Stable target-sensitive immunoliposomes

Membrane adhesion and the formation of heterogeneities: biology, biophysics, and biotechnology

Membrane adhesion is essential to many vital biological processes. Sites of membrane adhesion are often associated with heterogeneities in the lipid and protein composition of the membrane. These heterogeneities are thought to play functional roles by facilitating interactions between proteins. However, the causal links between membrane adhesion and membrane heterogeneities are not known. Here we survey the state of the field and indicate what we think are understudied areas ripe for development.

Improving intracellular doxorubicin delivery through nanoliposomes equipped with selective tumor cell membrane permeabilizing short-chain sphingolipids

PURPOSE

To improve nanoliposomal-doxorubicin (DoxNL) delivery in tumor cells using liposome membrane-incorporated short-chain sphingolipids (SCS) with selective membrane-permeabilizing properties. DoxNL bilayers contained synthetic short-chain derivatives of known membrane microdomain-forming sphingolipids; C₈-glucosylceramide (C₈-GluCer), C₈-galactosylceramide (C₈-GalCer) or C₈-lactosylceramide (C₈-LacCer).

METHODS

DoxNL enriched with C₈-GluCer or C₈-GalCer were developed, optimized and characterized with regard to size, stability and drug retention. In vitro cytotoxic activity was studied in a panel of human tumor cell lines and normal cells. Intracellular Dox delivery was measured by flow cytometry and visualized by fluorescence microscopy. For a further understanding of the involved drug delivery mechanism confocal microscopy studies addressed the cellular fate of the nanoliposomes, the SCS and Dox in living cells.

RESULTS

C₈-LacCer-DoxNL aggregated upon Dox loading. In tumor cell lines SCS-DoxNL with C₈-GluCer or C₈-GalCer demonstrated strongly increased Dox delivery and cytotoxicity compared to standard DoxNL. Surprisingly, this effect was much less pronounced in normal cells. Nanoliposomes were not internalized, SCS however transfered from the nanoliposomal bilayer to the cell membrane and preceded cellular uptake and subsequent nuclear localization of Dox.

CONCLUSION

C₈-GluCer or C₈-GalCer incorporated in DoxNL selectively improved intracellular drug delivery upon transfer to tumor cell membranes by local enhancement of cell membrane permeability.

Cholesterol as a bilayer anchor for PEGylation and targeting ligand in folate-receptor-targeted liposomes

Phospholipids have been extensively evaluated as an anchor for both PEGylation and receptor-targeting in liposomal formulations. However, cholesterol, another important component in biomembranes, has not been fully investigated as an alternative anchor. In this study, the potential role of cholesterol for anchoring PEG and folate was investigated. Cholesterol derivatives were synthesized for PEGylation (mPEG-cholesterol) and folate receptor (FR) targeting (folate-PEG-cholesterol) and incorporated into the bilayer of FR-targeted liposomal doxorubicin. The colloidal stability of these cholesterol derivative-containing liposomes was superior to non-PEGylated liposomes, indicating that steric barrier provided by mPEG-cholesterol can efficiently inhibit aggregation of liposomes. FR-targeting activity of these liposomes was demonstrated by in vitro cell-binding studies on FR-overexpressing KB cells. In addition, in vivo circulation of cholesterol-anchored liposomes was prolonged compared to non-PEGylated liposomes. These studies suggest that cholesterol is a viable bilayer anchor for synthesis of PEGylated and FR-targeted liposomes.

Structure of self-assembled liposome-DNA-metal complexes

We have studied the structural and morphological properties of the triple complex dioleoyl phosphatidylcholine (DOPC)-DNA-Mn2+ by means of synchrotron x-ray diffraction and freeze-fracture transmission electron microscopy. This complex is formed in a self-assembled manner when water solutions of neutral lipid, DNA, and metal ions are mixed, which represents a striking example of supramolecular chemistry. The DNA condensation in the complex is promoted by the metal cations that bind the polar heads of the lipid with the negatively charged phosphate groups of DNA. The complex is rather heterogeneous with respect to size and shape and exhibits the lamellar symmetry of the L(c)(alpha) phase: the structure consists of an ordered multilamellar assembly similar to that recently found in cationic liposome-DNA complexes, where the hydrated DNA helices are sandwiched between the liposome bilayers. The experimental results show that, at equilibrium, globules of the triple complex in the L(c)(alpha) phase coexist with globules of multilamellar vesicles of DOPC in the L(alpha) phase, the volume ratio of the two structures being dependent on the molar ratio of the three components DOPC, DNA, and Mn2+. These complexes are of potential interest for applications as synthetically based nonviral carriers of DNA vectors for gene therapy.

Thermosensitive polymer-modified liposomes

Temperature-sensitive liposomes are considered to be a promising tool to achieve site-specific delivery of drugs. These liposomes have been prepared using lipids whose membranes undergo a gel-to-liquid crystalline phase transition a few degrees above physiological temperature. However, recently, temperature-sensitization of liposomes has been attempted using thermosensitive polymers. So far, functional liposomes whose contents release behavior, surface properties, and affinity to cell surface can be controlled in a temperature-dependent manner, have been developed according to this strategy. The design and function of these thermosensitive polymer-modified liposomes have been outlined in this review.

Characterization, stability and in-vivo distribution of asialofetuin glycopeptide incorporating DSPC/CHOL liposomes prepared by mild cholate incubation

In this study, a small triantennary asialoglycopeptide of fetuin (A-F2) was used as a ligand to direct liposomes to hepatocytes. A-F2 was cleaved from asialofetuin, purified, conjugated with fatty acids and incorporated into pre-formed sonicated DSPC/Chol (2:1) liposomes. A mild cholate incubation method for incorporating the A-F2 ligand on pre-formed vesicles was used. In preliminary in vivo experiments 111In3+ encapsulated in A-F2/palmityl liposomes was seen to accumulate in the liver of mice significantly faster than when encapsulated in non-ligand bearing liposomes of the same lipid composition (studied before), justifying further investigation of this system. The presence of the A-F2/fatty acid conjugate in a functional form on the vesicle surface was confirmed by their reversible agglutination in the presence of Ricinus communis agglutinin (RCA120). Effects of ligand incorporation on the vesicle size distribution, z-potential, membrane integrity and stability were monitored. The results demonstrate that highest ligand incorporation was achieved when liposomes and ligand were co-incubated in the presence of 1 mM sodium cholate. Incorporation increased with the length of the fatty acid used for A-F2 conjugation. Ligand-bearing liposomes were demonstrated to be smaller in diameter (about 30%) with a more positive z-potential in comparison to control vesicles while ligand incorporation did not influence the liposome membrane integrity. The size of the ligand-incorporating vesicles was maintained after 24 hours of incubation in isotonic buffer, proving that the vesicles do not aggregate. Although the preliminary biodistribution results may suggest that ligand bearing liposomes are accumulating in the liver, further cell culture, in vivo distribution and especially liver fractionation studies are required in order to clarify the intrahepatic localization of these liposomes and the ability to target liver hepatocytes in vivo.

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.

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.

Clearance properties of liposomes involving conjugated proteins for targeting

This review examines methods of protein conjugation onto liposomes and the effects of surface bound protein on the liposomes' biological behavior. It is evident that the presence of a conjugated protein significantly alters the attributes of targeted liposomes. Specifically, protein conjugation can result in dramatic increases in liposome size, enhanced immunogenicity, and increased plasma elimination. Techniques are discussed for preventing some of the physical (size) and biological (immunogenic) alterations involving the use of PEG-lipids and drug loaded liposomes. In addition, the advantages of conjugating antibodies via carbohydrate moieties, to minimize changes in antibody binding and tertiary structure as well as effectively decreasing plasma elimination, are also discussed. It is, however, apparent that the accessibility of targeted liposomes to extravascular sites is a key step that will require further study and it is, therefore, anticipated that with the development of novel ligands and novel ligand-liposome interactions, the therapeutic utility of targeting strategies will likely be realized.

Use of circular dichroism spectroscopy in determining the conformation of a monoclonal antibody prior to its incorporation in an immunoliposome

Attachment of antibodies to liposomes endows target specificity to liposomes for a certain cell or organ that express the targeted antigenic determinant. These so-called immunoliposomes hold high promise as targeted drug carriers. One approach of immunoliposome preparation involves conjugating antibodies to hydrophobic anchors (e.g. fatty acids or phospholipid molecules) for incorporation into the liposome membrane. Often, these conjugation reactions are harsh and may result in undesirable chemical and structural changes in the antibody molecule. This necessitates confirmation of the target specificity of the derivatized antibody prior to its incorporation into the liposome. Our approach to this problem is to utilize circular dichroism spectroscopy, which can detect subtle structural differences in proteins with high reproducibility and accuracy in relatively short period of time. In addition, circular dichroism is a non-destructive technique. In this study, we demonstrate the ability of circular dichroism to confirm the conformation of a model antibody, HYB-241, conjugated to N-glutarylphosphatidylethanolamine, prior to its mixing with dioleoylphosphatidylethanolamine/dioleoylphosphatidic acid to form a target-sensitive immunoliposome.

Recognition of normal, neoplastic, and fetal airway epithelial cell membranes by two monoclonal antibodies

The reactivity of two rat monoclonal antibodies was studied. These antibodies, A2R and A2C, bind a 32 kDa alveolar type II cell membrane receptor for surfactant protein A. A2R and A2C also bind apical cell membranes of ciliated and nonciliated cells of the conducting airways. Because this reactivity suggested possible utility in targeting those cells for therapeutic gene transfer, the binding activity of these two antibodies was examined in human tissues. In conducting airways, A2R and A2C bound apical epithelial cell membranes throughout the embryologic period studied: from 15 weeks of gestation, through maturity. Reactivity was more restricted to ciliated cells of the airways as maturation progressed. In the peripheral lung, A2C and A2R only bound most cells in the early developing lung, but mainly type II cells in mature lungs. Other normal tissues recognized by these antibodies included crypt lining cells of the adult and fetal stomach, large bile duct epithelium, and pancreatic acinar cells. All of these cells derive from embryonic foregut endoderm. Other normal tissues, both of endodermal and nonendodermal origin, were negative. Pulmonary carcinomas were studied. A2C and A2R recognized all non-small cell carcinomas of the lung tested. In contrast, none of the small cell carcinomas or carcinoid tumors of the lung were recognized by these antibodies. The function of p32 in these diverse cell types is not clear, but whatever its role in these tissues, antibodies versus p32 may potentially be used to target gene or drug therapy to the normal or malignant cells they recognize.

Intracavitary liposome-mediated p53 gene transfer into glioblastoma with endogenous wild-type p53 in vivo results in tumor suppression and long-term survival

A cavitary glioblastoma model was created by injection of RT-2 cells, which express endogenous wild type p53, into the peritoneal cavity of nude mice. This model developed multiple layers of tumor cells invading the peritoneal surface and was used to mimic the postoperative surgical cavity remaining after glioblastoma (GBM) excision in patients. Rhodamine labeled DMRIE/DOPE + DNA complexes were found to penetrate at least 20 tumor cell layers. Injection of p53 gene/liposome complexes into the intraperitoneal cavity after the tumor was established resulted in massive tumor necrosis. Prominent staining of human p53 protein using the DO-1 antibody was found in tumor cells near the necrotic lesions. Tumor explants expressed human p53 protein and showed a 54% growth reduction in an in vitro growth assay. Further, DMRIE/DOPE mediated p53 gene transfection significantly increased the mean survival time of tumor bearing mice compared to vector control. These results demonstrate the efficiency of using exogenous wild type p53 to suppress glioblastoma cell with endogenous wild type p53 in vivo through liposome mediated transfection method.

In vitro interaction between a chloroplast transit peptide and chloroplast outer envelope lipids is sequence-specific and lipid class-dependent

Interaction of artificial lipid bilayers (liposomes) with the purified transit peptide (SS-tp) of the precursor form of the small subunit for ribulose-2,5-bisphosphate carboxylase/oxygenase (prSSU) has been studied using a vesicle-disruption assay (calcein dye release) and electron microscopy. Employing purified forms of Escherichia coli-expressed prSSU, mature small subunit, glutathione S-transferase-transit peptide fusion protein, and SS-tp in dye release studies demonstrated that lipid interaction is mediated primarily through the transit peptide. Using chemically synthesized peptides (20-mers), the lipid-interacting domain of the transit peptide was partially mapped to the C-terminal 20 amino acids of the transit peptide. Peptides corresponding to other regions of the transit peptide and control peptides promoted significantly less calcein release. Interaction between the transit peptide and the bilayer was very rapid and could not be resolved by stopped-flow fluorometry with a mixing time of <50 ms. Interaction between the peptides and bilayer was also lipid class-dependent. Disruption occurred only when the bilayer contained the galactolipid monogalactosyldiacylglycerol (MGDG). The extent of bilayer disruption directly correlated with the relative concentration of MGDG in the liposome, with maximum calcein release occurring in 20 mol % MGDG liposomes. Lipid bilayers with greater than 20 mol % MGDG could not be achieved as determined by calcein entrapment. Electron microscopy of the liposomes before and after addition of the transit peptide suggested that the transit peptide induced a dramatic reorganization of lipids. These results are discussed in light of a possible mechanism for the early steps in protein transport that may involve polymorphic changes in the envelope membrane organization to include localized non-bilayer HII structures.

Use of genetically engineered lipid-tagged antibody to generate functional europium chelate-loaded liposomes. Application in fluoroimmunoassay

Biosynthetically lipid-tagged single-chain antibody (Laukkanen et al., Protein Eng. 6 (1993) 449; Biochemistry 33 (1994) 11664) has been used to functionalize europium (Eu3+) chelate-loaded liposomes. The resulting Eu immunoliposomes displayed specific hapten-binding activity and little non-specific binding in time-resolved fluoroimmunoassay (TR-FIA). No loss of entrapped marker nor of binding activity was observed after storage of Eu immunoliposomes for 1 month at 4 degrees C. In comparison with Eu-labeled free single-chain antibody, Eu immunoliposomes produced a higher signal and provided increased sensitivity in a sandwich-type immunoassay. These results demonstrate the potential of Eu immunoliposomes as signal-amplifying reagents in TR-FIA.

New structures in complex formation between DNA and cationic liposomes visualized by freeze-fracture electron microscopy

Structures formed during interaction of cationic liposomes and plasmid DNA were studied by freeze-fracture electron microscopy and their morphology was found to be dependent on incubation time and DNA concentration. These structures were formed with liposomes composed of DC-Chol and DOPE after 30 min incubation at DNA:lipid concentrations encompassing maximal transfection activity. They resembled liposome complexes (meatballs) and additionally bilayer-covered DNA tubules (spaghetti), whereby the DNA-tubules were found to be connected to the liposome complexes as well as occurring free in the suspension. At later times and higher DNA-to-liposome ratios the complexes grow larger while their membranes become discontinuous, allowing the self-encapsulation of the DNA. The relative transfection potency of the various morphologically distinct structures is discussed.

Biosynthetic lipid-tagging of antibodies

Chemical conjugation of fatty acids to antibodies generates lipid-modified molecules which have found use in the targeting of liposome-mediated drug delivery and in liposome-based immunoassays. Alternatively, bacterial expression of antibodies as single-chain FV fragments fused to lipoprotein signal peptide and N-terminal sequence leads to in vivo enzymatic addition of a single glycerolipid group at the N-terminus of the molecule. This lipid-modification converts the antibody from a soluble protein into a functional membrane-bound molecule. These biosynthetically lipid-tagged antibodies may prove useful for immobilization of antibodies to membranes in various biotechnological applications.