New methods of therapy of B-cell leukemia in a murine model of prolymphocytic chronic lymphocytic leukemia (BCL?1?) have been evaluated. These have included cytoreductive therapy (splenectomy and total lymphoid irradiation) followed by immunotherapy with an immunotoxin composed of anti-delta and ricin A chain. Treatment of BCL?1?-bearing mice with this immunotoxin resulted in leukemia-free survival for a period of up to a year; however, the long-term follow-ups of these animals revealed that adoptive transfer of their tissues caused leukemia in naive recipients, suggesting that the """"""""cured"""""""" animals were still harboring small numbers of """"""""latent"""""""" tumor cells. In an effort to improve the therapeutic index of immunotoxins, we have developed an approach using ricin A and B chains coupled to different cell reactive antibodies. In additlon, we have coupled the ricin A chain to a cell reactive antibody and the ricin B chain to an anti-antibody. Attachment of B chain to an antibody greatly decreases its galactose-binding ability. Using this dual delivery system, we have demonstrated that we can greatly potentiate the specific cytotoxicity of A chain-containing immunotoxins in vitro. This approach will now be used in the BCL?1? tumor model to investigate the effect of such immunotoxins in vivo. In addition, we have also utilized deglycosylated ricin A and B chains attached to cell reactive antibodies in an effort to eliminate binding of the immunotoxin by cells of the reticuloendothelial system (RES). These immunotoxins effectively kill cells in vitro, and it has been demonstrated by Thorpe et al. that deglycosylated ricin is not cleared by the RES. Finally, recent experiments have been aimed at further attenuating the galactose binding site of the B chain by chemical modification. Preliminary results suggest that we can decrease galactose binding of the B chain without decreasing its ability to synergize with ricin A-containing immunotoxins when it is attached to the appropriate antibody. (HI)
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