Sven Engström

Cubic phases for studies of drug partition into lipid bilayers

Drug partition into lipid bilayers in a cubic liquid-crystalline phase was investigated. Glyceryl monooleate was used to form the lipid bilayer in a reversed bicontinuous cubic liquid-crystalline phase. The reason for using the cubic phase is that it may coexist with an external aqueous phase, and that the phase boundary (cubic phase/aqueous bulk) is well-defined due to the stiffness of the cubic phase. This makes the cubic phase a potential candidate for high throughput screening (HTS) of the lipophilicity and the dissociation constant (if any) of drug compounds. Clomethiazole (CMZ), lidocaine, prilocaine and 4-phenylbutylamine (4-PBA) were chosen as model drug compounds. It was shown that it is possible to determine a pH-dependent apparent partition coefficient, Kbl/w, of a drug compound using a lipid bilayer expressed as a cubic liquid-crystalline structure. Good agreement was found when the resulting Kbl/w vs. pH curves for CMZ, lidocaine and prilocaine were fitted to a mathematical expression. This included the bilayer/water partition coefficient for the unionised and ionised drug respectively and the pKa of the drug. The effect of different experimental conditions; such as amount of cubic phase, temperature, agitation, sample preparation and interfacial area between the cubic phase and the aqueous bulk on the partition kinetics were investigated as well. The studies reveal that the time needed to reach partition equilibrium was, as expected, substantially reduced (from days to hours) by decreasing the amount of cubic phase, increasing the interfacial area between the cubic phase and the aqueous phase, and increasing the temperature and the agitation of the sample. It was also shown that the bilayer affinity of 4-PBA was increased when a zwitterionic lipid (i.e. dioleoyl phosphatidylcholine, DOPC) was incorporated in the bilayer.

A novel approach to the understanding of human skin barrier function

The basis for externally caused skin disorders is penetration of the skin barrier. A recent model for the skin barrier, the domain mosaic model, based on current knowledge of the physics of lipid bilayer organization gave tentative explanations for several aspects of function. It is demonstrated here that a development of the model explains how the requirements are met for a water-tight structure that will still allow a controlled, minute loss of water, the perspiratio insensibilis, necessary for maintaining plasticity of the keratin. A major advantage of the extended model is that it allows an interpretation of the changes imposed on the structure when in contact with detergents and/or penetration enhancers.

In vitro release of timolol maleate from an in situ gelling polymer system

Abstract A thermogelling drug delivery system composed of a cellulose ether (ethyl(hydroxyethyl)cellulose - EHEC), an ionic surfactant and water was thoroughly characterized in the presence of timolol maleate with respect to phase and rheological behaviour, as well as in vitro drug release. The phase studies reveal that gelling systems may be formed with 0.34% (w/w) timolol maleate, and that the gelling behaviour is sensitive to the surfactant concentration and ionic strength of the solution. The release of timolol maleate from the gels is retarded compared to a non-gelling EHEC system. The release rate is about equal for systems with 1 and 2% (w/w) EHEC, implying that the release is controlled by a low convection in the gels and not by any drug-polymer interaction.

Phase coexistence in cholesterol–fatty acid mixtures and the effect of the penetration enhancer Azone

Small and wide-angle X-ray diffraction was used to study the phase behaviour of cholesterol-fatty acid mixtures in an attempt to understand lipid interaction occurring in the stratum corneum, the outermost layer of skin. The effect of the penetration enhancer Azone was investigated as well. It was found that equimolar mixtures of cholesterol, palmitic acid and oleic acid (with the acids neutralised to 41 mol%) in 25% (wt/wt) water typically showed three phases at room temperature, two crystalline and one gel phase. The crystalline phases consisted mainly of palmitic acid:soap and cholesterol, respectively. The water present was unevenly distributed and was associated with the gel phase. Both cholesterol and palmitic acid seemed to be depleted from their crystalline phases by Azone. The electrostatic effects on titration of fatty acids in lamellar aggregates were calculated in view of the present results, and the effects of phase separation were discussed.

Investigation of surfactant alkyl chain length and counterion effects on the thermogelling EHEC system

Ethylhydroxyethylcellulose (EHEC) forms a reversible thermogelling system in water in the presence of ionic surfactants. In an effort to minimise the amount of ionic surfactant needed to produce a thermogelling system with timolol in salt form, we have examined in detail the system with respect to the alkyl chain length and counterion of the ionic surfactant. Two salts of timolol were also investigated, timolol (hydrogen)maleate and timolol chloride. It was found that in order to form a gel with a small amount of ionic surfactant, a number of criteria have to be fulfilled, e.g., (i) the ionic drug should typically be a co-ion to the ionic surfactant, (ii) the counterion of the drug and the ionic surfactant should preferably be inorganic and with a low polarisability and (iii) the ionic surfactant should have a low CMC but a Krafft temperature not higher than room temperature.

Molecular dynamics simulation of the renin inhibitor H142 in water.

SummaryH142 is a synthetic decapeptide designed to inhibit renin, an enzyme acting in the regulation of blood pressure. The inhibiting effect of H142 is caused by a reduction of a-Leu-Val-peptide bond (i. e. C(=O)-NH→CH2-NH). The conformational and dynamical properties of H142 and its unreduced counterpart (H142n) was modelled by means of molecular dynamics simulations. Water was either included explicitly in the simulations or as a dielectric continuum. When water molecules surround the peptides, they remain in a more or less extended conformation through the simulation. If water is replaced by a dielectric continuum, the peptides undergo a conformational change from an extended to a folded state. It is not clear whether this difference is a consequence of a too short simulation time for the water simulations, a force-field artifact promoting extended conformations, or if the extended conformation represents the true conformational state of the peptide. A number of dynamic properties were evaluated as well, such as overall rotation, translational diffusion, side-chain dynamics and hydrogen bonding.