Compartmental Modeling of Transdermal Iontophoretic Transport II: In Vivo Model Derivation and Application

PURPOSE

This study was aimed to develop a family of compartmental models to describe in a strictly quantitative manner the transdermal iontophoretic transport of drugs in vivo. The new models are based on previously proposed compartmental models for the transport in vitro.

METHODS

The novel in vivo model considers two separate models to describe the input into the systemic circulation: a) constant input and b) time-variant input. Analogous to the in vitro models, the in vivo models contain four parameters: 1) kinetic lag time (tL), 2) steady-state flux during iontophoresis (JSS), 3) skin release rate constant (KR), and 4) passive flux in the post-iontophoretic period (Jpas). The elimination from the systemic circulation is described by a) the one-compartment and b) the two-compartment pharmacokinetic models. The models were applied to characterize the observed plasma concentration vs. time data following single-dose iontophoretic delivery of growth hormone-releasing factor (GRF) and R-apomorphine. Moreover, the models were also used to simulate the observed plasma concentration vs. time profiles following a two-dose transdermal iontophoretic administration of alniditan.

RESULTS

The time-variant input models were superior to the constant input models and appropriately converged to the observed data of GRF and R-apomorphine allowing the estimation of JSS, KR, and Jpas. In most cases, the values of tL were negligible. The estimated JSS and the in vivo flux profiles of GRF and R-apomorphine were similar to those obtained using the deconvolution method. The two-dose iontophoretic transport of alniditan was properly simulated using the proposed time-variant input model indicating the utility of the model to predict and to simulate the drug transport by a multiple-dose iontophoresis. Moreover, the use of the compartmental modeling approach to derive an in vitro-in vivo correlation for R-apomorphine was demonstrated. This approach was also used to identify the optimum in vitro model that closely mimics the in vivo iontophoretic transport of R-apomorphine.

CONCLUSIONS

The developed in vivo models demonstrate their consistency and capability to describe the in vivo iontophoretic drug transport. This compartmental modeling approach provides a scientific basis to examine in vitro-in vivo correlations of drug transport by iontophoresis.

Transdermal iontophoresis of the dopamine agonist 5-OH-DPAT in human skin in vitro

The feasibility of transdermal iontophoretic delivery of a potent dopamine agonist 5-OH-DPAT was studied in vitro in side by side diffusion cells across human stratum corneum (HSC) and dermatomed human skin (DHS) according to the following protocol: 6 h of passive diffusion, 9 h of iontophoresis and 5 h of passive diffusion. The influences of the following parameters on the flux were studied: donor solution pH, NaCl concentration, drug donor concentration, current density and skin type. A current density of 0.5 mA cm(-2) was used, except for one series of experiments to study the current density effect. Probably due to the influence of the skin perm-selectivity and the competition with H(+), increase in pH from 3 to 5 resulted in a significant increase in flux. Further increase in pH to 6 did not further increase the flux. The iontophoretic transport was found to increase linearly with concentration and current density, providing a convenient way to manage dose titration for Parkinson's disease therapy. Increase in concentration of NaCl dramatically reduced the flux of 5-OH-DPAT as a result of ion competition to the transport. When DHS was used, the iontophoretic transport was less. Also, with DHS the response in flux profile, by switching the current on and off, was shallower than that with HSC. With the optimum condition, a delivery of 104 microg of 5-OH-DPAT per cm(2) patch per hour is feasible, indicating that the therapeutic level could be achieved with a smaller patch size than required in case of rotigotine. Thus, based on this in vitro study, transdermal iontophoretic delivery of 5-OH-DPAT is very promising.

Vesicles as a tool for transdermal and dermal delivery

Transdermal and dermal drug delivery is problematic because the skin, as a natural barrier, has a very low permeation rate. Therefore several methods have been assessed to increase this rate locally and temporarily. One approach is the use of vesicle formulations. In this paper the effectiveness of conventional and deformable vesicles as drug delivery systems as well as their possible mode of action as permeation enhancers or transdermal drug carriers will be discussed.:

Quantitative Assessment of the Transport of Elastic and Rigid Vesicle Components and a Model Drug from these Vesicle Formulations into Human Skin In Vivo

The aim of this study was to quantitatively assess the distribution profiles of elastic and rigid vesicle material in human skin in vivo. Furthermore, the distribution profiles of the model drug ketorolac applied in these vesicle formulations was investigated. A deuterium-labelled phospholipid was incorporated into these vesicles to serve as a marker for the vesicle material. The vesicles were loaded with ketorolac at saturated concentrations. Vesicle solutions were applied non-occlusively onto the skin and the treated site was sequentially tape-stripped. Tape-strips were analyzed for vesicle material using attenuated total reflectance-Fourier transform infrared spectroscopy and for ketorolac by extraction of the tape-strips followed by high pressure liquid chromatography. Distribution profiles in the stratum corneum (SC) were obtained for the elastic and rigid vesicle material and for the ketorolac. These profiles have suggested that elastic vesicle material can rapidly enter the deeper layers of the SC and can reach almost the SC-viable epidermal junction. Rigid vesicle material, however, did not penetrate deep into the SC. Furthermore, the elastic vesicles were better than the rigid vesicles in the enhancement of ketorolac transport into human SC. The distribution profile of ketorolac in the deeper SC layers was, however, different from that of the vesicle material. This suggests that once the elastic vesicles partition into the SC, the ketorolac is released from the vesicles. The elastic vesicles are superior to the rigid vesicles both in terms of vesicular transport into the SC and in terms of therapeutic potential as a skin delivery vehicle.

Compartmental Modeling of Transdermal Iontophoretic Transport: I. In Vitro Model Derivation and Application

PURPOSE

The objective of this study was to develop a family of compartmental models to describe in a strictly quantitative manner the transdermal iontophoretic transport of drugs in vitro.

METHODS

Two structurally different compartmental models describing the in vitro transport during iontophoresis and one compartmental model describing the in vitro transport in post-iontophoretic period are proposed. These models are based on the mass transfer from the donor compartment to the acceptor compartment via the skin as an intermediate compartment. In these models, transdermal iontophoretic transport is characterized by 5 parameters: 1) kinetic lag time (tL), 2) steady-state flux during iontophoresis (Jss), 3) skin release rate constant (K(R)), 4) the first-order rate constant of the iontophoretic driving force from the skin to the acceptor compartment (I1), and 5) passive flux in the post-iontophoretic period (Jpas). The developed models were applied to data on the iontophoretic transport in human stratum corneum in vitro of R-apomorphine after pretreatment with phosphate buffered saline pH 7.4 (PBS) and after pretreatment with surfactant (SFC), as well as the iontophoretic transport of 0.5 mg ml(-1) rotigotine at pH 5 (RTG).

RESULTS

All of the proposed models could be fitted to the transport data of PBS, SFC, and RTG groups both during the iontophoresis and in the post-iontophoretic period. The incorporation of parameter I1 failed to improve the fitting performance of the model. This might indicate a negligible contribution of iontophoretic driving force to the mass transfer in the direction from the skin to the acceptor compartment, although it plays an important role in loading the skin with the drug. The estimated values of Jss of PBS, SFC, and RTG were identical (p > 0.05) to the values obtained with the diffusion lag time method. Moreover, time required to achieve steady-state flux can be estimated based on the parameter tL and the reciprocal value of parameter K(R). In addition, accumulation of drug molecules in the skin is reflected in a reduction of the value of the K(R) parameter.

CONCLUSIONS

The developed in vitro models demonstrated their strength and consistency to describe the drug transport during and post-iontophoresis.

Modelling the stratum corneum lipid organisation with synthetic lipid mixtures: the importance of synthetic ceramide composition

Cholesterol (CHOL), free fatty acids (FFA) and nine classes of ceramides (CER1-CER9) form the main constituents of the intercellular lipid lamellae in stratum corneum (SC), which regulate the skin barrier function. Both the presence of a unique 13-nm lamellar phase, of which the formation depends on the presence of CER1, and its dense lateral packing are characteristic for the SC lipid organisation. The present study focuses on the lipid organisation in mixtures prepared with CHOL, FFA and a limited number of synthetic CER, namely CER1, CER3 and bovine brain CER type IV (SigmaCERIV). The main objective is to determine the optimal molar ratio of CER3 to SigmaCERIV for the formation of the 13-nm lamellar phase. CER3 contains a uniform acyl chain length, whereas SigmaCERIV contains fatty acids with varying chain lengths. Using small angle X-ray diffraction (SAXD), it is demonstrated that the CER3 to SigmaCERIV ratio affects the formation of the 13-nm lamellar phase and that the optimal ratio depends on the presence of FFA. Furthermore, the formation of the 13-nm lamellar phase is not very sensitive to variations in the total CER level, which is similar to the in vivo situation.

Time and depth resolved visualisation of the diffusion of a lipophilic dye into the hair follicle of fresh unfixed human scalp skin

Visualising the penetration pathway of a lipophilic model dye into the hair follicle of fresh unfixed human skin would facilitate optimisation of drug formulations for local delivery to the pilosebaceous unit. A block of fresh human scalp skin was mechanically fixed in a newly designed combination of cutting device/on-line diffusion cell, manual cross-sectioned perpendicular to the skin surface and sealed to create the donor and acceptor compartment. The donor phase consisted of a saturated solution of Bodipy FL C(5) in a citric acid buffer solution. Images were obtained on-line by confocal laser scanning microscopy (CLSM) every 30 min for 16 h. For each time point and each skin region relative intensity values were calculated. The on-line visualisation showed a fast diffusion of the label into the gap of the hair follicle followed by a fluorescent staining in the gap itself. The data strongly indicate that the fluorescence in the cuticle originates mainly from the dye of the gap and not from the surrounding epidermis. The on-line visualisation provides a new and excellent tool to monitor simultaneous changes in distribution profiles in the various skin layers including the hair follicle. This information can be used to determine penetration pathways in the skin.

Skin penetration and mechanisms of action in the delivery of the D2-agonist rotigotine from surfactant-based elastic vesicle formulations

PURPOSE

This study was performed to investigate the effect of elastic and rigid vesicles on the penetration of the D2 dopamine agonist rotigotine across human skin and to further elucidate the mechanisms of action of the elastic vesicles.

METHODS

A series of rotigotine-loaded vesicles were prepared, ranging from very elastic to very rigid. The drug penetration from these vesicles across human skin was studied in vitro using flow-through diffusion cells. Micelle and buffer solutions were investigated as controls. For the most elastic vesicle composition, two additional variables were investigated. Coapplication of drug and vesicles was compared to pretreatment, and the effect of the drug entrapment efficiency was investigated.

RESULTS

The very elastic vesicle formulation L-595/PEG-8-L (50/50) gave steady-state fluxes of 214.4 +/- 27.8 ng/(h x cm2). This formulation was the most effective formulation and significantly better than the rigid vesicle formulations as well as the micelle and buffer controls. However, coapplication and a high drug entrapment efficiency were essential factors for an optimal drug delivery from elastic vesicle formulations.

CONCLUSIONS

Elastic vesicles are promising vehicles for transdermal drug delivery. It is essential that drug molecules are applied together with and entrapped within the vesicles themselves, suggesting that elastic vesicles act as drug carrier systems and not solely as penetration enhancers.

Novel lipid mixtures based on synthetic ceramides reproduce the unique stratum corneum lipid organization

Lipid lamellae present in the outermost layer of the skin protect the body from uncontrolled water loss. In human stratum corneum (SC), two crystalline lamellar phases are present, which contain mostly cholesterol, free fatty acids, and nine types of free ceramides. Previous studies have demonstrated that the SC lipid organization can be mimicked with model mixtures based on isolated SC lipids. However, those studies are hampered by low availability and high interindividual variability of the native tissue. To elucidate the role of each lipid class in the formation of a competent skin barrier, the use of synthetic lipids would offer an alternative. The small- and wide-angle X-ray diffraction results of the present study show for the first time that synthetic lipid mixtures, containing only three synthetic ceramides, reflect to a high extent the SC lipid organization. Both an appropriately chosen preparation method and lipid composition promote the formation of two characteristic lamellar phases with repeat distances similar to those found in native SC. From all synthetic lipid mixtures examined, equimolar mixtures of cholesterol, ceramides, and free fatty acids equilibrated at 80 degrees C resemble to the highest extent the lamellar and lateral SC lipid organization, both at room and increased temperatures.

On-Line Visualization of Dye Diffusion in Fresh Unfixed Human Skin

PURPOSE

The purpose of the current study was to develop a new method to examine the diffusion in fresh unfixed human skin on-line.

METHODS

Full thickness skin samples were cut perpendicular to the skin surface (cutting plane facing upwards) with a new cutting device forming part of the final diffusion cell. The donor solution contained 0.1 mg/ml Bodipy FL C5 (moderately lipophilic) dissolved in citric acid buffer, pH 5.0, and the acceptor phase consisted of phosphate-buffered saline, pH 7.4. Images were taken with confocal laser scanning microscopy (CLSM) every 10 min for 8 h.

RESULTS

This new method enabled for the first time visualization of concentration profiles in different skin layers simultaneously as a function of time. For this model penetrant, Bodipy FL C5 showed that the lower stratum corneum layer constitutes the greatest barrier to diffusion. Furthermore, there is preferred partitioning of this probe in epidermis vs. either stratum corneum or dermis.

CONCLUSIONS

The on-line diffusion cell in combination with CLSM is a promising tool to study diffusion processes of dyes in fresh unfixed skin on-line. The method has the potential to access deeper skin layers as well as to visualize diffusion processes in cells.

Transdermal Iontophoresis of Rotigotine Across Human Stratum Corneum in Vitro: Influence of pH and NaCl Concentration

PURPOSE

The aim of this study was to characterize the influence of pH and NaCl concentration on the transdermal iontophoretic transport of the dopamine receptor agonist rotigotine across human stratum corneum (HSC).

METHODS

Rotigotine transport was studied in vitro in side by side diffusion cells according to the following protocol: 6 h of passive diffusion, 9 h of iontophoresis, and 5 h of passive diffusion. A current density of 0.5 mA cm(-2) was used. The influence of donor phase pH (4, 5, and 6) and different concentrations of NaCl (0.07 and 0.14 M) on rotigotine iontophoretic flux were examined. The acceptor phase was phosphate-buffered saline (PBS) at pH 7.4 except in one series of experiments aimed to study the effects of rotigotine solubility on its iontophoretic transport. In this study, PBS at pH 6.2 was used. In separate studies. 14C-mannitol was used as a marker to determine the role of electro-osmosis during iontophoresis.

RESULTS

The estimated iontophoretic steady-state flux (Flux(ss)) of rotigotine was influenced by the pH of the donor solution. At a drug donor concentration of 0.5 mg ml(-1), the iontophoretic flux was 30.0 +/- 4.2 nmol cm(-2) h(-1) at pH 6 vs. 22.7 +/- 5.5 nmol cm(-2) h(-1) at pH 5. However, when the donor concentration was increased to 1.4 mg ml(-1), no significant difference in iontophoretic rotigotine transport was observed between pH 5 and 6. Increase of NaCl concentration from 0.07 M to 0.14 M resulted in a decrease of the rotigotine Flux(ss) from 22.7 +/- 5.5 nmol cm(-2) h(-1) to 14.1 +/- 4.9 nmol cm(-2) h(-1). The contribution of electro-osmosis was estimated less than 17%. Probably due to the lipophilic character of the drug, impeding the partitioning of rotigotine from HSC to the acceptor compartment, steady-state transport was not achieved during 9 h of iontophoresis.

CONCLUSIONS

Both pH and NaCl concentration of the donor phase are crucial on the iontophoretic transport of rotigotine. Electro-repulsion is the main mechanism of the iontophoretic transport of rotigotine.

Transdermal iontophoresis of rotigotine: influence of concentration, temperature and current density in human skin in vitro

Iontophoretic transport of rotigotine across human stratum corneum (HSC) was studied in vitro in side by side diffusion cells according to the following protocol: 6 h of passive diffusion, 9 h of iontophoresis followed by 5 h of passive diffusion. A current density of 0.5 mA cm(-2) was applied. The parameters studied were the influence of the rotigotine concentration in donor phase and the influence of the molecular weight of the co-ions. To this end, Na(+) was replaced by tetra ethyl ammonium (TEA(+)) or tetra butyl ammonium (TBA(+)) (both at pH 5 and 6). In addition, the influence of the acceptor phase temperature (32 degrees C versus room temperature), the replacement of HSC by dermatomed human skin (DHS), and the relation between drug transport and current density were examined. The estimated steady-state flux (Flux(ss)) gradually increased with the drug concentration in the donor phase in a linear manner. The flux was also linearly correlated with the applied current density providing a convenient approach to individual dose titration. The use of TEA(+) as co-ion increased the rotigotine iontophoretic flux significantly, while TBA(+) did not. Replacing HSC by DHS reduced the iontophoretic rotigotine transport, while an increase in temperature to 32 degrees C increased the rotigotine flux. The maximum Flux(ss) achieved was around 80 nmol cm(-2) h(-1) indicating that by means of iontophoresis, a therapeutic level of rotigotine might be achieved with a reasonable patch size.

Iontophoretic R-apomorphine delivery in combination with surfactant pretreatment: in vitro validation studies

To validate the efficacy and controllability of a newly developed transdermal delivery system for R-apomorphine in combination with the surfactant pretreatment, iontophoresis was performed in three-chamber continuous-flow-through diffusion cells in vitro. The transdermal iontophoretic transport of R-apomorphine was examined with both human SC and freshly dermatomed human skin, at room temperature and at 32 degrees C. Furthermore, the relationship between current density and iontophoretic flux was investigated. By increasing the temperature from 22 to 32 degrees C, the iontophoretic transport rate of R-apomorphine in human SC was increased 1.9-fold. Also the iontophoretic flux increased linearly with the increase in the current density from 100 to 500 microA/cm(2). When using dermatomed human skin instead of SC, the iontophoretic flux at a current density of 500 microA/cm(2) was decreased from 362+/-45 to 259+/-30 nmol/cm(2)h, and the corresponding lag time was prolonged from 0.8 to 2.8h. In conclusion, the combination of non-occlusive pretreatment with the surfactant formulation and iontophoresis has shown to substantially increase the transdermal transport rate of R-apomorphine. A linear relationship between current density and R-apomorphine flux indicates that the iontophoretic delivery combined with surfactant pretreatment allows a controlled and individualised administration of R-apomorphine.

The phase behaviour of skin lipid mixtures based on synthetic ceramides

The lipid lamellae present in the outermost layer of the skin, the stratum corneum (SC), form the main barrier for diffusion of molecules across the skin. The main lipid classes in SC are cholesterol (CHOL), free fatty acids (FFA) and at least nine classes of ceramides (CER), referred to as CER1 to CER9. In the present study the phase behaviour of four synthetic CER, either single or mixed with CHOL or CHOL and FFA, has been studied using small and wide angle X-ray diffraction. The lipid mixtures showed complex phase behaviour with coexistence of several phases. The results further revealed that the presence of synthetic CER1 as well as a proper composition of the other CER in the mixture were crucial for the formation of a phase with a long periodicity, characteristic for SC lipid phase behaviour. Only a mixture containing synthetic CER1 and CER3, CHOL and FFA showed similar phase behaviour to that of SC.

The in vivo transport of elastic vesicles into human skin: effects of occlusion, volume and duration of application

In the present study, several aspects of elastic vesicle transport into human skin were investigated in vivo. Surfactant-based elastic vesicles were applied onto human skin in vivo and subsequently a series of tape-strippings were performed, which were visualised by freeze fracture electron microscopy. Factors of investigation for non-occlusive treatment were the duration of application and the volume of application. In addition, occlusive vs. non-occlusive application was studied. The results have shown a fast penetration of intact elastic vesicles into the stratum corneum after non-occlusive treatment, frequently via channel-like regions. Intact vesicles could reach the ninth tape-strip after the 1-h non-occlusive treatment. After the 4-h treatment, vesicle material could be found in the 15th tape-strip. However, micrographs of the 4-h treatment showed extensive vesicle fusion, both at the skin surface as well as in the deeper layers of the stratum corneum. A higher volume of application resulted in an increase in the presence of vesicle material found in the deeper layers of the stratum corneum. Micrographs after occlusive treatment revealed very few intact vesicles in the deeper layers of the stratum corneum, but the presence of lipid plaques was frequently observed. Furthermore, we have proposed a hypothesis that the channel-like regions represent imperfections within the intercellular lipid lamellae in areas with highly undulating cornified envelopes.

Combined chemical and electrical enhancement modulates stratum corneum structure

In a previous in vitro study it has been shown that pretreatment with a water-based surfactant formulation results in a two-fold increase in transdermal iontophoretic transport of R-apomorphine compared to iontophoresis only. The aim of the study presented in this paper was to unravel the mechanisms involved in the increased iontophoretic delivery. Freeze fracture electron microscopy and cryo-scanning electron microscopy were used to visualise the ultrastucture of human stratum corneum after (i) application of the surfactant formulation, (ii) iontophoresis and (iii) application of the surfactant formulation followed by iontophoresis. Non-occlusive application of the surfactant formulation did not exert any detectable changes in the ultrastructure of the stratum corneum, except for swelling of the outermost corneocyte layers. Application of a current density of 0.5 mA/cm(2) for 9 h induced a swelling of the corneocytes and the formation of water pools that were occasionally present in the intercellular regions. Application of the surfactant formulation followed by iontophoresis resulted in a further swelling of the corneocytes and a frequent presence of water pools in the intercellular regions throughout the whole stratum corneum. The observed changes in the ultrastructure of the stratum corneum can explain the increased R-apomorphine transport during iontophoresis.

Water distribution and related morphology in human stratum corneum at different hydration levels

This study focused on the water distribution in human stratum corneum and on the swelling of the corneocytes. For this purpose stratum corneum was hydrated to various levels and used either for Fourier transform infrared spectroscopy or for cryo-scanning electron microscopy. The images were analyzed with respect to water localization and cell shape. The Fourier transform infrared spectra were measured to study the water-lipid interactions. The results show that water only slightly changes the lipid transitions in the stratum corneum even at a hydration level of 300% wt/wt compared to stratum corneum and that water is inhomogeneously distributed in the stratum corneum. No gradual increase in water level was observed in depth. At 57%-87% wt/wt water content the hydration level in the central part of stratum corneum is higher than in the superficial and deeper cell layers. Water domains are mainly present within the corneocytes and not in the intercellular regions. At a very high hydration level (300% wt/wt), the corneocytes are strongly swollen except for the deepest cell layers adjacent to the viable epidermis. The corneocytes in these layers are not swollen. At 300% wt/wt hydration level water domains are also present in intercellular regions. Between 17% wt/wt and 300% wt/wt the cell thickness increases linearly with the hydration level suggesting that swelling of cells mainly occurs in the direction perpendicular to the skin surface. At an increased hydration level, the corneocyte envelope more efficiently surrounds the cell content compensating for the increased cell volume. The changes in stratum corneum morphology with increasing water level have also been observed in dermatomed skin.

Permeant lipophilicity and vehicle composition influence accumulation of dyes in hair follicles of human skin

In skin and hair research drug targeting to the hair follicle is of great interest. Therefore the influence of permeant lipophilicity and vehicle composition on local accumulation has been examined using confocal laser scanning microscopy (CLSM). Formulations saturated with either Oregon Green 488, Bodipy FL C(5) or Bodipy 564/570 C(5) were prepared. The dyes were applied in citric acid buffer, 8% (w/v) surfactants in citric acid buffer or 8% (w/v) surfactants/20% (w/v) propylene glycol in citric acid buffer. Flow-through diffusion experiments were performed with fresh human scalp skin, after which the skin was imaged using CLSM. Diffusion studies showed for Oregon Green 488 (low lipophilicity) a higher flux when applied in citric acid buffer compared to surfactants. In contrast the fluxes of the more lipophilic dyes (Bodipy FL C(5) and Bodipy 564/570 C(5)) are highest when applied in surfactants/propylene glycol. CLSM studies revealed that follicular accumulation increased with (i) a lipophilic dye and (ii) application of lipophilic dyes in surfactants-propylene glycol. Therefore we conclude that targeting to the hair follicle can be increased by the use of lipophilic drugs in combination with surfactant solutions and propylene glycol.

Pretreatment with a water-based surfactant formulation affects transdermal iontophoretic delivery of R-apomorphine in vitro

PURPOSE

To further increase the transdermal transport rate of R-apomorphine, a nonocclusive pretreatment with an aqueous surfactant formulation in combination with iontophoresis was explored in vitro.

METHODS

The human stratum corneum was pretreated nonocclusively with formulations composed of laureth-3 oxyethylene ether (C12EO3), laureth-7 oxyethylene ether (C12EO7), and cholesterol sulfate (CSO4) prior to iontophoresis. The effect on the flux of the following parameters was examined: the composition, the charge, and the applied amount of surfactant formulations.

RESULTS

The iontophoretic flux of R-apomorphine was appreciably increased by pretreatment with surfactant formulations. A formulation containing C12EO3/C12EO7/CSO4 at a molar ratio of 70:30:5 was very stable and increased the iontophoretic flux of R-apomorphine from 92.2 +/- 13.9 nmol/cm2 x h to 181.5 +/- 22.6 nmol/cm2 x h. When further increasing the negative charge of this formulation the iontophoretic transport rate was slightly inhibited. A dose of 40 microL/cm2 of the formulation with a total surfactant concentration of 5% (w/w) was sufficient for a maximum enhancing effect.

CONCLUSIONS

The results obviously show that nonocclusive pretreatment with the surfactant formulation enhances the iontophoretic transport of R-apomorphine, and is a promising approach to achieve therapeutic concentrations of R-apomorphine.

The in vitro transport of pergolide from surfactant-based elastic vesicles through human skin: a suggested mechanism of action

This paper reports the in vitro transport of pergolide from L-595-PEG-8-L elastic vesicle formulations. Several aspects of vesicular delivery were studied in order to elucidate the possible mechanisms of action and to establish the optimal conditions and drug candidates for usage with L-595-PEG-8-L elastic vesicles. All studies were performed using human skin and flow-through Franz diffusion cells. Pergolide was chosen as model drug. The findings show that there was a strong correlation between the drug incorporation to saturated levels and the drug transport, both of which were influenced by the pH of the drug-vesicular system. The optimal pH was found to be 5.0, giving the highest drug incorporation as well as the highest drug transport. Non-occlusive co-treatment with elastic vesicles improved the skin delivery of pergolide compared to the non-occlusive buffer control by more than 2-fold. However, non-occlusive pre-treatment of skin with empty vesicles did not enhance drug transport. Occlusion improved drug transport from both elastic vesicle as well as buffer solutions due to the fact that water is an excellent penetration enhancer for pergolide. However, in contrast to non-occlusive application, the action of the elastic vesicles themselves was diminished, as occlusive treatments with elastic vesicles showed a lower flux compared to occlusive treatment with the buffer control. Hence, the highest pergolide skin permeation in this study was obtained from an occluded saturated buffer solution, giving a steady-state flux of 137.9 ng/h cm(-2). The volume of application did not have any effect on the drug transport. In conclusion, these results showed no evidence that a penetration enhancing effect is the main mechanism of action. The pH of the drug-vesicular system is an important factor to consider when optimising elastic vesicle delivery systems. Occlusion reduces the actions of elastic vesicles, but could increase the pergolide transport since water is a good penetration enhancer for this particular drug. Based on the results obtained, a mechanism of action for the elastic vesicles was proposed.

Structure of the skin barrier and its modulation by vesicular formulations

The natural function of the skin is to protect the body from unwanted influences from the environment. The main barrier of the skin is located in the outermost layer of the skin, the stratum corneum. Since the lipids regions in the stratum corneum form the only continuous structure, substances applied onto the skin always have to pass these regions. For this reason the organization in the lipid domains is considered to be very important for the skin barrier function. Due to the exceptional stratum corneum lipid composition, with long chain ceramides, free fatty acids and cholesterol as main lipid classes, the lipid phase behavior is different from that of other biological membranes. In stratum corneum crystalline phases are predominantly present, but most probably a subpopulation of lipids forms a liquid phase. Both the crystalline nature and the presence of a 13 nm lamellar phase are considered to be crucial for the skin barrier function. Since it is impossible to selectively extract individual lipid classes from the stratum corneum, the lipid organization has been studied in vitro using isolated lipid mixtures. These studies revealed that mixtures prepared with isolated stratum corneum lipids mimic to a high extent stratum corneum lipid phase behavior. This indicates that proteins do not play an important role in the stratum corneum lipid phase behavior. Furthermore, it was noticed that mixtures prepared only with ceramides and cholesterol already form the 13 nm lamellar phase. In the presence of free fatty acids the lattice density of the structure increases. In stratum corneum the ceramide fraction consists of various ceramide subclasses and the formation of the 13 nm lamellar phase is also affected by the ceramide composition. Particularly the presence of ceramide 1 is crucial. Based on these findings a molecular model has recently been proposed for the organization of the 13 nm lamellar phase, referred to as "the sandwich model", in which crystalline and liquid domains coexist. The major problem for topical drug delivery is the low diffusion rate of drugs across the stratum corneum. Therefore, several methods have been assessed to increase the permeation rate of drugs temporarily and locally. One of the approaches is the application of drugs in formulations containing vesicles. In order to unravel the mechanisms involved in increasing the drug transport across the skin, information on the effect of vesicles on drug permeation rate, the permeation pathway and perturbations of the skin ultrastructure is of importance. In the second part of this paper the possible interactions between vesicles and skin are described, focusing on differences between the effects of gel-state vesicles, liquid-state vesicles and elastic vesicles.

The in vivo and in vitro interactions of elastic and rigid vesicles with human skin

Elastic vesicles are the most novel development in vesicular systems design for dermal and transdermal drug delivery. However, interactions between these vesicles and human skin are not yet fully understood. In this study, the in vivo and in vitro interactions between elastic-, rigid vesicles and micelles with human skin were investigated. Vesicle and micelle solutions were applied onto human skin in vitro and in vivo. Subsequently, a series of tape strippings were performed, which were visualised by freeze fracture electron microscopy (FFEM). The results showed no ultrastructural changes in skin treated with rigid vesicles. Skin treated with elastic vesicles, however, showed a fast partitioning of intact vesicles into the deeper layers of the stratum corneum (SC), where they accumulated in channel-like regions. Only little vesicle material was found in the deepest layers of the SC, suggesting that the partitioning of intact vesicles from the SC into the viable epidermis is unlikely to happen. Treatment with micelles resulted in rough, irregular fracture planes. Similar results were obtained in vitro and in vivo, indicating an excellent in vitro/in vivo correlation. These results support the hypothesis that elastic vesicles have superior characteristics to rigid vesicles for the interaction with human skin. Elastic vesicles and micelles demonstrated very different interactions with human skin and hence probably also have different mechanisms of action for the enhancement of drug transport.

In vitro iontophoresis of R-apomorphine across human stratum corneum. Structure-transport relationship of penetration enhancement

To achieve a therapeutical effect of the anti-Parkinson's drug R-apomorphine via iontophoresis delivery, enhancement strategies in vitro were explored using three structurally related enhancers, lauric acid (LA), dodecyltrimethylammonium bromide (DTAB) and Laureth-3 oxyethylene ether (C(12)EO(3)). Human stratum corneum and shed snake skin were pretreated with 0.15 M each enhancer solution in propylene glycol (PG). Thereafter, passive diffusion, iontophoretic transport and post-iontophoretic passive diffusion were investigated. Compared to the control (PG pretreatment), a slight inhibition on both passive and iontophoretic delivery was observed with cationic surfactant DTAB pretreated stratum corneum. Pretreatment with anionic surfactant LA resulted in a great enhancement on passive delivery, but only a small enhancing effect on the iontophoretic delivery. Unlike the others, the nonionic surfactant C(12)EO(3) substantially increased iontophoretic transport rate of R-apomorphine by 2.3-fold, whereas passive delivery was basically unchanged or slightly affected. The magnitude of enhancing effect of C(12)EO(3) was dependent on the surfactant concentration and the pretreatment duration. Moreover, comparison of transport data through shed snake skin with human stratum corneum indicates that both shunt- and intercellular pathways are involved in the iontophoretic transport of R-apomorphine.

Skin structure and mode of action of vesicles

The natural function of the skin is to protect the body for unwanted influences from the environment. The main barrier of the skin is located in the outermost layer of the skin, the stratum corneum. Since the lipids regions in the stratum corneum form the only continuous structure, substances applied onto the skin always have to pass these regions. Therefore, in the first part of this paper, the barrier function has been explained, focusing on the lipid composition and organisation. The major obstacle for topical drug delivery is the low diffusion rate of drugs across the stratum corneum. Several methods have been assessed to increase the permeation rate of drugs temporarily. One of the approaches is the application of drugs in formulations containing vesicles. In order to unravel the mechanisms involved in increasing the drug transport across the skin, information on the effect of vesicles on drug permeation rate, the permeation pathway and perturbations of the skin ultrastructure is of importance. In the second part of this paper, the possible interactions between vesicles and skin are described, focusing on differences between the effects of gel-state, liquid-state, and elastic vesicles.

Penetration and distribution of three lipophilic probes in vitro in human skin focusing on the hair follicle

Fluorescent model substances of increasing lipophilicity (Oregon Green) 488, Bodipy, FL C5 and Bodipy 564/570 C5) were selected to enable the visualization in the skin using confocal laser scanning microscopy. After measuring the penetration for 18 h, the nonfixed human scalp skin was imaged from the bottom parallel to the stratum corneum and in a cross-section view perpendicular to the skin surface. The images were evaluated by calculating relative accumulation values for different penetrants. The studies indicate that the penetrated amount is highest for Bodipy FL C5 (medium lipophilicity) and lowest for Bodipy 564/570 C5 (high lipophilicity) whereas Bodipy 564/570 C5 (high lipophilicity) reveals the highest relative accumulation in parts of the hair follicle compared to Oregon Green 488 (low lipophilicity). The addition of 30% (v/v) ethanol to the donor phase of substance with a low lipophilicity increases the follicular delivery. From our results we conclude that delivery to the hair follicle can be improved by increasing the drugs lipophilicity and optimizing the composition of the donor phase. However, no conclusion can be drawn about the actual route of transport to the hair follicle.

Transdermal delivery of pergolide from surfactant-based elastic and rigid vesicles: characterization and in vitro transport studies

PURPOSE

The aim of this study was to investigate the effect of elastic and rigid vesicles on the penetration of pergolide across human skin.

METHODS

Vesicles used consisted of the bilayer-forming surfactant L-595 (sucrose laurate ester) and the micelle-forming surfactant PEG-8-L (octaoxyethylene laurate ester), together with the stabilizer sulfosuccinate. A series of L-595/PEG-8-L/sulfosuccinate vesicles were investigated, ranging from very rigid to very elastic. Pergolide-loaded elastic and rigid vesicles were visualized using Cryo-TEM and characterized for size and stability. Transdermal penetration of pergolide from different vesicle compositions was studied in vitro using flow-through Franz diffusion cells. A saturated buffer solution served as the control.

RESULTS

Vesicle composition had a major effect on the physicochemical characteristics, morphology and drug solubility of the vesicular system. L-595/PEG-8-L/sulfosuccinate (70/30/5) elastic vesicles gave the best balance between vesicle stability and elasticity, as well as the highest drug solubility. Transport studies clearly showed that elastic vesicles were superior to rigid vesicles. Elastic vesicles enhanced the drug transport compared to the buffer control, although rigid vesicles decreased the drug transport. The best drug transport was achieved from L-595/PEG-8-L/sulfosuccinate (70/30/5) elastic vesicles, resulting in a steady-state flux of 13.6 +/- 2.3 ng/ (h*cm2). This was a 6.2-fold increase compared to the most rigid vesicles.

CONCLUSIONS

This study supports the hypothesis that elastic vesicles are superior to rigid vesicles as vehicles for transdermal drug delivery.

Phase behavior of stratum corneum lipid mixtures based on human ceramides: the role of natural and synthetic ceramide 1

In a recent study the lipid phase behavior of mixtures of human ceramides, cholesterol, and free fatty acids has been examined. We observed in cholesterol: human ceramide mixtures a prominent formation of the 12.8 nm lamellar phase (referred to as the long periodicity phase). Addition of free fatty acids promoted the formation of a 5.6 nm lamellar phase (referred to as the short periodicity phase) and increased the subpopulation of lipids forming a fluid phase. In this study we focused on the role of human ceramide 1, as the presence of this ceramide appeared to be crucial for proper lipid phase behavior in mixtures prepared with ceramide isolated from pig stratum corneum. In order to do this, mixtures of cholesterol and free fatty acids were prepared with human ceramides, in which natural human ceramide 1 was replaced by either synthetic CER1-linoleate (CER1-lin), or CER1-oleate (CER1-ol), or CER1-stearate (CER1-ste). After substitution of natural human ceramide 1 by synthetic ceramide 1 the following observations were made. (i) In the presence of synthetic CER1-ste no long periodicity phase and no liquid phase could be detected. (ii) In the presence of HCER1-ol a liquid phase was more prominently formed than in the presence of HCER1-lin. (iii) In cholesterol:human ceramide mixtures in the presence of CER1-lin the long periodicity phase was more prominently present than in the presence of CER1-ol. (iv) In the presence of CER1-ste neither a long periodicity phase nor a liquid lateral packing could be detected. The results of these studies further indicate that for the formation of the long periodicity phase a certain (optimal) fraction of lipids has to form a liquid phase. When the fraction forming this liquid phase is either too low or too high, the formation of the short periodicity phase is increased at the expense of the formation of the long periodicity phase. Based on the results of this and previous studies we offer an explanation for the deviation in lipid organization in diseased and in dry skin compared to normal skin.

Phase behavior of lipid mixtures based on human ceramides: coexistence of crystalline and liquid phases

The lipid regions in the outermost layer of the skin (stratum corneum) form the main barrier for diffusion of substances through the skin. In this layer the main lipid classes are ceramides, cholesterol (CHOL), and FFA. Previous studies revealed a coexistence of two crystalline lamellar phases with periodicities of approximately 13 nm (referred to as long periodicity phase) and 6 nm (short periodicity phase). Additional studies showed that lipid mixtures prepared with isolated pig ceramides (pigCER) mimic lipid phase behavior in stratum corneum closely. Because the molecular structure of pigCER differs in some important aspects from that of human ceramides (HCER), in the present study the phase behavior of mixtures prepared with HCER has been examined. Phase behavior studies of mixtures based on HCER revealed that in CHOL:HCER mixtures the long periodicity phase dominates. In the absence of HCER1 the short periodicity phase is dominant. Addition of FFA promotes the formation of the short periodicity phase and induces a transition from a hexagonal sublattice to an orthorhombic sublattice. Furthermore, the presence of FFA promotes the formation of a liquid phase. Finally, cholesterol sulfate, a minor but important lipid in the stratum corneum, reduces the amount of cholesterol that phase separates in crystalline domains. From these observations it can be concluded that the phase behavior of mixtures prepared from HCER differs in some important aspects from that prepared from pigCER. The most prevalent differences are the following: i) the addition of FFA promotes the formation of the short periodicity phase; and ii) liquid lateral packing is obviously present in CHOL:HCER:FFA mixtures. These changes in phase behavior might be due to a larger amount of linoleic acid moiety in HCER mixtures compared with that in pigCER mixtures.

Lontophoretic delivery of apomorphine in vitro: physicochemic considerations

PURPOSE

To examine the mechanisms of transdermal iontophoretic delivery of apomorphine.

METHODS

Anodal iontophoresis of R-apomorphine across human stratum corneum was determined in vitro. The effects on the flux of the following parameters were studied: stability of drug, pH of donor solution, concentration of NaCl, and type of Na+ co-ions.

RESULTS

Ascorbic acid was effective to prevent apomorphine degradation. The iontophoretic transport of apomorphine was strongly influenced by the pH of the donor formulation. Increasing the pH from 3 to 6 resulted in an increase in the iontophoretic apomorphine flux from 27.9+/-4.4 nmol/cm2*h to 78.2+/-6.9 nmol/cm2*h. Upon decreasing NaCl concentration from 8 to 2 g/L, the iontophoretic flux was not significantly changed. Replacing NaCl in the donor formulation by tetraethylammonium chloride or tetrabutylammonium chloride resulted in 1.3 fold greater steady-state flux.

CONCLUSIONS

For optimized apomorphine iontophoretic delivery, a constant pH of the donor formulation is of great importance. The results suggest that although flux enhancement during iontophoresis is largely due to the electrical potential gradient, secondary effects, such as convective flow and electroosmosis may also contribute.

Aberrant lipid organization in stratum corneum of patients with atopic dermatitis and lamellar ichthyosis

There are several skin diseases in which the lipid composition in the intercellular matrix of the stratum corneum is different from that of healthy human skin. It has been shown that patients suffering from atopic dermatitis have a reduced ceramide content in the stratum corneum, whereas in the stratum corneum of lamellar ichthyosis patients, the amount of free fatty acids is decreased and the ceramide profile is altered. Both patient groups also show elevated levels of transepidermal water loss indicative of an impaired barrier function. As ceramides and free fatty acids are essential for a proper barrier function, we hypothesized that changes in the composition of these lipids would be reflected in the lipid organization in stratum corneum of atopic dermatitis and lamellar ichthyosis patients. We investigated the lateral lipid packing using electron diffraction and the lamellar organization using freeze fracture electron microscopy. In atopic dermatitis stratum corneum, we found that, in comparison with healthy stratum corneum, the presence of the hexagonal lattice (gel phase) is increased with respect to the orthorhombic packing (crystalline phase). In lamellar ichthyosis stratum corneum, the hexagonal packing was predominantly present, whereas the orthorhombic packing was observed only occasionally. This is in good agreement with studies on stratum corneum lipid models that show that the presence of long-chain free fatty acids is involved in the formation of the orthorhombic packing. The results of this study also suggest that the ceramide composition is important for the lateral lipid packing. Finally, using freeze fracture electron microscopy, changes in the lamellar organization in stratum corneum of both patient groups could be observed.

X-ray microanalysis of cryopreserved human skin to study the effect of iontophoresis on percutaneous ion transport

PURPOSE

To study at the ultrastructural level which part of the skin is associated with percutaneous iodide transport by passive diffusion and iontophoresis.

METHODS

Following passive diffusion or iontophoresis of iodide, the morphology and the ion distribution of the skin was preserved by rapid freezing. The skin was kept frozen until and during examination by transmission electron microscopy (TEM) and X-ray microanalysis (XRMA). The intrinsic electron absorbing characteristics of cryopreserved skin allow direct TEM examination without additional staining. XRMA can be used to obtain in a relatively nondestructive way in situ information on ion distributions across the skin.

RESULTS

After passive diffusion, iodide was mainly found in the stratum corneum (SC), whereas there was little iodide in the viable epidermis. Iontophoresis up to 300 microA/cm2 did not significantly affect this distribution. With iontophoresis at 1,000 microA/cm2, the amount of iodide increased dramatically and was equally distributed over the SC and viable epidermis. The presence of iodide in the SC suggests that iodide is present inside corneocytes.

CONCLUSIONS

Iontophoresis up to 300 microA/cm2 does not significantly perturb skin structures in contrast to iontophoresis at 1,000 microA/cm2. The presence of iodide inside corneocytes suggests the possibility of transcellular percutaneous iodide transport.

Elasticity of vesicles assessed by electron spin resonance, electron microscopy and extrusion measurements

The composition of vesicles determines the physical state and elasticity of their bilayers. Fatty acid spin labels were incorporated into vesicles, composed of the single chain non-ionic surfactant octaoxyethylenelaurate-ester (PEG-8-L), the sucrose laurate-ester L-595 and cholesterol sulfate (CS) to monitor local dynamic properties of lipid molecules in vesicle bilayers and to study the elasticity of vesicle bilayers. Studies with the spin label probes 5-, 12- and 16-doxyl stearic acid (DSA) indicated that both the order parameter and the rotational correlation times increased when the doxyl group was positioned closer to the headgroup region. These findings indicate that the fluidity of membranes decreased near the headgroup region. Comparing 16-DSA incorporated in vesicle formulations with either 30 or 70 mol% showed no difference in alkyl chain mobility as was reflected by the order parameter. The rotational correlation times, however, showed a slowdown from 0.38 to 0.71 and 1.13 ns when the PEG-8-L molar content was decreased from 100 to 70 and 30 mol% for PEG-8-L:L-595:CS vesicles, respectively. Extrusion measurements indicated an increase in elasticity of vesicle bilayers as the molar content of PEG-8-L was increased from 10 to 90 mol%. Incorporation of cholesterol sulfate stabilizes vesicles and thereby, decreases the elasticity. The increased elasticity correlated excellent with a reduction in the rotational correlation times observed. In conclusion, these results demonstrate that when the molar content of the single chain non-ionic surfactant PEG-8-L in vesicles is increased the elasticity is enhanced and the rotational correlation time is reduced. The enhanced elasticity might contribute to an optimal design of vesicles as drug carriers for transdermal application.

The influence of two azones and sebaceous lipids on the lateral organization of lipids isolated from human stratum corneum

The main problem with topical application of compounds to administer drugs to and regulate drug levels in a human body, is the barrier formed by the intercellular lipid matrix of the stratum corneum (SC). In a search for possibilities to overcome this barrier function, a good understanding of the organization and phase behavior of these lipids is required. SC lipid model studies especially provide a wealth of information with respect to the lipid organization and the importance of certain subclasses of lipids for the structure. Previously, we have shown that electron diffraction (ED) provides detailed information on the lateral lipid packing in both intact SC (G.S.K. Pilgram et al., J. Invest. Dermatol. 113 (1999) 403) and SC lipid models (G.S.K. Pilgram et al., J. Lipid Res. 39 (1998) 1669). In the present study, we used ED to examine the influence of two azones and sebaceous lipids on the lateral phase behavior of lipids isolated from human SC. We established that human SC lipids are arranged in an orthorhombic packing pattern. Upon mixing with the two enhancers the orthorhombic packing pattern was still observed; however, an additional fluid phase became more apparent. In mixtures with sebaceous lipids, the presence of the hexagonal lattice increased. These findings provide a basis for the mechanism by which these enhancers and sebaceous lipids interact with human SC lipids.

The lipid organisation in the skin barrier

The main function of the skin is to protect the body against exogenous substances. The skin barrier is located in the outermost layer of the skin, the stratum corneum. This layer consists of keratin enriched cells embedded in lipid lamellae. These lamellae form the main barrier for diffusion of substances through the skin. In diseased skin the barrier function is often impaired. For a full understanding of the properties of the human skin barrier, insight in the stratum corneum lipid organisation is of great importance. In this paper a short description of the lipid organisation in normal human stratum corneum will be given, after which the role the main lipid classes play in the stratum corneum lipid organisation will be described. In addition the effect of cholesterol sulfate and calcium on the lipid organisation will be discussed. Finally a new model, the "sandwich model", will be proposed that describe the localisation of the fluid phases in the stratum corneum.

Transdermal macromolecular delivery: real-time visualization of iontophoretic and chemically enhanced transport using two-photon excitation microscopy

PURPOSE

To investigate the transdermal delivery of a model macromolecule by passive and iontophoretic means following pretreatment with C12-penetration enhancers and to visualise transport across human stratum corneum (SC) in real time.

METHODS

Transport studies of dextran, labelled with fluorescent Cascade Blue (D-CB: M(R) = 3 kDa) across human stratum corneum, were conducted during passive and iontophoretic modes of delivery following pretreatment with either dodecyltrimethylammonium bromide (DTAB), sodium dodecyl sulphate (SDS) or Azone. Size-exclusion chromatography was used to assess maintenance of dextran structural integrity throughout experimental lifetime. Two-photon excitation microscopy was employed to visualise real-time dextran transport during current application.

RESULTS

The positively charged C12-enhancer DTAB elevated passive D-CB steady-state flux (J(ss)) and was the only enhancer to do so above control during iontophoresis. The negatively charged SDS had the least effect during both stages. On-line macromolecular transport was visualised, indicating both inter- and intra-cellular pathways across SC during current application. No transport was visible across untreated SC during passive transport.

CONCLUSIONS

Use of a positively charged enhancer may improve J(ss) of anionic macromolecular penetrants during passive and iontophoretic delivery. On-line visualisation of iontophoresis across SC was possible and can provide mechanistic insight into SC transport pathways.

The effect of two azones on the lateral lipid organization of human stratum corneum and its permeability

PURPOSE

Investigation of the relationship between changes in human SC lipid organization induced by N-alkyl-azocycloheptane-2-one and SC permeability to the model compound HgCl2.

METHODS

Human dermatomed skin was treated with propylene glycol (PG), oleyl-Azone (OAz) or dodecyl-Azone (DAz) in 0.15 M PG. Untreated skin served as control. The lateral lipid organization was studied by electron diffraction. Hg was measured on tape-strips by X-ray microanalysis and in the acceptor phase by atom absorption spectrometry.

RESULTS

In control and PG treated samples, the lipid packing was mainly orthorhombic, while a small fraction was hexagonal. In OAz and DAz treated samples, the orthorhombic lipid organization remained, however, the hexagonal packing was recorded less frequently. The amount of Hg decreased as a function of depth in all SC samples, however, the penetration profile increased significantly upon OAz treatment. The cumulative amount of Hg in the acceptor phase of OAz treated samples also increased significantly compared to control and PG treated samples.

CONCLUSIONS

The increased penetration of Hg into OAz treated skin could not be related to an orthorhombic-hexagonal phase transition. Alternatively, phase separation of OAz and/or formation of grain boundaries might affect SC permeability, hereby increasing Hg penetration. A similar mechanism is proposed for DAz.

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 3H(2)O 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 3H(2)O with an optimum application dose of 2.5 mg lipids/cm(2) compared to PBS pretreatment. No significant difference in the enhancement of the 3H(2)O-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 3H(2)O, 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.

Cholesterol sulfate and calcium affect stratum corneum lipid organization over a wide temperature range

The main diffusion barrier for drugs penetrating through the skin is located in the intercellular lipid matrix in the upper layer of the skin, the stratum corneum (SC). The main lipid classes in the SC are ceramides (CER), free fatty acids (FFA) and cholesterol (CHOL). The lipids in SC are organized into two lamellar phases with periodicities of approximately 13 and 6 nm, respectively. Similar lipid organization has been found with equimolar CHOL:CER:FFA mixtures in SAXD studies performed at room temperature. However, one may conclude that the phase behavior of the mixtures is similar to that in SC only when the lipid organization of the lipid mixtures resembles that in SC over a wide temperature range. Therefore, in the present study, the organization of the lipid mixtures has been studied in a temperature range between 20 degrees and 95 degrees C. From these experiments it appeared that at elevated temperatures in equimolar CHOL:CER:FFA mixtures a new prominent 4.3 nm phase is formed between 35;-55 degrees C, which is absent or only weakly formed in intact human and pig SC, respectively. As it has been suggested that gradients of pH and cholesterol sulfate exist in the SC and that Ca(2+) is present only in the lowest SC layers, the effect of pH, cholesterol sulfate, and Ca(2+) on the lipid phase behavior has been investigated with lipid mixtures. Both an increase in pH from 5 (pH at the skin surface) to 7.4 (pH at the SC;-stratum granulosum interface) and the presence of cholesterol sulfate promote the formation of the 13 nm lamellar phase. Furthermore, cholesterol sulfate reduces the amount of CHOL that is present in crystalline domains, causes a shift in the formation of the 4.3 nm phase to higher temperatures, and makes this phase less prominent at higher temperatures. The finding that Ca(2+) counteracts the effects of cholesterol sulfate indicates the importance of a proper balance of minor SC components for appropriate SC lipid organization. In addition, when the findings are extrapolated to the in vivo situation, it seems that cholesterol sulfate is required to dissolve cholesterol in the lamellar phases and to stabilize SC lipid organization. Therefore, a drop in cholesterol sulfate content in the superficial layers of the SC is expected to destabilize the lipid lamellar phases, which might facilitate the desquamation process.

Interactions of elastic and rigid vesicles with human skin in vitro: electron microscopy and two-photon excitation microscopy

Interactions between vesicle formulations and human skin were studied, in vitro, in relation to their composition and elasticity. The skin ultrastructure was investigated using transmission electron microscopy (TEM), freeze-fracture electron microscopy (FFEM) and two-photon fluorescence microscopy (TPE). The main difference between the vesicle formulations was their elasticity. Elastic vesicle formulations contained bilayer forming surfactants/lipids and single-chain surfactant octaoxyethylenelaurate-ester (PEG-8-L), whereas rigid vesicles contained bilayer surfactants in combination with cholesterol. TEM results showed three types of interactions after non-occlusive application of elastic PEG-8-L containing vesicle formulations on human skin: (1) the presence of spherical lipid structures containing or surrounded by electron dense spots; (2) oligolamellar vesicles were observed between the corneocytes in the upper part of the stratum corneum; and (3) large areas containing lipids, surfactants and electron dense spots were observed deeper down into the stratum corneum. Furthermore, after treatment with vesicles containing PEG-8-L and a saturated C12-chain surfactant, small stacks of bilayers were found in intercellular spaces of the stratum corneum. Rigid vesicles affected only the most apical corneocytes to some extent. FFEM observations supported the TEM findings. Major morphological changes in the intercellular lipid bilayer structure were only observed after treatment with PEG-8-L containing elastic vesicles. TPE showed a distinct difference in penetration pathways after non-occlusive application of elastic or rigid vesicles. After treatment with elastic vesicles, thread-like channels were formed within the entire stratum corneum and the polygonal cell shape of corneocytes could not be distinguished. Fluorescent label incorporated in rigid vesicles was confined to the intercellular spaces of the upper 2-5 micrometer of the stratum corneum and the cell contours could still be distinguished.

Niosomes as a novel peroral vaccine delivery system

The feasibility to develop a peroral vaccine delivery system based on non-ionic surfactant vesicles (niosomes) was evaluated using BALB/c mice. Ovalbumin was encapsulated in various lyophilized niosome preparations consisting of sucrose esters, cholesterol and dicetyl phosphate. Two different formulations were compared in this study. The specific antibody titres within serum, saliva and intestinal washings were monitored by ELISA on days 7, 14, 21 and 28 after intragastric administration. Only encapsulation of ovalbumin into Wasag7 (70% stearate sucrose ester, 30% palmitate sucrose ester (40% mono-, 60% di/tri-ester)) niosomes resulted in a significant increase in antibody titres. Administration of ovalbumin and empty niosomes did not exert a similar effect, neither did administration of any control formulation. In contrast to ovalbumin loaded Wasag7 niosomes, application of the more hydrophilic Wasag15 (30% stearate sucrose ester, 70% palmitate sucrose ester (70% mono-, 30% di/tri-ester)) niosome preparations did not result in an increase in antibody titres.

Electron diffraction provides new information on human stratum corneum lipid organization studied in relation to depth and temperature

The outermost layer of mammalian skin, the stratum corneum, provides the body with a barrier against transepidermal water loss and penetration of agents from outside. The lipid-rich extracellular matrix surrounding the corneocytes in the stratum corneum is mainly responsible for this barrier function. In this study (cryo-) electron diffraction was applied to obtain information about the local lateral lipid organization in the extracellular matrix in relation to depth in human stratum corneum. For this purpose, stratum corneum grid-strips were prepared from native skin in vivo and ex vivo. It was found that the lipid packing in samples prepared at room temperature is predominantly orthorhombic. In samples prepared at 32 degrees C the presence of a hexagonal packing is more pronounced in the outer layers of the stratum corneum. Gradually increasing the specimen temperature from 30 to 40 degrees C induced a further transition from an orthorhombic to a hexagonal sublattice. At 90 degrees C all lipids were present in a fluid phase. These results are in good agreement with previously reported wide angle X-ray diffraction and Fourier transformed infrared spectroscopy studies. We conclude that the lipids in human stratum corneum are highly ordered throughout the stratum corneum and that electron diffraction allows monitoring of the local lipid organization, which contributes to the understanding of stratum corneum barrier function.

The role of ceramide composition in the lipid organisation of the skin barrier

The lipid lamellae in the stratum corneum (SC) play a key role in the barrier function of the skin. The major lipids are ceramides (CER), cholesterol (CHOL) and free fatty acids (FFA). In pig SC at least six subclasses of ceramides (referred to as CER 1, 2-6) are present. Recently it was shown that in mixtures of isolated pig SC ceramides (referred to as CER(1-6)) and CHOL two lamellar phases are formed, which mimic SC lipid organisation very closely [J.A. Bouwstra et al., 1996, J. Lipid Res. 37, 999-1011] [1]. Since the CER composition in SC originating from different sources/donors often varies, information on the effect of variations in CER composition on the SC lipid organisation is important. The results of the present study with mixtures of CHOL including two different CER mixtures that lack CER 6 (CER(1-5) mixtures) revealed that at an equimolar molar ratio their lipid organisation was similar to that of the equimolar CHOL:CER(1-6) and CHOL:CER(1,2) mixtures, described previously. These observations suggest that at an equimolar CHOL:CER ratio the lipid organisation is remarkably insensitive toward a change in the CER composition. Similar observations have been made with equimolar CHOL:CER:FFA mixtures. The situation is different when the CHOL:CER molar ratio varies. While in the CHOL:CER(1-6) mixture the lamellar organisation hardly changed with varying molar ratio from 0.4 to 2, the lamellar organisation in the CHOL:CER(1-5) mixtures appeared to be more sensitive to a change in the relative CHOL content, especially concerning the changes in the periodicities of the lamellar phases. In summary, these findings clearly indicate that at an equimolar CHOL:CER molar ratio the lamellar organisation is least sensitive to a variation in CER composition, while at a reduced CHOL:CER molar ratio the CER composition plays a more prominent role in the lamellar phases. This observation may have an implication for the in vivo situation when both the CER composition and the CHOL:CER molar ratio change simultaneously.

Chemical enhancement of percutaneous absorption in relation to stratum corneum structural alterations

The outermost layer of the skin, stratum corneum (SC), provides an outstanding barrier against the external environment and is also responsible for skin impermeability toward most solutes. The barrier function is related to the unique composition of the SC lipids and their complex structural arrangement. The lipoidal matrix of the SC, therefore, is a target of penetration enhancer action. The literature on the skin barrier structure and function and on the mechanisms of action of some well established permeation promoters, with a focus on their impact on SC structural alterations, is reviewed. Data obtained from infrared, thermal, and fluorescence spectroscopic examinations of the SC and its components imply enhancer improved permeation of solutes through the SC is associated with alterations involving the hydrocarbon chains of the SC lipid components. Data obtained from electron microscopy and X-ray diffraction reveals that the disordering of the lamellar packing is also an important mechanism for increased permeation of drugs induced by penetration enhancers.

Application of vesicles to rat skin in vivo: a confocal laser scanning microscopy study

A major problem in (trans)dermal drug delivery is the low penetration rate of most substances through the barrier of the skin, the stratum corneum. One of the methods to increase the penetration rate across the skin is encapsulation of a (model) drug in lipid vesicles. In this study fluorescently labelled liposomes were applied on rat skin, in vivo. Bilayer labelled gel-state and liquid-state liposomes (conventional or with flexible bilayers) were non-occlusively applied on the dorsal area in the neck of the rat for 1, 3 or 6 h. Micelles were used as a control formulation. The penetration pathway and penetration depth of the lipophilic fluorescent label into the skin was visualised by confocal laser scanning microscopy (CLSM). During the first 3 h of application almost no differences in penetration depth were observed, when the label was applied in the various formulations. After 6 h application, it was clear that the label applied in micelles and gel-state liposomes did not penetrate as deep into the skin as the label applied in liquid-state vesicles. Among the liquid-state vesicles, the suspension with the most flexible bilayers showed the highest fluorescence intensity in the viable epidermis and dermis, 6 h post-application. Thus the vesicular form and the thermodynamic state of the bilayer and to a smaller extent the flexibility of the bilayer influence the penetration depth of the label into the skin at longer application periods. These results are in good agreement with CLSM results obtained from in vitro experiments with human skin.