Targeting myeloid leukemia with a DT₃₉₀-mIL-3 fusion immunotoxin: Ex vivo and in vivo studies in mice

The IL-3 receptor was expressed on a high frequency of myeloid leukemia cells and also on hematopoietic and vascular cells. We previously showed that a recombinant IL-3 fusion immunotoxin (DT₃₉₀IL-3) expressed by splicing the murine IL-3 gene to a truncated diphtheria toxin (DT₃₉₀) gene selectively killed IL-3R⁺ ex-pressing cells and was not uniformly toxic to uncommited BM progenitor cells. Thus, we explored the feasability of using DT₃₉₀IL-3 as an antileukemia agent. DT₃₉₀IL-3 was toxic when administered to mice at doses as low as 0.1 μg/day. The dose limiting toxicity appeared to be related to platelet and bleeding effects of the fusion toxin. Because of these effects, DT₃₉₀IL-3 was studied ex vivo as a means of purging contaminating leukemia cells from BM grafts in a murine autologous BM transplantation. In this setting, as few as 1000 IL-3R-expressing, bcr/abl transformed myeloid 32Dp210 leukemia cells were lethal. An optimal purging interval of 10 nM/l for 8 h eliminated leukemia cells from 32Dp210/BM mixtures given to lethally irradiated (8 Gy) C3H/HeJ syngeneic mice. Mice given treated grafts containing BM and a lethal dose of 32Dp210 cells survived over 100 days while mice given untreated grafts did not survive (P < 0.00001). DT₃₉₀IL-3 may prove highly useful for ex vivo purging of lethal malignant leukemia cells from autologous BM grafts.