Design of immuno-enzymosomes with maximum enzyme targeting capability: effect of the enzyme density on the enzyme targeting capability and cell binding properties

Immuno-enzymosomes have been proposed for the targeting of enzymes to cance r cells to achieve site specific activation of anticancer prodrugs. Previou sly, we reported that the enzyme beta-glucuronidase (GUS), capable of activ ating anthracycline-glucuronide prodrugs, can be coupled to the surface of inmunoliposomes directed against human ovarian cancer cells (OVCAR-3). This study aimed at the design of an immuno-enzymosome formulation with maximum enzyme targeting capability. By purification of the commercially available enzyme beta-glucuronidase (GUS), a 2-fold increase in the enzyme specific activity and a 4-fold increase in the enzymatic activity of immuno-enzymoso mes was achieved. As a result, upon incubation with human ovarian cancer ce lls (OVCAR3), cell-associated enzymatic activity increased correspondingly. The optimized immuno-enzymosomes were shown to bind to the target cells in a specific fashion. Above a GUS/Fab' molar ratio of 0.5, impairment of the target cell binding ability of the immuno-enzymosomes was observed. This w as likely due to a steric hindrance effect mediated by the presence of larg e amounts of bulky GUS molecules on the liposome surface. Nevertheless, inc reasing the GUS density on the surface of the immuno-enzymosomes to levels by far exceeding the GUS/Fab' molar ratio of 0.5, yielded a considerably Im proved enzyme targeting capability. (C) 1999 Elsevier Science B.V. All righ ts reserved.