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.

Method A. Conjugation of maleimide-modified antibody to a PDP-modified liposome.

Method B. Conjugation of a thiol-modified antibody to a PDP-modified liposome.

ImmunoFluor™-PDP is a PEGylated product. For other reactive (PEGylated and non-PEGyalated products) ImmunosFluor™ products suitable for other types of conjugation method see here.