, and cohesion decreased markedly. We further show that solar UV radiation poses a double threat to skin by both increasing the biomechanical driving force for damage while simultaneously decreasing the skin's natural ability to resist, compromising the critical barrier function of the skin.

Cellular changes that accompany shedding of human corneocytes

Corneocyte desquamation has been ascribed to either: 1) proteolytic degradation of corneodesmosomes (CD); 2) disorganization of extracellular lamellar bilayers; and/or 3) ‘swell-shrinkage-slough’ (SSS) from hydration/dehydration. To address the cellular basis for normal exfoliation, we compared changes in lamellar bilayer architecture and CD structure in DSquame® strips from the 1st vs. 5th stripping (‘outer’ vs. ‘mid’-stratum corneum [SC], respectively) from 9 normal adult forearms. Strippings were either processed for standard EM or for ruthenium (Ru-V)- or osmium-tetroxide (Os-V) vapor fixation, followed by immediate epoxy embedment, an artifact-free protocol that to our knowledge is previously unreported. CDs are largely intact in the mid-SC, but replaced by electron-dense (hydrophilic) clefts (lacunae) that expand laterally, splitting lamellar arrays in the outer SC. Some undegraded DSG1/DSC1 redistribute uniformly into corneocyte envelopes (CEs) in the outer SC (shown by proteomics, Z-stack confocal imaging and immunoEM). CEs then thicken, likely facilitating exfoliation by increasing corneocyte rigidity. In vapor-fixed images, hydration only altered the volume of the extracellular compartment, expanding lacunae further separating membrane arrays. During dehydration, air replaced water, maintaining the expanded extracellular compartment. Hydration also provoked degradation of membranes by activating contiguous acidic ceramidase activity. Together, these studies identify several parallel mechanisms that orchestrate exfoliation from the surface of normal human skin.

The structure and function of the stratum corneum

Over the past 150 years the skin's structure and function has been the subject of much investigation by scientists. The stratum corneum (SC), the skin's outermost layer and interface with the outside world is now well recognized as the barrier that prevents unwanted materials from entering, and excessive loss of water from exiting the body. This review summarizes the major advances in our understanding of this formidable membrane. The structure of the SC is outlined as well as techniques to visualize the barrier. The lipid organization and ionic gradients, as well as the metabolic responses and underlying cellular signalling that lead to barrier repair and homeostasis are discussed. Finally, a brief overview of the molecular and genetic factors that determine the development of a competent permeability barrier is provided.

Skin barrier disruption: a requirement for allergen sensitization?

For at least half a century, noninvasive techniques have been available to quantify skin barrier function, and these have shown that a number of human skin conditions and disorders are associated with defects in skin permeability. In the past decade, several genes responsible for skin barrier defects observed in both monogenetic and complex polygenic disorders have been elucidated and functionally characterized. This has led to an explosion of work in the past 6 years that has identified pathways connecting epidermal barrier disruption and antigen uptake, as well as the quality and/or magnitude of the antigen-specific adaptive immune response. This review will introduce the notion that diseases arise from the dynamic crosstalk that occurs between skin barrier and the immune system using atopic dermatitis or eczema as the disease prototype. Nevertheless, the concepts put forth are highly relevant to a number of antigen-driven disorders for which skin barrier is at least transiently compromised, such as psoriasis, allergic contact dermatitis, and blistering disorders.

Corneobiology and corneotherapy--a final chapter

The text obtained for this review from Professor Albert Kligman was drawn posthumously from a variety of notes that he had been planning to use to write a review on corneobiology and corneotherapy. It was a review that he had dearly hoped to complete--his final 'magnum opus' with reflections on the subject.

A proton NMR study on the hydration of normal versus psoriatic stratum corneum: linking distinguishable reservoirs to anatomical structures

The NMR behaviour of normal and psoriatic stratum corneum (SC) was investigated as a function of hydration with the aim of obtaining a better understanding of the role of water in the SC structure. Time domain NMR techniques were employed to identify the signal from water and that from nonaqueous components of the SC, such as lipids and proteins. The signals were investigated as a function of water content. The free induction decay was separated into mobile signal (from water and mobile lipids) and solid signal (from protein and 'solid' lipids). Spin-spin relaxation (T(2)) measurements further separated the mobile domains within the SC. The results suggested that, when water is added to dry SC, it first enters the corneocytes; then, at a hydration of 0.24-0.33 g H(2)O/g SC (normal SC) or 0.12-0.24 g H(2)O/g SC (psoriatic SC), water begins to accumulate in hydrated lipid regions. Water was found to exchange between these two domains on the time scale of a few hundred milliseconds. When compared with normal SC, psoriatic SC had a looser corneocyte structure, a larger mobile lipid component at low hydration and a smaller capacity for corneocyte water.

Percutaneous penetration modifiers and formulation effects: thermal and spectral analyses

The study investigated the formulation effects of laurocapram and iminosulfurane derived penetration modifiers on human stratum corneum using thermal and spectral analyses. Firstly, formulations of penetration modifiers were assessed as enhancers/retardants using the model permeant, diethyl-m-toluamide followed by investigation of their mechanisms of action using differential scanning calorimetry (DSC) and attenuated total reflectance Fourier-transform infra-red spectroscopy. The penetration modifiers investigated were laurocapram, 3-dodecanoyloxazolidin-2-one (N-0915), S,S-dimethyl-N-(4-bromobenzoyl) iminosulfurane (DMBIS), S,S-dimethyl-N-(2-methoxycarbonylbenzenesulfonyl) iminosulfurane (DMMCBI) and tert-butyl 1-dodecyl-2-oxoazepan-3-yl-carbamate (TBDOC) that were formulated in either water, propylene glycol (PG), ethanol or polyethylene glycol 400 (PEG 400). The results explain the mechanism for the first time why an enhancer can become a retardant or vice versa depending upon the vehicle in which it is applied to the skin. DSC indicated that penetration modifier formulations enhanced permeation of active mainly by disruption and fluidization of the stratum corneum lipid bilayers while IR data indicated characteristic blue shifts with decreases in peak intensity. On the other hand, DSC of penetration modifier formulations showing retardation depicted elevated T (m2) with a strengthening of lipid-protein complex while IR results indicated formation of multiple peaks around 1,738 cm(-1) transition in stratum corneum spectra suggesting retardation may be caused by organization of SC lipids by increased H-bonding.

Structure and functions of keratin proteins in simple, stratified, keratinized and cornified epithelia

Historically, the term 'keratin' stood for all of the proteins extracted from skin modifications, such as horns, claws and hooves. Subsequently, it was realized that this keratin is actually a mixture of keratins, keratin filament-associated proteins and other proteins, such as enzymes. Keratins were then defined as certain filament-forming proteins with specific physicochemical properties and extracted from the cornified layer of the epidermis, whereas those filament-forming proteins that were extracted from the living layers of the epidermis were grouped as 'prekeratins' or 'cytokeratins'. Currently, the term 'keratin' covers all intermediate filament-forming proteins with specific physicochemical properties and produced in any vertebrate epithelia. Similarly, the nomenclature of epithelia as cornified, keratinized or non-keratinized is based historically on the notion that only the epidermis of skin modifications such as horns, claws and hooves is cornified, that the non-modified epidermis is a keratinized stratified epithelium, and that all other stratified and non-stratified epithelia are non-keratinized epithelia. At this point in time, the concepts of keratins and of keratinized or cornified epithelia need clarification and revision concerning the structure and function of keratin and keratin filaments in various epithelia of different species, as well as of keratin genes and their modifications, in view of recent research, such as the sequencing of keratin proteins and their genes, cell culture, transfection of epithelial cells, immunohistochemistry and immunoblotting. Recently, new functions of keratins and keratin filaments in cell signaling and intracellular vesicle transport have been discovered. It is currently understood that all stratified epithelia are keratinized and that some of these keratinized stratified epithelia cornify by forming a Stratum corneum. The processes of keratinization and cornification in skin modifications are different especially with respect to the keratins that are produced. Future research in keratins will provide a better understanding of the processes of keratinization and cornification of stratified epithelia, including those of skin modifications, of the adaptability of epithelia in general, of skin diseases, and of the changes in structure and function of epithelia in the course of evolution. This review focuses on keratins and keratin filaments in mammalian tissue but keratins in the tissues of some other vertebrates are also considered.

Penetration pathways induced by low-frequency sonophoresis with physical and chemical enhancers: iron oxide nanoparticles versus lanthanum nitrates

Low-frequency sonophoresis (LFS) has been shown to disrupt the structure of stratum corneum (SC) lipid bilayers and enhance SC permeability. In this study, we examined the penetration pathway of lanthanum nitrate (LaNO(3)) tracer in viable epidermis after combined treatment of LFS and tape stripping (TS), as a physical enhancer, or oleic acid (OA) application, as a chemical enhancer, using transmission electron microscopy (TEM). As a positive control, we visualized the passive diffusion pathway of LaNO(3) and iron oxide (Fe(3)O(4)) nanoparticles after the incision of hairless mouse skin. Next, we applied LFS immediately after TS or OA application and visualized the penetration pathway of LaNO(3). Each treatment showed restricted penetration to the SC-stratum granulosum (SG) interface or upper SG layer. However, the additional application of LFS induced diffuse intracellular distribution of LaNO(3) throughout the viable epidermis. Quantitative analysis also revealed that combined treatment significantly increases LaNO(3) penetration into viable epidermis when compared with each treatment. Our ultrastructural findings show the synergistic effect of LFS and TS or OA application on transdermal drug delivery. We also found that this combined treatment enhances the penetration of LaNO(3) through the viable epidermis through an intracellular pathway.

Filaggrin deficiency confers a paracellular barrier abnormality that reduces inflammatory thresholds to irritants and haptens

BACKGROUND

Mutations in the human filaggrin gene (FLG) are associated with atopic dermatitis (AD) and are presumed to provoke a barrier abnormality. Yet additional acquired stressors might be necessary because the same mutations can result in a noninflammatory disorder, ichthyosis vulgaris.

OBJECTIVE

We examined here whether FLG deficiency alone suffices to produce a barrier abnormality, the basis for the putative abnormality, and its proinflammatory consequences.

METHODS

By using the flaky-tail mouse, which lacks processed murine filaggrin because of a frameshift mutation in the gene encoding profilaggrin that mimics some mutations in human AD, we assessed whether FLG deficiency provokes a barrier abnormality, further localized the defect, identified its subcellular basis, and assessed thresholds to irritant- and hapten-induced dermatitis.

RESULTS

Flaky-tail mice exhibit low-grade inflammation with increased bidirectional, paracellular permeability of water-soluble xenobiotes caused by impaired lamellar body secretion and altered stratum corneum extracellular membranes. This barrier abnormality correlates with reduced inflammatory thresholds to both topical irritants and haptens. Moreover, when exposed repeatedly to topical haptens at doses that produce no inflammation in wild-type mice, flaky-tail mice experience a severe AD-like dermatosis with a further deterioration in barrier function and features of a T(H)2 immunophenotype (increased CRTH levels plus inflammation, increased serum IgE levels, and reduced antimicrobial peptide [mBD3] expression).

CONCLUSIONS

FLG deficiency alone provokes a paracellular barrier abnormality in mice that reduces inflammatory thresholds to topical irritants/haptens, likely accounting for enhanced antigen penetration in FLG-associated AD.

Permeability of model stratum corneum lipid membrane measured using quartz crystal microbalance

The stratum corneum (SC) is the outermost layer of the epidermis. Stacked intercellular lipid membranes found in the SC play a crucial role in regulating water transport through the skin. Despite the importance of this role of the SC lipid membranes, only a few studies have presented quantitative methods to measure the permeability of water in SC lipid membranes. In this work, we present a new method to determine the water permeability of a model SC lipid membrane using a quartz crystal microbalance (QCM). We investigate a model SC lipid membrane comprising an equimolar mixture of brain ceramide (CER), cholesterol (CHO), and palmitic acid (PA), and use QCM to determine the diffusivity (D), solubility (S,) and permeability (P) of water vapor in the model SC lipid membrane.

Evaluation of the in vitro skin permeation of antiviral drugs from penciclovir 1% cream and acyclovir 5% cream used to treat herpes simplex virus infection

BACKGROUND

Herpes simplex virus infection (HSV) is a common and ubiquitous infection of the skin which causes mucocutaneous lesions called cold sores (herpes labialis) or fever blisters. It is estimated that approximately 80% of the population worldwide are carriers of the Herpes simplex virus, approximately 40% suffer from recurrent recurrent infections. This study evaluates the in vitro skin permeation and penetration of penciclovir and acyclovir from commercialized creams for the treatment of herpes labialis (cold sores), using non viable excised human abdominal skin samples, which were exposed to 5 mg/cm2 of acyclovir 5% cream or penciclovir 1% cream.

METHODS

After 24 h of cream application, excess cream was washed off and layers of stratum corneum were removed by successive tape stripping. Amounts of active ingredients having penetrated through the skin were measured, as well as the amounts in the washed-off cream, in skin strips and creams remaining in the skin. Molecular modelling was used to evaluate physico-chemical differences between the drugs. Western blot analysis enabled to determine whether the marker of basal cells keratin 5 could be detected in the various tape strips.

RESULTS

Application of penciclovir 1% cream yielded higher concentration of drug in the deeper layers of the epidermis as well as a higher drug flux through the skin. Molecular modelling showed two higher hydrophobic moieties for acyclovir. Presence of the basal cell marker keratin 5 was underscored in the deeper tape strips from the skin, giving evidence that both drugs can reach their target cells.

CONCLUSION

Penciclovir 1% cream has the tendency to facilitate the diffusion of the drug through the stratum corneum into the deeper epidermis layers, in which it could reach the target basal cells at effective therapeutical concentration. The small difference in the surface properties between both molecules might also contribute to favour the passage of penciclovir through the epidermis into the deeper basal cells.

Effect of cold storage on in vitro human skin absorption of six 14C-radiolabeled environmental contaminants: benzo[a]pyrene, ethylene glycol, methyl parathion, naphthalene, nonyl phenol, and toluene

Dermal absorption of human breast skin obtained fresh from a local hospital was tested before and after freezer storage at -19 degrees C for 30 or 60 d. Dermatomed skin (0.4-0.5 mm) was tested in vitro using the Bronaugh flow-through diffusion cells perfused at 1.5 ml/h with receiver solution (Hanks HEPES buffered basal saline containing 4% bovine serum albumin [BSA]). Six 14C-radiolabeled chemicals ranging in lipophilicity were tested, including benzo[a]pyrene (BaP), ethylene glycol (EG), methyl parathion (MP), naphthalene (Nap), nonyl phenol (NP), and toluene (Tol). There was significantly lower percent dermal absorption into the receiver solution for two of the six chemicals (BaP and Tol) with the skin depot excluded. However, with percent dermal absorption defined as that including the skin depot, with the exception of the BaP data for skin frozen 30 d, there was no significant difference between percent dermal absorption data for fresh unfrozen controls and those stored frozen for all 6 test chemicals for both 30 and 60 d freezer storage times. These results suggested with skin depot included that freezer storage may have potential for preserving human skin for in vitro absorption tests of environmental contaminants; however, optimal freezer storage conditions such as temperature and storage duration and their effects on skin viability and dermal metabolism need to be determined.

Evaluation of the porosity, the tortuosity, and the hindrance factor for the transdermal delivery of hydrophilic permeants in the context of the aqueous pore pathway hypothesis using dual-radiolabeled permeability experiments

The aqueous pore pathway hypothesis has been modified to include both transient and steady-state domains of diffusive transport to evaluate the porosity, the tortuosity, and the hindrance factor of the skin aqueous pore channels from an individual dual-radiolabeled permeability experiment. Using these theoretical and experimental methods, the porosity (epsilon), the tortuosity (tau), and the hindrance factor (H) of the skin aqueous pore channels were evaluated as a function of: (i) the radius of the selected model hydrophilic permeants (urea, mannitol, raffinose, and inulin), and (ii) the extent of skin perturbation present in untreated skin, skin pretreated at a low dose, and a high dose, with a simultaneous application of 20 kHz ultrasound and the surfactant sodium lauryl sulfate (SLS), and the dermis. The results of this investigation revealed that the tortuosity decreased, and only the hindrance factor for inulin was significantly less than 1, over the range of permeant radii examined. Furthermore, only the porosity increased over the range of skin perturbation examined (over 100-fold), suggesting that a surface-related phenomenon is primarily responsible for the observed enhancement in the transdermal permeability of hydrophilic permeants induced by the simultaneous application of ultrasound and SLS.

Dual-Channel Two-Photon Microscopy Study of Transdermal Transport in Skin Treated with Low-Frequency Ultrasound and a Chemical Enhancer

Visualization of transdermal permeant pathways is necessary to substantiate model-based conclusions drawn using permeability data. The aim of this investigation was to visualize the transdermal delivery of sulforhodamine B (SRB), a fluorescent hydrophilic permeant, and of rhodamine B hexyl ester (RBHE), a fluorescent hydrophobic permeant, using dual-channel two-photon microscopy (TPM) to better understand the transport pathways and the mechanisms of enhancement in skin treated with low-frequency ultrasound (US) and/or a chemical enhancer (sodium lauryl sulfate--SLS) relative to untreated skin (the control). The results demonstrate that (1) both SRB and RBHE penetrate beyond the stratum corneum and into the viable epidermis only in discrete regions (localized transport regions--LTRs) of US treated and of US/SLS-treated skin, (2) a chemical enhancer is required in the coupling medium during US treatment to obtain two significant levels of increased penetration of SRB and RBHE in US-treated skin relative to untreated skin, and (3) transcellular pathways are present in the LTRs of US treated and of US/SLS-treated skin for SRB and RBHE, and in SLS-treated skin for SRB. In summary, the skin is greatly perturbed in the LTRs of US treated and US/SLS-treated skin with chemical enhancers playing a significant role in US-mediated transdermal drug delivery.

The skin barrier as an innate immune element

Since life in a terrestrial environment threatens mammals continuously with desiccation, the structural, cellular, biochemical, and regulatory mechanisms that sustain permeability barrier homeostasis have justifiably comprised a major thrust of prior and recent research on epidermal barrier function. Yet, the epidermis mediates a broad set of protective 'barrier' functions that includes defense against pathogen challenges. Permeability and antimicrobial function are both co-regulated and interdependent, overlapping through the dual activities of their lipid/protein constituents. Most of the defensive (barrier) functions of the epidermis localize to the stratum corneum (SC), which limits pathogen colonization through its low water content, acidic pH, resident (normal) microflora, and surface-deposited antimicrobial lipids (1 degree free fatty acid). These various barrier functions are largely mediated by either the corneocyte or the extracellular matrix, and it is both the localization and the organization of secreted hydrophobic lipids into characteristic lamellar bilayers that is critical not only for permeability barrier function, but also for antimicrobial function through its contribution to the maintenance of SC integrity. Low constitutive levels of antimicrobial peptides under basal conditions emphasize the key role of epithelial structure in antimicrobial defense. But antimicrobial peptide synthesis and delivery to the SC interstices accelerates after external insults to the barrier.

An Integrated Pharmacokinetic and Imaging Evaluation of Vehicle Effects on Solute Human Epidermal Flux and, Retention Characteristics

OBJECTIVE

Our understanding of the differential effects of topically applied vehicles on solute partitioning and diffusion within the skin is presently limited. In this work, in vitro epidermal partitioning, penetration and multiphoton laser scanning microscopy (MPLSM) imaging studies were used to assess the distribution of 2-naphthol across human epidermis.

MATERIALS AND METHODS

Four commonly used liquid vehicles (100% water, 20% propylene glycol (PG)/water, 50% ethanol (EtOH)/water and 100% isopropyl myristate (IPM)) were used.

RESULTS AND DISCUSSION

The maximum flux and membrane retention of 2-naphthol from 50% EtOH/water was almost an order of magnitude or larger than from the other vehicles evaluated whereas IPM resulted in the highest membrane retention and lowest membrane penetration for 2-naphthol than other vehicles. MPLSM studies showed that 2-naphthol solute partitioned favourably into the intercellular lipids and that there was a vehicle-dependent uptake of 2-naphthol into corneocytes.

CONCLUSIONS

The integrated evaluation using in vitro penetration, epidermal retention and MPLSM imaging has shown that vehicle effects on skin penetration occurs by an alteration in the distribution of solutes between the corneocytes and intercellular lipids in addition to the well known mechanisms of altered partitioning into the stratum corneum and enhanced epidermal diffusion.

Nanoparticles: health effects--pros and cons

With the advent of nanotechnology, the prospects for using engineered nanomaterials with diameters of < 100 nm in industrial applications, medical imaging, disease diagnoses, drug delivery, cancer treatment, gene therapy, and other areas have progressed rapidly. The potential for nanoparticles (NPs) in these areas is infinite, with novel new applications constantly being explored. The possible toxic health effects of these NPs associated with human exposure are unknown. Many fine particles generally considered "nuisance dusts" are likely to acquire unique surface properties when engineered to nanosize and may exhibit toxic biological effects. Consequently, the nuisance dust may be transported to distant sites and could induce adverse health effects. In addition the beneficial uses of NPs in drug delivery, cancer treatment, and gene therapy may cause unintentional human exposure. Because of our lack of knowledge about the health effects associated with NP exposure, we have an ethical duty to take precautionary measures regarding their use. In this review we highlight the possible toxic human health effects that can result from exposure to ultrafine particles (UFPs) generated by anthropogenic activities and their cardiopulmonary outcomes. The comparability of engineered NPs to UFPs suggests that the human health effects are likely to be similar. Therefore, it is prudent to elucidate their toxicologic effect to minimize occupational and environmental exposure. Highlighting the human health outcomes caused by UFPs is not intended to give a lesser importance to either the unprecedented technologic and industrial rewards of the nanotechnology or their beneficial human uses.

Transcellular route of diffusion through stratum corneum: results from finite element models

Insight into the stratum corneum (SC) permeation pathway for hydrophilic compounds is gained by comparing experimental measurements of permeability and lag time (tlag) with the predictions of a finite element (FE) model. A database of permeability and lag time measurements (n=27) of hydrophilic compounds was compiled from the literature. Transcellular and lateral lipid diffusion pathways were modeled within a brick-and-mortar geometry representing fully hydrated human SC. Modeled tlag's for the lipid pathway are too brief to account for the experimental quantities, whereas the transcellular pathway with preferential corneocyte partitioning does account for them. Measured tlag's are highly correlated (p<0.0001) with the compound's octanol-water partition coefficient, supporting the hypothesis of an aqueous-lipid partition mechanism in the permeation of hydrophilic compounds. The importance of the lag time for identifying the diffusion pathway is demonstrated.

Transdermal drug delivery: Basic principles for the veterinarian

The use of topical pharmaceutical formulations is increasingly popular in veterinary medicine. A potential concern is that not all formulations are registered for the intended species, yet current knowledge strongly suggests that simple extrapolation of transdermal drug pharmacokinetics and pharmacodynamics between species, including humans, cannot be done. In this review, an overview is provided of the underlying basic principles determining the movement of topically applied molecules into and through the skin. Various factors that may affect transdermal drug penetration between species, between individuals of a particular species and regional differences in an individual are also discussed. A good understanding of the basic principles of transdermal drug delivery is critical to avoid adverse effects or lack of efficacy when applying topical formulations in veterinary medicine.

A multiphase microscopic diffusion model for stratum corneum permeability. I. Formulation, solution, and illustrative results for representative compounds

A two-dimensional microscopic transport model of the stratum corneum (SC) incorporating corneocytes of varying hydration and permeability embedded in an anisotropic lipid matrix is presented. Results are expressed in terms of a dimensionless permeability (P(SC/w)(comp), which is a function of two dimensionless parameters, R and sigma. R is a ratio of transbilayer to lateral molecular flows within a lipid bilayer and sigma is the ratio of (lateral) permeability in the lipid phase, D(lip)K(lip/w), to that in the corneocyte phase, D(cor)K(cor/w.) The shape of the dimensionless permeability surface is also governed by the arrangement of the SC lipids, where Model 1 represents the extreme in which lipid-phase transport can occur with no transbilayer transport, whereas Model 2 entails maximum transbilayer transport. Model calculations are exemplified by characterizing the skin permeability of four representative permeants: water, ethanol, nicotinamide, and testosterone. A comparison with experimental steady state permeability and partition data supports that the transport properties of the SC lipids are highly anisotropic, with lateral diffusivities several orders of magnitude higher than the equivalent diffusivity calculated from transbilayer hopping. Nevertheless, the calculations suggest that corneocyte-phase transport plays a major role for all four permeants. These results confirm our previous calculations on water permeability and present a marked contrast to the commonly stated doctrine that the SC transport pathway is primarily intercellular.

Low-frequency sonophoresis: ultrastructural basis for stratum corneum permeability assessed using quantum dots

Low-frequency sonophoresis (LFS) has been well documented to enhance the permeability of skin to macromolecular drugs via induction of localized transport regions. However, the organizational details of epidermis, specifically stratum corneum (SC), during sonophoresis are beyond the resolution limit of common histo-optical microscopy tools, which fail to reveal any notable structural alterations in these regions at a submicroscopic scale. Here we report, using quantum dots (QDs) as a tracer and confocal microscopy and transmission electron microscopy (TEM) (with OsO(4) and RuO(4) post-fixation) as visualization methods, on LFS-induced permeation pathways in the SC. QDs (20 nm diameter) penetrated well beyond the SC. TEM revealed that ultrasound significantly increased the frequency of occurrence of the otherwise scattered and separated lacunar spaces in the SC. A significant increase in lacunar dimensions was observed when 1% w/v sodium lauryl sulfate was added to the coupling medium. These studies show that LFS induces dilatation and higher connectivity of voids in the SC, possibly leading to formation of a three-dimensional porous network, which is capable of transporting QDs as well as macromolecules across the SC. This contention is consistent with previously conceived theoretical mechanistic understanding of LFS-induced enhanced transport across the skin.

Transdermal drug delivery using microemulsion and aqueous systems: Influence of skin storage conditions on the in vitro permeability of diclofenac from aqueous vehicle systems

The objective of this study was to evaluate the transdermal delivery potential of diclofenac-containing microemulsion system in vivo and in vitro. It was found that the transdermal administration of the microemulsion to rats resulted in 8-fold higher drug plasma levels than those obtained after application of Voltaren Emulgel. After s.c. administration (3.5 mg/kg), the plasma levels of diclofenac reached a peak of 0.94 microg/ml at t=1 h and decreased rapidly to 0.19 microg/ml at t=6 h, while transdermal administration of the drug in microemulsion maintained constant levels of 0.7-0.9 microg/ml for at least 8 h. The transdermal fluxes of diclofenac were measured in vitro using skin excised from different animal species. In three rodent species, penetration fluxes of 53.35+/-8.19 (furry mouse), 31.70+/-3.83 (hairless mouse), 31.66+/-4.45 (rat), and 22.89+/-6.23 microg/cm(2)/h (hairless guinea pig) were obtained following the application of the microemulsion. These fluxes were significantly higher than those obtained by application of the drug in aqueous solution. In contrast to these results, a 'flip-flop' phenomenon was observed when frozen porcine skin (but not fresh skin) was significantly more permeable to diclofenac-in-water than to the drug-in-microemulsion. In fact, the drug penetration from the microemulsion was not affected by the skin storage conditions, but it was increased when an aqueous solution was applied. However, this unusual phenomenon observed in non-freshly used porcine skin places a question mark on its relevancy for in vitro penetration studies involving aqueous vehicle systems.

Additive impairment of the barrier function by mechanical irritation, occlusion and sodium lauryl sulphate in vivo

BACKGROUND

The interaction between potential irritants in the workplace might be important because workers are not usually exposed to a single irritant, but to multiple potentially harmful substances. Physical irritant contact dermatitis caused by friction or mechanical abrasion is a common occupational dermatosis. Prolonged water exposure by occlusion is also common in the workplace. Several studies have revealed the negative effect of the common anionic detergent sodium lauryl sulphate (SLS) on permeability barrier function.

OBJECTIVES

To study the additive impairment of permeability barrier function by mechanical irritation combined with 0.5% SLS or prolonged water exposure by occlusion, as models of mild irritation.

METHODS

The volar forearms of 20 healthy volunteers were exposed to mechanical irritation and occlusion with water or 0.5% SLS for four consecutive days in a combined tandem repeated irritation test (TRIT). Permeability barrier function was measured with a Tewameter TM 210. Irritation was assessed with a Chromameter CR 300 and a visual score.

RESULTS

Barrier disruption in our model was rated as follows: occlusion with SLS and mechanical irritation > occlusion with SLS > occlusion with water and mechanical irritation > mechanical irritation and occlusion with water > occlusion with a glove and mechanical irritation > mechanical irritation > occlusion with water. Barrier disruption caused by occlusion or mechanical irritation was enhanced by the tandem application. The choice of irritant under occlusion, time of occlusion and order of tandem application all affected the degree of barrier disruption. Evaporimetry was able to detect early stages in the development of an irritant reaction before it became visible. Chromametry was not able to detect this early response.

CONCLUSIONS

Physical irritants (friction, abrasive grains, occlusion) and detergents such as SLS represent a significant irritation risk and should be minimized, especially when acting together, as shown in our TRIT model.

Stratum corneum defensive functions: an integrated view

Most epidermal functions can be considered as protective, or more specifically, as defensive in nature. Yet, the term "barrier function" is often used synonymously with only one such defensive function, though arguably its most important, i.e., permeability barrier homeostasis. Regardless of their relative importance, these protective cutaneous functions largely reside in the stratum corneum (SC). In this review, I first explore the ways in which the multiple defensive functions of the SC are linked and interrelated, either by their shared localization or by common biochemical processes; how they are co-regulated in response to specific stressors; and how alterations in one defensive function impact other protective functions. Then, the structural and biochemical basis for these defensive functions is reviewed, including metabolic responses and signaling mechanisms of barrier homeostasis. Finally, the clinical consequences and therapeutic implications of this integrated perspective are provided.

In vitro delivery of novel, highly potent anti-varicella zoster virus nucleoside analogues to their target site in the skin

PURPOSE

To determine the in-vitro dermal delivery of a new class of lipophilic, highly potent and uniquely selective anti-VZV nucleoside analogues in comparison with aciclovir.

METHODS

Three test compounds (Cf1698, Cf1743, Cf1712) and aciclovir were formulated into propylene glycol/aqueous cream BP formulations and finite doses applied to full-thickness pig ear skin for 48 hours in vertical Franz-type diffusion cells. Receptor phase samples were taken at specific intervals to determine permeation, and depth profiles were constructed following tape stripping and membrane separation.

RESULTS

All three test compounds reached the target basal epidermis in concentrations suggesting they would be highly efficacious in reducing viral load. Furthermore, the data showed that each of the test compounds would perform in a far superior manner to aciclovir, the current treatment of choice.

CONCLUSIONS

The dermatomal site of viral replication during secondary infection--the basal epidermis--was successfully targeted. Topical delivery of these compounds is highly promising as a new first line treatment of VZV infections. By attacking the virus at the first sign of reactivation, it is proposed that the extent of damage caused by the virus would be significantly lowered, thereby limiting the extent and severity of post-herpetic neuralgia.

Nanoparticles - known and unknown health risks

Manmade nanoparticles range from the well-established multi-ton production of carbon black and fumed silica for applications in plastic fillers and car tyres to microgram quantities of fluorescent quantum dots used as markers in biological imaging. As nano-sciences are experiencing massive investment worldwide, there will be a further rise in consumer products relying on nanotechnology. While benefits of nanotechnology are widely publicised, the discussion of the potential effects of their widespread use in the consumer and industrial products are just beginning to emerge. This review provides comprehensive analysis of data available on health effects of nanomaterials.

Transdermal delivery of macromolecules by erbium:YAG laser

The aim of this study was to assess the effect of molecular weight (MW) on the transdermal delivery of macromolecules by erbium:yttrium-aluminum-garnet (Er:YAG) laser treatment. Fluorescein isothiocyanate (FITC)-labeled dextran (FD) of increasing MWs (4.4, 19.4, 38, and 77 kDa) was used as the model macromolecules to investigate the skin permeation in vitro. Fluorescence microscopy and scanning electron microscopic (SEM) images were utilized to examine the transport mechanisms of the macromolecules via the skin after laser treatment. The results indicate a significant increase in the permeation of FITC and FD across skin treated by the laser. The MWs of macromolecules and laser fluences were found to play important roles in controlling macromolecular absorption. Transdermal delivery of FD with a MW of at least 77 kDa could be achieved with laser treatment. Follicular routes were significant for FITC permeation, whereas intercellular pathways played important roles on the delivery of FD. Ablation of the stratum corneum (SC) layer, photomechanical stress on intercellular regions, and alterations of the morphology and arrangement of corneocytes are possible mechanisms of how the Er:YAG laser promotes macromolecular delivery. No alteration of viable skin morphology was observed after laser treatment and the partly ablation of the SC may be reversible. Hexameric insulin showed higher skin permeation than did FD with similar MWs (38 kDa) with laser enhancement. From the study presented herein, it is concluded that the Er:YAG laser can be effective for transdermal delivery of macromolecules and hydrophilic permeants such as peptides and protein-based drugs.

Transdermal drug delivery with a pressure wave

Pressure waves, which are generated by intense laser radiation, can permeabilize the stratum corneum (SC) as well as the cell membrane. These pressure waves are compression waves and thus exclude biological effects induced by cavitation. Their amplitude is in the hundreds of atmospheres (bar) while the duration is in the range of nanoseconds to a few microseconds. The pressure waves interact with cells and tissue in ways that are probably different from those of ultrasound. Furthermore, the interactions of the pressure waves with tissue are specific and depend on their characteristics, such as peak pressure, rise time and duration. A single pressure wave is sufficient to permeabilize the SC and allow the transport of macromolecules into the epidermis and dermis. In addition, drugs delivered into the epidermis can enter the vasculature and produce a systemic effect. For example, insulin delivered by pressure waves resulted in reducing the blood glucose level over many hours. The application of pressure waves does not cause any pain or discomfort and the barrier function of the SC always recovers.

Effect of solute lipophilicity on penetration through canine skin

OBJECTIVE

To investigate the effect of lipophilicity on the percutaneous penetration of a homologous series of alcohols through canine skin.

DESIGN

Skin harvested from Greyhound thorax was placed in Franz-type diffusion cells and the in vitro passage of radiolabelled (14C) alcohols (ethanol, butanol, hexanol and octanol (Log P 0.19-3.0)) through separate skin sections was measured in replicates of five. Permeability coefficient (kP, cm/h), maximum flux (Jmax, mol/cm2/h) and residue remaining within the skin were determined.

RESULTS

The kP increased with increasing lipophilicity (6.2 x 10(-4) +/- 1.6 x 10(-4) cm/h for ethanol to 1.8 x 10(-2) +/- 3.6 x 10(-3) cm/h for octanol). Alcohol residues remaining within each skin sample followed a similar pattern. An exponential decrease in Jmax with increasing lipophilicity was observed.

CONCLUSION

Changes in canine skin permeability occur with increasing alcohol lipophilicity. This finding has practical consequences for the design of topical formulations and optimisation of drug delivery through animal skin.

Evaluation of Fluorescent Probe Surface Intensities as an Indicator of Transdermal Permeant Distributions Using Wide-Area Two-Photon Fluorescence Microscopy

The application of high-speed two-photon fluorescence microscopy (HTPM) to examine transdermal transport processes has enabled the noninvasive visualization of permeant spatial distributions over a larger, more clinically relevant wide area of the skin. Earlier studies demonstrated that the transdermal fluorescent probe distribution over a 2 x 2 mm skin area was well represented by a significantly reduced sampling of the 400 microscale skin sites (100 x 100 microm) constituting the wide area. In the present study, the 400 microscale skin sites are considered individually, and the site-to-site variability in permeant distributions is used as a model to reflect the range in experimentally measured skin permeabilities resulting from the inherent stratum corneum structural heterogeneity. The correlation established between the permeant surface intensity and the corresponding permeant intensity gradient at each skin site provides an indication of the potential for screening transdermal permeant distributions solely based on the evaluation of microscale permeant surface intensities. The strong linear correlation between the intensity gradient and the surface intensity for the hydrophilic model permeant, sulforhodamine B, demonstrated that surface intensities provide a robust indicator of the corresponding transdermal probe distributions at the microscale. For the hydrophobic model permeant, rhodamine B hexyl ester, however, weak correlations were observed between these two parameters. This result suggests that the stratum corneum microscale surface intensity does not validly capture the corresponding intensity gradients for the entire range of skin permeabilities typically encountered as a result of the inherent stratum corneum heterogeneity.