In vitro interleukin-3 binding to leukemia cells predicts cytotoxicity of a diphtheria toxin/IL-3 fusion protein

Patients with acute myeloid leukemia frequently develop chemotherapy resist ant blasts. To overcome multidrug resistance, a diphtheria toxin fusion pro tein (DTIL3) was engineered by fusing the catalytic and translocation domai ns of diphtheria toxin (DT) to human interleukin-3 (IL-3). However, when bl asts were isolated from patients and tested for colony growth inhibition by DTIL3, only a third of the samples showed sensitivity to the fusion protei n. Prior to clinical development, we need to be able to identify which pati ents are likely to respond to therapy with DTIL3. In this report, we compar ed the inhibition of thymidine incorporation in human leukemia cell lines b y DTIL3 to the IL-3 receptor number and affinity. We found DTIL3 was cytoto xic to four of the eight cell lines tested with half-maximal inhibition of thymidine incorporation (IC50) from 1 to 50 pM. The IL-3 receptor density f or these cell lines ranged from 0 to 2635 receptors per cell. The dissociat ion constant for an IL-3 high-affinity receptor agonist was 0.5 nM for cell lines with receptors. We found a correlation for the cell lines between th e presence of high-affinity IL-3 receptors and sensitivity to DTIL3 (p = 0. 03). These results suggest the variability in sensitivity of patient leukem ic progenitors to DTIL3 may be due in part to the presence or absence of hi gh-affinity IL-3 receptors.