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Cross-linking treatment of loaded erythrocytes increases delivery of encapsulated substance to macrophages.

Previous investigation has shown that osmotically loaded erythrocytes can act as drug carriers in systemic circulation, whereas chemically modified erythrocytes can be targeted to organs of the mononuclear phagocytic system because of changes introduced in the membrane that are recognized by macrophage cells. In this study we have examined the delivery of 125I-labelled carbonic anhydrase (125I-CA) carried by mouse erythrocytes, either loaded, or loaded and cross-linked with bis(sulphosuccinimidyl)suberate (BS3) and 3,3'-dithiobis-(sulphosuccinimidyl propionate), into homologous peritoneal macrophages maintained in culture. The hypotonically loaded mouse erythrocytes show a slight recognition by macrophages, similar to native erythrocytes. CA loaded into erythrocytes is thus delivered to a limited extent into macrophages. Neither the number of recognized loaded 51Cr-labelled erythrocytes nor the amount of delivered 125I-CA is affected by the presence of serum components or IgG. In contrast, cross-linking these loaded erythrocytes results in a greater phagocytosis by macrophages as assessed by microscopic observations, producing a markedly increased amount of targeted enzyme. The amount of CA delivered into macrophages, after BS3 cross-linker treatment of erythrocytes, is dependent on the presence of serum components in the incubation medium. Thus these cross-linking treatments improve the capacity of loaded mouse erythrocytes to deliver significant amounts of targeted enzyme to macrophage cells, increasing the therapeutic potential of carrier erythrocytes.