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

Macroscopic Skin Examination Can Determine the Number of Strips Necessary to Study the stratum corneum in Dogs

To evaluate the skin barrier, the stratum corneum (SC) must be isolated and extracted. Currently, skin biopsy is the gold standard method to investigate skin immunology and the presence of biomarkers in dogs. However, a standardized, non-invasive tool to exclusively remove the SC would be of great interest to study healthy and atopic dogs. In this study, we performed D-squames® tape stripping with standardized pressure on seven healthy beagle dogs. A control site was defined and then 25 strips, 50 strips and as many strips as needed to achieve a shiny appearance of the skin were performed on three different experimental sites. After stripping, blinded histopathological examination of a skin biopsy from each site was performed. The number of tape strips required for the skin to become shiny varied between individuals, with a mean of 40 (29-50) strips. There was no significant difference in SC depth between the control site and the site that underwent 25 tape strips. In contrast, the use of 50 strips removed almost all of the SC, with a mean remaining SC depth of 7.82 µm. These data suggest that this non-invasive method can effectively remove the SC, with individual variability, and that a shiny appearance of the skin after stripping can be used as an accurate marker of SC removal.

Dermal Delivery of Diclofenac Sodium-In Vitro and In Vivo Studies

Previously, we reported the use of confocal Raman spectroscopy (CRS) as a novel non-invasive approach to determine drug disposition in the skin in vivo. Results obtained by CRS were found to correlate with data from the well-established in vitro permeation test (IVPT) model using human epidermis. However, these studies used simple vehicles comprising single solvents and binary or ternary solvent mixtures; to date, the utility of CRS for monitoring dermal absorption following application of complex marketed formulations has not been examined. In the present work, skin delivery of diclofenac sodium (DFNa) from two topical dermatological drug products, namely Diclac® Lipogel 10 mg/g and Primofenac® Emulsion gel 1%, was determined by IVPT and in vivo by both CRS and tape stripping (TS) methodologies under similar experimental conditions. The in vivo data were evaluated against the in vitro findings, and a direct comparison between CRS and TS was performed. Results from all methodologies showed that Diclac promoted significantly greater DFNa delivery to the skin (p < 0.05). The cumulative amounts of DFNa which permeated at 24 h in vitro for Diclac (86.5 ± 9.4 µg/cm2) were 3.6-fold greater than the corresponding amounts found for Primofenac (24.4 ± 2.7 µg/cm2). Additionally, total skin uptake of DFNa in vivo, estimated by the area under the depth profiles curves (AUC), or the signal intensity of the drug detected in the upper stratum corneum (SC) (4 µm) ranged from 3.5 to 3.6-fold greater for Diclac than for Primofenac. The shape of the distribution profiles and the depth of DFNa penetration to the SC estimated by CRS and TS were similar for the two methods. However, TS data indicated a 4.7-fold greater efficacy of Diclac relative to Primofenac, with corresponding total amounts of drug penetrated, 94.1 ± 22.6 µg and 20.2 ± 7.0 µg. The findings demonstrate that CRS is a methodology that is capable of distinguishing skin delivery of DFNa from different formulations. The results support the use of this approach for non-invasive evaluation of topical products in vivo. Future studies will examine additional formulations with more complex compositions and will use a wider range of drugs with different physicochemical properties. The non-invasive nature of CRS coupled with the ability to monitor drug permeation in real time offer significant advantages for testing and development of topical dermatological products.

Advancement through epidermis using tape stripping technique and Reflectance Confocal Microscopy

The tape stripping technique is increasingly used in research regarding skin barrier function. However, number of tape strips varies between studies, and literature considering advancement into stratum corneum/epidermis in relation to number of tape strips is scarce. The aim of this pilot study was to assess the advancement through epidermis using tape stripping technique in healthy volunteers. A total of ten healthy volunteers were included. From all volunteers 0, 5, 15 and 35 consecutive tape strips (D-squame) were taken from four adjacent skin areas on the middle volar forearm, followed by Reflectance Confocal Microscopy (RCM) of the four areas to assess epidermal thickness. Squame Scan was used to determine amount of protein removed. Stratum corneum was completely removed in all volunteers after 35 tape strips. Advancement into epidermis was predominantly achieved by the first 15 tape strips, removing 25% of the total epidermis, whereas 35 tape strips removed 33% of epidermis. Protein removal per tape decreased with increasing depth. Information on advancement into the epidermis according to number of tape strips taken, is a significant step forward. The possibility to obtain samples from different layers of epidermis may lead to an improved understanding of skin barrier properties.

Mass Spectrometry in Cosmetic Science: Advanced Ionization Techniques for Detecting Trace Molecules in or on Human Skin

To provide safe and effective products to customers in the cosmetic industry, mass spectrometry (MS) is an indispensable analytical tool. In addition to its outstanding sensitivity and specificity, the method is applicable to a wide variety of compounds, which makes it irreplaceable for the development of cosmetics, which requires the analysis of complex systems. Because most cosmetic products are applied directly to the skin and function as they are designed, monitoring the molecular compositions of endogenous or exogenous compounds in or on the skin is crucial to ensure the safety and efficacy of a cosmetic product. Recent advancements in MS and ionization techniques, such as MS imaging and ambient ionization, now provide access to richer and deeper molecular information with less time and effort. This brief review discusses advanced ionization techniques that are currently used in the field of cosmetic science using two examples, namely, the use of desorption electrospray ionization and zero-volt paperspray ionization to detect trace molecules in or on human skin.

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

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

In vivo methods for the analysis of the penetration of topically applied substances in and through the skin barrier

The efficacy of a drug is characterized by its action mechanism and its ability to pass the skin barrier. In this article, different methods are discussed, which permit this penetration process to be analysed non-invasively. Providing qualitative and quantitative information, tape stripping is one of the oldest procedures for penetration studies. Although single cell layers of corneocytes are removed from the skin surface, this procedure is considered as non-invasive and is applicable exclusively to the stratum corneum. Recently, optical and spectroscopic methods have been used to investigate the penetration process. Fluorescence-labelled drugs can be easily detected in the skin by laser scanning microscopy. This method has the disadvantage that the dye labelling changes the molecular structures of the drug and consequently might influence the penetration properties. The penetration process of non-fluorescent substances can be analysed by Raman spectroscopy, electron paramagnetic resonance, CARS and multiphoton microscopic measurements. Using these methods, the concentration of the topically applied formulations in different depths of the stratum corneum can be detected by moving the laser focus from the skin surface deeper into the stratum corneum. The advantages and disadvantages of these methods will be discussed in this article.

In vitro vs. in vivo tape stripping: validation of the porcine ear model and penetration assessment of novel sucrose stearate emulsions

Porcine ear skin is frequently used as a substitute for human skin in dermatological research and is especially useful for tape stripping experiments where the penetration of active substances into the uppermost skin layers is investigated. However, certain differences between the surface properties of these skin types exist, and reports on the comparability of tape stripping data obtained in vitro using porcine ear skin and data obtained in vivo on human forearm skin are scarce. Thus, we performed comparative tape stripping experiments in which the skin penetration of curcumin and fluorescein sodium from conventional microemulsions and hydrogels was investigated. In this context, the skin penetration potential of novel semi-solid macroemulsions and fluid nanoemulsions based on sucrose stearate was evaluated as well. The removed corneocytes were quantified by NIR-densitometry using recent correlation data for human and porcine proteins. The trends observed for the skin penetration into porcine ear skin were highly representative for the in vivo situation on human skin, confirming that the porcine ear is an excellent in vitro model for tape stripping experiments. Moreover, the validity of the NIR-densitometric approach for the quantification of both human and porcine stratum corneum proteins was confirmed in this study for the first time.

In vivo investigations on the penetration of various oils and their influence on the skin barrier

BACKGROUND

The skin represents a potent barrier to the environment, which can be enhanced by the topical application of skin care products, such as oil and oil-based formulations by moisturizing the skin.

METHODS

The aim of this study was the investigation of the penetration behaviour of four vegetable oils and of paraffin oil into the stratum corneum by laser scanning microscopy. In addition, the occlusion capacity of these substances was assessed by transepidermal water loss (TEWL) measurements. Petrolatum served as a positive control for skin occlusion. The study was conducted in vivo and included six healthy volunteers.

RESULTS

Paraffin oil, as well as the vegetable oils, penetrated only into the first upper layers of the stratum corneum. TEWL measurements indicated that the application of the vegetable oils (except jojoba oil) as well as paraffin oil, led to a similar occlusion of the skin surface. The most effective occlusion was found for petrolatum.

CONCLUSION

For the investigated oils, a deeper penetration than into the first upper layers of the stratum corneum could be excluded. The decreased TEWL values indicate that the application of the oils leads to a semi-occlusion of the skin surface as it is intended by the use of oils to retain moisture in skin.

Pathways of lateral spreading

In the case of topically applied substances, usually both lateral spreading and competitive penetration into the skin occur in parallel. In the present study, the pathways of lateral spreading were studied quantitatively and visually. The local distribution and lateral spreading of the UV filter substance butyl methoxydibenzoylmethane applied in an o/w emulsion was studied on the forearm and the back. The tape stripping procedure was used to determine the recovery rates inside and outside the area of application. The skin characteristics of transepidermal water loss, pH value, hydration of the stratum corneum and sebum rate were determined at both anatomic sites. Photography and laser scanning microscopy were used to visually investigate the lateral spreading of topically applied dyes. On the back, a preferred direction of lateral spreading parallel to the body axis was observed. This result was caused by differences in the network of furrows. The furrows functioned as a pathway for lateral spreading, whereas the follicles formed a reservoir for the topically applied substance.

Fluorescence induction of protoporphyrin IX by a new 5-aminolevulinic acid nanoemulsion used for photodynamic therapy in a full-thickness ex vivo skin model

An ex vivo porcine skin model was utilized to analyse the penetration of 5-aminolevulinic acid (5-ALA) contained in a nanoemulsion-based formulation BF-200 ALA (10% 5-ALA-hydrochloride) versus 16% aminolevulinate methyl ester-hydrochloride in a commercially cream (MAL cream) by fluorescence microscopy of their common metabolite protoporphyrin IX (PpIX) after 3, 5, 8 and 12 h. Fluorescence signals of PpIX in pig skin treated with BF-200 ALA were stronger than those for MAL cream. At 8 and 12 h, the PpIX fluorescence signals were 4.8- and 5.0-fold higher than those measured after MAL cream application. Fluorescence signals of PpIX after application of BF-200 ALA were detected in deeper tissue layers of the epidermis than after application of MAL cream (97.2 +/- 5.7 microm for BF-200 ALA vs 42.0 +/- 4.2 microm for MAL cream). These data implicate that BF-200 ALA in photodynamic therapy might lead to a superior therapeutically effect of intraepidermal (in situ) squamous cell carcinomas.

Characteristics of electrical skin resistance at acupuncture points in healthy humans

OBJECTIVES

The aim of this study was to evaluate the phenomenon of electrical skin resistance (ESR) changes at different acupuncture points (APs).

SETTING

This single-blinded study was performed at the hospital of the University of Munich.

DESIGN

Six common APs were measured (TE5, PC6, LU6, ST36, SP6, GB39) in 53 subjects. Subgroups were formed with varying time intervals for follow-ups (1 minute, 1 hour, 1 week) and a varying grade of reduction of the stratum corneum.

METHODS

Electrical skin resistance measurements (ESRMs) were taken from a skin area of 6 x 6 cm using an array consisting of 64 (8 x 8) electrodes. The electrodes corresponding to the AP were located and the ESRM results were compared to those of the surrounding electrodes. The methodological setting made it possible to minimize major influence factors on electrical skin impedance measurements.

RESULTS

A total of 631 ESRMs was evaluated: In 62.8% of the measured APs, no significant ESR difference was found. In 234 (37.2%) of the ESRMs, the ESR at the AP was significantly different from the surrounding skin area, with 163 (25.9%) points showing a lower and 71 (11.3%) points showing a higher ESR. Reproducibility was extremely high after 1 minute but was low after 1 hour and 1 week.

CONCLUSIONS

This study shows that electrical skin resistance at APs can either be lower or higher compared to the surrounding area. The phenomenon is characterized by high short-term and low long-term reproducibility. Therefore, we conclude that APs might possess specific transient electrical properties. However, as the majority of the measured APs did not show a changed ESR, it cannot be concluded from our data that electrical skin resistance measurements can be used for acupuncture point localization or diagnostic/therapeutic purposes.

Effect of formulation components on the in vitro permeation of microemulsion drug delivery system of fluconazole

The purpose of this study was to evaluate the effect of formulation components on the in vitro skin permeation of microemulsion drug delivery system containing fluconazole (FLZ). Lauryl alcohol (LA) was screened as the oil phase of microemulsions. The pseudo-ternary phase diagrams for microemulsion regions were constructed using LA as the oil, Labrasol (Lab) as the surfactant and ethanol (EtOH) as the cosurfactant. The formulation which showed a highest permeation rate of 47.15 +/- 1.12 microg cm(-2) h(-1) and appropriate physicochemical properties was optimized as containing 2% FLZ, 10% LA, 20% Lab/EtOH (1:1), and 68% double-distilled water (w/w). The efficiency of microemulsion formulation in the topical delivery of FLZ was dependent upon the contents of water and LA as well as Lab/EtOH mixing ratio. It was concluded that the percutaneous absorption of FLZ from microemulsions was enhanced with increasing the LA and water contents, and with decreasing the Lab/EtOH ratio in the formulation. Candida albicans was used as a model fungus to evaluate the antifungal activity of the best formula achieved, which showed the widest zone of inhibition as compared to FLZ reference. The studied microemulsion formulation showed a good stability for a period of 3 months. These results indicate that the studied microemulsion formulation might be a promising vehicle for topical delivery of FLZ.

Cytokines at different stratum corneum levels in normal and sodium lauryl sulphate-irritated skin

BACKGROUND/PURPOSE

Cytokines play an important role in inflammatory and repair processes occurring in the skin. The objectives of this study were to determine the amounts of cytokines and protein isolated by tape stripping in the different layers of the stratum corneum (SC), and to compare normal skin with skin exposed in vivo to the irritant sodium lauryl sulphate (SLS).

METHODS

In eight volunteers, we determined the amount of total and soluble protein and also interleukin-1alpha (IL-1alpha) in pooled tape strips obtained from the upper, intermediate and lower parts of the SC. Three different types of tape were compared (Diamond , D-squame or Sentega tape). In a separate study, 20 volunteers were repeatedly exposed to 0.1% SLS over a 3-week period. The amounts of IL-1alpha, IL-1RA and IL-8 in strips obtained from the three different SC levels of SLS-exposed skin were compared with an unexposed site.

RESULTS

For normal skin, the amounts of soluble protein and IL-1alpha were similar for the three tapes. Diamond tape showed the highest yield of total protein. The total protein yield per strip decreased to lower SC levels, whereas soluble protein and IL-1alpha normalized by soluble protein did not change across the SC. After SLS induced skin irritation, IL-1alpha decreased and IL-1RA and IL-8 increased at increasing depth into the SC.

CONCLUSIONS

Tape stripping is a suitable method to determine SC cytokine concentrations in human skin. With this technique, it is possible to study changes in cytokine concentrations at different SC layers after skin irritation.

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.

Penetration of Pollen Proteins into the Skin

Atopic dermatitis is a chronic remittent skin disease. In the extrinsic form of atopic dermatitis, type IgE-mediated reactions play an important pathophysiological role. The aim of the present study was to examine whether type I allergens can penetrate into the skin. Therefore, pollen proteins were labeled with fluorescein isothiocyanate (FITC), and their penetration profile was studied qualitatively. Solutions of FITC-labeled pollen proteins were applied in vitro on porcine skin and in vivo on human skin. In vitro, the FITC-labeled proteins were observed within the complete stratum corneum (SC) and inside the hair follicles even 15 min after application. They were also distributed inside the dermis around the hair follicles. In vivo, a similar pattern of distribution within the SC and the hair follicles was observed. These results indicate penetration via the SC lipid layers and a faster penetration via the hair follicles. The FITC-labeled proteins entered the dermis via the follicular pathway. Therefore, the follicular penetration should be considered in the development of skin protection strategies. To evaluate such strategies, the developed method can be used, and further studies in atopic dermatitis patients are necessary to determine whether the penetration of type I allergens is increased.

Acute barrier disruption by adhesive tapes is influenced by pressure, time and anatomical location: integrity and cohesion assessed by sequential tape stripping. A randomized, controlled study

BACKGROUND

Tape stripping is an established procedure in stratum corneum (SC) physiology research. Adhesive films are pressed to the surface of the skin and then removed. The superficial layers of the SC adhere to the film and are accessible for further investigations. Although this method is widely used, only scant information about standardization is known. Various protocols are used but are difficult to compare.

OBJECTIVES

The aim of the present study was to investigate the effects of the type of tape, pressure, time, anatomical site and type of applied pressure.

METHODS

Twelve healthy volunteers (age range 20-31 years) were entered in a randomized, controlled study with sequential tape stripping at the volar forearm, upper arm, cheek and back. Different methods (roller, stamp, thumb, stretched skin), total duration of applied pressure (2 s, 10 s), degrees of pressure (2 N stamp, 7 N stamp) and different tapes (D-Squame, Corneofix, Blenderm) were investigated and the impact on barrier function assessed by transepidermal water loss measurements. Furthermore, measurements of SC hydration, skin colour and skin surface pH were performed. Spectroscopic measurements and a Bradford protein assay to determine the mass of removed SC were carried out in parallel.

RESULTS

The degree of barrier disruption, irritation and SC cohesion is influenced by the character of adhesive tapes, total duration of applied pressure (2 s, 10 s; 2 N, 7 N), the kind of method for pressure application (roller, stamp, thumb, stretched skin), anatomical site and condition before stripping (occlusion vs. nonocclusion). The spectroscopic assessment and Bradford protein assay determination showed a significant correlation (P < 0.0001; r = 0.7041).

CONCLUSIONS

The present study showed significant differences between different factors on controlled barrier disruption. The results indicate the importance of defining these factors when a study is initiated and when results of different studies should be compared. Based on our data we propose using a 2 N stamp for a duration of 2 s on 15 sequential D-Squame tape strips on the volar forearm and then discarding the first and second strips. This approach allows the performance of a standardized study with a reasonable amount of resources.

Comparison of stratum corneum penetration and localization of a lipophilic model drug applied in an o/w microemulsion and an amphiphilic cream

Vehicle dependent effects on the penetration behavior of drugs following topical application are well known from the literature. In this context, many reports concerning the enhancing activities for hydrophilic as well as lipophilic substances by colloidal drug carrier systems, particularly microemulsions, are available. However, there is little knowledge about the localization of the drugs within the skin and the stratum corneum, respectively. In the present study, the lipophilic dye curcumin incorporated in an oil-in-water microemulsion and in an amphiphilic cream was applied onto the skin of human volunteers. Using the method of tape stripping to remove the stratum corneum (SC), the depth profiles of the dye within the horny layer were compared. Applying the microemulsion, a deeper part of the SC was accessible by a number of 20 tapes removed and significantly smaller amounts of curcumin were found on the skin surface. Also differences in the distribution and localization of the dye within the stratum corneum were observed by laser scanning microscopy. Furthermore, curcumin was detected in hair follicles. It was obvious that the microemulsion led to a penetration into the complete follicular infundibula, whereas, following application of the cream, a fluorescence signal was only received from the follicular orifices.

Topical Bioavailability of Triamcinolone Acetonide: Effect of Occlusion

BACKGROUND/AIMS

Occlusion by covering the skin with an impermeable wrap enhances skin hydration, affects drug absorption and can induce the formation of a drug reservoir within the stratum corneum. This is desired in local therapy with topical corticosteroids. The aim of the study was to investigate the effect of occlusion before (experiment 1) and after (experiment 2) application on the penetration of triamcinolone acetonide (TACA) into the stratum corneum.

METHODS

The experiments were conducted on the forearms of 10 healthy volunteers. In experiment 1, 100 microg/cm(2) TACA in acetone were applied on 3 sites per arm, one arm having been pre-occluded for 16 h. In experiment 2, the same dose was applied on 2 sites per arm, and one arm was occluded after application until skin sampling. Stratum corneum samples were removed by tape stripping at 0.5, 4 and 24 h (experiment 1) and 4 and 24 h (experiment 2) after application. Corneocytes and TACA were quantified by ultraviolet-visible spectroscopy and HPLC, respectively. The total TACA amount penetrated into the stratum corneum was evaluated by multifactor ANOVA.

RESULTS

TACA penetration into the stratum corneum with and without pre-occlusion (experiment 1) showed no significant difference and decreased with time. Occlusion after application (experiment 2) produced a marked TACA accumulation within the stratum corneum, which persisted for 24 h.

CONCLUSION

Pre-occlusion showed no effect on the topical bioavailability of TACA in the stratum corneum. In contrast, post-occlusion enhanced the TACA penetration by a factor of 2, favouring the development of a drug reservoir.

Topical bioavailability of triamcinolone acetonide: effect of dose and application frequency

The application frequency of topical corticosteroids is a recurrently debated topic. Multiple-daily applications are common, although a superior efficacy compared to once-daily application is not unequivocally proven. Only few pharmacokinetic studies investigating application frequency exist. The aim of the study was to investigate the effect of dose (Experiment 1) and application frequency (Experiment 2) on the penetration of triamcinolone acetonide (TACA) into human stratum corneum (SC) in vivo. The experiments were conducted on the forearms of 15 healthy volunteers. In Experiment 1, single TACA doses (300 microg/cm(2) and 100 microg/cm(2)) dissolved in acetone were applied on three sites per arm. In experiment 2, single (1 x 300 microg/cm(2)) and multiple (3 x 100 microg/cm(2)) TACA doses were similarly applied. SC samples were harvested by tape stripping after 0.5, 4 and 24 h (Experiment 1) and after 4, 8 and 24 h (Experiment 2). Corneocytes and TACA were quantified by UV/VIS spectroscopy and HPLC, respectively. TACA amounts penetrated into SC were statistically evaluated by a paired-sample t-test. In Experiment 1, TACA amounts within SC after application of 1 x 300 microg/cm(2) compared to 1 x 100 microg/cm(2) were only significantly different directly after application and similar at 4 and 24 h. In Experiment 2, multiple applications of 3 x 100 microg/cm(2) yielded higher TACA amounts compared to a single application of 1 x 300 microg/cm(2) at 4 and 8 h. At 24 h, no difference was observed. In conclusion, using this simple vehicle, considerable TACA amounts were retained within SC independently of dose and application frequency. A low TACA dose applied once should be preferred to a high dose, which may promote higher systemic exposure.

Follicular Penetration: Development of a Method to Block the Follicles Selectively against the Penetration of Topically Applied Substances

Investigations into the penetration pathways of topically applied substances through the skin play an important role in dermatological science. Recently, the hair follicles have increasingly been recognized as an important pathway for percutaneous penetration, although the role of the follicles has still not been clarified in detail. Therefore, the aim of the present study was to develop an in vivo method for the analysis of the follicular rate of penetration processes. In order to reach this goal, the follicles were excluded from the penetration process of topically applied substances. Different study designs were evaluated, and we were able to show that the application of nail varnish to each follicular orifice represents an effective method of blocking the follicles selectively against the penetration of topically applied substances. In dermatological science, there is new information on the physiology of skin penetration processes, thus allowing examination of the follicular rate of penetration and improvement of the pharmacokinetics of topically applied substances.

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.

How does the type of vehicle influence the in vitro skin absorption and elimination kinetics of terpenes?

Terpenes are widely used in the topical dermal preparations, cosmetics and toiletries and also in the experimental dermopharmacy, as penetration enhancers. Terpenes do not need to penetrate into viable skin tissue and this event is not even desired. The aim of this study was to investigate skin absorption and elimination kinetics of two terpenes, namely linalool and terpinen-4-ol, incorporated in three different dermatological vehicles: oily solution, hydrogel and o/w emulsion. The preparations were applied onto the human skin in vitro, and after 1-4 h the content of terpenes in the stratum corneum layers and in the epidermis/dermis was determined using GC. Similarly, the amounts of terpenes in the skin were analysed during 4 h elimination process following 1 h absorption. The highest skin absorption was observed when terpenes were applied in hydrogel--their total content in the skin after 4 h was 385 and 705 microg/cm2 for linalool and terpinen-4-ol, respectively. After 1 h of the elimination process about 10-20% drop of the total content of both terpenes in the skin was noted for all formulations. The skin penetration of both terpenes from the vehicles is increasing in the following order: emulsion < oily solution < hydrogel, while the elimination phase is relatively slower for terpenes applied in hydrogel.

Cutaneous distribution and localization of dyes affected by vehicles all with different lipophilicity

The penetration of topically applied substances into the stratum corneum (SC) depends on several factors, e.g., the physicochemical properties of the vehicle used for application. The penetration of highly hydrophilic and lipophilic dyes into the skin was studied using a pure oil (o) or water (w) for the application compared to an o/w emulsion. The penetration and localization of both dyes, the lipophilic curcumin and the hydrophilic Patent blue V, was investigated in vivo using the method of tape stripping and microscopy. In addition, histological sections of biopsies, removed from porcine ear skin were studied using microscopy. Differences in the distribution and the localization of both dyes within the SC were observed. These differences depend on the physicochemical properties of both the vehicles and the dyes. The vehicle appears to affect, in particular, the pathways of penetration.

Effect of size of TiO2 nanoparticles embedded into stratum corneum on ultraviolet-A and ultraviolet-B sun-blocking properties of the skin

Recently there has been a strong demand to protect human skin against negative effects of the UV solar light. This problem is interesting due to the increased frequency of human diseases caused by such radiation. We aim to evaluate how the optical properties of the horny layer of skin can be effectively changed by imbedding TiO2 fine particles to achieve the maximal attenuation of the UV solar radiation. In-depth distribution of TiO2 particles embedded into the skin by multiple administration of sunscreens is determined experimentally using the tape-stripping technique. A computer code implementing the Monte Carlo method is developed to simulate photon migration within the 20-microm-thick horny layer filled with nanosized TiO2 spheres, 25 to 200 nm in diameter. Dependencies of the UV radiation of two wavelengths (310 and 400 nm) absorbed by and totally reflected from, as well as transmitted through the horny layer on the size of TiO2 particles are obtained and analyzed. The most attenuating particles are found to be 62 and 122 nm in diameter for 310- and 400-nm light, respectively. The former could be suggested as the main fraction to be used in sunscreens to prevent erythema.

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.

Comparison of transepidermal water loss and spectroscopic absorbance to quantify changes of the stratum corneum after tape stripping

The objective and quantitative application of tape stripping in pharmaceutics and dermatopharmacokinetics requires the determination of the exact position of each removed tape strip inside the stratum corneum (SC) and/or the determination of the relative SC thickness. In this study, transepidermal water loss (TEWL) and the optical spectroscopic data of the corneocytes were measured simultaneously during the complete removal of the SC by tape stripping. The spectroscopic data quantitatively reflect the amount of corneocytes removed by the individual tape strips, whereas TEWL and 1/TEWL are not sensitive enough to measure the relatively small changes in the SC thickness realized by the removal of the individual strips. The relative SC thickness can be determined directly by the spectroscopic data, while the 1/TEWL values require a second independent method. The results demonstrate the importance of tape stripping characterizing the behaviour of topically applied substances.

Reservoir Function of the Stratum corneum: Development of an in vivo Method to Quantitatively Determine the Stratum corneum Reservoir for Topically Applied Substances

Investigations on the stratum corneum (SC) reservoir for topically applied substances are of importance in dermatologic science in order to assess the pharmacokinetics of these substances. In the present study, an in vivo method was developed to determine the SC reservoir quantitatively and to investigate the temporal behavior of this reservoir. Therefore, increasing amounts of an oil-in-water emulsion (o/w emulsion) containing 4% of a chemical UV filter were topically applied onto the flexor forearms of 5 healthy volunteers. The saturation of the SC reservoir was determined utilizing the tape stripping technique 1 and 6 h after application. The capacity of the SC reservoir for the o/w emulsion was found to be approximately 2.7 mg/cm(2). Furthermore, a correlation of the capacity of the SC with transepidermal water loss was observed. Extending the time between the topical application and SC removal did not affect the distribution or the recovery rate of the UV filter in the SC. The results indicate that the reservoir of the SC is limited. This is reflected by the saturation level, which depends on the individual volunteer and, presumably, the topically applied substances and formulations used. The results show that the method developed is suited to quantitatively determine in vivo the SC reservoir for topically applied substances.

Determination of penetration profiles of topically applied substances by means of tape stripping and optical spectroscopy: UV filter substance in sunscreens

Penetration profiles of topically applied drugs and cosmetic products provide important information on their efficacy. The application of tape stripping in combination with UV/VIS spectroscopy is checked to determine the local position of topically applied substances inside the stratum corneum, the penetration profile. The amount of corneocytes removed with each tape strip is quantified via the particle-dependent absorption, the pseudoabsorption, in the visible spectral range. The concentration of a typical UV filter substance, 4-methylbenzylidene camphor, is determined by optical spectroscopy using the tape strips removed originally. In this case, a time-dependent increase in the absorbance must be taken into account. Laser scanning microscopic investigations confirm that the nonhomogeneous distribution of the filter substance, on the strips, can explain this spectroscopic behavior. When reaching a homogeneous distribution, the UV spectroscopic signal reflects the correct concentration. These spectroscopic values are compared with high performance liquid chromatography (HPLC) data. The values obtained with both methods for the concentrations of 4-methylbenzylidene camphor are in good agreement. The data obtained are used to illustrate the determination of a penetration profile of a UV filter substance. The results demonstrate that the described protocol is well suited to characterize, in a simple manner, topically applied substances that have a characteristic UV/VIS absorption band.

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.

The "strip" patch test: results of a multicentre study towards a standardization

BACKGROUND

The "strip" patch test (SPT) is a variant of patch testing which is used for substances with a poor percutaneous penetration. Penetration of the substances is enhanced by repeated applications of adhesive tape prior to their application to the skin. However, no guidelines exist for standardized performance of the SPT.

OBJECTIVES

The aim of this multicentre study was to obtain a first practical approach towards a standardized SPT procedure.

METHODS

Intact noninflamed skin of the upper back of 83 healthy volunteers was tape-stripped. For sequential strips, a 25-mm diameter 3M Blenderm surgical tape was vertically applied and gently pressed downward using the fingertips for about 2 s. The tape was removed in one quick movement at an angle of 45 degrees in the direction of adherence. Each strip was performed with a new piece of tape on exactly the same skin area. RESULTS. In each subject, we first determined the number of strips (A) until the skin surface started to glisten and calculated the median number of strips (A) in the sample (A=26 strips). We then ascertained the median number of strips (a) in the sample that was necessary to achieve a statistically significant and twofold increase in TEWL (a=11 strips), revealing a "critical" stratum corneum strip depth. The unknown number of strips (a) for each subject was finally calculated from the formula a/A=a/A, i.e. the individual number of strips (A) until the skin surface started to glisten was multiplied by a derived tape-specific correction factor (cf=a/A=11/26=0.4). The increase in percutaneous penetration in strip patch testing by performing "a" strips versus conventional patch testing was shown by scoring of clinical and subjective SLS irritant reactions.

CONCLUSIONS

The present multicentre study outlines an experimentally derived approach for a uniform SPT procedure, which does not require the use of complex technical equipment. This first approach now requires validation by a study involving the application of allergens to obtain evidence of enhancement in the sensitivity of patch testing.