Treatment of acute lymphoblastic leukemia with an rGel/BLyS fusion toxin

Acute lymphoblastic leukemia (ALL) is the most common malignancy affecting children and a major cause of mortality from hematopoietic malignancies in adults. A substantial number of patients become drug resistant during chemotherapy, necessitating the development of alternative modes of treatment. rGel (recombinant Gelonin)/BlyS (B-lymphocyte stimulator) is a toxin-cytokine fusion protein used for selective killing of malignant B-cells expressing receptors for B-cell-activating factor (BAFF/BLyS) by receptor-targeted delivery of the toxin, Gelonin. Here, we demonstrate that rGel/BLyS binds to ALL cells expressing BAFF receptor (BAFF-R) and upon internalization, it induces apoptosis of these cells and causes downregulation of survival genes even in the presence of stromal protection. Using an immunodeficient transplant model for human ALL, we show that rGel/BLyS prolongs survival of both Philadelphia chromosome-positive and negative ALL-bearing mice. Furthermore, we used AMD3100, a CXCR4 antagonist, to mobilize the leukemic cells protected in the bone marrow (BM) microenvironment and the combination with rGel/BLyS resulted in a significant reduction of the tumor load in the BM and complete eradication of ALL cells from the circulation. Thus, a combination treatment with the B-cell-specific fusion toxin rGel/BLyS and the mobilizing agent AMD3100 could be an effective alternative approach to chemotherapy for the treatment of primary and relapsed ALL.