During the past five decades, various types of chemistries have been used for conjugation of molecules such as antibodies to the surface of the liposomes. In general, the conjugation can be achieved through the N-terminus, the C-terminus or the available sulfur (e.g. Fab’ fraction or thiolated Ab). Not all chemistries have the same yield and efficiency of conjugation and often reproducing biocompatible batches can be a challenge. The liposomes containing pyridyldithiopropionate (PDP) lipids are used to conjugate proteins, antibodies and other molecules containing the reactive moiety. PDP lipids are not as widely used as maleimide lipids, but they do have their own niche application. The PDP group contains disulfide, which can react with sulfhydryl or thiolated proteins/antibodies. Therefore, PDP-functionalized liposomes can be used in two ways: Method A. In this approach, the pyridyldithio group on the distal ends of the PEG chains contains PDP is forest reduced by a reducing agent (dithiothreitol, DTT). Maleimide-containing antibodies are then efficiently coupled to the surface of liposomes. The thiol-maleimide procedure is one of the most desirable reactions in bioconjugate chemistry due to its simplicity and high coupling efficiency in aqueous solution. The reaction, which is based on the stable thioether linkage between a thiol group (reduced form of PDP-liposome) and the corresponding maleimide group, occurs selectively and irreversibly at neutral pH (6.5-7.5), and the formed bonds are not cleaved by reducing agents. In addition, due to the presence of two different oxidation states of sulfur residues (oxidized and reduced states as a disulfide bond and sulfhydryl group, respectively) on the two conjugating components (i.e., the liposome and protein/antibody), the probability of the crosslinking of the homologous agents is low. Therefore, protein-protein and liposome-liposome crosslinking does not usually happen. Method B. Alternatively, the PDP group can participate in disulfide exchange reactions with thiols present on targeting proteins/antibodies. The coupling reaction is fast and conducted under mild conditions. However, the formed disulfide bonds have been reported to be less stable than thioether bonds. Moreover, even in an alkaline medium (pH 8.0), thiol groups are oxidized. The disulfide bond formed between the protein/antibody and liposomes can also be broken in the presence of a reducing agent and therefore, the conjugation reaction is reversible.

During the past five decades, various types of chemistries have been used for conjugation of molecules such as antibodies to the surface of the liposomes. In general, the conjugation can be achieved through the N-terminus, the C-terminus or the available sulfur (e.g. Fab’ fraction or thiolated Ab). Not all chemistries have the same yield and efficiency of conjugation and often reproducing biocompatible batches can be a challenge. The liposomes containing pyridyldithiopropionate (PDP) lipids are used to conjugate proteins, antibodies and other molecules containing the reactive moiety. PDP lipids are not as widely used as maleimide lipids, but they do have their own niche application. The PDP group contains disulfide, which can react with sulfhydryl or thiolated proteins/antibodies. Therefore, PDP-functionalized liposomes can be used in two ways: Method A. In this approach, the pyridyldithio group on the distal ends of the PEG chains contains PDP is forest reduced by a reducing agent (dithiothreitol, DTT). Maleimide-containing antibodies are then efficiently coupled to the surface of liposomes. The thiol-maleimide procedure is one of the most desirable reactions in bioconjugate chemistry due to its simplicity and high coupling efficiency in aqueous solution. The reaction, which is based on the stable thioether linkage between a thiol group (reduced form of PDP-liposome) and the corresponding maleimide group, occurs selectively and irreversibly at neutral pH (6.5-7.5), and the formed bonds are not cleaved by reducing agents. In addition, due to the presence of two different oxidation states of sulfur residues (oxidized and reduced states as a disulfide bond and sulfhydryl group, respectively) on the two conjugating components (i.e., the liposome and protein/antibody), the probability of the crosslinking of the homologous agents is low. Therefore, protein-protein and liposome-liposome crosslinking does not usually happen. Method B. Alternatively, the PDP group can participate in disulfide exchange reactions with thiols present on targeting proteins/antibodies. The coupling reaction is fast and conducted under mild conditions. However, the formed disulfide bonds have been reported to be less stable than thioether bonds. Moreover, even in an alkaline medium (pH 8.0), thiol groups are oxidized. The disulfide bond formed between the protein/antibody and liposomes can also be broken in the presence of a reducing agent and therefore, the conjugation reaction is reversible.

Numerous techniques have been developed to prepare immunoliposomes based on the nucleophilic reactivity of free amine groups of proteins or peptides. However, the most common, versatile and straightforward activation chemistry for creating reactive acylating reagents and labeling peptides/proteins is to form NHS ester with primary amines. A single step nucleophilic substitution reaction between NHS ester derivative and alpha amines at the N terminal or the beta amines of lysine side chains leads to the formation of a stable amide bond. N-Hydroxysuccinimide (NHS) esters of DSPE-PEG-NHS liposomes react with the primary amine groups on the peptides, proteins, antibodies or other functional ligands for targeted drug delivery applications. The nucleophilic attack of the amine groups on the NHS-activated carbonyl group of the DSPE-PEG-NHS results in the elimination of the NHS group and formation of amide linkage. Immunodox-NHS is a PEGylated product. For other amine reactive (PEGylated and non-PEGyalated products) and also Immunodox products suitable for other types conjugation methods see here.

Numerous techniques have been developed to prepare immunoliposomes based on the nucleophilic reactivity of free amine groups of proteins or peptides. One of the most popular and commonly used methods is to covalently couple free carboxylic groups to primary amines through activation of the carboxyl groups with EDC (1-ethyl-3-[3-dimethylaminopropyl] carbodiimide). EDC, which is a so-called zero-length crosslinking agent, reacts with the carboxyl to form an amine reactive intermediate (O-acylisourea). The produced O-acylisourea can be easily displaced by nucleophilic attack from the primary amino groups in the reaction mixture. However, this intermediate is unstable and hydrolyzed in aqueous solutions. In order to prevent the intermediate hydrolysis, sulfo-NHS (N-hydroxysulfosuccinimide) is added to EDC to produce a significantly more stable and more soluble active intermediate (NHS ester). Consequently, the immunoliposomes are prepared by a two-step coupling procedure: first, activating the free carboxyl group of the linker lipid incorporated in the liposomes with EDC and sulfo-NHS, and then covalently conjugating the antibodies to the lipids through displacement of sulfo-NHS groups by antibody amines, as depicted below. EDC/sulfo-NHS coupling reactions are highly selective and highly efficient, and the biological activity of the protein or peptide is preserved.

In order to develop a rapid and straightforward coupling procedure at the PEG terminus, a method of direct coupling antibodies to the PEG terminus of liposomes was introduced by Bendas et al. [1]. In this methodology, antibodies are simply attached to the PEG terminus of liposomes, which had been end-group functionalized with cyanuric chloride, in mild basic conditions (pH 8.8) without prior antibody derivatizations. It has been shown that in order to obtain a stable attachment of proteins on liposome, the DSPE-PEG-cyanur was added into the liposomes to chemically conjugate with proteins to form a stable complex and minimize the denaturation of proteins. Proteins can be covalently coupled to the liposomes via amine-reactive cyanur-groups, either directly to the vesicle surface using cyanuric chloride-activated DSPE (cyanur-DSPE) or to the distal ends of PEG-spacers using activated cyanur-PEG-PE (ammonium salt). Cyanuric chloride at the PEG terminus functions to link peptides, antibodies and other amine-containing biomolecules or nanoparticles via a nucleophilic substitution reaction under basic conditions. Antibodies or other proteins can be conjugated without any previous derivatization.