The CXCR4 Antagonist BL-8040 Efficiently Induces Apoptosis and Inhibits The Survival Of AML Cells

The chemokine CXCL12 and its receptor CXCR4 are key players in mediating the interactions between the bone marrow (BM) microenvironment and Acute Myeloid Leukemia (AML) cells. CXCL12, which is constitutively secreted from the BM stroma and AML cells, is critical for the survival and retention of AML cells within the BM. CXCR4 expression is associated with poor prognosis in AML patients with or without a mutated FLT3 gene. Antagonists to CXCR4 inhibit migration of AML cells, induce mobilization of AML cells into the circulation and enhance anti-leukemic effects of chemotherapy in mice models. The hypothesis that CXCL12/CXCR4 interactions contribute to the resistance of AML cells to signal transduction inhibitor, and chemotherapy-induced apoptosis, is currently being tested in a series of clinical trials in humans.

In the present study, the effect of the high affinity CXCR4 antagonist BL-8040 (BKT140) and AMD3100 (Mozobil) alone and in combination with ARA-C or the FLT-3 inhibitor AC220 on the survival and proliferation of AML cells in-vitro was examined. In the in-vitro study HL60 (FLT3-WT), MV4-11 (FLT3-ITD) cell lines and human primary AML cells from patients with FLT3-ITD mutations and FLT3-WT genewere used. Cells were incubated for 48 hrs in the presence of BL-8040 (8µM-20µM), AMD3100 (20µM), ARA-C (10-200 ng/ml) and AC220 (0.5-50nM). The level of viable cells, percentage of apoptosis and cell cycle were evaluated by FACS using propidium iodide and 7-AAD. In-vivo, we used NOD scid gamma (NSG) mice engrafted with human primary AML blasts and explored the effects of single injection of BL-8040 on the mobilization and survival of the blasts in the blood and the BM of the engrafted mice.

In-vitro, treatment of MV4-11 cells (FLT3-ITD) with BL-8040, unlike treatment with AMD3100, directly inhibited cell growth by 35% and increased cell death by 39%. Furthermore, in-vitro, treatment of primary AML cells (FLT3-ITD) with BL-8040 directly inhibited cell growth by 28-47% and increased cell death by 75-100%. A combination of BL-8040 with AC220 or ARA-C further increased the apoptotic effect of these agents achieving a 96% reduction in cell viability and inducing cell death by 70- 90% of AML cells. When we studied the in-vitro effect of these agents on FLT3-WT cells (HL-60 cell line and primary AML cells), we found that BL-8040 inhibits cell growth by 16-50%. Unlike the FLT3-ITD cells, in the FLT3-WTcells we did not observe additive effects on cell growth for the combined treatments of BL-8040 with AC220. The combined treatment of BL-8040 with ARA-C was found to further increase the percentage of AML cell death. Moreover, BL-8040 decreases the percent of cycling cells by reducing the number of cells in G2/M+S phase while increasing the number of apoptotic cells in sub-G0 phase. It is interesting to mention that the migration of all tested AML cells toward CXCL12 was entirely inhibited by BL-8040. In-vivo, we found that a single injection of BL-8040 (100μg/mice) into NSG mice engrafted with human AML cells, induces rapid mobilization of AML cells to the periphery within 4 hrs after injection (an 8 fold increase from the control). When mice were administered with 5 consecutive injections of BL-8040 (400μg/mice), a reduction in the number of AML blasts in the blood was observed (40-60% reduction) with induction of AML cell apoptosis within the BM and in the blood by 10-20-fold, compared to the control.

The CXCR4 antagonist BL-8040 was found to rapidly and efficiently induces cell death of AML cells both in-vitro and in-vivo. These results suggest potential therapeutic advantages of BL-8040 in both FLT3-positive and negative AML patients by targeting not only AML anchorage in the BM but AML survival as well. Furthermore, it could provide a rational basis for BL-8040 therapy in combination with ARA-C and the FLT3 inhibitor AC220.

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