Diphtheria toxin fused to human interleukin-3 is toxic to blasts from patients with myeloid leukemias

Leukemic blasts from patients with acute phase chronic myeloid leukemic and refractory acute myeloid leukemia are highly resistant to a number of cyto toxic drugs. To overcome multi-drug resistance, we engineered a diphtheria fusion protein by fusing human interleukin-3 (IL3) to a truncated form of d iphtheria toxin (DT) with a (G(4)S)(2) linker (L), expressed and purified t he recombinant protein, and tested the cytotoxicity of the DTLIL3 molecule on human leukemias and normal progenitors. The DTLIL3 construct was more cy totoxic to interleukin-3 receptor (IL3R) bearing human myeloid leukemia cel l lines than receptor-negative cell lines based on assays of cytotoxicity u sing thymidine incorporation, growth in semi-solid medium and induction of apoptosis. Exposure of mononuclear cells to 680 pM DTLIL3 for 48 h in cultu re reduced the number of cells capable of forming colonies in semi-solid me dium (colony-forming units leukemia) greater than or equal to 10-fold in 4/ 11 (36%) patients with myeloid acute phase chronic myeloid leukemia (CML) a nd 3/9 (33%) patients with acute myeloid leukemia (AML). Normal myeloid pro genitors (colony-forming unit granulocyte-macrophage) from five different d onors treated and assayed under identical conditions showed intermediate se nsitivity with three- to five-fold reductions in colonies. The sensitivity to DTLIL3 of leukemic progenitors from a number of acute phase CML patients suggests that this agent could have therapeutic potential for some patient s with this disease.