Heterogeneity Effects on Permeability–Partition Coefficient Relationships in Human Stratum Corneum

The relationship between the permeability of solutes undergoing transport via the lipid pathway of the stratum corneum and the degree to which the same solutes partition into the stratum corneum has been explored by measuring the permeability coefficients and stratum corneum/water partition coefficients of a series of hydrocortisone esters varying in lipophilicity. Isolated human stratum corneum, used in both the permeability and the uptake experiments, was shown to resemble full-thickness skin in its overall resistance and selectivity to solute structure. As with full-thickness skin, delipidization destroys the barrier properties of isolated stratum corneum. Although a linear relationship is frequently assumed to exist between permeability coefficients and membrane/water partition coefficients, a log–log plot of permeability coefficients versus the intrinsic stratum corneum/water partition coefficients for the series of hydrocortisone esters studied is distinctly nonlinear. This nonlinearity arises from the fact that the transport of these solutes is rate limited by a lipid pathway in the stratum corneum, while uptake reflects both lipid and protein domains. From the relative permeability coefficients of 21-esters of hydrocortisone varying in acyl-chain structure, group contributions to the free energy of transfer of solute into the rate-limiting barrier microenvironment of the stratum corneum lipid pathway are calculated for a variety of functional groups including the –CH_2–, –CONH₂, –CON(CH₃)₂, -COOCH₃, –COOH, and –OH groups. These are compared to contributions to the free energies of transfer obtained for the same functional groups in octanol/water, heptane/water, and stratum corneum/water partitioning experiments. The group contributions to transport for polar, hydrogen-bonding functional groups are similar to the values obtained from octanol/water partition coefficients. This similarity suggests that complete loss of hydrogen bonding does not occur in the transition state for passive diffusion via the lipid pathway.     

Parallel artificial membrane permeability assay: a new membrane for the fast prediction of passive human skin permeability

This work was devoted to the search for new artificial membranes allowing a rapid evaluation of passive human skin permeation of compounds with a parallel artificial membrane permeability assay (PAMPA). Effective permeability coefficients (Pe) determined for a set of compounds using the PAMPA technique with isopropyl myristate (IPM) and silicone oil, alone or in mixture, were compared to the corresponding human skin permeability coefficient values (Kp). A good correlation between Pe and Kp was found for compounds tested through a membrane consisting of 70% silicone and 30% IPM. Moreover, positive correlation between the membrane retention of compounds and stratum corneum/water partition coefficients (PSC) was established. These results showed that this new artificial membrane, defined as PAMPA-skin, is able to mimic the main barrier properties of human stratum corneum and can be used for the fast prediction of passive human skin permeability coefficients.     

Prediction of skin permeability of drugs: comparison of human and hairless rat skin.

Relationships between skin permeability and physicochemical properties of drugs were examined to establish a predictive method for the steady-state permeation rate of drugs through human skin. Human skin permeation properties fell into two categories: one in which the permeability coefficient is correlated to the partition coefficient, revealed with lipophilic drugs; and the other in which the permeability coefficients are almost constant, shown with hydrophilic drugs. The stratum corneum, the main barrier in skin, could be considered as a membrane with two parallel permeation pathways: lipid and pore pathways, and an equation for predicting the steady-state permeation rate of drugs was derived. The skin permeabilities of drugs for man were compared with those for hairless rat. The species difference in skin permeability found was suggested to be due to the difference in skin permeation pathways, since lipid content and water uptake of the stratum corneum varied between human and hairless rat skin.     

The prediction of percutaneous absorption: I. Influence of the dermis on in vitro permeation models

In percutaneous penetration in vitro techniques, excised full-thickness skin with its stratum corneum, viable epidermis and dermis is often used. Since penetrants can be absorbed in vivo immediately below the viable epidermis, the dermal layer could act as an additional barrier in the in vitro experiments relative to the actual in vivo process. In the present paper, in vitro penetration studies through excised hairless rat skin devoid of its dermal layer are reported and compared with those previously carried out with the excised full-thickness skin of the same animal. A homologous series of 4-alkylanilines was used in all the studies, and the correlations found between permeability coefficients and n-octanol partition coefficients were analyzed. Correlations are bilinear in nature in both cases, but in the absence of the dermal layer the correlation line seems to tend to hyperbolicity, as assessed by a significant increment in permeability coefficients for the highly lipophilic compounds of the series and by a displacement of the optimal lipophilicity value (vertex of the correlation line) to a higher partition coefficient. It can be concluded that the heterogeneous nature of the skin, as far as absorption is concerned, may be due to the presence of the two anatomical hydrophilic layers, dermis and viable epidermis, rather than to the stratum corneum itself. A critical review of the results reported in the literature showed good agreement with these conclusions. The biophysical penetration model was identical and the optimal lipophilicity values very similar, so it may be that these features are independent of the type of epidermis used (rat, mouse or man) and also of the chemical composition of the penetrants.     

Age-related changes in skin permeability of hydrophilic and lipophilic compounds in rats

The effect of age on intact and stripped skin permeability of lipophilic (ketoprofen and isosorbide dinitrate) and hydrophilic permeants (deuterium oxide and diclofenac sodium) was investigated using STD: Wistar male rats aged 5 to 180 days. The permeability of permeants through intact skin increased with increasing lipophilicity of the permeants at each age, indicating that the permselective property of rat skin is revealed even at 5-days-old. The permeability coefficients through intact skin decreased with increasing age, and the extent of these decreases was higher for lipophilic permeants than that for hydrophilic permeants. On the other hand, the stripped skin permeability of permeants was almost the same at each age, and with aging each permeability coefficient through stripped skin decreased up to 21 days, dramatically during 21-90 days and then gradually again to 180 days. The thickness of the stratum corneum and stripped skin increased according to age with faster growth during 21-90 days. The reciprocal of the mean thickness of stratum corneum and stripped skin correlated well with intact skin and stripped skin permeability (r > 0.9), respectively. These results clarified that the permselectivity of rat skin against lipophilicity of permeant exists at the latest from 5 days after birth. In addition, it is speculated that the thickness of skin is a large factor in the decrease of its permeability with age.     

Structure-permeability relationship analysis of the permeation barrier properties of the stratum corneum and viable epidermis/dermis of rat skin

The purpose of this study was to evaluate structure-permeability relationships for chemicals through stratum corneum (SC) and viable epidermis/dermis (VED). In vitro skin permeation of ten compounds through excised rat skin was analyzed based on a two-layer diffusion model and the diffusion coefficients in SC (D(SC)) and VED (D(VED)) were determined. The relationships between the permeation parameters and the physicochemical parameters (octanol-water partition coefficient (log K(o/w)), and hydrogen bond donor number (HBD)) of the compounds were analyzed. D(SC) increased as lipophilicity increased, whereas D(VED) decreased for log K(o/w) > 2. Increases in log K(o/w) caused a decrease in the permeability coefficient from SC through VED (P(VED/SC)) for log K(o/w) > 1. The simulation study suggests that the in vitro skin permeation of a highly lipophilic compound is strongly controlled by skin thickness due to low diffusivity in VED. The present study suggests that VED act as a considerable permeation barrier for highly lipophilic compounds due to low diffusivity.     

Enhanced skin permeability for transdermal drug delivery: physiopathological and physicochemical considerations

To deliver drugs through the skin for systemic medication, the skin permeability needs enhancing by either modifying the drug molecules, or applying skin permeation enhancers to reduce the barrier property of the skin. Traditionally, the enhancement of skin permeability is considered as the result from the improvement of the lipophilicity of drugs and the partition of drugs into the skin, or from the direct actions of skin permeation enhancers on the chemical structure and/or composition of lipids and proteins in the stratum corneum. However, on the other hand, the skin also responds to drugs and/or skin permeation enhancers physiopathologically via its inflammatory and immune reaction. The physiopathological responses of the skin can also induce the changes in the chemical structure and composition of lipids and proteins in the skin. Therefore, the possible role of physiopathological responses of the skin in the enhanced skin permeability should be concerned.     

The Role of Protein and Lipid Domains in the Uptake of Solutes by Human Stratum Corneum

The uptake of a series of hydrocortisone esters varying in lipophilicity from water into untreated and delipidized human stratum corneum has been determined. The partition coefficients of solutes into fully hydrated stratum corneum are postulated to represent the separate contributions of three structurally distinct domains—the extractable lipids, protein, and the solvent domain. The solvent domain was assumed to have the properties of bulk water. The relative affinities of the protein and lipid domains of stratum corneum for solutes varying in structure were determined by comparing solute uptake in untreated and delipidized stratum corneum. Partitioning into the extracted lipids was also examined. Solute uptake into stratum corneum may be governed by the protein domain, the lipid domain, or a combination of the two, depending on solute lipophilicity. Due to differences in the selectivity of the two domains, a change in uptake mechanism occurs with increasing solute lipophilicity from protein-dominated uptake for hydrophilic solutes to lipid domain-dominated uptake for lipophilic solutes. The stratum corneum lipid content, which varies dramatically from individual to individual (3–46% in this study), is an important determinant of the affinity of the stratum corneum for highly lipophilic solutes but has no effect on the uptake of hydrophilic solutes.Prakash V. Raykar: In partial fulfillment of the Ph.D. degree of pharmaceutics, College of Pharmacy, University of Utah.     

Biophysical study of porcine ear skin in vitro and its comparison to human skin in vivo

The goal of this work was to establish, using biophysical characterization, that porcine ear skin in vitro is a valid model for its human counterpart. Specifically, stratum corneum (SC) barrier function was evaluated during its progressive removal by adhesive tape-stripping using the techniques of transepidermal water loss (TEWL) and impedance spectroscopy. TEWL increased slowly at first and then more rapidly with the degree of SC impairment. In contrast, low-frequency skin impedance declined exponentially as a function of progressive SC removal. The methods provide complementary and correlated information about SC barrier function. Biophysical parameters, including the diffusivity and permeability coefficient of water across the SC, and the thickness of the barrier were determined from the TEWL data using Fick's first law of diffusion. Furthermore, an ionic partition coefficient-mobility product was estimated from the skin impedance measurements. Comparison of the results with those previously reported for human skin in vivo strongly supports the validity of the porcine membrane as an in vitro model.     

Skin Solubility Determines Maximum Transepidermal Flux for Similar Size Molecules

Purpose  The maximum flux of solutes penetrating the epidermis has been known to depend predominantly on solute molecular weight. Here we sought to establish the mechanistic dependence of maximum flux on other solute physicochemical parameters. Methods  Maximum fluxes, stratum corneum solubilities and estimated diffusivities through human epidermis were therefore determined for 10 phenols with similar molecular weights and hydrogen bonding but varying in lipophilicity. Results  Maximum flux and stratum corneum solubilities of the phenolic compounds both showed a bilinear dependence on octanol-water partition coefficient (P), with solutes having a maximum solubility in the stratum corneum when 2.7<log P<3.1. In contrast, lag times and diffusivities were relatively independent of P. Stratum corneum-water partition coefficients and epidermal permeability coefficients were consistent with previously reported data. Conclusion  A key finding is that the convex dependence of maximum flux on lipophilicity arises primarily from variations in stratum corneum solubility, and not from diffusional or partitioning barrier effects at the stratum corneum–viable epidermis interface for the more lipophilic phenols. Our data support a solute structure-skin transport model for aqueous solutions in which permeation rates depend on both partitioning and diffusivity: partitioning is related to P, and diffusivity to solute size and hydrogen bonding. (199 words)     

Permeability of benzoic acid derivatives in excised guinea pig dorsal skin and effects of l-menthol

Penetration through excised guinea pig dorsal skin was examined for nine mono-substituents of benzoic acid derivatives. Permeability coefficients of the derivatives correlated well with their n-octanol/water partition coefficients. Since the regression coefficient was similar to the value obtained in human skin, it is suggested that the lipid lamellae of guinea pig skin resembled that of human skin in lipophilicity. Addition of penetration enhancer, 1% l-menthol in 15% ethanol markedly increased the flux and permeability coefficients of relatively hydrophilic derivatives and decreased the dependency of the permeability coefficients on the partition coefficients. Electron spin resonance analysis using 5-doxylstearic acid revealed the presence of a strongly immobilized component of the spin label in the skin and its disappearance in the presence of 1% l-menthol in 15% ethanol. These results suggest that the rigid lamellar structure of the stratum corneum was disrupted by l-menthol with ethanol, and caused the enhancement of penetration of relatively hydrophilic benzoic acid derivatives.     

Effect of vehicles and sodium lauryl sulphate on xenobiotic permeability and stratum corneum partitioning in porcine skin

Dermal contact with potentially toxic agricultural and industrial chemicals is a common hazard encountered in occupational, accidental spill and environmental contamination scenarios. Different solvents and chemical mixtures may influence dermal absorption. The effects of sodium lauryl sulphate (SLS) on the stratum corneum partitioning and permeability in porcine skin of 10 agricultural and industrial chemicals in water, ethanol and propylene glycol were investigated. The chemicals were phenol, p-nitrophenol, pentachlorophenol, methyl parathion, ethyl parathion, chlorpyrifos, fenthion, simazine, atrazine and propazine. SLS decreased partitioning into stratum corneum from water for lipophilic compounds, decreased partitioning from propylene glycol and did not alter partitioning from ethanol. SLS effects on permeability were less consistent, but generally decreased permeability from water, increased permeability from ethanol and had an inconsistent effect on permeability from propylene glycol. It was concluded that, for the compounds tested, partitioning into the stratum corneum was determined by the relative solubility of the solute in the donor solvent and the stratum corneum lipids. Permeability, however, reflected the result of successive, complex processes and was not predictable from stratum corneum partitioning alone. Addition of SLS to solvents altered partitioning and absorption characteristics across a range of compounds, which indicates that partition coefficients or skin permeability from neat chemical exposure should be used with caution in risk assessment procedures for chemical mixtures.     

Effects of sebum on drug transport across the human stratum corneum in vivo

The objective of this study was to investigate the effect of sebum on drug transport across the human stratum corneum (SC) in vivo for two model compounds, 4-cyanophenol (CP) and cimetidine (CM), of different lipophilicity and molecular size by utilizing noninvasive tape-stripping techniques, in conjunction with an unsteady-state diffusion model for data analysis. The results demonstrated that the SC permeability of the relatively hydrophilic CM on the forehead may be as much as four times the permeability on the forearm. The administration of sebum supplementation to the forearm increased the SC permeability of CM more than threefold, but did not have the same effect with regard to CP. Removal of sebum from the forehead demonstrated a small but significant effect (-22%) on the SC permeability of CM. The presence of sebum on the forehead or forearm increased the diffusion of both molecules, but the effect on partition varied between sites and drugs. The change in the SC permeability of the relatively hydrophilic drug using sebum treatment may be attributable to the altered barrier function of the SC due to the disordering structures of the intercellular lipid molecules.     

Membrane-coated fiber array approach for predicting skin permeability of chemical mixtures from different vehicles.

A membrane-coated fiber (MCF) array approach was developed for quantitative assessment of skin absorption from chemical mixtures, which was based on the similarity in the absorption mechanisms of the MCF membrane and the stratum corneum of the skin. A set of probe compounds were used to detect the relative molecular interaction strengths of chemicals with the vehicle and the membranes, which provided a linkage between the skin permeability (log k) and MCF partition coefficients (log KF). A predictive model was established via multiple linear regression analysis of the data matrix of experimentally measured log k value and log KFm values; log k=a0+a1 log KF1+a2 log KF2+...+an log KFm, where m is the number of diverse MCFs. Twenty-five probe compounds and three MCFs (polydimethylsiloxane for lipophilic, polyacrylate for polarizable, and CarboWax for polar interactions) were used to demonstrate the model development processes in the MCF array approach. The skin permeability of the probe compounds was measured with conventional diffusion cell experiments using dermatomed porcine skin. Three predictive models were established for skin permeability prediction from chemical mixtures in water, 50% ethanol, and 1% sodium lauryl sulfate (SLS) with R2 values of 93, 91, and 83, respectively. The log k and log KF values were considerably altered by the addition of ethanol or SLS into the dose vehicle; however, their correlations to skin permeability remained strong under various conditions. These results suggested that the experimentally based MCF array approach can be used to predict skin absorption from chemical mixtures in different vehicles or formulations.     

The effects of serine palmitoyltransferase inhibitor, ISP-I, on UV-induced barrier disruption in the stratum corneum

We examined the effects of ISP-I (myriocin, thermozymocidin) - a potent inhibitor of serine palmitoyltransferase (SPT) which is involved in the ceramide synthetic pathway-on skin barrier function in post-UVB-irradiated hairless mouse skin. Disruption of the skin barrier function after UVB irradiation as represented by the increase in transepidermal water loss (TEWL) was significantly suppressed with ISP-I treatment. In the ISP-I-treated skin, the peak of cell proliferation was observed 24 h earlier than in vehicle-treated skin. In addition, the number of apoptotic cells in ISP-I-treated skin showed a sharp decrease at 48 and 72 h post-irradiation. The number of stratum corneum cell layers was increased in ISP-I-treated skin at 72 h after UVB irradiation; at this time, TEWL in ISP-I-treated skin was lower than that in the vehicle-treated skin. We suggest ISP-I treatment altered cell proliferation and apoptosis after UVB exposure by modulating ceramide synthesis in epidermal cells, resulting in an increase of stratum corneum layers which lessened the effects of irradiation-induced barrier disruption.     

Skin permeability enhancement by low frequency sonophoresis: lipid extraction and transport pathways

The objective of this study was to shed light on the mechanism(s) by which low-frequency ultrasound (20 KHz) enhances the permeability of the skin. The physical effects on the barrier and the transport pathway, in particular, were examined. The amount of lipid removed from the intercellular domains of the stratum corneum following sonophoresis was determined by infrared spectroscopy. Transport of the fluorescent probes nile red and calcein, under the influence of ultrasound, was evaluated by laser-scanning confocal microscopy. The results were compared with the appropriate passive control data and with data obtained from experiments in which the skin was exposed simply to the thermal effects induced by ultrasound treatment. A significant fraction ( approximately 30%) of the intercellular lipids of the stratum corneum, which are principally responsible for skin barrier function, were removed during the application of low-frequency sonophoresis. Although the confocal images from the nile red experiments were not particularly informative, ultrasound clearly and significantly (again, relative to the corresponding controls) facilitated transport of the hydrophilic calcein via discrete permeabilized regions, whereas other areas of the barrier were apparently unaffected. Lipid removal from the stratum corneum is implicated as a factor contributing the observed permeation enhancement effects of low-frequency ultrasound. However, microscopic observations imply that sonophoresis induces localized (aqueous?) permeation pathways at discrete sites.     

Synthesis and evaluation of N-acetylprolinate esters - novel skin penetration enhancers

A series of N-acetylproline esters (alkyl side chain length, 5-18) were synthesized and tested for potential skin penetration enhancement activity using modified Franz diffusion cells and hairless mouse skin as the penetration barrier. Benazepril and hydrocortisone were used as model drugs and were applied as saturated solutions in propylene glycol. The enhancers were added at a concentration of 5% (w/v). Drug flux, permeability coefficient and enhancement ratios for permeability coefficient were determined. Azone was used as the positive control. While all the compounds tested increased the skin penetration of hydrocortisone, the 5- and 8- carbon esters had no significant effect on the skin penetration of benazepril. The highest fluxes were obtained with 11, 12, and 18-carbon esters and they were comparable to Azone. There was no significant difference between the fluxes obtained with 2 and 5% (w/v) concentrations of the 12-carbon ester on hydrocortisone permeation. The 16-carbon ester, where ethanol was used as a cosolvent, significantly increased the fluxes of both the drugs compared to the control. Differential scanning calorimetric studies suggested that the enhancers may be acting on the lipids of the stratum corneum and their effect was similar to that of Azone. The membrane/vehicle partition coefficient studies indicated an increase in benazepril partition coefficient with enhancer treatment compared to the control. Maximum flux increase was obtained with the 11 and 12 carbon (alkyl chain length) esters for both benazepril and hydrocortisone. The 18- carbon ester which has a cis-double bond in the alkyl side chain, also increased the flux significantly.     

A Method to Predict the Percutaneous Permeability of Various Compounds: Shed Snake Skin as a Model Membrane

Penetration of various compounds through shed snake skin was measured in vitro to examine the effect of lipophilicity and molecular size of a compound on permeability through this model membrane. The permeabilities were found to be controlled by the lipophilicity and the molecular size of the permeant. The smaller and the more lipophilic the compound, the greater the permeability. Equations have been developed to predict the permeability from the molecular weight and the distribution coefficient of a compound. Further, the lipophilicity of shed snake skin is similar to that of human skin and the response of shed snake skin to the molecular size of a permeant is more similar to human skin than to hairless mouse skin. Considering the similarities between shed snake skin and human stratum coraeum in terms of structure, composition, and permeability characteristics, the same considerations may apply to permeability through human stratum corneum.