Dr. models induced rapid malignant cell mobilization into the peripheral blood (PB). These findings support evaluation of this antibody in AML therapy, with particular appeal to patients resistant to chemotherapy and to unfit patients, unable to tolerate intensive chemotherapy. Introduction CXCR4 is a chemokine receptor highly expressed on multiple cell types including hematopoietic stem cells (HSC), and cancer cells. CXCL12 (also designated as stromal cell-derived factor-1 or SDF-1) is a homeostatic chemokine constitutively secreted by marrow stromal cells, acting as a potent chemo-attractant for immature and mature CXCR4 positive hematopoietic cells, while NIBR189 stimulating their adhesion through integrin activation1C4.CXCL12 also plays an important role in the development and organization of the immune system by regulating the architecture of the lymphoid tissues5, 6. During development, one of the main roles of CXCL12 in myelopoiesis is the migration of progenitors from the fetal liver to the BM. In adults, the CXCL12/CXCR4 pathway mediates retention and homing of hematopoietic stem cells in the BM microenvironment and lymphocyte trafficking7, ITGA2 8. Disruption of CXCL12/CXCR4 interactions results in mobilization of hematopoietic progenitors9C12. Besides its role in cell trafficking, the CXCL12/CXCR4 pathway plays a crucial role in the regulation of cell proliferation and apoptosis13, 14. Indeed, it was shown that knockout of CXCR4 or CXCL12 resulted in HSC NIBR189 proliferation and exhaustion7, 15C17. Acute myeloid leukemia (AML) represents a heterogeneous group of hematopoietic malignancies with different genetic, morphological and clinical characteristics. AML is characterized by the accumulation of malignant precursors of the myeloid lineage in the BM, interfering with the production of normal blood cells. Despite important advances in myelosuppressive chemotherapy and allogeneic transplantation, the majority of adults with AML succumb due to resistant or relapsed disease. In addition, a large number of patients currently experience unacceptable toxicity from currently available chemotherapy which, in many cases, leads patients to opt out or delay receiving treatment. This underscores the need for alternative treatment options for AML patients, with increased tolerability and improved efficacy. Several studies have shown that similarly NIBR189 to normal HSC, primary immature AML cells survival is dependent NIBR189 on the chemokine and growth factor rich microenvironment in the BM, which may prove to be the Achilles heel for AML18. Importantly, this cross-talk with the microenvironment was also demonstrated to play a role in acquired resistance to chemotherapy in minimal residual disease. Overexpression of CXCR4 occurs in approximately 25C30% of AML patients. Interestingly, patients with a high CXCR4 expression in the CD34+ subset of cells have a significantly reduced overall survival and have a greater risk of leukemia relapse19, 20. Therefore, inhibition of CXCR4 has emerged as a potent therapeutic strategy. A small molecule CXCR4 antagonist (AMD3100 or Plerixafor) was approved as a stem cell mobilization agent. When evaluated in combination with cytotoxic chemotherapy in a Phase 1/2 AML studies, AMD3100 mobilized malignant cells from the BM, increasing their sensitivity to chemotherapy. The combination resulted in increased remission, suggesting that long-term diseaseCfree survival after chemotherapy could be improved by this novel combination strategy21. Using patient derived xenograft (PDX) models, in which immunodeficient mice are reconstituted with cells from primary AML patients, it was demonstrated for the first time, that the use of CXCR4 antagonists AMD3100, or the peptide TN140, both known to mobilize cells from the BM as single agents, significantly inhibited AML tumor burden22. Recently, a similar study also demonstrated that a novel peptidic CXCR4 antagonist, LY2510924, administered as a monotherapy, induced mobilization of leukemic cells into the circulation followed by reduction in leukemia tumor burden23. Overall, the main mechanism of action described for the small molecules or peptides antagonists of CXCR4, evaluated in either preclinical or clinical studies, is centered on their ability to mobilize malignant cells from the BM,.