Enhanced delivery and antitumor activity of doxorubicin using long-circulating thermosensitive liposomes containing amphipathic polyethylene glycol in combination with local hyperthermia

Therapeutic efficacy and microSPECT/CT imaging of 188Re-DXR-liposome in a C26 murine colon carcinoma solid tumor model

Nanocarriers can selectively target cancer sites and carry payloads, thereby improving diagnostic and therapeutic effectiveness and reducing toxicity. The objective of this study was to investigate the therapeutic efficacy of a new co-delivery radiochemotherapeutics of (188)Re-N,N-bis (2-mercaptoethyl)-N',N'-diethylethylenediamine (BMEDA)-labeled pegylated liposomal doxorubicin (DXR) ((188)Re-DXR-liposome) in a C26 murine colon carcinoma solid tumor model. To evaluate the targeting and localization of (188)Re-DXR-liposome in C26 murine tumor-bearing mice, biodistribution, microSPECT/CT imaging and pharmacokinetic studies were performed. The antitumor effect of (188)Re-DXR-liposome was assessed by tumor growth inhibition, survival ratio and histopathological hematoxylin-eosin staining. The tumor target and localization of the nanoliposome delivery radiochemotherapeutics of (188)Re-DXR-liposome were demonstrated in the biodistribution, pharmacokinetics and in vivo nuclear imaging studies. In the study on therapeutic efficacy, the tumor-bearing mice treated with bimodality radiochemotherapeutics of (188)Re-DXR-liposome showed better mean tumor growth inhibition rate (MGI) and longer median survival time (MGI=0.048; 74 days) than those treated with radiotherapeutics of (188)Re-liposome (MGI=0.134; 60 days) and chemotherapeutics of Lipo-Dox (MGI=0.413; 38 days). The synergistic tumor regression effect was observed with the combination index (CI) exceeding 1 (CI=1.145) for co-delivery radiochemotherapeutics of (188)Re-DXR-liposome. Two (25%) of the mice treated with radiochemotherapeutics were completely cured after 120 days. The therapeutic efficacy of radiotherapeutics of (188)Re-liposome and the synergistic effect of the combination radiochemotherapeutics of (188)Re-DXR-liposome have been demonstrated in a C26 murine solid tumor animal model, which pointed to the potential benefit and promise of the co-delivery of nanoliposome radiochemotherapeutics for adjuvant cancer treatment on oncology applications.

Polymeric micelles for drug targeting

Polymeric micelles are nano-delivery systems formed through self-assembly of amphiphilic block copolymers in an aqueous environment. The nanoscopic dimension, stealth properties induced by the hydrophilic polymeric brush on the micellar surface, capacity for stabilized encapsulation of hydrophobic drugs offered by the hydrophobic and rigid micellar core, and finally a possibility for the chemical manipulation of the core/shell structure have made polymeric micelles one of the most promising carriers for drug targeting. To date, three generations of polymeric micellar delivery systems, i.e. polymeric micelles for passive, active and multifunctional drug targeting, have arisen from research efforts, with each subsequent generation displaying greater specificity for the diseased tissue and/or targeting efficiency. The present manuscript aims to review the research efforts made for the development of each generation and provide an assessment on the overall success of polymeric micellar delivery system in drug targeting. The emphasis is placed on the design and development of ligand modified, stimuli responsive and multifunctional polymeric micelles for drug targeting.

Controlling liposomal drug release with low frequency ultrasound: mechanism and feasibility

The ability of low-frequency ultrasound (LFUS) to release encapsulated drugs from sterically stabilized liposomes in a controlled manner was demonstrated. Three liposomal formulations having identical lipid bilayer compositions and a similar size ( approximately 100 nm) but differing in their encapsulated drugs and methods of drug loading have been tested. Two of the drugs, doxorubicin and methylpredinisolone hemisuccinate, were remote loaded by transmembrane gradients (ammonium sulfate and calcium acetate, respectively). The third drug, cisplatin, was loaded passively into the liposomes. For all three formulations, a short exposure to LFUS (<3 min) released nearly 80% of the drug. The magnitude of drug release was a function of LFUS amplitude and actual exposure time, irrespective of whether irradiation was pulsed or continuous. Furthermore, no change in liposome size distribution or in the chemical properties of the lipids or of the released drugs occurred due to exposure to LFUS. Based on our results, we propose that the mechanism of release is a transient introduction of porelike defects in the liposome membrane, which occurs only during exposure to LFUS, after which the membrane reseals. This explains the observed uptake of the membrane-impermeable fluorophore pyranine from the extraliposomal medium during exposure to LFUS. The implications of these findings for clinical applications of controlled drug release from liposomes are discussed.

Encapsulation of doxorubicin into thermosensitive liposomes via complexation with the transition metal manganese

In the present study, doxorubicin was encapsulated into two thermosensitive liposome formulations which were composed of DPPC/MSPC/DSPE-PEG(2000) (90/10/4 mole ratio) or DPPC/DSPE-PEG(2000) (95/5 mole ratio). Doxorubicin loading was achieved through the use of a pH gradient or a novel procedure that involved doxorubicin complexation with manganese. Regardless of the initial drug-to-lipid ratios (D:L), the final D:L reached a maximum of 0.05 (w/w) when doxorubicin was encapsulated via a pH gradient for both thermosensitive liposome formulations. In contrast, the final maximum D:L achieved through manganese complexation was 0.2 (w/w), and this loading method did not affect temperature-induced drug release, with 85% of drug released from MSPC-containing liposomes within 10 min at 42 degrees C but <5% released over 60 min at 37 degrees C. When the thermosensitive liposomes prepared via the two different loading methods were injected into mice, similar plasma elimination profiles were observed. Cryo-transmission electron microscopy analysis indicated the presence of doxorubicin fiber bundles in liposomes loaded via pH gradient, compared to a stippled and diffuse morphology in those loaded via manganese complexation. To investigate the effect of intraliposomal pH on drug precipitate morphology, the A23187 ionophore (mediates Mn(2+)/H(+) exchange) was added to liposomes loaded with doxorubicin-manganese complex, and the stippled and diffuse appearance could be converted to one exhibiting fiber bundles after acidification of the liposome core. This suggests that the formation of doxorubicin-manganese complex is favored when the intraliposomal pH is >6.5. During the conversion to the fiber bundle morphology, no doxorubicin release was observed when A23187 was added to liposomes exhibiting a 0.05 (w/w), whereas a significant release was noted when the initial D:L was 0.2 (w/w). Following acidification of the liposomal interior and establishment of an apparent new D:L equilibrium, the measured D:L ratio was 0.05 (w/w). In conclusion, the manganese complexation loading method increased the encapsulation efficiency of doxorubicin in thermosensitive liposomes with no major impact on temperature-triggered drug release or pharmacokinetics.

Prednisolone retention in integrated liposomes by chemical approach and pharmaceutical approach

The purpose of this study is to demonstrate a stable retention of prednisolone (PLS) in the unique liposomes integrated by lipophilic derivative approach and PEGylation approach. Palmitoyl prednisolone (Pal-PLS) was newly synthesized and used as a lipophilic derivative. The liposomes were composed of egg phosphatidylcholine (EggPC)/cholesterol (Chol) and L-alpha-distearoylphosphatidylcholine (DSPC)/Chol with or without L-alpha-distearoylphosphatidylethanolamine-polyethylene glycol 2000 (DSPE-PEG 2000) or -PEG 5000 (DSPE-PEG 5000). The retentions of PLS and Pal-PLS in the various liposomes were examined by ultrafiltration and gel filtration. Although PLS showed high trapping efficiency by all liposomes after ultrafiltration, low incorporation efficiency was observed in gel filtration. It indicates that PLS was released from the liposomes by a dilution with elution medium in gel filtration. Pal-PLS showed high incorporation into all liposomes after both ultrafiltration and gel filtration. The high incorporation of Pal-PLS into EggPC/Chol liposomes, however, was reduced by incubation with rat plasma in gel filtration. The reducing effect of rat plasma on drug incorporation into liposomes was inhibited by using DSPC and DSPE-PEGs. Thus, we systemically examined the drug retention in various liposomes and demonstrated the high retention of PLS in the liposomes integrated by lipophilic derivative approach and pharmaceutical approach using special lipids.

Direct 99mTc labeling of pegylated liposomal doxorubicin (Doxil) for pharmacokinetic and non-invasive imaging studies

Pharmacokinetic and organ distribution studies of liposomal drugs in humans are a challenge. A direct labeling method using (99m)Tc-N,N-bis(2-mercaptoethyl)-N',N'-diethyl-ethylenediamine (BMEDA) complex to label the commercially available pegylated liposomal doxorubicin, Doxil, has been introduced. Biodistributions of (99m)Tc-Doxil in normal rats were performed to evaluate the feasibility of using it for monitoring the pharmacokinetics of liposomes encapsulating drugs. Labeling efficiency of (99m)Tc-Doxil was 70.6 +/- 0.8% (n = 3). In vitro incubation of (99m)Tc-Doxil in 50% fetal bovine serum or 50% human serum at 37 degrees C showed good labeling stability with 72.3 +/- 3.6% or 78.6 +/- 1.8% of activity associated with Doxil at 24 h, respectively (n = 3). There was a two-phase blood clearance with half-clearance times of 2.2 and 26.2 h after bolus intravenous injection in normal rats. Distribution of (99m)Tc-Doxil at 44 h after injection had 19.8 +/- 1.3% of injected dose in blood, 14.1 +/- 1.7% in liver, 2.6 +/- 0.3% in spleen, 9.0 +/- 0.8% in bone with marrow, 6.0 +/- 0.5% in skin, and 15.3 +/- 4.3% in bowel (n = 5). Unencapsulated (99m)Tc-BMEDA had a very rapid blood clearance with a half-clearance time of only 0.12 h (n = 4). By using this (99m)Tc labeling method, biodistribution and pharmacokinetics of ammonium gradient liposomes encapsulating drugs can be determined by noninvasive scintigraphic imaging. This labeling method may be extended to (186)Re and (188)Re labeling to combine chemotherapy and radionuclide therapy for tumor treatment.

Disc formation in cholesterol-free liposomes during phase transition

Cryogenic transmission electron microscopy (cryo-TEM) images of lysolipid-containing thermosensitive liposomes (LTSL) revealed that open liposomes and bilayer discs appeared when liposomes were cycled through the gel (Lbeta') to liquid-crystalline (Lalpha) phase transition. The amount of bilayer discs generated was dependent on the combined presence of PEG-lipid and lysolipid in the membrane. We hypothesize that micelle-forming membrane components stabilize the rim of bilayer openings and membrane discs that form when liposomes are cycled through TC.

Alteration in the Temperature-Dependent Content Release Property of Thermosensitive Liposomes in Plasma

The effect of plasma components on the temperature-dependent content release property of thermosensitive liposomes has been described. Temperature-sensitive liposomes containing mitomycin C (MMC) were prepared from dipalmitoylphosphatidylcholine (DPPC liposomes) and a 7 : 3 mixture of DPPC and dipalmitoylophosphatidylglycerol (DPPC/DPPG liposomes). We defined in this study the difference in the content release between 38 degrees C and 44 degrees C as an index of the temperature-dependent content release efficiency (Delta% release). In the absence of rat plasma, the Delta% release of the DPPC liposomes and the DPPC/DPPG liposomes was 83% and 71%, respectively. However, when the release study was conducted with rat plasma, the Delta% release increased to about 96% for both liposomes. In addition, while the DPPC liposomes were destabilized by rat plasma below the gel-to-liquid crystalline phase transition temperature (T(m)), MMC leakage from the DPPC/DPPG liposomes below T(m) was suppressed by rat plasma. Moreover, the plasma protein binding onto lipid bilayer was concomitant with the gel-to-liquid crystalline phase transition and then enhanced the temperature-dependent release from the DPPC/DPPG liposomes. The possible mechanism of interaction between liposomes and plasma proteins, especially serum albumin, was discussed based on differential scanning calorimetry and protein binding experiments.

Improved retention of idarubicin after intravenous injection obtained for cholesterol-free liposomes

To date there has been a focus on the application of sterically stabilized liposomes, composed of saturated diacylphospholipid, polyethylene glycol (PEG) conjugated lipids (5-10 mole%) and cholesterol (CH) (>30 mole%), for the systemic delivery of drugs. However, we are now exploring the utility of liposome formulations composed of diacylphospholipid conjugated PEG mixtures prepared in the absence of added cholesterol, with the primary objective of developing formulations that retain encapsulated drug better than comparable formulations prepared with cholesterol. In this report the stability of cholesterol-free distearoylphosphatidylcholine (DSPC):distearoylphosphatidylethanolamine (DSPE)-PEG(2000) (95:5 mol/mol) liposomes was characterized in comparison to cholesterol-containing formulations DSPC:CH (55:45 mol/mol) and DSPC:CH:DSPE-PEG(2000) (50:45:5 mol/mol/mol), in vivo. Circulation longevity of these formulations was determined in consideration of variables that included varying phospholipid acyl chain length, PEG content and molecular weight. The application of cholesterol-free liposomes as carriers for the hydrophobic anthracycline antibiotic, idarubicin (IDA), was assessed. IDA was encapsulated using a transmembrane pH gradient driven process. To determine stability in vivo, pharmacokinetic studies were performed using 'empty' and drug-loaded [(3)H]cholesteryl hexadecyl ether radiolabeled liposomes administered intravenously to Balb/c mice. Inclusion of 5 mole% of DSPE-PEG(2000) or 45 mole% cholesterol to DSPC liposomes increased the mean plasma area under the curve (AUC(0-24h)) 19-fold and 10-fold, respectively. Cryo-transmission electron micrographs of IDA loaded liposomes indicated that the drug formed a precipitate within liposomes. The mean AUC(0-4h) for free IDA was 0.030 micromole h/ml as compared to 1.38 micromole h/ml determined for the DSPC:DSPE-PEG(2000) formulation, a 45-fold increase, demonstrating that IDA was retained better in cholesterol-free compared to cholesterol-containing liposomes.

Efficiency of liposomes surface-modified with soybean-derived sterylglucoside as a liver targeting carrier in HepG2 cells

We investigated the interaction of liposomes surface-modified with soybean-derived sterylglucoside (SG) (SG-liposomes) with HepG2 cells in the point of involvement of asialoglycoprotein receptor (ASGP-R) mediated endocytosis and examined the efficiency of SG-liposomes as drug carriers using 1,1'-dioctadecyl-3,3,3',3'-tetramethylindocarbocyanine perchlorate (DiI) as a maker of liposome, carboxylated polystyrene microspheres (Fluoresbrite) as a model drug not taken up in cells and doxorubicin (DXR). SG-liposomes were composed of dipalmitoylphosphatidylcholine (DPPC), cholesterol (Ch) and SG (DPPC/Ch/SG=6:3:1, molar ratio) and DiI, Fluoresbrite and DXR were entrapped in SG-liposomes, respectively. Each SG-liposome was incubated with HepG2 cells at 4 or 37 degrees C, and co-incubated with asialofetuin (AF) as a competitor of ASGP-R. The association of DiI, Fluoresbrite or DXR entrapped in SG-liposomes with HepG2 cells at 37 degrees C was significantly higher than that in liposomes containing no SG. That of DiI and Fluoresbrite was reduced significantly by the incubation with AF, but that of DXR was not affected. These findings suggest that Fluoresbrite behaves like the lipid component of SG-liposomes, but DXR in SG-liposomes does not behave similar to the lipid component of SG-liposomes, thus, its drug behavior released from liposomes may be due to its physicochemical properties. SG-liposomes are potentially useful drug carriers to the liver, because the glucose residue may work as a kind of ligand for ASGP-R.

Determination of incorporated amounts of poly(ethylene glycol)-derivatized lipids in liposomes for the physicochemical characterization of stealth liposomes

We describe a method for determining incorporated amounts of poly(ethylene glycol) (PEG)-derivatized lipids in liposomes for the physicochemical characterization of PEG-coated liposomes. This method is based on the spectrophotometric determination of complexes of polyethers with sodium ions after their extraction as picrates into 1,2-dichloroethane, developed by Favretto for measuring levels of polyoxyethylene alkylphenyl-ether non-ionic surfactants in waste water. The same assay was applied to the estimation of PEG-derivatized lipids in liposomes and percent incorporation of PEG-derivatized lipids into liposomes was successfully determined. To prevent the interference from liposomal lipids other than PEG-derivatized lipids in this assay, liposomal samples were diluted at least to a concentration of less than 0.2 mM. The percent incorporation of PEG-lipids varied, depending on the molecular weight of PEG and anchor acyl chain length in PEG-lipids and it was suggested that the percent incorporation of PEG-lipids into liposomes would be a good parameter of quality control of PEG-liposomes in manufacturing facility and the picrate method used in the present study allows for the determination of this parameter without the need for hazardous radioisotopes.

Targeting chemotherapy to solid tumors with long-circulating thermosensitive liposomes and local hyperthermia

The effectiveness of the combination of long-circulating, thermosensitive liposomes and hyperthermia is described. Small-sized, thermosensitive liposomes that encapsulate doxorubicin (DXR-PEG-TSL (SUV)) have a prolonged circulation time and are extravasated to targeted solid tumors in vivo, where they preferentially release the agent in an anatomical site subjected to local hyperthermia. Liposomes were prepared by the incorporation of amphipathic polyethyleneglycol (PEG) to prolong their circulation time. DXR-PEG-TSL (SUV) was retained longest and was accumulated most efficiently in solid tumors in Balb/c mice. The combination of DXR-PEG-TSL (SUV) and hyperthermia at the tumor sites 3 h after injection, gave high concentrations of doxorubicin in tumor tissue and resulted in more effective tumor retardation and increased survival time. A large amount of DXR-PEG-TSL (SUV) was extravasated into the tumors during circulation for 3 h after injection, suggesting that the encapsulated drug was released into the interstitial spaces of the lesions by local hyperthermia. This system is expected to be clinically valuable for the delivery of a wide range of chemotherapeutic agents in the treatment of solid tumors.

Possibility of active targeting to tumor tissues with liposomes

In terms of active targeting by immunoliposomes, two anatomical compartments are considerable for targeting sites. One is located a readily accessible site in intravascular, and another is a much less accessible target site located in the extravascular. However, it was made clear that the active targeting with immunoliposomes is determined by two kinetically competing processes, such as binding to the target site and uptake by the RES. To overcome these contradictions, we have designed a new type of long-circulating immunoliposome, which was PEG-immunoliposome attached antibodies at the distal end of PEG chain, so called the pendant type immunoliposome. The pendant type immunoliposome showed much higher targetability than the ordinary immunoliposomes to both targeting sites of lung endothelial cells and solid tumor tissue. This is due to the free PEG chains (not linked to the antibody) effectively avoiding the RES uptake of liposomes, resulting in elevated the blood concentration and enhanced the target binding of immunoliposomes. The presence of free PEG does not interfere with the binding of the terminally linked antibody to the antigen. For targeting to the vascular endothelial surface in the lung, 34A antibody, which is highly specific to mouse pulmonary endothelial cells, was conjugated to make the pendant type immunoliposomes (34A-PEG-ILP). 34A-PFG-ILP showed significantly higher targeting degree than the ordinary type of immunoliposomes. For targeting to the solid tumor tissue, Fab' fragment of 21B2 antibody which is anti-human CFA and transferrin (TF) were used. Both pendant type immunoliposomes (Fab'-PFG-ILP and TF-PEG-ILP) showed the low RES uptake and the long circulation time, and resulted in enhanced accumulation of the liposomes in the solid tumor. TF-PEG-ILP was internalized into tumor cells with receptor mediated endocytosis, after extravasation into tumor tissue. The pendant type immunoliposome can escape from the gaps between adjacent endothelial cells and openings at the vessel termini during tumor angiogenesis by passive convective transport much rather than ligand directed targeting. Active targeting to tumor tissue with the pendant type immunoliposome is particularly important for many highly toxic anticancer drugs for cancer chemotherapy. An ultimate goal of pendant type immunoliposome is the incorporation of a fusogenic molecule that would induce fusion of liposome following their binding to the target cells or their internalization by endocytosis. Such liposomal formulations should be useful for endocytotic internalization of plasmid DNA and other bioactive materials.

Triggerable plasmalogen liposomes: improvement of system efficiency

A photoactivated liposome release system that is generally applicable for triggered release of encapsulated hydrophilic materials is described. This approach to phototriggered release, derived from the known effects of plasmalogen photooxidation on membrane permeability in whole cells and model membrane systems, relies on producing a lamellar phase change or increase in permeability upon cleaving its constitutive lipids to single-chain surfactants using 630-820 nm light to sensitize the photooxidation of the plasmalogen vinyl ether linkage. Semi-synthetic plasmenylcholine liposomes containing encapsulated calcein and a membrane-bound sensitizer, such as zinc phthalocyanine, tin octabutoxyphthalocyanine, or bacteriochlorophyll a, were prepared by extrusion. Irradiation of air-saturated liposome solutions enhanced membrane permeability toward calcein and Mn2+, and promoted membrane fusion processes compared to non-irradiated or anaerobic controls. Bacteriochlorophyll a sensitization produced the fastest observed photoinitiated release rate from these liposomes (100% calcein release in less than 20 min; 800 nm irradiation at 300 mW); the observed release rate was two orders of magnitude slower for egg lecithin liposomes prepared and irradiated under identical experimental conditions. Liposome aggregation, interlipidic particle formation, and membrane fusion between adjoining liposomes was observed by 31P-NMR, freeze-fracture/freeze-etch TEM, and cryo-TEM as a function of irradiation time. The use of near-infrared sensitizers and the capacity of photolyzed plasmenylcholine liposomes to undergo membrane fusion processes make photodynamic therapy with these liposome-borne sensitizers an attractive adjunct to biochemical targeting methods.

Evaluation of dipalmitoylphosphatidylcholine liposomes containing a soybean-derived sterylglucoside mixture for liver targeting

We investigated multilamellar vesicle (MLV) liposomes composed of dipalmitoylphosphatidylcholine (DPPC) and a soybean-derived sterylglucoside mixture (SG) (DPPC/SG-liposomes) for targeting liver parenchymal cells after administration via the tail vein in mice, using liposome-entrapping calcein. The accumulation of DPPC/SG(7:2, mole ratio, DPPC:SG = 7:2)-liposomes in the liver was the highest among DPPC/SG-liposomes. About 80% and 40% of the dose of DPPC/SG(7:2)-liposomes accumulated in the liver at 15 min and 2 h, respectively, but about 20% of DPPC/SG(7:3.5, 7:7)-liposomes accumulated at 2 h after an intravenous administration. However, the uptake of DPPC/SG(7:2, 7:3.5, 7:7)-liposomes by liver parenchymal cells was about 7 times greater than that in non-parenchymal cells irrespective of the SG concentration in liposomes. The uptake of DPPC/SG(7:2)-liposomes in liver was almost the same level as that by liposomes containing lactocylceramide (LC) (LC-liposomes) that were already known to be taken up in liver parenchymal cells by the asialoglycoprotein receptor. DPPC/SG(7:2)-liposomes effectively targeted liver, having optimal stability and SG.

Recent advances in liposomal drug-delivery systems

Liposomal drug-delivery systems have come of age in recent years, with several liposomal drugs currently in advanced clinical trials or already on the market. It is clear from numerous pre-clinical and clinical studies that drugs, such as antitumor drugs, packaged in liposomes exhibit reduced toxicities, while retaining, or gaining enhanced, efficacy. This results, in part, from altered pharmacokinetics, which lead to drug accumulation at disease sites, such as tumors, and reduced distribution to sensitive tissues. Fusogenic liposomal systems that are under development have the potential to deliver drugs intracellularly, and this is expected to markedly enhance therapeutic activity. Advances in liposome design are leading to new applications for the delivery of new biotechnology products, such as recombinant proteins, antisense oligonucleotides and cloned genes.

Thermosensitive sterically stabilized liposomes: formulation and in vitro studies on mechanism of doxorubicin release by bovine serum and human plasma

PURPOSE

To formulate thermosensitive sterically stabilized liposomes and to study the effects of plasma and serum components in vitro.

METHODS

The rate of release of encapsulated doxorubicin (Dox) from liposomes of various compositions was followed by fluorometric assay at 37 degrees, 42 degrees and 45 degrees C, in buffer and also in both calf serum and human plasma up to 50% by volume.

RESULTS

The optimal composition for the maximal differential release of doxorubicin between 37 degrees C and 42 degrees C in human plasma was a mixture of dipalmitoylphosphatidylcholine/hydrogenated soy phosphatidylcholine/cholesterol and distearoylphosphatidylethanolamine derivatized with polyethylene glycol at a molar ratio of 100:50:30:6. In experiments designed to study the mechanism causing increased permeability of liposomes in bovine serum, we found two different distinct release patterns: a slow linear rise of rate of Dox release for fluid liposomes and fast exponential rise reaching plateau within 5 minutes for solid phase (rigid) liposomes. This release of Dox from rigid but not fluid liposomes was inhibited by pre-heating serum at 55 degrees C for 30 minutes or by addition of EDTA (but not EGTA) or antiserum to the C3 component of complement.

CONCLUSIONS

A formulation of sterically stabilized liposomes with the proper thermal sensitivity in human plasma has been obtained. In addition, the results suggest that complement may play an important role in the interaction of rigid but not fluid liposomes with bovine serum. Human plasma did not show this effect.