Serum independent liposome uptake by mouse liver

Development of PET Radioisotope Copper-64-Labeled Theranostic Immunoliposomes for EGFR Overexpressing Cancer-Targeted Therapy and Imaging

Combining standard surgical procedures with personalized chemotherapy and the continuous monitoring of cancer progression is necessary for effective NSCLC treatment. In this study, we developed liposomal nanoparticles as theranostic agents capable of simultaneous therapy for and imaging of target cancer cells. Copper-64 (64Cu), with a clinically practical half-life (t1/2 = 12.7 h) and decay properties, was selected as the radioisotope for molecular PET imaging. An anti-epidermal growth factor receptor (anti-EGFR) antibody was used to achieve target-specific delivery. Simultaneously, the chemotherapeutic agent doxorubicin (Dox) was encapsulated within the liposomes using a pH-gradient method. The conjugates of 64Cu-labeled and anti-EGFR antibody-conjugated micelles were inserted into the doxorubicin-encapsulating liposomes via a post-insertion procedure (64Cu-Dox-immunoliposomes). We evaluated the size and zeta-potential of the liposomes and analyzed target-specific cell binding and cytotoxicity in EGFR-positive cell lines. Then, we analyzed the specific therapeutic effect and PET imaging of the 64Cu-Dox-immunoliposomes with the A549 xenograft mouse model. In vivo therapeutic experiments on the mouse models demonstrated that the doxorubicin-containing 64Cu-immunoliposomes effectively inhibited tumor growth. Moreover, the 64Cu-immunoliposomes provided superior in vivo PET images of the tumors compared to the untargeted liposomes. We suggest that nanoparticles will be the potential platform for cancer treatment as a widely applicable theranostic system.

New liposome-radionuclide-chelate combination for tumor targeting and rapid healthy tissue clearance

Radionuclide imaging and therapy are promising methods for controlling systemic cancers; however, their clinical application has been limited by excessive radionuclide accumulation in healthy tissues. To minimize radionuclide accumulation in non-cancerous tissues while ensuring sufficient build up in tumors, we aimed to develop a method that controlled the in vivo dynamics of radionuclides post-administration. To this end, we describe a novel strategy that combines liposomes, a potent carrier system for drug delivery, with unique radionuclide-ligand complexes based on 111In-ethylenedicysteine. Conventional 111In-ligand-complexes-carrying liposomes delivered substantial amounts of radionuclides to tumors; however, they also accumulated in the liver and spleen. In contrast, 111In-ethylenedicysteine-carrying liposomes greatly reduced non-specific accumulation, while being retained selectively at high doses within tumors. Liposomes were rapidly broken down in the liver, releasing encapsulated 111In-ligand complexes. Among the chelates used, only 111In-ethylenedicysteine could escape from the liver and be excreted in the urine. Instead, most liposomes remained intact in tumors, retaining the radionuclide-ligand complexes within them. Therefore, high tumor accumulation was obtained regardless of the type of 111In-ligand complexes in the liposomes. In vivo single photon emission computed tomography/computed tomography imaging with 111In-ethylenedicysteine-carrying liposomes accurately revealed tumor-selective radionuclide retention with little background. Hence, our new strategy could greatly enhance tumor-to-healthy tissue ratios, improve diagnostic imaging, boost therapeutic efficacy, reduce toxicity to healthy tissues, and facilitate radionuclide imaging and therapy.

Complex Factors and Challenges that Affect the Pharmacology, Safety and Efficacy of Nanocarrier Drug Delivery Systems

Major developments in nanomedicines, such as nanoparticles (NPs), nanosomes, and conjugates, have revolutionized drug delivery capabilities over the past four decades. Although nanocarrier agents provide numerous advantages (e.g., greater solubility and duration of systemic exposure) compared to their small-molecule counterparts, there is considerable inter-patient variability seen in the systemic disposition, tumor delivery and overall pharmacological effects (i.e., anti-tumor efficacy and unwanted toxicity) of NP agents. This review aims to provide a summary of fundamental factors that affect the disposition of NPs in the treatment of cancer and why they should be evaluated during preclinical and clinical development. Furthermore, this chapter will highlight some of the translational challenges associated with elements of NPs and how these issues can only be addressed by detailed and novel pharmacology studies.

siRNA therapeutics for breast cancer: recent efforts in targeting metastasis, drug resistance, and immune evasion

Small interfering RNA (siRNA) has an established and precise mode of action to achieve protein knockdown. With the ability to target any protein, it is very attractive as a potential therapeutic for a plethora of diseases driven by the (over)expression of certain proteins. Utilizing siRNA to understand and treat cancer, a disease largely driven by genetic aberration, is thus actively investigated. However, the main hurdle for the clinical translation of siRNA therapeutics is to achieve effective delivery of siRNA molecules to tumors and the site of action, the cytosol, within cancer cells. Several nanoparticle delivery platforms for siRNA have been developed. In this Review, we describe recent efforts in developing siRNA therapeutics for the treatment of cancer, with particular emphasis on breast cancer. Instead of conventionally targeting proliferation and apoptosis aspects of tumorigenesis, we focus on recent attempts in targeting cancer's metastasis, drug resistance, and immune evasion, which are considered more challenging and less manageable in clinics with current therapeutic molecules. siRNA can target all proteins, including traditionally undruggable proteins, and is thus poised to address these clinical challenges. Evidence also suggests that siRNA can be superior to antibodies or small molecule inhibitors when inhibiting the same druggable pathway. In addition to cancer cells, the role of the tumor microenvironment has been increasingly appreciated. Components in the tumor microenvironment, particularly immune cells, and thus siRNA-based immunotherapy, are under extensive investigation. Lastly, multiple siRNAs with or without additional drugs can be codelivered on the same nanoparticle to the same target site of action, maximizing their potential synergy while limiting off-target toxicity.

Manipulating Cellular Interactions of Poly(glycidyl methacrylate) Nanoparticles Using Mixed Polymer Brushes

There is a growing need for the development of nanoparticles, with imaging and drug delivery capabilities, to maintain cellular uptake but avoid protein attachment and recognition. In this study we have demonstrated that nanoparticles consisting of a poly(glycidyl methacrylate) (PGMA) core and a mixed brush architecture of methoxypoly(ethylene glycol) and poly(ethylenimine) (mPEG-PEI) on the surface can meet this need. Surface functionalization with PEI alone results in cellular uptake, but rapid protein attachment whereas PEG alone can avoid protein attachment but to the detriment of cellular uptake. A mixed copolymer brush of both PEI and mPEG provides the ideal balance.

PEGylation as a strategy for improving nanoparticle-based drug and gene delivery

Coating the surface of nanoparticles with polyethylene glycol (PEG), or "PEGylation", is a commonly used approach for improving the efficiency of drug and gene delivery to target cells and tissues. Building from the success of PEGylating proteins to improve systemic circulation time and decrease immunogenicity, the impact of PEG coatings on the fate of systemically administered nanoparticle formulations has, and continues to be, widely studied. PEG coatings on nanoparticles shield the surface from aggregation, opsonization, and phagocytosis, prolonging systemic circulation time. Here, we briefly describe the history of the development of PEGylated nanoparticle formulations for systemic administration, including how factors such as PEG molecular weight, PEG surface density, nanoparticle core properties, and repeated administration impact circulation time. A less frequently discussed topic, we then describe how PEG coatings on nanoparticles have also been utilized for overcoming various biological barriers to efficient drug and gene delivery associated with other modes of administration, ranging from gastrointestinal to ocular. Finally, we describe both methods for PEGylating nanoparticles and methods for characterizing PEG surface density, a key factor in the effectiveness of the PEG surface coating for improving drug and gene delivery.

Stealth nanotubes: strategies of shielding carbon nanotubes to evade opsonization and improve biodistribution

Carbon nanotubes (CNTs) have recently been in the limelight for their potential role in disease diagnostics and therapeutics, as well as in tissue engineering. Before these medical applications can be realized, there is a need to address issues like opsonization, phagocytosis by macrophages, and sequestration to the liver and spleen for eventual elimination from the body; along with equally important issues such as aqueous solubility, dispersion, biocompatibility, and biofunctionalization. CNTs have not been shown to be able to evade such biological obstacles, which include their nonspecific attachments to cells and other biological components in the bloodstream, before reaching target tissues and cells in vivo. This will eventually determine their longevity in circulation and clearance rate from the body. This review article discusses the current status, challenges, practical strategies, and implementations of coating CNTs with biocompatible and opsonin-resistant moieties, rendering CNTs transparent to opsonins and deceiving the innate immune response to make believe that the CNTs are not foreign. A holistic approach to the development of such "stealth" CNTs is presented, which encompasses not only several biophysicochemical factors that are not limited to surface treatment of CNTs, but also extraneous biological factors such as the protein corona formation that inevitably controls the in vivo fate of the particles. This review also discusses the present and potential applications, along with the future directions, of CNTs and their hybrid-based nanotheranostic agents for multiplex, multimodal molecular imaging and therapy, as well as in other applications, such as drug delivery and tissue engineering.

The shape/morphology balance: a study of stealth liposomes via fractal analysis and drug encapsulation

PURPOSE

Fractal analysis was used as a tool in order to study the morphological characteristics of PEGylated liposomes. We report on the morphological characteristics of stealth liposomes composed of DPPC and DPPE-PEG 3000 in two dispersion media using fractal analysis.

METHODS

Light scattering techniques were used in order to elucidate the size, the morphology and the surface charge of PEGylated liposomes as a function of PEGylated lipid concentration and temperature. Fluorescence spectroscopy studies revealed a microenvironment of low polarity inside the liposomal membranes.

RESULTS

All formulations were found to retain their physicochemical characteristics for at least 3 weeks. The hydrodynamic radii (Rh) of stealth liposomes were stable in the process of heating up to 50°C; while the fractal dimension values (df) which correspond to their morphology, have been changed during heating. Hence, these results are a first indication of the presence of a heterogeneous microdomain structure of the stealth liposomal system. The amphiphilic drug indomethacin (IND) was successfully encapsulated within the liposomes and led to an increased size of stealth liposomes, while the morphology of liposomal vectors changed significantly at the highest molar ratio of PEGylated lipid.

CONCLUSIONS

We can state that this approach can promote a new analytical concept based on the morphological characteristics and quantify the shape of drug carriers complementary to that of the conventional analytical techniques.

Clearance and biodistribution of liposomally encapsulated nitroxides: a model for targeted delivery of electron paramagnetic resonance imaging probes to tumors

Electron paramagnetic resonance (EPR) imaging using nitroxides as molecular probes is potentially a powerful tool for the detection and physiological characterization of micrometastatic lesions. Encapsulating nitroxides in anti-HER2 immunoliposomes at high concentrations to take advantage of the "self-quenching" phenomenon of nitroxides allows generation of robust EPR signals in HER2-overexpressing breast tumor cells with minimal background from indifferent tissues or circulating liposomes. We investigated the in vivo pharmacological properties of nitroxides encapsulated in sterically stabilized liposomes designed for long circulation times. We show that circulation times of nitroxides can be extended from hours to days; this increases the proportion of liposomes in circulation to enhance tumor targeting. Furthermore, nitroxides encapsulated in sterically stabilized anti-HER2 immunoliposomes can be delivered to HER2-overexpressing tumors at micromolar concentrations, which should be imageable by EPR. Lastly, after in vivo administration, liposomally encapsulated nitroxide signal also appears in the liver, spleen, and kidneys. Although these organs are spatially distinct and would not hinder tumor imaging in our model, understanding nitroxide signal retention in these organs is essential for further improvements in EPR imaging contrast between tumors and other tissues. These results lay the foundation to use liposomally delivered nitroxides and EPR imaging to visualize tumor cells in vivo.

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.

Anti-HER2 immunoliposomes for selective delivery of electron paramagnetic resonance imaging probes to HER2-overexpressing breast tumor cells

Electron paramagnetic resonance (EPR) imaging is an emerging modality that can detect and localize paramagnetic molecular probes (so-called spin probes) in vivo. We previously demonstrated that nitroxide spin probes can be encapsulated in liposomes at concentrations exceeding 100 mM, at which nitroxides exhibit a concentration-dependent quenching of their EPR signal that is analogous to the self-quenching of fluorescent molecules. Therefore, intact liposomes encapsulating high concentrations of nitroxides exhibit greatly attenuated EPR spectral signals, and endocytosis of such liposomes represents a cell-activated contrast-generating mechanism. After endocytosis, the encapsulated nitroxide is liberated and becomes greatly diluted in the intracellular milieu. This dequenches the nitroxides to generate a robust intracellular EPR signal. It is therefore possible to deliver a high concentration of nitroxides to cells while minimizing background signal from unendocytosed liposomes. We report here that intracellular EPR signal can be selectively generated in a specific cell type by exploiting its expression of Human Epidermal Growth Factor Receptor 2 (HER2). When targeted by anti-HER2 immunoliposomes encapsulating quenched nitroxides, Hc7 cells, which are novel HER2-overexpressing cells derived from the MCF7 breast tumor cell line, endocytose the liposomes copiously, in contrast to the parent MCF7 cells or control CV1 cells, which do not express HER2. HER2-dependent liposomal delivery enables Hc7 cells to accumulate 750 μM nitroxide intracellularly. Through the use of phantom models, we verify that this concentration of nitroxides is more than sufficient for EPR imaging, thus laying the foundation for using EPR imaging to visualize HER2-overexpressing Hc7 tumors in animals.

Cellular uptake of electron paramagnetic resonance imaging probes through endocytosis of liposomes

Electron paramagnetic resonance imaging (EPRI) allows detection and localization of paramagnetic spin probes in vivo and in real time. We have shown that nitroxide spin probes entrapped in the intracellular milieu can be imaged by EPRI. Therefore, with the development of a tumor-targetable vehicle that can efficiently deliver nitroxides into cells, it should be possible to use nitroxide spin probes to label and image cells in a tumor. In this study, we assess the potential of liposomes as a delivery vehicle for imaging probes. We demonstrate that liposomes can stably encapsulate nitroxides at very high concentrations (>100 mM), at which nitroxides exhibit concentration-dependent quenching of their EPR signal-a process analogous to the quenching of fluorescent molecules. The encapsulating liposomes thus appear spectroscopically "dark". When the liposomes are endocytosed and degraded by cells, the encapsulated nitroxides are liberated and diluted into the much larger intracellular volume. The consequent relief of quenching generates a robust intracellular nitroxide signal that can be imaged. We show that through endocytosis of nitroxide-loaded liposomes, CV1 cells can achieve intracellular nitroxide concentrations of approximately 1 mM. By using tissue phantom models, we verify that this concentration is more than sufficient for in vivo EPR imaging.

Liposome Opsonization

Adsorption of serum proteins to the liposomal surface plays a critical role in the clearance of liposomes from the blood circulation. In this review, we will discuss the role of the liposomal opsonins proposed so far in liposome clearance. Additional, related topics that will be addressed are the cell-surface receptors that might be involved in liposome elimination from the blood compartment and the effect of poly(ethylene glycol) (PEG) modification on prevention of liposome opsonization.

Surface-active liposomes for targeted cancer therapy

An overview of liposome-based drug-delivery carriers to cancer cells is presented. Properties related to interfacial interactions between liposomes and the biological milieu that determine the fate of liposomes in vivo are discussed. Original approaches to improve specificity for the target and to control the structural responsiveness of liposomes, depending on their immediate environment, with the aim of enhancing the delivered therapeutic doses, are summarized. This review is not exhaustive on research examples of liposomes as carriers for cancer therapy but, rather, aims to describe major directions of designs and strategies over recent years. The current therapeutic trends that exhibit increasingly higher complexity in structures and responses are also discussed.

Mechanism of naked DNA clearance after intravenous injection

BACKGROUND

Injection of naked DNA has been viewed as a safer alternative to current gene delivery systems; however, the rate of clearance from the circulation has been a constant barrier in developing these methods. Naked DNA after intravenous (i.v.) injection will be taken up by the liver and depredated by serum nucleases.

MATERIALS AND METHODS

Our study examines the mechanisms involved in clearance of naked DNA by each compartment, the blood and the liver, in an in vivo mouse model. Confocal microscopy and transmission electron microscopy were employed to identify the type of cells taking up DNA and the barrier to DNA access to hepatocytes, respectively.

RESULTS

Our data showed the liver could take up over 50% of 5 microg perfused pDNA, with a maximum 25 microg of pDNA during a single pass, and a slower clearance rate compared to that of liver uptake. It was demonstrated that naked DNA is primarily taken up by the liver endothelial cells and this endothelial barrier to transfection could be overcome by manually massaging the liver, which enlarges the fenestrae.

CONCLUSIONS

This study clarifies the mechanism by which naked DNA is eliminated from the circulation after i.v. injection, focusing on the role of both the liver and blood compartments in vivo (i.e. mouse). With this knowledge, we can more clearly understand the mechanism of naked DNA clearance and develop more efficient strategies for DNA transfer in vivo.

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.

Uptake of phosphatidylserine-containing liposomes by liver sinusoidal endothelial cells in the serum-free perfused rat liver

We studied the kinetics of hepatic uptake of liposomes during serum-free recirculating perfusion of rat livers. Liposomes consisted of phosphatidylcholine, cholesterol and phosphatidylserine in a 6:4:0 or a 3:4:3 molar ratio and were radiolabelled with [3H]cholesteryl oleyl ether. The negatively charged liposomes were taken up to a 10-fold higher extent than the neutral ones. Hepatic uptake of fluorescently labelled liposomes was examined by fluorescence microscopy. The neutral liposomes displayed a typical Kupffer cell distribution pattern, in addition to weak diffuse staining of the parenchyma, while the negatively charged liposomes showed a characteristic sinusoidal lining pattern, consistent with an endothelial localization. In addition, scattered Kupffer cell staining was distinguished as well as diffuse parenchymal fluorescence. The mainly endothelial localisation of the negatively charged liposomes was confirmed by determining radioactivity in endothelial and Kupffer cells isolated following a 1-h perfusion. Perfusion in the presence of polyinosinic acid, an inhibitor of scavenger receptor activity, reduced the rate of uptake of the negatively charged liposomes twofold, indicating the involvement of this receptor in the elimination mechanism. These results are compatible with earlier in vitro studies on liposome uptake by isolated endothelial cells and Kupffer cells, which showed that in the absence of serum also endothelial cells in situ are able to take up massive amounts of negatively charged liposomes. The present results emphasize that the high in vitro endothelial cell uptake in the absence of serum from earlier observations was not an artifact induced by the cell isolation procedure.

Biodisposition of PEG-coated lipid microspheres of indomethacin in arthritic rats

Conventional lipid microspheres (LM) were prepared using soybean oil and lipid at a 5.5:1 weight ratio with lipid phase consisting of PC (phosphatidyl choline):CH (cholesterol) (1:0.5) by molar ratio. The average diameter of the particles was 150 nm. Long-circulating microspheres (S-LM) were also prepared similarly but the lipid phase consisted of PC:CH:DSPE-PEG (phosphatidyl choline:cholesterol:distearoyl phosphatidyl ethanolamine-polyethylene glycol) 1:0.5:0.16 by molar ratio. A comparative biodistribution study was conducted between free indomethacin and lipo-indomethacin (LM and S-LM) in the arthritic rats by administering the formulations at a dose equivalent to 12 mg of indomethacin/kg. It was observed that the free drug as well as the encapsulated drug followed biphasic clearance from the blood. Pharmacokinetic parameters, such as AUC(0-t), terminal half-life, MRT increased significantly when the drug was used in encapsulated form (p < 0.05). Clearance of the drug was reduced 1.4 times with the conventional lipid microspheres and was reduced three-fold when encapsulated in polyethylene glycol-coated lipid microspheres. The overall drug targeting efficiency (T(e)) with the PEG-coated lipid microspheres was 7.5-fold higher than the conventional lipid microspheres. The high accumulation of the drug in arthritic paw with S-LM system may be accounted for by the reduced uptake by RES cells, and thereby, availability for extravascularization in the inflammatory tissues.

Liposome clearance in mice: the effect of a separate and combined presence of surface charge and polymer coating

The purpose of our work was to compare the biodistribution of liposomes with different surface properties. Phosphatidylcholine (PC)/cholesterol (Chol) liposomes were prepared containing 6% mol of a charged lipid (stearylamine, SA; phosphatidic acid, PA; or phosphatidyl serine, PS) and/or polyethylene glycol (PEG)-PE of different MW (750 and 5000). zeta-Potentials and liposome clearance in mice were investigated. In vitro, the attachment of PEG in a similar fashion neutralizes the effect of any charged component. In vivo, the chemical nature of a charged lipid becomes important. Both short PEG750 and longer PEG5000 inhibit the clearance of positively charged SA-liposomes, while only longer PEG5000 inhibits the clearance of negatively charged PA-liposomes and none of the PEGs inhibit the clearance of negatively charged PS-liposomes. The opsonins with different molecular size may be involved in the clearance of liposomes containing different charged lipids.

Surface-associated serum proteins inhibit the uptake of phosphatidylserine and poly(ethylene glycol) liposomes by mouse macrophages

Serum proteins, acting as opsonins, are believed to contribute significantly to liposome-macrophage cell association and thus regulate liposome uptake by cells of the mononuclear phagocytic system (MPS). We studied the effect of serum protein on binding and uptake of phosphatidylglycerol-, phosphatidylserine-, cardiolipin-, and N,N-dioleyl-N,N-dimethylammonium chloride- (DODAC) containing as well as poly(ethylene glycol)- (PEG) containing liposomes by mouse bone marrow macrophages in vitro. Consistent with the postulated surface-shielding properties of PEG, protein-free uptake of liposomes containing 5 mol% PEG and either 20 mol% anionic phosphatidylserine or 20 mol% cationic DODAC was equivalent to uptake of neutral liposomes. In contrast to previous reports indicating that protein adsorption to liposomes increases uptake by macrophages, the presence of bound serum protein did not increase the uptake of these liposomes by cultured macrophages. Rather, we found that pre-incubating liposomes with serum reduced the uptake of liposomes containing phosphatidylserine. Surprisingly, serum treatment of PEG-containing liposomes also significantly reduced liposome uptake by macrophages. It is postulated that, in the case of phosphatidylserine liposomes, the bound serum protein can provide a non-specific surface-shielding property that reduces the charge-mediated interactions between liposomes and bone marrow macrophage cells. In addition, incubation of PEG-bearing liposomes with serum can result in a change in the properties of the PEG, resulting in a surface that is better protected against interactions with cells.

The role of hepatocytes in the clearance of liposomes from the blood circulation

In this chapter we summarize literature and describe in more detail our own observations over a period of nearly two decennia on the role of hepatocytes in the hepatic clearance of intravenously administered liposomes. Evidence is presented indicating that, although size is an important parameter, it is not decisive in determining access of liposomes to the hepatocytes. Also lipid composition is an important parameter, including charge, rigidity and headgroup composition. The role of the fenestrated sinusoidal endothelial cells in determining liposome accessibility of hepatocytes is discussed as well as the involvement of opsonizing plasma proteins such as apolipoprotein E. Our observations led us to postulate the existence of at least four different mechanisms of interaction of liposomes with hepatocytes, i.e. an endocytic and a non-endocytic one for both neutral and negatively charged vesicles

Recognition by macrophages and liver cells of opsonized phospholipid vesicles and phospholipid headgroups

The interaction of liposomes with blood proteins is believed to play a critical role in the clearance pharmacokinetics and tissue distribution of intravenously injected liposomes. In this article we have focused our discussion on the interaction of liposomes with key blood proteins, which include immunoglobulins, complement proteins, apolipoproteins, fetuin, von Willebrand factor, and thrombospondin, and their role in liposome recognition by professional phagocytes and nonmacrophage hepatic cells. Alternatively, macrophages as well as hepatocytes and liver endothelial cells may phagocytose/endocytose liposomes via direct recognition of phospholipid headgroups. A number of plasma membrane receptors such as lectin receptors, CD14, various classes of scavenger receptors (e.g., classes A, B, and D), Fc-gammaRI and FcgammaRII-B2 may participate in phospholipid recognition. These concepts are also discussed.

Biodistribution of liposomes and C3 fragments associated with liposomes: evaluation of their relationship

The biodistribution of liposomes with two different kind phospholipids (hydrogenated egg phosphatidylcholine and egg phosphatidylcholine) plus cholesterol (CHOL) were investigated after intravenous administration to rats. Elimination of liposomes from blood circulation was affected by the lipid composition. It appeared that the inclusion of CHOL in liposomes accelerates the rate of liposome uptake by liver, resulting in rapid elimination of liposomes. The amount of C3 fragments bound to liposomes was quantitatively determined to assess the contribution of the complement system to liposome accumulation into organs and liposome destabilization in vivo and in vitro. The amount of bound C3 fragments was directly proportional to CHOL content, and the amount was also proportional to the CLh, CLs as well as CLrel. This relationship suggests that the complement system is responsible for the elimination of liposomes from blood circulation, presumably as a consequence of opsonization by C3 fragments and assembly of membrane attack complex (MAC) onto liposomes. In addition, substitution of cholesteryl methyl ether into the liposome formulation for CHOL significantly diminished not only the binding of C3 fragments but also the CLh, CLs and CLrel, resulting in increased mean resident time (MRT) of the liposomes. This result suggests that the hydroxyl-group on CHOL is a binding site for C3 fragments on the liposomes and that CHOL in a liposome formulation promotes the accumulation of liposomes into the liver and spleen, probably due to their uptake by phagocytic cells, and impairs the stability of the liposomes in blood circulation, via a mechanism involving the complement system.

Antileishmanial activities of stearylamine-bearing liposomes

Here we report the activity of liposomes comprising egg phosphatidylcholine (PC) and stearylamine (SA) against Leishmania donovani parasites. Both promastigotes and intracellular amastigotes in vitro and in vivo were susceptible to SA-PC liposomes. A single dose of 55 mg of SA-PC liposomes/animal could significantly reduce the hepatic parasite burden by 85 and 68% against recent and established experimental visceral leishmaniasis, respectively, suggesting their strong therapeutic potential.

Metabolic stability of glutaraldehyde cross-linked peptide DNA condensates

The stability of peptide DNA condensates was examined after introducing glutaraldehyde to cross-link surface amine groups. A 20 amino acid peptide (CWK(18)) was used to condense DNA into small (70 nm) condensates. The reaction between glutaraldehyde and peptide DNA condensates was indirectly monitored using a fluorescence-based assay to establish reaction completion in 4-5 h when using glutaraldehyde-to-peptide ratios of 1 to 4 mol equiv. Higher levels of glutaraldehyde cross-linking led to significant increases in particle size. The improved stability imparted by glutaraldehyde cross-linking was demonstrated by the increased resistance of DNA condensates to shear stress induced fragmentation. The cross-linked condensates were also significantly more resistant to in vitro metabolism by serum endonucleases. A decrease in the magnitude of transient gene expression was determined for cross-linked DNA condensates which also resulted in a 10-day steady-state expression when cross-linking with 4 mol equiv of glutaraldehyde. The results suggest that cross-linking DNA condensates may provide a means to alter the time course of transient gene expression by inhibiting DNA metabolism.

Pharmacokinetics and urinary excretion of amikacin in low-clearance unilamellar liposomes after a single or repeated intravenous administration in the rhesus monkey

Liposomal aminoglycosides have been shown to have activity against intracellular infections, such as those caused by Mycobacterium avium. Amikacin in small, low-clearance liposomes (MiKasome) also has curative and prophylactic efficacies against Pseudomonas aeruginosa and Klebsiella pneumoniae. To develop appropriate dosing regimens for low-clearance liposomal amikacin, we studied the pharmacokinetics of liposomal amikacin in plasma, the level of exposure of plasma to free amikacin, and urinary excretion of amikacin after the administration of single-dose (20 mg/kg of body weight) and repeated-dose (20 mg/kg eight times at 48-h intervals) regimens in rhesus monkeys. The clearance of liposomal amikacin (single-dose regimen, 0.023 +/- 0.003 ml min-1 kg-1; repeated-dose regimen, 0.014 +/- 0.001 ml min-1 kg-1) was over 100-fold lower than the creatinine clearance (an estimate of conventional amikacin clearance). Half-lives in plasma were longer than those reported for other amikacin formulations and declined during the elimination phase following administration of the last dose (from 81.7 +/- 27 to 30.5 +/- 5 h). Peak and trough (48 h) levels after repeated dosing reached 728 +/- 72 and 418 +/- 60 micrograms/ml, respectively. The levels in plasma remained > 180 micrograms/ml for 6 days after the administration of the last dose. The free amikacin concentration in plasma never exceeded 17.4 +/- 1 micrograms/ml and fell rapidly (half-life, 1.47 to 1.85 h) after the administration of each dose of liposomal amikacin. This and the low volume of distribution (45 ml/kg) indicate that the amikacin in plasma largely remained sequestered in long-circulating liposomes. Less than half the amikacin was recovered in the urine, suggesting that the level of renal exposure to filtered free amikacin was reduced, possibly as a result of intracellular uptake or the metabolism of liposomal amikacin. Thus, low-clearance liposomal amikacin could be administered at prolonged (2- to 7-day) intervals to achieve high levels of exposure to liposomal amikacin with minimal exposure to free amikacin.

Liposomes: from biophysics to the design of peptide vaccines

Liposomes (lipid-based vesicles) have been widely studied as drug delivery systems due to their relative safety, their structural versatility concerning size, composition and bilayer fluidity, and their ability to incorporate almost any molecule regardless of its structure. Liposomes are successful in inducing potent in vivo immunity to incorporated antigens and are now being employed in numerous immunization procedures. This is a brief overview of the structural, biophysical and pharmacological properties of liposomes and of the current strategies in the design of liposomes as vaccine delivery systems.

Altered tissue distribution and elimination of amikacin encapsulated in unilamellar, low-clearance liposomes (MiKasome)

PURPOSE

Amikacin in small unilamellar liposomes (MiKasome) has prolonged plasma residence (half-life > 24hr) and sustained efficacy in Gram-negative infection models. Since low-clearance liposomes may be subject to a lower rate of phagocytic uptake, we hypothesized this formulation may enhance amikacin distribution to tissues outside the mononuclear phagocyte system.

METHODS

Rats received one intravenous dose (50 mg/kg) of conventional or liposomal amikacin. Amikacin was measured for ten days in plasma, twelve tissues, urine and bile.

RESULTS

Liposomal amikacin increased and prolonged drug exposure in all tissues. Tissue half-lives (63-465 hr) exceeded the plasma half-life (24.5 hr). Peak levels occurred within 4 hours in some tissues, but were delayed 1-3 days in spleen, liver, lungs and duodenum, demonstrating the importance of characterizing the entire tissue concentration vs. time profile for liposomal drugs. Predicted steady-state tissue concentrations for twice weekly dosing were >100 microg/g. Less than half the liposomal amikacin was recovered in tissues and excreta, suggesting metabolism occurred. Amikacin was not detected in plasma ultrafiltrates. Tissue-plasma partition coefficients (0.2-0.8 in most tissues) estimated from tissue-plasma ratios at Tmax were similar to those estimated from tissue AUCs.

CONCLUSIONS

Low-clearance liposomal amikacin increased and prolonged drug residence in all tissues compared to conventional amikacin. The long tissue half-lives suggest liposomal amikacin is sequestered within tissues, and that an extended dosing interval is appropriate for chronic or prophylactic therapy with this formulation.

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.

Effect of polyethyleneglycol-phospholipids on aggregate structure in preparations of small unilamellar liposomes

Phospholipids with covalently attached poly(ethylene glycol) (PEG lipids) are commonly used for the preparation of long circulating liposomes. Although it is well known that lipid/PEG-lipid mixed micelles may form above a certain critical concentration of PEG-lipid, little is known about the effects of PEG-lipids on liposome structure and leakage at submicellar concentrations. In this study we have used cryogenic transmission electron microscopy to investigate the effect of PEG(2000)-PE on aggregate structure in preparations of liposomes with different membrane compositions. The results reveal a number of important aggregate structures not documented before. The micrographs show that enclosure of PEG-PE induces the formation of open bilayer discs at concentrations well below those where mixed micelles begin to form. The maximum concentration of PEG-lipid that may be incorporated without alteration of the liposome structure depends on the phospholipid chain length, whereas phospholipid saturation or the presence of cholesterol has little or no effect. The presence of cholesterol does, however, affect the shape of the mixed micelles formed at high concentrations of PEG-lipid. Threadlike micelles form in the absence of cholesterol but adapt a globular shape when cholesterol is present.

Co-existence of serum-dependent and serum-independent mechanisms for liposome clearance and involvement of non-Kupffer cells in liposome uptake by mouse liver

The effect of serum on liver uptake of liposomes with different compositions was investigated using a single-pass liver perfusion technique. Among the liposomes tested are those containing CL, PA, DPGS, PE or glycolipids such as PI, GD, GT1b and aGM1. Liposomes containing PA, CL and DPGS showed high level of liver uptake in the absence of serum. Presence of serum decreased the total liver uptake for liposomes containing CL and PA by 50% and did not affect the level of liver uptake for DPGS-containing liposomes. The presence of serum, however, significantly increased the liposome uptake by the perfused liver for PG, PE and aGM1 liposomes. Liposomes containing PI showed a minimal liver uptake regardless of serum presence. Fluorescence microscopic studies using a dual fluorescence label system in combination with Kupffer cell elimination technique showed that, in addition to the dominant role of Kupffer cells in taking up liposomes, non-Kupffer cells may also be involved in taking up CL and DCP-containing liposomes. Competition experiments using various liposome compositions indicated that liposome uptake by the liver cells may involve different receptors. Serum activity in enhancing the liver uptake for PE- and aGM1-containing liposomes can be blocked by treatment of serum with EDTA, EGTA/Mg2+ and high temperature (56 degrees C), suggesting the involvement of complement system. Results from this study support the conclusion that blood clearance of liposomes by the liver involves two independent mechanisms, one requires serum opsonins and the other does not.