Accumulation Of Liposomes Research Articles

Determinants for in vivo anti-tumor effects of PEG liposomal doxorubicin: Importance of vascular permeability within tumors

To elucidate the determinants of the in vivo anti-tumor efficacy of polyethylene glycol (PEG)-modified liposomal doxorubicin (DOX), we examined its anti-tumor effect against three different tumor cell lines (Lewis lung cancer (LLC), Colon-26 (C26) and B16BL6 melanoma (B16)) in vitro and in vivo. In vitro, LLC was the most sensitive tumor to DOX and liposomal DOX based on the MTT assay. However, the strongest in vivo anti-tumor effect was observed in the C26 tumor-bearing mice. The in vivo accumulation of radiolabelled PEG liposome in the C26 tumor after intravenous injection was significantly larger than in other tumors. The extent of vascularity assessed by immunohistochemical staining of CD31 was not directly related with the tumor accumulation of PEG liposome. On the other hand, Evans blue extravasation and secretion of VEGF in C26 tumors were higher than in LLC tumors, clearly demonstrating that the vasculature permeability was higher within C26 tumors. These results indicated that the vascular permeability within the tumor substantially affects the tumor accumulation of PEG liposome and may be one of the important determinants in the in vivo anti-tumor efficacy of PEG liposomal DOX.

Inhibition of proinflammatory genes in anti-GBM glomerulonephritis by targeted dexamethasone-loaded AbEsel liposomes

E-selectin-directed targeted drug delivery was analyzed in anti-glomerular basement membrane glomerulonephritis. Liposomes conjugated with anti-E-selectin antibodies (Ab(Esel) liposomes) were internalized by activated endothelial cells in vitro through E-selectin-mediated endocytosis. At the onset of glomerulonephritis in mice, E-selectin was expressed on glomerular endothelial cells, which resulted in homing of Ab(Esel) liposomes to glomeruli after intravenous administration. Accumulation of Ab(Esel) liposomes in the kidney was 3.6 times higher than nontargeted IgG liposomes, whereas the accumulation of both liposomes in the clearance organs liver and spleen and in heart and lungs was comparable. In glomeruli, the Ab(Esel) liposomes colocalized with the endothelial cell marker CD31. Quantitative RT-PCR analysis of laser-microdissected arterioles, glomeruli, and postcapillary venules demonstrated that targeted delivery of dexamethasone by Ab(Esel) liposomes reduced glomerular endothelial expression of P-selectin, E-selectin, and vascular cell adhesion molecule-1 by 60-70%. The expression of these genes was not modulated in endothelial cells in nontargeted renal microvasculatures. Decrease of glomerular endothelial activation at disease onset was followed by reduced albuminuria at day 7. This study demonstrates the potential of vascular bed-specific drug delivery aimed at disease-induced epitopes on the microvascular endothelial cells as a therapeutic strategy for glomerulonephritis.

Targeted delivery of doxorubicin via estrone-appended liposomes

Estrone-appended liposomal formulation of doxorubicin was designed to enhance the capability of clinically used liposomal doxorubicin formulation with the added advantage of delivery of doxorubicin to its destination site, i.e. cancerous cells over-expressing estrogen receptors (ERs). Estrone was conjugated with distearoyl phosphatidylethanolamine (DSPE) using succinic anhydride as a linker and the conjugate was characterized by IR and mass spectroscopies. Estrone-coupled liposomes were prepared with the composition of egg phosphatidylcholine/cholesterol/distearoyl phosphatidylethanolamine–estrone (PC/CHOL/DSPE–ES) at the molar and drug–lipid ratios of 7:3:0.5 and 0.1:1 (w/w), respectively. The average vesicle sizes of the conventional and estrone-appended liposomes were found to be 193 ± 24 and 207 ± 28 nm, respectively. The fluorescent microscopy studies were performed with estrone-appended liposomes loaded with 6-carboxyfluorescein (6-CF). Results of in vivo biodistribution studies showed that estrone-appended liposomes were effectively taken up by cells expressing ERs. The drug uptake study showed that accumulation of ligand-appended liposomes in the breast and uterus was 13.9 and 12.7 times higher when compared with plain drug, and 11.05 and 10.3 times higher when compared with conventional liposomes, respectively, after 8 h of tail vein intravenous administration. The findings are seminal for selective targeting of antineoplastic agents to the ER, which are frequently over-expressed on carcinoma of breast and uterine origin, and opens the promising possibilities for non-immunogenic, site-specific delivery of bioactive(s) to these sites.

Abstract 545: Polyethylene-glycol Coated Liposomal Adenosine Attenuates Hypotension and Bradycardia and Augments Cardioprotection Against Ischemia/reperfusion Injury in Rats - Nano-size Drug Delivery System Targeted to Infarcted Myocardium -

Background Liposome is a nano-size particle that can delay the drug degradation and induce the drug to the specific tissues including ischemia/reperfused myocardium. Coating liposome with polyethylene-glycol (PEG) prolongs the residence time in the blood stream. Although adenosine reduces myocardial infarct size in clinical trials, its half-time is extremely short and a high-dose of adenosine administration causes both hypotension and bradycardia. In this study, we investigated whether PEGylated liposomal adenosine minimizes systemic hemodynamic effects of adenosine and maximize cardioprotective effects against ischemia/reperfusion injury. Methods and Results We obtained PEGylated liposomal adenosine (average diameter: 126.2+/−42.9 nm) by the remote loading method. In rats, the intravenous administration of free adenosine (450 μg/kg/min for 10 minutes) decreased mean blood pressure and heart rate by 22.7% and 15.4% compared with the baseline, respectively. PEGylated liposomal adenosine at the same dose did not significantly change either mean blood pressure or heart rate compared with the baseline. Next, we produced myocardial infarction (MI) in male rats by 30-minutes of ligation of the left anterior descending coronary artery followed by reperfusion. We evaluated the accumulation of liposome in the hearts and infarct size 72 hours after the MI. Fluorescence microscopy revealed the accumulation of fluorescent-labeled PEGylated liposomes in ischemic, but not non-ischemic area. PEGylated liposomal adenosine (450 μg/kg/min), free adenosine (450 μg/kg/min) or free-PEGylated liposome was intravenously administered for 10 minutes starting 5 minutes before the onset of reperfusion. PEGylated liposomal adenosine significantly (p<0.05 each) reduced infarct size (11.4±3.1 %) compared with free adenosine (21.7±3.0%) or free PEGylated liposome (23.6±2.2%). Conculusion PEGylated liposomal adenosine attenuated the systemic hemodynamic effects of adenosine and augmented cardioprotection against ischemia/repefusion injury. Nano-size PEGylated liposomal adenosine is a promising candidate for the adjunctive therapy for AMI.

Cyclic Arg-Gly-Asp Peptide-Labeled Liposomes for Targeting Drug Therapy of Hepatic Fibrosis in Rats

Targeting hepatic stellate cells (HSCs) has been challenging due to the lack of specific receptors or motifs on the cells. The aim of the present study was to develop a HSC-specific system for improving drug delivery to HSCs. The affinity of a cyclic peptide containing Arg-Gly-Asp (cRGD) to collagen type VI receptor on HSCs was examined in both in vitro and in vivo experiments. Sterically stable liposomes (SSLs) were modified with this peptide to yield a new carrier, cRGD-SSL. The targeting efficiency of this carrier in delivering interferon (IFN)-alpha1b was investigated in a rat model of liver fibrosis induced by bile duct ligation (BDL). When incubating HSCs or hepatocytes with cyclic RGD peptide, the peptide was bound preferentially to activated HSCs. Biodistribution study showed that the accumulation of cRGD peptide-labeled liposomes in HSCs isolated from BDL rats was 10-fold more than unlabeled SSLs. BDL rats receiving injections of IFN-alpha1b entrapped in cRGD-SSL exhibited significantly reduced extent of liver fibrosis compared with BDL control rats or BDL rats treated with IFN-alpha1b entrapped in SSLs. Thus, cRGD-SSL is an efficient drug carrier, which selectively targets activated HSCs and improves drug therapy for liver fibrosis to a significant extent. This liposomal formulation represents a new means of targeting drug carrier for the treatment of liver fibrosis, and it may have potential clinical applications.

Use of avidin/biotin-liposome system for enhanced peritoneal drug delivery in an ovarian cancer model

The goal of this study was to determine the distribution of the avidin/biotin-liposome system in an ovarian cancer xenograft model. Optimal avidin/biotin-liposome injection sequence with enhanced liposome accumulation to the peritoneum was determined. Two weeks after NIH:OVCAR-3 cell inoculation, rats were divided into three groups. Group 1 (B-A) ( n = 4), received an intraperitoneal injection of 99mTc-blue-biotin-liposomes 30 min before an intraperitoneal injection of avidin. Group 2 (A-B) ( n = 4), received an intraperitoneal injection of avidin 30 min before an intraperitoneal injection of 99mTc-blue-biotin-liposomes. Group 3 (A-B 2h) ( n = 5), received an intraperitoneal injection of avidin 2 h before an intraperitoneal injection of 99mTc-blue-biotin-liposomes. Three additional non-tumor nude rats served as controls in each group, and were subjected to the same injection sequences. Scintigraphic imaging commenced at various times post 99mTc-blue-biotin-liposome injection. After imaging, rats were euthanized at 23 h post-liposome injection for tissue biodistribution. Images showed no apparent difference in liposome distribution between control and tumor animals. Regional uptake analysis at 4 h for tumor rats showed significantly higher lymphatic channel uptake in the A-B 2h group ( p < 0.05) and a trend of increased peritoneal uptake in A-B group. By 22 h, peritoneal and lymphatic channel uptake was similar for all groups. At necropsy, most activity was found in blue-stained omentum, diaphragm, mediastinal and abdominal nodes. Bowel activity was minimal. These results correlate with previous normal rat studies, and demonstrate potential use of this avidin/biotin-liposome system for prolonging drug delivery to the peritoneal cavity and associating lymph nodes in this ovarian cancer xenograft model.

Accumulation of liposome with Sialyl Lewis X to inflammation and tumor region: Application to in vivo bio-imaging

We prepared the liposome binding Sialyl Lewis X (SLX) on the surface in order to specifically and efficiently deliver substances (fluorescent materials, chemical substances, proteins, genes, etc.) to inflammation or tumor regions. The liposome with SLX (SLX-Lipo-Cy5.5), in which fluorescent substance Cy5.5 was included, was administered intravenously to arthritis or Ehrlich Ascites Tumor (EAT) bearing mouse, and the accumulation of liposome was observed using two types of in vivo fluorescent imaging equipment. The result was that the accumulation of SLX-Lipo-Cy5.5 to inflammation or tumor regions was significantly higher than the control liposome without sugar chain (Lipo-Cy5.5) at 24 and 48 h after administration. In addition, it was confirmed that this accumulation showed a shift of liposome from blood vessels to the surrounding tissues. Thus, it was proven that this liposome is useful not only as an in vivo bio-imaging reagent but also as a drug delivery system (DDS).

Direct comparison of two pegylated liposomal doxorubicin formulations: Is AUC predictive for toxicity and efficacy?

Rationally designed liposomes could improve the therapeutic indexes of chemotherapeutic drugs, which is due to alterations in the pharmacokinetics and biodistribution of encapsulated drugs. For traditional drug delivery systems, the accumulation of drugs in healthy and malignant tissues could be correlated with toxicity and efficacy. Some previous studies also indicate that the higher tumor AUC, the better therapeutic efficacy, suggestive of the possible existence of positive correlation. Are AUC values of liposomal drugs really predictive? For the purpose to address this question, we designed two pegylated liposomal doxorubicin formulations (PLD-75 and PLD-100), which had the same lipid/drug ratio and bilayer composition, but different size and internal ammonium sulfate concentration. In vitro drug retention experiments revealed that drug was released at a faster rate from PLD-75, a small size formulation. The plasma pharmacokinetics of PLD-75 was similar to that of PLD-100, regardless of whether the mice were tumor-free or not. It should be noted, though, that in tumor-bearing mice the plasma doxorubicin level in PLD-75 group was only about 59% of that in PLD-100 group at 48 h post injection. Furthermore, their biodistribution behavior in S-180 tumor-bearing KM mice was significantly different. Compared with animals receiving PLD-100, those receiving PLD-75 showed a 19.2%, 27.8%, and 23.5% decrease in liver ( p < 0.01), spleen ( p < 0.001) and lung ( p < 0.05) AUC, respectively. In other healthy tissues except kidney, the drug deposition also reduced by 10–15%, but the difference was not significant. The tumor AUC after administration of PLD-100 and PLD-75 were 1285.3 ugh/g and 762.0 ugh/g, respectively ( p < 0.001). Maximum drug levels achieved in the tumors were 33.80 μg/g (for PLD-100) and 20.85 μg/g (for PLD-75), and peak tumor concentration was achieved faster in PLD-75 group. However, enhanced drug accumulation does not mean increased antineoplastic effect, and at the same doxorubicin dose level, PLD-75 was more efficacious. As for toxicity studies, PLD-75 caused more rapid and severe body weight loss despite the fact that drug accumulation in healthy tissues was reduced. Our data indicate that liposomal systems are more complicated than conventional drug delivery systems, and it is hard to predict the toxicity and efficacy of liposomal drugs through the measure of liposomal drug accumulation.

A Tracer Dose of Technetium-99m–Labeled Liposomes Can Estimate the Effect of Hyperthermia on Intratumoral Doxil Extravasation

A noninvasive method to monitor intratumoral Doxil delivery in individual patients during targeted tumor therapy is important to predict treatment response. The purpose of this study was to determine if a small tracer dose of technetium-99m (99mTc)-labeled liposomes could be used to quantify the effect of local hyperthermia on intratumoral Doxil extravasation. Experiments were carried out in a rat fibrosarcoma model with transplanted thigh tumors. Liposomes of approximately same size and composition as Doxil were radiolabeled using [technetium-99m (99mTc)]exametazime. Eight treatment groups received either Doxil, a tracer dose or a large dose of 99mTc-labeled liposomes, or a combination of tracer and Doxil, with or without hyperthermia. This design was chosen to assure that coadministration of both liposomal formulations did not influence their intratumoral distribution. Hyperthermia was done for 45 minutes. Scintigraphic images were obtained at 5 and 18 hours. At 18 hours, tumors were removed and gamma counts as well as doxorubicin concentrations were measured. Intratumoral extravasation of the 99mTc-labeled tracer could be imaged scintigraphically under normothermic and hyperthermic conditions. The thermal enhancement ratio was slightly higher for radiolabeled liposomes than for doxorubicin concentration. However, there was a significant positive correlation of intratumoral doxorubicin concentration and intratumoral uptake of the radiolabeled tracer (expressed as percentage of the injected dose per gram of tissue). Coadministration of radiolabeled liposomes did not negatively influence the amount of drug delivered with Doxil. The use of a radiolabeled tracer has potential value to monitor drug delivery and estimate the effect of an intervention aimed to increase liposomal accumulation, such as local hyperthermia.

Enhanced accumulation of long-circulating liposomes modified with the nucleosome-specific monoclonal antibody 2C5 in various tumours in mice: gamma-imaging studies

To further improve tumour targeting and delivery of imaging agents by long-circulating liposomes via the coupling of the anti-cancer monoclonal antibody 2C5 with nucleosome-restricted activity, which can recognize the surface of various tumours but not normal cells and can specifically target pharmaceutical carriers to tumour cells in vitro and in vivo. The 2C5 antibody was attached to the surface of long-circulating PEG-liposomes (LCL) by the post-insertion technique after antibody modification with a single-terminus activated PEG-lipid derivative to yield nucleosome-specific tumour-targeted liposomes. Tumour cell binding of the targeted liposomes was verified both by fluorescence microscopy and by flow cytometry in several cell lines using fluorescently labelled liposomes. 111In-radiolabelled liposomal formulations (prepared using membrane-anchored chelating groups) were used to examine in vivo biodistribution and tumour accumulation of liposomes by direct gamma scintigraphy. The 2C5 antibody-modified LCL demonstrated a three- to eightfold increase in in vitro specific cell binding to various cancer cell lines of diverse origin. 111In-labelled tumour-targeted liposomes demonstrated prolonged circulation and doubled tumour accumulation compared with that of control formulations. Whole-body gamma scintigraphic imaging of mice implanted with different tumours revealed markedly faster (6 h post injection for 2C5-LCL vs 24 h for non-specific analogues) and superior in vivo tumour visualization with 111In-2C5-LCL than with the 2C5-free formulations in tested tumour models. The 2C5 antibody-modified LCL effectively and specifically accumulate in various tumours and can serve as delivery vehicles for imaging agents, allowing for fast and efficient tumour visualization.

Effective Targeting of Liposomes to Liver and Hepatocytes In Vivo by Incorporation of a Plasmodium Amino Acid Sequence

Several species of the protozoan Plasmodium effectively target mammalian liver during the initial phase of host invasion. The purpose of this study was to demonstrate that a Plasmodium targeting amino acid sequence can be engineered into therapeutic nanoparticle delivery systems. A 19-amino peptide from the circumsporozoite protein of Plasmodium berghei was prepared containing the conserved region I as well as a consensus heparan sulfate proteoglycan binding sequence. This peptide was attached to the distal end of a lipid-polyethylene glycol bioconjugate. The bioconjugate was incorporated into phosphatidylcholine liposomes containing fluorescently labeled lipids to follow blood clearance and organ distribution in vivo. When administered intravenously into mice, the peptide-containing liposomes were rapidly cleared from the circulation and were recovered almost entirely in the liver. Fluorescence and electron microscopy demonstrated that the liposomes were accumulated both by nonparenchymal cells and hepatocytes, with the majority of the liposomal material associated with hepatocytes. Accumulation of liposomes in the liver was several hundredfold higher compared to heart, lung, and kidney, and more than 10-fold higher compared to spleen. In liver slice experiments, liposome binding was specific to sites sensitive to heparinase. Incorporation of amino acid sequences that recognize glycosaminoglycans is an effective strategy for the development of targeted drug delivery systems.

High Expression of Antioxidant Proteins in Dendritic Cells

Dendritic cells (DCs) display the unique ability to activate naive T cells and to initiate primary T cell responses revealed in DC-T cell alloreactions. DCs frequently operate under stress conditions. Oxidative stress enhances the production of inflammatory cytokines by DCs. We performed a proteomic analysis to see which major changes occur, at the protein expression level, during DC differentiation and maturation. Comparative two-dimensional gel analysis of the monocyte, immature DC, and mature DC stages was performed. Manganese superoxide dismutase (Mn-SOD) reached 0.7% of the gel-displayed proteins at the mature DC stage. This important amount of Mn-SOD is a primary antioxidant defense system against superoxide radicals, but its product, H(2)O(2), is also deleterious for cells. Peroxiredoxin (Prx) enzymes play an important role in eliminating such peroxide. Prx1 expression level continuously increased during DC differentiation and maturation, whereas Prx6 continuously decreased, and Prx2 peaked at the immature DC stage. As a consequence, DCs were more resistant than monocytes to apoptosis induced by high amounts of oxidized low density lipoproteins containing toxic organic peroxides and hydrogen peroxide. Furthermore DC-stimulated T cells produced high levels of receptor activator of nuclear factor kappaB ligand, a chemotactic and survival factor for monocytes and DCs. This study provides insights into the original ability of DCs to express very high levels of antioxidant enzymes such as Mn-SOD and Prx1, to detoxify oxidized low density lipoproteins, and to induce high levels of receptor activator of nuclear factor kappaB ligand by the T cells they activate and further emphasizes the role that DCs might play in atherosclerosis, a pathology recognized as a chronic inflammatory disorder.

ATP-loaded liposomes effectively protect mechanical functions of the myocardium from global ischemia in an isolated rat heart model

ATP-loaded liposomes (ATP-L) infused into Langendorff-instrumented isolated rat hearts protect the mechanical functions of the myocardium during ischemia/reperfusion. The left ventricular developed pressure (LVDP) at the end of the reperfusion in the ATP-L group recovered to 72% of the baseline (preservation of the systolic function) compared to 26%, 40%, and 51% in the groups treated with Krebs–Henseleit (KH) buffer, empty liposomes (EL), and free ATP (F-ATP), respectively. The ATP-L-treated group also showed a significantly lower left ventricular end diastolic pressure (LVEDP; better preservation of the diastolic function) after ischemia/reperfusion than controls. After incubating the F-ATP and ATP-L with ATPase, the protective effect of the F-ATP was completely eliminated because of ATP degradation, while the protective effect of the ATP-L remained unchanged. Fluorescence microscopy confirmed the accumulation of liposomes in ischemic areas, and the net ATP in the ischemic heart increased with ATP-L. Our results suggest that ATP-L can effectively protect myocardium from ischemic/reperfusion damage.

Novel mechanism of hybrid liposomes‐induced apoptosis in human tumor cells

Hybrid liposomes can be prepared by simply ultrasonicating a mixture of vesicular and micellar molecules in a buffer solution. The physical properties of these liposomes, such as size, membrane fluidity, phase transition temperature and hydrophobicity can be controlled by changing the composition. Hybrid liposomes composed of dimyristoylphosphatidylcholine and polyoxyethylene (10) dodecyl ether were found to inhibit the growth of human promyelocytic leukemia (HL-60) cells without using any drugs. Induction of apoptosis by hybrid liposomes in HL-60 cells was verified on the basis of fluorescence microscopy and flow cytometry analysis, after fusion and accumulation of hybrid liposomes, which was revealed on the basis of microphysiometer. We elucidated the pathways of apoptosis induced by the hybrid liposomes. That is, hybrid liposomes fused and accumulated in tumor cell membranes, and the apoptosis signal first passed through mitochondria, caspase-9 and caspase-3, second through Fas, caspase-8, caspase-3 and then reached the nucleus. Hybrid liposomes themselves can induce apoptosis in human tumor cells along with high inhibitory effects on the growth of tumor cells.

Induction of apoptosis of human lung carcinoma cells by hybrid liposomes containing polyoxyethylenedodecyl ether

Hybrid liposomes can be prepared by simply ultrasonicating a mixture of vesicular and micellar molecules in aqueous solution. A clear solution of hybrid liposomes composed of 90 mol% dimyristoylphosphatidylcholine (DMPC) and 10 mol% polyoxyethylene(23)dodecyl ether (C 12(EO) 23) having a hydrodynamic diameter of 100–120 nm was obtained. Highly inhibitory effects of hybrid liposomes of 90 mol% DMPC/10 mol% C 12(EO) 23 on the growth of human lung carcinoma (RERF-LC-OK and A549) cells without any drugs were obtained. Induction of apoptosis by hybrid liposomes in RERF-LC-OK and A549 cells was verified on the basis of fluorescence microscopy, agarose gel electrophoresis of DNA and flow cytometry. We elucidated the pathway of apoptosis induced by hybrid liposomes as follows: (a) accumulation of hybrid liposomes in tumor cell membrane was revealed using microphysiometer. (b) Reduction of mitochodrial membrane potential and activation of caspase-9, -3 and -8 were obtained, indicating that apoptotic signal by hybrid liposomes should pass through mitochondria and these caspases. It is worthy to note that such a novel mechanism of apoptosis induced by hybrid liposomes without any drugs was performed for the first time in human lung carcinoma cells.

Delivery of liposomes generated from proliposomes using air-jet, ultrasonic, and vibrating-mesh nebulisers

Abstact In this study, particulate-based proliposomes were investigated as a formulation which would form, an isotonic liposome dispersion in situ when nebulised within a medical nebuliser. Transmission electron microscopy and particle size analysis indicated that liposomes were generated from soyaphosphatidylcholine/cholesterol/sucrose proliposomes in air-jet (Pari LC Plus), ultrasonic (Liberty) and vibrating-mesh (Omron NE- U22) nebulisers, without the need for a prolonged hydration step. With jet and vibrating-mesh nebulisers, liposomes were efficiently delivered to the second (lower) stage of a two-stage impinger, indicating such liposomes would be predicted to be delivered to the peripheral airways. Phos- pholipid output from these nebulisers was less than the total mass output, suggesting some accumulation of liposomes in the nebuliser reservoir. The ultrasonic nebuliser delivered less than 6% of available phospholipid to the impinger, demonstrating that this device was inappropriate for delivering a therapeutic proliposome formulation for pulmonary administration. These results show that proliposomes offer a means of producing stable formulations which can readily generate isotonic liposome formulations in situ for delivery to the airways using either an air-jet or vibrating-mesh nebuliser.

Anti-neovascular therapy by use of tumor neovasculature-targeted long-circulating liposome

For the purpose of cancer anti-neovascular therapy (ANET), we previously isolated 5-mer peptide Ala–Pro–Arg–Pro–Gly (APRPG) that specifically bound to the tumor angiogenic site and observed that APRPG-modified liposomes encapsulating adriamycin were effective for the suppression of tumor in tumor-bearing mice. Since polyethylene glycol (PEG) modification of liposomes endows them with a future of long circulation, we modified liposomes with PEG and APRPG-conjugated distearoylphosphatidylethanolamine (DSPE-PEG-APRPG) and examined the applicability of the liposomes on ANET. Liposomes containing DSPE-PEG-APRPG not only specifically bound to vascular endothelial growth factor-stimulated human umbilical vein endothelial cells in vitro, but also showed long-circulating characteristic and enhanced accumulation in tumor in vivo. Furthermore, adriamycin-encapsulated liposomes modified with APRPG-PEG caused more efficient tumor growth suppression than adriamycin-encapsulated liposomes modified with PEG alone in Colon 26 NL-17 carcinoma-bearing mice, despite not so much different accumulation of both liposomes in the tumor. These data suggest that tumor neovasculature-targeted long-circulating liposomes encapsulating anti-cancer drugs effectively eradicate cancerous cells through damaging of angiogenic endothelial cells. ANET promises no drug resistance and is expected to be effective against essentially any kind of solid tumors. The present results demonstrate the beneficial usage of APRPG-PEG for the active-targeting of drug carriers to angiogenic site in the novel modality of tumor treatment, namely ANET.

Preparation, characterization, and biodistribution study of technetium-99m -labeled leuprolide acetate-loaded liposomes in Ehrlich ascites tumor-bearing mice.

The purpose of this study was to prepare conventional and sterically stabilized liposomes containing leuprolide acetate in an attempt to prolong the biological half life of the drug, to reduce the uptake by reticuloendothelial system (RES), and to reduce the injection frequency of intravenously administered peptide drugs. The conventional and sterically stabilized liposomes containing leuprolide acetate were prepared by reverse phase evaporation method and characterized for entrapment efficiency and particle size. Radiolabeling of leuprolide acetate and its liposomes was performed by direct labeling with reduced technetium-99m. Its biodistribution and imaging characteristics were studied in ehrlich ascites tumor (EAT)-bearing mice after labeling with technetium-99m. The systemic pharmacokinetic studies were performed in rabbits. A high uptake by tumor was observed by sterically stabilized liposome containing leuprolide acetate compared with free drug and conventional liposomes. The liver/tumor uptake ratio of free drug, conventional (LL), and sterically stabilized liposomes (SLL5000 and SLL2000) was found to be 20, 7.99, 1.63, and 1.23, respectively, which showed the increased accumulation of sterically stabilized liposomes in tumor compared with the free drug and conventional liposomes at 24 hours postinjection. Liver uptake of sterically stabilized liposomes was still 7-fold less than the conventional liposomes. The marked accumulation of liposomes in the tumor-bearing mice was also documented by gamma scintigraphic studies. The findings demonstrate the distribution of these liposomes within solid tumor and prove that the sterically stabilized liposomes experience increased tumor uptake and prolonged circulation half life. Hence these findings will be relevant for the optimal design of long circulating liposomes for the peptide drugs and for targeting of liposomes toward tumor.

Protamine enhances uptake of cationic liposomes in angiogenic microvessels of solid tumours

Cationic liposomes have been shown to target angiogenic endothelial cells of solid tumours. Supposing a charge-related mechanism might be responsible for liposome-endothelial interaction, we investigated the effect of intravenous pre-injection of the charged molecules protamine, a polycationic protein, and fucoidan, a polyanionic polysaccharide on the accumulation of cationic liposomes within the blood vessels of a solid tumour. Experiments were performed using the amelanotic hamster melanoma A-Mel-3 growing in a dorsal skinfold chamber of hamsters. Accumulation of fluorescently-labelled cationic liposomes was quantified by intravital macroscopy and digital image analysis of tumour (t) and surrounding normal host tissue (n) over an observation period of 6 h. All animals received an i.v. injection of cationic liposomes. Animals of the control group were pre-treated with an i.v. injection of 0.9% saline, while animals of group 2 received positively charged protamine and animals of group 3 negatively charged fucoidan prior to liposome injection. In control animals i.v. injection of cationic liposomes revealed a preferential targeting of the tumour vessels, indicated by a maximal t/n ratio of 2.2 +/- 0.24 and a maximal fluorescence intensity (fmax) corresponding to the tumour of 66 +/- 12 [% standard]. While there were no significant differences of liposome accumulation within normal host tissue, accumulation of cationic liposomes within the tumour was significantly enhanced after the pre-administration of protamine (fmax: 117 +/- 12 [% standard]). The t/n ratio was significantly increased in protamine pre-treated animals (5.3 +/- 1.7) in comparison to control and fucoidan treated animals. In contrast, pre-injection of fucoidan resulted in reduced maximal fluorescence intensities in tumour (47 +/- 8 [% standard]) and normal surrounding host tissue. Pre-administration of protamine increases the accumulation of cationic liposomes in a solid tumour animal model causing an increased selectivity of cationic liposomes in targeting angiogenic microvessels.

Application of purging biotinylated liposomes from plasma to elucidate influx and efflux processes associated with accumulation of liposomes in solid tumors

Although small, 100-nm liposomes are known to selectively accumulate in solid tumors, the individual contributions of liposome influx and egress rates are not well understood. The aim of this work was to determine influx and efflux kinetics for 100-nm, 1,2-distearoyl- sn-glycero-3-phosphocholine (DSPC)/cholesterol (Chol) liposomes by inducing aggregate formation of biotinylated liposomes upon administering avidin. Injecting 50 μg of neutravidin intravenously to mice that had previously been administered 100 mg/kg DPSC/Chol liposomes containing 0.5 mol% biotin-conjugated lipid resulted in >90% elimination of the liposomes from plasma within 1 h. This rapid removal by the reticuloendothelial system (RES) permitted the determination of the tumor efflux kinetics due to negligible tumor influx after neutravidin injection. The tumor efflux rate constant ( k −1) was determined to be 0.041 h −1 when neutravidin was injected 4 h after liposome injection. This allowed the determination of the tumor influx rate constant ( k 1), which under these conditions was 0.022 h −1. Therefore, DSPC/Chol liposomal accumulation, in LS180 solid tumors, is dictated primarily by plasma liposome concentrations and liposome egress is comparable or slightly faster than influx into the tumors. This method is applicable for a wide range of lipid doses, and can be used to characterize influx and efflux parameters at different time points after accumulation. The application, therefore, has the potential to be used to fully characterize the impact of different liposome parameters such as lipid composition, steric stabilization, size and dose on tumor accumulation kinetics.