DT₃₈₈-GM-CSF, a novel fusion toxin consisting of a truncated diphtheria toxin fused to human granulocyte–macrophage colony-stimulating factor, prolongs host survival in a SCID mouse model of acute myeloid leukemia

Abstract

Despite significant advances in the treatment of acute myeloid leukemia (AML), the majority of patients will succumb to drug-resistant AML. To overcome this resistance, we have developed a novel fusion toxin consisting of the catalytic and translocation subunits of diphtheria toxin (DT₃₈₈) linked to human granulocyte–macrophage colony-stimulating factor (GM-CSF). In vitro, DT₃₈₈-GM-CSF demonstrated significant activity against numerous AML cell lines and fresh AML blasts. To determine its in vivo efficacy, we developed an in vivo model of human AML in severe combined immunodeficiency (SCID) mice injected intravenously with 1 × 10⁷ HL-60 cells (AML-M2 cell line). The SCID mice developed abdominal masses, infiltration of the liver and bone marrow, and peripheral blasts with a median survival of 42.5 days. We tested DT₃₈₈-GM-CSF, ara-C, human GM-CSF, and DAB₃₈₉IL-2, which were injected intraperitoneally on days 2–6 in this model. DT₃₈₈-GM-CSF significantly improved survival of the SCID mice over Ara-C, DAB₃₈₉IL-2, or control (P < 0.001). dt₃₈₈-GM-CSF-treated mice who developed leukemia exhibited no difference in the number of GM-CSF receptors (P = 0.39), ligand affinity (P = 0.77), or sensitivity (P = 0.56) to DT₃₈₈-GM-CSF as compared to the controls. Frank leukemia in DT₃₈₈-GM-CSF-treated mice may be due to incomplete penetration of drug into tissues rather than cellular resistance. DT₃₈₈-GM-CSF is an active therapeutic agent in our SCID mouse model of AML with a unique mechanism of action and differing toxicities than current cytotoxic agents.