PEG-12-acyloxystearates constitute a novel class of pharmaceutical solubilizers and are synthesized from polyethylene glycol and 12-hydroxystearic acid, which has been esterified with a second acyl chain. The hemolytic activity of these surfactants decreases drastically with increasing pendant acyloxy chain length, and surfactants with an acyloxy chain of 14 carbon atoms or more are essentially nonhemolytic. In this paper, the interactions of PEG-12-acyloxystearates (acyloxy chain lengths ranging from 8 to 16 carbon atoms) with phosphatidylcholine vesicles, used as a model system for erythrocyte membranes, were studied in search of an explanation for the large variations in hemolytic activity. Surfactant-induced alterations of membrane permeability were investigated by studying the leakage of vesicle-entrapped calcein. It was found that all of the surfactants within the series interact with the vesicle membranes and cause slow leakage at elevated surfactant concentrations, but with large variations in leakage kinetics. The initial leakage rate decreases rapidly with increasing pendant acyloxy chain length. After prolonged incubation, on the other hand, the leakage is not a simple function of acyloxy chain length. The effect of the surfactants on membrane integrity was also investigated by turbidity measurements and cryo-transmission electron microscopy. At a surfactant/lipid molar ratio of 0.4, the vesicle membranes are saturated with surfactant. When the surfactant/lipid molar ratio is further increased, the vesicle membranes are progressively solubilized into mixed micelles. The rate of this process decreases strongly with increasing acyloxy chain length. When comparing the results of the different experiments, it can be concluded that there is no membrane permeabilization below saturation of the vesicle membranes. The large variations in the kinetics suggest that several steps are involved in the mechanism of leakage induced by PEG-12-acyloxystearates and that their relative rates vary with acyloxy chain length. The slow kinetics may in part be explained by the low critical micelle concentrations (CMCs) exhibited by the surfactants. The CMCs were found to be in the range of 0.003-0.025 microM.

The interaction of the cellular delivery vector penetratin with a model system consisting of negatively charged phospholipid vesicles has been studied. Above a certain peptide to lipid molar ratio, the cationic oligopeptide induces vesicle aggregation. Interestingly, the aggregation is followed by spontaneous disaggregation, which may be related to membrane translocation of the peptide. Circular dichroism (CD) measurements indicate a conformational transition, from α-helix to antiparallel β-pleated sheet, which is simultaneous with the aggregation process. The potential influence of spectroscopic artifacts on CD data due to the drastically increased turbidity during aggregation is discussed.

The potential use of polypeptides and oligonucleotides for therapeutical purposes has been questioned because of their inherently poor cellular uptake. However, the 16-mer oligopeptide penetratin, derived from the homeodomain of Antennapedia, has been reported to enter cells readily via a non-endocytotic and receptor- and transporter-independent pathway, even when conjugated to large hydrophilic molecules. We here present the first study where penetratin is shown to traverse a pure lipid bilayer. The results support the idea that the uptake mechanism involves only the interaction of the peptide with the membrane lipids. Furthermore, we conclude that the translocation does not involve pore formation.

Starting from the pharmaceutically interesting Winsor III system of water, l-propanol, soybean phosphatidylcholine, and medium-chain triglycerides (MCT), the influence of two active drug compounds, felodipine and (R)-N-2-(diphenylacetyl)-N-[(4-hydroxyphenyl)methyl]argininamide (BIBP3226), on the phase behavior and microstructure was studied by means of phase studies, NMR self-diffusion measurements, and measurements of drug solubility in the aqueous phase and the oil phase. Felodipine, being practically insoluble in water and slightly soluble in MCT, was found to act as a nonpenetrating oil. With increasing concentration of felodipine in the oil phase, the polarity of the oil phase increases, which in turn curves the surfactant film toward water. Thus, water is expelled from the microemulsion phase, and oil is incorporated as felodipine is added. With the composition used here, the microstructure remains bicontinuous, however, even at high felodipine concentrations. The increase in the polarity of the oil phase also has the effect of increasing the partitioning of 1-propanol in the oil phase, which increases the solubility of felodipine, The maximum solubility of felodipine in the system was 9 wt %, defined as the weight percent of felodipine/(felodipine + MCT). This value should be compared to 3 wt %, which is the solubility in pure MCT. BIBP3226 on the other hand, is a charged molecule and practically insoluble in MCT but slightly soluble in water. Furthermore, it has an affinity far the lecithin monolayer and is therefore partitioned between the water phase and the surfactant film. Mainly because of solubilization of BIBP3226 in the surfactant film and the entropy of the accompanying counterions, the excess water is incorporated in the microemulsion at a very low concentration of BIBP3226. At the drug concentration at which the water phase as well as the surfactant film is saturated with drug, the microstructure has changed from a bicontinuous structure to oil-swollen micelles (oil-in-water microemulsion). At this point, approximately 60% of the drug molecules are located in the surfactant film.