Skin drug delivery using lipid vesicles: A starting guideline for their development

Lipid vesicles can provide a cost-effective enhancement of skin drug absorption when vesicle production process is optimised. It is an important challenge to design the ideal vesicle, since their properties and features are related, as changes in one affect the others. Here, we review the main components, preparation and characterization methods commonly used, and the key properties that lead to highly efficient vesicles for transdermal drug delivery purposes. We stand by size, deformability degree and drug loading, as the most important vesicle features that determine the further transdermal drug absorption. The interest in this technology is increasing, as demonstrated by the exponential growth of publications on the topic. Although long-term preservation and scalability issues have limited the commercialization of lipid vesicle products, freeze-drying and modern escalation methods overcome these difficulties, thus predicting a higher use of these technologies in the market and clinical practice.

Clove Oil Endorsed Transdermal Flux of Dronedarone Hydrochloride Loaded Bilosomal Nanogel: Factorial Design, In vitro Evaluation and Ex vivo Permeation

The goal of this study was to develop a bilosomal gel formulation to enhance transdermal permeability of dronedarone hyrdrochloride (DRN) which suffers from poor oral absorption and limited bioavailability. To overcome this obstacle, bilosomes were successfully prepared using 2³ full-factorial design. Span®40, cholesterol, sodium deoxycholate (bile salt), clove oil (permeability enhancer), and either Tween® 60 or Tween® 80 (edge activator) were used in bilosome preparation by ethanol injection method. In this design, independent variables were X1, edge activator type; X2, edge activator amount (mg); and X3, permeability enhancer concentration (% w/v). Optimal formula (B2) of the highest desirability of (0.776) demonstrated minimum vesicle size (VS) of 312.4 ± 24.42 nm, maximum absolute value of zeta potential (ZP) - 36.17 ± 2.57 mV, maximum entrapment efficiency (EE %) of 80.95 ± 3.01%, maximum deformability Index (DI) of 8.24 ± 1.26 g and maximum drug flux after 12 h (J₁₂) of 21.23 ± 1.54 µg/cm² h upon ex vivo permeation study. After 12 h, 70.29 ± 6.46% of DRN was released from B2. TEM identification of B2 showed spherical shaped nanosized vesicles which were physically stable for 3 months at different temperatures. B2 was incorporated into carboxymethylcellulose gel base for easiness of dermal application. B2 gel demonstrated good physical properties, non-Newtonian psuedoplastic flow, and enhanced release (57.0 ± 8.68% of DRN compared to only 13.3 ± 1.2% released from drug suspension after 12 h) and enhanced skin permeation.

Cyanocobalamin Ultraflexible Lipid Vesicles: Characterization and In Vitro Evaluation of Drug-Skin Depth Profiles

Atopic dermatitis (AD) and psoriasis are the most common chronic inflammatory skin disorders, which importantly affect the quality of life of patients who suffer them. Among other causes, nitric oxide has been reported as part of the triggering factors in the pathogenesis of both conditions. Cyanocobalamin (vitamin B₁₂) has shown efficacy as a nitric oxide scavenger and some clinical trials have given positive outcomes in its use for treating skin pathologies. Passive skin diffusion is possible only for drugs with low molecular weights and intermediate lipophilicity. Unfortunately, the molecular weight and hydrophilicity of vitamin B₁₂ do not predict its effective diffusion through the skin. The aim of this work was to design new lipid vesicles to encapsulate the vitamin B₁₂ to enhance its skin penetration. Nine prototypes of vesicles were generated and characterized in terms of size, polydispersity, surface charge, drug encapsulation, flexibility, and stability with positive results. Additionally, their ability to release the drug content in a controlled manner was demonstrated. Finally, we found that these lipid vesicle formulations facilitated the penetration of cyanocobalamin to the deeper layers of the skin. The present work shows a promising system to effectively administer vitamin B₁₂ topically, which could be of interest in the treatment of skin diseases such as AD and psoriasis.

Skin tissue sample collection, sample homogenization, and analyte extraction strategies for liquid chromatographic mass spectrometry quantification of pharmaceutical compounds

Quantification of pharmaceutical compounds in skin tissue is challenging because of low expected concentrations, small typical sample volumes, and the hard nature of the skin structure itself. This review provides a comprehensive overview of sample collection, sample homogenization and analyte extraction methods that have been used to quantify pharmaceutical compounds in skin tissue, obtained from animals and humans, using liquid chromatography-mass spectrometry. For each step in the process of sample collection to sample extraction, methods are compared to discuss challenges and provide practical guidance. Furthermore, liquid chromatographic-mass spectrometry considerations regarding the quality and complexity of skin tissue sample measurements are discussed, with emphasis on analyte recovery and matrix effects. Given that the true recovery of analytes from skin tissue is difficult to assess, the extent of homogenization plays a crucial role in the accuracy of quantification. Chemical or enzymatic solubilization of skin tissue samples would therefore be preferable as homogenization method.

IgG1+ B-cell immunity predates IgE responses in epicutaneous sensitization to foods

BACKGROUND

The generation of IgE-mediated food allergy in humans is silent and only diagnosed upon manifestation of clinical symptoms. While experimental models have been used to investigate some mechanisms of allergic sensitization, the generation of humoral immunity and memory remains to be elucidated. Here, we defined the evolution of allergen-specific B-cell responses during epicutaneous sensitization to foods.

METHODS

Wild-type and genetic knockout animals, and drug or antibody strategies for cell depletion and immunoglobulin signaling blockade were used to investigate epicutaneous sensitization and disease progression; we analyzed allergen-specific germinal centers and IgG1⁺ memory B cells by flow cytometry, evaluated humoral responses, and determined clinical reactivity (anaphylaxis).

RESULTS

Epicutaneous sensitization caused microscopic skin damage, inflammation, and recruitment of activated dendritic cells to the draining lymph nodes. This process generated allergen-specific IgG1⁺ germinal center B cells, serum IgG1, and anaphylaxis that was mediated by the alternative pathway. Whether we used peanut and/or ovalbumin from the egg white for sensitization, the allergen-specific IgG1⁺ memory compartment predominantly exhibited an immature, pro-germinal center phenotype (PDL-2^- CD80^- CD35⁺ CD73⁺ ). Subsequent subclinical exposures to the allergen induced IgE⁺ germinal center B cells, serum IgE, and likely activated the classical pathway of anaphylaxis.

CONCLUSIONS

Our data demonstrate that IgG1⁺ B-cell immunity against food allergens in epicutaneous sensitization precedes the generation of IgE responses. Therefore, the assessment of allergen-specific cellular and humoral IgG1⁺ immunity may help to identify individuals at risk of developing IgE-mediated food allergy and hence provide a window for therapeutic interventions.

Application of an Optimized Tape Stripping Method for the Bioequivalence Assessment of Topical Acyclovir Creams

This study indicates the application of tape stripping (TS) for bioequivalence (BE) assessment of a topical cream product containing 5% acyclovir. A TS method, previously used successfully to assess BE of topical clobetasol propionate and clotrimazole formulations, was used to assess BE of an acyclovir cream (5%) formulation as well as a diluted acyclovir formulation (1.5%) applied to the skin of healthy humans. An appropriate application time was established by conducting a dose duration study using the innovator product, Zovirax® cream. Transepidermal water loss was measured and used to normalize thicknesses between subjects. The area under the curve (AUC) from a plot of amount of acyclovir/strip vs cumulative fraction of stratum corneum (SC) removed was calculated for each application site. BE was assessed using Fieller's theorem in accordance with FDA's guidance for assessment of BE of topical corticosteroids. Adco-acyclovir cream (5%) was found to be BE to Zovirax® cream, where the mean test/reference (T/R) ratio of the AUC's was 0.96 and the bioequivalence interval using a 90% confidence interval was 0.91-1.01 with a statistical power > 95%, whereas the diluted test product fell outside the BE acceptance criteria with T/R ratio of AUC of 0.23 and a 90% CI of 0.20-0.26. This study indicates that the data resulting from the application of this TS procedure has reinforced the potential for its use to assess BE of topical drug products intended for local action, thereby obviating the necessity to undertake clinical trials in patients.

Is the Skin Absorption of Hydrocortisone Modified by the Variability in Dosing Topical Products?

Fingertip units have been proposed as a tool to standardize topical therapy with semisolid formulations. However, no studies to date have characterized the variability in dosing by patients using this concept and whether this variability ultimately affects the topical absorption of drugs. This work aimed to answer these two questions. A first study determined the dose measured, the area of spread and the area-normalized dose for a 1% hydrocortisone cream and ointment applied by members of the public using this dosing approach before and after brief counselling. Then, in vivo tape-stripping and in vitro permeation studies investigated whether the variability in the area-normalized dose altered the skin absorption of hydrocortisone. Participants applied greater doses and spread them over larger areas after a short counselling intervention leading to smaller area-normalized doses. In vivo hydrocortisone uptake by the stratum corneum was significantly greater for the higher normalized dose and the differences were further supported by the in vitro permeation studies. However, these differences were relatively small and not proportional to the increase in normalized dose. This work shows that, following brief advice, patients and carers can apply consistent and sufficient doses of corticosteroids whilst minimizing risks and variability in hydrocortisone absorption.

The application of local hypobaric pressure - A novel means to enhance macromolecule entry into the skin

The local application of controlled hypobaric stress represents a novel means to facilitate drug delivery into the skin. The aims of this work were to understand how hypobaric stress modified the properties of the skin and assess if this penetration enhancement strategy could improve the percutaneous penetration of a macromolecule. Measurements of skin thickness demonstrated that the topical application of hypobaric stress thinned the tissue (p<0.05), atomic force microscopy showed that it shrunk the corneocytes in the stratum corneum (p<0.001) and the imaging of the skin hair follicles using multiphoton microscopy showed that it opened the follicular infundibula (p<0.001). Together, these changes contributed to a 19.6-fold increase in in vitro percutaneous penetration of a 10,000 molecular weight dextran molecule, which was shown using fluorescence microscopy to be localized around the hair follicles, when applied to the skin using hypobaric stress. In vivo, in the rat, a local hemodynamic response (i.e. a significant increase in blood flow, p<0.001) was shown to contribute to the increase in follicular transport of the dextran to produce a systemic absorption of 7.2±2.81fg·mL(-1). When hypobaric stress was not applied to the rat there was no detectable absorption of dextran and this provided evidence that this novel penetration enhancement technique can improve the percutaneous penetration of macromolecules after topical application to the skin.

Solid lipid nanoparticles (SLN) as carriers for the topical delivery of econazole nitrate: in-vitro characterization, ex-vivo and in-vivo studies

Solid lipid nanoparticles (SLN) designed for topical administration of econazole nitrate (ECN), were prepared by o/w high-shear homogenization method using different ratios of lipid and drug (5:1 and 10:1). SLN were characterized in terms of particle size, morphology, encapsulation efficiency and crystalline structure. After incorporation of SLN into hydrogels, rheological measurements were performed, and ex-vivo drug permeation tests were carried out using porcine stratum corneum (SC). In-vivo study of percutaneous absorption of ECN as a function of application time and composition of gels was carried out by tape-stripping technique. Penetration tests of the drug from a conventional gel were performed as comparison. High-shear homogenization method resulted in a good technique for preparation of ECN-loaded SLN. Particles had a mean diameter of about 150 nm and a regular shape and smooth surface. The encapsulation efficiency values were about 100%. Ex-vivo tests showed that SLN were able to control the drug release through the SC; the release rate depended upon the lipid content on the nanoparticles. In-vivo studies demonstrated that SLN promoted a rapid penetration of ECN through the SC after 1 h and improved the diffusion of the drug in the deeper skin layers after 3 h of application compared with the reference gel.

Surface charged temoporfin-loaded flexible vesicles: in vitro skin penetration studies and stability

In order to increase topical delivery of temoporfin (mTHPC), a highly hydrophobic photosensitizer with low percutaneous penetration, neutral, anionic and cationic flexible liposomes (i.e. flexosomes) were prepared and investigated for their penetration enhancing ability. The in vitro skin penetration study was performed using human abdominal skin mounted in Franz diffusion cells. Besides the effect of surface charge of flexosomes on skin penetration of mTHPC, also its effect on physical properties (particle size, polydispersity index, lamellarity) and physicochemical stability of vesicles was investigated. Photon-correlation spectroscopy revealed that vesicles had after preparation a small particle size and low polydispersity index, while cryo-electron microscopy confirmed that these vesicles were mostly unilamellar and of a spherical shape. Regarding stability, contrasting to anionic flexosomes showing lack of long-term stability, neutral and cationic flexosomes were stable during 9 months storage at 4 degrees C. As to the penetration enhancing ability, cationic flexosomes possessed the highest, i.e. they delivered the highest mTHPC-amount to stratum corneum and deeper skin layers compared to conventional liposomes, neutral and anionic flexosomes. In conclusion, mTHPC-loaded cationic flexosomes could be a promising tool for delivering mTHPC to the skin, which would be beneficial for the photodynamic therapy of cutaneous malignant or non-malignant diseases.

Molecular charge mediated transport of a 13 kD protein across microporated skin

Transport of proteins across the skin is highly limited owing to their hydrophilic nature and large molecular size. This study was conducted to assess the skin transport abilities of a model protein across hairless rat skin during iontophoresis alone and in combination with microneedles as a function of molecular charge. The effect of microneedle pretreatment on electroosmotic flow was also investigated. Skin permeation experiments were carried out in vitro using daniplestim (DP) (MW, 12.76 kD; isoelectric point, 6.2) as a model protein molecule. The effect of molecular charge on protein transport was evaluated by performing studies in two different buffers--TRIS (pH 7.5) and acetate (pH 4.0). Iontophoretic transport mechanisms of DP varied with respect to molecular charge on the protein. The combination approach (iontophoresis and microneedles) gave much higher flux values compared to iontophoresis alone at both pH 4.0 and pH 7.5, however, the delivery in this case was also found to be charge dependent. The findings of this study indicate that electroosmosis persisted upon microporation, thus retaining skin's permselective properties. This enables us to explore the combination of microneedles and iontophoresis as a potential approach for delivery of proteins.

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.

Estimation of atrazine in the stratum corneum and its toxic effects in skin following topical application to rabbits

ABSTRACT For risk assessment, a workable non-invasive method for the estimation of atrazine in skin was needed. Moreover, the dermato-toxic effects of different concentrations of atrazine have not been studied so far. Fifteen milligrams of 35% solution of atrazine in ethanol was topically applied to each of six different clipped sites on the back of rabbits (n = 6). Each site was tape-stripped with 10 consecutive adhesive tapes at 0.5, 1, 2, 3, 4, and 6 h of topical dosing to remove the stratum corneum (SC). Atrazine in tapes was extracted with methanol and analyzed with HPLC. The amount of atrazine detected in the SC at 6 h (1955.79 +/- 47.22 mug) is equivalent to 13.03% of the total applied dose. In dermal toxicity studies, 500 mul of 35%, 70% atrazine in ethanol and a commercial preparation of atrazine (Balance(R)) was dosed on clipped backs of rabbits (n = 4) for 4 days under occluded vs non-occluded conditions. Ethanol alone served as control. On the 5th day, rabbits were euthanized and skin was scored for erythema and then examined microscopically. Significant differences (p < 0.05) in erythema scores were observed with 70% atrazine and Balance(R) as compared to the control under occluded conditions. Significant differences in epidermal thickness and cell layers were observed with Balance(R) and 74% atrazine as compared to control in both dosing conditions. There were non-significant differences in erythema, epidermal thickness, or cell layers in occluded vs non-occluded applications of atrazine in ethanol, indicating that atrazine is equally toxic regardless of its application procedures. Moreover, under non-occluded application, very slight erythema was observed but microscopically significant epidermal hyperplasia was noticed. This indicates that even if there are no significant gross skin manifestations of atrazine this can still produce significant damage to the epidermal barrier and hence can result in increase in penetration of self or other toxic substances.

Semiquantitative Determination of the Penetration of a Fluorescent Hydrogel Formulation into the Hair Follicle with and without Follicular Closure by Microparticles by Means of Differential Stripping

Recently, hair follicles were shown to be of great importance concerning the penetration and reservoir behavior of topically applied substances. Especially microparticles are known to be efficient drug carriers into the hair follicles. It has been shown in various investigations that, depending on their size, microparticles can be utilized to enhance the penetration of substances into the hair follicles. In the present study, it was investigated whether they could also be used to block the hair follicles to stop penetration. For this purpose, the follicular penetration of a sodium fluorescein-containing hydrogel and follicular reservoir depletion were investigated by means of differential stripping in vivo. In a second study design, the skin areas were pretreated with microparticles 5 microm in diameter. Subsequently, the follicular penetration of sodium fluorescein-containing hydrogel was likewise investigated by differential stripping. The results showed that the hair follicle infundibula contained a significant fluorescent signal when sodium fluorescein in hydrogel was applied. In contrast, in the case of pretreatment with microparticles, only a very low fluorescence signal was measurable in the hair follicle infundibula. The microparticles clearly blocked the follicular orifices so that a penetration of sodium fluorescein was almost completely inhibited. This was confirmed by histological investigations. Thus, depending on their size, microparticles act as efficient drug carriers or can be utilized as follicle blockers to stop the penetration of topically applied substances. This could be very useful in skin protection to prevent the storage of harmful topically applied substances in the hair follicles, known to represent long-term reservoirs.

An in vivo model to evaluate the efficacy of barrier creams on the level of skin penetration of chemicals

The reservoir function and the barrier function are important properties of the skin. The reservoir function is dependent on the barrier function which, however, needs support by protective measures, in particular under working conditions. Barrier creams represent a possibility to protect the skin. In the present study, a method was developed to investigate the effectiveness of reservoir closure by different formulations. Patent Blue V in water was used as a model penetrant. Its penetration, with and without barrier cream treatment, was analyzed by tape stripping in combination with UV/VIS spectroscopic measurements. The investigations showed that the stratum corneum represents a reservoir for topically applied Patent Blue V in water. Furthermore, the barrier investigations showed that vaseline and bees wax form a 100% barrier on the skin surface. The third barrier cream, containing waxes and surfactant, only partially showed a protective effect against the penetration of Patent Blue V in water. Strong interindividual differences were observed for this barrier product. In conclusion, it was assumed that the application of barrier creams cannot replace other protective measures and should be maximally used to inhibit low-grade irritants or in combination with other protectants or in body areas where other protective measures are not applicable.

Differential Stripping: Determination of the Amount of Topically Applied Substances Penetrated into the Hair Follicles

The determination of penetration pathways of topically applied substances into the skin is the subject of several investigations. Recently, follicular penetration has become a major focus of interest. To date, a direct, non-invasive quantification of the amount of topically applied substance penetrated into the follicles had not been possible. The development of such a method was the aim of this study. Therefore, the advantages of both stripping techniques, tape stripping and cyanoacrylate skin surface biopsy, were combined and evaluated. Tape stripping was used to remove the part of the stratum corneum that contained the topically applied dye. Subsequently, the follicular contents were ripped off by cyanoacrylate skin surface biopsy. The combined method termed "differential stripping" was evaluated in vitro and in vivo, and the amount of topically applied fluorescent dye penetrated into the hair follicles was quantified after different penetration times. After 30 min, 5% of the recovered concentration of sodium fluorescein was found in the follicular infundibula, where it was still detectable after 48 h. Altogether, the results of this investigation revealed that differential stripping is a new method that can be used to study the penetration of topically applied substances into the follicular infundibula non-invasively and selectively.

Estimation of the relative stratum corneum amount removed by tape stripping

BACKGROUND/PURPOSE

The tape stripping procedure is a suitable minimal invasive tool to study, e.g. the penetration and dermatopharmacokinetics of topically applied substances. In the present study, this procedure was used to remove the stratum corneum (SC) completely and to study the penetration of the UVA filter substance butyl methoxydibenzoylmethane after application in two different vehicles.

METHODS

The amount of corneocytes removed by each tape strip from the flexor forearm of human volunteers was determined via their pseudo-absorption. In a second part, the penetration profiles of a UVA filter substance applied in two different vehicles were determined following the developed standard protocol using the tape stripping procedure in combination with UV/VIS spectroscopy.

RESULTS

The amount of corneocytes removed by each tape strip was related to the number of tape strips used for removal. Mean values with a deviation of less than 20% concerning the relative amount of SC removed by a constant number of tape strips were obtained. For instance, a relative amount of 66 +/- 12% was removed with the first 20 tape strips, while nearly the complete SC (95 +/- 3%) was removed using 50 tape strips. In addition, these results were used to estimate the relative SC amounts removed, studying the penetration of the UVA filter substance after application in two different vehicles. No significant differences between the distributions of the UV filter substance applied in both emulsions were obtained (P > 0.05).

CONCLUSIONS

The reported procedure for the estimation of the removed SC amount provides the possibility to avoid the complete removal of the SC and to compare the penetration characteristics obtained for different volunteers and different products in relation to the relative horny layer profile.

Artificial sweat enhances dermal transfer of chlorpyrifos from treated nylon carpet fibers

The dermal transfer and absorption of 14C-ring-chlorpyrifos from nylon carpet fibers was measured in skin penetration-evaporation cells with excised pig skin. Prior to application, synthetic sweat was applied to skin in half of the cells. Radioactivity was measured in receptor fluid, dermis, epidermis, tape stripping samples, and vapor trap samples during a 24-h period. The sum of radiolabel recovered from the dermis and receptor fluid represented the absorbed dose. There was no significant difference (p > 0.05) in percutaneous absorption between cells that received the synthetic sweat application and "dry" cells (1.3 +/- 0.3% of applied dose). There was significantly more (p < 0.05) radiolabel recovered from tape stripping (5.4 +/- 2.1 vs. 2.8 +/- 0.6%) and in the epidermis (4.5 +/- 0.8 vs. 3.1 +/- 0.3%) from cells that received the synthetic sweat application, which indicated synthetic sweat facilitated transfer of chlorpyrifos from a treated substrate to the skin surface. The measured value for percutaneous absorption of chlorpyrifos agreed with the value predicted from an empirical model previously developed for nitro compound-containing soil.

The Number of Stratum corneum Cell Layers Correlates with the Pseudo-Absorption of the Corneocytes

The removal of the stratum corneum (SC) using adhesive tapes is a common technique in cutaneous studies. The determination of the varying amounts of the SC removed would be a helpful tool in such investigations. In the present study, the cell layers of porcine SC were counted before and after removal of several tape strips using histological techniques. In addition, the pseudo-absorption of the corneocytes reflecting the amount of these cells was determined using spectroscopy. Different amounts of SC were removed using 20 tape strips. The spectroscopically determined data correlate linearly with the number of removed cell layers. Based on these results, the pseudo-absorption of the corneocytes can be used to calculate the absolute number of cell layers removed with a standard deviation of less than 11%. In this way, the SC can be quantified using the procedure of tape stripping in combination with the spectroscopic determination of the corneocytes.

Effect of the vehicle on the amount of stratum corneum removed by tape stripping

BACKGROUND

The penetration of topically applied substances into the stratum corneum can non-invasively be studied using the tape stripping procedure. This method was applied to investigate in vivo the penetration of a fragrance, vanillin, applied in ethanol and a w/o emulsion.

METHODS

Twenty tape strips were removed from each skin area treated with vanillin in ethanol or w/o emulsion, respectively. The concentration of vanillin was determined for each tape strip. In addition, the pseudo-absorption of the corneocytes was determined to calculate the SC profile.

RESULTS

The vanillin concentration was correlated both with the tape number and with the stratum corneum profile. Depending on whether the tape number or the profile of the stratum corneum were correlated with the vanillin concentration, different distributions within the stratum corneum were obtained. Different amounts of stratum corneum were removed with 20 tape strips dependent on the vehicle applied previously. The application of the w/o emulsion led to the removal of nearly the half the amount of corneocytes stripped from the ethanol-treated area.

CONCLUSIONS

The results obtained underline the general necessity to correlate the amount of stratum corneum with the amount of substance in penetration studies.