Chemotherapy is the primary treatment modality for AML patients. Although most patients show initial responses to chemotherapeutic agents, the majority of patients relapse, suggesting that more primitive cells in AML are resistant to chemotherapy-induced apoptosis. XIAP, a potent cellular caspase inhibitor, is highly expressed in various tumor cells and leukemic cells and contributes to chemoresistance. However, the expression levels of XIAP in early progenitor/stem cells of AML patient are unknown. To address this question, we collected blasts from bone marrow or peripheral blood of AML patients (n=11) and separated them into CD34+/CD38+ and CD34+/CD38− populations by fluorescent-activated cell sorter after immunostaining. RNA was isolated and XIAP mRNA levels were determined by Taq-Man quantitative RT-PCR in these two cell populations and compared with levels from normal CD34+/CD38+ and CD34+/CD38− cells (n=13). Our results demonstrated that XIAP is highly expressed in both progenitor (CD34+/CD38+) and stem cell (CD34+/CD38−) compartments in primary AML blasts, with XIAP mRNA levels higher in these two cell populations in AML than in the corresponding normal bone marrow progenitor/stem cells (p=0.05 and 0.08 for CD34+/CD38+ and CD34+/CD38− compartments, respectively). Our previous studies showed that inhibition of XIAP expression by antisense oligonucleotide and its suppression by small molecule chemicals induce apoptosis of AML cells. 1396–11 and 1396–34, polyphenylurea-based small molecule XIAP inhibitors bind to and inhibit the BIR2 domain of XIAP. 1396–34 was previously tested in AML (

Carter, et al. Blood 105: 4043, 2005
). Previous studies of 1396–11 demonstrated anti-tumor activity against human solid tumor xenografts in mice with no apparent hematopoietic or liver toxicities (
Schimmer, et al. Cancer Cell 5: 25, 2004
;
Berezovskaya, et al. Cancer Research 65: 2378, 2005
). We treated OCI-AML3, HL-60, Jurkat, and U937 leukemia cells with 1396–11 and found it to be more potent than 1396–34 in all leukemia cell lines tested. In addition, we treated AML blasts from 5 patients with 1396–11 and its inactive control 1396–28. Apoptosis was determined in total blasts and in both the CD34+/CD38+ and CD34+/CD38− compartments. Our results showed that 1396–11 induced cell death equally in bulk blasts, CD34+/CD38+ progenitor, and CD34+/CD38− stem cells. Four out of 5 AML samples showed significant apoptosis with ≤ 10 μM 1396–11. The control compound 1396–28 had minimal effect on cell survival in all 5 patient samples tested. Our studies illustrated that XIAP is overexpressed in AML progenitor/stem cells and that these cells are sensitive to a small molecule XIAP inhibitor, suggesting XIAP is a target for AML therapy, with the potential of eradicating primitive leukemic progenitor/stem cells.

Topics:
antineoplastic agents, antisense oligonucleotides, birc4 gene, caspase inhibitors, chemotherapy regimen, leukemia, leukemic cells, neoplasms, quantitative real-time polymerase chain reaction, rna

Disclosures: NIH AML PO1 CA55164.

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2006, The American Society of Hematology
2006
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