Hematopoietic stem cell mobilization for gene therapy: superior mobilization by the combination of granulocyte-colony stimulating factor plus plerixafor in patients with β-thalassemia major

Abstract

Successful stem cell gene therapy requires high numbers of genetically engineered hematopoietic stem cells collected using optimal mobilization strategies. Here we focus on stem cell mobilization strategies for thalassemia and present the results of a plerixafor-based mobilization trial with emphasis on the remobilization with granulocyte-colony stimulating factor (G-CSF)+plerixafor in those patients who had previously failed mobilization. Plerixafor rapidly mobilized CD34(+) cells without inducing hyperleukocytosis; however, 35% of patients failed to reach the target cell dose of ≥6×10(6) CD34(+) cells/kg. Four subjects who failed on either plerixafor or G-CSF were remobilized with G-CSF+plerixafor. The combination proved highly synergistic; the target cell dose was readily reached and the per-apheresis yield was significantly increased over initial mobilization, ultimately resulting in single-apheresis collections, despite a more than 50% reduction of the dose of G-CSF in splenectomized patients to avoid hyperleukocytosis. The total stem and progenitor cells mobilized in G-CSF+plerixafor patients were higher than in patients treated by plerixafor alone. Importantly, the G-CSF+plerixafor-mobilized cells displayed a primitive stem cell phenotype and higher clonogenic capacity over plerixafor-mobilized cells. G-CSF+plerixafor represents the optimal strategy when very high yields of stem cells or a single apheresis is required. The high yields and the favorable transplantation features render the G-CSF+plerixafor-mobilized cells the optimal CD34(+) cell source for stem cell gene therapy applications.

Figures

FIG. 1.

Mean CD34+ cell yield per apheresis in splenectomized and nonsplenectomized thalassemic subjects mobilized with plerixafor alone or G-CSF+plerixafor. Each data point represents the mean yield per apheresis of each patient in all evaluable subjects treated with plerixafor and/or G-CSF+plerixafor. The black horizontal line represents the mean CD34+ cell yield per apheresis per mobilization group. *p=0.0003 versus plerixafor. G-CSF, granulocyte-colony stimulating factor.

FIG. 2.

Kinetics of PB CD34+ cells and WBCs in splenectomized patients mobilized by plerixafor and/or G-CSF+plerixafor. The mean values of WBCs (total nucleated cells including erythroblasts,×103/μl) and peripheral blood CD34+ cells (/μl) during mobilization of splenectomized patients are depicted (mean±SEM). PB CD34+ cell counts were not performed on days 2 and 3 in remobilized patients. All evaluable splenectomized patients mobilized with plerixafor alone (n=11) and remobilized with G-CSF+plerixafor (n=3) were included. PB, peripheral blood; WBCs, whole blood cells.