THE TARGETING OF IMMUNOLIPOSOMES TO TUMOR-CELLS (A431) AND THE EFFECTS OF ENCAPSULATED METHOTREXATE

Immunoliposomes have been prepared from lipid mixtures of dipalmitoylp hosphatidylcholine, wheat germ phosphatidylinositol and a reactive lip id (the m-maleimidobenzoyl-N-hydroxysuccinimide derivative of dipalmit oylphosphatidylethanolamine) which was conjugated to the N-succinimidy l-S-acetylthioacetate (SATA) derivative of a monoclonal antibody (H17E 2) raised to human placental alkaline phosphatase (PLAP). The immunoli posomes were prepared by the extrusion technique (VETs) and by reverse phase evaporation (REVs) and were found to effectively target to immo bilised PLAP and to PLAP or PLAP-like enzyme on the surface of a tumou r cell line (A431) using an ELISA and autoradiography. The extent of b inding to immobilised PLAP was a function of immunoliposomal lipid con centration, the weight-average number of antibody molecules per liposo me (P(w)BAR) and the liposome size. The effectiveness of methotrexate- loaded immunoliposomes in inhibiting the proliferation of A431 cells w as investigated relative to equivalent levels of the free drug. The im munoliposomes prepared by the extrusion technique (VETs) inhibited gro wth of A431 cells but had no effect on the growth of a normal human fi broblastic cell line. Immunoliposomes prepared by reverse phase evapor ation (REVs) were less effective in inhibiting A431 cell proliferation . The immunoliposomes probably enter the tumour cells largely by recep tor-mediated endocytosis although other mechanisms of uptake cannot be excluded.