The characterisation of liposomes with covalently attached proteins

Liposome-ligand conjugates: a review on the current state of art

Early researchers focussed on developing stimuli-responsive liposomes in order to manipulate drug release at the site of action or under certain conditions. In recent times, a great deal of efforts has been made to modify the surface of liposomes with ligands for the purpose of achieving targeted drug delivery. Due to the morphology of liposomes, their surfaces can be engineered by attaching molecules such as oligosaccharides, peptides, antibodies, antigens and oligonucleotides to the bilayer structure. Over the years, a number of techniques including the use of covalent and non-covalent linkages have been utilised in designing ligand-liposome conjugates. In this review, various strategies for the functionalisation of liposomes as well as the different types of ligand-liposome conjugates have been discussed. Finally, the pros and cons of conjugation in liposomes are concisely summarised.

Preparation of cefquinome sulfate cationic proliposome and evaluation of its efficacy on Staphylococcus aureus biofilm

Staphylococcus aureus (S. aureus) has the propensity to form biofilms, which eventually cause antibiotic resistance and treatment failure. Cefquinome sulfate (CS) is an animal-specific antibacterial agent for S. aureus infection. In this work, CS cationic proliposomes (CSCPs) were prepared by solid-dispersion method combined with effervescent hydration to eradicate bacterial biofilm and improve the antibacterial effect of the drug. CSCPs were readily dispersed in water, thereby forming CS cationic liposomes (CSCLs) as a white, uniform suspension. The CSCLs had an encapsulation efficiency (EE) of 63.21%, a drug loading of 4.04%, an average particle size of 201.5 nm, and a positive zeta-potential of 65.29 mV. In vitro release studies showed that CSCLs had good sustained-release behavior. The CS and CSCL minimal inhibitory concentration (MIC) of S. aureus type culture strain were 1 and 0.48 g/mL, respectively. The eradication effect of CS on bacterial biofilm (BBF) was relatively weak during culture in drug-containing medium for 8 h-24 h. However, the CSCL eradication effect on BBF increased gradually, and the clearance rate of CSCLs on BBF was about twice that of CS. The clearance rate reached 81.30% with 2.5 × MIC in 24 h. All these results indicated that CSCLs can significantly improve the eradication effect of cefquinome on biofilm to inhibit bacterial growth.

In vivo theranostics at the peri-infarct region in cerebral ischemia

The use of theranostics in neurosciences has been rare to date because of the limitations imposed on the free delivery of substances to the brain by the blood-brain barrier. Here we report the development of a theranostic system for the treatment of stroke, a leading cause of death and disability in developed countries. We first performed a series of proteomic, immunoblotting and immunohistological studies to characterize the expression of molecular biomarkers for the so-called peri-infarct tissue, a key region of the brain for stroke treatment. We confirmed that the HSP72 protein is a suitable biomarker for the peri-infarct region, as it is selectively expressed by at-risk tissue for up to 7 days following cerebral ischemia. We also describe the development of anti-HSP72 vectorized stealth immunoliposomes containing imaging probes to make them traceable by conventional imaging techniques (fluorescence and MRI) that were used to encapsulate a therapeutic agent (citicoline) for the treatment of cerebral ischemia. We tested the molecular recognition capabilities of these nano-platforms in vitro together with their diagnostic and therapeutic properties in vivo, in an animal model of cerebral ischemia. Using MRI, we found that 80% of vectorized liposomes were located on the periphery of the ischemic lesion, and animals treated with citicoline encapsulated on these liposomes presented lesion volumes up to 30% smaller than animals treated with free (non-encapsulated) drugs. Our results show the potential of nanotechnology for the development of effective tools for the treatment of neurological diseases.

Effect of human serum albumin upon the permeabilizing activity of sticholysin II, a pore forming toxin from Stichodactyla heliantus

Sticholysin II (St II) is a haemolytic toxin isolated from the sea anemone Stichodactyla helianthus. The high haemolytic activity of this toxin is strongly dependent on the red cell status and the macromolecule conformation. In the present communication we evaluate the effect of human serum albumin on St II haemolytic activity and its capacity to form pores in the bilayer of synthetic liposomes. St II retains its pore forming capacity in the presence of large concentrations (up to 500 μM) of human serum albumin. This effect is observed both in its capacity to produce red blood cells haemolysis and to generate functional pores in liposomes. In particular, the capacity of the toxin to lyse red blood cells increases in the presence of human serum albumin (HSA). Regarding the rate of the pore forming process, it is moderately decreased in liposomes and in red blood cells, in spite of an almost total coverage of the interface by albumin. All the data obtained in red cells and model membranes show that St II remains lytically active even in the presence of high HSA concentrations. This stubbornness can explain why the toxin is able to exert its haemolytic activity on membranes immersed in complex plasma matrixes such as those present in living organisms.

Self-assembling micelle-like nanoparticles based on phospholipid-polyethyleneimine conjugates for systemic gene delivery

With few exceptions, where local administration is feasible, progress towards broad clinical application of gene therapies requires the development of effective delivery systems. Here we report a novel non-viral gene delivery vector, 'micelle-like nanoparticle' (MNP) suitable for systemic application. MNP were engineered by condensing plasmid DNA with a chemical conjugate of phospholipid with polyethylenimine (PLPEI) and then coating the complexes with an envelope of lipid monolayer additionally containing polyethylene glycol-phosphatidyl ethanolamine (PEG-PE), resulting in spherical 'hard-core' nanoparticles loaded with DNA. MNP allowed for complete protection of the loaded DNA from enzymatic degradation, resistance to salt-induced aggregation, and reduced cytotoxicity. MNP also demonstrated prolonged blood circulation and low RES accumulation. Intravenous injection of MNP loaded with plasmid DNA encoding for the Green Fluorescence Protein (GFP) resulted in an effective transfection of a distal tumor. Thus, MNP provide a promising tool for systemic gene therapy.

Use of Liposomes to Deliver Bactericides to Bacterial Biofilms

Methods are described for the preparation of anionic and cationic liposomes and proteoliposomes with covalently linked lectins or antibodies by the extrusion technique (vesicles by extrusion, VETs). The liposomes are prepared from the phospholipid dipalmitoylphosphatidylcholine (DPPC), together with the anionic lipid phosphatidylinositol (PI) or the cationic amphiphile dioctadecyldimethylammonium bromide (DDAB) together with the reactive lipid DPPE-MBS, the m-maleimidobenzoyl-N-hydroxysuccinimide (MBS) derivative of dipalmitoylphosphatidylethanolamine (DPPE). Proteins (lectin or antibody), after derivatization with N-succinimidyl-S-acetylthioacetate (SATA), can be covalently linked to the surface of the liposomes by reaction with the reactive lipid, DPPE-MBS. The physical and chemical characterization of the liposomes and proteoliposomes by photon correlation spectroscopy (PCS) and protein analysis, to determine the number of chemically linked protein molecules (lectin or antibody) per liposome, are described. The liposomes can be used for carrying oil-soluble bactericides (e.g., Triclosan) or water-soluble antibiotics (e.g., vancomycin or benzylpenicillin) and targeted to immobilized bacterial biofilms of oral or skin-associated bacteria adsorbed on microtiter plates. Techniques for the preparation of immobilized bacterial biofilms, applicable to a wide range of bacterial suspensions, and for the analysis of the adsorption (targeting) of the liposomes to the bacterial biofilms are given. The mode of delivery and assessment of antibacterial activity of liposomes encapsulating bactericides and antibiotics, when targeted to the bacterial biofilms, by use of an automated microtiter plate reader, are illustrated, with specific reference to the delivery of the antibiotic benzylpenicillin encapsulated in anionic liposomes to biofilms of Staphylococcus aureus. The methods have potential application for the delivery of oil-soluble or water-soluble bactericidal compounds to a wide range of adsorbed bacteria responsible for infections in implanted devices such as catheters, heart valves, and artificial joints.

The antibacterial properties of solid supported liposomes on Streptococcus oralis biofilms

A novel system for the delivery of drugs to bacterial biofilms has been developed. The system is based on the use of anionic and cationic liposomes as drug carriers adsorbed on the surface of zinc citrate particles. The adsorption process results in the formation of solid supported vesicles (SSVs) which aids the stabilisation of the liposomes. Anionic liposomes have been prepared by incorporation of phosphatidylinositol (PI) into dipalmitoylphosphatidylcholine (DPPC) liposomes and cationic liposomes have been prepared by incorporation of dioctadecyldimethylammonium bromide (DDAB) into DPPC plus cholesterol liposomes. The liposomes were adsorbed onto zinc citrate particle and targeted to immobilised biofilms of the oral bacterium Streptococcus oralis. The liposomes were used to carry the bactericides, Triclosan, a lipid-soluble agent, and the aqueous-soluble penicillin-G, and their ability to inhibit bacterial growth from immobilised biofilms was accessed. Zinc citrate is itself a bactericide and is used in the formulation of toothpastes. The SSVs carrying the drugs have therapeutic properties. To trace the origin of these properties, each component of the SSV was investigated alone and in combination in binary systems. Some combinations showed synergistic (or additive) antibacterial effects while others showed regressive effects compared with their components.

The transfection of Jurkat T-leukemic cells by use of pH sensitive immunoliposomes

A gene transfer vector has been developed utilising anionic liposomes as a carrier of plasmid DNA (pEGlacZ, 7.6 kb) to transfect CD3+ T lymphocytes (Jurkat cells). The plasmid DNA that contained the Escherichia coli beta-galactosidase reporter gene was condensed using poly-l-lysine of molecular mass 20,700 (PLK99) to form a polyplex which was interacted with several anionic liposome formulations to form lipopolyplexes. The liposome formulations where based on dioleoylphosphatidylethanolamine (DOPE) in combination with cholesterol and dioleoylphosphatidylcholine (DOPC) and oleic acid, or dimyristoylphosphatidylethanolamine (DMPE). For targeting to the Jurkat cells distearoylphosphatidylethanolamine (DSPE) linked to poly (ethylene glycol) molecular mass 2,000 and coupled to anti-CD3 antibody was incorporated. The polyplexes and lipopolyplexes were characterised in terms of size, zeta potential, agarose gel electrophoresis and electron microscopy and the permeability of the lipopolyplexes to liposome-encapsulated glucose was determined. The polyplexes consisted of a mixed population of rod-like structures (53-160 nm long and 23-31 nm diameter) and spheres (18-30 nm diameter). The lipopolyplexes retained a permeability barrier although were more permeable to glucose than their component liposomes. The poly-l-lysine condensing agent was still susceptible to pronase digestion suggesting that the polyplex was associated with the outer surface of the liposome. The lipopolyplexes with lipid composition DOPE/cholesterol/OA/DSPE-PEG2000 anti-CD3+ PLK99-plasmid DNA had significant gene transfer activity, as monitored by beta-galactosidase expression, that depended on the charge ratio of the component polyplex and the lipid/DNA weight ratio. The anti-CD3 antibody, the liposomal lipid and pH sensitivity were essential for transfection activity.

Prevention of antibody-mediated elimination of ligand-targeted liposomes by using poly(ethylene glycol)-modified lipids

One of the major obstacles in the development of ligand-targeted liposomes is poor liposome circulation longevity as a result of antibody-mediated elimination of these highly immunogenic carriers. Because studies from our laboratory suggest that it is not possible to reduce the immunogenicity of ligand-conjugated liposomes by using surface-grafted poly(ethylene glycol) (PEG), we investigated the usefulness of PEG in protecting hapten-conjugated liposomes from elimination by an existing immune response that was previously established against the hapten. Using biotin as a model hapten, a strong biotin-specific antibody response was generated in mice by using bovine serum albumin-biotin. When these animals were challenged with liposomes containing biotin-conjugated lipid (1 or 0.1%), these liposomes were rapidly eliminated. Incorporation of PEG-lipids into these liposomes substantially reduced biotin-specific antibody binding as measured using an in vitro antibody consumption assay. However, depending on the hapten concentration, significant reductions in antibody binding through the use of PEG-lipids may not be sufficient to protect these liposomes from rapid elimination in vivo. Complete protection of liposomes was only achieved when the biotin concentration on liposome surface was low (0.1%) and with 5 mol% of either 1,2-distearoyl-sn-glycero-3-phosphoethanolamine-n-[methoxy(polyethylene glycol)-2000] or 1,2-dipalmatoyl-sn-glycero-3-phosphoethanolamine-n-methoxy(polyethylene glycol)-2000]. The use of 1,2-dimyristoyl-sn-glycero-3-phosphoethanolamine-n-[methoxy(polyethylene glycol)-2000] (up to 15 mol%) was not effective in protecting liposomes from rapid elimination in vivo, indicating the limited usefulness of this highly exchangeable PEG-lipid. In conclusion, our in vivo and in vitro data indicate that liposomes can be protected from antibody-mediated elimination by using the right type and concentration of PEG-lipids. This result has important implication in the development of ligand-targeted liposomes.

Biofilm consortia on biomedical and biological surfaces: delivery and targeting strategies

Microbial biofilms have been observed as congregates and attached communities on a diverse range of microecosystems of medicinal and industrial importance. Until recently, most investigations have been performed on planktonic (floating or fluid phase) microorganisms. After realization of the biofilm existence and their recalcitrance toward conventionally adopted preventive strategies and antimicrobial agents, research has been shifted toward novel therapeutics based drug delivery and targeting approaches. With the emergence of various biofilm models and methods to assess biofilm formation and physiology, it is pivotal to discuss various novel strategies that may become the therapeutic tools and clinically adaptable strategies of the future. This review explores various novel research strategies studied to date for their potential in effective biofilm eradication.

The interaction of phospholipid liposomes with bacteria and their use in the delivery of bactericides

Liposomes have been prepared from dipalmitoylphosphatidylcholine (DPPC) incorporating the cationic lipids stearylamine (SA), dimethyldioctadecylammonium bromide (DDAB) and dimethylaminoethane carbamoyl cholesterol (DCchol) and the anionic lipids dipalmitoylphosphatidylglycerol (DPPG) and phosphatidylinositol (PI). Their adsorption to biofilms of skin-associated bacteria (Staphylococcus epidermidis and Proteus vulgaris) and oral bacteria (Streptococcus mutans and sanguis) has been investigated as a function of mole % cationic and anionic lipid. Targeting (adsorption) was most effective for the systems DPPC-chol-SA, DPPC-DPPG and DPPC-PI liposomes to S. epidermidis. The effect of extracellular mucopolysaccharide on targeting was investigated for S. epidermidis biofilms. It was found that targeting increased with the level of extracellular mucopolysaccharide for all liposome compositions studied. The delivery of the oil-soluble bactericide Triclosan and the water soluble bactericide chlorhexidine was studied for a number of liposomal compositions. Superior delivery of both bactericides relative to the free bactericide occurred for DPPC-chol-SA liposomes and for Triclosan delivery by DPPC-DPPG and DPPC-PI liposomes targeted to S. epidermidis at low bactericide concentrations. DPPC-chol-SA liposomes were also effective for delivery of Triclosan to S. sanguis biofilms. Double labelling experiments using [14C]-chlorhexidine and [3H]-DPPC suggested that there was exchange between adsorbed liposomes which had delivered bactericide to the biofilm and those in the bulk solution implying a diffusion mechanism for bactericide delivery.

The specificity and affinity of immunoliposome targeting to oral bacteria

Immunoliposomes have been prepared using antibodies raised to an antigenic determinant on the cell surface of the oral bacterium Streptococcus oralis (S. oralis) in an investigation of their potential to reduce dental plaque. The N-succinimidyl-S-acetylthioacetate (SATA) derivative of the antibodies were conjugated through the reactive m-maleimidobenzoyl-N-hydroxysuccinimide (MBS) derivative of dipalmitoyl-phosphatidylethanolamine (DPPE) incorporated into liposomes. The degree of antibody conjugation to the liposomes was controlled by the percentage of DPPEMBS incorporated into the liposomes. The chemical modification of the antibodies did not affect the ability of the antibodies to bind to a S. oralis biofilm. However, the affinity of the immunoliposomes for S. oralis was much lower than that of the free antibody. The anti-oralis antibodies were highly specific for S. oralis. The anti-oralis immunoliposomes showed the greatest affinity for S. oralis, when targeted to a range of different oral bacterial biofilms. The immunoliposome targeting affinity for S. oralis was largely unaffected by the number of antibodies conjugated to the liposomal surface or by the net charge of the liposomal lipid bilayer. The immunoliposomes showed a greater affinity for S. oralis than 'naked' (bearing no antibody) liposomes. However, positively charged liposomes, incorporating stearylamine, adsorbed to S. oralis with greater affinities than the immunoliposomes. The immunoliposomes appeared to be physically stable over a period of 18 months, as judged by particle-size measurements.

Reactive liposomes encapsulating a glucose oxidase-peroxidase system with antibacterial activity

Liposomes were prepared from phospholipid mixtures of dipalmitoylphosphatidylcholine (DPPC) and phosphatidylinositol (PI), encapsulating the enzymes glucose oxidase (GO) and GO in combination with horse radish peroxidase (HRP) by both extrusion (VET) and reverse-phase evaporation (REV). The optimum level of PI in DPPC/PI liposomes for targeting to biofilms of the oral bacterium Streptococcus gordonii has been established. The liposomes were characterised in terms of the content and activity of the encapsulated enzymes. The antibacterial activity of these 'reactive' liposomes arising from hydrogen peroxide and oxyacids in the presence of the substrates glucose and iodide ions, after targeting to the biofilms, were measured both as a function of liposome-biofilm incubation time and incubation time with the substrates. Bacterial inhibition increases with both liposome-biofilm and substrate-biofilm incubation time and with the extent of enzyme encapsulation. The reactive liposomes also display antibacterial activity in the presence of saliva. The reactive liposomes have potential value in the context of oral hygiene.

Reconstitution of the hexose phosphate translocator from the envelope membranes of wheat endosperm amyloplasts

Amyloplasts were isolated and purified from wheat endosperm and the envelope membranes reconstituted into liposomes. Envelope membranes were solubilized in n-octyl beta-D-glucopyranoside and mixed with liposomes supplemented with 5.6 mol% cholesterol to produce proteoliposomes of defined size, which showed negligible leakage of internal substrates. Transport experiments with proteoliposomes revealed a counter-exchange of glucose 1-phosphate (Glc1P), glucose 6-phosphate (Glc6P), inorganic phosphate (Pi), 3-phosphoglycerate and dihydroxyacetone phosphate. The Glc1P/Pi counter-exchange reaction exhibited an apparent K(m) for Glc1P of 0.4 mM. Glc6P was a competitive inhibitor of Glc1P transport (Ki 0.8 mM), and the two hexose phosphates could exchange with each other, indicating the operation of a single carrier protein. Glc1P/Pi antiport in proteoliposomes showed an exchange stoichiometry at pH 8.0 of 1 mol of phosphate transported per mol of sugar phosphate.

The interaction of cationic liposomes with the skin-associated bacterium Staphylococcus epidermidis: effects of ionic strength and temperature

Cationic liposomes have been prepared from dipalmitoylphosphatidylcholine (DPPC), cholesterol (Chol) and stearylamine (SA). These phospholipid vesicles were exposed to adsorbed biofilms of the skin-associated bacteria Staphylococcus epidermidis, to which they showed a strong affinity. The interaction (as assessed by the apparent monolayer coverage of the biofilms by liposomes) was described in terms of a Langmuir adsorption isotherm which enabled determination of the maximum theoretical coverage of the bacterial surface and association/dissociation constants. The interaction was shown to be dependent on the ionic strength of the surrounding medium; on increasing the ionic strength the biofilm-vesicle dissociation constant decreased. This suggested that the adsorption was mediated by electrostatic effects. The adsorption of the vesicles was examined at various temperatures, enabling determination of thermodynamic parameters for the interaction. The adsorbed state of the liposomes was energetically favoured and the interaction was enthalpy driven. The Gibbs energies of adsorption were in a range from -15 to -19 kJ mol-1 and the enthalpies of adsorption from -26 to -22 kJ mol-1. Studies using cell populations of different hydrophobicity showed that the hydrophobic character of the bacterial cells also had an effect on the adsorption of the vesicles to the biofilm.

Preparation and characterization of conjugates of (modified) human serum albumin and liposomes: drug carriers with an intrinsic anti-HIV activity

Human serum albumin (HSA) derivatized with cis-aconitic anhydride (Aco-HSA) that was earlier shown to inhibit replication of human immunodeficiency virus type 1 (HIV-1), was covalently coupled to conventional liposomes, consisting of phosphatidylcholine, cholesterol and maleimido-4-(p-phenylbutyryl)phosphatidylethanolamine, using the heterobifunctional reagent N-succinimidyl-S-acetylthioacetate (SATA). The amount of HSA that could be coupled to the liposomes depended on derivatization of the HSA and ranged from 64.2 +/- microgram HSA/micromol total lipid for native HSA to 29.5 +/- 2.7 microgram HSA/micromol total lipid for HSA in which 53 of the epsilon amino groups of lysine were derivatized with cis-aconitic anhydride (Aco53-HSA). Incorporation of 3.8 mol% of total lipid of a poly(ethylene glycol) derivative of phosphatidylethanolamine (PEG-PE) in the liposomes resulted in a lower coupling efficiency of Aco-HSA. The elimination and distribution of the liposomal conjugates in rats in vivo was largely dependent on the modification of the HSA coupled to the liposomes. With native HSA-liposomes, more than 70% of the conjugate was still found in the blood plasma 30 min after i.v. injection in rats, while at this time Aco-HSA-liposomes were completely cleared from the circulation. The rapid clearance of conventional Aco-HSA-liposomes was due to a rapid uptake into the liver and could be considerably decreased by incorporating PEG-PE in the liposomal bilayer. After 3 h 60% of Aco-HSA-PEG-liposome conjugates were found in the blood. In an in vitro anti-HIV-1 assay, the 50% inhibitory concentrations (IC50) for Aco39-HSA-liposomes and Aco53-HSA-liposomes expressed as protein weight, were 2.87 microgram/ml and 0.154 microgram/ml, respectively. When PEG-PE was incorporated, the Aco53-HSA-liposomes retained anti HIV-1 activity (IC50:3.13 microgram/ml). The possibility to modulate the residence time in the bloodstream of Aco-HSA-liposomes and the potent anti-HIV-1 activity of these conjugates, may allow the development of an intrinsically active drug carrier system. By incorporating anti HIV-1 drugs such as AZT into such liposomes a drug delivery system can be designed that might act simultaneously on the virus/cell binding by virtue of the coupled Aco-HSA and on the RNA/DNA transcription of the HIV-1 replication cycle through the nucleoside analogue.

Encapsulation of vancomycin and gentamicin within cationic liposomes for inhibition of growth of Staphylococcus epidermidis

Liposomes have been prepared from dipalmitoylphosphatidylcholine (DPPC), cholesterol (Chol) and dimethyldioctadecylammonium bromide (DDAB). The cationic vesicles adsorb to biofilms of the skin-associated bacteria Staphylococcus epidermidis, which have a negative charge. Encapsulation of the antibacterial drug vancomycin into such liposomes enhanced its activity relative to the free agent. The effectiveness of the preparation was dependent on the fluidity of the liposomal membrane and on the level of drug entrapment within the aqueous core of the vesicles. The aminoglycoside antibiotic gentamicin was also encapsulated within similar liposomes but was less effective, possibly due to its slow passage through the membrane. The liposomal vancomycin preparation has potential medical use in treating bacterial infections of foreign body biomedical devices (e.g. catheters), with either topical or intravenous administration.

The targeting of phospholipid liposomes to bacteria

Phospholipid liposomes have been prepared from phospholipid mixtures including dipalmitoylphosphatidylcholine/phosphatidylinositol (DPPC/PI) and DPPC/dipalmitoylphosphatidylglycerol (DPPC/DPPG) mixtures and targeted to adsorbed biofilms of the skin-associated bacteria Staphylococcus epidermidis and Proteus vulgaris and the oral bacterium Streptococcus sanguis. The effects of time, liposome concentration and density of bacteria in the biofilm have been studied in detail for Staphylococcus epidermidis. The targeting (as assessed by the apparent monolayer coverage of the biofilms by liposomes) to the biofilms was found to be sensitive to the mol% of PI and DPPG in the liposomes and optimum levels of PI were found for targeting to each bacterium. The use of PI and DPPG-containing liposomes for the delivery of the bactericide, Triclosan, to biofilms of Staphylococcus epidermidis was studied as a function of the amount of Triclosan carried by the liposomes. All the liposome systems tested inhibited the growth of bacteria from the biofilms after brief (2 min) exposure to Triclosan-carrying liposomes. At low Triclosan levels bacterial growth inhibition by Triclosan-carrying liposomes exceeded that by an equivalent level of free Triclosan. After short periods (min) of exposure of biofilms to Triclosan-carrying liposomes the bactericide was shown to preferentially concentrate in the biofilms relative to its liposomal lipid carrier. The results suggest that phospholipid liposomes with appropriately chosen lipid composition have potential for the targeting and delivery of bactericide to bacteria.

The use of phospholipid liposomes for targeting to oral and skin-associated bacteria

Phospholipid (dipalmitoylphosphatidylcholine (DPPC) plus phosphatidylinositol (PI)) proteoliposomes with surface bound lectins (succinylated concanavalin A (s con A) and wheat germ agglutinin (WGA)) have been prepared covering a range of size and surface density of lectin. Negatively charged phospholipid liposomes from DPPC-PI mixtures covering a range of PI mole % and positively charged liposomes from DPPC-cholesterol-stearylamine (SA) mixtures covering a range of SA mole % have been prepared. The targeting of the liposomes and proteoliposomes to a range of oral and skin-associated been prepared. The targeting of the liposomes and proteoliposomes to a range of oral and skin-associated bacterial biofilms has been investigated. The oral bacteria Streptococcus mutans and gordonii and the skin-associated bacterium Coryneform hofmanni can be targeted with s con A bearing proteoliposomes while the skin associated bacterium Staphylococcus epidermidis can be targeted with WGA bearing proteoliposomes. Both oral and skin-associated bacteria can be targeted with positively charged liposomes although the extents of adsorption to the biofilm are low except for Staphylococcus epidermidis. In the case of negatively charged liposomes targeting is critically dependent on the PI content of the liposomes and for all the bacteria studied optimum levels PI for targeting have been found. The adsorption of the oral bacterium Streptococcus gordonii to immobilised monolayers having the optimum PI level for adsorption has been studied by total internal reflection microscopy (TIRM). Both the phospholipid and proteoliposomes have been used to deliver the bactericide Triclosan to biofilms. All the systems studied inhibited bacterial growth to varying degrees.(ABSTRACT TRUNCATED AT 250 WORDS)

Small angle X-ray scattering: possibilities and limitations in characterization of vesicles

The use of small angle X-ray scattering (SAXS) for characterization of lipid vesicle dispersions is described. The effect of curvature of the membrane, the presence of proteins in the core and on the surface of the membrane, variations in membrane thickness and distribution in the number of bilayers of the vesicles in the dispersion on the scattering curve is discussed. Concerning unilamellar vesicles, either the membrane curvature of vesicles smaller than 50 nm or variations in membrane thickness result in a disappearance of the first node in the scattering curve, even if the bilayer is symmetric with respect to the electron density distribution. In the case of dispersion in which unilamellar as well as multilamellar vesicles are present it is shown that a small fraction of multilamellar liposomes changes the scattering curve dramatically. Liposomes were prepared from various compositions of dipalmitoylphosphatidylcholine (DPPC) and cholesterol hemisuccinate (CHEMS) by the film method. The electron density profile of the bilayers and distribution in the number of bilayers of the liposome dispersions were determined. The average number of bilayers increased as a function of the decrease in CHEMS content. Liposomes with higher CHEMS content than 10 mol% were unilamellar. It seems that increase in charge intercalated in the bilayers resulted in unilamellar vesicles.

Targeting and delivery of bactericide to adsorbed oral bacteria by use of proteoliposomes

Proteoliposomes having surface-bound succinylated concanavalin A (s-conA) have been prepared from a range of phospholipid mixtures by sonication (SUV) and reverse phase evaporation (REV) covering a range of size (weight-average diameter (dw)) from approx. 35 to 310 nm and weight-average number of protein molecules per liposomes (Pw) from approx. 50 to 3000. The targeting of the proteoliposomes to adsorbed biofilms of the bacteria Streptococcus sanguis and Streptococcus mutans has been assessed from the extent of inhibition of an enzyme-linked immunosorbent assay (ELISA) for bacterial cell surface antigens. The surface-bound lectin enhances targeting relative to 'naked' liposomes of comparable concentration by factors of 2-50 depending on the liposomal lipid composition and Pw. The effect of the bactericide Triclosan on the thermal properties and permeability characteristics of liposomes has been studied. At and above a molar ratio of Triclosan to lipid of 0.6, Triclosan eliminates the gel to liquid-crystalline phase transition in dipalmitoylphosphatidylcholine (DPPC) containing liposomes and increases the bilayer permeability of both liposomes and proteoliposomes to D-glucose. The proteoliposomes have been used to deliver Triclosan to S. sanguis biofilms and the inhibition of growth of the bacteria after treatment with liposomally delivered Triclosan has been determined using a microtitre plate re-growth assay and compared with growth inhibition by 'free' Triclosan. It is shown that for short exposure times (1 to 2 min) proteoliposomally delivered Triclosan is a more effective growth inhibitor than free Triclosan. The results are discussed in terms of the targeting, retention and subsequent release of Triclosan into the bacterial biofilms.

Incorporation of a bacterial membrane-bound hydrogenase into proteoliposomes

Membrane-bound nickel-iron hydrogenases from diverse genera of bacteria have been previously characterized and they are closely related. We report the reconstitution of purified Bradyrhizobium japonicum hydrogenase into proteoliposomes by a detergent dialysis method followed by two or three cycles of freeze-thaw. Sedimentation experiments revealed that more than 60% of the H2-uptake activity was particulate when reconstituted into Escherichia coli phospholipids. Sucrose-gradient centrifugation separated hydrogenase activity into two peaks, the less dense of which was phospholipid-associated and turbid, thereby showing successful incorporation. Purified enzyme did not bind to performed phospholipid vesicles, and 1.0 M NaCl failed to remove incorporated hydrogenase. The optimal micellar detergent:phospholipid ratio (rho) value for hydrogenase incorporation was 2.0. Proteoliposomes containing acidic phospholipids were the most effective for incorporation as well as for activity. The artificial electron acceptor specificity was similar for proteoliposomes and for H2-oxidizing membranes from B. japonicum. Proteoliposomes formed under optimal conditions had a broad size distribution centered around 400 nm diameter. Hydrogenase activity in proteoliposomes was partially protected from inactivation by the protein modification reagent diazobenzene sulfonate (DABS) (inactivation t1/2 = 30 min), whereas DABS rapidly inactivated the purified enzyme (t1/2 = 4 min). The latter result indicates protection of a catalytically important site by the phospholipid bilayer. This experimental system should be useful in addressing questions regarding the in vivo situation of bacterial membrane-bound hydrogenases.

The effect of blood serum on the size and stability of phospholipid liposomes

Liposomes have been prepared by sonication (SUV) and reverse phase evaporation (REV) from dipalmitoylphosphatidylcholine (DPPC) and its mixtures with phosphatidylinositol (PI), stearylamine and cholesterol. The effect of rat and human blood serum on the liposomes has been investigated by measurement of the particle size in the serum-liposome mixtures by photon correlation spectroscopy in the range of serum protein concentration up to approx. 25 mg ml-1. At low serum protein concentrations the measured particle sizes exceed those calculated from the known sizes and concentrations of liposomes and serum particles in the mixtures: a result consistent with serum-induced aggregation of the liposomes, but the aggregates dissociate at higher serum protein concentration. The effect of serum on the release of encapsulated [14C]glucose from REV liposomes has been investigated over a range of serum protein concentration by gel filtration. At low serum concentration a proportion of the liposomes remain intact but as the serum concentration is increased the size of the liposomes decreases with concomitant release of encapsulated glucose. At high serum concentrations (approx. 24 mg protein per ml) the larger liposomes in the distribution are disrupted and some of the liposomal lipid becomes associated with serum protein. The results are discussed with reference to the effect of blood on the uptake of liposomes by rat liver.

The effect of blood on the uptake of liposomal lipid by perfused rat liver

Liposomes have been prepared from dipalmitoylphosphatidylcholine (DPPC) and its mixtures with phosphatidylinositol (PI) and stearylamine. The absorption of the liposomes by perfused rat liver has been studied as a function of blood level (0-7% haematocrit). It has been found that the rate constant for uptake of liposomes (perfusion constant, kp) is markedly reduced by addition of blood to the perfusate particularly in the haematocrit range 0-3%. The perfusion constant is dependent on the liposome composition and decreases with incorporation of PI and increases with incorporation of stearylamine into DPPC liposomes, but is independent of the initial size of the liposomes in the range of weight-average diameter from 40-400 nm. The possible effects of blood components on the liposomes and their subsequent absorption by the liver are discussed.

The effect of surface-bound protein on the permeability of proteoliposomes

Proteoliposomes have been prepared from mixtures of dipalmitoylphosphatidylcholine and phosphatidylinositol by sonication (SUV) and reverse phase evaporation (REV) and conjugated with succinyl concanavalin A (sConA). The proteoliposomes were characterised in terms of size and composition and covered a range of size (weight-average diameter) from approx. 80 to 300 nm and surface-bound sConA (weight-average number of protein molecules per liposome) from approx. 200 to 1800. The permeabilities of the proteoliposomes to encapsulated D-glucose have been measured and found to increase linearly with protein conjugation. The D-glucose permeability also increases with temperature and passes through a maximum in the region of the gel to liquid-crystalline phase transition temperature. Conjugation has no effect on the chain-melting temperature but slightly decreases the enthalpy of the transition consistent with the withdrawal of some phospholipid participation in chain-melting. The D-glucose permeabilities and thermotropic properties of the proteoliposomes are discussed in terms of the dislocation of the bilayer by the possible off-axis motion of the lipid which anchors the protein to the liposomal surface.

Antibody-coated liposomes as a particulate solid phase for immunoassays. Measurement of urinary 'micro-albumin'

A novel use of liposomes as a solid phase material achieving separation in immunoassays is described. Antibody-coated liposomes were prepared and used as a particulate solid phase in a radioimmunoassay procedure for urinary albumin. The assay was compared to a liquid phase albumin radioimmunoassay. The potential benefits of liposomes over other particulate solid phases are discussed. The use of liposomes in this manner need not be restricted to radioimmunoassay but should also be applicable to other immunoassays using alternative non-isotopic labels.

Characterization of in vivo immunoliposome targeting to pulmonary endothelium

Two rat monoclonal antibodies, 34A and 201B, which specifically bind to a surface glycoprotein (gp112) of the pulmonary endothelial cell surface, have been coupled to unilamellar liposomes of approximately 0.25 microns in diameter. The 34A- and 201B-liposomes (monoclonal antibodies 273-34A and 411-201B, respectively), but not antibody-free liposomes and liposomes coupled to 14, a nonspecific monoclonal antibody, accumulate efficiently (approximately 30% injected dose) in the lung of mice which have been injected via the tail vein. Immunoliposome targeting to lung is demonstrated both by using a 125I-labeled lipid marker and an entrapped water-soluble marker. Lung accumulation of 34A-liposomes is completely blocked by a preincubation of free antibody 34A, but not antibody 14, indicating that the immunoliposome accumulation at the target site is immunospecific. Time course studies have revealed that 34A-liposomes bind to lung antigens within 1 min after injection, indicating that the target binding takes place during the first few passages of immunoliposomes through the lung capillary bed. Unbound immunoliposomes are taken up by liver and spleen within 3-5 min after injection. The level of lung accumulation increases significantly as the protein:lipid ratio of the immunoliposome increases. Approximately 50% of injected dose is accumulated in lung for 34A-liposomes, with an average of 935 antibody molecules per liposome. Immunoliposomes of larger size accumulate in lung more significantly than those of smaller size. Injection with higher doses also enhances the level of lung accumulation.(ABSTRACT TRUNCATED AT 250 WORDS)