High Dose Plerixafor Is Safe and Mobilizes Higher Numbers of CD34+ Cells Compared with Standard Dose Plerixafor

Plerixafor is a bicyclam compound that inhibits the binding of stromal cell derived factor-1 (SDF-1) to its cognate receptor CXCR4. This results in therapid release of CD34+ cells into circulation, which can then be collected by apheresis. Plerixafor is FDA approved at the 240 μg/kg dose to be used in conjunction with G-CSF to mobilize autografts for transplantation. Allogeneic grafts can also be mobilized using single agent plerixafor without G-CSF, and following transplantation, result in sustained donor derived hematopoiesis. However, when the 240 μg/kg dose is used, 1/3 of donors fail to mobilize minimally acceptable doses of CD34+ cells. Recently, we demonstrated the safety of administration of a single dose of 480 μg/kg of subcutaneous (sc) plerixafor in humans. We subsequently conducted a randomized cross-over trial comparing CD34+ mobilization in healthy subjects mobilized with a single dose of sc plerixafor given at either a high dose (480 μg/kg) or a conventional dose (240 μg/kg).

Twenty normal healthy volunteers were randomized and received either a 240 or 480 μg/kg dose of sc plerixafor followed by at least a 2 week wash out period then were administered the other dose of plerixafor. Circulating numbers of leukocytes and CD34+ cells/μlwere measured at multiple time points for 24 hours following each plerixafor injection and the CD34+ AUC over 24 hours was calculated for each subject at each dose level. Peripheral blood colony forming unit (CFU) assays were performed at baseline and 6 hours after plerixafor dosing. Adverse events were graded using CTCAE version 3.A sample size of 20 subjects was determined to have over 90% power to detect an absolute CD34+ count difference of 10/μl using this crossover design and a two-sidedpaired t-test at the 0.05 level.

Twenty-three subjects were enrolled and 20 completed administration of both doses. Peak circulating CD34+ cell numbers (median 31.5 vs 25, p=0.0009), circulating CD34+ cell numbers at 24hrs (median 15.5 vs 9, p<0.0001), and the CD34+ AUC over 24 hours (median 543 vs 411, p<0.0001) were all significantly higher following the administration of the 480 μg/kg plerixafor dose compared to the 240 μg/kg dose. The time to peak CD34+ was also slightly longer after the 480 μg/kg dose (median 10 vs 8 hrs, p=0.011). These differences were not related to the order of administration of the 2 different plerixafor doses. Although GM-CFUs from the peripheral blood at 6hrs following plerixafor were significantly higher compared to baseline levels at both plerixafordoses, there was no dose-effect relationship observed between drug dose and fold increase in GM-CFUs. The incidence and severity of AE's did not differ between lower and higher doses of plerixafor and no grade 3 or greater adverse events occurred at either dose level.

These preliminary data suggest high dose plerixafor can be administered safely and may mobilize more CD34+ cells than standard dose plerixafor. Furthermore, these data suggest mobilization following a single dose of plerixafor and a single apheresis procedure would result in graft collections containing higher CD34+ cell numbers when allogeneic stem cell donors are mobilized with high-dose plerixafor compared to standard-dose.

Off Label Use: Plerixafor, a hematopoietic stem cell mobilizer, is indicated in combination with granulocyte-colony stimulating factor (G-CSF) to mobilize hematopoietic stem cells to the peripheral blood for collection and subsequent autologous transplantation in patients with non-Hodgkin's lymphoma (NHL) and multiple myeloma (MM).