Preparation and characterization of polymer coated small unilamellar vesicles

Low-molecular-weight heparin-conjugated liposomes with improved stability and hemocompatibility

Multilamellar vesicles (MLV) containing phosphatidyl choline (PC), cholesterol (CHOL), and stearylamine (SA) in the molar ratio of 7:2:0.2 were prepared by the thin film hydration method. Low-molecular-weight heparin (LMWH, MW: 3000) was conjugated with the MLV using carbodiimide (EDC). Infrared, Raman, and nuclear magnetic resonance spectra and DSC of each sample (MLV, LMWH, and MLV-LMWH) were obtained, enabling the authors to determine the chemical changes that occurred in the MLV structure at the end of the conjugation step. In addition, the changes in the chemical structures of the conjugated samples were revealed by the use of elemental analysis. Particle size analysis was used to determine the difference between the sizes of MLV and MLV-LMWH. In order to study the effect of LMWH on the behavior of MLV-LMWH in blood, osmotic fragility (in saline and plasma), hemolytic activity, and plasma recalcification time tests were carried out. These tests showed that it was possible to construct liposomes that would not induce reactions in the blood and would have potentially longer half-lives in the circulation.

Biocompatible polymer vesicles from biamphiphilic triblock copolymers and their interaction with bovine serum albumin

The self-assembly of the biamphiphilic triblock copolymer poly(ethylene oxide)-b-poly(caprolactone)-b-poly(acrylic acid) into polymer vesicles is studied. The vesicles provide both biocompatibility and biodegradability. Moreover, the biamphiphilic nature of the triblock copolymer provides different surface properties in the interior and in the outer interface of the vesicles. Preparation of the aggregates by direct dissolution of the copolymer in a solution of albumin does not alter the morphology of the aggregates, and thus, they have the potential to immobilize protein molecules. Since a part of the protein is encapsulated in the interior of the vesicles, they can be used as nanocontainers. A further fraction of the protein is bound to the outer interface, which is primarily composed of the poly(acrylic acid) tails. Immobilization of protein on the outer interface can stabilize the colloidal particles and also provide them with a biofunctional component.

Liposome-encapsulated actin–hemoglobin (LEAcHb) artificial blood substitutes

A new approach to enhance the circulation persistence of liposomes has been applied to develop liposome-encapsulated actin-hemoglobin (LEAcHb) dispersions as potential blood substitutes by introducing an actin matrix into the liposome aqueous core. Asymmetric flow field-flow fractionation coupled with multi-angle static light scattering was used to study the shape, size distribution, and encapsulation efficiency of liposome-encapsulated hemoglobin (LEHb) and LEAcHb dispersions. By polymerizing monomeric actin into filamentous actin inside the liposome aqueous core, LEAcHb particles transformed into a disk-like shape. We studied the effect of an encapsulated actin matrix on the size distribution, hemoglobin (Hb) encapsulation efficiency, oxygen affinity, and methemoglobin (MetHb) level of LEAcHb dispersions, and compared them with plain LEHb dispersions (without actin). LEHb, and LEAcHb dispersions extruded through 400 nm membranes were injected into rats and it was observed that LEAcHb dispersions with 1mg/mL of actin enhanced the circulatory half-life versus LEHb dispersions. The circulatory characteristics of empty PEGylated and non-PEGylated actin-containing liposomes (without Hb) were studied as controls for the LEHb and LEAcHb dispersions in this paper, which displayed maximum circulatory half-lives greater than 72 h. Taken together the results of this study supports our hypothesis that a lipid membrane supported by an underlying actin matrix will extend the circulatory half-life of LEHb dispersions.

Mechanism of calcium ion induced multilamellar vesicle-DNA interaction

The effect of Ca2+ on the DNA interaction with anionic and neutral multilamellar vesicles (MLV) has been investigated. DNA from wheat (Triticum aestivum L. Gerek) was introduced to a suspension of MLV, composed of phosphatidylcholine (PC):dicetylphosphate (DCP):cholesterol (CHOL) at different molar ratios, to which Ca2+ (5-75 mM) was subsequently added. Indication of aggregation and/or fusion was obtained via light-scattering examination following the addition of Ca2+ and DNA to the MLV medium. Using a UV spectrophotometric assay, it was observed that although DNA alone has no effect on negatively charged MLV, it enhances liposomal interaction in the presence of calcium ions. The minimal Ca2+ concentration required to promote the interaction was detected to be 10 mM, and the highest level of interaction was observed at 75 mM. The aggregation/fusion of vesicles was detected for uncharged MLV (with no DCP in their structure), as well as for the anionic ones containing c. 10% CHOL, but not for anionic MLV containing 40% CHOL. This is explained in terms of cholesterol decreasing the membrane fluidity (above the Tc of components) as a result of which more rigid vesicles become less prone to aggregation/fusion interactions.

Imaging of liposomes by scanning tunneling microscopy

In this study, unilamellar and multilamellar liposomes consisting of dipalmitoylphosphatidylcholine, cholesterol and dicetylphosphate were prepared. Scanning tunneling microscopy images of these liposomes on highly oriented pyrolytic graphite surfaces were obtained. It was observed that scanning tunnelling microscopy can be successfully used for high-resolution, three-dimensional structure analysis of liposomes under very mild conditions.

Biotechnological developments in Turkey

Turkey is a country not usually associated with industrial biotechnology. However, when current research potential in universities and other R & D centers and particularly contributions made to the international literature since the mid-1980s are taken into account, high-quality international-level work is now commonplace, especially in areas such as industrial microbiology, enzyme technology, biomaterials and biological wastewater treatment. Work in plant biotechnology is at a relatively early stage, but is expected to become a rapidly developing force in the near future. The present article documents current potential in Turkey, based on significant publications produced during the last 8 years.

Influence of membrane components on the stability and drug release properties of reverse phase evaporation vesicles (REVs): light sensitive all-trans retinal, negatively charged phospholipid dicetylphosphate and cholesterol

Incorporation of a negatively charged phospholipid, dicetylphosphate, initially increased encapsulation efficiency (from 12 to 24%) but beyond 5% (molar) a detrimental effect was observed. Rate of drug release from REVs was, for most cases, found to be bi-phasic implying partitioning between the lipid bilayer and the aqueous compartment. It was not possible to prepare liposomes with more than 1% (molar) all-trans retinal (ATR) as a membrane component. When ATR was reduced to 0.5% (molar), encapsulation efficiency increased to 7.76%. Upon exposure to long wave UV (365 nm), release from ATR containing REVs was increased and this was attributed to the formation of 13-cis isomer as indicated by HPLC and UV spectroscopy data.

Surface-modified liposomes: assessment and characterization for increased stability and prolonged blood circulation

Advances in therapeutic applications of liposomes have been achieved through surface modifications increasing their biological stability: reduced constituent exchange and leakage as well as reduced unwanted uptake by cells of the mononuclear phagocytic system. The recent conclusions obtained from in vivo and in vitro studies are reviewed with an emphasis on evaluating the methods used and thus the kinds of conclusions which can be drawn. A number of issues are raised as to the limitations of the methods employed. Steric stabilization, meaning reduction in particle interactions by a surface steric barrier, has been proposed as a theoretical basis for the results and some of the initial results testing this hypothesis are reviewed here with respect to identification of the extent to which physical properties of the surface coatings correlate with the biological properties. At this time it seems that no one method is ideal so that multiple measures give the best characterization.

Sterically stabilized liposomes

Many recent reports have demonstrated that rapid uptake of liposomes in vivo by cells of the mononuclear phagocytic system (MPS), which has restricted their therapeutic utility, can be overcome by incorporation of lipids derivatized with the hydrophilic polymer polyethylene glycol (PEG). The structure-function relationship of PEG-derivatized phosphatidylethanolamine (PEG-PE) has been examined by measurement of blood lifetime and tissue distribution in both mice and rats. The results are reviewed and contrasted with those from liposomes without PEG-PE or other surface modifications. With a PEG molecular weight in the range of 1000 to 5000, prolonged circulation and reduced MPS uptake is achieved. After 24 h, up to 35% of the injected dose remains in the blood and less than 10% is taken up by the two major organs of the MPS, liver and spleen, compared with 1% and up to 50%, respectively, for liposomes without PEG-PE. Other important advantages of PEG-PE have been identified: prolonged circulation is independent of liposome cholesterol content, degree of hydrocarbon chain saturation in either the PC or the PE lipid anchor, lipid dose, or addition of most other negatively charged lipids. This versatility in lipid composition and dose is important for controlling drug release in a liposome-based therapeutic agent. Steric stabilization has been proposed as a theoretical basis for the results and some initial results testing this hypothesis have been reported. A description of a theoretical model is presented here and evaluated with the data available. The results are compared with other particulate drug carriers and the range of potential applications are considered.