Surface Charge Of Liposomes Research Articles

Disposition of Aerosolized Liposomal Amphotericin B

Amphotericin B (AmB) is an important drug for the treatment of fungal infection, but toxicity limits the lung tissue doses which may be achieved through intravenous administration. Although incorporation of AmB in liposomes reduces these effects and increases the therapeutic index for intravenous administration, targeted delivery to lung tissues via inhaled liposomal AmB aerosol may be a more effective approach. Aerosolization of liposomal amphotericin B targets the lungs, the organs first infested by many fungi. Development of optimal aerosolized liposomal AmB therapies requires a better understanding of the effect that liposome surface charge has on lung clearance kinetics. In this work we evaluated the clearance kinetics and organ distribution of inhaled liposomal AmB in male Balb/C mice. Mice were exposed via nose only to AmB-containing liposomal aerosols having positive, negative, or neutral surface charge characteristics. The formulations were aerosolized using a Collison nebulizer. Groups of animals were euthanized at predetermined times and the lungs and other organs were analyzed for AmB. AmB was not detected in serum and other organs such as kidneys, liver, and brain. The disposition of neutral and positive liposomal amphotericin B in lungs followed biexponential kinetics. The α and β phase half-lives for positive liposomes were 1.3 and 15.1days, respectively, and 2.3 and 22days for neutral liposomes. AmB delivered via negative liposomes exhibited monoexponential clearance with a half-life of 4.5days. These results suggest that toxic side effects in nontarget tissues are minimal and may indicate a potential for long term protection against fungal infections.

Abstract

This paper describes the preparation and characterization of microencapsulated liposome systems (MELs) for the controlled delivery of liposome-associated macromolecules. Liposomes were encapsulated within microspheres of calcium-crosslinked alginate, with an additional membrane of alginate-poly(L-lysine) (PLL). The membrane permeability to liposomes was highly dependent on PLL molecular weight, concentration and reaction time with the microspheres. Membranes formed from PLL of molecular weights ranging between 25 and 87 kDa retained more than 90% of the liposomes within MELs, while those of PLL of 111 kDa and above allowed liposome release. The release was characterized with an initial liposome burst, followed by a continuous release phase. It is suggested that the burst occurs as a result of membrane rupture upon liquefaction of the internal core of the microsphere, in phosphate-buffered saline. After re-establishment of the membrane, MELs released their liposomes, at a rate determined by the permeability properties of alginate-PLL membranes, and liposome surface charge. Direct imaging of the released media, using cryo-transmission electron microscopy (cryo-TEM), revealed that liposomes maintained their supramolecular structure. MELs, coated with PLL of different molecular weights, showed different liposome release rates, after s.c. injection into mice. Twenty-two days after injection, 71% of liposome-associated radioactivity was recovered in mice injected with MELs coated with 25 kDa, while only 6% was recovered in mice receiving MELs coated with 214 kDa. The release pattern of a model antigen, [3H]-labeled bovine serum albumin, from MELs was correlated with that of liposomes, indicating that the protein is released mainly in the context of liposomes. These studies show the potential of MELs as controlled release systems for liposome-associated macromolecules.

Characterization of microencapsulated liposome systems for the controlled delivery of liposome-associated macromolecules

This paper describes the preparation and characterization of microencapsulated liposome systems (MELs) for the controlled delivery of liposome-associated macromolecules. Liposomes were encapsulated within microspheres of calcium-crosslinked alginate, with an additional membrane of alginate-poly( L-lysine) (PLL). The membrane permeability to liposomes was highly dependent on PLL molecular weight, concentration and reaction time with the microspheres. Membranes formed from PLL of molecular weights ranging between 25 and 87 kDa retained more than 90% of the liposomes within MELs, while those of PLL of 111 kDa and above allowed liposome release. The release was characterized with an initial liposome burst, followed by a continuous release phase. It is suggested that the burst occurs as a result of membrane rupture upon liquefaction of the internal core of the microsphere, in phosphate-buffered saline. After re-establishment of the membrane, MELs released their liposomes, at a rate determined by the permeability properties of alginate-PLL membranes, and liposome surface charge. Direct imaging of the released media, using cryo-transmission electron microscopy (cryo-TEM), revealed that liposomes maintained their supramolecular structure. MELs, coated with PLL of different molecular weights, showed different liposome release rates, after s.c. injection into mice. Twenty-two days after injection, 71% of liposome-associated radioactivity was recovered in mice injected with MELs coated with 25 kDa, while only 6% was recovered in mice receiving MELs coated with 214 kDa. The release pattern of a model antigen, [ 3H]-labeled bovine serum albumin, from MELs was correlated with that of liposomes, indicating that the protein is released mainly in the context of liposomes. These studies show the potential of MELs as controlled release systems for liposome-associated macromolecules.

STABILITY STUDIES OF LIPOSOMES AS A FUNCTION OF LIPID COMPOSITION, LIPOSOMAL TYPE, SURFACE CHARGE AND STORAGE CONDITIONS

The effect of lipid composition, liposomal type, surface charge and storage conditions (time and temperature) on their stability was studied. The time course of lipid peroxidation for different liposomal formulations was chemically followed, during different storage conditions, using the thiobarbituric acid assay. Multilamellar and small unilamellar liposomes were prepared from egg lecithin and varying amounts of cholesterol with or without a charge inducing agent. Dicetyl phosphate or stearylamine was used to impart either a negative or a positive surface charge to liposomes. The results showed that, incorporation of cholesterol into the liposomal bilayer structure, generally, reduced the rate of lipid peroxidation and adjusting its content to 33 mole% could provide an appropriate molar ratio for maximum stability of liposomal system. Also, comparing between the two liposomal types of the same charge and storage conditions, small unilamellar liposomes showed significantly higher peroxidation values than multilamellar type Regarding the liposomal surface charge, negatively charged liposomes showed the lowest peroxidation values and neutral liposomes showed the highest values, as compared to others of the same type and storage conditions. Thus, inclusion of charged lipid results in an structral stabilizing effect, which could be attributed to the steric barrier on the liposomal surface presented by this charge. In addition, concerning the effect of storage temperature on lipid peroxidation, liposomes showed higher stability when stored at low temperature than when stored at high temperature. Finally, for choosing the proper storage conditions to optimize liposomal stability freeze-dried and freeze-thawed liposomes were prepared and compared with untreated liposomes. Freeze-dried negatively charged multilamellar liposomes could provide an optimal storage condition for maintaining liposomal stability. In conclusion, it should be stressed that lipsomal lipid composition, type, surface charge and storage conditions can be modified in a variety of ways, and so can potentially be tailored to satisfy the needs of the formulation for solving the stability problems of liposomes.

Determinants of liposome partitioning in aqueous two-phase systems: Evaluation by means of a factorial design

This article evaluates the influence of five parameters on liposome partitioning in aqueous two-phase systems (ATPSs), composed of poly(ethyleneglycol) (PEG)/dextran (Dx), using the factorial experimental design together with a multiple regression. Mathematical models to quantify the influence of these parameters, individually and/or jointly, on liposome partitioning in ATPS were developed. The models were statistically tested and verified by experimentation. This approach was then used to define the conditions for the preferential accumulation of liposomes in the top PEG-rich phase. The models predicted a significant effect of liposome surface charge, PEG molecular weight, phase-forming polymer concentration, and phosphate ion concentration on the partition behavior of liposomes. For negatively charged liposomes, it was found that the smaller the molecular weight of PEG and polymer concentration and the larger the phosphate ion concentration, the greater the partition coefficient of the liposomes. No significant effect of pH, at the range of 6-8, on liposome partitioning was noted. This approach has led to the development of an optimal two-phase system where 90% of negatively charged liposomes accumulated in the PEG phase. In addition to the general scientific value of this research, it has a technological importance as ATPSs may be useful for removing the unentrapped drug from liposomes during their preparation for pharmaceutical applications. (c) 1996 John Wiley & Sons, Inc.

OCULAR CONTROLLED DELIVERY OF PREDNISOLONE USING LIPOSOMES AS EFFECTIVE CARRIERS SYSTEM

The potential use of liposomes for ocular delivery of encapsulated prednisolone was evaluated in the rabbit's eye. Liposomes used in this study, multilamellar and unilamellar, were prepared from dipalmitoyl phosphatidylcholine and cholesterol, with or without a charge inducing agent. Stearylamine and dicetyl phosphate have been incorporated so as to import either a positive or a negative surface charge to liposomes. Sequential extrusion of liposomes through polycarbonate membranes was used to improve the homogeneity of the preparations. Results demonstrated that the maximal encapsulation of drug was achieved by using multilamellar liposomal formulations prepared with negatively charged lipid. The in-vivo performance of various formulations of the drug was assessed on the basis of the influence of the drug on the intra-ocular pressure. The data were interpreted in terms of area under the intra-ocular pressure/time curve, duration of action, the maximum response, and time of maximum response which have been taken in consideration as paraeters of drug activity. This study showed that the intra-ocular activiy of the drug using various liposomal form was greater than that of solution form. In addition, regarding the liposomal types, multilamellar liposomes produced significantly higher drug activity than the small unilamellar ones. Also, in both liposomal types, the parameters of drug activity depends to great extent on the liposomal surface charge and its influence was generally greatest with positively harged multilamellar liposomes as compared to other formulations. This may be attributed to electrostatic bindings between corneal surface bears a net negative charge and the positively charged liposomes. Accordingly, positively charged multilamellar liposomes represent an optimal delivery system which appeared to be suitable in modulating drug action for selective targeting in the ocular therapy.

Impregnation of collagen corneal shields with liposomes: Uptake and release of hydrophilic and lipophilic marker substances

Liposomes and collagen corneal shields (CCS) have been used as ophthalmic drug delivery devices. With regard to a possibly combined application, we studied the effects of surface charge and bilayer fluidity of liposomes on their uptake and release by CCS. 12-hours-CCS were soaked in large unilamellar liposomes, which had been labelled with 4,5-carboxyfluorescein (CF) and N-(lissamine rhodamine B sulfonyl)-diacyl-phosphatidylethanolamine (PE-RhB) in the aqueous space and in the liposome bilayer, respectively. Released fluorophores were determined fluorometrically in the elution buffer at intervals from 1 to 240 min after immersion. The CF concentration in the CCS soaked in a CF solution was two to seven times higher than immersion in the liposome suspensions. Among those, the negatively charged, cholesterol-containing preparation led to the highest CF concentration in the CCS. The PE-RhB concentration was highest after soaking the CCS in neutral, cholesterol-free liposomes. All types of liposomes were found inside the CCS by freeze fracture electron microscopy. The release kinetics data indicate a first order release. More than 90% of CF was released by the CCS within the first 30 min. This was equal after soaking the CCS in the CF solution or in liposomes. With DOPC-liposomes, the maximal release was already attained after 10 min. In general, the differences in the release kinetics of both hydrophilic and lipophilic markers, obtained by the various liposome types were small. Our results indicate that surface charge and bilayer fluidity are of minor importance for the interaction with collagen corneal shields. However, since the release kinetics of a liposome-encapsulated hydrophilic or lipophilic substance are similar to the release of a non-encapsulated drug, the combination of liposomes with collagen shields may be useful mainly with respect to the encapsulation of drugs which do not penetrate the ocular surface as well as to prolong corneal contact time of the liposomes.

Effect of Calcium Ions on the Surface Charge and Aggregation of Phosphatidylcholine Liposomes

The aggregation behaviour of multilamellar liposomes composed of egg phosphatidylcholine (EPC) and dipalmitoylphosphatidylcholine (DPPC) in the presence of varying concentrations of calcium ions has been studied by measuring changes in the particle size of liposome suspensions. Reversible aggregation occurred with both liposome preparations, showing a maximum rate at approximately 0.9 to 1.8 mM calcium chloride. At higher or lower concentrations aggregation was inhibited in the case of EPC suspensions or, for DPPC systems, occurred only slowly. Furthermore, the EPC systems showed more marked increases in size when aggregation occurred. The zeta potentials of both systems were negative in water, becoming positive on increasing the concentration of calcium ions. A reasonable correlation was found between calcium concentration corresponding to the minimum in initial zeta potential and that producing the greatest aggregation rate.

Liposomal presentation of antigens for human vaccines.

Liposomes are considered prime candidates to improve the immunogenicity of both antigens with hydrophobic anchor sequences and soluble, nonmembrane proteins or synthetic peptides. During the 20 years since liposomes were first demonstrated to have adjuvant potential, studies have shown that variation in liposomal size, lipid composition, surface charge, membrane fluidity, lipid-protein composition, anchor molecules, and fusogenicity can significantly influence results. In addition, antigen location (e.g., whether it is adsorbed or covalently coupled to the liposome surface or encapsulated in liposomal aqueous compartments) may also be important. Analysis of these variables as well as a comparison of the various techniques used to ensure the efficacy, stability, homogeneity, and safety of liposomal vaccine have been discussed.

Pharmacokinetics and tissue distribution of methotrexate after intravenous injection of differently charged liposome-entrapped methotrexate to rats

Liposome-entrapped [ 3H]MTX was prepared by modification of reverse-phase evaporation vesicle (REV) methods. Neutral liposomes were prepared with a mixture of phosphatidylcholine (PC), cholesterol (CH) and α-tocopherol (α-T) (8:4:0.1, molar ratio). Positively and negatively charged liposomes were also prepared by incorporation of stearylamine (SA) (8:4:0.1:1, molar ratio) and dicetyl phosphate (DCP) (8:4:0.1:1, molar ratio) into neutral liposomes, respectively. The release profiles of [ 3H]MTX from liposomes in the presence of rat plasma were the same shape regardless of surface charges, showing initial fast release followed by much slower release. The release profiles of [ 3H]MTX from liposomes were dependent on the surface charge of liposomes. Negatively charged liposomes showed much greater [ 3H]MTX release compared to positively charged and neutral liposomes. The enhanced in vitro release of [ 3H]MTX from negatively charged liposomes in the presence of rat plasma may be due to the interaction of incorporated lipid compositions of liposomes with plasma components, resulting in a change in drug release through the lipid bilayers by destabilization of the liposomal membranes. After intravenous (i.v.) injection of free and liposome-entrapped [ 3H]MTX to rats, the elimination of [ 3H]MTX from the blood stream showed biphasic patterns indicating rapid declining disposition up to 30 min followed by a slower elimination phase. Liposome-entrapped [ 3H]MTX maintained a higher and longer plasma concentration, and mainly intact form in plasma compared to free [ 3H]MTX for 2 h. Liposome-entrapped [ 3H]MTX enhanced bioavailability in plasma due to the retardation of drug release and protection of drug clearance by lipid bilayers of liposomes. Negatively charged liposome-entrapped drug was cleared more rapidly from the blood, resulting from the rapid uptake by liver to a greater extent, possibly via the reticuloendothelial system (RES), since liposome-entrapped drug cannot be eliminated by the kidney, the main eliminating organ for MTX. The tissue distribution of liposome-entrapped [ 3H]MTX was widely different when compared to free [ 3H]MTX. There was a markedly increased uptake of drug in spleen from neutral and negatively charged liposomes. Negatively charged liposomes also increased localization of drug in liver, lung and lymph nodes compared to neutral and positively charged liposomes at 2 h after i.v. injection to rats. From these findings, liposomes containing anticancer drugs would appear to be an effective carrier system for targeting to these sites. Although liposome-entrapped [ 3H]MTX was widely distributed in tissues without the exact role of the liposomal surface charge being known, it was evident that the surface charge of liposomes altered the biodistribution and membrane permeation of drug.

The effects of liposome surface charge and size on the intracellular delivery of clodronate and gallium in vitro

Clodronate (dichloromethylene bisphosphonate) and gallium in liposomes are macrophage suppressive agents, and thus possibly useful in inflammatory diseases. The effects of liposome surface charge, liposome type, and size on the delivery of these compounds to RAW 264 macrophages and L929 fibroblasts were evaluated by growth inhibition assay in vitro. Unilamellar liposomes with neutral surface charge (100 mol% DSPC) were unable to deliver clodronate or gallium to macrophages, but inclusion of negatively charged DSPG in liposomes provided effective delivery of the compounds. Large multilamellar liposomes with neutral surface charge were also ineffective in delivering clodronate to macrophages. A decrease in the size of unilamellar DSPG liposomes by extrusion did not affect the delivery of the compounds to macrophages, but drastically increased their potency for fibroblasts. For macrophage delivery, unextruded DSPG liposomes prepared by the REV method were found to be the best formulation studied, while extrusion of liposomes broadened the effects to other cell types. Various lysosomotropic agents did not affect the potency of liposomal clodronate for macrophages, but decreased the potency of liposomal gallium, indicating that, despite their similarities in mechanism of action in macrophages, the intracellular processing of liposomal clodronate differs from that of gallium.

Uptake and biodistribution of free and liposomally incorporated lipopolysaccharide of aeromonas salmonicida administered via different routes to rainbow trout: (Oncorhynchus mykiss)

AbstractThe effects on uptake and biodistribution of radiolabelled lipopolysaccharide (LPS) due to changing routes of administration, encapsulation of LPS within liposomes and altering liposomal surface charge were examined in rainbow trout (Oncorhynchus mykiss). 3H-labelled LPS, positively- and negatively-charged (14C-labelled) liposomes or 14C-labelled liposomes containing 3H-LPS were administered to trout via intravenous, intraperitoneal, intramuscular, or oral routes. Twenty-four hours following administration, relative uptake of LPS and multilamellar vesicles (MLV) based on detection of 3H and 1AC, respectively, was determined in samples taken from the kidney, spleen, liver, plasma, blood cells and skeletal muscle. In general, regardless of the route of administration, 3H-LPS, 1AC-MLV and liposomally encapsulated LPS were recovered primarily in the kidney and spleen. Intravenous administration resulted in the greatest uptake of radiolabel by the kidney and spleen, followed by the intraperitoneal and ...

Inclusion of ganglioside GM1 into liposome encapsulated hemoglobin does not extend circulation persistence at clinically relevant doses.

This investigation has evaluated the substitution of ganglioside GM1 for dimyristoyl phosphatidylglycerol (DMPG) in the preparation of liposome encapsulated hemoglobin (LEH), with the intention of increasing the circulation persistence of this potential oxygen carrier. Although equivalent yields of each formulation were produced by microfluidization, the hemoglobin encapsulation efficiency was greater for GM1-LEH than DMPG-LEH. Similar particle sizes, phospholipid content, methemoglobin levels, and oxygen-carrying capacity were observed for both formulations. Zeta potential measurements to monitor liposomal surface charge showed GM1-LEH to be more electropositive than DMPG-LEH. Using differential scanning calorimetry, similar enthalpy values and hemoglobin structural transition temperatures were determined for both LEH formulations. Circulation persistence of each LEH formulation was determined following a 0.25 ml (1 g phospholipid/Kg body weight) or 0.5 ml (2 g phospholipid/Kg body weight) injection in mice. During the first 18 hours, GM1-LEH was cleared at a faster rate than DMPG-LEH at both dosages studied. Then the remaining liposomes of each formulation were removed with identical circulation profiles until no liposomes were remaining in circulation at either 50 hours (0.25 ml) or 72 hours (0.5 ml) post-injection. These data reveal that the use of ganglioside GM1 to solely increase the circulation persistence of LEH was of little benefit.

Cytochrome c-catalyzed membrane lipid peroxidation by hydrogen peroxide

Cytochrome c 3+ -catalyzed peroxidation of phosphatidylcholine liposomes by hydrogen peroxide (H 2O 2) was indicated by the production of thiobarbituric acid reactive substances, oxygen consumption, and emission of spontaneous chemiluminescence. The iron chelator diethylenetriaminepentaacetic acid (DTPA) only partially inhibited peroxidation when H 2O 2 concentrations were 200 μ m or greater. In contrast, iron compounds such as ferric chloride, potassium ferricyanide, and hemin induced H 2O 2-dependent lipid peroxidation which was totally inhibitable by DTPA. Cyanide and urate, which react at or near the cytochrome—heme, completely prevented lipid peroxidation, while hydroxyl radical scavengers and superoxide dismutase had very little or no inhibitory effect. Changes in liposome surface charge did not influence cytochrome c 3+ plus H 2O 2-dependent peroxidation, but a net negative charge was critical in favoring cytochrome c 3+ -dependent, H 2O 2-independent lipid auto-oxidative processes. These results show that reaction of cytochrome c with H 2O 2 promotes membrane oxidation by more than one chemical mechanism, including formation of high oxidation states of iron at the cytochrome—heme and also by heme iron release at higher H 2O 2 concentrations. Cytochrome c 3+ could react with mitochondrial H 2O 2 to yield “site-specific” mitochondrial membrane lipid peroxidation during tissue oxidant stress.

Liposomes as carriers of 99mTc glucoheptonate for liver imaging

Liposomes were prepared by the detergent dialysis method and 99mTc-GHA, a radiopharmaceutical of choice for kidney imaging, was entrapped in SUV of different surface charges. The organ specificity of the agent with and without encapsulation in liposomes was studied in experimental animals. Results obtained show that 99mTc-GHA entrapped SUV can be targetted to liver. The effect of surface charge of liposomes on its targetting is discussed.

A radiolabel and electron microscopy study of the interaction of liposomes with synaptosomal membranes

The quantitative and qualitative interaction of liposomes with synaptosomes isolated from rat brain was examined using radiolabeled phospholipids and electron microscopy. Liposomes were prepared by sonication and detergent dialysis. Binding (adsorption) of radiolabeled phospholipid to synaptosomes was saturable when liposomes were in the liquid-crystalline state, were electrically neutral (egg-phosphatidylcholine), or carried increasing fractions (10:2 and 10:4 molar ratio) of negatively charged phosphatidic acid. Analysis using the Langmuir isotherm equation indicated a biphasic adsorption behavior. Adsorption increased with increasing temperature (4°C and 37°C). Binding was nonsaturable when liposomes were positively charged with stearylamine or composed of dimyristoylphosphatidylcholine and phosphatidylinositol (10:2 molar ratio). Due to the latter composition's solid state at 4°C, temperature dependency was inverse. Electron micrographs revealed disc-shaped areas of adsorption that were free of integral membrane particles which appeared to form a condensed layer surrounding the areas of liposome adsorption. Following interaction with stearylamine-containing liposomes the vesicular structure of synaptosomes appeared largely destroyed. It is concluded that both liposome surface charge and membrane fluidity determine the extent of interaction with biological membranes.

Pulmonary absorption of liposome-encapsulated 6-carboxyfluorescein

The pulmonary absorption of liposome-encapsulated 6-carboxyfluorescein (CF) has been investigated in the rat. Neutral dimyristoylphosphatidylcholine (DMPC)/cholesterol (1:1 mole ratio) and negatively-charged DMPC/cholesterol/dicetylphosphate (1:1:0.2) liposomes, each containing 0.25 M CF, were prepared by probe sonication of multilamellar vesicle suspensions. After removal of unentrapped CF by gel chromatography and ultrafiltration, the vesicle preparations had narrow but not monodisperse size distributions, with mean diameters of 105–113 nm (neutral) and 139–150 nm (negatively-charged). The liposome preparations were administered as fluid instillates to the lungs of pentobarbitone-anaesthetised rats. The pulmonary absorption of CF was lipid dose-dependent, with higher doses producing faster absorption. Both extent and rate of pulmonary absorption were influenced by liposomal surface charge and CF absorption did not involve the transfer of intact liposomes into the blood. CF was absorbed from the negatively-charged vesicles at least twice as rapidly as from neutral liposomes. The fraction available was also greater from the negatively-charged carriers. The differences in CF bioavailability could not be accounted for by the small differences of in vitro latency of CF between the two liposome species.

Use of 9-aminoacridine in the evaluation of liposome surface charge density: Role of the adsorption on lipidic sites

The procedure of surface charge density evaluation using 9-aminoacridine as a fluorescent probe implies that cations do not interact with the negatively charged lipids. From surface potential measurements on lipid monolayers coupled with fluorescence measurements on liposomes, we demonstrate that the cations present in the aqueous phase, including the fluorescent probe itself, interact with the lipidic sites. These association reactions should not be neglected in the evaluation of the surface charge density associated with model membranes.

Tumor uptake of 67Ga-carrying liposomes.

The in vivo distribution, excretion, and tumor localization of liposome-encapsulated 67Ga in normal and Ehrlich tumor (solid form)-bearing mice were studied. In normal mice, multilamellar vesicles (MLVs) were taken up mainly by the liver and spleen, whereas small unilamellar vesicles (SUVs) exhibited a broader tissue distribution. When 67Ga was encapsulated in MLVs or SUVs, the excretion of the radiotracer in the urine and feces was less than that observed for free tracer at 72 h after i.v. administration. In tumor-bearing mice, SUVs were found to accumulate preferentially in tumors. The tumor uptake of neutral, positive, and negative SUVs was 10%-13% of the administered dose per gram of tumor tissue at 24 h after their injection. These values were about three times higher than those found for free 67Ga-nitrilotriacetic acid (67Ga-NTA) or 67Ga-citrate. Significant differences in tumor uptake due to different surface charges of liposomes were not observed. Enhanced tumor-to-blood and tumor-to-muscle ratios were also observed at 24 h after injection. These results suggest that 67Ga-carrying liposomes may be a useful for tumor imaging.

Effect of surface modification of liposomes with sialoglycopeptide on their clearance from the circulation.

The effect of covalent binding of sialoglycopeptide (GP) derived from fetuin to the liposome surface on the clearance of liposomes from the circulation of rats was investigated. Small unilamellar vesicles (SUV) and two types of multilamellar liposomes (MLV, LMLV) were prepared and coupled with GP without changing the size distribution of the liposomes. The leakage of carboxyfluorescein (CF) from these GP-modified liposomes in plasma was measured in vitro. The clearance of intravenously injected GP-modified liposomes was followed by monitoring entrapped CF and incorporatd [3H]cholesteryl oleate ([3H]CHOL) in the liposomes.It was shown that CF release from GP-modified SUV increased slightly with increasing amount of GP-modification. On the other hand, CF was released more rapidly from GP-modified MLV and LMLV. The circulation times of MLV and LMLV were reduced by the surface modification of the liposomes with GP. In contrast, the clearance of SUV was inhibited significantly by incorporation of GP on the surface of the liposomes. The clearance pattern of negatively charged liposomes containing dicetyl phosphate, which had equivalent ζ potential and similar size distribution to GP-modified SUV, suggests that the prolonged circulation time of GP-modified SUV is not attributable to the negative surface charge of liposomes. These results suggest that surface modification of SUV with sialoglycopeptide derived from fetuin may be a useful technique for designing liposomes with a long circulation time.