Erythrocytes as carriers of chemotherapeutic agents for targeting the reticuloendothelial system

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Abstract
The object of this work was to define a model using hypotonically loaded erythrocytes as a vehicle to target drugs to the reticuloendothelial system (RES). The optimum hemolytic event was found to occur at 100 mOsm/kg using a 0.5-min exposure at 0°C. Approximately one third of the total volume of the cells could be replaced with hypotonic drug solutions under these conditions. Although cytosine-β-D-arabinofuranoside, ara C, is membrane permeable and could not be entrapped in the erythrocytes, phosphorylation of this nucleoside antimetabolite enabled it to be loaded efficiently. Actinomycin D could be loaded and retained within the cells at 0°C, but 90% of this loaded drug leaked out of the erythrocytes in 1 min at 37°C. Actinomycin D-DNA complexes, however, could be loaded and retained for longer periods. In this case, 50% of the DNA-bound drug was retained in the cells for one hour at 37°C. It was found that the glycopeptide antitumor antibiotic, bleomycin, could be entrapped and retained in the cells without appreciable leakage. It was possible to load a human therapeutic dose of this drug in 1–2 ml of packed cells. Furthermore, it was demonstrated that bleomycin entrapped in erythrocytes was significantly more effective than the same dose of free drug in suppressing the phagocytic function of the RES in Balb/C and C3H mice. The rationale is discussed for the possible use of these drugs, entrapped in erythrocytes, for the production of RES blockade in the treatment of disorders in man.