While imatinib and other tyrosine kinase inhibitors (TKIs) are effective in the treatment of chronic myeloid leukemia (CML), patients can fail all of these therapies. One reason for this is that TKIs have only limited activity against CD34+CD38- leukemic stem cells in CML. Recently, we demonstrated that the IL3 receptor (IL3R) is highly expressed on CD34+38- Bcr-Abl(+) CML stem cells and represents an attractive target for therapeutic intervention (ASH 2009 114: Abstract 2172). IL3R overexpression has also been demonstrated on leukemic stem cells of other hematological malignancies including acute myeloid leukemia (AML) and myelodysplastic syndrome (MDS). Moreover, IL3R is a clinically validated target, as demonstrated by anti-tumor activity including complete and partial responders with the IL3R targeting agent, SL-401, in a Phase I clinical trial of patients with advanced AML and MDS. However, IL3R has not yet been fully investigated as a therapeutic target in CML. Accordingly, in this study, we examined whether targeting IL3R with SL-401 and SL-501, two recombinant biologic agents, could eradicate CML stem cells. SL-401, which is comprised of human IL3 recombinantly conjugated to diphtheria toxin (DT), has been shown to eradicate AML stem cells in both in vitro and in vivo experimental systems. SL-501 is a second generation IL3R targeting agent that contains an optimized IL3 sequence, which increases its affinity for IL3R. Both SL-401 and SL-501 are directed to IL3R on the leukemic cell surface, thereby triggering receptor-mediated endocytosis, inhibition of protein synthesis, and induction of apoptosis. In this study we demonstrated that SL-401 and SL-501 significantly inhibited the growth of CML cells (p ≤ 0.00009) and induced apoptosis (p ≤ 0.003) in 14 primary CML samples. In six primary CML samples, these agents reduced the absolute numbers of viable CD34+/CD38-/CD123+ CML progenitor cells (p ≤ 0.03) by inducing apoptosis (52.2 ± 9.3% and 65.5 ± 10.7% for SL-401 and SL-501, respectively). To evaluate the effect of these agents on the growth of the most primitive stem cells, CML blasts were pretreated with IL3R targeted agents for 24 h and grown in the Long-Term Culture-Initiating Cell assay. SL-401 and SL-501 significantly reduced formation of hematopoietic colonies from primary CML samples in a dose-dependent manner (colony formation reduced by 54.4–80.5% and 66.6–75.1% for SL-401 and SL-501, respectively; p ≤ 0.009). Similar results were obtained when growth of CML progenitor cells was assessed in the Colony Forming Cell assay (colony formation reduced by 54.0–70.2% and 71.1–84.4% for SL-401 and SL-501, respectively; p ≤ 0.0002; N = 6). Notably, the majority of primary samples were obtained from patients resistant to TKIs (12 of 15) and/or harboring an abl mutation including T315I (7 of 15). Moreover, the combination of these IL3R targeted agents with imatinib demonstrated synergistic effects (CI < 1.0) against the KBM5 CML cell line and its TKI resistant KBM5STI subline, which harbors the T315I mutation. In addition, combination of these IL3R directed agents with imatinib further enhanced the apoptotic rate of primary CML cells (N = 5). Importantly, both SL-401 and SL-501 demonstrated high anti-leukemic activity in vivo when NOD/SCID/IL2Rg-KO mice were transplanted with leukemic cells from patients with primary myeloid blast crisis CML (median survival: vehicle = 37 d, SL-401 = 48 d, and SL-501 = 57 d; p = 0.0005). In conclusion, these data indicate that SL-401 and SL-501, two active IL3R directed agents, represent a novel therapeutic modality for the selective targeting of CML stem cells. Combinations of these agents with TKIs may also benefit CML patients who are resistant to TKI treatment by reducing and/or eliminating leukemic stem cells.
Off Label Use: We will discuss the use of SL-401 and SL-501 drugs in CML. Frankel:Stemline Therapeutics: Research Funding. Konopleva:Stemline Therapeutics: Research Funding.
Asterisk with author names denotes non-ASH members.