Percutaneous penetration of 2-phenoxyethanol through rat and human skin

Penetration pathways, influencing factors and predictive models for dermal absorption of exobiotic molecules: A critical review

This review provides a comprehensive summary of the skin penetration pathways of xenobiotics, including metals, organic pollutants, and nanoparticles (NPs), with a particular focus on the methodologies employed to elucidate these penetration routes. The impacts of the physicochemical properties of exogenous substances and the properties of solvent carriers on the penetration efficiencies were discussed. Furthermore, the review outlines the steady-state and transient models for predicting the skin permeability of xenobiotics, emphasizing the models which enable realistic visualization of pharmaco-kinetic phenomena via detailed geometric representations of the skin microstructure, such as stratum corneum (SC) (bricks and mortar) and skin appendages (hair follicles and sebaceous gland units). Limitations of published research, gaps in current knowledge, and recommendations for future research are highlighted, providing insight for a better understanding of the skin penetration behavior of xenobiotics and associated health risks in practical application contexts.

4-Aminobenzoic acid, 2-phenoxyethanol and iodine used as tracers in a short-term in vivo-kinetics study for tattoo ink ingredients: Mass spectrometry method development and validation

Tattoos have been gaining popularity in recent years, leading to a growing interest in researching tattoo inks and the tattooing process itself. Since the exposure to soluble tattoo ink ingredients has not yet been investigated, we here present the method validation for a short-term biokinetics study on soluble tattoo ink ingredients. The three tracers 4-aminobenzoic acid (PABA), 2-phenoxyethanol (PEtOH) and iodine will be added to commercially available tattoo inks, which will subsequently be used on healthy study participants. Following the tattooing process, blood and urine will be sampled at specific time points and analysed for these tracers. For this purpose, a method using liquid chromatography separation coupled to a quadrupole time-of-flight mass spectrometer (LC-QTOF-MS) in positive and negative ESI mode for the quantification of PABA, PEtOH and selected metabolites and an inductively-coupled plasma (ICP)-MS method for the determination of iodine were developed and validated. For LC-QTOF-MS analysis, the most applicable additives for LC eluents (0.01 % formic acid for positive and 0.005 % acetic acid for negative mode) were identified. Protein precipitation with acetonitrile was chosen for sample preparation. The methods were validated for selectivity, specificity, carryover, linearity, limit of detection (LOD) and quantification (LOQ), matrix effects, accuracy and precision, stability under different conditions and dilution integrity according to national and international guidelines with an allowed maximum variation of ±15 %. The LC-QTOF-MS method met the imposed guideline criteria for most parameters, however, some metabolites showed strong matrix effects. Validation of the ICP-MS method revealed that the KED-H2 collision mode is superior to the standard analysis mode due to enhanced method accuracy. The methods were validated for the relevant matrices plasma, urine, tattoo ink and tattoo consumables and proved to be applicable for the main target substances in the short-term biokinetics study. A proof-of-concept study showed successful quantification of iodine and PABA metabolites. The PEtOH metabolite was also quantified, but showed strong matrix effects in urine. Therefore standard addition was selected as an alternative quantification method.

Differential effects of aquatic anaesthetics on the pharmacokinetics of antibiotics: Examples using florfenicol in Nile tilapia (Oreochromis niloticus)

Anaesthetics are commonly applied in pharmacokinetic (PK) studies to assure smooth handling of experimental procedures or to promote animal welfare. However, the influence of anaesthetics on the PK of co-administered drug is generally unknown but assumes ignorable. The goal of the study was to investigate the effect of tricaine methanesulfonate (MS-222), 2-phenoxyethanol (2-PE) and eugenol (EUG) on the PK of florfenicol (FF) in Nile tilapia. Twenty-eight fish were repeatedly exposed to 90 ppm EUG, 300 ppm MS-222 or 900 ppm 2-PE before FF oral administration (15 mg/kg) and each successive blood sampling. The serum concentration-time profiles were analysed by a 2-compartmental model, and the generated parameters in the control (without anaesthetic) and anaesthetic groups were statistically compared. The results demonstrated that the serum concentrations of each anaesthetic were similar at every FF sampling times (70 μg/ml for MS-222; 277 μg/ml for 2-PE; and 61 μg/ml for EUG). In comparison with the control group, the repeated use of MS-222 did not result in a statistical difference in most of the PK parameters. In contrast, the elimination half-lives of the 2-PE and EUG groups were significantly longer whereas the absorption and distribution half-lives of the 2-PE group were significantly shorter than the control, resulting in altered optimal dosages in the simulation modelling. Whether or not the numbers and extent of PK parameters change mitigate subsequent estimations of other PK-derived secondary values such as dosing regimen and withdrawal time remains to be elucidated, but the auxiliary use of anaesthetics in PK studies should not assume uninfluential.

Pharmacokinetics of 2-phenoxyethanol and its major metabolite, phenoxyacetic acid, after dermal and inhaled routes of exposure: application to development PBPK model in rats

2-Phenoxyethanol (PE), ethylene glycol monophenyl ether, is widely used as a preservative in cosmetic products as well as in non-cosmetics. Since PE has been used in many types of products, it can be absorbed via dermal or inhaled route for systemic exposures. In this study, the pharmacokinetic (PK) studies of PE and its major metabolite, phenoxyacetic acid (PAA), after dermal (30 mg and 100 mg) and inhaled administration (77 mg) of PE in rats were performed. PE was administered daily for 4 days and blood samples were collected at day 1 and day 4 for PK analysis. PE was rapidly absorbed and extensively metabolized to form PAA. After multiple dosing, the exposures of PE and PAA were decreased presumably due to the induction of metabolizing enzymes of PE and PAA. In dermal mass balance study using [¹⁴C]-phenoxyethanol ([¹⁴C]PE) as a microtracer, most of the PE and its derivatives were excreted in urine (73.03%) and rarely found in feces (0.66%). Based on these PK results, a whole-body physiologically-based pharmacokinetic (PBPK) model of PE and PAA after dermal application and inhalation in rats was successfully developed. Most of parameters were obtained from the literatures and experiments, and intrinsic clearance at steady-state (CL_int,ss) were optimized based on the observed multiple PK data. With the developed model, systemic exposures of PE and PAA after dermal application and inhalation were simulated following no-observed-adverse-effect level (NOAEL) of 500 mg/kg/day for dermal application and that of 12.7 mg/kg/day for inhalation provided by the Environmental Protection Agency. The area under the concentration-time curve at steady state (AUC_ss) in kidney and liver (and lung for inhalations), which are known target organs of exhibiting toxicity of PE, as well as AUC_ss in plasma of PE and PAA were obtained from the model.

Safety review of phenoxyethanol when used as a preservative in cosmetics

Phenoxyethanol, or 2-phenoxyethanol, has a large spectrum of antimicrobial activity and has been widely used as a preservative in cosmetic products for decades. It is effective against various Gram-negative and Gram-positive bacteria, as well as against yeasts, and has only a weak inhibitory effect on resident skin flora. According to the European Scientific Committee on Consumer Safety, phenoxyethanol is safe for all consumers - including children of all ages - when used as a preservative in cosmetic products at a maximum concentration of 1%. Adverse systemic effects have been observed in toxicological studies on animals but only when the levels of exposure were many magnitudes higher (around 200-fold higher) than those to which consumers are exposed when using phenoxyethanol-containing cosmetic products. Despite its widespread use in cosmetic products, phenoxyethanol is a rare sensitizer. It can be considered as one of the most well-tolerated preservatives used in cosmetic products.

The application and limitations of mathematical modelling in the prediction of permeability across mammalian skin and polydimethylsiloxane membranes

OBJECTIVES

Predicting the rate of percutaneous absorption of a drug is an important issue with the increasing use of the skin as a means of moderating and controlling drug delivery. One key feature of this problem domain is that human skin permeability (as K(p)) has been shown to be inherently non-linear when mathematically related to the physicochemical parameters of penetrants. As such, the aims of this study were to apply and evaluate Gaussian process (GP) regression methods to datasets for membranes other than human skin, and to explore how the nature of the dataset may influence its analysis.

METHODS

Permeability data for absorption across rodent and pig skin, and artificial membranes (polydimethylsiloxane, PDMS, i.e. Silastic) membranes was collected from the literature. Two quantitative structure-permeability relationship (QSPR) models were used to compare with the GP models. Further performance metrics were computed in terms of all predictions, and a range of covariance functions were examined: the squared exponential (SE), neural network (NNone) and rational quadratic (QR) covariance functions, along with two simple cases of Matern covariance function (Matern3 and Matern5) where the polynomial order is set to 1 and 2, respectively. As measures of performance, the correlation coefficient (CORR), negative log estimated predictive density (NLL, or negative log loss) and mean squared error (MSE) were employed.

KEY FINDINGS

The results demonstrated that GP models with different covariance functions outperform QSPR models for human, pig and rodent datasets. For the artificial membranes, GPs perform better in one instance, and give similar results in other experiments (where different covariance parameters produce similar results). In some cases, the GP predictions for some of the artificial membrane dataset are poorly correlated, suggesting that the physicochemical parameters employed in this study might not be appropriate for developing models that represent this membrane.

CONCLUSIONS

While the results of this study indicate that permeation across rodent (mouse and rat) and pig skin is, in a statistical sense, similar, and that the artificial membranes are poor replacements of human or animal skin, the overriding issue raised in this study is the nature of the dataset and how it can influence the results, and subsequent interpretation, of any model produced for particular membranes. The size of the datasets, in both absolute and comparative senses, appears to influence model quality. Ideally, to generate viable cross-comparisons the datasets for different mammalian membranes should, wherever possible, exhibit as much commonality as possible.

The percutaneous permeation of a combination of 0.1% octenidine dihydrochloride and 2% 2-phenoxyethanol (octenisept®) through skin of different species in vitro

Background

A water based combination of 0.1% octenidine dihydrochloride and 2% 2 - phenoxyethanol is registered in many European countries as an antiseptic solution (octenisept^®) for topical treatment with high antimicrobial activity for human use, but octenidine based products have not been registered for veterinary use yet. The aim of the present study was to investigate whether octenidine dihydrochloride or 2 -phenoxyethanol, the two main components of this disinfectant, permeate through animal skin in vitro. Therefore, permeation studies were conducted using Franz-type diffusion cells. 2 ml of the test compound were applied onto 1.77 cm² split skin of cats, dogs, cows and horses. To simulate wounded skin, cattle skin was treated with adhesive tapes 100 times, as well. Up to an incubation time of 28 hours samples of the acceptor chamber were taken and were analysed by UV-HPLC. Using the method of the external standard, the apparent permeability coefficient, the flux J_max, and the recovery were calculated. Furthermore, the residues of both components in the skin samples were determined after completion of the diffusion experiment.

Results

After 28 hours no octenidine dihydrochloride was found in the receptor chamber of intact skin samples, while 2.7% of the topical applied octenidine dihydrochloride permeated through barrier disrupted cattle skin. 2 - phenoxyethanol permeated through all skin samples with the highest permeability in equine, followed by bovine, canine to feline skin. Furthermore, both components were found in the stratum corneum and the dermis of all split skin samples with different amounts in the examined species.

Conclusion

For 2-phenoxyethanol the systemic impact of the high absorption rate and a potential toxicological risk have to be investigated in further studies. Due to its low absorption rates through the skin, octenidine dihydrochloride is suitable for superficial skin treatment in the examined species.

Absorption of [14C]-tetrabromodiphenyl ether (TeBDE) through human and rat skin in vitro

The skin is the largest organ in the human body and has the potential to come into contact with a variety of xenobiotics both intentionally (e.g., drugs and cosmetics) or accidentally (e.g., agrochemicals and industrial chemicals). These chemicals may then cross the skin barrier (the stratum corneum) and enter into the systemic circulation where they may produce a desired or an undesired effect, or even no systemic effect at all. Tetrabromodiphenyl ether (TeBDE) is one congener in a mixture of polybrominated diphenyl ethers that makes up a flame-retardant commercial product called pentabromodiphenyl ether (PeBDE). TeBDE was used as a surrogate to assess the potential dermal absorption of this product. The physicochemical properties, including lipophilicity, of TeBDE and PeBDE are similar. Operator exposure of PeBDE product to human skin is possible during production and use. However, during these activities, operators wear protective clothing to protect from or minimize exposure. This study was designed to assess the rate and extent of absorption of [14C]-tetrabromodiphenyl ether ([14C]-TeBDE) through human and rat skin in vitro. [14C]-TeBDE was applied to human and rat split thickness skin membranes in vitro in a single test preparation: [14C]-TeBDE in acetone (ca. 20%, w/v). Dermal delivery and absorbed dose of TeBDE applied to human skin was 3.13% (313 microg equiv/cm(2)) and 1.94% (194 microg equiv/cm(2)) of the applied dose, respectively. Dermal delivery and absorbed dose of TeBDE applied to rat skin was 17.94% (1804 microg equiv/cm(2)) and 14.81% (1489 microg equiv/cm(2)) of the applied dose, respectively. These results confirm that the risk of systemic exposure due to external dermal exposure of the PeBDE product is low in the human. Consequently, based on the toxicological profile of these materials, the potential for undesirable effects is also quite low. The results also confirm that the rat is a conservative model overpredicting human absorption about eight fold.

In vitro studies-how good are they at replacing in vivo studies for measurement of skin absorption?

Measures of percutaneous penetration are required for risk assessment of exposure of man to chemicals. In vitro approaches and QSAR predictions can be used and reduce the use of in vivo animal experiments. The OECD Guidelines on in vitro dermal absorption studies were recently accepted but progress was hampered by a lack of direct in vitro/in vivo comparisons in humans or in rodents. Either flow through diffusion or static cell systems with full thickness, dermatomed skin or membranes can be used. In a study of the robustness of in vitro techniques, inter-skin variability was greater than inter-laboratory or between cell variability. Recent studies with a number of chemicals have shown a reasonably good prediction but the difference between in vitro and in vivo results was greater for lipophilic molecules as lipophilic molecules which were retained in the stratum corneum. The experimental flux obtained in vitro using conditions that reflect the potential occupational exposure may be the most appropriate figure for risk assessment purposes. A database of in vitro and in vivo dermal penetration has been established. Dermal absorption data using infinite doses has been combined in a number of databases used for predictive QSAR modelling approaches to dermal absorption. However, absorption values derived from QSAR may over estimate the actual absorption from a finite dose.

Cutaneous metabolism of glycol ethers

The toxicity of glycol ethers is associated with their oxidation to the corresponding aldehyde and alkoxyacetic acid by cytosolic alcohol dehydrogenase (ADH; EC 1.1.1.1.) and aldehyde dehydrogenase (ALDH; 1.2.1.3). Dermal exposure to these compounds can result in localised or systemic toxicity including skin sensitisation and irritancy, reproductive, developmental and haemotological effects. It has previously been shown that skin has the capacity for local metabolism of applied chemicals. Therefore, there is a requirement to consider metabolism during dermal absorption of these compounds in risk assessment for humans. Cytosolic fractions were prepared from rat liver, and whole and dermatomed skin by differential centrifugation. Rat skin cytosolic fractions were also prepared following multiple dermal exposure to dexamethasone, ethanol or 2-butoxyethanol (2-BE). The rate of ethanol, 2-ethoxyethanol (2-EE), ethylene glycol, 2-phenoxyethanol (2-PE) and 2-BE conversion to alkoxyacetic acid by ADH/ALDH in these fractions was continuously monitored by UV spectrophotometry via the conversion of NAD+ to NADH at 340 nm. Rates of ADH oxidation by rat liver cytosol were greatest for ethanol followed by 2-EE >ethylene glycol >2-PE >2-BE. However, the order of metabolism changed to 2-BE >2-PE >ethylene glycol >2-EE >ethanol using whole and dermatomed rat skin cytosolic fractions, with approximately twice the specific activity in dermatomed skin cytosol relative to whole rat skin. This suggests that ADH and ALDH are localised in the epidermis that constitutes more of the protein in dermatomed skin than whole skin cytosol. Inhibition of ADH oxidation in rat liver cytosol by pyrazole was greatest for ethanol followed by 2-EE >ethylene glycol >2-PE >2-BE, but it only inhibited ethanol metabolism by 40% in skin cytosol. Disulfiram completely inhibited alcohol and glycol ether metabolism in the liver and skin cytosolic fractions. Although ADH1, ADH2 and ADH3 are expressed at the protein level in rat liver, only ADH1 and ADH2 are selectively inhibited by pyrazole and they constitute the predominant isoforms that metabolise short-chain alcohols in preference to intermediate chain-length alcohols. However, ADH1, ADH3 and ADH4 predominate in rat skin, demonstrate different sensitivities to pyrazole, and are responsible for metabolising glycol ethers. ALDH1 is the predominant isoform in rat liver and skin cytosolic fractions that is selectively inhibited by disulfiram and responds to the amount of aldehyde formed by the ADH isoforms expressed in these tissues. Thus, the different affinity of ADH and ALDH for alcohols and glycol ethers of different carbon-chain length may reflect the relative isoform expression in rat liver and skin. Following multiple topical exposure, ethanol metabolism increased the most following ethanol treatment, and 2-BE metabolism increased the most following 2-BE treatment. Ethanol and 2-BE may induce specific ADH and ALDH isoforms that preferentially metabolise short-chain alcohols (i.e. ADH1, ALDH1) and longer chain alcohols (i.e. ADH3, ADH4, ALDH1), respectively. Treatment with a general inducing agent such as dexamethasone enhanced ethanol and 2-BE metabolism suggesting induction of multiple ADH isoforms.

Percutaneous penetration and metabolism of 2-butoxyethanol

2-Butoxyethanol (2-BE) is widely used as an industrial solvent, which may result in human dermal exposure within the workplace. This study compares in vivo and in vitro skin absorption of 2-BE using similar application regimes and determines the potential of skin to metabolise this chemical prior to entering the systemic blood circulation. Following topical application of undiluted [1-14C] 2-BE to occluded rat skin in vivo, 28% of the dose was absorbed after 24 h. The major routes of excretion included the urine (19%), expiration as carbon dioxide (6%) and faeces (0.4%) whilst little of the dose remained in the carcass (1.3%). Free 2-BE (0.5%), butoxyacetic acid (8%), glucuronide conjugate (3%), sulphate conjugates (0.7%) and ethylene glycol (0.6%) were detected in urine. Permeation rates of 2-BE through unoccluded rat dermatomed skin (16%) were greater than rat whole skin (8%) whilst absorption through human dermatomed skin (4%) was lower than the rat. Absorption of undiluted 2-BE through occluded rat dermatomed skin in vitro (18%) most accurately predicted absorption through rat skin in vivo. However, 2-BE absorption (23%) was enhanced by application in methanol. Distribution analysis and microautoradiography demonstrated the lack of 2-BE accumulation within the skin in vitro or in vivo. This was reflected in the absence of first pass metabolism of 2-BE during percutaneous penetration through viable human or rat skin in vitro or rat skin in vivo, despite rat skin cytosol having the potential to metabolise 2-BE. In conclusion, the in vitro system provided a reasonable estimate of dermal absorption in vivo for the rat. Therefore, by extrapolation of the comparative in vitro data for human and rat skin in vitro, dermal absorption of 2-BE in man was about one-fifth of that in the rat. However, the rapid penetration through skin in vitro prevented local metabolism and systemic exposure after skin contact with 2-BE in vivo was likely to be to the parent compound. Thus, in vitro skin systems can be used to model dermal absorption of volatile glycol ethers, to predict how much compound enters the circulation and allows the toxicologist to evaluate the body burden of a chemical and potential systemic toxicity.

In vitro predictions of skin absorption of caffeine, testosterone, and benzoic acid: a multi-centre comparison study

To obtain better insight into the robustness of in vitro percutaneous absorption methodology, the intra- and inter-laboratory variation in this type of study was investigated in 10 European laboratories. To this purpose, the in vitro absorption of three compounds through human skin (9 laboratories) and rat skin (1 laboratory) was determined. The test materials were benzoic acid, caffeine, and testosterone, representing a range of different physico-chemical properties. All laboratories performed their studies according to a detailed protocol in which all experimental details were described and each laboratory performed at least three independent experiments for each test chemical. All laboratories assigned the absorption of benzoic acid through human skin, the highest ranking of the three compounds (overall mean flux of 16.54+/-11.87 microg/cm(2)/h). The absorption of caffeine and testosterone through human skin was similar, having overall mean maximum absorption rates of 2.24+/-1.43 microg/cm(2)/h and 1.63+/-1.94 microg/cm(2)/h, respectively. In 7 out of 9 laboratories, the maximum absorption rates of caffeine were ranked higher than testosterone. No differences were observed between the mean absorption through human skin and the one rat study for benzoic acid and testosterone. For caffeine the maximum absorption rate and the total penetration through rat skin were clearly higher than the mean value for human skin. When evaluating all data, it appeared that no consistent relation existed between the diffusion cell type and the absorption of the test compounds. Skin thickness only slightly influenced the absorption of benzoic acid and caffeine. In contrast, the maximum absorption rate of testosterone was clearly higher in the laboratories using thin, dermatomed skin membranes. Testosterone is the most lipophilic compound and showed also a higher presence in the skin membrane after 24 h than the two other compounds. The results of this study indicate that the in vitro methodology for assessing skin absorption is relatively robust. A major effort was made to standardize the study performance, but, unlike in a formal validation study, not all variables were controlled. The variation observed may be largely attributed to human variability in dermal absorption and the skin source. For the most lipophilic compound, testosterone, skin thickness proved to be a critical variable.

Use of 2% 2-phenoxyethanol and 0.1% octenidine as antiseptic in premature newborn infants of 23-26 weeks gestation

In preterm newborn infants, topical iodine-containing antiseptics disturb thyroid hormone regulation while alcohol-based disinfectants may cause local burns. We therefore investigated the use of an aqueous solution containing 0.1% octenidine and 2% 2-phenoxyethanol for skin disinfection during the first seven days of life in premature newborns with a gestational age <27 weeks who were consecutively admitted to our level III neonatal intensive care unit between November 1, 2000 and December 31, 2001 (N=24). In boys. (N=13) the renal excretion of absorbed 2-phenoxyethanol and its metabolite 2-phenoxyacetic acid was quantitated by high-pressure liquid chromatography. In the most immature newborn (gestational age 23 6/7 weeks), a transient erythematous reaction was observed following application of the octenidine/phenoxyethanol solution prior to umbilical vessel catheterization. No other local reactions were observed. The urinary concentration of 2-phenoxyethanol was <2 ppm in all samples, while urinary 2-phenoxyacetic acid concentrations reached 5-95 ppm (median 24 ppm). One infant had a culture-proven septicaemia (Bacillus species) during the first seven days of life. We conclude that, in contrast to alcohol-based antiseptics, an aqueous solution of 0.1% octenidine and 2-phenoxyethanol does not cause major skin damage in premature newborn infants <27 weeks' gestation. 2-Phenoxyethanol is readily absorbed by the newborn's skin but apparently undergoes extensive oxidative metabolization to 2-phenoxyacetic acid.

Percutaneous penetration and metabolism of 2-ethoxyethanol

Percutaneous absorption and cutaneous metabolism of 2-ethoxyethanol were assessed in vivo and with an in vitro flow-through diffusion system. Topical application of undiluted (14)C-ethoxyethanol to occluded rat skin in vivo resulted in 25% of the dose being absorbed after 24 h. The major routes of excretion included the urine (15%), expiration as carbon dioxide (6%), and feces (1.2%), while little of the dose remained in the carcass (1.3%). Free ethoxyethanol, ethoxyacetic acid, and glycine conjugate were detected in urine. Permeation rates of ethoxyethanol through unoccluded rat split skin (20%) were greater than rat whole skin (11%), while absorption through human split skin (8%) was lower than the rat. Absorption of undiluted ethoxyethanol through occluded rat split skin in vitro (22%) most accurately predicted absorption through rat skin in vivo. However, ethoxyethanol absorption (29%) was enhanced by application in methanol. First pass metabolism of ethoxyethanol was not detected during percutaneous penetration through viable human or rat skin in vitro or rat skin in vivo. However, rat skin cytosol had the potential to metabolize ethoxyethanol, suggesting that the rapid penetration through skin in vivo prevented metabolism and that systemic exposure after skin contact with 2-ethoxyethanol is likely to be to the parent compound. In conclusion, the in vitro system provided a reasonable estimate of dermal absorption for the rat in vivo and comparison of human and rat skin in vitro indicated 2-ethoxyethanol absorption in humans is about one-third of that in the rat.

Percutaneous penetration and dermal metabolism of triclosan (2,4, 4'-trichloro-2'-hydroxydiphenyl ether)

triclosan is widely used in many products that contact the skin of consumers. This study compares in vivo and in vitro skin absorption of triclosan and determines the potential of skin to metobolize it prior to entering the blood stream. After in vivo topical application of a 64.5mM alcoholic solution of [(3)H]triclosan to rat skin, 12% radioactivity was recovered in the faeces, 8% in the carcass 1% in the urine, 30% in the stratum corneum and 26% was rinsed from the skin surface at 24 hours after application. Free triclosan and the glucuronide and sulfate conjugates of triclosan were found in urine and faeces. triclosan penetrated rat skin more rapidly and extensively than human skin in vitro. 23% of the dose had penetrated completely through rat skin into the receptor fluid by 24 hours, whereas penetration through human skin was only 6.3% of the dose. Chromatographic analysis of the receptor solutions showed that triclosan was metabolized to the glucuronide, and to a lesser extent to the sulfate, during passage through the skin. triclosan glucuronide appeared rapidly in the receptor fluid whereas triclosan sulfate remained in the skin. Although the major site of metabolism was the liver, conjugation of triclosan in skin was also demonstrated in vitro and in vivo, particularly to the glucuronide conjugate which was more readily removed from the skin. The in vitro system provides a reasonable estimate of dermal absorption in vivo for the rat. Therefore by extrapolation of the comparative in vitro data for human and rat skin it is reasonable to deduce that dermal absorption in human of triclosan applied at the same dose is about one-third of that in the rat in vivo.

Percutaneous penetration studies for risk assessment

During the last few years the general interest in the percutaneous absorption of chemicals has increased. It is generally accepted that there is very few reliable quantitative and qualitative data on dermal exposure to chemicals in the general population and in occupationally exposed workers. In order to predict the systemic risk of dermally absorbed chemicals and to enable agencies to set safety standards, data is needed on the rates of percutaneous penetration of important chemicals. Standardization of in vitro tests and comparison of their results with the in vivo data could produce internationally accepted penetration rates and/or absorption percentages very useful for regulatory toxicology. The work of the Percutaneous Penetration Subgroup of EC Dermal Exposure Network has been focussed on the standardization and validation of in vitro experiments, necessary to obtain internationally accepted penetration rates for regulatory purposes. The members of the Subgroup analyzed the guidelines on percutaneous penetration in vitro studies presented by various organizations and suggested a standardization of in vitro models for percutaneous penetration taking into account their individual experiences, literature data and guidelines already in existence. During the meetings of Percutaneous Penetration Subgroup they presented a number of short papers of up to date information on the key issues. The objective was to focus the existing knowledge and the gaps in the knowledge in the field of percutaneous penetration. This paper is an outcome of the meetings of the Percutaneous Penetration Subgroup and reports the presentations on the key issues identified throughout the 3-year duration of the Dermal Exposure Network (1997-1999).

Diffusion modeling of percutaneous absorption kinetics. 1. Effects of flow rate, receptor sampling rate, and viable epidermal resistance for a constant donor concentration

A diffusion model for the percutaneous absorption of a solute through the skin is developed for the specific case of a constant donor concentration with a finite removal rate from the receptor due to either perfusion rate or sampling. The model has been developed to include a viable epidermal resistance and a donor-stratum corneum interfacial resistance. Numerical inversion of the Laplace domain solutions were used for simulations of solute flux and cumulative amount absorbed and to model specific examples of percutaneous absorption. Limits of the Laplace domain solutions were used to define the steady-state flux, lag time, and receptor concentration. Steady-state approximations obtained from the solutions were used to relate the steady-state flux and the effective permeability coefficient to the viable epidermis resistance, a donor-stratum corneum interfacial resistance, receptor removal rate, and partitioning between the receptor and donor phases. The lag time was shown to be dependent on these parameters and on the volume of the receptor phase. It is concluded that curvilinear cumulative amount and flux-time profiles are dependent on the processes affecting percutaneous absorption, the shapes of the profiles reflecting the processes most determining transport.

Conceptual model for assessment of dermal exposure

Dermal exposure, primarily to pesticides, has been measured for almost half a century. Compared with exposure by inhalation, limited progress has been made towards standardisation of methods of measurement and development of biologically relevant exposure measures. It is suggested that the absence of a consistent terminology and a theoretical model has been an important cause of this lack of progress. Therefore, a consistent terminology based on a multicompartment model for assessment of dermal exposure is proposed that describes the transport of contaminant mass from the source of the hazardous substance to the surface of the skin. Six compartments and two barriers together with eight mass transport processes are described. With the model structure, examples are given of what some existing methods actually measure and where there are limited, or no, methods for measuring the relevant mass in a compartment or transport of mass. The importance of measuring the concentration of contaminant and not mass per area in the skin contaminant layer is stressed, as it is the concentration difference between the skin contamination layer and the perfused tissue that drives uptake. Methods for measuring uptake are currently not available. Measurement of mass, concentration, and the transport processes must be based on a theoretical model. Standardisation of methods of measurement of dermal exposure is strongly recommended.