Alfonso de la Maza

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.

Direct formation of mixed micelles in the solubilization of phospholipid liposomes by Triton X-100

The vesicle to micelle transition which results in the interaction of the Triton X‐100 surfactant with phosphatidylcholine vesicles was studied by means of dynamic light scattering (at different reading angles) and by freeze‐fracture electron microscopy techniques. Vesicle solubilization was produced by the direct formation of mixed micelles without the formation of complex intermediate aggregates. Thus, vesicle to micelle transformation was mainly governed by the progressive formation of mixed micelles within the bilayer. A subsequent separation of these micelles from the liposome surface (vesicle perforation by the formation of surfactant‐stabilized holes on the vesicle surface) led to a complete solubilization of liposomes.

Conformational Changes in Stratum Corneum Lipids by Effect of Bicellar Systems

Attenuated total reflectance-Fourier transform infrared (ATR-FTIR) spectroscopy was applied to study the effects of the bicelles formed by dimyristoyl-glycero-phosphocholine (DMPC) and dihexanoyl-glycero-phosphocholine (DHPC) in porcine stratum corneum (SC) in vitro. A comparison of skin samples treated and untreated with bicelles at different temperatures was carried out. The analysis of variations after treatment in the position of the symmetric CH2 stretching, CH2 scissoring, and CH2 rocking vibrations reported important information about the effect of bicelles on the skin. Bicellar systems caused a phase transition from the gel or solid state to the liquid crystalline state in the lipid conformation of SC, reflecting the major order-disorder transition from hexagonally packed to disordered chains. Grazing incidence small and wide X-ray scattering (GISAXS and GIWAXS) techniques confirmed this effect of bicelles on the SC. These results are probably related to with the permeabilizing effect previously described for the DMPC/DHPC bicelles.

Bicelles: Lipid Nanostructured Platforms with Potential Dermal Applications

Bicelles emerge as promising membrane models, and because of their attractive combination of lipid composition, small size and morphological versatility, they become new targets in skin research. Bicelles are able to modify skin biophysical parameters and modulate the skins barrier function, acting to enhance drug penetration. Because of their nanostructured assemblies, bicelles have the ability to penetrate through the narrow intercellular spaces of the stratum corneum of the skin to reinforce its lipid lamellae. The bicelle structure also allows for the incorporation of different molecules that can be carried through the skin layers. All of these characteristics can be modulated by varying the lipid composition and experimental conditions. The remarkable versatility of bicelles is their most important characteristic, which makes their use possible in various fields. This system represents a platform for dermal applications. In this review, an overview of the main properties of bicelles and their effects on the skin are presented.

Application of Bicellar Systems on Skin: Diffusion and Molecular Organization Effects

The effect of bicelles formed by dipalmitoylphosphatidylcholine (DPPC)/dihexanoylphosphatidylcholine (DHPC) on stratum corneum (SC) lipids was studied by attenuated total reflectance-Fourier transform infrared (ATR-FTIR) spectroscopy at different temperatures. Analysis of the lipid organization in terms of chain conformational order and lateral packing shows that the use of bicelles hampers the fluidification of SC lipids with temperature and leads to a lateral packing corresponding to a stable hexagonal phase. Grazing incidence small- and wide-angle X-ray scattering (GISAXS and GIWAXS) techniques confirm these results and give evidence of higher lamellar order after treatment with these bicelles. Additionally, the effects of DPPC/DHPC and dimyristoylphosphatidylcholine (DMPC)/DHPC bicelles at different SC depths were compared. The combination of ATR-FTIR spectroscopy and the tape-stripping method was very useful for this purpose.

Barrier function of intact and impaired skin: percutaneous penetration of caffeine and salicylic acid

Background  Normally, percutaneous absorption tests are carried out using skin biopsies for an apparent and acceptable physiological condition. However, under different pathological conditions, the stratum corneum (SC) barrier function is impaired.

Influence of the fluidity of liposome compositions on percutaneous absorption.

The penetration into the stratum corneum of fluorescein, as the acid form or as a sodium salt, encapsulated in liposomes formed by liquid- or gel-state phospholipids, with or without cholesterol, was investigated in humans by the stripping method. Liposomes prepared by extrusion were applied to the forearms of healthy human volunteers and 30 min later, strippings were performed. Fluorescein was extracted and determined by spectrofluorimetry. The skin penetration of sodium fluorescein was higher from fluid liposomes (phosphatidylcholine) than from rigid liposomes (hydrogenated phosphatidylcholine), but it was independent of the content of cholesterol. It seems that the liquid-crystalline state of the lipids is the main aspect involved in the fluidity of the liposome bilayer itself as well as in the interaction with the lipids of the stratum corneum. The similar enhanced penetration behavior obtained for unsaturated liposomes containing sodium or acid fluorescein seems to support the hypothesis of a previous destruction of the vesicles during its passage through the lipid intercellular pathway in the stratum corneum.

Vesicle to micelle phase transitions involved in the interaction of sodium cholate with phosphatidylcholine liposomes

Abstract The interaction of sodium cholate surfactant with phosphatidylcholine unilamellar liposomes was investigated. Alterations in bilayer permeability were detected as a change in 5(6)-carboxyfluorescein (CF) released from the interior of vesicles to the bulk aqueous medium and bilayer solubilization as a decrease in the static light-scattered by liposome suspensions. The value corresponding to the effective surfactant to phospholipid molar ratio parameter (Re) for each interaction step investigated progressively increased as the surfactant concentration rose, this rise being more pronounced during bilayer solubilization. At the subsolubilizing level, a maximum bilayer/water surfactant partitioning (K) was reached at 30% of CF release, correlated with the saturation of the outer vesicle leaflet, followed by an abrupt decrease in K up to the complete saturation of these bilayer structures (ReSAT). At the solubilizing level, the K parameter also drastically increased up to the complete bilayer solubilization (ReSOL). The fact that the free surfactant concentration at subsolubilizing and solubilizing levels showed values respectively lower than and similar to its critical micelle concentration (CMC) indicates that permeability alterations and bilayer solubilization were determined respectively by the action of surfactant monomer and by the formation of mixed micelles.

Changes in phosphatidylcholine liposomes caused by a mixture of Triton X-100 and sodium dodecyl sulfate

The mechanisms governing the interaction of equimolecular mixtures of Triton X-100 (Tx-100) and sodium dodecyl sulfate (SDS) with phosphatidylcholine liposomes were investigated. Permeability alterations were determined as a change in 5(6)-carboxyfluorescein released from the interior of vesicles and bilayer solubilization as a decrease in the static light-scattered by liposome suspensions. At subsolubilizing level, a maximum bilayer/water partitioning of surfactant mixture was reached at 30% CF release, which correlated with the increased presence of SDS in the bilayers. However, transition stages between 70% CF release and 100% light-scattering corresponded to the increased presence of Tx-100 in these structures. These findings may be correlated with the reduced deleterious effects caused by this mixture in different tissues versus pure SDS, given that the presence of Tx-100 may modulate the level of SDS partitioning in the human stratum corneum. At subsolubilizing level, the mixture showed higher affinity with bilayers than those reported for single components, whereas at solubilizing level this affinity was slightly lower and higher than those reported for Tx-100 and SDS respectively. A direct relationship was established in the initial interaction steps between the growth of vesicles, the leakage of entrapped CF and the effective molar ratio of surfactant to phospholipid in bilayers (Re). This dependence was also detected during solubilization, where the decrease in the vesicle size and in the scattered light of the system depended on the Re parameter and hence on the bilayer composition. The fact that the free surfactant concentration at subsolubilizing and solubilizing levels showed respectively lower and similar values than the critical micelle concentration (c.m.c.) of the surfactant mixture indicates that permeability alterations and solubilization were determined respectively by the action of surfactant monomer and by the formation of mixed micelles. This finding supports the generally admitted assumption, for single surfactants, that the concentration of free surfactant must reach the c.m.c. for solubilization to occur and highlights the influence of the negative synergism of this surfactant mixture on the free surfactant concentration needed to saturate or solubilize liposomes.

A rhenium tris-carbonyl derivative as a model molecule for incorporation into phospholipid assemblies for skin applications.

A rhenium tris-carbonyl derivative (fac-[Re(CO)3Cl(2-(1-dodecyl-1H-1,2,3,triazol-4-yl)-pyridine)]) was incorporated into phospholipid assemblies, called bicosomes, and the penetration of this molecule into skin was monitored using Fourier-transform infrared microspectroscopy (FTIR). To evaluate the capacity of bicosomes to promote the penetration of this derivative, the skin penetration of the Re(CO)3 derivative dissolved in dimethyl sulfoxide (DMSO), a typical enhancer, was also studied. Dynamic light scattering results (DLS) showed an increase in the size of the bicosomes with the incorporation of the Re(CO)3 derivative, and the FTIR microspectroscopy showed that the Re(CO)3 derivative incorporated in bicosomes penetrated deeper into the skin than when dissolved in DMSO. When this molecule was applied on the skin using the bicosomes, 60% of the Re(CO)3 derivative was retained in the stratum corneum (SC) and 40% reached the epidermis (Epi). Otherwise, the application of this molecule via DMSO resulted in 95% of the Re(CO)3 derivative being in the SC and only 5% reaching the Epi. Using a Re(CO)3 derivative with a dodecyl-chain as a model molecule, it was possible to determine the distribution of molecules with similar physicochemical characteristics in the skin using bicosomes. This fact makes these nanostructures promising vehicles for the application of lipophilic molecules inside the skin.