Penetration Of Caffeine Research Articles

PENETRATION TEST OF CAFFEINE IN ETHOSOME AND DESMOSOME GEL USING AN IN VITRO METHOD

Objectives: Caffeine has many functions including its use in the field of cosmetics. Nonetheless, the percutaneous absorption of caffeine is very low(9%), and the penetration of a substance such as caffeine in the skin is not desirable. Ethosomes and desmosomes are lipid vesicles created by themodification of liposomes containing phospholipids and ethanol or dimethyl sulfoxide (DMSO) as the penetration enhancer. The purpose of this studywas to compare the effectiveness of ethosomes and desmosomes as vesicles in increasing the penetration of caffeine.Methods: Ethosomes and desmosomes were prepared using phosphatidylcholine, ethanol/DMSO, and caffeine. Phosphatidylcholine was used in theform of phospholipon 90 g that was obtained from soybeans. Observations were done including the characteristic of ethosomes and desmosomes,organoleptic observation, homogeneity observation, and in vitro penetration test using Franz diffusion cell method.Results: The cumulative penetration of caffeine ethosome gel is 3316.46±218.51 μg/cm2, with flux 249.45±30.06 μg·cm−2·hr−1, and 62.35±4.52%. Thecumulative penetration of the desmosome gel is 2954.95±222.87 μg/cm2 with flux 381.68±34.91 μg·cm−2·hr−1 and 53.4±3.65%.Conclusions: It can be concluded that ethosome is more effective than desmosome in increasing the penetration of caffeine.

Non-ionic surfactant based vesicular drug delivery system for topical delivery of caffeine for treatment of cellulite: design, formulation, characterization, histological anti-cellulite activity, and pharmacokinetic evaluation

Cellulite is a common topographical alteration where skin acquires an orange peel or mattress appearance with alterations in adipose tissue and microcirculation. This work aims to develop and evaluate a topical niosomal gel formulae with good permeation to reach the subcutaneous fat layer. Several caffeine niosomal dispersions were prepared and incorporated into gel formulae using Carbopol 940 polymer, chemical penetration enhancers, and iontophoresis, then the prepared gels were applied onto the skin of rats and anticellulite activity of caffeine from the prepared gels compared to that of the commercial product Cellu Destock® was evaluated by histological study of the skin and measurement of plasma level of caffeine passing through the skin using liquid chromatography (LC/MS–MS). Results of histology revealed reduction of size and thickness of fatty layer of rat skin in the following order: FVII > FXIV > Cellu Destock® > FVII + Iontophoresis > FXIV + Iontophoresis. Pharmacokinetic results of caffeine in plasma revealed that Cmax, Tmax, and AUC0–12h decreased in the following order: FXIV > FVII > Cellu Destock®. These results conclude that incorporation of caffeine niosomal dispersion into gel matrix with penetration enhancers and iontophoresis resulted in improvement in penetration of caffeine through the skin into the underlying fatty layer in treatment of cellulite.

A 3D-psoriatic skin model for dermatological testing: The impact of culture conditions

BackgroundInadequate representation of the human tissue environment during a preclinical screen can result in inaccurate predictions of compound effects. Consequently, pharmaceutical investigators are searching for preclinical models that closely resemble original tissue for predicting clinical outcomes. MethodsThe current research aims to compare the impact of using serum-free medium instead of complete culture medium during the last step of psoriatic skin substitute reconstruction. Skin substitutes were produced according to the self-assembly approach. ResultsSerum-free conditions have no negative impact on the reconstruction of healthy or psoriatic skin substitutes presented in this study regarding their macroscopic or histological appearances. ATR-FTIR results showed no significant differences in the CH2 bands between psoriatic substitutes cultured with or without serum, thus suggesting that serum deprivation did not have a negative impact on the lipid organization of their stratum corneum. Serum deprivation could even lead to a better organization of healthy skin substitute lipids. Percutaneous analyses demonstrated that psoriatic substitutes cultured in serum-free conditions showed a higher permeability to hydrocortisone compared to controls, while no significant differences in benzoic acid and caffeine penetration profiles were observed. ConclusionsResults obtained with this 3D-psoriatic skin substitute demonstrate the potential and versatility of the model. It could offer good prediction of drug related toxicities at preclinical stages performed in order to avoid unexpected and costly findings in the clinic. General significanceTogether, these findings offer a new approach for one of the most important challenges of the 21st century, namely, prediction of drug toxicity.

Synergistic Skin Penetration Enhancer and Nanoemulsion Formulations Promote the Human Epidermal Permeation of Caffeine and Naproxen

We examined the extent of skin permeation enhancement of the hydrophilic drug caffeine and lipophilic drug naproxen applied in nanoemulsions incorporating skin penetration enhancers. Infinite doses of fully characterized oil-in-water nanoemulsions containing the skin penetration enhancers oleic acid or eucalyptol as oil phases and caffeine (3%) or naproxen (2%) were applied to human epidermal membranes in Franz diffusion cells, along with aqueous control solutions. Caffeine and naproxen fluxes were determined over 8 h. Solute solubility in the formulations and in the stratum corneum (SC), as well as the uptake of product components into the SC were measured. The nanoemulsions significantly enhanced the skin penetration of caffeine and naproxen, compared to aqueous control solutions. Caffeine maximum flux enhancement was associated with a synergistic increase in both caffeine SC solubility and skin diffusivity, whereas a formulation-increased solubility in the SC was the dominant determinant for increased naproxen fluxes. Enhancements in SC solubility were related to the uptake of the formulation excipients containing the active compounds into the SC. Enhanced skin penetration in these systems is largely driven by uptake of formulation excipients containing the active compounds into the SC with impacts on SC solubility and diffusivity.

A Comparison of the Penetration and Permeation of Caffeine into and through Human Epidermis after Application in Various Vesicle Formulations

Background/Aims: A range of vesicles is now widely used to carry various solutes into and through the epidermis. These usually have the active solute encapsulated within and may be modified to confer flexibility and skin penetration enhancement. Here, we compared the ability of five different vesicle systems to deliver a model hydrophilic drug, caffeine, into and through excised human skin. Methods: In addition to lipids, the vesicle excipients included eucalyptol or oleic acid as penetration enhancers, and decyl polyglucoside as a non-ionic surfactant. Vesicle particle sizes ranged between 135 and 158 nm, and caffeine encapsulation efficiencies were between 46 and 66%. Caffeine penetration and permeation were measured using high-performance liquid chromatography. Results: We found that niosomes, which are liposomes containing a non-ionic surfactant, and transferosomes (ultraflexible vesicles) showed significantly greater penetration into the skin and permeation across the stratum corneum. Significant enhancement of caffeine penetration into hair follicles was found for transferosomes and those liposomes containing oleic acid. Conclusions: We conclude that targeted delivery of the hydrophilic drug caffeine into the skin compartments can be modified using optimized vesicular formulations.

Caffeine, an in vivo oxidation protectant in the lens

SummaryThe major avoidable cause of cataract is ultraviolet radiation (UVR). We aimed at investigating topical caffeine in UVR‐induced cataract.Topical caffeine and a placebo were applied to the eyes of separate groups of Sprague Dawley rats that were exposed to sub‐doses of UVR and protective effect was evaluated. Penetration of topical caffeine in the rats to lens and blood was analysed by HPLC. Influence of topical caffeine on pupil diameter was measured in ketamine/xylazine anesthetized rats.Topically administered caffeine protected against UVR‐induced cataract development with a Protection Factor, an objective relative measure of protective properties, of 1.23 and inhibited UVR‐induced apoptosis. Topical caffeine peaked at 30 min in the lens, increased up to 120 min in the blood and antagonized ketamine/xylazine‐induced mydriasis. Eyes treated with caffeine reacted with quick dilatation after tropicamide application.In conclusion, topically applied caffeine protects against ultraviolet radiation cataract, reducing lens sensitivity 1.23 times. Considering that caffeine is a powerful antioxidant that is easily available and well tolerated our finding implicates that caffeine may be a clinically useful anticataract agent.

New easy handling and sampling device for bioavailability screening of topical formulations.

Biopharmaceutical assessment of topical drug formulations is widely carried out by using vertical diffusion cells (Franz type cell). Although Franz diffusion cell model is well designed for percutaneous absorption studies, the extent of drug penetration within the skin requires more adapted device. Recently, we have developed a new patented versatile, easy-to-use, and disposable diffusion cell called VitroPharma. In this study we have assessed the cutaneous bioavailability of caffeine as hydrophilic compound model using Franz diffusion cell and VitroPharma. The percutaneous absorption of caffeine assessed with Franz diffusion cell and VitroPharma was characterized by using (i) finite dose model and (ii) classical pharmacokinetic analysis. Furthermore, the follow-up of caffeine penetration within the skin was determined by sequential measurements of tissular drug concentration throughout the time of skin exposure with VitroPharma. However, classical experimental design using Franz diffusion cell involved unique determination of tissular concentration at the final point of skin exposure protocol. Finally, device equivalence between Franz diffusion cell and VitroPharma was claimed from percutaneous absorption data analysis. Concomitant assessment of dual penetration and permeation kinetics by using VitroPharma reinforced the understanding of skin drug delivery.

Differential effects of some natural compounds on the transdermal absorption and penetration of caffeine and salicylic acid

Many natural products have the potential to modulate the dermal penetration of topically applied drugs and chemicals. We studied the effect of five natural compounds (hydroxycitronellal, limonene 1,2-epoxide, terpinyl acetate, p-coumaric acid, transferrulic acid) and ethanol on the transdermal penetration of two marker drugs (14C-caffeine and 14C-salicylic acid) in a flow through in vitro porcine skin diffusion system. The parameters of flux, permeability, diffusivity, and percent dose absorbed/retained were calculated and compared. The dermal absorption of 14C-caffeine was significantly higher with terpinyl acetate and limonene 1,2-epoxide as compared to ethanol; while dermal absorption of 14C-salicylic acid was significantly greater with hydroxycitronellal and limonene 1,2-epoxide as compared to ethanol. A 10-fold increase in flux and permeability of caffeine with terpinyl acetate was observed while limonene increased flux of caffeine by 4-fold and permeability by 3-fold. Hydroxycitronellal and limonene increased salicylic acid’s flux and permeability over 2-fold. The other natural compounds tested did not produce statistically significant effects on dermal penetration parameters for both caffeine and salicylic acid (p≥0.05). These results emphasize the differential effects of natural substances on the transdermal penetration of hydrophilic (caffeine) and hydrophobic (salicylic acid) drugs.

In vitro-to-in vivo correlation of the skin penetration, liver clearance and hepatotoxicity of caffeine

This work illustrates the use of Physiologically-Based Toxicokinetic (PBTK) modelling for the healthy Caucasian population in in vitro-to-in vivo correlation of kinetic measures of caffeine skin penetration and liver clearance (based on literature experiments), as well as dose metrics of caffeine-induced measured HepaRG toxicity. We applied a simple correlation factor to quantify the in vitro and in vivo differences in the amount of caffeine permeated through the skin and concentration-time profiles of caffeine in the liver. We developed a multi-scale computational approach by linking the PBTK model with a Virtual Cell-Based Assay to relate an external oral and dermal dose with the measured in vitro HepaRG cell viability. The results revealed higher in vivo skin permeation profiles than those determined in vitro using identical exposure conditions. Liver clearance of caffeine derived from in vitro metabolism rates was found to be much slower than the optimised in vivo clearance with respect to caffeine plasma concentrations. Finally, HepaRG cell viability was shown to remain almost unchanged for external caffeine doses of 5–400 mg for both oral and dermal absorption routes. We modelled single exposure to caffeine only.

Pengaruh Natrium Hialuronat terhadap Penetrasi Kofein Sebagai Antiselulit dalam Sediaan Hidrogel, Hidroalkoholik Gel, dan Emulsi Gel

Anticellulite topical gel preparation with caffeine as active ingredient needs a penetration enhancer to reach subcutaneous layer. Sodium hyaluronate (NaHA), the sodium salt of hyaluronic acid, is a hydrophilic polysaccharide derivative polymer. It has ability to enhance percutaneous penetration by loosening the dense of the compact substance stratum corneum. The aim of this research was to observe the effects of NaHA on caffeine penetration as anticellulite active agent in three types of gel preparation: hydrogel, hydroalcoholic gel, and gel emulsion. Each gel type contained caffeine 1,5% and was varied into three formulas. Formula 1 contained HPMC 2% as gel basis; formula 2 contained HPMC 2% and NaHA 0,5%; formula 3 contained NaHA 2% as gel basis. Caffeine penetration properties were analyzed by Franz diffusion cell in vitro test using rat skin as membrane. Percent caffeine penetration of hydrogel formula 1, 2, 3 were 9,41 ± 0,01%; 11,74 ± 0,13%; 16,32 ± 0,03%, respectively. Percent caffeine penetration of hydroalcoholic gel formula 1, 2, 3 were 19,54 ± 0,02%; 22,99 ± 0,23%; 7,42 ± 0,08%, respectively. Percent caffeine penetration of gel emulsion formula 1, 2, 3 were 10,47 ± 0,19%; 13,41 ± 0,12%; 18,42 ± 0,06%, respectively. The result showed that NaHA enhanced the caffeine percutaneous penetration properties in various gel preparations, except hidroalkoholic gel formula 3.

Vibrational spectroscopies for the analysis of cutaneous permeation: experimental limiting factors identified in the case of caffeine penetration

Caffeine is utilised as a reference for permeation studies in dermatology and cosmetology. The present work aimed to monitor the permeation of a caffeine solution through the skin. For this purpose, Raman and infrared studies were performed. Raman microspectroscopy permitted a dynamic follow-up of the caffeine diffusion. In complementary, infrared microimaging provided information of the caffeine localization in the skin by applying multivariate statistical processing on skin tissue sections. Herein, we prove the possibility of tracking low concentrations of caffeine through the skin and we highlight some experimental limitations of vibrational spectroscopies.

Assessment of the Lateral Diffusion and Penetration of Topically Applied Drugs in Humans Using a Novel Concentric Tape Stripping Design

To determine the extent of lateral spread and stratum corneum (SC) penetration of caffeine (CAF), hydrocortisone (HC) and ibuprofen (IBU) using a novel concentric tape stripping technique. Ethanolic solutions of CAF, HC or IBU were applied to the forearm of 8 volunteers. At various time points, 10 successive layers of SC were removed by stripping with tapes perforated into concentric rings and analysed for drug concentration and mass of SC protein. In vitro permeation studies assessed the percutaneous absorption of these compounds across human skin. CAF and IBU showed significant lateral spreading across the SC while HC formed a drug depot at the site of application. Relative to the applied dose, the in vivo recovery of all compounds from the combined 10 strips at 3 mins ranged between 83.0 and 92.9 % and decreased to between 64.5 and 66.9 % at 3 h. IBU recovery further decreased to 47.7 ± 5.6 % at 6 h, correlating with greater in vitro penetration relative to CAF and HC. Drug concentration decreased with increased lateral distance from the application site. The lower recovery of IBU in the upper tape strip regions compared to CAF and HC may be a consequence of greater penetration into the SC with time.

Hair follicles contribute significantly to penetration through human skin only at times soon after application as a solvent deposited solid in man

The aim of this study was to define the underlying relative penetration of caffeine through hair follicles and through intact stratum corneum with time in vivo through pharmacokinetic modelling. Caffeine plasma concentration-time profiles after topical application into skin with or without hair follicle blocking were modelled using the Wagner-Nelson method or a compartmental model with first order absorption and elimination. Pharmacokinetic parameters describing absorption rate and extent of absorption through hair follicles or the stratum corneum were determined separately and compared with each other. The obtained pharmacokinetic parameters from the two methods were similar. The absorption rate constant of caffeine for hair follicles was nearly 10 times higher than that for the stratum corneum and the percentage of absorption from hair follicles was more than half of that of the stratum corneum. In addition, the absorption from the stratum corneum showed an approximately 10 min delay while there was no delay for absorption from hair follicles. All caffeine absorbed by hair follicles occurs within 30 min of application and accounts for 10.5 to 33.8% of the total amount absorbed across the skin for all subjects, whereas absorption of caffeine through the stratum corneum can occur over several hours. Hair follicles contribute significantly to percutaneous absorption of caffeine after topical application in man in vivo only at times soon after application.

Barrier function of intact and impaired skin: percutaneous penetration of caffeine and salicylic acid

Normally, percutaneous absorption tests are carried out using skin biopsies for an apparent and acceptable physiological condition. However, under different pathological conditions, the stratum corneum (SC) barrier function is impaired. The barrier function of the SC was assessed by correlation between the number of repeated applications of tape strips on the skin and its transepidermal water loss (TEWL), as well as by in vitro percutaneous absorption studies of different compounds, using Franz diffusion cells and porcine skin previously stripped. A progressive diminution of the skin barrier function has been detected by TEWL both in vitro and in vivo as the number of skin tape strips increases. On the other hand, the percutaneous absorption of the compounds tested increases in a different way as the number of strips increases. Salicylic acid increases linearly depending on the barrier disturbance. However, percutaneous absorption of caffeine exponentially increased with barrier disturbance. Our results indicate that the barrier impairment of skin always increases the penetration behavior of a given compound; however, the hydrophilic-lipophilic balance of the compounds or formulations used could greatly modify its penetration profile, especially when a modified skin is used. This in vitro protocol may be useful to simulate the percutaneous absorption profile of some drugs applied onto skin with an impaired SC barrier function and could be used to avoid, to some extent, the use of in vivo experimental animal models in the dermopharmaceutical field.

Storage Conditions of Skin Affect Tissue Structure and Subsequent in vitro Percutaneous Penetration

Storage of skin at low temperatures may affect its structure. There is no report in the literature on the correlation between spatially resolved skin structure and percutaneous penetration after different storage conditions. The present study applies imaging techniques (multiphoton excitation fluorescence microscopy) and in vitro percutaneous penetration of caffeine under four different storage conditions using skin samples from the same donors: fresh skin, skin kept at –20°C for 3 weeks (with or without the use of polyethylene glycol) and at –80°C. Our results show a correlation between increasing permeation of caffeine and tissue structural damage caused by the storage conditions, most so after skin storage at –80°C. The presented approach, which combines imaging techniques with studies on percutaneous penetration, enables the link between tissue damage at selected depths and penetration into the upper layers of the epidermis to be investigated.

Analysis of the penetration of a caffeine containing shampoo into the hair follicles by in vivo laser scanning microscopy

In previous in vitro investigations, it was demonstrated that caffeine is able to stimulate the hair growth. Therefore, a penetration of caffeine into the hair follicle is necessary. In the present study, in vivo laser scanning microscopy (LSM) was used to investigate the penetration and storage of a caffeine containing shampoo into the hair follicles. It was shown that a 2-min contact time of the shampoo with the skin was enough to accumulate significant parts of the shampoo in the hair follicles. A penetration of the shampoo up to a depth of approx. 200 μm could be detected, which represents the detection limit of the LSM. At this depth, the close network of the blood capillaries surrounding the hair follicles commences. Even after 24 h, the substance was still detectable in the hair follicles. This demonstrates the long-term reservoir function of the hair follicles for topically applied substances such as caffeine.

Permeation of topically applied caffeine through human skin – a comparison of in vivo and in vitro data

Due to ethical reasons, in vivo penetration studies are not applicable at all stages of development of new substances. Therefore, the development of appropriate in vitro methods is essential, as well as the comparison of the obtained in vivo and in vitro data, in order to identify their transferability. The aim of the present study was to investigate the follicular penetration of caffeine in vitro and to compare the data with the in vivo results determined previously under similar conditions. The Follicular Closing Technique (FCT) represents a method to investigate the follicular penetration selectively. In the present study, FCT was combined with the Franz diffusion cell in order to differentiate between follicular and intercellular penetration of caffeine into the receptor medium in vitro. Subsequently, the results were compared with the data obtained in an earlier study investigating follicular and intercellular penetration of caffeine in vivo. The comparison of the data revealed that the in vitro experiments were valuable for the investigation of the follicular penetration pathway, which contributed in vivo as well as in vitro to approximately 50% of the total penetration, whereas the kinetics of caffeine penetration were shown to be significantly different. The combination of FCT with the Franz diffusion cell represents a valuable method to investigate follicular penetration in vitro. Nevertheless, in vivo experiments should not be abandoned as in vitro, structural changes of skin occur and blood flow and metabolism are absent, probably accounting for reduced penetration rates in vitro.

The role of hair follicles in the percutaneous absorption of caffeine

* In recent years, it has been suggested that hair follicles represent important shunt routes into the skin for drugs and chemicals [1-3]. * In vitro studies have shown the importance of skin appendages for skin penetration by hydrophilic compounds [4]. Investigation of follicular penetration in vivo has been difficult due to the absence of appropriate analytical methods or suitable animal model systems. * Recently, a new method was described that quantifies follicular penetration in vivo by using selective closure of hair follicles [5]. * Caffeine is frequently used in skin penetration experiments as a model for highly water-soluble compounds. Occlusion [6] and skin thickness [7] seem to have little influence on the penetration of caffeine. However, percutaneous absorption rates for caffeine exhibit regional skin differences in humans in vivo[1]. * The results of the present study demonstrate that a fast drug delivery of caffeine occurs through shunt routes. Therefore, hair follicles are considerable weak spots in our protective sheath against penetration into the body by hydrophilic substances. * We showed that there is a quantitative distinction between follicular penetration and interfollicular diffusion of caffeine in vivo. * These findings are of importance for the development and optimization of topically applied drugs and cosmetics. In addition, such properties must be considered in the development of skin protection measures. The skin and its appendages are our protective shield against the environment and are necessary for the maintenance of homeostasis. Hypotheses concerning the penetration of substances into the skin have assumed diffusion through the lipid domains of the stratum corneum. It is believed that while hair follicles represent a weakness in the shield, they play a subordinate role in the percutaneous penetration processes. Previous investigation of follicular penetration has mostly addressed methodical and technical problems. Our study utilized a selective closure technique of hair follicle orifices in vivo, for the comparison of interfollicular and follicular absorption rates of caffeine in humans. Every single hair follicle within a delimited area of skin was blocked with a microdrop of a special varnish-wax-mixture in vivo. Caffeine in solution was topically applied and transcutaneous absorption into the blood was measured by a new surface ionization mass spectrometry (SI/MS) technique, which enabled a clear distinction to be made between interfollicular and follicular penetration of a topically applied substance. Caffeine (3.75 ng ml(-1)) was detected in blood samples, 5 min after topical application, when the follicles remained open. When the follicles were blocked, caffeine was detectable after 20 min (2.45 ng ml(-1)). Highest values (11.75 ng caffeine ml(-1)) were found 1 h after application when the follicles were open. Our findings demonstrate that hair follicles are considerable weak spots in our protective sheath against certain hydrophilic drugs and may allow a fast delivery of topically applied substances.

Follicular Penetration of Topically Applied Caffeine via a Shampoo Formulation

Aims: Follicular drug delivery is the prerequisite for an effective treatment of androgenetic alopecia or other reasons of premature hair loss. Methods: The follicular penetration of caffeine, applied topically in a shampoo formulation for 2 min, was measured with highly sensitive surface ionization in combination with mass spectroscopy, a selective method for the detection of very small quantities of transcutaneously absorbed substances in the blood. An experimental protocol, developed to selectively block the follicular pathway within the test area, was used. Based on this principle, a clear distinction between interfollicular and follicular penetration of topically applied caffeine was feasible. Results: After 2 min, caffeine penetrated via the hair follicles and stratum corneum. Conclusion: It was found that the penetration via hair follicles was faster and higher compared with the interfollicular route and that hair follicles are the only pathway for fast caffeine absorption during the first 20 min after application.

Interactions of skin thickness and physicochemical properties of test compounds in percutaneous penetration studies

To determine the effect of skin thickness on the percutaneous penetration and distribution of test compounds with varying physicochemical properties using in vitro systems. Studies were carried out in accordance with OECD guidelines on skin absorption tests. Percutaneous penetration of caffeine (log P -0.01), testosterone (log P 3.32), propoxur (log P 1.52) (finite dose in ethanol to water vehicle ratio) and butoxyethanol (log P 0.83) (undiluted finite dose or as an infinite dose 50% [v/v] aqueous solution) through skin of varying thicknesses under occluded conditions was measured using flow through cells for 8-24 h. Saline (adjusted to pH 7.4) was used as receptor fluid, with BSA added for studies with testosterone and propoxur. Following exposure, the remaining surface dose was removed by swabbing and the skin digested prior to scintillation counting. The maximum flux of caffeine was increased with decreasing skin thickness, although these differences were found to be non-significant. The presence of caffeine in the skin membrane was not altered by skin thickness. Maximum flux and cumulative dose absorbed of testosterone and butoxyethanol (in both finite and infinite doses) were markedly reduced with full thickness (about 1 mm thick) skin compared with split thickness skin (about 0.5 mm). Maximum flux of propoxur (dissolved in 60% ethanol) was clearly higher through skin of 0.71 mm than through skin of 1.36 mm, but no difference was found between 0.56 and 0.71 mm. The proportion of propoxur present in the membrane after 24 h increased significantly over the complete range of thicknesses tested (0.56-1.36 mm). A complex relationship exists between skin thickness, lipophilicity and percutaneous penetration and distribution. This has implications for risk assessment studies and for the validation of models with data from different sources.