Antibodies liposomes

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.

Liposomes with PEG 0.13 of antibody-liposomes, and (b) nonspecific accumulation of long-circulating liposomes resulting from impaired filtration. Relative targeting (target-to-normal ratio) is much better for immunoliposomes (22) than for PEG-liposomes (8) because of low accumulation of immunoliposomes in normal tissues (fast clearance) and high accumulation of PEG-liposomes (slow clearance). 

Determination of Antibody-Liposome Immunoreactivity by Direct Binding of Radiolabeled Antibody-Liposome and Antibody-Liposome-Polymer Conjugates... 

For antibody conjugation, the liposomal phospholipid concentration needs to be determined using Bartlett assays (94) (see Subheading 3.3). Both antibodies of the matched set should be conjugated to liposomes to determine which serves as the better detection antibody. Liposomes should be mixed with antibodies with the latter at 0.05 mol% of the total lipid input. Then, just before use, dilute EDC to a concentration of lOOmg/mL with 0.1 M MES buffer, pH 4.65 then add five molar equivalents (based on total W-glutaryl DPPE lipid input) to the liposome/ antibody mixture. Note that EDC is very hydroscopic - use care... 

---