Determination of the pH gradient across the stratum corneum

Non-Invasive Delivery of Negatively Charged Nanobodies by Anodal Iontophoresis: When Electroosmosis Dominates Electromigration

Iontophoresis enables the non-invasive transdermal delivery of moderately-sized proteins and the needle-free cutaneous delivery of antibodies. However, simple descriptors of protein characteristics cannot accurately predict the feasibility of iontophoretic transport. This study investigated the cathodal and anodal iontophoretic transport of the negatively charged M7D12H nanobody and a series of negatively charged variants with single amino acid substitutions. Surprisingly, M7D12H and its variants were only delivered transdermally by anodal iontophoresis. In contrast, transdermal permeation after cathodal iontophoresis and passive diffusion was Collapse

Key Words

• anti-EGFR 7D12 nanobody
• electroosmosis
• iontophoresis
• nanobodies
• protein expression and purification
• skin topical and transdermal delivery

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Affiliation(s)

Phedra Firdaws Sahraoui School of Pharmaceutical Sciences, University of Geneva, CMU-1 Rue Michel Servet, 1211 Geneva, Switzerland; Institute of Pharmaceutical Sciences of Western Switzerland, University of Geneva, CMU-1 Rue Michel Servet, 1211 Geneva, Switzerland
Oscar Vadas Department of Microbiology and Molecular Medicine, Faculty of Medicine, University of Geneva, CMU-1 Rue Michel Servet, 1211 Geneva, Switzerland;
Yogeshvar N. Kalia School of Pharmaceutical Sciences, University of Geneva, CMU-1 Rue Michel Servet, 1211 Geneva, Switzerland; Institute of Pharmaceutical Sciences of Western Switzerland, University of Geneva, CMU-1 Rue Michel Servet, 1211 Geneva, Switzerland

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Screen-printed wearable skin surface pH sensor for real-time monitoring of the buffering capacity of human skin

This study demonstrated for the first time that skin surface pH can be monitored in real-time, using a screen-printed wearable pH sensor, to evaluate the buffering capacity of the human skin. The screen-printed pH sensor was composed of a polyaniline-based pH-sensitive electrode and a nitrocellulose membrane-based liquid junction type of Ag/AgCl reference electrode. This sensor showed a reliable and reversible potentiometric response to pH with long-term potential stability. Intermittent monitoring of the buffering capacity of skin surface pH demonstrated the reliability of the proposed wearable pH sensor, which was comparable to that of a commercially available flat-tip pH sensor. We found that contact of the wearable pH sensor with the subject's skin via aqueous electrolyte solutions was necessary for the sensor to continuously monitor the skin surface pH while sustaining the natural buffer capacity of the human skin surface.

The skin barrier: An extraordinary interface with an exceptional lipid organization

The barrier function of the skin is primarily located in the stratum corneum (SC), the outermost layer of the skin. The SC is composed of dead cells with highly organized lipid lamellae in the intercellular space. As the lipid matrix forms the only continuous pathway, the lipids play an important role in the permeation of compounds through the SC. The main lipid classes are ceramides (CERs), cholesterol (CHOL) and free fatty acids (FFAs). Analysis of the SC lipid matrix is of crucial importance in understanding the skin barrier function, not only in healthy skin, but also in inflammatory skin diseases with an impaired skin barrier. In this review we provide i) a historical overview of the steps undertaken to obtain information on the lipid composition and organization in SC of healthy skin and inflammatory skin diseases, ii) information on the role CERs, CHOL and FFAs play in the lipid phase behavior of very complex lipid model systems and how this knowledge can be used to understand the deviation in lipid phase behavior in inflammatory skin diseases, iii) knowledge on the role of both, CER subclasses and chain length distribution, on lipid organization and lipid membrane permeability in complex and simple model systems with synthetic CERs, CHOL and FFAs, iv) similarity in lipid phase behavior in SC of different species and complex model systems, and vi) future directions in modulating lipid composition that is expected to improve the skin barrier in inflammatory skin diseases.

Wearable Glucose Monitoring and Implantable Drug Delivery Systems for Diabetes Management

The global cost of diabetes care exceeds $1 trillion each year with more than $327 billion being spent in the United States alone. Despite some of the advances in diabetes care including continuous glucose monitoring systems and insulin pumps, the technology associated with managing diabetes has largely remained unchanged over the past several decades. With the rise of wearable electronics and novel functional materials, the field is well-poised for the next generation of closed-loop diabetes care. Wearable glucose sensors implanted within diverse platforms including skin or on-tooth tattoos, skin-mounted patches, eyeglasses, contact lenses, fabrics, mouthguards, and pacifiers have enabled noninvasive, unobtrusive, and real-time analysis of glucose excursions in ambulatory care settings. These wearable glucose sensors can be integrated with implantable drug delivery systems, including an insulin pump, glucose responsive insulin release implant, and islets transplantation, to form self-regulating closed-loop systems. This review article encompasses the emerging trends and latest innovations of wearable glucose monitoring and implantable insulin delivery technologies for diabetes management with a focus on their advanced materials and construction. Perspectives on the current unmet challenges of these strategies are also discussed to motivate future technological development toward improved patient care in diabetes management.

Application of ultrasound-mediated adapalene-coated lysozyme-shelled microbubbles in UVA-induced skin photoaging

Photoaging, the premature aging of skin induced by ultraviolet rays, is characterized by wrinkling, roughness, laxity, and pigmentary changes. Various natural and synthetic retinoids have been explored for the treatment of aging. Among retinoids, adapalene (Ada, 0.3%) is one of the most potent and widely used drugs to treat photoaging. However, it causes irritant reactions that limit its acceptance by patients. Several studies have shown the applicability of Lysozyme (Lys)-shelled microbubbles (MBs) for drug delivery through sonophoresis, and recently we have shown its efficiency to treat inflammatory skin disease. Here, we report the construction of novel Ada-LysMBs based on opposite electric charges for combined effects to treat photoaging. The Ada-LysMBs were self-assembled and had a mean diameter of 2857 nm. The maximum loading efficiency of Ada onto LysMBs was 13.99 ± 0.59%. An acoustic power density of 3 W/cm2 for 1 min revealing maximum penetration depth of LysMBs was optimized for further in vitro and in vivo studies of Ada-LysMBs. It was observed that in vitro Ada release from Ada-LysMBs at 6 h after ultrasound (US) treatment was more rapid at pH 7.4 (82%) than at pH 5.5 (73%). Franz diffusion experiments on isolated porcine skin indicated that US approximately doubled Ada delivery by Ada-LysMBs and Ada + LysMBs at 12 h and six-fold Lys permeation by LysMBs at 6 h, compared to these treatments alone. A 5-week in vivo study in mice identified significant wrinkle reduction in animals treated with US plus Ada-LysMBs. Our findings indicate that US may be used with Ada-LysMBs in the water phase to treat photoaging by normalizing hyperkeratinization and promoting collagen synthesis.

State of the art in Stratum Corneum research: The biophysical properties of ceramides

This review is summarizing an important part of the state of the art in stratum corneum research. A complete overview on discoveries about the general biophysical and physicochemical properties of the known ceramide species' is provided. The ceramides are one of the three major components of the lipid matrix and mainly govern its properties and structure. They are shown to exhibit very little redundancy, despite the minor differences in their chemical structure. The results are discussed, compared to each other as well as the current base of knowledge. New interesting aspects and concepts are concluded or suggested. A novel interpretation of the 3-dimensional structure of the lipid matrix and its influence on the barrier function will be discussed. The most important conclusion is the presentation of a new and up to date theoretical model of the nanostructure of the short periodicity phase. The model suggests three perpendicular layers: The rigid head group region, the rigid chain region and, a liquid-like overlapping middle layer. The general principle of the skin barrier function is highlighted in regard to this structure and the ceramides biophysical and physicochemical properties. As a result of these considerations, the entropy vs. enthalpy principle is introduced, shedding light on the function as well as the effectiveness of the skin barrier. Additionally, general ideas to effectively overcome this barrier principle for dermal and transdermal delivery of actives or how to use it for specific targeting of the stratum corneum are proposed.

Drug permeation and barrier damage in Leishmania-infected mouse skin

OBJECTIVES

Pathological disorder can disrupt the barrier integrity of the skin, thereby altering the drug delivery from topical formulations to the target site. Cutaneous leishmaniasis (CL) is an infection of the dermal layers of the skin and manifests as a variety of skin lesions from defined nodular forms to plaques and chronic ulcers. The aim of this work was to characterize the physiology and barrier integrity of the Leishmania-infected BALB/c mouse skin and how they impacted delivery of drugs into the skin.

METHODS

A histological evaluation of the structural differences between uninfected and infected skin was performed using haematoxylin/eosin, elastic Van Gieson and Iba-1 stains. As a CL nodule developed and progressed, the skin pH, hydration and trans-epidermal water loss (TEWL) were recorded. Finally, Franz diffusion cells were used to evaluate the influence of the infection on drug delivery through the skin.

RESULTS

We found: (i) structural changes in both the epidermal and dermal layers due to the ingress of inflammatory cells, as shown by immunohistochemistry; (ii) a significant increase in TEWL; and (iii) significantly higher permeation of the model permeants caffeine and ibuprofen and the antileishmanial drugs buparvaquone and paromomycin, for Leishmania-infected skin compared with uninfected skin. The infection had no measurable influence on skin pH and hydration.

CONCLUSIONS

We report profound changes in the skin barrier physiology, function and permeability to drugs of Leishmania-infected skin.

Comparing in vivo biodistribution with radiolabeling and Franz cell permeation assay to validate the efficacy of both methodologies in the evaluation of nanoemulsions: a safety approach

The Franz cells permeation assay has been performed for over 25 years. However, the advent of nanotechnology created a whole new world, especially with regard to topical products. In this new global scenario an increasing number of nanostructure-based delivery systems (NDSs) have emerged and a global warning relating to the safety of these NDSs is arising. This work studied the efficacy of the Franz cells assay, comparing it with the radiolabeling biodistribution test. For this purpose a formulation of sunscreen based on an NDS was developed and characterized. The results demonstrated both that the NDS did not present in vitro cytotoxicity and that the radiolabeling biodistribution test is more precise for the evaluation of NDS cosmetics than the Franz cells assay, since it detected the permeation of the NDS at a picogram order. Due to this fact, and considering all the concerns related to NDSs and nanoparticles in general, more precise methods must be used in order to guarantee the safe use of these new classes of products.

Interaction of fengycin with stratum corneum mimicking model membranes: a calorimetry study

Based on its outstanding antifungal properties, it is reasonable to believe that fengycin might be efficient to topically treat localized dermatomycoses. Since most of the fungi species involved in the formation of those mycotic skin diseases colonize primarily the stratum corneum (SC), studying the interaction between fengycin and SC-mimicking lipid membranes is a primary step to determine the potential of fengycin to overcome the physical barrier of the skin. In this respect, multilamellar lipid vesicles (MLVs), with a lipid composition mimicking that of the SC, were prepared and characterized by differential scanning calorimetry (DSC). The critical micelle concentration (CMC) of fengycin was also assessed under skin conditions and found to be 1.2±0.1μM. The molecular interactions of fengycin with SC-mimicking MLVs were investigated by both DSC and isothermal titration calorimetry (ITC). Results showed that the interactions were considerably affected by changes in lipid phase behaviour. At 40°C and below, fengycin induced exothermic changes in the lipid structures suggesting that less-ordered lipid domains became more-ordered in presence of fengycin. At 60°C, clearly endothermic interaction enthalpies were observed, which could arise from the "melting" of remaining solid domains enriched in high melting lipids that without fengycin melt at higher temperatures.

Development and evaluation of a novel topical treatment for acne with azelaic acid-loaded nanoparticles

Azelaic acid (AzA) is used in the treatment of acne. However, side effects and low compliance have been associated with several topical treatments with AzA. Nanotechnology presents a strategy that can overcome these problems. Polymeric nanoparticles can control drug release and targeting and reduce local drug toxicity. The aim of this study was to produce and evaluate an innovative topical treatment for acne with AzA-loaded poly-DL-lactide/glycolide copolymer nanoparticles. A soft white powder of nanoparticles was prepared. The mean size of loaded nanoparticles was < 400 nm and zeta potential was negative. Spherical nanoparticles were observed by scanning electron microscopy. Encapsulation efficiency was around 80% and a strong interaction between the polymer and the drug was confirmed by differential scanning calorimetric analysis. In vitro drug release studies suggested a controlled and pulsatile release profile. System efficacy tests suggested similar results between the loaded nanoparticles and the nonencapsulated drug against the most common bacteria associated with acne. Cytotoxicity of AzA-loaded nanoparticles was concentration dependent, although not pronounced. The occluded patch test seemed to indicate that the formulation excipients were safe and thus AzA-loaded nanoparticles appear to be an efficient and safe treatment for acne.

The epidermal Ca(2+) gradient: Measurement using the phasor representation of fluorescent lifetime imaging

Ionic gradients are found across a variety of tissues and organs. In this report, we apply the phasor representation of fluorescence lifetime imaging data to the quantitative study of ionic concentrations in tissues, overcoming technical problems of tissue thickness, concentration artifacts of ion-sensitive dyes, and calibration across inhomogeneous tissue. We used epidermis as a model system, as Ca(2+) gradients in this organ have been shown previously to control essential biologic processes of differentiation and formation of the epidermal permeability barrier. The approach described here allowed much better localization of Ca(2+) stores than those used in previous studies, and revealed that the bulk of free Ca(2+) measured in the epidermis comes from intracellular Ca(2+) stores such as the Golgi and the endoplasmic reticulum, with extracellular Ca(2+) making a relatively small contribution to the epidermal Ca(2+) gradient. Due to the high spatial resolution of two-photon microscopy, we were able to measure a marked heterogeneity in average calcium concentrations from cell to cell in the basal keratinocytes. This finding, not reported in previous studies, calls into question the long-held hypothesis that keratinocytes increase intracellular Ca(2+), cease proliferation, and differentiate passively in response to changes in extracellular Ca(2+). The experimental results obtained using this approach illustrate the power of the experimental and analytical techniques outlined in this report. Our approach can be used in mechanistic studies to address the formation, maintenance, and function of the epidermal Ca(2+) gradient, and it should be broadly applicable to the study of other tissues with ionic gradients.

Large scale study of epidermal recovery after stratum corneum removal: dynamics of genomic response

The stratum corneum (SC) is a superficial skin compartment that protects the body from the outside environment. Any disturbance of this function induces cascading steps of molecular and cellular repair in the whole epidermis. The aim of this study was to investigate epidermal gene expression following SC removal by tape stripping. Twenty-nine healthy male volunteers were included (27 +/- 4 years old). Tape stripping was processed on one inner forearm, the other unstripped forearm served as a control. Epidermis samples were collected at 2, 6, 19, 30 and 72 h after tape stripping. Trans-epidermal water loss measurements were performed at each step to monitor barrier restoration. Total RNA was extracted from collected epidermis samples and analysed by using DermArray cDNA microarrays. Among 4000 genes under investigation, we found that the expression of 370 genes varied significantly at least once during the time following stripping. Using an original clustering method, the modulated genes were gathered into eight groups. A functional characterization of the clusters enabled us to get a dynamic and global view of the main molecular processes taking place during epidermal recovery.

Epidermal ceramidase activity regulates epidermal desquamation via stratum corneum acidification

The acidic pH of the outer surface of the mammalian skin plays several important roles in the epidermal barrier function. The 2 endogenous pathways that are currently known to elicit this acidic pH are the generation of free fatty acids from phospholipids and the exchange of protons for sodium ions by non-energy-dependent sodium-proton exchangers. In this study, we propose a third endogenous pathway, i.e. epidermal ceramidase activity, generating free fatty acids from ceramides. By topical application of N-oleylethanolamine, a well-known ceramidase inhibitor, we could demonstrate a significant increase in the stratum corneum pH and a corresponding decrease in the epidermal free fatty acid content. Moreover, we could show that the resulting change in the apparent skin pH also provoked a delay in early barrier recovery and an increased epidermal desquamation, corresponding to earlier observations made for the already known endogenous mechanisms.

Optimization of submerged keratinocyte cultures for the synthesis of barrier ceramides

Epidermal differentiation results in the formation of the extracellular lipid barrier in the stratum corneum, which mainly consists of ceramides, free fatty acids, and cholesterol. Differentiating keratinocytes of the stratum granulosum synthesize a series of complex long-chain ceramides and glucosylceramides with different chain lengths and hydroxylation patterns at intracellular membranes of the secretory pathway. Formation of complex extracellular ceramides parallels the transition of keratinocytes from the stratum granulosum to the stratum corneum, where their precursors, complex glucosylceramides and sphingomyelin, are secreted and exposed to extracellular lysosomal lipid hydrolases. Submerged cultures used so far showed a reduced ceramide content compared to the native epidermis or the air-exposed, organotypic culture system. In order to investigate the sphingolipid metabolism during keratinocyte differentiation, we optimized a simple cell culture system to generate the major barrier sphingolipids. This optimized model is based on the chemically well-defined serum-free MCDB medium. At low calcium ion concentrations (0.1mM), keratinocytes proliferate and synthesize mainly Cer(NS) and a small amount of Cer(NP). Supplementation of the MCDB cell culture medium with calcium ions (1.1mM) and 10 microM linoleic acid triggered differentiation of keratinocytes and synthesis of a complex pattern of free and covalently bound ceramides as found in native epidermis or air-exposed organotypic cultures, though at a reduced level. The mRNA levels of the differentiation markers keratin 10 and profilaggrin increased, as well as those of ceramide glucosyltransferase and glucosylceramide-beta-glucosidase. The described culture system was thus suitable for biochemical studies of the sphingolipid metabolism during keratinocyte differentiation. The addition of serum or vitamin A to the medium resulted in a decrease in ceramide and glucosylceramide content. Lowering the medium pH to 6, while maintained cell viability, led to an increase in the processing of probarrier lipids glucosylceramide and sphingomyelin to free ceramides and protein-bound ceramide Cer(OS).

Incontinence Induces Stratum Corneum Vulnerability and Impairs the Skin Barrier Function in the Perianal Region

BACKGROUND

Incontinence leads to a reduced skin barrier function, while also increasing the risk of skin breakdown.

AIM

To investigate the perianal skin barrier function of elderly patients with or without incontinence.

METHODS

We compared the skin barrier function by investigating the skin hydration, the skin pH, the amount of ceramide and by making a dermatological skin inspection of the perianal region in patients with and without incontinence.

RESULTS

Twenty-one incontinent patients (70.0%) showed a glossy skin, which indicates impairment of the skin barrier while none of the patients in the continence group did so. In the incontinence group, those who had a glossy skin showed a significantly greater skin pH and total amount of ceramide than those with a normal skin (p = 0.0099, p = 0.0179, respectively).

CONCLUSIONS

These results demonstrate that a glossy skin may be a useful indicator of a reduced perianal skin barrier function.

Water relations of tetrapod integument

The vertebrate integument represents an evolutionary compromise between the needs for mechanical protection and those of sensing the environment and regulating the exchange of materials and energy. Fibrous keratins evolved as a means of strengthening the integument while simultaneously providing a structural support for lipids, which comprise the principal barrier to cutaneous water efflux in terrestrial taxa. Whereas lipids are of fundamental importance to water barriers, the efficacy of these barriers depends in many cases on structural features that enhance or maintain the integrity of function. Amphibians are exceptional among tetrapods in having very little keratin and a thin stratum corneum. Thus, effective lipid barriers that are present in some specialized anurans living in xeric habitats are external to the epidermis, whereas lipid barriers of amniotes exist as a lipid-keratin complex within the stratum corneum. Amphibians prevent desiccation of the epidermis and underlying tissues either by evaporating water from a superficial aqueous film, which must be replenished, or by shielding the stratum corneum with superficial lipids. Water barrier function in vertebrates generally appears to be relatively fixed, although various species have`plasticity' to adjust the barrier effectiveness facultatively. While it is clear that both phenotypic plasticity and genetic adaptation can account for covariation between environment and skin resistance to water efflux, studies of the relative importance of these two phenomena are few. Fundamental mechanisms for adjusting the skin water barrier include changes in barrier thickness, composition and physicochemical properties of cutaneous lipids,and/or geometry of the barrier within the epidermis. While cutaneous lipids have been studied extensively in the contexts of disease and cosmetics,relatively little is known about the processes of permeability barrier ontogenesis related to adaptation and environment. Advances in such knowledge have didactic significance for understanding vertebrate evolution as well as practical application to clinical dermatology.

Characterization of the transdermal transport of flurbiprofen and indomethacin

Transdermal permeation of two types of NSAIDs, [(3)H] flurbiprofen and [(14)C] indomethacin, was examined by use of the Ussing-type chamber method. We found that the transdermal permeability in the absorptive direction (P(abs)) of [(3)H] flurbiprofen was significantly higher than that of [(14)C] indomethacin. A lower pH (5.0) on the epidermal side increased the accumulation and the P(abs) of [(3)H] flurbiprofen (18-fold and 50-fold, respectively) and [(14)C] indomethacin (18-fold and 22-fold, respectively), compared with pH 7.4. Coadministration of unlabeled flurbiprofen and indomethacin increased P(abs) of [(3)H] flurbiprofen and [(14)C] indomethacin, respectively, in a concentration-dependent manner. Similar high-affinity transport was also observed in the uptake of [(3)H] flurbiprofen by human epidermal keratinocytes (HEK001 cells). RT-PCR analysis revealed the expression of mRNA of numerous transporters including MRP, OATP, MCT and OCTN family members in hairless mouse skin, human skin and HEK001 cells. These findings support the novel hypothesis that transdermal permeation of NSAIDs is mediated by saturable transport mechanisms, which may be candidates as targets for transdermal delivery of drugs.

Extracellular pH Controls NHE1 expression in epidermis and keratinocytes: implications for barrier repair

We have previously shown that the Na+/H+ antiporter (NHE1) is an essential endogenous pathway responsible for stratum corneum (SC) acidification. Since the epidermis must re-establish its epidermal barrier after acute barrier perturbations, we asked whether the NHE1 was, in turn, regulated by changes in barrier status. We found that in vivo epidermal NHE1 expression was upregulated within hours of barrier disruption. We next asked whether NHE1 was regulated by barrier status per se, or by the SC alkalinization that accompanies barrier perturbation. NHE1 was upregulated by alkalinizing SC pH, whereas this antiporter was downregulated by acidifying SC pH, independent of changes in barrier status. Moreover, acidifying SC pH overrode the effects of barrier break in regulating NHE1 expression, suggesting that SC alkalinization is the major stimulus for increased NHE1 expression. Finally, we confirmed that the keratinocyte NHE1 antiporter is regulated by extracellular pH independent of barrier status, by demonstrating that NHE1 was upregulated in cultured keratinocytes exposed to pH 8.3 medium and downregulated in cultured keratinocytes exposed to pH 6.3 medium. These data suggest that the keratinocyte NHE1 is regulated by extracellular pH. SC barrier break also upregulates NHE1 expression, but this response seems to be mediated by concomitant changes in SC pH.

The nature of ultrasound–SLS synergism during enhanced transdermal transport

Ultrasound and sodium lauryl sulfate (SLS) exhibit a synergistic effect on transdermal transport, when applied simultaneously on the skin. The synergistic mechanism is not fully understood. Previous studies have shown that application of ultrasound simultaneously with SLS, results in enhanced mass transfer and improved penetration and dispersion of the surfactant. In this study we demonstrate that simultaneous application of ultrasound and SLS leads to modification of the pH profile of the stratum corneum. This pH modification within the stratum corneum's microenvironment, can affect both the structure of the lipid layers and the activity of SLS as a chemical enhancer due to its improved lipophilic solubility. The altered pH profile that results in improved SLS lipophilic solubility, together with improved SLS penetration and dispersion, can explain the synergistic enhancing effect of ultrasound and SLS on transdermal transport.

Visualization of skin penetration using confocal laser scanning microscopy

The use of skin as an alternative route for administering systemically active drugs has attracted considerable interest in recent years. However, the skin provides an excellent barrier, which limits the number of drug molecules suitable for transdermal delivery. Thus, in order to improve cutaneous delivery, it is necessary to adopt an enhancement method, either (i) passively using novel formulations, e.g. microemulsions, liposomes, and colloidal polymeric suspensions, or more conventional skin permeation enhancers, or (ii) with a physical approach, such as, iontophoresis, sonophoresis or electroporation. Although there has been much progress, the precise modes of action of the different techniques used are far from well-understood. The objective of this review, therefore, is to evaluate how confocal laser scanning microscopy may contribute to the determination of the mechanisms of diverse skin penetration enhancement strategies.

NHE1 regulates the stratum corneum permeability barrier homeostasis. Microenvironment acidification assessed with fluorescence lifetime imaging

The outermost epidermal layer, the stratum corneum (SC), exhibits an acidic surface pH, whereas the pH at its base approaches neutrality. NHE1 is the only Na(+)/H(+) antiporter isoform in keratinocytes and epidermis, and has been shown to regulate intracellular pH. We now demonstrate a novel function for NHE1, as we find that it also controls acidification of extracellular "microdomains" in the SC that are essential for activation of pH-sensitive enzymes and the formation of the epidermal permeability barrier. NHE1 expression in epidermis is most pronounced in granular cell layers, and although the surface pH of NHE1 knockout mice is only slightly more alkaline than normal using conventional pH measurements, a more sensitive method, fluorescence lifetime imaging, demonstrates that the acidic intercellular domains at the surface and of the lower SC disappear in NHE1 -/- animals. Fluorescence lifetime imaging studies also reveal that SC acidification does not occur through a uniform gradient, but through the progressive accumulation of acidic microdomains. These findings not only visualize the spatial distribution of the SC pH gradient, but also demonstrate a role for NHE1 in the generation of acidic extracellular domains of the lower SC, thus providing the acidification of deep SC interstices necessary for lipid processing and barrier homeostasis.

A comparative review of cutaneous pH

This review describes the role of pH in cutaneous structure and function. We first describe the molecules that contribute to the acidity or alkalinity of the skin. Next, differences in cutaneous pH among species, among individuals of the same species and within individuals are described. The potential functions of cutaneous pH in normal and diseased skin are analysed. For example, cutaneous pH has a role in the selection and maintenance of the normal cutaneous microbiota. In addition, cutaneous acidity may protect the skin against infection by microbes. Finally, there is evidence that a cutaneous pH gradient activates pH-dependent enzymes involved in the process of keratinization.

Two-photon fluorescence lifetime imaging of the skin stratum corneum pH gradient

Two-photon fluorescence lifetime imaging is used to identify microdomains (1-25 microm) of two distinct pH values within the uppermost layer of the epidermis (stratum corneum). The fluorophore used is 2',7'-bis-(2-carboxyethyl)-5-(and-6)-carboxyfluorescein (BCECF), whose lifetime tau (pH 4.5, tau = 2.75 ns; pH 8.5, tau = 3.90 ns) is pH dependent over the pH range of the stratum corneum (pH 4.5 to pH 7.2). Hairless mice (SKH1-hrBR) are used as a model for human skin. Images (< or =50 microm x 50 microm) are acquired every 1.7 microm from the stratum corneum surface to the first viable layer (stratum granulosum). Acidic microdomains (average pH 6.0) of variable size (~1 microm in diameter with variable length) are detected within the extracellular matrix of the stratum corneum, whereas the intracellular space of the corneocytes in mid-stratum corneum (25 microm diameter) approaches neutrality (average pH 7.0). The surface is acidic. The average pH of the stratum corneum increases with depth because of a decrease in the ratio of acidic to neutral regions within the stratum corneum. The data definitively show that the stratum corneum acid mantle results from the presence of aqueous acidic pockets within the lipid-rich extracellular matrix.

The acidic milieu of the horny layer: new findings on the physiology and pathophysiology of skin pH

The acidic pH of the horny layer, measurable on the skin surface, has long been regarded as a result of exocrine secretion of the skin glands. The 'acid mantle' was thought to regulate the bacterial skin flora and to be sensitive primarily to skin cleansing procedures. In recent years, an increasing number of investigations have been published on the changes in, and constituents and functions of, the pH of the deeper layers of the stratum corneum, as well as on the influence of physiological and pathological factors. A central role for the acidic milieu as a regulating factor in stratum corneum homeostasis is now emerging. This has relevance to the integrity of the barrier function, from normal maturation of the stratum corneum lipids through to desquamation. Changes in the pH and the organic factors influencing it appear to play a role, not only in the pathogenesis, prevention and treatment of irritant contact dermatitis, but also of atopic dermatitis and ichthyosis and in wound healing. On the basis of these findings, a broader concept, exceeding the superficial 'acid mantle' theory, has been formulated.

Effect of lipophilicity on in vivo iontophoretic delivery. II. Beta-blockers

The objective of this study was to investigate the relationship between drug lipophilicity and the transdermal absorption processes in the iontophoretic delivery in vivo. Anodal iontophoresis of beta-blockers as model drugs having different lipophilicity (atenolol, pindolol, metoprolol, acebutolol, oxprenolol and propranolol) was performed with rats (electrical current, 0.625 mA/cm2; application period, 90 min), and the drug concentrations in skin, cutaneous vein and systemic vein were determined. Increasing the lipophilicity of beta-blockers caused a greater absorption into the skin. Exceptionally, it was found that pindolol had high skin absorption, irrespective of its hydrophilic nature. Further, the drug transfer rate from skin to cutaneous vein (R(SC)) was evaluated from the arterio-venous plasma concentration difference of drug in the skin. Normalized R(SC) by skin concentration showed a negative correlation with the logarithm of n-octanol/buffer partition coefficient (Log P, pH 7.4), suggesting the partitioning between stratum corneum and viable epidermis was a primary process to determine the transfer properties of beta-blockers to local blood circulation. Pindolol exhibited both high skin absorption and high transfer from skin to cutaneous vein. These characteristics of pindolol could be explained by the chemical structure, molecular size and hydrophilicity. These findings for pindolol should be valuable for the optimal design of drug candidates for iontophoretic transdermal delivery.

Effect of lipophilicity on in vivo iontophoretic delivery. I. NSAIDs

The effect of drug lipophilicity on in vivo iontophoretic transdermal absorption was evaluated. Non-steroidal anti-inflammatory drugs (NSAIDs) were selected as model drugs with a wide range of lipophilicity: salicylic acid (SA), ketoprofen (KP), naproxen (NP) and indomethacin (IM). Cathodal iontophoresis of NSAIDs was conducted in rats (0.625 mA/cm2; 90 min), and drug concentrations in skin, cutaneous vein and systemic vein were determined. Skin concentrations of NSAID were higher in the case of lipophilic drugs (SA=KP=NP<IM), whereas cutaneous plasma concentrations decreased with an increase in lipophilicity (SA>KP=NP>IM). Additionally, the dependence of drug lipophilicity on systemic plasma concentration was similar to cutaneous plasma concentration. The transfer rate from skin to cutaneous vein (R(SC)) was calculated from the arterio-venous plasma concentration difference of drug in the skin. Normalized R(SC) by skin concentration (R(SC)/X(S)) yielded a negative correlation with the logarithm of n-octanol/buffer partition coefficient (Log P at pH 7.4), suggesting that transfer of NSAIDs from skin to cutaneous vein decreased with increasing lipophilicity (SA>KP=NP>IM). This correlation means that drug partitioning between stratum corneum and viable epidermis might be a dominant step.