Johan Engblom

A study of polar lipid drug systems undergoing a thermoreversible lamellar-to-cubic phase transition

Abstract The polar insoluble, but swelling, lipid monoolein (glyceryl monooleate) forms a highly ordered cubic phase in excess water, which can be used to sustain the release of different types of drugs. One problem with the cubic phase, however, is its stiffness, and in order to make the administration of the cubic phase easier, precursors in the form of a lamellar phase were investigated. This phase undergoes a phase transition to the cubic phase on temperature increase; thus the lipid system formed in this way acts similar to in situ activated gel-forming polymer systems, e.g., Pluronic D-127, Gelrite and EHEC. Lipid drug carrier systems can be made from a wide variety of lipids. They can also be used in the application of a wide variety of drugs, and are very versatile as regards the site of administration. They can, therefore, be an interesting alternative to polymer systems.

A water gradient can be used to regulate drug transport across skin

At normal conditions there is a substantial water gradient over the skin as it separates the water-rich inside of the body from the dry outside. This leads to a variation in the degree of hydration from the inside to the outside of skin and changes in this gradient may affect its structure and function. In this study we raise the question: How do changes in the water gradient across skin affect its permeability? We approach this problem in novel diffusion experiments that permit strict control of the gradient in the chemical potential of water and hence well-defined boundary conditions. The results demonstrate that a water gradient can be used to regulate transport of drugs with different lipophilic characteristics across the skin barrier. It is shown that the transport of metronidazole (log P(o/w)=0.0) and methyl salicylate (log P(o/w)=2.5) across skin increases abruptly at low water gradients, corresponding to high degrees of skin hydration, and that this effect is reversible. This phenomenon is highly relevant to drug delivery applications due to its potential of temporarily open the skin barrier for transdermal drug delivery and subsequently close the barrier after treatment. Further, the results contribute to the understanding of the occlusion effect and indicate the boundary conditions of the water gradient needed to make use of this effect.

Glycerol and urea can be used to increase skin permeability in reduced hydration conditions.

The natural moisturizing factor (NMF) is a group of hygroscopic molecules that is naturally present in skin and protects from severe drying. Glycerol and urea are two examples of NMF components that are also used in skin care applications. In the present study, we investigate the influence of glycerol and urea on the permeability of a model drug (metronidazole, Mz) across excised pig skin membranes at different hydrating conditions. The degree of skin hydration is regulated by the gradient in water activity across the membrane, which in turn depends on the water activity of the formulation in contact with the skin membrane. Here, we determine the water activity of all formulations employed using an isothermal calorimetric method. Thus, the gradient in water activity is controlled by a novel experimental set-up with well-defined boundary conditions on both sides of the skin membrane. The results demonstrate that glycerol and urea can retain high steady state flux of Mz across skin membranes at dehydrating conditions, which otherwise would decrease the permeability due to dehydration. X-ray diffraction measurements are performed to give insight into the effects of glycerol and urea on SC molecular organization. The novel steady state flux results can be related to the observation that water, glycerol, and urea all affect the structural features of the SC molecular components in a similar manner.

Effects of water gradients and use of urea on skin ultrastructure evaluated by confocal Raman microspectroscopy.

The rather thin outermost layer of the mammalian skin, stratum corneum (SC), is a complex biomembrane which separates the water rich inside of the body from the dry outside. The skin surface can be exposed to rather extreme variations in ambient conditions (e.g. water activity, temperature and pH), with potential effects on the barrier function. Increased understanding of how the barrier is affected by such changes is highly relevant for regulation of transdermal uptake of exogenous chemicals. In the present study we investigate the effect of hydration and the use of a well-known humectant, urea, on skin barrier ultrastructure by means of confocal Raman microspectroscopy. We also perform dynamic vapor sorption (DVS) microbalance measurements to examine the water uptake capacity of SC pretreated with urea. Based on novel Raman images, constructed from 2D spectral maps, we can distinguish large water inclusions within the skin membrane exceeding the size of fully hydrated corneocytes. We show that these inclusions contain water with spectral properties similar to that of bulk water. The results furthermore show that the ambient water activity has an important impact on the formation of these water inclusions as well as on the hydration profile across the membrane. Urea significantly increases the water uptake when present in skin, as compared to skin without urea, and it promotes formation of larger water inclusions in the tissue. The results confirm that urea can be used as a humectant to increase skin hydration.

Chemical penetration enhancers in stratum corneum - Relation between molecular effects and barrier function.

Skin is attractive for drug therapy because it offers an easily accessible route without first-pass metabolism. Transdermal drug delivery is also associated with high patient compliance and through the site of application, the drug delivery can be locally directed. However, to succeed with transdermal drug delivery it is often required to overcome the low permeability of the upper layer of the skin, the stratum corneum (SC). One common strategy is to employ so-called penetration enhancers that supposedly act to increase the drug passage across SC. Still, there is a lack of understanding of the molecular effects of so-called penetration enhancers on the skin barrier membrane, the SC. In this study, we provide a molecular characterization of how different classes of compounds, suggested as penetration enhancers, influence lipid and protein components in SC. The compounds investigated include monoterpenes, fatty acids, osmolytes, surfactant, and Azone. We employ natural abundance (13)C polarization transfer solid-state nuclear magnetic resonance (NMR) on intact porcine SC. With this method it is possible to detect small changes in the mobility of the minor fluid lipid and protein SC components, and simultaneously obtain information on the major fraction of solid SC components. The balance between fluid and solid components in the SC is essential to determine macroscopic material properties of the SC, including barrier and mechanical properties. We study SC at different hydration levels corresponding to SC in ambient air and under occlusion. The NMR studies are complemented with diffusion cell experiments that provide quantitative data on skin permeability when treated with different compounds. By correlating the effects on SC molecular components and SC barrier function, we aim at deepened understanding of diffusional transport in SC, and how this can be controlled, which can be utilized for optimal design of transdermal drug delivery formulations.

Azone@ and the formation of reversed mono- and bicontinuous lipid-water phases

The interfacial properties of the skin penetration enhancer Azone® were investigated as well as the effect the substance has on the phase behaviour of two bilayer forming lipid-water systems, i.e., lecithin-water and monoolein-water. Interfacial studies revealed a change in Azone packing on the water surface as the area per molecule decreased below 62 A2. It is suggested that this change reflects a situation where the polar amide bond of Azone loses its direct contact with water and the molecule adopts a straighter conformation normal to the surface. Phase studies show that Azone promotes the formation of reversed types of lipid-water phases such as bicontinuous cubic, reversed hexagonal and reversed micellar phases. From this phase behaviour and recent literature data, we suggest that the formation of reversed types of phases should be considered as one important mechanism behind the increased skin penetration of drugs caused by lipophilic substances such as Azone.

The effect of the skin penetration enhancer Azone® on fatty acid-sodium soap-water mixtures

Abstract The effect of Azone on the phase behaviour of a mixture of partly neutralised fatty acids in water was investigated by X-ray diffraction. The lipid blend consisted of saturated palmitic chains and unsaturated oleic chains. It was found that the system consisted of both crystalline and liquid crystalline phases at body temperature and below. The solid phases consisted essentially of the saturated lipids, while the unsaturated lipids formed the liquid crystalline phase. Azone had a minor effect on the crystalline phase, but partitioned into the liquid crystalline phase. The addition of Azone caused the original reversed hexagonal phase to be transformed into a reversed micellar phase. The findings presented here are in line with our previous observations on the effect of Azone on the soya bean lecithin-water and monoolein-water system, but in conflict with another study on a similar fatty acid system where it was assumed that Azone promoted the formation of the lamellar phase.

Effect of hydration on structural and thermodynamic properties of pig gastric and bovine submaxillary gland mucins

One of the essential functions of mucous gel is protection of tissues against dehydration. The effect of hydration on the structural and thermodynamic properties of pig gastric mucin (PGM) and bovine submaxillary gland mucin (BSM) have been studied using atomic force microscopy (AFM), sorption, and differential scanning calorimetry (DSC). The analysis of sorption isotherms shows the higher water sorption capacity of PGM compared to BSM at RH levels lower than about 78%. The value of the hydration enthalpy at zero water content at 25 °C for both biopolymers is about -20 kJ/mol. Glass transitions of BSM and PGM occur at RH levels between 60 and 70% for both mucins. AFM indicates the presence of a dumbbell structure as well as a fiber-like structure in PGM samples. The experimental volume of the dry dumbbell molecule obtained by AFM is 3140 ± 340 nm(3). Using DSC data, the amount of nonfreezing water was calculated to be about 0.51 g/g of PGM. The phase diagram of PGM demonstrates two regions of different Tg: dependent and independent of hydration levels. In particular, at mucin concentrations from 0 to 67 wt %, the glass transition occurs at a constant temperature of about -15 °C. At higher concentrations of mucin, Tg is increasing with increasing mucin concentrations.

The skin barrier from a lipid perspective.

This contribution summarises the results from a number of investigations undertaken in the spirit of the Domain Mosaic Model proposed by Forslind in 1994. Atomic Force Microscopy (AFM) studies on the two-dimensional phase behaviour of some stratum corneum lipids revealed phase separation of the lipids in the typical case and the ability of cholesterol to reduce the line tension between phases. A theoretical model was developed describing the response of an oriented stack of polar lipid bilayers in the presence of a gradient in water chemical potential (water solution to humid air). The gradient gives rise to an inhomogeneous water swelling, and presumably to a liquid crystal-to-gel transition in the lamellar region closest to humid air. Skin penetration enhancers such as Azone and oleic acid cause phase transformations in lipid bilayer systems which may be relevant in the context of skin permeation.

The use of micro- and nanoparticles in the stabilisation of pickering-type emulsions for topical delivery.

This review describes the use of Pickering emulsions for topical drug delivery. The focus is on Pickering emulsions and how to formulate these. However, a short description of the challenges of topical drug delivery is also given. The article describes how Pickering emulsions might have other properties than traditional topical creams. It is our believe that Pickering emulsions could give added value to topical formulations as it is surfactant free, has new properties, and may alter the transport of drugs across the skin barrier.