2-STEP TARGETING APPROACH FOR DELIVERY OF DOXORUBICIN-LOADED LIPOSOMES TO TUMOR-CELLS IN-VIVO

A two-step targeting approach was used to deliver doxorubicin-loaded l iposomes to a murine tumour cell (P388 leukaemia) grown in culture and , more importantly, in vivo. Targeting was mediated through the use of an antibody specific for the Thy 1.2 antigen that is highly expressed on P388 cells. Briefly, the approach consists of prelabeling target c ells with biotinylated anti-Thy 1.2 antibody prior to administration o f drug-loaded liposomes that have streptavidin covalently attached to their surface. Results from in vitro studies demonstrate that a 30-fol d increase in cell-associated lipid and a 20-fold increase in cell-ass ociated doxorubicin can be achieved over control liposomes using this two-step procedure. Flow-cytometry and fluorescent-microscopy data wer e used to confirm that P388 cells can be stably labeled with the bioti nylated anti-Thy 1.2 antibody in vivo. Subsequently, liposome-targetin g studies were initiated in vivo, where target cell binding was assess ed following i.p. or i.v. injection of doxorubicin-loaded liposomes in to animals bearing P388 tumours prelabeled with biotinylated antibody. A streptavidin-mediated 3.7-fold increase in cell-associated lipid an d drug was achieved when the Liposomes were given i.p. When doxorubici n-loaded streptavidin liposomes were injected i.v., P388 cells located in the peritoneal cavity were specifically labeled, although the effi ciency of this targeting reaction was low. Less than a 2-fold increase in cell-associated lipid was achieved through the use of target-speci fic (streptavidin-coated) liposomes. These studies demonstrate that th e presence of a well-labeled target cell population within the periton eal cavity will not promote accumulation of an i.v. injected, targeted liposomal drug. Furthermore, the importance of separating target-cell -specific binding from non-specific uptake by tumour-associated macrop hages is discussed.