Molecular size as the main determinant of solute maximum flux across the skin

In silico prediction of dermal penetration rate of chemicals from their molecular structural descriptors

The dermal penetration rate of some volatile and non-volatile organic compounds was estimated by quantitative structure-activity relationship approaches by using interpretable molecular descriptors. Linear and nonlinear models were developed using multiple linear regressions (MLR) and artificial neural network (ANN) methods. Robustness and reliability of the constructed MLR and ANN models were evaluated by using the leave-one-out cross-validation method, which produces the statistics of Q(MLR)(2)=0.786, Q( ANN)(2)=0.833 for non-volatiles and Q(MLR)(2)=0.639, Q( ANN)(2)=0.712 for volatile compounds. Furthermore, the chemical applicability domains of these models were determined via leverage approach. The results of this study indicated the ability of developed QSAR models in the prediction of dermal penetration rate of various chemicals from their calculated molecular descriptors.

The comparability of in vitro and ex vivo studies on the percutaneous permeation of topical formulations containing Ibuprofen

In order to avoid in vivo experiments and to gain information about the suitability of surrogates for skin replacement, Franz-type diffusion cell experiments were conducted by using three ibuprofen-containing formulations (cream, gel and microgel) on bovine split-skin samples and cellophane membranes. Moreover, ex vivo examinations were performed on the isolated perfused bovine udder, to study the comparability of in vitro and ex vivo experimental set-ups. Depending on the formulation, noticeable differences in the permeation of Ibuprofen occurred in vitro (udder skin) and ex vivo (isolated perfused bovine udder), but not in the cellophane membrane. The rates of ibuprofen permeability (cream > gel > microgel) and adsorption into the skin (gel > microgel > cream) varied with the formulation, and were probably caused by differences in the ingredients. Furthermore, different storage conditions and seasonal variation in the collection of the skin samples probably led to differences in the amounts of ibuprofen adsorption apparent in the isolated bovine udder and udder skin. In vitro diffusion experiments should be preferred to experiments on isolated organs with regard to the costs involved, the throughput, and the intensity of labour required, unless metabolism of the drug in the skin, or cell-cell interactions are of particular interest.

Estimation of maximum transdermal flux of nonionized xenobiotics from basic physicochemical determinants

An ability to estimate the maximum flux of a xenobiotic across skin is desirable from the perspective of both drug delivery and toxicology. While there is an abundance of mathematical models describing the estimation of drug permeability coefficients, there are relatively few that focus on the maximum flux. This article reports and evaluates a simple and easy-to-use predictive model for the estimation of maximum transdermal flux of xenobiotics based on three common molecular descriptors: logarithm of octanol-water partition coefficient, molecular weight and melting point. The use of all three can be justified on the theoretical basis of their influence on the solute aqueous solubility and the partitioning into the stratum corneum lipid domain. The model explains 81% of the variability in the permeation data set composed of 208 entries and can be used to obtain a quick estimate of maximum transdermal flux when experimental data is not readily available.

The effect of lipid and aqueous solubilities on flux of nicotinic acid esters from water through silicone membrane

OBJECTIVE

The maximum fluxes (J(M)) of nicotinic acid esters (NAE) across silicone membranes from water (J(MPAQ)) have been measured to determine how well they correlate with J(M) of NAE across human skin from water in vitro (J(MHAQ)) and in vivo (J(MHAQ1)) and with J(M) of NAE across hairless mouse skin from water (J(MMAQ)).

MATERIALS AND METHODS

The NAE were all commercially available. Solubilities in water (S(AQ)), isopropyl myristate (S(IPM)) and octanol (S(OCT)) were obtained from literature sources. J(MPAQ) were measured at saturation for all the esters except the methyl ester. In that case, flux was measured at a concentration (C) less than saturation (J(PAQ)) and converted to J(MPAQ) = (J(PAQ))(S(AQ)/C(AQ)).

RESULTS AND DISCUSSION

J(MPAQ) values predicted from the previously reported coefficients to the parameters in the Roberts-Sloan (RS) equation (PRE J(MPAQ)) were substantially lower than the experimental J(MPAQ) values (EXP J(MPAQ)) values obtained here. The EXP J(MPAQ) were incorporated into the previous J(MPAQ) database and new coefficients were obtained: x = -1.837; y = 0.742; z = 0.00435; r² = 0.86. Correlation of J(MPAQ) values with J(MHAQ), J(MHAQ1), and J(MMAQ) values show the same trend as the J(MPAQ) values.

CONCLUSIONS

The inclusion of the NAE n = 6 data into the previous n = 32 database for the permeation of the prodrugs through a silicone membrane from water (J(MPAQ)) greatly improved the fit of the n = 38 database to the RS equation: r² = 0.86 vs r² = 0.77. The correlation between log J(MHAQ) and log J(MPAQ) gave r² = 0.98. This suggests that J(MPAQ) values are good predictors of J(MHAQ) values.

An evaluation of the potential of linear and nonlinear skin permeation models for the prediction of experimentally measured percutaneous drug absorption

Objectives

The developments in combinatorial chemistry have led to a rapid increase in drug design and discovery and, ultimately, the production of many potential molecules that require evaluation. Hence, there has been much interest in the use of mathematical models to predict dermal absorption. Therefore, the aim of this study was to test the performance of both linear and nonlinear models to predict the skin permeation of a series of 11 compounds.

Methods

The modelling in this study was carried out by the application of both quantitative structure permeability relationships and Gaussian process-based machine learning methods to predict the flux and permeability coefficient of the 11 compounds. The actual permeation of these compounds across human skin was measured using Franz cells and a standard protocol with high performance liquid chromatography analysis. Statistical comparison between the predicted and experimentally-derived values was performed using mean squared error and the Pearson sample correlation coefficient.

Key findings

The findings of this study would suggest that the models failed to accurately predict permeation and in some cases were not within two- or three-orders of magnitude of the experimentally-derived values. However, with this set of compounds the models were able to effectively rank the permeants.

Conclusions

Although not suitable for accurately predicting permeation the models may be suitable for determining a rank order of permeation, which may help to select candidate molecules for in-vitro screening. However, it is important to note that such predictions need to take into account actual relative drug candidate potencies.

Liquid-crystalline membrane permeation ability for selected nitro hair dyes

AIM

A comparison of permeation ability of selected semi-permanent hair dyes and an attempt to estimate the influence of fundamental physicochemical parameters on dyes' epidermal penetration rate.

METHODS

Dyes' permeation ability through liposome membrane (as a model of stratum corneum) with side-by-side cells was assessed.

RESULTS

It has been shown that the chosen dyes are capable of permeating the membrane. High penetration coefficients (Kp) were obtained for a simple nitrophenylenediamines and nitroaminophenols. Their N-, O-hydroxyalkyl substitution significantly limits penetration. H-bonding capability has a major impact on the investigated dyes' permeation ability. Substituents with H-bonding properties can significantly limit dyes' penetration, even in the case of lipophilic structures. Special attention should be placed into compounds with strong intramolecular H-bonding properties, which improve transmembrane transport.

CONCLUSION

Substitution patterns have an influence on selected nitro dyes' permeation through a model stratum corneum. Permeation is limited by dyes diffusive properties (mostly by its H-bonding properties). Hydroxyalkylation results in hindered dyes permeation: purple, violet and blue nitrophenylenediamine or nitroaminophenol derivatives are less permeable than its not substituted analogues.

The effect of formulation vehicles on the in vitro percutaneous permeation of ibuprofen

Background

The transdermal application of substances represents an elegant approach to overcome side effects related to injections or oral treatment. Due to benefits like a constant plasma level, no pain during application and a simple therapeutic regime, the optimization of formulations for transdermal drug delivery has gained interest in the last decades. Ibuprofen is a non-steroidal anti-inflammatory compound which is nowadays often used transdermally. The objective of this work was to conduct a study on the effect of different 5% ibuprofen containing formulations (Ibutop^® cream, Ibutop^® gel, and ibuprofen solution in phosphate buffered saline) on the in vitro-percutaneous permeation of ibuprofen through skin to emphasise the importance of the formulation on percutaneous permeation and skin reservoir.

Methods

The permeation experiments were conducted in Franz-type diffusion cells according to OECD guideline 428 with 2 mg/cm² ibuprofen formulation on each skin sample. Ibuprofen was analysed in the receptor fluid and extracted skin samples by UV-VIS high-performance liquid-chromatography at 238 nm. The plot of the cumulative amount of ibuprofen permeated versus time was employed to calculate the apparent permeability coefficient, the maximum flux and the lagtime, all of which were statistically analysed by One-way ANOVA.

Results

Although ibuprofen permeation out of the gel increases rapidly within the first four hours, the cream produced the highest ibuprofen delivery through the skin within 28 hours, followed by the solution and the gel. A significant shorter lagtime was found after gel treatment compared with the cream and the solution. After 28 hours 59% of the applied ibuprofen was found in the receptor fluid of the cream treated samples, 26% in the solution treated samples and 21% in the samples treated with the gel. Fourfold higher ibuprofen reservoirs were found in the solution and gel treated skin samples compared to the cream treated skin samples.

Conclusion

The present study demonstrates the importance of the formulation on transdermal drug delivery of ibuprofen and emphasises the differences of drug storage within the skin due to the formulation. Thus, it is a mistaken assumption that formulations comprising the same drug amount are equivalent regarding skin permeability.

Mechanism and determinants of nanoparticle penetration through human skin

The ability of nanoparticles to penetrate the stratum corneum was the focus of several studies. Yet, there are controversial issues available for particle penetration due to different experimental setups. Meanwhile, there is little known about the mechanism and determinants of their penetration. In this paper the penetration of four model gold nanoparticles of diameter 6 and 15 nm, differing in surface polarity and the nature of the vehicle, through human skin was studied using multiphoton microscopy. This is in an attempt to profoundly investigate the parameters governing particle penetration through human skin. Our results imply that nanoparticles at this size range permeate the stratum corneum in a similar manner to drug molecules, mainly through the intercellular pathways. However, due to their particulate nature, permeation is also dependent on the complex microstructure of the stratum corneum with its tortuous aqueous and lipidic channels, as shown from our experiments performed using skin of different grades of barrier integrity. The vehicle (toluene-versus-water) had a minimal effect on skin penetration of gold nanoparticles. Other considerations in setting up a penetration experiment for nanoparticles were also studied. The results obtained are important for designing a new transdermal carrier and for a basic understanding of skin-nanoparticle interaction.

Nanoparticles and their interactions with the dermal barrier

The dermal application of drugs is promising due to the ease of application. In this context nano-scale carrier systems were already evaluated in several studies with respect to the skin interaction and the impact on drug penetration. At the same time the upcoming production of engineered nano-scale materials requires a thorough safety evaluation. Drug delivery as well as risk assessment depends crucially on the ability of such carriers to overcome the skin barrier and reach deeper tissue layers. Therefore, the interaction of nanoparticles with skin and especially skin models is an intriguing field. However, the data obtained do not show a clear image on the effect of nano-carriers. Especially the penetration of such particles is an open and controversially discussed topic. The literature reports different results mainly on pig or murine skin showing strong penetration (pig and mouse) or the opposite. Looking only at the sizes of the particles also no conclusive picture can be obtained. Nevertheless, size is regarded to play an important role for skin penetration. Furthermore, the state of the skin influences penetration (hydration) and the mechanical stress is of outmost importance.

Three-dimensional skin models as tools for transdermal drug delivery: challenges and limitations

INTRODUCTION

Transdermal drug delivery has several known advantages over the oral route and hypodermic injections. The number of drugs that can be taken up transdermally is, however, limited owing to the innate barrier function of the skin. New transdermal drug candidates need to be tested extensively before being used on humans. In this regard, in vitro permeation methods are highly important to predict in vivo permeation of drugs.

AREAS COVERED

This review illustrates how different types of reconstructed skin models are being used as alternatives to human and pig skin for in vitro permeation testing of drugs. Insights into how various factors (including the physicochemical nature of molecules and formulations) or skin properties might affect the permeability of drugs in reconstructed skin models are provided. Also, opportunities and pitfalls of reconstructed skin models are highlighted.

EXPERT OPINION

Many studies have revealed that the permeability of reconstructed skin models is much higher compared with human excised skin. This is in accordance with the incomplete barrier found in these models. Nevertheless, the reconstructed skin models available today are useful tools for estimating the rank order of percutaneous absorption of a series of compounds with different physicochemical properties. A major challenge in the further development of reconstructed skin models for drug delivery studies is to obtain a barrier function similar to in vivo skin. Whether this goal will be achieved in the near future is uncertain and will be, in the authors' opinion, a very difficult task.

Modern management of mild-to-moderate joint pain due to osteoarthritis: a holistic approach

Osteoarthritis (OA) is the most common form of arthritis and it is often associated with significant disability and impaired quality of life. Once thought to be caused by an age-related 'wearing out' of articular cartilage, it is now recognized to be a dynamic process in which cartilage degradation alternates with repair. Several expert guidelines for the management of OA exist, which concur in their recommendations for a stepwise approach to the employment of pharmacological agents and the introduction of suggestions to extend the use of agents such as topical non-steroidal anti-inflammatory drugs, especially for mild-to-moderate forms of the disease. They also emphasize the importance of non-pharmacological measures, such as nutraceuticals, education, diet, exercise and the use of aids in improving signs and symptoms and slowing progression. In many countries, effective medicinal and nutraceutical agents are available 'over-the-counter'. This review explains the modern approach to the management of mild-to-moderate osteoarthritic pain.

Drug structure-transport relationships

Malcolm Rowland has greatly facilitated an understanding of drug structure–pharmacokinetic relationships using a physiological perspective. His view points, covering a wide range of activities, have impacted on my own work and on my appreciation and understanding of our science. This overview summarises some of our parallel activities, beginning with Malcolm’s work on the pH control of amphetamine excretion, his work on the disposition of aspirin and on the application of clearance concepts in describing the disposition of lidocaine. Malcolm also spent a considerable amount of time developing principles that define solute structure and transport/pharmacokinetic relationships using in situ organ studies, which he then extended to involve the whole body. Together, we developed a physiological approach to studying hepatic clearance, introducing the convection–dispersion model in which there was a spread in blood transit times through the liver accompanied by permeation into hepatocytes and removal by metabolism or excretion into the bile. With a range of colleagues, we then further developed the model and applied it to various organs in the body. One of Malcolm’s special interests was in being able to apply this knowledge, together with an understanding of physiological differences in scaling up pharmacokinetics from animals to man. The description of his many other activities, such as the development of clearance concepts, application of pharmacokinetics to the clinical situation and using pharmacokinetics to develop new compounds and delivery systems, has been left to others.

Synthesis and transdermal permeation of novel N4-methoxypoly(ethylene glycol) carbamates of cytarabine

BACKGROUND

Cytarabine is a deoxycytidine analogue commonly used in the treatment of hematological malignant diseases. Its clinical utility, however, is severely limited by its short plasma half-life because of the catabolic action of nucleoside deaminases.

METHOD

In this study, N(4)-carbamate derivatives of cytarabine (1) were synthesized and evaluated for transdermal penetration because this mode of administration may circumvent its limitations. The synthesis of these compounds was achieved in a two-step process. First, the methoxypoly(ethylene glycol) was activated by p-nitrophenyl chloroformate. Second, the activated intermediates were reacted with cytarabine in the presence of N-hydroxysuccinamide to give the N(4)-methoxypoly(ethylene glycol) carbamate derivatives. The transdermal flux values of the N(4)-carbamates of cytarabine were determined in vitro by Franz diffusion cell methodology. Aqueous solubility and log D (pH 7.4) values were determined and assessed for correlation with transdermal flux values.

RESULTS

The synthesized carbamates, particularly, (9)-(13), showed increased solubility in both aqueous and lipid media. Log D values decreased as the oxyethylene chain lengthened.

CONCLUSION

Although none of the derivatives showed significantly higher transdermal penetration than cytarabine (1), it should be mentioned that the mean for cytarabine N(4)-methoxyethyleneoxycarbamate (8) was 10 times higher and the median was 2 times higher.

Human skin permeation of 3-O-alkyl carbamate prodrugs of naltrexone

N-Monoalkyl and N,N-dialkyl carbamate prodrugs of naltrexone (NTX), an opioid antagonist, were synthesized and their in vitro permeation across human skin was determined. Relevant physicochemical properties were also determined. Most prodrugs exhibited lower melting points, lower aqueous solubilities, and higher oil solubilities than NTX. The flux values from N-monoalkyl carbamate prodrugs were significantly higher than those from NTX and N,N-dialkyl carbamates. The melting points of N-monoalkyl carbamate prodrugs were quite low compared to the N,N-dialkyl carbamate prodrugs and NTX. Heats of fusion for the N,N-dialkyl carbamate prodrugs were higher than that for NTX. N-Monoalkyl carbamate prodrugs had higher stratum corneum/vehicle partition coefficients than their N,N-dialkyl counterparts. Higher percent prodrug bioconversion to NTX in skin appeared to be related to increased skin flux. N,N-Dialkyl carbamate prodrugs were more stable in buffer and in plasma than N-monoalkyl carbamate prodrugs. In conclusion, N-monoalkyl carbamate prodrugs of NTX improved the systemic delivery of NTX across human skin in vitro. N,N-Dialkyl substitution in the prodrug moiety decreased skin permeation and plasma hydrolysis to the parent drug. The cross-sectional area of the carbamate head group was the major determinant of flux of the N-monoalkyl and N,N-dialkyl carbamate prodrugs of NTX.

An investigation into the influence of binary drug solutions upon diffusion and partition processes in model membranes

Few studies have assessed the impact of binary systems on the fundamental mathematical models that describe drug permeation. The aim of this work was to determine the influence of varying the proportions of prilocaine and lidocaine in a binary saturated solution on mass transfer across synthetic membranes. Infinite-dose permeation studies were performed using Franz diffusion cells with either regenerated cellulose or silicone membranes, and partition coefficients were determined by drug loss over 24 h. There was a linear relationship between the flux of prilocaine and lidocaine through regenerated cellulose membrane (R2 ≥ 0.985, n = 5) and their normalised ratio in solution. This linear model was also applicable for the permeation of prilocaine through silicone membrane (R2 = 0.991, n = 5), as its partition coefficient was independent of the drug ratio (15.84 ± 1.41). However, the partition coefficient of lidocaine increased from 27.22 ± 1.68 to 47.03 ± 3.32 as the ratio of prilocaine increased and this resulted in a non-linear relationship between permeation and drug ratio. Irrespective of the membrane used, the permeation of one drug from a binary system was hindered by the presence of the second, which could be attributed to a reduction in available membrane diffusion volume.

In-vitro and in-vivo evaluation of oligoethylene esters as dermal prodrugs of 18β-glycyrrhetic acid

Novel polyoxyethylene esters of 18 β-glycyrrhetic acid (GA) were synthesized and evaluated as potential dermal prodrugs. The permeation of these prodrugs (1a-e) was studied in-vitro, using excised human skin membranes (SCE; stratum corneum/epidermis) mounted in Franz type cells, and in-vivo, evaluating the ability of these compounds to inhibit methyl nicotinate (MN)-induced skin erythema in healthy human subjects. All the esters synthesized showed a good water stability, while the enzymatic hydrolysis rate was significantly affected by the length of the polyoxyethylenic chain used as promoiety. In in-vitro percutaneous absorption studies, only esters 1b and 1c (respectively triethylen- and tetraethylenglycol derivatives) showed an increased flux through SCE membranes compared with GA. Furthermore, we observed an appreciable and sustained in-vivo topical anti-inflammatory activity of esters 1b and 1c compared with the parent drug.

Design, synthesis and characterization of captopril prodrugs for enhanced percutaneous absorption

Most drugs are designed primarily for oral administration, but the activity and stability profiles desirable for this route often make them unsuitable for transdermal delivery. We were therefore interested in designing analogues of captopril, a model drug with poor percutaneous penetration, for which the sustained steady-state blood plasma level associated with transdermal delivery (and which is unattainable orally) would be particularly beneficial. Quantitative structure—permeability relationships (QSPRs) predicted that ester and thiol prodrug derivatives of captopril would have lower maximal transdermal flux (Jm) than the parent drug, since the increases in permeability coefficient (kp) of prodrugs would be outweighed by the reductions in aqueous solubility. Therefore, the aim of this study was to synthesize a series of prodrugs of captopril and to determine if a QSPR model could be used to design therapeutically viable prodrugs. Molecules with the highest predicted kp values were synthesized and characterized, and Jm measured in Franz diffusion cells from saturated aqueous donor across porcine skin (fresh and frozen). In-vitro metabolism was also measured. Captopril and the prodrugs crossed the skin relatively freely, with Jm being highest for ethyl to butyl esters. Substantial first-order metabolism of the prodrugs was observed, suggesting that their enhanced percutaneous absorption was complemented by their metabolic performance. The results suggested that QSPR models provided excellent enhancements in drug delivery. This was not seen at higher lipophilicities, suggesting that issues of solubility need to be considered in conjunction with any such use of a QSPR model.

Transdermal behaviour of the N-alkylamide spilanthol (affinin) from Spilanthes acmella (Compositae) extracts

AIM OF THE STUDY

N-Alkylamides are a large group of bioactive molecules found in several plants from the genera Echinacea, Xanthoxylum and Spilanthes. Extracts and formulations derived from these plants are not only orally used, but also applied on the skin as well. However, there is currently no specific information available about the intrinsic local pharmacokinetics of N-alkylamides after topical application on human skin, questioning the role of this mode of administration. The present study investigates the transdermal behaviour of spilanthol, a prominent N-alkylamide.

MATERIALS AND METHODS

Two pharmaceutically accepted dose solutions (ethanol and propylene glycol based aqueous donor vehicles), combined with three different receptor fluids (PBS, PBS+0.5% HPbetaCD, EtOH/H(2)O (30:70, v/v)), were applied on split-thickness human skin in a Franz diffusion cell (FDC) system. Fundamental permeation characteristics of spilanthol in a solvent-independent way (100% aqueous dose solution) were also obtained using an extrapolation approach with different organic solvent/H(2)O ratios.

RESULTS AND CONCLUSIONS

We demonstrated for the first time that spilanthol permeates the skin. The following aqueous-extrapolated primary transdermal parameters were obtained (mean+/-SEM): K(p,aq)=3.31 (+/-0.29)x10(-3)cm/h, D(m,aq)=1.86 (+/-0.09)x10(-4)cm(2)/h and K(m,aq)=7.28 (+/-1.59)x10(-1). Partitioning (K(m)) was strongly dependent on the donor solution composition, while diffusion (D(m)) was mainly influenced by the receptor fluid composition.

Efficacy and side effects of diclofenac patch in treatment of patients with myofascial pain syndrome of the upper trapezius

Locally administered nonsteroidal anti-inflammatory drugs have been widely used in acute soft-tissue damage and articular musculoskeletal pain. This double-blind, placebo-controlled, randomized study was designed to evaluate the efficacy and safety of a topical diclofenac sodium patch in the relief of pain and inflammation as a result of myofascial pain syndrome (MPS) in the upper trapezius. After sample size calculations indicated that 147 patients would be needed to detect a 25% difference between drug and control, 153 patients with MPS were recruited and randomized to receive either a diclofenac sodium patch or control (menthol) patch. Visual analog scale (VAS), cervical active range of motion, pressure pain threshold of the myofascial trigger point (MTrP), patient global assessment, Neck Disability Index, and the occurrence of adverse events were assessed on Day 0 (baseline), Day 4, and Day 8. Use of the diclofenac sodium patch elicited favorable responses for the VAS, cervical active range of motion, and Neck Disability Index by the end of the treatment course (P<0.05), and was consistently superior to the control patch at all time intervals. No significant differences were observed for the pressure pain threshold of the MTrP for either patch. Tolerability assessment similarly showed the diclofenac patch to be comparatively superior. When assessed at the end of the study, 20 diclofenac patch patients, but only four control patients, considered the tolerability of treatment to be "very good." Significant differences in adverse reactions were observed between the diclofenac and control patches, with the control patch more likely to produce overall skin irritation. This study demonstrate that the diclofenac sodium patch was superior to the control patch in terms of reducing pain and improving functional outcomes, and did not result in significant adverse effects.

Metabolism of fatty acids and lipid hydroperoxides in human body monitoring with Fourier transform Infrared Spectroscopy

BACKGROUND

The metabolism of dietary fatty acids in human has been measured so far using human blood cells and stable-isotope labeled fatty acids, however, no direct data was available for human peripheral tissues and other major organs. To realize the role of dietary fatty acids in human health and diseases, it would be eager to develop convenient and suitable method to monitor fatty acid metabolism in human.

RESULTS

We have developed the measurement system in situ for human lip surface lipids using the Fourier transform infrared spectroscopy (FTIR) - attenuated total reflection (ATR) detection system with special adaptor to monitor metabolic changes of lipids in human body. As human lip surface lipids may not be much affected by skin sebum constituents and may be affected directly by the lipid constituents of diet, we could detect changes of FTIR-ATR spectra, especially at 3005 to approximately 3015 cm(-1), of lip surface polyunsaturated fatty acids in a duration time-dependent manner after intake of the docosahexaenoic acid (DHA)-containing triglyceride diet. The ingested DHA appeared on the lip surface and was detected by FTIR-ATR directly and non-invasively. It was found that the metabolic rates of DHA for male volunteer subjects with age 60s were much lower than those with age 20s. Lipid hydroperoxides were found in lip lipids which were extracted from the lip surface using a mixture of ethanol/ethylpropionate/iso-octane solvents, and were the highest in the content just before noon. The changes of lipid hydroperoxides were detected also in situ with FTIR-ATR at 968 cm(-1).

CONCLUSION

The measurements of lip surface lipids with FTIR-ATR technique may advance the investigation of human lipid metabolism in situ non-invasively.

A simple dermal absorption model: derivation and application

This paper deals with the derivation of a QSAR for the estimation of: the skin permeation coefficient from aqueous solutions in cm h(-1), the stratum corneum/water partition coefficient. These QSARs enable the estimation of: the aqueous permeation coefficient in cm h(-1), the maximum dermal absorption in mg cm(-2) h(-1) from a saturated aqueous solution at steady state, the lag time in hours (h), the diffusivity of a substance in the stratum corneum in cm2 h(-1). By using the independent variables: the log(octanol/water partition coefficient), the molecular weight,the water solubility. The estimated maximum dermal absorption and the lag time were compared with some recent measured data of substances, which were not used for developing the QSARs. The estimates were generally in the same order of magnitude as the measured absorption and lag time. These QSARs are recommended for risk assessment of chemicals in the scope of the European REACH legislation.

A comparison of the fit of flux through hairless mouse skin from water data to three model equations

Data for the delivery of total species containing parent drugs from water through hairless mouse skin by prodrugs, logJ(MMAQ), has been fitted to the Roberts-Sloan, RS, the Kasting-Smith-Cooper, KSC, and Magnusson-Anissimov-Cross-Roberts, MACR, equations. The RS model which contains a parameter for the dependence of flux on solubility in water, S(AQ), as well as solubility in the lipid isopropyl myristate, S(IPM), gave the best fit: logJ(MMAQ)=-2.30+0.575 logS(IPM)+0.425 logS(AQ)-0.0016MW, r(2)=0.903. The values for the coefficients to the parameters are quite similar to those obtained when the RS model was fit to flux of solutes from water through human skin, logJ(MHAQ). There was no trend in predicting the under or over-performance of prodrugs based on their fit to the RS model and whether they were more or less soluble than their parent drugs. There was an inverse dependence of logJ(MMAQ) on partition coefficients or permeability coefficients similar to that observed for logJ(MHAQ). The similarities in trends for results for logJ(MMAQ) and logJ(MHAQ) suggests that design directives obtained from mouse skin can be extended to design new prodrugs or select new drugs for delivery through human skin.

The usual suspects-influence of physicochemical properties on lag time, skin deposition, and percutaneous penetration of nine model compounds

The influence of physicochemical properties of nine model compounds on lag time, skin deposition, and percutaneous penetration was evaluated. Static diffusion cells mounted with human skin were used as the experimental model, and experiments were carried out in accordance with Organization for Economic Cooperation and Development (OECD) guidelines. The model compounds were chosen to cover a wide spectrum of solubilities and molecular weights. The pesticides included were glyphosate, dimethoate, pirimicarb, malathion, paclobutrazol, methiocarb, prochloraz, and benzoic acid, with the ninth model compound being caffeine. The fastest dermal penetration was observed for compounds with log Pow values between 1.5 and 4. Malathion did not fit into this generalization. No clear relationship was observed between molecular weight and Kp values. The shortest lag time was observed for the most hydrophilic model compounds. With increasing molecular weight, the lag time rose. Thus, the lag time for the smallest model compound was close to 1.5 h, while the lag time exceeded 20 h for a model compound with a molecular weight of 377 g. A difference in lag time of this magnitude inevitably produces differences in the amounts of a chemical able to penetrate the skin within a limited period of time. The relative deposition in the skin was highest for the lipophilic model compounds. For log Pow values between -1 and 2, a linear relationship was observed between log Pow and log Kp. Comparisons between theoretical Kp values based on the Potts-Guy equation and experimental Kp values demonstrated good agreement. Experimental Kp values based on experiments using full-thickness human skin generally underestimated the calculated Kp values by 25%. The observations are relevant to industrial hygienists and regulatory agencies using experimental data for assessing systemic toxicity following dermal exposures as for development of structure activity relationships for dermal absorption of chemicals.

Grey Gooon the Skin? Nanotechnology, Cosmetic and Sunscreen Safety

Many modern cosmetic or sunscreen products contain nano-sized components. Nanoemulsions are transparent and have unique tactile and texture properties; nanocapsule, nanosome, noisome, or liposome formulations contain small vesicles (range: 50 to 5000 nm) consisting of traditional cosmetic materials that protect light-or oxygen-sensitive cosmetic ingredients. Transdermal delivery and cosmetic research suggests that vesicle materials may penetrate the stratum corneum (SC) of the human skin, but not into living skin. Depending on the physical/chemical properties of the ingredient and the formulation, nano-sized formulations may enhance or reduce skin penetration, albeit at a limited rate. Modern sunscreens contain insoluble titanium dioxide (TiO(2)) or zinc oxide (ZnO) nanoparticles (NP), which are colorless and reflect/scatter ultraviolet (UV) more efficiently than larger particles. Most available theoretical and experimental evidence suggests that insoluble NP do not penetrate into or through normal as well as compromised human skin. Oral and topical toxicity data suggest that TiO(2) and ZnO NP have low systemic toxicity and are well tolerated on the skin. In vitro cytotoxicity, genotoxicity, and photogenotoxicity studies on TiO(2) or other insoluble NP reporting uptake by cells, oxidative cell damage, or genotoxicity should be interpreted with caution, since such toxicities may be secondary to phagocytosis of mammalian cells exposed to high concentrations of insoluble particles. Caution needs to be exercised concerning topical exposure to other NP that either have characteristics enabling some skin penetration and/or have inherently toxic constituents. Studies on wear debris particles from surgical implants and other toxicity studies on insoluble particles support the traditional toxicology view that the hazard of small particles is mainly defined by the intrinsic toxicity of particles, as distinct from their particle size. There is little evidence supporting the principle that smaller particles have greater effects on the skin or other tissues or produce novel toxicities relative to micro-sized materials. Overall, the current weight of evidence suggests that nano-materials such as nano-sized vesicles or TiO(2) and ZnO nanoparticles currently used in cosmetic preparations or sunscreens pose no risk to human skin or human health, although other NP may have properties that warrant safety evaluation on a case-by-case basis before human use.

Review: Takagi–Sugeno Fuzzy Modeling of Skin Permeability

The skin is a major exposure route for many potentially toxic chemicals. It is, therefore, important to be able to predict the permeability of compounds through skin under a variety of conditions. Available skin permeability databases are often limited in scope and not conducive to developing effective models. This sparseness and ambiguity of available data prompted the use of fuzzy set theory to model and predict skin permeability. Using a previously published database containing 140 compounds, a rule-based Takagi-Sugeno fuzzy model is shown to predict skin permeability of compounds using octanol-water partition coefficient, molecular weight, and temperature as inputs. Model performance was estimated using a cross-validation approach. In addition, 10 data points were removed prior to model development for additional testing with new data. The fuzzy model is compared to a regression model for the same inputs using both R2 and root mean square error measures. The quality of the fuzzy model is also compared with previously published models. The statistical analysis demonstrates that the fuzzy model performs better than the regression model with identical data and validation protocols. The prediction quality for this model is similar to others that were published. The fuzzy model provides insights on the relationships between lipophilicity, molecular weight, and temperature on percutaneous penetration. This model can be used as a tool for rapid determination of initial estimates of skin permeability.

The use of reconstructed human epidermis for skin absorption testing: Results of the validation study

A formal validation study was performed, in order to investigate whether the commercially-available reconstructed human epidermis (RHE) models, EPISKIN, EpiDerm and SkinEthic, are suitable for in vitro skin absorption testing. The skin types currently recommended in the OECD Test Guideline 428, namely, ex vivo human epidermis and pig skin, were used as references. Based on the promising outcome of the prevalidation study, the panel of test substances was enlarged to nine substances, covering a wider spectrum of physicochemical properties. The substances were tested under both infinite-dose and finite-dose conditions, in ten laboratories, under strictly controlled conditions. The data were subjected to independent statistical analyses. Intra-laboratory and inter-laboratory variability contributed almost equally to the total variability, which was in the same range as that in preceding studies. In general, permeation of the RHE models exceeded that of human epidermis and pig skin (the SkinEthic RHE was found to be the most permeable), yet the ranking of substance permeation through the three tested RHE models and the pig skin reflected the permeation through human epidermis. In addition, both infinite-dose and finite-dose experiments are feasible with RHE models. The RHE models did not show the expected significantly better reproducibility, as compared to excised skin, despite a tendency toward lower variability of the data. Importantly, however, the permeation data showed a sufficient correlation between all the preparations examined. Thus, the RHE models, EPISKIN, EpiDerm and SkinEthic, are appropriate alternatives to human and pig skin, for the in vitro assessment of the permeation and penetration of substances when applied as aqueous solutions.

Modeling of flux through silicone membranes from water

Do the Roberts-Sloan (RS) or modified Kasting-Smith-Cooper (KSC) equations that provide good fit to data for maximum flux, from water through mouse or human skin also provide a good fit to data for maximum fluxes through silicone membranes (polydimethylsiloxane, PDMS). The maximum fluxes through silicone membranes from water (J(MPAQ)), molecular weights (MW), solubilities in isopropyl myristate (S(IPM)) and water (S(AQ)) of 31 prodrugs and one parent drug have been fitted to the RS equation, which includes a parameter for dependence on S(AQ), and the KSC equation, which does not, to determine which equation gave the better fit. In addition, the J(MPAQ), MW, S(AQ) and solubilities in octanol (S(OCT)) of 26 diverse molecules from other laboratories were collected and fitted to the RS and KSC equations to determine if the choice of lipid parameter (S(IPM) or S(OCT)) had an effect on which equation gave the better fit. RS gave the better fit to the present prodrug database where: logJ(MPAQ)=-2.454+0.716 logS(IPM)+0.284 logS(AQ)+0.00208 MW, r(2)=0.77. RS also gave the better fit to the database from other laboratories where: logJ(MPAQ)=-2.046+0.667 logS(OCT)+0.333 logS(AQ)-0.00374 MW, r(2)=0.878 after four obvious outliers were removed to give n=22. Thus, data for J(MPAQ) can be fitted to the RS equation, which also provides the best fit to maximum flux from water through mouse or human skin and includes a dependence on S(AQ).

The Physicochemical Parameters of Marker Compounds and Vehicles for Use in In Vitro Percutaneous Absorption Studies

In order to prepare for a validation study to compare percutaneous absorption through reconstructed human epidermis with ex vivo skin absorption through human and animal skin, nine test compounds, covering a wide range of physicochemical properties were selected, namely: benzoic acid; caffeine; clotrimazole; digoxin; flufenamic acid; ivermectin; mannitol; nicotine; and testosterone. The donor and receptor media for the test substances, the addition of a solubiliser for the lipophilic compounds, as well as the stability and solubility of the test substances in the vehicles, were systematically analysed. Hydrophilic molecules, being freely soluble in water, were applied in buffered saline solutions. In order to overcome solubility restrictions for lipophilic compounds, the non-ionic surfactant, Igepal® CA-630, was added to the donor vehicle, and, in the case of clotrimazole and ivermectin, also to the receptor fluid. The model molecules showed a suitable solubility and stability in the selected donor and receptor media throughout the whole duration of the test.

Improving the applicability of (Q)SARs for percutaneous penetration in regulatory risk assessment

The new regulatory framework REACH (Registration, Evaluation, and Authorisation of Chemicals) foresees the use of non-testing approaches, such as read-across, chemical categories, structure–activity relationships (SARs) and quantitative structure–activity relationships (QSARs). Although information on skin absorption data are not a formal requirement under REACH, data on dermal absorption are an integral part of risk assessment of substances/products to which man is predominantly exposed via the dermal route. In this study, we assess the present applicability of publicly available QSARs on skin absorption for risk assessment purposes. We explicitly did not aim to give scientific judgments on individual QSARs. A total of 33 QSARs selected from the public domain were evaluated using the OECD (Organisation for Economic Co-operation and Development) Principles for the Validation of (Q)SAR Models. Additionally, several pragmatic criteria were formulated to select QSARs that are most suitable for their use in regulatory risk assessment. Based on these criteria, four QSARs were selected. The predictivity of these QSARs was evaluated by comparing their outcomes with experimentally derived skin absorption data (for 62 compounds). The predictivity was low for three of four QSARs, whereas one model gave reasonable predictions. Several suggestions are made to increase the applicability of QSARs for skin absorption for risk assessment purposes.