RICIN FUSION TOXIN TARGETED TO THE HUMAN GRANULOCYTE-MACROPHAGE COLONY-STIMULATING FACTOR-RECEPTOR IS SELECTIVELY TOXIC TO ACUTE MYELOID-LEUKEMIA CELLS

Treatment failure of patients with acute myelogenous leukemia (AML) is frequently due to the development of multidrug resistance phenotype b lasts. We have expressed a fusion protein consisting of human granuloc yte-macrophage colony stimulating factor (GMCSF) fused to the N-termin us of a lectin-deficient ricin toxin B chain (RTB) in Spodoptera frugi perda insect cells. The fusion protein was purified by immunoaffinity chromatography and reassociated with chemically deglycosylated ricin t oxin A chain (RTA). The resulting fusion toxin was found to react with antibodies to GMCSF, RTB and RTA and had the predicted molecular mass of 80 kDa. GMCSF-ricin bound poorly to asialofetuin (Kd = 10(6) M-1) and receptor negative cells indicating toss of lectin activity, but bo und strongly to GMCSF receptor positive HL60 cells. Ligand displacemen t assays showed fusion toxin affinity 2.6-fold less than native GMCSF. Selective inhibition of protein synthesis was observed on receptor po sitive cells. Induction of apoptosis was also observed on receptor pos itive cells. Cells expressing multidrug resistance gene products (P-gp , Bcl2 and BclX(L)) were also sensitive to fusion toxin. These results suggest that GMCSF-ricin deserves further preclinical development. (C ) 1997 Elsevier Science Ltd.