Antibody—liposome conjugates

Figure 22.17 Antibody-liposome conjugates may be used as targeting reagents for detection or therapeutic applications. The liposome may be constructed to contain fluorescent molecules for detection purposes or bioactive agents for therapy. The antibody component targets the complex for binding to specific antigenic determinants.

However, there are problems associated with the use of antibody—liposome conjugates for drug delivery in vivo. Particularly, since lipid vesicles are huge compared to... 

Modification of the protein amine groups is the procedure most frequently used to produce antibody-liposome conjugates. Early procedures used crosslinking agents, such as l-ethyl-3-(3-dimethylaminopropyl)carbodiimide hydrochloride (EDC) (27,28) in the presence of preformed liposomes containing a lipophilic carboxylic acid. Condensing agents like these tend to form protein-protein polymers. Control of these reactions is typically difficult and complex, and as a result separation of the liposomes from protein polymers is a... 

Reliable lipid and protein analysis of the prepared antibody-liposome conjugate is essential for proper characterization and subsequent interpretation of results obtained in the application of the conjugates. In addition, we strongly advise that the liposome conjugate size be determined with a particle sizer, if one is available, since liposome size plays such an important role in the pharmacokinetics of the system in vivo. 

Fig. 4. Aggregation reactions associated with different classes of antibody-liposome conjugates. (A) Antibody-liposome conjugates may react further with other liposomes to form aggregates. (B) The presence of PEG-lipids prevents these crosslinking reactions through steric hindrance. (C) Individual proteins may penetrate the PEG cloud to react with the liposome surface. (D) Antibodies tethered on the distal end of PEG may react with the distal ends of PEG molecules on a second liposome, resulting in crosslinking.

K., Abe, O., and Saito, K. (1987). Antitumor effect of adria-mycin entrapped in liposomes conjugated with anti-human alfa-fetoprotein monoclonal antibody, Cancer Res., 47, 4471-4477. 

Liposome conjugates may be used in various immunoassay procedures. The lipid vesicle can provide a multivalent surface to accommodate numerous antigen-antibody interactions and thus increase the sensitivity of an assay. At the same time, it can function as a vessel to carry encapsulated detection components needed for the assay system. This type of enzyme-linked immunosorbent assay (ELISA) is called a liposome immunosorbent assay or LISA. One method of using liposomes in an immunoassay is to modify the surface so that it can interact to form biotin-avidin or biotin-streptavidin complexes. The avidin-biotin interaction can be used to increase detectability or sensitivity in immunoassay tests (Chapter 23) (Savage et al., 1992). 

Figure 14.17. Liposome fluoroimmunoassay depiction, in which antigen-liposome conjugates containing fluorescent dye (concentration quenched) compete with analyte antigen for antibody binding sites, followed by wash and detergent lysis, to release the fluorophore for fluorescence measurement.

Figure 350 Antibodies may be conjugated to liposomes using an indirect approach incorporating an avidin—biotin system. Biotinylated liposomes may be complexed with biotinylated antibodies using avidin as a bridging molecule or may be complexed with an antibody—avidin conjugate.

Onuma, M. Yasutomi, Y. Yamamoto, M. Watarai, S. Yasuda, T. Kawakami, Y. Antitumor effect of adriamycin entrapped in liposomes conjugated with anti-bovine tumor antigen monoclonal antibody in leukemic cows. Zentralbl. Veterinarmed. 1989, 36, 139-147. 

Mix lipids (as showing on Table 2 for preparing 30 mg of appropriate sample) in a 250 mL round bottom flask according to the desired lipid molar ratio. For liposomal conjugation with an antibody or similar targeting agent, MPB-PE or PEG-PE is to be added. 

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