Polymerized surfactant vesicles: novel membrane mimetic systems

A new haemocompatible phospholipid polyurethane based on hydrogenated poly(isoprene) soft segment

A new haemocompatible phospholipid polyurethane based on hydrogenated poly(isoprene) glycol (HPIP) and 4,4'-methylendiphenyl diisocyanate (MDI) was synthesized using 2-[bis(2-hydroxyethyl)methyl-ammonio]ethylstearylphosphate (BESP) and 1,4-butanediol (BD) as chain extender. The bulk and surface characteristics of this material was investigated by differential scanning calorimetry (DSC), dynamic viscoelasticity and tensile property measurements, attenuated total reflectance-Fourier transform infrared spectroscopy (ATR-FTIR), and contact angle measurement. This polymer possessed a hydrophobic surface revealed by contact angle measurement. The haemocompatibility of this polyurethane was evaluated by platelet rich plasma (PRP) contacting studies and scanning electron microscopy (SEM) observation using medical grade poly(vinyl chloride) (PVC) as the reference. The results show that this new polyurethane had relatively lower platelet adhesion and limited shape change for the attached platelets compared to PVC. The clotting time of the materials in contact with platelet poor plasma (PPP) was 99, 75, and 62 s and in contact with PRP was more than 240, 100, and 86 s for new polyurethane, PVC, and glass, respectively. This new phospholipid polyurethane is expected to have wide applications as coating or structural material for blood-contacting medical equipment due to its outstanding haemocompatibility and excellent mechanical strength.

Paramagnetic polymerized liposomes as new recirculating MR contrast agents

We describe a well-tolerated blood pool contrast agent with extended recirculatory half-life based on paramagnetic polymerized liposomes (PPLs). PPLs were constructed from a new type of polymerizable lipid molecule that has a derivative of gadopentetate dimeglumine as the hydrophilic head group and diacetylene groups in the hydrophobic acyl chains, which cross-link when irradiated with ultraviolet (UV) light. Biodistribution, blood pool half-life, and MR image enhancement were determined for PPLs composed of 10% of the gadopentetate dimeglumine lipid and 90% of ditricosadiynoyl tricosadiynayl phosphatidylcholine (DAPC) at a dose of 0.015 mmol Gd+3/kg in rats. In T1-weighed MR images (TR/TE = 400/18 msec), an average signal enhancement of 34% in the kidneys and 20% in the liver was observed, which persisted for at least 90 minutes after administration of the PPLs. Biodistribution studies using radiolabeled PPLs confirmed that 80% of the injected dose remained in the blood pool after 2 hours. The half-life of elimination from the blood pool was 19 hours. The preparation was well tolerated in rats and produced similar MR contrast enhancement of the blood pool as produced by other liposome contrast agents. However, the half-life of PPL elimination from the blood pool was prolonged relative to other liposome systems.

Preparation and characterization of polymer coated small unilamellar vesicles

Glucose oxidase was entrapped in small unilamellar vesicles composed of phosphatidylcholine, dicetyl phosphate and cholesterol. Prediction of the enzyme content of liposomes by calculations based on input concentrations of lipid and protein, dimensions of the lipids and the liposomes yielded one protein per vesicle. The entrapment efficiency was experimentally determined to be about 13%. On the other hand the entrapment efficiency for the small chromate ions was found to be significantly lower (0.1%). The liposomes were then coated with a polymer, poly(1,4-pyridinium diylethylene salt). It was possible to remove the lipoid material from underneath the polymer layer with various techniques. The effect of sonication and treatment with organic solvents (tested for this purpose) on enzyme activity were found to be very significant and Triton X-100 was chosen for this purpose. It was shown that the enzyme within the remaining net has 89% of its original activity.

Phase transitions of polymerizable phospholipids

Polymerizable lipids have received considerable attention in the last ten years as polymerization of lipids in vesicle systems is a possibility to increase the stability of lipid bilayers. Lipids with various polymerizable groups have been synthesized in the last years. This paper is focussed on those lipids which are closely related to natural phospholipids, i.e. molecules which have two hydrophobic chains and a head group containing a phosphate moiety. The phase behaviour of polymerizable phospholipids as lipid monomers and in the polymerized state is reviewed and discussed.

Preparation and characterization of long chain amino acid and peptide vesicle membranes

Four amino acid dicarboxylic amphiphiles which contain cysteine or homocysteine were synthesized. Each forms synthetic bilayer membranes upon hydration. Extensive sonication above the lipid phase transition temperature, 61 to 82 degrees C, produced 1000 A diameter vesicles. Treatment of the vesicles with water-soluble carbodiimides during and after sonication induced oligopeptide formation at the vesicle surface with retention of vesicle size and shape. Size exclusion chromatography indicates the products are predominantly di- to decapeptides. The permeability characteristics of the amino acid and peptide vesicles to [3H]glucose and 6-carboxyfluorescein are reported. The amino acid vesicles are among the least permeable nonpolymerized bilayer vesicles described in the literature to date. Formation of the peptide vesicles increases the membrane permeability, whereas in other polymerizable lipid vesicles the permeability decreases upon polymerization. The amino acid vesicles can be immobilized on Sephadex beads by reaction with carbodiimide. The impermeability, biodegradability, and ease of immobilization make this class of vesicles attractive materials for the encapsulation of reagents.

In vivo behavior of polymerized lipid vesicles

We have investigated the blood clearance kinetics and tissue distribution of large multilamellar liposomes (MLVs) and small unilamellar liposomes (SUVs) composed of the photopolymerizable lipid bis[12-(methacryloyloxy)dodecanoyl]-L-alpha-phosphatidylcholine. Polymerized MLVs or SUVs are cleared from the bloodstream more rapidly than nonpolymerized vesicles with similar size profiles. Both polymerized and nonpolymerized vesicles primarily accumulate in liver and spleen. However, the polymerized vesicles display an enhanced biostability as revealed by an elevated level of radioactive lipid marker remaining in the tissues.

Transmission electron microscopy observations of sonication-induced changes in liposome structure

Freeze-fracture Transmission Electron Microscopy (TEM) was used to show that sonication does not homogeneously disrupt liposome dispersions to form vesicles. Many large multilamellar particles remain intact after sonication and small, unilamellar vesicles are present after just 10 s of exposure. Small vesicles appear to coexist with large liposomes even before sonication. The mechanical and thermal stresses induced by sonication nucleate liquid crystalline defects in the liposomes, including edge and screw dislocations and +1 disclinations, but the Dupin cyclide structure of unsonicated liposomes is still recognizable in the larger particles after sonication. Defects in the bilayer organization may provide pathways for enhanced transport within the liposome, as well as from the liposome interior to exterior. A screw dislocation-catalyzed mechanism of liposome-to-vesicle conversion is proposed that accounts for the TEM observations.

Polymeric phospholipids as new biomaterials

Phospholipid polymers form a new class of biomaterials with many potential applications in medicine and research. The development of these compounds is based upon the mimicry of cell surfaces and reflects our current understanding of the properties of membrane lipids. Physicochemical characterization of the monomeric, diacetylenic phospholipids illustrates the similarities to naturally occurring lipids, similarities that are confirmed by the capacity to enrich the membranes of A. laidlawii to the level of 90% diacetylenic lipid. Polymerization of diacetylenic phospholipids is easily attained by irradiation and produces a stable, crystalline array. The ability to link membrane lipids covalently permits the isothermal restriction in their motion, and is useful in basic studies of biomembranes. The thromboresistance of polymeric phosphatidylcholines in vitro may be a consequence of the inability of phosphatidylcholines to participate in coagulation. The restricted lateral diffusion of proteins along a polymeric lattice will also inhibit the formation of coagulation complexes. Existing polymers may be altered by a coating of polymeric lipid obtained by the Langmuir-Blodgett method. Polymerized vesicles display significant reductions in permeability and aggregation. Entrapment of soluble materials and reconstitution of membrane proteins may be exploited in controlled and site-directed drug delivery. Polymerization of cells in situ produces "cellular capsules" with entrapped membrane and cellular components. Polymeric hemosomes are capable of gas transport and may function as red cell surrogates. The hybrid qualities of biomembranes (polar surfaces, nonthrombogenic, low antigenic potential, and low permeability) and synthetic polymers (chemical and physical stability) suggest that polymeric phosphatidylcholines may serve as models for biomaterials design.

Ocular drug bioavailability from topically applied liposomes

During the past decade liposomes have been investigated extensively for their ability to improve drug utilization by the body, first in the area of chemotherapeutics and most recently in the area of ophthalmology. Liposomes are vesicle-like structures with a concentric series of alternating compartments of aqueous spaces and phospholipid bilayers. To date, liposomes have been found to both promote and reduce ocular drug absorption, indicating that a definite need exists for further studies to evaluate the interplay of drug, liposomes, and the corneal surface in determining the effectiveness of liposomes as vehicles for topically applied ophthalmic drugs. The purpose of this review is to place in perspective the role of liposomes in topical ocular drug delivery. As background material, the factors influencing ocular drug bioavailability and the features of liposomes pertinent to their effectiveness as drug carriers are reviewed.