Leukemic blasts from patients with acute myeloid leukemia (AML) frequently express high levels of the interleukin-3 receptor α chain (IL-3Rα). In the present study, we have explored the sensitivity of primary leukemic blasts obtained from 34 patients with AML to a diphtheria toxin (DT) composed of the catalytic and translocation domains of DT (DT388) fused to IL-3 (DT388IL-3) and to DT388 fused to a variant IL-3 with increased binding affinity (DT388IL-3[K116W]). On a molar basis, DT388IL-3[K116W] was significantly more active than DT 388IL-3 in mediating leukemic cell killing. The rate of cell killing induced by the 2 DT/IL-3 fusion proteins was significantly correlated with the level of IL-3Rα/IL-3Rβ expressed on leukemic blasts. These observations support a potential use of DT388IL-3[K116W] in the treatment of refractory AMLs and provide a simple biochemical parameter for the selection of eligible patients.
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