Liposome-complement interactions in rat serum: implications for liposome survival studies

Characterization and in vivo testing of a heterogeneous cationic lipid-DNA formulation

PURPOSE

To identify characteristics of lipid-DNA complexes that correlate with in vivo expression data.

METHODS

DOTIM:cholesterol liposomes (1:1 mole ratio) and DNA expressing chloramphenicol acetyl transferase (CAT) were complexed at a 4.2:1 mass ratio (cationic lipid:DNA). Complexes were fractionated by density gradient centrifugation. analyzed for particle size and zeta potential and quantitated using HPLC methods. The unfractionated complexes, "purified" fractions of the complexes, and purified complexes supplemented with liposomes were administered to mice by intravenous injection (i.v.) and intratracheal instillation (i.t.) and their ability to express gene product was assessed.

RESULTS

Centrifugation separated two distinct populations within complexes one consisting of free liposomes and the other of lipid complexed with DNA. The vesicle diameter and zeta potential among separated fractions varied from 113 to 354 nm. and + 24 to + 34 mV respectively. Re-centrifugation of the 'purified' fractions containing the lipid-DNA population produced a single band. CAT expression in lung tissue 24 hr post-i.v. was observed with the unfractionated complex, but not the purified form. Some activity was 'restored' with the liposome-supplemented complexes. In contrast, the same series of complexes administered by i.t. resulted in no significant difference in lung expression (p=0.16 ANOVA).

CONCLUSIONS

An uncomplexed liposome population exists within DOTIM:cholesterol-DNA complexes that influences the expression of complexes administered i.v. but not i.t..

The complement system in liposome clearance: Can complement deposition be inhibited?

The activation of complement results in the opsonization of particles for removal by the reticuloendothelial system. Experimental evidence suggests that complement-mediated clearance of liposomal systems may significantly contribute to liposome biodistribution. Because of the multiplicity of complement activation mechanisms and the large number of proteins in the pathway, there are multiple opportunities to reduce or eliminate the opsonic effects of complement activation. This review addresses the state of our understanding of the interaction of liposomes with complement proteins and suggests some approaches to minimize complement activation.

Structural and metabolic consequences of liposome-lipoprotein interactions

The two major proposed uses for liposomes, i.e., drug delivery and mobilization of peripheral deposits of cholesterol, each impose requirements and restrictions on liposomal structure, particularly as it affects interactions with lipoproteins. This chapter focuses on the role of lipoproteins and apolipoproteins in (1) disrupting membrane structure and causing the leakage of liposomal contents by inducing disc formation and (2) marking liposomes for whole-particle uptake by receptors involved in lipoprotein metabolism. Control of membrane stability and whole-particle half-life can be achieved by several strategies, such as membrane stiffening, shielding the membrane surface, and increasing the dose or predosing with "empty" liposomes. The rationales and applicabilities of these strategies are discussed in the contexts of liposomes as drug delivery vehicles and as antiatherogenic particles. Directions for further basic and applied research are also presented.

Serum-mediated recognition of liposomes by phagocytic cells of the reticuloendothelial system - The concept of tissue specificity

The mechanisms responsible for the clearance and recognition of intravenously injected liposomes by phagocytic cells of the reticuloendothelial system are presently not well understood. Here, we address a multiplicity of physicochemical and physiopathological factors which influence the clearance kinetics and tissue distribution of liposomes administered into the circulation. Emphasis is particularly drawn towards the concept of liposome-blood protein interaction and its role in liposome recognition by various phagocytes of the reticuloendothelial system.

Identification of proteins mediating clearance of liposomes using a liver perfusion system

The objective of this paper is to identify the principal blood components governing the fate of liposomes in blood circulation. Information based on an isolated perfused liver system in rats has revealed the central role of the complement system in enhancing the uptake of liposomes by the liver. A species difference was an important factor in determining the uptake mechanisms of liposomes by the liver. Limited evidence revealed the tendency that opsonin-dependent hepatic uptake is principal in rats, while opsonin-independent or dysopsonin-dependent uptake governs in mice, although there are some exceptions. These studies provide us with important information for understanding the uptake mechanisms of liposomes by the liver, and useful insights in predicting the in vivo disposition of liposomes in humans.

Interactions of liposomes and lipid-based carrier systems with blood proteins: Relation to clearance behaviour in vivo

Liposomes and lipid-based drug delivery systems have been used extensively over the last decade to improve the pharmacological and therapeutic activity of a wide variety of drugs. More recently, this class of carrier systems has been used for the delivery of relatively large DNA and RNA-based drugs, including plasmids, antisense oligonucleotides and ribozymes. Despite recent successes in prolonging the circulation times of liposomes, virtually all lipid compositions studied to date are removed from the plasma compartment within 24h after administration by the cells and tissues of the reticuloendothelial system (RES). Plasma proteins have long been thought to play a critical role in this process but only a few efforts were made to evaluate the relevant importance of plasma protein-liposome interactions in the clearance process. Strategies to increase the bioavailability of liposomal drugs have included altering lipid compositions and charge, increasing lipid doses, and incorporating surface coatings. All of these modifications can influence membrane-protein interactions. In this article, we will focus on our experiences with liposome-blood protein interactions and how alterations in the chemical and physical properties of the carrier system influence the interactions with blood proteins and circulation times.

Clearance properties of liposomes involving conjugated proteins for targeting

This review examines methods of protein conjugation onto liposomes and the effects of surface bound protein on the liposomes' biological behavior. It is evident that the presence of a conjugated protein significantly alters the attributes of targeted liposomes. Specifically, protein conjugation can result in dramatic increases in liposome size, enhanced immunogenicity, and increased plasma elimination. Techniques are discussed for preventing some of the physical (size) and biological (immunogenic) alterations involving the use of PEG-lipids and drug loaded liposomes. In addition, the advantages of conjugating antibodies via carbohydrate moieties, to minimize changes in antibody binding and tertiary structure as well as effectively decreasing plasma elimination, are also discussed. It is, however, apparent that the accessibility of targeted liposomes to extravascular sites is a key step that will require further study and it is, therefore, anticipated that with the development of novel ligands and novel ligand-liposome interactions, the therapeutic utility of targeting strategies will likely be realized.

Receptor versus non-receptor mediated clearance of liposomes

Numerous studies have appeared over the years dealing with liposome-cell interaction mechanisms, most of them performed under in vitro conditions with isolated cell populations or cell lines. It is remarkable that, nonetheless, there hardly seem to exist established and generally accepted views on how precisely liposomes interact with cells and by what parameters this is influenced. In this article we will summarize and discuss the most relevant studies (in our opinion) on this matter in relation to in vivo conditions and with special attention to the relation between scavenger, complement and PS receptors.Researchers in the field have long been aware of the interaction of liposomes with blood proteins and their potential involvement in the process of liposome elimination from the blood circulation. A few of these 'opsonizing' proteins have been identified, but it is not clear to what extent each of them determines the fate of the liposome in the blood stream and how liposomal parameters such as size, charge and rigidity play a role in this process. We will include in this article our own recent observations on a thus far largely ignored class of such liposomal 'opsonins', the apolipoproteins. This class of plasma proteins, which physiologically are instrumental in hepatic lipoprotein clearance and processing, has been shown to contribute specifically to hepatocyte-mediated uptake of liposomes.Separately, as opposed to the fate of plain liposomes, we briefly touch on the clearance of surface-modified liposomes, which are designed to actively target specific cells or tissues. Plasma proteins are not usually supposed to play a significant role in the clearance of such liposomes. We will summarize these studies and address in this connection the question of how plasma proteins may interfere with such active targeting attempts.

Effect of serum components on the physico-chemical properties of cationic lipid/oligonucleotide complexes and on their interactions with cells

The interactions among serum components and cationic lipid-nucleic acid complexes are central to the understanding of how serum inhibits cellular delivery of oligonucleotides in vitro and in vivo. In this study, we show that several serum proteins, in particular bovine serum albumin (BSA), lipoproteins (HDL and LDL) and macroglobulin, interact with cationic lipid/oligonucleotide complexes, alter the complex diameter and zeta potential (from positive to negative values), and significantly interfere with the ability of 1,2-dioleoyl-3-trimethylammonium-propane (DOTAP) to deliver phosphorothioate oligonucleotides (ODN) into cells. Serum and BSA do not dissociate the ODN and lipid components, therefore inhibition of delivery cannot be attributed to a displacement of cationic lipid from the ODN. Rather BSA at 2.5 mg/ml, comparable to the amount found in 10% serum, decreases the cell association of ODN by about 5-fold and nuclear uptake of ODN by greater than 20-fold. In contrast, immunoglobulin G, the other major serum component, alters the zeta potential from positive to near neutral, has a modest effect on the diameter of the complex but does not affect cell association or nuclear delivery of the ODN at amounts found in 10% serum. Other molecules found in serum, specifically oleic acid and heparin, displace the ODN from the complex and thus interfere with delivery. This displacement is attenuated by first incubating the complex with BSA. Another manifestation of serum-complex interactions is that ODN significantly and cationic liposomes slightly, activate complement. However, formation of the complex markedly reduces the complement activation of the ODN. Finally, the effect of serum can be partially counteracted by the selection of the helper lipid (DOPE or cholesterol). Inclusion of a helper lipid reduces the effective charge ratio (cationic groups/anionic thioates) required to deliver ODN into cells and permits delivery in the presence of greater percentages of serum in the culture medium. These results support the current view that the binding of serum proteins to the complex is a significant factor in modulating the activity of cationic lipid-ODN complexes in culture and after intravenous administration.

Effects of complement depletion on the pharmacokinetics and gene delivery mediated by cationic lipid-DNA complexes

We show that lipoplexes activate complement in human serum in vitro and deplete complement when administered intravenously (i.v.) to mice. This raised the possibility that complement proteins might alter gene expression mediated by lipoplex in animals. To investigate this phenomenon, complement levels were depleted to less than 5% in ICR mice by intraperitoneal (i.p.) injection of cobra venom factor and anti-C3 antibodies. The pharmacokinetics and distribution of radio labeled DOTAP-cholesterol (1.0:0.9 molar ratio)-DNA (5:1 positive charge ratio) complexes containing 131I-labeled p-hydroxybenzamidine phosphatidylethanolamine and 125I-labeled DNA were measured in mice after i.v. administration. Greater than 75% of the injected lipoplex appeared in the lungs 5 min following injection. The lipid and DNA were eliminated from the lungs at a constant ratio. Distribution in various organs was not affected by complement depletion. Expression of luciferase was highest in the lungs and showed a dose-dependent increase as the amount of DNA injected increased from 3 to 60 microg. Reporter gene expression was not affected by complement depletion. In addition, complement depletion had no effect on either the distribution or gene expression in the heart, spleen, or liver. We conclude that cationic lipid-DNA complexes interact with serum complement proteins upon i.v. injection in mice, but this interaction does not influence the lipofection efficiency or systemic distribution of the lipoplex.

Influence of surface properties at biodegradable microsphere surfaces: effects on plasma protein adsorption and phagocytosis

OBJECTIVE

The objective of this work was to determine plasma protein adsorption and macrophage phagocytosis of biodegradable polyanhydride, polylactic acid and polylactic-co-glycolic acid microspheres prepared by both spray-drying and solvent evaporation techniques.

METHODS

Microspheres were characterized by scanning electron microscopy (SEM), confocal laser microscopy, particle size distribution and zeta (zeta) potential determination. Plasma protein adsorption onto the microspheres was determined using a fluoroaldehyde reagent. Phagocytosis was evaluated by incubating microspheres containing the angiotensin II antagonist, L-158,809, with the macrophages in the presence or absence of the phagocytosis inhibitor cythochalasin D. The extent of phagocytosis was established by fluorescence determination of L-158,809 and by optical microscopy. The effect of amphiphilic poly(ethylene glycol) (PEG) derivatives on phagocytosis was determined using PEG-distearate incorporated into the microspheres.

RESULTS

The average diameter of the microspheres, which depended on the polymer and the initial formulation, ranged from 0.9 to 3.2 micrometers. Zeta potential studies showed strong negative values irrespective of the polymer used for the spray-dried formulations. The zeta potential was masked by the incorporation of PEG 400- or PEG 1,400-distearate in the formulation. Confocal laser microscopy showed a homogenous dispersion of PEG (measured as PEG-fluorescein) in the microspheres. Protein adsorption was not observed for any of the microsphere formulations following incubation with bovine serum. Incubation of microspheres with murine macrophages showed that PEG-distearate inhibited phagocytosis at appropriate levels (0.1% w/w). Higher levels > 1% w/w of PEG-distearate) resulted in enhanced association with macrophages, despite the presence of the phagocytosis inhibitor cytochalasin D, indicating fusion between the microspheres and the plasma membrane.

CONCLUSIONS

These results demonstrate that spray-dried PEG-containing microspheres can be manufactured and that an appropriate concentration of this excipient in microspheres results in decreased phagocytosis.

Cationic lipids, phosphatidylethanolamine and the intracellular delivery of polymeric, nucleic acid-based drugs (review)

Polymeric, nucleic acid drugs must be protected from endogenous nucleases and delivered to target cell nuclei in order to maximize their activity. Constructs expressing therapeutic genes, antisense oligonucleotides and ribozymes can be delivered into cells by viral vectors, but concerns over safety and clinical utility have led to research into the development of alternative, non-viral delivery systems. Antisense and ribozyme drug development has focused upon modifications to the natural oligonucleotide chemistry which make the molecules resistant to nuclease degradation. These novel oligonucleotides cannot be generated by transgenes and must be administered in similar fashion to conventional drugs. However, oligonucleotides cannot cross membranes by passive diffusion and intracellular delivery for these drugs is very inefficient. Here we review the recent advances in forming lipid-DNA particles designed to mimic viral delivery of DNA. Most evidence now supports the hypothesis that lipid-DNA drugs enter target cells by endocytosis and disrupt the endosomal membrane, releasing nucleic acid into the cytoplasm. The mechanisms of particle formation and endosome disruption are not well understood. Cationic lipids are employed to provide an electrostatic interaction between the lipid carrier and polyanionic nucleic acids, and they are critical for efficient packaging of the drugs into a form suitable for systemic administration. However, their role in endosome disruption and other aspects of successful delivery leading to gene expression or inhibition of mRNA translation are less clear. We discuss the propensity of lipid-nucleic acid particles to undergo lipid mixing and fusion with adjacent membranes, and how phosphatidylethanolamine and other lipids may act as factors capable of disrupting bilayer structure and the endosomal pathway. Finally, we consider the challenges that remain in bringing nucleic acid based drugs into the realm of clinical reality.

Complement component 3 interactions with coxsackievirus B3 capsid proteins: innate immunity and the rapid formation of splenic antiviral germinal centers

Innate immunity is central to the clearance of pathogens from hosts as well as to the definition of acquired immune responses (D. T. Fearon, and R. M. Locksley, Science 272:50-53, 1996). Coxsackievirus B3 (CVB3), a human cardiopathic virus, was evaluated for the ability to activate the alternative and classical pathway of complement. CVB3 proteins interact with complement component 3 (C3, a soluble protein effector of innate immunity) after either in vitro exposure to mouse serum or in vivo murine infection and activate the alternative pathway of complement. In addition, we demonstrate that viral antigen retention and localization in germinal centers is dependent on C3, while virus antigen retention in extrafollicular regions in the spleen is not. In vivo depletion of native C3 abolished the rapid formation of virus-specific germinal centers (by day 3 post-CVB3 infection) in the absence of serum anti-CVB3 antibodies. These studies demonstrate that innate immune mechanisms, such as C3 interaction with CVB3, are essential for splenic antiviral germinal center formation in naive (antigen nonsensitized) mice resistant (C57BL/6J strain) and susceptible (A/J strain) to CVB3-induced myocarditis.

Enhancement of the in vivo circulation lifetime of L-alpha-distearoylphosphatidylcholine liposomes: importance of liposomal aggregation versus complement opsonization

Incorporation of N-(omega-carboxy)acylamido-phosphatidylethanolamines (-PEs) into large unilamellar vesicles (LUVs) of L-alpha-distearoylphosphatidylcholine (DSPC) was found to dramatically increase the in vivo liposomal circulation lifetime in rats, reaching a maximal effect at 10 mol.% of the total phospholipid. Neither pure DSPC liposomes nor those with the longest circulating derivative, N-glutaryl-dipalmitoylphosphatidylethanolamine (-DPPE), were found to significantly bind complement from serum. Therefore, the relatively short circulation time of pure DSPC liposomes did not appear to be related to greater complement opsonization leading to uptake by the reticuloendothelial system. However, N-(omega-carboxy)acylamido-PEs were particularly efficient inhibitors of a limited aggregation detected for pure DSPC liposomes. The aggregation tendency of DSPC liposomes incorporating various structural analogs of N-glutaryl-DPPE correlated inversely with the circulation lifetimes. Therefore, it is concluded that such PE derivatives enhance the circulation time by preventing liposomal aggregation and avoiding a poorly understood mechanism of clearance that is dependent on size but is independent of complement opsonization. At high concentrations of N-glutaryl-DPPE (above 10 mol.%), the liposomes exhibited strong complement opsonization and were cleared from circulation rapidly, as were other highly negatively charged liposomes. These data demonstrate that both the lack of opsonization and the lack of a tendency to aggregate are required for long circulation. Liposomal disaggregation via N-(omega-carboxy)acylamido-PEs yields a new class of large unilamellar DSPC liposomes with circulation lifetimes that are comparable to those of sterically stabilized liposomes.

Role of complement in rats injected with liposome-encapsulated hemoglobin

Previous studies have documented that liposome-encapsulated hemoglobin (LEH) can cause a rapid and transient thrombocytopenia following intravenous injection into small animals. The present study evaluated the role of complement during the LEH-induced thrombocytopenia in rats. We have compared changes in platelet levels in the blood, platelet organ distribution, and total hemolytic complement levels following intravenous administration of LEH in control and complement-depleted rats. Changes in platelet organ distribution at various times after LEH administration were monitored by labeling autologous platelets with indium-111 (111In)-oxine and imaging the 111In-platelets with a gamma camera after reinjection. Platelet counts were determined by light-scattering methods and by following 111In radioactivity at various times after LEH administration. Platelet levels did not significantly change for the complement-depleted rats during the 60 min following an injection of LEH, whereas thrombocytopenia (40% decrease) was noted within 4 min post-LEH-injection for control rats with a gradual return to baseline circulating platelet levels within 60 min. This drop in circulating platelets was correlated with a rapid redistribution of 111In-platelets from the circulation to the lungs and liver, whereas complement-depleted rats showed no transient movement of the 111In-platelets from the circulation. Baseline complement levels of 21.6 +/- 2.2 CH50/ml for control rats and 0.2 +/- 0.1 CH50/ml for complement-depleted rats did not significantly change during the 60 min following LEH administration. This study suggests that complement must be present during LEH-induced transient thrombocytopenia, as complement-depleted rats underwent no thrombocytopenia, and that the transient LEH-induced thrombocytopenia may be associated with complement activation.

Complement consumption by poly(ethylene glycol) in different conformations chemically coupled to poly(isobutyl 2-cyanoacrylate) nanoparticles

There is an increasing interest to develop injectable drug polymeric carriers not recognizable by the body as foreign particles and eliminated very quickly from the bloodstream. A polyethylene glycol (PEG)-coating onto injectable particles showed to reduce either protein adsorption and complement consumption, as a function of the PEG density. In this work we compared the complement rejecting ability of PEG in different conformations coupled to polyisobutylcyanoacrylate (PIBCA) nanoparticles, through the analysis of the residual hemolytic capacity of the human serum after contact with the particles. Nanoparticles were formed by chemical coupling of PEG during emulsion/polymerization of isobutylcyanoacrylate (IBCA). Nanoparticles characterization included an investigation of their surface properties, such as hydrophilicity and conformational mobility of the PEG chains grafted on the nanoparticles surface, and PEG total content. The polymerization kinetics of IBCA in presence of PEG or MePEG were also studied. Complement consumption was observed to be very sensitive to the number of particles in contact with human serum, as well as to the PEG conformation, suggesting PEG configuration could affect the particle exposed surface.

Effect of sterical stabilization on macrophage uptake in vitro and on thickness of the fixed aqueous layer of liposomes made from alkylphosphocholines

A serious problem using liposomes for therapeutic purposes is the fast removal from blood circulation by components of the reticuloendothelial system (RES) most likely after opsonization of the vesicles. This study was performed to quantify the reduction in macrophage uptake in vitro of sterically stabilized liposomes (PEG-liposomes) prepared from hexadecylphosphocholine, cholesterol and poly(ethylene glycol2000) distearoylphosphoethanolamine (PEG2000DSPE) for the first time. The uptake was determined using HPC-liposomes of different defined size (125, 250 and 1000 nm) without and with sterical stabilization by incorporating 5 mol% of PEG2000DSPE. HPTS was used as fluorescence marker allowing the discrimination between general uptake and the part of liposomes internalized into the low pH-compartment (Daleke, L.D., Hong, K. and Papahadjopoulos. D. (1990) Biochim. Biophys. Acta 1024, 352-366). Liposomal uptake by J774 mouse macrophage-like cells was time-dependent. Both the uptake and internalization were clearly reduced for PEG-liposomes compared to plain liposomes. Sterical stabilization reduced the general uptake of liposomes in vitro by more than 50% and the internalization by about 50-60%. PEG-liposomes additionally showed a delay in internalization into the macrophages during the first 6 h. Size of used liposomes had only a minor influence on liposomal uptake but highest concentration of lipid was found for large multilammelar vesicles (MLV). The fixed aqueous layer thickness (FALT) was determined by zeta potential measurements of plain and sterically stabilised HPC-liposomes (100 nm) in solutions of different ion concentrations. The calculation of the thickness was based on the linear correlation between ln zeta (zeta-potential) and kappa (Debye Hückel-Parameter). FALT was calculated and found to be for plain HPC-liposomes 0.83 +/- 0.17 nm and for PEG-HPC-liposomes 3.57 +/- 0.17 nm. Exchange of the HPC by an alkylphospholipid with different head group has no or only minor effect (PEG-OPP-liposomes 3.44 +/- 0.31 nm). Thus the reduced uptake of HPC-LUVET correlates with an increased thickness of the fixed aqueous layer around these liposomes and could support the hypothesis that the thickness is an important property responsible for preventing opsonization and resulting finally in a reduced macrophage uptake.

Altered tissue-specific opsonic activities and opsono-recognition of liposomes in tumour-bearing rats

Reticuloendothelial phagocytic and serum opsonic activity was evaluated at terminal stages of tumour growth in rats transplanted subcutaneously with chondrosarcoma in an attempt to evaluate the role of opsonic protein(s) in governing liposome recognition and clearance by the macrophage system. The liver of the tumour-bearing animals manifested a decline in the uptake of multilamellar vesicles composed of egg phosphatidylcholine: cholesterol: dicetyl phosphate (mole ratio 7:2:1) from the blood when compared to healthy animals. In contrast, an increase in splenic clearance of liposomes was encountered in tumour-bearing rats. Studies with isolated liver non-parenchymal cells suggested that liposome recognition in both health and at terminal stages of cancer growth is influenced by a serum opsonin, which can be precipitated by 35-50% ammonium sulphate, as well as the concentration of calcium levels in serum. Serum of healthy animals equally enhanced liposome recognition by the hepatic macrophages of both normal and tumour-bearing rats. In contrast, both cell populations manifested poor liposome recognition in the presence of serum pooled from tumour-bearing animals and the results were comparable to the corresponding liposome-cell interaction in the absence of serum. The opsonic activity of serum derived from tumour-bearing rats could be demonstrated either by prior dialysis of serum against de-ionized water or by addition of EGTA. Liver phagocytes of healthy animals recognized more liposome in the presence of dialysed or EGTA-chelated tumour-serum than that of liver cells derived from tumour transplanted rats. A significant increase in serum calcium concentration was found in all tumour-bearing rats. When the concentration of calcium in the serum of normal animals was increased to the level that is encountered in tumour-bearing rats, a sharp drop in liposome recognition by liver phagocytes was observed. This drop in opsonic activity was not related to changes in the ionic strength of serum. The ammonium sulphate precipitated opsonin was also calcium-sensitive and its opsonic activity was abolished in the presence of calcium. Studies with isolated splenic phagocytes suggested that an increase in the opsonic activity of serum, but not the elevated calcium level, was responsible for hyperphagocytosis of liposomes by the splenic phagocytes of tumour-transplanted animals. The opsonic molecule which enhanced liposome recognition by liver non-parenchymal cells failed to enhance liposome clearance by the splenic phagocytes. These findings suggest that the alteration in macrophage clearance of liposomes during the terminal growth of cancer may be mediated in part by changes in the opsonic capacity of serum.

Synergistic effect between size and cholesterol content in the enhanced hepatic uptake clearance of liposomes through complement activation in rats

PURPOSE

The effect of liposome size and cholesterol (CH) content on the pharmacokinetics of liposomes was investigated in rats.

METHODS

The pharmacokinetics of liposomes was examined using 5(6)-carboxyfluorescein (CF) as an aqueous phase marker. The extent of complement activation (ECA) was also measured by the release of CF from liposomes in serum.

RESULTS

Both the size and the CH content influenced the mean residence time, total body clearance, and the hepatic uptake clearance (CLh) of liposomes. The increase of the size of liposomes increased the CLh at each CH content. There was no CH dependency of CLh in small liposomes (200 nm in diameter), although the CLh increased with the increase in the CH content in large (800 nm) and medium (400 nm) liposomes. A significant interaction effect was observed between liposome size and the CH content on CLh according to the analysis of variance. The good correlation between CLh and ECA indicated the role of complements as opsonins in enhancing the hepatic uptake of liposomes. The interaction effect between the size and CH content on CLh was explained principally by the product of the size and CH content.

CONCLUSIONS

A synergistic effect was observed between the size and the CH content on CLh. An underlying hypothesis of the synergistic effect was postulated based on the size dependent recognition of liposomes by complement system.

Comparative pharmacokinetics, distributions in tissue, and interactions with blood proteins of conventional and sterically stabilized liposomes containing 2',3'-dideoxyinosine

The pharmacokinetics and distribution in tissue of 2',3'-dideoxyinosine (ddI) encapsulated in sterically stabilized liposomes have been evaluated in rats. Most of the sterically stabilized liposomes concentrated in the spleen with a peak level at 24 h after their intravenous injection. An extended half-life in plasma was observed for sterically stabilized liposomes (14.5 h) compared with that of conventional liposomes (3.9 h). The systemic clearance of ddI incorporated in sterically stabilized liposomes was 180 times lower than that of the free drug. The levels of in vitro and in vivo protein binding on both conventional and sterically stabilized liposomes were also evaluated. Results suggest that the amount of proteins associated with liposomes might not be the only factor involved in the in vivo clearance of liposomes, as this process may also be influenced by the nature of the bound blood proteins.

An investigation on the role of plasma and serum opsonins on the internalization of biodegradable poly(D,L-lactic acid) nanoparticles by human monocytes

We demonstrate here that polyethylene glycol (PEG) 6,000 protects biodegradable poly(D,L-lactic acid) nanoparticles (PLA NP) from extensive uptake by monocytes in plasma. These results are in agreement with those previously obtained with PEG 20,000 which reduced the uptake of PLA NP by human monocytes in phosphate buffered saline and plasma, and prolonged the NP circulation time in vivo. The coating efficiency of PEG 6,000 and 20,000 was substantially decreased in serum. The difference between the uptake of plain and coated NP clearly reappeared for PEG 20,000-coated NP in heat inactivated serum and in IgG-depleted serum. We suggest that typical plasma proteins, heat labile serum proteins (e.g. complement components) and IgG are involved in the opsonization of plain and coated PLA NP. Other proteins previously found to adsorb onto these NP, namely albumin and apolipoprotein E, did not appear to directly influence the uptake process.

Endogenously opsonized particles divert prostanoid action from lethal to protective in models of experimental endotoxemia

We report that, in rats, the lethal consequences of high-dose endotoxin challenge are exacerbated by the intravascular administration of prostaglandin E1 but attenuated by the intravascular administration of endocytosable particles. This protection is mediated by opsonins. Nonopsonizable particles were unable to provide protection unless first pseudoopsonized with antibody directed against the CR3 (CD11b/CD18) phagocyte receptor. We show that endogenously opsonized particles can act in concert with prostaglandin E1 (putatively by elevation of neutrophil intracellular cAMP and the resultant downregulation of CR3) to completely rescue animals from the lethal late-stage sequelae of experimental endotoxemia. These data illustrate that the interaction of particles with cellular receptors can transform the overall systemic response to prostaglandin E1 from pro- to antiinflammatory. This suggests a role for multiple receptor engagement events in defining the systemic prostaglandin response and offers a rationale for developing new therapeutic modalities in the treatment of sepsis and other inflammatory diseases.

Size dependent liposome degradation in blood: in vivo/in vitro correlation by kinetic modeling

The degradation of liposomes in blood circulation is important in regulating the releasing rate of encapsulated compounds. In this study, the effect of liposome size--one of the principal determining factors in liposome disposition--on their degradation in serum/blood was evaluated quantitatively both in vitro and in vivo. In the in vitro study, the time courses of the degradation of liposomes in fresh rat serum were measured continuously using 5(6)-carboxyfluorescein (CF) as an aqueous phase marker and were described by the kinetic model with the lag time (tau), first order degradation rate constant (k), and the maximum degradation (alpha). Both k and alpha increased with the increase of liposome size, which indicated a higher affinity of larger liposomes for complement activation. In the in vivo study, the degradation of liposomes was evaluated sensitively by a first order degradation rate constant (kd) in blood circulation. The kd was obtained by kinetically modeling the liposome degradation in vivo using 3H-inulin as an aqueous phase marker. The size dependent kd correlated well with the hepatic uptake clearance, which suggests an underlying complement activation mechanism common to both degradation and hepatic uptake of liposomes. There was a good correlation in the degradation rate constant between in vitro and in vivo trials. These kinetic analyses validate the quantitative evaluation of liposome degradation in blood circulation and provide a useful way to predict the degradation of liposomes in vivo from in vitro experiments.

Liposome-induced activation of the classical complement pathway does not require immunoglobulin

We have investigated the contribution of immunoglobulin to the liposome-induced activation of complement in human serum. Liposomes containing the negatively charged phospholipids cardiolipin, phosphatidylglycerol or phosphatidylinositol, in addition to phosphatidylcholine and cholesterol, were used to activate complement in a whole serum system. The contribution of immunoglobulin was studied by comparing normal human serum (NHS) to serum depleted of IgG and IgM (DDS). Using hemolytic assays of complement function, greater concentrations of phospholipids were required to activate complement in the absence of immunoglobulins. Activation of the classical pathway was confirmed using a C1q ELISA which showed that activation was dependent on the presence of C1q and confirmed that greater concentrations of phospholipids were required to activate complement in the absence of immunoglobulins. Complement activation was also assessed using crossed immunoelectrophoresis of C3 activation fragments. Using immunoblot analysis, iC3b was detected on the surface of liposomes exposed to NHS or DDS. These studies demonstrate that when liposomes, containing anionic phospholipids at an equivalent charge to cardiolipin 20 mol%, are exposed to immunoglobulin depleted serum they become opsonized by complement proteins.