Two methods are used in our laboratory for the preparation of antibody-targeted liposomes. Both methods depend on the generation of a phosphatidylethanolamine (PE)-antibody or PE-F(ab')2 complex.

In the first method, the PE-F(ab')2 conjugate is associated with preformed small unilamellar vesicles (SUV). The preparation of SUV, discussed in detail later on, involves prolonged sonication. 1 This step limits the technique to the entrapment of substances that are not destroyed during the sonication. The entrapment capacity of SUV is also low because of their small internal volume. 2 On the other hand, SUV have been shown to remain in the circulation for a longer time than large multilamellar vesicles (ML V)3 and are more likely than any other liposomal preparations to cross anatomical barriers. 4

The second method of the preparation of targeted liposomes involves the use of a detergent. The PE-F(ab')2 conjugate is added to lipids suspended in the presence of a detergent and an internal marker, e.g., fragment A of the diphtheria toxin (DT-A) or a drug, e.g., methotrexate (MTX). Liposomes are formed upon the removal of the detergent by gel filtration, as described by Enoch and Strittmatter. 5 The entrapment is about tenfold greater than that of SUV and the preparations are subjected to sonication for a very short time. The liposomes are as stable as the lipids that form them. 5 •6 There are no reported in vivo studies with these liposomes and, therefore, the rate of clearance is not known. In our laboratory both preparation are used at the present time only for in vitro studies.