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

Reciprocal Interaction with Neutrophils Facilitates Cutaneous Accumulation of Liposomes

Liposomes are versatile drug delivery systems in clinical use for cancer and many other diseases. Unfortunately, PEGylated liposomal doxorubicin (sLip/DOX) exhibits serious dose-limiting cutaneous toxicities, which are closely related to the extravascular accumulation of sLip/DOX in the dermis. No clinical interventions have been proposed for cutaneous toxicities due to the elusive transport pathways. Herein, we showed that the reciprocal interaction between liposomes and neutrophils played pivotal roles in liposome extravasation into the dermis. Neutrophils captured liposomes via the complement receptor 3 (CD11b/CD18) recognizing the fragment of complement component C3 (iC3b) deposited on the liposomal surface. Uptake of liposomes also activated neutrophils to induce CD11b upregulation and enhanced the ability of neutrophils to migrate outside the capillaries. Furthermore, inhibition of complement activation either by CRIg-L-FH (a C3b/iC3b targeted complement inhibitor) or blocking the phosphate negative charge in mPEG-DSPE could significantly reduce liposome uptake by neutrophils and alleviate the cutaneous accumulation of liposomes. These results validated the liposome extravasation pathway mediated by neutrophils and provided potential solutions to the devastating cutaneous toxicities occurring during sLip/DOX treatment.

Mechanisms of selective monocyte targeting by liposomes functionalized with a cationic, arginine-rich lipopeptide

Stimulation of monocytes with immunomodulating agents can harness the immune system to treat a long range of diseases, including cancers, infections and autoimmune diseases. To this end we aimed to develop a monocyte-targeting delivery platform based on cationic liposomes, which can be utilized to deliver immunomodulators and thus induce monocyte-mediated immune responses while avoiding off-target side-effects. The cationic liposome design is based on functionalizing the liposomal membrane with a cholesterol-anchored tri-arginine peptide (TriArg). We demonstrate that TriArg liposomes can target monocytes with high specificity in both human and murine blood and that this targeting is dependent on the content of TriArg in the liposomal membrane. In addition, we show that the mechanism of selective monocyte targeting involves the CD14 co-receptor, and selectivity is compromised when the TriArg content is increased, resulting in complement-mediated off-target uptake in granulocytes. The presented mechanistic findings of uptake by peripheral blood leukocytes may guide the design of future drug delivery systems utilized for immunotherapy. STATEMENT OF SIGNIFICANCE: Monocytes are attractive targets for immunotherapies of cancers, infections and autoimmune diseases. Specific delivery of immunostimulatory drugs to monocytes is typically achieved using ligand-targeted drug delivery systems, but a simpler approach is to target monocytes using cationic liposomes. To achieve this, however, a deep understanding of the mechanisms governing the interactions of cationic liposomes with monocytes and other leukocytes is required. We here investigate these interactions using liposomes incorporating a cationic arginine-rich lipopeptide. We demonstrate that monocyte targeting can be achieved by fine-tuning the lipopeptide content in the liposomes. Additionally, we reveal that the CD14 receptor is involved in the targeting process, whereas the complement system is not. These mechanistic findings are critical for future design of monocyte-targeting liposomal therapies.

Anchoring Property of a Novel Hydrophilic Lipopolymer, HDAS-SHP, Post-Inserted in Preformed Liposomes

Polyethylene glycol (PEG)-phospholipids in long-circulating liposomes cause non-specific immune reactions; mainly attributable to negatively-charged phosphoryl s at the interface of PEG and phospholipid. We investigated a novel lipopolymer, by which a superhydrophilic polymer (SHP) is conjugated to a non-phospholipid N¹-(2-aminoethyl)-N⁴-hexadecyl-2-tetradecylsuccinamide (HDAS). The modification of preformed liposomes HDAS-SHP, HDAS-PEG₂₀₀₀, and DSPE-PEG₂₀₀₀ were performed by post-insertion techniques. The efficiency of post-insertion and desorption rates, from the liposome surface, were determined. HDAS-SHP micelles showed highly positive zeta potential (+28.4 mV); zeta potentials of DSPE-PEG₂₀₀₀ and HDAS-PEG₂₀₀₀ micelles were −34.4 mV, and −3.7 mV, respectively. Critical micelle concentration predicted amphiphilicity of HDAS-SHP (CMC 2.58 µM) as close to that of DSPE-PEG₂₀₀₀ (CMC 2.44 µM). Both HDAS-SHP and HDAS-PEG₂₀₀₀ post-inserted with comparable efficiency (79%, and 73%, respectively), but noticeably lower than DSPE-PEG₂₀₀₀ (90%). The desorption rate of HDAS-SHP was close to that of DSPE-PEG₂₀₀₀ (0.53%/h, and 0.45%/h, respectively); the desorption rate for HDAS-PEG₂₀₀₀ was slightly more at 0.67%/h. Compared to plain liposomes, both HDAS-SHP- and DSPE-PEG₂₀₀₀-liposomes showed significant leakage of encapsulated Na-fluorescein isothiocyanate (FITC) upon incubation with serum. At the same time, both modified liposomes were found to suppress serum levels of the complement proteins, Bb and C4d. We infer that HDAS-SHP is a viable alternative to commonly-used PEG-phospholipid derivatives for stealth purposes.

Biokinetics of Nanomaterials: the Role of Biopersistence

Nanotechnology risk management strategies and environmental regulations continue to rely on hazard and exposure assessment protocols developed for bulk materials, including larger size particles, while commercial application of nanomaterials (NMs) increases. In order to support and corroborate risk assessment of NMs for workers, consumers, and the environment it is crucial to establish the impact of biopersistence of NMs at realistic doses. In the future, such data will allow a more refined future categorization of NMs. Despite many experiments on NM characterization and numerous in vitro and in vivo studies, several questions remain unanswered including the influence of biopersistence on the toxicity of NMs. It is unclear which criteria to apply to characterize a NM as biopersistent. Detection and quantification of NMs, especially determination of their state, i.e., dissolution, aggregation, and agglomeration within biological matrices and other environments are still challenging tasks; moreover mechanisms of nanoparticle (NP) translocation and persistence remain critical gaps. This review summarizes the current understanding of NM biokinetics focusing on determinants of biopersistence. Thorough particle characterization in different exposure scenarios and biological matrices requires use of suitable analytical methods and is a prerequisite to understand biopersistence and for the development of appropriate dosimetry. Analytical tools that potentially can facilitate elucidation of key NM characteristics, such as ion beam microscopy (IBM) and time-of-flight secondary ion mass spectrometry (ToF-SIMS), are discussed in relation to their potential to advance the understanding of biopersistent NM kinetics. We conclude that a major requirement for future nanosafety research is the development and application of analytical tools to characterize NPs in different exposure scenarios and biological matrices.

Harnessing Nanoparticles for Immunomodulation and Vaccines

The first successful use of nanoparticles (NPs) for vaccination was reported almost 40 years ago with a virus-like particle-based vaccine against Hepatitis B. Since then, the term NP has been expanded to accommodate a large number of novel nano-sized particles engineered from a range of materials. The great interest in NPs is likely not only a result of the two successful vaccines against hepatitis B and Human Papilloma Virus (HPV) that use this technology, but also due to the versatility of those small-sized particles, as indicated by the wide range of applications reported so far, ranging from medicinal and cosmetics to purely technical applications. In this review, we will focus on the use of NPs, especially virus-like particles (VLPs), in the field of vaccines and will discuss their employment as vaccines, antigen display platforms, adjuvants and drug delivery systems.

Influence of acyl chain fluidity on the lipopolysaccharide-induced activation of complement

Lipopolysaccharides (LPSs, endotoxins) are the major amphiphilic constituents of the outer leaflet of the outer membrane of Gram-negative bacteria. They are known to activate the complement cascade to form lytic membrane pores. Here, we study the influence of the fluidity of the acyl chains of LPSs and lipid As on the formation of lytic pores. To this end, we have performed electrical measurements on asymmetric planar endotoxin/phospholipid bilayers as a reconstitution model of the outer membrane using two deep rough mutant LPSs (from Escherichia coli strains WBB01 and WBB25) and two lipid As (from E. coli WBB25 and Rhodobacter sphaeroides). The two LPSs and the two lipid As each differ in their acylation pattern which is correlated with the fluidity. The addition of human serum to the endotoxin side of the bilayers led to the formation of membrane pores, and pore formation correlated in each case with acyl chain fluidity, i.e. time required for the first lytic pore to be formed was shorter for the more fluid endotoxin. Furthermore, in the case of LPSs, the activation rate was higher for the more fluid membrane and the respective bacteria had a higher susceptibility to the growth inhibitory action of serum.

Delayed hepatic uptake of multi-phosphonic acid poly(ethylene glycol) coated iron oxide measured by real-time magnetic resonance imaging

We report on the synthesis, characterization, stability and pharmacokinetics of novel iron based contrast agents for magnetic resonance imaging (MRI).

Biodistribution and delivery efficiency of unmodified tumor-derived exosomes

The use of exosomes as a drug delivery vehicle has gained considerable interest. To establish if exosomes could be utilized effectively for drug delivery, a better understanding of their in vivo fate must be established. Through comparisons to liposomal formulations, which have been studied extensively for the last thirty years, we were able to make some comprehensive conclusions about the fate of unmodified tumor-derived exosomes in vivo. We observed a comparable rapid clearance and minimal tumor accumulation of intravenously-injected exosomes, PC:Chol liposomes, and liposomes formulated with the lipid extract of exosomes, suggesting that the unique protein and lipid composition of exosomes does not appreciably impact exosomes' rate of clearance and biodistribution. This rapid clearance along with minimal tumor accumulation of unmodified exosomes limits their use as an anti-cancer drug delivery vehicle; however, when delivered intratumorally, exosomes remained associated with tumor tissue to a significantly greater extent than PC:Chol liposomes. Furthermore, experiments utilizing mice with impaired adaptive or innate immune systems, revealed the significance of the innate immune system along with the complement protein C5 on exosomes' rate of clearance.

Biological evaluation of liposome-encapsulated hemoglobin surface-modified with a novel PEGylated nonphospholipid amphiphile

Traumatic injury is often associated with hemorrhagic shock. Liposome-encapsulated hemoglobin (LEH) is being developed as an artificial oxygen carrier to address post-hemorrhage oxygen and volume deficit. Here, we report a new composition of LEH based on the use of polyethylene glycol (PEG2K ) conjugated with nonphospholipid hexadecylcarbamoylmethylhexadecanoate (HDAS) to modify the surface of LEH particles. LEH was manufactured by the high-pressure homogenization method using dipalmitoylphosphatidylcholine (∼38 mol%), cholesterol (∼38 mol%), HDAS (∼20 mol%), and highly purified stroma-free human hemoglobin. HDAS-PEG2K was postinserted into the resultant LEH to generate HDAS-PEG2K -LEH. We investigated the potential immune response to HDAS-PEG2K -LEH in a mice model. At the same time, the preparation was tested in a rat model to study the effect of repeated HDAS-PEG2K -LEH injection over 4 weeks. We found that HDAS-PEG2K modification substantially reduced the circulating levels of anaphylatoxins C3a and C5a, as well as plasma levels of thromboxane B2, in mice. Repeated injections of HDAS-PEG2K -LEH in rats did not appear to alter its clearance profile after 4 weeks of treatment. No antibody response against human hemoglobin or PEG was detected in rat plasma. Histological observations of lung, liver, spleen, and kidney were not significantly different between saline-treated rats and HDAS-PEG2K -LEH-treated rats. Immunohistochemical staining for rat heme oxygenase-1 (HO-1) did not show induced expression of HO-1 in these organs. These results suggest that the new surface modification of LEH is immune-neutral and does not adversely affect histology even after repeated administration.

Accelerated drug release and clearance of PEGylated epirubicin liposomes following repeated injections: a new challenge for sequential low-dose chemotherapy

BACKGROUND

Sequential low-dose chemotherapy has received great attention for its unique advantages in attenuating multidrug resistance of tumor cells. Nevertheless, it runs the risk of producing new problems associated with the accelerated blood clearance phenomenon, especially with multiple injections of PEGylated liposomes.

METHODS

Liposomes were labeled with fluorescent phospholipids of 1,2-dipalmitoyl-snglycero-3-phosphoethanolamine-N-(7-nitro-2-1,3-benzoxadiazol-4-yl) and epirubicin (EPI). The pharmacokinetics profile and biodistribution of the drug and liposome carrier following multiple injections were determined. Meanwhile, the antitumor effect of sequential low-dose chemotherapy was tested. To clarify this unexpected phenomenon, the production of polyethylene glycol (PEG)-specific immunoglobulin M (IgM), drug release, and residual complement activity experiments were conducted in serum.

RESULTS

The first or sequential injections of PEGylated liposomes within a certain dose range induced the rapid clearance of subsequently injected PEGylated liposomal EPI. Of note, the clearance of EPI was two- to three-fold faster than the liposome itself, and a large amount of EPI was released from liposomes in the first 30 minutes in a complement-activation, direct-dependent manner. The therapeutic efficacy of liposomal EPI following 10 days of sequential injections in S180 tumor-bearing mice of 0.75 mg EPI/kg body weight was almost completely abolished between the sixth and tenth day of the sequential injections, even although the subsequently injected doses were doubled. The level of PEG-specific IgM in the blood increased rapidly, with a larger amount of complement being activated while the concentration of EPI in blood and tumor tissue was significantly reduced.

CONCLUSION

Our investigation implied that the accelerated blood clearance phenomenon and its accompanying rapid leakage and clearance of drug following sequential low-dose injections may reverse the unique pharmacokinetic-toxicity profile of liposomes which deserved our attention. Therefore, a more reasonable treatment regime should be selected to lessen or even eliminate this phenomenon.

Post-modification of preformed liposomes with novel non-phospholipid poly(ethylene glycol)-conjugated hexadecylcarbamoylmethyl hexadecanoic acid for enhanced circulation persistence in vivo

We report synthesis and characterization of a novel PEG2000-conjugated hexadecylcarbamoylmethyl hexadecanoate (HDAS-PEG) as a PEG-phospholipid substitute for enhancing circulation persistence of liposomes. HDAS-PEG showed critical micelle concentration of 4.25 μM. We used post-insertion technique to introduce HDAS-PEG in outer lipid layer of the preformed liposomes. The presence of surface HDAS-PEG was confirmed by altered electrophoretic mobility, confocal microscopy and PEG estimation by ELISA. The post-inserted HDAS-PEG desorbed at approximately half the rate at which post-inserted DSPE-PEG desorbed from the liposome surface. HDAS-PEG significantly reduced liposome-induced complement activation (C4d, Bb and SC5b); HDAS-PEG was more effective than more commonly used DSPE-PEG in this capacity. For studying circulation persistence, the liposomes were labeled with (99m)Tc radionuclide and administered in rats. (99m)Tc-HDAS-PEG-liposomes showed prolonged persistence in blood as compared to that shown by (99m)Tc-plain liposomes. After 24 h of administration, <1% of (99m)Tc-plain liposomes remained in blood, whereas approximately 28% of injected (99m)Tc-HDAS-PEG-liposomes were present in blood. In comparison, only 4.8% of (99m)Tc-DSPE-PEG-liposomes were measured in blood after 24 h. As expected, the clearance route of the liposomes was through liver and spleen. These results demonstrate the potential of a novel non-phosphoryl HDAS-PEG for surface modification of preformed liposomes with a goal of prolonging their circulation persistence and more effective inhibition of complement activation.

Peptide ligand and PEG-mediated long-circulating liposome targeted to FGFR overexpressing tumor in vivo

Background and methods

Paclitaxel, a widely used antitumor agent, has limited clinical application due to its hydrophobicity and systemic toxicity. To achieve sustained and targeted delivery of paclitaxel to tumor sites, liposomes composed of egg phosphatidylcholine, cholesterol, and distearolyphosphatidyl ethanolamine-N-poly(ethylene glycol) (PEG₂₀₀₀) were prepared by a lipid film method. In addition, the liposomes also contained truncated fibroblast growth factor fragment-PEG-cholesterol as a ligand targeting the tumor marker fibroblast growth factor receptor. Physicochemical characteristics, such as particle size, zeta potential, entrapment efficiency, and release profiles were investigated. Pharmacokinetics and biodistribution were evaluated in C57BL/6 J mice bearing B16 melanoma after intravenous injection of paclitaxel formulated in Cremophor EL (free paclitaxel), conventional liposomes (CL-PTX), or in targeted PEGylated liposomes (TL-PTX).

Results

Compared with CL-PTX and free paclitaxel, TL-PTX prolonged the half-life of paclitaxel by 2.01-fold and 3.40-fold, respectively, in plasma and improved the AUC_0→t values of paclitaxel by 1.56-fold and 2.31-fold, respectively, in blood. Biodistribution studies showed high accumulation of TL-PTX in tumor tissue and organs containing the mononuclear phagocyte system (liver and spleen), but a considerable decrease in other organs (heart, lung, and kidney) compared with CL-PTX and free paclitaxel.

Conclusion

The truncated fibroblast growth factor fragment-conjugated PEGylated liposome has promising potential as a long-circulating and tumor-targeting carrier system.

Active methods of drug loading into liposomes: recent strategies for stable drug entrapment and increased in vivo activity

INTRODUCTION

The use of liposomes increases the therapeutic index of many drugs, and also offers drug targeting and controlled release. The commercial impact of liposomes is strengthened by the invention of several active drug encapsulation methods, allowing the encapsulation of several weak base or weak acid drugs with very high drug-to-lipid ratios.

AREAS COVERED

In recent years, there have been reports on several new approaches to retain more hydrophobic drugs inside liposomes, in the circulation. Most of these methods apply drug precipitation inside preformed liposomes, as low soluble complexes with ions or chemicals. In some cases, drug derivatization was applied to enable active encapsulation of hydrophobic drugs, previously not reported to encapsulate, by active or remote loading. This review presents and compares most of the existing methods of active drug encapsulation and outlines recent strategies to achieve stable drug encapsulation in vivo.

EXPERT OPINION

At present, there is no single universal encapsulation method that offers stable encapsulation of most drugs; each drug requires a different approach to manage all of its properties. Now is the time to combine all these strategies to achieve the goal of a complex, but successful, anticancer therapy.

Longer-acting factor VIII to overcome limitations in haemophilia management: the PEGylated liposomes formulation issue

Injected factor VIII (FVIII), the current treatment for haemophilia A, leads to major improvements in the quality of life and life expectancy of individuals with this disorder. However, because injected FVIII has a short half-life in vivo, this strategy has major limitations for highly demanding regimens (e.g. prophylaxis, immune tolerance induction, surgery). Newer formulations of longer-acting FVIII are presently under investigation. The use of low molecular weight polyethylene glycol (PEG)-containing liposomes as carriers for recombinant FVIII (rFVIII) results in the prolongation of haemostatic efficacy. Data from preclinical experiments in mice, early clinical evaluations, and pharmacokinetics and pharmacodynamics results indicate that an rFVIII pegylated liposomal formulation may provide potential clinical benefit to patients with severe haemophilia A by prolonging the protection from bleeding. In light of this potential clinical benefit, a multicentre, randomized, active-controlled, non-inferiority phase II trial with two parallel treatment arms and equal randomization after stratification for the presence or absence of target joints in patients and for ages >/=18 years vs. <18 years is currently being conducted. The study will test the hypothesis that rFVIII-Lip once-weekly prophylaxis is not inferior to rFVIII-water for injection thrice-weekly prophylaxis. A total of 250 patients will be enrolled with severe haemophilia A (<1% FVIII) on on-demand or secondary prophylaxis treatment and with documented bleeds or injections during the 6 months before study entry. Sixty-four centres in 14 different countries are involved in the study; recruitment is underway. In Italy, six centres have already included 15 patients (no screening failure). Eight of these patients have completed the run-in phase and have begun the home treatment. No unexpected serious adverse events have been reported thus far. Data emerging from this phase II study will help collect relevant data to overcome current limitations in haemophilia management by employing treatment with longer-acting rFVIII.

Cationic liposomes as carriers for aerosolized formulations of an anionic drug: safety and efficacy study

This study tests the hypothesis that pegylated cationic liposomes are a viable carrier for inhalable formulations of low molecular weight heparin, an anionic drug. Cationic liposomal formulations of low molecular weight heparin were prepared by the hydration method using 1,2-dioleoyl-3-trimethylammonium-propane (chloride salt), cholesterol and 1,2-distearoyl-sn-glycero-3-phosphoethanolamine-N-[methoxy(polyethyleneglycol)-2000]. The formulations were characterized for particle size, entrapment efficiency, pulmonary absorption and pharmacological efficacy. For absorption studies, the formulations were administered to anesthetized male Sprague-Dawley rats via the pulmonary route and drug absorption was monitored by measuring plasma anti-factor Xa activity. The pharmacological efficacy of the formulations was studied in rodent models of pulmonary embolism and deep vein thrombosis. The mean particle size of the liposomes was 104.8+/-20.7 nm and the drug entrapment efficiency was 90.3+/-0.1%. The half-life of the cationic liposomal formulation was 10.6+/-0.2h, a 2.2-fold increase compared to low molecular weight heparin formulated in saline, and the relative bioavailability was approximately 73.4+/-19.1% when compared to subcutaneously administered drug. A once-every-other-day inhaled dose of the formulation showed similar efficacy in reducing thrombus weight as a once-daily dose of subcutaneously administered drug. Likewise, cationic liposomal formulations administered via the pulmonary route 6h prior to embolization in the lungs showed a thrombolytic effect comparable to that of low molecular weight heparin administered subcutaneously 2h before embolization. Histological examination of lung tissue and measurement of injury markers in bronchoalveolar lavage fluid suggest that the formulations did not produce extensive damage. The results demonstrate that pegylated cationic liposomes could be a viable carrier for an inhalable formulation of low molecular weight heparin.

Physicochemical characterization and study of in vitro interactions of pH-sensitive liposomes with the complement system

Complement activation is an important step in the acceleration of liposome clearance. The anaphylatoxins released following complement activation may motivate a wide variety of physiologic changes. We performed physicochemical characterization and in vitro studies of the interaction of complement system with both noncirculating and long-circulating pH-sensitive and nonpH-sensitive liposomes. The liposomes were characterized by diameter, zeta potential, and atomic force microscopy (AFM). The study of liposome interactions with complement system was conducted using hemolytic assay in rat serum. All liposomes presented a similar mean diameter (between 99.8 and 124.3 nm). The zeta potential was negative in all liposome preparations, except in liposomes modified with aminopoly (ethyleneglycol) 2000-distearoylphosphatidylethanolamine (aPEG(2000)-DSPE), which presented positive zeta potential. Atomic force microscopy images showed that non-long-circulating pH-sensitive liposomes are prone to vesicles aggregation. Non-pH-sensitive liposomes complement system activates, while pH-sensitive liposomes showed to be poor complement activators in rat serum.

[Accelerated blood clearance (ABC) phenomenon induced by administration of PEGylated liposome]

PEGylated liposomes (approximately 100 nm in diameter) lose their long-circulating characteristic upon repeated injection at certain intervals in the same animal (referred to as the "accelerated blood clearance (ABC) phenomenon"), as described by our group and by researchers in the Netherlands. Recently, it was demonstrated by our group that anti-PEG IgM, induced by the first dose of PEGylated liposomes, is responsible for the ABC phenomenon. The IgM produced in this manner then selectively bound to the surface of subsequently injected PEGylated liposomes, leading to substantial complement activation. It is generally believed that nanocarriers coated with a polymer, such as PEG, have no immunogenicity. However, unexpected immune responses occurred even in response to polymer-coated liposomes. This immunogenicity to PEGylated liposomes presents a serious concern in the development and clinical use of liposomal formulations. In this review, we demonstrate our recent observations regarding with the ABC phenomenon against liposomes.

Development of novel poly(ethylene glycol)-based vehicles for gene delivery

The purpose of this research was to develop and characterize a gene delivery vehicle with a poly(ethylene glycol) (PEG) backbone with the aim of overcoming limitations, such as cytotoxicity and rapid clearance, associated with current commonly used non-viral carriers. PEG was functionalized with DNA-binding peptides (DBPs) to make a vehicle (DBP-PEG) capable of condensing DNA. Complexes of plasmid DNA and DBP-PEG were formed and characterized by measuring particle size, zeta potential, and transfection efficiency as a function of N:P charge ratios (DBP-PEG amino groups:DNA phosphate). Dynamic light scattering showed that DBP-PEG was able to condense DNA efficiently resulting in a population of particles in the range of 250-300 nm. Neutral or slightly positive zeta potentials were measured for charge ratios of 3.5:1 and greater. DBP-PEG/DNA complexes, made with plasmids encoding the green fluorescent protein (GFP) and beta-Galactosidase (beta-Gal) genes, were used to transfect Chinese hamster ovary (CHO) cells. DBP-PEG/DNA was capable of transfecting cells and maximum transfection efficiency was observed for N:P ratios from 4:1 to 5:1, corresponding to zeta potentials from -4 to +1.6 mV. The effect of the DBP-PEG vehicle on cell viability was assayed. DBP-PEG was associated with a higher percentage of viable cells ( approximately 95%) than either polyethylenimine (PEI) or poly-L-lysine (PLL), and with transfection efficiency greater than PLL, but with somewhat lower than PEI. The results of this work demonstrate that PEG can be used as the backbone for gene delivery vehicles.

Parameters influencing the stealthiness of colloidal drug delivery systems

Over the last few decades, colloidal drug delivery systems (CDDS) such as nano-structures have been developed in order to improve the efficiency and the specificity of drug action. Their small size permits them to be injected intravenously in order to reach target tissues. However, it is known that they can be rapidly removed from blood circulation by the immune system. CDDS are removed via the complement system and via the cells of the mononuclear phagocyte system (MPS), after their recognition by opsonins and/or receptors present at the cell surface. This recognition is dependent on the physicochemical characteristics of the CDDS. In this study, we will focus on parameters influencing the interactions of opsonins and the macrophage plasma membrane with the surface of CDDS, whereby parameters of the polymer coating become necessary to provide good protection.

Evaluation of pegylated lipid nanocapsules versus complement system activation and macrophage uptake

This work consisted in defining the in vitro behavior of pegylated lipid nanocapsules (LNC) toward the immune system. LNC were composed of an oily core surrounded by a shell of lecithin and polyethylene glycol (PEG) known to decrease the recognition of nanoparticles by the immune system. The "stealth" properties were evaluated by measuring complement activation (CH50 technique and crossed-immunoelectrophoresis (C3 cleavage)) and macrophage uptake. These experiments were performed on 20-, 50-, and 100-nm LNC before and after dialysis. A high density of PEG at the surface led to very low complement activation by LNC with a slight effect of size. This size effect, associated to a dialysis effect in macrophage uptake, was due to differences in density and flexibility of PEG chains related to LNC curvature radius. Thanks to a high density, 660-Da PEG provided LNC a steric stabilization and a protective effect versus complement protein opsonization, but this protection decreased with the increase of LNC size, especially versus macrophage uptake.

Investigation of antibody-coated liposomes as a new treatment for immune thrombocytopenia

Immune thrombocytopenia (ITP) is an autoimmune disease that is mediated by anti-platelet antibodies. It is believed that anti-platelet antibody-opsonized platelets are eliminated through Fcgamma receptor-mediated and complement-mediated phagocytosis by macrophages of the reticuloendothelial system (RES). Polyclonal pooled immunoglobulin with high titer for the D-antigen of erythrocytes (i.e., anti-D) has been successfully used to ameliorate ITP. Based on the pathogenesis of ITP and based on the successful application of anti-D for the treatment of ITP, we hypothesized that antibody-coated liposomes may be used to inhibit Fcgamma receptor-mediated and complement-mediated phagocytosis, thereby increasing platelet counts in ITP. To test this hypothesis, we have developed a liposome preparation that is coated with a model monoclonal IgG1 antibody. Antibody-coated liposomes were found to inhibit complement deposition and macrophage phagocytosis in vitro. Furthermore, antibody-coated liposomes were also found to attenuate thrombocytopenia in a rat model of ITP, in a dose-dependent manner. The results suggest that antibody-coated liposomes may be used as 'decoy particles' to competitively inhibit the destruction of antibody-coated platelets; thus, antibody-coated liposomes may have value in the treatment of ITP.

Neutral and anionic liposome-encapsulated hemoglobin: effect of postinserted poly(ethylene glycol)-distearoylphosphatidylethanolamine on distribution and circulation kinetics

To prepare long-circulating liposomes, poly(ethylene glycol) (PEG)-lipid is usually mixed with other lipid components before vesicle formation. PEG-lipids can also be postinserted in the outer layer of liposomes after the preparation. In this study, PEG-distearoylphosphatidylethanolamine was incorporated by postinsertion technique into liposome-encapsulated hemoglobin (LEH) carrying neutral or negative charge. Postinsertion technique improved the encapsulation efficiency of hemoglobin from about 0.0017 to 0.017 (hemoglobin/phospholipid, molar ratio) for a similar lipid composition. Thus, neutral, anionic, PEG-neutral, and PEG-anionic LEHs were made and labeled with technetium-99m to follow their biodistribution. A small dose of LEH (approximately 15 mg of phospholipid) was injected intravenously in rabbits, and its distribution was monitored by blood sampling, gamma camera imaging, and tissue radioactivity counting on necropsy. The 24-h blood levels of neutral, PEG-neutral, anionic, and PEG-anionic LEHs were 14, 40.3, 13.1, and 35.7% of injected dose, respectively; calculated T(1/2) values of circulation were 8.9, 19.3, 9.6, and 16.5 h, respectively. PEGylation also influenced accumulation of LEH in the reticuloendothelial system. Liver uptake of neutral LEH dropped from 52.1 to 19.1%, whereas that of anionic LEH came down from 35.3 to 11.5% on PEGylation. In contrast, PEGylation increased the spleen uptake by 8.5- and 2.5-fold for neutral and anionic LEH, respectively. The results demonstrate that PEGylation by postinsertion not only improves the circulation t1/2 of LEH but also enhances hemoglobin content inside the vesicles for better oxygen-carrying capacity.

Rational design and engineering of delivery systems for therapeutics: biomedical exercises in colloid and surface science

Engineering delivery systems of therapeutic agents has grown into an independent field, transcending the scope of traditional disciplines and capturing the interest of both academic and industrial research. At the same time, the acceleration in the discovery of new therapeutic moieties (chemical, biological, genetic and radiological) has led to an increasing demand for delivery systems capable of protecting, transporting, and selectively depositing those therapeutic agents to desired sites. The vast majority of delivery systems physically reside in the colloidal domain, while their surface properties and interfacial interactions with the biological milieu critically determine the pharmacological profiles of the delivered therapeutic agents. Interestingly though, the colloidal and surface properties of delivery systems are commonly overlooked in view of the predominant attention placed on the therapeutic effectiveness achieved. Moreover, the development and evaluation of novel delivery systems towards clinical use is often progressed by serendipity rather than a systematic design process, often leading to failure. The present article will attempt to illustrate the colloid and interfacial perspective of a delivery event, as well as exemplify the vast opportunities offered by treating, analysing and manipulating delivery systems as colloidal systems. Exploring and defining the colloid and surface nature of the interactions taking place between the biological moieties in the body and an administered delivery vehicle will allow for the rational engineering of effective delivery systems. A design scheme is also proposed on the way in which the engineering of advanced delivery systems should be practiced towards their transformation from laboratory inventions to clinically viable therapeutics. Lastly, three case studies are presented, demonstrating how rational manipulation of the colloidal and surface properties of delivery systems can lead to newly engineered systems relevant to chemotherapy, gene therapy and radiotherapy.

Synthesis and formulation of neoglycolipids for the functionalization of liposomes and lipoplexes

Novel carbohydrate-based agents for the stabilization of ternary liposome:mu:DNA (LMD) nonviral vector systems are described. LMD vector systems comprise plasmid DNA (pDNA; D,7.5 kb) expressing a reporter gene (in this instance beta-galactosidase expressing gene) that is precondensed with the adenoviral core peptide mu (mu, M; MRRAHHRRRRASHRRMRGG) and then further packaged by means of DC-Chol:DOPE (3:2; m/m) cationic liposomes. Final optimized lipid:mu:pDNA ratio is typically 12:0.6:1 (w/w/w). We report the synthesis of a series of nine neoglycolipids prepared by coupling completely unprotected sugar monomers or oligomers (mannose, glucose, galactose, glucuronic acid, maltose, lactose, maltotriose, maltotetraose, and maltoheptaose) through their reducing-residue termini to an aminoxy-functionalized cholesterol-based lipid. Characterization of these novel neoglycolipids by (1)H NMR reveals that the coupling reaction has a major configurational preference for the beta-anomer. Unusually, even mannose coupling results in a neoglycolipid product with a predominantly beta-anomeric conformation (>85%). Formulation of neoglycolipids into LMD vector systems by incubation of LMD particles with neoglycolipid micelles results in the formation of a range of potential stabilized-LMD (sLMD) vector systems. Those potential sLMD systems prepared with longer chain neoglycolipids are found to have enhanced stabilities, with respect to aggregation in high ionic strength buffers, and enhanced transfection efficacies in comparison to the transfection properties of the naked first generation LMD vector system (i.e., gene delivery and expression). By contrast, when LMD vector systems are incubated with poly(ethylene glycol) DSPE-PEG micelles, resulting PEG-LMD vector systems are very stable with respect to colloidal instablility and aggregation in high ionic strength buffers and in serum, but are completely refractory to transfection. These data suggest that oligosaccharides could represent an alternative to PEG as a stealth polymer able to stabilize synthetic nonviral vector systems in some fluids but without impairing transfection efficiency. Furthermore, sLMD systems prepared with longer chain neoglycolipids appear to have sufficient useful characteristics to form the basis of viable second-generation LMD vector systems after further development.

Evaluation of lipid-based reagents to mediate intracellular gene delivery

We characterized different cationic lipid-based gene delivery systems consisting of both liposomes and nonliposomal structures, in terms of their in vitro transfection activity, resistance to the presence of serum, protective effect against nuclease degradation and stability under different storage conditions. The effect of lipid/DNA charge ratio of the resulting complexes on these properties was also evaluated. Our results indicate that the highest levels of transfection activity were observed for complexes prepared from nonliposomal structures composed of FuGENE 6. However, their DNA protective effect was shown to be lower than that observed for cationic liposome formulations when prepared at the optimal (+/-) charge ratio. Our results suggest that lipoplexes are resistant to serum up to 30% when prepared at a 2:1 lipid/DNA charge ratio. However, when they were prepared at higher (+/-) charge ratios, they become sensitive to serum for even lower concentrations (10%). Replacement of dioleoyl-phosphatidylethanolamine (DOPE) by cholesterol enhanced the resistance of the complexes to the inhibitory effect of serum. This different biological activity in the presence of serum was attributed to different extents of binding of serum proteins to the complexes, as evaluated by the immunoblotting assay. Studies on the stability under storage show that lipoplexes maintain most of their biological activity when stored at -80 degrees C, following their fast freezing in liquid nitrogen.

Exploitation of intracellular pH gradients in the cellular delivery of macromolecules

Most cellular components such as the cytoplasm, endosomes, lysosomes, endoplasmic reticulum, Golgi bodies, mitochondria, and nuclei are known to maintain their own characteristic pH values. These pH values range from as low as 4.5 in the lysosome to about 8.0 in the mitochondria. Given these proton gradients around a neutral pH, weak acids, and bases with a pKa between 5.0 and 8.0 can exhibit dramatic changes in physicochemical properties. These compounds can be conjugated as such to macromolecules or incorporated into polymeric or liposomal formulations to promote the efficient cellular delivery of macromolecules. Mechanistically, the carrier molecules can facilitate favorable membrane partition, membrane fusion, transient pore formation, or membrane disruption. Drug carriers equipped with such pH-sensitive triggers and switches are able to significantly enhance the cellular delivery of macromolecules in vitro. However, the successful application of these molecules for efficient delivery in vivo requires the design of noncytotoxic, nonimmunogenic, serum compatible and biochemically labile carriers, systematic analysis of their mechanisms of action, and extensive animal studies.

Species difference in correlation between in vivo/in vitro liposome-complement interactions

The objective of this study was to investigate the correlation between in vitro and in vivo liposome-complement interactions. Third component of the complement (C3) fragments associated with hydrogenated egg phosphatidylcholine (HEPC)-based liposomes in vivo and complement-dependent destabilization in vitro were determined as an indication of liposome-complement interaction in vivo and in vitro, respectively. C3 fragments on the liposomes were detected in both rats and guinea pigs. Pretreatment with K76COOH (K76), a complement inactivating agent, reduced the binding of C3 fragments. These findings indicated that the liposomes remarkably activated the complement system in both animals in vivo. Interestingly, significant complement-dependent liposome destabilization was observed in rat serum, but not in guinea pig serum, indicating that the liposomes activated the complement system in rats, but not in guinea pigs in vitro. Taken together, it is apparent that in vitro complement activation by the liposomes is not in agreement with in vivo complement activation in ginea pigs. This discrepancy in the liposome-complement interaction would suggest the need for further investigation to utilize the information obtained from the liposome-complement interaction to predict in vivo behavior of the liposomes.

The complement system enhances the clearance of phosphatidylserine (PS)-liposomes in rat and guinea pig

In this study, we investigated the contribution of the complement system to the biodistribution of phosphatidylserine (PS)-containing liposomes in rat and guinea pig. It appeared that the inclusion of PS in the liposome formulation accelerates the rate of liposome uptake by liver, resulting in rapid elimination of the liposomes from blood circulation. Pretreatment with K76COOH (K76), an anti-complement agent, decreased the rapid uptake of PS-containing liposomes by guinea pig liver, resulting in increasing blood concentration of the liposomes. Significant complement-dependent liposome destabilization was observed in vitro in both animals, whereas the complement-dependent destabilization in vivo was likely only a part of the process of the clearance of the PS-containing liposomes. This discrepancy suggests that the rate of complement-dependent liposome uptake by liver is much faster than the rate of complement-dependent liposome destabilization in vivo. Pretreatment of K76 dramatically inhibited the binding of C3 fragments, one of dominant opsonins, to PS-containing liposomes in guinea pig under both in vivo and in vitro conditions. This finding suggests that the C3 fragments in the system are responsible for the clearance of the PS-containing liposomes in guinea pig. In rat, in contrast to guinea pig, in vivo binding of C3 fragments was not inhibited by K76-pretreatment, while in vitro binding was inhibited. This discrepancy may be due to different experimental conditions between in vitro and in vivo assay. Nevertheless, based on the observations in this study, the complement components are most likely involved in the clearance of the PS-containing liposomes in rat. Taken together, the activity of PS in enhancing the liposome clearance appears to be mediated by the complement components, presumably C3 fragments, in both guinea pig and rat. This is a first report showing the mechanism on the hepatic uptake of the PS-containing liposomes in guinea pig.

C1q binding to liposomes is surface charge dependent and is inhibited by peptides consisting of residues 14-26 of the human C1qA chain in a sequence independent manner

Complement activation by anionic liposomes proceeds by antibody-independent, C1q-initiated activation of the classical pathway. Purified C1q bound to anionic liposomes in an acidic lipid concentration-dependent manner. Saturation binding, but not the apparent association constant, was enhanced by increasing the cardiolipin content of the liposomes or decreasing either the pH or ionic strength of the reaction mixture. These observations indicate the involvement of electrostatic factors in the binding. A highly cationic region in the collagen-like domain of C1q comprised of residues 14-26 of the C1qA polypeptide chain was assessed for involvement in liposome binding. This region has previously been shown to mediate C1q binding to other immunoglobulin-independent activators of the classical pathway of complement. Peptides containing residues 14-26 of C1qA, denoted C1qA14-26, inhibited C1q binding to and complement activation by anionic liposomes. The inhibitory capacity of these cationic peptides had no sequence or conformation specificity. Rather, the amount of positive charge on the peptides was the determining factor. When present in excess, peptides with five cationic residues inhibited C1q binding and complement activation; however, C1q peptides with only two cationic residues did not. In addition to the C1qA14-26 region, other parts of C1q that contain cationic residues may also be involved in C1q binding to anionic liposomes.

Inhibition of liposome-induced complement activation by incorporated poly(ethylene glycol)-lipids

Complement activation causes opsonization of foreign particles leading to particle elimination from the blood. Complement-mediated opsonization of charged and large liposomes presents a fundamental problem in their use to deliver therapeutic agents in vivo. To prolong the circulation half-lives of such liposomes, complement activation must be curtailed. The aim of this study was to assess the ability of poly(ethylene glycol)-lipids (PEG-lipids) to inhibit the in vitro activation of the classical pathway of complement in human serum by anionic liposomes. Incorporation of cholesterol-PEG600 (CH-PEG600), cholesterol-PEG1000 (CH-PEG1000), or phosphatidylethanolamine-PEG2000 (PE-PEG2000) resulted in dose-dependent inhibition of C1q binding and complement activation. The dose of PEG-lipid at which complement activation was blocked was inversely related to the PEG chain length. Complement activation was strongly inhibited when 15 mole% of CH-PEG600, 10 mole% CH-PEG1000, or 5 mole% PE-PEG2000 was incorporated into 100-nm anionic liposomes. PEG-lipid incorporation into larger liposomes (240 nm) was also successful in blocking C1q binding and complement activation. Radiolabeled cholesterol-PEG approximately 1400 was prepared and used to determine both the percentage of CH-PEG incorporated into the liposomes and the percentage maintained in the liposomes in the presence of 50% human serum at 37 degrees C for up to 24 h.