The influence of coupling transferrin to liposomes or minibeads on its uptake and fate in leukemic L2C cells

WT1 shRNA delivery using transferrin-conjugated PEG liposomes in an in vivo model of melanoma

The global incidence of melanoma is increasing. Mortality from melanoma is influenced primarily by metastasis in advanced stages of the disease. Current treatments are largely ineffective; thus, novel gene delivery approaches that target tumor-specific markers may be useful for the treatment of melanoma. Systemic administration of encapsulated RNA-interference plasmids targeted against tumor cells is a potential alternative therapy for cancer. Formulations of transferrin (Tf)-conjugated polyethylene glycol (PEG) liposomes loaded with short hairpin RNA (shRNA) against WT1 (Lip + RNAi + Tf), PEG liposomes loaded with shRNA against WT1 (Lip + RNAi), Tf-conjugated PEG liposomes loaded with pEGFP-N3 (Lip + GFP + Tf) and saline solution as negative control (untreated) were administered systemically to C57BL/6 mice implanted subcutaneously with a melanoma cell line. Tumor volume, body weight, tumor weight, survival and relative expression of WT1 were evaluated. No significant differences in net body weight were identified between groups. The tumor volume decreased from 7,871 mm³ (SD±2,087) in the untreated group to 5,981 mm³ (SD±2,099) in the Lip + RNAi + Tf group. The tumor weight was reduced, from 8.8 g (SD±0.30) in the untreated group to 5.5 g (SD±0.87) in the Lip + RNAi + Tf group. An increase of 37% in survival was also observed in the group treated with Lip + RNAi + Tf in comparison to the untreated group. Tumors treated with Lip + RNAi + Tf also showed a decrease in the mean relative expression of WT1 of 0.21 (SD±0.28) folds compared with 1.8 (SD±2.49) folds in untreated group, 1.34 (SD±0.43) folds in Lip + RNAi group and of 1.89 (SD±0.69) folds in Lip + GFP + Tf group. Systemic administration of transferrin-conjugated PEG liposomes loaded with shRNA against WT1 reduced WT1 expression and tumor size and increased survival.

Designing of 'intelligent' liposomes for efficient delivery of drugs

The liposome- vesicles made by a double phospholipid layers which may encapsulate aqueous solutions- have been introduced as drug delivery vehicles due to their structural flexibility in size, composition and bilayer fluidity as well as their ability to incorporate a large variety of both hydrophilic and hydrophobic compounds. With time the liposome formulations have been perfected so as to serve certain purposes and this lead to the design of "intelligent" liposomes which can stand specifically induced modifications of the bilayers or can be surfaced with different ligands that guide them to the specific target sites. We present here a brief overview of the current strategies in the design of liposomes as drug delivery carriers and the medical applications of liposomes in humans.

Therapeutic efficacy of anti-ErbB2 immunoliposomes targeted by a phage antibody selected for cellular endocytosis

Many targeted cancer therapies require endocytosis of the targeting molecule and delivery of the therapeutic agent to the interior of the tumor cell. To generate single chain Fv (scFv) antibodies capable of triggering receptor-mediated endocytosis, we previously developed a method to directly select phage antibodies for internalization by recovering infectious phage from the cytoplasm of the target cell. Using this methodology, we reported the selection of a panel of scFv that were internalized into breast cancer cells from a nonimmune phage library. For this work, an immunotherapeutic was generated from one of these scFv (F5), which bound to ErbB2 (HER2/neu). The F5 scFv was reengineered with a C-terminal cysteine, expressed at high levels in Escherichia coli, and coupled to sterically stabilized liposomes. F5 anti-ErbB2 immunoliposomes were immunoreactive as determined by surface plasmon resonance (SPR) and were avidly internalized by ErbB2-expressing tumor cell lines in proportion to the levels of ErbB2 expression. F5-scFv targeted liposomes containing doxorubicin had antitumor activity and produced significant reduction in tumor size in xenografted mice compared to nontargeted liposomes containing doxorubicin. This strategy should be applicable to generate immunotherapeutics for other malignancies by selecting phage antibodies for internalization into other tumor types and using the scFv to target liposomes or other nanoparticles.

Binding and uptake of transferrin-bound liposomes targeted to transferrin receptors of endothelial cells

The use of liposomes as carriers for site-specific delivery is an attractive strategy, especially for the vascular endothelium that by position is an accessible target for drug and gene delivery via the blood circulation. The aim of this study was to detect whether liposomes coupled to transferrin (Tf)-bound and are taken up by aortic endothelial cells (EC) following the pathway of Tf interaction with transferrin receptors, reportedly expressed on their cell membrane. To this purpose, small unilamellar liposomes of different compositions, either classical (C) or sterically stabilized (SS), have been prepared, characterized and coupled with transferrin (Tf-liposomes). To assess the binding and uptake, cultured EC were incubated with fluorescently labelled Tf-liposomes for various times intervals (from 5 min to 24 h) at 4 and 37 degrees C, and further investigated by flow cytometry, fluorimetry and fluorescence microscopy. The results showed that: (i) binding of Tf-liposomes to EC was specific; (ii) the EC binding of SS-Tf-liposomes was lower than that of C-Tf-liposomes; and (iii) after 30 min of incubation, both C- and SS-Tf-liposomes appeared localized in the acidic compartments of the cells. Together, the data indicate that transferrin-bound liposomes are specifically taken up by EC by a receptor-mediated mechanism employing the pathway of surface-exposed Tf receptors.

Targeting of ultrasmall superparamagnetic iron oxide (USPIO) particles to tumor cells in vivo by using transferrin receptor pathways

Human transferrin was covalently coupled to ultrasmall superparamagnetic iron oxide (USPIO) particles, and the transferrin-USPIO obtained was investigated in vivo in experimental SMT/2A tumor-bearing rats (rat mammary carcinoma). Physicochemical characterization showed an overall size of 36 nm (DLS) with a core size of 5 nm (TEM). Relaxivities were R1 = 23.6 and R2 = 52.1 liter/mmol.s (0.47 T). Bound transferrin was 280 micrograms/mg of iron. Pharmacokinetic investigations revealed a half-life of 17 min in normal rats. The MR evaluation of tumor signal intensity over time showed a 40% (range 25-55%) signal reduction 150 min after injection with the reduction persisting for at least 8 h. Control experiments using the parent USPIO compound or USPIO labeled with a nonspecific human serum albumin (HSA-USPIO) showed a change of only 10% (range 5-15%) in tumor signal intensity over time. The results demonstrate that a combination of the USPIO relaxivity properties with the specificity of transferrin-mediated endocytosis allows in vivo detection of tumors by MR imaging.

Sterically stabilized anti-HER2 immunoliposomes: design and targeting to human breast cancer cells in vitro

Liposomes (70-100 nm) of 1-palmitoyl-2-oleoylphosphatidylcholine, cholesterol, and poly(ethylene glycol) (PEG)-modified phosphatidylethanolamine (PEG-DSPE) were conjugated to Fab' fragments of a humanized recombinant MAb against the extracellular domain of HER2/neu to create sterically stabilized immunoliposomes (anti-HER2 SL) as a drug carrier targeting HER2-overexpressing cancers. Conjugation employed maleimide-terminated membrane-anchored spacers of two kinds: a short spacer, providing attachment of Fab' close to the liposome bilayer, or a long spacer, with Fab' attachment at the distal terminus of the PEG chain. Confocal microscopy and spectrofluorometry of HER2-overexpressing breast cancer cells incubated with fluorescently labeled anti-HER2 SL prepared with either spacer showed binding of liposomes (8000-23000 vesicles/cell) followed by endocytosis (rate constant ke = 0.012-0.033 min-1) via the coated-pit pathway, evidenced by intracellular acidification and colocalization with transferrin. Uptake of anti-HER2 immunoliposomes by breast cancer cells with low HER2 expression, or after preincubation of cells with free anti-HER2 Fab', was less than 0.2% and 4.3%, respectively, of the uptake by HER2-overexpressing cells. Increasing PEG-DSPE content (up to 5.7 mol %) in anti-HER2-SL prepared with the short spacer decreased liposome-cell binding affinity 60-100-fold, while ke decreased only 2-fold; however, when Fab' fragments were conjugated via a PEG spacer, both binding affinity and ke were unaffected by PEG-DSPE content. Cell binding and internalization of anti-HER2 immunoliposomes increased at higher surface density of conjugated Fab' fragments, reaching plateaus at approximately 40 Fab'/liposome for binding and approximately 10-15 Fab'/liposome for internalization. Uptake of anti-HER2 immunoliposomes correlated with the cell surface density of HER2 and significantly (p < 0.005) correlated with the antiproliferative effect of the targeting antibody but not with the total level of cellular HER2 expression. The results obtained were used to optimize in vivo preclinical studies of anti-HER2 SL loaded with antineoplastic drugs.

Characterization of in vivo immunoliposome targeting to pulmonary endothelium

Two rat monoclonal antibodies, 34A and 201B, which specifically bind to a surface glycoprotein (gp112) of the pulmonary endothelial cell surface, have been coupled to unilamellar liposomes of approximately 0.25 microns in diameter. The 34A- and 201B-liposomes (monoclonal antibodies 273-34A and 411-201B, respectively), but not antibody-free liposomes and liposomes coupled to 14, a nonspecific monoclonal antibody, accumulate efficiently (approximately 30% injected dose) in the lung of mice which have been injected via the tail vein. Immunoliposome targeting to lung is demonstrated both by using a 125I-labeled lipid marker and an entrapped water-soluble marker. Lung accumulation of 34A-liposomes is completely blocked by a preincubation of free antibody 34A, but not antibody 14, indicating that the immunoliposome accumulation at the target site is immunospecific. Time course studies have revealed that 34A-liposomes bind to lung antigens within 1 min after injection, indicating that the target binding takes place during the first few passages of immunoliposomes through the lung capillary bed. Unbound immunoliposomes are taken up by liver and spleen within 3-5 min after injection. The level of lung accumulation increases significantly as the protein:lipid ratio of the immunoliposome increases. Approximately 50% of injected dose is accumulated in lung for 34A-liposomes, with an average of 935 antibody molecules per liposome. Immunoliposomes of larger size accumulate in lung more significantly than those of smaller size. Injection with higher doses also enhances the level of lung accumulation.(ABSTRACT TRUNCATED AT 250 WORDS)

Mechanisms of delivery of liposome-encapsulated cytosine arabinoside to CV-1 cells in vitro. Fluorescence-microscopic and cytotoxicity studies

Fluorescence microscopy and assays of the cytotoxicity of liposome-encapsulated cytosine arabinoside (araC) have been used to examine the interactions of CV-1 cells with pH-sensitive liposomes, combining phosphatidylethanolamine (PE) with oleic acid or with double-chain protonatable amphiphiles, and with pH-insensitive liposomes combining phosphatidylcholine (PC) and phosphatidylglycerol (PG). Fluorescence-microscopic observations indicate that double-chain protonatable amphiphiles remain tightly associated with pH-sensitive liposomes during incubations with CV-1 cell monolayers, and that cellular uptake of liposomes is strongly promoted by transferrin coupled to the liposome surface. Liposome-encapsulated araC showed much greater cytotoxicity toward CV-1 cells than did the free drug at equivalent concentrations under the same conditions. The cytotoxicity of encapsulated araC was strongly enhanced by liposome-conjugated transferrin and was maximal using pH-sensitive liposomes combining PE with the double-chain protonatable amphiphile N-(N'-oleoyl-2-aminopalmitoyl)serine. However, the drug was also markedly more cytotoxic when encapsulated in other types of transferrin-conjugated liposomes, including pH-insensitive PC/PG/cholesterol liposomes, than in the free form. The cytotoxicity of liposome-encapsulated araC is significantly attenuated by the nucleoside transport inhibitor nitrobenzothioinosine, and fluorescence microscopy using calcein-containing liposomes provides no evidence for efficient fusion between cellular membranes and any of the types of liposomes examined here. Based on these observations, we suggest that the major mechanism for cytoplasmic delivery of liposome-encapsulated araC is the carrier-mediated transport of drug that has been released from liposomes into the endosomal and/or the lysosomal compartments.

Covalent modification of serum transferrin with phospholipid and incorporation into liposomal membranes

A method is described for incorporation of water-soluble proteins into liposomal membranes using covalent protein-phospholipid conjugates in detergent solution. A disulfide derivative of phosphatidylethanolamine containing a reactive N-hydroxysuccinimide ester group is synthesized, and the derivative is reacted with serum transferrin in deoxycholate-containing buffer. Disulfide-linked transferrin-phosphatidylethanolamine conjugates containing up to 6 mol phospholipid/mol protein are prepared. The amphiphilic conjugates have solubility properties very similar to integral membrane proteins. The conjugates self-associate to form protein micelles of narrow size distribution (Stokes radii 6-7 nm), and in the presence of excess phospholipid (egg phosphatidylcholine), they readily incorporate into liposomal membranes upon removal of detergent. Stable incorporation into liposomes requires the introduction of two molecules of phosphatidylethanolamine into the transferrin. Using the disulfide linker to release transferrin from the liposomes, evidence is presented for a function of the phosphatidylethanolamine as an anchor-molecule into the liposomal lipid. Optimal conditions for preparation of homogeneous liposomes with diameters in the range 30-125 nm and with a varying content of transferrin are defined. The liposomes appear well suited for studies on liposome-cell membrane interactions.