Uptake and processing of immunoglobulin-coated liposomes by subpopulations of rat liver macrophages

Interplay of protein corona and immune cells controls blood residency of liposomes

In vivo liposomes, like other types of nanoparticles, acquire a totally new 'biological identity' due to the formation of a biomolecular coating known as the protein corona that depends on and modifies the liposomes' synthetic identity. The liposome-protein corona is a dynamic interface that regulates the interaction of liposomes with the physiological environment. Here we show that the biological identity of liposomes is clearly linked to their sequestration from peripheral blood mononuclear cells (PBMCs) of healthy donors that ultimately leads to removal from the bloodstream. Pre-coating liposomes with an artificial corona made of human plasma proteins drastically reduces capture by circulating leukocytes in whole blood and may be an effective strategy to enable prolonged circulation in vivo. We conclude with a critical assessment of the key concepts of liposome technology that need to be reviewed for its definitive clinical translation.

Pharmacologic Studies of a Prodrug of Mitomycin C in Pegylated Liposomes (Promitil(®)): High Stability in Plasma and Rapid Thiolytic Prodrug Activation in Tissues

PURPOSE

Pegylated liposomal (PL) mitomycin C lipid-based prodrug (MLP) has recently entered clinical testing. We studied here the preclinical pharmacology of PL-MLP.

METHODS

The stability, pharmacokinetics, biodistribution, and other pharmacologic parameters of PL-MLP were examined. Thiolytic cleavage of MLP and release of active mitomycin C (MMC) were studied using dithiothreitol (DTT), and by incubation with tissue homogenates.

RESULTS

MLP was incorporated in the bilayer at 10% molar ratio with nearly 100% entrapment efficiency, resulting in a formulation with high plasma stability. In vitro, DTT induced cleavage of MLP with predictable kinetics, generating MMC and enhancing pharmacological activity. A long circulation half-life of MLP (10-15 h) was observed in rodents and minipigs. Free MMC is either extremely low or undetectable in plasma. However, urine from PL-MLP injected rats revealed delayed but significant excretion of MMC indicating in vivo activation of MLP. Studies in mice injected with H3-cholesterol radiolabeled PL-MLP demonstrated relatively greater tissue levels of H3-cholesterol than MLP. MLP levels were highest in tumor and spleen, and very low or undetectable in liver and lung. Rapid cleavage of MLP in various tissues, particularly in liver, was shown in ex-vivo experiments of PL-MLP with tissue homogenates. PL-MLP was less toxic in vivo than equivalent doses of MMC. Therapeutic studies in C26 mouse tumor models demonstrated dose-dependent improved efficacy of PL-MLP over MMC.

CONCLUSIONS

Thiolytic activation of PL-MLP occurs in tissues but not in plasma. Liposomal delivery of MLP confers a favorable pharmacological profile and greater therapeutic index than MMC.

Association of liposome-encapsulated trivalent antimonial with ascorbic acid: an effective and safe strategy in the treatment of experimental visceral leishmaniasis

BACKGROUND

Visceral leishmaniasis (VL) is a chronic debilitating disease endemic in tropical and subtropical areas, caused by protozoan parasites of the genus Leishmania. Annually, it is estimated the occurrence of 0.2 to 0.4 million new cases of the disease worldwide. Considering the lack of an effective vaccine the afflicted population must rely on both, an accurate diagnosis and successful treatment to combat the disease. Here we propose to evaluate the efficacy of trivalent antimonial encapsulated in conventional liposomes, in association with ascorbic acid, by monitoring its toxicity and efficacy in BALB/c mice infected with Leishmania infantum.

METHODOLOGY/PRINCIPAL FINDINGS

Infected mice were subjected to single-dose treatments consisting in the administration of either free or liposome-encapsulated trivalent antimony (SbIII), in association or not with ascorbic acid. Parasite burden was assessed in the liver, spleen and bone marrow using the serial limiting dilution technique. After treatment, tissue alterations were examined by histopathology of liver, heart and kidney and confirmed by serum levels of classic biomarkers. The phenotypic profile of splenocytes was also investigated by flow cytometry. Treatment with liposome-encapsulated SbIII significantly reduced the parasite burden in the liver, spleen and bone marrow. Co-administration of ascorbic acid, with either free SbIII or its liposomal form, did not interfere with its leishmanicidal activity and promoted reduced toxicity particularly to the kidney and liver tissues.

CONCLUSIONS/SIGNIFICANCE

Among the evaluated posological regimens treatment of L. infantum-infected mice with liposomal SbIII, in association with ascorbic acid, represented the best alternative as judged by its high leishmanicidal activity and absence of detectable toxic effects. Of particular importance, reduction of parasite burden in the bone marrow attested to the ability of SbIII-carrying liposomes to efficiently reach this body compartment.

Nanoparticle targeting to diseased vasculature for imaging and therapy

Significant challenges remain in targeting drugs to diseased vasculature; most important being rapid blood flow with high shear, limited availability of stable targets, and heterogeneity and recycling of cellular markers. We developed nanoparticles (NPs) to target degraded elastic lamina, a consistent pathological feature in vascular diseases. In-vitro organ and cell culture experiments demonstrated that these NPs were not taken up by cells, but instead retained within the extracellular space; NP binding was proportional to the extent of elastic lamina damage. With three well-established rodent models of vascular diseases such as aortic aneurysm (calcium chloride mediated aortic injury in rats), atherosclerosis (fat-fed apoE-/- mice), and vascular calcification (warfarin + vitamin K injections in rats), we show precise NPs spatial targeting to degraded vascular elastic lamina while sparing healthy vasculature when NPs were delivered systemically. Nanoparticle targeting degraded elastic lamina is attractive to deliver therapeutic or imaging agents to the diseased vasculature.

FROM THE CLINICAL EDITOR

This novel work focuses on nanoparticle targeting of degraded elastic lamina in a variety of diseases, including atherosclerosis, vascular calcification, and aneurysm formation, and demonstrates the feasibility to deliver therapeutic or imaging agents to the diseased vasculature.

Liposome encapsulation of zoledronic acid results in major changes in tissue distribution and increase in toxicity

BACKGROUND

Zoledronic acid (Zol) is a potent inhibitor of farnesyl-pyrophosphate synthase with broad clinical use in the treatment of osteoporosis, and bone metastases. We have previously shown that encapsulation of Zol in liposomes targeted to the folate receptor (FR) greatly enhances its in vitro cytotoxicity. To examine whether targeted liposomal delivery of Zol could be a useful therapeutic approach, we investigated here the in vivo pharmacology of i.v. administered liposomal Zol (L-Zol) in murine models.

METHODS

Zol was passively entrapped in the water phase of liposomes containing a small fraction of either dipalmitoyl-phosphatidylglycerol (DPPG) or a polyethylene-glycol (PEG)-conjugated phospholipid with or without insertion of a folate lipophilic conjugate. Radiolabeled formulations were used for pharmacokinetic (PK) and biodistribution studies. Toxicity was evaluated by clinical, hematological, biochemical, and histopathological parameters. Therapeutic studies comparing free Zol, nontargeted and folate targeted L-Zol were performed in FR-expressing human tumor models.

RESULTS

Encapsulation of Zol in liposomes resulted in major PK changes including sustained high plasma levels and very slow clearance. DPPG-L-Zol was cleared faster than PEG-L-Zol. Grafting of folate lipophilic conjugates on liposomes further accelerated the clearance of Zol. L-Zol caused a major shift in drug tissue distribution when compared to free Zol, with a major increase (20 to 100-fold) in liver and spleen, a substantial increase (7 to 10-fold) in tumor, and a modest increase (2-fold) in bone. Liposomal formulations proved to be highly toxic, up to 50-fold more than free Zol. PEG-L-Zol was more toxic than DPPG-L-Zol. Toxicity was non-cumulative and appears to involve macrophage/monocyte activation and release of cytokines. Co-injection of L-Zol with a large dose of blank liposomes, or injection of a very low Zol-to-phospholipid ratio liposome formulation reduced toxicity by 2-4-fold suggesting that diluting macrophage exposure below a threshold Zol concentration is important to overcome toxicity. L-Zol failed to significantly enhance the therapeutic activity of Zol vis-à-vis free ZOL and doxorubicin. Folate-targeted L-Zol was marginally better than other treatment modalities in the KB tumor model but toxic deaths greatly affected the outcome.

CONCLUSIONS

Liposome delivery of Zol causes a major change in tissue drug distribution and an increase in tumor Zol levels. However, the severe in vivo toxicity of L-Zol seriously limits its dose and its utility for in vivo tumor cell targeting. This strategy is under evaluation using liposomes carrying less toxic bisphosphonates.

Uptake and intracellular processing of PEG-liposomes and PEG-immunoliposomes by kupffer cells in vitro 1 *

Specific targeting of drugs to for instance tumors or sites of inflammation may be achieved by means of immunoliposomes carrying site-specific antibodies on their surface. The presence of these antibodies may adversely affect the circulation kinetics of such liposomes as a result of interactions with cells of the mononuclear phagocyte system (MPS), mainly represented by macrophages in liver and spleen. The additional insertion of poly(ethylene glycol) chains on the surface of the immunoliposomes may, however, attenuate this effect. We investigated the influence of surface-coupled rat or rabbit antibodies and of PEG on the uptake of liposomes by rat Kupffer cells in culture with (3)H-cholesteryloleyl ether as a metabolically stable marker. Additionally, we assessed the effects of surface-bound IgG and PEG on the intracellular processing of the liposomes by the Kupffer cells, based on a double-label assay using the (3)H-cholesteryl ether as an absolute measure for liposome uptake and the hydrolysis of the degradable marker cholesteryl-(14)C-oleate as relative measure of degradation. Attachment of both rat and rabbit antibodies to PEG-free liposomes caused a several-fold increase in apparent size. The uptake by Kupffer cells, however, was 3-4 fold higher for the rat than for the rabbit IgG liposomes. The presence of PEG drastically reduced the difference between these liposome types. Uptake of liposomes without antibodies amounted to only about 10% (non-PEGylated) or less (PEGylated) of that of the immunoliposomes. In contrast to the marked effects of IgG and PEG on Kupffer cell uptake, the rate of intracellular processing of the liposomes remained virtually unaffected by the presence of these substances on the liposomal surface. These observations are discussed with respect to the design of optimally formulated liposomal drug preparations, combining maximal therapeutic efficacy with minimal toxicity.

Hepatoprotective and neuroprotective activity of liposomal quercetin in combating chronic arsenic induced oxidative damage in liver and brain of rats

CONTEXT

Arsenic is a naturally occurring toxicant that causes acute and chronic adverse health effects, including cancer.

OBJECTIVE

The study was performed to evaluate the therapeutic efficacy of liposome entrapped flavonoidal quercetin in combating arsenic toxicity mediated oxidative damage in hepatocytes and brain cells in rat model.

MATERIALS AND METHODS

Hepatic and neuronal cell damage in rats was made by daily arsenic (6 mg/kg b wt, 9 mg/kg b wt and 12 mg/kg b wt) treatment via oral route for four consecutive months. Liposomal quercetin (2.71 mg QC/kg b. wt) were injected s.c. on rats treated with 12 mg/kg b. wt. NaAsO(2) twice a week for four months.

RESULTS AND DISCUSSION

Inorganic arsenic deposition was found to be most significant in hepatic (9.32 ± 0.100 µg/g tissue) and neuronal (6.21 ± 0.090 µg/g tissue) cells of rats treated with 12 mg/kg b wt of arsenite. Antioxidant levels in hepatic and neuronal cells were reduced significantly by the induction of arsenic. Liposomal quercetin was found most potent for a complete prevention of arsenite-induced reduction in antioxidant levels in the liver and brain of rats. Arsenic induced a substantial increase in hepatic hydroxyproline (HP) and Liposomal quercetin treatment resulted in complete replenishment of the HP level to normal. Liposomal quercetin completely prevented the arsenite-induced upregulation of cytochrome c expression in liver and brain significantly suggesting that the protective effect of Liposomal quercetin could be related to the reduction of arsenic deposition in both the organs.

CONCLUSION

Thus, Liposomal quercetin might prove to be of therapeutic potential against arsenite-induced hepatic and neuronal cell damage in rats.

Intracellular targeting delivery of liposomal drugs to solid tumors based on EPR effects

The success of an effective drug delivery system using liposomes for solid tumor targeting based on EPR effects is highly dependent on both size ranging from 100-200 nm in diameter and prolonged circulation half-life in the blood. A major development was the synthesis of PEG-liposomes with a prolonged circulation time in the blood. Active targeting of immunoliposomes to the solid tumor tissue can be achieved by the Fab' fragment which is better than whole IgG in terms of designing PEG-immunoliposomes with prolonged circulation. For intracellular targeting delivery to solid tumors based on EPR effects, transferrin-PEG-liposomes can stay in blood circulation for a long time and extravasate into the extravascular of tumor tissue by the EPR effect as PEG-liposomes. The extravasated transferrin-PEG-liposomes can maintain anti cancer drugs in interstitial space for a longer period, and deliver them into the cytoplasm of tumor cells via transferrin receptor-mediated endocytosis. Transferrin-PEG-liposomes improve the safety and efficacy of anti cancer drug by both passive targeting by prolonged circulation and active targeting by transferrin.

Nanocarriers' entry into the cell: relevance to drug delivery

Nanocarriers offer unique possibilities to overcome cellular barriers in order to improve the delivery of various drugs and drug candidates, including the promising therapeutic biomacromolecules (i.e., nucleic acids, proteins). There are various mechanisms of nanocarrier cell internalization that are dramatically influenced by nanoparticles' physicochemical properties. Depending on the cellular uptake and intracellular trafficking, different pharmacological applications may be considered. This review will discuss these opportunities, starting with the phagocytosis pathway, which, being increasingly well characterized and understood, has allowed several successes in the treatment of certain cancers and infectious diseases. On the other hand, the non-phagocytic pathways encompass various complicated mechanisms, such as clathrin-mediated endocytosis, caveolae-mediated endocytosis and macropinocytosis, which are more challenging to control for pharmaceutical drug delivery applications. Nevertheless, various strategies are being actively investigated in order to tailor nanocarriers able to deliver anticancer agents, nucleic acids, proteins and peptides for therapeutic applications by these non-phagocytic routes.

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.

Testis specific lactate dehydrogenase as target for immunoliposomes

PROBLEM

Is it possible to deliver therapeutic agents to testis through specific targeting?

METHOD OF STUDY

Immunoliposomes are designed by incorporating antibodies to lactate dehydrogenase-C4 (LDH-C(4)), which is the product of a testis specific gene. Their targeting and delivering ability is investigated in vitro and in vivo.

RESULTS

It is observed that LDHC(4)-immunoliposomes are able to discriminate and recognize antigens on spermatozoa and testes both in vitro and in vivo.

CONCLUSION

Specific targeting through LDH-C(4) appears to be a feasible strategy for delivering therapeutic as well as anti-spermatogenic agents to testis.

The use of immunoliposome for nutrient target regulation (a review)

Although research on the role of genetically engineered antibodies and liposomes in the immunology or the nutrition field is extensive, there is no case for immunoliposome to nutrient target regulation. It is known that liposomes are spherical particles that encapsulate a fraction of the solvent, in which they freely diffuse (float) into their interior. Therefore, identification of immunoliposomes in hypothalamic site or intestinal epithelial cells that are differentially regulated by liposomes encapsulating nutrients or drugs will be an important step toward understanding the role of immunoliposomes in nutrition regulation progression and ingredient stability. Consequently, a useful model (immunoliposomal nutrient delivery system, ILNDS) of nutrient target regulation via immunoliposomes is designed to regulate the endocrine system effectively. This review focuses on antibody libraries' construction, display and selection, a brief introduction of immunoliposome, and how to use ILNDS for nutrient target regulation.

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.

Current methods for attaching targeting ligands to liposomes and nanoparticles

Liposomes and nanoparticles have emerged as versatile carrier systems for delivering active molecules in the organism. These colloidal particles have demonstrated enhanced efficacy compared to conventional drugs. However, the design of liposomes and nanoparticles with a prolonged circulation time and ability to deliver active compounds specifically to target sites remains an ongoing research goal. One interesting way to achieve active targeting is to attach ligands, such as monoclonal antibodies or peptides, to the carrier. These surface-bound ligands recognize and bind specifically to target cells. To this end, various techniques have been described, including covalent and noncovalent approaches. Both in vitro and in vivo studies have proved the efficacy of the concept of active targeting. The present review summarizes the most common coupling techniques developed for binding homing moieties to the surface of liposomes and nanoparticles. Various coupling methods, covalent and noncovalent, will be reviewed, with emphasis on the major differences between the coupling reactions, on their advantages and drawbacks, on the coupling efficiency obtained, and on the importance of combining active targeting with long-circulating particles.

Advanced strategies in liposomal cancer therapy: problems and prospects of active and tumor specific drug release

Tumor specific drug delivery has become increasingly interesting in cancer therapy, as the use of chemotherapeutics is often limited due to severe side effects. Conventional drug delivery systems have shown low efficiency and a continuous search for more advanced drug delivery principles is therefore of great importance. In the first part of this review, we present current strategies in the drug delivery field, focusing on site-specific triggered drug release from liposomes in cancerous tissue. Currently marketed drug delivery systems lack the ability to actively release the carried drug and rely on passive diffusion or slow non-specific degradation of the liposomal carrier. To obtain elevated tumor-to-normal tissue drug ratios, it is important to develop drug delivery strategies where the liposomal carriers are actively degraded specifically in the tumor tissue. Many promising strategies have emerged ranging from externally triggered light- and thermosensitive liposomes to receptor targeted, pH- and enzymatically triggered liposomes relying on an endogenous trigger mechanism in the cancerous tissue. However, even though several of these strategies were introduced three decades ago, none of them have yet led to marketed drugs and are still far from achieving this goal. The most advanced and prospective technologies are probably the prodrug strategies where non-toxic drugs are carried and activated specifically in the malignant tissue by overexpressed enzymes. In the second part of this paper, we review our own work, exploiting secretory phospholipase A2 as a site-specific trigger and prodrug activator in cancer therapy. We present novel prodrug lipids together with biophysical investigations of liposome systems, constituted by these new lipids and demonstrate their degradability by secretory phospholipase A2. We furthermore give examples of the biological performance of the enzymatically degradable liposomes as advanced drug delivery systems.

Glucuronate-Modified, Long-Circulating Liposomes for the Delivery of Anticancer Agents

Liposomes are useful as drug carriers in drug delivery systems, especially for drugs with severe side effects such as antitumor agents. The conventional formulations of liposomes are opsonized by plasma proteins in the bloodstream and trapped in the reticuloendothelial system (RES). Therefore, liposomes with reduced opsonization are expected to have prolonged circulation and to accumulate in tumor tissue due to the leaky endothelium of the tissue. To avoid RES trapping of liposomes, two approaches have been considered. Liposomes may mimic cells circulating in the blood to escape host recognition as foreign substances, or liposomes may be covered with a hydrophilic barrier to escape recognition. For the latter purpose, poly(ethylene glycol) is widely used. For the former purpose, here we focus on the characteristics, in vivo trafficking, and usage in cancer therapy of glucuronate-modified liposomes. Glucuronate-modified liposomes bind to a lower extent to macrophage-like cells in vitro and passively accumulate in tumor tissue evaluated by a technique using positron emission tomography. Glucuronate-modified liposomes with extended circulation are useful for delivering anticancer agents to tumors and reducing the toxic side effects of the agents.

Effect of sialyl Lewis X-glycoliposomes on the inhibition of E-selectin-mediated tumour cell adhesion in vitro

The aim of this study was to evaluate the potential of different types of sialyl Lewis X-conjugated liposomes as competitive inhibitors for tumour cell adhesion to endothelial E-selectin. Sterically stabilised liposomes with the sLeX ligand at the terminal end of the polyethyleneglycol (PEG) chain, as well as vesicles that had the ligand embedded within the PEG-layer, were compared to ligand-bearing liposomes without sterical stabilisation. First, 14 different tumour cell lines were characterised for their expression of sialyl Lewis X and/or A. Tumour cell adhesion was characterised in three static assays in vitro using: (i) immobilised E-selectin, (ii) CHO cells, transfected to express E-selectin and (iii) human umbilical vein endothelial cells (HUVEC). Sterically stabilised liposomes with the ligand at the terminal end of the polyethylene chain were the most effective inhibitors in all three assays and inhibited the adhesion of HT29 colon- and Lewis lung (LL) carcinoma cells by about 60-80%. The binding was not affected by a PEG-coating of the liposomes. Sterical stabilisation, on the other hand, completely prevented macrophage uptake (J774 cell line) independently of the presence of the ligand, while plain liposomes were taken up in an amount of 5.4 nmol liposomal lipids/10(6) macrophages.

Ligand-targeted liposomal anticancer drugs

Antibody or ligand-mediated targeting of liposomal anticancer drugs to antigens expressed selectively or over-expressed on tumor cells is increasingly being recognized as an effective strategy for increasing the therapeutic indices of anticancer drugs. This review summarizes some recent advances in the field of ligand-targeted liposomes (LTLs) for the delivery of anticancer drugs. New approaches used in the design and optimization of LTLs is discussed and the advantages and potential problems associated with their therapeutic applications are described. New technologies are widening the spectrum of ligands available for targeting and are allowing choices to be made regarding affinity, internalization and size. The time is rapidly approaching where we will see translation of anticancer drugs entrapped in LTLs to the clinic.

Efficient intracellular delivery of 5-fluorodeoxyuridine into colon cancer cells by targeted immunoliposomes

Immunoliposomes, liposomes with monoclonal antibodies attached, are being developed for targeting the anti-cancer drug 5-fluoro-2'-deoxyuridine (FUdR) to colon cancer cells. A monoclonal antibody against the rat colon carcinoma CC531 was covalently coupled to liposomes containing a dipalmitoylated derivative of the anti-cancer drug FUdR (FUdR-dP) as a prodrug in their bilayers. We studied the association with the tumor cells of different types of immunoliposomes varying in the position and orientation of the antibody at the liposome surface. We also assessed the in vitro anti-tumor activity of these liposomes and the mechanism by which the active drug FUdR is delivered intracellularly. Specific binding of the immunoliposomes to the tumor cells was observed. Immunoliposomes containing FUdR-dP caused a much stronger inhibition of CC531 cell growth in vitro than FUdR-dP in non-targeted liposomes. After binding to the cell surface only limited amounts of the immunoliposomes were internalized. By contrast, already within 24 h immunoliposome-incorporated FUdR-dP was hydrolyzed virtually completely to the parent drug FUdR, intracellularly. The mechanism of intracellular delivery of the drug most likely involves a selective transfer of the lipophilic prodrug from the liposomes to the cell membrane and subsequent intracellular processing. In conclusion, we developed a targeted liposomal formulation, which is able to deliver FUdR to colon carcinoma cells intracellularly with high efficiency, without the need for the cells to internalize the liposomes as such. This approach may be attractive for other lipophilic anti-cancer (pro)drugs. In this sense our system also serves as a model for the development of new lipid-based drug delivery systems for anti-cancer therapy.

Pharmacokinetics of differently designed immunoliposome formulations in rats with or without hepatic colon cancer metastases

PURPOSE

Compare pharmacokinetics of tumor-directed immunoliposomes in healthy and tumor-bearing rats (hepatic colon cancer metastases).

METHODS

A tumor cell-specific monoclonal antibody was attached to polyethyleneglycol-stabilized liposomes, either in a random orientation via a lipid anchor (MPB-PEG-liposomes) or uniformly oriented at the distal end of the PEG chains (Hz-PEG-liposomes). Pharmacokinetics and tissue distribution were determined using [3H]cholesteryloleylether or bilayer-anchored 5-fluoro[3H]deoxyuridine-dipalmitate ([3H]FUdR-dP) as a marker.

RESULTS

In healthy animals clearance of PEG-(immuno)liposomes was almost log-linear and only slightly affected by antibody attachment; in tumor-bearing animals all liposomes displayed biphasic clearance. In normal and tumor animals blood elimination increased with increasing antibody density; particularly for the Hz-PEG-liposomes, and was accompanied by increased hepatic uptake, probably due to increased numbers of macrophages induced by tumor growth. The presence of antibodies on the liposomes enhanced tumor accumulation: uptake per gram tumor tissue (2-4% of dose) was similar to that of liver. Remarkably, this applied to tumor-specific and irrelevant antibody. Increased immunoliposome uptake by trypsin-treated Kupffer cells implicated involvement of high-affinity Fc-receptors on activated macrophages.

CONCLUSIONS

Tumor growth and immunoliposome characteristics (antibody density and orientation) determine immunoliposome pharmacokinetics. Although with a long-circulating immunoliposome formulation, efficiently retaining the prodrug FUdR-dP, we achieved enhanced uptake by hepatic metastases, this was probably not mediated by specific interaction with the tumor cells, but rather by tumor-associated macrophages.

Stealth monensin immunoliposomes as potentiator of immunotoxins in vitro

Stealth monensin liposomes (SML) were prepared using dipalmitoyl phosphatidylcholine, cholesterol, distearoyl glycerophosphoethanolamine coupled to polyethylene glycol, stearylamine, and N-succinimidyl pyridodithiopropionate linked to stearyl amine, in the molar ratio of 10:5:1.4:1.4:1.5. SML was conjugated to the anti-MY9 antibody by a disulfide linkage to form stealth monensin immunoliposomes (SMIL) by an already established procedure. The encapsulation concentrations of monensin in SML and SMIL were 10(-7) and 4.9x10(-8) M, respectively. More than 20% of monensin remained in circulation after 24 h in BALB/c mice. The ability of SML and SMIL to potentiate the effect of anti-MY9 immunotoxin (anti-MY9-IT) was tested against human leukemia HL-60 sensitive and resistant tumor cells in vitro. SML and SMIL potentiated the activity of anti-MY9-IT by 10-20 times against HL-60 sensitive tumor cell lines. However, greater potentiation of anti-MY9-IT was observed in combination with SML and SMIL against HL-60 resistant tumor cells, found to be 200 and 500 times, respectively. The potentiation of anti-MY9-IT by SMIL was more than two-fold compared with SML against both HL-60 sensitive and resistant tumor cells. Transmission electron microscopy studies conducted with HL-60 resistant cells incubated with anti-MY9-IT and monensin liposomes showed significant dilation of the golgi, which was reversible after re-incubation in fresh medium. Our studies show that SML and SMIL can be successfully used to potentiate the activity of ricin based anti-MY9-IT in vitro, and further in vivo studies will demonstrate the usefulness of this approach.

Liposomes bearing polyethyleneglycol-coupled transferrin with intracellular targeting property to the solid tumors in vivo

PURPOSE

The purpose of this study was to determine the usefulness of transferrin (TF)-pendant-type polyethyleneglycol (PEG)-liposomes (TF-PEG-liposomes), in which TF was covalently linked to the distal terminal of PEG chains on the external surface of PEG-liposomes as a carrier for in vivo cytoplasmic targeting to tumor cells.

METHODS

Small unilamellar TF-PEG-liposomes (100-140 nm in diameter) were prepared from DSPC, CH, DSPE-PEG, and DSPE-PEG-COOH (2:1:0.11:0.021, molar ratio), and were conjugated to TF via the carboxyl residue of DSPE-PEG-COOH. The intracellular targeting ability of TF-PEG-liposomes to tumor cells was examined in vitro and in Colon 26 tumor-bearing mice.

RESULTS

TF-PEG-liposomes, bearing approximately 25 TF molecules per liposome, readily bound to mouse Colon 26 cells in vitro and were internalized by receptor-mediated endocytosis. TF-PEG-liposomes showed a prolonged residence time in the circulation and low RES uptake in Colon 26 tumor-bearing mice, resulting in enhanced extravasation of the liposomes into the solid tumor tissue. Electron microscopic studies in Colon 26 tumor-bearing mice revealed that the extravasated TF-PEG-liposomes were internalized into tumor cells by receptor-mediated endocytosis.

CONCLUSION

TF-PEG-liposomes had the capabilities of specific receptor binding and receptor-mediated endocytosis to target cells after extravasation into solid tumors in vivo. Such liposomes should be useful for in vivo cytoplasmic targeting of chemotherapeutic agents or plasmid DNAs to target cells.

Uptake of long-circulating immunoliposomes, directed against colon adenocarcinoma cells, by liver metastases of colon cancer

Radiolabeled ([3H]cholesteryloleyl ether) immunoliposomes directed against rat colon adenocarcinoma CC531 cells were prepared by random coupling of a tumor cell-specific antibody, CC52, via a thio ether bond. In vitro binding experiments demonstrated a saturable and specific interaction of CC52-immunoliposomes, which could be inhibited by free non-coupled CC52 but not by irrelevant antibodies. The in vivo targeting potential of CC52-immunoliposomes, which were pegylated to achieve prolonged circulation times, was tested in an established rat liver CC531 metastasis model. Twenty-four hours after injection of the liposomes, 25% of the CC52-immunoliposomes were still present in the blood, which was comparable with the control liposomes (either with or without antibody). Liposomes were mainly taken up from the blood by the liver and the spleen, although hepatic uptake of the immunoliposomes was higher and splenic uptake was lower as compared to liposomes without antibody. Within the metastatic tumor nodules in the liver, uptake of both the CC52-immunoliposomes and non-specific immunoliposomes was significantly higher than that of control liposomes without antibody. Visualization of fluorescently or gold labeled CC52-immunoliposomes revealed that, although targeting to liver metastases was achieved, the immunoliposomes were mostly not associated with tumor cells but rather localized in tumor associated cells, probably macrophages.

Liposome targeting to tumors using vitamin and growth factor receptors

Liposome-encapsulated anticancer drugs reveal their potential for increased therapeutic efficacy and decreased nonspecific toxicities due to their ability to enhance the delivery of chemotherapeutic agents to solid tumors. Advances in liposome technology have resulted in the development of ligand-targeted liposomes capable of selectively increasing the efficacy of carried agents against receptor-bearing tumor cells. Receptors for vitamins and growth factors have become attractive targets for ligand-directed liposomal therapies due to their high expression levels on various forms of cancer and their ability to internalize after binding to the liposomes conjugated to receptors' natural ligands (vitamins) or synthetic agonists (receptor-specific antibodies and synthetic peptides). This chapter summarizes various strategies and advances in targeting liposomes to vitamin and growth factor receptors in vitro and in vivo with special emphasis on two extensively studied liposome-targeting systems utilizing folate receptor and HER2/neu growth factor receptor.

The effects of polyethyleneglycol (PEG)-derived lipid on the activity of target-sensitive immunoliposome

In this study, serum stability and target-sensitivity of phosphatidylethanolamine (PE) immunoliposomes prepared with dioleoylphosphatidylethanolamine (DOPE), HYB-241 monoclonal antibody that targets p-glycoproteins, and various levels of polyethyleneglycol 2000 dioleoylphosphatidylethanolamine (PEG(2000)-DOPE) were determined. Incubation of calcein-laden pegylated immunoliposomes prepared with different levels of PEG(2000)-DOPE (0.3, 0.5 and 1.0 mol%) with p-glycoprotein rich bovine brain microvessel endothelial cells in 10% serum cell culture medium, all resulted in time-dependent release of calcein from the liposomes. The release of calcein was greatest for immunoliposomes prepared with 0.3 mol% PEG(2000)-DOPE (66% in 1 h). Contrarily, the release of calcein from the other two immunoliposomes reached only approximately 10-3% after same period of incubation. When serum-induced leakage of calcein was investigated for the above liposome preparations, liposomes prepared with 0.3 and 0.5 mol% PEG(2000)-DOPE had the highest leakage level (10% in 1 h). Contrarily, the release of calcein from liposomes prepared with 1.0 mol% PEG(2000)-DOPE reached only 3% after same period of incubation. Together, it would appear that release of calcein from the immunoliposomes prepared with 0.3 mol% PEG(2000)-DOPE is a result of both serum-induced and target-induced destabilization of liposomes. The net release of calcein due to target-induced destabilization of liposomes is calculated to be at approximately 56%. In contrast, there is no target-induced leakage of calcein from immunoliposomes prepared with either 0.5 or 1.0 mol% PEG(2000)-DOPE.

Possibility of active targeting to tumor tissues with liposomes

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

Immunoliposomes for the targeted delivery of antitumor drugs

This review presents an overview of the field of immunoliposome-mediated targeting of anticancer agents. First, problems that are encountered when immunoliposomes are used for systemic anticancer drug delivery and potential solutions are discussed. Second, an update is given of the in vivo results obtained with immunoliposomes in tumor models. Finally, new developments on the utilization of immunoliposomes for the treatment of cancer are highlighted.

Selective transfer of a lipophilic prodrug of 5-fluorodeoxyuridine from immunoliposomes to colon cancer cells

A monoclonal antibody against the rat colon carcinoma CC531 was covalently coupled to liposomes containing a dipalmitoylated derivative of the anticancer drug FUdR as a prodrug in their bilayers. We investigated the in vitro interaction of these liposomes with CC531 target cells and the mechanism by which they deliver the active drug FUdR intracellularly to the cells by monitoring the fate of the liposomal bilayer markers cholesterol-[(14)C]oleate and [(3)H]cholesteryloleylether as well as the (3)H-labeled prodrug and colloidal gold as an encapsulated liposome marker. After binding of the immunoliposomes to the cell surface, only limited amounts were internalized as demonstrated by a low level of hydrolysis of liposomal cholesterol ester and by morphological studies employing colloidal gold-labeled immunoliposomes. By contrast, already within 24 h immunoliposome-incorporated FUdR-dP was hydrolyzed virtually completely to the parent drug FUdR intracellularly. This process was inhibited by a variety of endocytosis inhibitors, indicating that the prodrug enters and is processed by the cells by a mechanism involving an endocytic process, resulting in intracellular FUdR concentrations up to 3000-fold higher than those in the medium. Immunoliposomes containing poly(ethyleneglycol) (PEG) chains on their surface, with the antibody coupled either directly to the bilayer or at the distal end of the PEG chains were able to deliver the prodrug into the tumor cells at the same rate as immunoliposomes without PEG. Based on these observations, we tentatively conclude that during the interaction of the immunoliposomes with the tumor cells the lipophilic prodrug FUdR-dP is selectively transferred to the cell surface and subsequently internalized by constitutive endocytic or pinocytic invaginations of the plasma membrane, thus ultimately delivering the prodrug to a lysosomal compartment where hydrolysis and release of parent drug takes place. This concept allows for an efficient delivery of a liposome-associated drug without the need for the liposome as such to be internalized by the cells.

Antiproliferative effect of immunoliposomes containing 5-fluorodeoxyuridine-dipalmitate on colon cancer cells

We have investigated the antiproliferative action towards CC531 colon adenocarcinoma cells of target cell-specific immunoliposomes containing the amphiphilic dipalmitoyl derivative of 5-fluorodeoxyuridine (FUdR-dP). FUdR-dP incorporated in immunoliposomes caused a 13-fold stronger inhibition of CC531 cell growth in vitro, during a 72-h treatment, than FUdR-dP in liposomes without antibody, demonstrating that the prodrug is efficiently hydrolysed to yield the active drug, FUdR, intracellularly. The intracellular release of active FUdR was confirmed by determining the fate of 3H-labelled immunoliposomal FUdR-dP. Treatments shorter than 72 h with FUdR-dP in immunoliposomes resulted in anti-tumour activities comparable to, or even higher than, that of free FUdR. The shorter treatments reflect more closely the in vivo situation and illustrate the potential advantage of the use of immunoliposomes over non-targeted liposomal FUdR-dP or free FUdR. Association of tumour cell-specific immunoliposomes with CC531 cells was up to tenfold higher than that of liposomes without antibody or with irrelevant IgG coupled, demonstrating a specific interaction between liposomes and target cells which causes an efficient intracellular delivery of the drug. Since biochemical evidence indicates a lack of internalization or degradation of the liposomes as such, we postulate that entry of the drug most likely involves the direct transfer of the prodrug from the immunoliposome to the cell membrane during its antigen-specific interaction with the cells, followed by hydrolysis of FUdR-dP leading to relatively high intracellular FUdR-levels. In conclusion, we describe a targeted liposomal formulation for the anticancer drug FUdR, which is able to deliver the active drug to colon carcinoma cells with high efficiency, without the need for the cells to internalize the liposomes as such.

Targetability of novel immunoliposomes prepared by a new antibody conjugation technique

In order to develop long-circulating immunoliposomes (IL), which combine sterical stabilization with a superior targetability, we have introduced a new methodology for attaching monoclonal antibodies directly onto the distal ends of liposome-grafted polyethylene glycol (PEG) chains. Therefore, we have synthesized a new PEG-PE derivative, which had been endgroup-functionalized with cyanuric chloride. Antibodies can simply be coupled to this membrane anchor in mild basic conditions (pH 8.8) without the need for previous antibody derivatizations. The coupling results have been determined with consideration to various liposome parameters and have been compared to several established antibody coupling procedures, where antibodies had been linked directly to the liposome surface in the presence of PEG (conventional IL). To investigate the targetability of the resulting new IL, anti E-selectin mAb have been coupled and the degree of binding selectin-containing cells has been analyzed. The terminal coupled antibodies show a 1.8-fold higher degree of in vitro cell binding compared to conventional IL, which has been attributed to the antibody position being more easy accessible at the PEG termini. Furthermore, we have illustrated the liposome surface topology and the coupled antibodies by atomic force microscopy, which for such fluid IL has been used first. These images have finely corresponded to the cell binding results, and have been discussed in terms of antibody position and flexibility at the liposome surface.

Fusion of sendai virus with liposome depends on only F protein, but not HN protein

Sendai virus is able to fuse with liposomes even without virus receptors. To determine the roles of envelope protein, hemagglutinin-neuraminidase (HN) and fusion (F) protein, in Sendai virus-liposome fusion, we treated the virus with proteases and examined its fusion with liposomes and the conditions of HN and F protein. Sodium dodecyl sulfate-polyacrylamide gel electrophoresis and western blotting analysis showed that the virus treated with 150 units/ml of trypsin, which inactivated selectively hemolysis activity, maintained intact HN, F and partially digested F (32 kDa) protein, while virus treated with 15,000 units/ml of trypsin, which inactivated both hemolysis and neuraminidase activity, had only a 15-kDa digested HN protein and completely digested F protein. The former fused with liposomes, but the latter did not. In the virus treated with chymotrypsin, which lost both hemolysis and neuraminidase activity, F protein was intact, while HN protein was degraded to 15 kDa; in this case the virus fused with liposomes. As the virus with 15-kDa HN protein fused with liposomes and that with 20-kDa protein did not, HN protein does not appear to play any role in virus-liposome fusion. The virus that fused with liposomes had intact F protein. We conclude that Sendai virus-liposome fusion is strongly dependent on the presence of intact F protein, but not HN protein.

Ligand-targeted liposomes

Liposomes have gained increased attention as systemic drug delivery vehicles following recent regulatory approvals of several vesicle-formulated drugs. These products have demonstrated improved therapeutic indices over their corresponding conventional drugs by avoiding sensitive tissues and/or increasing delivery to specific targets in vivo. They have achieved these improvements primarily through physical means: (1) by retaining drug within vesicles while in the circulation, thus avoiding or minimizing uptake by sensitive normal tissues; and (2) by selectively extravasating into target tissues, releasing active drug. In order to improve upon these therapies in the future, clinically active liposome delivery systems most likely will need to include site-directed surface ligands to further enhance their selective delivery. This may be crucial for the in vivo transport and delivery of macromolecules, including antisense, oligonucleotide aptamers, and genes, which-unlike most conventional drugs-do not circulate well and often require cellular uptake by fusion, endocytosis, or other processes to reach their active sites. This manuscript reviews technologies applicable to directing liposomes and their contents to selected in vivo targets using surface-bound, site-specific ligands. Presented are the biological barriers to be overcome, criteria for selecting the determinants to be targeted, various targeting ligands and overall delivery system design considerations. Several novel targets as well as novel ligand constructs for site-directed therapy are reviewed and discussed. Systemic liposome therapy, which currently must be administered by the intravenous route, is the principal focus of this analysis.

Brain drug delivery of small molecules using immunoliposomes

Immunoliposomes (antibody-directed liposomes) were used in the present study for delivery of the antineoplastic agent daunomycin to the rat brain. A coupling procedure was introduced, which allows conjugation of a thiolated antibody to maleimide-grafted 85-nm liposomes sterically stabilized with PEG. Antibody was thereby coupled to the terminal end of a PEG-conjugated linker lipid. No brain uptake of PEG-conjugated liposomes carrying [3H]daunomycin was observed. However, brain targeting of immunoliposomes carrying [3H]daunomycin was mediated by the OX26 monoclonal antibody to the rat transferrin receptor, which is selectively enriched at the brain microvascular endothelium that comprises the blood-brain barrier in vivo. Coupling of 30 OX26 antibodies per liposome resulted in optimal brain delivery. Saturation of delivery was observed at higher antibody densities. Determination of brain levels of immunoliposomes over 24 h revealed that immunoliposomes accumulate in brain tissue. Brain targeting of immunoliposomes was not observed in immunoliposomes conjugated with a mouse IgG2a isotype control. In addition, coinjection of free OX26 saturated plasma clearance of immunoliposomes. Since a single liposome may carry > or = 10,000 drug molecules, the use of PEG-conjugated immunoliposomes increases the drug carrying capacity of the monoclonal antibody by up to 4 logarithmic orders in magnitude. In summary, specific OX26-mediated targeting of daunomycin to the rat brain was achieved by the use of an immunoliposome-based drug delivery system.

Targeted delivery of doxorubicin via sterically stabilized immunoliposomes: pharmacokinetics and biodistribution in tumor-bearing mice

PURPOSE

To evaluate benefits in tumor localization, availability, and noncancerous organ distribution of doxorubicin (DOX) delivered via small (< or = 120 nm) sterically stabilized immunoliposomes targeted against a tumor-associated antigen in fibrosarcoma-bearing mice.

METHODS

DOX-loaded liposomes were prepared with (i) specific monoclonal IgG3 antibody (32/2, D-SSIL-32/2); (ii) non-specific IgG3 (D-SSIL-IgG); or (iii) no IgG (D-SSL) on their surface. Equal DOX amounts were injected intravenously via each type of liposome into BALB/c mice carrying experimental lung metastases of a polyoma virus-induced fibrosarcoma (A9 ctc 220) expressing a polyoma virus-induced tumor-associated antigen (PAA) on their surface. Metastases occurred mainly in lung. Mice were treated at 3 stages of tumor development (micrometastases, medium-size metastases, and large, necrotic metastases). Performance evaluation was based on time-dependent quantification of DOX and DOX metabolites (DOX-M) in lung tumor, noncancerous organs, and plasma.

RESULTS

(i) DOX delivered via both SSIL retained the prolonged circulation time typical of DOX delivered via D-SSL. (ii) DOX accumulation in noncancerous organs was similar for all preparations. Low levels of DOX-M were obtained for all three preparations in all organs except liver, suggesting a similar processing. (iii) Preparations differed in behavior in lung tumor depending on tumor size and microanatomy. Only at the micrometastases stage were the specifically targeted D-SSIL-32/2 superior to D-SSL and D-SSIL-IgG, delivering 2-4 times more drug into the tumor. (iv) DOX-M level in all three tumor stages was in the following order: D-SSIL-32/2 > > D-SSL > > D-SSIL-IgG, suggesting that DOX delivered as D-SSIL-32/2 is most available to tumor cells.

CONCLUSIONS

The advantage of specific targeting of sterically stabilized liposomes is expressed mainly in increasing availability of DOX to tumor cells in a way which is dependent on tumor microanatomy. The impact of this advantage to therapeutic efficacy remains to be determined.

Heterogeneity in secretory responses of rat liver macrophages of different size

Four subpopulations of hepatic macrophages, differing in size, were isolated from rat liver. The secretion of nitric oxide (NO), tumor necrosis factor-alpha (TNF-alpha), prostaglandin E (PGE) and interleukin-1 (IL-1) by freshly isolated as well as cultured cells was studied after in vitro stimulation with the immunomodulator muramyl dipeptide (MDP) in free or liposome-encapsulated form. Freshly isolated liver macrophages could be induced to secrete significant levels of NO, TNF-alpha, PGE and IL-1. The extent of secretion, however, varied substantially between macrophages of different size. The highest levels of secretion of TNF-alpha, PGE and IL-1 were observed in the fraction containing the large-size macrophages, while progressively lower levels of secretion were observed with decreasing size. In contrast, the highest levels of NO secretion were observed by small macrophages and steadily decreased with increasing size. Hepatic macrophages of different size displayed differences in secretory potential during in vitro culture. The ability of small liver macrophages to secrete NO, TNF-alpha, or PGE, following activation with MDP, gradually increased with time in culture. In contrast, large liver macrophages gradually lost their secretory ability after 1-2 days and the intermediate-size cells after 2-3 days in culture. This functional heterogeneity in secretory properties among rat liver macrophages of different size is discussed with reference to their potential role and significance in host defense against metastatic tumor growth in the liver.

Histochemical and electron microscopic characterization of hepatic macrophage subfractions isolated from normal and liposomal muramyl dipeptide treated rats

Subfractions of the hepatic macrophage population, differing in cell size, were isolated from normal rats and rats treated with liposomal muramyl dipeptide (lipMDP) and analyzed histochemically and by ultrastructural peroxidase cytochemistry. The majority of cells in all subfractions of control rats displayed the ultrastructural endogenous peroxidase pattern of resident liver macrophages and showed positive staining with the general macrophage markers nonspecific esterase (NSE) and monoclonal antibody ED1. Heterogeneity in intensity of NSE and ED1 staining was observed among macrophages of different size. Generally, the intensity of NSE and ED1 staining decreased with decreasing cell size. After injection of lipMDP, we observed the appearance of a discrete subpopulation of cells in the liver in addition to the resident macrophages. These cells, containing a nucleus with a characteristic shape, were predominantly recovered in the small-sized fractions and were characterized by an immature ultrastructural macrophage morphology (no or only a few lysosomes and phagosomes) and a lack of ED1 reactivity, NSE, and endogenous peroxidase. We suggest an important role for these cells in lipMDP induced antitumor capacity of the liver.

Targetability of novel immunoliposomes modified with amphipathic poly(ethylene glycol)s conjugated at their distal terminals to monoclonal antibodies

Distearoyl-N-(3-carboxypropionoyl poly(ethylene glycol) succinyl)phosphatidylethanolamine (DSPE-PEG-COOH) was newly synthesized and used to prepare novel immunoliposomes carrying monoclonal antibodies at the distal ends of the PEG chains (Type C). Liposomes were prepared from egg phosphatidylcholine (ePC) and cholesterol (CH) (2;1, m/m) containing 6 mol% of DSPE-PEG-COOH, and a monoclonal IgG antibody, 34A, which is highly specific to pulmonary endothelial cells, was conjugated to the carboxyl groups of DSPE-PEG-COOH to give various amounts of antibody molecules per liposome. Other immunoliposomes with PEG coating (Type B) or without PEG coating (an earlier type of immunoliposome, Type A) were prepared for comparison. The average molecular weight of PEG in Type B or C immunoliposomes was 2000. Type B and Type C liposomes without antibodies showed prolonged circulation time and reduced reticulo-endothelial system (RES) uptake owing to the presence of PEG. These three different types of 34A-immunoliposomes with 30-35 antibody molecules per vesicle were injected into mice to test the immunotargetability to the lung. The efficiency of lung binding of 34A-Type B was one-half of that of 34A-Type A, though a large amount of 34A-Type B remained in the blood circulation for a long time, suggesting that the steric hindrance of PEG chains reduced not only the immunospecific antibody-antigen binding, but also the RES uptake. The degree of lung binding of 34A-Type C was about 1.3-fold higher than that of 34A-Type A, indicating that recognition by the antibodies attached to the PEG terminal was not sterically hindered and that the free PEG (i.e., that not carrying antibody) was effective in increasing the blood concentration of immunoliposomes by enabling them to evade RES uptake. The latter phenomenon was confirmed by using nonspecific antibody-Type C immunoliposomes (14-Type C), which showed a high blood level for a long time. Our approach provides a simple means of conjugating antibodies directly to the distal end of PEG which is already bound to the liposome membrane, and should contribute to the development of superior targetable drug delivery vehicles for use in diagnostics and therapy.

Enhanced hepatic uptake of liposomes through complement activation depending on the size of liposomes

The objective of this study was to differentiate the roles of opsonins and phagocytic cells in the size-dependent hepatic uptake of liposomes in the submicron region. The extent of opsonization decreased with the decrease in size of liposomes (from 800 to 200 nm in diameter) and no enhancement of uptake was observed at 200 nm. There was no effect of liposome size on the uptake of unopsonized liposomes. Serum was pretreated with empty liposomes of each size and its opsonic activity was measured in the perfused liver. The small liposomes could not consume the opsonic activity, while the larger ones did so substantially. These results suggest that opsonins bind to liposomes depending on the size of liposomes and phagocytic cells take up liposomes in proportion to the extent of opsonization. Size-dependent liposome degradation in serum was also found, which was consistent with the size-dependent complement activation, because liposomes with this composition have been shown to be degraded by complement. The mechanism of opsonization was examined by treating serum at 56 degrees C for 30 min or with anti-C3 antiserum. Since both treatments inhibited the opsonic activity, the hepatic uptake of liposomes is considered to occur via complement receptor. In conclusion, the size of liposomes affected complement recognition, and the liposomes were taken up by the liver depending on the extent of opsonization.

Surface-modified liposomes: assessment and characterization for increased stability and prolonged blood circulation

Advances in therapeutic applications of liposomes have been achieved through surface modifications increasing their biological stability: reduced constituent exchange and leakage as well as reduced unwanted uptake by cells of the mononuclear phagocytic system. The recent conclusions obtained from in vivo and in vitro studies are reviewed with an emphasis on evaluating the methods used and thus the kinds of conclusions which can be drawn. A number of issues are raised as to the limitations of the methods employed. Steric stabilization, meaning reduction in particle interactions by a surface steric barrier, has been proposed as a theoretical basis for the results and some of the initial results testing this hypothesis are reviewed here with respect to identification of the extent to which physical properties of the surface coatings correlate with the biological properties. At this time it seems that no one method is ideal so that multiple measures give the best characterization.

67Gallium-labeled liposomes with prolonged circulation: preparation and potential as nuclear imaging agents

A method is described for 67Ga-labeling liposomes containing a polyethylene glycol coating which exhibit prolonged blood circulation, reduced liver and spleen uptake and accumulation in tumors. Applications as agents for diagnostic imaging and delivery of therapeutic agents are considered. Previous methods were adapted to compensate for the presence of low temperature phase transition phospholipids resulting in consistent loading with low levels of residual unentrapped label.