Increased efficiency in selective elimination of leukemia cells by a combination of a stable derivative of cyclophosphamide and a human B-cell-specific immunotoxin containing pokeweed antiviral protein.

Leukemia cells were mixed with normal human bone marrow cells to simulate bone marrow from leukemia patients; the mixture was then treated with a combination of stabilized derivative of cyclophosphamide [Mafosfamide (ASTA Z 7557)] and pokeweed antiviral protein-containing immunotoxin. The ability of this protocol for selective elimination of B-ALL cells was evaluated by clonogenic assay. The monoclonal antibody (B43) portion of the immunotoxin was directed against human B-cells and was linked to pokeweed antiviral protein by a disulfide bond. The combination of ASTA Z 7557 and immunotoxin was superior to either ASTA Z 7557 or the immunotoxin alone and produced nearly 7 logs of elimination of leukemia cells from the cell mixtures. About 5 logs of contaminating tumor cells were eliminated from a 200-fold excess of normal marrow under conditions where fewer than 50% of pluripotent stem cells were lost. Moreover, this manipulation did not inhibit subsequent production of pluripotent stem cells in long-term bone marrow cultures, indicating that the more primitive progenitors were not harmed.