TARGETING OF STEALTH LIPOSOMES TO ERBB-2 (HER 2) RECEPTOR - IN-VITRO AND IN-VIVO STUDIES/

Long-circulating (stealth) liposomes coated with polyethylene glycol ( PEG), which show reduced uptake by the reticuloendothelial system (RES ) and enhanced accumulation in tumours, were used for conjugation to m onoclonal antibodies (MAbs) as a drug-targeting device. A MAb (N-12A5) directed against erbB-2 oncoprotein, a functional surface antigen, wa s used. Amplification and overexpression of the erbB-2 gene product, b eing unique to malignancy confer onto this antibody-mediated therapy h igh tumour specificity. In vitro binding of [H-3]cholesteryl ether ([H -3]Chol ether) labelled anti-erbB-2 conjugated liposomes to N-87 cells (erbB-2-positive human gastric carcinoma) was compared with the bindi ng of non-targeted liposomes and indicated a 16-fold increase in bindi ng for the targeted liposomes. No difference in binding to OV1063 cell s (erbB-2-negative human ovary carcinoma) was observed. These results indicate highly selective binding of antibody-targeted liposomes to er bB-2-overexpressing cells. Despite increased cell binding, doxorubicin (DOX) loaded in anti-erbB-2-conjugated liposomes did not cause increa sed in vitro cytotoxicity against N-87 cells, suggesting lack of lipos ome internalisation, In vivo, the critical factor needed to decrease t he non-specific RES uptake and prolong the circulation time of antibod y-conjugated liposomes is a low protein to phospholipid ratio (< 60 mu g mu mol(-1)). Using these optimised liposome preparations loaded wit h DOX and by monitoring the drug levels and the [H-3]Chol ether label, biodistribution studies in nude mice bearing subcutaneous implants of N-87 tumours were carried out. No significant differences in liver an d spleen uptake between antibody-conjugated and plain liposomes were o bserved. Nevertheless, there was no enhancement of tumour liposome lev els over plain liposomes. Both liposome preparations considerably enha nced DOX concentration in the tumour compared with free drug administr ation. Therapeutic experiments with N-87 tumour-bearing nude mice indi cated that anti-tumour activity of targeted and non-targeted liposomes was similar, although both preparations had an increased therapeutic efficacy compared with the free drug. These studies suggest that effic acy is dependent on drug delivery to the tumour and that the rate-limi ting factor of liposome accumulation in tumours is the liposome extrav asation process, irrespective of liposome affinity or targeting to tum our cells.