The targeting of immunoliposomes to tumour cells (A431) and the effects of encapsulated methotrexate.

Immunoliposomes have been prepared from lipid mixtures of dipalmitoylphosphatidylcholine, wheat germ phosphatidylinositol and a reactive lipid (the m-maleimidobenzoyl-N-hydroxysuccinimide derivative of dipalmitoylphosphatidylethanolamine) which was conjugated to the N-succinimidyl-S-acetylthioacetate (SATA) derivative of a monoclonal antibody (H17E2) raised to human placental alkaline phosphatase (PLAP). The immunoliposomes were prepared by the extrusion technique (VETs) and by reverse phase evaporation (REVs) and were found to effectively target to immobilised PLAP and to PLAP or PLAP-like enzyme on the surface of a tumour cell line (A431) using an ELISA and autoradiography. The extent of binding to immobilised PLAP was a function of immunoliposomal lipid concentration, the weight-average number of antibody molecules per liposome (Pw) and the liposome size. The effectiveness of methotrexate-loaded immunoliposomes in inhibiting the proliferation of A431 cells was investigated relative to equivalent levels of the free drug. The immunoliposomes prepared by the extrusion technique (VETs) inhibited growth of A431 cells but had no effect on the growth of a normal human fibroblastic cell line. Immunoliposomes prepared by reverse phase evaporation (REVs) were less effective in inhibiting A431 cell proliferation. The immunoliposomes probably enter the tumour cells largely by receptor-mediated endocytosis although other mechanisms of uptake cannot be excluded.