Attachment of immunoglobulin to liposomal membrane via protein carbohydrate

Introduction of Carbonyl Groups into Antibodies

Antibodies and their derivatives (scFv, Fabs, etc.) represent a unique class of biomolecules that combine selectivity with the ability to target drug delivery. Currently, one of the most promising endeavors in this field is the development of molecular diagnostic tools and antibody-based therapeutic agents, including antibody-drug conjugates (ADCs). To meet this challenge, it is imperative to advance methods for modifying antibodies. A particularly promising strategy involves the introduction of carbonyl groups into the antibody that are amenable to further modification by biorthogonal reactions, namely aliphatic, aromatic, and α-oxo aldehydes, as well as aliphatic and aryl-alkyl ketones. In this review, we summarize the preparation methods and applications of site-specific antibody conjugates that are synthesized using this approach.

Modulating antibody effector functions by Fc glycoengineering

Antibody based drugs, including IgG monoclonal antibodies, are an expanding class of therapeutics widely employed to treat cancer, autoimmune and infectious diseases. IgG antibodies have a conserved N-glycosylation site at Asn297 that bears complex type N-glycans which, along with other less conserved N- and O-glycosylation sites, fine-tune effector functions, complement activation, and half-life of antibodies. Fucosylation, galactosylation, sialylation, bisection and mannosylation all generate glycoforms that interact in a specific manner with different cellular antibody receptors and are linked to a distinct functional profile. Antibodies, including those employed in clinical settings, are generated with a mixture of glycoforms attached to them, which has an impact on their efficacy, stability and effector functions. It is therefore of great interest to produce antibodies containing only tailored glycoforms with specific effects associated with them. To this end, several antibody engineering strategies have been developed, including the usage of engineered mammalian cell lines, in vitro and in vivo glycoengineering.

Enzymatic Bioconjugation: A Perspective from the Pharmaceutical Industry

Enzymes have firmly established themselves as bespoke catalysts for small molecule transformations in the pharmaceutical industry, from early research and development stages to large-scale production. In principle, their exquisite selectivity and rate acceleration can also be leveraged for modifying macromolecules to form bioconjugates. However, available catalysts face stiff competition from other bioorthogonal chemistries. In this Perspective, we seek to illuminate applications of enzymatic bioconjugation in the face of an expanding palette of new drug modalities. With these applications, we wish to highlight some examples of current successes and pitfalls of using enzymes for bioconjugation along the pipeline and try to illustrate opportunities for further development.

Lipid-Based Nanomaterials for Drug Delivery Systems in Breast Cancer Therapy

Globally, breast cancer is one of the most prevalent diseases, inducing critical intimidation to human health. Lipid-based nanomaterials have been successfully demonstrated as drug carriers for breast cancer treatment. To date, the development of a better drug delivery system based on lipid nanomaterials is still urgent to make the treatment and diagnosis easily accessible to breast cancer patients. In a drug delivery system, lipid nanomaterials have revealed distinctive features, including high biocompatibility and efficient drug delivery. Specifically, a targeted drug delivery system based on lipid nanomaterials has inherited the advantage of optimum dosage and low side effects. In this review, insights on currently used potential lipid-based nanomaterials are collected and introduced. The review sheds light on conjugation, targeting, diagnosis, treatment, and clinical significance of lipid-based nanomaterials to treat breast cancer. Furthermore, a brighter side of lipid-based nanomaterials as future potential drug delivery systems for breast cancer therapy is discussed.

Dual α-1,4- and β-1,4-Glycosidase Activities by the Novel Carbohydrate-Binding Module in α-l-Fucosidase from Vibrio sp. Strain EJY3

Carbohydrates are structurally and functionally diverse materials including polysaccharides, and marine organisms are known to have many enzymes for the breakdown of complex polysaccharides. Here, we identified an α-l-fucosidase enzyme from the marine bacterium Vibrio sp. strain EJY3 (VejFCD) that has dual α-1,4-glucosidic and β-1,4-galactosidic specificities. We determined the crystal structure of VejFCD and provided the structural basis underlying the dual α- and β-glycosidase activities of the enzyme. Unlike other three-domain FCDs, in VejFCD, carbohydrate-binding module-B (CBM-B) with a novel β-sandwich fold tightly contacts with the CatD/CBM-B main body and provides key residues for the β-1,4-glycosidase activity of the enzyme. The phylogenetic tree analysis suggests that only a few FCDs from marine microorganisms have the key structural features for dual α-1,4- and β-1,4-glycosidase activities. This study provides the structural insights into the mechanism underlying the novel glycoside hydrolase activities and could be applied for more efficient utilization in the hydrolysis of complex carbohydrates in biotechnological applications.

Site-selective modification strategies in antibody-drug conjugates

Antibody-drug conjugates (ADCs) harness the highly specific targeting capabilities of an antibody to deliver a cytotoxic payload to specific cell types. They have garnered widespread interest in drug discovery, particularly in oncology, as discrimination between healthy and malignant tissues or cells can be achieved. Nine ADCs have received approval from the US Food and Drug Administration and more than 80 others are currently undergoing clinical investigations for a range of solid tumours and haematological malignancies. Extensive research over the past decade has highlighted the critical nature of the linkage strategy adopted to attach the payload to the antibody. Whilst early generation ADCs were primarily synthesised as heterogeneous mixtures, these were found to have sub-optimal pharmacokinetics, stability, tolerability and/or efficacy. Efforts have now shifted towards generating homogeneous constructs with precise drug loading and predetermined, controlled sites of attachment. Homogeneous ADCs have repeatedly demonstrated superior overall pharmacological profiles compared to their heterogeneous counterparts. A wide range of methods have been developed in the pursuit of homogeneity, comprising chemical or enzymatic methods or a combination thereof to afford precise modification of specific amino acid or sugar residues. In this review, we discuss advances in chemical and enzymatic methods for site-specific antibody modification that result in the generation of homogeneous ADCs.

Liposomal doxorubicin as targeted delivery platform: Current trends in surface functionalization

Liposomal delivery systems have significantly enhanced the efficacy and safety of chemotherapeutic agents compared to free (non-liposomal) formulations. Liposomes are vesicles made up of lipophilic bilayer and a hydrophilic core which provides perfect opportunity for their application as transport vehicle for various therapeutic and diagnostic agents. Doxorubicin is the most exploited chemotherapeutic agent for evaluation of different liposomal applications, as its physicochemical properties permit high drug entrapment and easy remote loading in pre-formulated liposomes. Pegylated liposomal doxorubicin clinically approved and, on the market, Doxil®, exemplifies the benefits offered upon the surface modification of liposome with polyethylene glycol. This unique formulation prolonged the drug residence time in the circulation and increased accumulation of doxorubicin in tumor tissue via passive targeting (enhanced permeability and retention effect). However, there is ample scope for further improvement in the efficiency of targeting tumors by coupling biological active ligands onto the liposome surface to generate intelligent drug delivery systems. Small biomolecules such as peptides, fraction of antibodies and carbohydrates have the potential to target receptors present on the surface of the malignant cells. Hence, active targeting of malignant cells using functionalised nanocarrier (liposomes encapsulated with doxorubicin) have been attempted which is reviewed in this article.

When liposomes met antibodies: Drug delivery and beyond

Drug encapsulated liposomes and monoclonal antibodies (Mabs) are two distinctively different classes of therapeutics, but both aim to become the ultimate "magic bullet". While PEGylated liposomes rely on the enhanced permeability and retention (EPR) effect for accumulation in solid tumor tissues, Mabs are designed to bind tightly to specific surface antigens on target cells to exert effector functions. Immunoliposome (IL) refers to the structural combination of liposomes and antibodies, whereas the antibodies are usually decorated on the liposome surface. ILs can therefore take advantage of interactions between antibodies and cancer cells for more efficient endocytosis and intracellular drug delivery. The antibody structure, affinity, density, as well as the liposome surface properties and drug to lipid ratios all contribute to the IL pharmacokinetic(PK) and pharmacodynamic(PD) behaviors. The optimal formulation parameters may vary for different target cells and tissues. Furthermore, besides the delivery of cytotoxic drugs to cancer cells, new ILs are being developed to interact with multiple target receptors, multiple target cells and trigger multiple therapeutic effects. We envision that the IL format can be a great platform for the molecular engineering of multi-valent, multi-specific interactions to achieve complex biological functions for therapeutic benefits, especially in the area of cancer immunotherapy.

Recent strategies towards the surface modification of liposomes: an innovative approach for different clinical applications

Liposomes are very useful biocompatible tools used in diverse scientific disciplines, employed for the vehiculation and delivery of lipophilic, ampiphilic or hydrophilic compounds. Liposomes have gained the importance as drug carriers, as the drugs alone have limited targets, higher toxicity and develop resistance when used in higher doses. Conventional liposomes suffer from several drawbacks like encapsulation inefficiencies and partially controlled particle size. The surface chemistry of liposome technology started from simple conventional vesicles to second generation liposomes by modulating their lipid composition and surface with different ligands. Introduction of polyethylene glycol to lipid anchor was the first innovative strategy which increased circulation time, delayed clearance and opsonin resistance. PEGylated liposomes have been found to possess higher drug loading capacity up to 90% or more and some drugs like CPX-1 encapsuled in such liposomes have increased the disease control up to 73% patients suffering from colorectal cancer. The surface of liposomes have been further liganded with small molecules, vitamins, carbohydrates, peptides, proteins, antibodies, aptamers and enzymes. These advanced liposomes exhibit greater solubility, higher stability, long-circulating time and specific drug targeting properties. The immense utility and demand of surface modified liposomes in different areas have led their way to the modern market. In addition to this, the multi-drug carrier approach of targeted liposomes is an innovative method to overcome drug resistance while treating ceratin tumors. Presently, several second-generation liposomal formulations of different anticancer drugs are at various stages of clinical trials. This review article summarizes briefly the preparation of liposomes, strategies of disease targeting and exclusively the surface modifications with different entities and their clinical applications especially as drug delivery system.

Liposome-ligand conjugates: a review on the current state of art

Early researchers focussed on developing stimuli-responsive liposomes in order to manipulate drug release at the site of action or under certain conditions. In recent times, a great deal of efforts has been made to modify the surface of liposomes with ligands for the purpose of achieving targeted drug delivery. Due to the morphology of liposomes, their surfaces can be engineered by attaching molecules such as oligosaccharides, peptides, antibodies, antigens and oligonucleotides to the bilayer structure. Over the years, a number of techniques including the use of covalent and non-covalent linkages have been utilised in designing ligand-liposome conjugates. In this review, various strategies for the functionalisation of liposomes as well as the different types of ligand-liposome conjugates have been discussed. Finally, the pros and cons of conjugation in liposomes are concisely summarised.

Glycans of Antibodies as a Specific Site for Drug Conjugation Using Glycosyltransferases

The therapeutic cargo molecules conjugated to a specific site on a monoclonal antibody (mAb), called antibody-drug conjugates (ADCs), are becoming powerful tools in cancer treatment. Generally, the cargo molecules conjugate at the cysteine or lysine residue of the mAb, which generally results in a highly heterogeneous ADC. Therapeutic cargo molecules need to be conjugated in a site-specific manner to the mAb so that the bioefficacy of these molecules is not compromised. The mAb (IgG1) are N-glycosylated at the conserved residue Asn(297), which is present in each heavy chain of the IgG1, near the CH2 domain of the Fc fragment. The mutant or wild-type glycosyltransferases transfer sugars with a chemical handle to the glycan molecule of IgG1, making the site-specific linking of cargo molecules possible via the chemical handle, and thus making the process an invaluable technique for the production of homogeneous ADCs.

Engineering liposomes and nanoparticles for biological targeting

Our ability to engineer nanomaterials for biological and medical applications is continuously increasing, and nanomaterial designs are becoming more and more complex. One very good example of this is the drug delivery field where nanoparticle systems can be used to deliver drugs specifically to diseased tissue. In the early days, the design of the nanoparticles was relatively simple, but today we can surface functionalize and manipulate material properties to target diseased tissue and build highly complex drug release mechanisms into our designs. One of the most promising strategies in drug delivery is to use ligands that target overexpressed or selectively expressed receptors on the surface of diseased cells. To utilize this approach, it is necessary to control the chemistry involved in surface functionalization of nanoparticles and construct highly specific functionalities that can be used as attachment points for a diverse range of targeting ligands such as antibodies, peptides, carbohydrates and vitamins. In this review we provide an overview and a critical evaluation of the many strategies that have been developed for surface functionalization of nanoparticles and furthermore provide an overview of how these methods have been used in drug delivery systems.

Site specific conjugation of fluoroprobes to the remodeled Fc N-glycans of monoclonal antibodies using mutant glycosyltransferases: application for cell surface antigen detection

The Fc N-glycan chains of four therapeutic monoclonal antibodies (mAbs), namely, Avastin, Rituxan, Remicade, and Herceptin, released by PNGase F, show by MALDI analysis that these biantennary N-glycans are a mixture of G0, G1, and G2 glycoforms. The G0 glycoform has no galactose on the terminal GlcNAc residues, and the G1 and G2 glycoforms have one or two terminal galactose residues, respectively, while no N-glycan with terminal sialic acid residue is observed. We show here that under native conditions we can convert the N-glycans of these mAbs to a homogeneous population of G0 glycoform using beta1,4 galactosidase from Streptococcus pneumoniae. The G0 glycoforms of mAbs can be galactosylated with a modified galactose having a chemical handle at the C2 position, such as ketone or azide, using a mutant beta1,4-galactosyltransferase (beta1,4Gal-T1-Y289L). The addition of the modified galactose at a specific glycan residue of a mAb permits the coupling of a biomolecule that carries an orthogonal reactive group. The linking of a biotinylated or a fluorescent dye carrying derivatives selectively occurs with the modified galactose, C2-keto-Gal, at the heavy chain of these mAbs, without altering their antigen binding activities, as shown by indirect enzyme linked immunosorbent assay (ELISA) and fluorescence activated cell sorting (FACS) methods. Our results demonstrate that the linking of cargo molecules to mAbs via glycans could prove to be an invaluable tool for potential drug targeting by immunotherapeutic methods.

Adhesion characteristics and stability assessment of lectin-modified liposomes for site-specific drug delivery

Carbohydrate moieties of the cellular glycocalyx have been suggested to play an important role in biological recognition processes during pathologic conditions, such as inflammation and cancer. Herein, we describe lectin-modified liposomes which might have potential for site-specific drug delivery during the therapy of such diseases. Specific interactions of plain (i.e., unmodified) and PEGylated, lectin-grafted liposomes with model membranes were investigated under real-time flow conditions using a quartz crystal microbalance. In addition, the morphology of the liposomal systems was assessed by atomic force microscopy. Plain liposomes exhibited only unspecific adhesion to glycolipid membranes and had a tendency to coalesce. The degree of membrane interaction was significantly increased when plain liposomes were modified with the lectin, Concanavalin A. However, vesicle fusion also markedly increased as a result of lectin modification. Additional PEGylation of liposomes reduced unspecific adhesion phenomena, as well as coalescence. Moreover, our studies enabled us to establish quartz crystal microbalance and atomic force microscopy as powerful and complementary methods to characterize adhesion properties of targeted drug delivery systems.

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.

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.

Attachment of antibodies to sterically stabilized liposomes: evaluation, comparison and optimization of coupling procedures

Several coupling methods for binding antibodies (Ab) to liposomes have previously been developed. We were interested in examining if some of these methods would be suitable for attaching Ab to long-circulating formulations of liposomes (SL), sterically stabilized with poly(ethylene glycol) (PEG). We studied three 'classical' coupling methods in which Ab was attached at the bilayer surface of SL, and two new coupling methods in which Ab was attached at the PEG terminus. Parameters examined including binding efficiency, antibody surface density, the ability of the immunoliposomes to remote-load the anticancer drug doxorubicin, and the specific binding of the resulting immunoliposomes to target cells. The non-covalent biotin-avidin coupling method resulted in low Ab densities at the cell surface, as did a coupling in method in which maleimide-derivatized Ab was attached to the liposome surface through a thiolated phospholipid incorporated into the liposomes. The low levels of Ab achieved in these method was likely due to interference by PEG with the access of the Ab to the liposome surface. However, when a maleimide-derivatized Ab was coupled to thiolated PEG, moving the coupling reaction away from the liposome surface, very high coupling efficiencies were achieved, and these immunoliposomes achieved good specific binding to their target cells. Oxidizing the Fc region of the Ab and coupling it to the PEG terminus through a hydrazone bond was a less efficient coupling method, but had the advantage of retaining Ab orientation. Efficient remote-loading of doxorubicin was found for immunoliposomes in which Ab was attached at the PEG terminus.

Immobilization of monoclonal antibodies for affinity chromatography using a chelating peptide

A procedure was developed for oriented immobilization of monoclonal antibodies on a solid support. The technique involves the specific oligosaccharide-directed covalent modification of the monoclonal antibody (mAb) with the chelating peptide, Lys-Gly-(His)6, in conjunction with immobilized metal ion affinity chromatography. Chelating peptide-mAb conjugates with a molar ratio of 2.2 retained full antigen binding activity. On immobilization of the modified antibodies on a nickel affinity resin, the molar antigen binding ratio was 1.4. The high antigen binding capacity is indicative of oriented immobilization providing maximum access for the antigen. The described method can be used for the preparation of high-capacity immunosorbents for affinity chromatography and it is applicable for all immunoglobulin classes.

The influence of periodate oxidation on monoclonal antibody avidity and immunoreactivity

Eight monoclonal antibodies of different classes and isotypes and rabbit IgG were oxidized under a variety of conditions with 5-50 mM periodate. The number of aldehyde groups generated per immunoglobulin were measured by reduction with tritiated sodium borohydride or coupling of fluoresceinthiosemicarbazide. There were up to 25.5 aldehyde groups detected on the periodate-oxidized antibody 96.5, measured by borohydride reduction whereas the same conditions led to only 9.6 aldehydes per IgG on the antibody L6 of the same IgG2A isotype. Fluoresceinthiosemicarbazide bound to oxidized antibodies but not to the same extent as tritium. On mildly oxidized IgMs it was possible to generate more than 200 aldehyde groups per antibody molecule. Depending on the conditions and the antibody used periodate oxidation could lead to antibody crosslinking. The avidities of the modified antibodies were determined by Scatchard analyses and inhibition assays. A new mathematical method to evaluate the immunoreactivities of modified antibodies relative to the unlabeled native antibody from inhibition binding data was established. Periodate concentrations higher than 50 mM decreased the avidities and immunoreactivities of all IgGs tested. This effect is more pronounced if the oxidation is performed at pH 5.6 and 25 degrees C instead of pH 4.6 and 0 degree C. The BR96 antibody is inactivated even under mild oxidation conditions.

A carbohydrate-directed heterobifunctional cross-linking reagent for the synthesis of immunoconjugates

A novel, highly water-soluble, heterobifunctional cross-linking reagent, S-(2-thiopyridyl)-L-cysteine hydrazide (TPCH), was synthesized which contains a hydrazide moiety for coupling to aldehyde groups generated in the carbohydrate residues of antibodies by mild periodate oxidation, and a pyridyl disulfide moiety for coupling to molecules with a free sulfhydryl group. Since the carbohydrate moieties are distal to the antigen binding region of antibodies, derivatization with this cross-linker minimizes impairment of the antigen binding function. Derivatization of the human monoclonal IgM antibody 16-88 against human colon carcinoma cells with as many as 16 TPCH cross-linker molecules did not impair its antigen binding capability. Using mild oxidation conditions for antibody derivatization, sialic acid residues were identified as attachment sites for the cross-linker molecules, since after desialylation of antibody 16-88 by neuraminidase virtually no cross-linker molecules could be incorporated. Comparison of TPCH with S-(2-thiopyridyl)mercaptopropionic acid hydrazide and S-(2-thiopyridyl)-L-cysteine, two related cross-linking reagents, revealed that TPCH is most efficiently incorporated into periodate-treated antibody. Based on the structural differences of the cross-linkers, the more efficient incorporation of TPCH appears to be a function of the presence of a hydrazide moiety with an adjacent amino group. When three to four molecules of pyridyl disulfide-derivatized barley toxin were coupled to TPCH-derivatized antibody 16-88, the antigen binding capability remained uncompromised. In addition, no significant impairment of toxin activity upon coupling to the antibody was observed. Based on these data, TPCH may be very useful for the synthesis of immuno-conjugates with no or only minimal impairment of the antigen binding function.

Enzymic oxidation of monoclonal antibodies by soluble and immobilized bifunctional enzyme complexes

Site-specific modification of monoclonal antibodies was achieved by oxidation of the carbohydrate moieties of antibodies which are located remote from the antigen binding sites. Sialic acid and galactose are terminal sugars of these carbohydrate chains. Concomitant treatment of the antibodies with neuraminidase and galactose oxidase generated aldehyde groups in the oligosaccharide moieties of immunoglobulins which reacted selectively with amino or hydrazide groups of the matrix. Subsequent immobilization of neuraminidase and galactose oxidase on Eupergit C-adipic dihydrazide proved to be an efficient and selective system for the enzymic oxidation of the monoclonal antibodies without impairing their immunological activity. Oriented immobilization of enzymically oxidized monoclonal antibodies on hydrazide or amino Eupergit C derivatives thus leads to the formation of antibody matrix conjugates which possess high antigen-binding activities.

Targeting of anti-Thy 1.1 monoclonal antibody conjugated liposomes in Thy 1.1 mice after intravenous administration

125I-labeled liposomes, conjugated to an anti-Thy 1.1 monoclonal antibody (MRCOX7), demonstrated up to 7.4-fold greater lymph node uptake than liposomes conjugated to non-specific monoclonal antibody (R-10) after intravenous injection into Thy 1.1 (AKR-J) mice. Uptake of anti-Thy 1.1-conjugated liposomes by the lymph nodes of AKR-J mice was 3-times greater than their uptake by lymph nodes of Thy 1.2 (AKR-Cu) mice. Lymph node localization of anti-Thy 1.1-liposomes was equal to that of control monoclonal antibody-liposomes in Thy 1.2 mice. Conjugation to either monoclonal antibody substantially increased liposome clearance by the liver, while decreasing liposome uptake in a number of organs outside the reticuloendothelial system. Changes in liposome size and phospholipid composition did not significantly alter these results. Administration of a large predose of unconjugated liposomes prior to injection of MRCOX7-conjugated liposomes increased blood levels and reduced liver uptake of the monoclonal antibody-liposome conjugates, but did not further enhance lymph node uptake. This study demonstrates that targeting of liposomes by conjugation to the appropriate monoclonal antibody, can significantly increase their uptake in lymph nodes which contain high levels of cells expressing the target antigen. However, conjugation to monoclonal antibody also increases clearance of liposomes by the liver. To increase the uptake of monoclonal antibody-conjugated liposomes in target tissue, substantial reduction of their clearance by the reticuloendothelial system will be required.

The role of multivalency in antibody mediated liposome targeting

We have investigated the role of multivalency in immunoliposome binding to cells displaying different amounts of surface antigen using liposomes with increasing numbers of palmitoyl anti-H2Kk antibodies incorporated into the bilayer. RDM-4 lymphoma cells were treated with proteinase k to generate a series of cells with various amounts of H2Kk antigen. Percent binding of immunoliposomes was related to the number of antigens displayed by the RDM-4 cell. Increasing liposome binding was observed with increasing number of antibody molecules per liposome. However, the ratio of binding of the high-antigen-density cells to that of the low-antigen-density cells was higher with immunoliposomes of lower antibody density than the ones with higher antibody density. This result suggests that for better discrimination between cells differing in antigen density, liposomes with lower numbers of antibody molecules per liposome may be more useful as a discriminatory tool for cells with a low level antigen expression than liposomes with greater antibody densities.

A new hydrophobic anchor for the attachment of proteins to liposomal membranes

The model enzyme alpha-chymotrypsin covalently modified by a phosphatidylethanolamine derivative has been attached to liposomal membranes in high yield. A maximal protein/lipid ratio of 5.4 X 10(-3) mol enzyme/mol lipid was achieved.

The development and application of protein-liposome conjugation techniques

Numerous techniques have been developed over the past 10 years for the conjugation of proteins to liposomes. Early procedures involved coupling with reagents such as glutaraldehyde or EDCI. Subsequently, more sophisticated approaches involving selective bifunctional coupling agents have been developed. These later procedures are also much more efficient for coupling in aqueous media. The techniques of coupling have become more rigorous because investigators have recognized the inherent problems of producing, purifying and characterizing protein conjugated liposomes. Protein-liposome coupling techniques were developed mainly for targeted drug delivery. The attachment of specific antibodies to the surface of the liposomes makes them able to bind to cells and to subsequently be internalised by the cells. Protein conjugated liposomes have also been used for various immunochemical and diagnostic purposes. These include the binding of labelled liposomes to cells and the agglutination of cells or latex particles by protein conjugated liposomes.

Site-specific covalent modification of monoclonal antibodies: in vitro and in vivo evaluations

A strategy for covalent modification of monoclonal antibodies utilizing the oxidized oligosaccharide moieties on the molecule was evaluated and compared to more conventional methods. As judged by quantitative in vitro measurements, a monoclonal antibody conjugate prepared via the oligosaccharides retained the homogeneous antigen binding property and affinity of the unmodified antibody. In contrast, conjugates of the same antibody, modified to the same degree on either lysines or aspartic and glutamic acid side chains, were heterogeneous in their antigen binding and had lowered affinity. In vivo biodistribution and nuclear-imaging experiments were also performed with a second monoclonal antibody and a tumor xenograft model. Antibodies modified on the oligosaccharides with either a peptide labeled with iodine-125 or a diethylenetriaminepentaacetic acid chelate with indium-111 localize into target tumors more efficiently than the same antibody radiolabeled on either tyrosines or lysines. These in vivo results, when compared to those reported in the literature for conventionally modified antibodies, suggest that oligosaccharide modification of monoclonal antibodies is a preferred method of preparing conjugates.

Cytolytic activity of liposomes containing stearylamine

In order to develop the cytotoxic liposome, the cytolytic effect of polycationic liposome was examined. Upon incubation of the stearylamine-containing liposome (stearylamine-liposome) with rabbit erythrocyte, a significant extent of hemolysis was observed. Hemolytic activity of the liposome depends on the amount of stearylamine in the liposome membrane. The plots of the initial rate of hemolysis versus the concentration of stearylamine-liposome showed a sigmoidal curve, suggesting that stearylamine-liposomes act cooperatively on the erythrocyte membrane. Hemolytic activity of stearylamine-liposome was markedly influenced by the composition of hydrocarbon chains of the phospholipids in the liposome membrane, suggesting that the membrane fluidity of stearylamine-liposome is important to evoke the hemolysis. Since the liposomes containing acidic phospholipids inhibited markedly the stearylamine-liposome-caused hemolysis, it is likely that the primary target of stearylamine-liposome is the negatively charged component(s) such as acidic phospholipids on the erythrocyte membrane. Furthermore, stearylamine-liposome induced the release of the intravesicular contents from the liposome made of acidic phospholipids but not from the liposome made of phosphatidylcholine only. These results suggest that stearylamine-liposome interacted with the negative charges of the erythrocyte membrane and eventually damaged the cell. Erythrocytes from rabbit, horse and guinea pig are highly susceptible to stearylamine-liposome but those from man, sheep, cow and chicken are less so.