Intercellular Lipid Domain Research Articles

Novel skin permeation enhancers based on amino acid ester ionic liquid: Design and permeation mechanism

This study developed novel ionic liquids (ILs) based on amino acids. We first screened 15 methyl amino acid ester hydrochlorides ([AAC1]Cl) for their skin permeation enhancements using 5-Fluorouracil (5-Fu) and Hydrocortisone (HC) as model drugs. Glycine methyl ester hydrochloride ([GlyC1]Cl), L-proline methyl ester hydrochloride ([L-ProC1]Cl), and L-leucine methyl ester hydrochloride ([L-LeuC1]Cl) were selected, and their ester sites were modified with different carbon chains (C8 and C12). The resulting ILs showed improved permeation to the two drugs. TEWL and CLSM assays revealed the moderation effects of the modified ILs on skin barrier function, whereas L-proline dodecyl ester hydrochloride ([ProC12]Cl) and L-leucine dodecyl ester hydrochloride ([L-LeuC12]Cl) exhibited the strongest activities. Permeation mechanisms were further investigated by ATR-FTIR, solid-NMR, SEM, and TEM analyses. The results suggested that [L-ProC12]Cl and [L-LeuC12]Cl combined the advantages of amino acid esters and IL solvent and could interact with the intercellular lipid domain by the multi-functions of lipid fluidization and lipid extraction, which were observed in a dosage- and time-dependent manner. Additionally, pathological examination suggested that the amino acid ester-based ILs (AAE-ILs) had good biocompatibility. In conclusion, this study has generated novel chemical penetration enhancers (CPEs) based on AAE-ILs and may be potentially utilized in drug transdermal delivery systems (TDDSs).

Enhancing effects of unsaturated fatty acids with various structures on the permeation of indomethacin through rat skin.

Effects of straight-chain, cis-type, unsaturated fatty acids with various structures (alkyl chain lengths, numbers of double bonds, position of double bonds, and cis- and trans-positional isomers) on the skin permeation of indomethacin were examined by using rat skins in-vitro. Furthermore, the disordering degrees of the intercellular lipid domain in the stratum corneum, which were treated with preparations of unsaturated fatty acids, were measured by the Fourier transform infrared (FT-IR) method using excised rabbit ear skins. Unsaturated fatty acids enhanced the permeation of indomethacin through rat skins. These permeation-enhancing effects by unsaturated fatty acids were affected by changes of their alkyl chain length from C14 to C22. The lag-times on the permeation of indomethacin were shortened by unsaturated fatty acids in the following order: C20 = C18 = C22 < C16 < C14. These fluxes were increased by unsaturated fatty acids in the following order: C20 > C22 = C18 = C16 > C14. Therefore, gondoic acid (cis-11-eicosenoic acid; C20H38O2) mostly enhanced the skin permeation of indomethacin. However, the enhancing effects of unsaturated fatty acids (C18 chain) were not affected by their differences of position and numbers of double bonds. These permeation-enhancing effects which were evaluated by flux were related to the degrees of wave-number shift in the frequency of the antisymmetric CH bond stretching absorbance (near 2920 cm-1) on FT-IR spectra of the fatty acid-treated stratum corneum. Therefore, the perturbation increase of lipid domain in the stratum corneum by these fatty acids probably was the cause of the enhancing effects of permeation of indomethacin.

Human skin sandwich for assessing shunt route penetration during passive and iontophoretic drug and liposome delivery.

This work explored the role of skin appendages (shunt route) in passive and iontophoretic drug and liposome penetration. The technique used an epidermis and stratum corneum sandwich from the same skin donor with the additional stratum corneum forming the top layer of the sandwich. Penetration was monitored during occluded passive and iontophoretic (0.5 mA cm(-2)) delivery of mannitol and estradiol solutions, and ultradeformable liposomes containing estradiol. The shunt route had a significant role during passive penetration of mannitol (hydrophilic compound), but was negligible during penetration of estradiol (lipophilic drug) and liposomes. In iontophoresis, the shunt route significantly contributed to the overall flux of all preparations, being highest for mannitol. However, shunts were not the only pathway for iontophoretic drug delivery and evidence was observed for the creation of new aqueous pathways via disorganization of the intercellular lipid domain of stratum corneum. The skin sandwich technique should prove valuable for general studies on routes of skin penetration.

Effects of solvent deposited enhancers on transdermal permeation and their relationship with Emax

Many topical pharmaceuticals such as aerosols, topical sprays, and hydro-alcoholic and polymer based gels contain chemical enhancers. The objectives of the present study were to (a) determine the enhancement effects induced by enhancers deposited from a volatile solvent on human epidermal membrane (HEM) upon transdermal permeation enhancement, (b) compare these enhancement factors with Emax, and (c) examine the relationship between enhancer-induced permeation enhancement and stratum corneum equilibrium uptake enhancement. In this study, HEM was treated with enhancer/ethanol (enhancer dissolved in ethanol). After the evaporation of ethanol, passive transport experiments were conducted using corticosterone (CS) as the model permeant. The uptake of another model corticosteroid, estradiol (E2β), into the intercellular lipid domain of stratum corneum after enhancer/ethanol treatment was also determined. The results show a correlation between Emax and the enhancement effect of most enhancers when the enhancers were deposited on the skin using the volatile solvent ethanol. The data suggest that the CS transport rate limiting domain was likely the same as the intercellular lipid domain probed by E2β uptake. The correlation between steady-state permeation enhancement and uptake enhancement into the intercellular lipid domain suggests that the permeation enhancement mechanism is primarily due to enhancement of permeant partitioning into the transport rate limiting domain.

Percutaneous permeation enhancement by terpenes: mechanistic view.

A popular approach for improving transdermal drug delivery involves the use of penetration enhancers (sorption promoters or accelerants) which penetrate into skin to reversibly reduce the barrier resistance. The potential mechanisms of action of penetration enhancers include disruption of intercellular lipid and/or keratin domains and tight junctions. This results in enhanced drug partitioning into tissue, altered thermodynamic activity/solubility of drug etc. Synthetic chemicals (solvents, azones, pyrrolidones, surfactants etc.) generally used for this purpose are rapidly losing their value in transdermal patches due to reports of their absorption into the systemic circulation and subsequent possible toxic effect upon long term application. Terpenes are included in the list of Generally Recognized As Safe (GRAS) substances and have low irritancy potential. Their mechanism of percutaneous permeation enhancement involves increasing the solubility of drugs in skin lipids, disruption of lipid/protein organization and/or extraction of skin micro constituents that are responsible for maintenance of barrier status. Hence, they appear to offer great promise for use in transdermal formulations. This article is aimed at reviewing the mechanisms responsible for percutaneous permeation enhancement activity of terpenes, which shall foster their rational use in transdermal formulations.

Mechanisms of action of novel skin penetration enhancers: Phospholipid versus skin lipid liposomes

Employing thermal analysis, we investigated the mechanism of action of novel enhancers and probed phospholipid (PL) versus stratum corneum lipid (SCL) liposomes as model membranes. The enhancers included octyl salicylate (OS), padimate O (PADO) and 2-(1-nonyl)-1,3-dioxolane (ND). The negative controls were the empty liposomes. Positive controls employed dimethylsulfoxide (DMSO) and Azone™ (AZ). For PL liposomes, DMSO sharpened the transitions. AZ abolished the pre-transition, broadened the main transition and linearly reduced its transition temperature ( T m). OS or PADO reduced T m and size of pre-transition, broadened the main transition and decreased its T m (non-linearly). ND abolished the pre-transition but increased T m of the main endotherm, suggesting retardation rather than enhancement. The results of SCL correlated with PL liposomes except for ND. In SCL liposomes, ND reduced T m and broadened the peaks indicating lipid disruption, which indicated its enhancing effects. In conclusion, OS, PADO and ND can enhance drugs by disrupting intercellular lipid domain but they differ from AZ in terms of the relationship between efficacy and concentration. Although PL liposomes are simple model membranes with sharp transitions which give detailed information about the effects of enhancers, they can provide misleading results. Simultaneous use of other models like SCL liposomes is recommended.

Amphiphilic Transdermal Permeation Enhancers: Structure-Activity Relationships

Transdermal drug delivery offers numerous advantages over conventional routes of administration; however, poor permeation of most drugs across the skin barrier constitutes a serious limitation of this methodology. One of the approaches used to enlarge the number of transdermally-applicable drugs uses permeation enhancers. These compounds promote drug permeation through the skin by a reversible decrease of the barrier resistance. Enhancers can act on the stratum corneum intracellular keratin, influence desmosomes, modify the intercellular lipid domains or alter the solvent nature of the stratum corneum. Even though, hundreds of substances have been identified as permeation enhancers to date, yet our understanding of the structure-activity relationships is limited. In general, enhancers can be divided into two large groups: small polar solvents, e.g. ethanol, propylene glycol, dimethylsulfoxide and amphiphilic compounds containing a polar head and a hydrophobic chain, e.g. fatty acids and alcohols, 1-dodecylazepan-2-one (Azone), 2-nonyl-1,3-dioxolane (SEPA 009), and dodecyl-2-dimethylaminopropanoate (DDAIP). In this review we have focused on structure-activity relationships of amphiphilic permeation enhancers, including the properties of the hydrophobic chains, e.g. length, unsaturation, and branching, as well as the polar heads characteristics, e.g. hydrogen bonding ability, lipophilicity, and size. We present over 180 examples of enhancers with different polar head to illustrate the structural requirements and the possible role of the polar head. We have given an overview of the methods used for investigation of the mechanisms of permeation enhancement, namely differential scanning calorimetry (DSC), infrared (IR) and Raman spectroscopy, X-ray diffraction and future perspectives in this field. Furthermore, biodegradability and chirality of the enhancers are discussed.

Model analysis of flux enhancement across hairless mouse skin induced by chemical permeation enhancers

Previous permeant partitioning studies with hairless mouse skin (HMS) in the presence of several chemical skin permeation enhancers have revealed that, when such enhancers induce significant skin permeability coefficient enhancement, it is accompanied by significant enhancement in the equilibrium uptake (partitioning) of the permeant into the intercellular lipid component of the stratum corneum (SC). Particularly, it was found that the 1-alkyl-2-pyrrolidones and the 1-alkyl-2-azacycloheptanones, at aqueous solution concentrations that gave skin permeation enhancement ( E) of 10 for corticosterone (CS, the permeant), enhanced the equilibrium uptake of β-estradiol (E2β, a surrogate permeant) from the aqueous phase into the intercellular lipids of HMS SC by a factor of 5–7. This finding raised the question of whether this uptake enhancement induced by the permeation enhancer under equilibrium conditions would be essentially the same as that determined kinetically from time-dependent permeation experiments utilizing appropriate SC membrane models and Fick's laws of diffusion to treat the data. HMS transport experiments were conducted with CS as the permeant and 1-octyl-2-pyrrolidone (OP) and 1-hexyl-2-azacyloheptanone (HAZ) as the enhancers. In treating the experimental data, a one-layer skin transport model (SC only) and a two-layer model (SC layer and the epidermis/dermis layer) were both investigated. Both the partition coefficient enhancement ( E K ) and the diffusion coefficient enhancement ( E D ) were deduced from the data treatment. The results showed that when the total transport enhancement of CS was around 11, E K was in the range of 6–8 and E D was in the range of 1.5–1.9 using both the one-layer and the two-layer models. This E K value was found to be in good agreement with the E2β partition enhancement obtained directly under equilibrium conditions in previous studies. This indicates that (a) the rate-limiting domain for the transport of the lipophilic permeants across HMS and the HMS SC intercellular lipid domain probed in the equilibrium partitioning experiments are essentially the same, and (b) the total flux enhancement ( E) of lipophilic permeants across HMS was driven mainly by enhancing the partitioning of the permeant into the rate-limiting domain ( E K ) and secondarily by enhancing the diffusion coefficients ( E D ) of the permeant in the domain. Comparison of the one-layer and two-layer skin model results revealed that non-steady-state transport of lipophilic compounds across HMS was better described by the two-layer model because the dermis/viable epidermis played a significant role in lipophilic permeant binding.

Ceramides and skin function.

Ceramides are the major lipid constituent of lamellar sheets present in the intercellular spaces of the stratum corneum. These lamellar sheets are thought to provide the barrier property of the epidermis. It is generally accepted that the intercellular lipid domain is composed of approximately equimolar concentrations of free fatty acids, cholesterol, and ceramides. Ceramides are a structurally heterogeneous and complex group of sphingolipids containing derivatives of sphingosine bases in amide linkage with a variety of fatty acids. Differences in chain length, type and extent of hydroxylation, saturation etc. are responsible for the heterogeneity of the epidermal sphingolipids. It is well known that ceramides play an essential role in structuring and maintaining the water permeability barrier function of the skin. In conjunction with the other stratum corneum lipids, they form ordered structures. An essential factor is the physical state of the lipid chains in the nonpolar regions of the bilayers. The stratum corneum intercellular lipid lamellae, the aliphatic chains in the ceramides and the fatty acids are mostly straight long-chain saturated compounds with a high melting point and a small polar head group. This means that at physiological temperatures, the lipid chains are mostly in a solid crystalline or gel state, which exhibits low lateral diffusional properties and is less permeable than the state of liquid crystalline membranes, which are present at higher temperatures. The link between skin disorders and changes in barrier lipid composition, especially in ceramides, is difficult to prove because of the many variables involved. However, most skin disorders that have a diminished barrier function present a decrease in total ceramide content with some differences in the ceramide pattern. Formulations containing lipids identical to those in skin and, in particular, some ceramide supplementation could improve disturbed skin conditions. Incomplete lipid mixtures yield abnormal lamellar body contents, and disorder intercellular lamellae, whereas complete lipid mixtures result in normal lamellar bodies and intercellular bilayers. The utilization of physiological lipids according to these parameters have potential as new forms of topical therapy for dermatoses. An alternative strategy to improving barrier function by topical application of the various mature lipid species is to enhance the natural lipid-synthetic capability of the epidermis through the topical delivery of lipid precursors.

Reconstitution of Liposomes inside the Intercellular Lipid Domain of the Stratum Corneum

Phosphatidylcholine (PC) liposomes were able to be reconstituted inside the pig stratum corneum (SC), the outermost layer of the skin. In addition, these vesicles protected the SC tissue against the octyl glucoside surfactant (OG) effect. These abilities were examined using high-resolution low-temperature scanning electron microscopy and freeze-substitution transmission electron microscopy. The treatment of the SC with OG led to the breaking of the corneocyte envelopes and to disorganization in the intercellular lipid structure. Previous incubation of SC with PC liposomes reduced the alterations induced by the OG, this protection being associated with the incorporation of about 10% of PC to the SC. Intact PC vesicles were found in the intercellular lipid domain of the SC samples treated first with PC liposomes and second with OG, after washing in water. The OG/PC molar ratio incorporated in these SC samples corresponded to that in which the transition micelle-lamellae occurs by dilution. Hence, the presence of PC vesicles inside the SC is explained by the incorporation of an OG/PC micellar system and the subsequent reconstitution in PC vesicles by continuous dilution during washing.

Physicochemical interaction of local anesthetics with lipid model systems—correlation with in vitro permeation and in vivo efficacy

In dermal/transdermal drug administration stratum corneum (SC) is often the rate-limiting step. Furthermore, the intercellular lipid domain of SC is nowadays widely accepted as the major contributor to the skin barrier. The current work investigates whether the difference in the level of topical efficacy of local anesthetic compounds correlates with the type of interaction between the drug and the intercellular lipids of SC. Therefore, local anesthetics of varying topical efficacy were evaluated with respect to their effect on the morphology of various model lipid systems using small and wide angle X-ray diffraction (SWAXD) and differential scanning calorimetry (DSC). The model lipids used were glyceryl monooleate, sphingomyelin and lipids isolated from human SC. Furthermore, partitioning into isolated human SC as well as permeation through isolated human SC and human tape-stripped skin were investigated in vitro. The results indicate that local anesthetics may act as their own permeation enhancers by increasing the degree of hydrocarbon chain fluidity of the intercellular lipids. Eventually these interactions may induce non-lamellar reversed types of liquid crystalline structures locally in SC, which further facilitate the drug mobility. The large difference in topical efficacy of the investigated local anesthetics could not be explained simply by looking at their effect on the phase behavior of lipid model systems. Despite the similarities in physicochemical properties of these substances, the in vitro skin permeability differed markedly (AD>EMLA>lidocaine>prilocaine>sameridine). Thus, it was concluded that sufficient drug permeability over SC is essential to obtain local anesthesia by blocking the superficial nociceptors.

Percutaneous drug delivery to the hair follicle

Unique biochemical and immunological events dictate the complex cyclic growth and differentiation patterns of hair follicles and their associated sebaceous glands (1), structures that are collectively referred to as pilosebaceous units. Once regarded as mere evolutionary remnants, hair follicles and sebaceous glands have been recognized increasingly as significant pathways for percutaneous transport (2). Percutaneous transport routes via the lipoidal domains of the stratum corneum have been well established (3,4), whereas comparatively less is known about the specific roles of hair follicles and sebaceous glands. Determination of the roles of these structures is complicated by the lack of adequate animal models and methodologies that can distinctly distinguish follicular and stratum corneumpathways.Moreover, it may be possible that a topically applied compound traverses more than one pathway simultaneously. The stratum corneum is acknowledged not only as the main barrier to skin penetration, but also as the major permeation pathway. The tightly packed, semicrystalline intercellular lipid domains and the extremely compact corneocytes of the stratum corneum create a barrier highly resistant to percutaneous transport (3,4). Modulation of stratum corneum lipid fluidity by topical agents has been thoroughly studied and is generally acknowledged as the major mechanism of percutaneous delivery (5). Passive percutaneous transport depends on several factors, including penetrant lipophilicity, charge, and molecular size (6). The upper limit of molecular size for permeation through the stratum corneum is still unknown. Early suggestions of a follicular pathway were based on the hypothesis that hair follicles act as shunts, resulting in the rapid transport of ions and large polar molecules. Scheuplein (7) and Scheuplein et al. (8) first described transient follicular delivery for small polar molecules and large polar steroids that ordinarily would not be expected to traverse the skin rapidly due to their charge or restrictive molecular size. Feldmann and Maibach (9) and Maibach et al. (10) observed increases in percutaneous transport through skin areas with greatest follicular densities in both animals and humans, which also hinted at the possibility of follicular delivery.

EFFECT OF JP-8 JET FUEL EXPOSURE ON THE ULTRASTRUCTURE OF SKIN

Fuel system maintenance usually requires direct, prolonged exposure to fuel. Thus, both military and commercial aircraft workers are at risk of dermal exposure and toxicity from JP-8 jet fuel. At present, there are no standards for U.S. Air Force personnel regarding dermal exposure to jet fuel. Hence, there are needs for data and approaches to understand the human hazard from dermal exposure to JP-8. In this study, we investigated the alteration in porcine skin at ultrastructural level, after a 24-h exposure to JP-8, with the help of transmission electron microscopy (TEM). Many ultrastructural modifications were noticed in the photomicrographs. Large-scale expansion in intercellular lipid domains of stratum corneum and disruption in its structural integrity; alteration in configuration of basal cells; rupture of hemidesmosomes and desmosomes; and diminished dendritic processes in Langerhans cells were observed. It is concluded that JP-8 exposure can cause alteration in skin anatomy, which may lead to derm...

Elasticity of vesicles affects hairless mouse skin structure and permeability

One of the possibilities for increasing the penetration rate of drugs through the skin is the use of vesicular systems. Currently, special attention is paid to the elastic properties of liquid-state vesicles, which are supposed to have superior properties compared to gel-state vesicles with respect to skin interactions. In this study, the effects of vesicles on hairless mouse skin, both in vivo and in vitro, were studied in relation to the composition of vesicles. The interactions of elastic vesicles containing the single chain surfactant octaoxyethylene laurate-ester (PEG-8-L) and sucrose laurate-ester (L-595) with hairless mouse skin were studied, in vivo, after non-occlusive application for 1, 3 and 6 h. The skin ultrastructure was examined by ruthenium tetroxide electron microscopy (TEM) and histology. The extent, to which vesicle constituents penetrated into the stratum corneum, was quantified by thin layer chromatography (TLC). The interactions of the elastic vesicles containing PEG-8-L and L-595 surfactants were compared with those observed after treatment with rigid vesicles containing the surfactant sucrose stearate-ester (Wasag-7). Furthermore, skin permeability experiments were carried out to investigate the effect of treatment with PEG-8-L micelles, elastic vesicles (containing PEG-8-L and L-595 surfactants) or rigid Wasag-7 vesicles on the 3H2O transport through hairless mouse skin, in vitro, after non-occlusive application. Treatment of hairless mouse skin with the elastic vesicles affected the ultrastructure of the stratum corneum: distinct regions with lamellar stacks derived from the vesicles were observed in intercellular spaces of the stratum corneum. These stacks disrupted the organization of skin bilayers leading to an increased skin permeability, whereas no changes in the ultrastructure of the underlying viable epidermis were observed. Treatment with rigid Wasag-7 vesicles did not affect the skin ultrastructure or skin permeability. TLC measurements showed that after 1 h of non-occlusive application of elastic or rigid vesicles, a six-fold increased amount of elastic vesicle material was present within the stratum corneum compared to rigid vesicle material. After 3 and 6 h of application the amount of PEG-8-L vesicle material in SC decreased to approximately three- and two-fold, respectively, compared to Wasag-7 vesicle material. Pretreatment of the hairless mouse skin with the elastic vesicles containing 70 mol% PEG-8-L increased the diffusion of 3H2O with an optimum application dose of 2.5 mg lipids/cm2 compared to PBS pretreatment. No significant difference in the enhancement of the 3H2O-diffusion was observed between PEG-8-L micelles or elastic vesicles containing 30 or 70 mol% PEG-8-L. Pretreatment with the rigid Wasag-7 vesicles decreased the diffusion rate of 3H2O, most probably by the formation of a lipid layer on the skin surface. The effect of the elastic vesicles on the skin permeability is supported by the ultrastructural changes observed by TEM in the intercellular lipid domains. The elastic vesicles containing 70 mol% PEG-8-L disorganize the lipid bilayers thereby creating or modifying pathways for possible drug penetration.

Cryo-electron diffraction as a tool to study local variations in the lipid organization of human stratum corneum.

The human skin provides the body with a barrier against transepidermal water loss and the penetration of harmful agents (e.g. microbes) from outside. This barrier function is produced mainly by the outermost, nonviable layer of the epidermis, the stratum corneum (s.c.). The s.c. consists of terminally differentiated corneocytes surrounded by a continuous intercellular lipid domain, which contains mostly ceramides, cholesterol and free fatty acids. Small- and wide-angle X-ray diffraction studies have elucidated the lamellar and lateral lipid organizations in these domains. However, these techniques require bulk quantities of SC, as a result of which local structure information on the lipids cannot be obtained. Insights to these local lipid arrangements are important when new transdermal drug delivery systems have to be developed. Therefore, the technique of electron diffraction arose as a tool to study the lateral packing of the lipids in the intercellular domains of SC, locally. In a previous study, the suitability of electron diffraction was demonstrated using a lipid model system that resembled the lipid composition of the SC. The spacings calculated from the electron diffraction patterns were in good agreement with the spacings revealed by wide-angle X-ray diffraction. The results presented here succeed this previous study. We improved the microscope settings and developed a new preparation method to study ex vivo human s.c. by cryo-electron diffraction. The method is based on the conventional tape-stripping method and offers the possibility to study depth-related changes in the lipid organization of human SC. Diffraction patterns of both hexagonal and orthorhombic lipid lattices have been recorded with spacings that resembled those found in human s.c. by wide-angle X-ray diffraction. After lipid extraction, such diffraction patterns could no longer be detected in the samples.

Effect of Menthol and Related Terpenes on the Percutaneous Absorption of Propranolol across Excised Hairless Mouse Skin

□ The potential use of terpenes/terpenoids as penetration enhancers in the transdermal delivery of propranolol hydrochloride (PL) was investigated. PL was chosen for the reasons of its extensive firstpass metabolism and short elimination half-life. The terpenes studied included L-menthol, (+)-limonene, (±)-linalool, and carvacrol at 1%, 5%, and 10% w/v concentrations. The diffusion of PL across excised hairless mouse skin was determined using side-by-side diffusion cells. Flux, permeability coefficient (Pm), and lag time (tL) were calculated. PL showed comparable lag times with menthol at all three concentration levels. At a 1% level of carvacrol, PL exhibited a 2.4- and 2.2-fold increase in lag time compared with 5 and 10% levels of enhancer, respectively. In the presence of limonene, PL had shown maximum lag time (between 3.0 and 3.3h) at all three levels. In the case of linalool, the lag times for PL with 5 and 10% levels of enhancer were 7.0- and 5.2-fold less compared with 1% level. A significant (p<0.05) concentration effect was observed only with linalool. Hydrogel-based patches were formulated with or without menthol as enhancer. Release profiles from the hydrogel formulations obeyed zero-order kinetics. The permeability of propranolol was significantly higher (p<0.05) from the test patch than the control (no enhancer) patch across the mouse skin. The mechanism of permeation enhancement of menthol could involve its distribution preferentially into the intercellular spaces of stratum corneum and the possible reversible disruption of the intercellular lipid domain. The results suggest the potential use of menthol as effective penetration enhancer in the delivery of significant amounts of PL through skin.

Mechanisms of oral permeation enhancement

This paper reviews the effects of a diverse range of penetration enhancers, including fatty acids, bile salts, surfactants, Azone® and ethanol, on oral mucosa. We describe in detail the effects on the intercellular lipid domains, the changes induced in proteinaceous regions and potential interactions between the enhancer and endogenous structures along the transport pathway. Since there have been few structure-based studies of the oral mucosa, which have discussed enhancer effect as a function of specific physicochemical properties, we have drawn upon analogous studies of skin and other mucosal membranes, as well as model lipid systems, e.g. liposomes.

Effect of phosphatidylglycerol on the in vitro percutaneous drug penetration through the dorsal skin of guinea pigs, and analysis of the molecular mechanism, using (ATR-FTIR) spectroscopy

Standard in vito skin percutaneous penetration methods using excised guinea pig dorsal skin were employed, to characterize the penetration of a water-insoluble drug: tenoxicam (TEX), and a water-soluble drug: diclofenac sodium salt (DFS), enhanced by phosphatidylglycerol (PG); and an attenuated total reflectance fourier transform infrared (ATR-FTIR) spectroscopy was used to analyze the molecular mechanism of the drug penetration route. The C-H bond stretching absorbance frequency shift in the stratum corneum (SC) induced a higher and a broader absorbance, and the shift was dependent on the PG concentrations. The percutaneous penetration of TEX was dependent on the PG concentration (up to 6%). The enhancing mechanism of PG to TEX may not only increase the diffusion coefficient (D) and the partition coefficient (K) in the percutaneous TEX penetration but also increase fluidity of the route (intercellular lipid domain) for TEX, while that of PG to DFS, excepting 1%, PG system, may be increasing the D value in the percutaneous DFS penetration only. The percutaneous penetration of DFS was not dependent on the PG concentrations. Furthermore, the percutaneous penetration of TEX was proportional to the C-H bond stretching absorbance frequency shifts. In contrast, the percutaneous penetration of DFS was not proportional to the C-H bond stretching absorbance frequency shifts. Furthermore, the accumulation of TEX in skin was proportional to the C-H bond stretching absorbance frequency shifts. The striking parallel between the enhancement of the percutaneous penetration of TEX and the measured SC lipid fluidity shifts caused by PG, suggests that the transdermal water-insoluble drug penetration may be ultimately related to the SC lipid structure. Overall, these results suggest that PG mainly affects the intercellular lipid pathway (lipid-rich domains).

A lamellar matrix model for stratum corneum intercellular lipids. V. Effects of terpene penetration enhancers on the structure and thermal behaviour of the matrix

The principal barrier to transdermal delivery of most drugs is the lamellar intercellular lipid domain of the stratum corneum (SC). Previously, we reported a lamellar matrix capable of modelling the structural and barrier properties of the SC lipids and effects of terpene penetration enhancers on the permeation of oestradiol and, to some extent, that of 5-fluorouracil (5-FU) through the SC. The present work is aimed to investigate the interactions of two terpenes (1,8-cineole and (+) limonene) with the matrix. Terpenes (5–40%, w/w) were added to the matrix and their effects studied by hot-stage light microscopy (25–160°C) and differential scanning calorimetry (10–100°C). Results revealed that cineole breaks the matrix into a dispersed system of a lamellar mesomorphic structure in an isotropic liquid at 25 and 32°C (SC temperature), in good correlation with human SC data. Increasing concentrations of limonene caused initially a decrease in the lamellar structure, then a lamellar-to-cubic phase transition and finally, changed the matrix to a dispersed system of crystalline and mesomorphic phases in a continuous liquid phase at 25 and 32°C. The effects of limonene on the structure of the model matrix are in agreement with those reported for SC lipids derived from X-ray diffraction results. Effects of terpenes on the structure of the matrix were related to their actions on the barrier performance of this lamellar model.

A lamellar matrix model for stratum corneum intercellular lipids IV. Effects of terpene penetration enhancers on the permeation of 5-fluorouracil and oestradiol through the matrix

It is widely accepted that the intercellular lipid domain of the stratum corneum (SC) is the main barrier to transdermal permeation of most drugs. Previously, we reported a lamellar lipid matrix capable of modelling the structural properties of the SC intercellular lipids and its barrier performance toward oestradiol (OE) and 5-fluorouracil (5-FU). To investigate the ability of the matrix in modelling the effects of terpene penetration enhancers on the barrier performance of the SC, permeation of model drugs (OE and 5-FU) through cineole or limonene treated matrices were investigated here. Results revealed that the matrix is able to model the effects of cineole and limonene on the permeability coefficients of OE through the SC. The effects of cineole on the permeability coefficient of 5-FU through the SC seem to be underestimated by the model matrix; the difference was attributed to the effects of cineole on the SC intracellular proteins or to differences in the hydrophilicities of the SC lipids and the matrix. The matrix failed to model the enhancement effect of limonene toward permeation of 5-FU through the SC which might indicate that limonene increases the permeation of 5-FU in part through interactions with SC proteins which are not modelled in the matrix.