CXCR4 Antagonists Mobilize Acute Lymphoblastic Leukemia Cells into the Peripheral Blood and Inhibit Engraftment in Mice.

The role of CXCL12 in the growth of B cell progenitor acute lymphoblastic leukemia (ALL) and the homing of these cells to the bone marrow has been well established. However the effect of modulating CXCL12/CXCR4 interactions on the growth of ALL cells in vivo has not been examined. In this study we used specific peptide and small molecule antagonists of CXCR4 to examine the importance of CXCL12/CXCR4 interactions in the development of leukemia in an in-vivo murine model of ALL. CXCR4 antagonists induced mobilization of human and murine B cell progenitor ALL cells into the peripheral blood, with a 3.8±1.9 and 6.5±3.3 fold increase in leukemic cells/ml one hour after administration of the antagonist respectively, similar to that observed for normal progenitors. Daily administration of AMD3100 commencing the day following the injection of cells and continuing for 21 days resulted in a mean reduction in peripheral blood white cell count of 50±12% and the leukemic cell count of 63±4%. There was also a significant reduction in both the total cells in the spleen of 58±1% and the leukemic cell number in this organ of 75±11%. A significant reduction in leukemic cell numbers in the bone marrow was observed in one (44% reduction) case. There was reduced infiltration of other organs including kidney, liver and skeletal muscle. This study demonstrates that disrupting the CXCL12/CXCR4 axis in B cell progenitor ALL reduces the tumor burden. Whether this is due to direct inhibitory effects on proliferation and survival, or results from disruption of the leukemic cell interactions within the bone marrow remains to be determined.