Sensitivity of human acute myeloid leukaemia to diphtheria toxin-GM-CSF fusion protein

The potential to selectively eliminate acute myeloid leukaemia (AML) cells with the GM-CSF-diphtheria toxin fusion protein (DT-GM-CSF) was studied under conditions of autonomous proliferation in vitro with no growth factors (GFs) added and after growth stimulation with a mixture of human (hu)G-CSF, huIL-3 and huSCE DNA synthesis was maximally inhibited after 48 h exposure to DT-GM-CSF, Cell viability and AML colony forming ability in vitro were reduced. 18/22 samples were found to be sensitive to DT-GM-CSF, with 50%, inhibition of DNA synthesis (ID 50 ) at concentrations ranging from 0.1 to 16 ng/ml, and four samples were minimally or not sensitive to DT-GM-CSF (ID 50 ≥ 99 ng/ml). From the 15 samples which showed autonomous proliferation. 13 were sensitive to inhibition of proliferation by DT-GM-CSF. The level of GM-CSF receptor (GM-CSFR) expression was determined by flow cytometry after labelling with specific antibodies for the alpha and beta subunits. Although the toxicity to DT-GM-CSF was specifically mediated by the GM-CSFR, no correlation was found between the level of expression of the GM-CSFR alpha or beta subunit and the sensitivity for DT-GM-CSF. These in vitro studies show that the DT-GM-CSF fusion protein can be used for specifically targeting and eliminating leukaemic cells in the majority of AML cases.